9.3.1 Coal devolatilization

Devolatilization occurs when the coal is heated above 400 °C (750 °F). During this period, the coal structure is altered, producing solid char, tars, condensable liquids, and light gases. The devolatilization products formed in an inert gas atmosphere are very different from those in an atmosphere containing hydrogen at elevated pressure. After devolatilization, char then gasifies at a lower rate. The specific reactions that take place during this second stage depend on the gasification medium.

After the rate of devolatilization has passed a maximum, another reaction occurs in which the semi-char is converted to char primarily through the evolution of hydrogen. In a hydrogen atmosphere at elevated pressure, additional yields of methane or other low molecular weight gaseous hydrocarbon can result during the initial coal gasification stage from reactions such as (1) direct hydrogenation of coal or semi-char because of active intermediate formed in coal structure after coal pyrolysis, and (2) the hydrogenation of other gaseous hydrocarbons, oils, tars, and carbon oxides.

9.3.2 Char gasification

Char gasification occurs as the char reacts with gases such as carbon dioxide (CO₂) and steam (H₂O) to produce carbon monoxide (CO) and hydrogen (H₂):

$2\mathrm{C}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{CO}+{\mathrm{H}}_2$

The resulting gas (producer gas or synthesis gas) may be more efficiently converted to electricity than is typically possible by direct combustion of coal. Also, corrosive ash elements such as chloride and potassium may be refined out by the gasification process, allowing high temperature combustion of the gas from otherwise problematic coal feedstocks.

Although the devolatilization reaction is completed in short order (typically in seconds) at elevated temperatures, the subsequent gasification of the char is much slower, requiring minutes or hours to obtain significant conversion under practical conditions. In fact, reactor design for commercial gasification processes is largely dependent on the reactivity of the char.

The reactivity of char produced in the pyrolysis step depends on the nature of the parent coal; it increases with oxygen content of the parent coal but decreases with carbon content. In general, char produced from low-rank coal is more reactive than char produced from high-rank coal. The reactivity of char from low-rank coal may be influenced by catalytic effect of mineral matter in char. In addition, as the carbon content of coal increases, the reactive functional groups present in coal decrease and the coal substance becomes more aromatic and cross-linked in nature (Speight, 2013a). Therefore, char obtained from high-rank coal contains a lesser number of functional groups and a higher proportion of aromatic and cross-linked structures, which reduce reactivity. The rate of gasification of the char decreases as the process temperature increases due to the decrease in active surface area of char. So, a change of char preparation temperature may change the chemical nature of char, which in turn may change the gasification rate (Johnson, 1979; Penner, 1987; Speight, 2013a).

9.3.3 Gasification chemistry

Coal gasification occurs under reducing conditions – coal (in the presence of steam and oxygen at high temperature and moderate pressure) is converted to a mixture of product gases. The chemistry of coal gasification is but can be conveniently (and simply) represented by the following reactions:

$\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2\mathrm{\Delta }\mathrm{Hr}=-393.4\mathrm{MJ}/\mathrm{kmol}$

$\mathrm{C}+\mathrm{½}{\mathrm{O}}_2\to \mathrm{CO}\mathrm{\Delta }\mathrm{Hr}=-111.4\mathrm{MJ}/\mathrm{kmol}$

$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{CO}\mathrm{\Delta }\mathrm{Hr}=130.5\mathrm{MJ}/\mathrm{kmol}$

$\mathrm{C}+{\mathrm{CO}}_2\leftrightarrow 2\mathrm{CO}\mathrm{\Delta }\mathrm{Hr}=170.7\mathrm{MJ}/\mathrm{kmol}$

$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_2+{\mathrm{CO}}_2\mathrm{\Delta }\mathrm{Hr}=-40.2\mathrm{MJ}/\mathrm{kmol}$

$\mathrm{C}+2{\mathrm{H}}_2\to {\mathrm{CH}}_4\mathrm{\Delta }\mathrm{Hr}=-74.7\mathrm{MJ}/\mathrm{kmol}$

