Mechanisms for the ignition of pulverized coal particles

Abstract

A detailed model for the steady-state combustion of a coal particle considering both surface oxidation and volatile combustion is presented. By solving the model equations in the absence of surface oxidation at various ambient gas temperatures, the homogeneous ignition temperature is determined as the gas temperature at which the mass consumption rate of the particle shows a jump, i.e., a transition from a low-temperature regime with little combustion of volatiles in kthe boundary layer to a high-temperature regime where essentially all volatiles burn. This ignition temperature is shown to be inversely related to particle size and oxygen concentration, as has been observed experimentally. The application of the flame sheet approximation is shown to overestimate the ignition temperature in most of the cases considered and to predict the incorrect dependence of the ignition temperature on the oxygen concentration. Solution of the model equations considering both surface oxidation and volatile combustion shows that surface oxidation influences the ignition temperature of only small particles or at high oxygen concentrations. When the surface oxidation rate of the particle is high, two ignition jumps, the first due to the heterogeneous mechanism and the second due to the homogeneous mechanism, are observed.