Lignites of North America

LIGNITES OF NORTH AMERICA COAL SCIENCE AND TECHNOLOGY Series Editor: Larry L. Anderson Department of Fuels Engineeri...

Author: Harold H. Schobert

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LIGNITES OF NORTH AMERICA

COAL SCIENCE AND TECHNOLOGY Series Editor:

Larry L. Anderson Department of Fuels Engineering, University of Utah, Salt Lake City, UT 84112, U.S.A. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol.

l: Geochemistry of Coal (Bou~ka) 2: Fundamentals of Coal Benefication and Utilization (Tsai) 3: Coal: Typology, Chemistry, Physics and Constitution (Van Krevelen) 4: Coal Pyrolysis (Gavalas) 5: Free Radicals in Coals and Synthetics Fuels (Petrakis and Grandy) 6: Coal Combustion Chemistry-Correlation Aspects (Badin) 7: The Chemistry of Coal (Berkowitz) 8: Natural Gas Substitutes from Coal and Oil (Qader) 9: Processing and Utilization of High-Sulfur Coals (Attia, Editor) 10: Coal Science and Chemistry (Volborth, Editor) 11:1987 International Conference on Coal Science (Moulijn, Nater and Chermin, Editors) Vol. 12: Spectroscopic Analysis of Coal Liquids (Kershaw, Editor) Vol. 13: Energy Recovery from Lignin, Peat and Lower Rank Coals (Trantolo and Wise, Editors) Vol. 14: Chemistry of Coal Weathering (Nelson, Editor) Vol. 15: Advanced Methodologies in Coal Characterization (Charcosset, Editorassisted by Nickel-Pepin-Donat) Vol. 16: Processing and Utilization of High-Sulfur Coals III (Markuszewski and Wheelock, Editors) Vol. 17: Chlorine in Coal (Stringer and Banerjee, Editors) Vol. 18: Processing and Utilization of High-Sulfur Coals IV (Dugan, Quigley and Attia, Editors) Vol. 19: Coal Quality and Combustion Performance: An International Perspective (Unsworth, Barratt and Roberts) Vol. 20: Fundamentals of Coal Combustion for Clean and Efficient Use (Smoot, Editor) Vol. 21: Processing and Utilization of High-Sulfur Coals V (Parekh and Groppo, Editors) Vol. 22: Atmospheric Fluidized Bed Coal Combustion. Research, Development and Application (Valk, Editor) Vol. 23: Lignites of North America (Schobert)

COALSCIENCEAND TECHNOLOGY23

L/GNITES OF NORTH AMERICA HAROLD H. SCHOBERT Fuel Science Program, The Pennsylvania State University, University Park, PA 16802-2303, U.S.A.

1995 ELSEVIER Amsterdam - Lausanne - New York-

Oxford-

Shannon - Tokyo

ELSEVIER SCIENCE B.V. Sara B urgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

ISBN: 0-444-89823-9

9 1995 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the USA. This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands

This book is dedicated to the memory of two lignite researchers who made numerous contributions to lignite science and technology, who freely gave of their time and knowledge to answer questions and to help me, and, most importantly, who were my friends--

Wayne Kube Merle Fegley

This Page Intentionally Left Blank

vii

PREFACE

This book is intended to provide a comprehensive survey of the origin, the fundamental properties, and the technology of utilization of the lignites of North America. I have presumed that the most likely users of this book will be professional scientists and engineers working in some field of coal research or coal technology. Thus I have made no attempt to explain terms that I consider likely to be part of the working vocabulary of such persons. I hope that most persons just coming into the field, with little or no prior exposure to the terminology and conventions of the field, will nevertheless find most of the material reasonably accessible. Coals of course display a continuum of properties, often with no sharp, step change between ranks. In writing about lignites of North America, it would have been all too easy to allow the manuscript to balloon to even larger size by including throughout the text comparisons of the properties and behavior of lignites with those for coals of other ranks. In an effort to keep an enormous amount of information under some sort of control, I have, to a great extent, restricted the discussion strictly to lignites, although I have included some occasional comparisons with other coals, especially brown coals and subbituminous coals. Although there are some interesting distinctions among coals based on their geographic distribution, in many situations coals of comparable rank and composition show similar behavior regardless of their geographic source. A second constraint, therefore, in writing about lignites of North America has been the exclusion of most (but not all) information about lignites from other parts of the world. As I have done in comparing lignites with other ranks of coals, I have occasionally made reference to results on lignites from elsewhere in the world, mainly to point out how North American lignites often show similarities to other lignites. Much excellent work on lignites is being done around the world, and the North American coal scientist or engineer could profit from following it. As this book began to take shape, it seemed to me that only the most ardent of lignitophiles would ever be likely to read it straight through. Rather, I suspect that many persons will dip into the selected chapters or sections most relevant to their immediate interests. For that reason, I have tried to make extensive references to the original work. This book has 1,830 citations to the literature. Unfortunately, much of that original work has often appeared previously only in "obscure government reports" (to borrow a charming phrase coined by a reviewer of one of my papers). I have tried to include sufficient information on experimental conditions and scale of work (that is, whether bench-, pilot-, or commercial-scale) so that the reader can judge whether or not the material is pertinent to his or her interest, and, if appropriate, follow it further into the original literature. The very extensive index is a further effort to make the contents of the book easily

viii accessible to the reader. We who work on coal suffer from having no systematic nomenclature for naming coals. Some authors name the lignite on the basis of seam, others on the basis of formation, others on the basis of its county of origin, and still others on the basis of the mine. Furthermore, some mines have been closed since the lignite was studied and reported on, and in some states or regions the names of geological formations and units have been changed over the years. In some papers, the authors refer simply to, e.g., "a North Dakota lignite," or to the even more awful "a lignite." As a result, the coal literature is a welter of confusing, occasionally colorful, and sometimes bizarre, names. One option for dealing with this mess would have been to impose my own system of order, to decide, for example, that all lignites would be named on the basis of the mine, or, say, on the county from which they come. It seemed to me, however, that the potential for the inadvertent introduction of error was enormous. I have instead reported the name of the lignite as it was used in the original literature. To be sure, this approach perpetuates a mess, but obviates the potential for generating an even more colossal mess by giving some lignites the names of things they are not. I have reported numerical data almost entirely in SI units, except for temperatures, which are generally given as degrees Celsius rather than kelvins. For readers who have been steeped in English units and find SI units strange, I would say simply that it's time to learn. The only situations in which English units have been retained are equations that have constants or coefficients that were derived from the fitting of original data that also had English units. In such cases I felt as I did with the issue of nomenclature--the prospects for inadvertent introduction of error by an attempted "correction" outweighed the discomfort of retaining the obsolete units. In those instances I have tried clearly to state in the accompanying text the appropriate units to use. The writing of this book has been a long and arduous undertaking. Throughout, I have been helped greatly by many persons who have given me access to materials in their personal collections, who have answered questions, and who have given advice on miscellaneous computer problems. I particularly thank my present and former colleagues at Penn State, and the members of what used to be the Coal Science Division ( a group for whom the term used above, "occasionally colorful, and sometimes bizarre," is most apt) at what used to be the University of North Dakota Energy Research Center. I am pleased to acknowledge a pleasant and, to me, quite helpful correspondence with Olav Schmitz of Elsevier, and to several generations of patient Elsevier editors, several of whom, I think, gave up in despair. This book has been prepared entirely on my own time with my own resources. Any statements of opinion, or any editorializing, are mine alone, and do not necessarily reflect the positions either of The Pennsylvania State University or of Elsevier. Of course I take full responsibility for any errors in the book. Harold H. Schobert

University Park, Pennsylvania

iX

CONTENTS

Preface .........................................................................................

C h a p t e r 1. T h e p r i n c i p a l lignite d e p o s i t s o f N o r t h A m e r i c a ............................. 1.1. 1.2.

1.3.

Introduction The

Gulf

....................................................................

region

1.2.1.

Texas

1.2.2. 1.2.3.

lignites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 4

.................................................................

4

Alabama

..............................................................

12

Arkansas

.............................................................

15

1.2.4.

Louisiana

.............................................................

16

1.2.5.

Mississippi

...........................................................

18

1.2.6.

Tennessee

The

Fort

Union

............................................................

region

1.3.1.

North

1.3.2.

Montana

Dakota

.........................................................

19 19

........................................................

19

..............................................................

28

1.3.3.

Saskatchewan

........................................................

34

1.3.4.

South

........................................................

35

Dakota

1.4. O t h e r l i g n i t e s o f the U n i t e d S t a t e s ............................................ 1.4.1.

California

.............................................................

1.4.2.

The

Rocky

Mountains

1.4.3.

The

Plains

states

The

Northwest

1.4.4. 1.5.

vii

Other

lignites

Ontario

1.5.2.

British

1.5.3.

The

1.5.4.

Nova

References

..............................................

41 42

.......................................................

42

.......................................................

43

................................................................

43

Columbia

Canadian Scotia

....................................................

Arctic

................................................

44 45

C h a p t e r 2. T h e d e p o s i t i o n a n d f o r m a t i o n o f lignite ........................................ Introduction

2.2.

Paleoflora

44

..........................................................

.............................................................................

2.1.

39

.....................................................

of Canada

1.5.1.

39

46

51

......................................................................

51

........................................................................

51

2.2.1. N o r t h e r n G r e a t P l a i n s p a l e o f l o r a ..................................

51

2.2.2.

52

Gulf Coast

paleoflora

...............................................

2.3.

Environments

of deposition

...................................................

2.3.1.

Fluvial

environments

...............................................

55

2.3.2.

Deltaic

environments

................................................

58

.............................................

59

2.3.3.

Lagoonal

2.3.4.

Backswamp

environments

2.3.5.

Marsh

2.3.6.

Lacustrine

environments

environments

.........................................

59

................................................

61

environments

...........................................

2.4. T r a n s f o r m a t i o n o f p l a n t material to lignite ................................... 2.4.1.

Introduction

2.4.2.

Loss

..........................................................

of cellulose

....................................................

2.4.3. S t r u c t u r a l e v o l u t i o n o f l i g n i n r e s i d u e s ............................ 2.4.4. E v o l u t i o n o f the c a r b o n s k e l e t o n .................................

63 65 66 68

2.4.6.

71

Sulfur

................................................................. o f m e t a l i o n s ........................................

.............................................................................

C h a p t e r 3. T h e o r g a n i c s t r u c t u r e o f l i g n i t e s ................................................

3.3.

63

7O

References

3.2.

62

2.4.5. T r a n s f o r m a t i o n s o f o x y g e n f u n c t i o n a l g r o u p s ...................

2.4.7. Accumulation

3.1.

54

Petrography

of lignites

3.1.1.

Lithologic

3.1.2.

Lithotypes

3.1.3.

Macerals

The

carbon

.........................................................

72 74

79 79

..................................................

79

............................................................

81

..............................................................

85

layering

skeleton

............................................................

92

3.2.1.

Aromatic

carbon

.....................................................

92

3.2.2.

Aliphatic

carbon

.....................................................

101

.....................................................................

109

Heteroatoms 3.3.1.

Introduction

..........................................................

109

.............................................

109

..........................................................

113

3.3.2.

Carboxylic

3.3.3.

Humic

acid groups

3.3.4.

Methoxyl

....................................................

114

3.3.5.

Phenolic

groups

.....................................................

115

3.3.6.

Carbonyl

groups

....................................................

116

acids

groups

.........................................

116

..........................................................

118

3.3.7. Ethers other than methoxyl 3.3.8. 3.3.9.

Ester

groups

C e l l u l o s e a n d l i g n i n r e s i d u e s .......................................

3.3.10.

Nitrogen

groups

3.3.11.

Sulfur

3.3.12.

Chlorine

118

....................................................

121

.......................................................

122

.............................................................

123

3.4. T h e t h r e e - d i m e n s i o n a l s t r u c t u r e o f lignite ...................................

123

groups

xi 3.4.1. E v i d e n c e for a colloidal structure ..................................

123

3.4.2. C r o s s l i n k i n g in lignite structures ..................................

124

3.4.3. M o l e c u l e s trapped in the lignite structure .........................

126

3.4.4.

D e p o l y m e r i z a t i o n r e a c t i o n s .........................................

129

.............................................................................

130

References

C h a p t e r 4. F u n d a m e n t a l organic reaction chemistry of lignites ...........................

139

4.1. R e a c t i o n s w i t h c a r b o n m o n o x i d e ..............................................

139

4.1.1. Reactions of c a r b o n m o n o x i d e with lignite .......................

139

4.1.2. S u p p o r t i n g m o d e l c o m p o u n d studies .............................

142

4.1.3. C a r b o n m o n o x i d e - h y d r o g e n m i x t u r e s ..........................

144

4.1.4. C a r b o n m o n o x i d e - steam m i x t u r e s ...............................

145

4.2.

Reactions

with

hydrogen

.......................................................

4.2.1. H y d r o p y r o l y s i s and hydrogasification reactions .................

149

4.2.2. R e a c t i o n s u n d e r liquefaction conditions ...........................

158

4.3. R e a c t i o n s in o t h e r g a s e o u s a t m o s p h e r e s ...................................... 4.3.1.

4.4.

149

Oxidation

160

...................................................

160

4.3.2. R e a c t i o n s with w a t e r or steam ......................................

164

4.3.3. R e a c t i o n s with sulfur c o m p o u n d s ..................................

169

Pyrolysis

reactions

of lignites

4.4.1.

Heats

4.4.2.

Kinetics

.............................................................

of reaction

.....................................................

of pyrolysis

172 172

................................................

173

4.4.3. C o m p a r a t i v e effects of gaseous a t m o s p h e r e s ....................

177

4.4.4. Effects of lignite moisture content on pyrolysis .................

179

4.4.5. Effects of cations on lignite pyrolysis ............................

179

4.4.6. E f f e c t s o f p e t r o g r a p h i c c o m p o s i t i o n ..............................

184

4.4.7. Changes in physical structure a c c o m p a n y i n g pyrolysis ........

186

4.4.8. C h a n g e s in functional groups during pyrolysis .................

188

4.4.9. E v o l u t i o n of products during pyrolysis ..........................

194

References

.............................................................................

207

C h a p t e r 5. T h e i n o r g a n i c c o n s t i t u e n t s o f lignites ..........................................

218

5.1. I n c o r p o r a t i o n o f m a j o r e l e m e n t s ..............................................

218

5.1.1. A c c u m u l a t i o n of inorganic c o m p o n e n t s ..........................

218

5.1.2.

Chemical

..............................................

220

5.1.3.

The

5.2.

Minerals

in

fractionation

major elements lignites

5.2.1.

Introductory

5.2.2.

Sulfide

.................................................

230

.............................................................

239

overview

..............................................

239

.....................................................

242

minerals

xii

5.3.

minerals

.......................................................

244

5.2.3.

Oxide

5.2.4.

Carbonate

minerals

..................................................

245

5.2.5.

Phosphate

minerals

.................................................

246

5.2.6.

Sulfate

minerals

.....................................................

246

5.2.7.

Silicate

minerals

.....................................................

247

5.2.8.

Oxalate

minerals

.....................................................

250

..................................................................

250

Trace

elements

5.3.1.

Introduction

5.3.2.

Alkali

5.3.3.

Alkaline

..........................................................

250

.........................................................

251

metals

.......................................................

253

5.3.4. F i r s t - r o w t r a n s i t i o n m e t a l s .........................................

254

5.3.5.

257

5.3.6.

earths

H e a v y t r a n s i t i o n m e t a l s ............................................. ..........................................................

258

.............................................................

259

Lanthanides

5.3.7.

Actinides

5.3.8.

Group

lib elements

.................................................

263

5.3.9.

Group

III e l e m e n t s

..................................................

263

5.3.10.

Group

IV e l e m e n t s

................................................

263

V elements

.................................................

265

.........................................................

265

............................................................

265

5.4. V a r i a b i l i t y o f i n o r g a n i c c o m p o s i t i o n .........................................

266

5.3.11.

Group

5.3.12.

Chalcogens

5.3.13.

Halogens

5.4.1.

Introduction

..........................................................

266

5.4.2. Factors affecting the inorganic composition of lignites .........

266

5.4.3. Vertical v a r i a b i l i t y within s e a m s ...................................

271

5.4.4.

...................................................

275

5.4.5. State and regional v a r i a b i l i t y .......................................

278

References

In-mine

variability

.............................................................................

C h a p t e r 6. B e h a v i o r o f inorganic c o m p o n e n t s of lignites .................................

281

290

........................................................

290

6.2. R e a c t i o n s o f e x c h a n g e a b l e cations .............................................

293

6.1.

Low-temperature

ashing

6.2.1. R e a c t i o n s during ashing or c o m b u s t i o n ...........................

293

6.2.2. B e h a v i o r of c a l c i u m in lignite liquefaction ........................

297

6.3. M i n e r a l reactions at e l e v a t e d t e m p e r a t u r e s ....................................

298

6.3.1.

Anhydrite

..............................................................

298

6.3.2.

Bredigite

...............................................................

299

..................................................................

299

6.3.3.

Calcite

..................................................

299

6.3.5. G e h l e n i t e - a k e r m a n i t e series ........................................

301

6.3.4.

Feldspathic

minerals

xiii 6.3.6.

Hematite

................................................................

301

6.3.7.

Hercynite

...............................................................

302

6.3.8.

Kaolinite

.................................................................

302

6.3.9.

Magnetite

...............................................................

303

6.3.10.

Pyrite

..................................................................

303

6.3.11.

Quartz

.................................................................

304

silicates

6.3.12.

Sodium

.....................................................

305

6.3.13.

Sulfur

......................................................

305

6.3.14.

V o l a t i l i z a t i o n d u r i n g a s h i n g ........................................

306

6.4. T h e r m a l p r o p e r t i e s o f ashes and slags .........................................

307

retention

6.4.1.

Specific

heat

6.4.2.

Radiative

heat transfer

6.5.

Sintering

6.6.

Ash

........................................................... ................................................

307 307

...........................................................................

309

.........................................................................

316

fusion

6.7. S l a g v i s c o s i t y a n d s u r f a c e tension ..............................................

320

6.7.1. L a b o r a t o r y data on slag viscosity ...................................

320

6.7.2. C a l c u l a t i o n o f viscosity f r o m c o m p o s i t i o n .........................

327

6.7.3. A p p l i c a t i o n s o f viscosity data in lignite processing ...............

335

6.7.4.

335

References

Surface

tension

........................................................

..............................................................................

337

C h a p t e r 7. P h y s i c a l p r o p e r t i e s o f lignites ...................................................

343

7.1.

Bulk

mechanical

7.1.1. 7.1.2.

Friability

Grindability

7.1.5.

7.3.

7.4.

Density

.....................................................

shrinkage

7.1.3. 7.1.4.

7.2.

properties

Volumetric

................................................

..............................................................

343 343 345

..........................................................

351

.............................................................

354

F r e e sliding s h e a r s t r e n g t h .........................................

355

Hardness

...........................................................................

357

7.2.1. D e n s i t y m e a s u r e m e n t s by i m m e r s i o n in fluids ...................

357

7.2.2.

Bulk

357

7.2.3.

Mercury

7.2.4.

Helium

Thermal

density

..........................................................

densities

....................................................

357

.....................................................

358

..............................................................

358

densities

properties

7.3.1.

Specific

heat

7.3.2.

Thermal

conductivity

Surface

358

................................................

359

.......................................................

360

o f s u r f a c e a r e a ........................................

360

area and porosity

7.4.1. M e a s u r e m e n t 7.4.2.

..........................................................

Porosity and pore volume

...........................................

365

xiv 7.4.3.

H y d r a u l i c conductivity ...............................................

368

7.4.4. Effects of heating on surface area and porosity ...................

368

7.4.5. The fractal structure of lignite .......................................

372

References

..............................................................................

C h a p t e r 8. Moisture in lignite .................................................................. 8.1.

Introduction

8.2.

Equilibrium

....................................................................... moisture

............................................................

378

383 383 385

8.3. Measurement of moisture by elevated-temperature methods ................

387

8.4. Evidence for bimodal incorporation of water .................................

391

8.5. Adsorption and desorption of moisture ........................................

396

8.6. Heat of wetting in water .........................................................

405

References

4O6

..............................................................................

Chapter 9. Mining, transportation, and storage ............................................. 9.1.

Lignite mining

....................................................................

410 410

9.1.1. Historical development of lignite mining ..........................

410

9.1.2. Overview of surface mining .........................................

412

9.1.3. Preliminary steps in opening a mine ...............................

415

9.1.4.

O v e r b u r d e n removal

.................................................

416

9.1.5.

L i g n i t e removal

.......................................................

421

9.1.6. M i n e d - l a n d reclamation ..............................................

423

9.1.7. Specific examples of mining practices ..............................

424

9.2.

Transportation

...................................................................

429

9.3.

H a n d l i n g and storage ...........................................................

432

9.3.1.

Handling of lignite ..................................................

432

9.3.2.

Stockpiling

433

...........................................................

9.3.3. Low-temperature air oxidation .....................................

436

9.3.4. Autogenous heating and combustion ..............................

436

9.3.5. Storage of dried lignite ..............................................

AAA

9.3.6.

446

Dust explosions

......................................................

.............................................................................

446

C h a p t e r 10. Beneficiation of lignite .........................................................

451

References

10.1.

Drying

..........................................................................

451

......................................................

451

10.1.2. Entrained and fluidized drying ...................................

457

10.1.3.

H o t - w a t e r drying ..................................................

457

10.1.4. Steam and hot-water drying ......................................

463

10.1.1.

Rotary

drying

xv 10.1.5.

Steam

drying

.......................................................

465

..................................................

467

in hot oil ...................................................

473

10.2. R e m o v a l o f m i n e r a l m a t t e r ...................................................

474

10.1.6.

Fleissner

10.1.7.

Drying

process

10.2.1.

Introduction

........................................................

10.2.2.

Gulf Coast lignites

474

.................................................

475

10.2.3. N o r t h e r n G r e a t Plains lignites ....................................

477

10.3. R e m o v a l of inorganic constituents by ion e x c h a n g e ......................

480

10.3.1.

Introduction

480

10.3.3.

Bench-scale

...................................................

483

10.3.4.

Pilot-scale

Desulfurization

10.5.

Size

10.7.

480

10.3.2. F u n d a m e n t a l s o f ion e x c h a n g e with lignites ....................

10.4.

10.6.

........................................................

tests

.....................................................

486

.................................................................

487

tests

................................................................

489

10.5.1. Size r e d u c t i o n at the m i n e .........................................

489

10.5.2. P u l v e r i z a t i o n for c o m b u s t i o n .....................................

491

10.5.3.

492

preparation

Micropulverization

Briquetting

.................................................

.....................................................................

493

10.6.1. L a b o r a t o r y and pilot-scale studies ................................

493

10.6.2.

............................................

496

..........................................................

497

Commercial

Lignite/water

briquetting

slurries

.............................................

497

10.7.2. R h e o l o g y of l i g n i t e / w a t e r slurries ................................

499

10.7.3. Solids loadings of lignite/water slurries .........................

499

10.7.1.

References

Pilot-scale preparation

.............................................................................

499

o f lignites .........................................................

505

11.1. F u n d a m e n t a l s of lignite c o m b u s t i o n ........................................

505

11.1.1. C a l o r i f i c v a l u e s of lignites ........................................

505

11.1.2. M e c h a n i s m s o f c o m b u s t i o n ......................................

507

11.1.3. I n o r g a n i c transformations during c o m b u s t i o n .................

515

Chapter

11. C o m b u s t i o n

11.2. S m a l l - s c a l e c o m b u s t i o n 11.2.1.

Historical

s y s t e m s ............................................ ............................................

521

11.2.2. D o m e s t i c use o f lignite ............................................

521

11.2.3.

........................................................

522

p r a c t i c e .................................................

527

Stoker

development

521

firing

11.3. C u r r e n t c o m b u s t i o n

11.3.1. Historical d e v e l o p m e n t of pulverized firing of lignite .........

527

11.3.2. Evaluation of lignites as potential fuels for p o w e r plants .....

528

11.3.3. C o m m e r c i a l firing for p o w e r generation ........................

531

xvi 11.3.4. S u r v e y of current commercial practice ..........................

534

11.3.5. Emission control technology

.......................................

543

11.4. A s h deposition, slagging and corrosion ....................................

544

11.4.1.

11.5.

........................................................

544

11.4.2. Factors affecting fouling and slagging ..........................

Introduction

546

11.4.3. T h e m e c h a n i s m of deposition ....................................

56O

11.4.4. G r o w t h of strength in deposits ...................................

572

11.4.5. Remedial measures for combatting fouling and slagging .....

575

11.4.6.

..............................................

582

....................................................

583

.........................................................

583

Ash-related corrosion

Fluidized-bed 11.5.1.

combustion

Introduction

11.5.2.

Combustion

behavior

..............................................

585

11.5.3.

I n - b e d s u l f u r capture

...............................................

585

11.5.4.

NOx

......................................................

588

11.5.5. References

Bed

formation

agglomeration

..................................................

589

.............................................................................

591

C h a p t e r 12. A l t e r n a t i v e uses of lignites .....................................................

602

12.1.

o f lignite .........................................................

602

12.1.1. F u n d a m e n t a l s of gasification .....................................

602

12.1.2. C o m m e r c i a l gasification in North A m e r i c a .....................

614

12.1.3. C o m m e r c i a l gasification outside North A m e r i c a ...............

622

Gasification

12.1.4. Pilot-scale gasification of lignites ................................

624

12.1.5. U n d e r g r o u n d gasification of lignite .............................

637

12.1.6. Biological gasification of lignites ................................

638

12.2. Direct l i q u e f a c t i o n of lignites ................................................

639

12.2.1. T h e E x x o n D o n o r Solvent process ..............................

639

12.2.2.

643

Project

Lignite

12.2.3. T h e C O - S t e a m

...................................................... process ............................................

648

............................................

651

12.2.5. Bench-scale studies for process i m p r o v e m e n t .................

652

12.3. C h e m i c a l products from lignites .............................................

655

12.2.4. T w o - s t a g e

12.3.1.

Montan

12.3.2.

Activated

liquefaction

wax

........................................................

655

...................................................

656

12.3.3. Direct uses of lignites in water treatment ........................

658

12.3.4.

658

Charcoal

carbon

............................................................

12.3.5. Applications in extractive m e t a l l u r g y ............................

659

12.3.6. R e c o v e r y of uranium from lignites ..............................

661

12.3.7.

663

Leonardite

..........................................................

xvii 12.3.8. Prospects for other chemical products from lignites ............

665

12.3.9. A p p l i c a t i o n s of lignite ashes .......................................

666

12.4. C o m b u s t i o n of lignite-water slurries .........................................

666

12.4.1. Slurry preparation for c o m b u s t i o n .................................

666

12.4.2.

Combustion

...........................................

667

12.4.3.

Ash

.........................................................

667

References

Index

performance

behavior

..............................................................................

669

............................................................................................

679

This Page Intentionally Left Blank

Chapter 1

THE P R I N C I P A L LIGNITE DEPOSITS OF NORTH A M E R I C A

1.1 I N T R O D U C T I O N This chapter describes the lignite deposits of North America, providing information on the estimated reserves and resources, the geological setting of the deposits, and the quality of the lignites. Two major deposits of lignites exist in North America. The Gulf Coast lignites occur in the southeastern United States, in deposits lying in a band roughly parallel to the coastline of the Gulf of Mexico, forming an arc stretching from southern Texas across Louisiana and Mississippi into Alabama. The second major deposit is the Fort Union lignite field, lying in the northern Great Plains in portions of North Dakota, Montana, and Saskatchewan. The Gulf Coast and Fort Union lignites represent the largest proportion of reserves and resources of lignites in North America, are the most important commercial sources of lignites, and are those which have been most extensively studied. Hence most of this chapter is devoted to a discussion of these lignites. Smaller deposits of lignites occur outside the Gulf and Fort Union regions. Some information on these lignites is also presented in this chapter. Even though the reserve base of these lignites is much smaller and in many cases the deposits have not been as well studied, some have commercial potential and all are worthy of additional investigation. Estimated lignite reserves and resources in the United States are shown in Table 1.1 [ 1]. In addition, the principal Canadian lignite deposit, in Saskatchewan, includes 35.2 Gt of in-place resources [2]. The remaining reserves of U. S. lignite able to be recovered by strip mining are shown, by state, in Table 1.2 [3]. The same information is shown by coal province in Table 1.3 [3].

TABLE 1.1 U.S. lignite reserves and resources, gigatonnes [1].

Region

Strippable Reserve Base

Fort Union North Dakota Montana South Dakota Total Gulf Texas Alabama Arkansas Louisiana Total Denver Other states Total ........................

Identified Resources

9.2 14.3 0.4 23.9

318.1 102.1 2.0 422.2

7.2 1.0 1.8 0.5 10.5 2.6

47.4 1.8 12.2 0.5 61.9 9.1 0.2 37.0 ............. 493.4

TABLE 1.2 Remaining U. S. lignite reserves recoverable by strip mining, Mt [3]. State Reserves California 23 Colorado 907 Montana 15,168 North Dakota 11,839 South Dakota 300 Texas 8,618 Washington 5 Wyoming 454 Total ............... 37,314

TABLE 1.3 Remaining lignite reserves in coal provinces recoverable by strip mining, Mt [3]. Province Reserves Coastal Plains 8,618 Great Plains 19,456 Pacific Coast- Sierra Nevada 27 Wyoming Basins 9,213 T otal ............................... 37,314

About 10% of all U. S. low-rank coals (including subbituminous) are classified as the strippable

reserve base [4]. For lignites, the strippable reserve base is 37.0 Gt, and the total identified resources are 493.4 Gt [4]. Data on the reserve base of lignites, organized by state, county, bed, and thickness, is available in [5]. The identified resources of U. S. lignites are classified by depth of seam in Table 1.4 and by seam thickness in Table 1.5 [6]. Identified resources are classified by sulfur content in Table 1.6 and by ash in Table 1.7 [6]. In both tables, the data are shown as weight percent on an as-received basis. In the sections which follow, the principal lignite deposits of each state or province are discussed. The first two sections discuss the Gulf and Fort Union region lignites, with shorter sections treating some of the other lignite deposits. Most of the discussion is devoted to the lignites of Texas, North Dakota, and Montana, since these are the most important deposits in commercial use. A few very small deposits of lignites, having no evident commercial potential but of interest for the geochemistry and organic structure of lignites, are treated briefly in other chapters. T A B L E 1.4 U.S. identified lignite resources (gigatonnes) classified by depth [6].

Depth of Lignite Seam (meters) Region 0-305 305-610 Unknown Total Fort Union 333.6 88.5 422.1 Gulf 39.0 23.0 62.0 Denver 9.1 9.1 Other states 0.2 0.2 Totals ..... 381.7 ............. 23.0 ........... 88.7 .......... 493.4

T A B L E 1.5 U.S. identified lignite resources (gigatonnes) classified by seam thickness [6].

Seam Thickness (meters) Region 0-3 3--6 >6 Unknown Total Fort Union 6.1 11.8 10.2 394.1 422.1 Gulf 21.5 2.4 0.2 37.9 62.0 Denver 9.1 9.1 Other states 0.2 0.2 Totals .......... 27.6 ........... 14.2 ........... 10.4 ........... 441.3 ......... 493.4

TABLE 1.6 U.S. identified lignite resources (gigatonnes) classified by sulfur content (weight percent asreceived basis) [6].

Region Fort Union Gulf Denver Other states Totals .........

0-- 1 422.1 58.4 9.1 489.6

Sulfur Content, as-received 1-3 >3 1.8

Unknown

1.8

........ 1.8 ............. 1.8 ...........

0.2 0.2 ...........

Total 422.1 62.0 9.1 0.2 493.4

TABLE 1.7 U.S. identified lignite resources (gigatonnes) classified by ash value (weight percent as-received basis) [6]. ...... Ash, as-received Region 5-10 > 10 Unknown Total Fort Union 422.1 422.1 Gulf 18.2 43.8 62.0 Denver 9.1 9.1 Other states 0.2 0.2 Totals .............. 440.3 ......... 43.8 .......... 9.3 ............ 493.4

1.2 T H E G U L F R E G I O N

LIGNITES

1.2.1 Texas (i) History. The first scientific description of lignite in Texas was published in 1839 [6]. Small-scale production for local use began in the 1850's [7]. Production reached 1.4 Mt in 1914 [8], but then declined steadily to 1950, when it dropped below 18,000 tonnes. Before World War II, lignite was mined in many counties, in a band from Medina County northeastward to Louisiana. Around the turn of the century, lignite mined near Calvert Bluff was blended with bituminous coal as a locomotive fuel [9]. A use of lignite probably unique to Texas was combustion for process heat needed in the production of salt [10]. Mining in the vicinity of Darco (Harrison County) for activated carbon production started in 1931. Over the years lignite was gradually supplanted as a fuel source by natural gas and petroleum. Mining of lignite as fuel ceased in 1946 [11]. Some mining continued, however, for lignite used as a feedstock for activated carbon production. Mining for fuel use resumed in 1955 with the opening of a large power plant at Sandow (Milam County) to supply power to Alcoa for aluminum refining. Production began increasing sharply in the 1960's. (ii) Occurrence. Texas possesses the largest lignite resources of the Gulf Coast states, as shown in Table 1.8 [ 12]:

TABLE 1.8 Gulf Coast lignite resources by state, gigatonnes [12].

State Alabama Arkansas Louisiana Mississippi Tennessee Texas

Resources* 1.27 2.27 1.54 4.54 0.91 21.23

*Estimated in-place tonnage of seams greater than 1 m thick to 60 m deep. The resources of Texas lignite are summarized by geologic unit and region in Table 1.9 [13]. In Texas, lignite occurs in three stratigraphic units of Eocene age: the Wilcox and Jackson Groups and the Yegua Formation [ 14,15]. TABLE 1.9 Texas lignite resources (megatonnes) [ 13]. Geologic Unit Wilcox

Resources Re~ion Near-surface Deep-basin East-Central 5,880 5~,940 Northeast 4,626 0 Sabine Uplift 3,549 4,959 South 978 4,201 subtotal .............. 15,032 ........ 15,100 ....... Jackson East 4,081 5,080 South 687 4,785 subtotal ................ 4,768 ....... 9,124 ....... Yegua East 1,407 0 T O T A L ........................ 21,208 ....... 24,966 .......

Total 11,820 4,626 8,508 5,179 30,133 9,162 5,472 14,634 1,407 46,174

The potential near-surface resources at depths less than 60 m [14], amenable to conventional surface mining, and in seams 0.9 m or thicker, are 21.2 Gt [15]. About 70% of the near-surface lignite occurs in the Wilcox, about 23% in the Jackson and 7% in the Yegua [14]. More than 90% of this lignite occurs in the Wilcox and Jackson Groups north of the Colorado River [ 14]. The average seam thickness is less than 1.5 m, with a seam of 3 m being exceptional [15]. Wilcox lignite is the best quality, having a calorific value of 15.1 MJ/kg, 33% moisture, 1% sulfur, and 15% ash [ 15]. Yegua lignite is intermediate in quality and Jackson is the poorest. Near-surface lignite occurs in three areas: a continuous band running approximately southwest to northeast from the Rio Grande River in Webb County, passing south of San Antonio, Austin and Waco to Texarkana; a second band which is roughly parallel to the first, from the Rio Grande in Starr County to the Angelina River; and an irregularly shaped deposit in Panola County

and the surrounding region [16]. Near-surface lignite occurs in the main lignite-bearing rocks in an area centered on Panola and adjacent counties of the Sabine Uplift. Near-surface lignite also occurs in two bands, a northern band continuous from the Rio Grande River in Webb County to the Red River in Bowie County; and a parallel, but discontinuous, band lying coastward and stretching from the Rio Grande River in Starr County to the Angelina River in Angelina County. The northern band is divided into three regions [17]: South, from Webb to Caldwell counties; Central, from Bastrop through Freestone counties; and East, from Henderson through Bowie counties and the Sabine Uplift. The coastward band has two regions [ 17]: South, Atascosa, LaSalle, McMullen, Starr, and Zapata counties; and Southeast, from Fayette County through Angelina County. The near-surface lignite resource strata are divided into seven units on a geographic basis; resources according to these units are shown in Table 1.10 [18]. The Wilcox Group accounts for 71% of the total tonnage and 77% of the resource on a calorific value basis. TABLE 1.10 Resources of near-surface lignite in Texas, megatonnes [18]. Region Central Wilcox Northeast Wilcox Sabine Uplift Wilcox South Wilcox East Jackson South Jackson East Claiborne Total ...........................

Resource 5,880 4,626 3,549 978 4,081 687 1,407 21,208

The thickest, most laterally continuous deposits are in the Wilcox of central Texas and the central region of the Sabine Uplift, and in the east Jackson. Some Wilcox seams are laterally continuous for 27 km; some Jackson seams, for up to 48 km [18]. Typical seam thickness is about 3 m, though in some areas seams may reach 4.6 m [ 18]. The largest resource blocks are 900 Mt in the east Jackson and 450 Mt in the central Wilcox. Seams in the northeast Wilcox and the northern Sabine Uplift are thinner, usually less than 2.4 m, but resource blocks of up to 450 Mt are present [18]. In the Yegua Formation seams are usually less than 1.8 m and are discontinuous; resource blocks are 136 Mt or less. South of the Colorado River the Jackson Group has resource blocks of up to 272 Mt [ 18]. Deep-basin lignite is that which occurs under more than 60 m of cover [14]. The resources lying between 60 and 1,524 m are 90 Gt [17]. About 32 Gt lie between 60 and 600 m [17]. Of this, 69% is in the Wilcox and 31% in the Jackson [18]. Deep-basin lignite is unimportant in the Yegua. Deep-basin lignite occurs coastward and downdip from the near-surface deposits. The principal deep-basin lignite occurs in central and eastern Texas, in an area running from Gonzales County to Cherokee County. In Cherokee County the deep-basin lignite splits into two bands

which curl around the sides of the Sabine Uplift. The deep-basin lignite in south Texas extends in a band from the common border of Jim Hogg, Starr, and Zapata counties to McMullen County [ 16]. Six small isolated areas of deep-basin lignite have also been mapped. The largest is in Washington County. The largest and most extensive deposits of deep-basin lignite are in the Wilcox Group of central Texas. Deep-basin lignites are thickest and most numerous in Bastrop, Fayette, Houston, Lee, Leon, and Madison counties. The eastern half of Texas is underlain by lignite deposits, which dip very slightly toward the coast. The lignite can be mined by stripping in regions close to the outcrops. The amount economically recoverable to 46 m depths is 6.1 Gt, and to 60 m is 8.1 Gt [19]. Although the grade of the lignite is variable, an average calorific value is about 15.1 MJ/kg [19]. Ash and moisture contents are both usually high. The strippable lignite lies between the two major metropolitan belts in Texas, and has potential for future use as a fuel for industrial process heat, in addition to its present use as a fuel for electric power generation. Strippable reserves are estimated at 1.2 Gt [8]. Lignite occurs in the Coastal Plain of Texas, in a band 965 km long and 80 to 160 km wide running southwest to northeast from the Rio Grande to the Sabine rivers [20]. The band tends to parallel the coast line, lying 160 to 240 km inland. The main lignite deposits in Texas occur in the Wilcox Group. Smaller deposits occur in the Y egua Formation and the Jackson Group. The most extensive resources occur in the Wilcox Group in eastern and central Texas generally north of the Colorado River. Essentially all of the lignites are Eocene age, although a few of the Wilcox lignites south of the Colorado River and in the southern part of the Sabine Uplift may be Paleocene [21]. The lignites occur throughout the lower Tertiary but are especially abundant in the lower and middle Eocene. Lignite occurs in 49 counties of eastern and central Texas [22]. The strata associated with the lignite beds are largely clays and unconsolidated sands. The region has a thick covering of soil, so that exposures of bedrock are rare; consequently, the extent and thickness of the lignite beds is usually determined from shallow drill holes or pits. Individual seams vary widely in thickness and quality. Thicknesses range up to 6 m [8]. Virtually all of the lignites contain less than 2% sulfur; most are below 1% [8]. Calorific values range from 16.3 to 20.9 MJ/kg (as-received basis) [8]. (iii)

Quality. Generally

the differences in grade and continuity of seams can be correlated

with the depositional environment [14]. North of the Trinity River the Wilcox lignite occurs in lenticular seams of moderately high ash value but low sulfur, and a maximum thickness of 4.6 m. This lignite accumulated on an ancient alluvial plain. Stream courses were closely spaced, and interchannel basins were fairly small. The associated peat swamps were of restricted area and were vulnerable to flooding, with attendant influx of sediments. South of the Trinity River and on the Sabine Uplift the interchannel basins were formed lower on the alluvial plain and thus were larger and less likely to experience flooding. The Jackson Group lignites accumulated in marshes on the lower delta plain. Blanket peats could form over extensive areas of the abandoned delta lobes. Because these blanket peats lay more to the seaward, they were more likely to be permeated by sulfur-rich waters. These factors result in the Jackson lignites being more laterally continuous and

of higher sulfur content than the Wilcox lignites. The highest quality lignite occurs in the Lower Eocene Wilcox Group north of the Colorado River. In this region the individual seams are thicker, of more uniform quality, and more persistent than in other locations in Texas. Lower quality lignite of the Wilcox, as well as Upper Eocene Yegua and Manning Formations, is found south of the Colorado River. Most Texas lignite is xyloid; over 50% derives from the woody parts of plants [7]. However, Texas lignite commonly contains considerable amounts of attrital material, from residual remains of plant matter chemically or mechanically broken down to microscopic fragments during the early stages of coalification, as well as some decay-resistant parts of plants. On a dry basis, the ash value of Texas lignites ranges from less than 10% to over 40% [7]. The average ash value is about 16% [7,23]. North of the Colorado River the average is less than 15%; some Wilcox lignites in the southern part of the Sabine Uplift yield less than 10% [7]. Lignites with very high ash, occasionally exceeding 40%, occur south of the Colorado River. The Upper Eocene lignites also show a geographic trend in ash value, ranging from 15-20% ash in east Texas to 35% in south Texas [7]. The average calorific value of Texas lignites is 24.2 MJ/kg on a dry basis [7]; it ranges from 27.9 MJ/kg for east Texas lignites to less than 23.2 MJ/kg for lignite in south Texas [7,23]. This change in calorific value tracks the trends discussed above for the geographic distribution of ash. As a rule, if lignites of comparable ash value are compared, those having the higher calorific value have higher fixed carbon and higher carbon and hydrogen contents. Lignites with calorific values above 25.6 MJ/kg are found in the Sabine Uplift, in Lee County to the Trinity River and in Houston and Trinity counties. Medium calorific values (23.2 to 25.2 MJ/kg) are found in lignites from Trinity River to Bowie County, and in Medina, Bastrop, and Fayette counties. South Texas lignites of Eocene age generally have calorific values less than 23.2 M.l/kg. The average fixed carbon content of Texas lignites is 37%, on a dry basis [7]. North of the Colorado River the fixed carbon of Wilcox lignites ranges from 30 to 50%, but south of the fiver these lignites contain less than 30% fixed carbon [7]. For lignites of Upper Eocene age, the fixed carbon is generally 30-40% in east Texas and decreases to less than 30% in south Texas [7]. The volatile matter content decreases toward the south, consistent with the trend for increasing ash content. Lignites having volatile matter exceeding 50% on a dry basis occur from the Trinity River to the Sabine River, in the southern part of the Sabine Uplift, and in Franklin, Bowie, Bostrop, Houston, and Trinity counties. Volatile matter contents in the range of 40 to 50% (dry basis) are found in lignites of Rains, Wood, Hopkins, Fayette, Medina and Caldwell counties, in the northern part of the Sabine Uplift, and from Lee County to the Trinity River. The South Texas lignites of Eocene age generally have less than 40% volatile matter (dry basis) [7]. Most Texas lignites have sulfur contents below 1.5% [23], although south of the Colorado River the sulfur content of the Wilcox lignite reaches 2% [7]. As a rule the Wilcox lignites north of the Colorado River contain less than 1% sulfur (as-received basis) [7]. Lignites occurring between the Brazos and Trinity Rivers may have sulfur contents in the range of 1-1.5% [7]. Sulfur is

lowest in the northeast, where lignite accumulated in ancient alluvial plains in fresh water swamps. Explanations for the higher sulfur content of the lignites of the Sabine Uplift and east central Texas include a greater degree of marine influence in the depositional environment, a slower rate of accumulation, or a deeper burial and consequent higher temperatures during coalification [24]. For lignites of Upper Eocene age, the sulfur content varies from under 1% in east Texas to 2% in south Texas [7]. On an as-received basis the average moisture content of Texas lignites is about 30% [7]. There is little regional variation in the moisture content. Several useful surveys of Texas lignites, which include extensive analytical data, are available in [7,17,20,24]. (iv) Wilcox Group lignites. The most extensive deposits of lignites, and those of greatest commercial importance, occur in the Wilcox Group. In central Texas and the northeastern Gulf Coastal Plain the Wilcox Group is divided into the Sabinetown, Calvert Bluff, Simsboro, Hooper, and Seguin Formations. The principal lignite deposits occur in the lower part of the Calvert Bluff. The Wilcox Group contains lignite interbedded with sand and clay. The Wilcox is early Eocene in age and non-marine in origin. The total thickness of the strata ranges from 183 to 457 m [25]. Lignite occurs in the upper two-thirds of the Wilcox [26], where it accumulated in hardwood swamps between alluvial ridges [27]. The commercial deposits, in seams 0.6 to 4 m thick, range from 23 to 363 Mt [26]. Generally the lignite seams are lenticular. Few seams extend over more than 26 km 2 [7]. In south Texas the seams are thinner and have more partings than the seams in east Texas. The partings consist of clay, shale, or very poor, highly mineralized grades of lignite called blackjack, bone, or rotten coal. The thicker and more extensive seams are generally more uniform and of better quality than the thinner, irregular seams. This same relationship between extent and thickness of seams and uniformity and quality of the lignite is observed for Montana lignites. The best grade of lignite, and the largest deposits, occur in the Wilcox Group north of the Colorado River [14]. The seam thickness ranges from 0.6 to 6.7 m (typically 0.9-3 m) and may be laterally continuous for up to 24 km [ 14]. North of the Trinity River the Wilcox lignite is of poorer grade and in seams which are both thinner (typically 0.6-1.2 m) and lenticular. Over the years there have probably been over a hundred mines working the Wilcox lignite [ 16]. In central Texas, Wilcox Group lignite is commercially mined at two large strip mines, one at Alcoa in Milam County and the other at the Big Brown electric generating plant in Freestone County. In this region the lignites were deposited in deltaic environments. Wilcox lignite is being mined in east Texas at Darco (Harrison County) for conversion to activated carbon. Here the Wilcox lignite is of fluvial origin. In east-central Texas, the Wilcox is divided into three formations, which are, in ascending order, the Hooper, the Simsboro, and the Calvert Bluff. Lignite is found at the top of the Hooper Formation, and at the bottom and top of the Calvert Bluff Formation. The estimated lignite in the lower and upper Calvert Bluff in east-central Texas is 5.4 Gt

10 [24], based on seams greater than 1 m thick with less than 60 m overburden. Calvert Bluff seams are typically 0.6-3 m thick with a maximum of about 6 m [24]. Some seams have been correlated for 32 km. Sixteen lignite seams are found in the Calvert Bluff [28]. The principal seams (from an economic point of view) are found in the lower Calvert Bluff within 91 m of the Simsboro Formation, and in upper 61 m of the Calvert Bluff beneath the Carrizo Sand Formation. In the lower Calvert Bluff, the thick seams tend to be both most numerous and laterally continuous in the region between the Navasota and the Colorado Rivers. An example is the commercially important Sandow deposit in Milam County. In the upper Calvert Bluff the seams are thickest and most extensive northeast of the Brazos River. The Big Brown deposit in Freestone county is an example. The important lignite deposits in Bastrop County are in the lower part of the Calvert Bluff. In the western part of Bastrop County the Hooper Formation contains numerous beds of lignite, although the beds are thin and lenticular and are unlikely to be of commercial importance. The principal lignite bed in Leon County is 2-3.3 m in thickness [7]. The upper 30--60 cm are rotten coal, lignite which is extremely friable and of otherwise poor quality. The most important of the Texas lignites occur in Milam County. The Calvert Bluff lignites in Milam County extend in a band running northeastward to the Brazos River. In some cases the seams may be up to 6 m thick, but the average thickness is 2.1 m [7]. Between the Colorado and Trinity Rivers in east-central Texas the Wilcox Group is 366 to 1067 m thick [26]. The only significant lignite in this region is in the Calvert Bluff Formation. The lignite occurs in the lower part of this formation above the Simsboro Sand. Irregular lignite occurrences are reported for the upper Calvert Bluff. The seams range in thickness from 0.6 to 7.6 m, although 1.5-3 m is typical [26]. Some are continuous for about 22 km [26]. Lignite in east-central Texas occurs between the Colorado and Trinity Rivers [24]. In this region the lignite is in the lower Calvert Bluff. The Calvert Bluff lignite occurs between belts of channel sand. In the Calvert Bluff lignites, the number of individual lignites increases and the thickness decreases in the direction of the ancient interchannel flood basins; the best lignite deposits occur at the juncture of the ancient alluvial and deltaic plains [24]. In northeast Texas near-surface lignite occurs in the upper two thirds of the Wilcox [24]. Seams in the Wilcox are less continuous than in the Calvert Bluff. This difference reflects the fact that the Wilcox lignites accumulated in smaller basins which did not remain intact over as long a time as in the deposition of the Calvert Bluff lignites. The lignite seams of greatest thickness and lateral continuity occur on the Sabine Uplift, the area of transition from the ancient alluvial to deltaic plain [24]. Simsboro lignites are found principally in sand-deficient interchannel areas [15]. Hooper lignites are most numerous and thickest in the upper portion of the formation, immediately below the Simsboro. Lignite is most common in the upper two-thirds of the Wilcox Group. The Wilcox Mount Pleasant Fluvial System occurs over a large area on the Texas-Louisiana border [29]. The lignite mined near Darco belongs to this Mount Pleasant Fluvial System. The Darco lignite is fairly

11 thick (2-3 m) and laterally extensive. The lower portion of the Darco lignite is primarily attrital lignite; a woody lignite constituting the second (and smaller) portion of the seam overlies the attrital lignite. (v) Yegua Formation lignite. Commercially, the Yegua lignite is second in importance to the Wilcox Group lignites. The Yegua lignites are not as extensive and are of poorer quality than the Wilcox lignites. Most of the important deposits of Yegua lignite occur north of the Brazos River, and tend to be concentrated in the middle and upper parts of the formation [7]. Lignite occurs in the upper part of the Y egua Formation and the lower Jackson Group in south Texas, south of the Colorado River. The deep-basin lignite occurs in a band running toward the northeast. At the northeast extremity, the lignite outcrops in central McMullen and Atascosa Counties, where it is currently being mined for the San Miguel electric power station. To the south, it occurs in strippable deposits in Zapata and Starr Counties in the Rio Grande valley. The lignites in south Texas amount to more than 180 Mt in seams of 0.6 to 4 m thickness [26]. Some of the seams extend for 22 km [26]. In Houston County the beds of Yegua lignite range from 0.6 to 1.8 m in thickness (averaging about 1.5 m) and generally occur at depths of 9-18 m [23]. These lignites are of commercial quality and have sufficient seam thickness for commercial exploitation. In south Texas lignite of a lagoonal origin is sometimes grouped as "Yegua-Jackson" lignite. The Caddell Formation (the lowest in the Jackson Group) is nominally the marker separating the Jackson and the Yegua, but in regions of south Texas the Caddell is either absent or, at best, difficult to recognize. The boundary between the Jackson and the Yegua becomes somewhat arbitrary and has to be established by paleontological evidence [ 16]. This lignite has been used commercially as drilling mud additive. (v) Jackson Group lignite. The poorest grade Texas lignite occurs in the Jackson Group [ 14]. Many of the seams are less than 1.5 m thick, though some may be laterally continuous for up to 48 km [ 14]. Most lignite beds in the Jackson Group are too local, too thin, of too poor quality, or suffer some combination of these factors, to be mined. In east Texas, the Jackson Group has been divided into four formations: the Caddell, Wellborn, Manning, and Whitsett [28]. The thickest lignite seams occur primarily in the Manning Formation. The more important lignites of the Jackson Group occur near the middle of the group and in the lower part of the Manning Formation. In Fayette County the Manning Formation lignites are notable for a very high ash value. Manning lignites have sulfur contents greater than 1.5% [ 17]. Compared to Wilcox lignite, Jackson lignites generally have higher moisture, ash, and sulfur contents, and lower calorific values [24]. Generally the older seams have the lower calorific values. The high sulfur contents and ash values are a result of the accumulation of lignite in swamps or marshes in the standplain-lagoonal regions or on the delta plain. The sulfur content also results from accumulation closer to the sea, the organic matter being affected by sulfate-rich marine waters. The high ash value reflects a greater contamination by clastics, either water-borne sediment spreading over and into the delta plain or strandplain, or airborne volcanic ash. The San

12 Miguel lignite interval contains four to six partings each 15-30 cm thick, which are believed to be of volcanic origin [24]. The best Jackson lignite occurs in east Texas. In southeast Texas, the Jackson Group lying between the Colorado and Neches rivers is about 300 m thick [26]. It consists of four formations: Caddell (at the base), Wellborn, Manning, and Whitsett. Lignite occurs in the upper Wellbom and upper Manning Formations. The Manning is the more important. Outcrops of Jackson lignites are numerous, but generally the seams are thin, usually less than 0.9 m [26]. In a 60 m section at Somerville, eighteen lignites are present, but only four are of greater thickness than 0.9 m [26]. In east Texas, lignite occurs as a continuous band in the Manning and Upper Wellborn Formations. In the Manning Formation seams are typically 0.9-2.4 m thick with a maximum of about 3.7 m. The seams are laterally continuous, commonly for 32 km and possibly up to 64 km [24]. The San Miguel lignite (lower Jackson) extends for about 40 km; the interval is typically 3-4.6 m thick [24]. Between the Colorado and Angelena rivers in east Texas, deltaic lignite occurs in the Jackson Group (upper Eocene) [24]. Lignite found in outcrops occurs in the Manning and upper Wellborn Formations [24]. The Jackson lignites have not been characterized by petrographic or palynological studies to the same extent as the Wilcox lignites, but arguments have been made for either marshy [27] or swampy [30] environments of deposition. The San Miguel lignite (upper Yegua or lower Jackson) ranges from reddish brown to brownish black in color with a dull to waxy luster [31]. Cores show no evident banding, and there is little or no hard or woody material. The Hardgrove grindability is 89 (at 28.7% moisture) and the lignite can generally be cut smoothly with a knife [31]. Pyritic sulfur averages 0.72% [31]. Seam partings range from over 60 cm in thickness down to the thickness of a knife edge [31 ]. Partings tend to be clays or carbonaceous shales. 1.2.2 Alabama (i) Occurrence. Alabama has a demonstrated reserve of lignite of 262 Mt lying under less than 76 m of overburden [32]. The inferred reserve is 1.8 Gt [32]. For the deep-basin lignites, which lie under 76-1,830 m of overburden, the demonstrated resources amount to 426 Mt, and the inferred resources, 3.4 Gt [32]. Over half the total estimated lignite resources are located in western Alabama in the Naheola Formation of the Midway Group. About 1.8 Gt occurs in nearsurface deposits of the Oak Hill lignite bed [33]. The near-surface Oak Hill lignite demonstrated resource is 925 Mt, and 4.3 Gt for the deep-basin resource [54]. The demonstrated reserves of Oak Hill lignite amount to 256 Mt, assuming an 85% recovery factor [33]. The subeconomic resources add another 205 Mt [33]. The total hypothetical resource in southwestern Alabama is 7 Gt [34]. Alabama lignites are Tertiary and late Cretaceous in age. The most important are of Eocene age (Wilcox Group). In Alabama the Wilcox is a band roughly 40 km wide running northwest to southeast across the state. This deposit includes almost the whole of Wilcox, Butler, and Crenshaw counties, and lesser portions of twelve other counties. The total demonstrated and inferred resources of Gravel Creek lignite are 386 Mt [34].

13 The oldest Gulf Coast lignites occur in the Naheola Formation (Upper Paleocene) of Alabama and Mississippi. The high sulfur contents of these lignites suggest that they may have been deposited under marine conditions [35]. The Wilcox and Claiborne Groups, which extend into Alabama from Texas, are Upper Paleocene to Lower Eocene. These lignites were deposited in a variety of environments, which ranged from fresh through brackish to saline [ 17]. Lignite is widely distributed in Paleocene and Eocene formations along the Gulf Coastal Plain in Alabama. In the Alabama-Tombigbee Rivers Region, the main lignite deposits occur in the Midway, Wilcox, and Claiborne Groups [33]. In the Midway, the lignite deposits occur in the Oak Hill Member of the Naheola Formation. The Naheola Formation is divided into the Oak Hill (lower) and Coal Bluff (upper) Members. Lignite seams ranging from 0.3 to 4.3 m thick are present near the top of the Oak Hill [33]. The Tertiary lignites run in a west-to-east band across the southern part of the state. The thickest and most persistent bed is the Oak Hill lignite. The Oak Hill Member of the Naheola Formation contains a remarkable lignite seam that can be traced for 120 km from the MississippiAlabama border in a southeastern direction to Wilcox County [18]. Over this distance the seam thickness varies from 0.3 to 4 m [18]. The inferred resources in Wilcox County are 188 Mt, underlying an area of 175 km2 [36]. Some unusual lignite deposits nearly 15 m thick have been formed in depressions created by the collapse of underground caverns. These deposits are highly lenticular (less than 1.6 km in width) and are of interest only as geological oddities. They have no evident commercial value. (ii)

Quality.

