| With increasing depletion and shortage of fossil resources,the conversion of heavy organics(for example,biomass,oil shale,heavy tar)to liquid fuels and high-value chemicals,to replace fossil products has aroused extensive attention.Moreover,the unique situation that China is rich in the reserve of coal but poor in the reserves of petroleum and natural gas,determines the necessity for China to develop the efficient and clean conversion technologies of heavy organics.Biomass is the only renewable heavy organics that exists widely in nature.The technology of conversion of biomass to liquid fuels(bio-oil)and high-value chemicals is still developing slowly,and the present utilization of biomass in China is still highly wasteful.Biomass is a kind of macromolecular polymer constituted by different covalent bonds.Its thermal depolymerization follows the free radical mechanism.That is,the process of covalent bonds cleavage to produce free radical fragments,and free radical fragments reaction to produce bio-oil,gas and bio-char.At present,the key scientific problem is that the pyrolysis reaction of biomass is extremely complex,and the understanding of the reaction is not comprehensive.Decoupling pyrolysis reaction of biomass is the basis of optimizing reactor design and pyrolysis technology,and achieving efficient utilization of biomass.Therefore,this dissertation has studied four aspects of work,and the scientific issues,research methods and main conclusions are the followings:(1)Volatiles reaction is unavoidable during biomass pyrolysis and significantly influences the product yields,bio-oil quality and coking on the reactor wall.In any reactor,biomass pyrolysis temperature is different from volatiles temperature and the latter is higher than the former(about 10-300?).However,it is difficult to decouple the pyrolysis reaction and volatiles reaction.Based on this,a two-stage reactor was designed in the chapter 2 to decouple pyrolysis reaction and volatiles reaction.The first stage is for corn stalk pyrolysis to generate volatiles at 600? and the second stage is for volatiles reaction at volatiles temperature of 440-650? and volatiles residence time of 1.5-4.7 s.Results show that the volatiles reaction also generates a few coke(tetrahydrofuran-insoluble compounds),which can be divided into coke-wall and soot-oil according to the deposition position.With increasing volatiles temperature and volatiles residence time,the coke-wall yield increases gradually,and it is much higher than the soot-oil yield,indicating that the coking behavior during volatiles reaction is serious.With increasing the volatiles temperature,the bio-oil quality is improved but yield decreases.Effect of volatiles residence time on the bio-oil quality depends on the volatiles temperature:becoming worse at 440? and better at 600? with increasing the volatiles residence time.(2)Biomass pyrolysis by solid heat carrier(SHC)is the SHC are preheated to a designated temperature and then mixed with the biomass particles to make them fast pyrolysis,which generally regarded to be efficient in heat transfer,high in bio-oil yield and easier for reactor scale-up.However,the transient behaviors in heat and mass transfer and complicate reactions,especially the volatiles reaction in SHC phase,make it difficult to understand the relations between operating conditions and the conditions of actual reactions and consequently the yield and composition of the products.Based on this,a small hermetic reactor,simulating a finite element in a large pyrolysis reactor and fully collecting all the pyrolysis products,is designed for a 4 mm layer of walnut shell(WS)pyrolysis with quartz sand(QS)as the SHC to decouple the WS pyrolysis and volatiles reaction.The average temperature of the WS layer(TES,WS pyrolysis temperature)and the highest temperature of the QS layer(TQS-o,volatiles temperature)under various QS preheating temperatures(PT,600-900?)and SHC-to-biomass mass ratios(Rs-b,5:1-9:1)are determined and decoupled the effects of TWS and TQs-o on WS pyrolysis and volatiles reaction.The results show that TWS and TQs-o depends linearly on PT and Rs-b,and their difference varies from 39 to 108?.A reaction matrix is proposed to decouple oil formation(WS pyrolysis)and conversion(volatiles reaction),and the oil yield is modeled by first-order kinetics,which decouple the roles of TWS in WS pyrolysis and TQs-o in oil reaction.It is found that the WS pyrolysis is strongly influenced by heat transfer from QS to WS with activation energy of 46.4 and 19.2 kJ/mol for varying PT and Rs-b,respectively.The oil reaction in the QS layer reduces oil yield and is dominated by the reaction of radical fragments that is weakly dependent on temperature with activation energy of only 4.8 and 2.9 kJ/mol for varying PT and/Rs-b,respectively.