THE TRANSIENT IGNITION OF ISOLATED COAL PARTICLE

A transient model for ignition of a coal particle is presented. Criteria for heterogeneous and homogeneous ignition for transient model are proposed. Using the inflection condition for particle temperature, the criterion for heterogeneous ignition is reduced in terms of critical mass loss rate. Ignition for smaller particles is primarily heterogeneous (HI), followed by secondary homogeneous ignition (SGI). Primary ignition is homogeneous (GI) at high gas temperatures and for larger particles. The boundary separating homogeneous ignition from heterogeneous ignition is estimated using the transient model, which changes with ambient temperature and oxygen concentration. The SGI is obtained for particle diameters below 400 mum at ambient gas temperature of 1500 K and oxygen concentration of 0.23. The ignition sequence of HI followed by SGI is also revealed by our recent video imaging of particle ignition. The heterogeneous ignition temperature (T(p, HI)) Of the particle is inversely related to particle diameter and oxygen concentration. While the minimum ambient gas temperature (T(infinity, GI)) required for homogeneous ignition shows a slight decrease with an increase in oxygen concentration, the local homogeneous ignition temperature (T(g, SG)) does not appear to change with oxygen concentration. The results are compared with those predicted with the steady state model of other researchers.