Applied Coal Petrology: The Role of Petrology in Coal Utilization

About the Editors Dr. Isabel Sua´rez-Ruiz is a scientific researcher at the Instituto Nacional del Carbon (INCAR-CSIC, Spain) working in the field of applied organic petrology. She received her Ph.D. in 1988 from the University of Oviedo (Spain) for her doctoral thesis on oil shales and source rocks. She has spent extensive periods of time carrying out research in petrology and organic geochemistry in well-known laboratories in France (Orle´ans) and the United States (Carbondale in Illinois and Lexington in Kentucky). Dr. Sua´rez-Ruiz has published extensively on topics of coal and organic petrology related to fundamental and applied aspects of this science, receiving the 2006 Organic Petrology Award from the International Committee for Coal and Organic Petrology (ICCP). Dr. John C. Crelling earned his B.A. in geology at the University of Delaware (1964) and his M.S. (1967) and Ph.D. (1973) degrees at the Pennsylvania State University. He started his professional career in 1972 at the Homer Research Laboratories of the Bethlehem Steel Corporation and joined the faculty at Southern Illinois University (U.S.) in 1977. As the leader of the Coal Characterization Laboratory, he established a research program in coal petrology and started the Maceral Separation Laboratory for the separation and characterization of pure coal macerals. He recently created an Internet-based petrographic atlas of coals, cokes, chars, carbons, and graphites. Professor Crelling has received numerous awards, including the Gilbert H. Cady Award from the GSA, the Joseph Becker Award from the Iron and Steel Society, and the Reinhardt Thiessen Medal from the ICCP.

Contributing Authors John C. Crelling (Chapters 7, 8, 11) Department of Geology, Southern Illinois University, Carbondale, USA Joan S. Esterle (Chapter 3) Mining Geoscience Group, CSIRO Exploration and Mining, Kenmore, Australia Robert B. Finkelman (Chapter 10) University of Texas at Dallas, Richardson, USA Stephen F. Greb (Chapter 10) Kentucky Geological Survey, University of Kentucky, Lexington, USA Gareth D. Mitchell (Chapter 6) Coal & Organic Petrology Labs, EMS Energy Institute, Pennsylvania State University, University Park, USA Jack C. Pashin (Chapter 9) Geological Survey of Alabama, Tuscaloosa, USA Isabel Sua´rez-Ruiz (Chapters 1, 2, 4, 8, 11) Instituto Nacional del Carbo´n (INCAR-CSIC), Oviedo, Spain Nicola J. Wagner (Chapter 5)* Coal & Carbon Research Group, School Chemical & Metallurgical Engineering, University of Witwatersrand, Wits, South Africa Colin R. Ward (Chapters 1, 2, 4) School of Biological, Earth & Environmental Sciences, University of New South Wales, Sydney, Australia

xiv Contributing Authors

*Assisting Dr. Wagner with Chapter 5 M. Coertzen Research and Development, Sasol Technology, R&D, Sasolburg, South Africa R. H. Matjie Research and Development, Sasol Technology, R&D, Sasolburg, South Africa J. C. van Dyk Research and Development, Sasol Technology, R&D, Sasolburg, South Africa

Acknowledgments The material covered in this book represents the contributions of highly qualified and well-known specialists in the coal field, particularly in the branch of coal petrology. We, the editors, are extremely grateful to the authors for their individual chapters and to the contributors of Chapter 5, M. Coertzen, R. H. Matjie, and J. C. van Dyk from SASOL (South Africa), for the great effort they made in preparing their contributions that have made this book possible. Thanks are also due to those colleagues who helped by reviewing chapters and sections of the original manuscript. Their opinions, remarks, and comments contributed greatly to the improvement of the final version of this book. Special thanks are given to J. M. Diez Tasco´n and to M. Granda (INCAR-CSIC, Spain) for their revision of Chapter 8; and to M. Granda, C. Blanco, and B. Ruiz (INCAR-CSIC, Spain); R. A. Creelman (Australia); R. H. Matjie (SASOL, South Africa); R. Gray (United States); Bill Huggett (SIU-C, Illinois, United States); CAER (UKY, United States), and Z. Baoshan (China) for providing us with the pictures included in Chapters 4, 5, 7, 8, 10, and 11. We would also like to thank A. Go´mez (INCAR-CSIC, Spain) for offering his support in the bibliographic section. Permissions to reproduce copyrighted material relating to figures, tables, and microphotographs were given by Elsevier, Blackwell, the American Chemical Society (ACS), Gebru¨der-Borntraeger (Germany), CSIRO (Australia), the Pittsburgh Coal Conference (University of Pittsburgh, United States), Taylor & Francis, the Energy Institute (London), Steel Publications, Association for Iron & Steel Technology (AIST), the Royal Microscopical Society (Wiley Blackwell Publishing), and the Society for Mining, Metallurgy, and Exploration, Inc. Finally, we want to acknowledge the cooperation of Elsevier for giving us the opportunity to publish this book, particularly for the help of Kenneth P. McCombs, Kristi Green, Diana Spencer, Jane Macdonald, Anne McGee and Mageswaran BabuSivakumar for providing the facilities and their support and help in preparing the manuscript.

Preface This book is intended as a reference book, providing extensive information on the applications of coal petrology and the importance of that discipline for characterization of coals in terms of composition and rank, for the assessment of coal quality, and for the prediction of coal properties and behavior during utilization. The book is especially recommended for coal producers, coal marketers, and others associated with the coal industry, as well as for coal researchers and teachers. It provides extensive background information not only on different coal characteristics and properties but also on the way those characteristics determine technological properties. The compilation begins with an introduction to coal petrology and an outline of the fundamental concepts, familiarizing the reader with the general nature of the discipline. This discussion is followed by a synthesis of the current geographical distribution of coal resources and coal reserves. The most recent estimates for world coal mining, coal production, and consumption are outlined, as are the likely trends for the near future. Coal mining and some of the environmental issues associated with coal are only briefly described here; these topics are dealt with in other chapters. The role that applied coal petrology has played in understanding coal and the history of coal petrology from the very beginning are also briefly described. Chapter 2 is devoted to a description of the basic factors, especially those related to coal composition and rank, which influence coal properties, coal quality, and technological behavior in various processes. Current coal petrographic nomenclature and the terminology of coal components, as well as the main coal classifications, are also discussed. Coal composition in terms of its organic/inorganic components, including trace elements, coal evolution during the coalification process, and the role of coal rank and its determination, are emphasized. The importance of blending coals to obtain the quality required by the final consumer is also stressed, since coal petrography as a tool used in coal petrology is the only means by which the individual coals that make up blends can be identified. Chapter 3 covers the coal mining issues and coal beneficiation processes prior to coal utilization. In this section the influence of

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Preface

coal composition during mining and beneficiation is discussed in depth, together with the influence of mining and beneficiation on the physical properties of the coal products. Chapters 4 to 7 deal with the main uses of coal in combustion as a source of electrical energy, and in gasification, liquefaction, and carbonization processes. Particular stress is placed on the specific properties of coal for each process and the role of petrology in the characterization and prediction of properties and coal behavior, as well as in the characterization of certain derived coal products or byproducts such as fly ash, gasification residues, and cokes. Because coals are also precursors of carbon materials, coal petrology, particularly petrography, has been recently applied to the characterization of synthetic carbon products. This is an issue that is discussed in Chapter 8, where examples from applied petrology are used in the study of well-known carbon materials such as carbon/carbon composites, graphites, and activated carbons. Two other significant aspects of coal—its importance as a petroleum source rock and its function as a reservoir rock due to its physical properties—are dealt with in Chapter 9. Environmental and health matters in relation to coal as a material resource and in coal utilization are covered in Chapter 10. This chapter also includes some well-known examples of the impact of coal on health and the environment in relation to coal mining and coal utilization in various parts of the world. Chapter 11 covers the role and potential role of organic petrology applied to wider fields, perhaps only partially related to coal, such as archaeology or forensic science. These topics are illustrated and highlighted with examples. Each of the subjects discussed in these chapters is a major topic in itself, and this book is able to offer only a short review of each, highlighting in particular the successful applications of coal petrology to coal utilization. The bibliographic material at the end of the book can be used as a further guide to the topics discussed in the chapters. We have attempted to provide comprehensive information that may serve as a reference for the reader and a basis for further understanding of this wide-ranging field. We hope that the book will provide an accurate and meaningful source of data and information for many years to come.

CHAPTER 1

Introduction to Applied Coal Petrology Colin R. Ward Isabel Sua´rez-Ruiz

1.1 Fundamental Concepts Coal is a combustible sedimentary rock, composed essentially of lithified plant debris. The plant debris was originally deposited in a swampy depositional environment to form a soft, spongy sediment called peat. However, physical and chemical processes brought about by compaction and elevated temperatures with prolonged burial at depths of up to several kilometers and over periods of up to several hundred million years then changed the peat into coal through a process referred to as coalification or rank advance. The properties of a given coal can be related to three independent geological parameters, each of which is determined by some aspect of the coal’s origin. As discussed more fully by authors such as Ward (1984), Diessel (1992a), Taylor et al. (1998), and Thomas (2002), these parameters are briefly defined as follows: l

l

Rank. Coal rank reflects the degree of metamorphism (or coalification) to which the original mass of plant debris (peat) has been subjected during its burial history. This depends in turn on the maximum temperature to which it has been exposed and the time it has been held at that temperature and for most coals reflects the depth of burial and geothermal gradient prevailing at the time of coalification in the basin concerned. Heat flow from nearby igneous intrusions, however, may also play a part. Type. Coal type reflects the nature of the plant debris from which the original peat was derived, including the mixture of plant components (wood, leaves, algae, etc.) involved and the degree of degradation to which they were exposed before burial.

Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

2 Applied Coal Petrology

l

The individual plant components occurring in coal, and in some cases fragments or other materials derived from them, are referred to as macerals (see Chapter 2); these form the fundamental starting point for many different coal petrology studies. Grade. The grade of a coal reflects the extent to which the accumulation of plant debris has been kept free of contamination by inorganic material (mineral matter), including the periods before burial (i.e., during peat accumulation), after burial, and during rank advance. A high-grade coal is therefore a coal, regardless of its rank or type, with a low overall proportion of mineral matter, and hence a high organic matter content.

Although organic matter derived from marine algae occurs in very old (Precambrian) sedimentary rocks, land plants capable of forming coal did not appear until the Silurian and Devonian periods. Major coal deposits occur in the Carboniferous strata (354–290 My) of Europe and North America, and in the Permian (290–248 My) sequences of Australia, India, South America, and the other land masses that made up the former continent of Gondwanaland. Coals of Carboniferous and Permian age also occur in China. Mesozoic coal occurs in a number of areas, notably the Jurassic (205–142 My) of Australia and China and the Cretaceous (142–65 My) of North America. There are also significant resources of Palaeogene and Neogene age (65–1.8 My) in various continents, including Europe, North America, Asia, and Australia.

1.2 Coal Resources, Mining, and Utilization Coal is a versatile fossil fuel that has long been used for a variety of domestic and industrial purposes. It currently provides around 25% of the world’s total primary energy (International Energy Agency, 2007) and, although subject to some possible variation with different policy developments, is expected to provide a similar share in future years (e.g., 23–26% in 2030; International Energy Agency, 2007). Most of the world’s coal is used for the production of electric power (see Chapter 4). The other main use is for production of coke as a reducing agent in the iron and steel industry (see Chapter 7). Coal is also used as fuel for a range of manufacturing processes, such as the production of heat in cement kilns and other industrial plants, gasification and petrochemical production (see Chapter 5), and heating domestic and commercial buildings. In addition, it is used as a raw material in a range of nonenergy applications (see Chapter 8), such as the production of carbon electrodes for the aluminum industry or as a precursor for a number of other carbon-based industrial materials.

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The availability of coal resources has been a major contributor to the economic growth of many countries, either directly through their own resources or indirectly through access to the international coal trade. In the late 19th and early 20th centuries, coal was mainly used as a transport fuel (such as for ships and railway locomotives) or as a source of heat and power for industrial and domestic applications. In the middle of the 20th century the use of coal decreased in some areas because of low oil prices, but the oil supply crisis of the 1970s reversed this trend and led to an increase in coal consumption. Another consequence of the oil supply crisis was a significant increase in coal liquefaction research and development (see Chapter 6), although much of this work was subsequently put on hold when oil prices stabilized. Though coal usage has continued to increase, environmental concerns and changes in the political climate have again begun to give coal an unfavorable public image. Increasing concerns about coal utilization as a contributor to greenhouse gas emissions, particularly CO2, have led to more intense questioning of the role of coal and a renewed search for alternative energy sources. According to International Energy Agency data (International Energy Agency, 2007), coal became the world’s principal source of anthropogenic CO2 emissions in 2004, moving ahead of emissions derived from oil and natural gas sources. This has led to another change in the focus of coal research, with the emphasis shifting toward increasing the efficiency of coal utilization and to integrating coal utilization with CO2 sequestration/ storage processes.

