Effect of Neodymium Laser Pulses of Nanosecond Duration on Brown Coal

The stages involved in the evaporation and ignition of brown coal under the influence of nanosecond laser pulses on tablets of brown coal of density 1 g/cm³ and particle size 63 μm were identified. At the first stage volatile substances are released during the pulse, and evaporation, vaporization, and nonlinear multiplication of the luminescence centers occur; the ignition threshold is ${\mathrm{H}}_{\mathrm{cr}}^{(1)}$ = 0.2 J/cm². The second stage includes the first stage and ignition of the coke residue with ignition threshold ${\mathrm{H}}_{\mathrm{cr}}^{(2)}$ = 3.5 J/cm². In the glow kinetics at H > ${\mathrm{H}}_{\mathrm{cr}}^{(1)}$ = 0.2 J/cm² two components were identified — a singlet state with duration ~20 ns and a second component in the microsecond region which quenches with second-order kinetics. At H > ${\mathrm{H}}_{\mathrm{cr}}^{(2)}$ ~ 200 μs after the laser pulse ignition and combustion of coke residue occur in the time interval of 200–1000 μs as a result of chemical reactions. The amplitude of the flame glow during the excitation pulse increases nonlinearly with increasing energy density, which indicates an avalanche-like process for the formation of the glow centers.