Mathematical modeling of combustion and gasification of a wet coal slab—II: Mode of combustion, steady state multiplicities and extinction

Abstract

The previous one-dimensional steady state model for gasification of wet coal is extended to include both homogeneous and heterogeneous combustion. The conditions under which the flame separates from the coal surface are presented and it is shown that these conditions are functions of the properties of the coal and the ambient gas. It is also shown that for a given bulk gas temperature, there is a limiting bulk oxygen concentration for combustion. Above the limiting bulk gas oxygen concentration, two possible solutions were found: the flame located at the char surface (heterogeneous combustion) and the flame located in the ash layer or in the gas film (homogeneous combustion). The heterogeneous combustion is shown to be unstable while the homogeneous combustion, which has been observed experimentally, is shown to be stable. The global rates, the mode of combustion, the multiplicity of steady state solution and the flame extinction points are discussed as a function of ambient gas temperature, ash layer thickness, coal moisture content and film transport coefficients.