Structure–property relationship of co-precipitated Cu-rich, Al2O3- or MgAl2O4-stabilized oxygen carriers for chemical looping with oxygen uncoupling (CLOU)

Chemical looping with oxygen uncoupling (CLOU) is a novel combustion technology that utilizes the decomposition reaction of an oxygen storage material to combust solid (hydro-)carbonaceous fuels. CuO is an ideal candidate for CLOU owing to its high oxygen uncoupling capacity (∼10 wt.%) and its ability to release oxygen at temperatures relevant for chemical looping combustion (CLC), i.e. 850–1000 °C. Here, we investigate the feasibility of Cu-rich, Al2O3 or MgAl2O4-stabilized oxygen carriers containing 70–90 wt.% CuO for the CLOU process. Al2O3- and MgAl2O4-stabilized oxygen carriers were synthesized using a co-precipitation technique and calcined either at 800 °C or 1000 °C. The CLOU performance of the oxygen carriers was evaluated at 950 °C in a fluidized bed reactor using a N2 atmosphere for the decomposition reaction and a mixture of 10.5 vol.% O2 and 89.5 vol.% N2 for the re-oxidation reaction. It was found that the calcination temperature and the surface area had no significant influence on the oxygen carrying capacity of the synthesized oxygen carriers. The oxygen uncoupling capacity of Al2O3-stabilized oxygen carriers was substantially below the theoretically expected values due to the formation of the mixed oxides CuAl2O4 and CuAlO2. Based on thermo-gravimetric measurements, a reaction pathway for the oxygen release and re-oxidation reactions of Al2O3-stabilized CuO was proposed. On the other hand, no interaction between MgAl2O4 and the active phase, i.e. CuO was observed. As a consequence, MgAl2O4-stabilized oxygen carriers possessed a high oxygen uncoupling capacity very close to the theoretical values. Owing to its stable and high oxygen release capacity, MgAl2O4-supported oxygen carriers containing 80 wt.% CuO were identified as a promising material for CLOU.