Typically the sulfur content of Alabama lignites is 1.6-6.3%, the ash value

is less than 16%, and the calorific value ranges from 9.35 to 26.1 MJ/kg (average 20.9), on a moisture-free basis [34]. An unusual aspect of the eastern Alabama lignites is a trend for increasing oxygen content with increasing depth [37]. Since it is normal to presume an increase in coalification, and hence a decrease of oxygen, with increasing depth, this finding is surprising, but appears to be real, despite the possible errors associated with calculating oxygen content by difference. A study of drill core samples has shown wide differences in sulfur contents [37], suggesting major differences in the accumulation of the precursor peats. The generally high levels of sulfur suggest a saline deposition, comparable to the environment of the Florida Everglades

(e.g. [38]) where peat is now accumulating. Analyses of some specific lignite samples have been tabulated in [20]. The Oak Hill lignite has an average calorific value of 20.9 MJ/kg (moisture-free basis), with a range of 9.35-26.0 MJ/kg [33]. Most Oak Hill lignite yields less than 16% ash; ash increases toward the pinchout in Wilcox County, and consequently the calorific value decreases in this area. Moisture ranges from 45 to 53%; sulfur, from 1.6 to 6.3% (moisture-free basis) [33]. The sulfatic sulfur amounts to 12% of the total sulfur; the balance being almost equally divided between organic and pyritic [33]. The lignite of the Coal Bluff seam is of unusually high ash. On a moisture-free basis, this lignite contains 23.1% carbon, 2.5% hydrogen, 0.6% nitrogen, 2.3% sulfur, 10.1% oxygen, and 61.4% ash [36]. The ash fusion properties are 1560 ~ initial

14 deformation, 1599" softening, and 1599" C fluid temperatures [36]. The average composition of Gravel Creek lignite is 44.05% moisture, 14.9% fixed carbon, 26.2% volatile matter, 19.79% ash, and 1.6% sulfur [34]. The average calorific value is 11.05 MJ/kg (as-received basis) and 22.4 MJ/kg (maf basis) [34]. Tuscahoma lignite (Wilcox) has an average 41% ash, 2.9% sulfur, and calorific value of 13.5 MJ/kg [33,34]. The range of calorific values of Tuscahoma lignites is 5.6 to 22.6 MJ/kg. The range of sulfur contents is 2.0-6.1%; of this, sulfatic sulfur has the uncommonly high proportion of 24.8%, while the organic sulfur is 44.6% and the pyritic, 30.6% [33]. (iii)

Midway Group. Lignite having possible commercial value occurs in the Oak Hill

Member of the Naheola Formation (middle Paleocene). The bed is generally 0.5--4 m in thickness, and represents the most extensive lignite deposit in southwestern Alabama [39]. This lignite accumulated in the interchannel areas of coastal marshes lying on a delta plain. The marshes lay behind a barrier system. The lignite itself occurs in a sequence dominated by clay. The upper portion of the Tuscahoma Sand Member (upper Paleocene) contains lignite having no commercial value. This lignite does not occur over wide areas, and the beds are generally less than 1 m thick [39]. Up to six lignite seams occur in the Tuscahoma Sand, but their combined thickness is only about 1.5 m [39]. The Oak Hill Member is 25--50 m thick. It contains fine-grained sand, sandy silts, and silty clays [40]. The lignites typically occur near the top of the Oak Hill, and are enclosed in clays. In some localities the Oak Hill seams may have been removed, wholly or in part, by erosion. The lignite is 1.2-1.5 m thick, and extends from Kemper County in eastern Mississippi to the Alabama River (Wilcox County, Alabama) [35]. Two seams of lignite occur in the upper Oak Hill Member. The upper, which is not everywhere present, is typically about 15 cm thick [33]. The interseam parting, about 1 m thick, is composed of carbonaceous sand, clay, and silt. The lower seam is 0.3 to 4.2 m thick and is the most continuous lignite seam in Alabama [33]. It extends in a belt up to 13 km wide from the Mississippi border southeasterly through Sumter, Choctaw, Marengo, and Wilcox counties. The bed finally pinches out in Wilcox County on the east side of the Alabama River, near Camden. Oak Hill lignite is enclosed in nonmarine sediments. The low ash, moderate to high sulfur, large areal extent, and tabular shape of the lignite suggest that deposition was not influenced by active streams or by the Gulf of Mexico, although a high sulfur content suggests brackish water [33]. A depositional environment having these characteristics is an inactive delta lobe. The Coal Bluff seam, in the Naheola Formation, averages 1.2 to 2.4 m in thickness [36]. In Wilcox County it averages 1.8 to 2.1 m [36]. It underlies an area of about 28 km2 between County Line and Kimbrough [36]. The measured and indicated lignite reserves in this area amount to about 66 Mt [36]. The overburden is unconsolidated sands and clays. Deep-basin lignites occur at the top of the Midway Group, in the southern portion of the Alabama-Tombigbee Rivers Region. Throughout most of the region the cover is less than 610 m thick, but in the southwestern portion of this region the Midway lignite lies beneath 910-1,200 m

15 of sediments [33]. The Midway lignites in Choctaw, Clarke, Monroe, and Washington counties comprise the most extensive and largest of the deep-basin lignites. Seam thickness can exceed 4.3 m [33]. The demonstrated reserves are 426 Mt, with inferred resources of over 3.4 Gt [33]. (iv) Wilcox Group. In Alabama the Wilcox is divided into the Nanafalia, Tuscahoma Sand, and Hatchetigbee Formations. Thin seams of lignite occur in the Nanafalia Formation, the Tuscahoma Sand, and the Hatchetigbee Formation. The principal occurrences of Wilcox lignite are in the upper (unnamed) member of the Tuscahoma Sand. This member contains six seams of fairly thin lignite. The Tuscahoma lignite occurs in a belt roughly paralleling the Oak Hill lignite, but about 27 km to the south [33]. The Tuscahoma lignite is continuous under large areas of Choctaw, Clarke, and the western portion of Wilcox counties, and in some cases may be over 1.5 m thick [33]. The demonstrated reserves of Tuscahoma lignite amount to 6.6 Mt, assuming an 85% recovery factor [33,34]. The demonstrated subeconomic resources are about 43 Mt [33]. The stacked, lenticular beds (also described as thin and tabular), high ash value, and low sulfur content all suggest deposition between distributary channels of an active delta lobe [33]. Thin seams of lignite also occur in the upper member of the Hatchetigbee Formation. The Tallahatta Formation is the lowest formation in the Claiborne Group; the Lisbon Formation overlies the Tallahatta and contains some deposits of lignite and lignitic clay. The Gosport Sand overlies the Lisbon and contains two very thin, lenticular beds of lignite. The lignites of eastern Alabama are in the Gravel Creek Sand Member of the Lower Nanafalia Formation (Wilcox Group). These lignites are younger than those of western Alabama. In contrast to the western Alabama lignite which is laterally extensive, the eastern Alabama lignite tends to occur in "pods" which may be fairly thick but are of limited areal extent [35]. The seams are enclosed by silty or micaceous sands. These lignite pods may have arisen from peat deposition in abandoned stream channels [41]. Deep-basin lignites also occur in the Wilcox Group. However, they appear to be less extensive than the Midway deep-basin lignites, as well as somewhat thinner (rarely exceeding 1.5 m) [33]. The deposits are thin and lenticular. 1.2.3 Arkansas (i) History. Arkansas lignites have been used as sources of montan wax, dyes, and heating oils, as well as fuel for local domestic heating. The lignite makes an acceptable domestic and industrial fuel when dehydrated and shaped into briquettes. Early attempts to use the lignite as a fuel for blacksmith forges were unsuccessful [42]. (ii) Occurrence. Lignites occur in the southwest and northeast corners of Arkansas and in the south-central region, in an area of about 15,900 km2 [43]. The major deposit lies in a 8-32 km wide belt running from Little Rock to the Texas border [25]. The total lignite to a depth of 45 m is estimated to be 12.2 Gt [18], irrespective of seam thickness. In some cases, lignite deposits exceeding 1 m of thickness are encountered at depths of less than 45 m [1]. Most seams are less than 2 m thick and are lenticular. In addition, the seams may be broken up by faults or ancient

16 channels. The total resources of Wilcox lignite in Arkansas are 3.9 Gt [34]. Pulaski County has the largest reserves, with approximately 40% of the estimated total. Other counties containing large deposits, expressed as a percentage of the total state reserves, are Saline, 20%; Ouachita, 16%; and Dallas, 15%. The most extensive lignite area in Arkansas is about 155 km2, northwest of Camden in Ouachita County [44]. The lignite is exposed along the banks of the Ouachita River and its tributaries. The average seam thickness is about 75-90 cm, with a maximum of 1.8 m [44]. The deposit contains about 68 Mt [44]. A single deposit of an extent of about 24 hectares, but containing a lignite seam 2.5 m thick, occurs near Manning in Dallas County [44]. This lignite was used to manufacture brown pigment. Lignite near Sweet Home in Pulaski County has a maximum thickness of 8 m [44]. Arkansas lignites are found in the Wilcox, Claiborne, and Jackson Groups (Tertiary). Most of the lignite occurs in the Wilcox Formation (Eocene) in the Gulf Coastal Plain. The lignite is present in Cleveland, Dallas, Grant, Hot Spring, Ouachita, Pulaski, and Saline counties. Many of the beds are thin and lenticular, with no real commercial value. Over 85% of the strippable reserves are in the Wilcox Group, which contains about 19.6 Mt [8]. The Crowley's Ridge area contains lignite of the Wilcox Group in Clay, Craighead, Greene, and Poinsett counties. The most abundant lignite and the thickest seams generally occur in the Wilcox Group. The average thickness of Wilcox Group strata in this region is about 275 m. Lignite occurs in the Wilcox in beds up to 4 m thick within 45 m of the surface [34]. The thickest seams exceed 7 m. The beds are generally lenticular. The most important beds in the group occur in the lower and middle Wilcox. These lignites were deposited in a fluvial environment. The lignite has been most extensively studied in Pulaski and Saline counties. The Claiborne Group overlies the Wilcox. Claiborne lignites occur in lenticular beds of limited extent, with maximum thickness of 3 m [34]. The estimated resources are 4.3 Gt [34]. The lignites originated as attrital deposits in deltaic and shallow marine environments [34]. Some lignite occurs in the Tokio Formation [42]. (iii)

Quality. The

calorific value of Arkansas lignite is about 14 MJ/kg [44]. The lignites are

high in ash, and therefore have low calorific values on an as-received basis. The sulfur content of Wilcox Group lignites is less than 1% [8]. The calorific value ranges from 6.44 to 17.6 MJ/kg [8]. The lignites of the Wilcox Group average 35.7% moisture, 17.6% ash, 0.57% sulfur, and 13.4 MJ/kg calorific value on an as-received basis [34]. On a moist, mineral-matter-free basis the Wilcox lignites average 16.5 MJ/kg, corresponding to an ASTM rank of lignite A [34]. These lignites contain 64-90% translucent attritus [25]. The average composition of Claiborne lignites, on an as-received basis, is 38.7% moisture, 17.9% ash, 0.62% sulfur, with a calorific value of 12.7 MJ/kg [34]. The ASTM rank is lignite A, based on a moist, mineral-matter-free calorific value of 15.7 M.l/kg. 1.2.4 Louisiana (i) History. A lignite bed about 1.5 m thick is exposed on the east bank of the Sabine River

17 [45]. The first attempt at mining this lignite occurred in the 1870's, but the barge transporting the lignite downriver to market sank, and the attempt was apparently never repeated. The lignite was considered to be a marketable fuel only in the local area, where it was in competition with pine wood. A 5.5 m thick bed of lignite is exposed at Iron Mine Run on Avery Island [46]. An attempt to exploit the lignite for local fuel use never came to fruition. (ii) Occurrence. The lignite in Louisiana is early Tertiary in age. Outcrops occur north of a line running from Natchitoches to Sabinetown and from the Red to the Sabine River. Underground deposits occur between the Red and the Ouachita rivers, along a line from Natchitoches to Catahoula Parish. The area of the state underlain by lignite is estimated to be 23,000 kin2 [20]. The resources likely to be of commercial significance are located in the Sabine Uplift area in the northwestern portion of the state [47], and are of the same age as the Wilcox lignites of Texas [18]. The estimated lignite resources are 1.5 Gt, but, because of the limited amount of data available for some areas, the actual total may be higher by as much as 450 Mt [18]. Most of the seams are less than 2.5 m thick [ 18]; many are less than 1.5 m thick [8], and tend to be lenticular. The Chemard Lake Lentil (DeSoto Parish) is continuous for a distance of 24 km. This single deposit may contain over 450 Mt. It is about 3 m thick and contains lignites and lignitic clays [48]. The Lentil marks the contact of the Naborton and Dolet Hills Formations, and is also sometimes known as the Blue Bed. A second bed of lignite, sometimes called the Red Bed, lies 8-15 m below the Chemard Lake Lentil. The two beds are separated by shales, clays, siltstones, and sandstones. The Green Bed is about 34 m below the Chemard Lake Lentil. The Green Bed has a maximum thickness of 1.5 m [48]. The Sabine Uplift is centered in De Soto Parish and extends from there into Louisiana and Texas. Lignite occurs principally in the Wilcox Group (Eocene); it is found to a lesser extent in the overlying Claiborne and Jackson Groups. The principal lignite bed is the Chemard Lake, which occurs at the top of the Naborton Formation of the Wilcox Group. The Naborton Formation includes lignitic silts, as well as clays and calcareous sands [48]. Chemard Lake lignite occurs over a fairly wide area of Louisiana. It is laterally extensive for over 80 km [49]. The lignite thickness ranges from 2.4 m to less than 30 cm; the overburden averages about 23 m [47]. The estimated reserves of Chemard Lake lignite amount to 495 Mr, based on an assumption of 1.8 m average seam thickness and specific gravity of 1.29 [47]. Chemard Lake lignite has a woody structure, low sulfur content, moderate calorific value, and high ash value, characteristics which suggest deposition from a fresh water environment. In Sabine Parish an area of about 40 km2 is underlain with lignite having an average seam thickness of 1 m [47]. The reserves are estimated to be 45 Mt [47]. Lignite outcrops at the surface in nearly all the formations of the Wilcox and Claiborne groups in Sabine Parish. However, the only ones which seem to have commercial potential are the Bayou San Miguel and the Sabine River Stone Coal Bluff beds [45]. The Bayou San Miguel bed is typically 1 m thick; the lignite is hard and black with little woody structure [50]. The Sabine River - Stone Coal Bluff lignite seam is of comparable thickness. Other lignite deposits of potential commercial value occur in the Beinville,

18 Bossier, Caddo, Natchitoches, and Red River parishes. The Porters Creek Formation is the topmost formation in the Midway Group. It consists of lignitic shales as well as calcareous shales and clays [48]. The Porters Creek Formation directly underlies the Naborton Formation in the Wilcox Group. (iii) Quality. Chemard Lake lignite has a low sulfur content (0.53%) and a calorific value of 14.7 MJ/kg [49]. The average maceral content is 60.8% vitrinite, 26.3% exinite, and 12.9% inertinite [49]. This lignite consists of bright, banded, and banded-bright lithotypes deriving mainly from forest-moor paleoflora which accumulated on an upper deltaic plain. The presence of duller lithotypes correlates with increased pyrite content. Sunrise lignite (also called the Green Seam or Green Bed) lies about 30 m below the Chemard Lake lignite. The sulfur and calorific values of the Sunrise lignite are slightly higher than the Chemard Lake lignite, 0.62% and 16 MJ/kg, respectively [49]. Lignites from De Soto and Sabine parishes have an average calorific value of 16.6 MJ/kg, with 0.63% sulfur, 16.1% ash, and 30% moisture [47]. Analyses of lignite and ash samples have been tabulated in [20]. 1.2.5 Mississippi (i) Occurrence. Lignite in Mississippi is found mainly in the area north of a line through Meridian, Jackson, and Vicksburg and to the east of the "bluff." In Mississippi the so-called bluff is actually a line of bluffs paralleling the Mississippi River and running on the east side of the river from Kentucky to Louisiana. The seams tend to be lenticular and generally less than 1.5 m thick [8]. Lignites outcrop in an area starting at the border with Tennessee in the north, curving through the central part of Mississippi, and then extending to the border with Alabama in the east [51]. Lignite occurrences cover 32,600 km2 in the state, with outcrops in 33 of the 82 counties [34]. The lignite beds are generally tabular and discontinuous. Mississippi lignite reserves are found in the Wilcox, Claiborne and Midway Groups. The total estimated resource is estimated to be 4.5 Gt [ 18], predominantly in the Wilcox, and is of Eocene age. The principal lignite beds occur in the Ackerman Member of the Wilcox Group. The Wilcox lignites are of principal significance in east-central Mississippi, in a belt running from Calhoun through Lauderdale counties. The best deposits lie in Webster, Calhoun, and Lafayette counties. Other counties containing appreciable deposits are Yalobusha, Choctaw, and Jasper. Lignites of commercial potential in the Wilcox were deposited in fluvial or delta-plain environments. These lignites are irregular in shape, erratic in thickness, and of limited extent [ 1]. The shapes vary from narrow deposits typical of meandering, fluvial systems to elliptical deposits of deltaic origin. At the outer edges, the seams may be only a few centimeters thick, but at the center of the deposit the seams may be up to 3.7 m thick [1]. Generally the Claiborne lignites are similar to those of the Wilcox Group. The Claiborne lignites are found mainly in the Cockfield (Yegua) Formation. Lignite is abundant in this formation. Seams up to 1.5 m thick have been observed [51]. Some sands in the

19

Cockfield are lignitic, and some lignitic clays are also observed. Some lignites are also found in the Cook Mountain, Kosciusko, and Zilpha Formations. The depositional environment for the Claiborne lignites was similar to that for the Wilcox lignites. (ii) Quality. The moisture content of Mississippi lignite is in the range of 40-45%, and may exceed 50% [34]. The sulfur content runs up to 4%, but is generally less than 2% [34]. On a moisture-free basis the calorific value is in the range of 16 to 26 MJ/kg [34]. A summary of analyses of some Mississippi lignite samples has been compiled in [20]. The Wilcox lignites average about 16% ash and less than 1% sulfur [51]. The sulfur content increases geographically as lignite is sampled more and more closely to the ancient marine environment of Kemper and Lauderdale counties. The moisture is in the range of 40--45%; the calorific value, 11-13 MJ/kg (as-received basis) [51]. The Claiborne lignites are generally high in ash and low in calorific value. 1.2.6 Tennessee Lignite is found in western Tennessee in the Wilcox and Claiborne Groups. The total resource is estimated to be 900 Mt [18]. The seam thickness is generally less than 2.7 m [18]. 1.3 THE F O R T UNION REGION

1.3.1 North Dakota (i) History. The first use of North Dakota lignite as a fuel was made by the members of the Lewis and Clark expedition, which wintered in North Dakota in 1804 [52,53]. These explorers reported on the abundant deposits of lignite observed along the Missouri River when they passed through the area in 1805. Extensive exploration of the lignite deposits, particularly in the Missouri Valley, was conducted by the Hayden expedition in 1854 [54]. The lignite-bearing strata were referred to as the Great Lignite Group [55]. Hayden's chief colleague, F. B. Meek, substituted the name Fort Union for Great Lignite [56]. The name Fort Union was taken from a United States army fort on the banks of the Missouri River near the Montana-North Dakota border [55], although the fort was in fact in Montana. The first commercial mining was carried out by the Northern Pacific Railroad in 1884, across the Little Missouri River from Medora [52]. All North Dakota lignite is currently recovered by strip mining. The production was 3 Mt in 1950 and then gradually declined to 2.1 Mt by 1958; however, the production increased again through the 1960's and reached 4.1 Mt in 1968 [8]. (ii) Occurrence. The largest reserves of lignite in the United States occur in western North Dakota and adjacent areas of eastern Montana and northwestern South Dakota. The age of the lignite ranges from late Cretaceous through Eocene [57]. Lignite occurs in the Hell Creek Formation (Upper Cretaceous), the Fort Union Group (Paleocene), and the Golden Valley Formation (Eocene). The lignite deposits of North Dakota generally lie west of the 100th meridian. The lignite field can be divided into three provinces, based on the overburden characteristics [58].

20 North of the Missouri River the overburden is primarily glacial drift. Between the Missouri River and the limit of glaciation, the overburden is mainly fine-grained clastic sediments with some scattered areas of glacial drift. In the unglaciated area (the southwestern region of the state) the overburden is clastic rocks. The overburden on most of the lignite in North Dakota contains higher quantities of sodium than does that on the subbituminous coals further to the west in Montana and Wyoming [58]. The high sodium content has consequences for possible sodium toxicity problems during mined land reclamation, and may also represent the source of sodium in the lignite, which is related to ash deposition problems during combustion in power plant boilers (Chapter 11). More than one hundred lignite beds have been described in western North Dakota, but because few have been traced laterally for more than 40 km, it is possible that many of these hundred beds may be at the same, or nearly the same, horizon as others [59]. Exposures of lignite beds are particularly good southwest of the Missouri River, since overlying glacial deposits are fairly thin and discontinuous. In contrast, northeast of the Missouri the exposures of lignitebearing series are very poor because of a widespread cover of glacial deposits, which in some places are 60 m thick [59]. Good exposures of the lignite beds are observed in bluffs along stream valleys. An excellent example is the Little Missouri River in Slope, Billings, and Golden Valley counties, where the relief in places reaches 300 m, exposing the entire section of lignite-beating rocks [59]. A total of 22 lignite beds has been noted in these counties; the thickness of individual beds ranges from 1 to 11 m, and the aggregate thickness of all the beds is 58 m [59]. The lignite deposits of North Dakota cover an area of about 83,000 km2, of which about 72,000 km2 are underlain by beds over 1.4 m thick [58,60]. However, only about 2800 km2 contain lignite recoverable by surface mining [58]. The demonstrated strippable reserve base of lignite in North Dakota is 11.8 Gt [3], based on the criteria of a minimum seam thickness of 1.5 m, a maximum overburden of 30 m, and a maximum stripping ratio of 10:1 [ 1]. (The economic limits for a reserve to be considered strippable are a minimum seam thickness of 1.5 m, 15 m of overburden, maximum stripping depth of 67 m, and minimum calorific value of 14 MJ/kg [8]). The total estimated resources amount to 318 Gt [58,60-62] (other sources have estimated 544 Gt [59]). In addition, there are an estimated 163 Gt of lignite in the category of hypothetical resources

(i.e., resources in unexplored and unmapped areas) [58,60,62]. Of the total identified resource, 89.3% is subeconomic, a category which includes the inferred resources and any indicated or measured resources in seams less than 1.5 m thick [58]. The remainder (10.7%) represents demonstrated, measured, or indicated resources in seams 1.5 m or greater in thickness [58]. Approximately 14 Gt lie within 30 m of the surface [58]. If a 90% recovery factor is allowed for strip mining, the recoverable reserve is 13.1 Gt [58]. Future improvements in mining technology, to increase the depth at which strip mining is economically feasible and to increase the recovery factor, will bring more of the total resource into the category of recoverable reserves. The largest amounts of recoverable reserves, in units of megatonnes, occur in the following counties: Slope, 1825; Dunn, 1633; Mercer, 1621; Stark, 1041; and Williams, 923 [58]. The remainder is distributed among eighteen other counties.

71 Resource estimates come mainly from outcrop information. Lignite present at some distance from the exposure might not be included in the estimate. In some areas only the most prominent bed is included in the estimate, neglecting lignite in other beds. Furthermore, only lignite in beds at least 75 cm thick is included in the estimate. Thus the actual amount of lignite in the ground is likely to be somewhat greater than the estimated amount. Lignite recoverable by strip mining has been estimated to be 17 Gt [61]. A useful summary, with measured, indicated, and inferred resources divided by seam thickness (0.75-1.5, 1.5-3, and >3 m) for each county has been published [62]. Reserves recoverable by strip mining are summarized in Table 1.11 [3]. TABLE 1.11 North Dakota lignite reserves recoverable by strip mining [3]. Area Reserves Mt Adams County 91 Beach- Wibaux 454 Beaver Creek 136 Beulah-Zap 816 Bowman 1,089 Center 635 Dickinson 1,633 Dunn Center 1,633 Falkirk- Washburn 1,089 Gascoyne- Scranton 272 Grant County 272 Hazen 726 Hettinger County 181 Niobe 272 Noonan - Columbus 136 Renners Cove 907 Stanton 272 Stony Creek 635 Velva 544 Wilton 45 Total .................. 11,838

The oldest lignite-bearing unit is the Hell Creek Formation, consisting mainly of about 150 m of nonmarine sandstones, siltstones, and mudstones [58]. Lignites in the Hell Creek are generally too thin for successful commercial mining, discontinuous, and of poor quality. The Hell Creek is the youngest of the Cretaceous formations, and contacts the Ludlow, the oldest of the Tertiary formations. The top of the Hell Creek was recognized as the base of the Paleocene sediments [63]. Some lignite also occurs in the Lance Formation (Cretaceous), which underlies the Fort Union. The Lance Formation outcrops in a belt running southwestward from central North Dakota to northwestern South Dakota; the belt of outcrops is roughly 80 km [59]. The Fort Union Group contains essentially all of the lignite included in the estimated North

22 Dakota resources. The Fort Union Group is included in the rocks of the Zuni Sequence (Cenozoic). The Fort Union Group derived from erosion of the Laramide Rocky Mountains [64]. Both the lignites and the elastic sediments in the Fort Union are predominantly derived from terrestrial origins [65], except for the Cannonball Formation in the lower part of the Fort Union. (The Cannonball is a marine deposit, while the other formations are non-marine fluvial, alluvial, or swampy origin.) Lignites occur in the Ludlow, Slope, Bullion Creek, and Sentinel Butte Formations [1], all but the Cannonball. Most of the lignite recoverable by strip mining is in the Bullion Creek and Sentinel Butte Formations [28]. The reserves and resources of the lignite in the Fort Union have been estimated at, respectively, 18 Gt and 318 Gt [66]. The vertical range of beds in the Fort Union is about 400 m [20]. In North Dakota the eastern border of the Fort Union runs southeastward from Renville County on the northern boundary roughly to the center of the state. In mid-state the border begins running southwesterly to Adams County on the southern boundary. South and east from Bismarck the Fort Union is thin, and in many places entirely missing. Some of the lignite beds extend over 3,900 km2, with thicknesses ranging from 1.5 to 12 m [93]. However, the lignites vary considerably in thickness, down to a few millimeters [68]. Lignite seams comprise less than 5% of the total thickness of these formations, the majority of the thickness being interbedded silt and clay and silty fine- to medium-grained sand [73]. Most of the strippable lignite deposits are found in the Williston Basin, the largest occurring on the southwestern flank of the Basin, and smaller deposits on the eastern flank. Here the bed thickness ranges widely, from less than 25 mm to over 7.6 m [8]. In some cases two or more beds of thickness appropriate for commercial mining may occur in the same area. Most of the mineable lignite is under less than 300 m of overburden, and 70% is under less than 150 m [60]. There is very little structural variation in the lignite in the Fort Union. The beds are generally flat, with an estimated dip of only 4 m/km [69]. Both the Cannonball and Ludlow Formations are about 90 m thick [67]. The Ludlow contains lignites that range in thickness from less than 30 cm to about 7 m [1], with aggregate thickness up to 12 m [62,70]. The Ludlow is best exposed in southwestern North Dakota, especially in Bowman County. The Ludlow underlies the Cannonball in the western portions of North Dakota. The Ludlow Formation is the lowermost of the Fort Union Group, and is similar to the underlying Hell Creek Formation. The Slope Formation contains strata that had previously been assigned to the upper Ludlow or to the Tongue River. Lignite makes up about 10% of the rocks in the Slope Formation, and occurs in beds having a maximum thickness of 4 m [1]. Like the Slope Formation, the Bullion Creek has recently been defined and contains many strata formerly assigned to the Tongue River Formation. The Bullion Creek is also similar to the Slope Formation in that lignite beds comprise about 10% of the strata [1]. The lignites average about 1 m in thickness, with a maximum bed thickness of 4 m [1]. The Harmon and Hansen beds are the lowest lignite members of the Bullion Creek Formation. The Harmon bed underlies small areas in themnorthern part of Bowman County. It reaches a maximum thickness of 10 m [60].

23 Overburden is 36 m or less [3]. At the Gascoyne mine (Bowman County) two seams are separated by about 90 cm of partings. The Gascoyne mine supplies lignite for use in a power plant in Big Stone City, South Dakota. The Weller Slough bed is also in the Bullion Creek Formation. It is about 3.7 m thick in Mercer County [71 ]. The Coal Lake Coulee bed generally ranges from 60 to 120 cm thick, with a maximum of 2.4 m [71]. The Tavis Creek bed, a relatively thin (ca. 90cm) bed, lies just below the contact between the Bullion Creek and Sentinel Butte Formations. Lignite beds in the Sentinel Butte range in thickness from less than 30 cm to, in some instances, over 6 m [ 1]. Sentinel Butte lignites are less continuous than those of the Bullion Creek. Sentinel Butte sediments tend to be darker, relative to the Bullion Creek, and are described as "somber" [58]. Generally, lignite seams occurring in the Sentinel Butte are lenticular and often split into multiples. The Sentinel Butte Formation is primarily silt, clay, and lignite, with sands at both base and top. The Hagel bed is the lowest lignite bed in the Sentinel Butte [71]. The average thickness in Dunn County is about 2.4 m, although in some locations the Hagel bed splits into several minor seams separated by inter-seam partings up to about 1 m thick [71]. The Hagel bed dates to the late Paleocene. It is extensive throughout the central and south-central portions of Oliver County along Square Butte Creek and its various tributaries [72]. The Hagel bed occurs throughout the Knife River drainage basin and into adjacent McClean and Oliver counties [71,73]. Currently the Hagel bed is mined at the Center mine near Center (Oliver County), the Glenharold mine near Stanton (Mercer County), and the Falkirk mine near Falkirk (McClean County). At the Center mine the seam is about 3.4 m thick under 15 m of overburden [3]. Here the Hagel seam contains partings of dark carbonaceous clay or gray clays. The underclay is a graygreen clay with some fragments of lignite; the overburden is gray and consists mainly of clayey silts and silty clays [74]. This lignite is burned at the Square Butte power plant. The BaukolNoonan mine supplies lignite for an adjacent power plant. The seam thickness ranges from 3 to 4.3 m [75]. The Keuther seam, about 11-12 m above the Hagel in this location [58,75], is mined if local conditions permit; however, much of the seam has been removed by erosion or has oxidized to leonardite. Baukol-Noonan lignite is generally of low sodium content. This characteristic is related to the permeable overburden containing abundant carbonate minerals; sodium in groundwater could be replaced by calcium through ion exchange. The overburden is mainly glacial till and poorly consolidated sandstones and shale. At the Glenharold mine the seam thickness ranges from 60 cm to 2.4 m, under 12 m of overburden [3]. The overburden is primarily soft sandstone, shale, and clay with a veneer of glacial drift. However, a layer of a hard, well cemented sandstone is immediately above the Stanton lignite. This lignite is also used as fuel for an electric power plant. The Falkirk mine operates in two seams, one of 90 cm and the other of 2.4 m thickness, separated by a 1.5 m parting [3]. The overburden thickness is 24 m [3]. The lignite is used as fuel in a power plant in nearby Underwood. The Kinneman Creek bed is one of the most persistent lignites throughout the Knife River Basin. The main seam is about 2.4 m thick [71]. Associated seams about half as thick lie within 2.4 to 12 m of the main seam. Lignites along the Little Missouri River northward from the vicinity of Yule are sometimes

24 referred to as the Great Bend group of beds [76]. Of these, Bed I (also called the Sand Creek bed) is the thickest bed (about 11 m) that outcrops anywhere in North Dakota. This outcrop was located about 8 km southwest of the town of Amidon [76]. The Sand Creek bed can be traced for over 64 km, from the old Yule Post Office at the mouth of Williams Creek nearly to the Harmon ranch, 13 km south of Medora. The Antelope Creek bed usually occurs as two seams separated by a clay parting, but in some locations may consist of several seams separated by clay, sand, or silt. The sequence of multiple seams with interbedded sand, silt, or clay may be up to 15 m [71]. It is generally traceable throughout the Knife River Basin. The Jim Creek bed is a thin seam generally less than 1 m thick which occasionally splits into two [71]. The Spaer bed has a maximum thickness of about 1.8 m in the vicinity of Beulah and Zap and generally is of 60 cm to 1.8 m thickness in that region [71]. Elsewhere in the Knife River Basin it is thinner and even pinches out in places. In some locations it separates into four very thin beds separated by silt, sand, or clay. In Billings County the D and E seams range up to 4 m in thickness [3]. This lignite is notable in being the first commercial discovery of uraniferous lignite in the United States, the discovery occurring in the vicinity of Rocky Ridge in Billings County. Mining of lignite for its uranium content was carried out in 1956-8 and again in 1966-7 [3]. Northeast of Dickinson, the D and E seams are 2.7 to 5.5 m thick, under a maximum of 30 m of overburden. The Beulah-Zap deposit in Mercer and Oliver counties is comprised of three separate lignite fields: North Beulah, South Beulah, and Zap. In the North Beulah and Zap fields lignite is found in the Zap bed. In the South Beulah field the lignite is in the Schoolhouse bed. Both beds are part of the Sentinel Butte Formation. At the Indian Head mine the Zap bed is typically 3-3.7 m thick [58]. The maximum thickness is 5.2 m [28]. The overburden is mainly composed of clay and sandstone, and averages 23 m in thickness [58]. The Beulah-Zap bed averages about 3.6 m in thickness; however, it frequently splits into as many as five seams with clay, silt, or sand partings and in such cases the thickness of the entire interval may be as much as 9 m [71]. North of Beulah the Beulah-Zap merges with the Spaer to form a single seam about 6 m thick [71]; in western Dunn County the Beulah-Zap merges with the Schoolhouse bed. The Beulah-Zap bed underlies a large area of Mercer County. In places it reaches a maximum thickness of 6.7 m [60]. The Beulah-Zap also occurs in southeastern Dunn County, where it crops out along both sides of the Little Missouri River. The maximum thickness of the Beulah-Zap in Dunn County is 4.3 m [60]. In the Renner's Cove area the Beulah-Zap is 5.5 m thick, with overburden ranging from 15-30 m [3]. Currently active mining occurs at the Indian Head and Beulah mines. The latter enjoys a thicker seam, 4.9 overburden, 15

vs.

vs.

2.7 m on average, and less

20 m. At the Indian Head mine the Beulah-Zap seam has an average thickness

of about 4.3 m [75]. The Freedom mine was established to supply lignite to the Antelope Valley power station and the adjacent coal gasification plant near Beulah. The seam thickness here is 4.9 m, with maximum overburden of 30 m [3]. An extension of the Zap bed, running northward from

25 the Zap lignite field, is found south of Lake Sakakawea in Mercer County. The average thickness here is 5.5 m, with up to 30 m of overburden [58]. The Zap bed also makes up the Hazen deposit in Mercer County. The average thickness of the bed is about 3.7 m, but is often split by a shale parting [58]. The overburden is mainly sandstone and clay with a thin cap of glacial till. The Dakota Star mine was operated in this deposit until 1966. The Schoolhouse bed in the South Beulah field consists of three seams which average 3.7, 3.4, and 1.4 m in thickness [58]. The overburden thickness is an average 19 m and is primarily clay and sandstone [58]. The Schoolhouse bed splits into two seams separated by 2.4-3.7 m of clay, sand, and silt; the two seams themselves may be in the range of 1.5 m in thickness [71]. The Twin Buttes bed is of highly variable quality, ranging from a fairly thick single seam of good quality lignite to a thin seam of a carbonaceous clay. In some locations it consists of a single, 1.5 m thick seam while in others it occurs as two seams of this thickness separated by clay up to 1.2 m thick [71]. The Dunn Center deposit in Dunn County ranges from 3.4 to 7.3 m thick with less than 15 m of shale overburden [58]. The Stanton bed forms the Washburn deposit in McLean County. The average thickness of the Stanton bed is 2.4 m, with an average overburden thickness of 15 m [58]. The overburden is mainly clays, sands, and sandy clays with a thin veneer of glacial drift. The Stanton bed also comprises the Stanton deposit in Mercer and Oliver counties. In Burke County the Bonus seam is about 2.6 m thick, and overlies a 1.8 m thick Niobe seam. The overburden covering the Bonus seam is 46 m or less [3]. In the Noonan-Columbus area, the Noonan seam is 2-3 m thick [3]. Lignite is actively mined at the Baukol-Noonan or Larsen mine, where the seam is 2.4 m feet thick with a maximum overburden thickness of 15 m [3]. In the Dickinson area a small amount of lignite is mined from the Lehigh seam at the Husky No. 2 mine. This lignite is used for the production of charcoal briquettes. The seam is about 3 m thick, under 27 m of overburden [3]. Lignite is mined from the Coteau seam in Ward County at the Velva mine. Seam thickness at the mine is about 3.7 m [3]. The maximum overburden thickness in Ward County is 30 m, with the lignite ranging from 3 to 4.6 m thick [3]. The Coteau bed was named because of its proximity to the rough, hilly moraine belt known as the Missouri Coteau [76]. The Harnisch bed is the uppermost lignite in the Sentinel Butte. It occurs just below the contact with the Golden Valley Formation. It is typically 1.2 m thick. In Dunn County the bed separates into a series of seams separated by clay; the entire interval may be up to 5.5 m thick [71]. The Golden Valley Formation (Eocene) lies above the Fort Union. The Golden Valley contains thin beds of lignite, but the amount is insignificant, and the lignite has little commercial potential [67]. The principal members are a kaolinitic claystone interbedded with sandstone and a micaceous sandstone/siltstone. The former is capped with a thin layer of lignite which seldom exceeds 1.8 m in thickness [58]. The Golden Valley is also included in the Zuni Sequence, which terminated following deposition of the Golden Valley. fiii)

Quality.

North Dakota lignite is mainly brown, though in some deposits it appears

26 black and lustrous. Many samples are conspicuously woody in appearance, often exhibiting the grain or cells of the wood. Breakage along the grain occurs readily, but less easily in other directions. Sometimes flattened trunks or branches are found in the lignite. The extent of the "woodiness" varies with location in the seam, with some portions of the seam consisting of alternating layers of a tough, brown lignite and a black, lustrous, brittle material. Weathered surfaces are generally black, with a bright, vitreous luster. The luster of the weathered surfaces seems inversely related to the ash value; as the yield of ash increases, the luster of the weathered surface is duller. The high moisture content, which may exceed 40% in some locations, makes freshly mined pieces sometimes feel moist. The lignite normally cleaves parallel to the bedding plane but seldom cleaves vertically. Consequently the lignite can sometimes be mined in thick slabs often more than 1 m long. The lignite degrades rapidly on exposure to the air (a process called slacking or slackening). Storage of the lignite occasionally leads to spontaneous combustion. The beds are sometimes marked by clinkers where the lignite has bumed from natural causes (the characteristic color of this material leads to the term "red dog"). Severely weathered exposures of lignite contain a soft, earthy, medium-brown material known as leonardite. Extensive compilations of analyses of North Dakota lignites have been published [20,58,7779]. Moisture and ash values are the most variable. For 413 samples, the average moisture content was 37.5%, with a standard deviation of 2.3 and range of 28.1-45.1% [79]. The average ash value was 6.4% on an as-received basis, with standard deviation of 1.9 [79]. The ash value increases in a westerly direction. Calorific values are virtually constant, 28 MJ/kg on an maf basis [79]. The average sulfur content was 1.1% (maf basis), with a range of 0.3-4.5% [79]. The average CaO content of the ash 31% [79]. The average Na20 content is 6.5%, but the standard deviation is 5.0 and the range is a remarkable 0.1-27.0%. SiO2 is highly variable and averages 27% [79]. Both A1203 and Fe203 show little variability; they average 14% and 12%, respectively [79]. Structural deformation in the Fort Union has been very slight except in the extreme southwestern part of North Dakota (the Badlands) and in the Black Hills region of western South Dakota. Consequently there is little variation in rank throughout the Fort Union, particularly in North Dakota. In eastern Montana the coals of the Fort Union become subbituminous in rank, but the change is so gradual with westward distance that there is no apparent change in North Dakota. In fact, the variation of lignite composition and properties from one horizon to another is about the same magnitude as the variation within a single lignite bed. North Dakota lignites show large within-mine variability of most components. An extreme case is the Gascoyne mine (Harmon Bed, Bullion Creek Formation), for which the following ranges are reported [79]: moisture, 32.444.8%; ash, 5.0-14.3%; SiO 2, 18.0-61.6%; A1203, I0.1-16.2%; Fe203, 2.8-15.1%; CaO, 14.7-44.7%; and Na20, 1.1-10.2%. The average quality of Gascoyne lignite, on an as-received basis, is a calorific value of 14.4 MJ/kg, with a proximate analysis of 40% moisture, 25% volatile matter, 26% fixed carbon, and 6% ash [3]. The average as-received sulfur content is 0.75% [3].

27 The quality of lignite from the Center mine (Hagel bed, Sentinel Butte Formation), on an asreceived basis, is calorific value 15.3-16.4 MJ/kg, 6--7% ash, 0.6-0.9% sulfur, 35% moisture, 29% volatile matter, and 29% fixed carbon [3,58]. The lignite is black to brownish black and slacks rapidly when exposed to air [74]. Lignite from the Glenharold mine in this seam is slightly better with regard to calorific value, ash, and sulfur, being, on an as-received basis, 16-16.5 M.l/kg calorific value; 38% moisture, 26-27% volatile matter, 30-31% fixed carbon, and 4-5% ash; and 0.4-0.5% sulfur [3,58]. From the Falkirk mine, also operating in the Hagel seam, the average quality is 14.9 MJ/kg calorific value; proximate analysis of 39% moisture, 29% volatile matter, 28% fixed carbon, and 7% ash; and a sulfur content of 0.6% (as-received basis). Typically the lignite of the Dunn Center deposit of the Sentinel Butte is 38-39% moisture, 27% volatile matter, 27% fixed carbon, 7-8% ash, 0.8-1% sulfur with a calorific value of 14.6 MJ/kg, on an as-received basis [58]. The average quality of lignite from the Leigh seam, used commercially for the manufacture of briquettes, is, on an as-received basis, 15.1 MJ/kg calorific value; 35% moisture, 25% volatile matter, 32% fixed carbon, and 7% ash; and 1.2% sulfur. The average qualities of lignites from the Dakota Star, Indian Head, and Beulah mines (Beulah-Zap bed) are quite similar. On an as-received basis the calorific value is 16-16.5 MJ/kg. The proximate analysis is 35-37% moisture, 26-29% volatile matter, 28-31% fixed carbon, 6 8% ash, and a sulfur content ranging from 0.6% at the Indian Head mine to 0.8% at Beulah [3,58]. Indian Head lignite is characterized by abundant, thick vitrain. Coalified tree stumps near the top of the seam have been observed, ranging in diameter from 15 to 75 cm [75]. Indian Head lignite is high in sodium, containing typically 8.5% Na20 in the ash [75]. The overburden is a highly impermeable clay low in carbonate minerals, the reverse of the situation for the low-sodium Hagel seam at the Baukol-Noonan mine. The composition of lignite from the Zap bed south of Lake Sakakawea, on an as-received basis, is 34% moisture, 30% volatile matter, 30% fixed carbon, 6% ash, 0.5% sulfur with a calorific value of 16.4 MJ/kg [58]. Velva lignite, mined from the Coteau seam, on an as-received basis, averages 15.8 M,l/kg calorific value; 37% moisture, 27% volatile matter, 31% fixed carbon, and 5% ash; with 0.2% sulfur [3]. It is remarkable for a very high calcium content, sometimes exceeding 40% reported as calcium oxide in the ash. The lignite in the Coteau bed is black and less woody in appearance than most of the lignite elsewhere in North Dakota. The lignite is intermediate in appearance between a cannel coal and the brown lignite typical of much of the North Dakota lignite [76]. It readily splits along the bedding plane, and tends to slack when exposed to sun and air movement, or other storage conditions that allow the moisture to evaporate. The lignites in the Fort Union Group frequently serve as aquifers. (In fact, any rock with 35% moisture arguably is an aquifer [58].) The yield of water is generally 4 to 40 L/min [58]. The chemical quality of the water from the lignite aquifers varies greatly in hardness, being extremely hard in some cases, and frequently shows a reddish-brown coloration due to dissolved or suspended organic materials. The water is used for both for domestic purposes and for watering livestock. Groundwater flowing into the pits can be a problem in many of the lignite mines in the

28 region. Bedrock aquifers consist mainly of consolidated sandstones and limestones with some interbedding of shales, siltstones, or claystones. However, some of the sands and clays are associated with lignites. Both the Fort Union and Hell Creek Formations contain major lignite beds that are aquifers. 1.3.2 Montana (i) Occurrence. About 200 Gt of mineable coal are available in Montana, and, of this, about three-fourths is lignite or subbituminous in rank. The lignite reserves of Montana are estimated to be 79 Gt [80,81], generally found in the northeastern part of the state. The area underlain by lignite exceeds 18,000 km2 [20]. Many of the strippable coal beds are up to 12 m thick. Six continuous lignite beds are present in northeastern Montana; four are fairly thick [82,83]. The seams range from a few cm to over 6 m in thickness [20], but tend to be in the range of 4 to 6 m, with a maximum of about 26 m [8]. The vertical range of lignite seams exceeds 450 m [20]. The Fort Union Formation contains over 90% of the coal reserves of Montana [81]. The formation derives its name from Old Fort Union, which was located near the confluence of the Missouri and Yellowstone Rivers [84]. In Montana the Fort Union Formation has been divided into three members, which are, from oldest to youngest, the Tullock, Lebo, and Tongue River Members [85]. The three members are of Paleocene age. The base of the Tullock was defined to be the first persistent bed of lignite above the Cretaceous dinosaur-bearing beds [63]. The contact with the Lebo Member is taken to be the base of the Big Dirty lignite seam [85]. The Lebo shale member is distinguished from the Tongue River on the basis of lithologic differences [81]. Near the North Dakota border the rocks occurring at the same stratigraphic position as the Lebo are known as the Ludlow Formation. The coals of the Tongue River Member are lignite or subbituminous in rank. In the eastern part of the state the Tongue River is 335 m thick [81]. Some lignites of Tertiary age occur in former lake beds which are now basins lying between mountain ranges in the southwestern portion of the state [81]. Strippable deposits of lignite occur in the eastern part of the state, in the Tongue River Member. The demonstrated reserve of strippable lignite in Montana is 14.3 Gt, based on the criteria of a minimum seam thickness of 1.5 m, a maximum overburden of 38 m, and a maximum stripping ratio of 8:1 [1]. The lignites occur in the Tongue River member are found in the Powder River and Williston Basins, which respectively contain 8.8 and 6.4 Gt of reserves recoverable by strip mining [3]. The reserves in the Powder River Basin are shown in Table 1.12 [3]. The Tongue River contains most of the economically important coal deposits of eastern Montana, with total strippable reserves of all ranks estimated to be 25 Gt [81,85]. The Knobloch Bed contains the largest strippable reserves in the Tongue River, 6.8 Gt [85]. Here the lignite seams may be up to 12 m in thickness, and much of the lignite is within 30 m of the surface [3]. A summary of the reserves recoverable by strip mining is given in Table 1.13 [3].

29 TABLE 1.12 Powder River Basin lignite reserves recoverable by strip mining [3]. Area Reserves, Mt Ashland 1633 Beaver-Liscom Creek 272 Broadus 590 Cache Creek 18 Diamond Butte 227 East Moorhead 318 Fire Creek - Pinto Creek 136 Foster Creek 984 Goodspeed Butte 272 Pumpkin Creek 1452 Sand Creek 168 Sonnette 454 Threemile Butte 91 West Moorhead 1361 Yager Butte 816 Total ..................................... 8792

TABLE 1.13 Williston Basin reserves recoverable by strip mining [3].

Area Reserves, Mt Beach- Wibaux 544 Breezy Flat 272 Burns Creek 1360 Coalridge 72 Ekalaka 18 Fort Kipp 113 Four Buttes 45 Fox Lake 136 Glendive 726 Hardscrabble Creek 181 Knowlton 454 Lame Jones - Milk Creeks 91 Lanark 45 Lane 181 Little Beaver Creek 82 Little Sheep Mountain 109 North Fork- Thirteenmile Creeks 544 O'Brien - Alkali Creeks 318 Pine Hills 45 Poplar River 45 Redwater Creek 454 Reserve 27 Smith - D r y - Parsons Creeks 91 Weldon - Timber Creeks 318 Wolf Creek 136 Total .................................... 6407

30 Cii) Quality. The coals of Montana show a progressive increase in rank westward from the border with North Dakota. Generally the lowest rank occurs along the North Dakota border and the highest, along the Wyoming border. In easternmost Montana the coal is lignite; near Miles City it is subbituminous; west of Miles City it becomes subbituminous C in rank; and reaches bituminous rank near Red Lodge. The coal in the Fort Union Formation in eastern Montana ranges in calorific value from 16.3 to 20.9 MJ/kg on an as-received basis [86]. With the westward increase in rank is a concomitant decrease in moisture content. Montana lignites generally have as-mined moisture contents in excess of 35%, with about 30% fixed carbon and 25-30% volatile matter [79]. Lignite in eastern Montana contains up to 43% moisture [81]. The ratio of fixed carbon to volatile matter averages 1.18 [79]. Ash values are typically in the range of 5-14%; and sulfur, 0.5-1.0%, although sulfur contents up to 1.7% have been reported [79,81]. On an maf basis the sulfur content averages 1.0% [79]. The average as-received calorific value is 15.8 MJ/kg [79]. The heating values show a tendency to increase from east to west. In the ash the CaO content averages 27%; MgO, 10%. The SiO2 content averaged 31%; A1203, 20%; and Fe203, 8% [79]. The iron content of the ash is very variable. Lignites formed in lake bed deposits of Tertiary age are much higher in ash value, about 20%, but of comparable sulfur content, relative to the Fort Union lignites [81]. As a rule, beds of thickness greater than 1.5 m have lower ash and are more uniform in quality than the smaller beds [87]. However, the quality is highly variable laterally; some lignite seams up to 4.6 m thick which are low ash may become high in ash value in a lateral distance of a few hundred meters to a kilometer [87]. The lateral variability originates from unstable conditions of deposition in the bogs or marshes where the original organic matter was accumulating. Lignites from Richland County show subconchoidal fractures and a brown streak [87]. Transverse to the bedding plane, a fresh surface usually shows compact, alternating layers of dull and shiny material. The woody texture of the lignite is preserved, with only a moderate flattening of the cells. On exposure to air, the lignite loses moisture, and the surface luster dulls; small cracks appear, which gradually become deeper and more numerous until the sample slacks to a powder. Fresh samples of lignite from the Knobloch bed show a bright luster, attributed to previtrain, a material derived from the coalification of single, relatively large fragments of ancient plants. Lignite occurs along Coal Creek near the North Fork of the Flathead River in Flathead County, as well as along the Middle and South Forks of the Flathead River [88]. Only the deposit along the North Fork of the fiver was actively worked. The lignite from the Coal Creek region contains nodules of mineral resin [89]. Tables of analyses of Montana lignites have been published [20,79,90] including data on ash composition [79]. A compilation of analyses of samples of the major deposits in seven Tongue River coal beds has been published [85]. (iii) Powder River Basin lignites. In the Moorhead area the Anderson, Dietz, and Canyon beds have commercial potential [91]. Dietz coal generally ranks as subbituminous C [91], but is classified as lignite in the regions of Decker and West Moorhead. In the Decker area the Dietz

31 seams combine with the Anderson to produce a multiple seam up to 24 m in thickness [3]. The lignite contains about 5% ash and 0.35% sulfur and has a calorific value of 17.9 MJ/kg, on an asreceived basis [3]. The quality is very similar in the West Moorhead region, where the seam thickness is about 5 m [3]. The West Moorhead coal is classified at the exact upper limit of lignite, having a calorific value of 19.3 MJ/kg on a moist, mineral-matter-free basis [3]. The Canyon, one of the most widespread beds in Montana, is not commercially mined. Much of the coal is classified as subbituminous C, but some lignite and some subbituminous B coals have been reported [91]. Lignite occurs near Diamond Butte and Threemile Butte. At Diamond Butte, the average seam thickness is 3.5 m [3]. On an as-received basis, the lignite quality is 4 - 5 % ash, 0.3-0.4% sulfur, and 17-17.4 MJ/kg calorific value [3]. In the Threemile Butte area, the seam has an average thickness of 2.6 m and the lignite is of lower quality: 6.2% ash, 0.9% sulfur, and 15.8 MJ/kg (as-received basis) [3]. Of the Tongue River coals in Montana, the Canyon bed shows both the greatest ash value and the greatest variation in ash value. The maximum thickness of the Knobloch bed is 20 m in the Otter Creek and Ashland deposits. The Knobloch is not currently mined. The rank varies from lignite to subbituminous. Lignite is found in the Foster and Sand Creeks areas. The Sand Creek lignite has a calorific value of 17 MJ/kg, with 6.5% ash and 0.3% sulfur [3]; Foster Creek, a calorific value of 17.7 MJ/kg with 7.5% ash and 0.9% sulfur [3]. At Foster Creek the seam is about 3.4 m thick [3]. The magnitude of the Foster Creek deposit is illustrated by the estimate that sufficient reserves exist to fuel thirty 200 MW electric generating stations (i.e., 6000 MW total) for forty years [92]. In the Beaver Creek-Liscom Creek area, the Knobloch is about 4.6 m thick; the lignite is about 9% ash and 1% sulfur [3]. The coal in the Knobloch bed reaches a thickness of 14 m on Otter Creek; the rank ranges from lignite to subbituminous C [86]. The Otter Creek deposit is low in ash and sulfur. Characteristics of the Knobloch coals are summarized in Table 1.14 [90]. TABLE 1.14 Characteristics of Knobloch bed coals [90]. Coal deposit Ashland Beaver - Liscom Creeks Foster Creek Otter Creek Poker Jim O'Dell Creeks Sand Creek

Cal. value, MJ/kg 17.8-21.1 17.1-19.6 17.2-18.2 18.6-21.2 19.5--21.2 16.8-17.3

Sulfur,% 0.1-0.5 0.2-0.9 0.3-1.6 0.1-0.4 0.1-0.6 0.3

Ash, % 3.7--6.8 5.1-13.8 6,7-8.7 3.0-10.6 3.7-6.4 5.1-8.3

The Roland seam represents the top of the Tongue River Member of the Fort Union Formation. The rank ranges from lignite in the Squirrel Creek area to subbituminous in the Roland area. The seam thickness ranges from 1.8 to 4.3 m [3]. In general the coal in the Roland deposit ranges in calorific value from 16.3 to 21.2 MJ/kg, with sulfur content of 0.2-0.7% and ash value

32 of 3.8--9.7% [90]. In the Squirrel Creek deposit the calorific value ranges from 15.4 to 19.3 MJ/kg; sulfur, 0.2-0.6%; and ash, 3.0-14.2% [90]. The Roland bed is about 60 m above the Smith bed [91]. The Cook bed lies below the Canyon and above the Wall beds. The bed occurs in Powder River County in the regions around Goodspeed Butte, Sonnette, and Yager Butte. The seam occurs as two benches. In all three areas the calorific value is 15.6-15.8 MJ/kg and the ash content is 10-12% on an as-received basis [3]. In the Yager Butte deposit the benches are 12-23 m apart, and thicknesses are 0-5.8 m for the upper bench and 2-3.4 m for the lower [90]. The as-received sulfur content is 0.5% [3]. The Goodspeed Butte lignite benches average 5 and 4 m, but the sulfur content is 1.5% [3]. The benches lie 10-14 m apart [90]. At Sonette the bench thicknesses and the sulfur content are intermediate between these extremes. Here the benches are 6.6 to 12 m apart [90]. Characteristics of the Cook Bed coals are shown in Table 1.15 [90]. TABLE 1.15 Characteristics of Cook bed coals [90]. Coal deposit Goodspeed Butte Sonnette Yager Butte

Cal. value, M.l/kg 15.5-16.0 15.2-16.7 13.7-17.9

Sulfur,% 1.2-2.1 0.7-1.9 0.3-0.7

Ash,% 8.9-12.4 6.5-13.3 3.8-20.7

The Elk bed is also in the Yager Butte area. It is about 4.6 m thick [3], with a range of 3 to 6.4 m [90]. Lignite quality, on an as-received basis, is 17.4 MJ/kg calorific value, 5.5% ash, and 0.35% sulfur [3]. The Dunning bed lies below the Elk. The average thickness of the Dunning is 5 m [3], with a range of 4.3-6 m [90]. On an as-received basis, the calorific value is 17.9 MJ/kg; ash, 5%; and sulfur, 0.3% [3]. The Sawyer bed occurs in Powder River County around Ashland and Pumpkin and Little Pumpkin Creeks. The thickness reaches 11 m [90]. Sulfur content is low, 0.4-4).6%. In the Ashland area the ash is 5% and the calorific value is 18.1 MJ/kg; the ash value is higher near Pumpkin Creek, 8.5%, and thus the calorific value is somewhat lower, 17 MJ/kg [3]. In Powder River County the Broadus bed ranges up to 7.6 m thick [3,90]. The maximum thickness is 8 m at the Peerless mine [86]. The calorific value is 17.2 MJ/kg; ash, 7%; and sulfur, 0.3% [3]. The reserve estimated to be recoverable by strip mining is 590 Mt [3]. The Broadus bed lies 30 m above the base of the Tongue River [90]. The coal in the Flowers-Goodale seam varies in rank. In the Foster Creek area the rank is lignite, with a calorific value of 17.7 MJ/kg, 8% ash and 0.5% sulfur [3]. The seam thickness in the Foster Creek area ranges from 60 cm to 4.3 m [92]. In the Beaver Creek-Liscom Creek area the Terret seam is about 2.4 m thick; the average quality is 16 MJ/kg, 9% ash, and 1.1% sulfur, on an as-received basis [3]. In the Foster Creek area the coal is at the border of classification between lignite A and subbituminous C, with a moist, mineral-matter-free calorific value of 19.4 MJ/kg. This coal has a 6% ash value and a remarkably

33 low sulfur content of 0.2% [3]. Here the seam is about 2.7 m thick [3,92]. In the East Moorhead area the T seam ranges in thickness to 7.6 m; the lignite quality is 6% ash, 0.6% sulfur, and 16.5 MJ/kg on an as-received basis [3]. At East Moorhead the ash ranges up to 13.2% and the sulfur to 1.2% [90]. The T bed lies about 80 m above the Broadus [90]. The Pawnee bed is about 6 m thick in the regions of Cache Creek and Pumpkin Creek near Sonnette [86,90]. The Pawnee forms two benches which may be up to 14 m apart [79]. Characteristics of the coal in the Sonnette field are 12.9-18.4 MJ/kg calorific value, 0.2-2.7% sulfur, and 3.925.3% ash [90]. In the Fire Gulch deposit, the calorific value is 17.8 MJ/kg with 0.2% sulfur and 6.0% ash [3]. The Brewster-Arnold bed lies 72-84 m below the Wall bed [90]. The maximum thickness is 6 m [90]. In the Birney field the characteristics of the Brewster-Arnold coal are 18.6-21.9 MJ/kg calorific value, 0 . 2 - 0 . 7 % sulfur, and 3.1-8.2% ash [90]. (iv) Williston Basin lignites. The Dominy bed contains two or three benches, the lower averaging 6 m and the upper 2 m in thickness [90]. In the Knowlton deposit the coal quality is 14.6-15.9 MJ/kg calorific value, 0.2-0.9% sulfur, and 3.8-10.5% ash [3,90]. Comparable values for the Pine Hills deposit are 16.9-17.2 M.l/kg, 0.4-0.6% sulfur, and 6.6-8.1% ash [3,90]. In the Knowlton area the bed occurs as three benches, the upper being 8 m; the middle, 4 m; and the bottom, 3 m. In the Pine Hills area the lower bench is 6 m thick [3]. In the Poplar River area these seams are 2.5 to 3 m thick. The calorific value is low, 13.7 MJ/kg on an as-received basis, with 9% ash and 0.5% sulfur. In the Beach-Wibaux area, the C seam ranges to 9 m in thickness, with 36 m or less of overburden. The maximum thickness is 12 m, at the Black Diamond mine [3]. In some places the overburden is less than 18 m thick. The lignite quality, on an as-received basis, is 14 MJ/kg calorific value, 8% ash, and 0.9% sulfur [3]. In the Four Buttes area, the seam is much thinner, only about 3 m on average. The lignite quality is much the same, being about 14.2 MJ/kg calorific value, 10% ash, and 1.1% sulfur on an asreceived basis [3]. In the Burns Creek area the Pust seam ranges up to 6 m in thickness, with less than 60 m of overburden [3]. The lignite quality is 14.2 MJ/kg calorific value, 8% ash, and 0.65% sulfur [3]. In the North Fork-Thirteenmile Creek area, the seam is 8 m thick, and the quality has improved. Here the average calorific value is 16 M.l/kg, with 7% ash and 0.5% sulfur [3]. Seam thickness is 3.4 m in the Fox Lake area, with 15.8 MJ/kg calorific value, 6% ash, and 0.5% sulfur. In the Redwater Creek area the S seam can reach 6 m in thickness, usually under less than 46 m of overburden [3]. On an as-received basis the lignite has a calorific value of 15.8 MJ/kg, with 11% ash and 0.4% sulfur [3]. In the Weldon-Timber Creek area, the seam thickness is only 3.6 m, but the quality has improved to 17.4 MJ/kg calorific value, 6% ash, and 0.3% sulfur [3]. In the Breezy Flat area, lignite is mined commercially at the Savage mine, the output supplying a power plant at Sidney. At the Savage mine the seam thickness is about 6 m. The calorific value on an as-received basis is 15.1 MJ/kg. The proximate analysis averages 38% moisture, 27% volatile matter, 27% fixed carbon, and 7% ash. The sulfur content is 0.5% [3]. In the Hardscrabble Creek area the F and H seams range from 2 to 3 m in thickness, with

34 overburden less than 60 m [3]. On an as-received basis the lignites have a calorific value of 15.1 MJ/kg, ash of 7% and sulfur content of 0.6% [3]. The Fort Kipp and Peck seams are each about 1.5 m thick, separated by a parting of about 6 m. In the Fort Kipp area the average calorific value is 15.8 MJ/kg on an as-received basis, with 5% ash and 0.3% sulfur. In the Reserve area the Timber Coulee seam is about 3 m thick. Quality is 15.8 MJ/kg calorific value, 7% ash, and 1% sulfur on an as-received basis [3]. The Smith bed lies about 34-46 m above the Anderson bed [91]. The Smith is generally thin, but is remarkable for the large number of petrified tree stumps it contains. Many of the stumps remain in an uptight position. Two samples of the Smith bed lignite from the Decker deposit have calorific values of 17.7 and 19.2 MJ/kg, sulfur 0.6 and 1.0%, and ash 6.8 and 30.2% [90]. Silica was a major constituent of the ash, amounting to 38.5 and 78.3% of the ash. 1.3.3 Saskatchewan Lignite provides about a third of Canada's recoverable coal reserves on a weight basis, and about a fourth when the amount of coal is expressed on a calorific value basis [93]. The province of Saskatchewan contains the major Canadian lignite deposits, which are of Tertiary age. The resources amount to 3.8 Gt, of which 1.36 Gt are measured and the remaining 2.43 Gt are indicated [94]. The total recoverable reserves of Saskatchewan lignite are estimated to be 2.1-2.4 Gt, all extractable by surface mining [93,95]. On an as-mined basis, the Saskatchewan lignites tend to fall in the range of 20-32% moisture, 15-25% volatile matter, and 13-35% ash [94]. Sulfur contents are low, in the range of 0.3-0.8% (daf basis). The calorific value ranges from 12-15.4 MJ/kg. Some of the properties of lignites from the major areas in Saskatchewan are summarized in Table 1.16 [94]. TABLE 1.16 Quality of Saskatchewan lignites [94].

Area Cypress Estevan Willow Bunch Wood Mountain

Proximate Analysis Moisture,% Vol. Mat.,% 20-28 22-45 27-32 22-25 25-32 20-25 24-28 15-23

Ash,% 15-23 13-25 14-30 22-35

Calorific val. MJ/kg, moist 8.8--13.7 13.0-15.4 12.1-15.1 10.7-13.7

The sulfur content of the lignites from all four areas lies in the range 0.2-0.6% [94]. Data on the quality of lignites in Saskatchewan is available in [96]. Lignite occurs principally in four basins: Cypress, Estevan, Willow Bunch, and Wood Mountain; these deposits are collectively known as the Ravenscrag Formation. A small deposit, in seams too thin for profitable mining (i.e., less than about 1.5 m), occurs in the area of Wapawekka near Lac La Ronge. Not only are the seams thin, but this lignite is very high ash.

35 Seams in the Ravenscrag Formation range from 1 to 3 m; generally the lignite is shallow and level [95]. Lignite of the Ravenscrag Formation is mined in a 2.5 m seam in the Klimax Mine of the Butte River Coal Company near Estevan [97]. The Ravenscrag Formation is equivalent in age (Paleocene) to the Fort Union lignites in the United States. At Estevan there are eight lignite seams in a stratigraphic interval of 230 m [97]. Of these, the four upper seams amount collectively to 8 m of lignite, and occur in an interval of 38 m [97]. The annual production of Saskatchewan lignite is about 10 Mt [95], used almost entirely for electric power generation. Most is burned in minemouth plants, but about 1 Mt per year is exported to Ontario. The lignite in Saskatchewan is a major contributor to the electric power production in that province. In 1982 the Saskatchewan Power Corporation consumed 71% of the lignite produced in the province [98]. Saskatchewan Power operates the Souris Valley and Poplar River mines. Other mines which provide lignite to Saskatchewan Power are the Utility and Costello mines (Manalta Coal Ltd.) and the Boundary and Bienfait mines (Luscar Ltd.). The Saskatchewan lignite deposit extends into the southwestern portion of Manitoba, but in Manitoba the seams are so thin as to not be currently considered for mining. 1.3.4 South Dakota (i) History. The total recorded lignite production in South Dakota is only 1.23 Mt, through January 1, 1964 [ 103]. Production peaked in 1941, when 63.5 kt of lignite were mined [79,103]. Most of the production has come from strip mines in Dewey and Corson counties. Virtually all the lignite mined in South Dakota for fuel use was used for domestic heating. During the settlement of the Dakotas, the lignite deposits in the southeastern quarter of South Dakota were of considerable local interest, because of the great distances to other supplies of fuel. The only gun battle ever reported in the lignite literature is said to have occurred as a result of a dispute over a lignite seam in Big Sioux Valley [ 104]. The lignites of South Dakota have been of some commercial interest due to their uranium content. During active mining in 1955-8 and again in 1962--6, 320 t of uranium were recovered from lignites of the Slim Buttes and Cave Hills areas of Harding County. At one time, the most important lignite mining area of South Dakota was the Isabel-Firesteel district in Dewey and Ziebach counties. The lignite in this district is neither as thick nor as extensive as other deposits in the state, but commercial activity was greatly facilitated by the availability of a railroad line which allowed the lignite to be transported and marketed away from the immediate area of mining [3]. (ii) Occurrence. Lignite occurs in a rectangular area in the northwestern part of the state, primarily in Perkins and Harding counties. Lignite having commercial potential occurs in Harding, Dewey, Perkins, and Corson counties, and the northern portion of Meade County; small portions of five other counties may contain lignite. The lignites are an extension of the larger deposits in North Dakota. They occur in the Lance and Fort Union Formations (Tertiary). Bed thickness is typically 1.2 to 1.8 m [8]. The Fort Union Formation does not extend very far into South Dakota, and little lignite is found among the Fort Union sediments in this state. The predominant lignite

36 beds occur in the Lance Formation, particularly in the Ludlow and Hell Creek members [18]. The lignite-bearing area of South Dakota is at the southern edge of the Fort Union region, near the edges of the areas in which these lignites were deposited. Thus the beds tend to be thinner and more lenticular than the lignite deposits of North Dakota and Montana. The oldest lignite-bearing formation is the Fox Hills. The Fox Hills Formation is the first shore deposit of the Cretaceous sea as it began its retreat. The Stoneville lignite-bearing member lies in the upper third of the Fox Hills [ 104]. The Hell Creek Member (late Cretaceous) of the Lance Formation overlies the Fox Hills. The Hell Creek underlies most of the northwestern quarter of the state [103]. It contains much of the South Dakota lignite. The lignite outcrops form a semicircle in Corson, Dewey, Harding, and Perkins counties. The Hell Creek lignite generally occurs in thin, lenticular beds less than 300 m in diameter and 60-100 cm thick [105], and in fact it is better known as a good source of dinosaur fossils than as a lignite source. Some of the Hell Creek lignite occurs in beds of 1.2 to 1.8 m in thickness [ 104]. The lignite beds generally occur 12-24 m above the base of the formation. Seams thick enough to support commercial mining are found in the Isabel-Firesteel area, Dewey and Ziebach counties; Gopher, Corson County; and the Slim Buttes area of Harding and Perkins counties. Mining has also occurred in northern Meade County. The Fort Union Formation comprises, in ascending order, the Ludlow, Cannonball, and Tongue River Members. The Cannonball Member is a marine unit and does not contain lignite; however, the Ludlow and Tongue River Members are non-marine and are lignite-beating. The Ludlow is the most prolific lignite-bearing unit in South Dakota [106]. The Tongue River is present only in the northern parts of Harding and Perkins Counties. Although the seams range up to 2.7 m in thickness [103], the total amount of lignite in this member in South Dakota is small because of its limited areal extent. The Ludlow Member of the Fort Union Formation overlies the Hell Creek. In South Dakota the Ludlow contains the greatest amount of lignite. In Harding County the Ludlow may be up to 107 m thick. The Ludlow is thought to be equivalent to the Lebo Shale and the Tullock Member of the Fort Union in Montana [106]. The beds containing the largest reserves of South Dakota lignite are the Widow Clark and the Giannonatti, both of which occur in the Ludlow member of the Fort Union in Harding County [ 106]. The abundant lignite seams in Harding County occur in terrestrial and shore-line deposits of sand and sandy shales. These lignite are the most persistent and thickest in South Dakota, with a maximum thickness of 4.3 m [ 104]. The two beds make up the Cave Hills field. The Giannonatti is the thickest bed in South Dakota, having a maximum thickness of 4.2 m [ 106], but it tends to thin rapidly. The Giannonatti seam has an average thickness of 1.8 m and a maximum of 4 m [3]. The maximum overburden thickness is 37 m [3]. The lignite in the Slim Buttes area averages about 1 m in thickness [3]. The youngest lignite is in the Tongue River Member of the Fort Union. The Tongue River overlies the Ludlow. Despite the importance of the Tongue River as a source of lignite in North

37 Dakota, Montana, and Wyoming, it contains little lignite in South Dakota. In Perkins County the thickest bed is the Lodgepole, one of several beds of lignite in the Tongue River Member, which ranges from 1.5 to 2.7 m thick [3,106]. The lignite reserves of South Dakota are estimated to be 1.8 Gt [80,106]. The area of the state underlain by known lignite deposits is 20,000 km2 [103,106]. About 84% of the reserves are found in Harding County; most of the rest in Perkins County (about 9%) and Dewey County (about 6%). Very small amounts of lignite are found in Corson, Meade, and Ziebach counties. The demonstrated strippable reserve base in South Dakota is 360 Mt, based on the criteria of a minimum seam thickness of 1.5 m, a maximum overburden of 30 m, and a maximum stripping ratio of 12:1 [1]. The major reserves recoverable by strip mining are shown in Table 1.17 [3]. TABLE 1.17 Reserves of South Dakota lignite recoverable by strip mining [3]. Area Reserves, Mt Cave Hills 159 Isabel - Firesteel 91 Lodgepole 27 Slim Buttes 23 Total .............................. 300

The total resources have been estimated at 2.7 Gt [8]. Over 900 Mt of lignite are estimated to occur in seams more than 86 cm thick [107]. This estimate does not take into account the entire lignitebearing region of South Dakota [ 104]. Based on the original known resources estimate of 1.8 Gt [106], 63% occurs in seams of 0.8-1.5 m thickness and only 3% in seams greater than 3 m [103]. The Stoneville area is underlain by at least 45 Mt of lignite [108]. However, only 1.1 Mt occurs in deposits thick enough, and accessible enough, for mining [104]. Perkins and Harding counties contain 995 kt of mineable lignite, excluding the Hell Creek and Fox Hills lignites [ 107]. In the southeastern quarter of South Dakota some lignite seams 5-10 cm in thickness are exposed in the Dakota Formation of Big Sioux Valley and in the Niobrara Formation near Springfield [ 104]. Some lignite has been encountered by water well drillers at depths of 18-30 m from the surface [104]. These occurrences have been reported near Stickney, Aurora County; Geddes, Charles Mix County; and Fulton, Hanson County. These lignites occur in or near the top of the Dakota Sandstone and the base of the sand in the Graneros Formation. (iii)

Quality.

The lignite beds range from a few cm to over 5 m in thickness [20]. In

general, the seams in South Dakota are not as thick as lignites of the same strata in Montana and North Dakota. Most of the lignite is close to the surface, with overburden thickness typically in the range of 1.8 to 6 m [20]. The lignite beds are generally flat-lying in regions of little to moderate topographic relief. The average calorific value of the lignites in the Lance and Fort Union Formations is 14-16.3 MJ/kg and the average sulfur content is less than 1% [8,104]. On an as-

38 received basis, the Fort Union lignite is 34.3% moisture, 29.1% volatile matter, 29.5% fixed carbon, and 7.1% ash [ 104]. Proximate analyses of some South Dakota lignite samples have been tabulated in [20]. The average analyses of Ludlow lignites are shown in Table 1.18 [ 104]. TABLE 1.18 Average analyses of lignite from Ludlow member, Lance Formation, South Dakota [ 104].

Proximate, % Moisture Volatile Matter Fixed Carbon Ash Ultimate, % Carbon Hydrogen Nitrogen Sulfur Oxygen Calorific Value, MJ/kg

As-received

Moisture-and-ash-free

37.92 28.43 26.25 10.55

-49.59 50.41 --

3 5.01 6.33 0.50 1.25 45.20 13.4

68.68 4.16 0.99 2.58 23.25 25.7

The average quality of the lignite in the Cave Hills field, on an as-received basis, is a calorific value of 13.5 MJ/kg; proximate analysis of 41% moisture, 24% volatile matter, 26% fixed carbon, and 9% ash; with 0.9% sulfur and 0.1% U308 [3]. Average as-received quality of lignite in the Lxxtgepole seam is 16.3 MJ/kg calorific value, 11% ash, and 0.75% sulfur [3]. The Firesteel seam is 1 to 2 m in thickness [3]. Average as-received quality is 16.3 MJ/kg calorific value, 6% ash, and 0.5% sulfur [3]. The Hell Creek Formation lignite is close to subbituminous in rank. It has been classified as lignite by virtue of the high moisture content, which is sufficient to cause slacking on air drying, and a brown streak test [ 105]. Distinct horizontal laminations are observed in the Hell Creek lignite exposed on the Standing Rock and Cheyenne River Reservations. These laminations are a result of alternation of a dull, dark brown lignite which constitutes most of the seam with a bright, black socalled glance coal occurring in laminae or lenses of about 2.5 cm in thickness. The Stoneville lignite, and the Hell Creek lignite found near Isabel, contain unusual amounts of transparent pale lemon-yellow, brittle, irregularly shaped resin bodies. These resin bodies range from the size of specks to about 6 mm in diameter [105], with reports of some up to 12 mm across [104]. The average as-mined analysis of the Hell Creek lignite is 36.8% moisture, 26.6% volatile matter, 7.1% ash, and 29.4% fixed carbon [104]. The maximum moisture content is about 42% [104]. The average sulfur content, on an as-received basis, is 0.52% [ 104]. The calorific value averages 15.7 MJ/kg on an as-received basis, and 24.9 MJ/kg air-dried [104]. The lignite quality in the Slim Buttes area averages, on an as-received basis, 14.9 MJ/kg

39 calorific value, 7% ash, and 2% sulfur [3]. The ultimate analysis of lignite from the Phillips mine, near Strool, is 72.6% carbon, 4.84% hydrogen, 1.28% nitrogen, 2.39% sulfur, and 18.9% oxygen, on a moisture-and-ash-free basis [104]. The calorific value, on the same basis, is 28.5 MJ/kg. As a rule, the Fox Hills lignites are the thinnest of any of the South Dakota lignites. The seam thickness ranges from a few centimeters up to 1.4 meters, although over most of the area the average is less than 60 cm [ 104]. 1.4 O T H E R L I G N I T E S OF T H E UNITED STATES

1.4.1 California (i) History. The discovery of lignite in California dates from the placer mining of the gold rush days. The first discoveries were in the areas of Big Bar, Cox Bar, and along Hayfork Creek. The existence of the Big Bar deposit on the Trinity River was first noticed in 1896 [109]. For many years the lignite was used locally as a fuel in blacksmith forges in the mining camps. A lignite interbedded with fine-grained sandstone and shale was mined near Big Bar from 1929 to 1934 and was used locally as a fuel [110]. The lignite of the Carmel Mine (Monterey County) was discovered in 1874 [111]. Some of the lignite was shipped to San Francisco for use as a fuel. In 1879 a railroad was constructed to transport the lignite from the mine to the coast (a distance of about 8 km) for loading onto ships [112]. Nevertheless, mining ceased the next year, and then continued in an on-again, off-again manner, apparently subject to various law suits. The total production through 1893 amounted to only 18 kt [113]. By 1925 the mine had been idle for many years [114]. Lignite reserves recoverable by strip mining in Amador County in the region of lone were used a century ago in the locality as a domestic fuel. Occasional use of the lignite in this way continued sporadically until about the time of World War II. Since the late 1940's, the lignite has been mined as a source of montan wax, rather than as a fuel. A lignite seam 1.4 m thick was uncovered by the gold mining operations of the Wiltshire mine, which was a hydraulic mine. Some of this lignite was recovered in 1926 by driving adits into the seam and was used locally as a fuel for blacksmith forges [115]. By 1929 some of the lignite was being transported to Redding and Eureka for use as domestic fuel, but the mine closed long before the onset of World War II [ 116]. The Reese deposit (Trinity County) was mined commercially in 1936-38, the lignite being used locally and transported to Redding. In 1949 the feasibility of using the Reese lignite for the manufacture of barbecue briquettes was investigated, but commercial production was never begun. (ii) Occurrence. The area of the state underlain by lignite is estimated to be 1,300 km 2, with a total of about 900 Mt of lignite [20]. The most extensive deposit of lignite occurs in Amador County. The lignite occurs in the Ione Formation (Eocene) [3]; lignite beds range from a few cm to 9 m in thickness [25]. Only one mine is currently in operation in the area of Ione. The lignite beds

40 and associated clay and sand strata comprise the Ione Formation. The lignites are lenticular and discontinuous. They are believed to have formed in manner similar to the Gulf Coast lignites [ 117]. Interest in these lignites stems from the fact that they are commercial sources of waxes. Small lignite deposits occur near Covelo (Mendocino County), 220 km north of San Francisco, and near Hayfork and Douglas City in Trinity County. The Covelo lignite is Eocene, and the others are probably Pliocene coals [ 118]. Lignite of Eocene or Oligocene age is found in Hayfork Valley and Hyampom Valley, at Poison Camp, and near Redding Creek. The commercial development of these lignites has suffered because of the distance to markets, as well as competition from other fuels and hydroelectric power. (iii)

Quality.

