Combustion of a porous char particle in an incipiently fluidized bed

Abstract

A simple single particle model has been developed for the combustion of carbon. The heterogeneous reactions of carbon oxidation and gasification are assumed to take place inside the porous char, while the homogeneous oxidation of carbon monoxide is assumed to occur in the film surrounding the burning particle. Depending on the extent of consumption of oxygen by the primary product CO, oxygen may or may not reach the char surface. Hence, the formulation leads to a single film porous particle model in the case where oxygen reaches the particle surface, and a double film porous particle model when oxygen does not reach the char particle surface. Analytical expressions are derived for the concentration profiles for the different gaseous species. The application of the model is illustrated through calculations relevant for combustion of carbon in an incipiently fluidized bed. These simulations, then, reflect the combustion behavior in the dense phase of a bubbling fluidized bed. The model successfully predicts the experimental observation of two extrema in CO/CO 2 product ratio at the surface of a burning particle. Parametric studies were conducted to assess the effect of char and inert particle sizes, moisture content and the bulk species concentration on the CO/CO 2 product ratio, as well as the average rates of species consumption (or generation). The results show that char gasification reaction influences the carbon consumption behavior significantly at high temperatures. The CO/CO 2 product ratio at the surface increases—as a consequence of a decrease in the net film oxidation rate—with an increase in the size of bed particles. The CO/CO 2 product ratio is also enhanced by a decrease in char size and gas phase moisture content. The temperature corresponding to a minimum in CO/CO 2 ratio increases in a similar fashion. An increase in particle porosity and specific internal surface area led to a decrease in this temperature. The model results also permit identification of the three regimes of combustion.