Research On The Free Radical Reaction Mechanism And Directional Regulation Of Pyrolysis Of Lignocellulosic Biomass Into Liquid Fuels

Biomass has been extensively studied as the only renewable energy source that can be converted into carbon-containing liquid fuels.Among the technological pathways for the production of liquid fuels from biomass,pyrolysis is the technology that can efficiently convert dispersed biomass into liquid bio-oil that can be easily stored and transported in a continuous and industrialized production mode.The bio-oil can be further refined to produce high-quality liquid fuels such as aviation,gasoline and diesel.However,due to the disadvantages of low calorific value,poor thermal stability,high corrosiveness and inability to be directly utilized of the bio-oil produced by traditional pyrolysis technology,more advanced pyrolysis technologies have been developed,and the current problems still include unclear fundamental issues such as the thermal depolymerization characteristics of biomass macromolecules in inert or hydrogen atmosphere,the conversion of reactive radical intermediates,the low carbon yield of the system,and the serious deactivation of the catalyst.By studying the key radical intermediates in biomass pyrolysis,it can help to reveal the reaction mechanism in biomass pyrolysis,gradually construct the complete pathway of reactants-intermediates-products,explore a more effective way of product regulation,and provide an important theoretical basis for the study of the key technologies required for the preparation of high-quality liquid fuels from biomass pyrolysis.With the lignin fraction accounting for about 10-35%of the weight of the biomass feedstock and about 40%of the energy of the biomass feedstock,the conversion of lignin into chemicals or fuels would greatly improve the economics of the whole component utilization process of biomass.Focusing on the important monomer model of lignin,guaiacol,combined with a fluidized sand bath system and a microreactor,based on the characteristics of EPR low-temperature measurement that can extend the life of free radicals,reduce line width,and improve detection accuracy,pyrolysis-Experimental detection and analysis of pyrolytic free radicals of guaiacol using freeze trapping（PFT）and pyrolysis-spin trapping（PST）methods.The complete guaiacol pyrolysis process was divided into three main reaction stages.Through mutual verification of freeze capture and spin capture,the changes in free radicals at each stage of the guaiacol pyrolysis process were observed,and the spectrum was successfully analyzed..The main free radicals observed in the pyrolysis of guaiacol are:methyl,hydroxyl,methoxy,o-phenylsemiquinone,cyclopentadienyl,phenyl and phenoxy,among which the semiquinone free radical is thermal The key free radicals that form coke in the middle stage of pyrolysis,and the cyclopentadienyl free radicals are the key free radicals that form tar in the middle and late stages of pyrolysis.In addition,the hydrogen supply source plays a key role in the deconstruction of methoxy groups and the inhibition of semiquinone free radicals during the pyrolysis of guaiacol,which is beneficial to the ideal yield and quality of the pyrolysis liquid phase product.The monomer model used in the microscopic level study was reasonably enlarged to explore the cleavage mechanism of the main connecting bonds of lignin with the dimer model that is closer to the real component.By modeling the typical dimer models ofα-O-4 andβ-O-4,the most abundant linkages in lignin,BPE（α-O-4）,PPE（β-O-4）and GGE（β-O-4）EPR in-situ pyrolysis free radical detection and analysis,combined with the distribution rules of pyrolysis products,reveals the free radical reactions of relevant side chain groups（methoxy,etc.）in the pyrolysis reaction path from the microscopic molecular level mechanism.The results show that benzyl and phenoxy groups can abstract hydrogen at the benzyl position of BPE or bibenzyl to form toluene and phenol,respectively.The EPR signal is mainly related to carbon-centered benzyl radicals and oxygen-centered phenoxy radicals.Phenoxy radicals are considered to be the precursors to the major pyrolysis products via hydrogen abstraction reactions.When abundant hydrogen radicals are present in the pyrolysis system,these homolytic radicals can be further hydrogenated to form stable products,otherwise they would polymerize to form coke.During the preliminary pyrolysis process,the O-CH₃ bond homolysis reaction occurs preferentially.The free radicals generated by the homolysis will undergo hydrogen transfer reactions with the functional groups of the initial reaction product during the secondary reaction to generate stable hydrogenation products,hydrogen radicals.It will promote the removal reaction of the benzene ring side chain substituent.The cleavage of the C-O single bond in the methoxy group dominates at high temperatures,and the methoxy substitution on the aromatic ring increases the reactivity of the homolytic cleavage of the C-O single bond.Methoxy-substituted structures can produce many different free radical species and participate in a variety of reaction mechanisms,leading to complex reaction pathways and pyrolysis products.Among them,methoxy substitution on the aromatic ring significantly promotes oligomerization to form large molecular weight compounds.The free radical reaction mechanism during the pyrolysis of actual biomass poplar wood,enzymatic lignin,and microcrystalline cellulose was further studied,and the free radical stability in bio-oil and biochar was also discussed.The results show that enzymatic lignin pyrolysis is mainly divided into three stages（initial stage,primary stage and carbonization stage）,in which chemical bonds such as C-C and C-O-C bonds can be cleaved to produce phenolic and aromatic compounds.Hydrocarbons and functional groups such as carboxyl,carbonyl,and methoxy groups will escape from the benzene ring to form some light gases.Finally,lignin biochar with rich functional groups（such as-OH and-OCH₃）is generated through carbonization.The carbonization mechanism of lignin pyrolysis shows that the methoxy group,as an electron-donating group,does not promote the repolymerization reaction,while the phenolic hydroxyl group,as an electron-donating group,promotes the repolymerization reaction by affecting the electron cloud density;methoxyl group and phenolic hydroxyl group The intramolecular reaction between them significantly promotes repolymerization,and the promotion of char formation is proportional to the amount of the ortho-methoxy group of the phenolic hydroxyl group.Based on the in-depth study of the mechanism of biomass pyrolysis,the research on the catalytic mechanism of hydrogenation pyrolysis was explored.Nine kinds of doped catalysts with Al₂O₃-Ti O₂-Si O₂（ATS）as composite carrier and different metal loading amounts of Co,Ni,and Mo were prepared.The changes and content of free radicals during poplar pyrolysis under different working conditions were studied through EPR.A doped catalyst（Me-2）with high conversion efficiency and resistance to coking was screened out.A long-term continuous laboratory-scale HPVU system was built to conduct experimental verification and product analysis on three routes of poplar hydrocatalytic pyrolysis,hydropyrolysis online upgrading and hydrocatalytic pyrolysis online upgrading under different pressures.The results showed that the carbon conversion efficiency of bio-oil ranged from 32.24%to 41.19%under all test conditions of the three hydrogenation routes.The CHP-1.0 condition has the highest carbon yield,reaching 41.19%,and the mass yield of C4+hydrocarbons is also the highest,reaching23.81%.Under CHPVU-2.0,the carbon deposit on the catalyst was as low as 1.56 wt.%,the hydrogen consumption was 55.94 grams of hydrogen per kilogram of biomass,the average HHV value of bio-oil after hydrogenation reaction was 44.47MJ/kg,and the TAN value was also lower.The biomass hydropyrolysis upgrading route has the significant advantages of high hydrocarbon yield and slow catalyst deactivation rate.