Fundamental Research On Chemical Looping Hydrogen Production And CO₂ Reduction Via Calcium Ferrite Oxygen Carrier

With the development and progress of human society,the demand for energy is increasing dramatically.The combustion of fossil fuels such as coal and petroleum emits a large amount of CO₂ gas,which is one of the most important causes of global warming.In addition,human activities like power generation,metallurgy,petrochemicals,and textile printing have caused many environmental problems such as acid rain and PM2.5,which seriously affects the environmental quality and human health.Chemical looping combustion（CLC）,as a promising combustion technology,has many advantages such as high energy conversion efficiency,low NO_x and dioxin emissions with inherent CO₂ separation.Chemical looping hydrogen generation（CLHG）and chemical looping CO₂ re-utilization technology are developed based on the technology of chemical looping combustion,which is of great significance for achieving carbon dioxide negative emissions and efficient energy conversion and utilization.The research contents of this paper can be divided into these parts:i)thermodynamic calculation using HSC Chemistry and ASPEN Plus to understand the redox activity of the oxygen carriers as well as the effect of oxygen carriers and reducing agents on the gaseous product distribution from CLHG and chemical looping CO₂ splitting;ii)calcium ferrite preparation,characterization,and synthesizing parameters optimization;iii)experimental investigation on two-step chemical looping hydrogen generation;iv)experimental study on chemical looping biomass gasification with hydrogen-rich syngas production as well as the analysis of tar abatement mechanisms;v)experimental research on chemical looping CO₂reduction with biomass cascade utilization as well as synergistic promotion mechanism;vi)exploration on CO₂ biochar gasification activity and kinetic investigation under Ca₂Fe₂O₅.The main research contents and conclusions are summarized as follows:Thermodynamic calculation based on Gibbs free energy.According to the Gibbs free energy distributions,the deep reduction performance of Fe-based oxygen carriers(Fe³⁺→Fe⁰)is initially analyzed.Results show that with the introduction of calcium,it becomes more difficult to reduce Ca₂Fe₂O₅ and CaFe₂O₄ compared with Fe₂O₃.The influence of reducing agents is investigated,taking Ca₂Fe₂O₅ as an example,it is found that the reduction of Ca₂Fe₂O₅by H₂ and CO is not thermodynamically spontaneous,while the complete oxidation or partial oxidation of methane can be carried out spontaneously,and the partial oxidation of methane is more prone to occurr.Combined with the simulation results from ASPEN Plus,Ca₂Fe₂O₅performs weak oxidizing ability to H₂ and CO,but it has a better activating effect on the conversion of CH₄ or CH₄-containing syngas.Calcium ferrite preparation and investigation on two-step chemical looping hydrogen generation process using calcium ferrite as the oxygen carrier.Ca₂Fe₂O₅ and CaFe₂O₄ were prepared by citric acid assisted sol-gel method.The preparation parameters,Ca/Fe mole ratio,citric acid amount,calcination temperature,were optimized and analyzed.Characterizations such as SEM,TEM,M?ssbauer,TGA,XPS,CO₂-TPD,H₂-TPR,etc.were performed to study the particle morphology,Ca and Fe element distributions,unit cell arrangement,oxygen species at the surface,thermal stability,CO₂ adsorption performance,and the redox activity of lattice oxygen.The effect of OC types,reducing agents,and temperatures on the migration rate of lattice oxygen were studied.The phase evolution,cyclic stability,as well as the changes of particle morphology,BET surface area,and the pore volume were further studied at the critical stage of redox looping.XRD results show that the deep reduced Ca₂Fe₂O₅ oxygen carrier（Fe⁰）can be oxidized by steam to Ca₂Fe₂O₅(Fe³⁺)in one step,and the hydrogen yield can be theoretically increased by 12.5%compared with the same molar amount of Fe₂O₃ as the oxide carrier.According to the above exploration,a two-step chemical-looping hydrogen production process based on calcium ferrite oxygen carrier was proposed.Research on the hydrogen-riched syngas production from calcium ferrite-based chemical looping biomass gasification.The performances with applied Fe₂O₃,CaO-Ca₂Fe₂O₅,Ca₂Fe₂O₅,CaFe₂O₄,and FeCO₃ were initially studied in biomass pyrolysis and biochar gasification.It is found that CaO-Ca₂Fe₂O₅ and Ca₂Fe₂O₅ perform good catalytic biomass conversion performance and their phases remain stable