Reduction kinetics of Fe₂O₃/Al₂O₃ by ultralow concentration methane under conditions pertinent to chemical looping combustion

Fe₂O₃/Al₂O₃ is found to be a suitable oxygen carrier candidate for the chemical looping combustion with ultralow methane concentration in a previous study by our team. In order to facilitate the fundamental reactor design and understand the energy consumption, the reduction kinetics mechanism of Fe₂O₃ (hematite) with 0.5 vol % CH4 was determined and the kinetic parameters were estimated based on the thermogravimetric analysis. Two oxygen carriers (i.e., Fe25Al and Fe45Al) were prepared and used in the TGA experiment. It was observed that the reduction of Fe₂O₃ was a two-steps process. Initially, Fe₂O₃ is transformed into Fe₃O₄ (magnetite) at a fast reaction rate and followed by a slow step corresponding to the reduction from Fe₃O₄ to FeAl₂O₄. A topochemical approach associated with Hancock and Sharp's method was therefore applied to determine the most suitable kinetic model for the reduction process. It was found that the initial fast step can be described by the Avrami-Erofe'ev phase change model, the A2 model for low conversion, and the A3 model for high conversion, whereas the reaction for the second step was in diffusion control. It also can be concluded that within the Fe₂O₃ content of 25-45 wt %, there is no difference on the reduction kinetic mechanism and similar activation energy was obtained, which can be comparable with the findings in the literature