Kinetics of redox reactions of CuO@TiO2–Al2O3 for chemical looping combustion and chemical looping with oxygen uncoupling

Abstract

Cu-based materials present as promising oxygen carrier candidates in chemical looping processes, in which CuO can either react directly with fuel gas via heterogeneous gas–solid reaction, or release gaseous O2 under suitable temperature and O2 partial pressure, and then react with fuel gas (even solid fuel) via combustion reactions. For comprehensive understandings of the complex reactions of the oxygen carrier in chemical looping processes, as well as for chemical looping reactor design and computational fluid dynamic (CFD) simulation, it is necessary to acquire the reaction kinetics of the oxygen carrier involved therein. This work aims to establish the redox reaction kinetics of a hieratically-structured and high-performance CuO@TiO2–Al2O3 oxygen carrier under the context of chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU). For the investigation, careful efforts have been made to ensure that the kinetics tests were chemical reaction controlled, and theoretical analysis further confirmed negligible effects of gas diffusion in the tests. Well-organized isothermal tests in a thermal gravimetric analyzer (TGA) were then conducted to obtain the solid conversion data needed for reaction kinetics analysis. The oxygen decoupling and subsequent oxygen uptake kinetics of the CuO@TiO2–Al2O3 oxygen carrier was first studied. Afterwards, the reactivity of the oxygen carrier with respect to three kinds of fuel gases, i.e., H2, CO, and CH4 was evaluated individually. Therefore, kinetics for the relevant reactions of the oxygen carrier in both CLC and CLOU conditions were determined. The conversion of the CuO@TiO2–Al2O3 oxygen carrier under different reacting atmosphere was found to subject to different reaction mechanisms. The global activation energies attained for the oxygen decoupling, oxygen uptake, and reduction by H2, CO, and CH4 were 217.2 kJ/mol, 87.5 kJ/mol, 44.5 kJ/mol, 40.1 kJ/mol, and 112.2 kJ/mol, respectively. The kinetics parameters were then used to estimate the solid inventory required by the fuel reactor when using the CuO@TiO2–Al2O3 as oxygen carrier in both CLC of gaseous fuels and CLOU of coals. For the case of CLC, the minimum solid inventory was 16.0 kg/MWth, 23.6 kg/MWth, and 39.4 kg/MWth when using H2, CO, and CH4 as fuel gas, respectively. Moreover, typical low oxygen carrier inventory, i.e., 124.2 kg/MWth, was needed to fully combust a Chinese lignite at 950 °C under CLOU condition. The high redox reactivity of the CuO@TiO2–Al2O3 demonstrated the feasibility of using this material as oxygen carrier in chemical looping processes.