Ignition and combustion of coal particle streams

Abstract

This paper presents a model for group combustion of a cylindrical stream of coal particles and preliminary experimental data. Results are obtained for transient ignition and combustion behaviour. It is found that homogeneous ignition (ignition of volatiles) occurs for a dense cloud, while heterogeneous ignition (ignition of carbon) occurs for a dilute cloud. Once ignited, the flame moves towards the cloud, reaches the cloud surface and then bifurcates into two flames, called the inner and outer flames. The inner flame propagates into an unburnt volatile-air mixture in the cloud, while the outer diffusion flame moves away from the cloud. The inner flame propagates at the laminar burning velocity and rapidly heats the particles. There is only a diffusion flame outside the cloud after all the oxygen in the cloud is consumed. The flame moves toward the cloud as the volatiles are depleted. The cloud, with the volatiles exhausted and starved of oxygen, burns slowly, controlled by diffusion into the cloud rather than to each particle in the cloud. Qualitative comparisons are given with preliminary experimental data obtained using a digital imaging system.