Gasification, coal enthalpies

M. Greenfeld described unique laboratory experiments designed to stimulate and understand the complex chemistry of in-situ coal gasification. Developed at the Alberta Research Council, the gasification simulator was heavily instrumented with calorimeters and gas chromatographs to determine the enthalpy, composition, and kinetics of formation of the product gases. Computer techniques were used to calculate mass and heat balances and to test kinetic models. 

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

The complex reaction of coal gasification may be described by the heterogeneous and homogeneous reactions listed in Table 1.3 and by its pyrolysis [1.4]. It will generally be sufficient to have a look at these reactions, since the reaction enthalpies of all other equations can be derived from them by simple arithmetic manipulation. The course of the individual reaction has no influence on the total heat input/output of a reaction sequence as long as the same feedstocks lead to the same products (Hess s law). 

Very few refinery operations operate about 800°F. The coal gasification processes, on the other hand, start at about 1300 F and some operate up to nearly 3000°F. Many of the new processes operate at pressures of about 1000 psla. Reliable zero pressure enthalpies up to these temperatures are available for most of the materials found in gasifiers, but the mixing rules and pressure corrections which the trade has been using for nonpolar mixtures are probably inadequate. 

If any thermodynamic assessment involving coal is intended, it is necessary to define its heat of formation. In particular, temperature calculation during the modeling of gasification or pyrolysis is extremely sensitive to consistent enthalpy calculation. The problem is that normally only clearly defined substances such as carbon (graphite) can be consistently treated in chemical reaction systems. Therefore, coal must be theoretically decomposed to its elements and a correctly assigned standard enthalpy of formation is required. 

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