Propagation of laminar pulverized coal-air flames

Abstract

The propagation of laminar, coal-air flames has been studied experimentally and theoretically. Measurements of flame velocity have been made for coal-air mixtures in a downward flowing, 4 in. diameter, flatflame burner at atmospheric pressure. Flame velocities of 14–22 cm/sec and 16–33 cm/sec were obtained with 33 micron and 10 micron Pittsburgh coal respectively, depending upon coal concentration. Probe temperature profiles and concentration samples on each side of the flame about 1 cm thick suggest a rapid devolatilization process, while photomicrographs show softening, slight swelling and increased proosity of the coal during reaction. A generalized model for a flame propagating in laminar, premixed particle-gas mixtures was formulated, assuming particle-gas dynamic equilibrium and uniform pressure. Effects of gaseous diffusion, coal pyrolysis, char oxidation, and gaseous reaction were considered. The unsteady state equations were solved numerically using finite-difference techniques. Flame velocity predictions, neglecting radiation, were generally within 25% of measured coal values. The theory suggests that in a laminar coal-dust flame, gas phase diffusion and conduction, gas-particle conduction, and coal pyrolysis are importnat rate-determining steps, while hydrocarbon oxidation and char oxidation may not be rate-limiting. The importance of gas-phase diffusional processes in such flames is suggested.