Characteristics of the Laser Ignition of Brown Coal Microparticles Depending on Their Sizes

The ignition of brown coal microparticles in a size range of 0.23-67 pm under the action of laser pulses in the free-running mode (1064 nm, 120 mu s, and 1.3 J) was studied. The kinetic characteristics of sample glowing were measured. Three stages of ignition were recognized. The duration of the first stage coincided with the duration of a laser pulse. The duration of the second was in a time interval to 3-5 ms. The duration of the third stage was in a time interval from 3 to 100 ms. The times of burning at different stages were similar within the statistical spread for all of the particle sizes studied. It was found that the dependences of ignition thresholds on particle sizes were nonmonotonic. The minimum values of Hcr were reached at a particle size of 2-4 mu m: H-cr((1)) = 0.23 J/cm(2); H-cr(()2) = 0.9 J/cm(2); and H-cr(()3) = 1.7 J/cm(2). At larger and smaller particle sizes, an increase in Hcr was observed. The results were interpreted in accordance with the data of the proximate analysis of grade 2B coal.