The chemistry of coal gasification is usually depicted to involve the following reactions of carbon, oxygen, and steam (cf. Bodle and Schora, 1979). The standard enthalpy change (gram molecules) at 298 К is shown for each reaction.

(9) C02 + 4H2 -» CH4 + H20 — 165.0 kj

(10) C + 2H2 -* CH4 — 74.8 kj.

In theory, gasification processes can be designed so that the exothermic and endothermic reactions are thermally balanced. For example, consider reactions 2 and 5. The feed rates could be controlled so that the heat released balances the heat requirement. In this hypothetical case, the amount of oxygen required is 0.27 mol/mol of carbon, the amount of steam required is 0.45 mol/ mol of carbon, and the oxygen-to-steam molar ratio is 0.6:

C 4- H20 —* CO + H2 4- 131.3 kj 1.2C 4- 0.6O2 -* 1.2CO — 131.3 kj Net: 2.2C + H20 4- 0.6O2 -» 2.2CO 4- H2.

Many reactions occur simultaneously in coal gasification systems and it is not possible to control the process precisely as indicated here. But by careful selection of temperature, pressure, reactant and recycle product feed rates, reaction times, and oxygen-steam ratios, it is often possible to maximize certain desired products. When high-energy fuel gas is the desired product, selective utilization of high pressure, low temperature, and recycled hydrogen can result in practically all of the net fuel gas production in the form of methane.

The oxygen-steam ratios required to maintain zero net enthalpy change are given in Table 9.3 for several temperatures and pressures (Parent and Katz, 1948). With increased pressure, the ratio necessary to preserve a zero net enthalpy change diminishes. The decrease is most pronounced at low pressures. The effect of temperature change at constant pressure is also shown in Table 9.3. At lower temperatures, the oxygen-steam ratio doubles for each temperature

interval of 100 K. At higher temperatures, the increase diminishes and finally becomes very small.

The thermodynamic equilibrium compositions and enthalpy changes for the carbon-oxygen-steam system are graphically illustrated at several represen­tative temperatures and pressures in Figs. 9.1 to 9.4 (Parent and Katz, 1948). Increasing pressures tend to lower the equilibrium concentrations of hydrogen and carbon monoxide and increase the methane and carbon dioxide concentra­tions (Fig. 9.1). Methane and carbon dioxide formation are favored at lower temperatures, and at higher temperatures, carbon monoxide and hydrogen are the dominant equilibrium products (Figs. 9.2 and 9.3). At high temperatures, the reactions occurring in the system are thermodynamically equivalent to reactions 2 and 5. It is also apparent that hydrogen-to-carbon monoxide molar ratios of 1.0 or more are thermodynamically feasible at lower feed ratios of oxygen to steam and low pressure (Fig. 9.4).

Although the utility of thermodynamic data to optimize the operating condi­tions of a gasification process is of considerable importance, thermodynamics ignore kinetic and catalytic effects and the mechanisms by which processes occur. The data presented here, however, provide valuable guidelines for the design of gasification processes. For coal gasification, the type of coal and reactant contact conditions in the gasifier produce large differences in the raw product gas compositions. In general, the same principles and conclusions apply to biomass gasification. Where experimental conditions are favorable, equilibrium may be approached by prolonged contact of the reactants or by use of catalysts. Where neither of these conditions offers a convenient solution, a compromise between idealized equilibrium and kinetics is necessary.