5.2.1 Devolatilization

Devolatilization occurs rapidly as the coal is heated above 400 °C (750 °F). During this period, the coal structure is altered, producing solid char, tars, condensable liquids, and low molecular weight gases. Furthermore, the products of the devolatilization stage in an inert gas atmosphere are very different from those in an atmosphere containing hydrogen at elevated pressure. 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. Again, the kinetic picture for such reactions is complex due to the varying composition of the volatile products which, in turn, are related to the character of the coal feedstock and the process parameters, including the reactor type.

5.2.2 Char gasification

After the rate of devolatilization has passed, another reaction becomes important. In this reaction, the semi-char is converted to char (sometimes erroneously referred to as stable char) primarily through the evolution of hydrogen. Thus, the gasification process occurs as the char reacts with gases such as carbon dioxide and steam to produce carbon monoxide and hydrogen. The resulting gas (producer gas or synthesis gas) may be more efficiently converted to electricity than is typically possible by direct combustion of the. Also, corrosive ash elements such as chloride and potassium may be refined by the gasification process, allowing high temperature combustion of the gas from otherwise problematic coal feedstocks (Speight, 2013a, 2013b).

Oxidation and gasification reactions consume the char, and the oxidation and the gasification kinetic rates follow Arrhenius-type dependence on temperature; the kinetic parameters are coal-rank-specific and there is no true global relationship to describe the kinetics of coal (char) gasification. The complexity of the reactions makes the reaction initiation and the subsequent rates subject to many factors, any one of which can influence the kinetic aspects of the reaction.

Although the initial gasification stage (devolatilization) is completed in seconds or even less at elevated temperature, the subsequent gasification of the coal char produced at the initial coal-gasification stage is much slower, requiring minutes or hours to obtain significant conversion under practical conditions. Reactor designs for commercial gasifiers are largely dependent on the reactivity of the coal char and also on the gasification medium rate (Johnson, 1979; Penner, 1987; Sha, 2005). Thus, the distribution and chemical composition of the products are also influenced by the prevailing conditions (i.e., temperature, heating rate, pressure, residence time, etc.) and, last but not least, the nature of the feedstock. Also, the presence of oxygen, hydrogen, water vapor, carbon oxides, and other compounds in the reaction atmosphere during pyrolysis may either support or inhibit numerous reactions with coal and with the products evolved.

The reactivity of char produced in the pyrolysis step depends on the nature of parent coal. It increases with oxygen content of 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 reactivity of char also depends on the thermal treatment it receives during formation from the parent coal. The gasification rate of char decreases as the char preparation temperature increases due to the decrease in active surface areas of char. Thus, a change of char preparation temperature may change the chemical nature of char, which, in turn, may change the gasification rate.

Typically, char has a higher surface area compared to the surface area of the parent coal. The surface area changes as char undergoes gasification, increasing the surface area with carbon conversion, reaching maximum, and then decreasing. These changes, in turn, affect gasification rates. In general, reactivity increases with the increase in surface area. The initial increase in surface area appears to be caused by clean-up and widening of pores. The decrease in surface area at high carbon conversion may be due to coalescence of pores, which ultimately leads to collapse of the pore structure.

Furthermore, char reactivity is also influenced by the catalytic effect of mineral matter in char. The reactivity of lignite char, which had been initially treated with acid to remove mineral constituents, was much lower than the corresponding reactivity exhibited by untreated char. However, this phenomenon has not been observed with char from bituminous and sub-bituminous coal (Speight, 2013a, 2013b, and references cited therein). The behavior of the lignite char may be the result of the catalytic effect of sodium or calcium combined with carboxyl functional groups in the organic structure of the lignite. Given that the concentration of carboxyl functional groups decreases significantly with increasing coal rank, this catalytic effect would predominate in lignite and would decrease rapidly with increasing coal rank.

Heat transfer and mass transfer processes in fixed or moving bed gasifiers are affected by complex solids flow and chemical reactions. Coarsely crushed coal settles while undergoing heating, drying, devolatilization, gasification, and combustion. Coal particles change in diameter, shape, and porosity – non-ideal behavior may result from coal bridges, gas bubbles, and channel, and a variable void fraction may also change heat and mass transfer characteristics.

An important issue is the significance of the pyrolysis temperature as a major factor in the thermal history, and consequently in the thermodynamics of the coal chars. However, the thermal history of a char should also depend on the rate of temperature rise to the pyrolysis temperature and on the length of time the char is kept at the pyrolysis temperature (soak time), which might be expected to reduce the residual entropy of the char by employing a longer soak time.

