A Review on Hydrothermal Biomass Conversion: Byproducts and Valorization for a Circular Bioeconomy

The increasing global energy demand and the continued reliance on fossil fuels have intensified environmental pressures, highlighting the need for sustainable energy technologies. Among renewable conversion routes, hydrothermal processing of biomass offers a versatile pathway for transforming wet organic matter into fuels and valuable chemicals without prior drying. This review provides a comprehensive assessment of hydrothermal carbonization (HTC), liquefaction (HTL), and gasification (HTG) with a particular focus on the valorization of their coproducts hydrochar, bio-crude, aqueous phase, and syngas. Emphasis is placed on how reaction parameters and feedstock characteristics influence product distribution and composition and on recent advances in green solvent extraction, hydrochar upgrading, and syngas utilization. Furthermore, the integration of techno-economic analysis (TEA) and life cycle analysis (LCA) is reviewed to benchmark environmental and economic feasibility. The review concludes with key technological challenges and research perspectives that can enhance process scalability and circular bioeconomy performance.