1.2.1 Coal Resources and Production According to data reported by the World Energy Council (2007), the total proven recoverable reserves of coal worldwide (all ranks) are currently estimated at 847 Mt, made up of 431 Mt of bituminous coal and anthracite, 267 Mt of subbituminous coal, and 150 Mt of lignite. The reserves are located on every continent and in over 70 countries, with major proportions identified in the United States, the Russian Federation, China, India, Australia, South Africa, Ukraine, and Kazakhstan. World coal production in 2006 was 6,284 Mt (International Energy Agency, 2007), represented by 5,370 Mt of hard coal (bituminous coal and anthracite) and 914 Mt of subbituminous coal and lignite (brown coal; Table 1.1). This continues the trend previously reported by the World Coal Institute (2005), indicating an overall increase of about 40% in coal production during the past 20 years. China is now the largest single producer, with 2,841 Mt of hard coal in 2006. Other major producers, especially of hard coal, include the United States, India, Australia, South Africa, Russia, and Indonesia.

Coal Production People’s Republic of China United States of America India Australia South Africa Russia Indonesia Poland Kazakhstan Colombia Rest of world Total

Hard Coal (Mt)

Brown Coal (Mt)

2,481

(a)

Australia

231

Japan

178

990

76

Indonesia

129

Korea

80

427 309 244

30 71 0

92 69 63

Taiwan United Kingdom Germany

64 51 41

233 169

76 0

60 45

61

92 64 266 5,370

5 0 595 914

India People’s Republic of China United States of America Russia Italy Rest of world Total

41 37

95

Russia South Africa People’s Republic of China Colombia United States of America Canada

(a) Included with hard coal production.

Coal Exports

Kazakhstan Vietnam Rest of world Total

Hard Coal (Mt)

27 26 22 51 815

Coal Imports

Hard Coal (Mt)

33 26 25 243 819

4 Applied Coal Petrology

TABLE 1.1 Coal production, exports, and imports by country, 2006 (Data compiled from International Energy Agency, 2007.)

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5

The size and extent of the world’s coal reserves suggest that there is enough available to meet demands for the next 150–190 years at current production rates. The life of the reserves could be extended still further through discovery of new reserves and through upgrading less well-known deposits as a result of further exploration activities. Reserves may also be effectively increased by advances in mining technology that would allow previously inaccessible resources to be reached. On the other hand, limitations on coal mining and use due to higher costs, increased regulatory restrictions, carbon penalties, and land-use conflicts may reduce the recoverable reserves below levels that would otherwise be available for economic use. The great magnitude, widespread distribution and relatively long projected life of the world’s coal resources, compared to those of oil and gas, and the fact that many economies still depend on coal for a significant part of their energy needs (Thomas, 2002; Mills, 2004) mean that coal is expected to continue as a major energy resource for the next few decades at least. This does not, however, remove the need to establish new technologies or to improve those already in existence, to control and where possible reduce the emissions from coal utilization so that they are in accord with emerging environmental regulations and trading agreements.

1.2.2 Coal Mining and Utilization Coal is mined via two basic methods, surface and underground mining, with the choice being mainly determined by the geology of the deposits involved. Coal seams close to the ground surface, where the overlying strata as well as the coal itself can be safely and economically removed, may be won by open-cut mining techniques (see Figure 1.1). Although such operations may have a more significant environmental impact than coal extraction by underground methods, a higher proportion of the in situ coal (usually more than 90%) is recovered for use, including in many cases seams that are either too thin or too thick for effective recovery in underground operations. With some possible exceptions, open-cut methods usually provide coal at a lower overall cost than underground mines; they also avoid some of the safety hazards, such as roof falls, gas outbursts, and coal-dust explosions, that can occur in the underground mining environment. Most of the world’s coal resources, however, occur at depths where only underground mining is likely to be possible. Underground mining currently accounts for about 60% of world coal production (World Coal Institute, 2005), and open-cut mining the remaining 40%. The coal extracted from an open-cut or underground mine, referred to as run-of-mine (ROM) coal, often contains impurities such as rock from the roof and floor or layers of noncoal material occurring

6 Applied Coal Petrology

FIGURE 1.1. Aerial view of open-cut coal mine, Queensland, Australia, showing overburden removal to expose the coal seam. (Photo: C.R. Ward.)

within the seam. A coal preparation process (see Chapter 3) is commonly used to reduce the proportion of this material, ensuring a consistent standard of coal quality and enhancing the suitability of the mine product for specific end uses. Coal currently supplies fuel for 39% of the world’s electricity generation, a proportion that is expected to remain at a similar level for at least the next 30 years (International Energy Agency, 2007). The demand for coal in the iron and steel industry is expected to increase by almost 1% per year over the same period of time. The biggest market for coal, particularly for steam-raising (power generation) and coking purposes, is found in the developing Asian countries, which account for 54% of total coal consumption. China is one of the most significant coal users, and the United States, India, Japan, and Russia are large coal consumers. Coal is also traded all over the world. Australia is the world’s largest coal exporter (Table 1.1), with almost 75% of the country’s coal exports going to the Asian market. Other major exporters include Indonesia, Russia, South Africa, China, and Colombia. Certain characteristics of coal ensure its place as an efficient and competitive energy source and that it contributes to stabilizing energy prices. As reported by the World Coal Institute (2005), key factors include (1) the very large reserves without associated geopolitical or safety issues, (2) the availability of coal from a wide variety of sources, (3) the facility with which coal can be stored in normal conditions, and (4) the nonspecial and relatively inexpensive protection required for the main coal supply routes.

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7

1.3 Environmental Issues and Options Both coal mining and coal consumption have a significant impact on the natural environment. Thus, although coal is an important contributor to the economic and social development of many countries, there is a strong need to minimize and where possible reduce the negative impacts associated with its mining and use. Some of these impacts and their implications, especially to human health, are further discussed in Chapter 10.

1.3.1 Impacts Associated with Coal Mining Coal mining, particularly in the case of open-cut mines, requires that large areas of land are temporarily disturbed, and perhaps withdrawn from other productive uses such as agriculture, while the process of coal extraction is under way. The land may, however, be returned to productive use after mining or even while mining in other parts of the area is still in progress (Figure 1.2). With underground operations, subsidence may result in a lowering of the ground surface above and around the mined-out coal bed. Other environmental risks associated with coal mining may include increased generation of noise and dust around the mine installations, soil erosion, water pollution (including in some cases acid mine drainage), and potential impacts on local biodiversity (wildlife and vegetation). An associated problem may be the release of methane (referred to as coal mine methane, or CMM), which, as well as being potentially explosive, is also a significant greenhouse gas. If present in the subsurface coal seam, the methane has to be drained before underground

FIGURE 1.2. A golf course developed on rehabilitated overburden dumps at an open-cut coal mine in northern Thailand. (Photo: C.R. Ward.)

8 Applied Coal Petrology

mining operations, to maintain safe working conditions. Ideally, however, the methane should be used rather than simply released into the atmosphere, even though, if mixed with the ventilation exhausts from underground mines, it may only be at very low concentrations. Methane accounts for 18% of the overall global-warming effect associated with human activities (World Coal Institute, 2005), although methane from coal accounts for only 8% of the world’s major methane emissions. Utilization of methane produced during coal mining would clearly reduce the environmental impact its release might otherwise generate and could also have a commercial benefit through uses such as additional power generation. Before a coal mine is allowed to commence operations, thorough studies must be carried out to identify all the potential risks to the surrounding environment and to minimize any negative impacts. Such a study, especially for an open-cut mine, should include a final land rehabilitation plan aimed at returning the land to other acceptable uses once the mining operations are completed.

1.3.2 Impacts of Coal Combustion Coal consumption for power generation and heat production is of growing environmental concern, due mainly to emissions of CO2 associated with the combustion process. The release of CO2 into the atmosphere as a consequence of human activities, especially those related to fossil fuel combustion, has been reported to be linked to increased global warming and associated climate change. According to the World Coal Institute (2005), CO2 emissions from all sources, including coal, account for around 50% of the overall global-warming effect associated with human activities. Although coal is only one of many sources represented by this anthropogenic CO2, the coal industry is searching for and developing technological options to mitigate its contribution to the problem. As part of this process, new technologies and improvements to existing technologies have been developed to increase the efficiency of combustion and power generation and reduce the CO2 and other emissions per unit of electrical energy produced. As well as CO2, the emissions released during coal combustion (described more fully in Chapters 4 and 10) may include oxides of sulphur and nitrogen (SOx and NOx), fine solid particulates, and possibly a range of trace elements, among which mercury is of special interest in some areas (U.S. Environmental Protection Agency, 2005a; Commission of the European Communities, 2005). Clean coal technologies offer a series of technological options for improving the environmental performance of coal, reducing emissions and at the same time increasing the amount of useable energy derived

Introduction to Applied Coal Petrology

9

from each ton of coal. These can include more efficient coal preparation (see Chapter 3), which serves not only to increase the heating value of the coal and therefore the efficiency of the combustion process but may also reduce the levels of sulphur and ash-forming mineral matter. This may in turn help to reduce the amount of waste as well as, perhaps, SOx and potentially harmful trace elements associated with the combustion process. Sulphur and nitrogen oxides released into the atmosphere from coal combustion (as well as from other noncoal sources) may chemically react with water vapor and other substances to form acids that are finally deposited as acid rain. Use of feed coals with a low sulphur content is in many cases the most economical way of reducing SOx emissions. An alternative, however, is the incorporation of flue gas desulphurization (FGD) systems in power plants (see Chapter 4), which can remove as much as 99% of the SOx emissions otherwise released and therefore help considerably in the prevention of acid rain problems. The technologies developed to reduce SOx emissions from coal combustion in power plants are also effective in some cases for reducing emissions of sulphur-related trace elements such as mercury. Emissions of nitrogen oxides contribute not only to the formation of acid rain but also to the development of photochemical smog. In the case of coal combustion these emissions may be controlled by the use of improved burner designs (low NOx burners) and possibly by using selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) technologies to treat the flue gas stream. Although such an approach may reduce NOx emissions by up to 80–90% (World Coal Institute, 2005), there may be a trade-off due to less efficient combustion of the coal, resulting in higher levels of unburnt carbon in the ash from the power plant. This may in turn impact the usefulness of the ash for processes such as cement and concrete manufacture. Another alternative is to use fluidized-bed combustion (FBC), an advanced, high-efficiency technology that may reduce both nitrogen and sulphur oxide emissions by 90% or more (World Coal Institute, 2005). The release into the atmosphere of fine particulates from coal combustion can be reduced, if not totally eliminated, by incorporation of electrostatic precipitators (ESP) or baghouses with fabric filters (FF) in the combustion stream. These may recover the suspended coal ash (fly ash) and other fine particulates with an efficiency of up to at least 99.5%.

1.3.3 Amelioration of Combustion Impacts Because of CO2’s association with global warming, the reduction of CO2 emissions from coal utilization is one of the biggest present-day challenges faced by the world coal industry. In the case of coal

10

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combustion a significant step has been taken with the development of supercritical and ultra-supercritical steam cycle technologies, which can achieve thermal efficiency levels of 43–45% (supercritical) to 50% in the case of ultra-supercritical power plants (World Coal Institute, 2005). Integrated gasification combined cycle (IGCC) technology (see Chapters 4 and 5) is another possible option, producing gas from coal for use in a gas turbine rather than using direct coal combustion. IGCC-based power plants may reach high efficiency levels (50%) and can also be designed to capture CO2 emissions more effectively for input to subsurface storage systems. Carbon capture and storage technologies are perhaps the most promising options for substantially reducing CO2 emissions from coal utilization, and a great deal of research is being carried out in this field (see Chapter 9). This research is directed toward both the procedure for CO2 capture from the gases produced in different utilization plants and the identification of appropriate geological environments and sites where CO2 can be permanently stored in a way that prevents its escape back into the atmosphere. Although there are other sources of CO2 emissions from coal, such as the iron and steel industry, cement plants, and domestic usage, and there are also numerous sources of CO2 unrelated to coal utilization (e.g., motor vehicles, air transport, and charcoal production), the initial focus of such activities is mainly on coal combustion in power plants. This is partly because they represent large but stationary sources of CO2 and partly because the organizations involved in their operation might be expected to have a capacity for implementing any remediation measures that may be developed.

1.4 The Role of Applied Coal Petrology For as long as coal has been used in industry it has been important to assess the quality of the coal and determine the chemical and physical properties that influence its suitability for the purpose in question. Some coals can be sold and used in the as-mined state (i.e., as ROM coal), whereas others may require quality improvement through coal preparation processes. Coals from different sources can also be blended to obtain a product that has particular quality characteristics. Although some of the tests and analyses applied to coal have changed in response to new technological developments and improvements in analytical techniques, coal science still has a strongly traditional basis. Many of the tests that are used in coal characterization, such as proximate and ultimate analysis and heating value determination (see Chapter 2), are little changed from those applied to coal testing over 100 years ago.

