Hydrothermal liquefaction of algal feedstocks: The effect of biomass characteristics and extraction solvents

Hydrothermal liquefaction of algal biomass is growing as a promising technology for the production of liquid biofuels in the renewable energy sector. The potential of this technology under varied process conditions has been extensively demonstrated worldwide. Recent developments have revealed that the feedstock quality and extraction solvents have a considerable effect on the biocrude quality and yield. Algal lipids and proteins contribute significantly to biocrude formation, while carbohydrates contribute mainly to the production of gaseous products. However, the challenges associated with high N/C and O/C ratios in biocrude derived from proteinaceous biomass need to be addressed. Recently, co-liquefaction of carbonaceous biomass with different biochemical compositions has proven promising in biocrude upgradation. The most contemporary research also revealed that solvents as a co-solvent and/or extraction agent have the most significant effect on biocrude quality and yield in the HTL mixture. These solvents could also intensely influence the properties and distribution of energy-dense compounds in the biocrude. The present paper extensively reviews the effect of feedstock composition on HTL biocrude formation and its characteristics based on the information collected from more than 150 scientific resources published between 2010 and 2021. Furthermore, the role of solvents as reaction medium and extraction agent on biocrude yield and composition has been critically reviewed. Future perspectives and challenges in the use of approaches for improving biocrude formation are also discussed.