Synergy effects of combined red muds as oxygen carriers for chemical looping combustion of methane

Iron-containing natural ores or solid wastes (e.g., red mud) are considered as ideal candidate as oxygen carrier for large-scale chemical looping combustion technology due to their high content of Fe2O3 and low cost. However, these oxygen carriers usually show low activity for fuel conversion because of the special structure and components. In the present study, two types of red mud (V-RM with rich Fe2O3 and W-RM with rich inert and alkaline components) were combined to modify the structure and the distribution of different components in the red mud oxygen carrier for the chemical looping combustion of methane. The synergy effects of the active ingredient (i.e., Fe2O3) in the Fe2O3-rich red mud (V-RM) and the inert/alkaline components (i.e., Al2O3, CaO and Na2O) in the W-RM strongly improved the activity and stability of the oxygen carrier. Among the different samples, the W7V3 shows the best property for methane conversion (the average CH4 conversion and CO2 selectivity were 81% and 89%, respectively). In the multiple redox testing, the CH4 conversion decreases in the first ten cycles but the CO2 selectivity maintains at a high level (ca. 90%). The associated reduction kinetics parameters were also estimated based on isothermal thermogravimetric analysis (TGA) in a reducing atmosphere of methane. It was observed that the reduction of Fe2O3 in the red mud was a two-steps process. The reduction mechanism for the first stage reaction (i.e., Fe2O3 to Fe3O4) can be described by the two-dimensional growth of nuclei model A2. The activation energy of V-RM, W3V7, W5V5, W7V3 and W-RM oxygen carriers are 95.9, 93.5, 85.2, 77.5 and 84.2 kJ/mol, respectively.