Kinetic determination of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier for use in gas-fueled Chemical Looping Combustion

The objective of this work was to determine the kinetic parameters for reduction and oxidation reactions of a highly reactive Fe-based oxygen carrier for use in Chemical Looping Combustion (CLC) of gaseous fuels containing CH4, CO and/or H2, e.g. natural gas, syngas and PSA-off gas. The oxygen carrier was prepared by impregnation of iron on alumina. The effect of both the temperature and gas concentration was analyzed in a thermogravimetric analyzer (TGA). The grain model with uniform conversion in the particle and reaction in grains following the shrinking core model (SCM) was used for kinetics determination. It was assumed that the reduction reactions were controlled by two different resistances: the reaction rate was controlled by chemical reaction in a first step, whereas the mechanism that controlled the reactions at higher conversion values was diffusion through the product layer around the grains. Furthermore, it was found that the reduction reaction mechanism was based on the interaction of Fe2O3 with Al2O3 in presence of the reacting gases to form FeAl2O4 as the only stable Fe-based phase. The reaction order values found for the reducing gases were 0.25, 0.3 and 0.6 for CH4, H2 and CO, respectively, and the activation energy took values of between 8 kJ mol−1 (for H2) and 66 kJ mol−1 (for CH4). With regard to oxidation kinetics, the reacting model assumed a reaction rate that was only controlled by chemical reaction. Values of 0.9 and 23 kJ mol−1 were found for reaction order and activation energy, respectively. Finally, the solids inventory needed in a CLC system was also estimated by considering kinetic parameters. The total solids inventory in the CLC unit took a minimum value of 150 kg MW−1 for CH4 combustion, which is a low value when compared to those of other Fe-based materials found in the literature.