Studies On Chemical Looping Dry Reforming Of CH₄ With Ni-doped Fe/Ce Oxygen Carriers Prepared By Flame Spray Pyrolysis Method

Chemical looping dry reforming(CLDR)is developed with the technology based on chemical looping combustion(CLC),which can divide the reforming of methane(DRM)into two-step reactions.The greenhouse gases(CH₄ and CO₂)can be used in CLDR as raw materials to produce syngas and it helps to alleviate the greenhouse effect.Compared to conventional preparation methods,flame spray pyrolysis(FSP)has unique advantages of one-step synthesizing high-efficient oxygen carriers.However,there are few studies used by FSP to synthesize oxygen carriers.In this thesis,Fe/Ce oxygen carriers with Ni doping were prepared by FSP and the reaction performance of as-prepared oxygen carriers for CLDR was studied through thermodynamic simulation and experiments.Firstly,four kinds of Fe/Ce oxygen carriers with varied amounts of Ni doping were prepared by FSP.The chemical characterization showed that the main components of fresh oxygen carrier are Ce O₂ and Fe₂O₃ and a little amount of Fe-Ni alloy and Ce-Ni alloy are also found.Fresh oxygen carriers have high specific surface area and their particle size are within the range of 60 nm.Thermodynamic parameters of the iron-based oxygen carrier participating in the chemical looping dry reforming process were studied through the HSC Chemistry software.The results show that the reaction of Fe₂O₃ with CH₄ is first reduced to Fe₃O₄,companying with generation of H₂O and CO₂,and then it is reduced to Fe,with H₂ and CO generated.Fe could be oxidized to Fe₃O₄ and Fe O by CO₂,and the mole ratio of Fe O to Fe₃O₄ could be around 5.The calculation of the equilibrium product in the reactor indicates that:in the reduction reactor,with the increase of the initial CH₄content,the product is converted from CO₂ and H₂O to CO and H₂.When CH₄ is excessive,the cracking happens and it will produce carbon deposits;in the reduction reactor inside,CO₂ can remove carbon deposits while restoring lattice oxygen in the oxygen carrier.Oxygen carriers with different levels of doped Ni are used in the methane chemical looping dry reforming process.The experimental results show that with the increase of Ni doping,the reactivity of the oxygen carrier with CH₄ is significantly enhanced,and more carbon deposits are generated.2Ni Fe(the mole ratio of Ni/Fe is 2:98)sample showed the best CO selectivity in the experiment,and has higher reactivity and lower carbon deposition rate,which is more suitable for the methane chemical looping dry reforming process oxygen carrier.The reaction performance of 2Ni Fe oxygen carrier for chemical looping dry reforming was further studied.As the reaction temperature increases,the reaction activity of the oxygen carrier gradually increases,and the CO selectivity and CH₄conversion both increase significantly.However,as the temperature increases,more carbon deposits are produced,which is not conducive to the formation of syngas.Lower the flow rate of streaming gas can increase the gas residence time and increase the CH₄conversion.During the 5 redox cycles,although the particle size increases and the specific surface area decreases in some extent,the reaction performance remains stable.After the reaction,the main components of the oxygen carrier become Ce Fe O₃,Ce O₂and Fe₃O₄,indicating that Fe O_x is the main active component in CLDR,Ce O₂ also provides parts of the lattice oxygen,and the functional Ni site can catalyze the reaction of the oxygen carrier with CH₄.