Theory of Idealized Coal Devolatilization

Abstract

The aerothermochemical interaction between a devolatilizing coal particle and its ambient is studied by means of an idealized model consisting of two solid-phase species, which thermally decompose to yield a single gas-phase species. Devolatilization is described by a classical linear theory involving both equilibrium and kinetic characteristics of each solid-phase species. The temperature history of the particle is treated in two different ways reflecting different experimental and applied situations. One simulates the direct heating of a particle, e.g., on a wire grid, so that the temperature history is prescribed. The second relates to injection of a cold particle into a hot ambient either under experimental conditions or in an entrained flow reactor. In the second situation both a frozen gas phase corresponding to devolatilization in inert ambients and equilibrium chemistry in the gas phase as described by a flame-sheet model are discussed. It is concluded that, despite the high degree of idealization involved in the analysis, a large number of thermochemical parameters, some with uncertain values, enter the description of particle behavior. It is difficult to determine kinetic data from direct heating experiments. The thermal response of a cold particle injected into a hot ambient is sensitive to the kinetics of devolatilization and to the heats of pyrolysis.