Reactions (9.1) and (9.2) are exothermic oxidation reactions and provide most of the energy required by the endothermic gasification reactions (9.3) and (9.4). The oxidation reactions occur very rapidly, completely consuming all of the oxygen present in the gasifier, so that most of the gasifier operates under reducing conditions. Reaction (9.5) is the water-gas shift reaction, where water (steam) is converted to hydrogen. This reaction is used to alter the hydrogen/carbon monoxide ratio when synthesis gas is the desired product, such as for use in Fischer-Tropsch processes. Reaction (9.6) is favored by high pressure and low temperature and is therefore important mainly in lower temperature gasification systems. Methane formation is an exothermic reaction that does not consume oxygen and so it increases the efficiency of the gasification process and the final heat content of the product gas. Overall, ~ 70% of the heating value of the product gas is associated with the carbon monoxide and hydrogen, but this can be higher depending on the gasifier type (Chapter 2; Chadeesingh, 2011).

Depending on the gasifier technology employed and the operating conditions (Chapter 2), significant quantities of water, carbon dioxide, and methane can be present in the product gas, as well as a number of minor and trace components. Under the reducing conditions in the gasifier, most of the organically bound sulfur in the coal feedstock is converted to hydrogen sulfide (H₂S), but a small amount (3-10% w/w) is converted to carbonyl sulfide (COS). Organically bound nitrogen in the coal feedstock is generally converted to gaseous nitrogen (N₂), but small amounts of ammonia (NH₃) and hydrogen cyanide (HCN) are also formed. Any chlorine in the coal (which typically originates from tine in the coal seam) is converted to hydrogen chloride (HCl) with some chlorine present in the particulate matter (fly ash). Trace elements, such as mercury and arsenic, are released during gasification and partition among the different phases, such as fly ash, bottom ash, slag, and product gas.

9.3.5 Process optimization

The output and quality of the gas produced is determined by the equilibrium established when the heat of oxidation (combustion) balances the heat of vaporization and volatilization plus the sensible heat (temperature rise) of the exhaust gases. The quality of the outlet gas (BTU/ft.³) is determined by the amount of volatile gases (such as hydrogen, carbon monoxide, water, carbon dioxide, and methane) in the gas stream.

In a gasifier, the coal particle is exposed to high temperatures generated from the partial oxidation of the carbon. As the particle is heated, any residual moisture (assuming that the coal has been pre-fired) is driven off and further heating of the particle begins to drive off the volatile gases. Discharge of these volatiles will generate a wide spectrum of hydrocarbons ranging from carbon monoxide and methane to long-chain hydrocarbons comprising tars, creosote, and heavy oil. At temperatures above 500 °C (930 °F), the conversion of the coal to char and ash is completed. In most of the early gasification processes, this was the desired by-product, but for gas generation the char provides the necessary energy to effect further heating. Typically, the char is contacted with air or oxygen and steam to generate the product gases.

Gasification of coal/char in a carbon dioxide atmosphere can be divided into two stages. The first stage is due to pyrolysis (removal of moisture content and devolatilization), which is comparatively at lower temperature and the second stage is char gasification by different oxygen/carbon dioxide mixtures at high temperature. In nitrogen and carbon dioxide environments from room temperature to 1000 °C (1830 °F), the mass loss rate of coal pyrolysis in nitrogen is lower than that of carbon dioxide due to the difference in properties of the bulk gases. The gasification process of pulverized coal in the oxygen/carbon dioxide environment is almost the same as compared with that in oxygen/nitrogen at the same oxygen concentration, but this effect is little bit delayed at high temperature. This may be due to the lower rate of diffusion of oxygen through carbon dioxide and the higher specific heat capacity of carbon dioxide. However, with the increase of oxygen concentration, the mass loss rate of coal also increases and hence it shortens the burnout time of coal. The optimum value oxygen/carbon dioxide ratio for the reaction of oxygen with the functional group present in the coal sample was found to be about 8%.

