Research On Hydrothermal Liquefaction Of Lignocellulosic Biomass With In-situ Hydrogen Donors And Product Characteristic

Biomass resources are regarded as an ideal alternative to traditional fossil fuels due to their advantages such as abundant reserves,renewable,carbon neutral,wide global distribution and low sulfur and nitrogen content.As a traditional agricultural country,China is rich in agricultural and forestry waste resources,which have great application potential.Biomass liquefaction has the advantages of high energy efficiency,mild reaction conditions,and can save the energy consumption from the pre-drying process of biomass feedstocks with high moisture,which has been considered as one of the most promising biomass conversion technologies for the production of bio-based renewable energy and high value chemicals.However,the bio-oil products from biomass liquefaction usually have defects of low bio-oil yield,high oxygen content,low energy density,high viscosity and corrosion,which are difficult to be directly applied in downstream applications.Therefore,some typical lignocellulosic biomass feedstocks are taken as the research objects in this paper,focusing on achieving in-situ hydro-liquefaction of biomass using in-situ hydrogen donors to produce bio-oil products with high yield and quality.To standardize the extraction and recovery process of bio-oil products in this work,the effects of extraction solvents（acetone,ethanol,dichloromethane and ethyl acetate）on biomass liquefaction in reaction medium（water,ethanol and water-ethanol mixed solvent）are studied,and the product separation steps are optimized according to the characteristics of various extraction solvents.It is found that the yield and quality of bio-oil are affected by both reaction medium and extraction solvents.When pure reaction medium（pure water or ethanol）is used in the experiment,there is no significant difference in bio-oil yield and elemental composition of the bio-oil products recovered by acetone,dichloromethane or ethyl acetate,indicating that the selection of optimal extraction solvent is flexible.When using water-ethanol mixed solvent as the liquefaction reaction medium,the bio-oil yield recovered by acetone is significantly higher than those from the other three extraction solvents.Therefore,although the bio-oil extracted by acetone has the highest oxygen content and the lowest heating value,the highest energy recovery and carbon recovery of bio-oil are achieved by acetone,showing the advantages of acetone as the extraction solvent of bio-oil product.Moreover,the analysis results show that the higher bio-oil yield obtained by acetone is mainly due to its stronger ability to dissolve the macromolecular components in bio-oil products,while the while the molecular weight and boiling point distribution of bio-oil extracted by dichloromethane are the lowest,indicating its best application quality.Considering the research objects of this paper,acetone is finally selected as the extraction solvent of bio-oil products in the subsequent chapters.After determining the optimal extraction solvent and products separation procedure,the effects of formic acid addition and reaction conditions on the products distribution from biomass liquefaction products are investigated in this paper.The effects of different reaction media and typical hydrogenation catalysts are also studied along with formic acid to construct an optimal reaction system for one-step hydro-liquefaction of biomass.It is found that formic acid can significantly improve the yields of bio-oil products,and the promotion mechanism of formic acid is obviously different from that of high-pressure gaseous hydrogen.The combination of water-alcohol mixed solvent and formic acid can obtain the highest bio-oil yields.Besides,the addition of hydrogenation catalyst can further increase the bio-oil yields and the biomass conversion rate.When FHUDS-2catalyst is used with formic acid,the highest bio-oil yield of 62.38 wt.%is obtained,while the biomass conversion can be over 99 wt.%under appropriate reaction conditions.The analysis results show that formic acid can increase the C/H content and heating value of bio-oil through in-situ hydrogen donation.The combination of formic acid and hydrogenation catalysts can obviously improve the C/H content and the hydrocarbon content in the bio-oil products through catalytic hydrodeoxygenation,reducing the average molecular weights and boiling point distribution of bio-oils and improving their application quality.The catalytic performance of Ru/C catalyst on catalytic hydrodeoxygenation is better than FHUDS-2 catalyst during biomass liquefaction.Hydrothermal co-liquefaction of lignite and lignocellulosic biomass in the presence of formic acid is investigated in this work.The effects of experimental parameters and the synergistic interaction between lignite and biomass on products distribution and characteristics are studied in details.The results indicate that oil production is significantly enhanced by the addition of FA,while the oil yields obtained from co-liquefaction of L/CS and L/PW reach 29.34 wt.%and28.36 wt.%under the optimal condition,respectively.With the assist of formic acid,the synergistic effects between lignite and biomass not only increase the oil yield but also promote the carbon and energy recovery in oil products.In addition,the produced oil products from co-liquefaction have similar boiling point fractions as those of the biomass-derived oil,and contain mainly hydrocarbons,phenols and ketones.The in-situ hydro-liquefaction process of biomass in pure water or water-ethanol mixed-solvent with the assist of zero-valent metal iron（Fe）is studied in this work,exploring the mechanism of Fe in promoting the bio-oil production.The results show that the addition of Fe can obviously increase the bio-oil yield,while the bio-oil yield and the biomass conversion rate reach 50.46 wt.%and95.29 wt.%under the optimal reaction condition.Moreover,the analysis results reveal that the presence of Fe can improve the C/H content and the heating value of bio-oil products.When using water-ethanol mixed solvent as the reaction medium,the positive role of metallic Fe during biomass liquefaction is believed to be related to the in-situ hydrogen formed from the reaction of Fe and water,but Fe could not react with ethanol in the reaction medium and the generated Fe₃O₄exhibits negligible catalytic effects on the bio-oil production.The effects of the combination of Fe and typical liquefaction catalysts on the hydrothermal liquefaction of pine sawdust are investigated in this work.The experimental results show that the promotion effect of metallic Fe on the bio-oil production is far greater than that of gaseous hydrogen,indicating that the in-situ hydrogen produced through the reaction between Fe and subcritical water can directly interact with biomass fragments and thus give better hydrogenation efficiency.With the same additive dosage（10 wt.%）,the bio-oil yield obtained with Fe+alkaline catalysts or Fe+FeS is higher than those from single additive,while the maximum bio-oil yield（45.58 wt.%）and energy recovery（81%）from bio-oil are given by Fe+Na₂CO₃.In addition,using Fe can improve the C/H contents of bio-oil,while the combination of Fe and Ru/C produce bio-oil product with the highest C/H contents and the heating value（30.93 MJ/kg）,indicating the catalytic role of Ru/C in hydrodeoxygenation reaction with the assist of Fe.Furthermore,the combination of Fe and alkaline catalysts can increase the contents of aromatic and phenolic compounds in the bio-oil products,and the addition of Fe+Ru/C or Fe+FeS is beneficial to the formation of hydrocarbon and can reduce the content of oxygenated compounds in bio-oil products.Compared with the blank group,the bio-oils produced with Fe and catalysts totally have lower average molecular weights and higher volatile component contents,and thus the application quality of bio-oil products is greatly improved.