On an as-received basis, the lignite of the Ione Formation is 42% moisture,

32% volatile matter, 14% fixed carbon, and 11% ash, with 1 to 1.3% sulfur and a calorific value ranging from 13 to 14.2 MJ/kg [3]. A discussion of California lignite on a county-by-county basis has been published, with a tabulation of a few analyses [20]. The Ione lignites occur in an Eocene sequence which is mainly sandstone [118]. The lignites have very high in liptinite content, derived from spores, cuticles, resins, and waxes. Much of the liptinite consists of a fine, broken detritus. Cutinite occurs in both thin and massive bands. Blebs of cuticular wax and dispersed microspores can be observed. Resin occurs both in isolated, oval-shaped bodies and in vesiculated rodlets. Although there is a distinct microscopic structure parallel to the contacts, the Ione lignite does not contain massive amounts of woody lignite nor fine attritus, both of which are characteristic of other lignite deposits. Layers of huminite derived from wood, bark, or root material are few. Thus most of the plant organs and tissues have been preserved, but most of the humic material has experienced severe decay. The lignite is still highly liptinitic (as indicated by a high hydrogen index on programmed pyrolysis) even after wax extraction. Furthermore, the liptinitic fragments are not visibly altered by the extraction process. These observations suggest that the wax product is derived from cuticular waxes and not from 9resins or spores [ 118]. The lignites in Trinity County are entirely woody. The Covelo lignite contains abundant remains of fungal sclerotia. The lignite in the Reese deposit (Trinity County) has an ash value of about 15- 20% and a calorific value of 15.8-16 MJ/kg [116]. The seam thickness in the area of Ione is about 3.7 m with about 18 m of overburden [3]. Thicker, lenticular seams reaching 7.6 m in thickness occur at depths to 46 m [3]. The lignite beds near Ione and Buena Vista in Amador County vary in thickness from 1.2 to 9 m [ 154]. The Reese Deposit in Trinity County is 2.6 m thick [155]. The Temblor Formation (Miocene) contains four thin beds of lignite, each less than 90 cm thick [156]. The lignite seams lie in the upper 46 m of the formation. Despite the fact that California lignite deposits are small, the wax industry based on the solvent extraction of these lignites is an important one. The wax-from-lignite operation in California serves as an important reminder that lignites have potential uses other than combustion,

41 and that an assessment of their potential as sources of chemicals rather than as boiler fuels is overdue. 1.4.2 The Rocky Mountains (i) Colorado. Years ago lignite was obtained from underground mines in the areas around Ramah and Scranton. In some of the lignite-bearing areas east of Denver, mining started in 1887 [8]. Lignites occur about 24 km east of Denver, and continue southward into Adams, Arapahoe, and Elbert counties. Reserves of lignite accessible by strip mining are present in the Denver and Cheyenne basins [3]. The reserves remaining in the Laramie Formation are estimated at 18 Gt, and, in the Denver Formation, 9 Gt [1]. The Denver Formation may also contain up to another 900 Mt at depths greater than 300 m [1]. Large reserves of lignite occur in the Denver Formation (Paleocene) beneath the Dawson Arkose [3]. Adams, Arapahoe, Elbert and E1 Paso counties contain an estimated 900 Mt of lignite in areas with less than 60 m of overburden [3]. In the Denver Basin east and southeast of Denver there are estimated to be several Gt of lignite [120]. The Laramie Formation (Cretaceous) contains seven seams, of which No. 6 is the one most readily exploited. Strip mining potentially could occur in portions of Adams, Arapahoe, Boulder, and Weld counties, in a region roughly bounded by Eldorado Springs, Frederick, and Henderson. The coal seams in the Laramie Formation are lenticular, but may range up to 3 m in thickness [3]. The No. 6 seam contains coal which is at the lignite A / subbituminous C boundary, having a calorific value of 17.4-20 MJ/kg (as-received basis) [3]. Other properties of this coal, on an asreceived basis, are 22% moisture, 35% volatile matter, 8% ash, and 0.5% sulfur [3]. The lignite in the Denver Formation is brownish black. Like most lignites, it readily undergoes weathering, slacking, and disintegration. The moisture content is 22-40% and the calorific value is 9.3-17.4 MJ/kg [1]. Most of the coal is classified as lignite A, but some of the beds contain intervals that rank as subbituminous C. The lignite quality in the Denver Formation shows a wide range, having, on an as-received basis, 22-40% moisture, 8-30% ash, and 0.2-0.6% sulfur [3]. Lignite also occurs in the Denver Formation (Upper Cretaceous to Paleocene) [ 1]. Denver Formation lignites were deposited in swampy areas, and occur in sequences with shales, claystones, and sandstones. A useful compilation of lignite analyses from the Denver Region is available [ 121]. The overburden is composed of thick sandstones and conglomerates. Many of the lignite beds contain partings of kaolin which may range from about one cm in thickness to over 60 cm [ 1]. The seams become thinner southward in the Denver Formation, being 3-4.5 m thick in the north, and about 1.5-3 m thick in the south [3]. The multiple beds in the Denver Basin are relatively thick. The total thickness of lignite is 18 to 24 m [ 120], with the lignite beds separated by mudstones. The lignites lie at depths of 76 to 460 m [120]. (ii) Idaho. Lignites of poor quality occur in Cassia County southwest of Oakley [3]. The lower Salt Lake Formation (Tertiary) contains six seams, each 0.6 to 1.5 m thick, under less than 24 m of overburden [3]. The lignite quality, on an as-received basis, shows 20-35% moisture,

42 30- 60% ash, and 1-1.3% sulfur, with calorific values less than 11.6 MJ/kg [3]. 1.4.3 The Plains States (i) Kansas. The lignite reserves in Kansas are estimated to be 179 Mt [80]. The estimate is somewhat uncertain because the lignite beds are less than 76 cm in thickness, the minimum bed thickness accepted by the U. S. Geological Survey for estimating lignite reserves. Consequently the estimate is derived from the State Geological Survey of Kansas [ 122]. Lignite occurs in the Dakota Formation (Cretaceous) in the north-central part of the state. Lignite occurs in 19 counties. The lignite has been mined in at least 12 counties in the years 18751940. The original reserve of lignite in these 12 counties was 180 Mt [8]. The last active lignite mine was in Cloud County, and closed in 1940 [8]. The lignite was used for domestic heating and as a locomotive fuel. The lignite beds in Kansas are extremely variable in thickness. Seam thickness ranges from 5 cm to 3.7 m [8]. The sulfur content ranges from 0.48 to 6.28% [8]. The highest reported calorific value is 17 MJ/kg. (ii) Oklahoma. Lignite is found in the Purgatoire Formation in Cimmaron County. The Purgatoire Formation runs along the northwestern portion of the county, where the lignite seams are exposed in areas about 5 km apart [8]. The seam thickness is about 46 cm. The lignite has been mined on a local basis for domestic fuel. Two analyses of Oklahoma lignite indicate sulfur contents of 0.40-4).50% and calorific values of 17.6-19.8 MJ/kg [8]. 1.4.4 The Northwest (i) Washington. Small reserves of lignite occur in the Cedar Creek area of Lewis County. The reserves that could be recovered by strip mining amount to about 5 Mt [3]. The total identified resources of lignite in the Kelso-Castle Rock area are 114 Mt [1]. Some lignite occurs in the Skookumchuck Formation (late Eocene) in the Centralia-Chehalis area. Most of the coal in this formation is subbituminous C, and some is classified as subbituminous B. Most seams in the Skookumchuck Formation are 1.8-2.4 m thick, although the range of thickness runs from less than 30 cm to over 12 m [1]. The total identified resource in the Centralia-Chehalis Area (that is, including both lignite and subbituminous coal) is 3.4 Gt [ 1]. Coal occurs in the Cowlitz Formation (Eocene) and Toutle Formation (Oligocene) in the Kelso-Castle Rock Area. Most of the coal in the Cowlitz Formation is subbituminous C rank, though some lignite also occurs. All of the coal in the Toutle is lignite. Small reserves of lignites occur in the Cedar Creek No. 1 and No. 2 seams of the Toutle Formation. The seam thickness is in the range of 1.5 to 3 m [3]. The overburden thickness is 18 m, and the parting between the seams is about 3 m thick [3]. The analysis of one sample of Toutle Formation lignite is reported as 27.9% moisture, 28.0% volatile matter, 27.6% fixed carbon, 16.5% ash, 1.4% sulfur, and a calorific value of 15.7 MJ/kg, all on an as-received basis [ 1]. (ii) Alaska. The Kenai Formation in the Homer district includes about 30 coal beds that

43 range in thickness from 0.9 to 2 m, as well as additional, thinner beds [ 1]. The rank ranges from lignite through subbituminous B; most is subbituminous C. The Broad Pass coal field contains two districts, Broad Pass Station and Costello Creek. The latter contains subbituminous coal; the Broad Pass Station coal is lignite. Mine samples from the Broad Pass field show the following ranges of composition: 8.7-18.8% moisture, 32.043.4% volatile matter, 23.3--42.2% fixed carbon, 6.0--21.2% ash, 0.3-0.6% sulfur, and calorific values of 18.6-24.6 MJ/kg, all expressed on an as-received basis [1]. Alaskan lignites are generally of low sulfur content [ 123]. 1.5 O T H E R L I G N I T E S OF C A N A D A

1.5.1 Ontario Early reports, dating from the late 19th century, mention deposits of lignite and peat along the Missinaibi, Mattagami, and Abitibi rivers in the Moose River Basin, as well as some outlying deposits along the Kwataboahegan River. The lignite deposits near Coal River and at Blacksmith Rapids were considered to be of superior quality [ 134]. Ontario possesses 198 Mt of measured lignite resources [94]. The lignite deposit in the Mattagami Formation is estimated at 180 Mt [125]. The proven resources of the Onakawana field are 190 Mt [ 125]. A lignite deposit estimated to be less than 450 Mt is found in the James Bay area of northern Ontario [2]. A lignite deposit occurs in the southern Moose River Basin in the Onakawana field. It is not well studied; there is not as yet even a reliable estimate of reserves. In the Onakawana coal field the various lignite beds are fairly continuous, and are enclosed by clay. The lignite in the Onakawana field is low in ash and sulfur, but contains about 50% moisture [125]. The lignite in the Onakawana field between the Abitibi and Mattagami rivers seems to have the greatest potential for exploitation. The lignite-containing sequence includes a basal clay, a lower lignite seam of relatively constant thickness, a seam parting of clay, and an upper seam of variable thickness, overlain by clay. The average thickness of the lower seam is 4.2 m, reaching a maximum of 6 m along its southeastern limit near the Onakawana River [125]. The average thickness of the upper seam is 5.4 m [125]. The lignite consists of coalified wood, including recognizable tree trunks or branches, as well as more earthy lignite containing spores and resin inclusions. Some lignite may also occur on the east side of the Mattagami River, in the region running from the southeastern boundary of the Onakawana field toward Grand Rapids, with another potential deposit running westward from Onakawana. The Mattagami Formation (lower Cretaceous) underlies the southern part of the Moose River Basin adjacent to the Canadian Shield in northern Ontario. The lignite is relatively thick and of low sulfur content, but generally of high moisture. The age of the lignite in the Moose River Basin (Mesozoic) was established from fossil plants [126]. The Mattagami Formation is 119 meters thick [ 125]. Limited data suggest that the lignite in the southern Moose River Basin is about

44 50% moisture, 22% volatile matter and 6% ash on an as-mined basis, 0.9% sulfur (daf basis) and provides 14 MJ/kg calorific value (moist basis) [94]. The principal lignite beds in the Mattagami Formation occur on Coal River at the juncture with the Missinaibi River, near the confluence of Adam Creek with the Mattagami River, at Blacksmith Rapids on the Abitibi River near Onakawana, and at Portage Island at the confluence of the Missinaibi and Mattagami rivers. Early studies of the lignite exposures at Coal River indicated a bed 0.9 m thick [127]. laalynological studies of the lignite confirmed the early Cretaceous age, either Early Albian or Middle Albian or Aptian [128]. 1.5.2 British Columbia Measured resources of lignite in British Columbia and the Yukon are 1,670 Mt, classified as indicated resources [94]. The reserves recoverable by surface mining are 360 Mt [94]. The coal reserves in the Hat Creek deposit amount to 360 Mt [94]. The Hat Creek deposit contains large reserves of low-rank coal, both lignite and subbituminous. The lignite is variable in quality. The ranges for the proximate analyses are 20-23% moisture, 24-30% volatile matter, and 8-23% ash, on an as-mined basis [94]. Sulfur ranges from 0.5-0.8% on a daf basis. The calorific value is 11.6-22.1 M.l/kg on a moist basis. The lignite ash has high fusion temperatures [94].

1.5.3 The Canadian Arctic The existence of coal-bearing strata in the Tintina Trench has been known since the gold rush era in the late nineteenth century [ 129]. Early in the 20th century, the coal near Dawson was mined, some of the coal being used in an electric power plant. The Gates Mine worked the lignite along Coal Creek in 1898 to 1903, and again briefly in 1937 [130]. Another mine on Coal Creek supplied fuel until 1914 for a power station providing electricity for the gold dredges operating in the Klondike. Mining also occurred along Cliff Creek from 1898 to 1903. More recently, however, there has been little interest in mining the lignite because of its low rank, the complexity of the geologic structures, and the distance from potential markets. Some interest was sparked in the 1970's by the proposed construction of a natural gas pipeline from the Mackenzie Delta and Prudhoe Bay gas fields. It was thought that the lignite could be used to fire local electric power stations which would provide the necessary power for the gas pumping stations. Lignite in the Yukon and Northwest Territories exists in seams up to 9 m thick [95]; however, most of the data comes only from outcrops. Exploration of the lignite resource has so far been limited. The strata of the Eureka Sound Formation contain major deposits of lignite. This formation occurs in the Canadian Arctic Archipelago, running northward from Banks and Baffin Islands to Ellesmere Island. The Remus Basin on Ellesmere Island contains over 90 seams of coal in a section approximately 3,200 m thick [131]. The coals in this section are lignites and subbituminous and high volatile bituminous coals. Some lignite seams on Axel Heiberg Island are 7 m thick. In the southern part of the Remus Basin the lignite is the most resistant unit in the stratigraphic

45 sequence. In some places ridges are capped with lignite seams up to 25 m thick [ 131 ]. The total inferred coal resources of the Eureka Sound Formation are about 50 Gt [131]. On the basis of limited analytical data, the coal of the Eureka Sound Formation appears to be low sulfur ( 1.5 m) occur between fluvial channel sand belts [26]. These lignites cap crevasse splays that are 9 to 12 m thick and coarsen upward. The peat deposits accumulating in Borneo between dendritic stream courses may be a modern analogue of the East Texas Wilcox lignite depositional environments [3]. Interpretations of the deposition of the Yegua lignites suggest a wave-dominated delta [27] or a fluvial-dominated system [28]. Both interpretations suggest that the lignite deposited in interchannel positions [27], accumulating in hardwood swamps, and that the original peat accumulated in a fluvial environment on an ancient coastal deltaic plain. Thin marine deposits, glauconitic sands, marls, or muds separate the thick lignite-bearing units [27]. Compared to the lignites in the northeast, accumulation in east-central Texas was lower on the coastal plain and the deposit was consequently subjected to greater marine influence. This accounts for the higher sulfur contents of the east-central lignites compared to those of the northeast. (Other factors causing a difference in sulfur contents may have been higher temperatures resulting from deeper burial, or a slower rate of accumulation [25].) The Calvert Bluff and Yegua lignites were deposited in a fluvial-deltaic system [11,29]. The palynology of the Calvert Bluff suggests primary accumulation in hardwood swamp environments, and to a lesser extent in marshes [4,27,30-32]. The Mississippi delta system is a modem analogue for the Calvert Bluff depositional environment [27,33,34]. The relationship between swamps and marshes to channels on the delta is similar to the Calvert Bluff lignite and its associated channel facies. The A tchafalaya and Des Allemands-Barataria basins are modern analogues of the interchannel basins in the Calvert Bluff [17,27,35]. The Des Allemands-Barataria

56 Basin is part of the fluvial-deltaic complex of the Mississippi, shallow with low relief, broadening and opening to the south. The Des Allemands-Barataria Basin lies between levee and meanderbelt deposits and an older, abandoned course of the Mississippi. The basin contains a distinct zonation which includes, proceeding coastward, a cypress-gum swamp, freshwater marsh, saline marsh, and interconnected lake system. Moving toward the Gulf, the number of basins increases but their size diminishes. Modem peat deposits in the Mississippi delta are thickest and most extensive where the alluvial and delta plains meet. The Calvert Bluff lies just above the Simsboro Sand. The lignite shows no relationship to the underlying sand; however, the sand was the platform for accumulation of the lower Calvert Bluff lignites, as well as being a fresh-water conduit and aquifer

[25]. Wilcox Group lignites in the Sabine Uplift area were deposited by rivers flowing southwesterly from the Ouachita Mountains [36]. These lignites are divided into two informal units: the Upper Wilcox, an aggradational, fluvial unit, overlying the Lower Wilcox progradational, deltaic unit. Lower Wilcox lignites of Texas extend eastward into Louisiana. These lignites formed in a very large interdeltaic area which itself formed as fluvial sedimentation moved both to the east and west of the Sabine Uplift [36]. Abundant kaolinite and quartz, with minor amounts of dickite, suggest a fresh water fluvial environment of deposition for the Chemard Lake lignite (Louisiana) [19]. Taxodium and Thomsonipollis palynomorphs also indicate a fluvial environment between a lower fluvial plain and an upper delta plain. Upper Wilcox lignites are found in small floodbasins between sand belts filling fluvial channels in large alluvial floodplains, analogous to the modern Des Allemands-Barataria Basin. In the Wilcox Mount Pleasant Fluvial System on the Texas-Louisiana border the environment was one of greatly meandering fresh water streams, and peat was deposited in, or adjacent to, abandoned stream channels, along stream terraces, or between overbank deposits. South and east from this region (i.e., toward the sea) peat also accumulated in the Rockdale delta system, which was characterized by marsh distributary channels. Wilcox lignites of Mississippi were deposited in a low-lying environment near the coast [37]. During the Eocene, the embayment area was a humid, subtropical coastal plain supporting strand and lowland swamp flora [38]. Further inland, the Appalachian foothills supported a temperate, upland forest. These lignites are characterized by small deposits of erratic seam thickness, and by multiple seams. The seams, generally of irregular shape, range from long, narrow fluvial deposits to more elliptical deposits typical of a deltaic origin. The seams may be up to 3.7 m thick, but at the margins of the deposits the thickness may be only a few centimeters [37]. Mississippi lignites deposited in an area created by southward movement of the Mississippi Embayment toward the Gulf of Mexico. Many features of the coastal plain--swamps and marshes, estuaries, landlocked bays, and lagoons--provided basins for accumulation of plant remains. These lignites occur as small, stacked deposits in contrast to large, blanket-like seams. The configurations of the ancient swamps, bays, and other features of the coastal plain determined the shapes of Mississippi Wilcox lignite deposits.

57 The Golden Valley Formation (Early Tertiary) in North Dakota deposited in a fluvial environment [2]. The existence of sandstone lenses typical of channel facies, interbedded with deposits typical of backswamp or overbank conditions, provides the basis for this deduction [2]. The Fort Union Group in southwestern North Dakota consists of deltaic (the Slope and Ludlow Formations) and marine (the Cannonball Formation) deposits overlain by alluvial plain deposits (the Bullion Creek and Sentinel Butte Formations). The lower Sentinel Butte in the Knife River area of west central North Dakota was deposited in swamp and fluvio-lacustrine conditions [39]. The lignite was deposited in the swamp environments and the elastic sediments in the fluviolacustrine conditions. These two systems alternately dominated an alluvial plain that drained to the east and southeast toward the Cannonball Sea. The continuity of the lignite suggests that similar environmental conditions prevailed across wide areas of the Williston Basin during the time of deposition. The thickness of the lignites shows that the environmental conditions prevailed for long periods of time. The times required for deposition were about 4,500 years for lignite 1 m thick to 14,000 years for 3 m thick seams [39]. An alluvial flood plain depositional model has been proposed to fit the Tongue River and Sentinel Butte [40]. The sand beds represent channel fills or deposits formed by migration of the channels. Sandy and clayey silt beds deposited as natural stream levees, while other clays and silts deposited in back basin areas of the flood plain. Because the lignite beds in the Bullion Creek and Sentinel Butte are relatively thick and laterally continuous for long distances, there must have been lengthy periods of fairly uniform conditions in the late Paleocene in North Dakota. The environment of deposition was similar to the modern Mississippi River delta [41]. Tongue River lignites were deposited by eastward-flowing, sluggish streams draining a source area to the west [42]. A stable fluvial system allowed development of protected back-swamps. As this depositional episode ended, elevation of the source area was reduced to the extent that the rate of subsidence exceeded the rate of sedimentation. Swampy conditions then became widespread throughout western North Dakota. Here the lignite beds are thin without extensive lateral persistence. The proximity of the lignite to other sediments of clearly fluvial origin strongly indicates that the lignites were deposited in fluvial conditions. These lignites are generally high in moisture and ash. In central and eastern Montana, and adjoining areas of the Dakotas and Wyoming, a broad, low-lying coastal plain existed during the Cretaceous and Early Tertiary. Extensive peat swamps formed on the plain. The peat beds were subjected to recurrent burial by sediments spreading out from the uplifting of the Rocky Mountains. Continual repetition of this process resulted in numerous buried beds of peat that eventually became beds of lignite. For example, near Roundup twenty-six beds of coal have been identified [43]. Moose River Basin (Ontario) lignite was deposited in a floodplain environment [44]. Freshwater alluvial floodplain deposition of this lignite is reflected in its syngenetic mineral content; the primary clays are kaolinite and illite, and pyrite is less common than in coals deposited in marine or marine-influenced environments [45]. The Ravenscrag Formation in Saskatchewan is equivalent of the upper Fort Union in North

58 Dakota [46]. The lignite was deposited in an alluvial plain-fluvial environment [46]. This lignite shows significant banding, which, together with the variations in petrography, suggest the accumulation of the precursor peat in a forest moor environment. Water level variations in the swamp contributed to a diverse sequence of plant communities, which in turn resulted in a diversity in the peat. Periodic low water levels may expose the peat to air, with oxidation forming inertinite. At other times, deep water levels contributed to the accumulation of attrital material. During periods in which the water level was relatively stable, preservation of woody plant parts was favored. These combined effects lead to formation of a banded lignite seam with a variety of macerals [46]. Coal of the Tintina Trench in the Canadian Arctic was deposited in fluvial and lacustrine environments [47]. No influence of marine sources is apparent. Sub-environments included lakes, alluvial fans, fan deltas, braided and meandering streams, and associated flood plain environments [47]. Variations in the sedimentary loading, and particularly variations in the geothermal gradient, complicated the coalification processes. The geothermal gradient was abnormally high during much of the burial period, a result of the emplacement of shallow magmatic intrusions. Coals of the Tintina Trench display the highest rank gradients--0.1 to 0.2% Ro/100 m - - o f any coals of western Canada [47]. A further complication of the stratigraphic pattern is strike-slip faulting, which has juxtaposed areas of different thermal histories. The Alaskan lignites and subbituminous coals in the Beluga, Nenana, and Yentra coal fields formed in various environments, including alluvial plains of non-marine continental-fluvial systems, and forest moor backswamp environments on valley flats [48]. Peats formed mainly from tree vegetation. As in the case of the Saskatchewan lignites [46], drier conditions led to coals with higher amounts of inertinites [48]. 2.3.2 Deltaic environments Upper deltaic plain coals were deposited in swamps, and formed mainly from the arborescent plants growing in the region [49]. As a result, large amounts of massive vitrinite formed, with lesser amounts of cutinite, inertinite, resinite, and sporinite. In comparison, vegetation characterizing bay or estuarine facies is generally less arborescent [49]. Abundant amounts of amorphinite, as well as framboidal pyrite, reflect a significant contribution of microbial action to degradation of the plant remains. Deltaic lignite probably originated in marshes. Although deltaic lignite is non-woody (particularly in comparison with fluvial lignite), woody material occurs with some deltaic lignites. The Alcoa seam (Milam County, Texas) palynology suggests an alternation of flesh-water marsh and hardwood swamp environments. Deltaic Wilcox lignites are characterized by low ash, moderate sulfur content, and a wide lateral extent, consistent with original deposition as a blanket peat, that is, peat which formed on inactive delta lobes [50]. Wave-dominated deltaic facies are characteristic of the Eocene lignites of South Texas [25,51]. A modern analogue is found in Malaysia, in the delta of the Klang and Langat rivers.

59 Deltaic lignite is found in the Wilcox in Central Texas and in the Yegua and Manning formations in the southeast. Three types of sedimentation patterns have been identified in deltaic lignite: sedimentation alternating between distributary channels and interchannel areas, repetitive delta front sequences, and meanderbelt deposits [22]. Commercially important deltaic lignites occur with interdistributary deposits in delta plains. East Texas deltaic lignite occurs in the Jackson Group between the Colorado and Angelina Rivers [22]. Jackson Group lignite in Southeast Texas also accumulated in deltaic environments, originating from blanket peats that spread across abandoned distributary channels in lower delta plain environments [27]. Lignite in the Manning and upper Wellborn Formations accumulated in either swamps or marshes [4,52]. Laterally extensive and relatively thick lignites derive from blanket peats originating in a variety of inactive environments from distributory channel fills to bay or lake fills [25]. Lignites that are more restricted areally may have accumulated in interdistributary basins while deltation was active [25]. 2.3.3 Lagoonal environments Lagoonal lignite is characterized by high sulfur contents and high ash values. These characteristics suggest an origin in a salt marsh that experienced frequent introduction of clastics [22]. Deposits of lagoonal lignite in Texas occur mainly in the Wilcox and Jackson Groups and Yegua Formation in the south, and in the Wilcox in East Texas. The sedimentation pattern coarsens upward; lignites are associated with lagoonal muds. Lagoonal lignite occurs in South Texas in the Jackson Group south of the Atascosa River [25]. Lignite occurrences are elongate, extending in a belt roughly 16 to 24 km wide and 200 km long [25]. South of the Atascosa River, Jackson lignite is strandplain or lagoonal [25]. Modem analogues of this system occur in western Mexico, along the Nayarit coast. There the peats are accumulating in a strandplain-lagoonal system in which peat is being laid down on top of a strandplain-barrier bar clastic sediment [53]. The resulting stratigraphic sequence of clastics topped by peat is similar to sequences that can be recognized in the lower Jackson [25]. 2.3.4 Backswamp environments Martin Lake (Texas) lignite was deposited in an arboreal (Nyssa) swamp, as indicated by high ulminite and terrestrially derived exinites [54]. Areas in the Martin Lake seam that show high inertinite contents indicate regions where the peat swamp was drying. Darco (Texas) lignite formed from backswamp peats on flood plains that were separated by meander belts [22]. The depositional environment was a forested, fresh-water swamp, inferred on the basis of a low sulfur content [22], woody appearance, and accumulated palynoflora [55]. Chemard Lake lignite is the product of upper deltaic arborescent flora that accumulated in a fresh-water swamp [56]. The forest vegetation is indicated by bright, banded, and banded-bright lithotypes. Clarains are the dominant lithotypes with clarite-V and vitrite the dominant

60 microlithotypes [19]. These lithotypes contain significant amounts of pollen of Carya and

Triporopollenites bituitus. Dull macerals such as duroclarite and carbominerite occur in association with the floral assemblage containing higher amounts of Engelhardia and Microreticulatosporites [19]. The vitrinite (60.8% average, moisture-and-ash-free (maf) basis) and inertinite (12.9% average, maf basis) contents also indicate an origin from woody vegetation. This lignite has a low content of pyrite, an indication of deposition in a fresh-water environment. Some clarite bands contain high pyrite, indicative of a marine transgression that covered parts of the peat [57]. Chemard Lake lignite palynology suggests an upper deltaic interdistributary environment [57]. The lignite is dominated by arborescent paleovegetation that accumulated in a fresh-water swamp on the upper deltaic plain [ 19]; the lignite contains minor amounts of hardwood pollen grains. Lignite in the Knife River Basin of North Dakota formed in swamps in the vicinity of the Cannonball Sea, which was in retreat during the late Paleocene [41]. The environment of deposition of the Hagel bed, inferred from palynomorphs, was a laterally extensive, aborescentdominated swamp forest characterized by cypress trees and associated herbaceous ferns and mosses, with a minor deciduous flora [58]. The dominance of Taxodiaceae--Cupressaceae pollen indicates the swampy terrain with abundant taxodium trees. Abundant xylitic material in the lignite indicates that plant remains accumulated under standing water, and that the surface was covered by standing water for most of the year. This environment was similar floristically and physically to the present Okenfenokee Swamp [59]. The Okefenokee currently has an area of 1700 km2 depositing a thick, continuous layer of peat [60]. The Beulah-Zap (North Dakota) depositional swamp was about 1050 km2 [61]. Evidence for lacustrine deposition of the Beulah-Zap lignite includes fine-grained (clay and silt size) sediments in finely laminated underclays and partings within the lignite [62]. The fine grained, finely laminated partings represent transgression-regression sequences in a marsh-lake system. No evidence of a meandering fluvial system, such as sand deposits, peat erosion, or channel structures, exists. The rate of peat deposition in the Okefenokee is about 1 cm per 20 years [63]. The compaction ratio of peat to soft brown coal is about 2 [6]. These data applied to the Hagel seam suggest a rate of deposition of approximately 4000 yr/m [58], which is very rapid on a geological time scale. The alternation of lignite and clastic deposition also took place rapidly, probably controlled by shifting of associated lacustrine and fluvial depositional environments. Pollen grains from Falkirk lignite indicate a more upland flora and are, in part, detritus. The Falkirk also has the coarsest grained clastics of three mines (Falkirk, Center, and Glenharold) at which the Hagel was studied. Lignite of the Falkirk mine was more closely associated with fluvial environments, possibly the system that supplied water to the taxodium swamp and lake environments of the Center and Glenharold mines [58]. The Center and Glenharold mines were more likely associated with lacustrine deposition [3]. The lower sodium content of the Falkirk relative to the Center and Glenharold lignites may be due to a higher mobility of the sodium ion in the hydrologically more dynamic fluvial system [58].

61 In Montana, the Tongue River Member of the Fort Union Formation was deposited by slow-moving streams on a low gradient. Sediments were transported eastward in suspension. Near the close of the depositional event, the altitude of the sediment source to the west was reduced. As a result, the rate of subsidence exceeded that of deposition, and a swamp was created. The coal beds were deposited in this backswamp [64]. 2.3.5 Marsh environments Texas lignites in the Manning Formation (Jackson Group) accumulated in marshes on the lower delta plain of a fluvial-deltaic environment [35]. These lignites derive from blanket peats that accumulated on foundering lobes of the delta and spread into inactive environments such as lake or bay fills and distributary channels [28]. Lignites of the lower seam at the Big Brown mine and the lower and middle portions of the Sandow mine were deposited in reed marsh complexes, as indicated by high humodetrinite and liptodetrinite contents [54]. Lignite from the San Miguel mine also deposited in a reed marsh. Gelification is more pronounced than in the Big Brown or Sandow lignites, and there is greater evidence of bacterial activity, suggesting that the conditions in the marsh that produced the San Miguel lignite were at various times both oxic and anoxic [54]. Deposition of some organic material in regions of subaquatic bacterial activity is indicated by the presence of dinoflagellates and sapropelinites. The high gelinite content indicates an oxic environment, which may have derived from aqueous oxidation of peat. The presence of eugelinite indicates that the water was at times brackish. Martin Lake lignite formed in fresh or slightly brackish water. The lignite has a low sulfur content, and a sulfur isotope ratio of +9.5 [65], indicative of a freshwater or brackish environment. The coarsening-upward sequence in the overburden contains pyrite nodules and siderite bands and concretions, indicative of highly reducing, anoxic, non-sulfidic conditions in the bottom sediments, and further indicating that the pore fluid in the sediments was not sea water [66,67]. Coexistence of siderite and pyrite indicates brackish water deposition. North Dakota lignites have been suggested to have been deposited from peat bogs similar to the modern swamps of Wisconsin and Michigan [68]. Beulah-Zap lignite was deposited in a marshdominated lacustrine system [62]. The depositional basin had extremely low relief. The rates of peat accumulation and basin subsidence were in equilibrium, resulting in a thick and laterally continuous lignite free of carbonaceous shales. Beulah-Zap deposition started in moderately deep water. Vegetation growth was halted twice by major increases in the water level that also resulted in deposition of clay and silt in the southern portion of the basin. Deposition ended with a major drying episode, followed by extensive flooding that terminated peat accumulation throughout the depositional basin. The sequence of depositional conditions was deduced from the distribution and abundance of the lithotypes, using the technique of seam formation analysis [62]. The lowest seam formed in deep water, with a gradually, but steadily, shallowing water level. Deposition of the middle seam began in relatively shallow water. The water level then increased throughout the depositional basin, so that the middle portion of the middle seam formed in moderate to deep

62 water. As deposition continued, a sequence of deepening followed by shallowing occurred. The uppermost Beulah-Zap seam fluctuates in thickness and is not laterally extensive. The depositional environment was unstable, with the majority of deposition occurring in moderate to shallow water with an overall shallowing trend preceding the end of deposition. The most important coalification process in the Beulah-Zap lignite was humification, as indicated by the dominance of huminite macerals in the lignite [62]. Gelification was important in horizons overlying inorganic-rich zones or carbonaceous shales. Fusinitization was a minor process occurring mainly at the end of deposition as the shallowing water allowed periodic subaerial exposure. Good preservation of fusinite macerals suggests that more fusinitization occurred by fire than by subaerial exposure. 2.3.6 Lacustrine environments In a lacustrine setting, marshes and swamps will form along the shores of the lake [49]. Vitrinite will form in abundance from the woody parts of plants. Alginite will occur to a lesser extent than in lignites deposited at the depocenter, because algae would be the dominant source of plant material only toward the center of the lake. In lignites formed near the depocenter, alginite will be more dominant, along with amorphinite, the degradation product of alginite [49]. The Chemard Lake lignite paleoflora have been interpreted as representing peat accumulation in fresh water lakes of an upper deltaic environment [19]. The fresh water environment is indicated by spores of fresh water algae, Schizosporis texus and Schizosporis

parvus. Other vegetation indicative of a lake or swamp environment included algae, fungi, ferns, Sphagnum, and hardwood trees such as Carya and Engelhardia [ 19]. Similar assemblages have been reported for the Mount Pleasant and Rockdale Delta systems of Texas [31]. Infilling of the swamp is demonstrated by the vertical distribution of palynomorphs. Algal spores are dominant near the base of the seam. In the middle of the seam the spores of ferns and mosses are abundant. Hardwood tree pollen becomes more abundant near the top of the seam, the development of deeply rooted vegetation being consistent with a reduction of surface water. Taxodium is rare in Chemard Lake lignite. Evidence for deposition in an ancient oxbow lake exists for Claiborne Group lignite in Tennessee [4]. Arboreal pollen is common at the base of the lignite. Herbaceous pollen becomes increasingly abundant upward. As the source of clastic sediments to the lake was cut off, the lake filled with organic debris from the surrounding area, which was wooded. However, herbaceous plants became established around the lake margins and gradually came to dominate the flora as the lake filled with organic debris. Similarly, lignites in the Jackson Purchase region of Kentucky may have formed in abandoned oxbows of Eocene rivers [ 16]. Large changes in peat thickness and paleofloral content over relatively short distances would occur in such situations, and produce significant variations in petrographic and palynological composition of the lignite.

63 2.3.7 Summary Figure 2.1, adapted from [69], summarizes some of the principal features of paleoflora, environments and depositional settings, and the resulting microscopic features of the lignites.

Paleoflora

Sequoia

Environment

Setting

Microscopic features

Lithotype

NyssaTaxodium

Swampmarsh complex

Marsh

Aquatic

Upper delta plain

Junction of upper and lower delta plain or lower delta plain

Lakes on delta plain

Mixed humotelinite (ulminite), Abundant humodetrinhumotelinite ite, lipto(mainly uldetrinite, minite), sporinite, sporinite, cutinite, resinite, and alginite, and suberinite resinite

Mainly humodetrinite and liptodetrinite with minor humotelinite alginite

Mainly humocollinite, humodetrinite, sapropelite and major alginite

Moderate to finely banded xylitic to non-xylitic coal

Unbanded non-xylitic to unbanded detrital coal

Unbanded, tough non-xylitic or detrital coal

Swamp

Alluvial plain backswamp or swamp at junction of lower alluvial plain and upper delta plain

Mainly humotelinite with textinite A, suberinite, resinite

Moderate to highly banded xylitic coal

Nyssa BetalaceaeMyricaceae flotant

Botyrococcus Schizoporis, Pediastrum Engelhardtia Degraded stem, Arecipites bark tissues, Liliacidites marsh plants

Moderately banded to unbanded non-xylitic coal

Fig. 2.1. Summary of relationships among paleoflora, depositional environment and setting, microscopic and macroscopic features of resulting coal. Adapted from [69].

2.4 TRANSFORMATION OF PLANT MATERIAL TO LIGNITE 2.4.1 Introduction Peat represents the first stage of coal formation. At some point a change in the environment results in the end of peat formation and burial of the existing peat with clay, sand, or other sediments. This point represents the transition from diagenesis, or the biochemical phase of coal

64 formation, to catagenesis, or the geochemical phase. A typical peat contains 80--90% moisture and, on a dry basis, 50--60% volatile matter and 40-50% carbon. The transition from peat to lignite is marked by reduction in moisture content and volatile matter and by an increase in carbon content. North Dakota lignite contains about 35% moisture and, on a dry basis, 45% volatile matter and 70% carbon. These changes are accompanied by an increased calorific value, from 16-21 MJ/kg (dry basis) for peat up to 28 MJ/kg (dry) for North Dakota lignite [ 18]. Pollen metamorphism in lignites suggests that temperature is the primary factor in coalification, and that pressure may retard coalification [73]. At slow heating rates lignites begin to evolve volatiles at temperatures as low as 200~ [74], indicating that lignites could not have been exposed to temperatures higher than this during coalification. Hydrocarbons evolved from lignites on controlled pyrolysis reflect the geochemical origin of the coal. Sapropelic lignites from the Wilcox, Claiborne, and Jackson Groups have a bimodal nalkane distribution with domination by n-alkanes >C 25, whereas humic lignites show n-alkanes 70% of the lignite. Upon subsequent liquefaction at 410~

for 1 h in 6.7 MPa hydrogen, oil yields increase and preasphaltenes and

residue decrease. An increase in liquefaction reactivity relates to increased severity of pretreatment. The hydrolysis presumably occurs at ether linkages. If the liquefaction reaction is hydrogendeficient, condensation reactions between phenols rapidly increase the number of ether crosslinks. Formation of phenols may result from reaction of hydrogen with lignin structures. The relative concentrations of six phenols--o-cresol, m-cresol, p-cresol, 2-ethylphenol, 2,4dimethylphenol, and 4-ethylphenol -- produced in the liquefaction of Beulah lignite [77] showed the same pattern as observed for pine lignin [78], Loy Yang (Victoria, Australia) brown coal [78], and Wyodak (Wyoming) subbituminous coal [77]. Reduction of the phenols is favored by high partial pressures of hydrogen or high concentrations of hydrogen donors [79]. Hydrogenation of lignin residues produces phenols and aromatic compounds in ratio of about 1:1 [80]. Cleavage of ether bonds to form phenols and alkylindans is favored over cleavage to alkylbenzenes and alkylindanols. The liquid yield from Hagel lignite, as well as a Turkish and a Spanish lignite, increases as a function of organic sulfur content, regardless of whether the lignite had been impregnated with a catalyst [81,82]. Furthermore, the increase in liquids yield obtained in a hydrogen atmosphere relative to that obtained by thermolysis of the same lignite in nitrogen, was also proportional to the organic sulfur content of the lignite. These experiments were carried out in the absence of a liquid hydrogen donor vehicle. 4.3 R E A C T I O N S IN O T H E R G A S E O U S A T M O S P H E R E S

4.3.1 Oxidation reactions This subsection treats reactions of lignites with air or oxygen at relatively mild conditions. Further discussions of oxidation of lignites are found in Chapter 9, relating to spontaneous heating and combustion, and in Chapter 11, on combustion processes. Reactions of lignites with oxidants in the liquid phase, primarily for structural studies, have been treated in Chapter 3. Chemisorption of oxygen and formation of peroxides is the first step in the low-

161 temperature oxidation of lignites [83-85]. Subsequent formation of carboxyl groups may be responsible for reducing the rate as oxidation progresses [16]. At low temperatures oxygen accumulates on the lignite, whereas above 70-80~

gaseous oxidation products evolve [16,86].

Using tetralin and diphenylmethane as examples, the initial reactions can be written: OOH

OOH

I

+ 0 2

[84]. These equations highlight the importance of reaction at the benzylic carbon. Once the peroxide has formed, free radical chain reactions can proceed, leading ultimately to the formation of carbon monoxide, carbon dioxide and water [84]. An increase in the number of crosslinks in the lignite accompanies these changes [86]. A reaction sequence, shown on the top of page 162, first elucidated for phenol-formaldehyde resins [87] demonstrates formation of the reaction products and increased crosslinking. Increased crosslinking affects other properties of the lignite, including the amount of extractable material and the volatile matter content [84]. A sequence of oxidation reactions has been proposed as follows (here Ar represents a general aromatic system and ArH2 a general hydroaromatic system): 4 Lignite-COH + 2 Ar + 02 + 2 H20 ~ 4 Lignite-COOH + 2 ArH2 4 Lignite-COOH + 2 Ar ----4 Lignite + 2 ArH2 + 4CO2 4 Lignite + O2 + 2 I-I20--* 4 Lignite-OH (i.e., Lignite-CHzOH) 4 Lignite-CHzOH + 4 Ar ---, 4 Lignite-COH + 4 ArH2

[85,88]. Reaction of lignite with oxygen produces -OH groups (as in the third equation above) that oxidize sequentially to aldehydes and acids; the acids decarboxylate, resulting in loss of some of the carbon from the lignite structure and generating sites at which the reaction sequence could begin anew. Although the initial stage of oxygen reaction with lignite is a chemisorption [85], in later stages diffusion becomes important [89]. The rate of oxygen uptake at a given temperature decreases as a function of time. The abundant phenolic structures enhance the susceptibility of lowrank coals to oxidation [90]. X-ray photoelectron studies of air oxidation of Beulah lignite vitrain provided results summarized in Table 4.4 [91]. The data are shown as the ratio of the intensity of the indicated peak

162 OOH I

O II +

0 II

H20

0 II ,C ~

+ 0

il

C9 ~-

~

o

+

O II 1C ~

co O II

4- "OH

+

~

~

C~OH

+ CO 2

r

to the total intensity of all carbon ls peaks. Air oxidation of Pust (Montana) lignite increases the number of hydroxyl groups [92], suggesting that autoxidation proceeds via alkoxy and hydroxyl radical intermediates [93]. Diffuse reflectance infrared Fourier transform (DRIFT) spectra TABLE 4.4 Oxidation of vitrain in air, expressed as ratio of peak intensity to total carbon ls peak intensities [91]

Time Temp., ~ (start) (ambient) 24 hr 95 24 hr 150 24 hr 200 5 days 25

Ether or Hydroxyl 0.04 0.03 0.04 0.27 0.08

Carbonyl 0.00 0.03 0.04 0.01 0.03

.Carboxyl 0.20 0.01 0.02 0.09 0.00

163 of Beulah lignite that had been vacuum dried and then exposed to water-saturated air at 220C for 1 week and 1 month showed large increases in - - O H [94]. A slight increase in absorption around 1100 cm-1 was attributed to ethers or alcohols. Absorption in the carbonyl region of 1500-1800 cm-1 also increased. Infrared evidence indicated chemisorption of water onto the oxygen functional groups by hydrogen bonding. During oxidation of a North Dakota lignite in a fluid bed reactor at 2000C, total acidity (phenolic plus carboxylic, measured by barium cation exchange at pH 12.5) increased from 5 meq/g to an asymptotic value of 9 meq/g after 12 hours [95]. At 230~

the total acidity had

increased to 13 meq/g after 12 hours, but had not yet reached an asymptote. The formation of carbon oxides and phenolic and carboxylic groups represent parallel processes that occur at similar rates.

J

CO + CO2

Lignite --COOH + -OH

The rate of oxidation depends on many many factors, which include rank, temperature, oxygen concentration, particle size, moisture in the coal and the oxidizing atmosphere, and the extent of any prior oxidation. Oxidation is first order with respect to oxygen partial pressure at 25, 65, and 95~

[96]. The rate increases with temperature; in the range of 5--33% moisture, the

average activation energy for five lignites was 11.8 kcal/mol 0 2 [96]. Over the range 25-950C oxidation rates are a maximum at moisture contents near the equilibrium moisture of 20%. Rates at 5 and 36% moisture are similar to each other and are about half the value at 20% moisture [96]. For lignites dried in an inert atmosphere, the effect of moisture on oxidation rate is proportional to 1 + 0.674M - 0.01634M2

where M is the moisture content in percent [96]. The oxidation rate correlates with the cube root of the specific surface area S, where S is given by S = 1.22/m

and m is the mean sieve size in inches [96]. Oxidation starts at the surface and falls off exponentially with depth from the particle surface, in which case r = (k/m)[ 1 - exp(-5.5m)]

164 [96]. Here r is the oxidation rate in ppm O2/hr, k the rate constant, and m is defined as above. At a constant temperature below 70~

the oxidation rate decreases with time and with

cumulative oxidation [96]. For different lignites, the initial rates and the time decay vary randomly with respect to source of the lignite and the position in the seam from which the samples were obtained. (Apparently no petrographic data to accompanied these samples.) The decrease in rate of oxidation does not occur above 70~

conditions at which appreciable amounts of carbon dioxide

form [96]. The activation energy for reaction with oxygen is 50 kJ/mol [96]. For comparison, the activation energy for oxidation of lignite with potassium permanganate solution is 19 kJ/mol, based on tests using 0.1N KMnO4 solution, 6x8 mesh lignite, and temperatures of 24, 57, and 100~ [96]. The heat of reaction increased from 314 kJ/mol O2 at 20~ to 377 ld/mol O2 at 900C [96]. Moisture content, particle size, and extent of cumulative oxidation had no apparent effect on the heat of reaction. The effect of temperature on heat of reaction and oxygen consumption rate is shown in Figs. 4.5 and 4.6 [96]. The incremental carbon conversion of Onakawana lignite in air decreased with increasing temperature [95]. That is, the increased carbon conversion obtained by raising the reaction temperature from 715 to 805~ was essentially the same as obtained from the much larger increase in reaction temperature from 805 to 1000~

As reaction temperature increases from 8050C to

1000~ carbon monoxide formation increases while carbon dioxide formation slows. Both untreated and acid-washed Fort Union (Montana) lignite lost about 90% of its initial weight on reaction in air [97]. The presence of the metal cations decreases the rate of weight loss in air, relative to the acid-washed sample from which the cations had been removed. Oxidation of lignite by air during storage affects reactivity in subsequent processing. Lignite stored in air for 70 weeks showed a decreased yield of solvent refined lignite (37%, maf basis) compared to fresh lignite (45%) or lignite stored under nitrogen or under water for the same time [21]. The gas yields (25%), and light oil yields (21%) were identical regardless of whether fresh lignite or lignite exposed to air for 70 weeks were tested, and the conversions were very similar (91% for fresh, and 88% for air-oxidized lignite) [21]. 4.3.2 Reactions with water or steam This subsection discusses results of reactivity experiments in which the starting material was a raw lignite; in general, the discussion proceeds in order of increasing reaction temperature. The gasification reactions of lignite chars with steam are discussed in Chapter 12. Catechol-like structures decompose at relatively low-severity conditions. This effect has been observed in the hydrous pyrolysis reaction of Patapsco (Maryland) lignite at 100-350~

for

reaction times of 30 minutes to 10 days [98]. The catechol structures, characteristic of lignin remnants, transform to phenol structures. Loss of aliphatic components also occurs [98]. Alkanes appear among the reaction products of Rapponmatsu (Japanese) lignite treated at

165 500

x o o

400

fl

300

"~ 2oo o

~

lOO

m ''''

I ' ' ' '

20

I''''

40

I''''

60

I''''

80

100

Temperature, ~ Figure 4.5. The heat of reaction (IO/mol oxygen consumed) as a function of temperature for reaction of Baukol-Noonan lignite in oxygen (adapted from[97]). The curve shown is the best visual fit of data representing samples of various particle sizes, states of activation, and moisture contents.

a~ 0.08

~9 0 . 0 7 ~

0.06

0.05" .~" 0 . 0 4 " 4","

.

~'

0.03

oO

0.02

a~ 0.01 " X

o

o

'''~

I'''' 20

I '''~ 40

I'''' 60

I'''' 80

100

Temperature, ~ Figure 4.6. Moles of oxygen consumed per hour, per kilogram of maf lignite, as a function of temperature, for the reaction of Baukol-Noonan lignite with oxygen (adapted from [97]). The curve shown is the best visual fit of data representing samples of various particle sizes, states of activation, and moisture contents.

166 20(O500C in 0--4 M aqueous sodium hydroxide [99]. The other gaseous products were carbon dioxide and hydrogen. Hydrogen and the alkanes increase with increasing reaction temperature, above 350"C. Phenols, along with cresols, naphthols, and other aromatics, appear in the liquid products of the reaction. Hydrolysis reactions are important below 300"C. The alkanes form by decarboxylation reactions, the rate of which begins to increase around 350~ pitch, and significant gas yields, becomes important above 400~

Formation of tar or

as a result of dehydration

reactions [99]. The reduction of oxygen functional groups also accompanies aqueous reaction of Zap lignite (250-350~

28 MPa, and 5-300 min reaction time) [100]. The methane yield increases

with increasing reaction time. A noticeable increase in tar yield occurs at short reaction times under these conditions, but does not persist at longer times. Beulah-Zap lignite shows a maximum in oil yield at 340~ 250-360~

for hydrous pyrolysis at

72 h [101]i The yield amounts to 30 mg/g [101]. In comparison, a lignite of Miocene

age from the Far East had a much higher oil yield (101 mg/g) with maximum oil production occurring above 360~

[ 101]. The distinction reflects the different petrographic compositions of

the two samples, the Beulah-Zap containing about 3% liptinite (so-called liptinite-poor), but the Far Eastern lignite being liptinite-rich with about 32% liptinite. Reaction of the Beulah-Zap lignite below 310~ forms an oil containing low molecular weight aromatics (e.g., alkyltetralins and alkylnaphthalenes), phenols, and n-alkanes in the range C14-C35 [101]. The dominant n-alkane is C 25. The alkanes and aromatics occur in comparable amounts. Increasing the reaction temperature to 350-3600C increases the yield of n-alkanes, and they become more abundant than the aromatics. The range of n-alkanes extends from benzenes > phenols. Besides changing the relative order of the benzenes and phenols, nitrogen and sulfur compounds (e.g., pyridines, pyrroles, and thiophenes) were produced in greater quantities during hydropyrolysis, suggesting that the hydrogen acts to stabilize such compounds which would otherwise, in nitrogen, deposit as char. The effects of heating rate on volatile losses are essentially reversed for nitrogen and hydrogen atmospheres. Five Louisiana lignites heated in nitrogen at 11, 25, and 435~

in the

range 10(0-800~ showed that the volatile losses are greater the higher the heating rate [35]. The trend is exactly opposite in hydrogen. The total volatiles yield is always greater in hydrogen than in nitrogen, for comparable heating rates and reaction temperatures. There is little variation in activation energy among various combinations of heating rates and atmospheres; for example, in nitrogen the activation energy ranges from 198 kJ/mol at 435~

to 218 kJ/mol at 1 l~

while in hydrogen the comparable values are 221 and 209 kJ/mol, respectively. (iii) Nitrogen vs. air. Pyrolysis of Richland County lignite in an entrained flow reactor gave a 30% maximum weight loss in nitrogen [97]. In comparison, pyrolysis of either untreated or acid-washed samples in air, in otherwise similar reaction conditions, resulted in a 90% weight loss for both. (iv) Nitrogen vs. steam. Pyrolysis of Alcoa (Texas) lignite falling freely through a countercurrent stream of nitrogen or steam causes no diminution of the sulfur content (originally 1.09% in the untreated lignite) for pyrolysis in nitrogen, but a reduction in sulfur as function of pyrolysis temperature when a steam atmosphere is used [107]. In steam, no change in sulfur is noted for pyrolysis temperatures up to 565~ char drops, reaching 0.66% at 868~

but at higher temperatures the sulfur content of the

Production of gases is a linear function of temperature for

pyrolysis in nitrogen; in steam, a sharp increase in gas production as a function of temperature occurs above 700~ Vacuum pyrolysis of a North Dakota lignite which had been treated with steam at 320~ and 7.6 MPa for 15 minutes showed that fewer total volatiles were emitted on pyrolysis to 7400C than in the case of the untreated lignite [146]. However, the tar yield increased slightly, from 2.1 to 4.2%. (v) Hydrogen vs. synthesis gas. Reaction of a North Dakota lignite in a CO-H2 mixture showed higher reaction rates than in hydrogen showed apparent reaction orders of 2.9 for thermal decomposition at 450~ and 4.7 at 575 ~ [133]. Comparable experiments showed essentially no change for the reaction of a bituminous coal in the two atmospheres [133].

179 4.4.4 Effects of lignite moisture content on pyrolysis The necessary air/lignite ratio doubles for carbonizing high-moisture lignite, without an increase in the carbonization temperature, for entrained flow carbonization in an internally heated reactor [147]. For example, carbonization of Sandow (Texas) lignite dried to 2.9% moisture was effected at an air/lignite ratio of 2.35 x 10-4 m3/kg at 525~

for Glenharold (North Dakota) lignite

at 20.6% moisture the corresponding data are 4.33 x 10-4 m3/kg and 593~ [147]. The higher air rate is required by the additional thermal requirement associated with drying the moist lignite. The practical implication is that operating problems with carbonization of lignites at their full moisture content--specifically, an increase in gas volume and velocity due to the additional steam, and condensation with consequent bridging in storage and feed bins--show that all lignites should be partially dried before carbonizing [147]. Thermogravimetric analyses of Indian Head lignite in argon demonstrated effects of airdrying and sample aging [148]. Freshly air-dried lignite showed a variety of transitions in the first derivative TGA curve, with reactions occurring at 200, 400, 500, and 750~ occurring in the regions of 600-700 and 800-1000~

and smaller changes

After three to four days of the lignite's

standing in the laboratory, the first-derivative TGA indicated much simpler pyrolysis behavior, with major reactions only at 200 ~ and 450~

and small changes at 550 ~ and 850~

weeks, the only evident changes occurred at 200 ~ and 450~ the first derivative TGA plot up to 1100~

After two

with no other reactions apparent in

The reasons for this behavior were not elucidated, nor,

insofar as is known, was this work ever followed up. However, these observations show that drying and subsequent aging may significantly affect the pyrolysis behavior of lignite. Moisture can have a significant effect on the extent of pyrolysis. An increased weight loss observed for dried Hagel lignite, relative to the moist lignite, is due to faster heating of the dried particles and faster release of volatiles away from the particles [ 145]. As-received and vacuum-dried Gascoyne (North Dakota) lignite produce similar yields of water-soluble organics upon pyrolysis [ 149]. The conditions of the experiment were a heating rate of 450C/min to 850~

in a helium atmosphere. Although the yields of organic compounds were

similar, substantial differences in the rate of gas evolution were noticed over a wide temperature range. 4.4.5 Effects of cations on lignite pyrolysis An important feature distinguishing low-rank coals from higher rank coals is the presence, in low-rank coals, of abundant cations associated with the carboxyl groups. (The association of inorganic components with the carbonaceous portion of the coal is treated in Chapter 5.) The cations affect many aspects of lignite behavior. They can have chemical or physical roles in affecting pyrolysis of lignites [ 150]. Cations catalyze secondary reactions that increase the amount of char formed. The cations also catalyze condensation of tar species into higher molecular weight material (even if this increased molecular weight material is not char), reducing the volatility of these species. Either the cations themselves or their reaction products formed in early stages of

180 pyrolysis can block the apertures of pores, impeding escape of volatiles. (i) Effects of cations on weight loss. The effect of cations on entrained flow pyrolysis of Montana lignite, in helium and nitrogen atmospheres at 973 and 1173 K, was studied using raw lignite, lignite acid washed to remove exchangeable cations, and acid-washed lignites backexchanged with calcium, magnesium, or sodium [97,142,151]. All forms showed the same qualitative behavior, in that as a function of time a rapid initial weight loss is followed by a period in which there is essentially no further weight loss [142]. Further, the maximum weight loss increased with pyrolysis temperature. Despite these qualitative similarities, substantial quantitative differences were observed between the raw and acid-washed lignites. The former showed a weight loss of 30% (on a dry, inorganic-cation-free basis) in 0.15 s, while the acid-washed lignite showed a weight loss of 50% in 0.05 s at 1173 K [97,142]. The calcium-loaded and raw lignites behaved alike. The calcium-loaded lignite had a cation content of 3.1%; the raw lignite, 2.8%. In 0.15 s at 1173 K, the calcium-loaded sample lost 30% of its weight while the raw lignite lost 29% [142]. (The similarity of behavior indicates the reversibility of the ion exchange process; the acid washing and back-exchange of calcium induced no alterations that affected the pyrolysis behavior.) The maximum weight loss decreased as calcium content increased. Cation loading has no significant effect on ASTM volatile matter content. In comparison, at extended residence times in an entrained flow reactor, the presence of cations decreased weight loss for all temperatures, cations, and degrees of cation loading tested [ 142]. This difference in behavior derives from the degree of secondary char-forming reactions, which are favored in fixed beds, but have a much lower likelihood in entrained flow reactors. The cations, in entrained flow pyrolysis, promote formation of secondary pyrolysis products that deposit on the char, either by cracking of primary products to carbon-rich solids and light gases or by polymerizing tars to molecular sizes too large to remain volatile. Thus in entrained flow pyrolysis (1173 K, 0.03-0.3 s residence time) a lignite treated with hydrochloric acid to remove the exchangeable cations and put all of the carboxyl groups into the acid form experiences about a 50% weight loss. In comparison, acid-treated lignite back-exchanged with calcium or magnesium experiences only a 30% weight loss at similar experimental conditions [ 152]. Cations also reduce weight loss at residence times longer than obtained in entrained flow reactors [ 150]. (ii) Effects of cations on pyrolysis kinetics. The rate of volatile release, the apparent activation energy for volatile release, and the tar yield are reduced by the cations [150]. Cations also reduce the rate of decomposition of the carboxyl groups. For pyrolysis in air, the initial rate of weight loss is lower for lignite containing cations than for comparable acid-washed samples [ 150]. As examples of the effect of cations on pyrolysis rate, the activation energy and preexponential factors for entrained flow pyrolysis of Montana lignite were 58 kJ/mol and 8 x 103 s-l, respectively [142]. Acid washing increased these values to 148 kJ/mol and 2 x 108 s-l, while backexchange of calcium resulted in activation energy of 99 kJ/mol with preexponential of 5 x 105 s-1 [1421. The rate constants for decarboxylation of Montana lignite in entrained flow pyrolysis

181 increase from 0.91 s-1 at 973 K to 9.1 s-1 at 1173 K [151]. Acid washing increases these values by roughly a factor of four, to 3.9 s-1 at 973 K to 31.5 s-1 at 1173 K [151]. Decarboxylation of the raw lignite has an activation energy of 110 "lcJ/mol and pre-exponential factor of 7 x 105 s-l; the respective values for acid-washed lignite are 97 kJ/mol and 6 x 105 s-1 [151]. The temperature of maximum rate of hydrogen evolution is lowered in the presence of cations, whereas the maximum temperature of carbon monoxide release in increased during slow heating rate pyrolysis of Darco lignite in nitrogen [153]. Rates of gas evolution from raw and calcium-loaded samples of Darco lignite are similar [ 154]. The maximum rate of hydrogen evolution from Darco lignite shifts to lower temperatures for cation-loaded lignite [154]. (iii) Effects of cations on volatiles yields and quality. Pyrolysis of demineralized lignites produces a carbon dioxide yield equivalent to the concentration of carboxyl groups in the lignite. However, back-exchange of cations onto demineralized lignite enhances carbon dioxide evolution relative to that of demineralized lignite. The maximum increase in carbon dioxide production amounted to 67%, for lignite back-exchanged with potassium [153], due to catalysis of the water gas shift reaction. Volatilization of water and carbon dioxide decreases the average number of oxygens around a calcium ion, but bulk species containing calcium do not form under these conditions. In Falkirk (North Dakota) lignite loaded with calcium by ion exchange, the radial structure function (RSF) of the calcium, as observed by x-ray absorption fine structure, is typical of a non-crystalline and highly dispersed calcium [155]. The RSF does not change on rapid pyrolysis, indicating that the calcium is still in a highly dispersed, non-crystalline state [ 155,156]. The calcium remains bound to oxygen atoms from the original carboxyl groups. As the pyrolysis severity is increased, by increasing either the time, temperature, or both, long-range order begins to develop. The formation of long-range order presumes some mobility of the calcium atoms. The RSF begins to resemble that of calcium oxide, although some calcium remains in a highly dispersed state. Significant amounts of bulk calcium oxide form in slow heating pyrolysis. Mild pyrolysis of Richland County lignite shows no change in the immediate coordination sphere around the calcium [157]. The calcium-oxygen bond distance decreases by 2-3 % from 25 ~ to 420~ Hydrogen production is enhanced during slow heating pyrolysis of cation-loaded lignites. The maximum enhancement relative to demineralized lignite was obtained for sodium-loaded lignite, a 42% increase [ 153]. The increased hydrogen may result from the water gas shift reaction or catalysis of the steam-carbon reaction. A reduction in the water yield accompanies the increased production of hydrogen in the presence of cations. The reduced water yield is consistent with the occurrence of the water gas shift reaction, carbon-steam reaction, or both. A slight decrease in hydrogen yield accompanies acid washing of Estevan lignite [ 158]. Untreated, ion-exchanged, and demineralized Zap lignite all evolve carbon monoxide between 400 ~ and 800~

[ 159]. Evolution of carbon monoxide is lower below 750 ~ but is higher

above this temperature, for the untreated and ion-exchanged samples relative to the demineralized

182 lignite. The fraction of total carbon monoxide evolving below 750~ increases with increasing tar yield [ 159]. The increased CO evolution below 750~ from the demineralized lignite, and greater tar yield, indicate a greater proportion of oxygen functional groups decomposing to CO without attendant cross-linking. On the other hand, the reduced CO evolution below 750~ for the ionexchanged sample results from a retention of oxygen in cross-links. Acid-washing Velva lignite gave comparable char and tar yields for the acid-washed and untreated lignites, 64.6 vs 62.5% char yields (daf basis) and 3.48 vs 3.65% tar yields, respectively [ 160]. The gas composition from the acid-washed lignite showed increased carbon monoxide and ethane and decreased carbon dioxide and hydrogen compared to the gas from the untreated lignite. A higher yield of carbon monoxide from demineralized samples could relate to an increase in the number of oxygen functional groups able to evolve as CO without instead being incorporated into cross-links [159]. In comparison, more tar and less carbon monoxide and dioxide were evolved from demineralized Zap lignite than from untreated lignite [ 161]. A higher tar yield with accompanying lower gas yield (mostly resulting from reduced carbon dioxide yield) is also observed for acid-washed Estevan lignite, relative to the untreated material [158]. Higher tar yields, with lower carbon dioxide yields, similarly result from acid washing of Coronach (Saskatchewan) lignite [158]. Cation-loaded lignites generally seem to give greater amounts of carbon dioxide, and hydrogen, than acid-washed lignites [154]. Pyrolysis of Velva lignite with and without added potassium carbonate or trona showed significant differences in the production of catechol, methylcatechols, guaiacol, and methylguaiacols [155]. Samples treated with potassium carbonate or trona resulted in a >90% reduction of catechol and a >80% reduction in methylcatechols. Similarly, the amounts of catechol and methylcatechol increase in tars from acid-washed Coronach lignite, relative to the untreated lignite [ 158]. (Phenols are much less affected.) The removal of the cations somehow affects the reactivity of the structures that are the precursors to catechol and its derivatives. Demineralized lignite generally gives a higher tar yield [159,161]. Tar yield from Zap lignite increased with increasing extent of ion exchange removal of cations [ 159]. Metal cations reduced tar yield during entrained flow pyrolysis of Montana lignite by 70 to 94% [151]. With Estevan lignite, tar yield drops as severity of acid washing is increased [158]. Tars from untreated lignite are more aliphatic than those from comparable experiments using acid-washed lignite [ 150]. Slow heating rate pyrolysis of Darco lignite in a nitrogen atmosphere showed little effect, up to 1000~

of metal cations on weight of char produced [153]. The maximum char yield for

lignites back-exchanged with various cations was only 3.4% greater than the char obtained from demineralized lignite. However, the cations increased the total weight of gases formed, by up to 17% with calcium-loaded lignite [153]. Acid washing reduced char yields from Saskatchewan lignites [ 158]. The role of the cations in promotion of secondary char-forming reactions can be shown in part by the difference in physical properties and chemical constitution (as deduced from F r l R spectra) of tar from untreated and acid-washed Montana lignite [151]. The tar from the untreated

183 lignite was a black, gummy material with about three times as much aliphatic hydrogen as the brown, powdery tar from the acid-washed lignite. Removal of the cations from lignite significantly reduced the deposition of carbon in the lignite char from methane cracking reactions [ 162]. Carbon deposition reduces the surface area and open pore volumes, and therefore adversely affects the reactivity of the char. Cross-linking behavior of ion-exchanged Zap lignite resembles that of untreated lignite [161]. However, the cross-linking behavior of the demineralized lignite differs; specifically, shifting to higher temperatures for the same loss in solvent-swelling capability (which would presumably represent the same extent of cross-linking). (iv) Comparative effects of different cations. When compared on a similar molar basis, calcium and magnesium had nearly identical effects on the maximum weight loss in entrained flow pyrolysis of Montana lignite (helium and nitrogen atmospheres, 973 and 1173 K) [97,142,151]. Sodium-loaded lignite that contained about two-thirds of the amount of calcium or magnesium (in meq/g) showed the highest or second highest maximum weight losses of all the cation-form lignites. The important factor is the number of carboxyl groups involved in bonding to the cations (or, from the other perspective, the equivalents of cations present). The effect of cations on carbon monoxide production, relative to demineralized lignite, depends on the type of cation. Potassium- and sodium-loaded lignites form less carbon monoxide, and less methane, than lignites loaded with divalent cations [154]. Back-exchanging lignite with calcium or barium increases carbon monoxide formation, while sodium or potassium decrease it [153]. This effect may be a result of the relative activities of these cations to catalyze the water gas shift reaction vs. activities for the carbon-steam reaction. Divalent cations, Ca, Mg, and Ba, produced greater gas yields than the monovalent K or Na during slow heating rate pyrolysis of Darco lignite in nitrogen [153]. Methane formation is reduced when the lignite is loaded with sodium or potassium cations, but is enhanced if the lignite is back-exchanged with divalent cations [153,154]. This effect represents a balance between methane cracking and methane generation via carbon gasification or gas-phase methanation. Cation-loaded lignites produce more carbon dioxide and hydrogen than demineralized lignite [ 154]. The order of cations in enhancing carbon dioxide and hydrogen yields is K ~ Na > Ba > Ca > Mg > demineralized lignite [154]. A similar order exists for ranking the reduction in carbon monoxide yield. Release of water during pyrolysis is less from lignite loaded with divalent cations than for the demineralized lignite, or lignite loaded with monovalent cations [154]. Any cation-loaded lignite releases less total amount of water than the demineralized lignite. Treating Velva lignite with aqueous magnesium chloride decreased tar yield and increased char formation during carbonization [160]. Similar treatment with iron(III) chloride also decreased tar yield, which in this instance was accompanied by increases in both char and gas. With aluminum chloride, a reduction of tar yield again occurs, but the compensating increase in gas and char was mainly as increased gas yield. In comparison, calcium carbonate as an additive during

184 carbonization gave a very small decrease in tar yield (2.6% vs. 10.3% for aluminum chloride and 38.5% for iron(Ill) chloride) but a significant increase in gas yield (17.9%). The effects of added potassium carbonate or trona on production of catechols and guaiacols from Velva lignite could not be observed for treatment of the lignite with calcium carbonate [155]. Tar yield is higher from potassium-exchanged Zap lignite than from samples exchanged with calcium or barium [ 159] The divalent cations may "tighten" the lignite structure by enhancing the cross-linking of molecular fragments in the structure, increasing the difficulty of tar formation. Treatment of Montana lignite with aqueous solutions of sodium, calcium, iron, magnesium, and aluminum salts all increased char yield [ 160]. In these experiments the lignite-salt mixture was heated to 500~ in 15 minutes (i.e., a heating rate of 320/min) and held at 50(O510 ~ for 1.5 h. The largest increase in char yield was obtained when aluminum sulfate was used as the additive. 4.4.6 Effects of petrographic composition For coals of various ranks shows that the initial volatilization temperature, T i, and the average volatilization rate, kv, measured by thermogravimetry, correlates with vitrinite reflectance (as well as volatile matter) [137]. For lignite, Ti is lowest (about 250~

and kv is at a maximum

compared to other ranks of coal. Durain, fusain and vitrain lithotypes separated from Beulah lignite were pyrolyzed in helium at a rate of 450C/min to a final temperature of 850~

Gas chromatographic analysis of the

water-soluble organic products gave the results shown in Table 4.10 [163]: TABLE 4.10 Yields* of water-soluble organics from three Beulah lithotypes [ 163]. Compound Methanol Acetone Acetonitrile 2-Butanone Propionitrile Phenol o-Cresol p-Cresol m-Cresol Catechol

Vitrain 2680 1910 280 480 110 2680 590 720 680 6940

Attritus 580 1340 310 380 160 1690 340 460 420 990

Fusain 730 1390 270 380 370 3040 780 850 880 1010

*Yields are reported in micrograms per gram of lignite (mat" basis)

In addition to the variations in yields from the various lithotypes, most notable for methanol, acetone, phenol, the cresols, and catechol, the total yield of compounds from the vitrain was about 2.5 times that from durain (17,070

vs.

