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Such solid fuels also include fossil energy sources: anthracite, bituminous and subbituminous coal, lignite, oil shale, tar sands, and peat. Finally the wastes being burned in commercially valuable quantities include mixed municipal solid waste, hospital and medical waste, and various forms of hazardous waste. Overview of Solid FueL· Combustion 59 A model can be constructed describing the combustion of solid fuels and wastes. This model involves a four stage process including: 1) drying the fuel particle, and heating it to reaction temperature, defined as T onset by Hertzberg; 2) pyrolysis of the solid fuel particle into noncondensible and condensible volatiles, including tars, and a carbon-rich char, or other volatilization mechanisms such as boiling and evaporation and sublimation; 3) homogeneous gas phase oxidation of the volatiles generated by pyrolysis or volatilization of the solid fuel particle, and gas phase oxidation of partial oxidation products from char-oxygen and char-moisture reactions; and 4) heterogeneous gas-solids reactions to accomplish char oxida­ tion.

C. Acid Gas Emissions Acid gases formed by the combustion of solid fuels include sulfur dioxide (S0 2 ), sulfur trioxide (S0 3 ), sulfuric acid mists, hydrogen chloride (HC1), and hydrogen fluoride (HF). All of these airborne emissions are formed as a function of the quantity of sulfur, chlorine, and fluorine in the fuel (see, for example, Table IV). Consequently they are treated largely in Chapters 2 - 5 . In general, however, the halogens act as oxidants. Consequently the formation of HC1 and HF are homogeneous gas phase reactions.

This transition zone has applicability to both fluidized bed systems and to pile burning systems. In such cases, a mass transfer limited model assuming pore diffusion of oxygen has applicability. g. T< 1,500°F), can one consider the entire pore system including micropores to be actively involved in char oxidation. Under such circumstances char oxidation can be considered as kinetically controlled. 3. Char oxidation summary Char oxidation, then, involves consumption of the solid particle by heterogeneous gas-solids reactions.