Lignocellulosic and algal biomass for bio-crude production using hydrothermal liquefaction: Conversion techniques, mechanism and process conditions: A review

Thermochemical conversion is an effective process in production of biocrude. It mainly includes techniques such as torrefaction, liquefaction, gasification and pyrolysis in which Hydrothermal Liquefaction (HTL) has the potential to produce significant energy resource. Algae, one of the third-generation feedstocks is placed in the top order for production of bio-oil compared to the first and second-generation feedstock, as the algae can get multiplied in shorter time with the uptake of greenhouse gases. In HTL, the subcritical water helps the biomass to undergo thermal depolymerisation and produce various chemicals such as nitrogenates, alkanes, phenolics, esters, etc. The produced "biocrude" or "bio-oil" may be further upgraded into value-added chemicals and fuels. In addition, the bio-gas and bio-char are also synthesized as by-products. This review provides an overview of different routes available for thermochemical conversion of biomass. It also provides an insight on the operating parameters such as temperature, pressure, dosage of catalyst and solvent for lignocellulosic and algal biomass under HTL environment. In extent, the article covers the conversion mechanism for these two feedstocks and also the effects of the operating parameters on the yield of biocrude are studied in detail.