6,670 ~tg/g, respectively) and almost double that from

fusain (9700 l~g/g). The results show that the yields vary from one lithotype to another, and

185 suggest that a petrographic analysis of lignite might be useful in helping to predict pyrolysis yields. Since the proportions of the different lithotypes vary within a seam, the composition of pyrolysis products in a commercial operation (e.g., gasification) may vary depending on the place in the mine from which the specific batch of lignite had been extracted. Four distinct lithologic layers have been observed in the Freedom (North Dakota) mine [164]. Pyrolysis of samples of each of the four lithologic layers under the same conditions described above also results in significant differences in the water-soluble organics [ 165-167]. TABLE4.11 Yields* of water-soluble organics from lithologic layers, Freedom mine, North Dakota [ 165-167] Compound Methanol Acetone Acetonitrile 2-Butanone Propionitrile Phenol o-Cresol p-Cresol m-Cresol Catechol

Layer 1 990 1350 240 360 70 2110 610 680 710 990

Layer 2 1010 1320 250 340 130 1720 520 570 630 1010

Layer 3 940 1490 260 420 280 1800 580 600 720 1200

Layer 4 1590 1420 190 350 190 3820 980 1190 1420 31 50

*Yields are in micrograms per gram of lignite (maf basis).

In general, the yields from the top three layers (i.e., layers 1-3) are reasonably similar, while the fourth differs considerably, particularly in the yields of phenol, the cresols, and catechol. This distinction among layers is also borne out in the petrography, the first three layers being similar while the fourth shows a much larger amount of corpohuminite (the submaceral phlobaphenite), present as 6.4 volume percent in layer 4 but ranging from 1.0 to 2.6% in the other three layers [167,168]. Phlobaphenite derives from catechoi and tannins and may be responsible for the substantially higher yields of the phenolic compounds. Catechol yields correlate with the amount of corpohuminite in each layer, with a linear least squares correlation coefficient of 0.92 [167]. The total yields of water-soluble organics and catechol correlate directly with the volatile matter (maf basis) of the four samples. The four layers have also been examined by Fischer assay. The data are shown in Table 4.12 [167,169]. Pyrolysis of Zap and Darco lignites in a steam-nitrogen mixture to temperatures up to 2400 K and pressures to 1.3 MPa showed only a weak correlation between petrographic composition and volatiles yield [ 170].

186 TABLE4.12 Fischer assay yields (as-received basis) for lithologic layers in Freedom lignite [167,169].

Tar

(kg/t) (l_Jt) API gravity Gas (kg/t) (m3/t) Water (kg/t) (L/t) Char (kg/t)

Layer 1 49.4 50.5 12.9 142.7 85.6 225.3 225.6 662.9

Layer 2 28.2 28.8 12.9 139.9 94.2 241.6 241.9 590.3

Layer 3 40.1 40.9 12.9 127.2 87.5 288.2 288.2 544.6

Layer 4 32.8 33.4 12.9 113.0 78.3 333.0 333.2 521.0

4.4.7 Changes in physical structure accompanying pyrolysis Pyrolysis of Monticello lignite in a drop tube reactor, in nitrogen, increased the open pore volume by about an order of magnitude [ 139]. The unreacted sample had a total open pore volume of 0.078 cm3/g; at 53% pyrolysis weight loss in a 1450"C atmosphere, the total open pore volume increased to 0.980 cm3/g [ 139]. The BET surface area increased from 3 to 117 m2/g. As temperature increases, the loss of volatile constituents increases the microporosity, making the resulting char readily accessible to reactants. While the volatiles are being driven off, realignment of aromatic regions occurs concomitantly. The rearrangement of aromatic structures decreases pore volume. Up to about 900"C volatile loss predominates relative to pore volume loss, so that the net effect is an "opening up" of the char structure [171]. At higher temperatures, the pore volume loss accompanying structural realignment becomes predominant. Formation of carbon in the pores by thermal decomposition of gaseous pyrolysis products on carbonaceous or mineral matter surfaces may also result in loss of pore volume. Lignite chars are aperture-cavity materials [172]. The pores have a narrow opening leading to a cavity of larger volume inside the material. Deposition of 2.6% (by weight) of carbon via chemical vapor decomposition of methane (as would be representative of cracking of hydrocarbon volatiles on the carbon surface) decreased the open porosity in the char from 35.6 to 25.3% [171]. Thus some of the apertures become so reduced in size that helium can not penetrate at room temperature. The amount of carbon deposited by cracking methane is much less than anticipated from the open pore volume of the char; this is consistent with the existence of an aperture-cavity pore structure [171]. Deposition of carbon can reduce subsequent reactivity both by decreasing the accessible active surface area and by coating the surfaces of potentially catalytically active inorganic species. Specific surface areas measured by nitrogen and carbon dioxide adsorption both increase as a function of isothermal pyrolysis time. The largest changes occur after about 200 ms [136]. Using Savage (Montana) lignite with an isothermal pyrolysis time of 0.16 s as an example, the measured surface areas (daf basis) were 9.5 m2/g in nitrogen and 271 m2/g in carbon dioxide. This

187 difference is attributed to a molecular sieve effect, suggesting the presence of large amounts of pores with apertures in the range 0.49--0.52 nm [136]. Specific

surface

areas

of

chars

of

three

lignites--Darco,

Savage,

and

Glenharold--pyrolyzed in ambient pressure nitrogen in an entrained flow reactor at heating rate 8000~

temperature of 808~

and isothermal pyrolysis times between 18 and 1025 ms

increased with increasing pyrolysis time, and the increase in surface area became more appreciable with times greater than 200 ms [173]. Helium density of the chars gradually increases with pyrolysis time, but the mercury density decreases. The decrease in mercury density is initially rapid, but becomes slower at longer times. The changes in both densities become nil with pyrolysis times greater than 600 ms. The total open pore volume and the porosity both increase rapidly with time, but the change is minimal above 600 ms. Release of volatile matter develops of internal porosity as a result of the opening of previously closed pores, forming new pores, or enlarging either existing or new pores, or through some combination of these factors. Helium and mercury densities become constant after about 600 ms of isothermal pyrolysis; however, at short times the helium density increases and the mercury density decreases. These changes are consistent with an increase in porosity and total pore volume [ 136]. This behavior contrasts with that of chars produced in a relatively slow heating fluidized bed, where there is negligible difference between the mercury densities of the lignite and char, but the helium density of the char is much higher than that of the lignite. The pore volumes of chars from the slow heating are less than half those of chars from rapid heating [ 136]. Whether a more open or less open pore structure than that of the original lignite is produced in pyrolysis depends on the balance of two processes: creation of additional pore volume by release of volatiles, and removal of pore volume by the alignment and growth of aromatic and hydroaromatic structural units. Rank, heating rate, maximum heat treatment temperature, and time held at the maximum temperature affect this balance. With lignites, volatiles release dominates, a situation typical of low-temperature pyrolysis [136]. Formation of high-temperature (12000C) semicoke from Beulah-Zap lignite, as well as four coals of higher rank, in a flat-flame burner shows the effects of pyrolysis on a variety of the char properties [ 174]. Micropores of 20 nm) range. Pore volumes increased by factors of five to ten. Rosin-Rammler parameters [ 175] for lignite chars show that pyrolysis produces a broader distribution of the particle size than existed in the lignite before pyrolysis [136]. In addition, pyrolysis decreases the weight-mean particle size. Particle shrinkage and density changes occur simultaneously during entrained flow pyrolysis [ 176]. Reduction in particle size is independent of temperature, but significant particle decrepitation does not occur. Wilcox lignite particles of 50x60

188 mesh pyrolyzed at heating rates of 0.1, 10, and 1000~

showed no visible change in the char

particles as heating rate was increased, other than formation of a few relatively large cracks or fractures [177,178]. Similarly, heating rate had little effect on porosity and macropore surface area. A decrease in average particle size radius accompanies increasing heating rate, a result of increased fragmentation of the particles as they undergo devolatilization [178] 4.4.8 Changes in functional groups during pyrolysis (i) Carboxyl groups. Carbon dioxide arises mainly from thermal decarboxylation; formation of some carbon dioxide from carbonyl groups has been suggested [ 179]. The carboxyl content of char from Beulah lignite at 225~ is comparable to that of unheated lignite; for char produced at 500~

the carboxyl content is equivalent to loss of about 55--60% of the original

carboxyl groups [180]. Decarboxylation of Russian brown coals begins at 3500C, along with the loss of some phenolic --OH [181]. In the presence of traces of oxygen, these reactions start about 100~ lower. Decarboxylation and loss of phenolic --OH are complete by about 5000C. Pyrolysis of 17 coals of various ranks, including Velva lignite, by Curie point pyrolysis at 3000*C/s shows a good linear relationship (correlation coefficient 0.93) between the carbon dioxide yield and the stretching vibration of the carbonyl band in the carboxyl group at 1760 cm-1 [ 182]. Observations of the yield of carbon dioxide as a function of time for pyrolysis of a North Dakota and an Montana lignite, and of the rate of carbon dioxide formation as a function of temperature for a Texas lignite, suggest that carbon dioxide formed during lignite pyrolysis has two sources, one weakly bound to the structure and the other tightly bound [183]. In an entrained flow reactor 35% of the total weight loss from Montana lignite could be attributed to decarboxylation [151]. Loss of carboxyl occurs both by direct thermal scission of carboxyl groups and by loss of tar molecules which themselves may contain some carboxyls. Decarboxylation kinetics are first order. The rate constants ranged from 0.91 s-1 at 973 K to 9.1 s-1 at 1173 K [151]. The activation energy was 110 k.l/mole with a pre-exponential of 7 x 10-5. The activation energy is lower than might be expected for chemical bond breaking, and reflects the fact that a significant portion of the carboxyls are actually released with the tar, the tar in turn being held in the lignite by bonds of low strength with low activation energies for cleavage. The loss of carboxyl groups on mild pyrolysis is catalyzed by copper [ 184]. For example, loading 1% copper onto Megalopolis (Greek) lignite by treatment with aqueous copper sulfate results in a 40% increase in calorific value, from 24.25 to 34.07 MJ/kg, after mild pyrolysis to 300~

So far as is known, this work has not been extended to North American lignites. Entrained flow pyrolysis of Savage lignite at 800~

showed rapid reduction of carboxyl,

along with hydroxyl and aliphatic hydrogen, by FTIR analysis of the char [176]. C - - O bonds are retained and aromatic hydrogen increases. Water and carbon dioxide are the primary products below 400~ [179]. This is in contrast to reactions of bituminous coal under the same conditions, in which case hydrocarbons are the dominant product. The yields from lignite are not affected greatly by pressure or particle size.

189 The first evidence of tar evolution during pyrolysis of a North Dakota lignite on a wire grid heater coincides with the first evidence of weight loss and the first evidence of carbon dioxide release [ 185]. This reaction system used 5 lxm particles heated at 950-1300~

in 172 kPa helium.

The decomposition of functional groups leading to the formation of carbon dioxide occurs essentially simultaneously with the cleavage of bonds leading to tar formation under these conditions. (ii) Hydroxyl groups. Some evolved water comes from the moisture in the lignite; however, additional water can be liberated by thermal dehydration of hydroxyl groups [176,186]. Pyrolysis of Beulah lignite in a heated grid cell at various temperature increments from 350 ~ to 9000C showed a decrease in --OH, as indicated by decreases in the broad FFIR peak in the region 3600-2200 cm-1 and sharp peak at 1600 cm-] [187]. Formation of water on pyrolysis of hydroxyl groups occurs via the reactions R-OH + R'-OH ---, R-O-R' + H20 R-OH + R"-H ~ R-R" + H20

where R, R', and R" are various organic structural fragments of the lignite [ 154]. In the presence of water, dehydration and decarboxylation of German brown coal above 300~ are suppressed in favor of release of humic acids [ 188]. However, other work has indicated that the changes in functional group composition were independent of whether the lignite was pyrolyzed in a wet or dry condition [ 189]. Beulah lignite exposed to D20 vapor for about 15 minutes readily undergoes deuterium exchange [ 183]. Pyrolysis of the deuterated lignite to 5000C produced deuterated gas species D20 and DHO. D - - O bonds were also present in the tar. These results suggest that the water produced during pyrolysis and the hydroxy groups found in the tar derive from hydroxy groups in the lignite. If the hydroxy groups were abstracting hydrogen and leaving as water, with no other structural changes, a comparable amount of D - - O and H - - O bonds would be expected in the pyrolysis products of the deuterated lignite. In fact, the amount of D - - O bonds was greater than H - - O bonds, suggesting that some ether formation takes place between two --OD functional groups with the liberation of D20 and retention of an ether group in the char. Water liberated on pyrolysis of Spanish lignite correlates with the amount of evolved carbon dioxide [ 190]. Several water molecules form per CO2 molecule, showing that the hydroxyl groups other than those that are part of the carboxyl functional group contribute significantly to water production. The correlation between water and carbon dioxide fails for pyrolysis temperatures above 5000C; some of the CO2 produced above 5000C comes from decomposition of other functional groups without concomitant water formation. Polyhydroxy aromatic compounds readily undergo self-condensation reactions; examples are 1,3-dihydroxynaphthalene and resorcinol. The existence of polyhydroxy aromatic structures in

190

lignite may also contribute, along with the thermal decarboxylation, to the facile crosslinking of lignites. (iii) Other oxygen functional groups. The quinone concentration in Russian brown coals decreases steadily over the temperature range 350--500"C [181]. Pyrolysis of guaiacol yields methane, carbon monoxide, catechol, and phenol as the only products at low conversions [191]. At higher conversions a solid char is formed, accompanied by reduced yields of catechol. Guaiacol decomposition proceeds via two parallel reaction pathways:

q- CH 4 OH

OH

+ OH

CO

OH

The major products of pyrolysis of benzaldehyde are carbon monoxide and benzene.

+

CO

2,6-Dimethoxyphenol and isoeugenol decompose analogously to produce methane, carbon monoxide, and liquid products. Vanillin produces carbon monoxide and methane. At low conversions of vanillin, the major products are guaiacol and dihydroxybenzaldehyde. At higher conversions the products include catechol and phenol, and, with extensive conversion, a carbonaceous char is produced. The decomposition of vanillin is illustrated by HC d'

O~

"-

+ OH

OH

HC ~0~

o"

OH

co

HC ~OH

o*

+

CH 4

OH

Guaiacyl substituents enhance decarbonylation of aryl aldehydes, but demethanation of guaiacyl structures seems unaffected by the presence of the carbonyl group.

191 A summary of the functional group composition of Beulah lignite and the rate expressions for the decomposition of the functional groups or formation of pyrolysis products using a heated grid cell in a Fourier transform infrared spectrometer is given in Table 4.13 [187]. TABLE 4.13 Kinetic rates and functional group composition for Beulah lignite [187]. Composition parameter (dmmf) Carbon Hydrogen Nitrogen Sulfur (organic) Oxygen Sulfur (mineral) Carboxyl Hydroxyl Ether loose Ether tight Nitrogen loose Nitrogen tight Aliphatic hydrogen Aromatic hydrogen Nonvolatile carbon Organic sulfur Total Tar Olefins Acetylene Soot

Composition 0.726 0.044 0.010 0.001 0.219 0.028* 0.050 0.045 0.02 0.23 0.011 0.009 0.129 0.015 0.49 0.001 1.000 0.07

Kinetic expressions

kl = 5400 exp (-8850 / T) k2 = 5400 exp (-8850 / T) k3 = 5400 exp (-8850 / T) k4 = 2.15 ~ 1016 exp (-57000 / T) k5 = 5400 exp (-8850 / T) k6 = 290 exp (-13000 / T) k7 19000 e x p (- 11000 / T) k8 = 40644 exp (- 14085 / T) k9=0 "

-

kt = 5400 exp (-8850 / T) ko = 2 9 109 exp (-24000 / T) ka = 1~ 1016 exp (-50000 / T) ks = 4 ~ 1019 exp (-60(900 / T)

*Dry basis A large concentration of ether groups results in the large temperature dependence of volatiles at high temperatures. This effect is not observed in pyrolysis of bituminous coals under comparable conditions. Pyrolysis of Center (North Dakota) lignite (helium atmosphere, 45~ 850~

heating rate,

maximum temperature) was compared with that of a second sample of the same lignite

treated with potassium hydroxide to form non-volatile potassium phenoxides, to discriminate between phenols arising from cleavage of alkylaryl ethers and those liberated from pre-existing phenolic structures. The yields of the three cresols were essentially the same from the two samples, while the phenol yield was reduced from 1310 to 630 lag/g (maf basis) [165,192]. About half the phenol yield from untreated lignite is due to cleavage of alkylaryl ethers and the remainder arises from phenolic structures already in the lignite. Dialkyl ethers form hydrocarbons and carbon monoxide on pyrolysis, and diaryl ethers are stable at high temperatures [193]. Thus the only type

192 of ether groups likely to contribute to phenol formation during pyrolysis under these conditions would be the alkylaryl ethers. This experiment resulted in about a 50% increase in the yield of acetone (from 850 to 1310 rtg/g) from the KOH-treated sample compared to the untreated sample [ 165]. No mechanism was offered for this change. Anisole pyrolyzes to form methane, carbon monoxide, and hydrogen [191]. The major liquid products include o-cresol, phenol, and benzene, with some toluene, xylenes, and xylenols formed as well. Anisole undergoes rearrangement to o-cresol, to form methane plus phenol, and to form benzene plus carbon monoxide. Methoxyl groups may be a source of some of the methane produced during pyrolysis [154]. The formation of water from hydroxyl groups, shown by the reactions discussed above, does not account for all of the water evolved during lignite pyrolysis. Water not accounted for by hydroxyl group reactions may arise from cleavage of ethers, postulated to produce aldehydes as intermediates [ 154]. In a subsequent step, the aldehydes could react with mobile hydrogens in the lignite to form water. Analogously, the mobile hydrogens could also react with carbonyl groups in the lignite structure to produce water. The yield of methanol from eleven samples of coals of ranks from lignite through high volatile C bituminous correlates with the methoxyl content of these coals. The correlation coefficient is 0.95 [163]. The yields of both 2-butanone and acetone for six coals, lignite through hvC bituminous, correlated inversely with the carbon/oxygen ratios. In both cases the correlation coefficient was 0.99 [163]. Methanol yields from the separated lithotypes of Beulah lignite, pyrolyzed at 850"C (helium atmosphere, 45~

heating rate) show a good correlation with the methoxyl contents of the

lithotypes, a linear least squares fit having a correlation coefficient of 0.95 [ 163]. Both acetone and 2-butanone, correlated with the C/O ratios of the lithotypes, with least squares correlation coefficients of 0.99 [163]. Carbon monoxide can be formed in a cheletropic extrusion of a carbonyl unit from such structures as coniferaldehyde [191 ]. Carbon dioxide can be formed from cycloreversion of lactones and aryl carboxylic acids. Elimination of water can result from the retro-ene reactions of guaiacylglycerol units in residual lignin structures. Pyrolytic decomposition of trans-cinnamaldehyde produces carbon monoxide and smaller amounts of hydrogen, methane, and acetylene [191]. A wide variety of liquid products form, including phenols, cresols, styrene, toluene, benzene, biphenyl, alkylated benzenes, and dimers produced in condensation reactions. The reaction proceeds via CH :CH

0

+

CO

The major gaseous products of acetophenone pyrolysis are carbon monoxide and methane, in

1 93 approximately 2:1 ratio, along with some hydrogen [191]. A very diverse mixture of liquid products was formed, including benzene, toluene, xylenes, styrene, cresols, benzaldehyde, biphenyl, as well as an assortment of aromatic ethers and dimers. Carbon monoxide is formed in benzaldehyde pyrolysis via cheletropic extrusion reactions [191]. Elimination of methane from guaiacol during pyrolysis occurs via a pericyclic group transfer elimination reaction. (iv) Aliphatic and aromatic hydrogen. Pyrolysis of Beulah lignite in a heated grid cell at various temperature increments from 350 ~ to 900~

showed (by FFIR analysis of the chars)

decreasing concentration of aliphatic C-H bonds, as noted by a decrease in the aliphatic stretch at 2900 cm-1 [187]. Loss of aromatic C-H begins only above 500~ Entrained flow pyrolysis of Beulah lignite in 0.1 MPa N2 showed that aliphatic species (and hydroxyl groups) were removed during primary pyrolysis, followed by char condensation with loss of aromatic hydrogen during secondary pyrolysis [ 189]. Aliphatic hydrogen occurs in labile methylene or polymethylene bridges between aromatic units in the structure. The labile bridges are key reaction centers during pyrolysis [194]. Their abundant occurrence in lignites facilitates conversion of lignites to high yields of non-condensible products [ 194]. (v) Sulfur and nitrogen. The distribution of sulfur in the flash pyrolysis products of Alcoa lignite is summarized in Table 4.14 [195]. The pyrolysis experiments were conducted at 850~ TABLE 4.14 Distribution of sulfur in flash pyrolysis products of Alcoa lignite [195]. Fraction Gas Tar Char Trapped on sand and reactor

Yield, wt. % 36.3 5.5 31.1 29.1

% of total sulfur 48.4 8.8 31.4 1.3

A correlation between desulfurization occurring during pyrolysis and the relative amounts of the different sulfur forms in the lignites could not be developed for Turkish lignites containing high concentrations of organic sulfur [ 196]. The first trace of hydrogen sulfide is observed at 200~

during low-temperature

carbonization of Sandow lignite, but formation of this gas does not become significant until temperatures of 300 ~ are reached [ 197]. The temperature dependence of the amount of sulfur liberated as hydrogen sulfide as a percentage of the total sulfur in the lignite is shown in Table 4.15 [197]. Isothermal pyrolysis of lignite over 300--1000~ and residence times of 5--80 s showed that the nitrogen was initially released almost entirely into the tar [198]. The composition and 13C NMR and FFIR spectra of the tar closely resemble those of the parent lignite. The nitrogen structures in the tar are very similar to those of the lignite. The initial release of nitrogen is described by the tar

194 TABLE 4.15 Sulfur liberation (sulfur in H2S as percentage of total sulfur) from Sandow lignite as a function of temperature [ 197]. Temp., ~ 150 200 250 300 400 500

% of Total Sulfur as S in H2~_S 0.00 0.31 0.47 2.51 20.1 37.4

release rate constant, given by [ 198] k - 81 exp (-5800/T) s-k The observed rate is smaller than those measured for nitrogen-containing heterocycles such as pyridine and pyrrole derivatives. Thus the nitrogen is likely contained in thermally stable heterocyclic ring systems that resist cleavage during pyrolysis. At higher temperatures a secondary release of nitrogen into the gases occurs, presumably the result of ring-opening reactions at high temperature. Pyrolysis of Montana lignite in an electrically heated grid reactor also shows that the nitrogen exists in strongly bonded compounds that are among the most thermally stable structures in the lignite [198]. Nitrogen compounds are similarly released without cleavage of the C - - N bonds into the tar during initial devolatilization, the amount of nitrogen released from the lignite being proportional to the amount of tar released. At high temperatures, decomposition of nitrogencontaining structures releases nitrogen from the char into the gas. The rate for this decomposition of nitrogen compounds is about two orders of magnitude lower than the rate of release of nitrogen compounds into the tar. 4.4.9 Evolution of products during pyrolysis (i)

Effects of pyrolysis temperature. Devolatilization of Montana lignite occurs in an

electrically heated grid apparatus in five main phases: evolution of moisture at 100~ carbon dioxide and a small amount of tar around 450~

evolution of water (from thermal

dehydration reactions) and carbon dioxide in the range 500-700~ hydrogen, hydrocarbon gases and tar at 700-900~

evolution of

evolution of the carbon oxides,

and finally formation of additional amounts of

the carbon oxides at very high temperatures [199]. In helium at rates of 1000~

the tar yield

increased with temperature to an asymptotic maximum at 5.4% above 8000C. The maximum yield of hydrogen and light hydrocarbon gases was 3.3%. The principal components of this product fraction are methane (1.3% yield), ethylene (0.6%), and hydrogen (0.5%), along with smaller amounts of ethane, propane, propylene, and benzene. Methane and ethylene yields reach an

195 asymptotic value in the range 600-7000C, but upon further heating the yields of these gases again increase, with a second asymptote around 900~

In contrast, the hydrogen yield shows only a

"one-step" behavior with the major evolution occurring at high temperatures. The yields of water, carbon monoxide, and carbon dioxide reach asymptotic values of 16.5%, 7.1%, and 8.4%, respectively. By holding at 1000~ for 5-10 s, instead of immediately cooling the sample when the desired pyrolysis temperature had been reached, the yields of the carbon oxides were increased to 9.4% for the monoxide and 9.5% for the dioxide. The occurrence of pyrolysis reactions at high temperatures independently of those taking place at lower temperatures indicates the existence of multiple parallel independent reactions. Assay yields and gas compositions for low-temperature carbonization of Sandow lignite are shown in Table 4.16 [197]. TABLE 4.16 Low temperature carbonization yields and gas compositions for Sandow lignite [197].

Yields, % maf Char Water Tar Light oil Gas Hydrogen Sulfide Gas Composition, % Carbon Dioxide Illuminants Carbon Monoxide Hydrogen Methane Ethane

Temp. ~ 250 300

150

200

400

99.5 0 0 0 0.5 0

99.2 0 0 0.2 0.7 0

98.0 0 0 0.7 1.4 0

93.8 1.8 0.2 1.4 3.0 0.1

74.6 6.8 7.3 1.6 9.4 0.6

95.9 0 0 0 4.1 0

90.3 0.3 6.5 0 2.9 0

88.7 0.4 8.8 0 2.1 0

78.2 0.8 12.7 0.8 7.3 0.2

67.6 1.0 13.3 0.8 16.9 0.4

Elbistan (Turkish) lignite pyrolyzed in a Fisher retort (220-600~ Abundant gas formation starts above 300~

N2) behaves similarly.

increasing temperature enhances gas formation [200].

North Dakota lignite pyrolyzed at 450 ~ and 600~

in a Parr reactor shows only minor

change in the distribution of n-alkanes [201]. This suggests that there is little additional cracking of the alkanes at 6000C relative to 450 ~ and that these alkanes are products of volatilization (or "thermal extraction") rather than cracking. The major n-alkanes were between C14 and C21 [201]. The effect of carbonization temperature on gas composition is shown in Table 4.17, for Sandow lignite [197]. In low-temperature reactions of Sandow lignite, the first indication of disruption of the coal structure occurred at 200~

a yield of 0.2% of light oils, along with small

amounts of carbon monoxide and illuminants in the gas [197]. At this temperature, 3.1% of the original amount of moisture in the lignite was still present, determined by xylene distillation (the

196 TABLE 4.17 Effect of carbonization temperature on gas composition from Sandow lignite [ 197].

Gas yield, % maf Gas composition, % Carbon dioxide Illuminants Carbon monoxide Hydrogen Methane Ethane

5OO 12.6

Temp., ~ 600 900 18.1 25.3

44.5 1.5 10.7 14.7 27.0 1.6

33.3 1.2 12.1 23.2 28.3 1.9

20.2 0.4 16.5 42.2 19.9 0.8

untreated lignite contained 32.1% moisture). At 250"C the first trace of tar was observed. A small amount of moisture, 1.2% of the original amount, still remained. At 3000C the amounts of gas and light oil were doubled compared to yields at 250 ~, and the first significant amounts of tar were noted. The maximum increase in yields occurs between 300 and 4000C. Tar yield is maximized at 500~

above that temperature the main carbonization products are gases. Heavy hydrocarbons

volatilized from a North Dakota lignite, treated by pyrolysis-GC, between 500 ~ and 7000C [201]. Maximum yield of volatiles was below 600 ~ [201]. Weight loss during pyrolysis of Darco lignite in an entrained flow reactor increased with temperature in the range 700-10000C for a given residence time [135]. The maximum yield of volatiles was 66.7% on a dry, ash-free basis, a factor of 1.3 greater than the ASTM proximate analysis volatile matter content [ 135]. A similar factor of 1.3 has been observed for RWE-Ki31n (German) lignite pyrolyzed at 80(01300 K [202]. The increased volatiles yield is a consequence of the reduction of secondary char-forming reactions and not a result of the substantially greater heating rate in the entrained flow reactor (over 104 *C/s) relative to the heating rate of the ASTM volatile matter test (about 20~ The effect of maximum pyrolysis temperature was determined by pyrolysis of -60 mesh Indian Head lignite at heating rates of 45~

to final temperatures of 850* and l l00~

in

helium. The yields of the smaller aliphatic compounds--methanol, acetone, acetonitrile, 2butanone, and propionitrile--were similar in the two experiments methanol at 850 ~ and 1100~

(e.g.,

1490 vs. 1460 tag/g for

respectively) [195,203]. However, the yields of phenol and o- and p-

cresols were higher, by 40--47%, at 1100~ (e.g., 2300 vs 3220 ~tg/g phenol) [ 195,203]. The mcresol yield at 1100~

was 79% greater than the yield at 850~

1270 vs. 710 ~tg/g respectively

[ 195]. This suggests that pyrolytic evolution of phenol and the cresols is probably not complete at the lower temperature. The phenol and cresols evolved at the lower temperatures may originate from cleavage of alkylaryl ethers [203]. The additional phenol and cresols formed at 1100~ may originate from cleavage of diaryl ethers, which have higher C - - O bond energies than the alkylaryl ethers [ 163].

197 As the pyrolysis temperature of Alcoa lignite is increased, in flash pyrolysis experiments, the polymethylene content in the tar decreases [204]. Hydroaromatic and benzylic species behave similarly. The aromatic species in the tar increase rapidly as a function of temperature, becoming the dominant components at high temperatures. At 2127 ~

rapid pyrolysis in nitrogen-steam atmospheres showed the total volatile yield

from all ranks of coal to be much higher than the proximate analysis volatile matter yield [170]. At 1327 ~

this is true only for lignite. (ii) Effects of heating rate. During pyrolysis of Center lignite to 380 ~ and 8500C at rates of 5

and 450C/min the yields of phenol and the cresols were higher with slower heating rate [163], regardless of the final pyrolysis temperature. However, the slower heating rate experiments are also substantially longer (170 vs. 20 minutes for a final pyrolysis temperature of 850~

the

effects attributed to heating rate may be confounded by residence time effects. The data are summarized in Table 4.18. TABLE4.18 Effects of heating rate on yields* of water-soluble organics from Center lignite [ 163].

Compound Acetone Acetonitrile 2-B utanone o-Cresol m-Cresol p-Cresol Methanol Phenol F'ropionitrile

380~ Maximum Temp. 5~ 45~C/min 110 40 ND ND 20 10 50 10 80 10 180 40 570 270 460 100 ND ND

850~ Maximum Temp. 5~ 45~C/min 1190 1490 300 280 310 440 1300 780 1920 1400 1740 1360 1590 1700 4380 3460 30 380

*Micrograms/Gram maf lignite. ND = not detected.

For Indian Head lignite in argon, the rate of mass loss per degree above 700~ was higher for the sample heated at 5~

than for a comparable sample heated at 100~

suggests that above 700 ~ (at least up to 1100~

[15]. This

pyrolysis weight loss is not only a function of

temperature but also of time. Louisiana lignites pyrolyzed in a thermogravimetric analyzer (in nitrogen at atmospheric pressure) show higher volatile losses at higher heating rates [35]. TMRWL also increased with increasing heating rate. In hydrogen, heating rate effects are the opposite of those in nitrogen, so that volatiles production is greater at lower heating rates. However, the temperature of maximum rate of weight loss in hydrogen increases with temperature as in nitrogen. The total volatiles produced are greater in hydrogen for similar reaction conditions. Hydropyrolysis begins at about 2000C [35]. The increased weight loss in hydrogen relative to nitrogen ranged from 18.9% at heating rates of 11 ~

to 5.8% at 4350C/min [35].

198 Heating rate was the most important variable affecting the weight loss behavior from North Dakota lignite for pyrolysis in a thermogravimetric analyzer to 1000"C in helium [131]. Pyrolysis at 160*C/min showed a TMRWL 25-50"C higher than for pyrolysis at 40~

However,

heating rate did not have a significant effect on volatiles yield. Heating rate had no evident effect on activation energies for pyrolysis of a North Dakota lignite, values of 224 kJ/mol and 230 kJ/mol being observed at heating rates of 160~

and 40~

respectively. With Montana lignite,

some differences in kinetic parameters were observed at the different heating rates. At 160~ the activation energy was 223 kJ/mol with a pre-exponential of 1.67 x 1013; whereas at the lower heating rate, the respective values were 175 kJ/mol and 1.07 x 1010 [131]. Tar yield increases with increased heating rate [205,206]. Tar yields for rapid heating rates at atmospheric pressure are 10-20% [207]. Field ionization mass spectrometry (FIMS) of tars produced at low heating rates indicates a marked fall-off at higher molecular weights [208]. For higher heating rates, FIMS shows an increased contribution from larger molecules, consistent with a reduction in the extent of crosslinking during pyrolysis [208]. High values of number average molecular weights of tars reflect suppressed fragmentation of molecules [207]. (iii) Effects o f residence time. Weight loss is a function of both isothermal pyrolysis time and particle size for Savage and Glenharold lignites pyrolyzed in an entrained flow reactor [209] in nitrogen at 808~ [136]. For example, the weight loss from Savage lignite after 0.16 s ranged from 22% (daf basis) of the original coal for 200x270 mesh lignite to 6% for 70x100 mesh. The weight loss depended on the isothermal exposure time (that is, the time during which the sample was at 808~

with no significant weight loss occurring during either very rapid heating or very rapid

cooling. Pyrolysis of Monticello lignite in a drop tube reactor illustrates the effects of residence time and temperature. At 0.2 s residence time in nitrogen, the pyrolysis weight loss increased from 16% to 51% with an increase in temperature from 788~ to 1455~ [139]. At 788~

pyrolysis

weight loss increases from 3% to 34% as residence time increases from 0.05 to 0.8 s. At 1455~ pyrolysis is essentially complete within 0.2 s. The difference in weight loss vs. time for various particle sizes persists to longer heating times (to 1.0 s), but with a distinct change in the slope of the weight loss vs. time curves. For Glenharold lignite the time at which the slope change occurred, 0.2 s, was independent of particle size [136]. These sharp breaks in the curve are characteristic of the occurrence of two parallel reactions: rapid release of volatiles by decomposition of oxygen-containing functional groups and methyl substituents, and slower release of volatiles as the hydroaromatic structures convert first to aromatic structures and then to small carbon crystallites. Loss of oxygen functional groups and side chain methyl groups produces only volatiles. However, decomposition of hydroaromatic structures and subsequent formation of carbon crystallites produces significant amounts of char. The relative proportions of volatiles and char depend on the amount of functional groups and the size and shape of the aromatic and hydroaromatic structures--which are governed by the rank of the coal--and by the pyrolysis conditions.

199 Pyrolysis of Wilcox lignite in a drop tube furnace at temperatures of 1460~

showed

pyrolysis was complete in 0.2 s [140]. In this reactor system, volatile evolution was 12% higher than the ASTM volatile matter yield. Weight loss during entrained flow pyrolysis of Glenharold, Darco, and Savage lignites depends both on the particle size and the residence time in the reactor, with smaller particle sizes and longer residence times producing greater weight losses [173]. Prolonged heating at 1000~

in a heated grid reactor gave a total volatiles yield from

Montana lignite of 44.0% [199]. Only about 22% of the carbon was volatilized, indicating that most of the volatiles result from loss of hydrogen and oxygen. The reduction in sulfur was about 70%; that of nitrogen, 25%. At a heating rate of 1000~

thermal dehydration reactions leading to

the formation of water are about 90% complete before significant tar formation begins. The mass lost from Indian Head lignite in a thermogravimetric analyzer at 1100~ in argon at ambient pressure agrees quite well with the standard ASTM volatile matter determination [ 15]. The range of values observed in pyrolysis in the TGA was 38.2 to 42.0%, while the ASTM volatile matter determination on the same samples indicated 39.0 and 42.2%, respectively. Values from another laboratory, obtained by heating to 1000~

were 38.6 and 41.2% [131]. The amount

of volatiles determined by this TGA method depends on heating rate, since at lower heating rates the residence time will be longer and the volatiles release will be higher. The effect of residence time at maximum temperature was evaluated for Indian Head lignite at two residence times, 0 and 30 minutes, and at two temperatures, 380* and 850~ lignite pyrolyzed in helium at a heating rate of 45~

for --60 mesh

At 380~ the effect of residence time was

marked, with yields for a holding time of 0 minutes being below the limits of detection by gas chromatography, but with significant yields observed at 30 minutes [ 163]. In contrast, at 850~ there was essentially no difference in yields between the two reaction times. The results are summarized in Table 4.19 [163]. TABLE4.19 Effects of time at temperature on yield* of water-soluble organics from Indian Head lignite [163].

Compound Acetone Acetonitrile 2-Butanone Catechol o-Cresol m-Cresol p-Cresol Methanol Phenol Propionitrile

380"C Maximum Temp. 0 Minutes 30 Minutes ND 480 ND 30 ND 140 ND 1480 ND 350 ND 450 ND 550 ND 900 ND 1370 ND ND

*Micrograms/Gram maf lignite. ND = not detected.

850~ Maximum Temp. 0 Minutes 30 Minutes 1420 1460 230 240 400 370 2210 2120 510 530 710 820 760 820 1490 1740 2300 2360 130 270

200 The amount of light hydrocarbon gases formed during carbonization reactions in slagging fixed-bed gasification increases with increased the gas residence time in the reactor [210]. (The residence time was increased by increasing the pressure or by decreasing the oxygen feed rate to the gasification reactions.) In either case, a mass balance showed increased weight of gases equivalent to a decrease in tars. This suggests that at least portions of the methane, ethane, propane, and butane derive from cracking or dealkylation reactions of the tar components. Pyrolysis of Elbistan lignite for 60-180 min shows that liberation of both tar and gas slows with increasing residence time [200]. The residence time at which char yield becomes constant is in this range, and depends on temperature. Two factors account for the tar formation occurring during the early stages of pyrolysis [207]. First, the tar precursors initially present in the lignite are readily swept away by the flux of non-condensible gases being produced at the same time. Second, however, as pyrolysis--and associated cross-linking--proceed, fewer and fewer bridges remain to be cleaved to replenish the supply of tar precursors. (iv) Effects of pressure. Pressure effects diminish for lignites, compared to coals of higher rank, because few tar precursors are generated on decomposition of the lignite structure [207], and because the tar yield is low [211]. As an example, a lignite giving a 7% tar yield (daf basis) at 1 Pa still produced a yield of 5% at 0.1 MPa [211]. Although the tar yield decreases slightly with pressure, the gas yield increases to compensate [211]. Montana lignite, pyrolyzed in vacuum to temperatures of 800~ in a heated grid reactor, displayed no differences from pyrolysis to the same temperatures in 0.1 MPa helium [ 199]. Some differences were observed above 800~

Vacuum pyrolysis results in higher yields of heavy

hydrocarbons and lower yields of light gases, as opposed to pyrolysis in 0.1 MPa helium [ 199]. The total weight loss during vacuum pyrolysis exceeds that in helium, suggesting that secondary cracking reactions and char-forming reactions influence the observed yields. The effect of pressure on yields of water-soluble organics was minimal with Gascoyne and Center lignites pyrolyzed at 8500C (heating rate of 230C/min) in a 50 cm3/min stream of nitrogen. Little difference in the yields of methanol, acetone, acetonitrile, 2-butanone, propionitrile, phenol, and the three cresols occurred at ambient and 2.8 MPa pressure [212]. The yield of catechol from Center lignite decreased from 1860 to 210 ~tg/g. (v) Effects ofparticle size. Entrained flow pyrolysis of Darco lignite in the temperature range 700-10000C shows that weight loss is independent of particle size in the range 41-201 ~tm [176]. At heating rates of 10,000~

pyrolysis rate is independent of particle size in the range

41-201 ~m [176]. This observation suggests that physical factors do not control the pyrolysis. Particle size in the range 60-1000 ~tm does not influence total pyrolysis yield from Greek lignites [213]. The maximum potential weight loss to be expected from Darco lignite pyrolysis in an entrained flow reactor is 66% [176]. This value is much higher than the volatile matter content obtained during the proximate analysis (51%, daf basis). The maximum potential weight loss is independent of particle size, suggesting that secondary reactions leading to carbon or char

201 deposition inside the pores are not important. The effects of experimental conditions on pyrolysis of Indian Head lignite in a thermogravimetric analyzer are summarized in Table 4.20 [ 15]. TABLE 4.20 Effect of experimental conditions on pyrolysis of Indian Head lignite [ 15].

Increasing Particle size Sample weight Gas flow rate Heating rate

(vi)

TMRWL No trend No trend Decrease Increase

Effect on , ~ Decrease Decrease No trend Decrease

V* No trend Decrease Decrease Decrease

Comparative pyrolysis of different lignites. A comparison of product yields from three

North Dakota lignites is shown in Table 4.21 [163,195]. The experimental conditions were a heating rate of 45~

maximum temperature of 8500C, -60 mesh particles, and helium

atmosphere. TABLE 4.21 Yields* of polar organics from three North Dakota lignites [163,195]. Compound Methanol Acetone Acetonitrile 2-Butanone Propionitrile Phenol o-Cresol p-Cresol m-Cresol

Indian Head 1490 1420 230 400 130 2300 510 760 710

Gascoyn.e 730 1600 220 510 230 2900 590 1080 910

Center 1700 1480 280 440 370 3440 780 1360 1400

*Yields are reported in micrograms per gram of lignite (maf basis).

(vii)

Evolution of pyrolysis products. Carbon dioxide and water are the major products

during the initial stages of pyrolysis [206]. As the number of methyl groups in phenol-formaldehyde resins or in methylated phenols increases, the temperature at which methane formation is at a maximum shifts to lower temperatures [214]. As would be expected, the methane yield increases with increasing methyl groups. Japanese lignites show a similar carbonization behavior to that of resins of methylated phenols. Methane could be formed by a concerted group transfer elimination from a guaiacyl structure [191]:

202 %

~

0

O~CH3 ~--

-t- CH 4

OH

Pyrolysis of various materials--including lignin, methylated and perdeuteromethylated coals, and polymers with methoxy or methyl groups--has shown that when methoxyl groups are present, methane evolution occurs consistently at 4500C, independently of other reactions involved with tar formation [215]. Cleavage of the C--O bond forms methyl radicals, which then produce methane via hydrogen abstraction reactions. For substances containing methyl, rather than methoxyl groups, methane evolution coincides with tar formation at 500oC, and results from cleavage of methyl groups from the structure. Heated grid pyrolysis of a North Dakota lignite, at heating rates of 950-1300 ~

and

particle sizes of 54-74 ~tm in helium, indicated that all hydrocarbon gases appear at higher temperatures than carbon dioxide [185]. Some carbon monoxide was observed below 577~ [185]. The appearance of CO suggests the presence of some of the oxygen in the lignite in thermally labile functional groups other than carboxyl. When lignites are heated in hydrogen-containing gases the initial reactions involve loss the oxides of carbon, water, tars and oils, and some gaseous hydrocarbons [216]. The solid product of this reaction has been referred to as an "intermediate semi-char" [216]. In a second stage, decomposition of the semi-char with evolution of hydrogen forms a comparatively unreactive char. The yields of methane and ethane (if corrected for those deriving directly from the decomposition of the lignite and for those obtained cracking of the other light hydrocarbon gases) relate stoichiometrically to the amount of hydrogen evolved during the second stage of devolatilization. The transition from semi-char to the final, relatively unreactive char passes through an intermediate stage in which the solid is potentially quite reactive. The ratio of methane and ethane to char is proportional to the hydrogen partial pressure. Pyrolysis of Beulah-Zap lignite in the Rock-Eval test showed the following results [217]: free hydrocarbons evolved during the initial 3 minutes at 300~

1.3 mg/g (as received basis); total

amount of free hydrocarbon evolved during pyrolysis, 60 mg/g; carbon dioxide yield below 390~

13.9 mg/g; and temperature of maximum rate of hydrocarbon evolution, 1150C. The kinetic

parameters for this lignite in the Rock-Eval test, as determined by nonlinear regression, were a preexponential factor of 1.2 x 1015, and an activation energy of 232 kJ/mol with o of 6.1%. For coals ranging in rank through low volatile bituminous, carbon dioxide evolution is highest and total hydrocarbon evolution lowest for the lignite. Pyrolysis of this same lignite in a fluidized bed reactor at a heating rate of 4~

in argon atmosphere resulted in the following gas yields

(expressed as mg/g, daf basis): 14.6 H2, 24.1 CH4, 2.4 C2H4, 3.5 C2H6, 2.6 C3H8, 4.7 C4H10, 93 CO2, 82 CO, and 120 H20 (above 2000C) [202]. Carbon dioxide evolution below 520~ was

203 70 mg/g. Carbonization of Dakota (South Dakota) lignite at 500~ at a heating rate of 6-7~ produced gas containing 89.8% CO2, 4.2% CH4, 4.5% C2H6, and 1.5% illuminants [197]. Flash pyrolysis of Alcoa lignite at 8500C for 0.5-1 s produced the following gas yields (on a moisture-free basis): methane, 2.4%; ethylene, 3.2%; ethane, 0.33%; propylene, 0.74%;, propane, 0.04%; 1,3-butadiene, 0.41%, 1-butene, 0.13%; butane, nil; and benzene, 0.8% [218]. The concentration of polymethylene groups was about 5.3%, based on evolution of ethylene and other hydrocarbons [219]. Flash pyrolysis of lignite at temperatures of 700~ or above produces low molecular weight aliphatic hydrocarbons from cracking longer polymethylene chains. The polymethylene units are part of the coal structure or, possibly, molecules trapped in the coal matrix [220]. Reaction at lower temperatures (e.g., 600~

preserves some of the long chain (C 17 and higher) alkanes and alkenes,

which form part of the tar. Some lignite and subbituminous coals may contain as much as 10% (CHz)x [220]. Pyrolysis of Onakawana lignite in a spouted bed reactor, in nitrogen, showed a maximum liquid yield occurring about 5000C [221 ]. The liquid, however, amounted to about 80% water; the yield of dry tar was 6 - 9 % . Above 400~ the volatile content of the char decreases slowly with increasing temperature, but above 525~ a rapid decrease begins. The methane yield is about 1% at temperatures to 5500C, but increases rapidly above that temperature. About 50% of the gas yield is carbon dioxide, with much smaller amounts of methane, ethylene, and hydrogen. Production of alkenes during carbonization in a slagging fixed-bed gasifier decreases with increasing operating pressure of the gasifier [ 138]. Alkylphenols, alkyldihydroxybenzenes, and alkylmethoxyphenols dominate the products from Curie point pyrolysis of lignites [222]. For laser pyrolysis, major products include toluene, phenol, the cresols, C2-phenols, naphthalene, and methylguaiacol [223]. Most of the tar generation in lignite pyrolysis precedes evolution of light liquids [217]. Tar generation seems to be associated with thermal breakdown of oxygen and sulfur functional groups; the light liquids are formed by the cleavage of stronger bonds. Elimination of carbon dioxide, carbon monoxide and water may occur during crosslinking reactions, the extent being affected by heating rate [134]. The amount of tar formed in devolatilization correlates with the daf volatile matter content by X = 0.95 VM + 0.025 [224]. This equation is applicable for the following conditions: temperatures 400-1000~ sizes 25-1000 ~tm, heating rates 5.6x10-3-104 ~

particle

and pressures 10-3-10 MPa. The ultimate

weight loss was unaffected by heating rate in the range 650 to 104 ~

[224].

Evolution of tar from Texas lignite (pyrolyzed at maximum temperature of 400~

began at

204 about 300" [225]. Alkanes as large as C30 were observed in the products, along with some alkenes. Comparison of these results for lignite with model compound studies suggests that the alkanes and attendant alkenes are formed from cleavage of alkylaromatics. Mass spectroscopic analyses of the tars produced from Indian Head and Baukol-Noonan (North Dakota) lignites during slow heating rate carbonization in a slagging fixed-bed gasifier were compared with analyses of benzene:methanol extracts of lignite [226,227]. Seventeen of the same compound types, based on Z-number and molecular weights, occur in both the tar and the extract [227]. The major difference was that highly alkylated compounds found in the extract, such as C 5 tetralins and C 10 benzenes [228], were missing from the tar. For thirteen compounds found in the tars, the Spearman rank correlation between their concentration in the tar and their thermal stabilities, as measured by liquid phase pyrolysis reactions [229], was 0.725 (the probability of r = 0.725 occurring by chance is less than 0.01). During carbonization the long alkyl groups were thermally cleaved. Solvent extraction had shown that pyridine was the only heteroaromatic nitrogen or sulfur structure [226]. Quinoline derivatives were also identified in the tar, but it was not determined whether the quinoline structures were present in the original lignite or were artifacts of the carbonization process [228]. Methylation of the carboxyl groups increases tar yield on pyrolysis [205]. The molecular weight distribution of the tar resembles that from pyrolysis of bituminous coals [205]. Texas lignite reacted with tetrabutylammonium hydroxide followed by methyl iodide, and then pyrolyzed by heating at 5-10~

to 620~

showed an increased tar yield well above the amount calculated

simply from the added methyl groups [230]. That is, methylation of the lignite made the tars more volatile. Evolution of tars from untreated lignite could be inhibited by crosslinking reactions of the type Lignite-OH + HO-Lignite ---, Lignite-O-Lignite + H20 Methylation eliminates inhibition of volatiles release by preventing formation of ether crosslinks. Methanol evolution from the methylated lignite is significant, and suggests methylation of the carboxyl groups as well as aliphatic hydroxyl groups. (viii)

Crosslinking during pyrolysis. Low-rank coals are non-caking, at least at heating

rates typical of fixed-bed carbonization or the free swelling index test. Humovitrains of lignites do exhibit plasticity under normal carbonization conditions [231]. The high oxygen content of lignite provides the possibility of crosslinking to occur as the oxygen-containing functional groups react during pyrolysis [232]. The extent of crosslinking during pyrolysis can be reduced by using very high heating rates. Such heating rates also produce a melting and swelling of the char, as well as higher yields of soluble products. Onset of crosslinking in lignite pyrolysis coincides with the release of carbon dioxide [ 161,232] and is related to the decomposition of carboxyl groups [ 161,190,205]. A decrease in the ability of lignites to swell in organic solvents, observed during early stages of pyrolysis, correlates

205

with the evolution of carbon dioxide [233]. Big Brown lignite shows a slower rate of crosslinking (as determined by measurements of the volumetric swelling ratio) than does Indian Head lignite; Big Brown evolves less carbon dioxide under identical conditions than Indian Head, and the area of the carbonyl peak in the FTIR spectrum of Big Brown is also less than for Indian Head. The ability to swell in solvents is inversely related to extent of cross-linking [233]. These observations suggest that crosslinking relates directly to the loss of carboxyl groups [205]. Cross-linking of Zap lignite accompanied a decrease of carboxyl and hydroxyl groups in the lignite structure [ 161 ]. An increase in cross-linking for oxidized lignites also demonstrates the relationship between crosslinking and oxygen functional groups [161]. Furthermore, cross-linking begins almost immediately upon drying [233]. During pyrolysis, low-rank coals crosslink at a much lower temperature than high volatile bituminous coals [234]. Zap lignite crosslinks before tar evolution becomes significant [161,235] and before any rapid loss of aliphatic hydrogen [ 161 ]. Lignite crosslinking occurs at temperatures around 380~

[205], and possibly as low as 200~

[161,236], much lower than that at which

crosslinking takes place in bituminous coals. The tendency of lignites to crosslink under relatively slow heating conditions may be a reason that lignites are sometimes thought (erroneously) to be less desirable for liquefaction than bituminous coals. However, both volatile release and crosslinking depend on heating rate. With slow heating, e.g., 0.5~

lignite crosslinking begins

before tar evolution becomes significant. If the heating rate is very high, e.g., 20,000~

tar

evolution coincides with crosslinking and, as a consequence, the tar yield is much higher. These very high heating rates also provide visual evidence for the development of thermoplasticity in the lignite. The char shows definite visual evidence of fluidity, bubbling, and swelling, consequences of the reduced crosslinking.The occurrence of thermoplasticity indicates that the rate of bond cleavage must be competitive with the rate of crosslinking. Furthermore, this so-called ultra-rapid heating substantially decreases the crosslinking [ 161 ]. The tendency to crosslink at lower temperatures prevents softening of the coal, and is in part responsible for the relatively low tar yields from low-rank coals, compared with tar yields of bituminous coals [237]. Even though the bridges between aromatic ring systems in lignites are labile, to propensity of oxygen functional groups to enhance cross-linking results in the formation strong linkages in the char [238]. The molecular species that would be the precursors to tar participate in the crosslinking before they are able to escape into the vapor phase and consequently become part of the char. For pyrolysis of 53-88 ~tm North Dakota lignite in a heated grid reactor at heating rates of 1000 ~

(and subsequent cooling rates of 200-400 ~

in vacuum or helium,

the number average molecular weight of the tar produced at 464~ is 323; for maximum pyrolysis temperatures of 860~

the value is 279 [234, 237]. This decrease in molecular weight of the tar

with increasing pyrolysis temperature is consistent with a higher degree of crosslinking accompanying more severe thermal treatment. Structures that would be tar precursors participate in crosslinking so rapidly that they are incorporated into the char before being able to escape. Heated grid pyrolysis of Beulah lignite at heating rates of 1000 ~

to 464~ produced a 3.0% yield of

206 tar. Retrogressive reactions leading to char formation are quite facile in lignite. The crosslinking behavior can be addressed without even necessarily considering specific functional groups. In the FLASHCHAIN theory of pyrolysis, coal components are classified into four pseudo-components on the basis of the ultimate analysis, carbon aromaticity, proton aromaticity, aromatic cluster size, and pyridine extract yield [ 194]. All of the aliphatics and oxygen are assigned to labile bridges. Such labile bridges abound in lignites, large yields of noncondensible products arise from lignite pyrolysis. However, the abundant oxygen sites promote cross-linking, which in turn suppresses tar formation [194,207]. Cleavage of the labile bridges and associated cross-linking produces the non-condensible gases, but does not significantly break down the lignite macromolecule into tar precursors [207]. Hence most of the aromatic structures remain associated with the char. Methane yields from various ion-exchanged samples of Zap lignite relate inversely to tar yields [ 159]; as potential tar precursors are incorporated into char structures, they can yield volatiles only by cracking off relatively small molecular fragments. With an increase in temperature, residence time, or both, the aromaticity of the char relative to the parent lignite increases, and the amount of aliphatic carbon remaining in the char decreases [239]. For Beulah-Zap lignite, the size of aromatic clusters increases monotonically with final temperature, for slow heating. The effect of rapid heating (104 ~

on char structure is

summarized in Table 4.22 [239]. TABLE 4.22 Changes in aromatic cluster size with pyrolysis conditions, BeulahZap lignite, 104 ~ heating [239] Final temp.,~

Time at temp.

Average carbons per cluster 8.5 0.5 min 11.4 2.4 s 13.3 60 ms 9.8 160 ms 17.8 -

800 800 1600 1600

-

Methylation makes lignite behavior during pyrolysis similar to that of high-rank caking coals [208]. Methylation decreases retrogressive reactions during pyrolysis (and liquefaction) [240]. The yields and molecular weight distributions of the tar, as well as visual evidence that the lignite softened and passed through a plastic stage, are consistent with a substantial reduction in crosslinking. Methylation of hydroxyl and carboxyl groups prevents their participation in crosslinking. The molecular weight distributions are comparable to those of tars from bituminous coals. FTIR analysis of the tars from very rapid heating rate pyrolysis indicates that the tar molecules have not lost the oxygen functional groups. At lower heating rates, the oxygen functional groups crack to carbon dioxide, carbon monoxide, or water. The oxygen groups thus do not appear in the tar components. The reactions that cleave these light gaseous products appear to

207 be those accompanied by crosslinking. Cross-linking of ion-exchanged Zap lignite resembles that of untreated lignite [161]. Loading lignite with calcium by ion exchange and subsequent pyrolysis shows no change relative to the untreated lignite [205]. However, pyrolysis of demineralized Indian Head lignite shows that a shift to higher temperatures is needed to achieve the same extent of crosslinking. Demineralization reduces cross-linking [240]. Compared to the untreated lignite, tar evolution was higher from the demineralized lignite, with less carbon monoxide and carbon dioxide evolution. These results suggest that the ion-exchangeable cations may have an important role in pyrolysis reactions. Demineralized Zap lignite behaves in the same way as the demineralized Indian Head [161]. REFERENCES

10 11 12 13 14 15 16 17

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P.R. Solomon, D.G. Hamblen, J.R. Markham, and P.E. Best, An investigation of vaporization and devolatilization of coal/water fuels, U.S. Dept. of Energy Rept., DOE/PC/50254-8, (1984). J.V. Ibarra, I. Cervero, M. Garcia, and R. Moliner, Influence of cross-linking on tar formation during pyrolysis of low-rank coals, Fuel Proc. Technol., 24 (1990) 19-25. M.T. Klein and P.S. Virk, Model pathways for gas release from lignites, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(4) (1980) 180-190. S.F. Ross, Pyrolysis and devolatilization, University of North Dakota Energy Research Center monthly report, April 1985. M. Siskin and T. Aczel, Pyrolysis studies on the structure of ethers and phenols in coals, Fuel, 62 (1983) 1321-1326. S. Niksa and A.R. Kerstein, FLASHCHAIN theory for rapid coal devolatilization kinetics. 1. Formulation, Energy Fuels, 5 (1991) 647-665. W.H. Calkins, Investigation of organic sulfur-containing structures in coal by flash pyrolysis experiments, Energy Fuels, 1 (1987) 59-64. K. Gueruez and S. Celebi, Sulfur removal by pyrolysis of Turkish lignites, Fuel, 58 (1979) 893-895. J.B. Goodman, M. Gomez, and V.F. Parry, Laboratory carbonization assay of low-rank coals at low, medium, and high temperatures, U.S. Bur. Mines Rept. Invest. 5383, (1958). P.R. Solomon and M.B. Colket, Evolution of fuel nitrogen in coal devolatilization, Fuel, 57 (1978) 749-755. E.M. Suuberg, W.A. Peters, and J.B. Howard, Product composition and kinetics of lignite pyrolysis, Amer. Chem. Soc. Div. Fuel Chem. F'reprints, 22(1) (1977) 112-136. K. Ceylan, Low-temperature carbonization of Elbistan lignite, Chim. Acta Turc., 19 (1992) 197-206. M.I.M. Chou, C.W. Kruse, J.M. Lytle, and K.K. Ho, Organic volatile matter and its sulfur-containing compounds produced by coal pyrolysis, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 38 (1993) 655-661. P.J.J. Tromp, F. Kapteijn, and J.A. Moulijn, Determination of coal behavior for practical coal conversion processes, in: Y. Ytirtim (Ed.) Clean Utilization of Coal, Kluwer Academic Publishers, Dordrecht, Netherlands, 1992, pp. 75-84. S.F. Ross, Pyrolysis and devolatilization, University of North Dakota Energy Research Center monthly report, August 1984. W.H. Calkins and R.J. Tyler, Coal flash pyrolysis. 2. Polymethylene compounds in lowtemperature flash pyrolysis tars, Fuel, 63 (1984) 1119-1124. G.V. Deshpande, P.R. Solomon, and M.A. Serio, Crosslinking reactions in coal pyrolysis, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(2) (1988) 310-316. S. Niksa and A.R. Kerstein, Predicting the transient devolatilization of various coals with FLASHCHAIN, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 37 (1992)410-416. S. Niksa, FLASHCHAIN theory for rapid coal devolatilization kinetics. 3. Modeling the behavior of various coals, Energy Fuels, 5 ( 1991) 673-683. P.R. Solomon and K.R. Squire, Experiments and modeling of coal depolymerization, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 30(4) (1985) 346-351. S. Badzioch and P.G.W. Hawksley, Kinetics of thermal decomposition of pulverized coal particles, Ind. Eng. Chem. Proc. Des. Devel., 9 (1970) 521-530. J.K. Olson and H.H. Schobert, Effect of operating conditions on production of light hydrocarbon gases in slagging fixed-bed coal gasification, U.S. Dept. of Energy Rept. GFETC/RI-80/2, (1980). T.H. Fletcher, A.R. Kerstein, R.J. Pugmire, M.J. Solum, and D.M. Grant, Chemical percolation model for devolatilization. 3. Direct use of 13C NMR data to predict effects of coal type, Energy Fuels, 6 (1992) 414-431. S.F. Ross, Pyrolysis and devolatilization, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. of Energy Rept. DOE/UNDERC/QTR-85/3-4, (1986), pp. 18-1 - 186. A.A. Lappas and I.A. Vasalos, Evaluation and kinetic modeling of Greek lignites, Fuel, 68 (1989) 1243-1247. Y. Yamashita and K. Ouchi, Carbonization reaction of phenol resins and a brown coal, J.