(3)Lignin is a three-dimensional network of polyphenolic macromolecular,which is cross-linked by phenyl propane units through aryl ether bonds and aliphatic carbon-carbon bond.The structural characteristics of lignin make it possible to prepare high-value monophenols.The selectivity of lignin pyrolysis to produce monophenols is low,and the hydro-liquefaction technology can be used to selectively produce monophenols by directional control reaction.However,the mechanism of lignin hydro-liquefaction is still unclear,which makes it difficult to form a systematic step depolymerization method of lignin and understand the formation mechanism of monophenols.Therefore,DHP(030 mg)was used as hydrogen donor solvent for hydro-liquefaction of DAL(5 mg)in a closed glass tube reactor(3 mm in inside diameter and 33 mm in length)in the chapter 4.The product yields,quantity of hydrogen donated by DHP and liquid composition(guaiacols)was analyzed under different reaction temperature(320-440?),reaction time(1-30 min)and DHP-to-DAL mass ratios(Rs-d,0:1-6:1)to explore the formation path of guaiacols products.Moreover,guaiacol was used for hydro-liquefaction experiments to explore the conversion path of guaiacols intermediate products.Results show that the formation of guaiacols originated from two pathways:direct pyrolysis of DAL and re-pyrolysis of some intermediate products.The hydrogen donation mainly promotes guaiacols formation under QH?5.7 mmol/g and will inhibit guaiacols formation under QH>5.7 mmol/g.However,the hydrogen donation is conducive to the stability of guaiacol under QH>0.19 mmol/g and will promote guaiacol conversion under QH?0.19 mmol/g,and more guaiacol conversion into catechol at reaction temperature>350?,little guaiacol conversion into phenols and cyclohexanone at reaction temperature>400?.(4)There are two main paths for the generation of solid resuidues from DAL hydro-liquefaction:the condensation of solid-phase or volatile radical fragments and coupling of volatile radical frogments with hydrogen radicals.The effect of hydrogen donation on the two reactions and the difference in the composition and structure of the solid residues produced by the two reactions need to be further understood.Therefore,Chapter 5 adopts the experimental scheme of Chapter 4,and the solid residues were divided into CS2-ins(carbonaceous solids and pitch)and THF-ins(carbonaceous solids)by CS2 and THF as extraction solvents to compare the generation mechanism and structure differences of the same components(carbonaceous solids)and different components(pitch)in CS2-ins and THF-ins under the action of hydrogen donation.Results show that hydrogen donation mainly affects the condensation reaction of voaltile radical fragments(volatiles reaction),and has a little effect on the condensation reaction of solid-phase radical fragments.Moreover,the main product of the condensation reaction of solid-phase or volatile radical fragments is carbonaceous solids,and the main products of the coupling of volatile free radical fragments and hydrogen radicals(pitch and CS2-s)vary with the change of reaction conditions.When the reaction temperature is 350? and reaction time is 10 min,the product of the coupling of volatile radical fragments and hydrogen radicals is mainly pitch and CS2-S when QH?5.7 mmol/g(Rs-d ? 2:1)and 5.7 mmol/g<QH<6.9 mmol/g(2:1<Rs-d?6:1),respectively.The increasing of reaction temperature and reaction time mainly aggravated the thermal cracking reaction of DAL.When the Rs-d is 2:1,reaction temperature is 350? and reaction time is within 1-30 min,the main product of the coupling of volatile radical fragments and hydrogen radicals is CS2-s;when the Rs-d is 2:1,reaction time is 10 min and reaction temperature is less than 400?,the coupling product of the volatile radical fragments and hydrogen radicals is mainly pitch;when the reaction temperature is higher than 400?,due to the limitation of hydrogen supply,the condensation reaction between the volatile radical fragments is mainly generated carbonaceous solids.Compared with carbonaceous solids,the content of hydroxyl(phenol/alcohol)is higher and the content of aromatic ring structure and aryl ether bond(Car-O-C)is lower in pitch,and the condensation degree of pitch is smaller. |
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