Continuous regime of chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) reactivity of CuO oxygen carriers

Abstract

Chemical-looping with CuO and CH4 presents an interesting reaction system, in which CuO participates in heterogeneous reduction reactions with the gaseous fuel, as well as oxygen uncoupling and catalytic reactions. Therefore, there exists a temperature dependent competition between the processes typically termed as chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU). The objective of this work is to carry out an assessment of the rates of all the relevant reactions involved at various temperatures and thus to show a continuous regime, where the CLC reactions domination proceeds to CuO oxygen uncoupling dominance, as temperature increases. The possible reactions occurring in CuO-based chemical-looping include heterogeneous reactions with Cu oxides, CuO oxygen uncoupling and Cu-catalyzed reactions. The significance of each individual reaction and the corresponding kinetic parameters are experimentally examined in a bench-scale fixed-bed, alternating-flow chemical-looping reactor at conditions relevant to both CLC and CLOU. In situ XRD characterization is used to determine the phases in which Cu exists during reduction and oxidation and possible interactions with the SiO2 oxygen carrier support. A kinetic scheme is proposed, applicable to the entire regime of CLC and CLOU, which is validated with in-house experiments and experimental data from the literature, at conditions where heterogeneous reduction, oxygen uncoupling, Cu-catalyzed, and gas phase oxidation reactions present significant contributions.