Integration of Hydrothermal Liquefaction of Algae to Biofuel Production

Abstract Hydro Thermal Liquefaction (HTL) is a transformative process capable of converting algal biomass into four distinct phases: biocrude, gaseous, aqueous products, solid residue. Algal biofuel is regarded as the promising "third" generation biofuel, with three primary routes for liquid biofuel production from algae: biodiesel extraction or transesterification, bio-oil through pyrolysis, and biocrude via hydrothermal liquefaction (HTL). Both pyrolysis and HTL fall under the umbrella of thermochemical liquefaction technologies. HTL, specifically, involves the direct liquefaction of algal biomass into biocrude oil within a closed, oxygen-free reactor. This process utilizes pressurized inert gases like N2 or He, or reducing gases such as H2 or CO, at temperatures ranging from 250 to 380°C and pressures from 5 to 28 MPa. Notably, HTL employs hot compressed water, functioning as both a solvent and a reaction medium, with the advantage of being near-critical water, which is abundant, non-toxic, non-flammable, cost-effective, and naturally present in biomass. The paper further considers key challenge of HTL with organic solvents, which is the relative high cost, the use of hot compressed water which offers significant advantages. HTL with hot compressed water eliminates the need for an expensive solvent, and it has the flexibility to process wet algal feedstock directly, as the total solids (TS) content in the feedstock typically ranges from 10 to 25%. Furthermore, HTL enables the conversion of the entire algal composition, including lipids, proteins, and carbohydrates, resulting in a higher biocrude yield. Unlike other liquid biofuel production technologies such as oil extraction or pyrolysis, HTL offers distinct benefits: elimination of the drying process, broader feedstock applicability, enhanced mass transfer facilitated by sub-/super-critical water acting as both a reaction medium and solvent, and improved energy efficiency due to reduced latent loss during phase change.