The effects of WO3 addition to NiO/ZrO2 oxygen carriers for chemical looping combustion of methane

Chemical looping combustion (CLC) is a promising combustion technology with inherent CO2 capture at low energy penalty. The success of CLC relies heavily on the development of an appropriate oxygen carrier. Mixed metal oxide supported oxygen carriers have been widely used to alleviate some of the disadvantages of monometallic based oxygen carriers. In this study, the effects of introducing WO3 into NiO/ZrO2 oxygen carriers were investigated for methane CLC using different synthesis methods and varying oxygen carriers' compositions. Several characterization techniques such as ICP-OES, N-2 adsorption-desorption, XRD, SEM/EDS, H-2-TPR, and XPS have been used to understand the effects of preparation method, and oxygen carrier composition on methane CLC. The dual NiO-WO3/ZrO2 oxygen carriers exhibited enhanced oxygen capacity and lower carbon formation compared to their NiO/ZrO2 monometallic counterpart. Introduction of WO3 into NiO/ZrO2 lattice structure led to formation of an intermediate phase (NiWO4) which enhanced oxygen carriers' reducibility and improved metal dispersion. This was mainly due to the high valence number of W6+ which created oxygen vacancy in the OC lattice structure due to charge balance difference. Coprecipitation and impregnation methods have been used to synthesize dual OCs, and impregnation method exhibited superior performance in terms of cyclic stability and coking resistance. Using impregnation synthesis method, WO3 and NiO loadings were optimized based on oxygen capacity and coke formation under cyclic redox experiments. Amongst the tested samples for methane CLC, NiO(60%)-WO3(25%)/ZrO2 showed the optimum performance with 15.6% oxygen carrying capacity and no carbon formation.