Valorization of waste biomass through hydrothermal liquefaction: A review with focus on linking hydrothermal factors to products characteristics

Hydrothermal liquefaction (HTL) is one of the important conversion technologies from biomass to value-added products. However, the product quality is directly related to the reaction conditions and properties of raw materials. As a result, the review thoroughly examined the linking hydrothermal factor (temperature, retention time, and pressure) to products characteristics in order to systematically improve the quality of HTL products. The improvement degree of various pretreatment techniques (acid, alkali, enzyme, ozone, ultrasonic, and mi-crowave) coupled with HTL was subsequently summarized, as well as their mechanism of breaking and depo-lymerizing long-chain polymers by hydrolysis or energy-assisted methods in such as lignin, cellulose, protein, and lipid. Finally, the contribution of components to calorific value and yield of products was emphasized, as well as the synergistic and antagonistic effects of model compounds. Therefore, the pretreatments of biomass are promising yet challenging in reducing N-/O-heteroatoms.

Automated summary

Hydrothermal liquefaction (HTL) is an important technique for converting biomass into value-added products, and the quality of the products is closely related to the reaction conditions and properties of the raw materials. This review examines the relationship between hydrothermal factors (temperature, retention time, and pressure) and product characteristics in order to improve the quality of HTL products. It also summarizes the different pretreatment techniques (acid, alkali, enzyme, ozone, ultrasonic, and microwave) coupled with HTL, and their mechanisms for breaking down long-chain polymers such as lignin, cellulose, protein, and lipid. The contribution of components to the calorific value and yield of the products is emphasized, as well as the synergistic and antagonistic effects of model compounds.