HIGH-ENERGY ND-YAG LASER IGNITION OF COALS - EXPERIMENTAL-OBSERVATIONS

Some experiments were Conducted to study the interaction of the Nd-Yag laser radiation with four different coals. All coals were ground and screened to 400 mesh and then pressed into cylindrical pellets of 3 mm diameter and 2 mm length. The coal pellets were prepared by cold pressing using a stainless-steel die to a pressure of about 500 kg/cm2. Laser intensities ranging from 0.5 x 10(3) to 1.5 x 10(4) W/cm2 at 1.064 mum wavelength and a pulse duration of 5 ms were used. For laser intensities less than 800 W/cm2, no ignition was observed for all coals. For laser intensities above this value, two ignition mechanisms were observed: the surface ignition followed by the gas-phase ignition when Wyoming subbituminous, Indian lignite, and North Dakota lignite coals were used. For the same range of the laser intensities, however, only the gas-phase ignition was observed when Pittsburgh bituminous coal was used. It was also noted that a significant amount of the external laser radiations were absorbed by the pyrolysis products during the early stages of the ignition period. This process leads to a series of nonlinear phenomena and dictates not only the processes occurring at the coal surface but also in the gas phase.