Kinetics of CO2 and steam gasification of Victorian brown coal chars

Abstract

This work reports a kinetic study of the CO2 and steam gasification of Victorian brown coal chars produced by rapid pyrolysis in a vertical drop tube furnace under entrained flow conditions. The study was performed by atmospheric pressure thermogravimetry over a temperature range of 650–1100°C. The intrinsic kinetic parameters for the chemical rate control regime were determined, and the onset temperature of mass transfer limitations observed. The experimental results were well described by the volumetric model, grain model and random pore model for char conversion from 10% to 50% due to the high porosity created during rapid pyrolysis. Activation energies ranging from 162 to 175kJ/mol and 119 to 165kJ/mol were determined for CO2 and steam gasification of Morwell, Loy Yang and Yallourn coal chars, respectively. The pre-exponential factors were of similar orders of magnitude, and increased for a smaller particle size range. The instantaneous relative reactivities, k(H2O)/k(CO2), at 800°C for the three Victorian brown coal chars ranged from 1.2 to 2.5, indicating a significant influence of the partial pressure of gasification reagent. The dominant influence in the case of CO2 gasification reactivity of these chars appears to be similar for all three coals and is morphological in nature. The order of increasing activation energies for char-steam gasification correlated well with the molar ratio Na/(Ca+Na) in the char.