A review on hydrothermal liquefaction of algal biomass on process parameters, purification and applications

• Conversion of algae biomass into bio-oil, biochar and biogas using hydrothermal liquefaction. • Algae is considered as a potential source due to its rapid growth and high flexibility. • The pathway of HTL involves de-polymerization, de-composition and recombination. • Carbohydrates, lipids and proteins which are the primary constituents of an algal biomass. Algae, a potential biomass feedstock with a faster growth rate and capability of greenhouse gas absorption, mitigates the limitations of the first- and second-generation feedstock in bio-oil production. hydrothermal liquefaction (HTL) is known to be an active method capable of producing substantial energy resources. In HTL, biomass undergoes thermal depolymerization in the presence of water, at around 280 °C–350 °C following subcritical and near supercritical conditions to produce chemical compounds such as alkanes, nitrogenates, esters, phenolics, etc. The primary product, “Biocrude/Bio-oil” obtained from the reaction, is identified as the essential fuel source after processing and also as a distinct value-added chemical source, along with biochar and biogas as co-products. This review outlines a range of routes available for thermochemical conversion of the algal biomass. It also provides a better understanding of the reaction mechanism like depolymerization, decomposition, and re-polymerization, operating conditions like temperature, pressure, the quantity of catalyst required, and the solvent used in the process. The review also highlights the yield achieved by altering the aforementioned parameters, comparing and presenting them as a collective result.