HIGH-ENERGY ND-YAG LASER IGNITION OF COALS - MODELING ANALYSIS

A modeling analysis of laser-induced ignition of coals is presented. T he model is one dimensional, transient, and incorporates the following : the surface ignition reaction, the in-depth pyrolysis, the gas-phase absorption of the laser radiation and by the pyrolysis products, and the gas-phase chemical reaction. The solution is obtained numerically using the method of lines. The results confirm that the pyrolysis prod ucts absorb a significant amount of the laser radiation. This importan t mechanism dictates complicated interactions among different processe s including heat and mass transfer and chemical reactions. It was foun d that, for laser energy fluxes ranging from 2500 to 7000 W/cm2, the i gnition of lignite and subituminous coals occurred first at the surfac e which was then followed by the ignition of the pyrolysis products in the gas-phase region. However, for bituminous coal, only single gas-p hase ignition mode was predicted by the model. The model also predicts the rapid decrease of both the surface ignition time and the gas-phas e ignition time with increasing laser energy fluxes. In general, a goo d agreement between predictions and experiments is obtained. A sensiti vity analysis, where the kinetic parameters and initial laser energy f luxes were varied over a wide range, was also carried out. This analys is establishes an upper limit for I0, E(c), E(p), and a lower limit fo r E(g). Within these limits, the laser-induced ignition of coals appea rs as an integration of two consecutive ipition modes: a surface ignit ion mode followed by the gas-phase ignition mode. Beyond these limits, a single gas-phase ignition mode is the only ignition mode that preva ils.