Hydrothermal liquefaction of biomass for bio-crude production: A review on feedstocks, chemical compositions, operating parameters, reaction kinetics, techno-economic study, and life cycle assessment

An in-depth, comprehensive, and critical review of the hydrothermal liquefaction (HTL) technology for maximum yield of biocrude with enhanced properties has been undertaken. The feedstocks, process parameters, kinetic of HTL for bio-crude production have been systematically examined and collated from a board range of research. In the coverage of Techno-Economic Analysis (TEA) and Life Cycle Assessment (LCA) of HTL of feedstocks, this review overcomes the failure of existing literature to give due weightage to those crucial issues. HTL is introduced by the direct use of wet biomass that eliminates the drying unit, reduction in total expenditure, and conversion of materials into solid and liquid fuels at moderate temperatures (250–400 °C) and pressures (10–35 MPa). The introduction of hot pressurized water results in a lower yield of tar with higher energy proficiency. The biomass is delineated via elemental composition, renewable feedstock potential, and evaluation of dry versus wet biomass feedstocks. HTL is comprehensively reviewed through the process mechanisms, depolymerization reactions of biomass, hydrothermal liquefaction process of dry and wet lignocellulose feedstocks, and the effect of operating parameters. The product(s) description and evaluation, process advancements, energy efficiency, and kinetic investigation in relation to HTL form the final parts of this review. This review is aimed at optimization and commercialization of the HTL technology to meet the demands of the biofuel sector, the researcher working in the thermochemical conversion of feedstocks, and the development of new HTL reactors.