THE IGNITION OF SINGLE PULVERIZED COAL PARTICLES - MINIMUM LASER POWER REQUIRED

The ignition of pulverized fuel particles in air under laser irradiation with an optical fibre was studied. Six coals of different rank were used, in three size fractions. An Nd-YAG laser (continuous wave) with radiation wavelength 1.06 mum and variable output power was used as the energy source to heat single particles on the end of the optical fibre (100 mum core diameter). Two laser power levels were defined: (1) the critical laser power passing through the fibre, as measured; (2) the minimum laser power absorbed by the particles, estimated using the optical properties of particles. The experimental results indicate that there is a minimum laser power required to ignite a fuel particle, dependent on coal type and particle size. The measured critical (or minimum) laser power (P(f,c)) for ignition to occur, which comes from the optical fibre end, decreases with increasing particle size, whereas the minimum laser power (P(ab)) predicted to be absorbed by the particle increases. Three different ignition mechanisms under laser heating, depending on the volatile matter of the particle, were observed. Supporting theoretical analysis of the experimental data, based on a heterogeneous ignition assumption, is also presented and brief comparisons are made with other ignition techniques.