Research On The Gas-Solid Reaction Mechanisms Over The Changed Iron Based Oxygen Carrier Surfaces Resulted By Chemical Looping Combustion

Chemical-looping combustion?CLC?is such a technology by using oxygen carrier?OC?rather than air to combust with fuel,therefore,it can separate CO₂ inherently,which provides a benefit for reducing the cost of CO₂ capture.In this work,density functional theory?DFT?calculations were conducted to detailedly investigate the correlation between reduction degree???of iron oxide and the interaction of CO\H₂S\Hg⁰ on OC surface during coal syngas CLC process,as well as the synergetic effect of H₂S\Hg⁰ on CO adsorption,oxidation,and decomposition behaviors on deep reduction of iron oxide surface.CO can chemically adsorb on Fe site of the prefect and reduced ?-Fe₂O₃?001?surfaces,showing a monotone increasing functional relationship between adsorption energy?Eads?and the reduction degree???.Partial reduction of iron oxide with reduction degree lower than 33.3% contributes to effecient CO oxdiation and inhibiting carbon deposition.Further reduction of iron oxide decreases CO oxdiation reaction rate sharply whereas strongly benefits the dcompositon of CO.Because of the orbital hybridization between S and Fe,H₂ S chemisorbs on the Fe site of the perfect and reduced surfaces.Reduction of iron oxide benefits the H₂ S adsorption and when ? = 50% the adsorption energy of H₂ S on surface is largest.On surface at high reduction state?? > 33%?,two adsorption modes of molecular and dissociative adsorption occur and the latter becomes the main adsorption type.The first dehydrogenation step is very facile on the perfect and reduced ?-Fe₂O₃?001?surfaces.The maximum energy barrier for H₂ S decomposition presents a wave curve with the ? of iron oxdie increasing,and the mimnum energy barriers can be obtained at ? = 33.3% and 100%,which suggests these surfaces exhibit very strong catalytic capacity for H₂ S dissociation.S atom derived from decomposition of H₂ S binds on Fe top site of the perfect ?-Fe₂O₃?001?surface,while on the reduced surface,S atom prefers to adsorb on the hollow site of Fe atoms.The reduction behavior of iron oxide promotes S adsorption on surface.Then the adsorbed S atom alters the electronic properties of surface,leading to a firstly promoting and then inhibiting effect on CO adsorption.The presence of S increases the Ea for CO oxidation reaction thus decreases the reaction rate.Similarly,on high reduction degree surface,the CO decomposition reaction can be inhibited.Hg⁰ physically adsorbs on the perfect ?-Fe₂O₃?001?surface,and then chemically binds on a series of reduced surfaces.A Monotone increasing function between adsorption energy of CO and reduction degree of Fe₂O₃ can be found.Hg⁰ adsorption inhibits the interaction of CO on surface with ?>45.8%,whereas promotes the interaction of CO on further reduced surfaces and decreases the energy barrier for CO decomposition reaction,which hence accelerates carbon deposition reducing CLC efficiency.The present study reveals fundermantal understanding of gas – solid reaction machanisms over iron based OC surface with multi-factor synergetic effect,provides important intrinsic reaction kinetic parameters for the establishment of cross scale kinetic model of CLC,and offers theoretical guidance for the control and optimization of CLC process.