Overlapping of the devolatilization and char combustion stages in the burning of coal particles

The oxygen content at the surface of a fuel particle can significantly exce ed zero during the devolatilization stage of combustion, despite the flux o f volatiles from the surface and also gas phase reactions. This implies tha t char oxidation can take place simultaneously. This overlapping of the dev olatilization and char combustion stages is studied by modeling. The rates of gas phase reactions around the particle influence the availability of ox ygen at the surface of a burning particle and they are accounted for by usi ng a two-step global model for combustion of volatiles. The effects of part icle size, ambient temperature, and oxygen concentration on the degree of o verlap are studied. The study provides theoretical and experimental evidenc e that the combustion time of a particle does not always increase with its size at constant ambient conditions, but there can be a specific particle s ize giving a maximum combustion rate. (C) 1998 by The Combustion Institute.