The combination of pyrolysis and gasification process can be the unique and fruitful technique, as it can save the prior use of gasifying medium and the production of fresh char simultaneously in one process. With the increase of heating rate, coal particles are faster heated in a short period of time and burned in a higher temperature region, but the increase in heating rate has almost no substantial effect on the combustion mechanism of coal. The increase of heating rate causes a decrease in activation energy value (Irfan, 2009).

9.4.1 Low Btu gas

Low Btu gas (low-heat content gas) is the product when the oxygen is not separated from the air and, as a result, the gas product invariably has a low-heat content (150-300 Btu/ft³). Several important chemical reactions (Table 9.1), and a host of side reactions, are involved in the manufacture of low-heat content gas under the high temperature conditions employed. Low Btu gas (low-heat content gas) contains several components (Table 9.2). In medium-heat content gas, the H₂/CO ratio varies from 2:3 to ~ 3:1, and the increased heating value correlates with higher methane and hydrogen content, as well as with lower carbon dioxide content.

The nitrogen content of low-heat content gas ranges from somewhat less than 33% v/v to slightly more than 50% v/v and cannot be removed by any reasonable means, which limits the applicability of the gas to chemical synthesis. Two other noncombustible components, water and carbon dioxide, further lower the heating value of the gas. Water can be removed by condensation, and carbon dioxide by relatively straightforward chemical means.

The two major combustible components are hydrogen and carbon monoxide; the hydrogen/carbon monoxide ratio varies from ~ 2:3 to about 3:2. Methane may also make an appreciable contribution to the heat content of the gas. Of the minor components, hydrogen sulfide is the most significant and the amount produced is, in fact, proportional to the sulfur content of the feed coal. Any hydrogen sulfide present must be removed by one, or more, of several available on-stream commercial processes (Speight, 2007, 2013a, 2014).

9.4.2 Medium Btu gas

Medium Btu gas (medium-heat content gas) has a heating value in the range 300-550 Btu/ft³, and the composition is much like that of low-heat content gas, except that there is virtually no nitrogen. The primary combustible gases in medium-heat content gas are hydrogen and carbon monoxide (Kasem, 1979).

Medium-heat content gas is considerably more versatile than low-heat content gas; as with low-heat content gas, medium-heat content gas may be used directly as a fuel to raise steam, or used through a combined power cycle to drive a gas turbine, with the hot exhaust gases employed to raise steam. Medium-heat content gas, however, is especially amenable to synthesizing methane by methanation, higher hydrocarbons by Fischer-Tropsch synthesis, methanol, and a variety of synthetic chemicals (Chadeesingh, 2011; Davis and Occelli, 2010).

The reactions used to produce medium-heat content gas are the same as those employed for low-heat content gas synthesis, with the major difference being the application of a nitrogen barrier, such as the use of pure oxygen, to keep diluent nitrogen out of the system.

9.4.3 High Btu gas

High Btu gas (high-heat content gas) is essentially pure methane and often referred to as synthetic natural gas (SNG) (Kasem, 1979; Speight, 1990, 2013a). However, to qualify as a synthetic natural gas, a product must contain at least 95% methane, and the energy content of synthetic natural gas is 980-1080 Btu/ft³. The commonly accepted approach to the synthesis of high-heat content gas is the catalytic reaction of hydrogen and carbon monoxide.

$3{\mathrm{H}}_2+\mathrm{CO}\to {\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}$

During this process, the hydrogen is usually present in slight excess to ensure that the toxic carbon monoxide is reacted; this small quantity of hydrogen will lower the heat content to a small degree.

The carbon monoxide/hydrogen reaction is somewhat inefficient as a means of producing methane because the reaction liberates large quantities of heat. In addition, the methanation catalyst is troublesome and prone to poisoning by sulfur compounds, and the decomposition of metals can destroy the catalyst. Thus, hydrogasification may be employed to minimize the need for methanation.

${\mathrm{C}}_{\mathrm{coal}}+2{\mathrm{H}}_2\to {\mathrm{CH}}_4$

The product of hydrogasification is not pure methane. Additional methanation is required after hydrogen sulfide and other impurities are removed.

