Porosity effects on catalytic char oxidation. Part I: A catalyst deposition model

Abstract

Six char samples generated from two coals were impregnated by solutions with different concentrations of NazC03 and/or KzCO3. The catalyst solution penetration was studied, the catalyst uptake was measured, and the effects of impregnation on the char pore structure were determined experimentally. The results indicate that the impregnated catalysts reside only on particle exteriors without significant penetration. The overall rate of reaction is therefore the combination of catalytic reaction on particle exteriors and noncatalytic reaction on the pore surfaces inside the particle. A kinetic model is presented that takes these findings into account. J. L. SU and D. D. PERLMUllER Department of Chemical EngineeringUniversity of Pennsylvania Philadelphia, PA 19104 SCOPESix char samples generated from two coals were impregnated by solutions containing different concentrations of NazC03 and/or KzCO3. The effects of catalyst impregnation on the char pore structure were examined by COz adsorption and mercury porosimetry. The catalyst uptakes were determined by atomic absorption. Pycnometry tests were also conducted to determine the degree of catalyst penetration during impregnation. Catalyzed and uncatalyzed samples were examined under a scanning electron microscope at both unreacted and partially reacted stages. The experimental results were used to formulate a kinetic model that accounts for the several findings. CONCLUSIONS AND SIGNIFICANCEThe experimental work of this study focused on the degree of catalyst solution penetration that occurs during impregnation of chars. The catalyst uptake was determined as well as the effects of impregnation on the char pore structure. The impregnating solution was found unable to penetrate into the interior pore structure of a char. The deposited catalysts resided mainly on particle exteriors without affecting the pore structure. As a consequence, rapid catalytic reaction occurs on a char's exterior surfaces, reducing particle size, while a slower noncatalytic reaction occurs simultaneously inside the pore structures. The observed reaction rate is then the combination of the contributions from catalytic and noncatalytic mechanisms.From such a physical picture a model was developed for reaction kinetics of catalytic gasification. The model produces an expectation that the enhanced reactivity would be more significant at low levels of conversion, and predicts that the overall rate would decrease to that of the noncatalytic reaction at high conversion. The contributions to the overall reaction rate of catalytic and noncatalytic steps are discussed.

Pore structural effects in catalytic gasification of coal chars

“…Since the isotherms obtained for the impregnated and nonimpregnated samples are almost identical, yielding similar accessible micropore volumes, one can conclude that the catalyst did not block the micropores of these samples to any appreciable extent. Su and Perlmutter (1985a) conducted similar experiments on chars impregnated with sodium and potassium carbonates from aqueous solutions and also found that catalyst addition caused insignificant blockage of the micropores.…”