216 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239

Fuel Soc. Japan, 53 (1974) 1064-1072. R.M. Carangelo, P.R. Solomon, and D.J. Gerson, Application of TG-FT-i.r. to study hydrocarbon structure and kinetics, Fuel, 66 (1987) 960-967. J.L. Johnson, Kinetics of initial coal hydrogasification stages, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 22(1) (1977) 17-29. A.K. Bumham, M.S. Oh, R.W. Crawford, and A.M. Samoun, Pyrolysis of Argonne premium coals: Activation energy distributions and related chemistry, Energy Fuels, 3 (1989) 42-55. W.H. Calkins, E. Hagaman, and H. Zeldes, Coal flash pyrolysis. 1. An indication of the olefin precursors in coal by CP/MAS 13C nmr spectroscopy, Fuel, 63 (1984) 1113-1118. W.H. Calkins, Coal flash pyrolysis. 3. An analytical method for polymethylene moieties in coal, Fuel, 63 (1984) 1125-1129. W.H. Calkins, Coal structure vs. flash pyrolysis products, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 28(5) (1983) 85-105. K.C. Teo and A.P. Watkinson, Rapid pyrolysis of Canadian coals in miniature spouted bed reactor, Fuel, 65 (1986) 949-959. H. Huai, R. Lo, Y. Yun, and H.L.C. Meuzelaar, A comparative study of 8 U.S. coals by several different pyrolysis / mass spectrometry techniques, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 35 (1990) 816-823. W.S. Maswadeli, Y. Fu, J. Dubow, and H.L.C. Meuzelaar, Structure / reactivity studies of single coal particles at very high heating rates by laser pyrolysis GC/MS, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 37 (1992) 699-706. L.H. Chen and C.Y. Wen, A model for coal pyrolysis, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 24(3) (1979) 141-146. R.M. Roberts and K.M. Sweeney, Low-temperature pyrolysis of Texas lignite, basic extracts, and some related model compounds, Fuel, 63 (1984) 904-908. R. Hayatsu, R.E. Winans, R.G. Scott, L.P. Moore, and M.H. Studier, Trapped organic compounds and aromatic units in coal, Fuel, 57 (1978) 541-548. D.J. Miller, J.K. Olson, and H.H. Schobert, Organic structural studies of lignite coal tars, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(1) (1980) 256-263. D.J. Miller, J.K. Olson, and H.H. Schobert, Mass spectroscopic characterization of tars from the gasification of low-rank coals, Fuel, 60 (1981) 370-374. A.G. Sharkey Jr., J.L. Schultz, and R.A. Friedel, Mass spectra of pyrolyzates of several aromatic structures identified in coal extracts, Carbon, 4 (1966) 365-374. C.J. Chu, S.A. Cannon, R.H. Hauge, and J.L. Margrave, Studies of the effects of methylation on the pyrolysis behavior of four brown coals, Fuel, 65 (1986) 1740-1749. F.T.C. Ting, The characterization of coking properties of selected components of lignite and subbituminous coals, Univ. North Dakota Dept. of Geol., Unpublished report, 1972. F.J. Derbyshire, A. Davis, and R. Lin, Considerations of physicochemical phenomena in coal processing, Energy Fuels, 3 (1989) 431-437. E.M. Suuberg, Y. Otake, and S.C. Deevi, Solvent swelling as a measure of the breakdown of the macromolecular structure of coal, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 36 (1991) 258-266. E.M. Suuberg, P.E. Unger, and J.W. Larsen, The relation between tar and extractables formation and crosslinking during coal pyrolysis, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 30(4) (1985) 291-295. P.R. Solomon, D.G. Hamblen, G.V. Deshpande, and M.A. Serio, A general model of coal devolatilization, Proceedings 1987 International Conference on Coal Science, pp. 601604. P.R. Solomon, S. Charpenay, Z.Z. Yu, M.A. Serio, E. Kroo, M.S. Solum, and R.J. Pugmire, Network changes during coal pyrolysis: Experiment and theory, Proceedings 1991 International. Conference on Coal Science, pp. 484-487. E.M. Suuberg, P.E. Unger, and J.W. Larsen, Relation between tar and extractables formation and cross-linking during coal pyrolysis, Energy Fuels, 1 (1987) 305-308. S. Niksa and A.R. Kerstein, Modeling the devolatilization behavior of various coals, Proceedings 1991 International Conference on Coal Science, pp. 488-491. M.S. Solum, R.J. Pugmire, D.M. Grant, T.H. Fletcher, and P.R. Solomon, Solid state 13C NMR studies of coal char structure evolution, Amer. Chem. Soc. Div. Fuel Chem.

217 240

Preprints, 34 (1989) 1337-1344. M.A. Serio, P.R. Solomon, E. Kroo, R. Bassilakis, R. Malhotra, and D. McMillen, Studies of retrogressive reactions in direct liquefaction, Proceedings 1991 International Conference on Coal Science, pp. 656-659.

218

Chapter 5

THE I N O R G A N I C CONSTITUENTS OF LIGNITES

5.1 INCORPORATION OF MAJOR ELEMENTS 5.1.1 Accumulation of inorganic components The average ash value of peat, on a dry basis, is 8.8% [1]. On the same basis, the ash value of Fort Union lignites ranges from 8 to 9% [1]. The similarity suggests that, for these lignites, most of the accumulation of inorganic constituents occurred during the formation of the precursor peat. Separated samples of anthraxylon yielded 2.5-4% ash on an as-received basis. The mean value of ash of common tree woods is 3.6% [2]. The agreement of ash values for lignitederived anthraxylon and modern wood, and the fact that most lignites have ash values well above 4%, suggest that most of the inorganic components associate with the non-anthraxylous (i.e., nonwoody) portion of the lignite. Silicified samples of lignites were found in the Wood Mountain uplands of south-central Saskatchewan [3]. The silicified plant debris consists mainly of compressed stems and grass blades. In Rosenbach (German) lignite, silicification arises from colloidal silica derived from sedimentary rocks [4]. The variation of elemental concentration as function of ash value indicates both the source and the fate of elements in the depositional system. Coals appear to be closed systems for most inorganic elements; that is, once an element enters the depositional system, it tends to stay and become incorporated in the coal [5,6]. Comparison of elemental concentrations in lignites with those in higher rank coals determines the elements for which the concept of a closed system is valid. Compared to higher rank coals, lignites are relatively enriched in Ba, B, Ca, Mg, Na, and Sr [5,7]. These elements are associated with the organic portion of the lignite and their concentrations in coals decrease as a consequence of the increase in rank. All of these elements, except boron, associate largely or wholly with carboxyl groups in lignite; loss of these groups on coalification to higher ranks removes the ion-exchange sites that are the principal means of incorporating these elements. As a cautionary note, the organic affinity of a given element may vary among coals from the eastern United States, Illinois Basin, and the western United States [8]; a generalization about the organic affinity of an element in a coal sample that has not been analyzed may be inaccurate. Data for a wide range of elements, including such chemically diverse species as Ba, Cu, Ga, Hf, Pb, Mo, K, Sc, Ag, Ta, Th, Ti, V, Y, Zr, and the rare earths, show that elemental concentration increases with ash value. This relationship indicates a detrital source for these

219 elements [5]. Calcium and strontium, on the other hand, decrease in concentration with increasing ash value. These two elements derive from the original plant material; consequently, their original concentration in the coalifying organic matter becomes "diluted" with increasing influx of detrital inorganic matter [5]. In some South Australian lignites, the ash value and mineral matter content are highest where the detrital inertinite is highest [9]. In this instance, the local depositional environment had a direct influence on the accumulation of inorganic species by the lignite, particularly the inherent mineral matter and syngenetic minerals. The organic functional groups in lignite act as traps in concentrating elements from groundwater, mainly by ion-exchange processes. In addition, some coordination or chelation occurs, as evidenced by the presence of acid-soluble forms of elements which cannot be accounted for as carbonates or other acid-soluble minerals. The association of exchangeable cations with carboxyl groups can be inferred from pKa data. Lignites contain two acidic functional groups: carboxyl and phenolic. The pKa values of the carboxyl groups range from 4 to 5, and thus they undergo ion-exchange, whereas the pKa's of phenols are in the range of 10.5-12, hence phenols more likely participate in complex formation [ 10]. Ion exchange of cations in groundwater with carboxylate groups in lignite may be affected by the specificity of cations for coordinating with carboxylate as a ligand [ 11]. If two cations occur in solution (e.g., groundwater) at equal activities, one will be preferred to the other; consequently the amount of one held in association with the carboxylate groups will be greater than the amount of the other. The ionic potential (charge to radius ratio) of the cations governs this selectivity. A cation of large ionic potential will displace one having smaller ionic potential. The practical implication is that, assuming equal activities in solution, a higher quantity of an alkaline earth cation, such as Ca+2, will be incorporated relative to an alkali cation such as Na+. If only a series of alkali metal cations, or only alkaline earth cations, are compared (so that ionic charge is not a factor) then the cations undergo exchange in order of radii, i.e., Li+ Ba+2 [17]. For the divalent elements, the percentages remaining in the lignite after three extractions with ammonium acetate are

221 coal

-325 mesh coal Vacuum dried 48h

Analysis

~ cc~al Analysis

~,M filtrate

1 extraction 100 m L H20

250C - 24 h ~ residue

Analysis

~,9 filtrate t

3 extractions 100 mL 1M NH4OAc 70~ 24 h Analysis

residue~ 2 extractions 100 m L 1M HCI

70~ - 24 h I

Analysis ~

filtrate

I

I

I

v

v_

residue

Analysis

Figure 5.1. Flow chart for chemical fractionation procedure [15].

Mg, 3%; Ca, 10%, Sr, 9%; and Ba, 50%. Removal of alkaline earth elements by extraction of three lignites with 1Mammonium acetate produced the results shown in Table 5.1, where the data show the percentage of the initial concentration of each element extracted from the lignite by the ammonium acetate solution [ 17]. Assuming that two carboxyl groups associate with each divalent cation and one with each monovalent cation, the percentage of total carboxyl groups associated with cations ranges from 43% for the Hagel (North Dakota) lignite to 54-60% for the Montana lignite [16,18]. The effect of ammonium acetate extraction on both major and trace elements in BaukolNoonan lignite is shown in Table 5.2 [19]. The twelve elements shown in Table 5.2 are those for which the greatest extractability in ammonium acetate solution was observed; data for 31 other major and trace elements are given in the original literature [19]. A comparison of the amounts of ion-exchangeable cations in lignite sampled from two pits of the same mine (Gascoyne) is provided in Table 5.3 [20]. (Additional data on these samples are shown later in this chapter.)

222 TABLE 5.1 Percentage of elements extracted by aqueous ammonium acetate [ 17]. Source of lignite State Seam N. Dakota Hagel Texas Darco Montana Fort Union

Percentage extracted C___~a M_M_g B___~a Sl" 70 60 100 78 79 62 68 82 79 84 71 74

TABLE 5.2 Percentage of elements extracted from Baukol-Noonan lignite [19]. Element % Extracted AI 10.6 Ba 20.0 Br 59.8 Ca 63.4 K 33.3 Mg 80.8

Element Mn Na Ni P Rb Sr

% Extracted 27.1 98.5 62.5 27.5 29.6 58.3

TABLE 5.3 Comparison of exchangeable cations in two Gascoyne lignite samples from different pits [20]

Element Aluminum Barium Calcium Iron Magnesium Manganese Potassium Silicon Sodium Titanium Carboxyl, meq/g

Red Pit Blue Pit Initial conc. % Removed Initial conc. % Removed lxg/g mf NH4Cg_H_3Q2 ~ / g mf N H 4 ~ Q 2 8740 0 7300 0 593 39 1268 61 17370 78 22790 85 3890 0 2540 0 2991 74 2588 75 123 34 163 39 1260 22 1430 19 28640 0 10920 1 1317 100 2694 100 1180 0 546 0 2.46

2.65

A concern in chemical fractionation is the potential solubility of various minerals in ammonium acetate solution, since cations contributed by these soluble minerals would be counted as exchangeable cations. Observed weight losses for common lignite minerals treated with 1M

223 ammonium acetate solution are shown in Table 5.4 [ 16]. Furthermore, exchangeable ions occur in clay minerals. The potential contribution of exchangeable cations from clays is shown in Table 5.5 [16]. TABLE 5.4 Weight loss of minerals treated in ammonium acetate solution [ 16]. Mineral Calcite Dolomite Gypsum Illite Kaolinite Montmorillonite Pyrite Quartz

% Weight loss 9 10

100 4 6 18 1 0

TABLE 5.5 Exchangeable cations in clays [ 16].

Clay Kaolinite Illite Montmorillonite

Exchange capacity 10-3 mol/g clay 0.02-0.10 0.13-0.42 0.80-1.50

Contribution to exchangeable cation content l_._Q0-6mol/g dmmf coal 0.50-2.50 3.25-10.5 20.0-37.5

Optical microscopy and mineralogical analysis of three lignites indicated that gypsum amounted to less than 10% of the total mineral matter [ 16]. The possible error contributed by the complete solubilization of gypsum in ammonium acetate solution for a lignite of 10% mineral matter and 1.5% actual exchangeable calcium is about 10% of the measured exchangeable calcium [16]. Since gypsum forms during lignite weathering, the amount of exchangeable calcium contributed by gypsum in unweathered lignite would be less than this estimate of 10%. The role of 0.1M hydrochloric acid in extracting elements from co3rdination complexes is not clear. The suitability of hydrochloric acid in this role depends on the stability constants of the complexes to be disrupted. Some complexes may not even dissociate at pH 1 [ 10]. Acid extraction is by no means selective for complexed metal ions, since dilute hydrochloric acid will also dissolve carbonates and some oxides and sulfides, as well as leach cations from clays. Chemical fractionation data for a high-sodium Beulah (North Dakota) lignite and its constituent lithotypes are shown in Table 5.6 [21]. Vitrains from a single layer in the Beulah-Zap seam have uniform elemental composition;

224 TABLE 5.6 Chemical fractionation data for Beulah lignite and its constituent lithotypes, ~g/g dry basis [21].

Element Na Mg A1 Si K Ca Ti Mn Fe Ni Ba

Lignite Removed by NH4OAc HCI 5412 26 1913 179 0 2323 13 423 31 166 3135 7136 0 11 13 32 5 2144 0 1 346 428

Attritus Removed by NH4OAc HCI 2685 18 1644 410 0 3064 66 1418 0 0 5434 6536 0 23 14 70 5 1659 0 1 156 668

Fusain Removed by NH4OAc HCI 3854 28 2355 312 0 2292 53 963 31 0 8041 4876 0 7 17 60 3 1613 0 2 299 451

Vitrain Removed by NH4OAc HCI 5569 80 2095 104 0 3044 42 318 31 0 7132 2747 0 3 13 37 0 1411 0 2 338 350

but samples of vitrain from different levels in the seam have different inorganic compositions, especially with regard to calcium, magnesium, and sulfur [22]. Fusain and attritus have inorganic compositions significantly different from the vitrain, particularly higher quantities of calcium, magnesium, and silicon, and lower amounts of aluminum, iron, and sulfur. Microprobe analysis of ulminite from the Beulah-Zap bed showed 1.2% S, 0.7% Ca, 0.4% Na, 0.3% Fe, 0.3% Mg and smaller amounts of Si, K, Ba, and Sr [23]. The lithotypes of Beulah lignite show strong differences in inorganic composition [24]. Vitrain contains major amounts of calcium and sulfur, significant sodium and magnesium, and lesser amounts of aluminum, silicon, potassium, strontium, and barium. Fusain also contains major amounts of calcium, but with significantly lesssodium and sulfur compared to vitrain. Attritus contains major amounts of calcium, magnesium, and sodium. Various grains of vitrain from Beulah lignite showed a consistent chemical pattern of high sulfur and calcium, moderate sodium, magnesium, and aluminum, and low silicon, potassium, iron, and strontium [25]. Several subtypes of vitrain grains have been identified on the basis of S/Ca and Mg/AI ratios. Major differences exist among ulminite macerals, both from within a particular lignite seam, as well as in different seams. The data in Table 5.7 illustrate these differences [22]. Generally, the ulminites of the Beulah-Zap have high iron and sodium and low sulfur. Hagel seam ulminites have high calcium but low sodium and sulfur. Martin Lake (Texas) ulminites have low sodium and high magnesium and sulfur. These differences reflect the effects of hydrogeochemical processes during diagenesis. Maceral samples from eastern Alabama lignite showed definite changes in inorganic composition [11]. Calcium and sulfur were homogeneously dispersed, suggesting that both elements are organically associated. Humodetrinites showed S/Ca ratios of 2. Fusinites showed S/Ca of nearly 1, but with some variations in X-ray intensity for Ca from one particle to another.

225 TABLE 5.7 Inorganic compositions of ulminites (weight percent) determined by electron microprobe analysis [22]. Lignite Beulah-Zap Hagel Martin Lake

Sample 1 2 1 2 1 2

AI 0.22 0.18 0.16 0.24 0.32 0.16

Ca 0.49 1.54 1.03 2.40 1.69 1.67

Fe 0.51 0.79 0.24 0.68 0.45 0.36

Mg 0.11 0.38 0.15 0.42 0.38 0.42

Na 0.32 0.54 0.06 0.07 0.15 0.06

S 0.45 0.43 0.59 0.51 1.25 1.16

Si 0.10 0.12 0.08 0.17 0.28 0.07

Sr 0.04 0.06 0.01 0.05 0.06

High-calcium fusinites displayed Ca intensities exceeding any of the humodetrinites. Gelinite gave the highest observed S/Ca ratio, 3, and a calcium intensity comparable to humodetrinite. Inertodetrinite gave the highest calcium intensity (about triple that of humodetrinite) but the lowest S/Ca ratio, 0.5. The content of organically bound calcium varies from one maceral to another and can be high in the inert macerals. Variations in the S/Ca ratio indicate chemical differences among macerals. A high concentration of ion-exchangeable calcium in fusinite is unusual because fusinite is usually considered to have low oxygen content [26], so it would be expected that fusinite would have few functional groups capable of acting as ion-exchange sites. (This observation relates to suggestions that lignites may contain highly reflecting materials that may be identified as fusinite or inertodetrinite, but that are also highly oxidized, thus containing large amounts of oxygen functional groups [27,28].) Chemical fractionation of two samples of Beulah lignite, a high-sodium lignite with 2.32 meq/g carboxyl, and a low-sodium lignite with 1.46 meq/g carboxyl, are shown in Tables 5.8 and 5.9, respectively [29]. Although the absolute amounts of the various elements change markedly from one lignite to the other, the percentages of each element in exchangeable, acid-soluble, or residual forms are quite similar. TABLE 5.8 Chemical fractionation of high-sodium Beulah lignite [29].

Element Aluminum Barium Calcium Iron Magnesium Manganese Silicon Sodium Titanium

Initial conc. opm (dry) 2880 720 7770 4880 980 24 7760 4620 104

% Removed by .NH4_Q2_~O~ HCI 0 81 48 40 73 18 0 45 85 15 22 75 0 6 100 0 0 11

% in Residue 19 12 8 55 0 3 94 0 89

226 TABLE 5.9 Chemical fractionation of low-sodium Beulah lignite [29].

Element Barium Calcium Iron Magnesium Manganese Silicon Sodium Titanium

Initial conc. oom (dry) 370 7500 10560 1480 52 7990 1380 280

% Removed by NH4_~2_~Q2 HCI 27 24 78 11 0 36 77 16 29 66 0 16 97 1 0 9

% in Residue 49 11 64 7 5 84 2 91

Additional data on the comparative behavior of high- and low-sodium samples of the same lignites (Beulah and Gascoyne) is presented in [30]. A higher percentage of barium was removed by ammonium acetate from the high-sodium lignites than from the low-sodium lignites. With hydrochloric acid, a higher percentage of aluminum was removed from the high-sodium than from the low-sodium lignites. The difference in the behavior of aluminum seems related to the presence of micaceous clays, with the aluminum removed by hydrochloric acid resulting from selective attack on gibbsite layers in the micaceous clays. However, in comparing the two pairs of lignites, no significant differences exist between the modes of occurrence of the inorganic constituents, the major differences being in the amounts of the inorganic elements present in each lignite. Chemical fractionation data for lignite from the Blue and Red pits of the Gascoyne mine are shown in Tables 5.10 and 5.11, respectively [31]. TABLE 5.10 Chemical fractionation of Gascoyne lignite, Blue pit [31].

Element Aluminum Calcium Copper Iron Magnesium Manganese Potassium Silicon Strontium Titanium

Initial conc. % Extracted % Extracted ppm, mf by NH4OAc by HC1 7300 22.6 38.8 22790 85.1 14.3 11 0 0 2540 0 42.5 17890 98.0 2.0 25 28.0 72.0 1430 89.0 0 10920 11.6 16.7 313 94.6 5.4 546 0 57.0

% Remaining in Lignite 38.6 0.6 100 57.5 0 0 11.0 71.7 0 43.0

227 TABLE 5.11 Chemical fractionation of Gascoyne lignite, Red pit [31].

Element Aluminum Calcium Copper Iron Magnesium Manganese Potassium Silicon Strontium Titanium

Initial conc., ppm, mf 8740 17370 10 3890 20540 18 1260 28640 276 1180

% Extracted % Extracted by NH4OAc by HCI 22.2 27.6 78.1 19.8 0 0 0 55.8 95.4 4.6 22.0 78.0 62.8 3.7 17.2 8.1 77.5 22.5 0 13.6

% Remaining in Lignite 50.2 2.1 100 44.2 0 0 33.5 74.7 0 86.4

Chemical fractionation data for Choctaw (Alabama) lignite are shown in Table 5.12 [32]. This is one of the most extensive sets of data available on chemical fractionation results for trace elements. This sample of Choctaw lignite had been washed prior to shipment, but details of the washing process are not known. TABLE 5.12 Chemical fractionation of Choctaw lignite (dry basis) [32].

Element Aluminum Antimony Barium Bromine Calcium Cesium Cobalt Europium Gold Iodine Iron Lanthanum Magnesium Manganese Molybdenum Potassium Rubidium Samarium Scandium Selenium Silicon Sodium Thorium Titanium Vanadium

Initial conc. % Removed by ppm (dry) NI-t4_~2_H~Q2_._ HC1 5907 0 37 0.87 5 54 87 80 0 6 15 0 13958 48 44 0.15 0 0 3 52 36 0.17 6 24 0.0011 36 11 0.81 0 12 14092 8 70 2 0 22 1098 34 27 112 14 85 10 23 46 746 62 0 4 0 0 0.46 4 15 3 41 0 2 4 0 11305 0 0 864 87 1 0.80 0 0 356 0 0 13 0 42

% in Residue 63 42 20 85 8 100 12 70 54 88 22 78 38 1 31 38 100 81 59 96 100 12 100 100 58

228 Chemical fractionation data for Center (North Dakota) lignite are provided in Table 5.13 [20]. These data illustrate the effect of a water leaching prior to ammonium acetate extraction. This source [20] also contains additional data on Beulah and Gascoyne (Yellow pit) lignites. TABLE 5.13 Chemical fractionation of Center lignite (moisture-free basis) [20].

Element Barium Calcium Iron Magnesium Potassium Silicon Sodium Strontium Titanium

Initial conc. ~t~/lz mf 40 27301 19754 3747 691 11630 2329 400 138 -

v

_

H:,0 0 0 0 0 4 1 21 0 0

% Removed by

N'H4C~HH~O~ 32 82 0 78 21 1 76 94 0

HCI 68 18 26 8 15 21 1 16 8

% in Residue 0 0 74 14 60 77 2 0 92

The chemical fractionation data for Indian Head (North Dakota) lignite are shown in Table 5.14 [21]. TABLE 5.14 Chemical fractionation of Indian Head lignite [21].

Element Aluminum Barium Calcium Iron Magnesium Nickel Potassium Silicon Sodium Titanium

Initial conc .... % Removed by ~t~/~z, mf NHaOAc HC1 4940 0 51 519 38 61 15560 39 12 6810 0 40 9690 19 1 5 0 18 1150 2 1 7060 1 14 6221 76 0 402 0 2

% in Residue 49 1 49 60 80 82 97 85 24 98

Chemical fractionation results for Bryan (Texas) lignite are given in Table 5.15 [33]. Additional data on Beulah lignite are available in this source [33]. Despite differences in geographical source and ash content, extraction of alkali and alkaline earth elements is reasonably consistent between the Bryan lignite data shown in Table 5.15 and Beulah lignite. Larger differences between the lignites are observed when comparing the results of the hydrochloric acid extraction. The solubility of aluminum may reflect its occurrence as a complex with humic acid structures in ways analogous to the reported behavior of aluminum in peat [34] or soil [3 5].

229 TABLE 5.15 Chemical fractionation of Bryan lignite [33].

Element Aluminum Barium Calcium Chromium Copper Iron Magnesium Manganese Nickel Potassium Silicon Sodium Strontium Sulfur Titanium

Initial conc. ~ / ~ , mf 12360 190 7130 21 24 20950 2000 300 40 1970 69400 310 80 19500 1410

% Removed by NI-Lt.OAc HC1 0 22 28 55 62 18 14 43 0 42 0 68 94 6 43 51 0 52 9 8 0 0 75 3 81 19 0 0 0 29

% in Residue 78 17 20 43 58 32 0 6 48 83 100 22 0 100 71

Data for another Texas lignite, Monticello, are given in Table 5.16 [36]. TABLE 5.16 Chemical fractionation of Monticello lignite [36].

Element Aluminum Barium Calcium Iron Magnesium Potassium Silicon Sodium Strontium Titanium

Initial conc. ~t~,l~, mf 9110 100 7320 2200 1180 335 23600 315 130 395

HgO 0 0 0 0 0 21 0 31 0 0

% Removed by NH4OAc 0 52 77 0 70 5 0 31 72 0

HC1 5 26 13 53 4 0 1 0 11 15

% in Residue 95 22 10 47 26 74 99 18 17 85

A procedure for estimating the amount of alkali and alkaline earth oxides in ash which derived from cations associated with the organic matter defines [37] "net calcium oxide" as the amount of acid-soluble calcium not stoichiometrically equivalent to the amount of carbonate estimated from the CO2 yield, viz.,

CaOnet = CaOa.s.- 1.274 CO2

230 where the calcium oxide term on the right-hand side is determined from the acid-soluble calcium. Then the organically derived alkali and alkaline earth oxides is found from ALK = CaOnet + (MgO + Na20 + K20)a.s. where again the subscript a.s. indicates the acid-soluble amounts of the respective species. When a low-rank coal containing significant alkali or alkaline earth elements associated with the carboxyl groups is ashed, some of the oxygen in the ash will derive from the organic functional groups. The amount of this oxygen can be estimated by calculating the amount that would be associated with the exchangeable cations. Oxygen associated with ion-exchangeable elements, Oie, is found from

Ole- (Ocao)ne t + (OMgO + ONa20 + OK20)a.s.

[37] where the subscripts have the same meaning as before. 5.1.3 The major elements (i) Aluminum. A portion of the aluminum in Beulah and Savage (Montana) lignites was found to be organically bound, on the basis of electron microprobe line scans [38]. In a North Dakota lignite, approximately one-third of the aluminum was ion-exchangeable, the balance present in clays [ 11 ]. 27AI nuclear magnetic resonance may provide an opportunity for determining how aluminum is associated with the organic portion of the lignite. The data reported so far indicate the possibility of distinguishing between tetrahedral and octahedral coordination, but no specific details of the organic environment [39]. Chemical fractionation of specific gravity fractions of Glenharold lignite showed about a third of the total aluminum to be extractable with acid, the amount increasing as the gravity of the fraction decreased [10]. This suggests some aluminum complexation by the organic matter, since the aluminum leached from clays with dilute hydrochloric acid is small, and acid-soluble aluminum minerals such as gibbsite and boehmite have specific gravities in the range of 2.8-3.0 (thus concentrating in the heavier fractions). In some Alabama lignites the acid-soluble aluminum amounts to 50% of the total [ 11]. Acid-soluble aluminum does not correlate with acid-insoluble aluminum, indicating that the aluminum dissolving in acid does not represent acid leaching of the clays. Gascoyne lignite (previously treated with ammonium acetate to remove ion-exchangeable cations) was leached with 0.1M disodium EDTA at pH's of 4.5, 5.5, and 7.0. In addition, kaolinite, montmorillonite, halloysite, and illite were also treated with this reagent. The lignite extracts contained up to ten times more aluminum than the mineral extracts [40,41], with little variation due to pH. The higher levels of aluminum removed from the lignite represent aluminum contained in some type of complex with the organic structure. (Some potassium, calcium, and iron

231 were also removed from the lignite by the EDTA procedure, but the nature of the association of these elements was not elucidated.) Aluminum is associated with detrital minerals in Alabama lignite [ 11]. Aluminum occurs in clays in the Calvert Bluff (Texas) lignites [42] and Monticello lignite [36]. An electron microprobe study of Gascoyne and Beulah lignites showed aluminum present only in combination with silica in kaolinite, illite, and mica [30]. As in the case of silicon, some acid-insoluble aluminum is present in all specific gravity fractions, but occurs in maximum amount in the 1.5-1.8 sp. gr. fraction [ 10]. This fraction also contains the highest amount of clays. The acid-insoluble aluminum is accounted for by the kaolinite content [ 11]. (ii) Calcium. Electron microprobe line scans of Beulah and Savage lignites show the calcium to be predominantly organically bound [38]. The dispersion of calcium through lignite as salts of carboxyl groups has been confirmed by extended X-ray absorption fine structure (EXAFS) spectroscopy [43--45] and X-ray absorption near-edge spectroscopy (XANES) [45,46]. Infrared spectra of humic acids extracted from Spanish lignites show calcium ions complexed with the carboxyl groups [47]. The average bond distances in the calcium sites are 0.24 nm [45]. The calcium is coordinated by six oxygen atoms. There is little long-range order around the calcium ions, suggesting that the calcium sites are separated from each other and are well dispersed throughout the samples [48]. Calcium is present in ion-exchangeable form in an Alabama lignite, and is the major exchangeable cation in these lignites [11]. Among Alabama lignites the Ca+2/Mg+2 molar ratios vary from 2.0 to 9.1 [11]. Chemical fractionation of three lignites showed calcium to be the predominant cation [ 16]. This observation was substantiated in studies on a larger number of lowrank coals; calcium represented 1-2.5% of the dry coal, and 75-95% was exchangeable with ammonium acetate [ 14]. About 90% of the calcium in the lignite of the Calvert Bluff Formation occurs in organic association [42]. The vertical distribution of ion-exchangeable calcium in Alabama lignite varies little with position in the seam [14], but correlates with variations in petrographic composition. Specific gravity fractions of Glenharold lignite show that up to 92% of the calcium was extracted into ammonium acetate [ 11 ], but the proportion of calcium soluble in ammonium acetate decreased with increasing specific gravity. Some calcium in Alabama lignite is present in carbonate minerals (calcite) [11,16] and associated with detrital minerals. Although most of the ion-exchangeable calcium is present as the counterion of carboxyl groups, some exchangeable calcium may also be present in gypsum, which is partly soluble in ammonium acetate solution. The non-exchangeable calcium in Calvert Bluff lignites [42] occurs as calcite, clays and phosphates. About 8% of the total calcium in Glenharold lignite is present as gypsum or calcite. The acid-soluble and acid-insoluble calcium may not be different forms of the element, but rather residual material soluble, but incompletely dissolved, in ammonium acetate. (iii) Iron. Some ion-exchangeable iron was observed in a North Dakota lignite [11]. Analyses of Beulah and Savage lignites by electron microprobe line suggest that a portion of the

232 iron in these samples may be organically bound, on the basis of line scans that indicate an appreciable, nearly constant background in addition to occasional spikes indicative of discrete mineral grains [38]. No iron was detected in ammonium acetate extracts of specific gravity fractions of Glenharold lignite [ 11 ]. Supporting evidence also comes from studies of iron salts ionexchanged onto lignite, in investigations of dispersed iron catalysts for direct liquefaction [49]. M6ssbauer spectroscopy of lignite exchanged with iron(II) chloride solution shows about 55% of the iron in clusters of 0-2 nm [49]. After exchange with iron(II) acetate, about 45% of the iron is in clusters >6.5 nm, and about 30% in 0-2 nm clusters. XANES indicates a coordination number of 4.8_+1 in the third shell of atoms around a central iron atom, and 7.6-2_1 in the fourth [49]. Iron soluble in 2M HCI increases with increasing specific gravity of fractions of Glenharold lignite, and may represent dissolution of goethite, jarosite, and melanterite. Some acidsoluble iron, especially in the lighter specific gravity fractions, may exist as complexes which are dissociated by acid [11]. In the Calvert Bluff lignites iron is found as siderite, as well as pyrite and marcasite [42]. Chemical fractionation of an Alabama lignite showed iron to be associated with pyrite [11]. The iron in the 1.80 sp. gr. sink fraction of Glenharold lignite is more than triple the amount calculated from the pyritic sulfur (1854 and 560 ppm, respectively). This implies a second, highspecific gravity iron mineral in the lignite, but it was not specifically identified. Iron is present in this lignite in minerals that concentrate in the heaviest specific gravity fractions [ 11 ]. Most of the iron in Beulah and San Miguel (Texas) lignites is present as pyrite, as indicated by MOssbauer spectroscopy [46]; the same technique showed 100% of the iron in a sample of Beulah-Zap lignite to be present as pyrite, with no indication of iron sulfates, iron-bearing clays, or siderite [50]. Determination of pyritic sulfur by extraction with dilute nitric acid, evaporation of the extract to dryness, dissolution of the solid in hydrochloric acid, and analysis of the resulting solution for iron and sulfur [51] allows calculation of pyritic sulfur from the equation

Spy-" 1.145(FEN- Fell) where Spy is the pyritic sulfur, FeN the percentage of iron in the nitric acid extraction, and Fe H the percentage of iron soluble in the hydrochloric acid. For most coals the sulfur determined directly as pyritic sulfur and that calculated from the so-called pyritic iron agreed quite well. For lignites, however, the difference between determined and calculated pyritic sulfur was greater than for any other coals, leading to the inference that not all of the iron in lignite was in the form of pyrite, but that a portion of the iron is present in some other form soluble in dilute HNO3. This observation was not followed up to investigate the nature of the acid-soluble iron. In Alabama lignite the acid-soluble iron correlated with the pyrite, suggesting that the acidsoluble material was oxidation products of the pyrite [11 ]. However, the correlation does not exist for Darco (Texas) lignite, suggesting that in this lignite some other form of acid-soluble iron

Occurs.

233 (iv)

Magnesium. Electron microprobe line scans of Beulah and Savage lignites suggest the

magnesium to be present in organically bound form, based on a relatively constant composition across the sample rather than sharp peaks indicative of mineral grains [38]. Chemical fractionation of three lignites showed magnesium to be the second-most dominant cation [16]. Magnesium is present primarily in ion-exchangeable form in Alabama lignite, second only to exchangeable calcium in these lignites [11]. In a North Dakota lignite the ion-exchangeable form predominated [11]. Most of the magnesium in specific gravity fractions of Glenharold lignite is present as ionexchangeable form. In some lignites magnesium is present totally in an ion-exchangeable form [16]. Electron microprobe analyses of Gascoyne and Beulah lignites showed magnesium in combination with aluminosilicates and with calcium carbonate [30]. Some magnesium in Alabama lignites also occurred in carbonate minerals and with detrital minerals [11]. The presence of magnesium in detrital minerals results from its substitution in the clay structures. The partial insolubility of magnesium and its presence in heavier specific gravity fractions was also attributed to its presence in the lattices of clays [10]. The acid-insoluble magnesium in specific gravity fractions of Glenharold lignite had a similar distribution as the acid-insoluble silicon and aluminum, suggesting that it is tightly bound in clays. In Alabama lignite the acid-insoluble magnesium correlated with the clays [ 11]. Magnesium occurs mainly with clays and micas in the lignite of the Calvert Bluff [42]. (v)

Nitrogen. The possible occurrence of inorganic nitrogen as nitrate has been a subject of

controversy. Nitrates can be extracted from lignites with water. The extraction process appears to be complex, and depends on the interactions among the ion-exchange behavior and concentrations of the various ions present in the lignite, and on the adsorption properties of the lignite itself [52]. The difficulty of water extraction of nitrates suggests some special interaction, such as preferential adsorption of nitrate on clays or encapsulation of nitrates in coalified plant structures. X-ray powder patterns of Beulah lignite include several lines characteristic of calcium nitrate, corroborating the presence of alkaline earth nitrates in water extracts of Beulah lignites [52]. Nitrate has been reported in the low-temperature ash of lignites [53,54]. Sodium and alkaline earth nitrates have been identified in the low temperature ashes of lignites, but not in lowtemperature ashes of bituminous coals or anthracites [55]. The possibility of this nitrate being mainly an artifact of the ashing process was first raised because of the inability to detect sodium nitrate in the water extracts of North Dakota lignites [.56]. Fourier transform infrared spectroscopy (FTIR) of the low-temperature ashes of lignites, washed lignites, lignite extracts, and model compounds led to the assignment of the band at 1384 cm-1 as evidence that the fixation of organic nitrogen occurs during the ashing process to produce nitrate. The formation of nitrate from organic nitrogen during low temperature ashing was demonstrated by ashing a mixture of 3-hydroxyl-6methylpyridine, sodium carbonate, and graphite. The presence of nitrate in the resulting ash was confirmed by FTIR analysis, specifically by the presence of the 1384 cm-1 peak [57]. Fixation depends on the presence of the carboxyl groups in the lignite and is affected by the type of metal

234 ion associated with the acid groups. Spectra of water extracts show that a small quantity of nitrate may be present in the parent lignite, although most of the nitrate in the ash is formed as an artifact of the ashing process [57]. (vi) Phosphorus. The average phosphorus content in five Montana lignite samples was 0.32% on a dry basis, with a range of values of 0.004% to 0.058% [58]. For eight North Dakota lignites, the comparable values were 0.009% and 0.002-0.017%, respectively [58]. The low concentration of phosphorus in most lignites, and its evident lack of significant affect on any lignite technology, likely are the reasons why the geochemistry of phosphorus in North American lignites has been studied so little. (vii) Potassium. In many lignites potassium is present both in detrital minerals and in ionexchangeable form [11,16]. EXAFS shows that some of the potassium in lignite is bonded to carboxyl groups [48]. Chemical fractionation of specific gravity fractions of Glenharold lignite showed that ion-exchangeable potassium decreases sharply with increasing gravity, but the amount of acid-soluble potassium increased with gravity [10]. In this lignite about 44% of the total potassium was ion-exchangeable and 34% bound in minerals such as illite. Potassium is associated primarily with the detrital minerals in an Alabama lignite [ 11]. Acidinsoluble potassium in Alabama lignite correlated with the clays, suggesting that potassium is present in illite or in a mixed-layer clay [11]. Potassium also associates with clay minerals in Monticello lignite [36]. In the Calvert Bluff lignites potassium is found in clays, mica, and feldspars [42]. XANES of San Miguel and Beulah lignites shows potassium present almost entirely as illite clays [46] Potassium in Gascoyne and Beulah lignites was present in aluminosilicates, illite, mica, and potassium feldspar [30]. (viii) Silicon. Chemical fractionation of specific gravity fractions of Glenharold lignite showed some acid-insoluble silicon present in all gravity fractions [ 10]. The highest concentration of acid-insoluble silicon occurred in the 1.5-1.8 sp. gr. fraction; this fraction also contains the highest concentration of clays. The highest concentration of quartz was found in the 1.80 sp. gr. sink fraction. The acid-insoluble silicon is contained in quartz and kaolinite [11]. Silicon is associated with detrital minerals in an Alabama lignite [11]. Most of the silicon in Calvert Bluff lignites occurs in quartz and clays [42], as is also true for Monticello lignite [36]. (ix) Sodium. Sodium is the only element extracted from lignites in appreciable quantities by water leaching [59]. It was assumed to be present as the chloride, sulfate, or as water-soluble humate salts. Some data on the amounts of the total sodium extracted by water are shown in Table 5.17 [59]. Press dewatering of Falkirk (North Dakota) lignite at 1 l0 MPa expressed 20 mL of water from a 550 g sample. The water had a pH of 8.1 with sodium the dominant cation, at 3600 ppm [60--62]. (Other cations observed were Ca, at 430 ppm, and Mg, 210 ppm.) The dominant anion was sulfate (10480 ppm); carbonate was not analyzed. A similar experiment with Gascoyne lignite expressed water containing 3200 ppm Na, 510 ppm Ca, and 360 ppm Mg [63]. These water analyses, which presumably reflect the composition of water contained in the pores of the lignite,

235 TABLE 5.17 Water-extractable sodium in lignites [59]. Lignite Beulah Center Gascoyne (White) Gascoyne (Yellow)

Initial Na Content, ppm 5800 2300 5100 700

% Extracted by Water 14 21 18 42

are remarkably similar, especially in light of very significant differences in the ash value and ash composition between the lignites. Falkirk lignite had 8.2% ash containing 30.3% CaO, and supposedly 0.0% reported Na20, whereas the Gascoyne sample had 13.5% ash containing 12.4% CaO and 3.6% Na20. The complete analyses of the expressed pore water from the Falkirk and Gascoyne lignites are shown in Table 5.18 [59]. TABLE 5.18 Analyses of water expressed from Falkirk and Gascoyne lignites, ~tg/mL [62].

Aluminum Barium Boron Calcium Chloride Copper Iron

Magnesium Manganese Molybdenum Nitrate Potassium Silicon Sodium Strontium Sulfate Zinc pH

Falkirk 2 0.1 10 430 13 0.1

Gascoyne 28 24-28 46 38--46 30 nm 1.2 n m < Transition pores > 1, small angle scattering from a fractal surface in a porous solid is given by I(q) = n c 82 Ie F(5-D)sin[~t(D- 1)/2]q-(6-D)

[93]. In this equation 8 is the difference in electron densities between the two phases (e.g., the lignite substance itself and the air in the pores), Ie is the scattering intensity per electron, and F(5D) is a gamma function (see, e.g., [109]). Again provided that ql >> 1, the scattering intensity I(q) will be proportional to q-(6-D), so that plotting log[I(q)} as a function of log(q) should given a straight line from which D can be calculated readily. In this model the pore positions and radii are random variables with certain distributions. An example of a randomly generated computer representations of a shape function is shown in Figure 7.11 [80]. The set which results from removing from a uniform block of material the intersection with a set of cavity-generating spheres is called a trema set [ 110]. Small angle X-ray scattering data indicate that over a scale ranging from about 1 nm to microns the pore sizes follow a Korcak distribution N r ( R > r l ) = CrI-B

with the pores distributed uniformly in space, the distribution being

Nr (R >r I) = C / (E CE vIE)

[110] where E is a dimension (in normal Euclidian space E = 3) and C E is the volume of the Edimensional unit sphere. The constant C is given by C = E - D, where D is the fractal dimension which controls the scaling over the range of rl. The range of rl has two cutoffs, an outer cutoff f~, which is limited by the size of the sample particles themselves, and an inner cutoff ~ which is limited by the molecular (lattice) spacing in the solid. If V = 1, then f2 = 1 also, and the pore

375

Figure 7.11. Computer representation of a particular fractal pore structure in lignite. In this structure D is 1.9 and the inner cutoff is =0.01 [80].

distribution will then be defined by two parameters, e and D. Within the range of rl the pore distribution has perfect translation and dilation symmetry. The size of the pores and the tendency of the pores to percolate, i.e., to form channels to the edges of the sample, are not independent of D. (In the language of fractal geometry, the size of the pores and the tendency to percolate are known as lacunarity and succolarity, respectively.) Presumably every physical property of the lignite that is related to the pore distribution would be governed by D and e, which would be characteristics of the individual lignite. The scaling of individual measurements of physical properties could be correlated by various functions of D and e. To approach more realistically the anisotropic pores in lignites, the spherical pore cavities could be replaced by ellipsoidal cavities, but this extension of the model has not yet been done. Mercury porosimetry data for a Canadian lignite have been interpreted to follow a power law distribution having a fractal dimension of 2.84 [ 111]. This interpretation is valid for mercury pressures below 20 MPa. The drying of lignite and mass transfer of reactants in or products out are governed by the extent to which the pores inside the lignite particles connect to the surface. Water, reactants, and products move in and out of the lignite across an interface made up of the combined surface area of all the pores that communicate with the external environment. An extension of the application of

376 fractal geometry to lignite should examine how porosity, the fraction of pores connecting to the surface, and the effective surface area vary with e. Visual confirmation of the fractal structure of lignites was obtained by observation of dried slurries of hot-water-dried Indian Head lignite using transmission electron microscopy [ 112]. The degree of complexity of the edge structure of lignite particles did not appear to vary over several orders of magnitude. The invariance with scale of the structure is an important property of fractal structures. The transmission electron microscopy results present independent corroboration of the small angle X-ray scattering results. An alternative approach to visual examination of pores, and confirmation of a fractal model, is obtained by filling the pores with Wood's metal, followed by microscopy of the metalimpregnated sample [85]. (Wood's metal is an alloy of bismuth, tin, lead, and cadmium melting at 66"C 113].) Darco lignite showed a distinctly different texture from five samples of higher rank coals. The macropores are elliptical in cross-section, with rough boundaries between the pores and lignite matrix. (An elliptical-shaped cross-section observed in a "slice" through the sample is consistent with cylindrical macropores.) The macropore population was highest near the edge of the lignite particles, and non-existent near the centers. This shows that the alloy wetted a rough, and texturally complex, surface rather than penetrating an interconnected network of cylindrical pores. The surfaces of the Darco lignite appear very rough at all levels of magnification. These observations support the concept of a fractal nature for lignite particle surfaces [85]. A unified picture of lignite porosity and surface area and the relationship of these properties to other measurable properties, such as moisture content, behavior on drying, and mechanical properties would be a significant step toward a sound fundamental understanding of relationships between physical structure and properties. A difficulty in developing such a picture is that many measurement techniques used to study physical properties, such as gas adsorption, heat of immersion calorimetry, and friability, often lead to inferences or conclusions that contradict those from a different measurement. An understanding of pore sizes and their distribution is central to understanding equilibrium moisture, behavior on drying, mass transfer of reagents and reaction products, and the dependence of mechanical properties on physical structure. A start toward developing such a model is the approach via fractal geometry, based on observations of the fractal nature of lignite [80,104,114]. A trema fractal ("trema" referring to hole or cavity [110]) can be defined by assuming a distribution of spherical pores with the radii of the pores distributed as discussed above, where Nr is proportional to vl-B for some exponent B but in this model the spheres are allowed to overlap. The trema set is outside the union of the overlapping spheres. B = 3 is chosen to fulfill the fractal requirement that the pore size distribution appear the same on all length scales. The fractal model of lignite derives from its shape function F(r), which is one for the lignite substance and 0 for the pore cavities. A cube of lignite of volume V and side L is assumed to contain spherical pores of random positions and random radii. The function O(rl - r) has the value 0 when its argument is negative and the value 1 when its argument is positive. In the simplest case,

377 a lignite containing one pore of radius 1"11and position rl,

F(r) = 1 - O

[1"11 -

abs(r- rl)]

[80]. Then for a lignite with two pores an analogous equation would be

F(r) = 1 - 0

[1"]1 -

abs(r-rl)]

- 0[/]2-

abs(r-r2)]

[80]. However, this equation does not take pore overlap into account and thus would count the overlapping region of two pores twice. If the general equation for the k-th pore is written as

fk(r) = 0 [rl- abs(r-rk)]

then the correct form of the shape function for lignite having N pores would be

N

F(r) - ~ (

1 - fk(r) )

[80]. The small angle X-ray scattering intensity is determined by , where F(k) is the Fourier transform of F(r) and the use of angular brackets demotes the average over the pore distribution. Averaged products of this kind relate not only to the X-ray

scattering

but also relate

via percolation theory to other physical properties. A simple model of the mechanical properties can be developed by considering the special case of fracture in a plane, where the fracture stress is dependent on the relative cross sectional area of the pores to the total cross sectional area. In the extreme case of e = 0, the pores would cover the entire cross section. In realistic cases, where e has some finite value, the pores will account for some fraction of the total cross section. For a given value of D the fraction of the plane covered by pores will vary with ~. The trema fractal model, which depends only on D and e offers the prospect of a unified conceptual approach to the systematics of pore structure. Drying, fracture, combustion, and gasification all depend in some way on the pore structure. Thus gaining a general understanding of the pore structure characteristics will eventually lead to a more unified theoretical basis for many aspects of lignite processing.

378 R E F E R E N C E S

10 11 12 13 14 15 16 17 18

19 20 21

22 23 24

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382 100 101 102 103 104 105 106 107

108 109 110 111 112 113 114

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383

Chapter 8

M O I S T U R E IN LIGNITE

8.1 I N T R O D U C T I O N Unquestionably the most difficult and perplexing problem in the characterization of lignites is determination of the moisture content, and developing an understanding of the ways in which moisture is adsorbed into, retained in, and desorbed from lignites. Decades of research have been devoted to trying, first, to measure accurately the total moisture content of a sample and, second, to determine the ways in which water might be incorporated in lignite and how the total moisture content is apportioned among them. The problem remains unsolved. At least five categories of water have been defined to occur in coals: 1) surface adsorbed water; 2) capillary condensed water; 3) water of hydration of the inorganic species; 4) so-called combined water, released by decomposition of the organic components; and 5) dissolved water [ 1]. Considering the current state of knowledge of the general topic of moisture in lignite, it is safest to regard any moisture determination simply as an operational definition. That is, "moisture" determined by, for example, measuring weight loss during heating to 105~ certainly does not measure the total amount of moisture in the lignite. However, this, or some comparable analytical technique, may be taken as a standardized, reproducible method of comparison of various lignite samples. When samples of two lignites that are otherwise reasonably similar in composition and properties are treated under identical conditions at 105~

the difference in the results reasonably

indicates the amount of loosely held moisture in each [2]. Often this comparison provides adequate information for the user of the lignite. Customarily the moisture content of coals is considered as the water driven off at 105~ in a specified period of time, or, when the xylene moisture determination is used, the amount of water driven off at the boiling point of the xylenes, 138-144~

Accepting either of these methods as the

measurement of moisture content ignores water more tightly bound to the lignite structure, hydrogen bonded to functional groups or present as water of hydration and the so-called water of decomposition, (water formed during thermal dehydration of the lignite by, for example, loss of a hydroxyl group and a neighboring hydrogen). Most approaches to the determination of water represent so-called indirect methods, because they do not use a reagent that reacts specifically with water. (Direct methods have occasionally been suggested, an example being the reaction of lignite with calcium carbide and

384 measurement of the acetylene generated. Such methods are generally rather cumbersome and in some cases may involve expensive reagents or apparatus.) Indirect methods are not undesirable per se; often in analytical chemistry their use is convenient and relatively easy. The difficulty arises

when an indirect method is applied to a material as complex and heterogeneous as coal, which furthermore easily undergoes competing reactions, such as oxidation, that complicate analytical measurements. Specific problems have been summarized [2] as follows: 1) the moisture content is sensitive to prevailing atmospheric conditions, especially with fine particle sizes; 2) the possibility of volatizing substances other than water (e.g., carbon dioxide, methane, or nitrogen); and 3) reactions with oxygen, including adsorption of oxygen, its reaction with carbon to form carbon dioxide, and reaction with hydrogen to form water. The methods for determining moisture in coal originally developed by the U. S. Bureau of Mines and later standardized by the American Society for Testing and Materials were developed for coals in the eastern United States, that is, bituminous coals and anthracites. The problems mentioned in the preceding paragraph for the application of the indirect moisture determination to coal are especially severe in regard to the weight loss during air drying of lignites. For lignites, the loss in sample weight does not necessarily mean that an equivalent weight of water has been lost, and even if it were, the loss in weight does not mean that all the water has been removed. Ideally, a method for the determination of moisture in lignite should meet the following criteria: 1) it should be able to determine the total water content; 2) it should not cause oxidation of the sample; 3) it should be practical, that is, reasonably easily carried out in the laboratory; and 4) it must be accurate [3]. The inherent moisture in low-rank coals should be determined [4] by ASTM procedure D 1412 (as indicated in procedure D388 [5]). Research continues unabated on the development of new methods for determining moisture, often ones which rely on sophisticated analytical instrumentation. Several examples are cited here. Moisture can be determined by measuring the phase shift in a guided microwave train, the phase shift being related to the amount of moisture. For a suite of five Palana (India) lignites, the difference between the microwave method and oven drying ranged from +0.50 (i.e., oven drying indicating 18.40% moisture; microwave, 17.90%) to -0.40 (16.80% by oven drying vs. 17.20% by the microwave method) [6]. Water can be determined by measuring the free induction decay of a single rd2 pulse in the 1H nuclear magnetic resonance (NMR) spectrum. For a suite of nine German brown coals, agreement with oven drying ranged from + 1.9 (11.5% by oven drying, 9.5% by NMR) t o - 1 . 5 (9.8% by oven drying, 11.3% by NMR) [7]. Extraction of water from coal into pyridine, reaction of the water with organophosphorus compounds and subsequent determination by 31p NMR provides a relative error, compared to the ASTM method, of +__2%[8].

385 8.2 EQUILIBRIUM MOISTURE The equilibrium moisture content can be used as an indicator of the anticipated as-mined moisture content [9]. The equilibrium moisture is also used to determine the bed moisture. The bed moisture and estimated surface moisture provide an estimate of the total moisture in the lignite. Equilibrium moisture varies inversely with the ash content (expressed on a dry basis), because the inorganic components may have much lower moisture holding capacities than the lignite substance itself. For example, at 40% (dry basis) ash, a lignite had an equilibrium moisture of 28%, but had an equilibrium moisture of 36% at an ash content of 8% [9]. For lignites, the equilibrium moisture is invariably lower--sometimes significantly so--than the so-called inherent moisture as determined by the ASTM method [10]. Samples of lignite that appeared to be dry visually but which probably still contained all of the bed moisture showed, on comparison with samples of the same lignite deliberately wetted by shaking in water, that both eventually attained the same equilibrium moisture, but that the wetted sample generally required an additional 24 h to come to equilibrium. For the samples that appeared dry, a equilibrium value of 32.5% was attained in 48 h over saturated potassium sulfate solution; 72 h was required to achieve this value with the wetted samples [ 11]. For lignites the equilibrium condition preferably should be approached from the "wet state," that is, from moisture contents above the equilibrium moisture [11]. Recommended analytical procedures have been published [11]. The relationship between moisture on an as-received basis and oxygen content of the coal is shown in Figure 8.1 [12]. Moisture shows a positive correlation with oxygen content [13]. This is attributed to the formation of hydrogen bonds between water molecules and oxygen functional groups on the lignite surface. The principal factor governing the equilibrium moisture content of low-rank coals is the content of carboxyl and phenolic hydroxyl groups [14]. At 52% relative humidity and 20~

the equilibrium moisture content can be expressed by the regression equation

% Equilibrium moisture = 2.181(COOH) + 0.599(OH) + 4.90 for which the correlation coefficient is 0.957. Here the terms (COOH) and (OH) represent the carboxylic and phenolic hydroxyl contents expressed as meq/g of dry acid-form coal. (The socalled acid form coal is coal treated with hydrochloric acid to remove ion-exchangeable cations and convert all the carboxylic and hydroxyl groups to their protonated forms.) The strong correlation shows that the oxygen functional groups have a very powerful influence on the adsorption of water. The coefficients in this equation suggest that the carboxyl group has roughly three times as much water associated with it as does the phenolic group. If the phenolic contribution is ignored, then the simpler equation % Equilibrium moisture = 2.239(COOH) + 6.60

386 0

70!

60

504o~ 30: ~ 20: 100

''''I

0

''''I

5

''''I'

'''I''

''I'''

'I''''

10 15 20 25 30 Percent oxygen, Parr basis

35

Figure 8.1. Relationship between moisture and oxygen content. Data for 64 samples of Canadian lignites, American peats, German brown coals, and Upper Silesian bituminous coals lie in the band [12].

can be derived [14]. These two equations were derived for coals having carboxyl contents of 0.944.01 meq/g and phenolic hydroxyl contents of 2.51-3.95 meq/g. Extrapolation outside these ranges would be of questionable validity; for example, there are bituminous coals for which the carboxyl content is essentially zero but which certainly have measurable equilibrium moisture contents. Coals in the acid form can be converted to a salt form either by reaction with an aqueous solution of the desired cation or by ion-exchange reactions after first converting the coal to its sodium form. The equilibrium moisture content of various salt form coals varies in the order of the hydration of the cations in aqueous solution. Furthermore, the moisture content increases linearly with cation content [14]. Thus for as-mined low-rank coals the carboxyl content essentially determines the equilibrium moisture content, along with the amount of cations incorporated (that is, the amounts of carboxyl in the acid and salt forms) and the kinds of cations and relative amounts of each. The importance of knowing the kinds of cations derives from the strength of the hydration. For example, water is weakly associated with sodium or potassium cations and is probably evolved on drying at temperatures around 110~

other cations, such as magnesium, have much

stronger bonds to the water of hydration and not all of the water will be released on mild thermal drying. In such a case, the moisture content determined as weight loss on heating will be low and

387 subsequent corrections of ultimate analysis data to d ~ - or moisture-free bases will give high values of organic hydrogen and oxygen. Correlations of moisture contents of North American lignites with petrographic composition have not been developed. South Australian lignites show a correlation of moisture with desmocollinite and telovitrinite, in that an increased moisture content is observed as the abundance of these macerals increases [15]. The effects of air drying on the restoration of the bed moisture for Beulah (North Dakota), Sandow (Texas), and Alnador County (California) lignites is shown in Table 8.1 [ 11]. Regardless of whether the lignite is wetted again before the equilibrium moisture determination, air drying destroys some of the moisture capacity of the lignite.

T A B L E 8.1 Effect of different degrees of air-drying on moisture-holding capacity [11].

Moisture, percent Before equilibration Sample

As-received

Air-dried

North Dakota

34.7 35.8 37.7 -

25.1 19.8 14.6 25.5 19.4 15.2 23.9 19.0 14.3

Texas

California

Equilibrated at 30 ~ 97% RH* Unwetted 33.9 30.5 28.7 26.7 35.0 30.3 27.1 25.2 35.0 28.0 25.5 22.8

Wetted 34.3 33.4 33.1 32.1 34.8 33.5 32.2 30.6 36.4 33.1 32.6 31.0

*Relative humidity

8.3

MEASUREMENT

OF

MOISTURE

BY

E L E V AT E D - T E M P E R A T U R E

METHODS

One of the most straightforward techniques, at least in principle, for the determination of moisture is heating in a suitable inert atmosphere and measuring the resulting weight loss. In practice, however, the situation is not as simple as would appear at first sight. The potential processes that might affect the amount of moisture determined by a thermal method are: 1) water would actually be given off; 2) various gases such as methane or the carbon oxides may also be given off; 3) other gases--oxygen or nitrogen--may be adsorbed; 4) the coal may be oxidized and lose weight due to the loss of carbon dioxide or water (the water in this case being the product of

388 the oxidation reaction); and 5) some water may be retained in the coal. In most laboratories the detennination of moisture is done simply by measuring the weight loss after heating in a prescribed time-temperature regime. Equating the observed weight loss to the moisture content presumes that the only process occurring during the moisture determination is the first. That none of the others occur seems a very risky presumption. Exposing a dried sample of lignite to the laboratory air for 10, 60, and 120 seconds indicated moisture contents of 18.80, 18.41, and 18.27%, respectively [16]. On the basis of these results, and similar tests with bituminous coals, exposure of samples to the air during transfer from the oven to the desiccator should be for the minimum practical time [ 16]. Oxidation of low-rank coals at 105-110~

may introduce errors into the moisture

determination. The Stansfield-Gilbart method minimizes such errors [17]. In this procedure, 5 g samples o f - 1 4 mesh coal are placed in a desiccator over saturated potassium sulfate solution, which is then evacuated and placed in a constant temperature oven at 30 + 0.05~ for 48 h. Moisture is then determined by heating in a vacuum oven for 3 h at 105-110~ in a stream of methane at subambient pressure. The effects of passing dry nitrogen at 10 kPa pressure or of dry air at ambient pressure while drying -16 mesh lignite at 105-110~

have been compared [11]. For 1.5 h in air, the

moisture content as determined was affected by the thickness of the layer of lignite in the sample container. For example, using a 43 mm crucible (resulting in a 9 mm thick layer of lignite), the moisture content was determined to be 29.2%, whereas the same sample treated in a 70 mm bottle (2 mm thick layer) gave the result of 30.8% moisture. Variations in the thickness of the layer of the lignite had no evident effect when heating in nitrogen at reduced pressure. Longer heating times, up to 4.5 h, had a limited effect, except for samples in 43 mm crucibles in air. Using a 70 mm bottle, the moisture content was determined to be 30.8% with 1.5 h heating in air and 31.0% with 4.5 h heating. For equal heating times, the moisture content as determined in reduced pressure of nitrogen averaged 0.5 percent higher than that determined in air. Under other conditions, heating in nitrogen and in air show only very small changes between the two atmospheres. At 1 h in a flow rate equivalent to 90 volume changes of the oven per hour, the moisture contents of an unidentified lignite determined in nitrogen and air were 18.21 and 18.24%, respectively [16]. When the heating time was extended to 2 h and the flow rate increased to 180 volume changes per hour, a small but discernable difference was observed: 18.63% determined in nitrogen and 18.46% in air [16]. Thermogravimetric analysis of Beulah-Zap lignite, coupled with Fourier transform infrared determination of evolved products, showed reasonable agreement with the ASTM moisture determination. The TG-FFIR method indicated 30.6% moisture, compared with 32.2% determined by the ASTM method [18]. Thermogravimetric analysis of Gascoyne (North Dakota) lignite shows a steady moisture loss at temperatures to about 90~ [19]. A slow weight loss is still recorded in the temperature range 100-260~

This latter weight loss might be ascribed to tightly bound

moisture. The relative proportions of loosely bound moisture, which comes off at or below 90 ~

389 and the tightly bound moisture are 80:20, which agrees with studies of the same lignite conducted by dielectric relaxation spectroscopy (Section 8.4). Water continued to be released from Kincaid (North Dakota) lignite for heating times as long as 150 h at temperatures as high as 175 ~ [20,21]. The results are shown in Figure 8.2. The moisture release depends upon heating time, temperature, and gas atmosphere. The nature of the gas affecting water release indicates a diffusion-controlled mechanism. If the release of water were governed by the thermal decomposition of species within the lignite (e.g., hydrates) the atmosphere should have no effect, since then the vapor pressure of the water would eventually be sufficient to force its release. A similar argument can be lodged against the hypothesis that the rate of water release is controlled by temperature. The smooth curves, with no evident changes of slope, indicate that there is not likely a sudden switch from one mechanism to another at some point in the drying process, nor that there is a change in the way the released water had been bound to the lignite. Australian brown coal has shown similar behavior; when the coal was dried in a stream of inert gas, measurable moisture evolution persisted for several days [22]. Other studies, however, show a transition from one rate of moisture loss to another [23,24]. For Beulah-Zap lignite, a plot of the logarithm of water remaining as a function of time has a constant slope for about 85% of the moisture loss from the lignite. A transition to a slower rate, considered to be indicative of a structural rearrangement, then occurs.

1 He, 175"

4 3.5-

2 N2, 150"

3L-

1 2.5-

17t

~I

2-

u.,

~

2

1.51 "

5-~ 0"0"1

!

0

i

I I

20

I

I

'1

40

'''

I'

60

''

I1

I '

I

I I

80 100 Time, hours

I I

I I I

120

I I

140

,

I

160

Figure 8.2. Water recovery as a function of drying time, gas atmosphere, and temperature for Kincaid lignite [20]. The water recovery is actual water recovered during the drying minus the ASTM moisture, expressed as a percentage of sample weight.

390 Changes in the surface area or depth of the bed of sample being dried had no effect on the rate of water release from Kincaid lignite, indicating that diffusion of the water through the bed of lignite particles is not a limiting factor in removal of water. This, together with the process being diffusion-controlled within the particles, suggests that removal of moisture from lignite is similar to the drying of porous solids [21]. However, this conclusion must be contrasted with a result obtained as a by-product of a study of the effects of heating on the pore structure of Beulah lignite [25]. The small angle X-ray scattering behavior of a sample heated to 225~ in argon for 4 h was very similar to that of the unheated sample. Since the basis of the small angle X-ray technique is that the intensity of the scattering depends on the square of the density difference between two phases, if the pores had been filled with water in the untreated lignite and with argon in the dried lignite, the difference between the coal/water and coal/argon systems should produce a change by a factor of seven in the X-ray results for the two systems. That very little change was observed suggests that the moisture in the untreated sample was not in the pores [25]. The average latent heat of vaporization of water from lignite, calculated from the ClausiusClapeyron equation, was 2.55 kJ/g in the range 20-40 ~ The latent heat of evaporation of water from a plane surface, calculated by the same equation, is 2.43 kJ/g [26]. Both the vapor pressure of water and the calculated heat of vaporization vary with moisture content, as shown in Table 8.2 [26]. Measurement of the heat of evaporation of water from Romanian lignite has shown that the percentage increase in heat of evaporation, relative to the heat of evaporation of pure water, is greatest when the last gram of water is being evaporated from the lignite [27]. TABLE 8.2 Calculated heat of vaporization of moisture from lignite [26]. Moisture per 100 grams dry lignite 40 35 30 25 20 15 10

Vapor pressure, mm Hg Heat of vaporization, 20"C 90"C calories per gram 16.53 53.55 592.9 15.61 50.51 593.8 14.33 46.46 594.8 12.54 40.99 598.9 10.24 34.40 612.8 7.37 25.45 626.7 3.95 14.11 643.8 Average ....................... 609.1

Comparison of heating in inert atmosphere with the xylene moisture determination (discussed below) shows that, for the same lignite, moisture evolution ceases after 1.5-2.5 h in xylene. The xylene penetrates the pore system of the lignite and either reduces subsequent moisture loss to a rate limited by liquid phase diffusion or blocks water release entirely [21]. The utility of the xylene distillation method for moisture determination in lignites and its

391 superiority to the conventional air drying methods have been established by meticulous research [3]. using six coal samples that included two lignites--Dakota Star (North Dakota) and Sandow. The essence of the method is distilling xylene in the presence of the coal sample; the water in the coal and the xylene form an azeotrope that distills at about 140~ into a graduated receiver from which the volume of water can be read. Assuming a density of 1 g/cm3 immediately gives the weight of water, from which calculation of the moisture content is straightforward. For the most precise work, corrections can be introduced for the density of water at the ambient temperature and for the solubility of water in xylene. The xylene method has been compared with standard air drying, drying in a vacuum oven, and Schoch's oil dehydration technique [28]. The xylene distillation and oil dehydration gave comparable precision, but because the latter is very cumbersome (it involves heating a 250 g sample of coal in the presence of 500 mL kerosene to 225 ~ the xylene method is preferable. For coals having 30-40% moisture the ASTM oven drying method gave 95-96% of the moisture reported by the xylene method. The vacuum oven gave lower values than either the ASTM or xylene methods. The xylene method accounts for all the moisture in a coal except for the combined moisture [3]. When large particle sizes are used, the xylene method is not satisfactory. Eight samples of 6x 13 mm Wyodak (Wyoming) subbituminous coal gave results ranging from 26.7% to 28.6%, with a standard deviation of 0.62, but eleven samples of 6x0 mm Dakota Star lignite ranged from 39.2% to 39.8% with a standard deviation of 0.17 [3].

