Post-ignition transients in the combustion of single char particles

Abstract

The temperature-time histories of single char particles burning at temperatures above 1500 K have been measured by two-colour near infrared pyrometry techniques. Two coal chars and a synthetic char consisting of spherical glassy carbon particles of uniform size were used in the experiments. The results indicate that with lean combustion and low-to-medium temperatures, the char particles do not ignite over their whole external surface, but exhibit preferential ignition at specific sites. These ‘hot-spots’ probably involve regions in which the material exhibits strong inhomogeneities, either on or below the surface. Following ignition, the reaction may propagate over the particle surface. At elevated combustor temperatures and/or high oxygen partial pressures, ignition spreads quickly over the entire particle surface and the importance of the hot-spots diminishes. A simple model is used to estimate the rate of growth of a hot-spot under various experimental conditions.