Study On Microwave-induced Biomass Pyrolysis Catalyzed By Biochar-Based Catalysts

With the increase of energy demand,the massive use of fossil energy has caused the crisis of fossil energy depletion,pollutant emission and greenhouse effect.And biomass,which is abundant,renewable,clean and carbon neutral,is one of the most promising alternative energies today.Biomass pyrolysis technology can convert biomass into high value-added syngas and bio-oil,which is currently the most effective way to achieve high value utilization of biomass.How to improve the efficiency of biomass utilization and further improve the quality of biomass pyrolysis products by improving heating methods and developing new catalysts is an important issue requiring urgent study.Therefore,in this thesis,the biochar-based catalysts with both high microwave absorption performance and high catalytic activity were synthesized from pine sawdust,a typical lignocellulosic biomass,and coupled with microwave heating technology to realize the upgrading of biomass pyrolysis products.The specific work is as follows:（1）Thermogravimetric analysis and microwave pyrolysis experiments at different temperatures were carried out on pine sawdust to elucidate the mechanism of pine sawdust pyrolysis and the reaction mechanism of its three main components,cellulose,hemicellulose and lignin during the pyrolysis process,as well as to systematically analyze its pyrolysis products.The main components of the gaseous products are H₂,CO,CH₄ and CO₂,while the chemical composition of the bio-oil products is extremely complex,with phenols as the main component,and also including hydrocarbons,alcohols,esters,aldehydes and ketones,ethers,nitrogen-containing compounds,furans,acids and little other compounds.（2）The effect of biochar pore structure on microwave-induced heating and its catalytic cracking biomass pyrolysis properties was investigated.Four biochar-based catalysts cum microwave absorbers for catalytic microwave pyrolysis of biomass were prepared by conventional and microwave heating methods as well as one-step activation of typical activators by microwave heating,respectively.The microwave heating was found to promote the formation of porous structure of biochar,while the activators Zn Cl₂ and KOH both played a significant activating effect on biochar,which promoted the production of a large number of micropores greatly increasing the specific surface area to 1110.1 and 1055.6 m² g^-1,respectively.All four biochar have excellent microwave absorption properties,can achieve rapid temperature rise under microwave conditions,and have strong catalytic activity for microwave-assisted biomass pyrolysis process,which promoted the improvement of gas yield and simplifies the chemical composition of bio-oil.Both microwave heating and activation effectively enhanced the catalytic performance of biochar,with KOH activation being the most effective in promoting syngas production,while Zn Cl₂activation improved the selectivity of H₂.（3）Characterization of microwave induced biochar loaded Fe catalyst catalyzed biomass pyrolysis was studied.Three types of biochar-based Fe catalysts with different Fe contents were prepared by Fe（NO₃）₃ solution impregnation and microwave one-step pyrolysis,andα-Fe crystals with high catalytic activity were successfully loaded on the surface of biochar with high crystallinity and average particle size of 15.17 nm.The performance of the catalysts in microwave-assisted catalytic pyrolysis of pine sawdust was experimentally investigated,and it was found that the loading of Fe enhanced the microwave absorption performance and significantly increased the catalytic activity.The catalytic performance of the catalysts was the strongest at 0.06 mol Fe（NO₃）₃/20g pine sawdust,which promoted the catalytic pyrolysis of pine sawdust to produce the most gaseous products with CO and H₂ yields reaching 304.2 m L/g and 171.4 m L/g at 650°C,respectively.It also achieved a significant simplification of the chemical composition of the bio-oil products as well as a catalytic effect of promoting the enrichment of phenolic compounds resulting in the relative content of phenols reached up to 60.7%,which were mainly monophenols.（4）The selective catalytic properties of microwave-induced biochar-based Fe-series alloy catalysts for biomass pyrolysis were investigated.Two biochar-based Fe-series alloy catalysts,biochar-based Fe Ni and biochar-based Fe Co,were prepared by introducing a second active metal in addition to Fe.Characterization showed that Fe_0.64Ni_0.36 and Fe Co alloy crystals were formed on the surface of biochar with an average particle size of 17.33 nm and 12.55 nm,respectively.it was found that the introduction of bimetals further improved the microwave absorption performance and catalytic activity through the microwave-assisted catalytic pyrolysis experiments of pine sawdust.Fe_0.64Ni_0.36 had the best catalytic performance for the production of syngas,with the highest total yield of combustible gases（H₂,CO and CH₄）of 574.8m L/g at 650°C,and also promoted the enrichment of hydrocarbons in bio-oil.As for Fe Co,it had the most significant selective catalytic effect on H₂ production,with a yield of 230.8 m L/g of H₂ at 650°C,and promoted the selective conversion of bio-oil components to phenols,resulting in a maximum relative content of 74.0%of phenols including 17.5%of phenol selectivity.