8.4 E V I D E N C E F O R B I M O D A L I N C O R P O R A T I O N OF W A T E R

The incorporation of moisture in low-rank coals involves at least two fundamentally different mechanisms. The first "type" of moisture behaves as if it were free, the vapor pressure vs.

temperature behavior being that of pure water. Such moisture may be present in macroscopic

amounts in cracks or large pores. The second type of moisture is bound more tightly, having a lower vapor pressure than pure water at the same temperature. Such water may be hydrogenbonded to oxygen containing functional groups in the coal [ 13,29] or it may be present as water of hydration with various cations. Although various lines of evidence suggest that there may be several ways in which water is incorporated into lignites, it is important to note that the quantitative differentiation among these types or forms of moisture depends on the experimental technique used to make the measurements [30]. Dielectric relaxation is useful for studying bulk matter containing specific polar molecules. The graphs produced in a dielectric relaxation experiment are often called spectra, and the technique is sometimes called dielectric relaxation spectroscopy. It is not, however, spectroscopy in the quantum mechanical sense of resonant absorption of electromagnetic energy corresponding to the energy differences between two discrete quantum states. It is rather a classical phenomenon in

392

which energy in the dielectric medium is dissipated when the cycle time of an applied oscillating electric field is of the same order of magnitude as the relaxation time of the polar molecules in the medium. Useful background discussions of dielectric relaxation are provided in [31-33]. A parallel plate capacitor with a vacuum between its electrodes ceases to behave as a pure capacitance when a condensed matter dielectric is inserted between the electrodes [34]. If an alternating voltage is then applied, the current and voltage will no longer be precisely rd2 radians out of phase (rd2 being the value for a capacitor with a vacuum between its electrodes). The degree of departure from rd2 is expressed by the angle 8 and depends on the "lossyness" of the dielectric. If e represents the dielectric constant of the medium filling the capacitor, in complex notation

E* = e ' - i e"

The phase relationship between current and voltage is n / 2 - 6 . The detailed derivation of the relationship of 8 to E' and E" has been published [35,36]; condensing somewhat, tan 8 = E"/e'

and further,

t a n 8 = (E R - eLl) / (ER + 032 1;2 EU)

where ER is the low frequency limit (relaxed) dielectric constant,Eu is the high frequency limit (unrelaxed) dielectric constant, to the angular frequency, and x the extrinsic relaxation time. The quantitative interpretation of dielectric measurements requires that, ideally, the material being studied should consist of a single phase, or, if the system is multiphase, the arrangement of the phases in three dimensions be known. Obviously, neither condition exists for studies of water in coal. Thus calculated values of ER, EU, and x may not be accurate, and experimental studies have focused on calculation of tan 8. By measuring tan 8 vs. temperature at constant frequency, a quantity proportional to x can be determined. The large ionic double layer capacitance in coal containing water would seriously impede measurements made above 0~

Experimental results have been published for Gascoyne

lignite [35,36] and for San Miguel (Texas) lignite [36]. Gascoyne lignite is of particular interest because it can contain in excess of 40% moisture as mined, the highest moisture content of any commercially mined North American lignite. The sample is initially cooled to about-190"C. As it warms, the frequency F, capacitance C, conductance G and the temperature are measured [34]. The phase shift, tan 8, is obtained as a function of frequency from the equation

393 tan 6 = G/2nFC The tan 6 values plotted as a function of temperature yield the dielectric spectrum. The dielectric constant of brown coal has been measured; the dielectric constant as a function of water content showed a maximum around 50% moisture [37]. Nazarovo (Russian) brown coal displayed a non-linear tangent of the loss angle as a function of moisture content [38]. The dielectric spectrum of Gascoyne lignite shows two sets of peaks, as displayed in Figure 8.3 [34]. Depending on the frequency of measurement, one peak appears a t - 1 0 2 , - 9 2 , or -80~ the second appears at - 6 0 , - 5 0 , or -36~

The latter is attributed to the presence of macroscopic

crystals of ice [39]. After air drying to a constant weight (over several days) the high temperature peak disappeared from the spectrum. Upon vacuum drying of the air-dried sample, virtually all structure disappeared from the spectrum (Figure 8.4). Reconstituting the sample with water after freeze drying restored the spectrum to that of the original untreated lignite. The experiments suggest that this lignite contains about 80% of its moisture in a "loosely bound" form which freezes to ice below 0~

and the remaining 20% in a tightly bound form, possibly water of

hydration, which does not freeze to solid ice [40].

.

c3 4" X

~3 v

r

1O"

''

' I v-i i

''

'1'

''

~ ~ ~

i

I' (:2, O0 i

''1

''

'1

,t::) '4:) i

~ "~ i

'''

I' (:D 0,,,,I i

'' ,D

Temperature, ~

Figure 8.3. Dielectric relaxation spectrum of water-saturated Gascoyne lignite [35]. Data collected at 0.1 kHz.

The moisture distribution in Beulah-Zap lignite was measured by extracting the moisture into pyridine, followed by reaction of the water (now in the pyridine) with diphenylphosphinic

394

.

o4: .

~3 v

~2 .

1" ~

0 -200

-150

-100 -50 Temperature, *C

0

50

Figure 8.4. Dielectric relaxation spectrum of Gascoyne lignite after vacuum drying [35]. Virtually all the signal has been lost.

chloride and subsequent quantitation by 3]p nuclear magnetic resonance [41]. This work showed that 89% of the total moisture was "very loosely bound," and is probably surface moisture; 9% is "loosely bound," and 2% is "tightly bound." Unfortunately, the same lignite sample has not been examined by both techniques, and, further, one must use great caution in interpreting qualitative descriptive terms such as "loosely" and "tightly." However, the dielectric spectroscopy work suggests an 80:20 split between two forms of moisture [40]; 31p NMR suggests an 89:11 split [41], and drying measurements on the same lignite as used for the NMR work suggests an 85:15 differentiation [23,24]. San Miguel lignite, of about 30% moisture, showed only a single set of low-temperature dielectric relaxation peaks (Figure 8.5). These peaks actually appear as shoulders on a continuously rising ionic peak; the true, unperturbed maxima are 10 to 15~ lower, thus falling in the temperature region observed for other coals [36]. It was concluded that San Miguel lignite does contain chemically bound water [36]. Both Falkirk (North Dakota) and Martin Lake (Texas) lignites have large proportions of their moisture content in the tightly bound mode [42]. From the expression

"l;max = 1;o e x p (Ea/RTmax)

where Tmaxis the temperature (in kelvins) of the maximum in tan 6, it follows that

395 35 3025,,,==i

20"~ 15-

1o" 5

0 ' -140

'

'

I

'

'

'

I

'

'

'

I

'

'

'

-60

-120 -100 -80 Temperature, ~

Figure 8.5. Dielectric relaxation spectrum of San Miguel lignite at 0.1 kHz [36]. Only the low-temperature peaks are present.

log f = C2 - (Ea/2.3 RTmax) [40]. The activation energy for motion of the water molecules can be obtained from measurements at several frequencies by plotting log f

vs.

reciprocal temperature. For air-dried Gascoyne lignite, a

value of 67 kJ/mol was derived from the low-temperature peaks in the dielectric spectrum [40]. The activation energy for pure ice is 55.3 kJ/mol [33]. Since the low-temperature peaks in the dielectric relaxation spectra of lignites differ both in peak position and in activation energy for those in pure ice, it is unlikely that the moisture responsible for the low-temperature peak is present as ice. That the peaks occur at lower temperature than for pure ice suggests that the water molecules in lignite are in a less restricted environment than those in ice crystals. The less restricted environment, compared to that in ice crystals, enables significant molecular motion to occur at a lower temperature. These less restricted water molecules may be present as water of hydration of the cations present as counterions for the carboxyl groups [35,36]. From an estimated of 84 J/g for the heat of wetting of dried coal in water, and a molecular weight of a hypothesized structural unit in lignite of 450 [43], the energy associated with retention of water in lignite would be about 38 kJ/mol [44]. This value is in the range of bond energies normally associated with hydrogen bond formation [45]. This calculation does not prove that socalled bound water is held in the lignite via hydrogen bonding, but is not inconsistent with that idea. Oxygen functional groups have been suggested to be responsible for hydrogen bonding of

396 water in lignites [ 13,29,46]. That the organic oxygen functional groups play a role of some sort in the moisture content of lignite is also suggested by the correlation of the as-received moisture content with the humic acid content of peat and low-rank coals [26]. Hysteresis in the adsorption and desorption of moisture has also been attributed to the availability of hydroxyl groups for interaction with the water molecules--the availability of these groups must in some way be different during the desorption and adsorption processes [26]. Computer-aided molecular design also indicates differences in drying and rehydration. The shape of the Beulah-Zap lignite "molecule" configuration changes from an extended shape when fully hydrated to a more contracted shape as drying proceeds [47]. As drying proceeds, the number of intramolecular hydrogen bonds (i.e., hydrogen bonds entirely within the lignite structure, and not lignite-water bonds) increases from 4 to 10. As water is readsorbed (in the computer simulation) the number of intramolecular hydrogen bonds stays at about 10. This result suggests that the conformational change in lignite structure on drying is irreversible [47]. The heat of desorption of water from lignite, measured by differential scanning calorimetry, indicates that the interaction of lignite with readsorbed water is stronger than the interaction with the water molecules originally present [47].

8.5 A D S O R P T I O N

A N D D E S O R P T I O N OF M O I S T U R E

Three North Dakota lignites (Velva, Garrison, and Columbus) stored in desiccators over pure sulfuric acid required about 40 days to attain constant weight [48,49]. The three lignites showed very similar behavior, in that the dehydration could be divided into three distinct stages: first, a gradual decrease in relative vapor pressure from 100% to 80%; second, a rapid drop from 80% to 15%; and finally another gradual decrease from 15% to 0%. This behavior of lignite is very similar to the desorption of moisture from wood and peat [48,50,51]. The corresponding time for equilibration with the atmosphere in a sealed desiccator at 50~ is about 8 days [49]. The difference in equilibration time at the two temperatures reflects the increased diffusion rate of the moisture at the higher temperature. Hysteresis between the adsorption and desorption isotherms (discussed in more detail below) is also less at 50~ suggesting that a change in the physical structure of the lignite has already occurred even at this comparatively low temperature. The practical importance of these observations is that, upon exposure to the atmosphere, lignite will dry to a point at which it is in equilibrium with the prevailing humidity. For a typical relative humidity of 60%, no practical benefit would accrue from drying the lignite below about 16% total moisture, because the dried lignite would be hygroscopic and absorb water to reattain this moisture level, unless the drying process also effects some fundamental change in the lignite structure to prevent such absorption. Desorption isotherms of four North Dakota lignites are shown in Figure 8.6 [52]. The zero value of the ordinate represents a condition obtained by keeping the sample at 30~ and relative

397 50

1 Freedom 2 Beulah 3 Glenharold

o 'O ~0

ca ca

4 Gascoyne

35 30"

~ --m

.

ff

25

~o

2o

~

15-

.~1 o

10~

52 T''''I ''''I'''' I''''I''''I''''I''''I''''I''''I'''' .

.

C~

.

C~

.

.

C~

.

C~

.

C~

.

.

C)

C~

C~

P/Po, Relative humidity Figure 8.6. Moisture adsorption isotherms for four North Dakota lignites at 30~ [52].

humidity of 0 until an apparent equilibrium was attained. This condition is not the same as attained by thermal drying in nitrogen at 100~ for an hour. Drying these lignites under the latter conditions always causes a small additional mass loss--presumed to be moisture--compared to the former condition. Comparable behavior is observed with Australian brown coals. That different mass losses are observed when drying at 30 ~ and 0 relative humidity for many days

vs.

drying at 100~

in nitrogen for an hour suggests that there are different sites in the lignite responsible for retaining moisture, and the strength of retention is different for the different sites. Moisture contents below about 50% relative humidity are governed by some process, such as adsorption, other than simple capillary filling of pores [52]. That is, up to half the moisture content of lignites may be governed by adsorption behavior, again similar to that of Australian brown coals. Micron-sized porosity is not able to hold more than about 25% of the lignite moisture [52]. One micron pores are filled only at relative humidities above 99.9% [52]. Sorption of water on Texas lignite in the range of 1.2-2.3 kPa partial pressure water showed that Fickian diffusion combined with adsorption was the best theoretical model for describing the process [53], although the best correlation of the data was obtained via straightforward mass action kinetics. The desorption of water from lignite (as well as from peat and from birch wood) shows the same behavior as when water is reduced below its saturation value [50]. The vapor pressure

398 gradually decreases in the range 100-85%, followed by a rapid decrease from 85-15%, and then another gradual drying in the range 15--0%. Moisture absorption follows an S-shaped curve similar in shape to the absorption isotherm of a swelling gel. The first traces of moisture are adsorbed on the surface, and the vapor pressure increases slowly. A point will be reached at which continued absorption of water begins filling the smaller pores. With continued moisture absorption beyond this point, the vapor pressure increases rapidly, as a result of the changes in the radius of curvature of the water meniscus in the pores. When the smaller pores have been filled, the last stage of water absorption consists of filling the largest pores until the sample is saturated with water. On the other hand, inferences from heat of immersion experiments suggest that the initial reabsorption of moisture into dried lignite occurs in the largest diameter pores [ 19,54]. The heat of immersion of vacuum-dried lignite in cyclohexane was 3.9 J/g. However, after a comparable sample of vacuumdried lignite was allowed to stand in the laboratory for a short time, the heat of immersion became essentially zero. This observation suggests that the large pores, amenable to penetration by cyclohexane, quickly filled with water. In principle, water vapor can be taken into a solid adsorbent in three ways: by pure adsorption on active surfaces, by capillary adsorption and condensation to a liquid, or by adsorption or other penetration into the bulk interior of the solid. Analysis of the absorption isotherms indicates that lignite adsorbs water by two mechanisms: adsorption on active surfaces, and absorption in capillaries with subsequent condensation to liquid water [26,51]. This observation shows further analogy between moisture retention behaviors of lignite and silica gel. The physical structure of lignite can be regarded as a colloidal, sponge-like mass with "ultramicroscopic" capillaries of various radii [48]. The higher than normal vapor pressure of water may be due to water being present in a sponge-like structure of extremely small capillaries that retain the liquid with a convex meniscus (i.e., concave toward the vapor side) [51]. Hysteresis results from capillary collapse due to shrinkage during drying and from the difficulty of displacing gas molecules which had become adsorbed on the water-free surface [26]. The adsorption of moisture into the dried lignite in essence reverses the desorption process. There is first a gradual adsorption, then a very rapid increase in adsorption, followed by another gradual adsorption as the lignite approaches saturation. This behavior is typical of the adsorption isotherms of swelling gels [50]. The hydrogel or colloidal gel structural model of lignite seems generally agreed upon by investigators of moisture in lignite [55-57]. The first small amounts of moisture are adsorbed on surfaces. Further adsorption results in filling small capillaries. As more moisture is adsorbed larger capillaries will fill; this effect changes the radius of curvature of the water meniscus and hence increases the vapor pressure. In the very final stages the largest capillaries become filled until the lignite is fully saturated. The cumulative amount of moisture held within pores of various radii is shown in Table 8.3 [50]. The adsorption isotherm for brown coal, which shows the same general shape as that for lignite, shows monolayer adsorption from 0 to 20% relative vapor pressure, multi-layer adsorption in the range 20-50%, and then adsorption of water into the capillary structure above 50% [14].

399 TABLE 8.3 Moisture held in various pore radii [50] Radius interval, 10-7 cm

Cumulative moisture, %

>56.73 10.31-56.73 6.20-10.31 4.65--6.20 3.93-4.65 2.60-3.93 1.65--2.60 1.35-1.65 0.97-1.35 0.67--0.97 0.57-0.67 0.45-0.57 0.32-0.45 1040 ~

h)w

bw

Low Low-medium Medium

Low-medium

Low Medium HIGH

Figure 11.10. Matrix of the key variables affecting fouling potential [91]. Effect of :Ash content is based on constant 1040"C flue gas temperature; effect of flue gas tem~rature is based on constant 8% ash in coal (dry basis).

slowly accelerating rate. Removal of deposits by sootblowing seldom achieves a complete removal, with the result of a slow, net accumulation of deposits. As the deposits slowly accumulate, it will be necessary to increase the boiler temperature in order to achieve a constant heat-transfer rate. However, the rate of fouling also increases with boiler temperature, thus initiating a spiraling escalation of temperatures and fouling rates. In severe instances, the wall deposits will melt to a running slag. (ii) Sodiunr The association of sodium with fouling for large lignite-fired units was first established in cooperative work between Otter Tail Power Company and what was then the Lignite Research Laboratory of the Bureau of Mines (now the Energy and Environmental Research Center of the University of North Dakota). When a lignite having 9% Na20 in the ash was burned, only three days' operation could be achieved before it was necessary to shut down for cleaning the tubes. In contrast, fouling was not excessive when low-sodium-oxide lignite of the same total ash content was fired with similar boiler load. Pilot-scale tests established that lignite containing 8-10% Na20 in the ash results in a much higher deposition rate than a lignite with less than 2% Na20 [ 118], later confirmed in full-scale tests [125]. A general rule, derived from tests on 31 coals, including 25 lignites, is that coals containing less than 1% Na20 in the ash are low fouling [ 126]. Coals in the range of 1% to about 5% sodium

548 less than 1% Na20 in the ash are low fouling [126]. Coals in the range of 1% to about 5% sodium oxide are medium fouling. Above 5% sodium oxide in the ash, the lignite would be considered to be high fouling. These criteria are not absolute guidelines because factors other than sodium also affect deposition. For example, Rockdale (Texas) lignite has less than 1% sodium oxide in the ash and might therefore be expected to be low fouling; however, it also has about 39% silica in the ash, a relatively high value for lignites, and actually shows medium fouling behavior. Velva (North Dakota) lignite is not as severe a fouling lignite as would be predicted solely on the basis of sodium oxide content, because it also contains over 40% calcium oxide in the ash, an unusually high value. A coal in which the sodium content exceeds 5%, expressed as Na20 in ash, and the ash value exceeds 5% (dry basis) can be expected to result in high to severe fouling [127]. The single exception among the lignites tested was the medium-fouling Rockdale lignite with 39% SiO2 in the ash. Again with one exception, none of the coals with greater than 3% Na20 in the ash were low fouling. In this case the exception was Velva lignite. For coals having a lignitic type ash (i.e., CaO + MgO > Fe203) the effect of sodium on fouling is summarized by the data in Table 11.4 [126,128]. TABLE 11.4 Relationship between sodium oxide in ash and fouling [126,128]. % Na~O in Ash < 2.0 2-6 6-8 >8

Fouling Low Medium High Severe

Fouling is not solely a function of sodium content; for example, at a given level of sodium, increasing calcium content decreases fouling while increasing silicon increases fouling. The relationship between sodium content and rate of deposition is shown for Beulah lignite in Figure 11.11 [ 126]. This lignite typically contains about 20% CaO in the ash, which is close to the average value for North Dakota lignites. The fouling rate becomes essentially independent of sodium content above about 10% Na20; a similar effect was noted for other lignites. To fully understand fouling, however, it is not enough to have information on the oxide content of the ash, because only certain forms of sodium, before conversion to the ash, are the troublemakers; and other components besides sodium may have a significant role in fouling [ 124]. All other factors being equal, deposition rates in the superheater and reheater areas were a linear function of the sodium content up to 6% sodium oxide in the ash. Above that level, the deposition rate was essentially independent of higher sodium content. (see, e.g., Figure 11.12.) Pilot-scale combustion studies of Beulah lignite of various sodium contents established a so-called saturation

549 .

500

Beulah lignite (--20% CaO) 9 Velvalignite (--36% CaO)

400.

f

300

o

200lOO o

'

0

'

'

'

I

'

'

'

'

5

I

10

'

'

'

'

I

'

' '

15

'

20

Percent sodium oxide in ash

Figure 11.11. Comparison of fouling in a pilot-scale combustor for high- and normal-calcium content lignites as a function of sodium content [126].

effect above which deposition rate was independent of sodium content. The saturation effect occurred in the range of 8-10% sodium oxide in the ash. The fact that the saturation effect does not occur at a fixed, invariant level of sodium oxide in the ash for all conditions reflects the fact that other properties of the lignite and the boiler operating conditions also affect deposition rates. The threshold concentration of sodium necessary to produce the low-melting, sticky aluminosilicates which form the deposit matrix is 4--5% Na20 in the ash [129]. These sodium aluminosilicates are sticky enough to retain fly ash particles impacting the growing deposit. An early rule-of-thumb was developed which indicated that 0.4 to 0.5% sodium oxide in the coal (not ash) was the limit above which alkali-bonded deposits could occur. The chemical form of sodium was important, suggesting a rough distinction between "volatile" and "stable" sodium. In lignites of North America, the sodium is mainly incorporated as relatively mobile cations associated with carboxylic acid functional groups. The reason that empirical correlations which relate ash deposition to sodium oxide content of the ash provide a reasonable predictor of deposition without making a distinction between volatile and stable sodium is that for most lignites most of the sodium occurs in the cationic form. Sodium associated with the organic portion of the lignite is worse, from a fouling perspective, than sodium contained in minerals, because the organically associated sodium is both highly volatile and in a highly reactive form in the furnace. Support for the idea is derived from the

550 200

9

180 160~a0

~0 .),,4

to o

9

9

1014~ : 100-

9

9

80 60 40 20 [""l ,=,

,~.

....

r

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,~.

r,.-

,0q.

Sodium oxide in coal, % dry basis

Figure 11.12. Deposit weight in pilot-scale combustion of North Dakota lignites as a function of sodium content [130].

observation that the sodium concentration in deposits taken from convection surfaces in boilers is often higher than in the ash of the coal being burned [88]. The importance of organically bound sodium was demonstrated by first removing the sodium by ion exchange and then backexchanging sodium acetate. The fouling behavior of the lignite doped with sodium acetate in amount equal to the original sodium content was essentially identical with that of the untreated lignite. Pilot-scale data for Beulah lignites with sodium removed and back-exchanged with sodium bicarbonate are shown in Table 11.5 [ 130]. TABLE 11.5 Effects of changing sodium content on ash deposition behavior of Beulah lignite [130]. Condition natural, low-sodium natural, high-sodium ion-exchanged high-sodium ditto, back-exchanged with NaHCO3

% NagO in Ash 1.1 6.3 1.0 6.7

Deposit weight, g 118 399 127 438

The relationship between sodium, expressed as percent Na20 in the ash, and the percentage of annual output lost due to ash-related problems is shown in Figure 11.13 [ 130] for seven power stations. For six of the plants a remarkably linear relationship exists, highlighting the profound

551 25 o

20

0

N

d~

15

,n" ~n lO

,_q

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5

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0

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3

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'I

5

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6

7

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8

Percent sodium oxide in ash Figure 11.13. Total ash-related losses (as a percent of annual output) in electric power stations as a function of sodium content in ash [121]. The outlier is a coal of unusually low ash yield (5.7%, dry basis).

effect of sodium on ash behavior. The single point lying well off the line is data for a power station burning a subbituminous coal with ash content (5.7%) well below that of the coals burned in the other stations (all >8%). A survey of fouling losses at several power plants burning lignites or subbituminous coals confirmed the general relationship of sodium in ash to fouling [97]. Stations burning low-sodium Texas lignite had no fouling losses. A low-sodium subbituminous coal produced low fouling losses at the Jim Bridger station, and a subbituminous coal of somewhat higher sodium content had higher fouling losses (at the Four Comers power plant). The highest fouling losses in the study [97] were incurred at Big Stone and Leland Olds, which burn high-sodium lignites, the worst case being the Leland Olds Unit 2, which burns lignite of higher sodium content than is used at Big Stone. (iii) Flue gas temperature. The gas temperature is an important operational parameter affecting deposition. Indeed, the gas temperature at the point of deposition is second only to ash composition (particularly sodium) in importance in affecting fouling [ 131]. For lignite-fired units a rule of thumb is that there is a possibility of deposits forming on convection surfaces in all areas where the gas temperature is above 815~

As a rule, the amount of deposit formed in a given time

increases with increased flue gas temperature [126,127]. This effect is illustrated in Figure 11.14 [127], which presents data from a 34 kg/h pc-fired test combustor. The temperature of the gas entering the first bank of test probes (designed to simulate secondary superheater tubes in the convection section of a boiler) was varied from 980 ~ to 1150 ~

a threefold increase in deposit rate

552 700 600~0 5009r-I

400-

~9 3000

121 200" 100

o 900

'

'

I

'

'

'

1000

I

'

1100

'

'

1200

Flue gas temperature, ~

Figure 11.14. Ash deposit formation in a pilot-scale combustor as a function of the flue gas temperature, with deposition rates corrected to constant coal consumption rates [127].

occurred over this temperature range [ 126]. The effect of increasing the gas temperature is ascribed to the gas temperature approaching the ash softening temperature [126], although there appears from other studies to be no direct relationship between ASTM fusion data obtained on the laboratory ash and the formation of ash deposits. Rapid fouling certainly occurs when the temperature of the gases reaching the convection banks is well above design temperature. For example, in a unit designed for a flue gas temperature of 1137"C entering the secondary superheater, unusually heavy buildup of wall slag resulted in the gas temperature rising to 1315~

This condition resulted in rapid fouling of the superheater. Any

combination of high values of sodium (>5% Na20 in the ash), high ash levels (>8% ash on a dry coal basis) and flue gas temperatures above 10370C is a recipe for serious trouble. Comparable observations are made with Siberian lignites having 40% CaO in the ash; for example, exit gas temperatures of 1020-1050~ are recommended [132]. When the condensation of sodium sulfate, and its subsequent participation in formation of a sticky surface layer that accumulates particles, governs fouling, a temperature "window" may exist in which deposit accumulation is particularly severe. In this model, at gas temperatures above 9770C the ash particles will not have a molten surface layer, and any particles that have accumulated on the tube surface will also be dry, because the tube metal temperature is below the melting temperature of the sulfate [133]. Any sodium sulfate that condenses on the tube is removed by erosion; a deposit would not be expected to form. At gas temperatures of 977~ or

553 below, sodium sulfate will begin to condense on suspended particles; eventually enough sodium sulfate will accumulate to make a sticky surface layer on some impacting particles, the sticky layer being sufficiently thick that the rate of deposition and accumulation exceeds the rate of erosion [133]. Deposition should cease when the gas temperature drops to 884~

or below, this

temperature being the assumed melting temperature of the sulfate [ 133]. Because of the role of gas temperature in affecting fouling, it can be a serious mistake to increase firing rate to regain boiler output once the boiler has begun to foul [ 134]. Doing so will increase the outlet gas temperature, which will in turn increase the slag buildup even further. Outlet gas temperature can be reduced by increasing windbox pressure, reducing primary air temperatures, or lowering the burner tilts. Furnace exit gas temperatures are very important when burning high-fouling lignites [97]. If gas temperatures become too high, fouling will be a problem. Water blowers on furnace walls are the most cost-effective measure for controlling furnace exit temperatures [97]. When fouling becomes severe enough to affect the load, usually virtually all of the tubes in the convection section are heavily fouled. As the fouled area suffers reduced heat-transfer ability, the temperature will increase through the convection section. Consequently the zone in which gas temperatures are high enough to promote fouling steadily moves back through the convection section until eventually the fouling becomes so severe that draft losses or the temperature at the back of the convection section force a curtailment [97]. However, the accumulation of fouling throughout the convection section is slow, and often remedial measures such as sootblowing or hand lancing can be applied during periods of load reduction without requiring a curtailment. In cyclone furnaces, low slag viscosities, which are otherwise desirable for good slag tapping, could increase the carry-through of coal particles burned in suspension (as opposed to those trapped and retained by a sticky, high viscosity slag) again raising temperatures at the furnace outlet and exacerbating deposition. (iv) Tube metal temperature. The temperature of the tube surface has a moderate effect on ash deposition. As the temperature of the metal tubes increases, the total weight of deposit accumulated in a specified time also increases [126,127,131]. However, the weight of the inner white layer decreases with increasing metal temperatures. The effects of tube metal temperature in the temperature range 425-650~

are shown in Figure 11.15 [127]. In the test furnace used to

acquire these data, the deposit forms on metal tubes simulating the steam tubes in the convective pass of a utility boiler. The suggested explanation is that at lower temperatures whatever deposit is already on the tube solidifies faster and is less sticky, therefore being less able to trap or retain additional material. When particles solidify, at temperatures below the melting temperature of the sulfates, deposition should cease [ 133]. (v) Calcium.

A high calcium content in the ash reduces deposition [48,131]. Large

amounts of calcium may prevent formation of large amounts of glassy ash particles of high iron content, the calcium serving as a nucleating agent [135]. (High magnesium levels may have the same effect [48].) Large grains of caicite could "dilute" deposit strength [45]. Calcium could also

554 600

6

500

-5

~4oo-

-4

.~0 .

~1 9 3000

-3

"

K

,~ 200 -

~,4

-

1000

--1

. . . .

400

i

500

. . . .

I

. . . .

600

~

0

700

Tube metal temperature, ~

Figure 11.15. Deposit formation in pilot-scale combustion as a function of tube metal temperature [127].

increase the viscosity of the liquid phase in a deposit, decreasing sintering and hence reducing deposit strength [45]. However, with Nazarovo (Russian) brown coal, sulfation of calcium oxide is a causative factor for the formation of strongly bonded deposits on the convective heating surfaces [ 136]. Calcium sulfate has been implicated as the "glue" in some superheater deposits of Texas lignite ashes [ 137]. Sulfation of calcium occurs after the deposition of calcium-rich particles [138]. The propensity toward sulfation appears to depend on the nature of the association of the calcium with other elements in the deposit [45]. Calcium silicate particles are stickier at high temperatures than are other ash particles, and are more likely to stick to boiler surfaces [45]. The reaction between calcium-rich particles and aluminosilicates leads to a deposit matrix rich in calcium, aluminum, and silicon [137]. Fine calcite grains may react with silicates to depress liquid viscosities, thereby enhancing ash stickiness and deposit formation [45]. Velva lignite has an unusually high calcium content, about 40% CaO in the ash, double the average for North Dakota lignites. The deposition rote for this lignite is about half that of a lignite with the more typical 20% CaO [126]. Velva lignite has a lower ash value than some other North Dakota lignites, but even when deposition rates are corrected to an equal ash input basis the highcalcium lignite has a lower rate of fouling. Similarly, deposition is greater for a lignite of lowerthan-normal calcium con~,ent when compared with a lignite having normal calcium and comparable sodium and ash levels, The behavior of calcium relative to sodium may at first seem counterintuitive. Except 7or the obvious difference in ionic charge, the two elements have much in common. They form ionic compounds, their cationic radii are virtually identical, and their oxides

555 are strong bases. Increasing the amount of calcium in ash increases the ash softening temperature for lignitic ashes, but there does not seem to be a correlation between ash fusion temperature and fouling. Calcium affects the melting temperatures of the matrix material in the outer sinter layer of deposits. Sodium melilite has a melting point in the range 1120-1200~

In high-calcium systems,

there is a likelihood of forming gehlenite, Ca2AI2SiO7, which has a much higher melting point, 1590"C. A system high in calcium and magnesium could form akermanite, which also has a melting point much higher than sodium melilite, 1438~

The high melting temperatures of

gehlenite and akemlanite suggest that the role of the high calcium content in reducing fouling may result from increasing the melting temperature of the matrix. Reactions of kaolinite with sodium and calcium acetates, discussed in Chapter 6, have shown that kaolinite and sodium acetate produce nepheline, whereas kaolinite and calcium acetate produce gehlenite, suggesting that different mineral assemblages may fornl in the early stages of combustion of high-sodium and highcalcium lignites. Since the subsequent behavior of ash in the combustion system is dependent on composition and mineralogy of the ash particles, the different compounds formed early in combustion could have a profound effect on deposition behavior when the ash reaches the boiler tubes. (vi) Ash value. The ash value per se is not a predictor of fouling behavior. Some coals with high ash levels show pronounced fouling whereas others do not. However, when the total ash value is considered in conjunction with ash composition, an effect of ash value can be observed. At a given level of ash value, the fouling rate is a direct function of the sodium content. For any given level of sodium content, the fouling rate increases exponentially with ash value. Thus ash value can be an important predictor of fouling. The most severe fouling conditions occur when both the sodium and ash levels are high, as shown in Figure 11.16. However, a contrary view is that lignite having high sodium but low ash value is more troublesome than a high sodium lignite of high ash value. The role of the sodium as a flux or in promoting sintering may be diluted when larger quantities of ash are present [96]. In a survey of the economic effects of fouling, a plant burning a subbituminous coal having the highest sodium content of any low-rank coal included in the survey experienced only moderate fouling, and no fouling losses during the six month period of study. This uncharacteristic behavior of a high sodium coal was attributed to the very low ash levels in the coal, which were 4.1-5.1% on an as-received basis [97]. Blended coals show fouling strongly influenced by the sodium and ash values of the blend. However, these two parameters do not always vary linearly, because the fouling behavior of the blend was not necessarily that predicted from the weighted average of the sodium contents or ash values of the unblended coals. Thus fouling is a function of the characteristics of the blend itself regardless of the characteristics of the s~arting coals. (vii) Other factors of lignite composition. Moisture, sulfur content, and ash fusion temperatures have minor roles in affecting deposition behavior [ 126,131 ]. None have the same

556 800 700-

~,"

600-

9 l%Na2Oash

500-

9 5% Na20 in ash

.w-i

400-

o

10% Na20 in ash

oga~ 300 200 100 0 0

2

4

6

8

10

12

Ash content, % dry basis Figure 11.16. Relationship of deposit weight in pilotscale combustion of subbituminous coals as a function of the ash yield, for three levels of sodium in ash [130].

degree of importance as sodium content. Fouling from a coal with its normal 30% moisture content and the same coal dried to 15-20% moisture, show no significant differences.The silica content of the ash tends to increase fouling, but is generally has a minor role compared to sodium and total ash value. Ash deposits obtained after shutdown of the boiler usually contain a greater amount of sulfatic sulfur than would be indicated by the sulfur-to-ash ratio of the lignite. This suggests some interaction between the components of the ash and sulfur oxides. Further confirmation was provided by measurements of flue gas SO2 and SO3 when high- and low-sodium coals of comparable sulfur content were fired. The SO2 content was higher by about a factor of two for the flue gas from the low-sodium coal. For the high-sodium coals about 60% of the input sulfur was found as sulfur gases, whereas in the low-sodium case virtually 100% of the sulfur was in the flue gas. These observations suggest a minor role for sulfur in deposit formation, but large differences in sulfur content are not nearly as important a factor as sodium content in determining fouling behavior. (viii) Factors of boiler operation. Excess air level during firing appears to have a minor role in affecting deposition behavior [126,~t31].The relationship of excess air to fouling is not clearly understood. In some tests no effect of excess air levels in the range of 5-50% was observed. Tests in a drop-tube furnace showed no effects of oxygen concentration on the composition of adhering particles in the deposit [ 139]. Other work has shown an increase in deposition rate at low excess air levels. However, in these cases wall slagging also had increased, and thus the effect of the low excess air may have been to trigger first the increased wall slagging, which itself leads to higher

557 gas temperatures, and the higher gas temperatures then become responsible for increased deposition in the convective region. An increase in excess air level has been claimed as a method for controlling slagging problems [9]. When control of NOx emissions is crucial, it may not be feasible to use excess air adjustments as a control method for slagging. Change in damper positions from normal to overfire air conditions does not increase ash deposition [48]. Changes in burner tilt angle modify asia deposition to some extent, as well as affecting the NOx concentration in the flue gases. Boiler load factor can influence deposition. Lignite which causes severe fouling under high load conditions produces much less fouling at lower loads. However, the load factor may also be an indicator of changes in llue gas temperature, which affects deposition significantly. Variations in soot-blowing practice have been claimed to affect deposition; however, this topic does not appear to have been the subject of systematic research. At constant load, deposit rate can be a function of burner position. The deposit rate is more a function of the burner position than of the overfire air condition. Changing damper positions from normal to overfire air conditions does not appear to increase fouling. Deposit rate thus does not appear to be a function of air distribution. The effects of operating parameters in full-scale boilers are generally related to the gas temperature in the secondary superheater region. The effect of the boiler parameters is illustrated in Figure 11.17 [91].

BOILER PARAMETERS

Major operating factors affecting fouling.

Possibly affecting fouling.

Effect on fouling potential as parameter decreases.

Flue gas temp.

Decreases significantly as temperature drops from 1150 ~ to 870 ~ C.

Plant load

Decreases significantly from 100% to 70% of load.

Excess air

Increases - slightly to significantly - as 02 drops below design level.

Pulverized coal size

Decreases as size changes from 70% passing 200 mesh screen (74 0.m).

Volumetric heat release

Decreases as volumetric heat release is reduced from normal to 60% of normal for non-fouling coal.

Figure 11.17. Boiler operating factors related to ash fouling for a plant burning 5% Na20 in ash coal, 70% -200 mesh grind, 426 MJ/m3.h volumetric heat release rate [91].

Any variable affecting burning rate of the lignite will also affect fouling. A coarse grind from the pulverizers will mean that larger lignite particles may continue burning as they are carried upward through the furnace, thus increasing I:x)th the temperature in the convection section and, as a consequence, deposition. Changes of burner tilt, excess air, and the air and fuel distribution could have similar effects. Canadian lignites show a deposition rate greater with coarse

558 pulverization and with increased gas velocity [140]. (ix) Sintering behavior o f ash. Measurements of the sintering behavior of the ash (as determined from inflections in a resistance vs. reciprocal absolute temperature plot) have shown that ashes from high-fouling North Dakota lignites have lower sinter points (625-800~

and

generally lower resistances (6x 103 to 105 ohms) [ 141 ]. Ashes from low-fouling Alabama lignites had higher sinter points (about 950~

and higher resistances (about 2 x 106ohms).

The sintering behavior of ashes from five North Dakota lignites--Blue and Red pit Gascoyne, Velva, Center, and Indian Head--as well as Choctaw (Alabama) lignite and Navajo (Arizona) subbituminous coal were measured for comparison to observed deposition behavior [142]. Measurements were made by two techniques--observation of changes in shrinkage or electrical resistance with temperature--on ashes prepared by the standard ASTM technique from -60 mesh coals. The results are shown in Table 11.6. By either method the sinter point is determined as a change in slope of the shrinkage (or resistance) vs. temperature curve. The deposition behavior is a qualitative observation of fouling tendency based on tests in the 34 kg/h pilot-scale combustor.

TABLE 11.6 Sintering temperatures correlated with ash deposition behavior for low-rank coal ashes [142].

Lignite Center Choctaw Gascoyne (Blue pit) Gascoyne (Red pit) Indian Head Navajo Velva

Sinter Point,*C Shrinkage Resistance 890 825 900 * 760 975 840 * 890 850 900 945 910 960

Ash Fouling Tendency High Low High Low High Low Low/Moderate

*No distinct break was observed in the resistance vs. temperature curve.

In neither method does it appear that any useful correlation exists between the observed sinter point and the fouling tendency; compare for example the shrinkage data for Navajo and Center coals. However, the shrinkage observed above the sinter point correlates reasonably well with fouling. At 950"C the ashes from both the high-fouling Gascoyne Blue pit, Center and Indian Head lignites showed a shrinkage of 25-27% from those observed at the sinter point. In comparison, at the same temperature the ashes from the low-fouling Velva, Choctaw, and Navajo coals showed shrinkages of only 0.5-2% of the sinter point shrinkage, and the value for Gascoyne Red pit lignite was about 7.5% [142]. It may be noteworthy that the sinter points measured by the shrinkage test tend to cluster around the melting point of sodium sulfate (884"C), the single exception being the Blue pit

559 Gascoyne lignite. In a reducing atmosphere, viscosities of slags from seven lignites and three coals of higher rank decrease, while surface tensions increase [143]. This results in an increased sintering potential of ash in a reducing environment relative to an oxidizing environment. Increased silica content increases the viscosity at a greater rate than it does the surface tension, thus reducing sintering [143]. On the other hand, increased sodium oxide content decreases viscosity at a greater rate than surface tension, increasing sintering behavior [143]. The observation of the effect of sodium may extend to total alkali content. In general, ashes that tend not to sinter or agglomerate form a melt phase having high viscosity relative to surface tension at a given temperature, while ashes that tend to agglomerate have lower viscosities relative to surface tension. (x) Prediction from pilot-scale testiug. The Predicted Ash Collection Efficiency (PACE) Index was developed originally for Australian brown coals. A regression analysis of total ash collected in a test furnace as a function of composition led to the equation PACE INDEX = 0.67 + (0.13 Na + 0.08 Ca) 9(100/Ash) [12] where the Na and Ca are expressed as percentage of the element (not oxide) on a dry coal basis. Generally, low-fouling coals have a PACE Index less than 2 while high-fouling coals have an Index greater than 3 [12]. In applying the Index to lignites, it should be remembered that the lower moisture of the lignites, relative to brown coals, could result in higher flame temperatures which might make the fouling experienced by lignites worse than might be anticipated from the calculated Index. In a 34 kg/h pulverized-coal-fired furnace, the relative fouling potential is assessed on the basis of the weight of deposit collected on the first probe bank, with a deposit weight below 150 grams indicating a low fouling potential, 150 to 300 g medium, and weights greater than 300 g a high fouling potential [91]. A regression equation calculated from results of 44 ash deposition tests is log (FDW) = 1.21 + 0.45 log(Ti) + 1.46 log(S) + 0.38 log(ash) + 1.14 log(Ca/S) + 0.63 log(alk) [144] where FDW is the fouling deposit weight expressed in grams per million Btu fired, Ti is the percentage of titanium dioxide in the sulfate-free ash, S is the percentage of sulfur in the dry coal, ash is the percentage of ash in the dry coal, Ca/S is the ratio of the percentages of calcium and sulfur in the dry coal, and alk is the alkali ratio, which is calculated from alk = [(ash) 9(Na20 + 0.659 K20)] / (CaO + MgO)

where again the ash is expressed as percentage of the dry coal and the molecular formulas represent the percentages of each compound in the sulfate-free ash. The error associated with the predicted FDW is 20-30% [144]. The largest errors occur for coals giving the highest deposit weights; the

560 magnitude of the error for such coals is unimportant since in any case a severe fouling problem will exist. This work indicates that coal having a low ash value will have a low fouling behavior, while coals of high ash value could exhibit severe lbuling, provided that there is sufficient adhesive material to agglomerate the del:x)sit [ 144]. The term including titanium is an indicator of the amount of detrital contribution to the ash [ 144]. 11.4.3 The mechanism of deposition (i) The sulfate layer. Ash deposition is initiated by formation of a layer rich in sodium sulfate on the boiler tubes. This layer is called the inner white layer. The layer forms completely around the tube. Its formation appears to proceed via a diffusional process [36]. Thermal decomposition of sodium carboxylates in the coal starts a sequence of reactions which lead ultimately to the formation of sodium sulfate in the flame or flue gas. Convective mass transfer diffusion of sodium-containing species through a boundary layer around the tube deposits sodium sulfate on the tube surface [36,125,145]. Deposition of the white layer occurs when the metal temperature was raised (using a test probe of which the temperature of the metal surface could be varied) from 870 ~ to 980~ [125]. The inner white layer contains most of the elements present in the coal ash but with sodium and sulfur predominating. X-ray diffraction has shown that the material is crystalline with the major phase being ct-NazSO4. Calcium sulfate and silica have been identified in the inner white layer. Generally the concentrations of calcium and magnesium are higher than in the original coal ash, while those of silicon and sodium are lower. The amount of water-soluble material in the inner white layer increases with increasing sodium content in the coal, ranging from about 7% for a lignite with 2% sodium oxide in the ash to 54% for a lignite with 9% sodium oxide in the ash. In some deposits the total amount of sulfate is greater than can be balanced by the alkali and alkaline earth elements, suggesting the presence of iron or aluminum sulfates. Condensed sodium sulfate tends to be most concentrated in small particles with low impaction efficiencies. Thus the concentration of sCx:lium in a deposit is not useful in indicating the importance of sodium in the deposition process [ 133]. Combustion of Beulah lignite in a drop-tube furnace provided no unequivocal evidence for "gluing" of the deposit by sulfate or sodiumcontaining species [ 139]. Particles adhering to a boiler steel substrate contained significant quantities of calcium and sulfur (probably as calcium sulfate) [51]. These particles were produced in the combustion of Center lignite in a laboratory-scale drop-tube furnace. Adhesive strengths were 3.0-3.7MPa [51]. Center lignite that had been ion-exchanged to remove the cations produced adherent calcium-sulfur particles in greatly reduced numbers compared to the untreated lignite. The particles having greatest adhesive strength had high concentrations of iron and silicon, probably as fayalite. (Fayalite has a relatively low melting point of 1200~

With the ion-exchanged lignite, the iron-silicon particles

made up a greater proportion of the adherent particles, but the total number of adherent particles of all compositions was less. In similar tests with San Miguel and Martin Lake (Texas) lignites, most

561 of the adherent particles of high adhesive strength were rich in iron and silicon [51]. The adhesive strengths of the iron-silicon particles were in the range 7.1-76 MPa. In comparison, iron-sulfur particles showed adhesive strengths in the range 2.2-4 MPa, and the calcium-sulfur particles, 3.0-3.7 MPa. Strengths in excess of 50 MPa are comparable to those of enamel coatings on metals, and 40 MPa is comparable to a strong bond between epoxy resin and steel [146]. An adhesion strength of 0.1 kPa is adequate to support a 10 mm lightly sintered ash deposit of 1000 kg/m3 density [147]. The adhesive layer formed from combustion of a Texas lignite was very calcium-rich with abundant sulfur and some iron-rich particles [137]. The presence of a melt phase of calcium-magnesium-sodium sulfates on deposits is consistent with role for calcium sulfate in fouling at temperatures near 927~ [ 133]. X-ray diffraction analyses of white layers of deposits produced from combustion of various lignites in a pilot-scale combustor are shown in Table 11.7 [148]. TABLE 11.7 X-ray diffraction analyses of white layers from lignite ash deposits [148]. Lignite Baukol-Noonan Beulah Gascoyne

Major Phases Minor Ph~tses Trace Phases CaSO4 MgO Fe304 CaSO4 Fe203 SiO,,_ CaSO4

Iron and aluminum may assist in fomling a low-melting material that would allows sticking of ash particles. Thus ash deposits from Beulah lignite (produced in a drop-tube furnace) contained 1-3 ~m particles of calcium aluminosilicates enriched in iron, as well as sodium and magnesium [139]. Iron or aluminum sulfate can lower the melting point of alkali sulfate mixtures by several hundred degrees. As the inner white layer builds up, it acts as a porous mat into which fly ash particles striking the tube can sink. As the relatively hot fly ash particles sink into the inner white layer, they may partially melt the sodium sulfate or mixed metal sulfates, thereby improving bonding or sticking. Bench-scale studies of ash volatilization from the Red, Blue and White pits of the Gascoyne mine and a low-sodium Beulah lignite were conducted at heating rates of 25~ [149]. A water-cooled platinum disk 7.5 cm above the crucible was used to collect the vaporized material. The White and Blue pit Gascoyne lignites produced substantial deposits of potassium and sodium sulfates having the appearance of condensed droplets. These droplets were absent from the immediate vicinity of aluminosilicate particles, suggesting that the sulfates had been incorporated into the aluminosilicates. Reports in the literature differ considerably about the adhesion of the inner white layer to the tube surface, the layer being variously described as "tightly attached" and "loosely attached and

562 ... easily removed with a toothbrush." The gas and tube temperatures would determine whether the sodium sulfate condenses as a liquid or solid. Thickness is dependent on temperature. On a test probe for which the metal temperature varied over the range 425 ~ to 650~

the inner white layer

reduced in thickness with increasing temperature. In other work, the white layer "disappeared" (which evidently means that it did not form in the first place) when the tube temperature was held at 815~

Once the bulk of the deposit forms, the inner white layer cannot be identified on the

upstream side of the tube under the deposit. Formation of the inner white layer is important for the initiation of fouling but that it subsequently has little effect on the build-up of the deposit. During combustion, sodium present as carboxylates could react to form a number of sodium-containing compounds, depending on the temperature and other species available in the diffusion environment [52]. Above 1400 K vapor-phase sodium will occur either as sodium metal or sodium hydroxide [150]. Vapor-phase sodium that remains in the flue gas below 1400 K will condense on ash or boiler surfaces and can then react with sulfur species to form sodium sulfate [150]. Calculations based on a pilot-scale combustor with a gas temperature of 1093 ~ entering the convection section indicate a diffusivity of sodium sulfate of 9.55 x 10-5 m2/sec, a Sherwood number of 22.1 (assuming a Schmidt number of 1), and a mass transfer coefficient of 5.0 x 10-2 m/sec [ 151]. These values are very close to data reported for the deposition of material in oil-fired boilers [152]. Thus it was assumed that the formation of the inner white layer proceeded via a similar physical process, namely convective mass transfer diffusion of sodium-containing species through a boundary layer on the probe. (ii) Tile inner sinter layer. The inner sinter layer is characterized by discrete ash particles bonded to each other or to the tube via fairly weak bonds. There is no distinguishable melt phase. Deposition of the sulfate layer is followed by accumulation of a layer of ash particles which builds up via inertial impaction [151]. Some particles may be molten or partially molten when they strike the tube [145]. A significant increase in adhesion efficiency occurs as temperatures in the combustion system are increased [ 153]. If these particles do not freeze immediately upon impact, they provide a sticky surface for the subsequent trapping of additional particles, regardless of whether the newly arrived particles are molten themselves. Surface stickiness of the ash particles provides enough particle-to-particle adhesion to allow the inner sinter layer to form and grow. The inner sinter layer acts as an insulator, so that it takes longer and longer for the growing upstream surface to cool by conduction to the tube; eventually the inner sinter layer becomes a good enough insulator so that its upstream surface stays hot enough for a melt phase to form. Accumulation of particles gradually thickens the inner sinter layer until a point is reached at which heat loss to the tube is sufficiently slow to allow reactions with gas-phase sodium-containing species to occur. At that point a glassy matrix begins to form. This matrix material, and the ash particles subsequently trapped in it, constitutes the outer sinter layer, the bulk of the deposit. Several mechanisms have been suggested for transport of the particles to the surface, including thermal diffusion caused by the large temperature gradient between the tube and the bulk of the flue gas, and Brownian particle motion in the tx~undary layer. Larger-than-average or denser-

563 than-average particles may preferentially impact the surface. The smaller-than-expected deposits produced during combustion of micronized lignite have been attributed in part to the ash particles being much smaller than normal and thus better able to follow the flow streamlines around the tube. Particle viscosity dominates the factors affecting inertial impaction [154]. A critical viscosity for adhesion is in the range 105-108 Pa.s; below the critical viscosity, particles will adhere to a substrate, but will not do so above the critical viscosity [154]. The calculated critical viscosity for deposition of San Miguel lignite is 107 Pa.s [154]. A simplified physical picture of the events lk:~llowingparticle impaction is that the particles deposit in sublayers in a fashion in which the first particles to arrive cover the available surface of the probe, forming a sublayer which is then itself covered by a second sublayer of particles, and so on. It is likely that the particles in the first sublayer will cool very quickly because they are in contact with the relatively cold tube. Particles in the second sublayer will not cool quite as rapidly, because the conductive heat transfer to the probe must now proceed through the intervening first sublayer. With each successive accumulated sublayer, the particles in that sublayer will cool less and less rapidly because of the increasing insulating effect of the previously deposited sublayers. At some point the particles in the "n-th" sublayer will cool so slowly that time is available for crystallization or reactions with gas-phase sodium species to begin the fomaation of matrix-- i.e., to begin the formation of the outer sinter layer. If it is assumed that the thermal conductivity of the ash particles is independent of composition (or, in essence, is not coal-specific) and that the physical packing of particles in the various sublayers is also independent of the specific coal being burned, then the pathways for conductive heat transfer from the accumulating ash particles to the tube will be the same for all coals. This deduction leads further to the notion that the inner sinter layer would therefore be the same thickness regardless of the coal being burned (for a constant configuration of the combustor and temperatures at the inlet to the convection pass). Indeed six samples of deposits from a pilot-scale combustor showed very similar thicknesses of the inner sinter layer [151]. Measurements by two independent observers showed average thicknesses of 0.94 and 1.07 mm with relative standard deviations of 5.83 and 13.5%, respectively. Little work has been done on characterizing the inner sinter layer. Individual particles of silica, iron oxide, aluminum silicate, magnesium aluminum silicates, and calcium aluminum silicates have been identified. Virtually all particles have coatings of stx:lium sulfate; some particles have coatings of calcium sulfate. Detailed examination of the inner sinter layer of a deposit produced from Beulah lignite showed the following constituents, listed in approximate order of abundance: 1) calcium-sodium-l:x~tassium aluminosilicate glass spheres containing minor amounts of unfused clay minerals, quartz and other minerals; 2) quartz groins of various sizes; 3) anhydrite occurring both as a matrix and as a coating of spheres; 4) spheres of hematite and iron oxide glass; 5) sulfates of magnesiurn, aluminum and sodium; and 6) calcium- and magnesium-rich spheres of sintered carbonates [ 155]. X-ray diffraction analyses of the sintered layer from deposits produced from three lignites in a pilot-scale combustor are shown in Table 11.8 [ 148].

564 TABLE 11.8 X-ray diffraction analyses of sintered layers from lignite ash deposits [ 148]. Lignite Baukol-Noonan Beulah

Gascoyne

Major Phases Minor Phases CazAI2SiO7 Si02 CaSO4 Ca2AI2SiO7 SiO2 CaSO4 Fe203 (Na,Ca)8(Si,AI) 12024(SO4)2 Fe304 CaMgSi206 SiO2 Ca2AlzSiO7

Trace Phases

CaSO4

(iii) The outer sinter layer. For low-sodium, low-fouling lignites the entire deposit consists essentially of the inner sinter layer and the inner white layer. However, for the large, troublesome deposits by far the bulk of the deposit consists of an outer sinter layer. The outer sinter layer is a coarse, brownish, strongly integrated deposit. It consists of semicontinuous fused skeletal material (the melt phase, often referred to as matrix material) with seemingly unreacted ash particles embedded in and bonded to the matrix. In contrast to the sulfate layer, which forms all around the tube, the sinter layers form only on the side of the tube facing the gas stream. The outer sinter layer sometimes has a glassy appearance, from which it was inferred that the matrix material was probably amorphous. However, X-ray diffraction has shown that the matrix is largely crystalline. The crystallinity likely contributes to the strength of the deposit. Virtually all of the elements in ash are also found in the outer sinter layer. The dominant elements affecting formation of the outer sinter layer matrix are calcium, sulfur, iron, and sodium [58]. For example, decomposition of calcium sulfate (e.g., as gypsum in the lignite) occurs around 1200~ the CaO and SO 3 are able to recombine at the growing surface of the deposit. Magnesium, aluminum, iron, and silicon are usually enriched compared to their concentrations in the original coal ash, while sodium and sulfur are depleted. Pilot-scale tests of Bienfait (Saskatchewan) lignite showed a selective deposition of ash components to produce a eutectic composition enriched in silicon, iron and sulfur, and depleted in calcium and magnesium relative to the lignite ash [ 156]. These deposits also were rich in water-soluble alkali sulfates. The ranking of elemental compositions is Ca >> A1,Si > Mg,Fe > Na,S. Calcium, magnesium and silicon are evenly distributed throughout the deposit, as are sodium and sulfur. Iron is present in discrete particles, sometimes seen as small balls of iron oxide. In some deposits these balls are surrounded by a ring in which sodium is at a much higher concentration than in the bulk of the deposit. Examination of fragments of outer sinter layer material shows that there are high concentrations of sodium and sulfur on the surface, with calcium, aluminum, and silicon concentrations higher in the core. Calcium sulfate is also occasionally seen as a crystalline coating, possibly as a result of CaO and

565 SO3 (initially produced by thermal decomposition of gypsum in the lignite) recombination at the growing surface of the deposit. If vapor-phase sodium encounters a molten silicate or aluminosilicate grain, it will most likely react with the grain to form a variety of mineral species or glasses upon cooling [ 157]. If these particles collide with the ash deposit they may become part of the silicate-aluminosilicate matrix that binds the deposit. Sodium bound in clay or tectosilicate minerals will mainly be retained in the particle, although new minerals may be fomaed in reactions with the flue gas or by structural reordering Ul:X-mcooling from a molten state [ 158]. Several mechanisms transfer ash constituents from the vapor phase to the tube surface, including vapor phase diffusion, thermal diffusion, electrostatic interaction, and inertial impaction [48]. Inertial impaction is the principal mechanism, as suggested by the observation that deposits build up on the upstream face of the tube. Accumulation of the outer sinter layer will be affected by the rate of particle impaction and the retention of particles in the sticky layer. The efficiency of particle retention is influenced by the size, geometry, and melting behavior of the particles [48]. The inertial forces favoring impaction of an ash particle into the deposit are counteracted to some extent by fluid dynamic forces which keep ash particles moving with the flow streamlines around the tube. Of five coals of various ranks, a North Dakota lignite had the highest ash content and the highest sticking coefficient, but there was no general correlation of rank with sticking coefficient [159]. For self-regulated capture of super-micron ash to be enhanced by liquids, the inertial impaction rate depends on the steady-state sticking coefficient, s, but s in turn depends on the amount and physical properties of the liquid phase "glue" available to each particle in the surface layer of the deposit [ 159]. Values of s for steady-state deposition and of the particle deposition rate must be obtained from a set of coupled nonlinear equations. Iron, silicon, calcium, and especially aluminum all act to suppress "glue" formation. Predicted sticking values can be obtained by calculating the fraction of liquid phase by thermodynamic analysis and using the amount of glue as an input to the coupled nonlinear equations R-~rthe self-regulated capture model. The growth rates of ash deposits for North Dakota lignites in a drop-tube furnace are shown in Figures 11.18 through 11.21 [51]. The distinction between the Indian Head and Beulah on the one hand and Center and Velva on the other may arise from the higher concentration of sodium in the first pair. The sodium may form small particles more likely to follow the flow streamlines and thus less likely to be captured by the deposit. The accelerated growth rate of the White pit Gascoyne lignite is a result of the formation of abundant liquid at the top of the growing deposit. This liquid would create a captive surface that would retain most of the ash particles impacting it. The dependence of sticking fraction on time is illustrated in Figure 11.22 for San Miguel and Martin Lake lignites [51]. The San Miguel lignite has the higher sticking fraction. Zeolite minerals in San Miguel lignite contain significant amounts of sodium [160], which can reduce the melting temperature and enhance formation of a very sticky captive surface. In the Martin Lake lignite virtually 'all of the alkali and alkaline earth elements are associated with the carboxyl groups

566

.i,-I

9 Beulah

[~ 0.1-

9 Velva

or

A Indian Head A// 0.01 0

o

''''l''''l''''l''''l''''l''''l 5

10

15

20

25

Center

30

Time, minutes

Figure 11.18. Deposit weight in bench-scale drop-tube furnace combustion of North Dakota lignites as a function of time [51].

1

9 Beulah

0.9-

9 Velva

0.8a~

.r

0.7-

A Indian Head

0.6o ~

0.5

g

0.4

I~ 9

o.3

o

Center

.

i

0.2 0.1 0 0

''''1''''1''''1''''1''''1'''' 5

10

15

20

25

30

Time, minutes Figure 11.19. Sticking coefficients as a function of time for bench-scale drop-tube furnace combustion of North Dakota lignites [51]. Sticking coefficient is a ratio of deposition rate to rate of firing of ASTM ash.

567

,

Red pit 9

0.1-

White pit

A Blue pit

o

o

0.01

....

0

I''''1

5

....

10

I ....

15

I ....

20

Yellow pit

I ....

25

30

Time, minutes Figure 11.20. Deposit weight as a function of time in bench-scale drop-tube furnace combustion of Gascoyne lignites from different pits in the same mine [51].

0.9 ,

0.8 ~

Red pit 9 White pit

0.7

.r,,l

~

0.6"

o

0.5

A Blue pit o

.~ 0.4 ~9

~'1

Yellow pit

0.3" -

0.2 0.1

"

o

''''

0

I''''

5

I''''

I''''

lO 15 Time, minutes

I''''

20

25

Figure 11.21. Sticking coefficient as a function of time for bench-scale drop-tube furnace combustion of Gascoyne lignite from different pits [51].

568 1

.

0.9" a~

0.8

9 Martin Lake

0.7

9 San Miguel

..=.1

0.6 o

0.5"

.~ 0.4 ~9

o.3

0.2 II,'"

0.1-

A

0

'

0

' '

I"'

2

-

''~'

4

v

''

I'

6

''

I'

8

''

I'''

10

Time, minutes Figure 11.22. Sticking coefficient as a function of time for bench-scale combustion of Texas lignites [51].

rather than with aluminosilicate minerals. The factors that may affect the cohesion in the deposit include van der Waals interactions, liquid film effects, retention in a liquid matrix, or even trapping of particles by geometric effects, such as might be hypothesized for an ash particle being trapped in a mat of whiskery particles [48]. The factors which will determine whether a partial melt phase will form in the deposit include the fusion temperatures of the ash particles, the flue gas temperature, the boiler tube temperature, and the thickness of whatever ash layer has already been deposited. Once a melt phase has been formed, liquid-phase diffusion may also affect deposit growth [48,161]. The outer sinter layer is characterized by the presence of matrix material, which forms an effective, strong bond holding this layer together. Formation of the outer sinter layer depends on the temperature of the deposit surface becoming high enough either to allow the molten material striking the surface to CD'stallize slowly or to allow ash particles to react with some fluxing material in the gas phase (possibly sodium compounds) to form a new low-melting phase which can act as the matrix. The surface temperature rises because of the insulating effect of the inner sinter layer and interior regions of the outer sinter layer. The matrix consists of complex aluminosilicates, of which sodium melilite is a particularly important constituent [162]. Sodium melilite is a lowmelting material which could stay fluid long enough to react with other silicates, aluminosilicates, or sulfates and which could crystallize into a hard, strong deposit. The reaction anhydrite + quartz + AI,Fe,Na aluminosilicate glass --, melilite + Ca,Na,Mg,AI sulfate

569 has been postulated as a mechanism for the formation of melilite in the deposits [ 155]. Reaction of volatilized sodium with kaolinite particles can form sodium aluminosilicates melting in the range 90(O1100~

These compounds can react with molten sulfates in the accumulating ash deposit to

form complex melilites of the general formula (Na,Ca,K)2[Mg,Fe+2,Fe+3,A1,Si)307] [49]. The important role of sodium in these instances suggests a rough correlation: the more sodium in the coal, the more melilite which could form; the more melilite, the more matrix; and the more matrix, the worse the fouling problem. This is the chemical basis for the rule-of-thumb correlation between the amount of sodium in the coal and the severity of the fouling. Fly ash from Beulah lignite was heated to 90(O1000~ for 10 minutes, and then cooled to 5000C over a period of two hours [151] to approximate exposure of ash particles to the gas temperature entering the convection section of a pilot-scale combustor with subsequent cooling to the tube temperature. A weight loss of 1-2% was observed, in good agreement with the calculated weight of a 0.3 ~tm layer of sodium sulfate coating a 50 tan ash particle. The fly ash before heating was essentially amorphous, although traces of magnetite, magnesioferrite, and (Na0.8 Ca0.1)SO4 were observed. After heating, X-ray diffraction indicated the presence of melilite (gehlenite or sodium melilite), monticellite, and (Na0.8 Ca0.1)SO4. In igneous rock petrology, both melilite and monticellite are known to form in situations where high temperatures prevailed for a short time only, and cooling was relatively rapid [163]. Other crystalline phases which have been identified are pyroxene and plagioclase [164]. Crystallization of melilite, pyroxene, and plagioclase is most pronounced when the ash is rich in sodium and calcium. Crystallization is accompanied by formation of a residual liquid phase which may be the agent responsible for growth of the deposit and enhancement of deposit strength. If the residual liquid eventually solidifies into a glassy phase, then the relative amounts of crystalline and glassy material that form in a deposit may be important in determining deposit growth and strength [164]. Deposits collected on a test probe inserted into a boiler at the Hoot Lake station showed compositions of the matrix consistent with a mixture of sodium melilite, quartz, and a sodium aluminosilicate hypothesized to be NaAISi 206 [ 151]. Sulfur species were virtually non-existent in the matrix of these samples. The absence of sulfur species may be a result of the concentration being below detection limits or, in the case of X-ray diffraction, of the material being noncrystalline. X-ray diffraction is capable of identifying both sodium and calcium sulfate in a mixture of 2% of each of these salts with fly ash [151]. Crystalline melilite is the major strength component of the deposit [ 151]. The relative amount of melilite increases with increasing distance from the tube. Increased distance from the tube provides a longer time for cooling and therefore increased time for crystallization and for reactions of sodium with ash material to form melilite. Quartz particles can be quite large in lignite ash deposits [ 137]. Assimilation of quartz into a melt phase might be retarded because of slow reaction rates and, possibly, liquid-phase immiscibility. The slow assimilation of quartz results in the melt phase being more basic than would be predicted from bulk composition; in turn, the basicity of the melt enables crystallization

570 of phases such as melilite [ 137]. The gradual assimilation of quartz into the melt eventually allows the composition of the melt to shift toward a more acidic composition, and the major phase to be crystallizing becomes anorthite [ 137]. Quartz and clays are easily able to form glass at high temperatures. In the glass structures the SiO4-4 tetrahedra are joined by linkage of three corners. However, alkali or alkaline earth oxide addition provides a source of oxide ions that modify the glass structure because of the increased O/Si ratio. Disruption of the extensively linked glass structure by the added oxide ions (referred to in this context as "network modifiers") leads to crystallization of some of the components [165]. This argument is analogous to the role of oxide ions as "polymer breakers" in affecting the viscosity of slags. In the early 1970's the theory was advanced that organically bound cations react with finely disseminated clays or silica to form a type of ash particle called "matrix parent." The matrix parent particles deposited on the tube can then be fluxed with sodium to soften, flow, and collect and cement other ash particles to form the matrix. The flux was postulated to be sodium sulfate depositing from the vapor. Successful formation of matrix requires high temperature and long times to allow reactions to occur. According to this theory, the key variables affecting ash deposition would be the amount of sodium, the amount of matrix parent, and the melting properties of the reaction products. In recent years several pieces of evidence have been collected which relate closely to the original matrix parent theory. First, the plateau in the fouling

vs.

sodium content

curves can be shown to arise from straightforward stoichiometry. Above about 8% sodium oxide in the ash all of the available aluminum and silicon have been converted to sodium melilite, representing therefore the maximum amount of sodium melilite which can form. Since the relative amounts of sodium, calcium, aluminum, and silicon vary from coal to coal the point at which the curve levels off will vary also, in the range of about 6 to 10% sodium oxide. Second, studies with mixtures of kaolinite and sodium and calcium acetates (Chapter 6) suggest that an important reaction early in the combustion process is the reaction of organically bound alkali and alkaline earth elements with clay to form new phases. Analogous prcxzesses in lignite would give rise to the matrix parent. Third, tests with micronized coal have shown much less ash deposition than might be predicted for the same coal at a standard utility grind. A possible explanation is that the micronizing has removed more of the silica and clay particles from juxtaposition with the organically bound species, and thus there is less opportunity for the formation of matrix parent. X-ray diffraction indicated gehlenite, sodium melilite, and akermanite as principal phases constituting the matrix material. The amount of sodium melilite increases with increasing distance from the tube. X-ray diffraction of a deposit produced from Beulah lignite in a pilot-scale combustor showed a decrease or disappearance of quartz, anhydrite, and hematite in comparing the inner sinter layer with inner and outer zones of the outer sintcr layer [ 155]. (The inner zone of the outer sinter layer is transitional between the inner sinter layer and the outer layer of the outer sinter layer and contains a mixture of the constituents of those two layers.) In this same transition, from inner sinter layer to inner and then outer zones of the outer sinter layer, magnetite and melilite

$71 increased. In terms of bulk composition, silicon, aluminum, and iron show decreases in composition whereas calcium, magnesium, sodium and sulfur increase in an outward direction through the deposit [1551]. An iron-rich zone has been observed as the outer transition from the inner to outer zones of the outer sinter layer; the iron-rich zone shows a change in the oxidation state of iron [155]. Electron microprobe analysis of deposits from six North Dakota lignites showed that high concentrations of iron may be necessary for the formation of strong deposits, unless the sodium content is also extremely high [ 145]. Deposits produced from Beulah, Decker (Montana), Velva, and a Texas lignite in a pilot scale combustor had, as dominant phases,wollastonite, gehlenite, and anorthite [166]. Occasional regions of high calcium and sulfur (presumably anhydrite) were observed, as were particles of quartz and particles having high iron contents. The Texas lignite deposit showed substantial numbers of ash "bubbles" in the deposit, which could have arisen either from cenosphere formation in the fly ash or fusion of clusters of particles forming a pocket of entrapped gas. Cenospheres were also observed in the deposit from the Velva lignite. Electron microscopy of deposits from an unidentified North Dakota lignite showed the primary components to be calcitnn aluminosilicates (55~3%), along with small amounts of alkali sulfates mixed with calcium sulfates [ 167]. Other components identified included calcium- and ironrich aluminosilicates, silica, hematite, a calciunl iron femte, and calcium magnesium sulfate [167]. Further analysis by potassium K-edge X-ray absorption spectroscopy revealed characteristic X-ray absorption near-edge structures (XANES) useful for identifying the sulfate species. XANES of a secondary superheater detx~sit from a boiler was very similar to that of potassium hydrogen sulfate [167]. A deposit formed at 1090~ in a test combustor had a XANES spectrum typical of a 60:40 mixture of potassium sulfate and a potassium alurninosilicate glass [ 167]. Deposits formed from a Texas lignite showed a matrix rich in calcium aluminosilicates which appeared to have formed as a result of reactions of calcium with aluminosilicate minerals [137]. The most common crystalline phases in the matrix are melilite, Ca2(AI,Mg,Fe,Si)2SiO7, and anorthite, CaAI2Si2C~. The melilite occurs in the less agglomerated areas of the matrix, while the anorthite occurs in the areas of greater agglomeration. As indicated above, quartz particle assimilation into a melt phase may be slowed by unfavorable reaction kinetics or by liquid-liquid immiscibility [ 137]. Deposits at the entrance or front of the convective pass are enriched in iron [125,161]. Ash deposited further away from the furnace enclosure had a diminished iron content, and ash collected at 430~

had less than half the iron content of the original coal ash. This segregation may be an

effect of particle density. Ash particles containing iron are among the most dense particles, and are more likely to impinge on furnace walls or tubes rather than to follow the flow streamlines. Furthermore, iron-rich ash particles are among the most adhesive and are thus more likely than alkali aluminosilicates to adhere to surfaces once they do impinge.

