Kinetics of the chemical looping oxidation of H2 by a co-precipitated mixture of CuO and Al2O3

Chemical-looping combustion (CLC) has the inherent property of separating CO2 from flue gases. Instead of air, it uses an oxygen-carrier, usually in the form of a metal oxide, to provide oxygen for combustion. When used for the combustion of gaseous fuels, such as natural gas, or synthesis gas from the gasification of coal, the technique gives a stream of CO2 which, on an industrial scale, would be sufficiently pure for geological sequestration. An important issue is the form of the metal oxide, since it must retain its reactivity through many cycles of complete reduction and oxidation. Here, we report on the rates of oxidation of one constituent of synthesis gas, H2, by co-precipitated mixtures of CuO + Al2O3 using a laboratory-scale fluidised bed. To minimise the influence of external mass transfer, and also of errors in the measurement of [H2], particles sized to 355–500 μm were used at low [H2], with the temperature ranging from 450 to 900 °C. Under such conditions, the reaction was slow enough for meaningful measurements of the intrinsic kinetics to be made. The reaction was found to be first order with respect to H2. Above ∼800 °C, the reaction of CuO was fast and conformed to the shrinking core mechanism, proceeding via the intermediate, Cu2O, in: 2CuO+H2→Cu2O+H2O, ΔH1073 K0=−116.8 kJ/mol; Cu2O+H2→2Cu+H2O, ΔH1073 K0−80.9 kJ/mol. After oxidation of the products Cu and Cu2O back to CuO, the kinetics in subsequent cycles of chemical looping oxidation of H2 could be approximated by those in the first. Interestingly, the carrier was found to react at temperatures as low as 300 °C. The influence of the number of cycles of reduction and oxidation is explored. Comparisons are drawn with previous work using reduction by CO. Finally, these results indicate that the kinetics of reaction of the oxygen carrier with gasifier synthesis gases is very much faster than rates of gasification of the original fuel.

Research highlights▶ Chemical-looping combustion (CLC) has the inherent property of separating CO2 from flue gases. ▶ We report rates of oxidation of H2 by co-precipitated CuO + Al2O3 using a lab-scale fluidised bed. ▶ Above 800 °C, the reaction of CuO was fast and conformed to a shrinking core mechanism. ▶ The CuO/Al2O3 was found to react at temperatures as low as 300 °. ▶ The reaction of CuO/Al2O3 with syngas is much faster than rates of gasification of the original fuel.