Process Investigation On The High Efficient Conversion Of Poplar Sawdut To Bio-Oil Via Catalytic Hydrothermal Liquefaction

The depletion of fossil resources caused by over-exploitation and consumption has become one of the main problems threatening global energy security.Lignocellulosic biomass utilization is an effective way to reduce carbon emissions and improve energy structure in the world.Among them,the use of biomass thermochemical conversion technology can effectively prepare biofuels and chemicals.The hydrothermal liquefaction technology is favored by many researchers due to the advantages of no drying treatment and high liquefaction strength.In order to realize the preparation of high-yield and high-quality bio-oil,this paper uses poplar sawdust as raw material to explore the influence of parameters including temperature,time and catalyst on the hydrothermal liquefaction process.TGA,FT-IR and other analytical techniques to identify the product,and at the same time speculate the possible reaction path of the main components of poplar,including the following three aspects:（1）The influence of H₂O₂,NaOH and H₂O₂/NaOH catalytic systems on hydrothermal liquefaction process was investigated at different liquefaction temperatures.The results showed that under the catalysis of NaOH（35 g/L）/H₂O₂（30g/L）at 280℃,the highest yield of total bio-oil was obtained,which was 70.65%.Compared with the catalytic conditions of only NaOH（35 g/L）or H₂O₂（30 g/L）,the total bio-oil yield was increased by 22.96%and 42.45%,respectively.At the same time,the obtained bio-oil had the smallest average molecular weight,which was 237g/mol.Bio-oil mainly contains aromatics,such as 2,6-dimethoxyphenol,catechol,o-xylene,p-xylene,3-methoxy-1,2-benzenediol,etc.;acids such as lactic acid,2-hydroxy-2-methylpropionic acid and 2-hydroxy-2-methylbutanoic acid,etc.;alcohol compounds such as 3-pentanol,cyclopropanol,2,2-dimethyl-1,3-propylene glycol and 4-penten-2-ol.The synergistic catalysis of alkaline hydrogen peroxide promotes more small molecular acids and alcohols to stay in the oil phase,reducing the conversion of gaseous products,and a small number of nitrogen-containing compounds come from the Maillard reaction between reducing sugars and amino acids.（2）A comparative study on the gradient and direct hydrothermal liquefaction of poplar was carried out.The liquefaction process was divided into three temperature sections,220°C,260°C and 300°C,and the residence time（10 min,20 min and 30min）was optimized.In the gradient liquefaction for 20 min,Na₂CO₃and Na₂CO₃/Fe catalyzed the highest total oil yield,50.46%and 57.09%,respectively,and the highest energy recovery,74.72%and 77.10%,respectively.Compared with direct hydrothermal liquefaction,the use of gradient liquefaction technology can effectively improve the total bio-oil yield.The results of GC-MS analysis showed that under the condition of gradient liquefaction,Na₂CO₃catalysis significantly increased the content of dibutyl phthalate in bio-oil（the highest yield was 83.88%）,and Na₂CO₃/Fe catalysis was more favorable for phthalic acid,butyl 2-ethylhexyl ester（43.33%highest yield）and phthalic acid,diisobutyl ester（21.04%highest yield）were produced.Cellulose and hemicellulose undergo dehydration and isomerization of monosaccharides to form cyclic ketones,which are further dehydrated and condensed with lignin cracking products to form corresponding esters.According to the analysis results of GC-MS under various liquefaction conditions,a reasonable liquefaction reaction pathway is given.（3）According to the catalytic effect of metal Fe and Co on biomass degradation,Fe_x-Co_（1-x）/Al₂O₃catalyst was prepared by co-precipitation method,and 10%ethanol was used as auxiliary liquefaction agent.The reaction temperature（220℃,260℃and 300℃）and time（10 min,30 min and 60 min）were optimized,and the60Fe-40Co/Al₂O₃catalytic effect was the best under the reaction conditions of 260℃,30 min.The maximum total bio-oil yield and energy recovery were obtained,which were 67.35%and 93.07%,respectively.GC-MS detection results show that bimetallic catalysis not only aggravates the hydrodeoxygenation process,but also effectively reduces the N content in bio-oil,which is conducive to the formation of phenols and hydrocarbons,and also promotes the transfer of more bio-oil components to low-boiling substances.Comparing the three catalytic cycle effects,the60Fe-40Co/Al₂O₃catalyst still maintains a high conversion rate of 91.45%after 4catalytic cycles,while the catalytic activities of 100Fe/Al₂O₃and 100Co/Al₂O₃drop sharply,which fully reflects the synergistic effect of bimetals.Effectively reduce carbon deposition.