Particle Structure Design Of Iron-based Oxygen Carrier Toward To Chemical Looping Dry Reforming

Oxygen carrier(OC)is the core of chemical looping reforming(CLR).The particle structure and physicochemical properties of oxygen carrier directly affect the reaction performance during chemical looping reforming,which has always been the focus of chemical looping reforming technology research.Fe-based oxygen carriers are considered to be the most promising industrial oxygen carriers due to their low carbon deposition,large reserves and low price.However,in the process of redox cycles,iron ions constantly migrate from the bulk to the particle surface and accumulate on the particle surface subsequently,which leads to serious sintering of Fe-based oxygen carriers and has an unfavorable effect on the cycle stability.In recent years,researchers have improved the performance of oxygen carriers by adding the second component,composite metal oxides and particle structure design,but most of them are designed for fixed bed.In order to match with fluidized bed,a new design idea of oxygen carrier is put forward in this paper,that is,a core-shell like Fe2O3/MgO oxygen carrier is designed and synthesized through adjusting the concentration of polyvinylpyrrolidone(PVP),the concentration of urea and the reaction time to control the particle size which using the theory of the different solubility products of ferrous carbonate and magnesium carbonate.The results are as follows:(1)The key factors affecting the morphology and particle size of Fe2O3/MgO oxygen carriers with core-shell structure are investigated.It is found that the spherical morphology of oxygen carriers is controlled by PVP,and the particle size is related to the concentration of PVP,the concentration of urea and reaction time.The particle size of oxygen carriers is negatively related to the concentration of PVP and urea,while positively related to the reaction time.The chelation of lone pair electrons in PVP repeat unit changes the growth habit of primary Fe2O3/MgO particles.The subunit structure changes from two-dimensional plate to pyramid.The existence of PVP also promotes the self-assembly of pyramid subunits,which changes the oxygen carrier morphology from aggregated structure by irregular nano-plate to micron sized spherical particles(20～70 pm).However,the strong steric effect caused by long-chain structure of PVP hinders the growth rate of the crystals;the concentration of urea affects the supersaturation of CO32-in the solution,thus affecting the nucleation rate and finally reflected in the particle size.(2)The formation mechanism of Fe2O3/MgO oxygen carrier with core-shell-like structure is revealed:at the beginning of hydrothermal reaction,primary particles begin to form in the solution.With the extension of reaction time,under the influence of urea releasing CO32-,the agglomeration morphology of Fe2O3/MgO primary particles converts from irregular agglomerates to spherical particles,while grows up with time of process.Therefore,a core-shell-like structure is formed,in which iron is mainly concentrated in the center of the particle and magnesium is mainly wrapped outside.The primary particles formed in the reaction process are combined with the long chain of PVP,forming irregular agglomerates in a short reaction time,which then transformed into large-size particles with the extension of time.(3)The redox stability of core-shell-like Fe2O3/MgO oxygen carriers is evaluated in CLR.The results show that the unique core-shell-like structure,in which a large number of magnesium elements in the outer layer occupy the transferable vacancies of iron,inhibits the external diffusion of iron.The core-shell-like Fe2O3/MgO oxygen carriers perform good sintering resistance and better cycle stability than the oxygen carrier prepared by impregnation method.(4)Comparing the reactivity and cycle stability of oxygen carriers with different Fe content,it is found that the core-shell-like oxygen carriers with low Fe content have better reactivity and cycle stability due to their smaller grain size.This is because when the content of Fe2O3 in the core-shell oxygen carrier is low,the distribution of Fe2O3 grains is more dispersed,and it is more difficult to aggregate during redox cycles.This phenomenon is more obvious especially in the center of the sphere particles where the content of Fe2Ois higher than the edge of the particle.