5.2.3 Products

If air is used for combustion, the product gas will have a heat content on the order of 150-300 Btu/ft³ depending on process design characteristics and will contain undesirable constituents such as carbon dioxide, hydrogen sulfide, and nitrogen. The use of pure oxygen results in a product gas having a heat content of 300-400 Btu/ft³ with carbon dioxide and hydrogen sulfide as by-products, both of which can be removed from low-heat content or medium-heat content – low-Btu or medium-Btu gas (Table 5.2) – by any of several available processes (Mokhatab, Poe, & Speight, 2006; Speight, 2013a, 2014).

If high-heat content (high-Btu) gas (900-1000 Btu/ft³) is required, efforts must be made to increase the methane content of the gas. The reactions that generate methane are all exothermic and have negative values, but the reaction rates are relatively slow, and catalysts may therefore be necessary to accelerate the reactions to acceptable commercial rates. Indeed, it is also possible that the mineral constituents of coal and char may modify the reactivity by a direct catalytic mechanism. The presence of oxygen, hydrogen, water vapor, carbon oxides, and other compounds in the reaction atmosphere during pyrolysis may either support or inhibit numerous reactions with coal and with the products evolved.

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 the volatile products will generate a wide spectrum of hydrocarbons ranging from carbon monoxide and methane to long-chain hydrocarbons comprising tars, creosote, and heavy oil. The complexity of the products will also affect the progress and rate of the reaction, as each product is produced by a different chemical process at a different rate. At a temperature above 500 °C (930 °F), the conversion of the coal to char, and ash and char, 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.

Furthermore, with an increase in heating rate, coal particles are heated more rapidly and are burned in a higher temperature region, but the increase in heating rate has almost no substantial effect on the mechanism. Also, the increase in the heating rate causes a decrease in the activation energy value. Activation energy values were calculated by various well-known methods at different fractions from 90% to 15% of the original coal within the temperature range of about 400-600 °C (750-1110 °F), and the Coats-Redfern approach showed the highest value of activation energy, whereas the Freeman-Carroll method showed the least value of activation energy for every fraction of converted coal (Irfan, 2009).

The most notable effects in the physical chemistry of coal gasification are those effects due to coal character, and often those effects relate to the maceral type and maceral content (Speight, 2013a, 2013b). With regard to the maceral content, differences have been noted between the different maceral groups, with inertinite being the most reactive (Huang et al., 1991). In more general terms of the character of the coal, gasification technologies generally require some initial processing of the coal feedstock with the type and degree of pretreatment being a function of the process and/or the type of coal. For example, the Lurgi process will accept lump coal (1 in., 25 mm, to 28 mesh), but it must be non-caking coal with the fines removed. Caking or agglomerating coals tend to form a plastic mass in the bottom of a gasifier and subsequently plugs up the system, thereby markedly reducing process efficiency. Thus, some attempt to reduce caking tendencies is necessary and can involve preliminary partial oxidation of the coal, thus destroying the caking properties.

Another factor, often presented as a general rule of thumb, is that optimum gas yields and gas quality are obtained at operating temperatures of approximately 595-650 °C (1100-1200 °F). A gaseous product with a higher heat content (Btu/ft³) can be obtained at lower system temperatures but the overall yield of gas (determined as the fuel-to-gas ratiois reduced by the unburned char fraction.

With some coal feedstocks, the higher the amounts of volatile produced in the early stages of the process, the higher the heat content of the product gas. In some cases, the highest gas quality may be produced at the lowest temperatures, but when the temperature is too low, char oxidation reaction is suppressed and the overall heat content of the product gas is diminished. All such events serve to complicate the reaction rate and make derivative of a global kinetic relationship applicable to all types of coal subject to serious question and doubt.

Depending on the type of coal being processed and the analysis of the gas product desired, pressure also plays a role in product definition. In fact, some (or all) of the following processing steps will be required: (1) pretreatment of the coal (if caking is a problem); (2) primary gasification of the coal; (3) secondary gasification of the carbonaceous residue from the primary gasifier; (4) removal of carbon dioxide, hydrogen sulfide, and other acid gases; (5) shift conversion for adjustment of the carbon monoxide/hydrogen mole ratio to the desired ratio; and (6) catalytic methanation of the carbon monoxide/hydrogen mixture to form methane. If high-heat content (high-Btu) gas is desired, all of these processing steps are required because coal gasifiers do not yield methane in the concentrations.

5.3.2 Pretreatment

Some coals display caking, or agglomerating, characteristics when heated. These coals are usually not amenable to treatment by gasification processes employing fluidized-bed or moving-bed reactors; in fact, caked coal is difficult to handle in fixed-bed reactors. The pre treatment involves a mild oxidation treatment that destroys the caking characteristics of coals and usually consists of low-temperature heating of the coal in the presence of air or oxygen.

