Biocrude production from hydrothermal liquefaction and pyrolysis: Product characteristics, formation mechanisms, and upgrading strategies

Biomass conversion is an important route for producing renewable liquid fuels and reducing dependence on fossil resources. Pyrolysis (Py) and hydrothermal liquefaction (HTL) are two widely used thermochemical processes, but their differences in biocrude formation, product properties, and upgrading requirements have not been fully compared. Raw biocrude usually contains considerable oxygen- and nitrogen-containing compounds, leading to low heating value, poor stability, high viscosity, and limited direct fuel applicability. This review summarizes the main reaction pathways of biomass during Py and HTL, focusing on the conversion of cellulose, hemicellulose, lignin, lipids, proteins, and carbohydrates. Based on reported data, the preferred temperature ranges for Py and HTL are mainly 500–550 °C and 270–340 °C, respectively. Compared with pyrolysis-derived biocrude, HTL-derived biocrude generally shows a lower O/C ratio, indicating stronger deoxygenation under hydrothermal conditions. However, its N/C ratio and viscosity are often higher, especially for protein- and ash-rich feedstocks. To improve biocrude quality, common upgrading methods, including solvent addition, esterification, emulsification, catalytic cracking, and catalytic hydrogenation, are further discussed, with attention to catalyst composition, active sites, and reaction conditions. This review provides a comparative understanding of Py and HTL and offers useful references for selecting suitable biomass-to-liquid fuel pathways.