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1.4.1 History and Scope of Coal Petrology Coal petrology is also a branch of coal science that dates back to the beginning of the 20th century, and it was in 1913 that White and Thiessen laid down some of the fundamentals that underpin coal petrology today. The titles of Thiessen’s works in the 1920s, Under the Microscope Coal Has Already Lost Some of Its Former Mystery, reflected new discoveries in this emerging field (Thiessen, 1920a–c, 1921, 1926). However, the thin-section techniques Thiessen used and the resulting nomenclature developed at the U.S. Bureau of Mines (the Thiessen–Bureau of Mines system) are not used in modern coal characterization. At about the same time, Stach (1935) was developing the discipline in Europe, in this case using reflected-light microscopy techniques. These techniques are still used in modern coal petrography. Stopes (1919, 1935), who was also interested in the nature of coal, coined the term maceral and established the concept of lithotypes that is also used today (see Chapter 2). Cady (1939) introduced the lithotype concept to the North American coal community, although the concept of macerals was not adopted until later. The widespread use of coal petrology in the steel industry and the founding of the International Committee for Coal Petrology (now the International Committee for Coal and Organic Petrology, ICCP) in 1953 and later the North American Coal Petrographers group (succeeded by The Society for Organic Petrology, TSOP, in 1984) served to emphasize the use of reflected-light techniques in the study of coal, to unify maceral nomenclature, and to establish a classification of the coal components that can be identified using microscopic techniques. The currently accepted classification of coal components is a result of the work of the ICCP, expressed through the production of a number of editions of the International Handbook of Coal Petrology (ICCP, 1963, 1971, 1975, 1993). These have been replaced in part by more recent publications (ICCP, 1998, 2001; Sy´korova´ et al., 2005), detailing revised and expanded nomenclature and a new reclassification. Because coal is a complex rock, coal petrology is a broader subject than merely the simple study of its organic constituents, the macerals. In the present work, coal petrology is broken down into three fundamental components: (1) organic petrology, (2) inorganic petrology and geochemistry, and (3) coal rank, or the metamorphic transformation of the macerals and minerals in coal. Investigating a coal for the purpose of utilization involves knowing something about all these characteristics, none of which should be separated from the others. Coal quality is a function of these factors and their interactions, and coal petrology is the fundamental

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discipline that contributes to the knowledge of coal quality. The petrology of a coal may be expressed by a number of fundamental parameters, including (1) the nature of the organic constituents in terms of macerals or maceral groups (an indicator of coal type), (2) the mineral matter, including the major elements in the coal or oxides in the ash, the minerals in the coal, the forms of sulfur, and the trace elements that may also be present (indicators of coal grade), and (3) the vitrinite reflectance (which is usually taken as an indicator of coal rank). These parameters reflect the composition and rank of the coal and are the primary factors that contribute to the coal’s specific physical and chemical properties. The physical and chemical properties in turn determine the overall quality of the coal and its suitability for specific purposes. The analytical procedures used to determine the petrographic, physical, and chemical properties have been standardized in a number of international norms (ISO, ASTM, etc.) and additional discussion is given in compilations such as those of Peters et al. (1962), Karr (1978a,b; 1979), Ward (1984), van Krevelen (1993), and Thomas (2002).

1.4.2 Coal Characteristics for Utilization The basic chemical parameters of a coal are determined by proximate analysis (moisture, ash, volatile matter, and fixed carbon percentages) and ultimate analysis (carbon, hydrogen, nitrogen, sulphur, and oxygen contents). Other analyses that may be carried out include determining the forms of sulphur in the coal (pyritic, sulphate, organic) and the carbon (or CO2) content derived from the carbonate mineral fraction. The chlorine content, which is mainly associated with inorganic salts (relatively high proportions of chlorine may give rise to corrosion in coal utilization), and the phosphorous content (an undesirable element in coals to be used in the steel industry) may also be determined. The ash of the coal may be analyzed to determine the major and minor metal oxides (these influence coal and ash behavior during usage), and the proportions of a number of different trace elements, some of which could be potentially hazardous, may also be evaluated. In addition to the chemical properties, effective use of coal also requires knowledge of particular physical properties, such as the coal’s density (which is dependent on a combination of rank and mineral matter content), hardness, and grindability (both related to coal composition and rank). Other properties include the coal’s abrasion index (derived mainly from coarse-grained quartz) and the particle size distribution. Float-sink testing may also be integrated with the analysis process, separating the (crushed) coal into different density fractions as a basis for assessing its response to coal preparation processes. Float-sink techniques may also be used to provide a coal sample that

Introduction to Applied Coal Petrology

13

represents the expected product of a preparation plant, to assess the quality of the coal that will actually be sold or used rather than the in situ or run-of-mine material represented by an untreated (raw) coal sample. As well as proximate and ultimate analysis data, the coal quality parameters that need to be taken into account in coal combustion, such as in coal-fired power plants, include information from a number of specific tests, such as: l

l

l

l

The Hardgrove grindability index (HGI). This indicates the ease with which the coal can be ground to fine powder and is important for gauging the coal’s compatibility with the precombustion pulverization system of the plant concerned. This HGI is most directly related to the maceral and maceral group composition (see Chapter 3) but is also dependent on rank and mineral content. The heating value, calorific value, or specific energy. This indicates the amount of heat liberated per unit of mass of combusted coal and is of fundamental importance in setting the price of particular coals for combustion applications. Although generally regarded as a rank-related parameter, the calorific value is also dependent on the macerals in the coal and the mineral composition. The total sulfur content. This may be derived from a combination of the organic constituents and the mineral matter. As well as the overall percentage, it may be expressed in some cases as the sulfur dioxide emissions expected in relation to the heating (or calorific) value—for example, as kg SO2/GJ. The ash fusion temperatures. These indicate the behavior of the ash residues from the coal at high temperatures and are mainly related to the chemical composition of the ash and the nature of the coal’s mineral matter (see Chapter 4). They are used to indicate whether the ash will remain as a fine powder within the furnace system after the coal is burned or whether it might partly melt and form a slag on the boiler’s heat exchange surfaces.

Other tests that provide information about the potential behavior of coals during carbonization and coking processes (see Chapter 7) include: l

The free-swelling index (FSI) or crucible swelling number (CSN). This is a measure of the increase in volume of the coal when it is heated in a small crucible in the absence of air. This test is also used to characterize coals for combustion, especially

14

Applied Coal Petrology

l

l

l

where the coal is burned in beds of coarse-crushed material in stoker-based systems. The Roga index. This test provides information on the caking properties of the coal, in a similar way to the free-swelling index. The index itself is derived from the strength or cohesion of the coke produced in the crucible, as evaluated by a subsequent tumbler test. The Gray-King and Fischer assays. These determine the proportions of coke or char (carbonaceous solids), tar (organic liquids), liquor (ammonia-rich solutions), and gas produced when the coal is carbonized (heated in the absence of air) under particular laboratory conditions and hence provide a basis for estimating the yields of coke and coke byproducts obtained from the coal in an industrial coke oven or oil-shale processing plant. Giseler plastometer and Audibert-Arnu dilatomer tests. These monitor how the coal behaves as the different macerals melt, devolatilize, and resolidify at different temperatures during the carbonization process (see Chapter 7). The Gieseler plastometer evaluates the coal’s behavior by measuring the fluidity of a packed coal powder as it is heated, whereas the AudibertArnu dilatometer measures the contraction and expansion of a powdered sample pressed into a cylindrical coal “pencil.” Such properties are significant when different coals are blended for coke production, to ensure compatibility of the different blend components. Indeed, coal-blending strategies for coke production are generally decided from a combination of rheologic and petrographic parameters for individual coal samples, which are used to select coals to make up a blend with specific coking properties.

1.4.3 Petrological Controls on Coal Characteristics Although various combinations of these tests are used to evaluate the suitability of particular coals for a variety of industrial processes, the properties determined by the various tests are ultimately related to the coal composition (organic and inorganic components) and the coal rank (degree of metamorphism). Organic petrology therefore plays a fundamental role in determining coal behavior, and evaluation of petrographic properties (e.g., maceral percentages, vitrinite reflectance, mineral matter composition) should be an essential part of any coal analysis and testing program. Another factor that must be taken into account in determining coal quality is the degree of coal oxidation. Coal oxidation may result from exposure to weathering processes during handling and transport or when the coal is stockpiled under different environmental conditions.

Introduction to Applied Coal Petrology

15

Oxidation may affect both the organic and inorganic components and can give rise to deterioration in the coal properties, especially those relevant to coking applications (see Chapter 7). Petrographic examination may help to identify coals that have been oxidized and perhaps explain any anomalous behavior associated with the oxidation process. A different consequence of coal oxidation is the development of spontaneous combustion (see Chapters 10 and 11), when the heat generated by in-situ oxidation causes the coal to smolder and ultimately burn without any external heat source. The liability to oxidation is mainly determined by the coal’s rank, in conjunction perhaps with the maceral and mineral (e.g., pyrite) content (see Chapters 3 and 11). Low-rank coals are particularly prone to spontaneous combustion; other factors, such as access of air to coal stockpiles, may need to be controlled to reduce spontaneous combustion risk. Role of Organic Constituents The organic constituents of coal, including both the maceral groups (liptinite, inertinite, and huminite/vitrinite) and the individual macerals in those groups (see Chapter 2), are, singly and in combination (as natural associations or microlithotypes), fundamental to many coal properties. Vitrinite is the most common maceral group in many coals, especially the Carboniferous coals of the Northern Hemisphere, and it is the properties of the vitrinite in such coals, together with the variations in those properties with rank, that to a large extent determine the properties of the coal concerned. There are, however, major exceptions to this principle, such as with the Permian Gondwana coals of India, Australia, Southern Africa, South America, and Antarctica. These coals are commonly rich in inertinite-group macerals, with vitrinite in some cases forming only a relatively minor component. The different maceral assemblage reflects deposition in a cooler and drier climate and a more terrestrial environment than the Carboniferous coals of Europe or North America. The Gondwana coals are also more variable, and vitrinite-rich and inertinite-rich coals may occur in close proximity, and even as different parts of the same coal seam. Environmentally Significant Inorganic Components The inorganic constituents of coal are often expressed on the basis of simple parameters such as ash yield and sulfur content. It is, however, often convenient to express an inorganic constituent relative to another parameter, such as the expression of sulfur in terms of SO2 emissions per unit energy, with the latter being derived from the heating value.

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Applied Coal Petrology

Knowledge of the inorganic constituents in coal may take on a more complex role if emissions of the so-called hazardous air pollutants (abbreviated as HAPs) are regulated. HAPs generally include Sb, As, Be, Cd, Ni, Pb, Se, Hg, Co, Cr, and Mn, with Cl and the radionuclides, Th and U, also included in some assessments. In the United States, the focus has been on Hg, with the U.S. Environmental Protection Agency (2005a) mandating a reduction in Hg emissions to 15 short tons/year by 2018 (from early 21st-century levels of about 45 short tons/year). The European Union is also taking measures on this matter, as described in Chapter 4. Many of the elements that may be of concern are trapped with the fly ash after coal combustion (discussed in Chapter 4) and, in the case of power plants using flue-gas desulphurization (FGD) systems, with the FGD (or scrubber) byproducts. Coal beneficiation processes prior to utilization may also serve as a means of reducing the levels of at least some trace elements (e.g., Hower et al., 1998). Elements of concern that occur at significant levels in the processing or utilization residues may give rise to waste disposal or control problems that are different from air pollution, through processes such as leaching into the natural environment following ground or surface water infiltration (e.g., Jankowski et al., 2006) and the potential for such issues may also need to be investigated. The lower heating value of lower-rank coals means that more coal must be burned to produce one unit of electricity compared to higher-rank (e.g., bituminous) coals. Just as the amount of sulfur in a coal can be translated into kg of SO2 per GJ of energy produced, on the basis of the heating value, the abundance of HAPs and similar elements may also need to be expressed in energy terms. Evaluation and Significance of Coal Rank Coal rank is commonly expressed in terms of vitrinite reflectance (see Chapter 2). Because it is measured by optical microscopy and takes into account only one coal component, this parameter has the capacity to provide an indicator that is independent of other factors (e.g., coal type or grade). Unlike other indicators, such as total (organic) carbon, volatile matter, or calorific value, it is not dependent on the overall coal composition (e.g., relative proportions of different macerals); the only requirement for the determination is that vitrinite is present in the coal. Although vitrinite reflectance is widely used as a measure of coal rank, it is not always a truly independent rank indicator. As discussed further in Chapter 2, some vitrinites may have anomalously low reflectance ( due, for example, to the original depositional environment), a phenomenon known as reflectance suppression (Barker, 1991), which

Introduction to Applied Coal Petrology

17

may give misleading results if other indicators are not taken into account. Despite the advantages and simplicity of vitrinite reflectance, it is very difficult to find an indicator of coal rank that is totally independent of the organic and inorganic composition or of other influencing factors such as the depositional environment of the original peat deposit. Despite the difficulties in identifying a robust rank indicator, a number of coal properties progressively change with rank advance (Figure 1.3), and the rank of a coal is thus a major factor influencing its potential usage. For example, the heating value determines how much coal is required to produce a given amount of steam and hence to generate a given amount of electricity, and the rank thus represents the fundamental basis for assessing the values of coals, per tonne, on the steaming coal market. The free-swelling index (FSI), which is important for both metallurgical and steaming coals, is also at least in part a rank-dependent parameter, increasing with rank through the high-volatile bituminous range but decreasing again above the medium volatile bituminous range. The free-swelling index also depends on the maceral composition of the coal, with the vitrinite maceral group being the main contributor to swelling properties. Some of the inertinite group macerals,

FIGURE 1.3. Variation in some key coal properties with rank advance. (Source: Coal Geology and Coal Technology, by C. R. Ward (Ed.), “Blackwell Scientific Publications, Melbourne,” 345 pp., copyright 1984, with permission from Blackwell.)

