Catalytic Hydrothermal Liquefaction of Algal Biomass for Diesel Like Bio-Crude Oil

The green energy industry has recently focused its attention on algal biomass as a raw material for drop-in fuel generation. Algal biomass has been considered to be a third-generation feedstock for biofuel production. It has several advantages viz. fast-growing, high in lipid, and does not compete with food crops. Several technologies are available for converting biomass into bioenergy, including torrefaction, hydrothermal carbonization, pyrolysis, and gasification. Hydrothermal liquefaction (HTL) is an emerging technology that involves the transformation of biomass into a liquid energy carrier referred to as bio-oil using sub/supercritical water. Depending on the operating parameters, hydrothermal liquefaction of algal biomass produces several compounds. In this process, algal biomass is converted into bio-crude oil under moderate to high temperatures (250–550 °C) and pressures (5–25 MPa). Bio-crude oil has unique physicochemical properties i.e., its remarkable energy density, renewability, and sustainability, which can meet global environmental challenges and energy shortages. The only drawback of the HTL process is that bio-crude oil contains oxygenated compounds. Interestingly, catalyst plays a crucial role in the up-gradation of bio-crude oil produced by HTL. Both the homozygous and heterozygous catalysts are used in HTL to boost its activity. Several catalysts (alkaline salts, organic acids, metal oxides, Ni, Cu, and Fe) are tested to improve the bio-crude oil yield and quality. In this article, different catalytic mechanisms likely, hydrogenation, deoxygenation, decarboxylation, denitrogenation, desulfurization, and approaches for bio-crude oil enhancement are summarized and analyzed comprehensively. Moreover, extensive description has been provided on the factors affecting HTL, and the role of various catalysts on other HTL products viz. biochar, aqueous phase extract, and gas. Furthermore, future research and development challenges that may arise have been proposed.