Postprint version. Published in Fuel, Volume 74, Issue 3, March 1, 1995, pages 323-330. Copyright © 1995 Elsevier. The definitive version is available at http://dx.doi.org/10.1016/0016-2361(95)93463-N.
NOTE: At the time of publication, the author John Chen was not yet affiliated with Cal Poly.
Direct observation of pulsed-laser ignition and combustion of pulverized coals with a high speed video has been made. A dilute stream of particles was dropped into a laminar, upward-flow wind tunnel with a quartz test section. The gas stream was not preheated. A single pulse from a Nd:YAG laser was focused through the tunnel and ignited several particles. The transparent test section and cool walls allowed for optical detection of the reaction sequence. The ignition and combustion behaviours of amorphous carbon spheres and an anthracite were as expected for heterogeneous ignition and reaction. Surprisingly, the reaction of a high volatile bituminous and a subbituminous coal exhibited two broad emissions, as detected with a photomultiplier tube. For these coals, high speed videos revealed that two successive clouds of volatiles surrounded the particles, and each cloud was associated with one of the broad emissions. It is hypothesized that the first cloud was due to the evolution and reaction of tar, and the second resulted from low molecular weight, combustible gases. The subbituminous and bituminous coals apparently ignited heterogeneously, and possible explanations for this are discussed.