5.3.3 Primary gasification

Primary gasification involves thermal decomposition of the raw coal via various chemical processes. Many schemes involve pressures ranging from atmospheric pressure to high pressure (14.7-1000 psi). Air or oxygen may be admitted to support combustion to provide the necessary heat. The product is usually a low-Btu gas (low-heat content gas) ranging from a carbon monoxide/hydrogen mixture to mixtures containing varying amounts of carbon monoxide, carbon dioxide, hydrogen, water, methane, hydrogen sulfide, nitrogen, and typical products of thermal decomposition such as tar (themselves being complex mixtures), hydrocarbon oils, and phenol derivatives (Speight, 2013a, 2013b).

The solid char product is produced that may represent the bulk of the weight of the original coal. This type of coal being processed determines (to a large extent) the amount of char produced and the composition of the gas product.

5.3.4 Secondary gasification

Secondary gasification usually involves the gasification of char from the primary gasification step. This is usually achieved by reaction of the hot char with water vapor (steam gasification) to produce carbon monoxide and hydrogen:

${\left[\mathrm{C}\right]}_{\mathrm{char}}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{CO}+{\mathrm{H}}_2$

The reaction requires heat input (endothermic) in order to proceed in its forward direction. Usually, an excess amount of steam is also needed to promote the reaction. However, excess steam used in this reaction has an adverse effect on the thermal efficiency of the process. Therefore, this reaction is typically combined with other gasification reactions in practical applications. The hydrogen-carbon monoxide ratio of the syngas product depends on the synthesis chemistry as well as process engineering.

The mechanism of this reaction section is based on the reaction between carbon and gaseous reactants, not for the reactions between coal and gaseous reactants. Hence the equations may oversimpifly the actual chemistry of the steam gasification reaction. Even though carbon is the dominant atomic species present in coal, coal is more reactive than pure carbon. The presence of various reactive organic functional groups and the availability of catalytic activity via naturally occurring mineral ingredients can enhance the relative reactivity of coal – anthracite – which has the highest carbon content among all ranks of coal (Speight, 2013a) and is most difficult to gasify or liquefy.

Alkali metal salts are known to catalyze the steam gasification reaction of carbonaceous materials, including coal. The process is based on the concept that alkali metal salts (such as potassium carbonate, sodium carbonate, potassium sulfide, sodium sulfide, and the like) will catalyze the steam gasification of coal. The order of catalytic activity of alkali metals on coal gasification reaction is:

$\mathrm{Cesium}\left(\mathrm{Cs}\right)>\mathrm{rubidium}\left(\mathrm{Rb}\right)>\mathrm{potassium}\left(\mathrm{K}\right)>\mathrm{sodium}\left(\mathrm{Na}\right)>\mathrm{lithium}\left(\mathrm{Li}\right)$

Catalyst amounts on the order of 10-20% w/w potassium carbonate will lower bituminous coal gasifier temperatures from 925 °C (1695 °F) to 700 °C (1090 °F) and then the catalyst can be introduced to the gasifier impregnated on coal or char.

In addition, tests with potassium carbonate showed that this material also acts as a catalyst for the methanation reaction. In addition, the use of catalysts can reduce the amount of tar formed in the process (Cusumano, Dalla Betta, & Levy, 1978; McKee, 1981; Shinnar, Fortuna, & Shapira, 1982). In the case of catalytic steam gasification of coal, carbon deposition reaction may affect catalyst life by fouling the catalyst active sites. This carbon deposition reaction is more likely to take place whenever the steam concentration is low.

Ruthenium-containing catalysts are used primarily in the production of ammonia. It has been shown that ruthenium catalysts provide 5-10 times higher reactivity rates than other catalysts. But ruthenium quickly becomes inactive due to its necessary supporting material, such as activated carbon, which is used to achieve effective reactivity. However, during the process, the carbon is consumed, thereby reducing the effect of the ruthenium catalyst.

Catalysts can also be used to favor or suppress the formation of certain components in the gaseous product by changing the chemistry of the reaction, the rate of reaction, and the thermodynamic balance of the reaction. For example, in the production of synthesis gas (mixtures of hydrogen and carbon monoxide), methane is also produced in small amounts. Catalytic gasification can be used to either promote methane formation or suppress it.

5.3.5 Carbon dioxide gasification

The reaction of coal with carbon dioxide produces carbon monoxide (Boudouard reaction), and, like the steam-gasification reaction, is also an endothermic reaction:

$\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)\to 2\mathrm{CO}\left(\mathrm{g}\right)$

The reverse reaction results in carbon deposition (carbon fouling) on many surfaces, including the catalysts and results in catalyst deactivation.

