Handbook of Combustion, Volume 3 (Gaseous and Liquid Fuels)

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1 Overview of Gaseous Fuels Anuradda Ganesh 1.1 Introduction

Gaseous fuels are popular and have distinct advantages over solid and liquid fuels. They are easy and convenient to handle, generally free of any mineral impurities, require low or negligible maintenance of burners and result in good combustion efficiencies. Generally, in a highly populated industrial area, a distribution network is used to deliver gaseous fuels on an “on tap” basis, or some industries store gaseous fuels in “gas holders.” A few industries also produce the fuel “on-site” for their use. Gaseous fuels are either extracted from naturally occurring resources or are manufactured, and are composed mostly of one or a mixture of hydrocarbons (methane, propane, butane), carbon monoxide and hydrogen. Methane, is one of the most common constituents of gaseous fuels. The biogenic and thermogenic degradation of organic materials (be it fossils or waste) leads to the formation of methane. Depending on the number of years over which the degradation has occurred, the environmental conditions to which it has been subjected, and the composition of the precursors, the percentage of methane is seen to vary. Natural gas, coal bed methane, methane hydrates, biogas, and landfill gas all have methane as the main combustible component. Most of the synthesized gaseous fuels, however, mainly have carbon monoxide and hydrogen contributing to the calorific value. Hydrocarbons such as propane and butane are popular in the form of liquefied petroleum gas (LPG), used as a domestic or transportation fuel.

1.2 Classification of Gaseous Fuels

Gaseous fuels are mainly classified based on their mode of occurrence and can be grouped as follows: .

Naturally occurring gases (the gases maybe extracted directly, or obtained as a product of another process like refining).

Handbook of Combustion Vol.3: Gaseous and Liquid Fuels Edited by Maximilian Lackner, Franz Winter, and Avinash K. Agarwal Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32449-1

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. .

Synthesized gases (which are manufactured from other sources such as solid/ liquid fuel or water). By-products (these gases are by-products of specific coal processing or utilization reactors).

1.2.1 Naturally Occurring Gases 1.2.1.1 Natural Gas Natural gas in its raw form consists primarily of methane but is a mixture of hydrocarbons such as ethane, propane, butane and pentane. It is found in gas wells and as dissolved gas in oil wells. The natural gas that comes from oil wells is therefore also called “associated gas” [1]. In some locations, natural gas contains large amounts of nitrogen and carbon dioxide. In some places, small yet recoverable amounts of helium are also found. Similarly, hydrogen sulfide is also present, which may be treated for production of elemental sulfur [1]. Natural gas is considered as a fossil fuel, as scientists believe that it is formed by decay of sea plants and animals which died many hundred million years ago. It is believed that these dead plants and animals sank to the bottom of the oceans and were buried under layers of sedimentary rocks. With time, the rocks grew in thickness to thousands of meters, thereby subjecting the dead plants and animals to extreme pressure and temperature conditions. The petroleum and natural gas thus formed were trapped in the rock layers. Natural gas processing comprises separation of various condensates, hydrocarbons other than methane (small quantities of ethane, propane and butane are left behind), and impurities. It broadly involves the following four steps: . . . .

oil and condensate removal water removal separation of natural gas liquids (NGLs) sulfur and carbon dioxide removal.

The final product is a dry, pipeable, sweet (absence of hydrogen sulfide) gas. The byproducts are oil condensates (when raw natural gas is sourced from oil wells and exists as dissolved natural gas) and natural gas liquids (when raw natural gas is coming directly from gas wells) and are sent for further processing as they have commercial value. 1.2.1.2 Coal Bed Methane (CBM) Coal bed methane (CBM), similarly to natural gas, has a high percentages of methane and is found in seams of coal, and therefore is also called coal seam methane. Until recently, it was considered as a hazard and explosions due to lack of proper ventilation in mines occurred [2]. To reduce hazards and explosions, ventilation was provided, gases diluted and the mixture released to the atmosphere. Both to avoid release of a potent greenhouse gas and to tap the natural reserves of a gaseous fuel, it is now considered as a valuable source of gaseous fuel.

