Mass transfer processes around burning char particles in fluidized beds

Abstract

The detailed modelling of the fluidized bed combustion of coal depends on the elucidation of the mass transfer processes around large active particles in fluidized beds of smaller inert particles. Experimental techniques based on naphthalene sublimation as well as carbon combustion have been used to obtain mass transfer data for this configuration. In previous modelling studies the active particle has been considered to reside only in the emulsion phase. In this paper, the effect of active particle motion on mass transfer is considered. The active particle is assumed to reside alternately in the bubble and emulsion phases. By considering this particle motion in conjunction with the gas flow distribution, expressions are obtained for the appropriate Sherwood numbers in each phase. Using the emulsion phase residence probability, an expression for the overall Sherwood number is developed. The results of the model calculations are compared favorably with experimental data reported in the literature for fixed as well as fluidized beds.