9.4.4 Methane

Several exothermic reactions may occur simultaneously within a methanation unit (Seglin, 1975). A variety of metals have been used as catalysts for the methanation reaction, and the most common, and to some extent the most effective, methanation catalysts appear to be nickel and ruthenium, with nickel being the most widely used (Cusumano et al., 1978; Seglin, 1975; Tucci & Thompson, 1979; Watson, 1980). The synthesis gas must be desulfurized before the methanation step, because sulfur compounds will rapidly deactivate (poison) the catalysts (Cusumano et al., 1978). A problem may arise when the concentration of carbon monoxide is excessive in the stream to be methanated. Large amounts of heat must be removed from the system to prevent high temperatures and deactivation of the catalyst by sintering as well as the deposition of carbon (Cusumano et al., 1978). So, to eliminate carbon deposition, process temperatures should be maintained below 400^oC (750 °F).

9.4.5 Hydrogen

Hydrogen is also produced by coal gasification (Johnson, Yang, & Ogden, 2007). Although several gasifier types exist (Chapter 2), entrained-flow gasifiers are considered most appropriate for producing both hydrogen and electricity from coal. This is because they operate at temperatures high enough (~ 1500 °C, 2730 °F) to enable high carbon conversion and prevent downstream fouling from tars and other residuals.

In the process, the coal undergoes three processes in its conversation to synthesis gas. The first two processes, pyrolysis and combustion, occur very rapidly. In pyrolysis, char is produced as the coal heats up and the volatiles are released. In the combustion process, the volatile products and some of the char react with oxygen to produce various products (primarily carbon dioxide and carbon monoxide) and the heat required for subsequent gasification reactions. Finally, in the third process – gasification – the coal char reacts with steam to produce hydrogen (H₂) and carbon monoxide (CO).

$2{\mathrm{C}}_{\mathrm{coal}}+{\mathrm{O}}_2\to 2\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}$

${\mathrm{C}}_{\mathrm{coal}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{CO}$

$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{\mathrm{CO}}_2$

The resulting syngas is ~ 63% v/v carbon monoxide, 34% v/v hydrogen, and 3% v/v carbon dioxide. At the gasifier temperature, the ash and other coal mineral matter liquefy and exit at the bottom of the gasifier as slag, a sand-like inert material that can be sold as a co-product to other industries (such as the road-building industry). The synthesis gas exits the gasifier at high pressure and high temperature and must be cooled prior to the syngas cleaning stage.

Although processes that use the high temperature to raise high-pressure steam are more efficient for electricity production (Speight, 2013b), full-quench cooling, by which the synthesis gas is cooled by the direct injection of water, is more appropriate for hydrogen production and provides the necessary steam to facilitate the catalytic water gas shift reaction:

$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+{\mathrm{H}}_2$

Unlike pulverized coal combustion plants in which expensive emissions control technologies are required to scrub contaminants from large volumes of flue gas, smaller and less expensive emissions control technologies are appropriate for coal gasification plants because the clean-up occurs in the syngas. The synthesis gas is at high pressure and contains contaminants at high partial pressures, which facilitates gas cleaning.

As with other processes, the characteristics of the coal feedstock (e.g., heating value and ash, moisture, and sulfur content) have a substantial impact on plant efficiency and emissions. As a result, the cost of producing hydrogen from coal gasification can vary substantially, depending on the proximity to appropriate coal types.

9.4.6 Other products

There is a series of products that are called by older (even archaic) names that should also be mentioned here for clarification: (1) producer gas, (2) water gas, (3) town gas, and (4) synthetic natural gas.

Producer gas is a low-Btu gas obtained from a coal gasifier (fixed-bed) when air instead of oxygen is introduced into the fuel bed. The composition of the producer gas is ~ 28% v/v carbon monoxide, 55% v/v nitrogen, 12% v/v hydrogen, and 5% v/v methane with some carbon dioxide.

Water gas is a medium-Btu gas that is produced by the introduction of steam into the hot fuel bed of the gasifier. The composition of the gas is ~ 50% v/v hydrogen and 40% v/v carbon monoxide with small amounts of nitrogen and carbon dioxide.

