Study On Catalytic Performance Of Biochar-based Nickel Catalysts Prepared By Collaborative Pyrolysis Of Pine And Nickel

Biomass tar is one of the byproducts of biomass pyrolysis.It has the problems of high acidity,high viscosity and low utilization efficiency,which is easy to cause the damage of equipment and the erosion of pipelines.Catalytic cracking is an effective way to transform the difficult-to-utilize tar into high value synthetic gas.Biochar can catalyze the cracking of tar,but the catalytic efficiency is limited.Nickel metal has high catalytic efficiency for tar,but it is easy to deactivate.The composite catalyst prepared by nickel and biochar can improve the catalytic efficiency and stability of the catalyst.At present,the collaborative pyrolysis of biomass and nickel is one of the preparation methods of biochar-nickel catalysts.However,the pyrolysis mechanism and catalytic activity of catalysts for heavy biomass tar have not been studied enough.In this study,the catalyst precursor was prepared by impregnating the mixture of nickel nitrate and pine,and the carbon-based catalyst supported by nickel was prepared by pyrolysis precursor system for catalytic cracking of biomass heavy tar.The preparation mechanism of the catalyst,catalytic tar activity,stability and regeneration of the catalyst were thoroughly studied.The main research contents and conclusions are as follows:（1）The kinetic behavior of pine and nickel co-pyrolysis was studied.TGA was used to analyze the thermodynamic characteristics of the catalyst precursor,and the kinetics of pyrolysis reaction was studied.The kinetic mechanism of the collaborative pyrolysis of nickel and pine was investigated.The results show that nickel nitrate is converted into nickel in the pyrolysis process.Under the influence of nitric acid oxidation and carbothermic reduction,the activation energy of catalyst precursor is higher than that of pine raw material.The addition of nickel will change the pyrolysis mechanism at high conversion rate（α>0.5）from the diffusion mechanism of pine pyrolysis alone to the reaction order mechanism of collaborative pyrolysis.Pyrolysis is an endothermic reaction,and the reaction speed is fast within the main temperature range of pyrolysis.The overall reaction is in an unstable state,and the addition of nickel will increase the complexity of pyrolysis reaction.（2）The preparation mechanism of catalyst and its catalytic effect on biomass heavy tar at low temperature were investigated.The catalyst was characterized by XRD,BET,SEM and FTIR to explore the preparation mechanism of the catalyst.In addition,in situ catalysis of biomass heavy tar was carried out in a tube furnace to explore the catalytic mechanism of tar.The results show that Ni²⁺is reduced to Ni particles at temperatures above 550℃.The abundant oxygen-containing functional groups and well-developed pore structure of biochar contribute to the dispersion of Ni nanoparticles.The catalyst pore size and Ni particle size were adjusted by changing the Ni loading ratio and pyrolysis temperature,and the pyrolysis temperature had a more significant effect on the Ni particle size.The average Ni particle size of PSNI-P-10（10%nickel loading,pyrolyzing at 650℃and holding for 30min）was the minimum 5.83nm,and the mesoporous and macroporous areas were the most abundant,with specific surface areas of 158.36m²/g and 204.13m²/g respectively.The tar conversion and H₂yield of PSNI-P-10 reached the maximum value of 62.17%and 0.130L/g respectively.The small metal particle size and abundant mesoporous and mesoporous structure of catalysts have obvious catalytic effects on tar cracking,Water gas shift and methane cracking.（3）The deactivation and regeneration mechanism of catalysts were investigated.The catalyst was pyrolyzed and regenerated in a tube furnace for seven cycles at 600℃.The catalyst was characterized by XRD,BET,Raman,FTIR,XPS and TGA.The results show that the carbon deposition on catalyst surface is the main factor leading to the decrease of catalytic activity.The carbon deposit on the catalyst surface will increase the Ni particle size,block the pore,cover the surface functional groups,and lead to the decrease of catalyst activity.The oxidizing substances produced by tar pyrolysis will oxidize part of Ni to Ni O,reducing the active site of the catalyst.The tar conversion rate decreased from 62.17%after one reaction to 28.77%after seven reactions,the H₂ production decreased from 0.130L/g to 0.022L/g,and the H₂/CO decreased from 1.56 to0.37.Carbon deposition on catalyst surface reduces catalytic efficiency and water gas shift efficiency.Regeneration can eliminate some carbon deposits and reduce Ni O to Ni by carbothermal reduction,effectively restoring catalyst activity.Tar conversion rate increased from 28.77%to52.95%,H₂yield increased from 0.022L/g to 0.101L/g,H₂/CO increased from 0.37 to 1.25.