A new active site/intermediate model (ASIM) is formulated in terms of a simplified calcium-catalyzed mechanism of char gasification to characterize a conversion-dependent maximum in reaction rate. In a limited case, the model is retrogressed to a common volumetric model (VM). Two sets of kinetic data were experimentally gathered by steam gasification of two lignite chars in a fixed bed reactor with negligible mass diffusion limitation. The gasification behaviors of two chars were, respectively, representative of catalyzed and uncatalyzed gasification as a result of the influences of different mineral components in them. It was found that ASIM intimately fit the kinetic profiles over the entire range of conversion for both catalyzed and uncatalyzed char gasification, demonstrating the good adaptability to varying chars. The model could determine the activation energies for the formation of the total carbon-containing gases and of two individual gases (CO and CO2) from the oxygenated intermediate. Moreover, for the case of catalyzed gasification, ASIM reasonably predicted the formation, growth and decline of the intermediate with carbon conversion as well as the change in the intermediate concentration with gasification temperature. For the case of uncatalyzed gasification, the model portrayed essentially a linear decline in the intermediate concentration with increasing carbon conversion, similar to VM. (C) 2014 Elsevier Ltd. All rights reserved.