High-energy Nd-yag laser ignition of coals: Modeling analysis

A modeling analysis of laser-induced ignition of coals is presented. The 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 products absorb a significant amount of the laser radiation. This important mechanism dictates complicated interactions among different processes including heat and mass transfer and chemical reactions. It was found that, for laser energy fluxes ranging from 2500 to 7000 W/cm 2, the ignition of lignite and subituminous coals occurred first at the surface which was then followed by the ignition of the pyrolysis products in the gas-phase region. However, for bituminous coal, only single gas-phase ignition mode was predicted by the model. The model also predicts the rapid decrease of both the surface ignition time and the gas-phase ignition time with increasing laser energy fluxes. In general, a good agreement between predictions and experiments is obtained. A sensitivity analysis, where the kinetic parameters and initial laser energy fluxes were varied over a wide range, was also carried out. This analysis establishes an upper limit for I 0, E c, E p, and a lower limit for E g. Within these limits, the laser-induced ignition of coals appears as an integration of two consecutive ignition modes: a surface ignition mode followed by the gas-phase ignition mode. Beyond these limits, a single gas-phase ignition mode is the only ignition mode that prevails.