1.2 Classification of Gaseous Fuels

The difference between natural gas and CBM is the way in which the gas is actually trapped within the rock. In conventional reservoirs, natural gas is contained in the pore spaces between the sand grains, and in coal methane it is actually absorbed on the surface of the rock. Gas is part of the coal and is produced when coal is actually formed. Often, a coal seam is saturated with water and methane and is held in the coal by the pressure of water. CBM exists in areas where a coal seam is buried deeply enough to maintain sufficient water pressure to hold the gas in place. A coal seam has favorable reserves if it produces 1–2 m3 per tonne of coal. It is reported that CBM extraction is economical at >1 m3 t
1 of coal when a coal seam is 6 m or more thick [3]. 1.2.1.3 Methane Clathrates Clathrates are large molecules forming cage-like structures and when the molecules are water they are called hydrates. Hydrates are unstable and tend to dissociate rapidly due to the presence of a large, empty cavity at the core of their structure. When methane is added into the cavity, it stabilizes the hydrate structure and therefore the product is called a gas hydrate; it is also called a methane clathrate [4, 5]. Methane is by far the most commonly encountered guest in naturally occurring clathrates. Methane clathrates are also called natural gas clathrates. Cages are arranged in a body-centered type of packing wherein a unit cell contains 46 molecules of water and up to eight molecules of methane. Generally, not all cages are occupied. If all were occupied by methane, then 1 m3 of solid hydrate could contain 170.7 m3 of methane at standard temperature and pressure (STP). However, in Nature 1 m3 contains up to 164 m3 of methane [4]. Methane hydrate, when either warmed or depressurized, reverts back to water and natural gas. Gas hydrates require a cold temperature at moderate pressure or a warm temperature at higher pressure. They are reported to be stable at water depths below 500 m, where temperature and pressure are favorable for gas hydrate stability. Gas hydrates have been found at depths of over 4000 m within the ocean. Hydrate deposits may be several hundred meters thick and generally occur in two types of settings: under Arctic permafrost and beneath the ocean floor. Global estimates, although they vary, indicate that the energy content of methane as hydrates exceed the combined energy content of all other known fossil fuels. It is reported that as much as 1019 g of carbon is trapped, mostly as CH4, within solid gas hydrates [4, 5]. 1.2.1.4 Liquefied Petroleum Gas Liquefied petroleum gas (LPG) is one of the first end products upon refining crude petroleum. LPG consists of C3 and C4 compounds and is liquefied at room temperature by application of moderate pressure. LPG is available as commercial propane and also as commercial butane in various countries. When a mixture of two hydrocarbons is sold, then the LPG normally has higher proportions of C4 compounds [1]. It is also known as bottled gas. LPG products are high calorific value gases and are commonly used as domestic fuel. Since LPG is odorless, odorants such as thiols or sulfides are added to detect leakage. LPG burners are available for domestic and commercial use [1].

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1.2.2 Synthesis Gases 1.2.2.1 Biogas and Landfill Gas Biogas is the gas produced by anaerobic biological process (popularly called anaerobic digestion) of organic materials and is composed mainly of methane, carbon dioxide, and water vapor, along with small amounts of hydrogen sulfide, nitrogen, and hydrogen. The gas formed from the treatment of sewage and industry waste water is also called biogas. Biogas production takes place mainly in two stages – acidogenesis and methanogenesis. Acidogenesis involves breaking down of complex organic molecules into simple organic acids by microbes called acidogenic bacteria. These acids are later converted in the methanogenesis step into methane and carbon dioxide by the methanogenic bacteria. In the acidogenesis stage, hydrogen gas is also evolved; however, hydrogen is not seen in significant amounts in the final biogas because it is used up by the methanogenic bacteria in making methane. The digestible organic material is called the substrate. The yield and composition of the biogas depends on the nature of substrate. The percentage of fixed solid (FS), total solid (TS), and volatile solid (VS) is important for the yields; it is expected that a substrate having a large percentage of VS will give lot of biogas. Compounds having a high lignin content, however, behave stubbornly. The carbon to nitrogen (C/N) ratio is also important. In case of non-lignin substrate, a C/N ratio of 25–30 is considered good. In cases where the lignin content is higher, for example in woody material, a C/N of 40–50 is considered good [6]. The biogas production is sensitive also to pH and temperature. The acidogenic bacteria which produce fatty acids can tolerate low pH, whereas the methanogenic bacteria cannot survive below pH 5.5. It is generally preferable to maintain the pH between 6.5 and 8.5. There are two commonly known ranges of temperatures in which anaerobic bacteria survive – mesophilic (21–40  C) and thermophilic (40–60  C). Parameters such as loading rate (weight of volatile solids loaded each day divided by volumeofthedigester)andhydraulicretentiontime(HRT),whichistheaveragenumber of days a unit volume of the substrate stays in the digester, are also important [6, 7]. In addition to all these, agitation or mixing is advantageous as this helps control scum formation, maintains a uniform temperature throughout the digester, eliminates passive pockets, and so on. Large amounts of ammonia, urea, pesticides, herbicides, antibiotics, heavy metals, and synthetic detergents act as toxins and are harmful for the bacteria. Another source of methane is landfill gas. Similarly to anaerobic digestion in the biogas plant, the organic matter in the garbage dumped into landfills also biodegrades and methane is evolved. Although this gas contains methane, the percentages are appreciably lower than those in natural gas and biogas and are in the range 30–50%, the remainder mainly being carbon dioxide. Landfill gas also contains varying amounts of nitrogen, oxygen, sulfur, and so on. Mercury is also known to be present in landfill gas and sometimes the radioactive contaminant tritium has also been reported. Many toxic chemicals such as benzene, toluene, chloroform, vinyl chloride, and carbon tetrachloride have also been reported [8].

