| Biomass is an important renewable energy resource,whose development and utilization are of great significance for alleviating energy shortages and environmental pollution.However,the low-grade energy characteristics of biomass,such as high oxygen content,high water content,low energy density,high alkali metal content,high moisture absorption capability,and high tendency to spontaneous combustion,have restricted the development and commercialization of biomass energy.Deoxygenation and upgrading of biomass prior to its utilization,then achieving the high-value application of the upgrading products is an effective method for efficient utilization of biomass.Unlike conventional biomass deoxygenation and upgrading technologies?such as torrefaction and hydrothermal carbonization?,the proposed degradative solvent extraction method can simultaneously realize the deep deoxygenation and deashing of biomass,and the extraction products show superior properties such as high carbon content,low oxygen content,almost no ash and moisture,unique thermoplasticity,making this method highly promising.Up to now,few studies are available for degradative solvent extraction of biomass.In order to promote the development and industrial application of biomass degradative solvent extraction technology,this thesis conducted a comprehensive research based on three important processes?preparation of extraction products,storage or transportation of extraction products,and high-value utilization of extraction products?in degradative solvent extraction of biomass.The key scientific and technological issues involved are:?1?The chemical conversion mechanism of biomass in degradative solvent extraction process and detailed kinetic analysis and quantitative description remain unclear;?2?The moisture readsorption characteristic and spontaneous combustion propensity of extraction products during storage or transportation are unknown;?3?Based on the unique and superior physicochemical properties of the extracted products,the high-value utilization methods and application fields of the extracted products are not clear.To solve the above issues,the in-depth researches were carried out in this study:?1?Systematic experiments were conducted to investigate the effect of different operation condition on the yield variation of extraction products,and a variety of analysis methods were dopted to analyze the physicochemical properties and chemical structure changes of the extraction products.Based on these results,the reaction mechanism of the degradative solvent extraction process was proposed.The results showed that the conversion process of degradative solvent extraction of biomass can be divided into two stages.The first stage occurred at the heating-up stage from 250 oC to 350 oC and the start of the isothermal stage at 350 oC.Severe deoxygenation reactions and aromatization reactions took place at this stage,and the main product was the high-molecular-weight extract?Deposit?.At the second stage,part of Deposit was converted into low-molecular-weight extract?Soluble?.At this stage,Deposit experienced complex reactions such as the cleavage of oxygen-containing cross-links and further aromatization reactions,and the oxygen in Deposit was mainly removed as H2O.?2?On the basis of the experimental and qualitative reaction mechanism,the reaction model of degradative solvent extraction of biomass was established.The differential equations were established based on the model to simulate the process.The Matlab software was used to numerically solve the differential equations and to obtain the kinetic parameters of each reaction.The results showed that the modelling results correlated well with the experimental results,and the quality of the proposed model is as high as 96.59%.The degradative solvent extraction process of biomass under different temperatures belonged to different reaction stages:the dominant reaction at 300 oC was the conversion of unreacted biomass to high-molecular-weight extract?Deposit?,low-molecular-weight extract?Soluble?,and liquid product?Liquid?;while the conversion of Deposit to Soluble is the controlling reaction at 350 oC.?3?The moisture readsorption characteristic of Soluble and Deposit was investigated in constant temperature and humidity environment.The TGA-DSC method was adopted to explore the self-heating and spontaneous combustion tendency of Soluble and Deposit,and the kinetic parameters of spontaneous combustion of extraction products were obtained.The results showed that the moisture readsorption capacity and spontaneous combustion tendency of the extraction products were greatly inhibited.The extraction products had higher weight loss temperature range,exothermic onset temperature,greater ignition temperature,and greater activation energy for low-temperature oxidation reaction.The dense surface structure and low oxygen-containing functional groups content reduced the moisture reabsorption capacity and spontaneous combustion tendency of the extraction products,making the extraction product more safer during the storage or transportation process.?4?Based on the high carbon content and thermoplasticity of the extraction products,the extraction products was used as coke-making additives and the effect of the extraction products on the thermoplastic properties of coking coal and the quality of produced coke were investigated.The results showed that the higher H/C atomic ratio and the lower oxygen-containing functional group in the Soluble or Deposit make it provide more hydrogen donors to stabilize the free radicals generated by the thermal cracking of coking coal,leading to only 2%addition of Soluble or Deposit to coking coal can significantly improve the thermoplasticity of coking coal.Soluble or Deposit addition can notably increase the coke strength after reaction?CSR?,and reduce the coke CO2 reactivity index?CRI?,thereby markedly improving the quality of coke.?5?A two-stage conversion method?combining degradative solvent extraction with subsequent pyrolysis?to prepare bio-oil was proposed,and the effect of this two-stage conversion method on the properties and composition of bio-oil was studied.The results showed that the bio-oil prepared by Soluble pyrolysis had extremely higher carbon content?up to 90%?,calorific value?44.63 MJ/kg?,aromatic hydrocarbon content,extremely lower oxygen content,water content?less than 1%?and oxygen-containing compounds content than that of biomass.The quality and added-value of bio-oil were both significantly improved.Compared with traditional bio-oil preparation or upgrading technologies,the proposed two-stage conversion method produces bio-oil with high quality and added-value,requiring neither hydrogen nor catalysts,showing a good application prospect.In summary,this study provided new idea and theoretical foundations for the deoxygenation and upgrading of biomass waste and the efficient utilication of its products,laying the foundation for the development of the degradative solvent extraction method of biomass. |
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