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Applied Coal Petrology

and also the mineral matter, act as diluents, reducing the swelling properties that would otherwise apply to vitrinite-rich coals at the relevant rank level.

1.4.4 Integration of Coal Petrology in the Evaluation Process This book considers coal as the sum of all its components, organic and inorganic, together with the metamorphic changes they have undergone through the rank advance process. It is the integration of these fundamental factors that is important, rather than the features or percentages of any individual component. The interplay of coal characteristics and coal utilization has been important as long as coal has been used and will continue to be important (as discussed in Chapters 8 and 9) until coal is no longer a viable economic resource. Since the beginnings of the science, coal petrology has been found to be a powerful tool in the characterization of coals for both geological and industrial applications. This book focuses on the applications of coal petrology to coal mining, preparation, and (especially) utilization as well as to other related areas, such as archaeological studies (see Chapter 11). Each of these subjects is a major topic in itself, and the book is able to present only a brief review of each. Bibliographic references are provided, however, for additional information on specific aspects and applications.

CHAPTER 2

Basic Factors Controlling Coal Quality and Technological Behavior of Coal Isabel Sua´rez-Ruiz Colin R. Ward

2.1 Introduction Conventionally, coal is used in processes such as combustion, gasification, and liquefaction and in carbonization for the manufacture of metallurgical coke. Coal and its derivative products are also used as precursors of other materials and in the production of chemicals. Thus, a coal must be characterized before it is used, whether as a single or blended coal. Characterization is performed in order to find out the properties of a coal, to determine its quality, and to predict its technological behavior. Basically there are two characteristics that influence the use of coal: its composition and its rank. Coal composition is in turn represented by two essentially independent factors (Ward, 1984): type (nature of the organic components) and grade (extent of dilution by mineral matter).

2.2 Coal Composition: Organic Components Coal is a heterogeneous material, and evaluation of coal type may be approached on two different levels: the macroscopical and microscopical, both of which form a part of coal petrology. Macroscopically, coals can be classified into two broad categories based on coal type: (1) humic coals or banded coals, which are the more common in nature and are derived from a heterogeneous mixture of a wide range of plant debris, and (2) sapropelic, nonbanded, or massive coals (ICCP Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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Applied Coal Petrology

1963), which are homogeneous in appearance and require special conditions for accumulation and preservation of the original organic matter (Stach et al., 1982). Lithotypes are the macroscopically recognizable bands in humic coals, and four lithotypes—vitrain, clarain, durain, and fusain—have been described by the ICCP (1963). These lithotypes are the result of plant growth and the physicochemical conditions existing in the peat swamps in which the organic remains accumulated. The minimum thickness of bands described as lithotypes has been established between 3 mm and 10 mm. Lithotypes can be distinguished from one another, particularly in high volatile bituminous coals, on the basis of their physical properties, such as luster, fracture pattern, color, and streak (ICCP, 1963; Stach et al., 1982; and Taylor et al., 1998). Stach et al. (1982) extended the definition of lithotypes to include cannel and boghead coals, both of which have been identified in sapropelic coals.

2.2.1 Organic Petrography: Macerals and Microlithotypes Macerals Microscopically coal is composed of various constituents (macerals), which occur together in different associations (microlithotypes). Mineral matter is also present in different proportions. Thus, macerals are the coalified remains of various plant tissues or plant-derived substances existing at the time of peat formation. Due to variable and often severe alteration during the peatification and coalification processes, it is not always possible to recognize the plant material from which many macerals were originally derived (ICCP, 1971). The formation of macerals from plant remains during the early stages of peat accumulation depends on the type of plant community, climatic and ecological controls, and conditions of the depositional environment (Stach et al., 1982). When the processes of biochemical degradation cease and the organic material is buried at great depths in the sedimentary environment, geochemical coalification over a long period of time and under conditions of high temperature and pressure takes over. As a result, the sediment of the original peat swamp is transformed and passes through the progressive evolutionary stages of lignite, subbituminous, and bituminous coal to anthracite and meta-anthracite. Throughout these stages the physicochemical characteristics of the coal as well as its technological properties are modified (Stach et al., 1982, and Taylor et al., 1998). In polished sections under the microscope using incident light, macerals are identified on the basis of their optical properties. Universal acceptance is given to the ICCP classification and redefinition (ICCP, 1963, 1971, 1975, 1998, 2001; Sy´korova´ et al., 2005) of macerals into three groups: liptinite, inertinite, and huminite/vitrinite (Figure 2.1).

Basic Factors Controlling Coal Quality and Technological Behavior 21

Sf Tl Sp

Dt Sf

(a)

(b)

F F Dt Sp (c)

(d)

R

Mi T

Tl Mi

(e)

(f)

Ct

V V

(g)

Dt

(h) (Figure legend continues on next page)

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Applied Coal Petrology

FIGURE 2.1. Photomicrographs of coal macerals in bituminous coals (medium coal rank) taken in reflected white light and with a 32
oil immersion objective (long side of the pictures: 200 mm). (a) Coal with a vitrinite random reflectance of 0.97%. Liptinite (Sp: sporinite), vitrinite (Dt: detrovitrinite), and inertinite (Sf: semifusinite). (b) Coal with a vitrinite random reflectance of 0.90%. Inertinite (Sf: semifusinite) and vitrinite (Tl: telovitrinite). (c) Coal with a vitrinite random reflectance of 0.63%. Inertinite (F: fusinite with broken cell walls). (d) Coal with a vitrinite random reflectance of 0.63%. Inertinite (F: fusinite), liptinite (Sp: sporinite), and vitrinite (Dt: detrovitrinite). (e) Coal with a vitrinite random reflectance of 0.70%. Vitrinite (T: telinite), liptinite (R: resinite in cell cavities), and inertinite (Mi: micrinite generated from hydrogenate macerals such as the sporinite). (f) Coal with a vitrinite random reflectance of 0.97%. Vitrinite (Tl: telovitrinite) and inertinite (Mi: micrinite). (g) Coal with a vitrinite random reflectance of 0.65%. Liptinite (Ct: cutinite) and vitrinite (Dt: detrovitrinite). (h) Blend of coals with vitrinite (V) random reflectances of 1.40 and 0.88%, used in carbonization processes. (Photomicrographs: I. Sua´rez-Ruiz.)

These maceral groups are subdivided into a variety of macerals, submacerals, and maceral varieties on the basis of their reflectance, degree of destruction/preservation of original material, presence of cellular structure, gelification, and morphological features. The three maceral groups differ in both chemical composition and optical properties, and their names conventionally end in –inite. Macerals of the liptinite group (Table 2.1 and Figure 2.1a, d, e, g) include all the chemically distinct parts of plants such as spores, cuticles, suberine cell walls, resins and polymerized waxes, fats and oils of vegetable origin, some degradation products, and products of secondary generation during the coalification process (coal evolution). These macerals have the highest hydrogen content and contain compounds of mainly an aliphatic nature. Their color in reflected light is dark and so their reflectance is the lowest among the maceral groups. Most of the macerals of this group display a fluorescence of variable intensity when excited with short wavelength radiation, although this property disappears with increasing coal rank. During coalification most macerals of this group disappear due to thermal transformation, or they develop similar optical properties (reflectance) to those of the vitrinite group at the medium volatile bituminous coal rank stage. The influence of this maceral group in the technological properties of coal is related to the proportion in which it occurs. Because of their high hydrogen content, liptinite macerals yield high proportions of tars and gases during the carbonization process. Members of the liptinite group also have a high calorific value. The sensitivity to oxidation of liptinite macerals is low, and the hydrogenation capacity of

Maceral

Origin

Petrographic Characteristics

Sporinite

From pollen and spores

Cutinite Resinite

From cuticles of leaves Diverse origins: resins, waxes

Alginite

Algal or bacterial

Suberinite

From suberous tissues

Chlorophyllinite

From chlorophyllic pigments

Liptodetrinite

Fragments from the other liptinite macerals From vegetable oils

Individual bodies, usually compressed, well preserved, distinct botanical form, high relief, variable wall thickness and size. Most frequent maceral of this group in coals. Elongated bodies, serrated edges, well preserved, high relief. Individual ovoid, globular and irregular bodies, cell fillings, impregnations on vitrinite, relief þ / nul, red internal reflections, different properties according to its nature. Rare in humic coals, main component in sapropelic coals. Telalginite: Individual bodies (discs) or colonies, rounded, elongated, semicompressed morphologies, internal structure, intense fluorescence. Lamalginite: lamelae, thickness 10 microns), compact appearance, high reflectance Very fine-grained material. Fine particles of small size (2 microns), high reflectance

Secretinite Macrinite

Micrinite

Inertodetrinite

Without structure, fragments of size 95% V þ L >95% I þ L > 95% V þ I > 95% V > L, I (each >5%) L > V, I (each >5%) I > V, L (each >5%) Coal þ 20-60% (vol.) clays Coal þ 5-20% (vol.) sulfides Coal þ 20-60% (vol.) carbonates Coal þ 20-60% (vol.) quartz Coal þ 20*-60% (vol.) various minerals *5% if high pyrite

Group Monomaceralic

Bimaceralic

Trimaceralic

Carbominerite

Source: Organic Petrology, G. H. Taylor, M. Teichmu¨ller, A. Davis, C. F. K. Diessel, R. Littke, and P. Robert, 704 pp., copyright 1998, with permission from Gebru¨derBorntraeger (www.borntraeger-cramer.de).

and quantity of associated minerals (Stach et al., 1982). The degree of heterogeneity in a microlithotype is also important in its technological behavior, particularly in carbonization (see Chapter 7), combustion (see Chapter 4), and gasification (see Chapter 5) processes. The maceral, mineral, and microlithotype composition of a coal seam may change over short distances both vertically and laterally, in response to the conditions existing during the formation of the original peat swamps (Stach et al., 1982). These changes can be quantified by petrographic assessment of the microlithotypes in relevant coal samples. Microlithotype Analysis Microlithotype analysis is used to determine the relative proportions of the various microlithotypes and coal-mineral associations (carbominerites) present in a coal sample (ICCP, 1963). The procedure is standardized as indicated in the ISO 7404/4 (1988) norm. Although microlithotype analysis is carried out in a similar manner to maceral analysis, a suitable 20-point reticule must be placed in one of the oculars of the microscope as a substitute for the micrometer or cross-hairs. Two conventions (ICCP, 1963) must be observed: (1) the minimum bandwidth of the association to be measured must be 50 microns, and (2) macerals present in the association in amounts

Basic Factors Controlling Coal Quality and Technological Behavior 31

smaller than 5% should be disregarded (the 5% rule). Each observation on a 20-intersection reticule is regarded as one point in the analysis, and each intersection on the reticule represents 5% of the total number of intersections (20), providing guidance in use of the 5% rule. For a complete microlithotype analysis, at least 500 points should be measured, and the results should be expressed as volume percentages. Microlithotype analysis is less accurate than maceral analysis. The calculation of repeatability and reproducibility (ISO 7404/4, 1988) is made in the same way as for maceral analysis.

