On the mechanism of the potassium carbonate catalysed gasification of activated carbon: the influence of the catalyst concentration on the reactivity and selectivity at low steam pressures

Abstract

In this study a detailed description is given of reactivities, selectivities and kinetics in the potassium carbonate catalysed gasification of highly pure activated carbon with low pressures of steam. It is shown that the absolute reactivity increases at increasing initial catalyst loading and at increasing burn off, the latter leading to a maximum value between 40 and 80% burn off, dependent on the initial loading. The reactivity per unit weight of carbon reacting increases linearly with the catalyst loading up to 40 weight %, either initially or created by partial gasification. This implies that the catalyst is mobile under reaction conditions. Activation of the catalyst in the region up to 50% burn off is probably due to breaking up of K-O-C bonds. The lack of activity at low catalyst concentrations is explained by intercalate formation, which makes the catalyst inaccessible for the gaseous reactant. The results suggest that both CO and CO 2 are primary reaction products from different reaction pathways, in which potassium catalysis is basically controlled by the catalyst-carbon interaction. The reaction can be described by a redox mechanism in which the reduction step becomes more important at higher catalyst loadings. It is finally shown, that the assumption of water gas shift equilibrium is doubtful during the potassium catalysed low pressure steam gasification of activated carbon. A Langmuir type rate equation appears to be operative with an adsorption constant for water increasing with the catalyst loading.