Study On Methane Chemical Chain Combustion Performance Of Red Mud Oxygen Carrier

Chemical looping combustion?CLC?is a novel of combustion technology that utilizes cyclic reduction-oxidation?redox?of oxygen storage materials to provide oxygen for the combustion of fuels to generate CO₂ and H₂O,while obtaining high purity CO₂ gas for easy capturing and storaging.Oxygen carrier?OC?is the key factor for the development of chemical looping combustion technology.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.This paper proposes that the development of red mud as an oxygen carrier for chemical looping combustion of methane?CH₄?not only make effective resources reuse of red mud,but also greatly reduce the cost of oxygen carriers.In this study,a series of oxygen carriers were prepared by controlling the pretreatment temperature of red mud.The effects of different pretreatment conditions on the reactivity and stability of red mud oxygen carriers were investigated in detail.The physicochemical properties of the red mud are characterized by XRF,XRD,TG-DSC,H₂-TPR and BET technologies.It is found that high calcination temperature result in the decrease of the reactivity of oxygen carrier but improve the thermal stability due to the formation of composite salts(e.g.Na₆Al₄Si₄O₁₇ and Ca₂Al₂SiO₇)via the reactions between alkali-metal oxide?Na₂O?or alkaline-earth-metal oxide?CaO?with the inert materials?Al₂O₃ and SiO₂?.Among the different red mud oxygen carriers?e.g.,800-RM,850-RM,900-RM and 950-RM,which are obtained after calcination at 800,850,900 and 950 ^oC,respectively?,the 900-RM sample shows the highest CH₄ conversion?71%?and CO₂ selectivity?79%?during the performance testing,indicating high activity for CH₄ oxidation.This phenomenon suggests that 900 ^oC is the most suitable calcination temperature.No obvious changes on the reactivity and phase structure of the red mud is observed after 30 redox cycles,which indicates that the calcined red mud oxygen carrier owns excellent redox stability for chemical looping combustion of CH₄.However,the red mud 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 CH₄.The synergy effects of the active ingredient?i.e.,Fe₂O₃?in V-RM and the inert/alkaline components?i.e.,Al₂O₃,CaO and Na₂O?in W-RM strongly improved the activity and stability of the oxygen carrier.Among the different samples,W7V3 shows the best property for CH₄ 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 CH₄.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,which are comparable with those published results using synthetic Fe₂O₃-based oxides or iron ores oxygen carriers.This suggests that the reaction behavior of CH₄ with Fe₂O₃ in the red mud is similar with other Fe₂O₃-contained materials.Based on the study of the reactivity and stability of W-RM and V-RM,in this paper,the modification of the two types red mud with oxygen storage and release characteristics were studied,separately.The excessive content of alkaline components in W-RM,so dealkalization is the key factor to its modification,the modification of W-RM by hydrochloric acid leaching and citric acid leaching respectively.Through XRF chemical element analysis of the acid-leaching modified red mud,it was found that the Na-removal effect of both methods was significant,and the Na removal rate was over 94%.The comparison of activity evaluation experiments found that the activity of modified red mud oxygen carrier by hydrochloric acid leaching method is better than that of modified red mud oxygen carrier by citric acid leaching method.V-RM is characterized by high content of Fe₂O₃,but the utilization rate of the active ingredient is low?ie,the reduction rate of Fe₂O₃ is too slow and not complete?,and insufficient inert carrier components cause red mud particles to easily agglomerate when reacting with fuel at high temperatures.In view of the composition of V-RM,this study added Ce O₂ and Al₂O₃ to enhance the reactivity and stability of the red mud.It was found through experiments that the red mud oxygen carrier modified by addition of 10 wt%CeO₂+10 wt%Al₂O₃ had the best performance.