This gasification reaction is thermodynamically favored at high temperatures (> 680 °C, > 1255 °F), which is also quite similar to the steam gasification. If carried out alone, the reaction requires high temperature (for fast reaction) and high pressure (for higher reactant concentrations) for significant conversion. But as a separate reaction, a variety of factors come into play: (1) low conversion, (2) slow kinetic rate, and (3) low thermal efficiency.

Also, the rate of the carbon dioxide gasification of coal is different from the rate of the carbon dioxide gasification of carbon. Generally, the carbon-carbon dioxide reaction follows a reaction order based on the partial pressure of the carbon dioxide that is approximately 1.0 (or lower), whereas the coal-carbon dioxide reaction follows a reaction order based on the partial pressure of the carbon dioxide that is 1.0 (or higher). The observed higher reaction order for the coal reaction is also based on the relative reactivity of the coal in the gasification system.

5.3.6 Water gas shift reaction

The water gas shift reaction (shift conversion) is necessary because the gaseous product from a gasifier generally contains large amounts of carbon monoxide and hydrogen, plus lesser amounts of other gases. Carbon monoxide and hydrogen (if they are present in the mole ratio of 1:3) can be reacted in the presence of a catalyst to produce methane. However, some adjustment to the ideal (1:3) is usually required. To accomplish this, all or part of the stream is treated according to the water-gas shift (shift conversion) reaction. This involves reacting carbon monoxide with steam to produce a carbon dioxide and hydrogen whereby the desired 1:3 mol ratio of carbon monoxide to hydrogen may be obtained:

$\mathrm{CO}\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$

Even though the water-gas shift reaction is not classified as one of the principal gasification reactions, it cannot be omitted in the analysis of chemical reaction systems that involve synthesis gas. Among all reactions involving syngas, this reaction equilibrium is least sensitive to the temperature variation – the equilibrium constant is least dependent on the temperature. Therefore, the reaction equilibrium can be reversed in a variety of practical process conditions over a wide range of temperature.

The water-gas shift reaction in its forward direction is mildly exothermic. Although all the participating chemical species are in gaseous form, the reaction is believed to be heterogeneous insofar as the chemistry occurs at the surface of the coal and the reaction is actually catalyzed by carbon surfaces. In addition, the reaction can also take place homogeneously as well as heterogeneously. A general understanding of the water-gas shift reaction is difficult to achieve; even the published kinetic rate information is not immediately useful or applicable to a practical reactor situation.

Synthesis gas from a gasifier contains a variety of gaseous species other than carbon monoxide and hydrogen. Typically, they include carbon dioxide, methane, and water (steam). Depending on the objective of the ensuing process, the composition of syngas may need to be preferentially readjusted. If the objective of the gasification process is to obtain a high yield of methane, it would be preferred to have the molar ratio of hydrogen to carbon monoxide at 3:1:

$\mathrm{CO}\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\to {\mathrm{CH}}_4\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$

On the other hand, if the objective of generating syngas is the synthesis of methanol via a vapor-phase low-pressure process, the stoichiometrically consistent ratio between hydrogen and carbon monoxide would be 2:1. In such cases, the stoichiometrically consistent synthesis gas mixture is often referred to as balanced gas, whereas a synthesis gas composition that is substantially deviated from the principal reaction’s stoichiometry is called unbalanced gas. If the objective of synthesis gas production is to obtain a high yield of hydrogen, it would be advantageous to increase the ratio of hydrogen to carbon monoxide by further converting carbon monoxide (and water) into hydrogen (and carbon dioxide) via the water-gas shift reaction.

The water-gas shift reaction is one of the major reactions in the steam gasification process, where both water and carbon monoxide are present in ample amounts. Although the four chemical species involved in the water-gas shift reaction are gaseous compounds at the reaction stage of most gas processing, the water-gas shift reaction, in the case of steam gasification of coal, predominantly takes place on the solid surface of coal (heterogeneous reaction). If the product synthesis gas from a gasifier needs to be reconditioned by the water-gas shift reaction, this reaction can be catalyzed by a variety of metallic catalysts.

Choice of specific kinds of catalysts has always depended on the desired outcome, the prevailing temperature conditions, composition of gas mixture, and process economics. Typical catalysts used for the reaction include catalysts containing iron, copper, zinc, nickel, chromium, and molybdenum.

