Reduction kinetics of CoO-NiO/Al2O3 oxygen carrier for chemical-looping combustion

This communication reports the reduction mechanism and kinetics of CoO-NiO/Al2O3 oxygen carrier using methane as the reduction gas. Kinetics experiments are performed in a novel fluidized bed CREC riser simulator under turbulent fluidized bed reaction conditions. A shrinking-core model is used for the kinetic modeling assuming spherical grains of the oxygen carrier particles. The XRD analysis showed that in the bimetallic carrier, nickel is present mainly as NiO. Therefore, in particle conversion calculations NiO was considered as the only reducible oxide phase, under the selected reaction conditions. Mercury porosimetry revealed that the pore size of the carrier particle is slightly increased after reduction, which is an indication of a possible shrinking core model since the molar volume of Ni (6.6 cm/mol) is lower than that of NiO (7.54 cm/mol). The rate-controlling steps of the gas-solid reactions in CLC process is established by observing the reaction rates at different experimental conditions as well as using a number of the theoretical calculations i.e. Sherwood number, Weisz-Prater criterion for external and internal mass transfer limitations. It is shown that under the reaction conditions studied the chemical reaction between methane and the solid oxides controls the overall reduction rate. Temperature variation experiments show that the reduction reaction is a strong function of the thermal level, which further confirms the reaction controlling rate findings. The activation energy calculated from the shrinking-core model is 49 ± 10 kJ/mol with this being consistent with the literature values.