Metal-oxide stabilized CaO/CuO composites for the integrated Ca/Cu looping process

Abstract

An integrated Ca/Cu looping process has been proposed recently for CO2 capture. It uses the exothermic in-situ reduction of CuO with methane or natural gas to supply the heat required for the endothermic calcination of CaCO3 to regenerate CaO for the following CO2 sorption cycle via bifunctional CaO/CuO composites. CaO/CuO composites possess excellent redox characteristics, but the rapid decline in CO2 capture performance remains an unresolved problem. Two different types of stabilizers, i.e., Al2O3 that can form a mixed phase with CaO, and MgO that does not form a mixed phase with CaO under reaction conditions, were incorporated into CaO/CuO composites via a Pechini method, and investigated by thermogravimetry. The results showed that the incorporation of Al2O3 or MgO enhanced cyclic stability significantly, and the formation of calcium-aluminum mixed oxides improved the cyclic CO2 capture stability more than did MgO. The best performing Al2O3- and MgO-stabilized CaO/CuO composites exhibited CO2 uptake of 0.14 and 0.09 gCO2/gmaterial after ten repeated cycles, exceeding the unstabilized CaO/CuO composites by 100% and 29%, respectively. We observed that 15 mol. % Al2O3 is the optimal quantity for Al2O3-stabilized CaO/CuO composites, ensuring a high and cyclically stable CO2 uptake capacity (0.11 gCO2/gmaterial after 45 cycles). Moreover, the cyclic stability was enhanced remarkably with increasing carbonation/oxidation temperature (from 550 to 750 °C). In-situ X-ray powder diffraction results suggest that the intermediate phase Cu2O played an important role in the enhanced performance of the composites at high carbonation/oxidation temperatures.