2.2.2 Elemental Composition of Coal Macerals Chemical analysis of coal provides data gathered from “whole-coal” materials, embracing moisture and mineral matter as well as the organic constituents. The data from ultimate analysis (C, H, O, N, and S percentages) may be corrected to a moist, mineral matter-free (mmmf); dry, mineral matter-free (dmmf); or dry, ash-free (daf) basis to assess composition of the organic matter alone, but even so the composition of the organic matter determined in this way inherently represents an aggregation of the composition of the different maceral components. Variations in chemical composition indicated by ultimate analysis data derived from whole-coal samples therefore reflect variations in the coal type (i.e., the mixture of macerals present) as well as the rank of the coals concerned. As indicated previously coal is a heterogeneous solid, and it is the individual macerals within the coal that react, both independently and with each other, when the coal is used. In addition to providing further insights into the coalification process, knowledge of maceral chemistry may therefore be of value to understand the processes associated with factors such as burning rate, emission release, CO2 generation, fouling and slagging, as well as reactions during gasification and coking associated with the different coal components. Although some success has been achieved in maceral separation through density gradient centrifugation (see below), it is inherently difficult to cleanly isolate the individual macerals in a coal for separate chemical analysis without contamination by minerals or other organic components. The development of special techniques for light-element analysis using the electron microprobe (e.g., Bustin et al., 1993, 1996, and Mastalerz and Gurba, 2001) provides a mechanism for directly determining the elemental composition of the individual macerals in coal-polished sections by analyzing selected areas only a few micrometers in size, without the need for a prior maceral separation process. Electron microprobe techniques have been used to evaluate the elemental composition of the individual macerals in a number of North American and Australian coals (Mastalerz and

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Bustin, 1993, 1997; Ward and Gurba, 1998; Gurba and Ward, 2000; and Ward et al., 2005, 2007), helping to explain more fully the variations in coal composition indicated by whole-coal analysis as well as some of the geological processes associated with coal formation. Details of the procedures used for microprobe analysis of coal macerals are given by Bustin et al. (1993, 1996), Mastalerz and Gurba (2001), and Ward et al. (2005). Samples for electron microprobe study are prepared as polished sections in the same way as for optical microscopy, although specific-sized mounts may be needed to suit the microprobe’s sample handling facilities. The sections, and also the relevant calibration standards, are coated with a thin film of carbon, to provide a conductive surface (Bustin et al., 1993) prior to the analysis process. An accelerating voltage of 10 kV is used for the electron beam in most studies, with a filament current 20 nA. An overall magnification of 20,000x gives a spot size for the electron beam of around 5 to 10 mm in diameter on the sample for the actual measurement process. As discussed by Bustin et al. (1993), an independently analyzed anthracite sample provides a more effective calibration standard than graphite for carbon in microprobe analysis of coal macerals. Separately analyzed mineral samples are generally used as standards for other elements (Ward et al., 2005), such as O, N, S, Ca, Al, Si, and Fe. Care should be taken to avoid analyzing areas of the coal where visible minerals are also present. Points that include mineral contaminants may, for example, be indicated by high Si or unexpectedly high Fe and S percentages. Points that include some of the mounting epoxy resin may be indicated by unusual oxygen and high nitrogen contents. Figure 2.2 indicates the changes in elemental composition of the vitrinite and inertinite macerals in coals from the Bowen Basin in Australia (Ward et al., 2005) with variation in rank over a range, as indicated by vitrinite reflectance, from subbituminous coal to semianthracite. For a given rank level the vitrinite in these coals, especially the collotelinite, has the lowest carbon and highest oxygen contents, while the inertinite, especially the inertodetrinite, has the highest carbon and lowest oxygen contents. The difference in composition between the vitrinites and the inertinites decreases steadily as the rank (vitrinite reflectance) increases; the maceral groups have quite different compositions in lower rank coals, but only very small differences at the upper end of the rank range. Because it is applied directly to the organic material, electron microprobe analysis provides a mean of directly measuring the organic sulphur content of coal macerals, a parameter that is only determined indirectly for coals by conventional analysis techniques. As indicated in Figure 2.2 and also in other studies (e.g., Ward and Gurba, 1998), the vitrinite macerals have significantly higher organic

Carbon

Oxygen 30

Collotelinite Collodetrinite Semifusinite Fusinite Inertodetrinite

80

70

Oxygen in maceral %

90

Collotelinite

20

Collodetrinite Semifusinite Fusinite Inertodetrinite

10

60

0 0.0

0.5

(a)

1.0

1.5

2.0

2.5

3.0

3.5

0.0

4.0

0.5

(b)

Rv max (collotelinite) %

1.0

2.5

3.0

3.5

4.0

3.0

0.75

Collotelinite Collodetrinite Semifusinite Fusinite Inertodetrinite

0.50

0.25

0.00

Nitrogen in maceral %

Sulphur in maceral - %

2.0

Organic Nitrogen

Organic Sulphur 1.00

Collotelinite

2.0

Collodetrinite Semifusinite Fusinite Inertodetrinite

1.0

0.0 0.0

(c)

1.5

Rv max (collotelinite) %

0.5

1.0

1.5

2.0

2.5

3.0

Rv max (collotelinite) %

3.5

0.0

4.0

(d)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Rv max (collotelinite) %

FIGURE 2.2. Plots showing percentages of (a) carbon, (b) oxygen, (c) organic sulphur, and (d) organic nitrogen in different macerals of Australian (Bowen Basin) coals in relation to the coal rank as measured by vitrinite reflectance. (Source: International Journal of Coal Geology 63, by C. R. Ward, Z. Li, and L. W. Gurba, “Variations in coal maceral chemistry with rank advance in the German Creek and Moranbah Coal Measures of the Bowen Basin, Australia, using electron microprobe techniques,” 117–129, copyright 2005, with permission from Elsevier.)

Basic Factors Controlling Coal Quality and Technological Behavior 33

Carbon in maceral %

100

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Applied Coal Petrology

sulphur contents than the fusinite and inertodetrinite, with semifusinite typically having intermediate organic sulphur contents. Electron microprobe studies typically show that the vitrinite macerals have around twice the organic sulphur of the fusinite and inertodetrinite in the same coal samples. There is also evidence, especially in some high-sulphur coals, that the sulphur replaces oxygen in the vitrinite’s chemical structure (Ward et al., 2007). Figure 2.2 also indicates that the nitrogen content of the vitrinite macerals is generally higher than that of the inertinite components, especially fusinite and inertodetrinite, in the same coal samples. This is further supported by the work of Mastalerz and Gurba (2001). For the coals on which Figure 2.2 is based, however, the difference in nitrogen content between these maceral groups appears to decrease at high rank levels (semi-anthracite and above), possibly due to redistribution of the organic nitrogen into some of the clay minerals (Ward et al., 2005).

2.2.3 Organic Geochemistry Two significant works on the organic geochemistry of coal published in the last 25 years are the papers by Given (1984) and Hatcher and Clifford (1997). Because vitrinite is the dominant maceral in most coals, the present review is primarily confined, following the works of those authors, to the transformation of wood to vitrinite. Of the original components of wood, cellulose is preferentially lost, whereas lignin is retained. The preservation of plant structures in brown coals indicates that at least some of the chemical transformation is not accompanied by maceration of the wood structure. The transformation of lignin to lignite involves processes such as demethylation, dehydroxylation, and the cleavage of b-O-4 aryl ethers (Figure 2.3). More significant structural alteration of plant structures occurs at subbituminous rank, with some annealing of the plant cells. The chemical transformations involve the sidechain dehydroxylation and dehydroxylation of catechols. The latter involves condensation to hydroxylated diaryl ethers and the loss of water from the structure. With further coalification, the ether bond is cleaved, resulting in the formation of a catechol-like structure and a phenolic structure. The pathway to high volatile bituminous coal involves the condensation of phenols to aryl ethers or dibenzofuran-like structures. Overall, the aromaticity of vitrinite increases with increasing rank, implying the condensation of benzene-like structures with aliphatic functional groups to polycyclic aromatic structures. Density gradient centrifugation (DGC), a tool for obtaining narrow-density fractions from a coal (Dyrkacz et al., 1981), can be used

Reactions of lignin to form brown coal and lignite dehydroxylation CH2OH

CH2 CH

CH

alkylation

HCOH

CH2OH HC

OCH3 O

HCOH

CH2OH

β-O-4 ether cleavage

HC

OCH2 OH

HCOH

HCOH

demethylation

OCH3

OH OH

OH

Reactions of lignite to form subbituminous coal CH2

CH2

side-chain dehydroxylation

CH

CH CH2

HCOH

CH3

CH2OH HC

OH

HC

OH

OH

CH2

HCOH

dehydroxylation of catechols OH OH

OH

Reactions leading to bituminous coal CH2

CH2

CH

CH

CH2 CH3 HC CH2

CH2 CH3

OH

HC CH2

OH

FIGURE 2.3. Transformation of lignin to lignite to subbituminous coal to bituminous coal. (Source: Organic Geochemistry 27, by P. G. Hatcher and D. J. Clifford, “The organic geochemistry of coal: from plant materials to coal,” 251–274, copyright 1997, with permission from Elsevier.)

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to study chemical variation between macerals. Various studies have been conducted, including an investigation of a number of British and U.S. bituminous coals using Curie-point pyrolysis mass spectroscopy (Meuzelaar et al., 1984); a study of a high volatile C bituminous Pennsylvanian Indiana coal using Curie-point pyrolysis mass spectroscopy and the latter technique coupled with gas chromatography (Nip et al., 1992); and a study of a medium volatile bituminous Permian Australian coal using electron spin resonance (ESR), crosspolarization 13C nuclear magnetic resonance (NMR), and single-pulse 13 C NMR excitation (Maroto-Valer et al., 1998). In general, all these results demonstrated that macerals increased in aromaticity from liptinite to vitrinite to inertinite. Hower et al. (1994a) examined both the organic and inorganic geochemistry of a DGC sample taken from a 0.4%-ash high volatile A bituminous lithotype from a Pennsylvanian coal bed of Kentucky (U.S.). By means of Fourier-transform infrared (FTIR) spectroscopy, they demonstrated that the indicators of aliphatic bonds were strongest in the density concentrates dominated by liptinite macerals (no liptinite macerals were found in near-monomaceral concentrates, unlike the vitrinite and inertinite group macerals) and that the indicators of aromatic bonds were strongest in the >95% vitrinite concentrates but not as strong in the inertinite-rich fractions (Figure 2.4). Coincident with an increase in aromaticity, the Blue Gem (Rimmer et al., 2006) and the Australian (Maroto-Valer et al., 1998) maceral concentrates showed a decrease in atomic H/C with an increase in density (liptinite to vitrinite to inertinite for the Blue Gem coal; vitrinite to semifusinite for the Australian coal). Mastalerz and Bustin (1993, 1996), Walker and Mastalerz (2004), and Li et al. (2006) have used FTIR, sometimes in conjunction with other techniques (Mastalerz et al., 1998), to study macerals on a microscopic scale. They noted an increased aromatic character in the vitrinite and inertinites with an increase in rank and a shift toward greater aromaticity from liptinite to vitrinite to inertinite.

2.3 Coal Composition: Inorganic Components 2.3.1 Minerals and Mineral Matter As discussed by Ward (2002), the material classed as “mineral matter” embraces all the minerals and other inorganic elements occurring in coal, including (1) dissolved salts and other inorganic substances in the pore water of the coal, (2) inorganic elements incorporated within the organic compounds of the coal macerals, and (3) discrete inorganic particles (crystalline or noncrystalline) representing the actual mineral components.

Basic Factors Controlling Coal Quality and Technological Behavior 37

FIGURE 2.4. Maceral concentration versus aliphatic C-H and aromatic C=C for maceral density concentrates of the Blue Gem coal bed, Kentucky. Note that the liptinite concentrate contains little more than 80% litpinite, with much of the remainder being vitrinite. The vitrinite and inertinite concentrates exceed 95% purity. Mineral matter is negligible in most of the concentrates. (Source: Energy and Fuels 8, by J. C. Hower, D. N. Taulbee, S. M. Rimmer, and L. G. Morrell, “Petrographic and geochemical anatomy of lithotypes from the Blue Gem coal bed, southeastern Kentucky,” 719–728, copyright 1994, with permission from American Chemical Society [ACS].)

