Study On The Splitting Mechanism Of H₂O/CO₂ On The Surface Of Reduced CaFe₂O₄ And Reaction Kinetics Based On Rate Equation

Calcium ferrite(CaFe2O4)has become a popular oxygen carrier in biomass chemical looping gasification(CLG)due to its high syngas selectivity,sinter resistance and cheap availability.In the process of biomass chemical looping gasification with calcium ferrite as oxygen carrier,H2O or CO2 is injected into the oxidation reactor to achieve the oxidative regeneration of reduced CaFe2O4,while splitting H2O or CO2 to produce high-purity H2 or CO without additional energy consumption,which is a new technology that has attracted wide attention in recent years.However,the pyrolysis mechanism of H2O and CO2 on the surface of reduced CaFe2O4 remains unclear.In addition,the reaction mechanism calculated by DFT cannot be directly correlated with the conversion characteristics obtained by TGA analysis to explain the reaction phenomenon.Based on the DFT calculation,the adsorption characteristics,electronic structure,and reaction paths of H2O and CO2 on the surface of reduced CaFe2O4 were systematically studied to reveal the reaction mechanism.The reduction kinetics of CaFe2O4 and the oxidation kinetics of reduced CaFe2O4 were studied by using the microkinetic rate equation theory based on the first principles.(1)Based on XRD,HR-TEM,and EDS characterization of reduced CaFe2O4,a CaO(111)/Fe(110)composite interface was established to simulate the surface structure of reduced CaFe2O4,and the DFT was calculated.The results show that H2O and CO2 form stable complexes by chemisorption on the CaO(111)/Fe(110)surface with adsorption energies of-0.55 eV and-1.01 eV,respectively.PDOS results also show that H2O and CO2 molecules have strong interactions with Fe on CaO(111)/Fe(110)surface.H2O is split into H2 and adsorbed to O*by a twostep reaction with activation energies of 0.69 eV and 0.40 eV,respectively.In this process,the splitting of H2O into HO*and H*is the rate-determining step of the reaction.CO2 is split into CO*and O*by a one-step reaction,and the activation energy of CO2 pyrolysis is 1.08 eV.CO desorption from the surface needs to overcome the energy of 0.398 eV.CO desorption from the surface is ratedetermining step.H2O and CO2 adsorption after total charge-0.743 |e| and 0.954|e| respectively,the total charge of Fe atoms on the surface is 0.931 |e| and 0.433|e|.H2O and CO.are electron acceptors,and the surface Fe atoms are electron donors.The H2O and CO2 release heat during the cracking process of the reduced CaFe2O4 surface,which can realize the oxidation of the reduced CaFe2O4 and provide heat to the system.(2)The reduction kinetics of CaFe2O4 with CO and the oxidation kinetics of reduced CaFe2O4 with CO2 were studied by using the rate equation theory based on first principles.First,the adsorption energy of CO was calculated by DFT.CO adsorbed on the surface of OCs by chemical adsorption,and the reaction activation energy of this step was 1.110 eV.Desorption of the new formed CO2 molecules from the surface of CaFe2O4(001)demands an energy of 0.398 eV.Surface lattice oxygen breaks away from the surface by overcoming the reaction energy barrier and combines with CO to form a new C-O bond and form oxygen vacancy.In the experimental stage,when 20 vol.%CO was introduced,the change of reaction temperature had no noteworthy impact on the lattice oxygen conversion in CaFe2O4.At 900℃,the conversion rate of 20 vol.%CO is significantly faster than that of 10 vol.%CO.The results indicate that the oxygen conversion in the lattice of CaFe2O4 is mainly affected by the concentration of CO gas on the surface.The data obtained by using the rate equation theory model are in good agreement with the experimental data,showing the same trend as the experimental results,which also proves the rationality of the DFT calculation results.(3)In the stage of CO2 oxidation of reduced CaFe2O4,the DFT calculation results show that CO2 needs to overcome the activation energy of 1.909 eV after the adsorption of reduced CaFe2O4 surface to achieve the C-O bond fracture.The newly formed CO has to overcome 2.119 eV of the energy barrier to be released into the gas phase.The results show that the reduced CaFe2O4 can be completely oxidized within 10 to 20 min at the temperature range of 800 to 900℃.When the reaction temperature was further increased to 950℃,the oxidation rate did not increase significantly.The concentration of CO2 has a significant effect on the oxidation reaction of reduced CaFe2O4.With increases in CO2 concentration,the reaction rate becomes faster.When the concentration of CO2 exceeds 15%,the effect on the conversion rate is no longer significant.In the later stage of the reaction,the oxidation rate of reduced CaFe2O4 slowed down significantly,which may be because the surface of reduced CaFe2O4 was covered with a thick layer of newly generated CaFe2O4,which affected the lattice oxygen transport and resulted in a significant decrease in the oxidation rate in the later stage.The theoretical model is helpful to explore the reduction kinetics of CaFe2O4 and oxidation kinetics of reduced CaFe2O4.