and unchanged after the redox cycle（reduction and re-oxidation）.The effect of temperature,water vapor flow rate,catalyst loading amount,gasification time on product distribution,cumulative gas yield,carbon conversion,H₂/CO,and CO/CO₂ mole ratio was further investigated.Results indicate that the addition of Ca₂Fe₂O₅ can effectively promote the generation of hydrogen-rich syngas and enhance biochar conversion.In the first 10 min of gasification,the cumulative H₂ yield was 7.12 mol·H₂/kg·biomass with applied Ca₂Fe₂O₅,which was improved by 78.98%compared with the biomass gasification result without OC addition.Furthermore,the addition of Ca₂Fe₂O₅ increased the carbon conversion rate and total gas yield by 17.3%and 11.7%,respectively.Further,the biomass utilization pathway and the bio-tar conversion mechanism were proposed and thoroughly discussed for the chemical looping biomass gasification process with Ca₂Fe₂O₅ being the oxygen carrier.Synthesis of calcium ferrite aerogel and its utilization on the synergistic promotion of biomass cascade utilization and chemical looping-based CO₂ reduction.The Ca-Fe composite gel was prepared using the organic reagent polyacrylic acid and propylene oxide.The aerogel materials,Ca₂Fe₂O₅ and CaFe₂O₄,were further synthesized using a freeze-drying method.In-depth analysis of the gelation mechanisms was discussed,and the preparation parameters were optimized for the optimal preparation of calcium iron composite gels.Moreover,the properties of calcium ferrite aerogels were characterized by XRD,BET,and SEM-EDS mapping.TEM,XPS,M?ssbauer,and TGA etc.were also carried out to investigate the morphology,surface chemical state,the types of Fe species and its composition,as well as the lattice oxygen migration rate.Experimental investigation on the synergistic enhancement of biomass cascade utilization and chemical looping CO₂ reduction,the gaseous product distribution and yield,bio-oil component,as well as the CO concentration and yield were explored.Results reveal that the addition of Ca₂Fe₂O₅ increases the liquid yield by 10.7%.In addition,the generated bio-oil is composed of a high percentage of aromatic and phenolic compounds which accounts for 55.5%and 45.6%of the detected bio-oil.As for CO₂ reduction stage,the influence of oxygen carrier type and the OC loading amount on CO concentration and yield was studied.The highest conversion rate of CO₂ can be obtained when Ca₂Fe₂O₅ is used as the oxygen carrier with high-concentration CO production under the conditions of OC loading 10 wt.%and temperature at800oC.The cumulative CO production with applied Fe₂O₃,Ca₂Fe₂O₅,and CaFe₂O₄ increased by 20.3%,28.3%,and 11.1%,respectively compared with the experiment without OC addition.Finally,the mechanism for synergistic enhancement of biomass cascade utilization with CO₂reduction was discussed,that is,the preliminary products from biomass fast pyrolysis promotes the OC reduction;the products from OC reduction,CaO,H₂O,and CO₂,further enhance the bio-tar cracking and reforming;and the products from bio-tar further facilitates the deep reduction of Ca₂Fe₂O₅.Biochar gasification activity and kinetic investigation on CO₂ biochar gasification with the existence of OC.The biochar produced from 800oC pyrolysis was analyzed by TEM and Raman to analyze the type of biochar and the corresponding microscopic morphology.The biochar was found to have a high degree of graphitization and a worm-like morphology under high-resolution TEM.In the kinetic study,the random pore model（RPM）is adopted,and the best matching structural parameterΨis obtained according to the least square method.Then,the gasification reaction rate constant under different oxygen carriers is obtained by curve fitting.Finally,the activation energies of biochar gasification without OC,10 wt.%Fe₂O₃,10wt.%Ca₂Fe₂O₅,and 10 wt.%CaFe₂O₄ were calculated to be 171.1 kJ/mol,179.2 kJ/mol,178.2kJ/mol,and 180.5 kJ/mol,respectively.The reason for the slight increase of apparent activation energy is presumed to be a competitive reaction between the oxygen carrier and the biochar,which equivalently reduces the partial pressure of CO₂ for biochar gasification.It is found that the gasification time has a good fitting effect with the biochar conversion rate when the biochar conversion rate is lower than 0.8;while the fitting effect is relatively poor when the when the biochar conversion rate is higher than 0.8,which could be affected by the oxidation of the oxygen carrier by CO₂ at the period biochar is almost completely converted.