The first two forms, sometimes described as nonmineral inorganics, are typically most abundant in the mineral matter of lower rank coals (Kiss and King, 1977, 1979; Given and Spackman, 1978; Benson and Holm, 1985; Miller and Given, 1986; Given and Miller, 1987a,b; and Ward, 1991, 1992). Although there are some exceptions (e.g., Ward et al., 2007), the nonmineral inorganics usually disappear from the coal with an increase in rank. Discrete mineral particles, however, may occur in coal of any rank and are usually the dominant component of

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the mineral matter in higher rank materials (Rao and Gluskoter, 1973; Ward, 1977, 1978; Renton, 1986; and Ward et al., 2001). Coals produced from mines may also contain minerals derived from intra-seam noncoal bands or admixed roof or floor strata. This material (extraneous mineral matter) may be at least partly removed by cleaning processes in coal preparation plants. Mineral matter closely associated with the macerals (inherent mineral matter), however—including both intimately admixed minerals and nonmineral inorganics in the maceral components—remains a part of the clean coal product and must be taken into account when assessing the behavior of a coal with handling, storage, and use. Determination of Mineral Matter Content Many of the minerals occurring in coal undergo major chemical changes at the high temperatures associated with combustion and ash formation, including the loss of CO2 from carbonates, the loss of structural water from clay minerals, and the loss of sulfur from sulfides (Rees, 1966; Raask, 1985a; Vassilev et al., 1995; and Reifenstein et al., 1999). The nonmineral inorganics in the macerals may also react with some of the other coal components to form mineral artifacts, such as sulfates, in the ash residue. Because of such changes, the percentage of ash determined in routine coal analysis is usually less than the percentage of mineral matter contained in the original coal sample. The chemical composition and crystal structure of the ash may also be somewhat different from the chemical composition and structure of the original mineral matter. One of the most widely used methods for determining the percentage of mineral matter (as opposed to ash) involves removing the organic matter at low temperature (around 120 C) by exposing the coal to a reactive oxygen plasma produced by a radio-frequency electromagnetic field (Gluskoter, 1965; Frazer and Belcher, 1973; Miller, 1984; and Standards Australia, 2000). The residue remaining after oxidation of the organic matter consists of the essentially unaltered mineral components of the original coal, together in some cases with additional artifacts produced from the nonmineral inorganic components. Exposing the coal to air at around 370 C (Brown et al., 1959, and Ward et al., 2001) or treating the coal with hot concentrated hydrogen peroxide to oxidize the organic components (Nawalk and Friedel, 1972, and Ward, 1974) may also serve to isolate a mineral residue, although these techniques may irreversibly alter some of the mineral components. Calculations based on the ash percentage and ash composition, combined with other chemical data, may also be used to provide an estimate of the mineral matter content (e.g., King et al., 1936; Rees, 1966; Given and Yarzab, 1978; Pollack, 1979; and Scholz, 1980).

Basic Factors Controlling Coal Quality and Technological Behavior 39

Mineral Analysis in Coal and LTA Samples The identity of the crystalline minerals in coal or LTA residues can be evaluated by X-ray diffraction techniques (Rekus and Haberkorn, 1966; O’Gorman and Walker, 1971; Rao and Gluskoter, 1973; Ward, 1977, 1978; Russell and Rimmer, 1979; Renton, 1986; and Harvey and Ruch, 1986). Semiquantitative methods were used in many early studies, based on comparing key peak intensities with intensities associated with known proportions of added-in crystalline spike components. More recent XRD analyses, however, are based on the fullprofile analysis methods developed by Rietveld (1969). The Rietveld approach allows a calculated XRD profile of a sample to be generated from the structural parameters of each mineral present and to be adjusted iteratively by least-squares techniques to fit the observed XRD profile of the analysis sample (Taylor, 1991). Rietveld-based XRD techniques have been applied to the analysis of the minerals in both LTA and whole-coal samples (e.g., Mandile and Hutton, 1995; Ward and Taylor, 1996; Ward et al., 1999, 2001; and Ruan and Ward, 2002), with independent checks against ash analysis and other data confirming the consistency of the mineralogical evaluations. French et al. (2001a) used a Rietveld-based technique to determine the overall percentage of crystalline mineral matter in the coal, as well as the relative proportions of each mineral, by performing a direct XRD analysis of the whole-coal samples without a lowtemperature ashing step to concentrate the mineral components. Structure models were developed separately for the organic matter of coals at different rank levels on the basis of XRD traces derived from chemically demineralized coals, and these were incorporated into the Rietveld analysis to allow the organic matter to be quantified as if it was another “mineral” phase. The proportions of microscopically visible minerals in a coal sample may also be determined at the same time as the percentages of the different maceral components by means of the microscopic point-count analysis (Davis, 1984, and Taylor et al., 1998). Some mineral occurrences intimately associated with the macerals may, however, be inadvertently overlooked by the point-counting process. The mineral percentages determined by point counting are also volumetric percentages, whereas the mineral matter evaluated by low-temperature ashing and similar methods is expressed as a mass percentage. Conversion of volumetric percentages to mass percentages for silicate materials involves approximately doubling the volumetric percentage values (Davis, 1984) and multiplying by even higher factors for sulfides and other dense mineral materials. Mineral particles in coal can be investigated by using a scanning electron microscope and similar techniques (e.g., Stanton and Finkelman, 1979; Russell and Rimmer, 1979; Allen and Vander Sande, 1984;

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Martinez-Tarazona et al., 1992; Hower et al., 1994a, 2000a; and Ward et al., 1996). The automatic collection of element data by means of computer-controlled scanning electron microscopy (CCSEM) techniques (Straszheim and Markuszewski, 1990; Galbreath et al., 1996; and Gupta et al., 1998, 1999a), including element associations, serves to evaluate more fully the nature and distribution of minerals in coal samples. More advanced CCSEM techniques, including the QEM*SEM and QEMScan systems (Gottlieb et al., 1992; Creelman et al., 1993; and Creelman and Ward, 1996), allow the integration of SEM data with image analysis methods. In such operations the electron beam passes over relevant parts of a polished section, stopping at predetermined intervals to collect X-ray spectra. As each X-ray spectrum is collected it is processed through a species identification protocol (SIP) and the mineral represented at that point is identified from its chemical characteristics. Individual mineralogical identifications are thus made at each point (or pixel) in the area scanned by the electron beam, and these can be used to generate a digital map showing how the various minerals occur within the coal sample. Data from such systems can be used to provide a variety of mineral matter information, including the relative abundance of the different minerals in the coal; the particle size, shape, and mode of occurrence of particular minerals; the textural associations between minerals; and the degree of liberation of minerals and mineral aggregates from organic matter associated with coal-crushing and pulverization processes. More precise determination of the composition of particular minerals may also be obtained from electron microprobe analysis of polished coal sections, using methods outlined by Reed (1996). Electron microprobe and similar techniques have been applied to minerals in coal by authors such as Minkin et al. (1979), Raymond and Gooley (1979), Kolker and Chou (1994), Patterson et al. (1994), and Zodrow and Cleal (1999). They have also been used to investigate the elemental composition of individual coal macerals (Bustin et al., 1993, 1996; Gurba and Ward, 2000; and Ward et al., 2005), including the occurrence of nonmineral inorganic elements. The nature and relative abundance of iron-bearing phases in coal may be evaluated by Mo¨ssbauer spectroscopy (Gracia et al., 1999). A wide range of other methods have also been used to identify the minerals present in coal samples, including thermal analysis (Warne, 1964), Fourier-transform infrared (FTIR) spectrometry (Painter et al., 1978), and a range of other instrumental techniques (Ward, 2002; Huggins, 2002; and Vassilev and Tascon, 2002). Minerals in Coal and LTA Residue A list of the minerals that may be found in coals or LTA residues is given in Table 2.6. The most abundant of these are usually clay minerals, although quartz, pyrite, siderite, calcite, and dolomite or

Basic Factors Controlling Coal Quality and Technological Behavior 41

TABLE 2.6 Principal minerals found in coal and LTA (Data from Ward, 2002.) Silicates Quartz Chalcedony Clay minerals: Kaolinite Illite Smectite Chlorite Interstratified clay minerals Feldspar

SiO2 SiO2 Al2Si2O5(OH)4 K1.5Al4(Si6.5Al1.5)O20(OH)4 Na0.33(Al1.67Mg0.33)Si4O10(OH)2 (MgFeAl)6(AlSi)4O10(OH)8

Tourmaline Analcime Clinoptilolite Heulandite

KAlSi3O8 NaAlSi3O8 CaAl2Si2O8 Na(MgFeMn)3Al6B3Si6O27(OH)4 NaAlSi2O6H2O (NaK)6(SiAl)36O7220H2O CaAl2Si7O186H2O

Sulfides Pyrite Marcasite Pyrrhotite Sphalerite Galena Stibnite Millerite

FeS2 FeS2 Fe(1x)S ZnS PbS SbS NiS

Phosphates Apatite Crandallite Gorceixite Goyazite Monazite Xenotime

Ca5F(PO4)3 CaAl3(PO4)2(OH)5H2O BaAl3(PO4)2(OH)5H2O SrAl3(PO4)2(OH)5H2O (Ce,La,Th,Nd)PO4 (Y,Er)PO4

Carbonates Calcite Aragonite Dolomite Ankerite Siderite Dawsonite Strontianite Witherite Alstonite

CaCO3 CaCO3 CaMg(CO3)2 (Fe,Ca,Mg)CO3 FeCO3 NaAlCO3(OH)2 SrCO3 BaCO3 BaCa(CO3)2

Sulfates Gypsum Bassanite Anhydrite Barite Coquimbite Rozenite Szomolnokite Natrojarosite Thenardite Glauberite Hexahydrite Tschermigite

CaSO42H2O CaSO4½H2O CaSO4 BaSO4 Fe2(SO4)39H2O FeSO44H2O FeSO4H2O NaFe3(SO4)2(OH)6 Na2SO4 Na2Ca(SO4)2 MgSO46H2O NH4Al(SO4)212H2O

Others Anatase Rutile Boehmite Goethite Crocoite Chromite Clausthalite Zircon

TiO2 TiO2 AlOOH Fe(OH)3 PbCrO4 (Fe,Mg)Cr2O4 PbSe ZrSiO4

Source: International Journal of Coal Geology 50, by C. R. Ward, “Analysis and significance of mineral matter in coal seams,” 135–168, copyright 2002, with permission from Elsevier.

ankerite, together in some cases with phosphate minerals such as apatite, may also be found as significant components of the mineral matter in many coal seams (O’Gorman and Walker, 1971; Rao and Gluskoter, 1973; Ward, 1977, 1978, 2002; Davis et al., 1984; Vorres, 1986; Ward et al., 2001; Vassilev and Tascon, 2002; and Pinetown et al., 2007). Iron-bearing sulfate minerals, such as jarosite and coquimbite, may be formed by the oxidation of pyrite on the exposure of the coal to the atmosphere, a process which also liberates

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sulfuric acid when there is associated runoff water. Many other sulfates in oxidation residues, such as bassanite, glauberite, and hexahydrite, usually represent mineral artifacts formed by the interaction of nonmineral inorganics during the destruction of the organic matter. The crystalline mineral matter in coal may occur as bands, lenticles, fracture fillings, plant impregnations, mineral-rich nodules, and other masses visible at a macroscopic scale. Some of the nonmineral inorganics may also be precipitated when coal pore water evaporates on exposed outcrops, mine faces, or drill cores. Microscopically visible mineral matter in coal includes material intimately admixed within the macerals as well as discrete mineral fragments or crystals and a range of nodules, lenticles, veins, pore infillings, and cell replacement structures (Kemezys and Taylor, 1964; Taylor et al., 1998; and Ward, 2002). The minerals in coal may represent transformed accumulations of biogenic constituents such as phytoliths and skeletal fragments (Raymond and Andrejeko, 1983), or they may be of detrital origin, introduced as epiclastic or pyroclastic particles into the peat bed (Davis et al., 1984; Ruppert et al., 1991; and Bohor and Triplehorn, 1993). Other minerals are produced by authigenic precipitation, either syngenetically with peat accumulation or at a later stage in cleats and other pore spaces through epigenetic processes (Rao and Gluskoter, 1973; Cobb, 1985; Spears, 1987; Querol et al., 1989; Sykes and Lindqvist, 1993; Kortenski and Kostova, 1996; Faraj et al., 1996; Ward et al., 1996; and Rao and Walsh, 1997). The syngenetic minerals may represent solution and reprecipitation products of biogenic and detrital material, or they may be derived from solutions or decaying organic matter within the peat deposit.

2.3.2 Nonmineral Inorganic Components The nonmineral inorganics in coal occur either as dissolved constituents in the pore waters or as an inherent, though sometimes exchangeable, part of the maceral components. They may represent exchangeable ions attached to carboxylic, phenolic, or hydroxyl groups (Durie, 1991) as well as metalloporphyrins and other organometallic compounds (Kiss, 1982; Bunnett et al., 1987; Durie, 1991; and Saxby, 2000). Selective leaching with water, ammonium acetate, and hydrochloric acid may be used to determine the abundance and mode of occurrence of the principal nonmineral inorganic elements in lower rank coals (Miller and Given, 1986; Benson and Holm, 1985; and Ward, 1991, 1992). As an example, Figure 2.5 shows the percentage of various elements released by each process in a sequential leaching study.

Basic Factors Controlling Coal Quality and Technological Behavior 43

Water Washing

Acetate

Acid

Leachable % of Element

100 80 60 40 20 0 Al

Ca

Fe

K

Mg

Mn

Na

P

S

Si

Ti

FIGURE 2.5. Percentage of selected elements leached from a low rank coal by sequential treatment with water, ammonium acetate, and hydrochloric acid. (Source: International Journal of Coal Geology 50, by C. R. Ward, “Analysis and significance of mineral matter in coal seams,” 135–168, copyright 2002, with permission from Elsevier.)

