Investigation Of The Evolution And Coking Characteristics Of Bio-oil During Thermal Treatment Based On The Interactions Among Bio-oil Components

Most bio-oil conversion processes require heat treatment of bio-oil.The knowledge about the evolution of bio-oil during its thermal treatment is the foundation for the efficient use of bio-oil.Bio-oil has a complex composition,which consists of hundreds of compounds.Most of these compounds(e.g.acids,aldehydes,ketones,phenols,sugars,and lignin-derived oligomers)are quite reactive.When heated,the complex interactions among the bio-oil components will take place.Therefore,the interactions among bio-oil components cannot be ignored during the thermal treatment of bio-oil.Meanwhile,as the critical operational parameters,temperature and heating rata can affect the conversion of bio-oil significantly,and how the temperature and heating rata affect the interactions among bio-oil components is also not clear,which deserves the in-depth research.On the other aspect,coke is easy to be formed during the thermal treatment of bio-oil.For example,during the catalytic conversion of bio-oil,the coke deposition can block the reactor and deactivate the catalyst.Obviously,the coke formation during the thermal treatment of bio-oil is one of the bottleneck problem which hinders the large-scale use of bio-oil.Similarly,the interactions among the bio-oil components also affect the coke formation from bio-oil.The key to solve this problem is to understand the evolution of bio-oil during its thermal treatment,and to clarify the coke formation mechanisms.Therefore,it could be possible to optimize the process conditions or modify the bio-oil composition to control the coke formation.Therefore,this thesis paper aims to investigate the evolution and coking properties of bio-oil during the pyrolysis at different temperatures and heating rates,to analyse the effects of the interactions among bio-oil components on the evolution of bio-oil.Further to this,this paper also aims to clarify the reaction routes and coke formation mechanisms which involve the effects of the interactions among bio-oil components during the thermal treatment of biooil.The key findings of this thesis paper are as follows:In this study,a bio-oil sample was pyrolyzed in a fixed-bed reactor at temperatures(300-800°C)and heating rates(0.33-200°C/s)to investigate the effects of the temperature and heating rate on the evolution of bio-oil.The results indicate that slow heating rates promote polymerization of bio-oil components,particularly at low temperatures(< 500°C),resulting in higher primary coke yields than that of the fast heating rates.Therein the condensed aromatics are formed via the polymerization of small aromatics with one or two rings,especially at slow heating rates.At high temperatures(> 500°C),the secondary reactions were promoted,causing the decreases in the tar yields and abundance of light compounds,the increases in the yields of the secondary coke,and the formations of more condensed aromatic structures.The cokes formed from the pyrolysis of bio-oil at different temperatures and heating rates were characterized by using a range of advanced analytical instruments.The coke formation mechanisms were clarified.The results show that the cokes generated at slow heating rates are imporous with smooth surface but porous at high temperatures and fast heating rates.The radicals in the cokes can be active at high temperatures to promote the condensation reaction of aromatic systems to form larger ring structures with lower H/C and O/C ratios.The O-containing functional groups could be brought into the coke via the interactions between light and heavy components of bio-oil.The bio-oil sample was separated into aromatic-rich fraction(ARF)and aromatic-poor fraction(APF)via the n-hexane extraction.The bio-oil sample and its extracted fractions were pyrolyzed in a fixed-bed reactor at 300-800°C at different heating rates.The results show that the pyrolytic products(including the yields and aromatic structures)from raw biooil and its extracted fractions are significantly different,which proves the existence of the interactions between the aromatic components and light components of the bio-oil.Owing to the presence of the aromatic-poor fraction,the aromatic compounds(especially ≥ 2 rings)from the pyrolysis of bio-oil are less than that of the aromatic-rich fraction at relatively low temperatures(≤ 500°C),especially at slow heating rates.This is because the interactions among ARF and APF promotes the transformation of more aromatic compounds into coke.At fast heating rates,among the complex interactions,the self-gasification of bio-oil is intensified at high temperatures(≥ 700°C),resulting in lower secondary coke yields and tar yields as well as the concentration of aromatic compounds(especially ≥ 2 rings).The cokes formed from the pyrolysis of ARF and APF at different temperatures and heating rates were characterized.The coke structures of oil-coke,ARF-coke and APF-coke were compared to illuminate the effects of interactions on the coke formation.The results show that the interactions among light compounds and aromatic compounds could lead to the formation of the cross-linking structures of the coke,which caused the different structures of oil-coke,ARF-coke and APF-coke.At temperatures ≤ 400°C,the aromatics which cannot form coke individually can be transformed into coke via the intermediates among light compounds and aromatics.The APF-cokes are sponge-like while the ARF-cokes have a dense structure.The matrix of oil-cokes is similar to the matrix of ARF-cokes,while its surface is similar to that of APF-cokes,which should be due to the interactions between different biooil fractions.Moreover,the interactions between ARF and APF can promote more Ocontaining species to be transformed into the oil-cokes.In order to further understand the interactions among bio-oil components during thermal treatment,the interactions take place in gas-phase and liquid-phase are decoupled and studied separately.The results show that the gas-phase interactions could promote the formation of coke via the cross-linking polymerization between the light compounds and aromatic compounds during the pyrolysis at 300-400°C,and the tar yields and content of the aromatics in the tar are decreased.During the pyrolysis at 500-800°C,due to the cross-linking polymerization of the volatiles promoted by the gas-phase interactions,more large molecules are generated,thus the tar yields and the coke yields are increases,as well as the aromatics with more than 3 rings.Additionally,the increasing percentage is higher at higher temperature.The liquid-phase interactions can promote the cross-linking polymerization between the light compounds and aromatic compounds to form the non-evaporable large molecules,leading the increase in the heavy tar yields and decrease in the light tar yields.Meanwhile,the liquid-phase interactions can promote the formation of the primary coke and inhibit the formation of the secondary coke,and promote the formation of the O-containing functional groups in the coke.