Hydrothermal Liquefaction of Wet Microalgae for Bio-Crude Production: Process Parameters, Catalytic Strategies and Scale-Up Challenges

Algal biomass is being investigated as a renewable feedstock for liquid fuels because microalgae can grow rapidly, can be cultivated on non-arable land, and can use a range of water sources and carbon dioxide streams. A major barrier in conventional algal biofuel routes is the energy and cost associated with harvesting, dewatering and drying biomass before lipid extraction. Hydrothermal liquefaction (HTL) offers an alternative by converting wet biomass in hot compressed water into a bio-crude product without requiring complete drying. This review synthesises literature reported in the source material on the HTL of wet microalgae, focusing on operating conditions, feedstock composition, catalysts, product separation, aqueous-phase recycling and process integration. Across the studies summarised, reported bio-crude yields commonly fall in the approximate range of 30–70 wt% of dry feed, although values vary with algal strain, reactor configuration, temperature, residence time and product-recovery method. Temperatures around 300–350 °C are frequently reported to favour liquid products, while higher temperatures can increase gas formation and lower temperatures may favour solids. The review also identifies persistent challenges: nitrogen and oxygen in the bio-crude, catalyst cost and deactivation, high-pressure solids handling, uncertain commercial-scale economics, and the need for heat integration. Because the underlying project is literature-based, reported values are presented as results from prior studies rather than new experimental measurements. Further work should prioritise comparable reporting methods, continuous-reactor validation, upgrading performance and integrated techno-economic and environmental assessment.