Heat transfer behaviour and temepratures of freely moving burning carbonaceous particles in fluidized beds

Abstract

In fluidized bed combustion, relatively few large coal particles are fluidized along with more dense and smaller sulphur-sorbent bed particles. Predictions for drying, ignition and devolatilization, as well as particle temperatures during char combustion, require knowledge of external heat transfer coefficients. Several studies have dealt with mass transfer for freely moving large particles in fluidized beds; heat transfer to immobile tubes has also received wide research attention. Very few studies have dealt with heat transfer to freely moving large particles in fluidized beds and the correlations available are, at best, semi-empirical. In this paper a mechanistic model for heat transfer is proposed which takes into account the movement of the active particle within the bed. The average heat transfer coefficients predicted by the model are in good agreement with direct measurements reported in the literature. The average heat transfer coefficients are then used, in conjunction with a shrinking core formulation, to predict the temperature histories of burning coke and char particles. The model predicts that the coke and char particles experience rapidly changing external heat transfer conditions. In the case of the char particles the presence or absence of the ash layer will influence the temperature history. The implication of these variations on the temperature histories in batch/continuous operation and for drying, ignition and devolatilization require further consideration.