| The utilization of biomass is an important method to alleviate the shortage of fuel and chemical products as well as the exhaustion of fossil resources.Among all biorefinery technologies,the hydrothermal process is considered to be one of the most promising technologies due to its low energy consumption,high efficiency and abundant product.Hydrothermal hydrolysis and hydrothermal carbonization technologies are currently the most widely used and most beneficial processes.Their target products are often produced at the same time due to the similarity of reaction conditions.The ambiguity of the product generation mechanism and the regulation mechanism of reaction conditions restricts the production of the target product.In this study,a typical forest waste biomass poplar sawdust was used as the raw material,and the product generation mechanism of lignocellulose hydrothermal hydrolysis and carbonization were investigated respectively.For hydrothermal hydrolysis,the water solvent was replaced with a water-organic mixed solvent to promote the formation of the target products of furfural and 5-HMF（5-hydroxymethyl furfural）.The product generation mechanism in the mixed solvent system and organic solvent recycling efficiency were investigated.For hydrothermal carbonization,the acid-base characteristics of solvent were regulated to improve the generation of the target product humic acid and the quality of humic acid.A variety of analysis methods were carried out to explore the reaction mechanism of humic acid formation.The main conclusions are listed as follows:（1）Under the same conditions,the yields of furfural and 5-HMF increased by 30%and11%in H2O-THF（tetrahydrofuran）mixed solvent compared with the water solvent.The highest yields of furfural（90.77%）and 5-HMF（36.02%）were obtained at 190°C,1/3（v/v）H2O/THF ratio,40 min,and 1.0 wt%sulfuric acid concentration.（2）Through the two-factor orthogonal experiment,THF effectively reduced the loss of furfural at higher temperature,which enabled furfural and 5-HMF to reach the highest yield under the same conditions.The hydrothermal reaction experiment of furfural and xylan further proved that compared with water solvent,THF mixed solvent reduced the loss of furfural caused by the dehydration and condensation reaction of furfural with itself by 20%and effectively reduced the condensation of furfural with other small molecule compounds.（3）The results of continuous recycling of THF show that the recovery of THF will cause a little loss of furfural and 5-HMF.After five continuous recycling,the yields of furfural and5-HMF stabilized at 78.73%and 32.04%.The recovery rate of THF gradually decreased with the increase of the recycling times,and finally stabilized at 67.50%.（4）As for hydrothermal carbonization with water as the solvent,the highest yield（9.73%）of humic acid was obtained under alkaline conditions.Subsequent analysis results show that KOH-HA（humic acid generated under alkaline condition）possessed the advantages of high carbon content（61.09%）,high acidic functional group content（3.381 mmol/L）and high purity.Alkaline was the most suitable condition for humic acid formation.（5）The composition,chemical structure and thermochemical conversion characteristics of Kraft lignin,THF lignin and KOH-HA were investigated.The results show that THF lignin possessed the most complex composition,and its carbon storage and oxygen-containing functional groups were abundant.By contrast,KOH-HA had a single component,which was the product of migration and transformation of lignin fragments.In summary,using THF mixed organic solvent as the solvent for the hydrothermal reaction increase yields of liquid phase products,and THF possessed excellent recycling efficiency and the potential for large-scale commercial production due to its low boiling point.In the water solvent system,humic acid obtained under alkaline conditions had the highest yield and its quality was the best,which provided an important reference for the large-scale production of humic acid. |
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