Elements released by soaking in water are commonly taken as representing ions originally in solution in the coal’s pore water, whereas elements released by treatment with ammonium acetate are usually regarded as representing exchangeable ions attached to carboxylates and other functional groups in the maceral components. However, some of the carbonate minerals in the coal, if present, may also dissolve in ammonium acetate solutions (e.g., Matsuoka et al., 2002) and contribute to the elements liberated in this way. Elements released by hydrochloric acid treatment may include those elements incorporated as organometallic complexes into the maceral components as well as any calcite or dolomite not affected by the acetate treatment. Siderite has only limited solubility in cold acids, but iron occurring as oxide or hydroxide material—associated, for example, with iron staining—may be readily dissolved by acid treatment. The most abundant elements associated with the organic matter are usually Na, Ca, Mg, and, in some cases, Al and Fe (Figure 2.5). An electron microprobe analysis may also show measurable concentrations of such inorganic elements in the maceral components (Ward et al., 2003). More detailed element mapping by Li et al. (2007) has identified up to 1.5% Ca, 0.5% Al, and 0.7% Fe as consistent components of supposedly “clean” macerals, especially vitrinite, in several lower rank coals. These elements show a uniform distribution pattern within the macerals similar to that of the organic sulfur component. Huggins (2002) discusses a number of other techniques that have been used to investigate the inorganic elements in coal and ash. These include the proton microprobe using proton-induced X-ray emission (PIXE) for elemental analysis (Minkin et al., 1982; Hickmott and

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Baldridge, 1991; and Caridi et al., 1993), the ion microprobe mass analyzer (IMMA) (Finkelman et al., 1984), the laser microprobe mass analyzer (LAMMA) (Lyons et al., 1987, and Morelli et al., 1988), laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAMP-ICP-MS) (Chenery et al., 1995), and the sensitive high-resolution ion microprobe (SHRIMP) (Kolker et al., 2000a). Another technique that has been used to investigate the forms of occurrence of trace elements in coal and ash is X-ray absorption fine structure (XAFS) spectroscopy (Huggins and Huffman, 1996). Many of the inorganic elements in coal were probably inherited from the original plant tissues and fixed by processes such as carboxylation, metallation, and chelation during peat accumulation (Filby and van Berkel, 1987, and Given and Miller, 1987a, b). Subsurface waters may have subsequently redistributed the inorganic elements and possibly introduced additional material, leading in some cases to the distribution of element concentrations within individual coal beds that were controlled mainly by post-depositional ion migration processes (Brockway and Borsaru, 1985). The inorganic elements are usually expelled from the organic matter as the maceral structures change with rank advance (Filby and van Berkel, 1987). However, the expulsion may be inhibited in some cases. For example, microprobe studies by Ward et al. (2007) found significant concentrations of Ca and Al in the macerals of several bituminous coals together with perhydrous vitrinites and anomalously low reflectance properties. Elements occurring in nonmineral inorganic form are generally more reactive than the same elements occurring in crystalline mineral phases when the coal is used. The nonmineral inorganics may also interact with the mineral components in the coal to form new minerals if the coal is heated by igneous intrusions while still at low rank levels (Susilawati and Ward, 2006).

2.3.3 Trace Elements in Coal The total concentrations of the individual inorganic elements in coal, including both major and trace components, are usually determined by chemical analysis of the coal or coal ash material. Direct analysis of the coal is preferred for elements that may be partly volatilized at elevated temperatures, but for most elements ashing increases their concentration in the analysis sample, thereby helping the analysis process. Even if the element concentration is measured by ash analysis, the result is usually expressed as a fraction of the original coal sample. A number of techniques have been used to determine the concentration of individual inorganic elements in coal at major and trace levels (Karr, 1978a, b, 1979; Davidson and Clarke, 1996; Huggins,

Basic Factors Controlling Coal Quality and Technological Behavior 45

2002; and Vassilev and Tascon, 2002). These include X-ray fluorescence (XRF) spectrometry, neutron activation analysis (NAA), atomic absorption spectrometry (AAS), optical emission spectrometry (OES), and inductively coupled plasma vaporization, combined with atomic emission spectrometry (ICP-AES) or mass spectrometry (ICP-MS). Some techniques are more suitable than others for particular elements, depending in part on the concentration of that element and the matrix within which it occurs. Almost every element in the periodic table has been identified in coal (Swaine, 1990; Finkelman, 1994a; Swaine and Goodarzi, 1995; and Ren et al., 1999), and the extent of knowledge on trace elements is increasing as more sensitive analytical methods are developed. With the possible exception of selenium, boron, arsenic, and antimony, which appear to be more abundant in coal, most of the trace elements in coal occur at comparable to lower concentrations than the same elements in other rock and soil materials. Sixteen of the elements occurring in coal have been included in a list of potentially hazardous air pollutants (HAPs) under the U.S. Clean Air Act (Demir et al., 1997), namely As, Be, Cd, Cl, Cr, Co, F, Hg, Mn, Ni, P, Pb, Sb, Se, Th, and U. Some of these may have other impacts on coal utilization, such as the adverse effect of phosphorus on iron and steel production. Web-based summaries for a number of individual elements, including range of abundance, modes of occurrence, analytical methods, and behavior during combustion and environmental effects, are given by CSIRO Energy Technology (2005). Like the major elements, the trace elements in coal may be associated with either the organic components (macerals) or with the crystalline mineral materials. An analysis of different density fractions prepared from finely ground coal may be used to indicate the “organic affinity” of particular elements (Zubovic, 1966; Gluskoter et al., 1977; and Querol et al., 2001), i.e., the extent to which they are associated with the relatively clean, low-density macerals (organic affinity) or the denser, mineral-rich fraction (inorganic affinity). Elements with an organic affinity (such as boron) may be intimately bound to the organic structure (i.e., nonmineral inorganics), but they may also represent fine particles of minerals occurring within the maceral components. Most trace elements in coal tend to have a relatively strong inorganic affinity (Davidson and Clarke, 1996, and Kolker and Finkelman, 1998), representing either low concentrations of minerals with that element as a major constituent or higher concentrations of minerals containing minor proportions of the element in question. The sequential digestion of the coal in a series of solutions, including HNO3 (to dissolve pyrite) and HF (to dissolve silicates), may also be used to evaluate the association of the trace elements in coal with different mineral matter components (Finkelman et al., 1990;

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Dale et al., 1993; Palmer et al., 1993; Laban and Atkin, 1999; and Davidson, 2000). The effectiveness of such techniques, however, may be reduced if fine mineral particles are encapsulated by organic matter, limiting access for the relevant reagents. In other cases, because of the low solubility of siderite and dolomite in acid, the minerals present may be less soluble than expected in the reagent concerned. Direct association between element concentration and the abundance of particular minerals (Ward et al., 1999) in the coal may also help identify the mode of occurrence for some trace elements. The concentration of some mineral-related elements (elements with a high inorganic affinity) may be reduced by coal preparation, but the effectiveness of any such reduction depends at least in part on the particle size of the host minerals and the extent to which they are liberated from the organic matter when the coal is crushed. Elements with an organic affinity are also likely to be concentrated, rather than reduced, by coal-cleaning processes. Moreover, the removal of elements from the cleaned coal has the effect of concentrating them in the waste fraction, from which they may be released by a different route when the wastes are dumped at disposal sites. Knowledge of trace element occurrence in minerals and rocks, together with the help of a series of coal-specific studies, provides a basis for assessing the mineral associations of particular trace elements in coal samples (e.g., Finkelman, 1982, 1994a,b; Rimmer, 1991; Belkin et al., 1997a; Ward et al., 1999; Spears and Zheng, 1999; and Finkelman et al., 2002). Elements such as As, Cd, Se, Tl, Hg, Pb, Sb, and Zn, for example, sometimes referred to as chalcophile elements, are generally thought to be associated with sulfide minerals such as pyrite, either as solid-solution constituents or as discrete sulfide phases. Elements such as Rb, Ti, Cr, Zr, and Hf, often referred to as being among the lithophile elements, are more probably associated with aluminosilicates such as micas, feldspars, and the clay minerals. An indication of the possible mode of occurrence for some of the elements identified as hazardous air pollutants is given in Table 2.7. Further discussion of the associations of a number of elements is provided by Swaine (1990) and Finkelman (1994a).

2.4 Coal Metamorphism: Rank Determination Coal metamorphism involves the physical and chemical transformation from peat through bituminous coal through anthracite and meta-anthracite to graphite (albeit not necessarily a pure graphite). In general, coal metamorphism (or coalification), denoted as the coal rank, is marked by a progressive decrease in moisture and volatile

Basic Factors Controlling Coal Quality and Technological Behavior 47

TABLE 2.7 Indicative mode of occurrence of some potentially hazardous trace elements in coal. Element

Common Mode of Occurrence

Antimony Arsenic Beryllium Cadmium Chromium Cobalt Lead Manganese Mercury Nickel

Pyrite and accessory sulfides Pyrite and accessory sulfides Organic association Solid solution in sphalerite Organic and/or clay association Pyrite; some in accessory sulfides Galena Carbonates, especially siderite and ankerite Pyrite Unclear; perhaps sulfides, organics, or clay minerals Organic association; pyrite and accessory sulfides; selenides

Selenium

Level of Confidence* 4 8 4 8 2 4 8 8 6 2 8

*Level of confidence: A number between 1 (low) and 10 (high) expressing the consistency and predictability of the element’s indicated common mode of occurrence in coal. Source: Fuel Processing Technology 39, by R. B. Finkelman, “Modes of occurrence of potentially hazardous elements in coal: levels of confidence,” 21–34, copyright 1994, with permission from Elsevier.

functional groups with a consequent increase in the carbon content of the coal (see Table 2.8). Many of the fundamental properties of coal that are important for industrial use are rank dependent. Coal metamorphism is a function of heat and pressure acting over a period of time. Taylor et al. (1998) and Hower and Gayer (2002), among others, have reviewed the mechanisms of coal metamorphism. Among the three primary factors, heat is generally considered to be the most important. Traditionally, increased heat at greater depths of burial has been considered the primary factor (Hilt’s Law, after Hilt, 1873). Though this continues to be the primary argument in Taylor et al. (1998), it has been recognized that influences from tectonically driven geothermal fluids have also played an important role in coalification (Hower and Gayer, 2002, and Harrison et al., 2004). There is little doubt that time does play a role in coalification, with the amount of time necessary to achieve the coal rank varying from less than a year in contact metamorphism to 106–107 years for regional metamorphism. The role of pressure is now acknowledged as a hindrance to metamorphism in closed systems (Dalla Torre et al., 1997, and Carr, 1999), but it has always been considered a primary influence in the progression of coalification. Pressure causes physicostructural coalification, which influences the physical properties of coals.

Rank Stage

% Carbon % Volatile (daf) Matter (daf)

Gross Specific Energy % in situ (MJ/kg) Moisture

% Vitrinite Reflectance

(oil, 546 nm)

(Diessel, 1992a)

(oil, 546 nm)

(Teichmu¨ller, 1982)*

Rrandom

Rmax

Rank Subclass

Rrandom

Wood Peat Lignite Subbituminous

50 60 71 80

>65 >60 52 40

14.7 23 33.5

75 30 5

0.2 0.4 0.6

0.2 0.42 0.63

High volatile Bituminous

86

31

35.6

3

0.97

1.03

Medium volatile Bituminous Low volatile Bituminous Semianthracite Anthracite

90

22

36

0.6% R or >24 MJ/kg) in international trade. This new system was applicable to all coals of different origin and geological age from different type of deposits as well as to single seams and multiseam blends of runof-mine coals and washed coals. Medium and high rank coals were characterized by means of a 14-digit code number comprising eight coal-quality parameters: (1) mean random vitrinite reflectance, either measured directly or estimated as maximum vitrinite reflectance divided by 1.06; (2) the character of the reflectrogram; (3) maceral composition expressed as (a) the percentage of inertinite and as (b) the percentage of liptinite, which provides a means of distinguishing, in part, between Gondwana and Carboniferous coals; (4) the free-swelling index; (5) volatile matter (dry, ash-free); (6) ash percentage (dry basis); (7) the total sulfur (dry basis); and (8) gross calorific value (dry, ashfree) in megajoules per kilogram (MJ/kg). Other parameters may be appended to provide a more thorough description of coal quality, such as ash composition, ash fusion characteristics, and Hardgrove grindability index for steam coals or AudibertArnu dilation properties and phosphorous content for metallurgical coals. In 1998 the United Nations Economic Commission for Europe proposed an international classification of in-seams coals. This system was intended to serve as a means of classifying coals and ensuring a better characterization of coal deposits. Unlike the previous United Nations Economic Commission for Europe (1988) classification system, which was intended for commercial purposes, this new system was clearly described as not intended for use in commerce or trade. The 1998 coal classification is based on three fundamental coal characteristics to be used in combination: coal rank (or degree of coalification), petrographic composition, and grade or amount of impurities (ash yield). Figure 2.6 shows a scheme for the classification of in-seam

