Study On Perovskite-based Oxygen Carriers For Chemical Looping Steam Methane Reforming

Because of steadily increasing demands for global energy consumption,natural gas is expected to become the major energy in the future,natural gas utilization elicited increasing attention.With the development of global industry and the increasing demand of clean energy,the consumptions of synthesis gas and hydrogen are also increasing.Traditional methane steam reforming process produces syngas with cumbersome processes,and the energy cost is also high.The chemical-looping steam methane reforming(CL-SMR)method become a novel synthesis gas and hydrogen producing technology with potential advantages.In CL-SMR,selection of oxygen carrier is crucial because oxygen carriers must feature a redox property and a high cyclic stability.In this paper,the performance of CeO₂ supported Ba-Co based perovskite oxygen carriers for CL-SMR was systemically studied.The oxygen capacity,reaction selectivity,redox performance,synergistic effect between CeO₂ and perovskite,doping elements in perovskite,reaction mechanism and kinetic parameters were analyzed.The effect of A/B-site elements on oxygen capacity,oxygen releasing rate,reactivity and reaction selectivity were investigated.BaCoO_3-?showed the best oxygen capacity and oxygen releasing rate among the chosen perovskite-type oxygen carriers(A-site element:Ba,Sr,La;B-site element:Fe,Co,Ni).In CL-SMR,the reactivity and reaction selectivity of BaCoO_3-?is also better than the other samples,the H₂/CO ratio of syngas production is closed to ideal value of 2.To increase the oxygen capacity,a series of CeO₂ supported BaCoO_3-?perovskite-type oxides were prepared and evaluated as oxygen carriers in CL-SMR.The CeO₂ supported BaCoO_3-?oxygen carriers showed higher gas productions than pure BaCoO_3-?and CeO₂,it also showed a good cyclic stability.The oxygen carriers need a proper temperature to make a balance between reactivity with methane and oxygen mobility;a lower temperature led to a smaller and slower oxygen supply from the CeO₂support;and a higher temperature led to more decomposition of methane and CO impurity production.To improve the reaction selectivity,CeO₂-supported Ba_1-xSr_xCoO_3-?oxygen carriers with different A-site doping ratios(x=0,0.1,0.3,0.5,0.7,0.9)were synthesized,and the effect of Sr-doping on reactivity was analyzed.The Sr-doping suppressed side reactions like complete oxidation of methane and decomposition of methane,the H₂/CO ratio of syngas production was also closed to the ideal value of 2.During cyclic reactions,Ce ion reversibly shuttled between CeO₂ and ACeO₃(A=Ba,Sr)depending on the local redox condition,this synergistic effect of A-site element and CeO₂ not only enhanced the selectivity of partial oxidizing of methane,but also increased syngas production and hydrogen.To enhance the synergistic effect between CeO₂ and A-site element,the effect of chemical distribution on oxygen mobility and reaction performance of BaCoO_3-?/CeO₂oxygen carrier was investigated.The reaction mechanism of methane on BaCoO_3-?/CeO₂was also studied.The result indicated that the dense contract of CeO₂ support and perovskite layer could improve the oxygen mobility and enhanced the synergistic effect,so the uniformly adhered perovskite layer on CeO₂support could improve the gas productions because of the better lattice oxygen supply.In situ Diffuse Reflectance Infrared Fourier Transform spectroscopy(DRIFT)was applied to analysis the reaction intermediates and mechanism of partial oxidation methane on BaCoO_3-?/CeO₂,the result suggested that CH₄ was gradually dehydrogenated and react with lattice oxygen to produce CO and H₂,conversion of-O-CH₃?-CHO is the rate controlling step.Finally,Ba_0.3Sr_0.7CoO_3-?/CeO₂ oxygen carriers were loaded on honeycomb monolith and the reaction performance was investigated.The kinetic model of partial oxidation of methane was developed based on experimental data.The calculated kinetic parameter was applied in numerical simulation of the partial oxidation of methane on honeycomb oxygen carriers,the temperature distribution,velocity distribution and evolution of reactions were analyzed.The behavior of oxygen carriers was investigated to obtain a further understanding of application of perovskite-based oxygen carriers on CL-SMR.