Ignition of Coal Microparticles by Laser Pulses of the Second Harmonic of a Neodymium Laser in the Q-Switched Regime

The ignition of pelletized samples of hard coals of the long-flame gas (LG), gas (G), fat (F), coke (C) grades with particle sizes <= 63 mu m by laser pulses (lambda = 532 nm, tau(i) = 10 ns) is studied. When the critical radiation energy density H-cr((1)), specific for each grade of coal, is exceeded, the optical breakdown occurs and a dense plasma with a continuous emission spectrum is formed. As the plasma expands and rarefies, the spectra show the emission of carbon ions CII, excited nitrogen atoms N, excited carbon molecules C-2, and carbon monoxide CO. The plasma glow's intensity peaks at the end of the laser pulse, and the glow relaxation time is similar to 1 mu s. The plasma glow's amplitude increases nonlinearly with the increasing energy density of laser pulses. At a radiation energy density of H >= H-cr((2)), specific for each grade of coal, thermochemical reactions are initiated in the volume of microparticles and coal particles are ignited in the submillisecond time interval.