1.2 Classification of Gaseous Fuels

Landfill gas is obtained by carefully installing gas collection systems which include a series of wells and a flare system installed in the landfills during construction. The gas thus formed is diverted to a central point where it is processed and treated in accordance with its ultimate usage – to be flared off or used to fuel a generator. 1.2.2.2 Producer Gas, Synthesis Gas and, Blue Gas Producer gas is a product of partial combustion of biomass or coal with air or a mixture of air and steam. This process is called gasification. The reactor wherein this process occurs is called a gasifier. Urban waste and natural gas are also used as precursors. Carbon monoxide and hydrogen are the two principle combustible components of producer gas. Carbon dioxide and nitrogen are also present in relatively large quantities and consequently the resultant gas is of low calorific value. For specific application (to eliminate nitrogen in the gases), oxygen is used instead of air [7, 9]. Apart from the initial combustion reaction which takes place due to the presence of sub-stoichiometric amounts of oxygen, the other important reactions are as follows: Boudard reaction:
 CO2 þ C > 2CO DH ¼ 172:6 kJ mol
1

Water gas reaction (steam-carbon reaction):
 C þ H2 O > CO þ H2 DH ¼ 131:4 kJ mol
1
 C þ 2H2 O > CO2 þ 2H2 DH ¼ 88:0 kJ mol
1

ð1:1Þ

ð1:2Þ ð1:3Þ

Water gas shift reaction:
 CO þ H2 O > CO2 þ H2 DH ¼
41:2 kJ mol
1

ð1:4Þ


 C þ 2H2 > CH4 DH ¼
75:0 kJ mol
1

ð1:5Þ

These reactions take place at high temperatures of the order of 800–1000  C. The methane-forming reaction is favored at lower temperatures and higher pressures [10]. Many design variations exist in the gasifiers. The most popular types of gasifiers are based on the flow conditions and are the moving bed, the entrained flow, and the fluidized bed reactors. Recently, underground coal gasification (UCG) has been looked into as an alternate to convert unminable coal deposits into synthesis gas. This technology is also expected to be advantageous for gasification of high-ash coal, where problems with surface gasification are still being resolved. In most UCG trials the gases have been used for power generation. Recently, syngas production has also been investigated [11]. When the producer gas consists of varying amounts of hydrogen and carbon monoxide, and is used as intermediate for producing synthetic natural gas, methanol, or synthetic petroleum, then the gas is called synthesis gas. Fischer–Tropsch conversion is one such well-known process for conversion of synthesis gas to synthetic liquid fuels. Normally, synthesis gas has a higher calorific value than

