Hydrothermal liquefaction of macroalgae for the production of renewable biofuels

In all biorefinery systems, excess water represents a key challenge, and its removal by drying is often a necessary and crucial pre-treatment. Second-generation feedstocks have often fallen at this hurdle, particularly microalgae-derived biomasses, which require extensive (and costly) dewatering. Hydrothermal liquefaction (HTL) has gained increasing attention in recent years as a technology that uses the water present in the feedstock as a versatile reaction medium, which functions as a solvent, reactant, and catalyst for a cascade of organic reactions. Converting organic biomasses into oil, aqueous, solid, and gas fractions, the development of HTL provides the opportunity to exploit previously unsuitable biomasses in a versatile bio-refinery approach. Marine macroalgae (seaweeds) offer a sustainable source of renewable biomass, which require no land or freshwater to cultivate or harvest. With 70% of the surface of the planet covered in seawater and levels of eutrophication increasing, seaweeds are an underutilized resource with excellent potential for relieving the pressure on fossil resources. Hitherto, this exploitation has been hindered by a lack of suitable and economical processing tools. Here we review the potential for applying HTL to processing marine macroalgae and discuss the potential products and services that can be derived from this potential biorefinery system.