OCR Text |
Show original construction of the model. They also were used to govern selection of individual model elements and equations. 2.1. Overall Approach 12 Solids Combustion The generalized mechanistic approach to solids combustion underpinning SFCOMB is shown in Fig. 2. This set of combustion pathways is based upon the model for coal proposed by Edwards (14). This mcxlel is consistent with approaches to biomass combustion developed by Shafizadeh [47] and Tillman [56]. It has been shown to be appropriate for MSW as well [51]. In this model, the following reaction sequences occur: (1) solids heating and drying, (2) pyrolysis of the solid fuel into volatile components and a reactive char, (3) pyrolysis of the volatiles into radicals and fragments, (4) gas phase oxidation reactions, and (5) gas-solids char oxidation reactions. The reaction sequences which distinguish the combustion of solids from the combustion of liquids and gases are the solids pyrolysis reactions and the char oxidation reactions. Solid fuel panicle pyrolysis is readily manipulable. The products of solid fuel pyrolysis are variable and depend substantially on the temperature of pyrolysis, the minor fuel panicle dimension, the particle moisture content, and such properties of the fuel as thermal conductivity, heat capacity, and specific heat [48, 56]. Higher temperatures promote volatile formation as opposed to char formation (48). Similarly smaller fuel particles promote volatile formation over char formation [56]. Solid fuel pyrolysis is governed largely by heat transfer from the environment to the fuel particle. This has been demonstrated by the temperature gradient in fuel particles [65], and by the sharp reaction front which always occurs in the pyrolysis of a single piece of lignite, peat, biomass, or waste [see also 17, 18, 24). Char oxidation is a diffusion controlled reaction sequence which depends upon the availability of fuel surface area to the oxidant [see 46). Low grade fuel combustion system designers recognize the influence of pyrolysis and char oxidation reactions on the rate of combustion, and design accordingly. They accoIIUllodate the wide range in fuel particle sizes (see Fig. 3) associated with stoker coal, and the equally dramatic range of fuel particle sizes associated with wood waste. Funher, they accommodate the vastly different reactivities among such fuel particles [28], and recognize that all such particle sizes of a given fuel are being fed to the combustor simultaneously. This range of reactivities is shown, for wood fuels, in Table 1. It is equally significant for the other low grade fuels and wastes. Combustion system designs accommodate the wide range of fuel particle properties by using large furnaces. For example, grate fired biomass combustion systems typically have grate heat release rates of about 500,000 - 850,()()() Btu/sq. ft. - hr, and volumetric heat release rates of about 12,000 - 17,000 Btu/cu. ft. - hr. [53]. Similar heat release rates exist with coal fired spreader stokers as well [14. 43]. Grate heat release rates for MSW burners are typically about 300,000 Btu/sq. fL - hr and volumetric heat release rates for MSW incinerators are typically on the order of 10,000 Btu/cu. ft. - hr [51]. Such heat release rates commonly permit gaseous residence times in the furnace section of the boiler well in excess of 1 sec. Further, solids residence times on grates can exceed one hour. 4 |