Study On The Reaction Mechanism And Regulation Method Of Bio-oil Deoxygenation During Biomass Pyrolysis Process With Ca-based Additives

Fast pyrolysis is one of the most promising methods for the conversion of abundant biomass.However,the target product?bio-oil?obtained from fast pyrolysis is not suitable for direct application in the existing petroleum-based infrastructure.The addition of catalysts into the fast pyrolysis process can upgrade the quality of bio-oil by the oxygen removal in the form of H₂O,CO or CO₂,but will lead to an inevitable decrease in bio-oil yield at the same time.Thus,the key to economically produce high quality bio-oil is to examine how the oxygen can be removed at the cost of less bio-oil yield drop.This dissertation presented a novel method for the balance between the quality and yield of bio-oil through selective decarboxylation,of which the reaction mechanism,regulation method and subsequent application were further investigated.The detalis are listed as follows:Firstly,the influences of acidic?Al₂O₃?,basic?CaO and MgO?and transitional?CuO,Fe₂O₃,NiO,ZnO,ZrO₂ and TiO₂?metal oxides and zeolite catalysts?HZSM-5 and MCM-41?addition on biomass fast pyrolysis after the optimization of reaction conditions are studied.Results indicated CaO,NiO and Al₂O₃ showed low deoxygenation index?DI,which means the mass loss of organic compounds per unit mass oxygen removal?and less oxygen was removed in the form of H₂O in these cases.In particular,CaO favored the decarboxylation reaction,and about 41wt.%O was removed in the form of CO₂.Furthermore,CaO showed the best performance in balance between the quality and yield of bio-oil when considereing its lower DI value?0.93?as well as the lowest moisture content?37.79%?and highest pH?4.8?of modified bio-oil.Secondly,the products evolution characteristics from the pyrolysis of three main biomass model components?cellulose,hemicellulose and lignin?in the presence of CaO were examined using PY-GC/MS and TG-FTIR.Results indicated that CaO would react with H₂O,CO₂ and organics?such as acids and phenols from hemilcellulose pyrolysis,sugars from cellulose pyrolysis and phenols from lignin pyrolysis?at lower temperature?400-600^oC?.At higher temperature?700^oC-800^oC?,the catalytic effect of CaO was more obvious,leading to the promotion of decarbonylation reactions of ketones from hemicellulose pyrolysis and the formation of aromatics was promoted at the same time.For cellulose pyrolysis,the presence of CaO would enhance the ring-opening reactions of sugars,and thus promoted the foramtion of oxygen-containing organics with small molecule weight.As to the pyrolysis of lignin,the addition of CaO would promote the radical reactions,and meanwhile,monohydric phenols with lowever carbon mumbers increased.Thirdly,the evolution of pyrolysis products was examined to propose the various roles of Ca at a range of temperatures and CaO addition ratios during biomass pyrolysis on a fixed-bed reactor.It is found that upon the addition of CaO,the content of ketones produced increased,while that of acidic compounds decreased.Under similar conditions,the concentration of evolved H₂ and CH₄ increased,while that of CO₂ decreased.Thus,variation in the CaO/biomass?Ca/B?mass ratios and pyrolysis temperatures indicated that CaO could act as a reactent,an absorbent,and a catalayst at Ca/B ratios of<0.2,>0.2,and>0.4,respectively.Moreover,at temperatures>600°C,the roles of CaO as an absorbent and a reactant were less apparent,while its role as a catalyst was enhanced.Then,pyrolysis-gas chromatography/mass-spectrometry?PY-GC/MS?and fixed-bed were used to identify the variation of CaO's perfermace as well as its cyclic stability caused by the removal of AAEMs using water and HCl washing method.It was found that the removal of inherent AAEMs in corncob increased a lot on the yield of bio-oil,and as a result,the influence of CaO on reducing the bio-oil yield increased.Moreover,the performace of CaO on removal of acids deteriorated due to the worse pore characteristics of CaO during repeated pyrolysis-calcination process.The pretreatment of corncob by water and HCl washing method could slow down the decrease in specific surface area of CaO,leading to a better cyclic stability of CaO on improving product quality.In addition,the influence of the types of calcium precursors on the perfermance of CaO-based additives in the pyrolysis of biomass was investigated on a fixed-bed.It was found that upon the addition of CaO-based additives?except the case that forsterite was the precursors?,the quality of bio-oil and gaseous products increased by dereasing the content of acids and CO₂,respectively.Higher specific surface area was favourable for improving products quality.However,the pyrolysis-calcination process would lead to steadily worse performance of CaO-based additive caused by the sintering phenomenon of CaO.The exsitentce of MgO was unfavourable for improving products quality but it could improve the cyclic stability of CaO-based additive.In general,calcium acetate and dolomite were better precursors.Finally,combined with the advantages of calcium based additive and HZSM-5 in deoxidization measures and deoxidization efficiency,respecvively,the high grade liquid fuel was prepared by co-catalysis,and the process mechanism was further discussed.Results indicated that the addition of suitable calcium based additives can promote the increase of specific aromatic hydrocarbons compared with the catalytic pyrolysis with HZSM-5 alone.On one hand,although the mass of the condensable compounds in the volatile was reduced,the content of acids with high oxygen content decreased with the addition of calcium based additive at the same time,promoting the conversion of volatiles to aromatic hydrocarbons.On the other hand,the addition of calcium based additive could increase the partial pressure of the H₂,and thus inhibited the condensation reaction and increased the yield of aromatic hydrocarbons.