C ar bo na c

A

D

C

B

HIGH-RANK

A

C

B

A

50

L% 50

100

V% (L >I)

100

GRADE

m

ic

)

l

30

B

N on -b co an al de d

50

MEDIUM-RANK

LOW-RANK C

0

Ve

H Me Low ry ig lo g h diu w gr m rad gr ad e g B ad r (m an c e e co ade oa ai de l co nl d a c y l oa al H coa u l

80 Ash (HT) mass %, (db)

Coal ↔ non-coal rock washability test

eo us

ro ck

S co peli apr tic oal

R

oc k

PE TR

sh Oil al e

O G R d AP ox ry HI ae id ↔ C ro ati w CO bi on et M c ↔ PO ↔ SI an red TI ac uc O ro tio N bi n c

Basic Factors Controlling Coal Quality and Technological Behavior 57

20 10

50 50

o

et a M

a

rth O

pe r

Pa r

o

et a M

a

rth

Pa r

V% (I > L) 100

I%

Rr %

BITUMINOUS

SU

LIGNITE

RANK

B

B

O

IT U

M

et a M

rth O

Paleo B-time

o

0

0.6

1.0

1.4

2.0

ANTHRACITE

3.0

4.0

PETROGRAPHIC COMPOSITION

Maceral analysis (m/nf) vol.%

Not to be included in the classification

GCV (MJ/kg, m, af)

15

20

24

Rr% - Vitrinite mean Random Reflectance, per cent (ISO 7404-5 standard) GCV (MJ/kg, m, af) - Gross Calorific value in MJ/kg, recalculated to moist, ash-free basis (ISO 1928 and 1170 standards) Ash (HT) mass % db - Ash content (High temperature), mass per cent, recalculated to dry basis (ISO 1171, 331 and 1170 standards) V%. L%. I% - Vitrinite, Liptinite and Intertinite contents respectively, volume per cent, recalculated to mineral-matter-free basis (ISO 7404-3 standard)

FIGURE 2.6. Coal classification. (Source: United Nations Economic Commission for Europe, 1998; reprinted from International Journal of Coal Geology 50, by B. Alpern and M. J. Lemos de Sousa, “Documented international enquiry on solid sedimentary fossil fuels; coal: definitions, classifications, reserves-resources, and energy potential,” 3–41, copyright 2002, with permission from Elsevier.)

coals. Although not very frequent, this system has sometimes been referred to in the literature (e.g., Alpern and Lemos de Sousa, 2002). A recently devised classification method is that of the “classification of coals” by the International Organization for Standardization (ISO-11760, 2005). Its development has been guided by the classification system of the United Nations Economic Commission for Europe (1998) and, as in the previous case, it is not intended to be used for commercial purposes. It is also based on the three fundamental coal properties: vitrinite reflectance (mean random reflectance), vitrinite content in percent per volume on a mineral-free basis, and ash yield. The ISO-11760 (2005) system provides a simple classification method of descriptive categorization that can be applied to coals of all ranks,

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Applied Coal Petrology

a method of comparison of coals taking into account certain key characteristics, and guidance in selecting the appropriate ISO standard procedures for coal analyses.

2.6 Coal Blends Many of the installations that use coal, whether for carbonization (coking) or combustion purposes, use a feedstock blended from a number of different coals to obtain the appropriate quality specifications rather than coal of uniform rank from a single seam or deposit. Usually the blended coals originate from different sources, each having a different composition and/or coal rank (Figure 2.1h). In some cases the feedstock from a single source of supply (which may be a mine or a preparation plant) also encompasses coals having more than one rank level, possibly due to the incorporation of coal affected by igneous intrusions or other local heat sources or to the mixing of coals from different zones of the one deposit or region. Coal petrography (Figure 2.1h) is the only effective way to identify coal blends (as it is summarized in Sua´rez-Ruiz, 2004). This is particularly significant for blends that show similar chemical parameters (e.g., moisture, ash content, volatile matter), but they may differ in some of their technological properties or display a quite different technological behavior in processes such as coke production or even in coal combustion. Figure 2.7 shows the distribution of vitrinite reflectance values in five different coals in the form of a histogram illustrating the proportion of particles falling in the different reflectance intervals (V-steps). The coals have similar overall proportions of volatile matter and similar mean maximum vitrinite reflectances. At the same time each coal has quite different coking properties, as expressed by the respective free-swelling index values. The sample at the top of the diagram, for example, represents a single medium volatile bituminous coal with a vitrinite reflectance of 1.3%, whereas the sample in the lowest plot represents a mixture of two quite different coals, one with a vitrinite reflectance of around 0.9% and one with a vitrinite reflectance of around 1.9%. This figure shows that, although coals having particular overall properties can be prepared by blending two or more coals of different rank (and reflectance) characteristics, the resulting coal blends display a substantially different technological behavior in processes such as coke production. Careful petrographic analysis is required to analyze coal blends, with particular attention being paid to the distribution of vitrinite reflectance values as well as to the range of optical characteristics of other macerals. Both the histogram plots and the standard deviation (scattering) of the individual reflectance values (identified as S in Figure 2.7) may be relevant to the evaluation process.

Basic Factors Controlling Coal Quality and Technological Behavior 59

Volatile Matter

Swelling Index

24,5

9

25,2

9

24,2

S

1,31

0,063

20 15 10 5

1,26

0,117

7 1/2

10 5

1,31

0,234

24,2

4 1/2

10 5

1,26

(0,372)

25,0

1

15 10 5

1,41

(0,553)

Vol.-% 1/2 V-Step

Rm

40 35 30 25 20 15 10 5

0,5 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 % Reflectance =10 Vol.-%

40

35

30

25

20

15

10 % Vol. Matter Vitrinite

FIGURE 2.7. Reflectograms, mean reflectance and scatter, volatile matter, and swelling index of five different coals and coal blends. (Source: Stach’s Textbook of Coal Petrology by E. Stach, M.-Th. Mackowsky, M. Teichmu¨ller, G. H. Taylor, D. Chandra, and R. Teichmu¨ller (editors), 535 pp., copyright 1982, with permission from Gebru¨der-Borntraeger (www.borntraeger-cramer.de).

Petrographic analysis, vitrinite reflectance measurements, and the maceral analysis of coal blends may be used to obtain information on (1) the number of different component coals in the blend, (2) the proportion of each coal in the blend, (3) the overall mean (random or maximum) vitrinite reflectance of the coal blend, (4) the mean (random or maximum) vitrinite reflectance of each individual coal in the blend, (5) the overall maceral composition of the blend, and (6) the maceral composition of each individual coal in the blend. The identification and analysis of coal blends is especially relevant to metallurgical coke production. Authors such as Schapiro et al. (1961), Benedict et al. (1968a), Gray et al. (1979), and Taylor et al. (1998) have described methods for predicting coke strength and other properties from maceral composition and vitrinite reflectance (including the V-step) data that are applicable to coal blends. The evaluation of coal blends may also be significant to combustion processes as a means of identifying the sources of different types of unburned carbons.

CHAPTER 3

Mining and Beneficiation Joan S. Esterle

3.1 Introduction Similar to utilization behavior, the rank and composition of coal directly influence its material behavior during mining and beneficiation (van Aubel, 1928; Raub, 1937; Mackowsky and Abramski, 1943; Donahue and Leonard, 1967; Jansen, 1987; among others). Mining is designed to first liberate the coal from the host clastic rock and beneficiation to liberate higher grade coal and discard stone and poorer quality coal through a process of comminution, density separation, and flotation that is controlled by particle size and surface chemistry. Within a seam, trends in coal quality can often be predicted by understanding the distribution of coal lithotypes relative to the thickness and splitting characteristics of the coal seam that reflects original depositional controls (Moore, 1995; Staub, 2002; Greb et al., 2002; Moore et al., 2006; among others). This assists in optimizing coal quality through either selective mining of different areas or seam plies or processing and blending from different pits (for examples, see Gomez and Donaven, 1971; Clarkson, 1992; and Swanson and Mackinnon, 2003). Changes in maceral composition can also assist in predicting splitting in advance of mining (Esterle and Ferm, 1986, and Moore, 1991) and abrupt changes in coal rank or trace element content may forecast intrusions (Bostick and Pawlewicz, 1984; Stewart et al., 2005; Susilawati and Ward, 2006; and Golab et al., 2007) and in some cases faults by a change in vitrinite reflectance anisotropy that records changes in paleo stress (Stone and Cook, 1985; Levine and Davis, 1989; Cmiel and Idziak, 2003; and Langenberg and Kalkreuth, 1991). For mines producing thermal coals, keeping rank and grade to specification is the prime task, along with tracking deleterious trace elements in product and reject. For mines producing metallurgical coal products, coal type or vitrinite content is an added factor as are element distributions such as phosphorus and sulphur. The same Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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Applied Coal Petrology

factors that make a good coking coal, such as bituminous rank up to low volatile stages and high vitrinite contents, also make a coal friable or easy to cut, crush, or grind (Hardgrove, 1931; Evans and Pomeroy, 1966; Gomez and Hazen, 1970; MacGregor, 1983; and Hower, 1998). The difference in intact strength between lithotypes, along with their density, will control their fragmentation behavior and the resulting size and composition of daughter particles. Hard, dull coals tend to concentrate in the coarser fractions, whereas more friable, vitrain-rich coals tend to break down into finer particles. This allows different products—for example, a thermal and coking coal—to be produced from the same seam by crushing, screening, and density separation. For coking coals, the weak nature of vitrain-rich lithotypes can create mining problems such as weak pillars in underground mines, excessive dust generation during cutting, or fines generation during blasting that will increase the load on the flotation circuit downstream in the preparation plant. Understanding the distribution of coal lithotypes, in addition to rank and grade, can assist in predicting the behavior of a coal seam at all stages of the mining and beneficiation chain.

3.2 Coal Strength There are different ways of estimating coal strength and hardness— compressive strength, fracture toughness, or grindability—but all will show a trend relative to rank, type, and grade of the coal. The measurement of coal strength is affected by the size of the test specimen, the orientation of stress relative to banding, and the confining pressure of the test (Hobbs, 1964; Zipf and Bieniawski, 1990; Mark and Barton, 1996; and Medhurst and Brown, 1998). By its nature coal is a banded material which makes it weak by comparison to most other rocks. Intact rock strength is commonly defined as the strength of the rock material that occurs between discontinuities, which in coal are closely spaced and related to lithotype banding and cleat. Cleat spacing will increase as a function of rank (Law, 1993, and Laubach et al., 1998) and also proportion of vitrain banding (Smyth and Buckley, 1993, and Pattison et al., 1996). For a given rank, individual lithotypes can have large compressive strength differences owing to wide ranges in maceral composition, banding texture, and cleat density (Medhurst and Brown, 1998), as shown in Figure 3.1. The larger the test sample, the greater the presence of inherent flaws due to cleats and banding, which significantly reduces the strength of the coal (Bienawski, 1968). Mark and Barton (1996) found size effects more pronounced in “blocky” (dull) coals than in friable (more well-banded, vitrain-rich) coals which fail more readily. For a given rank, vitrain-rich coals will also fail more readily when stress

Mining and Beneficiation 63

Peak Strength (MPa at 0.2MPa confining pressure)

35 30 25 20 15 10 5 BB

IB DB DM Coal Lithotypes

D

FIGURE 3.1. Cross-plots of peak compressive strength at 0.2 MPa confining stress against coal lithotype for a single seam of Rvmax ¼ 0.8%. BB: bright banded coal >60–90% vitrain; IB: interbanded coal 40–60% vitrain; DB: dull banded coal 10–40% vitrain; DM: dull coal 1–10% vitrain; D: dull coal 60% vitrain; dull coal is pseudovitrinite (low Rmax) >

Mining and Beneficiation 81

vitrinite (high Rmax) > vitrinite (low Rmax) ¼ micrinite ¼ exinite (liptinite) ¼ semifusinite > resinite > fusinite. Note, this differs from the ICCP (1998) maceral definitions, but pseudovitrinite does have a usage precedent in the coal petrology literature (Benedict et al., 1968b). Arnold and Aplan (1989) also examined microlithotype relationships, finding vitrite to be concentrated in the faster floating fractions and inertite in slower floating fractions. In order of decreasing floatability, microlithotypes follow the general order vitrite > inertite > vitrinertite > clarite > duroclarite. A rank relationship exists, with hydrophobicity increasing sharply through the high volatile bituminous rank range (Aplan, 1993). Honaker et al. (1996) investigated differences in maceral partitioning in column flotation related to pH. Since different researchers have used a variety of coals with varying petrographic composition and varying rank, exact comparisons between studies are difficult (Arnold and Aplan, 1989). Overall, the behavior of particles is best understood by understanding the composition of the entire particle (Sarkar et al., 1984; Ofori et al., 2006), although it is really only the outer surface that is important in processing. Flotation, generally conducted on