5.3.7 Methanation

Several exothermic reactions may occur simultaneously within a methanation unit. A variety of metals have been used as catalysts for the methanation reaction; the most common, and to some extent the most effective methanation catalysts, appear to be nickel and ruthenium, with nickel being the most widely (Cusumano et al., 1978):

$\mathrm{Ruthenium}\left(\mathrm{Ru}\right)>\mathrm{nickel}\left(\mathrm{Ni}\right)>\mathrm{cobalt}\left(\mathrm{Co}\right)>\mathrm{iron}\left(\mathrm{Fe}\right)>\mathrm{molybdenum}\left(\mathrm{Mo}\right)$

Nearly all the commercially available catalysts used for this process are very susceptible to sulfur poisoning, so efforts must be taken to remove all hydrogen sulfide (H₂S) before the catalytic reaction starts. It is necessary to reduce the sulfur concentration in the feed gas to less than 0.5 ppm v/v in order to maintain adequate catalyst activity for a long period of time.

The synthesis gas must be desulfurized before the methanation step because sulfur compounds will rapidly deactivate (poison) the catalysts. 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. To eliminate this problem, temperatures should be maintained below 400 °C (750 °F).

The methanation reaction is used to increase the methane content of the product gas, as needed for the production of high-Btu gas.

$4{\mathrm{H}}_2+{\mathrm{CO}}_2\to {\mathrm{CH}}_4+2{\mathrm{H}}_2\mathrm{O}$

$2\mathrm{CO}\to \mathrm{C}+{\mathrm{CO}}_2$

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

Among these, the most dominant chemical reaction leading to methane is the first one. Therefore, if methanation is carried out over a catalyst with a synthesis gas mixture of hydrogen and carbon monoxide, the desired hydrogen-carbon monoxide ratio of the feed synthesis gas is around 3:1. The large amount of water (vapor) produced is removed by condensation and recirculated as process water or steam. During this process, most of the exothermic heat due to the methanation reaction is also recovered through a variety of energy integration processes.

Whereas all the reactions listed here are quite strongly exothermic except the forward water-gas shift reaction, which is mildly exothermic, the heat release depends largely on the amount of carbon monoxide present in the feed synthesis gas. For each 1% v/v carbon monoxide in the feed synthesis gas, an adiabatic reaction will experience a 60 °C (108 °F) temperature rise, which may be termed as adiabatic temperature rise.

5.3.8 Hydrogasification

Hydrogasification is the gasification of coal in the presence of an atmosphere of hydrogen under pressure (Anthony & Howard, 1976). Thus, not all high-heat content (high-Btu) gasification technologies depend entirely on catalytic methanation. In fact, a number of gasification processes use hydrogasification – that is, the direct addition of hydrogen to coal under pressure to form methane:

${\left[\mathrm{C}\right]}_{\mathrm{coal}}+{\mathrm{H}}_2\to {\mathrm{CH}}_4$

The hydrogen-rich gas for hydrogasification can be manufactured from steam by using the char that leaves the hydrogasifier. Appreciable quantities of methane are formed directly in the primary gasifier, and the heat released by methane formation is at a sufficiently high temperature to be used in the steam-carbon reaction to produce hydrogen so that less oxygen is used to produce heat for the steam-carbon reaction. Hence, less heat is lost in the low-temperature methanation step, thereby leading to higher overall process efficiency.

The hydrogasification reaction is exothermic and is thermodynamically favored at low temperatures (< 670 °C, < 1240 °F), unlike the endothermic steam gasification and carbon dioxide gasification reactions. However, at low temperatures, the reaction rate is inevitably too slow. Therefore, a high temperature is always required for kinetic reasons, which, in turn, requires high pressure of hydrogen, which is also preferred from equilibrium considerations. This reaction can be catalyzed by salts such as potassium carbonate (K₂CO₃), nickel chloride (NiCl₂), iron chloride (FeCl₂), and iron sulfate (FeSO₄). However, use of a catalyst in coal gasification suffers from difficulty in recovering and reusing the catalyst and the potential for the spent catalyst becoming an environmental issue.

In a hydrogen atmosphere at elevated pressure, additional yields of methane or other low molecular weight hydrocarbons can result during the initial coal gasification stage from direct hydrogenation of coal or semi-char because of the active intermediate formed in coal structure after coal pyrolysis. The direct hydrogenation can also increase the amount of coal carbon that is gasified as well as the hydrogenation of gaseous hydrocarbons, oil, and tar.

The kinetics of the rapid-rate reaction between gaseous hydrogen and the active intermediate depends on hydrogen partial pressure (P_H2). Greatly increased gaseous hydrocarbons produced during the initial coal gasification stage are extremely important in processes to convert coal into methane (SNG, synthetic natural gas).