Redox Investigation of Some Oxides of Transition-State Metals Ni, Cu, Fe, and Mn Supported on SiO2 and MgAl2O4

Chemical-looping combustion (CLC) and chemical-looping reforming (CLR) involve the use of a metal oxide as an oxygen carrier which transfers oxygen from combustion air to the fuel. Two interconnected fluidized beds, a fuel reactor, and an air reactor are used in both processes. In the fuel reactor, the fuel is oxidized by a metal oxide, and in the air reactor, the reduced metal is oxidized back to the original phase. In CLC, a high conversion of the fuel to CO2 and H2O is required in the fuel reactor, whereas only a partial oxidation of the fuel is desired in CLR. Oxides of Ni, Cu, Fe, and Mn supported on SiO2 and MgAl2O4 were prepared by dry impregnation and investigated under alternating reducing and oxidizing conditions in a thermogravimetric analyzer at 800−1000 °C using fuel (10% CH4, 10% H2O, and 5% CO2) and oxidizing gas (5% O2). NiO and CuO supported on both SiO2 and MgAl2O4 showed very high reactivity. However, the reactivity of NiO/SiO2 decreased as a function of the cycle number at 950 °C but was avoided below 850 °C. SiO2-supported Mn and Fe oxides may not be feasible oxygen carriers to be used in the process because of the formation of irreversible silicates at high temperatures. However, iron and manganese oxide supported on MgAl2O4 showed a rather high reactivity during reduction and oxidation and can possibly be used in the process. The amount of material needed in air and fuel reactors was estimated in the range of 127−350 kg/MWth, and the recirculation rate of the oxygen carrier necessary between the two reactors was 1.5−8.0 kg MWth -1 s-1 for CLC.