Catalytic and Non-Catalytic Hydrothermal Liquefaction of Microalgae

Hydrothermal liquefaction (HTL) is an attractive thermochemical pathway that converts microalgal cells into biocrude which can be upgraded and refined into drop-in transportation fuel. HTL is propitious from an environmental sustainability standpoint because the reaction requires medium temperatures (200–400 °C) and high pressures (5–25 MPa) (subcritical and supercritical conditions) for a relatively short period of time (10–60 min) without the need for dewatering and drying of the microalgal culture (wet microalgae with cultivation culture). Instead, water provides dual use to the reaction: as a solvent and as a catalyst. At HTL conditions, water is a reactive nonpolar species with high miscibility in organics. It solubilizes even the recalcitrant microalgal components such as lignin to produce biocrude, aqueous, gaseous, and solid products. This chapter discusses the process and chemistry of microalgae HTL, the role of water in the reaction, the difference between catalytic and non-catalytic HTL as it applies to microalgae, and perspectives and direction on the state of research for microalgae HTL.