Char Oxidation, Conversion, and Reaction Rate Processes

Abstract

The heterogeneous carbon oxidation and char gasification step is the second process to occur in the utilization of coal, and proceeds simultaneously or after devolatilization, depending on reaction conditions (Saito et al., 1991). The time required for the combustion of a char particle can be several orders of magnitude larger than that for devolatilization, ranging from 30 ms to over an hour, and is often the rate-determining step in the overall combustion of pulverized fuels (Essenhigh, 1981; I. Smith, 1982; Smoot and Smith, 1985). The processes of char oxidation are no less complex than those of devolatilization. The chemical structure of the coal does not control the reaction processes to the same extent as devolatilization, but, due to the high temperatures generally associated with char oxidation, pore diffusion and external diffusion often play a pronounced role. Thus, the physical structure of coal, including pore structure, surface area, particle size, and inorganic content, is important in understanding and modeling char oxidation processes. Intrinsic reactivity of char refers to the chemical reaction on the pore walls, after diffusion of gas through the pores inside the char. When intrinsic reactivity is the ratecontrolling process, oxygen migrates toward the center of the char particle, the particle size remains nearly constant during combustion, and the particle density decreases with conversion. However, if the reaction is fast, typically at high temperature, oxygen diffusion is the dominant process; the oxygen is consumed as it reaches the particle surface, and the density of the particle is near constant while the particle size decreases in a shrinking core mode.