Aspects of Combustion of Solid Fuels in Fluidized Bed Combustors

Abstract

The work reported here represents an experimental study of the mechanism of combustion of solid fuels in fluidized bed combustor. Many such combustors burn coal particles in a bed of sand typically at 1073 - 1173°K. Past work on mechanisms of combustion in fluidized bed combustors has concentrated on combustion of residual carbon. Study in the present work is concentrated on the combustion of volatile matter in the fuel. Initially some ancillary experiments were conducted at ambient temperature to obtain a little insight into the mixing of volatiles with air in the above-bed region. Combustion experiments were then performed using bituminous and non-bituminous coals and also wood in shallow ( circa 100 mm deep ) fluidized bed combustor. Both steady state and batch burning experiments were performed. The effect of fuel particle size on the time taken for emission of the volatile matter and combustion of it, was observed together with simultaneous measurements of temperatures, oxygen, carbon dioxide, carbon monoxide and unburnt hydrocarbon concentrations in the off-gas. When burning coal, it was found that feeding secondary air into the above bed region did not enhance combustion, but feeding the same amount of air into the bed did. When burning wood the reverse was found. In steady state tests, fuel, air and exhaust gas concentrations were used to estimate heat released by the fuel both in the bed and the above bed zone. These results were compared with predictions of a model developed by Merrick, and a different model developed in the course of this work. It showed that Merrick’s model could be extended into FBC field. Data showed that the amount of heat released in the above bed zone was very dependent upon the type of fuel and its particle size. The importance of this in relation to distribution of heat transfer surfaces is discussed.