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producer gas because of the absence of nitrogen. Oxygen instead of air is generally used as oxidant in such cases [12]. When superheated steam is used for the gasification reaction involving mainly the water gas reaction (Equation 1.2), then water gas is generated [1]. Theoretically, carbon monoxide and hydrogen should be produced in equal proportions. However, in practice it is impossible to exclude the other reactions (Equations ) and Boudard’s reaction (Equation 1.1), due to which carbon dioxide is also present. Owing to the high carbon monoxide content, this gas gives a blue flame and is therefore called blue water gas or blue gas. 1.2.2.3 Hydrogen Hydrogen is today considered to be an attractive source for replacement of conventional fossil fuels. Since hydrogen is a fuel which generates no pollutants upon combustion, it is considered to be a clean energy source. Hydrogen can be used as a fuel directly in internal combustion engines, and can be used to power a vehicle via fuel cells. Hydrogen for fuel cells can be generated either on-board through catalytic steam reforming of hydrogen or by storing it as hydrates or in carbon nanotubes as an intermediate step. Natural gas and naphtha have been the main source of commercial hydrogen. Gasification of coal and electrolysis of water are other industrial methods used for hydrogen production. Recently, alternative resources such as biomass are being exploited for production of biomass. For hydrogen production from biomass, various routes are being explored which include biomass gasification coupled with the water gas shift reaction; fast pyrolysis followed by reforming of the resultant oil is also being investigated [13–15]. Microbial conversion of biomass is being studied extensively and is fast gaining popularity. Various hydrogen-producing microorganisms have been used to produce hydrogen selectively upon breaking up of biomass carbohydrates at various stages. Molecular hydrogen can also be recovered as a product or co-product of the anaerobic fermentation of biomass. It is reported that photosynthetic algae are capable of generating molecular hydrogen by bio-photolysis. However, the microbial generation of hydrogen is still far from achieving commercial success [7, 15]. 1.2.3 By-Product Gases 1.2.3.1 Blast Furnace Gas When the combustion gases in the blast furnace move upwards through the descending mix of coke, iron ore and flux, the carbon dioxide is reduced to carbon monoxide, and the steam also decomposes to hydrogen and carbon monoxide. The reduction of iron oxide is achieved mostly by carbon dioxide and marginally by hydrogen. The gases leave the furnace top at temperatures of about 200  C. These gases are rich in carbon monoxide and poor in hydrogen content. The carbon dioxide content is also high. The gases also have a high dust content and have to be cleaned before any use [1].

1.3 Properties of Gaseous Fuels

1.2.3.2 Coke Oven Gas Coke oven gases are a by-product of the coal carbonization process. They are a result of secondary cracking reaction of the primary tar vapors at the high temperature in the coke oven. Methane and hydrogen form the principle combustible components of this gas. Table 1.1 gives the representative compositions of various gaseous fuels [1, 4, 6, 7, 9, 10, 16]. It must be noted that most of these gases are produced through various reactions either in Nature or by humans and are a mixture of gases. The composition may spread beyond the range given.

1.3 Properties of Gaseous Fuels

Gaseous fuels are characterized by their composition and the constituent gases contribute to the fuel-related properties. These properties are also dependent on many physical and chemical variables. The thermal and transport properties of the gaseous fuels are important for simulation and understanding of the combustion process. These include density, specific heat, viscosity, thermal conductivity and binary mass diffusivity. In most cases, the ideal gas laws are applicable. However, since most of the gaseous fuels are a mixture of various species, it is important to use the appropriate mixing rules for evaluating these properties, especially for fundamental studies such as aimed at understanding the combustion process. The subject had dealt with in detail in the book by Kuo [17]. Specific fuel-related properties are discussed here and below: Fuel properties such as calorific value, adiabatic flame temperatures, Weaver flame speed, flammability limits, Wobbe number, and methane number are specific to each gaseous fuel, depend on its composition, and play an important role in classifying the gaseous fuels for their application and interchangeability. 1.3.1 Calorific Value

Gaseous fuels are often classified as high, medium and low calorific value gases. High calorific value gases are also sometimes called rich gases and the low calorific value gases are called lean gases. The typical ranges of calorific values of gaseous fuels are shown in Table 1.1. 1.3.2 Wobbe Number (Wo)

The Wobbe number is an important factor when determining the interchangeability of the gases in a burner, that is, for the same burner, the pressure drop (DP) across the

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a)

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— — — — — — — — 100

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— — — — — — — — — — — 0–70 —

Acetylene (%)

Butane (%)

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