A Review of Hydrothermal Biomass Liquefaction: Operating Parameters, Reaction Mechanism, and Bio-Oil Yields and Compositions

Biomass hydrothermal liquefaction (HTL) is a conversion technology that utilizes high-temperature and high-pressure hydrothermal conditions to convert biomass into liquid fuels and chemicals. This review introduces all aspects of biomass HTL from system process parameters to reaction mechanism pathways and product control methods. First, the effects of key process parameters such as biomass composition, moisture content, heating rate, temperature, residence time, and pressure on the reaction process and product characteristics of HTL were discussed. Next, the reaction mechanism studies of typical components were reviewed. For typical components such as proteins, lipids, and carbohydrates, their reaction mechanisms and reaction pathways during the HTL process were explored. Understanding the reaction mechanisms of typical components can help to deeply understand the reaction characteristics of the entire HTL process. In terms of HTL product characteristics and product catalytic control, the properties and potential application values of the main components of the product, including bio-oil, the aqueous phase, and solid and gaseous products, were analyzed, and methods for product catalytic control were discussed. It introduced the methods and catalytic mechanism of adding additives and using catalysts to improve product characteristics. Finally, the challenges and technical difficulties faced by biomass hydrothermal liquefaction technology were discussed, and the future development direction was prospected.