Characteristic And Mechanism Investigation On Biomass Chemical Looping Gasification Based On Mn-Fe Composite Oxygen Carriers

Chemical Looping Gasification（CLG）is a new technology which could converts the solid into syngas、tar and coke by using oxygen carriers to provide lattice oxygen.Oxygen carrier is the key factor to realize the chemical looping concept so great efforts have been devoted on the performance improvement of oxygen carriers by different researchers.The combined oxides of Mn andFehad been considered as one of the most potential oxygen carriers for chemical looping process but their structural features should be promoted.Therefore,the reactivity in CLG of Mn-Fe composite oxygen carriers loaded on Al₂O₃、MnFeO₃ and Mn Fe₂O₄ was studied in this paper.Mn-Fe composite oxides were loaded on Al₂O₃ and their feasibility for CLG process were studied.Thermo gravimetric experiments and fixed bed tests were carried out to explore the effects different parameters on the gasification.It was found that Mn-Fe composite oxides promote the pyrolysis and gasification efficiency of biomass,the addition of Al₂O₃ could enhance the stability at relatively high temperature.Mn-Fe composite oxide with 60 wt.%Al₂O₃ was chosen as the ideal oxygen carrier.In addition,some of the Mn O transferred to Mn Al₂O₄ which was dystectic to resist sintering and it separated into manganese oxide and Al₂O₃ after the oxidation.The granular microstructure provided by Al₂O₃ was also contributed to reactivity maintain.However,the diameter of the individual grains increased and sintering phenomenon happened after cyclic reactions.Based on the research above,Mn-Fe composite oxides directly loaded on Al₂O₃ could be complex during preparation and the content of oxygen carriers participated in CLG was low.Therefore,Mn Fe₂O₄ with spinel structure and MnFeO₃ with nanostructured iron manganite structure were prepared as oxygen carriers.The in situ infrared results indicated that the ferromanganese composite oxides accelerated the pyrolysis process and contributed to reforming reaction of carbon and CH₄,which promoted the carbon conversion efficient.MnFe₂O₄ has better gasification characteristics without adding steam,but the yields of H₂ and CO in syngas could be greatly improved when steam was introduced,especially for MnFeO₃.The XRD analysis demonstrated that MnFeO₃ and Mn Fe₂O₄ transformed to a combination of（Fe,Mn）O during the reduction process and both of them exhibit recyclable crystalline phase variation.Moreover,MnFeO₃ formed a granular、porous structure which helped to maintain a stable reactivity in cyclic tests.However,Mn Fe₂O₄ exhibited a decreasing tendency of gasification efficient because of agglomeration,its typical surface morphology changed from scattered particles to blocky structure.Density Functional Theory（DFT）was used to describe the oxidation between ferromanganese composite oxides and combustible components produced in CLG process.The possible adsorption and reaction mechanisms between combustible gas（CO、H₂）and stable surface of oxygen carriers（Mn Fe₂O₄、MnFeO₃）were discussed.It was indicated that both CO and H₂ tend to absorbed on the Mn Fe₂O₄ instead of MnFeO₃.So we proceed to calculate the dissociation and oxidation mechanisms on Mn Fe₂O₄ surface and the results showed that the energy barrier was low so it would deteriorate the gasification process.Thus,oxygen carrier MnFeO₃ was considered to have better applicability in CLG process.