Laser ignition and flame propagation of pulverized coal dust clouds

Ignition and combustion behaviors were examined for uniformly sized, pulverized coal dust clouds that were suspended in a laminar upward flow and heated by a single-pulsed YAG laser. Effects of coal the, particle diameter, and oxygen concentration on lean limit of flame propagation and flame propagation velocity for hv and Iv bituminous coals and anthracite were examined. Lean limit of flame propagation of 22-63 mu m particles were examined by relation between particle concentration and flame propagation probability. The results were considered from the viewpoint of the distance between particles at which ignition of one particle from another was possible. Flame propagation velocity was examined by image processing of flame pictures. Observation with high-speed video revealed that first a few particles were heated by the laser pulse until they ignited, then the number of burning particles increased, which meant that the pulverized coal flame propagated. The maximum distance between particles for flame propagation increased with volatile content and oxygen concentration, and increased in proportion to the square root of the particle diameter. The effect of coal type on the maximum distance coincided qualitatively with the previous result obtained from millimeter-sized particles arranged in a row. However, the effect of particle diameter was larger than previously found. Flame propagation velocity was related to volatile content. Coals containing less-volatile matter had the lower maximum velocity. The effect was significant for lv bituminous coals. The flame propagation velocities of hv and iv bituminous coals were about 1.6 times the order of the oxygen concentration, which coincided with the previous result for sub-bituminous coal.