572 11.4.4 Growth of strength in deposits The strength of the deposits is variable and is a function of the composition, temperature, and age of the deposit [51]. The strength of sintcred ash specimens increases with corresponding increases in sintering time at any given temperature. Coals with high sodium levels and low ash values may produce small deposits but ones which are strong, while conversely low-sodium, highash coals may produce large but weak deposits which are easily removed. Time, composition, and temperature can all influence the crystallinity of the deposit; the crystallinity may in turn contribute to strength. The sintering strength of lignite ash is proportional to its sodium content [32]. This relationship is illustrated in Figure 11.23 [32]. There is also a direct relationship between the sinter strength of ash and the temperature [32]. For short periods of time (i.e., under 25 hours) the sinter strength does not change with time. However, lignite ashes increase in strength with longer periods of heating. Consequently the older (and thus nearer the tube) portions of a deposit on a superheater tube may be very hard while the leading edge is soft. 20

it,

16 14

d ~

12 1!

5-1

2 0

~ ....

I'"'l'"'i

....

I'"'l'"'l'"'

% Sodium oxide in ash Figure 11.23. Effect of sodium oxide content on the sintering strength of a North Dakota lignite ash [32].

Deposits from high-sodium lignites have a continuous melt phase that provides the material for a strongly bonded network in the deposit. In comparison, deposits from low-sodium lignites have no continuous melt phase, and seem to be held together only by particle-to-particle interactions. Deposit strength, expressed in units of MPa/mm, is shown as a function of sodium content (as Na20 in the ash) in Figure 11.24 [51]. The curve appears to level off around 8.8%

573 10

I

4,-'

0.1

. . . .

0

I

5

. . . .

I

10

'

'

'

'

I

....

15

20

Sodium oxide in ash, %

Figure 11.24. Ratio of deposit strength to height as a function of sodium oxide content for bench-scale droptube furnace combustion of various lignites [51].

Na20. When ash deposition ( e.g., weight of ash collected on probes in a pilot scale combustor) is plotted against sodium, that curve also tends to level off in the region of 8.0-8.5% Na20 [161]. The similarity suggests a correlation between the growth of strength in a deposit and the ability of the deposit to form a large mass on a boiler tube. For ash deposits produced in a laboratory-scale drop tube furnace, strength varies exponentially with the height at which the strength was measured [168]. This relationship had the form S = a exp(bH) where S is the strength and H the height. The values of a and b differ for different lignites, but for an 3, given lignite, the coefficient of determination, r2, for the exponential fit varied from 0.94 to 0.99. For the coefficient a, a negative Spearman rank correlation was found with calcium and aluminum and a positive correlation with sodium. For coefficient b, the same elements contributed to significant Spcarman rank correlations, but in the opposite sense, i.e., positive correlation with calciuln and aluminum and a negative correlation with sodium. The results confirm observations that calcium and sodiurn act as antagonists rather than acting together, and indicate that a relationship bctwecn gehlenite, CazAIzSiO7, and sodium melilite, NaCaAISi207, may be important in determining both the initial strength and the growth of strength of the deposits. An empirical characteristic of deposits, the relative rcfractory nature R, was defined as the mole fraction of SiO2 in the deposit to the mole fraction of SIC)-,_in the original ash of the lignite [168]. For deposits from differcnt lignites, as R increases the deposit is increasingly refractory relative to the ash from which it formed. For ash deposits formed in a drop-tube furnace, an

574 increase in R accompanies visual evidence that the top of the deposit has experienced less fusion, evidence for sintering in the lower portion of the deposit occurs further and further toward the base, and the base layer becomes thinner. In addition, when the strengths of a series of deposits are measured at a comparable height from the base, the strength decreases as R increases. For example, deposits produced from Indian Head and Velva lignites for which R = 1.00 and 1.67, respectively, showed that the strength measured 13 mm above the base was 7 MPa for the deposit from Indian Head but only 4.6 MPa for the Velva deposit [168]. In drop-tube furnace tests, the strongest deposits contained massive fused regions near the top, with abundant crystal growth [51]. Weak deposits are characterized by high porosity, suggestive of less extensive sintering or fusing of the deposit. With increased sintering, crystallization of aluminosilicates such as pyroxenes, melilites, and plagioclase from the melt begins. These aluminosilicates appeared to nucleate from regions of high iron content. Sodium was concentrated in the glass adjacent to crystals, suggesting that the development of deposit strength is due to viscous flow sintering and not, as suggested in earlier work [ 161] due to the formation of crystalline sodium melilite. Development of deposit strength being due to viscous flow sintering is consistent with the action of availability of alkali or alkaline earth elements to act as fluxes for aluminosilicates, and with the importance of gas temperature in the region of deposition as the primary factors affecting formation of strong deposits [51]. The degree of interaction between the consitiuents in a deposit is expressed to some extent by the formation of crystalline material. In drop-tube furnace tests some of the strongest deposits contained abundant pyroxene and melilite crystals [51]. In the strong deposits, sodium accumulated in the glass phase. The glass is primarily responsible for viscous flow sintering and thus for the development of deposit strength. The presence of sodium in the glass reduces its viscosity, improving the ability of the liquid to increase deposit strength via sintering. When strength development is due to viscous flow sintering, the sintering rate can be determined by Sr = kl(Rg/c)(1/V) [169] where Rg/c is the ratio of glass to crystalline material and V is the viscosity of the glass. Deposit strength is shown as a function of glass viscosity in Figure 11.25 [51]. Here the composition of the glass was determined by X-ray microprobe analysis and the viscosity then calculated from composition via the modified Urbain equation [170]. The inverse relationship of deposit strength and glass viscosity is consistent with viscous flow sintering theory. Sintering or melting of a deposit that contains abundant alkali or alkaline earth elements can produce highly crystalline deposits [164] containing melilite, plagioclase, and pyroxene. Crystallization is most pronounced for sodium- and calcium-rich deposits, but is accompanied by partitioning between the glassy and crystalline phases. For example, with deposits produced from Gascoyne lignite, sodium tends to concentrate in the glass phase [ 164]. As crystallization occurs, the residual liquid is responsible for the growth of size and of strength in the deposit.

575

7-

\

.

6-

4-

~9

3

o

2

'

'

'

I

200

'

'

'

I

'

'

'

400

I

600

'

'

'

800

Viscosity, Pa.s Figure 11.25. Strength of deposits produced in bench-scale drop-tube furnace combustion of low-rank coals as a function of the viscosity of the glass phase at 1300~ [51].

The quantity of liquid expected to exist at a given temperature can be predicted from thermodynamic calculations that minimize the free energy of the systern (e.g., [171]). Deposit strength plotted as a function of the predicted quantity of liquid is shown in Figure 11.26 [51]. As presumed from viscous flow sintering theory, deposit strength increases with increasing quantity of liquid.The formation of crystals could raise the viscosity of the residual liquid and retard the sintering rate. Additives that promote crystallization thus reduce the strength of the deposit.. The physical strength of the deposits increases as a result of continued exposure to sulfur oxides at high temperatures, a process known as sulfating [49]. The amount of anhydrite present was greater than the amount of melilites in sulfated deposits obtained from the Hoot Lake station [49]. 11.4.5 Remedial measures for combatting fouling and slagging The basic approaches for controlling fouling are conservative design of furnace height and area, to allow ample time for burnout thereby to minimize the furnace exit temperature; installation of an adequate number of sootblowers at spots likely to be troublesorne (and in fact to allow the ability for repositioning some of the sootblowers if necessary); to use fuel additives containing calcium or magnesium to reduce the fluxing ability of any molten ash phases, or containing aluminum to form high-melting point materials; and to limit the sodium content of the lignite by selective mining, blending, or ion-exchange [91. (i) Boiler design. The methods suggested for control of fouling include boiler design, sootblower placement, restricting the sodium level in the lignite, and using additives. For new

576

7-.

6"

9

3-

o =

1 ''''

0

0

I''''

0.1

I''''

0.2

I''''

0.3

I''''

0.4

0.5

Liquid-to-solid ratio

Figure 11.26. Strength of deposits produced in benchscale drop-tube furnace combustion of low-rank coals as a function of the predicted quantity of liquid in deposits [51].

construction, the most economically attractive option to combat fouling is modification of the boiler design to accommodate the high-fouling fuel [ 123]. The objective of conservative design is to limit the volumetric heat release rate to 250 MJ/h m3, about half the value which can be tolerated for a low-fouling fuel [9]. Further design aspects include ample spacing between the burners and between the tubes in the convection section, steeply sloping floors at the base of the convection section (to handle dislodged slag or ash deposits falling through the furnace), and large numbers of sootblowers. An example of conservative design practices is provided by the Antelope Valley station, which includes a furnace volume exceeding 2 lm3/MW; furnace exit gas temperature of 575~

minimum of 20 wall blowers and 23 soot blowers per 100 MW capacity; and tube spacings

of 60 cm in the secondary superheater, 23 cm in the reheater, 11 cm in the primary superheater, and 10 cm in the economizer [9]. Successful firing of high-fouling lignites eventually derives from conservative boiler design, particularly in the heat release rate [97]. The strategy is to produce very conservative designs which, in a sense, derate the boiler before it is built. A large height and large furnace volume provides a low volumetric heat release rate to give ample time for burnout of suspended particles and to have a low furnace exit gas temperature. For burning a high-fouling lignite, a conservative value of the volumetric heat release rate would be 268 MJ/m3 h [48]. This value will give a furnace exit gas temperature of about 1035~

In addition, wide spacing of tubes in the

convection section, steeply sloping floors under the superheaters (to help shed deposits into the main furnace) and ample provision for sootblowers can be incorporated into the boiler design.

577 While these design procedures can increase reliability and operability for combustion of high fouling lignites, they add about 15% to the capital cost of the boiler [48]. For new construction, boiler modification is economically attractive, since it provides cost savings in purchased replacement power during outages, and in operations and maintenance costs associated with additive injection systems or sootblowers. Recommended design practice for a new boiler burning a high-sodium North Dakota lignite would be the following: heat release at full load of 850 MJ/m2; plan heat release in the burner zone of 14.8-15.9 GJ/m2; gas velocities of 9 m/s in the platen superheater and 18 m/s in the convection reheater; 60 cm spacing in the superheater platen and 30 cm spacing elsewhere; and a division wall and upper furnace division panels in the furnace [123]. For high-fouling lignites, pulverized-coal firing appears preferable to cyclone firing. A good comparison is provided by Units 1 and 2 at the Leland Olds station. With both units burning the identical lignite, Unit 1 (pulverized-coal fired), has a smaller percentage loss and uses only steam sootblowing, whereas the cyclone-fired Unit 2 requires water sootblowers [97]. In addition, Leland Olds Unit 1 experiences less fouling than Big Stone, even though the Big Stone plant burns a lignite of lower sodium content [97]. (ii) Boiler load. Boiler load has a large effect on fouling. The most common practice to reduce problems associated with ash fouling is reduction of load on the boiler [ 123]. In severe cases, the unit must be derated or even shut down completely. It is preferable to reduce the nameplate rating of the boiler by a predetermined amount to assure continued reliable operation than to endure unplanned outages resulting from severe fouling. During an unscheduled outage, the cost of replacement power is at a premium, whereas with scheduled curtailments the replacement power can be negotiated in advance at a less-than-premium cost. A conservative estimate for deration to insure boiler availability and freedom from unscheduled outages is 23% [ 123]. Load reduction is a costly alternative to combatting fouling. However, since load reduction is preferable to paying a premium for replacement power during unscheduled outages, load reduction is the recommended alternative unless more sophisticated approaches can be taken. (iii) Fuel additives. Additive injection is also an economically attractive approach to combatting fouling [ 123]. For existing power plants, a combination of sootblowing and additive injection is the most economically competitive approach. New plants should have appropriately designed (i.e., conservative) furnaces with sootblowers installed, and should reasonably be expected to give satisfactory performance with high fouling lignite. Should any other actions be necessary, additive injection would be the most attractive economically. X-ray diffraction the matrix has shown high concentrations of sodium melilite, NaCaA1Si207 [ 172]. This material melts in the range of 1120-1200~

The prospect of forming

higher-melting phases and thus possibly altering the melting characteristics of the matrix is a basis to evaluate additives to reduce fouling [ 172]. Replacing the sodium with calcium should produce gehlenite, Ca2AlzSiO7, which has a melting point of 1600~

The use of calcium and magnesium

S78 to replace sodium would produce akermanite, Ca2MgSi207, which melts at about 14400C. Therefore addition of calcium or magnesium could reduce fouling by increasing the melting temperature of the matrix material [172]. The most promising fuel additives seem to be calcium or magnesium compounds or minerals containing these elements. Presumably the additive acts by converting sodium melilite either to gehlenite or to akermanite [88]. In pilot-scale combustion tests of a high-fouling lignite, pulsed injection of additives did not significantly affect the rate of deposition, but did reduce the deposit strength, comparable to that obtained by lowering the flue gas temperature from 1095~ to 980~ without use of additives. A survey of about twenty fuel additives showed that monticellite, Niobrara shale, calcium carbonate, and magnesium oxide appeared to have the most potential to reduce deposit strength [99], based on measurements of the friability (ASTM tumbler test D-44145) of ash deposits produced in a pilot scale combustor firing Beulah lignite. To meet the criteria of having low deposit weight and strength with high friability, the best additive was calcium carbonate, injected at a rate of 2.6 kg/t of lignite [172]. A deposit produced while injecting magnesium oxide at a dosage of 3.8 kg/t had a friability of 80.6%, the weakest deposit produced in the test series. Pilot-scale tests with magnesium oxide using an intermittent injection equivalent to 50 kg additive/t for 30 second intervals every six minutes reduced ash deposit weights by 35% [99]. Dosage of 50 kg/t at this interval is equivalent to a steady injection dosage of 2.5--5 kg/t. In full-scale tests, addition oi 270 kg calcium carbonate, in the form of Number 6 poultry grit, every four hours to a 216 MW wall-fired pulverized-lignite unit (Leland Olds Unit 1) proved to be very successful in maintaining a clean boiler [ 172]. Tests in a 53 MW tangential-fired and a 79 MW wall-fired pc units produced some improvement in operation and facilitated cleanup after a shutdown. Excellent results were achieved with the addition of lime or limestone at 23 kg every two hours (1.2 kg limestone/t lignite) in a 15 MW spreader-stoker unit [172]. Results are not as promising in cyclone-fired units. In the Leland Olds Unit 2, a 440 MW cyclone firing the identical coal as in Unit 1, addition of pulverized limestone at 540 kg every two hours was not particularly successful, an experience attributed to the enrichment of sodium in the ash deposits in cyclone boilers [172]. Similarly, there was no apparent effect on operability or cleanability of the Big Stone plant, which is also a 440 MW cyclone [172]. Limestone has been used successfully to reduce fouling in full-scale boilers fired with high-sodium Saskatchewan lignite. The success of the limestone additive is to decre~tse the compressive strength of the deposits, making them more easily removed by soot blowing [173]. The first commercial-scale experience which contradicted the then-prevailing conventional wisdom that additive injection v,,ould be costly and troublesome was obtained by Saskatchewan Power Corporation in 1977, demonstrating positive results of periodic shot-dosing calcium carbonate during the firing of a high-sodium Estevan (Saskatchewan) lignite [88,134]. Increasing the calcium or magnesium content (and, to a lesser extent, the aluminum content) increased the ash softening temperature [134]. A successful method was developed for shot-dosing calcium carbonate grit into the pulverizer inlet to prevent buildup of slag or fouling deposits and to soften

579 existing deposits. A boiler rated at 66 MW but reduced in load to 60 MW because of fouling deposits was restored in only 8 h to operation at 65 MW by lime addition. Routine addition of calcium carbonate reduced both the length and the severity of boiler cleaning during overhaul. With calcium carbonate addition, superheater and economizer deposits are more easily removed. Double dosing with calcium carbonate at the onset of observed deposition lowered the gas temperature and increased steam output. These results were substantiated by experience at Basin Electric Cooperative and Otter Tail Power Company. The remedial action of lime- or limestone-based additives appears to be limited to pulverized-lignite-fired units, and does not extend to cyclones. The range of conditions for best results with calcium-based additive injection appears to be 0.5-2 kg of additive per tonne of lignite, 1-4 hour intervals, and injection times of 15 s to 5 min. [88]. Injection of "LiquiMag" (a suspension of magnesium oxide in fuel oil) at a rate of 0.15 L/t substantially reduced ash deposition in plants operated by Winnipeg Hydro [174]. The plant in Winnipeg, Manitoba has two Babcock and Wilcox pulverized-lignite-fired boilers rated at 21 and 38 kg/s steam, and burn high-sodium lignite from Saskatchewan. Pilot-scale tests with Saskatchewan and James Bay (Ontario) lignites showed that injection of either magnesium oxide or dolomite ahead of the screen tubes could sufficiently alter the deposit structure to improve the prospects for removal by sootblowing [156]. Vermiculite has been used successfully as a fireside additive to reduce fouling in Unit 2 of the Leland Olds Station of Basin Electric Power Cooperative [ 175]. It is injected into the furnace on the front wall at the same elevation as the arch. As the vermiculite is heated by the gas stream, the particles expand and are swept into the superheater pendants by the gas. The expanded particles become incorporated into the deposits, increasing the friability to increase the susceptibility of the deposits to removal by sootblowing. Without vermiculite addition, the deposits in this boiler have been described as being "rock-like" [ 175]. Vermiculite addition rates of up to 2 kg/t of lignite have been used, the necessary feed rate varying with boiler load [175]. High temperatures attendant on sustained operation at high load can overwhelm any possible beneficial effect of vermiculite injection. Injection of vermiculite is not of itself adequate to eliminate deposition [9,175]. Other tactics include limiting the gas temperature to 1065~ using gas recirculation fans to temper the gas stream at high load; and selective sootblowing to concentrate the cleaning on problem areas in the superheater region. After the vermiculite addition program was adopted, only one outage for deslagging was necessary in 27 months, whereas before vermiculite addition 31 deslagging outages had occurred in 45 months [175]. Other benefits include improved heat transfer from the superheater; reduced sootblowing, rodding, and shotgunning to remove deposits; reduced shutdown, startup and (overtime) labor costs; and improved safety [ 175]. Perlite has also been tested in analogous fashion, also in the Leland Olds Unit 2. Perlite provides some benefits, but it is not as effective as vermiculite [ 175]. A small portion of alkalis vaporizes from clays as they convert to alkali and alkaline earth silicates in the temperature range 10(0)-1300~ [58]. The vaporization process might be retarded by using heavy metal additives, the reduction in vaporization of alkali translating to a reduction in

580 deposit formation [58]. (iv) Sootblowing. Economic analysis of potential solutions has shown, for existing plants, the most economical option is a combination of sootblowing with additive addition [123]. Sootblowers physically remove accumulated ash and slag deposits. In some cases it may be difficult to predict the exact locations in the boiler where fouling or slagging problems may occur; because of this, the necessary sootblowers may be mounted in the wrong locations or may even be lacking completely. In those cases, a retrofit relocation (or addition) of sootblowers is necessary. The need for sootblowing may be determined by monitoring the steam temperature; if the steam temperature is too low, this may indicate that the superheater tubes are fouled and are in need of cleaning. (An ash deposit 3 mm thick can reduce heat transfer to boiler tubes by 50% [123].) Alternatively, the low steam temperature may indicate that the gas temperatures are too low. The increase in the size of the boilers, as well as an increase in the number of sootblowers installed per boiler, has in some cases placed constraints on the number of sootblowing cycles per day due to limitations in the air or steam supply system for the sootblowers. An increase in the number of cycles per day can be attained by increasing the travel time of the sootblowers. For long retractable sootblowers, cleaning ability does not deteriorate even when the travel speed is doubled [123]. This finding negates the previously held concept that cleaning was proportional to nozzle dwell time. By establishing an appropriate sootblowing cleaning pattern on the furnace walls and superheater screens, it is possible to burn high-fouling lignites successfully. For existing plants, sootblowing can be an economically attractive option to combatting fouling [ 123]. The amount of water needed to produce sufficient thermal shock to break a deposit off a tube can be estimated from the viscosity-temperature behavior of the ash. Many lignite ashes have non-Newtonian (plastic) viscosities over a fairly narrow range of temperature, generally a range of 40-85"C. The non-Newtonian range also occurs at low temperatures, often below 1200~

[32].

These deposits will require much less water to cool to a point where they can be broken off the tube with a water lance or water blower than would the deposit from an ash having a plastic viscosity over a large temperature range. The mechanical or physical properties of ash at high temperatures seems to be a neglected area of research. More information is needed in the susceptibility of different kinds of ash deposits to sootblowing. Most units would eventually plug without some degree of sootblowing. However, at the Big Stone and Leland Olds plants a major problem with fouling is not so much the rate of deposition as the tenacity with which the deposit adheres to the tubes [97]. (v) Fine gri, ding ("micro, izi,g"). Comparison of deposition using a standard utility grind

(i.e., 80% _-98%), with efficiencies of about 85% for subbituminous coals and about 60% for anthracites [28]. The lower reactivity of high-rank coal chars, combined with the very short residence times of entrained flow gasifiers, are the factors responsible for the superior performance of lignites. 12.1.4 Pilot-scale gasification of lignites This section discusses some of the processes that have been tested at the pilot-plant scale, generally in continuous flow reactors, for the gasification of lignites. (i) The C02 Acceptor Process. The CO2 Acceptor Process is a fluidized-bed gasification

625 system designed to convert lignite or subbituminous coal into a substitute natural gas having a calorific value of 33.5-35.4 MJ/m3 [64]. Development of the CO2 Acceptor Process by the Consolidation Coal Company began in the early 1960's. The novelty of the process is that it uses the exothermic carbonation reaction of calcined limestone or dolomite with carbon dioxide to provide the heat necessary for the gasification process. The limestone or dolomite is re-calcined in a separate fluidized-bed reactor heated with char. The C O 2 Acceptor Process produces a substitute natural gas without using oxygen. Carbon utilization is over 99% [28,45]. The cold gas thermal efficiency is 77% [45]. Hydrocarbons other than methane are not present in the substitute natural gas, and concentrations of carbon dioxide and hydrogen sulfide are minimal. In addition, the H2/CO ratio in the product gas is greater than 3, eliminating the need for shift conversion prior to methanation. Since the steam-carbon reaction is endothermic, any gasification process must have a strategy for supplying enough heat to overcome the endothermicity of the steam-carbon reaction. The unique feature of the CO2 Acceptor Process was the use of the reaction between calcined dolomite or limestone (literally, the "CO2 acceptor") and carbon dioxide to serve as the exothermic heat source. At the same time, the capture, or acceptance, of carbon dioxide removes much of this gas and thus reduces the burden on downstream gas purification operations. The CO2 Acceptor Process used two fluidized bed reactors. In the first, the devolatizer/gasifier, lignite that has been ground (pre-screened to 8x100 mesh [45]), dried, and preheated was reacted with steam in the presence of hot, calcined dolomite. Operating conditions were 815~ and 1-2 MPa [64,65]. (This reactor temperature limits operation to low-rank coals, since feedstocks must have high reactivity at this temperature [45].) The lignite enters a bed of lignite char. The acceptor is then "showered" downward through the bed, and collects in the lower section of the gasifier, called the "boot." The heat needed to overcome the endothermic steamcarbon reaction was derived from the heat of recarbonization of the calcined acceptor and partly from the sensible heat it supplied. In the second reactor, the regenerator, char remaining from the partial gasification of the lignite was burned at about 1040~

[65] to supply the heat necessary to

calcine the spent acceptor. Since the gasifier uses no air (other than at start-up) the product gas has essentially no nitrogen. A flowsheet of the CO2 Acceptor Process is shown in Figure 12.11 [66]. The lignite is reduced to 8x100 mesh and dried to 5% moisture [64]. Lignite is fed into the bottom of the gasifier, where it experiences rapid hydrodevolatilization, followed by steam gasification of the char. The purpose of feeding the lignite deep into the bed is to provide the maximum retention time for volatiles, to allow them to crack to methane, hydrogen, and the carbon oxides in the bed. Steam is injected at the bottom of the gasifier. The char bed depth is about 7.6 m [64]. Heat for the endothermic gasification reaction comes from reaction of the acceptor (either calcined limestone or dolomite) with carbon dioxide. The reaction of calcium oxide with carbon dioxide liberates 177 kJ/mol at 250C [66]. The gasification rate is 0.0040 kg C gasified/kg C in the bed/min at 840~

626

"- FLUE GAS

I~ ASH

9

~

GAS

1-,

D \

~ ,

LIGNITE

~

GAS

MAKE-UP ACCEPTOR;,.d" "-I

MgO-CaC03

STEAM and AIR

L

LII~ GAS

S

ik REIECT ACCEPTOR

Figure 12.11. Block flow diagram of the CO2 Acceptor process [66].

[66].The purpose of the regenerator is to remove carbon dioxide from the spent acceptor. The regenerator operates at 1010~

heat is supplied by burning the residual char from the gasifier. The

acceptor gradually loses reactivity after multiple passes through the system. Some of the acceptor inventory is withdrawn from the gasifier and replaced by fresh makeup. The makeup is added as the acceptor is being returned to the regenerator. There is little evidence for ash fusion in the regenerator. Some problems were encountered as a result of partial conversion of calcium sulfide, present in char ash, to calcium sulfate, by reaction with either oxygen or carbon dioxide (or both). Above 925"C a mixture of calcium sulfide and calcium sulfate melts, the melt then trapping char ash and acceptor fines [64]. The sulfide and sulfate can react with each other to produce calcium oxide and sulfur dioxide; the calcium oxide resolidifies as a deposit on the wall. Devolatilization and gasification reactions in the gasifier consume about 63% of the carbon in the lignite [66].Total consumption of carbon in the process amounted to 95.6%, with 0.7% in the ash leaving the regenerator and 3.7% lost through the gasifier cyclone [66]. The product gas has a H2/CO ratio of 3.6. Since a value of 3 is needed for methanation, the excess hydrogen could be used to methanate some of the carbon dioxide. The only hydrocarbon in the product gas was

627 methane, obviating the need for removal of tars and oils. The only gas clean-up needed is removal of ammonia, hydrogen sulfide, and a portion of the carbon dioxide. The gasifier product gas flows through a steam-generating heat exchanger, a quench tower, a scrubber, acid gas removal system, and then into a packed-bed methanator. The raw product gas typically has very high hydrogen content and relatively high methane. The waste products from the CO2 Acceptor Process are the ash, spent limestone or dolomite acceptor, acid gases, and some sour water. Operating data obtained with an unspecified lignite feedstock are summarized in Table 12.3 [281. TABLE 12.3 Operating data for CO2 Acceptor Process [28] Gas Composition, mole % Carbon monoxide Carbon dioxide Hydrogen Methane Hydrogen sulfide + carbonyl sulfide Nitrogen + argon Raw Gas Heating Value, MJ/m3 Feed Requirements, kg/kg lignite Air (regenerator) Steam (gasifier) Operating Conditions Pressure, MPa Temperatures, ~ Regenerator Gasifier Gas velocities, m/s Cold Gas Thermal Efficiency, %

15.5 9.1 58.8 13.7 0.0 2.9 14.3 2.3 1.1 1 1010 815 0.55- 0.73 77

Among the advantages claimed for the C O 2 Acceptor Process are the ability to use fines in the gasifier, elimination of the need for oxygen, a lower water requirement than the Lurgi dry-ash gasifier, elimination of production of tars and oils due to the high operating temperatures, and minimal contamination by carbon dioxide, hydrogen sulfide, and higher hydrocarbons [28]. Water gas shift is not necessary if the product is to be methanated for pipeline gas production. The capture of significant amounts of CO2, along with H 2S, in the devolatilizer means that only small amounts of these gases need to be removed during a final gas purification. A disadvantage of the process is the need to dry the lignite to about 5% moisture, this being an expensive and difficult operation. Other disadvantages recognized for the process were 1) very low methane concentrations in the product gas require a large separate methanation operation; 2) the relatively low pressures of operation require compression of the gas for introduction to a pipeline; and 3) running two fluidized bed reactors in synchronization is a complex process and may have potential

628 problems in controlling the operation. The CO2 Acceptor Process was tested in a 36 t/day pilot plant in Rapid City, South Dakota. By 1975 the plant had been operated in a test in which the recycling acceptor supplied the entire heat requirements of the gasifier for 171 hours and an overall process efficiency of 77% was obtained [66]. The feedstock for this test was Velva lignite. (ii) The Wellman-Galusha gasifier. The Wellman-Galusha gasifier is an atmospheric pressure reactor that can be operated with either steam/air or steam/oxygen mixtures as the gasifying medium. A typical Wellman-Galusha gasifier is a cylindrical vessel 0.5-3 m in diameter [28]. The gasifier is refractory lined or cooled by a water jacket. Coal is fed through vertical feed pipes; the gasification medium is introduced through the bottom of the bed. The product gas exits the top of the gasifier at temperatures in the range of 480-650~ [28], though some temperatures as low as 200~ have been reported [67]. After removal of entrained particulate, the gas can be used directly as a fuel gas. The high moisture level of low-rank coals can cause problems in handling the crushed coal, and will lead to increased production of tar liquor. On the other hand, the high moisture level in the feed coal can reduce the temperature of the exit gas. Low-rank coals which have ashes of low softening temperature will certainly require careful temperature control. They may also require injection of additional steam to keep temperatures in the reaction zone below the fusion temperatures. If additional steam injection is required, gasification efficiency will be reduced and the production of tar liquor will be increased. The Wellman-Galusha gasifier is a countercurrent-flow, fixed-bed unit. The gasifier can be operated at atmospheric or elevated pressure and with air or oxygen blowing. Lignite is fed from a lock hopper into the gasifier, where, as typical of fixed-bed gasification processes, the lignite is successively dried, devolatilized, gasified, and combusted as it descends through the vessel. The feedstock size range for optimum performance is 50x6 mm [67]. If the amount of -6 mm material in the feed is large, some problems can be encountered during operation. Ash is removed through a grate. For an air-blown, atmospheric-pressure unit, air saturated with water vapor is injected through the grate; by varying the saturation temperature (i.e., the steam/air ratio) it is possible to maintain the temperature of the combustion zone below the point at which ash fusion would begin. For any given feedstock the product gas composition is determined by the steam/air ratio of the blast, As this ratio increases, the proportions of hydrogen and carbon dioxide in the product gas increase, and the carbon monoxide decreases. This behavior reflects attainment of equilibrium in the water gas shift reaction inside the gasifier. The proportion of carbon dioxide in the product gas increases as the rank of the feedstock decreases. As rank decreases, more gaseous sulfurcontaining products appear as mercaptans rather than as hydrogen sulfide. With lignites, up to 20% of the sulfur can be appear in the products as mercaptans [67]. With air blowing at atmospheric pressure the ash is generally low in carbon. Usually less than 1% of the carbon in the feed remains in the ash [67]. The unconsumed carbon in the ash increases as the steam/air ratio increases. Since a greater amount of steam entering the combustion

629 zone will lower the temperature there, at high steam/air ratios the carbon conversion may be kinetically controlled. The alkaline nature of lignite ashes provides some sulfur-capturing ability, but even with Ca/S molar ratios of 2.0 the amount of feedstock sulfur appearing in the ash never exceeded 25% [67]. Pilot-scale tests of lignites have been conducted in an air-blown atmospheric pressure unit [67]. Indian Head lignite gasified well in the Wellman-Galusha unit, and provided very high throughputs. However, the friability of this lignite resulted in some size degradation during handling. An unidentified Texas lignite provided the same problems with friability, and, in addition, caused problems in controlling ash removal because of a highly variable ash content. Moderate throughputs were achieved, although the quality of the product gas was very high. Benton (Arkansas) lignite also gasified well with very high throughputs. With this lignite the throughput was limited by the ability to supply steam to the gasifier. The friability of this lignite seemed markedly dependent on moisture, the lignite being very hard while wet, but very friable when dry. Some performance data obtained with Benton lignite are shown in Table 12.4 [67]. TABLE 12.4 Process behavior of Benton lignite in air-blown Wellman-Galusha gasifier [67]. Dry gas higher heating value, MJ/m3 Water in gas, kg/m3 Tar in gas, kg/m3 Tar yield, % of coal weight Hot, raw gas efficiency, % Maximum throughput, kg/h. m2

6.5 0.62 0.1(3" 11" 97 913

*Estimated

Of seven fuels tested, including four subbituminous coals and two peats, Benton lignite showed the highest maximum throughput rate. The limitation to the throughput rate was the capacity to deliver steam to the gasifier. Indian Head lignite has also been gasified in a Wellman-Galusha unit modified to operate at pressures to 2 MPa with oxygen blowing [68]. The results of these tests, which provide a comparison of air and oxygen blowing, and shown in Tables 12.5 and 12.6 [68]. (iii)

Slagging fixed-bed gasification. The

slagging, fixed-bed gasification of lignite was

examined in a pilot-scale reactor at the Bureau of Mines and Department of Energy laboratory in Grand Forks, North Dakota (now the Energy and Environmental Research Center of the University of North Dakota). The pilot-scale gasifier was the only one of its type in North America. Although the conventional Lurgi fixed-bed gasifier is successful on a commercial scale, a disadvantage of the conventional ("dry ash") Lurgi is that considerable amounts of excess steam must be fed to the unit to maintain the hearth temperature below the ash fusion temperature of the

630 TABLE 12.5 Process behavior of Indian Head lignite in pressurized Wellman-Galusha gasifier [68]. Oxygen-blown 182 Exit gas temperature, ~ 1.0 Gasifier pressure, MPa Steam/lignite, kg/kg 0.73 0.29 Air or oxygen/lignite, kg/kg 2.50 Steam/air or oxygen, kg/kg 46 Ash, kg/t lignite 20.7 Dust, kg/t lignite 2.22 Raw gas, Mm3/t lignite 12.2 Tars and oils, kg/t lignite 60 Illuminants, kg/t lignite Raw gas, Mm3/h 3.56 43 Water in raw gas, volume % 6.59 Raw gas calorific value, M.l/m3 10.8 Product gas calorific value, MJ/m3* 78 Raw gas efficiency, %

Air-blown 300 1.7 0.29 1.44 0.20 41 20.2 2.72 16.7 62 4.52 24 5.36 6.18 88

*After cleaning

TABLE 12.6 Average raw gas composition (vol. percent) from Wellman-Galusha gasification of Indian Head lignite [68].

Hydrogen Carbon dioxide Carbon monoxide Nitrogen Methane Hydrocarbon gases Tars and oils Water Hydrogen sulfide

Oxygen-blown 22.5 16.5 13.5 0.5 2.5 0.8 0.1 43 0.21

Air- blown 16.5 10.5 11.5 35.5 2 0.8 0.1 24 0.17

coal. Most of this excess steam serves only as a diluent to control hearth temperatures. It passes through the gasifier without reacting, and on condensation serves to dilute the gas liquor produced as by-product. Alternatively, the oxygen concentration in the steam-oxygen mixture can be increased to a point at which hearth temperatures are high enough to melt the ash, thus resulting in slagging rather than dry ash operation. Slagging operation significantly reduces the amount of steam that must be generated for operation. In turn, this reduces the volume of gas liquor and increases the concentration of dissolved by-products in this stream. Slagging operation also offers significant improvements in the rate of gas production per unit cross-section area of the gasifier.

631 The unit was able to operate at oxygen/steam molar ratios of 0.7 to 1.2 [34]. The limiting factors were too low a hearth temperature (and hence a slag viscosity too high for good operability in a continuous tapping mode) at 0.7 oxygen/steam ratio; and vaporization of ash components, causing fuel bed bridging, at 1.2 oxygen/steam ratio [34]. The gas production rate and the load on the gasifier depend entirely on the rate of oxygen supply [69]. For lignites, the maximum gasification rate at 2.8 MPa was determined to be 9,800 m3/h.m2 [69]. Operations were conducted at various operating pressures in the range 0.6--2.8 MPa. The pressure dependence of oxygen and fuel requirements per unit gas production are shown in Figure 12.12 [69]. Pressure had little effect on gas production rate and steam, oxygen, and lignite consumption [70]. The product gas composition is shown as a function of pressure in Figure 12.13 [69]. Generally, increasing operating pressures increased the carbon dioxide and methane contents of the gas and decreased carbon monoxide [70]. Operating pressure did not affect wastewater production appreciably, the main parameter affecting water production being the lignite moisture content [70]. 600 t"o

AA

~500

A

A

+ Oxygen

A

9 Steam

~" 4 0 0 -

A Fuel (maf)

.~ 300-

+ +

+

t#

2009

9

;

100 O" ' ' ' ' 1 .... 0 0.5

I ....

1

I''''1

1.5

....

2

i ....

2.5

3

Pressure, MPa Figure 12.12. Material requirements per unit gas production as a function of pressure in slagging fixed-bed gasification of lignite [69].

Operation with Indian Head lignite indicated that fuel rate, product gas rate, and oxygen and steam consumption were not significantly affected by pressure when comparing tests at 1.4 and 2.8 MPa [71]. The oxygen and steam requirements per amount of (CO + H2) produced were higher at the higher pressure. Cold gas efficiency and operational efficiency also increased with pressure. The cold gas efficiency is the ratio of potential heat in the product gas to potential heat of the fuel, usually expressed as a percentage [71]. The operational efficiency is the ratio of the sum of potential heats of product gas and tar to that of the fuel, usually expressed as a percentage [71].

632 80

+ CO

70"

9 1+

o

+

!

4

A CO2

50-

~ CH4

~ 40o

H2

30

9 9

9

9

O 2010, , , , i , , , , i , , , , i , , , , i , , , , i , , , ,

0

0

0.5

1

1.5

2

2.5

3

Pressure, MPa Figure 12.13. Product gas composition as a function of pressure in slagging fixed-bed gasification of lignite [69].

A comparison of the results from this pilot-scale unit with three other gasifiers is provided in Table 12.7 [69]. The lignite gasified in the Grand Forks unit was not identified in the original literature, but was very likely either Baukol-Noonan (North Dakota) or Velva. The major differences between slagging and non-slagging operation shown in Table 12.7 can be attributed to the lower steam consumption of the former, and the consequent higher capacity per unit hearth area. Fuel consumption and gas production rates are higher by factors of 3 to 6 for slagging gasifiers relative to non-slagging units. Operating data for four different North Dakota lignites are summarized in Table 12.8 [27,69]. Summary results on three of these lignites have also been published [70]. In comparing these data it should be noted that the data for Velva lignite [69] were obtained at a different pressure and oxygen rate (as well as being obtained about 15 years earlier) than the data for the other three lignites [27]. The effects of pressure on operation with a single lignite (Indian Head) are illustrated by the data in Table 12.9 [27]. The moisture content of the lignite feed is considered to be a limiting factor in operation. Operations with Gascoyne lignite, of 40% moisture, experienced significant difficulties attributed to the heat produced in gasification and combustion reactions deep within the fuel bed being insufficient to vaporize all the moisture in the incoming coal [27]. The effect is a steady reduction of temperatures in the gasifier hearth with eventual cessation of slag flow. However, entirely successful tests were performed with Indian Head lignite at 38.6% moisture [27]. A much more serious problem is bridging of the lignite particles in the upper portion of the fuel bed. The bridging causes the combustion and gasfication reaction zone to migrate upward, depriving the

633 TABLE 12.7. Comparison of slagging fixed-bed gasification of lignite with other fixed-bed gasification processes [69].

I_xxzation Grand Forks Type of operation Slagging O2/steam molar ratio 0.9 Operating pressure, MPa 2.8 Hearth area, m2 0.14 Fuel Rank Lignite Size, mm 19x10 Ash, % 6.9 Moisture, % 22.7 Calorific value, MJ/kg 20.2 Gas composition (N2 free) CO2 5.9 Illuminants 0.5 O2 0.4 H2 29.0 CO 58.7 C2H6 0.3 CH4 5.2 Gas calorific value, MJ/m3 13.1 Fuel rate, t (maf)/m2 hearth area. h 4.3 Gas rate, m3/m2 hearth area. h 7960 Oxygen consumption m3/Mm3 gas 192 m3/GJ of product gas 14.6 m3/Mm3 (CO + H2) 223 m3/kg maf fuel 0.30 Steam consumption kg/Mm3 gas 193 kg/GJ of product gas 12.3 kg/Mm3 (CO + H2) 224 kg/kg maf fuel 0.30 Fuel consumption, maf kg/Mm3 gas 640 kg/GJ gas produced 41.0 kg/Mm3 (CO+H2) 743 MJ fuel/Mm3 gas produced 1.56 MJ fuel/MJ gas produced 1.18 Cold gas efficiency, % 84.4 Operational efficiency, % 94.2

Morwella Dry ash 0.11 2.8 4.9 Brown coal 60x55x36d 1.7 15.3 22.2 34.4 0.3 0.2 36.6 14.6 0.6 13.3 11.6 1.2 5280

Westfieldb Dry ash 0.20 2.4 5.9

Solihullc Slagging

hvB 32x10 14.6 15.6 22.4

hvB 38x25 4.9 13.8 25.4

2.1 0.66

24.9 1.1 0 40.3 24.9 e 8.8 11.7 1.2 2760

2.7 0.5 0.1 28.1 60.9 e 7.7 14.0 7.0 13000

120 10.3 235 0.19

155 13.2 238 0.29

205 14.7 230 0.32

966 69.7 1890 1.51

697 49.9 1070 1.31

187 11.3 210 0.29

642 46.3 1255 1.44 1.24 80.7 86.9

530 38.1 813 1.45 1.24 80.5 88.0

642 38.6 722 1.38 1.21 82.7 90.0

Notes: aVictoria, Australia. bScotland, cEngland, dThis fuel was in briquette form. eNot available.

634 TABLE 12.8 Comparative performance of four North Dakota lignites in slagging fixed-bed gasification [27,69] Lignite Moisture content, % Operating pressure, MPa Oxygen rate, m3/h Oxygen/steam mol ratio Test duration, h Fuel rate, kg/h Fuel rate kg (maf)/h Product gas rate, m3/h Slag rate, kg/h Oxygen consumption m3/Mm3 gas m3/Mm3 (CO + H2) m3/kg (maf) fuel Steam consumption kg/Mm3 gas kg/Mm3 (CO + H2) kg/kg (maf) fuel Product gas composition, % CO2 H2 CO C2-C4 hydrocarbons CH4

Baukol-Noonan 30.3 1.4 95 1.0 11.7 535 340 518 43

Gascoyne 40.5 1.4 95 1.0 3.6 619 322 544 30

Indian Head 34.7 1.4 95 1.0 8.9 573 339 525 24

Velva 37.2 2.8 119 0.9 4.7 661 393 600 22

185 212 0.28

175 202 0.30

182 212 0.28

198 234 0.30

168 191 0.25

158 182 0.14

164 192 0.25

193 235 0.30

6.4 30.5 57.3 0.7 5.1

7.7 31.3 56.1 0.6 4.3

8.4 32.5 53.6 0.6 4.9

7.4 28.8 56.4 0.5 6.9

hearth of heat and resulting in slag freezeing [72,73]. This problem is very likely a result of the unusually narrow (41 cm) diameter of the gasifier vessel in the configuration used for several of the pilot plant campaigns, and not a generic problem of slagging gasification of lignites [72]. Subsequent enlargement of the interior diameter to 56 cm substantially increased reliability of operation [73]. There appears at present to be no interest in construction of a commercial-scale slagging gasifier for lignites. A useful summary of design considerations to be taken into account should the process be scaled up is available [73]. (iv) The Riley gasifier. The Riley gasifier is also a fixed-bed unit, but has the novel feature that the entire fuel bed is caused to rotate slowly, as a means of insuring an even distribution of fuel in the bed [74]. The Riley gasifier features a very thin fuel bed, at most 1.4 m, designed mainly as a means of varying the coal particle heating rate to cope with gasification of caking coals. Some pilot-scale data have been obtained for gasification of a northern Great Plains lignite [74]. The lignite was sized 50x12 mm and fuel bed depth was 1.2 m. The air requirement was 2.44 kg air/kg lignite (daf basis), with a steam/air mass ratio of 0.18. These conditions provide a gas offtake temperature of 270"C. The gas yield was 3.5 m3/kg lignite (daf basis), with a calorific value of 6.2 MJ/m3. The gas contained 44.5 N2, 28.1 CO, 17.3 H2, 6.1 CO2, 1.5 CH4, with the balance

635 TABLE 12.9. Effects of operating pressure on slagging fixed-bed gasification of Indian Head lignite [27]. All data were obtained using an oxygen rate of 113 m3/h and oxygen/steam molar ratio of 1.0. Pressure, MPa Moisture in lignite, % Fuel rate, kg/h Fuel rate, kg (maf)/h Product gas rate m3/h Slag rate, kg/h Oxygen consumption m3/Mm3 gas m3/Mm3 (CO + H2) m3/kg (maf fuel) Steam consumption kg/Mm3 gas kg/Mm3 (CO + H2) kg/kg (maf fuel) Product gas consumption, % CO2 H2 CO C2-C4 hydrocarbons CH4

0.7 22.9 476 331 569 33

0.88 36.4 559 319 588 25

1.4 35.7 571 331 623 30

2.8 38.6 588 330 596 23

199 224 0.29

193 219 0.30

182 212 0.29

190 230 0.29

174 196 0.26

169 191 0.27

159 186 0.26

166 202 0.27

5.0 29.1 59.6 1.0 4.3

6.6 31.3 57.0 0.6 4.5

8.4 31.5 54.2 0.6 5.3

10.5 28.5 54.4 0.5 6.1

being higher hydrocarbons, COS and H2S, and inert gases. By-product tar formed in amount equivalent to 0.02 kg/kg lignite. (v)

Entrained-flow gasification.

Shell conducted a four-year pilot-scale program on

entrained-flow gasification at a facility in Deer Park, Texas. This campaign included operation for a total of 647 h on Texas lignite from the Alcoa mine at Sandow (Milam County), the most reactive of all coals tested in this unit [75]. Carbon conversion of greater than 99.0% was achieved, with cold gas efficiencies up to 80% [75,76]. Methane is the only organic compound surviving the gasification reaction; tars, phenols, or other hydrocarbons do not occur in the product streams. The acid gas system removed up to 99.8% of the sulfur species in the gas [75]. The feed, gasification, and solids removal systems of the Shell process are shown in Figure 12.14, modified from [75]. Details of the coal receiving and handling system and the gas and water treatment systems have also been published [75]. Some of the key results, obtained for gasification of two different batches of Sandow lignite (designated as high-ash and low-ash), are summarized in Table 12.10 [75,76].

636 Lignite

t

Steam v

..~smam

Syngas cooler

r

Slag lock hoppers Lignite feed vessel

I J~rnite v

---

~lag dewatering

Flyslag

~ Slag

Figure 12.14. Coal feed, gasification, and solids removal systems for pilot-plant Shell entrained-flow gasification (adapted from [75]).

TABLE 12.10 Operating results for Shell gasification of Sandow lignite [75,76].

Lignite fed to plant, t/d Oxygen/maf lignite ratio Offgas composition, dry, vol% Carbon monoxide Hydrogen Carbon dioxide H2S + COS N2 + Ar + CH4 Sweet syngas production Mass basis, kg/h HHV energy basis, GJ/h Sulfur removal from syngas, % Carbon conversion, % Cold gas efficiency, %HHV Slag production, kg/h Flyslag production, kg/h Acid gas, kg/h

High-ash lignite 334 0.877 60.59 28.20 5.38 0.71 5.08 12,060 155 99.1 99.7 78.8 2,210 410 1,690

Low-ash lignite 248 0.865 61.82 28.21 4.47 0.80 4.83 10,380 131 99.8 99.4 80.3

637

The relatively high carbon dioxide level in the product gas relates to the high oxygen content of the lignite. The high exit gas temperatures (1300-1400"C) facilitate high carbon conversion [44]. Problems at the mill hampered early operations with the Sandow lignite [75]. The mill feed chute plugged with wet fines. As a result, mill outages limited feed rates to the gasifer to 60% of design. These problems were subsequently overcome.The drying gas entering the mill was preheated to 370-4000C because of the high moisture content of the lignite. Despite these temperatures, the hydrocarbon concentration in the vent from the drying loop was less than 100 ppb [75]. The Shell process has been selected for a plan for a nominal 500 MW integrated gasification combined cycle plant planned for a Houston Power and Lighting site in Malakoff, Texas [77]. The plan is based on use of lignite with 15.1 MJ/kg calorific value from the Trinity mine. Various design cases have been considered, including two gasification trains with no coproduct, two gasification trains with methanol as a co-product, and three trains with ammonia and urea as co-products. In contrast to the Shell system, the Texaco and Dow entrained-flow gasifiers are fed with a coal/water slurry. Using high-moisture lignite efficiently in these gasifiers depends on hydrothermal dewatering

(e.g., hot-water drying, Chapter 10) to produce a dense slurry of high

energy content [44,45]. Otherwise, excessive moisture in the lignite, in addition to the water used as the slurry vehicle, limits gas yield and efficiencies [45]. For the Texaco gasifier, a minimum of 50% dry solids loading in the slurry is needed to achieve low oxygen demand and efficient operation [45]. The Riser Cracking process was developed as an entrained-flow hydropyrolysis that produced both gas and liquid products from a helical tube reactor [78]. The hydropyrolysis occurred at 760-816~ and 10.3-13.8 MPa, with a residence time of 2--4 s. Depending on reaction severity, the liquids could be used as aromatic feedstocks for chemicals production or blending stock for high-octane gasoline. Low-severity operation favors the latter. Reaction of lignite at 14 MPa system pressure (hydrogen partial pressure of 11.2 MPa) and coil outlet temperature of 760~ provided a carbon conversion of 47.7% [78]. This was achieved with a residence time of 3.1 s and hydrogen feed of 35.3% by weight of the maf lignite. Gaseous products, expressed as weight percent of the maf lignite, were 10.5 CH4, 5.2 C2H6, 1 C3H8, 0.3 other light hydrocarbon gases, and 12.0 carbon oxides. Hydrocarbon liquid yield amounted to 11% of maf lignite; of this amount of liquid, 53% could be considered 2050C endpoint gasoline. The major components of the liquid included benzene, toluene, naphthalene, and phenol; other components included xylenes, ethylbenzene, indenes, cresols, biphenyl, and quinoline. 12.1.5 Underground gasification of lignite During the so-called synfuels boom of the late 1970's much of the interest, and experimental effort, in underground gasification of coal was directed to the subbituminous coal

638 deposits of the western United States, especially in Wyoming. (A fine overview discussion of underground gasification has been published [79].) The deep-basin Texas lignites were also thought to have considerable promise for underground gasification [80]. In Texas, deep-basin seams 61-610 m below the surface having thickness greater than 1.5 m contain 31.2 Gt of lignite, representing three times as much energy as the proven oil and natural gas reserves in the state [80]. Three potential uses of gas were envisioned: a) low calorific value gas for mine-mouth generation of electricity; b) medium calorific value gas transported off-site for process heating and steam raising; and c) conversion of the gas, either on- or off-site, to chemicals, such as methanol. Carbon dioxide could be a by-product for use in enhanced oil recovery in Texas and Louisiana. The lower calorific value of lignite, relative to the subbituminous coals, results in a higher gasification cost, assuming other factors are equivalent [80]. Furthermore, the candidate lignite seams in Texas are thinner than the subbituminous seams in Wyoming. These two factors combine to require a greater number of injection and production wells to achieve the same gas production as from a thicker subbituminous seam. The drilling costs represent a significant fraction of the total cost for underground lignite gasification. The higher moisture content of the lignite is also a disadvantage, in part because the moisture participates in the gasification reaction, thus giving a higher steam/oxygen ratio than had been desired. This effect results in poor oxygen utilization and poor gas quality [81 ]. Underground lignite gasification conducted near Fairfield, Bryan, and Alcoa, Texas in 1976-79 showed that air blowing produced gas of 3.2-3.7 MJ/m3 calorific value [80]. Oxygen blowing produced gas with calorific value of 8.5 MJ/rrd at a thermal efficiency of 60-70% [80]. 12.1.6 Biological gasification of lignites Bacterial enzymes break down the macromolecular structures of lignites to lower molecular weight products. Soluble material extracted from Vermont lignite with 1M sodium hydroxide, and subsequently precipitated at pH 5.5, had an apparent molecular weight (by gel permeation chromatography) of 167,000-174,000 daltons [82]. Treatment with aerobic, Gram-positive Bacillus or any of three strains of aerobic Gram-negative bacteria for 1-4 h produced cell-free

culture filtrates with a sharp chromatogram peak at 113,000 daltons [82]. The bacterial enzymes cleave ether or ester linkages. The breakdown of the structure may facilitate further conversion of the lignite to desired gaseous or liquid products. Microbial solubilization of lignite depends on the extent of oxidation of the lignite [83]. Increased solubilization correlates with increases in the oxygen content of the lignite, as well as with increases in the pH of the culture broth. Bioconversion of lignite to methane could occur in a two-stage process. The first stage would involve an aerobic alkaline oxidation and solubilization of the lignite, with the second stage involving anaerobic conversion to methane of both the products and the cell mass from the first stage [83,84]. Lignite solubilization is generally necessary prior to biogasification [85]. The formation of methane from pretreated Texas lignite using anaerobic bacteria can occur in concentrated brines [86]. In this process, the optimal salt concentration is

639 8--12% and the optimal temperature is 35--42"C [86]. Thus it is possible that the process could be carried out in underground salt caverns. Texas lignites are the most reactive substrates for bacterial consortia derived from termites [87]. Maximum methane production was 7195 ~mol/g lignite. Lignite particle size in the range 44-650 ~tm did not affect biogasification, but micronized lignite (=10 ~tm particles) enhanced methane production by 26.6% [87]. Biogasification in this system is a multistep process in which the lignite structure is degraded to compounds of 2-3 tings, which are suitable substrates for acetogenic organisms. This initial degradation is the rate-determining step. The acetogenic organisms convert the 2-3 ring compounds to volatile aliphatic carboxylic acids and short-chain aliphatic alcohols. Methanogenic bacteria then convert the acids, acetic acid in particular, along with carbon dioxide and hydrogen, to methane. Texas lignite in the 28-325 mesh size range produced methane at rates of 50(0700 ~tmol/day.g lignite [87].

12.2 DIRECT L I Q U E F A C T I O N OF LIGNITES

Direct liquefaction is the conversion of coal to a liquid product by hydrogenation of the coal, usually slurried in a solvent medium, with no intervening process steps between the coal and liquid (this of course does not rule out down-stream treatment of the liquid product). Indirect liquefaction is the formation of liquids from synthesis gas produced by coal gasification. Usually indirect liquefaction would employ the Fischer-Tropsch synthesis or some variant of the FischerTropsch as the liquefaction step. Neither the direct nor indirect liquefaction of lignites is current commercial practice in North America, and it does not seem likely that liquefaction will become commercial in the near future. This section provides information on some pilot-scale processes for direct liquefaction of lignites. Fundamental aspects of the chemical reactions of lignite relevant to liquefaction processing are treated in Chapter 4. 12.2.1 The Exxon Donor Solvent Process In both the Exxon Donor Solvent process and the Solvent Refined Coal II process (SRC II) lignites process more readily than subbituminous coals and produces higher yields on an maf basis [88]. With addition of pyrite, Texas lignite can be processed to >60% oil yields in the SRC II process. Lower yields of oil were obtained if no additional pyrite was added. Lignites have the potential of being excellent feedstocks for direct liquefaction. The Exxon Donor Solvent process, hereafter referred to as EDS, is based on the reaction of coal with hydrogen in a hydrogenated recycle solvent. Components of the solvent serve as hydrogen donors to the coal; the process name derives from this behavior. As originally conceived, a slurry of coal in solvent was fed to the liquefaction reactor with hydrogen. Typical operating conditions were 14 MPa total pressure, 450~

and upward plug flow in the reactor [89]. The

material leaving the reactor consists of a depleted recycle solvent, gaseous hydrocarbons, C4-

640 540"C distillate, and vacuum bottoms that contain 540*+ liquids, unconverted coal and mineral residues. The configuration of the EDS process is shown in Figure 12.15 [90]. The dried, crushed coal is fed to the liquefaction reactor with hydrogenated recycle solvent and hydrogen. Distillation of the reactor products provides a depleted recycle donor solvent stream, as well as light gases, C4-540"C liquids, heavy vacuum bottoms (540"C+) and the unconverted coal and mineral matter. Hydrogenation of the recycle solvent is performed in a conventional fixed bed catalytic reactor. The vacuum bottoms are fed to a Flexicoking unit (a fluidized bed process combining coking and gasification) which produces some liquid products along with fuel gas for process furnaces. The necessary hydrogen is obtained by steam reforming the light hydrocarbon gases. Several options also exist for the 540*+ vacuum bottoms: partial recycle to the liquefaction reactor, flexicoking to liquids and fuel gas; partial oxidation to produce hydrogen or fuel gas, or simply feeding directly to a furnace for process heat [91]. The vacuum gas oil, which boils in the range 425-540"C, can also be recycled to the liquefaction reactor.

HYDROGEN

I HYDROGEN .._1

GAS

SOLVENT HYDROGENATION

STEAM REFORMING

l LIQUEFACTION

HYDIOGEN

..._1

w,.---

|

~STEAM

DISTILLATION

..--I v

~0ITOMS FLEXICOKING

-.- FUELGAS

,,,...-

ASH Figure 12.15. Block flow diagram of Exxon Donor Solvent process, operating with bottoms treatment by Flexicoking [90].

641 Pilot-scale testing in the late 1970's suggested that lignites would be more difficult to process in EDS than bituminous coals [89]. Big Brown (Texas) lignite produced total liquid yields of about 37%, 28% from the liquefaction reactor and 9% from the Flexicoker. By comparison, three bituminous coals, Monterey, Burning Star (both Illinois), and Ireland (West Virginia), had total liquid yields of 44--46%, with yields from the liquefaction reactor in the range of 29-35% [89]. The EDS results for various ranks of coal show quite clearly that for each coal there is an optimum combination of operating parameters, such as residence time, for maximizing the yields of liquids. For example, the maximum liquid yield from Big Brown lignite occurs at 25--40 min. residence time. The total process yield (that is, from both liquefaction and flexicoking) to C4+ products is around 40% (daf basis) for Big Brown and Indian Head lignites. Of the total C4+ product from these lignites, roughly 15-18% is derived from Flexicoking, and the balance from the liquefaction process itself. Product yields from EDS as a function of rank show some correlations with the expected rank dependence of composition. For example, the yields of water and the carbon oxides from low-rank coals are higher than from bituminous coal, reflecting the higher oxygen content of the lower rank coals [90]. Coals that are higher in sulfur content yield a greater amount of hydrogen sulfide in the product gases. The C 1-C3 gas yield is also slightly higher for low-rank coals. With more of the carbon reporting to the carbon oxides and light hydrocarbons, the yield of

C4-540~ and 540~

liquids from low-rank coals is somewhat lower than from bituminous

coals. The effect of reactor conditions and mode of operation on the yields of gas, naphtha, and oil obtained from Big Brown lignite is shown in Figure 12.16 [91]. Using solvent, coal, and vacuum bottoms in proportion 1.6:1:1 increases the yield of C3-540~

liquids by about a third

compared to once-through operation. The naphtha yield also increases by about the same proportion, although the selectivity to naphtha is not improved. Changes in process variables allow optimizing the selectivity to naphtha. For example, by changing to a solvent:coal:bottoms mixture in proportion 3:1:1, the naphtha cut becomes 85% of the total liquid product [91]. Bottoms recycle can significantly improve yields of C3-5400C liquids compared to once-through operation. On a daf coal basis, the yield of C3-540~

liquids from Big Brown lignite was 49% with bottoms

recycle, but only 35% for once-through operation [91]. For Big Brown lignite processed at 450~ and 10.5 MPa, the total conversion to 540~ products increases with increasing residence time up to about 40 min [90]. Higher residence times, up to 110 minutes, have essentially no effect on lignite conversion to 540~ - products. However, the conversion to C4-540~ liquids drops steadily as residence time is increased, from 33% at 20 min to 27% at 110 minutes. At constant temperature, the yield of the C4-540~ liquids increases to a maximum and then decreases as a function of residence time. The time at which the maximum yield occurs is coal-specific; for Big Brown lignite processed at 450~ the maximum yield occurs in 20 min [28,91]. In comparison, the maximum for Illinois bituminous coal is reached in about 40 min. The residence time corresponding to the maximum yield changes as temperature changes. As

642

[~

50_

,-1 ~

4o_

GAS

~] NAPHTHA

30_ r I/-I I.-4 >'

20_

t..r

O' ~

xx

10..

O b ,

o

0

A

B

Figure 12.16. Comparative effects of bottoms recycle and once-through operation for liquefaction of Big Brown lignite [91]. Reaction conditions are defined as follows: A, 448"C. 40 min. residence time, 10.3 MPa, 1.6:1:0 solvent:coal:bottoms ratio; B, 427"C, 100 min. residence time, 10.3 MPa, same solvent:coal:bottoms ratio; C, 448~ 40 min. residence time, 14 MPa, 1.6:1:1 solvent:coal:bottoms; D, 448~ 40 min. residence time, 14 MPa, 1.3:1:0.5 solvent:coal:bottoms; and E, 448~ 40 min. residence time, 14 MPa, 3:1:1 solvent:coal:bottoms.

residence time increases at 450"C the viscosity of the heavy oils (bottoms) decreases sharply. Bottoms viscosity also drops with increasing temperature. Operation of the EDS process on low-rank coals has shown that problems arise from formation of less liquid product and correspondingly more carbon dioxide and water (from the higher oxygen content of low-rank coals), a higher bottoms viscosity for low-rank coals, and the formation of free flowing agglomerates and reactor wall scale of calcium carbonate [28,90]. (The calcium carbonate problem is discussed in Chapter 6.) The extent of calcium carbonate deposition relates to the amount of ion-exchangeable calcium in the coal. Hence these deposits were believed to originate from decomposition of calcium carboxylates in the lignite. The approaches to controlling the problem include removal of some of the free-flowing agglomerated particles during operation and acid washing of the wall scale during reactor downtime. Deposition can be alleviated by bottoms recycle operation. The higher fluid density and viscosity enabled more solids to be swept out of the reactor, and a moderation in reaction rate due to the dilution of the reactor contents with some of the recycled bottoms [91]. Indeed, recycling the vacuum bottoms, including the ash, appears to be a useful remedy for the precipitation of calcium salts [45]. Contacting the lignite with

643 appears to be a useful remedy for the precipitation of calcium salts [45]. Contacting the lignite with liquid sulfur dioxide at temperatures of 0-90~

and pressures of 70--350 kPa for 0.1 to 4 h

suppresses formation of calcium carbonate scale [92]. Ash from the residue of untreated lignite liquefied at 4500C in tetralin for 40 min contained 40% calcium carbonate. Treatment with liquid sulfur dioxide, followed by liquefaction at the same conditions, resulted in 2.9% calcium carbonate in the residue ash. Gaseous sulfur dioxide treatment also effectively suppresses calcium carbonate formation, but not as well as the liquid sulfur dioxide. Lignite treated with gaseous sulfur dioxide produced a residue containing 8% calcium carbonate in the ash. .The viscosity of the bottoms is higher for low-rank coals than for bituminous coals [90]. A comparison of bottoms viscosity for Big Brown and Monterey coals is shown in Figure 12.17 [89], for aperation at 4500C and 10 MPa H2. These viscosities represent a mechanical operability of the process since they relate directly to pumping the bottoms from a vacuum tower to a coking or gasification operation. The criterion of pumpability was a viscosity below 5 Pa.s [89]. The high viscosity is undesirable for good operation of the vacuum tower and for operation of the liquids feed system for bottoms recycle. For low-rank coals, the bottoms viscosity decreases with increasing residence time. For example, the viscosity of bottoms diluted with 10% 540~ - material decreased from about 16 Pa.s for material produced at a residence time of 20 min to about 1 Pa.s for material produced with a residence time of 100 min [90]. (The viscosity data were obtained at 290~

Bottoms dilution is also a strategy for decreasing bottoms viscosity; increasing the

concentration of the 5400C - from 10 to 15% decreases the viscosity by a factor of about four for material produced with a residence time of 20 min. Any 540~ - liquids used for bottoms dilution can be recovered in the Flexicoking operation. Bottoms recycle during the processing of Big Brown lignite reduces the proportion of preasphaltenes (pyridine-soluble, benzene-insoluble) in favor of increasing the proportion of asphaltenes (540~

benzene-soluble). Vacuum bottoms from

bottoms recycle operation show improved thermal stability on storage. For example, the relative viscosity of vacuum bottoms from Big Brown bottoms recycle operation increased only from 1 to 2 units in an accelerated aging test at 315~ for 28 h in nitrogen [91]. 12.2.2 Project Lignite The original concept, dating from the 1960's, that motivated work on Project Lignite, was the prospect of developing a "lignite refinery" that would produce a slate of useful products from lignite. The products envisioned were solvent-refined coal (or, more specifically, a solvent-refined lignite); distillate liquids from hydrogenation of the solvent-refined lignite; products from lignite carbonization, including solid char, liquid products, and gases; and further liquids from direct hydrogenation of the lignite [93]. Thus the products leaving the plant gate would include synthetic crude oil, fuel gases, a low-ash char, a solvent-refined lignite, and possibly even feedstocks for the chemical industry. As the project eventually evolved, however, the dominant focus was on the production of solvent-refined lignite at the pilot scale, with some supporting bench-scale work on direct hydrogenation of lignite. The pilot-scale operations processed 0.5 tonne/day of lignite.

