THE TRANSIENT IGNITION OF ISOLATED COAL PARTICLE

A transient model for ignition of a coal particle is presented. Criter ia 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 heterogene ous (HI), followed by secondary homogeneous ignition (SGI). Primary ig nition is homogeneous (GI) at high gas temperatures and for larger par ticles. The boundary separating homogeneous ignition from heterogeneou s 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 ignit ion. The heterogeneous ignition temperature (T(p, HI)) Of the particle is inversely related to particle diameter and oxygen concentration. W hile the minimum ambient gas temperature (T(infinity, GI)) required fo r homogeneous ignition shows a slight decrease with an increase in oxy gen concentration, the local homogeneous ignition temperature (T(g, SG )) does not appear to change with oxygen concentration. The results ar e compared with those predicted with the steady state model of other r esearchers.