Town gas is a medium-Btu gas that is produced in the coke ovens and has the following approximate composition: 55% v/v hydrogen, 27% v/v methane, 6% v/v carbon monoxide, 10% v/v nitrogen, and 2% v/v carbon dioxide. Carbon monoxide can be removed from the gas by catalytic treatment with steam to produce carbon dioxide and hydrogen.

Synthetic natural gas (SNG) is methane obtained from the reaction of carbon monoxide or carbon with hydrogen. Depending on the methane concentration, the heating value can be in the range of high-Btu gases.

9.5.1 Coal tar chemicals

Coal tar is a black or dark brown liquid or a high-viscosity semi-solid that is one of the by-products formed when coal is carbonized. Coal tars are complex and variable mixtures of polycyclic aromatic hydrocarbons (PAHs), phenols, and heterocyclic compounds. Because of its flammable composition, coal tar is often used for fire boilers in order to create heat. They must be heated before any heavy oil flows easily.

By comparison, coal tar creosote is a distillation product of coal tar and consists of aromatic hydrocarbons, anthracene, naphthalene, and phenanthrene derivatives. At least 75% of the coal tar creosote mixture is PAHs. Unlike the coal tars and coal tar creosotes, coal tar pitch is a residue produced during the distillation of coal tar. The pitch is a shiny, dark brown to black residue that contains PAHs and their methyl and poly-methyl derivatives, as well as heteronuclear aromatic compounds.

Primary distillation of crude tar produces pitch (residue) and several distillate fractions, the amounts and boiling ranges of which are influenced by the nature of the crude tar (which depends on the coal feedstock) and the processing conditions. For example, in the case of the tar from continuous vertical retorts, the objective is to concentrate the tar acids (phenol derivatives, cresol derivatives, and xylenol derivatives) into carbolic oil fractions. On the other hand, the objective with coke oven tar is to concentrate the naphthalene and anthracene components into naphthalene oil and anthracene oil, respectively.

The first step in refining benzole is steam distillation, which is employed to remove compounds boiling below benzene. To obtain pure products, the benzole can be distilled to yield a fraction containing benzene, toluene, and xylene(s). Benzene is used in the manufacture of numerous products, including nylon, gammexane, polystyrene, phenol, nitrobenzene, and aniline. On the other hand, toluene is a starting material in the preparation of saccharin, trinitrotoluene, and polyurethane foams. The xylenes present in the light oil are not always separated into the individual pure isomers because xylene mixtures can be marketed as specialty solvents. Higher boiling fractions of the distillate from the tar contain pyridine bases, naphtha, and coumarone resins. Other tar bases occur in the higher boiling range and these are mainly quinoline, iso-quinoline, and quinaldine.

Pyridine has long been used as a solvent in the production of rubber chemicals and textile water-repellant agents, and in the synthesis of drugs. The derivatives 2-benzylpyridine and 2-aminopyridine are used in the preparation of antihistamines. Another market for pyridine is in the manufacture of the non-persistent herbicides diquat and paraquat. Alpha-picoline (2-picoline; 2-methylpryridine) is used for the production of 2-vinylpyridine, which, when co-polymerized with butadiene and styrene, produces a used as a latex adhesive that is used in the manufacture of car tires. Other uses are in the preparation of 2-beta-methoxyethylpyridine (known as Promintic, an anthelmintic for cattle) and in the synthesis of a 2-picoline quaternary compound (Amprolium), which is used against coccidiosis in young poultry. Beta-picoline (3-picoline; 3-methylpryridine) can be oxidized to nicotinic acid, which, with the amide form (nicotinamide), belongs to the vitamin B complex; both products are widely used to fortify human and animal diets. Gama-picoline (4-picoline; 4-methylpyridine) is an intermediate in the manufacture of isonicotinic acid hydrazide (Isoniazide), which is a tuberculostatic drug. And 2,6-Lutidine (2,6-dimethylpyridine) can be converted to dipicolinic acid, which is used as a stabilizer for hydrogen peroxide and peracetic acid.

