Study On The Mechanism Of Using Ca-based Catalyst In Catalytic Biomass Pyrolysis For Hydrogen Production

In the process of H₂-rich gas production from biomass pyrolysis,CO₂and tar seriously affect the gas quality and reduce the conversion rate of production,which can be removed by using catalysts for high yield H₂production.The reaction mechanisms cannot be clearly explained due to the numerous reaction pathways and wide product distribution.It is of great significance for the development of H₂production technology from biomass to conduct in-depth study on the mechanism of H₂release by decoking and decarbonization.In this paper,molecular simulation and fixed bed experiments were used to study the mechanism of Ca-based catalysts for H₂production from biomass pyrolysis.First,the adsorption characteristics of CO₂,H₂O,CO,CH₄,H₂,and C₇H₈molecules on Ca O,10 wt%Ni/Ca O（NC）,10 wt%Fe/Ca O（FC）,and 10 wt%Ni-10 wt%Fe/Ca O（NFC）surfaces were explored by the simulation and CO₂-TPD experiments,and the adsorption performance of CO₂on the surface of different catalysts was obtained.Secondly,the molecular simulation and experiments of catalytic cracking of toluene,which is a model compound of tar,were carried out to investigate the catalytic H₂production and anti-carbon properties of the catalysts.Additionally,the molecular mechanism of toluene conversion was described.Then,the complex catalytic pyrolysis reaction network was analyzed.Based on the transition state（TS）theory,toluene catalytic cracking reaction,water gas shift reaction,methane reforming reaction and carbon reduction reaction were calculated to determine the rate determining step（RDS）and clarify the chemical reaction mechanism of different catalysts’surfaces.Finally,sawdust catalytic pyrolysis experiments were conducted to verify that NFC catalyst is suitable for efficient catalytic pyrolysis of biomass to produce H₂-rich gas,which is consistent with the results of simulation calculation.It provides theoretical guidance for the development of H₂production technology from biomass.The main research results are as follows:1.The studies on the adsorption performance of Ca-based catalysts for small molecules showed that the adsorption capacity of CO₂increased as Ca O<FC<NC<NFC.Ni and Fe doping in Ca O increased the activity of O_surfatom on the surface-active center,enhancing the adsorption capacity of CO₂.The CO₂-TPD experiment results were in accordance with the simulation results,indicating that the optimal adsorption temperature of CO₂for the four catalysts was 650℃.By comparing the adsorption energy of other molecules,it was found that H₂and CH₄were physically adsorbed on the surface of NFC,while other molecules were chemically adsorbed on the surface of NFC catalyst,and the order of adsorption capacity was CO<C₇H₈<CO₂<H₂O.2.The DFT simulation and experiments of tar catalytic cracking showed that Ni and Fe doping in Ca O improved the Ni electron activity and enhanced the catalyst activity.The C-H bond on the methyl of the toluene molecule in the NFC adsorption system was weakened and can be easily broken.The energy barrier of H₂production reaction of toluene cracking decreased,making the reaction more likely to process,while the carbon generation reaction is less likely to occur.At 650℃,the conversion rate of toluene cracking catalyzed by NC is 79.15%,while the conversion rate is increased by 23.95%under the catalytic action of NFC,and the H₂yield is increased by27.92%.When Fe was added to NC catalyst,it was not easy to form carbon deposition on the surface of catalyst and deactivation due to carbon deposition.3.The reaction network analysis of biomass catalytic pyrolysis for H₂production showed that the energy barrier of the four key reactions decreased after Fe doping in NC.These reactions were more likely to occur and therefore promoted the generation of H₂-dominated gas products.The order of the energy barrier of the RDS in NFC catalytic system is toluene cracking reaction<carbon reduction reaction<methane reforming reaction<water gas shift reaction.The reaction of toluene cracking is the easiest to happen.Compared with NC,and the energy barrier of carbon reduction reaction RDS on NFC surface decreased the most.4.The experimental results of NFC catalytic pyrolysis of sawdust for H₂production showed that the H₂concentration is the highest at 650℃in the range of 550～700℃,which is consistent with the results of thermodynamic equilibrium simulation.Compared with NC,the gas yield increased by 26.47%,tar yield decreased by 18.32%,while H₂yield and concentration increased by 18.29%and 4.50%,respectively,which verified the calculation results of DFT.NFC is suitable for H₂production because of the excellent performance of CO₂removal,catalytic tar cracking and anti-carbon deposition,the yield and concentration of H₂in H₂-rich gas production are 453.34 mL/g-biomass and 58.79 vol.%,respectively.