644 20 + Big Brown 9 Wyodak

15

A Burning Star ~100

"

5

0

' ' ~ '

0

I

' ~ ' '

I ~ w ~

I ~ ' ' '

I ' ' ' '

20 40 60 80 Residence Time, min.

100

Figure 12.17. The effect of liquefaction residence time (at 450~ 10 MPa Hz) on residual bottoms viscosity at 10 s-1 shear rate [91]. Operability problems are encountered at viscosities above 5 Pa.s. Wyodak is a Wyoming subbituminous coal; Burning Star, an Illinois high volatile C bituminous coal.

Broadly, the lignite feedstock was slurried, without drying, with a donor solvent. The slurry is then pressurized, preheated, and reacted in a synthesis gas atmosphere at temperatures to 510" and pressures to 18 MPa. At a process feed rate of 23 kg/h, the production of solvent-refined lignite was about 7 kg/h [93]. Temperatures in excess of 480 ~ in the dissolver caused excessive coking in the unit [93]. The effect of reaction temperature, for operation at 17.5 MPa of 1:1 H2/CO, is shown in Figure 12.18. The temperature data used to illustrate the effect are the maximum temperatures observed in the dissolver unit. The yield of oil and solvent-refined lignite passes through a maximum at 440~ At this condition, the amount of pyridine-insoluble unreacted lignite remaining is about 8% (ashfree basis). The change in slope represents the combination of two opposing effects. On the one hand, the conversion of lignite to solvent-refined lignite and oil increases as a function of temperature. Offsetting this, however, is the fact that the rates of retrogressive recombination reactions, or coking of the solvent, or both, also increase with temperature. Above =440~

the

recombination or coking reactions dominate and consequently the net conversion decreases. In Figure 12.18 the effects of temperature on the yields of solvent-refined lignite and oil are shown. The solvent-refined lignite yield decreases monotonically as a function of temperature. The oil yield increases as a function of temperature to about 440~

and appears to be independent of

temperature at higher temperatures. Distillation of the oil into a cut boiling below 2800C (so-called

645 70 ~60

t" x ~ - ~ . - - . . ~ _ ~+, +

+ +,

+ Product

+

++ ~ . , . . , . ~ +

-

9 Gas

+

,..150

+

-

9

A Unconverted

9

30 A

2o

A

z lO-

A

A

A

A AA AAA~x A

0

. . . .

430

I

440

. . . .

I

450

. . . .

460

Maximum Reactor Temperature, ~ Figure 12.18. Yields of product (oil plus solvent-refined lignite), gas, and unconverted lignite as a function of maximum reactor temperature [93].

light oil) and solvent (boiling between 280 ~ and the solvent-refined lignite) allows separation of the oil yield as shown in Figure 12.19. While production of light oil increases as a monotonic function of temperature, the solvent yield seems to pass through a maximum in the vicinity of 440~

It

should be noted that over much of the temperature range the solvent yield, when expressed on an maf coal basis, is actually negative, and it is only in the fairly narrow temperature range around 439-443~

that positive solvent yields are obtained.

The solvent-to-coal ratio could be varied in the range 1.4-3.0 [93]. At lower ratios, e.g., 1.33, problems occur with mixing and pumping the slurry. An increase in the solvent-to-coal ratio from 1.3 to 2.3 increases the gas make by about 5 percentage points, as a result of increased gas production from the solvent itself when the amount of solvent present in the system is increased. For successful long-term operation of a process such as this, establishing and maintaining "solvent balance" is critical [93,94]. That is, the process must produce enough solvent to maintain its own operability. If quantity of solvent is the only criterion, then in some cases it is possible to process lignite with solvent balances in excess of 100%; that is, more solvent is produced than is required for recycle to keep the process operating. However, combined with the issue of solvent balance is the question of solvent quality. In continuous runs in which all available liquid products were recycled, conversion of lignite decreased as a function of time, suggesting that the solvent quality was also decreasing. In comparison, if only liquids boiling above 2000C are recycled, then the solvent appears to remain effective. (Scouting studies in batch autoclaves of 200 g maf lignite capacity verified that light, low-boiling solvents produce lower solvent recoveries and reduced yields of light oils, relative to heavier solvents [93].) Under these circumstances, solvent balance

646 70

+ Oil 609

.~0 50-

SRL

4o~. 30+

- 20"

+-!-+

+ ..,_....,_,..,..,~

10" +

4--"

2;

O-

+

-10

'

+ '

'

'

I

'

'

'

'

I

'

'

'

'

440 450 460 4130 Maximum Reactor Temperature, ~ Figure 12.19. Yields of oil and solvent-refined lignite as a function of maximum reactor temperature [93].

was obtained only for reactions at 17.5 MPa. Successful operation of the process was achieved at pressures of 10.5, 14, and 17.5 MPa [93]. The effects of reactor pressure are illustrated in Figure 12.20, and the yields of solvent, light oil, and solvent-refined lignite are shown in Figure 12.21. The yield of solvent-refined lignite decreases slightly as pressure is increased above 14 MPa. On the other hand, the yield of light oil increases significantly at the high pressures. At low pressures solvent is actually lost; although the solvent loss decreases with pressure, for the conditions in which these data were generated (440~ 1.8-2.2 solvent-to-coal ratio, and 71-74% H2 in the gas feed) the system does not achieve solvent balance at any pressure. The best hope for achieving solvent balance would occur at higher pressures. Changes in the gas composition have only minor effects on the product slate, that is, on oil plus solvent-refined lignite, gases, water, and unconverted lignite [93]. However, among the desirable product fraction, the oil plus solvent-refined lignite, the yield of solvent-refined lignite decreases as the mole fraction of H2 in the gas increases. This effect is most pronounced as the H 2 increases from 25% to 50%. In comparison, the light oils increase with increasing H2. For reaction at 17.5 MPa, 4400C, 1.97 solvent-to-coal ratio, there is a very slight positive yield of solvent at H2 mole fractions 99% carbon burnout at the combustor exit [198]. However, complete carbon burnout in slurry combustion may require a longer residence time, because of the high water content of the slurries. Complete combustion of the low-rank coal slurries is easier to achieve than for slurries of bituminous coals because of the non-caking nature of the low-rank coals. The lack of caking behavior eliminates agglomeration of the coal particles during the evaporation of the slurry water and devolatilization of the coal. High levels of combustion air swirl were necessary to achieve a stable flame [198]. For South Hallsville (Texas) lignite, slurries with 55-58% solids produced clearly defined, stable flames. Flue gas NOx was invariably higher for slurry combustion than in comparable tests with the same lignite burned in the pulverized coal mode. This may be attributable, at least in part, to the increased heat input needed to evaporate the slurry water and maintain furnace exit temperatures (heat inputs are higher by about 7% in the slurry tests compared with pulverized firing of the same lignite). However, slurry combustion should occur with lower flame temperature, so that it would be reasonable to expect reduced thermal NOx levels associated with slurry combustion. Comparison of bituminous coal and lignite slurries in pilot-scale combustion showed that the combustion of the bituminous coal slurry proceeded with the abundant formation of "sparklers," expanded char particles which required long times for complete burnout, whereas the combustion of a lignite slurry under the same conditions (and same burner configuration) produced a diffuse flame with only a haze of finely burning sparks [199]. These results suggest that agglomeration of char particles is not a problem with lignite slurries. In turn, this finding means that the effectiveness of slurry atomization in the combustor is not as critical for lignite slurries as for bituminous coal slurries. 12.4.3 Ash Behavior The comparative behavior of ash in the combustion of South Hallsville lignite in the pulverized coal and slurry firing modes is compared in Table 12.12 [198]. The lignite slurry was prepared by hot-water drying (Chapter 10) at 330~ The principal difference for slurry firing appears to be a much greater accumulation of ash on the wall of the combustor and in the flue; apparently because of this wall accumulation less of the ash susceptible to removal in the precipitator actually reaches the precipitator. The fact that little ash

668 TABLE 12.12 Comparative ash behavior in pulverized (PC) and slurry firing of South Hallsville lignite (adapted from [ 198]).

Ash input, gin. % of ash collected Total Combustor wall Combustor bottom Probes* Flue gas duct Precipitator

PC 21024 44.5 2.4 1.1 0.1 7.3 33.7

Slurry_ 20598 67.3 26.6 3.2 0.03 22.8 14.7

*Simulating steam tubes

was deposited on the probes in either test is a reflection of the low sodium content in the South Hallsville lignite. Pilot-scale combustion of Beulah lignite slurry showed that the droplet size is not the only factor affecting the particle size distribution of the ash [200]. Processes of agglomeration and coalescence affected the ash particle size distribution. The agglomeration and coalescence behavior is in turn influenced by the kinds of inorganic species in the lignite and their size distributions [201]. Furthermore, the amounts of various inorganics and their size distributions are not necessarily the same as those in the parent, untreated lignite. Preparation of Beulah lignite slurry by hot water drying causes a significant reduction in the particle size distribution of the pyrite and removes organically associated sodium [201]. The coalescence and agglomeration are facilitated by the fluxing action of the iron when it becomes incorporated into aluminosilicates [202]. The aluminosilicates also react with organically bound alkali and alkaline earth elements, forming new complex aluminosilicates of lower melting point than the original phases [203]. These new, lowmelting alkali and alkaline earth aluminosilicates also contribute to coalescence [203]. Several factors could contribute to the increased deposition of ash on the combustor walls. These factors include the slurry spray pattern, the intense swirl near the burner, and the longer flame (compared to a pulverized coal flame). In general, the ash deposits produced by firing low-rank coal slurries are more friable and easily removed than those produced in pulverized coal firing, even though there was little difference in the amount of ash deposited. The ash deposition rate for both South Hallsville and Indian Head lignites was lower in slurry firing than in conventional pulverized firing. The difference in physical properties of the ash deposits from pulverized and slurry firing may be related to differences in the mineral composition of the deposits produced in the two cases. As an example, Table 12.13 provides X-ray diffraction analyses of ash deposits from pulverized and slurry firing of Indian Head lignite [198].

669 TABLE 12.13 Comparative X-ray diffraction analyses of ash deposits from pulverized- and slurry-fired Indian Head lignite (Adapted from[198]) Pc-fired White Layer Major components

Quartz Anhydrite

Slurry-fired Quartz Anhydrite

Minor components

Hematite

Trace components

Lime Periclase Hematite Magnetite

Magnetite

Anhydrite Quartz Periclase

Pyroxene Melilite

Melilite

Anhydrite

Sintered Layer Major components

Minor components

Quartz Trace components

Hematite Magnetite

A general observation, based on the presence of minerals such as the pyroxenes and melilites in the ash deposits (as well as plagioclase feldspars observed in ash deposited in the flue gas duct), is that many of the phases detected in the ash from slurry firing appear to have crystallized from a melt, whereas many of the phases in ash from pulverized firing may have arisen from a simple thermal decomposition of minerals [ 198].

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10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

28 29 30 31

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671 32 33 34 35 36 37 38 39 40 41 42 43 44

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679

INDEX

Abernathyite, 262 Abietic acid, 129 Absorptivity (of ash), 308 Acetic acid, 92, 103-104 Acetoguaiacone, 67 Acetone, 261 from lignite, 184, 192, 196, 200 Acetonitrile, 196, 200 Acetophenone, 148 Acetylene, 384 Activated carbon (from lignite), 4, 9, 656-657 Adipic acid, 101-102, 104 Air, reactions in, 161-163 Akermanite, 299, 301,518, 555, 570, 578, 590 Alamine, 658

Anhydrite, 240, 246, 298, 301,303-305, 314, 563,570, 575 Anisole, 94, 192 Anorthi te, 312, 571 Antelope Valley power station, 541-542, 576 Anthracene, 157 Anthracene oil, 141,170, 650-651,653 Anthracite, 66, 128, 433 Anthraquinone, 92 Anthraxylon, 52, 82, 90, 218 Anthrocyanins, 69 Antimony, 265, 268, 271-274 Antrim lignite, 655 Apatite, 246, 266 Arabinose, 120 Aragonite, 239, 245, 254

Alang iaceae, 54

Arecipites, 54

Albite, 236, 239, 249 Alcoa lignite mining, 9, 410 NMR of, 106 pyrolysis, 108, 123, 178, 192, 197 sulfur forms, 123 Alginite, 62 Alumina, 302,329, 589 Aluminum effect on pyrolysis, 184 in lignite, 230-231,268, 271-274, 277279 Aluminum chloride, 183-184 Aluminum oxide, 519 Aluminum silicate, 563 Aluminum sulfate, 563 Ammonia from lignite, 458-459, 511,619-620 for NOx control, 544 reaction with lignite, 488 Ammonium acetate, 220-223,226, 231-232, 236, 253, 255, 276, 291,293, 300, 514 Ammonium hydroxide, 496 Ammonium molybdate, 158 Ammonium tetrathiomolybdate, 488, 652-653 Ammonium tetrathiotungstate, 489

Arsenic, 265, 268, 271-273 Ash hemispherical temperature, 318 Ash softening temperature, 317-319 Atrazine, 665 Attrinite, 86-90 Attrital lignites, 82 Attritus, 16, 81-88, 90 aromaticity, 98 cellulose residues in, 120 friability, 346, 350 morganlcs in, 224 methoxyl groups, 115 reaction with hydrogen, 158 sodium in, 236 Augite, 301 Autogenous combustion, 26, 437-438, 442, 457 A utogenous heating, 433,436, 438, 440, 442 Autunite, 261

Amonoa, 54

Amorphinite, 58, 62 Analcime, 236 Anatase, 238-239, 244 Anderson lignite air oxidation, 436 Andesine, 239

Backhoes, 419-420, 425-426 Bagasse, 494-495 Baghouses, 543 Balakhovskii brown coal, 400 Barite, 239, 241,247, 254, 266, 277 Barium effect on air oxidation, 440 effect on pyrolysis, 183-184 in lignites, 253-254, 268, 271-274, 277, 279 Barium acetate, 110, 493 Barium chloride, 110 Barium hydroxide, 110 Barium sulfate, 520

680 Barnhardt-Williams sintering test, 309 Bassanite, 240-241,246-247, 292, 298, 301 Baukol-Noonan lignite aromaticity of core samples, 99 ash slag viscosity, 320, 324, 326 bulk density, 357 calcite in, 245 carbonization, 204 combustion, 535 drying, 463 microprobe analysis, 220 mine, 23, 25 minerals in, 241 Mohs hardness, 355 nacrite in, 249 pyrite in, 243 removal of cations, 221 stockpiling, 434 tetrapyrroles in, 121 Beckville lignite, 426 Benton lignite, 629 Benzaldehyde, 144, 190, 192 Benzene, 92, 192-193,261 extraction with, 128, 655 for heat of immersion, 364, 367 from lignite (see also BTX), 154, 157, 194, 203,620, 637 Benzenepentacarboxylic acid, 93-94 1,2,4,5-Benzenetetracarboxylic acid (see also pyromellitic acid), 104 1,2,3-Benzenetricarboxylic acid (see also hemimellitic acid), 106 1,2,4-Benzenetricarboxylic acid (see also trimellitic acid), 103, 106 Benzofluorene, 526 Benzofuran, 117 Benzophenone, 169 Benzo[a]pyrene, 526 Benzoxanthone, 117 Bergueda lignite, 654 Beryllium, 253,271,274 Betulaceae, 54 Betulaceae-Myricaceae, 53 Beulah lignite (also see Beulah-Zap) aluminum in, 230-231 aromaticity, 97-99 aromaticity of lithotypes, 97-98 ash deposition, laboratory, 560-561, 563,565, 570-571,578, 580581 ash formation, 519-520 ash fouling, 545, 548, 550 ash reactions, 569 ash sinter point, 309-310, 313 ash sinter strength, 312-314 ash slag surface tension, 336 ash slag viscosity, 322 barite in, 247

Beulah lignite (continued) calcite in, 245 carbonyl groups, 116 carboxyl groups, 110 carboxyl groups in lithotypes, 111 cellulose residues in, 120 cellulose in lithotypes, 120 char properties, 514 chemical fractionation, 223, 225-226, 228 clays in, 520 combustion, 515, 536-537 combustion rates, 512, 514 depolymerization, 129 dolomite in, 245 DRIFF of, 163 drying, 387 esters, 118 fluidized-bed combustion, 310, 315, 485, 586-588, 591 free radicals in, 159 friability of lithotypes, 350 FTIR of, 99, 107, 112, 116 gypsum in, 246 hematite in, 244 humic acids from, 113-114, 116 ignition, 509 ion-exchange beneficiation, 481,484, 486 iron in, 232 kaolinite in, 248 lignin residues in, 119 liquefaction, 160, 170, 297, 304, 648, 651,654 magnesium in, 233 magnetite in, 244 methoxyl groups, 115, 119 methoxyl groups in lithotypes, 115 methylation of, 103 microprobe analysis, 231,233,236-237 mine, 24, 428 minerals in, 241 montmorillonite in, 250 nacrite in, 249 nitrates in, 233 oxidation of lithotypes, 104 oxidation reactions, 70, 93, 101-103 quality, 27, 84-85 petrography, 87 polonium in, 265 potassium in, 234 pyrite in, 242-243 pyrolysis, 188-189, 191,193,205 pyrolysis of lithotypes, 184, 192 quartz in, 247 Raman spectroscopy of, 112 reaction with hydrogen, 159 SAXS of, 365, 369-370, 373-374, 390

681 Beulah lignite (continued) shrinkage, 403,452 siderite in, 246 slurry combustion, 668 sodium in, 236 solvent extraction of, 126-127 sulfur forms in, 122, 237-238 surface area, 363,402 swelling, 403-404 talc in, 250 titanium in, 238 uranium in, 262 XAFS of, 122 XANES of, 234 XPS of, 112, 369 zircon in, 244 Beulah-Zap lignite (see also Beulah and Zap

lignites) adsorption of metals antimony in, 265 arsenic in, 265 ash fusion behavior, 320 barium in, 254 beryllium in, 253 boron in, 263 bromine in, 265 cadmium in, 263 calcite in, 245 carbonyl groups, 116 carboxyl groups, 110 cerium in, 258 cesium in, 253 char properties, 511 chromium in, 255 cobalt in, 256 copper in, 257 deposit, 24-25 deposition, 60-62, 89 devolatilization, 510 dolomite in, 245 drying, 389 esters, 118 ether groups, 116 europium in, 259 FIMS of, 113, 118, 124, 128 float-sink separation, 477-478 fluorine in, 265 friability, 346 FTIR of, 112 gallium in, 263 genesis, 84 hafnium in, 257 heat of immersion, 364 hematite in, 244 hydration of, 396 hydraulic conductivity, 368 illite in, 249 inorganics in, 224

Beulah-Zap lignite (continued) iron in, 232 kaolinite in, 248 lanthanum in, 258 lead in, 264 liquefaction, 652-654 lithium in, 251 low-temperature ashing, 290 lutetium in, 259 magnesite in, 245 manganese in, 255 minerals in ash, 305 moisture, 393 molybdenum in, 258 monazite in, 246 nickel in, 256 neodymium in, 258 niobium in, 257 NMR of, 95, 100, 107 petrography, 79, 81, 83, 85-88, 90 phenolic groups in, 115 praseodymium in, 258 platinum group elements in, 259 pyrite in, 242 pyrolysis, 187, 202, 206 quartz in, 247-248 Rock-Eval analysis, 202 rubidium in, 253 rutile in, 244, 248 samarium in, 259 scandium in, 254 selenium in, 265 solvent extraction, 108-109, 113, 128 strontium in, 253 sulfur forms, 123 surface area, 361-362 swelling of, 367-368 tantalum in, 257 terbium in, 259 thermogravimetric analysis, 388 thorium in, 259 tin in, 264 tungsten in, 258 uranium in, 262 vanadium in, 255 XAFS of, 123 XANES of, 123 ytterbium in, 259 yttrium in, 257 zinc in, 263 zircon in, 244-245 zirconium in, 257 Beypazari lignite air oxidation, 436 liquefaction, 653-654 Bibenzyl, 169 Bienfait lignite ash fouling, 564

682 Bienfait lignite (continued) FTIR of, 107, 126 hydrogen bonding in, 126 NMR of, 96 BI-GAS process, 155 Big Brown lignite aromaticity, 97 carboxyl groups, 110 deposition, 61 humic acids from, 113-114 liquefaction, 141,147, 159, 170, 304, 641,643, 653-654 mining, 9, 417-418, 425 NMR of, 106 oxidation reactions, 102 petrography, 90-91 pyrolysis, 205 solvent extraction of, 126-128 solvent swelling, 125 Big Brown power station, 539, 545 Big game hunting, 249 Big Stone power station, 536, 545, 551,577578, 580 Bills method (viscosity), 329 Biotite, 239 Biphenyl, 192-193,637 Blackjack, 9 Blasting, 413,417, 420-421,428 Boehmite, 230, 244 Bombacaceae, 54 Bombacacidites claibornensis, 54 Bombapollis, 54 Bone coal, 9 Booker space heater, 522 Borax, 495 Boron, 263, 268 in soil, 424 Boron trifluoride, 109, 117, 120, 129, 489 Bottinga-Weill method (viscosity), 330 Boudouard reaction, 603,660 Boundary Dam lignite, 129 Boundary Dam power station, 542 B randon li gni te, 107 Bredigite, 299 Bromine, 265, 268, 271,273 Brookite, 238, 531 Brooms, mechanical, 421 Bryan lignite, 228 BTX (from lignite), 153, 155-157 Bucket wheel excavators, 413-414, 418-421, 425 Bulldozers, 413,415, 417, 419-421,423, 425, 433 Burleigh lignite, 478 Burning Star coal, 641 Burseraceae, 54 Butadiene, 108, 203 Butane, 202

Butanedioic acid, 103 2-Butanone, 192, 196, 200 1-Butene, 203 Butyraldehyde, 142 Butyric acid, 103 Cadalene, 127-128 Cadmium, 263,268, 271-273 sorption by lignite, 658 Cadmium oxide (see also cadoxen), 120 Cadoxen, 120 CalamuspoUenites, 54 Calamuspo Uenites-Are cipi tes, 53 Calcite (see also calcium carbonate) in ash deposition, 553-554 formation in liquefaction, 297 in lignites, 231,239-240, 245, 266, 268270, 292 manganoan, 246 reactions of, 296, 299, 301-303,305 Calcium in ash deposition, 553-555 behavior during combustion, 518-519 effect on pyrolysis, 183-184, 207 exchangeable, 223 as ignition promoter, 509 in lignite, 231,268, 271-274, 277-279 in macerals, 225 removal by ion exchange, 485-486 removal during drying, 465 in retrogressive reactions, 653 Calcium acetate, 296-297, 299, 302-303, 312, 314, 555, 570 Calcium carbide, 383 Calcium carbonate (see also calcite) effect on pyrolysis, 184 formation in liquefaction (see also oolites) 297-298, 642 suppression of ash fouling, 578-579 Calcium chloride, 483-484, 486 Calcium hydroxide, 495-496 Calcium magnesium sulfate, 571 Calcium montmorillonite, 250 Calcium nitrate, 233, 544 Calcium oxide, 298-299, 302-303,486, 518 effect on ash fusion, 317 gasification catalysis, 606 Calcium silicate, 554 Calcium sulfate (see also anhydrite, bassanite, gypsum) 299, 486, 515, 519-520, 536-537, 554, 560, 563-564, 584, 590 Calcium sulfide, 299, 306 Calcium sulfite, 299 Calvert Bluff lignite aluminum in, 230 barite in, 254 calcite in, 245

683 Calvert Bluff lignite (continued) chalcopyrite in, 257 cobalt in, 256 chromium in, 255 deposit, 9-10 gypsum in, 246 illite in, 249 iron in, 232 kaolinite in, 248 magnesium in, 233 manganese in, 256 mica in, 249 molybdenum in, 258 monazite in, 246 potassium in, 234 rubidium in, 252 scandium in, 254 siderite in, 246 silicon in, 234 strontium in, 253 titanium in, 238 rutile in, 244 vanadium in, 255 yttrium in, 257 zircon in, 245 Can lignite, 659 Canakkale-r lignite, 121 Cannel coal, 64, 105 Canneloid lignites, 82 Cannizzaro reaction, 140, 144 Carbominerite, 53, 60 Carbon County lignite, 175 Carbon dioxide from lignite, 155-156, 166, 181-183, 188, 195, 201-205, 458, 467, 470, 509, 620 reaction with lignite, 603 swelling effect, 362 Carbon dioxide index, 437 Carbon disulfide, 72 Carbon monoxide from lignite, 155, 166, 181-183, 195, 202, 445-446, 458, 509 reactions with, 139-149 Carbon tetrachloride, 239, 244-247, 249-250 Carbonyl groups in lignites, 116 loss in coalification, 68 Carboxyl groups association with cations, 219 determination, 110, 112 in lignites, 109-112 in lithotypes, 85 loss in coalification, 68 loss in pyrolysis, 180-181,188, 370 retention of moisture, 385 Carex, 53-54 Carnegieite, 296, 302-303, 311

Carya, 60, 62 Catechol, 68, 164, 182, 184-185, 190, 200 Catechols, 67, 184 Cayhiran lignite, 652 Celestite, 247, 253 Cellulose, 64-66, 84, 119-121 reactions of, 148 Center lignite ash deposition, laboratory, 560, 565 ash sintering, 558 chemical fractionation, 228 deposition, 60 float-sink separation, 477 gymnosperms in, 52 mine, 23,428 petrography, 91 pteridophytes in, 51 pyrolysis, 191, 197, 200 quality, 27 sodium vaporization, 516 surface area, 363 uranium in, 251 Central Otago lignite, 97 Cerium, 258, 268, 271-273 Cesium, 253,268, 271-273 Cesium fluorosulfonate, 94, 106 Chalcopyrite, 243,257 Charcoal (from lignite), 25, 27, 658 Cheirolepidiaceae, 52 Chemard Lake lignite deposit, 17 deposition, 56, 59, 62 palynology, 53-54, 60 quality, 18 Chemical fractionation, 220 Cheremkhovo brown coal, 660 Chlorine in lignites, 265, 268, 272-273 reaction with lignites, 487 Chlorite, 239, 241 Chlorobenzene, 125 Chloroform extraction with, 126, 655 heat of immersion in, 364, 367 Choctaw lignite ash sintering, 558 ash slag viscosity, 324 chemical fractionation, 227 Chromite, 255 Chromium, 255, 268, 271-274, 279 sorption by lignite Chrysene, 100 Cinnamaldehyde, 192 Clarain, 59 Clarite, 53, 59-60 Clarodurite, 53 Clausthalite, 243,264-265 Clinker, 426

684 CO-Steam process, 648-651 CO2 Acceptor process, 167, 624-628 Coal Creek power station, 537-538 Coastal Bermudagrass, 422 Cobalt, 256, 268, 271-273 Cobalt molybdenum catalysts, 619, 654 Coffinite, 261-262 Collinite, 89 Columbus lignite, 396 Coniferaldehyde, 192 Conveyor belts, 421,425, 432-433 Copper effect on pyrolysis, 188 in lignites, 256-257, 268, 271-274, 279 Copper(II) chloride, 488 Copper(II) oxide, 65, 94, 100, 118-119 Copper sulfate, 188 Coquimbite, 241 Coronach lignite pyrolysis, 182 reaction with carbon dioxide, 603 solvent extraction, 129 Corpohuminite, 86, 88, 90, 185 Corundum, 302, 312 Coumarins, 69 Coyote power station, 537 Crandallite, 246 Crawler tractors, 414 Creosote, 618, 620 Creosote oil, 652-653 m-Cresol, 160, 196 o-Cresol, 192 from lignite, 160, 196 p-Cresol, 160, 196 Cresols, 67, 94, 192-193 from gasification, 619-620, 637 from lignite, 184-185, 191, 197, 200 Cresylic acid, 476-477 (see also cresols) Crimson clover, 423 Cristobalite, 248, 302, 304 Critical oxidation temperature, 437-438, 440 Cupulifero idaepo llinites, 53 Custer, George A., 412 Cutinite, 40, 58, 86-87, 90-91 Cyclohexane, 364, 367, 398 Cyclopentane, 367 p-Cymene, 70 Cysteine, 71 Cystine, 71 Dakalite, 664-665 Dakota lignite, 203 Dakota Gasification plant, 614, 616-618 Dakota Star lignite drying, 472 gasification, 304 moisture, 391

Dakota Star lignite (continued) quality, 27 storage, 444 sulfur forms, 237, 304 Darco lignite activated carbon from, 4, 656 carboxyl groups, 111 density, 358 deposit, 4, 9-11 deposition, 59 devolatilization, 510 ilmenite in, 244 iron in, 232 mining, 4, 9, 410, 424 pores in, 376 pyrolysis, 174-175, 177, 181-183, 185, 187, 196, 199-200 reaction with hydrogen, 150 repeating units between crosslinks, 124 Decarboxylation in coalification, 70 in drying, 461,463,465, 467 Decker lignite ash deposition, laboratory, 571 ash slag viscosity, 325 oxidation reactions, 92, 94 solvent extraction, 127-128 Decker subbituminous coal, 516 Dehydration, 70-71 Dehydroabietic acid, 129 Dehydroxylation, 70 Desinite, 85-90 Desmocollinite, 89 Desulfovibrio, 71 Detrinite, 89 Deuterium oxide, 147, 189 Deuteroformate ion, 147 Diammonium phosphate, 619 Dibenzodioxane, 117 Dibenzofuran, 117 Dichlorodicyanoquinone, 102 Dickinson lignite, 657 Dickite, 56 1,2-Dihydronaphthalene, 143 9,10-Dihydrophenanthrene, 143, 146 as solvent, 170, 653 Dihydroxyaluminum aminoacetate, 658 Dihydroxybenzaldehyde, 190 Dihydroxybenzenes, 436 3,4-Dihydroxybenzoic acid, 118-119 1,3-Dihydroxynaphthalene, 189 2,6-Dimethoxyphenol, 190 N,N-Dimethylaniline, 169 Dimethyl disulfide, 651 Dimethyl ether, 487 2,4-Dimethylphenol, 160 Dimethyl sulfate, 70, 115 Dimethyl sulfoxide, 238, 261,364

685 Dinitrochlorobenzene, 658 Dinitrophenol, 658 Dinitrotoluene, 658 Dinoflagellates (in lignite), 61 Diphenyl disulfide, 149 1,1-Diphenylethane, 124 Diphenyl ether, 143 Diphenylmethane, 103, 161,169-170 Diphenylphosphinic chloride, 393 Diphenyl sulfide, 149, 169 Dolet Hills lignite, 426 Dolomite, 239, 245, 268, 270, 579, 623, 625, 656 Dolomite ratio, 334 Dopplerinite, 89 Dow gasification process, 637 Draglines, 412-420, 422-423,425-429 Dump-bottom trailers, 421 Durain, 184 Durite, 53 Duroclarite, 53, 60 Dyes (from lignite), 15 Dysprosium, 259, 268 Easi-Miner, 421,425 EDTA (see ethylenediaminetetraacetic acid) Elbistan lignite liquefaction, 653 pyrolysis, 195, 200 Electrodes, from lignite, 660 Electrostatic precipitators, 543 Elgin-Butler lignite briquetting, 495 friability, 349 gasification, 621-622 Emissivity (of ash), 308 Engelhardia, 53-54, 60, 62 Engelhardia dilatus, 53

Enopodios, 54

Epidote, 250 Eschka mixture, 237 Ester groups, 118 Estevan lignite anatase in, 244 aragonite in, 245 ash fouling, 578 barite in, 247 calcite in, 245 carbonyl groups, 116 carboxyl groups, 110 combustion, 542 co-processing, 171 dolomite in, 245 ethers, 116 illite in, 249 liquefaction, 145, 158, 653 mica in, 249 NMR of, 96, 116

Estevan lignite (continued) oxidation reactions, 92 petrography, 86, 89 phenolic groups, 115 pyrolysis, 181-182 reaction with sulfur dioxide, 480 siderite in, 245 uranium in, 274 Ethane from gasification, 637 from lignite, 153-157, 194, 202-203, 458-459 Ether groups, 116-117 Ethylbenzene, 637 Ethylene from hydropyrolysis, 154 from pyrolysis, 108, 194, 202-203 Ethylenediamine, 364 Ethylenediaminetetraacetic acid, 72, 230, 277 4-Ethylguaiacol, 67, 118 2-Ethylphenol, 160 4-Ethylphenol, 160 Ettringite, 666 Eudalene, 128 Eugelinite, 61 Eugenol, 66-67 Europium, 250, 268, 271-273 Eu-ulminite, 84, 86, 88-89, 120 Exinite, 64, 86, 105, 129 reaction with hydrogen, 153 Exxon catalytic gasification process, 167 Exxon Donor Solvent process, 144, 297, 473, 639-643, 648 Fairfield lignite drying, 463,465 removal of cations, 465 Falkirk lignite angiosperms in, 51-52 calcium in, 181 deposition, 60 dewatering, 234 dielectric relaxation, 394 mine, 23,428 petrography, 91 pyrolysis, 181 quality, 27 Fayalite, 560 Feldspar, 267 Fescue, 423 Flammability index, 508 FLASHCHAIN, 206 Flavones, 69 Fleissner, Hans, 468 Fleissner process, 467-472 Flexicoking, 640-641 Fluoranthene, 526 Fluorene, 526

686 9-Fluorenone, 92 Fluorine in lignite, 265 reaction with lignite, 99 Fly ash, 424 FMC process (briquetting), 496 Formaldehyde, 526 Formate ion, 140, 143-144, 148 Formcoke, 659 Formic acid, 140, 143 Freedom lignite calcite in, 245 friability, 346, 351 mine, 24 Mohs hardness, 355 petrography, 80, 86, 91 pyrite m, 243 pyrolysis, 185 quartz m, 248 selenite in, 246 shrinkage, 452 Freestone lignite, 487 Front end loaders, 414-415, 419-422, 425-427 Fundamental jelly, 64 Fusain, 81-85, 87-88, 90 aromaticity, 97-98 carboxyl groups, 111 cellulose residues in, 120 friability, 346, 350 ignition delay, 508 inorganics in, 224 methoxyl groups, 115 pyrolysis, 184 quartz in, 248 reaction with hydrogen, 158 sodium in, 236 Fusinite, 62, 85-88, 90, 129, 224-225 Fusinitization, 62 Galactose, 120 Galena, 243,264 Gallium, 263,274, 279 Garrison lignite moisture sorption, 396 stockpiling, 433 Gascoyne lignite aluminum in, 230 aromaticity, 98 aromaticity of lithotypes, 98 ash deposition, laboratory, 561,565, 574 ash formation, 300, 519 ash sintering, 558 ash slag viscosity, 330-331 barite in, 247 bio-deashing, 480 carboxyl groups, 111 chemical fractionation, 226, 228

Gascoyne lignite (continued) combustion, 536 dewatering, 234 dielectric relaxation, 392-393, 395 drying, 454-456 gasification, 608, 632 heat of immersion, 364, 367 microprobe analysis, 230 mine, 23 minerals in ash, 293 nacrite in, 249 petrography, 83, 91 polonium in, 265 potassium in, 234 pyrite m, 242 pyrolysis, 179, 200 quality, 26 reaction with EDTA, 230 removal of cations, 220-221 SAXS of, 365, 374 shrinkage, 403,452 sodium in, 236 sodium vaporization, 516 solvent swelling, 125 storage, A.A.A. surface area, 361,363-364 thermogravimetric analysis, 388 titanium in, 238 transportation, 430-431 uranium in, 262 Gasoline, 637 Gehlenite, 296-297, 299, 301-303, 311, 313314, 334, 518, 555, 569-571,573, 577-578, 590 Gelification, 61, 65 Gelinite, 61, 85-86, 88, 90, 225 Germanium, 263-264, 271,273-274 Gibbons Creek lignite, 487 Gibbons Creek power station, 542 Gibbsite, 230, 244 Gilsonite, 494 Glance coal, 38 Glenharold lignite aromaticity, 98 aromaticity of lithotypes, 98 barium in, 254 beryllium in, 253 calcite in, 245 calcium in, 231 calcium adsorption, 219 carbonization, 179 chemical fractionation, 230 chromium in, 255 combustion, 538 copper in, 256 deposition, 60 gypsum in, 246 iron in, 232

687 Glenharold lignite (continued) magnesium in, 233 manganese in, 255-256 mine, 23,418, 428 nacrite in, 249 nickel in, 256 potassium in, 234 pyrite in, 242-243 pyrolysis, 187, 198-199 quality, 27 quartz in, 248 scandium in, 254 selenite in, 246 shrinkage, 452 silicon in, 234 sodium adsorption, 219 sodium in, 236 strontium in, 253 titanium in, 237 vanadium in, 255 ytterbium in, 259 yttrium in, 257 zirconium in, 257 Glover-West process, 469 Glucose, 120 Glutaric acid, 101-102, 104, 128 Glutathione, 71 Goethite, 232, 243 Gold, 258 sorption by lignite, 658 Graphite, 143,233 Greigite, 243 Guaiacol, 66, 118, 182, 190, 192 Gun fights, 35 Gypsum decomposition, 564 in lignites, 73,223, 231,239, 242, 246-247, 254, 268-270, 292, 298, 305, 663 in scrubbers, 543 in soil, 424 Hafnium, 257, 268 Hagel lignite barite in, 247 carboxyl groups, 111 deposition, 60, 89 desulfurization, 488 float-sink separation, 477-478 methine carbons in, 124 hydropyrolysis, 152 inorgamcs in, 224 liquefaction, 159-160, 652, 654 MW between crosslinks, 125 oxidation reactions, 105 petrography, 79 pore volume, 365 pyrolysis, 177, 179

Hagel lignite (continued) reaction with hydrogen, 153-154 soot formation, 510 surface area, 362 swelling, 362 tetrapyrroles in, 121 Halloysite, 230, 248-249 Hardgrove grindability, 351-354, 490, 529530, 542, 583 Harmon lignite, 150 Harrington stoker, 522 Hat Creek lignite arsenic in, 265 boron in, 263 bromine in, 265 chlorine in, 265 chromium in, 255 cobalt in, 256 dolomite in, 245 gypsum in, 246 hafnium in, 257 illite in, 249 kaolinite in, 248 quartz in, 248 rubidium in, 252 scandium in, 254 siderite in, 246 strontium in, 253 thorium in, 259 uranium in, 262 vanadium in, 255 zinc in, 263 Hauyne, 293,296, 299-300, 303, 314 Hematite, 239, 244, 293, 301-304, 563,570571,660 Hemimellitic acid, 92 Heptanedioic acid, 103 Hercynite, 301-302 Herrington, John, 618 Hexamethylbenzene, 95 Hexane, 128, 655 Hexanedioic acid, 103 Highvale lignite, 606 Holmium, 259 Hoot Lake power station, 535-536, 544 Hornblende, 239 Hot-water drying, 457-463,465, 480, 497, 666-667 Howard, Jerome, 687 Hoy equation (viscosity), 334 Humic acids from lignites, 113, 116-117 sorption of cations, 73 Humification, 62 Huminite, 40, 45, 79, 82, 88 birefringence of, 65 reaction with hydrogen, 158 Humodetrinite, 88, 90, 224-225

688 Husky lignite charcoal from, 25 mine, 25 Hydrochloric acid, 220, 223,226, 232, 238, 246, 254-255, 277, 291-292, 304, 385, 485, 653 Hydrodarco, 657 Hydrofluoric acid, 291-292 Hydrogen from lignite, 166, 181-183, 194-195, 202-203, 446, 458, 509, 627 reactions with, 145-146, 149-155, 174 Hydrogen cyanide, 510-511 Hydrogen sulfide as catalyst, 145, 148, 169-171,488, 654 formation in coalification, 72 from lignite, 193,458-459 m-Hydroxybenzoic acid, 65 p-Hydroxybenzoic acid, 65, 118-119 Hydroxyl groups loss in coalification, 68 loss in pyrolysis, 189 3-Hydroxyl-6-methylpyridine, 233 Ignition delay, 508 Ignition index, 508 Illinois No. 6 coal, 167 Illite, 57, 230-231,234, 239-241,249, 266 Ilmenite, 238, 244, 660 Indenes, 637 Indian Head lignite aromaticity, 98 aromaticity of lithotypes, 98 ash deposition, laboratory, 565, 574 ash sintering, 558 ash slag viscosity, 322, 331 carbonization, 204 chemical fractionation, 228 dielectric relaxation, 401,458 flotation, 478 friability, 349, 351 (see also mechanical friability, thermal friability) gasification, 307, 604, 629, 631-632 heat of immersion, 364, 367 hot-water drying, 458-461,497 ion-exchange beneficiation, 485 liquefaction, 141,145, 159, 170, 641, 653-655 low-temperature ashing, 291 mechanical friability, 345-346 methoxyl groups, 114-115 mining, 429 Mohs hardness, 355 NMR of, 460 oil agglomeration, 479 petrography, 91 pulverization, 492

Indian Head lignite (continued) pyrolysis, 174, 176, 196-197, 199, 201, 205, 207, 304 quality, 27 reaction with CO, 141 reaction with hydrogen, 150, 157 reaction with steam, 167-169 SAXS of, 370, 461 shear strength, 356 shrinkage (on drying), 343-345 slurry combustion, 668-669 slurry properties, 497, 499 solvent extraction, 126 solvent swelling, 125 surface area, 460-461 TEM of, 376 thermal analysis, 172, 175-176, 179 thermal friability, 348 waxes from, 460 Inertinite, 58, 60, 79, 88, 346 Inertodetrinite, 85, 87-88, 90, 225 Ireland coal depolymerization, 129 liquefaction, 641 Iron effect on pyrolysis, 184 in lignites, 231-232, 268, 271-273,277-279 metallic, 301,326, 520 Iron(II) acetate, 232 Iron(Ill) acetate, 654 Iron carbonyl, 654 Iron(II) chloride, 232 Iron(Ill) chloride, 183-184 Iron(II) oxide, 324, 326-327 Iron(III) oxide (see also hematite), 324, 331, 334, 582 Iron(II) sulfate, 298 Iron(II) sulfide, 71,298, 520, 583 Isoeugenol, 66-67, 190 Isophthalic acid, 92 Isopropanol, 655 Isoquinoline, 146, 650 Itmann coal, 129 Jarosite, 232, 239, 243, 247 Jet fuel, 620 Jewett lignite, 426 JPL chlorinolysis process, 487 Jug landaceae , 53-54 Kansk-Atchinsk lignite, 654 Kaolin, 41,516 Kaolinite in lignites, 56-57, 230-231,234, 239241,248-249, 266, 268, 292 reactions of, 296-297, 299-303, 311312, 314-315, 555, 569-570 Karilova lignite, 653

689 Kashmir lignite, 363 Kemmerer coal, 326 Kincaid lignite carbonyl groups, 116 critical oxidation temperature, 437 drying, 389, 467, 470 methoxyl groups, 114 phenolic groups, 115 Koppelman process, 467 Koppers-Totzek gasification, 624 Lanthanum, 258, 268, 271-274, 279 Larsen lignite, 25 Lawsonite, 250 Lazurite, 301 Lead, 264, 274 sorption by lignite, 73,658 Ledbetter lignite, 97, 99 Lei gh lignite charcoal from, 27 combustion, 523-524 metamorphosed wood in, 64 quality, 27 Leland Olds power station, 538-539, 545, 551, 577-580 Leonard, Arthur G., 663 Leonardite, 23, 26, 416, 663-664 sorption of cations on, 72 Levigelinite, 87, 90 Lignin, 64-70, 94, 118-119, 129, 160, 164, 202 Lignite factor, 334 Liliacidites, 53-54 Limestone, 538, 543-544, 578, 585-587, 589591,623,625 d-Limonene, 70 Liptinite, 40, 88, 166 Liptodetrinite, 87-88, 91 Liquidambar, 54 LiquiMag, 579 Lithium, 251 Lithium aluminum hydride, 112 Loy Yang brown coal, 160 Lurgi gasification, 616, 618, 629 Lurgi methanators, 619 Lurgi-Spulgas process, 496-497 Lutetium, 259, 268 Macrinite, 90 Magnesioferrite, 560 Magnesite, 245 Magnesium effect on pyrolysis, 183-184 in lignites, 233,268, 271-274, 276277, 279 removal by ion-exchange, 486 removal during drying, 465 Magnesium chloride, 183

Magnesium oxide, 518, 578-579 Magnesium sulfate, 563 Magnetite, 244, 293,301-304, 569-570, 583 Magothy lignite, 246 Maissade lignite, 495 Malonic acid, 103-104 Manganese, 255-256, 268, 271-274, 279 Manganese nodules, 660 Marcasite, 232, 239, 243,264 Marl, 55 Martin Lake lignite ash deposition, laboratory, 560, 565 ash slag composition, 333 co-processing, 654 deposition, 59, 61 desulfurization, 487 dielectric relaxation, 394 float-sink separation, 475-476 inorganics in, 224 liquefaction, 147, 159, 651 mining, 421 oil agglomeration. 476 petrography, 89-91 pyrrhotite in, 243 reaction with steam, 167, 169 reaction with sulfur dioxide, 476 surface area, 363, 371 Mataura lignite, 150 Matrix parent, 570 Meek, F.B., 19 Megalopolis lignite, 188 Melanterite, 232, 243 Melilites, 293-294, 299-301, 314, 516, 569571,574-575, 590, 669 Mellitic acid, 93-94 Mellophanic acid, 93 Mequinenza lignite, 652 Mercury adsorption, 658 Metaautunite, 261 Metakaolinite, 302, 315 Metatorbernite, 261-262 Metauranocirite, 262 Metazeunerite, 261 Methane from biogasification, 639 from gasification, 604-605, 627, 637 from lignite, 153-157, 166, 183, 194, 201203,206, 446, 458-459, 509 from liquefaction, 650 Methanethiol, 72 Methanol adsorption on lignite, 363 m heat of immersion calorimetry, 363-364 from lignite, 184, 192, 196, 200 reaction with lignite, 653 swelling, 362, 653 Methoxyl groups, 65 decomposition, 192

690 Methoxyl groups (continued) demethylation, 67-69 determination, 114 in lignites, 114 loss in coalification, 66, 68 Methoxymethyldibenzofurancarboxylic acid, 128 2-Methyl- 1,4-butanedioic acid, 103 4-Methylguaiacol, 66, 118 6-Methylguaiacol, 118 Methyl iodide, 103, 204 Methyl isobutyl ketone, 655 2-Methyl- 1,5-pentanedioic acid, 103 4-Methylsterene, 105 Mica, 231,233-234, 248-249, 267 Micrinite, 90, 129 Microreticulatosporites, 53, 60 Mikro-pul hammer mill, 492 Millerite, 243, 256 Milton R. Young power station, 532, 534 Mohs scale, 354 Molasses, 495 Molybdenum, 257-258, 268 Molybdenum disulfide, 488 Momipites-En gelhardia, 53 Monazite, 246, 259 Montan wax, 15, 39, 655-656, 665 Monterey coal, 641,643 Monticellite, 569, 578 Monticello lignite aluminum in, 230 barium in, 254 chemical fractionation, 229 flammability index, 508 illite in, 249 mining, 421 pore volume, 186 potassium in, 234 pyrolysis, 175, 186, 198 quartz in, 247 silicon in, 234 surface area, 186 titanium in, 238 Monticello power station, 539-540, 545 Montmorillonite, 70, 230, 239-240, 250 Morton Mains lignite, 150 Morwell brown coal, 115 Moose River Basin lignite arsenic in, 265 cadmium in, 263 calcite in, 245 chromium in, 255 cobalt in, 256 copper in, 256 gibbsite in, 244 gold in, 258 gypsum in, 246 illite in, 249

Moose River Basin lignite (continued) kaolinite in, 249 lead in, 264 mica in, 249 molybdenum in, 258 nickel in, 256 platinum group elements in, 258 pyrite in, 242-243 quartz in, 247-248 rubidium in, 252 siderite in, 245 strontium in, 253-254 thorium in, 259 tungsten in, 258 uranium m, 262 zinc in, 263 zirconium in, 257 Mullite, 299, 302-303, 312, 315 Muscovite, 239 Myriceae-Betulaceae, 52 Nacrite, 249 Nahcolite as catalyst, 168 for SOx control, 585 Nakayama lignite, 100 Naphtha, 479, 618-619, 641 Naphthalene, 92 from gasification, 637 from lignite, 157 1-Naphthol, 141,146 Navajo coal, 558 Nazarovo brown coal ash fouling, 554 dielectric relaxation, 393 Neodymium, 258 Nepheline, 293,296-297, 300, 302-303, 311, 313-314, 334, 555 New Source Performance Standard, 475, 532, 586, 589 Newvale lignite, 362 Nickel, 268, 271-274, 279 catalyst, 619 Nickel nitrate, 608 Nickel sulfate, 488 Nickel sulfide, 298, 488 Nimz's procedure, 120 Niobrara shale, 578 Nipadites, 53 Nitrates, 233 Nitric acid, 94, 106, 232, 485 Nitrobenzene, 368 Nitrogen in lignites, 233 4-Nitroperbenzoic acid, 116-117 Nitrous oxide formation in combustion, 589 reaction with lignites, 487

691

NOx control, 538, 542, 544, 557, 585-586, 665 formation, 510, 531,583,588, 667

Nomapollis, 54 Nonanedioic acid, 103 Normapolle, 53 Nosean, 293,299-301, 314 Novacekite, 261 Novodmitrovskoe lignite, 108 Nuxpollenites, 54 Nymphaea, 54 Nyssa, 53-54, 59 Nyssapollenites, 54 Octanedioic acid, 103 Oil agglomeration, 476, 479 Oil shale, 106 Oligoclase, 239 Onakawana lignite drying, 470 pyrolysis, 203 reaction with air, 164 reaction with hydrogen, 158 reaction with steam, 168 Oolites, 297-298 Orhaneli lignite, 586 Orthoclase, 239 Osmunda, 52 Oxalic acid, 92 Oxaloglutaric acid, 112 Oxalomalonic acid, 112 Oxalosuccinic acid, 112 Oxygen absorption, 436, 438, 444, 606 reactions with, 164 Palana lignite moisture, 384 Vickers hardness, 355 Papakube process, 495-496 Paraffin, 99 Parry process (briquetting), 496 Patapsco lignite, 164 Patuxent lignite, 264 Peerless lignite grindability, 352 pore structure, 360 porosity, 366 Pentane, 129 Pentanedioic acid, 103 Perlite, 579 Peroxyacetic acid, 94 Peroxytrifluoroacetic acid, 94, 104, 114, 159 Phenanthrene, 97, 143 as solvent, 141,146 Phenol, 190, 192 depolymerization with, 109, 117, 129,

Phenol (continued) depolymerization (cont'd), 489 from gasification, 637 from lignites, 67, 94, 184-185, 191, 196197, 200, 459, 620 Phenolic groups in lignites, 115 retention of moisture, 385, 466 Phenols, 67, 192 from lignites, 166, 178, 436, 618-619 Phenolsolvan process, 619 1-Phenoxynaphthalene, 143 9-Phenoxyphenanthrene, 143 Phenylalanine, 68 Phenyl glycol, 148 Phillips lignite, 350 Phlobaphenite, 185 Phosam process, 619 Phosphorus, 234, 271,273,279 Photochemical oxidation, 94 Phthalic acid, 92-94, 102-104, 106 Pittsburgh coal, 124 Plagioclase, 239, 241,334, 569, 574, 669 Platinum group elements, 258 Podocarpaceae, 52

Pollienites laesius, 54 Polonium, 265 Poly(coniferyl alcohol), 119 Poly(ethylene terephthalate), 112 Poly(p-hydroxystyrene), 70 Polypodium, 54 Poly(vinyltoluene), 99 Poplar River lignite calcite in, 245 dolomite in, 245 greigite in, 243 gypsum in, 247 mica in, 249 quality, 33 siderite in, 245 Porphyrins, 121 Portlandite, 666 Potassium effect on pyrolysis, 183-184 in lignites, 234, 268, 271,273-274, 277, 279 removal during drying, 465 Potassium acetate, 037 Potassium carbonate, 146-147, 167-169, 182, 184, 037 Potassium chloride, 037 Potassium feldspar, 234 Potassium hydrogen sulfate, 571 Potassium hydroxide, 144, 485, 496 effect on pyrolysis, 191 Potassium nitrate, 607 Potassium permanganate, 92-94, 106, 115117, 164

692 Potassium sulfate, 519, 561,571,607 Poultry grit, 578 Power shovels, 413-414, 416-417, 420-422, 426-428 Praseodymium, 258 Predicted Ash Collection Efficiency Index, 559 Previtrain, 30 Project Lignite, 297, 643-648 Propane from gasification, 637 from lignite, 194, 202-203,458 1,2,3-Propanetricarboxylic acid, 103-104 Propionic acid, 103 Propionitrile, 196, 200 Propylene, 108, 194, 203 4-Propylguaiacol, 118 Pseudocoke, 659 Pseudomonas putida, 488 Pseudovitrinite, 153 Pseudowollastonite, 590 Ptolemais lignite, 657 Pust lignite aromaticity, 97 bio-deashing, 480 quality, 33 reaction in air, 162 Pyrene, 100, 526 Pyridine extraction of water with, 384, 393 heat of immersion in, 364 swelling with, 124-125, 368, 403-404, 653 Pyridinium iodide, 94, 105 Pyrite as catalyst, 145, 639, 654 decomposition, 488, 583 determination, 291 fragmentation, 520 framboidal, 58, 71,242-243 in lignites, 57, 60, 71, 73, 232, 239240, 242-243,246, 255, 264, 266, 268-269, 272, 274-275, 292, 529 molybdenum in, 257-258 nickel in, 256 nodules, 61,243 oxidation of, 293,303 reactions of, 296, 299, 304-305 zirconium in, 257 Pyrolusite, 256 Pyromellitic acid, 93 Pyroxene, 293,300, 334, 569, 574, 669 Pyrrhotite as catalyst, 653-654 in lignites, 243 Quartz agglomeration, 590

Quartz (continued) fragmentation, 520 in lignites, 56, 234, 238-241,247, 248, 266-267, 274, 292-293, 530, 563, 569, 663 reactions of, 296, 299, 301,304-305, 314, 316, 570 rutilated, 244 Quercus, 54 Quinoline, 637 Rapponmatsu lignite, 164, 166 Rasa lignite, 436 Ravenscrag lignite antimony in, 265 barium in, 254 cesium in, 251 cobalt in, 256 deposit, 35 deposition, 57-58 lithium in, 251 manganese in, 255 nickel in, 256 pyrite in, 242 rubidium in, 251-252 scandium in, 254 strontium in, 253 vanadium in, 255 Raymond bowl mills, 491 Raymond dryer, 457 Rectisol process, 479, 619 Red dog, 26 Reichert microhardness, 355 Relative refractory index, 573-574 Resinite, 58, 87, 91,461 reaction with hydrogen, 153 Resins (in lignite), 30, 38, 43, 45, 52, 64 Resorcinol, 189 Retene, 69, 97 Rhodochrosite, 246 Ribose, 120 Richland County lignite pyrolysis, 177-178, 181 reaction with hydrogen, 150 swelling, 362 Riley gasifier, 634 Riser Cracking process, 637 River King coal, 167 Rockdale lignite aromaticity, 97 ash fouling, 548 ash slag viscosity, 320-321,325 hydrogasification, 659 hydropyrolysis, 153, 155-157 NMR of, 106 oxidation reactions, 93, 103 solvent extraction, 128 Rosebud coal, 97

693 Rosenbach lignite, 218 Roto-Louvre drying, 354, 453-454, 456 Rotten coal, 9, 10 Rubidium, 251-253, 268, 273,279 Ruthenium, 271-273 Ruthenium tetroxide, 93, 101-104, 112, 124 Rutile, 238-239, 244, 248, 277, 531 RWE-K/31n lignite, 196 Sabugalite, 262 Saleeite, 262 Samarium, 259, 268, 271-273 Samarium iodide, 95 San Miguel lignite ash deposition, laboratory, 560, 563, 565 ash formation, 519 deposit, 12 deposition, 61 desulfurization, 488 dielectric relaxation, 392, 394 grindability, 12 iron in, 232 mining, 421,425 petrography, 90-91 potassium in, 234 pyritic sulfur content, 12 pyrolysis, 173, 177 XANES of, 234 zeolites in, 519, 565 San Miguel power station, 540-541 Sand, glauconitic, 55 Sandow lignite carbonization, 179, 193, 195 deposition, 61 drying, 387 gasification, 635-637 mining, 4,421,426 moisture, 391 petrography, 87

Sapindaceae, 54 Sapotaceae, 54 Sapropelinite, 61 Saran, 366 SASOL plant, 616 Savage lignite aluminum in, 230 ash formation from, 294 ball milling, 367, 492 calcite in, 245 calcium in, 231 dolomite in, 245 grindability of lithotypes, 352 magnesium in, 233 microprobe analysis, 231,233,236237 mine, 33 nacrite in, 249

Savage lignite (continued) polonium in, 265 pyrite in, 242 pyrolysis, 186-188, 198-199 quality, 33 sodium in, 236 sulfur in, 237 uranium in, 262 Scandium, 254, 268, 271-274, 279 Schizoporis parvus, 62 Schizosporis texus, 62 Schoch's method, 391 Sclerotia, 40 Sclerotinite, 90 Scoria, 426 Scranton lignite, 241 Scrapers (in mining), 413,415, 417-418, 420, 422-423,425, 428-429 Selenite, 246 Selenium, 265, 268, 271,273-274 Self-ignition temperature, 436, 441 Semicoke, 659 Semifusinite, 79, 85, 87-90, 129 Sernapollenites, 53 Seyitomer lignite, 585 Shell 324 catalyst, 652 Shell gasification process, 635-637 Sheridan lignite oxidation reactions, 64, 92-94, 105-106, 117, 119, 121 solvent extraction, 128 Shoe polish, 656 Siderite, 61,232, 239, 245, 256, 264, 266, 268 Silica, 296, 302, 329-330, 519, 530, 556, 560, 563, 571 Silicon, 234, 268, 271-272, 274, 277-279 Silicon monoxide, 296 Silver, 258, 268, 272-273 Silver oxide, 70, 94, 105 Simonellite, 128 Slack, 416, 421 Slackening (see slacking) Slacking, 26, 27, 30, 41,432-433,452, 468469, 474

SOx control, 543-544, 585-588, 665 formation, 584-585 Sodium in ash deposition, 530, 546-552, 555 in fluid bed agglomeration, 589-590 effect on pyrolysis, 183-184 in lignites, 234-236, 268-276, 278-279 in lithotypes, 236 metal, vapor, 516, 562 promotion of liquefaction, 655 removal by ion exchange, 481-484, 486487

694 Sodium (continued) removal during drying, 461-462, 465 vaporization during combustion, 515516, 591 Sodium acetate, 296-297, 302-303, 516, 555, 570 Sodium autunite, 262 Sodium bicarbonate, 302 Sodium carbonate, 233,516 as catalyst, 168-169 reactions, 302 retention of sulfur, 237 Sodium chloride, 516 Sodium dichromate, 64, 92, 94, 105, 117, 121 Sodium formate, 140, 143 Sodium hydroxide, 117, 128, 144, 166, 485, 488, 496, 515-516, 544, 562, 638 Sodium hypochlorite, 105 Sodium melilite, 311, 314, 518, 555, 568-570, 573-574, 577-578, 581 Sodium methoxide, 118 Sodium nitrate, 233,292, 495 Sodium oxide, 305 Sodium periodate, 115 Sodium silicates, 304-305 Sodium smectite, 270 Sodium sulfate in ash deposition, 552-553,560-562, 570, 581 in combustion products, 515, 519, 536, 563 in lignite, 247 reactions, 296, 302-303,305, 312, 314,516 in scrubbers, 543 Sodium sulfite, 312, 543 Sodium toxicity, 20 SOLGASMIX, 332-333 Solvent Refined Coal process SRC I, 144, 297, 473, 648 SRC II, 639, 648 Soma lignite, 659 South Hallsville lignite hot-water drying, 497 slurry combustion, 667-668 slurry properties, 497 Southland lignite, 97 Soya lignite hydropyrolysis, 156 oxidation reactions, 65, 106 Specific heat, of ash, 307 Sphagnum, 53, 62 Sphalerite, 243,263 cadmium in, 263 Spinel, 293 Spontaneous combustion (See autogenous

combustion) Sporinite, 58, 66, 86-88, 90-91

Sporinite (continued) fluorescence, 88 oxidation reactions, 93, 106, 116 solvent extraction, 128 synthetic, 66 Sporopollenin, 66, 71 Stansfield-Gilbert method, 388 Sterane, 105 Sterene, 105 Stereisporites, 53 Steriosporites antiquasporites, 52 Stretford process, 619 Strontium in lignites, 253-254, 268, 271,273-274 sorption by lignite, 73 Strontium sulfate, 254 Styrene, 192-193 Suberinite, 89 Succinic acid, 101-102, 104, 128, 159 Sulfite liquor, 496, 659 Sulfur by-product, 620 fixation during ashing, 305-306 as liquefaction promoter, 170 in lignites, 237, 268, 271-274

(elemental), 238 Sulfur balls, 242 Sulfur dioxide (see also SOx) in coalification, 72 reactions with lignites, 171,298, 476, 480, 643 Sulfuric acid, 298, 482-486 Sulfur tetrafluoride, 112 Sulfur trioxide, 292, 515, 582 (see also SOx) Sunrise lignite palynology, 53 petrography, 87 Surface Mining Control and Reclamation Act, 422, 424 Sweepers, 428 Szomolnokite, 239 Taconite, 659 Talc, 239, 250 Tantalum, 268 Tavsanli lignite, 586 Taxodiaceae, 52

Taxodiaceae pollinites, 53 Taxodiaceae-Cupressaceae, 51-52, 60 Taxodium, 53-54, 56, 62 Taylerton lignite, 119, 172 Telocollinite density, 357-3 58 pore volume, 365 Terbium, 259 Terephthalic acid, 92 Tetrabutylammonium hydroxide, 204, 653 n-Tetracontane, 97

695 Tetrahydrofuran extraction with, 128-129 swelling with, 125, 403,653 Tetralin, 161 heat of immersion calorimetry, 364 as solvent, 158, 643, 650-651,665 Texaco gasification process, 637 Texto-ulminite, 89 Thenardite, 247 Thermo lignite, 425 Thioacetic acid, 120 Thioanisole, 149 Thiobacillus ferrooxidans, 480 Thionine, 71 Thiophenol, 149 Thomsonipollis, 53, 56 Thomsonipollis magnificus, 54 Thorium in lignite, 259, 268, 271 sorption by lignite, 73 Thulium, 259 Tin, 264 Titanium, 238, 268, 271-273,277, 279, 531 Titus lignite FFIR of, 99, 107 glass transition temperature, 124 MW between crosslinks, 125 repeating units between crosslinks, 124 Toluene, 192-193 extraction with, 129, 655 from lignite, (see also BTX) 620, 637 swelling with, 367 p-Toluenesulfonic acid, 117, 129 Tourmaline, 263 Tree stumps, coalified, 27, 34, 43, 55, 82, 115, 120, 243 TriatriopoUenites arboratus, 54 Trichloroethane, 487 Tricolpopollenites hians, 54 Tricolporopollenites, 53 Tridymite, 248, 304 Triethylborane, 116 Trimellitic acid, 92, 94 Trimesic acid, 93 Trimethylamine, 487 Triplanospo ri te-De ltiospo rite, 53 Triporopollenites bituitus, 54, 60 Troilite, 303-304 Trona as catalyst, 168, 182, 184 for SOx control, 585 Tunqbilek lignite, 659 Tungsten, 258, 268 Tungsten sulfide, 489 Tyrosine, 68 Ulminite, 86-90, 224 Unit trains, 429

Uranitite, 261 Uranium from lignite, 24, 35. 661-662 in lignite, 38, 251,259-262, 268, 271272, 274, 279 sorption by lignite, 73 Uranyl sulfate, 73 Urbain equation, modified version, 327-328, 574 Vanadium, 254-255, 268, 271-274 Vandyke brown, 656, 665 Vanillic acid, 65 Vanillin, 67, 190 Vaterite, 297 Velva lignite, activated carbon from, 657 aromaticity, 98 aromaticity of lithotypes, 98 ash deposition, laboratory, 565, 571, 574 ash fouling, 548, 554 ash sintering, 558 ash slag viscosity, 335 barite in, 247 char reactivity, 605, 607 depolymerization, 129 drying, 463 gasification, 628, 632 hematite in, 244 moisture sorption, 396, 399 mine, 25 nacrite in, 249 petrography, 91 pyrolysis, 182-184, 188 quality, 27 reaction with hydrogen, 151 reaction with steam, 167-169 surface area, 363 Vermiculite, 579 Vickers microhardness, 355 4-Vinylguaiacol, 66 Vitrain, 80-85, 87-88 aromaticity, 97-98 carboxyl groups, 111 cellulose residues in, 120 friability, 346, 350 ignition delay, 508 lnorganics in, 223-224 methoxyl groups, 115 pyrite in, 242-243 pyrolysis, 184 quartz In, 248 reaction in air, 161 sodium in, 236 XPS of, 161 Vitrinite, 58, 60, 62, 89, 129 reaction with hydrogen, 153

696 Vitrinite (continued) reflectance, 88-89 Vitrite, 53, 59 Volcanic ash, 244, 249, 267 Volcanic glass, 239 Watt-Fereday equation (viscosity), 331 modified version, 334 Wax (see also montan wax), 40, 82, 113, 128, 460 Weddellite, 244, 250 Wellman-Galusha gasifier, 628-629 Wharen, G.B., 521 Whewellite, 250 Willow Branch lignite oil agglomeration, 479 petrography, 86 WinNer gasification, 622-623 Witherite, 246, 254 Wollastonite, 311,571 Wood's metal, 376 Worsham furnace, 522 Wyodak coal humic acids from, 113 liquefaction, 160 moisture, 391 Xanthone, 117 Xenotime, 246, 257 Xundian lignite, 514 Xylem, 65-66 p-Xylene, 479-480 Xylene method (moisture), 390-391 Xylenes, 192-193 from gasification (see also BTX), 620, 637 Xylenols, 94, 192, 620 Xylinite, 89 Xylinoids, 88 Xylite, 52, 60, 67-68, 494 Xyloid lignites, 8 Xylose, 120

YeguapoUis, 54 Yellow cake, 662 Ytterbium, 259, 268, 271-274, 279 Yttrium, 257, 268, 271,273-274, 279 Zap lignite (see also Beulah-Zap) ash formation, 519 desulfurization, 487 flame temperature, 515 hydrous pyrolysis, 166 liquefaction, 160, 169, 652, 654 oxidation reactions, 64, 92 porosity, 366 pyrolysis, 181-185, 205-207

Zap lignite (continued) quality, 27 reaction with hydrogen, 150 surface area, 362 Zeisel method, 114 Zeolites, 519, 565 Zinc, 263,268, 271-274, 279 Zircon, 239, 244-245, 257 hafnium in, 257 Zirconium, 257, 268, 271,273-274, 279

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