Kinetics of Char Gasification with CO2 under Regime II Conditions: Effects of Temperature, Reactant, and Total Pressure

There are relatively few studies reported in the literature characterizing the kinetics of high-pressure, high-temperature char gasification reactions. Of particular interest to the application of kinetic data to gasification models are studies that provide links between reaction rate data relevant to high-temperature conditions, where some extent of pore diffusion limitation may apply, and lower-temperature, intrinsic gasification reactivity data. This work describes the effects of temperature, reactant partial pressure, and total pressure on the kinetics of the char−CO2 reaction under conditions where reactant diffusion through the pores of the reacting char has been shown to impact the overall conversion rate. A global nth-order rate equation was used to describe these kinetics, in particular, the effect of temperature (via the activation energy) and reactant pressure (via the reaction order). As expected, activation energies at high temperatures (≥∼1300 K) were consistently less than those obtained under conditions free of diffusion limitations; the extent to which the measured activation energies were less than the “true” activation energy was dependent on the extent of diffusion limitation. The effect of CO2 partial pressure was less than expected, based on measurements of the intrinsic reaction orders, although increasing the CO2 partial pressure did lead to an increase in the measured reaction rate. Total pressure was not found to have a significant and systematic effect on the measured kineticsthis is consistent with Knudsen diffusion being the dominant mode of diffusion in these char samples, which have relatively small pore size (∼15−30 Å).