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 microwave) coupled with HTL was subsequently summarized, as well as their mechanism of breaking and depolymerizing 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.