Enhancement of char gasification in CO2 during chemical looping combustion

Abstract

Three chars were gasified in CO2 in a fluidised bed of inert sand or Fe2O3 as an oxygen carrier (either derived from an ore or prepared in the laboratory). Rates of gasification were found to increase with temperature and the presence of active oxygen carriers. The observed change varied both with the chars’ and carriers’ reactivity. A numerical model was developed to simulate char gasification. It accounts for: intrinsic kinetics of gasification, intraparticle mass transfer in the char, external mass transfer in the particulate phase of the fluidised bed and CO combustion in the bed of Fe2O3. At 1223 K, CO was removed from the vicinity of the char on being oxidised by an oxygen carrier; this was accompanied by a simultaneous increase in CO2 concentration, reducing the limitation imposed by mass transfer. It was concluded that the acceleration of gasification by oxygen carriers is significant only if gasification is limited by mass transfer in the first place. In addition, an analytical solution has been proposed, to combine all the processes into one simplified expression for estimating the apparent gasification rate. The influence of the intraparticle resistance was introduced with the effectiveness factor; the gasification rate was linearized with a Taylor series; finally the processes in the particulate phase were described with an enhancement factor, based on the Hatta number. This simple analytical model allows one to predict the influence of an oxygen carrier on the gasification of char during chemical looping combustion. The proposed expression for the effective rate was used to construct a map of the enhancement expected for various kinetic characteristics of an oxygen carrier.