Biomass to biofuels using hydrothermal liquefaction: A comprehensive review

The utilization of renewable fuel alternatives holds promise for reducing the financial burden of regulatory compliance and the social responsibility associated with greenhouse gas emissions. Hydrothermal liquefaction (HTL) is one of the most versatile technologies for converting renewable biomass feedstocks (especially in the wet state) into biofuel (biocrude oil) in a compact plant. Therefore, this review is devoted to thoroughly reviewing and critically discussing biocrude oil production from biomass feedstocks through the HTL process. This review starts by discussing the principles of biomass HTL processing and product upgrading, aiming to provide a grounded and broad understanding of current developments in this domain. The data reported in the published literature are analyzed and visualized in order to scrutinize the effects of the main process parameters on the quantity, quality, cost, and environmental impacts of resultant biofuels. Higher biocrude oil yields are obtained at temperatures, pressures, and residual times between 300 and 350 °C, 24–27 MPa, and 15–25 min, respectively. Concerning yield and calorific value, biocrude oil derived from homogeneous catalysts demonstrates figures of 23.6 % and 32.1 MJ/kg, whereas that from heterogeneous catalysts exhibits percentages of 66.8 % and 40 MJ/kg, respectively. The challenges and prospects for the future development of biocrude oil are also discussed. HTL has a long way to go before being used for biofuel production on a large scale. Future studies appear to be directed towards the use of HTL technology under the biorefinery framework to maximize the exploitation of biomass into value-added products, while minimizing waste generation.