Reduction and Oxidation Kinetics of a Copper-Based Oxygen Carrier Prepared by Impregnation for Chemical-Looping Combustion

Abstract

The kinetics of reduction with CH 4, H 2, and CO and oxidation with O 2 of a Cu-based oxygen carrier prepared by impregnation on alumina to be used in a chemical-looping combustion (CLC) system have been determined in a thermogravimetric analyzer. The oxygen carrier exhibited high reactivity in both reduction and oxidation with times for complete conversion lower than 40 s at 1073 K and 5−70 vol % of the fuel gas and 5−21 vol % of O 2 . The analysis of the sample carried out by scanning electron microscopy using energy-dispersive X-ray and chemisorption showed that the CuO was well dispersed in the porous surface of the alumina matrix and a uniform thin layer on the porous surface was considered. The shrinking-core model for platelike geometry of the reacting surface was used for the kinetic determination, in which the chemical reaction controlled the global reaction rate. No effect of the gas products (H 2 O and CO 2 ) on the reaction rate was detected. The reaction order depended on the fuel gas, and values of 0.4, 0.6, and 0.8 were found for CH 4, H 2, and CO, respectively. The order of the oxidation reaction was 1. The activation energies for the reduction and oxidation reactions varied between 14 and 60 kJ mol -1 . The reactivity data together with the operating variables were used to calculate some design parameters for a CLC system. It was found that the total solid inventory and the recirculation rate are linked. To optimize both parameters, conversion variations of the oxygen carrier in the fuel and air reactors, Δ X s, should be about 0.2−0.4. For a typical CLC operating condition, the total solids inventory for this Cu-based oxygen carrier (10 wt % CuO) was 133 kg/MW f if the fuel gas was CH 4, 86 kg/MW f if the fuel gas was H 2, and 104 kg/MW f if the fuel gas was CO, with recirculation rates of about 12 kg s -1 per MW f . The high reactivity of the material, in both reduction and oxidation, demonstrated the feasibility of this oxygen carrier to be used in a CLC system.