Reactivity of some metal oxides supported on alumina with alternating methane and oxygen-application for chemical-looping combustion

Chemical-looping combustion (CLC) is a combustion technology with inherent separation of the greenhouse gas CO2. The technique involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from the combustion air to the fuel. Two reactors are used in the process: (i) a fuel reactor where the metal oxide is reduced by reaction with the fuel, and (ii) an air reactor where the reduced metal oxide from the fuel reactor is oxidized with air. The possibility of using oxides of Cu, Co, Mn, and Ni as oxygen carriers was investigated. Particles were prepared by deposition of the metal oxides on gamma-Al2O3 particles by so-called dry impregnation. The reactivity of the oxygen carrier particles was evaluated in a thermogravimetric analyzer (TGA), where the alternating atmosphere which an oxygen carrier encounters in a CLC system was simulated by exposing the sample to alternating reducing (10% CH4, 5% CO2, 10% H2O) and oxidizing (10% O-2) conditions at temperatures between 750 and 950 degreesC. The particles of Ni and Cu showed high reactivity at all temperatures and cycles, with reduction rates of up to 100%/min for CuO and 45%/min for NiO and oxidation rates of up to 25%/min for the oxidation of both reduced metals. Oxides of Mn and Co showed a limited extent of reaction, which was explained by the chemical reaction of the metal oxide with the alumina, with the formation of highly irreversible phases which do not react with methane and oxygen. Thus, Mn and Co are not suitable as oxygen carriers for CLC when supported on Al2O3. From the reactivity data of the nickel and copper oxygen carriers, it was estimated that 460-620 kg/MW oxygen carrier would be needed in the reactors. Similarly, from the oxygen transfer capacity of the particles, the solids circulation rate between the fuel and air reactor would need to be between 1 and 8 kg MW-1 s(-1).