Solvent naphtha and heavy naphtha are the mixtures obtained when the 150-200 °C (300-390 °F) fraction, after removal of tar acids and tar bases, is fractionated. These naphtha fractions are used as solvents.

The tar-acid-free and tar-base-free coke-oven naphtha can be fractionated to give a narrow-boiling fraction (170-185 °C; 340-365 °F) containing coumarone and indene. This is treated with strong sulfuric acid to remove unsaturated components and is then washed and re-distilled. The concentrate is heated with a catalyst (such as a boron fluoride/phenol complex) to polymerize the indene and part of the coumarone. Unreacted oil is distilled off and the resins obtained vary from pale amber to dark brown in color. They are used in the production of flooring tiles and in paints and polishes.

Naphthalene and several tar acids are the important products extracted from volatile oils from coal tar. It is necessary to first extract the phenolic compounds from the oils and then to process the phenol-depleted oils for naphthalene recovery.

Tar acids are produced by extraction of the oils with aqueous caustic soda at a temperature sufficient to prevent naphthalene from crystallizing. The phenols react with the sodium hydroxide to give the corresponding sodium salts an aqueous extract known variously as crude sodium phenate, sodium phenolate, sodium carbolate, or sodium cresylate. The extract is separated from the phenol-free oils that are then taken for naphthalene recovery.

Naphthalene is probably the most abundant component in high-temperature coal tars. The primary fractionation of the crude tar concentrates the naphthalene into oils which, in the case of coke-oven tar, contain the majority (75-90% w/w) of the total naphthalene. After separation, naphthalene can be oxidized to produce phthalic anhydride, which is used in the manufacture of alkyd and glyptal resins and plasticizers for polyvinyl chloride and other plastics.

The main chemical extracted on the commercial scale from the higher-boiling oils (b.p. 250 °C, 480 °F) is crude anthracene. The majority of the crude anthracene is used in the manufacture of dyes after purification and oxidation to anthraquinone.

Creosote is the residual distillate oils obtained when the valuable components, such as naphthalene, anthracene, tar acids, and tar bases, have been removed from the corresponding fractions. Creosote is a brownish-black/yellowish-dark green oily liquid with a characteristic sharp odor, obtained by the fractional distillation of crude coal tars. The approximate distillation range is 200-400 °C (390-750 °F). The chemical composition of creosotes is influenced by the origin of the coal and also by the nature of the distilling process. As a result, the creosote components are rarely consistent in their type and concentration.

As a corollary to this section where the emphasis has been on the production of bulk chemicals from coal, a tendency-to-be-forgotten item must also be included. That is the mineral ash from coal processes. Coal minerals are a very important part of the coal matrix; it offers the potential for the recovery of valuable inorganic materials (Speight, 2013a). However, there is another aspect of the mineral content of coal that must be addressed, and it relates to the use of the ash as materials for roadbed stabilization, landfill cover, cementing (due to the content of pozzolanic materials), and wall construction.

9.5.2 Fischer-Tropsch chemicals

Fischer-Tropsch chemicals are those chemicals produced by conversion of the synthesis gas mixture (carbon monoxide, CO, and hydrogen, H₂) to higher molecular weight liquid fuels and other chemicals (Chadeesingh, 2011; Penner, 1987; Speight, 2013a). In principle, syngas can be produced from any hydrocarbon feedstock. These include natural gas, naphtha, residual oil, petroleum coke, coal, and biomass.

The synthesis of hydrocarbons from carbon monoxide and hydrogen (the Fischer-Tropsch synthesis) is a procedure for the indirect liquefaction of coal (Anderson, 1984; Dry & Erasmus, 1987). This process is the only coal liquefaction scheme currently in use on a relatively large commercial scale. South Africa is currently using the Fischer-Tropsch process on a commercial scale in their SASOL (South Africa) complex, although Germany produced roughly 156 million barrels of synthetic petroleum annually using the Fischer-Tropsch process during the Second World War.