The effect of volatile matter cloud surrounding a single coal particle on the ignition mechanism has been examined experimentally. The amount of volatiles in the vicinity of particles was varied by employing three different experimental conditions that had different intensities of convection. These conditions were microgravity with negligible convection, normal gravity with natural convection, and normal gravity with forced convection. Three types of coal having mean particle sizes of 0.8, 1.0, and 1.2 mm were ignited in air by heat radiation at heating rates of 770-900 K/s. Results showed that the amount of volatiles remaining in the vicinity of the particle is significantly affected by natural and/or forced convection, which in turn affects the ignition mechanism and temperature. Under negligible and natural convections, coal ignited homogeneously, while under forced convection, it ignited heterogeneously. Ignition temperatures were highest when a particle was surrounded by the largest amount of volatiles and lowest when the volatile amount was the least. Further, the ignition temperature decreased with the volatile matter content of coal under negligible and natural convection. Under forced convection, however, the ignition temperature remained more or less constant. The ignition temperatures under negligible and forced convections seem to converge at low volatile matter contents, suggesting a point of transition from homogeneous to heterogeneous ignition. The effect of particle size was almost the same in all three conditions where ignition temperatures increased with increasing particle size. The converging trend was not observed on varying particle size, indicating that the ignition mechanism is determined mainly by the volatile matter content.
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