Impact of nanosecond neodymium laser pulses on lignite

Abstract

Two stages are distinguished, which are connected with evaporation and ignition of lignite, at impact of nanosecond laser pulses on lignite tablets with a density of 1 g/cm³  and particle size of 63 μm. At the first stage during the pulse, volatiles, evaporation, vaporization and nonlinear multiplication of luminescence centers are released. The ignition threshold at this stage is H⁽¹⁾_cr=0.2 J/cm² . The second stage includes the first and ignition of the coke residue with the ignition threshold of which is H⁽¹⁾_cr=3.5 J/cm². In the kinetics of glow at H ˃ H⁽¹⁾_cr , two components are distinguished, namely, a singlet glow with a duration of ~20 ns and the second glow component in the microsecond range, which decays in the second order kinetics. At H ˃ H⁽²⁾_cr in ~200 μs after the laser pulse, as a result of chemical reactions, the ignition and combustion of the coke residue occurs in the time interval of 200–1000 μs. The amplitude of the flame glow during the excitation pulse increases nonlinearly with increasing energy density, which testifies an avalanche nature of the process of formation of glow centers.