Biocrude production via hydrothermal liquefaction of biomass: Recent advances and trends

Hydrothermal liquefaction (HTL) appears as a promising way to convert biomass into biocrude, which, in turn, can be upgraded to advanced transportation biofuels. In the last ten years, research on HTL has significantly increased. The necessity for sustainable fuel alternatives originated from the updated EU RED III targets has triggered this growth. This review aims to present the recent progress in HTL by focusing on feedstock diversity, solvent selection, reaction mechanism, and process optimization. A plethora of biomass types has been studied so far, including lignocellulosic residues, municipal and industrial waste, sewage sludge, animal manure, microalgae, and oleaginous yeasts. This fact highlights HTL compatibility and its significant impact towards circular bioeconomy. Furthermore, the major biomass components (cellulose, hemicellulose, lignin, lipids, and proteins) are examined comparatively, along with how water, organic solvents, and co-solvent reaction mediums can impact product distribution and fuel quality. Also, important operational factors like temperature, residence time, reactor loading, pressure, and heating rate are examined concerning biocrude yield and quality. Subsequently, the review dives into studies engaged with extractant selection, catalyst development, aqueous phase recirculation, and the valorization of HTL byproducts. Finally, a summary of recent advances in upgrading methods, scaling up technology, economic analysis, and life cycle assessment is presented to evaluate HTL commercial readiness and sustainability. Overall, this review aims to collectively present all the recent active HTL studies and stress the next steps towards process escalation.