Towards an Aviation Fuel Through the Hydrothermal Liquefaction of Algae

The hydrothermal liquefaction (HTL) of algal biomass is a promising route to viable third-generation biofuels that improves on the energy efficiency of previously examined algal lipid production. The process is highly suitable for both microalgae and seaweed (macroalgae), however, there still needs to be improvement of the Energy Return on Energy Invested before the technology can be commercially adapted. This can be achieved through the integration of biomass cultivation with other services, such as environmental remediation, as well as the valorization of alternative products to form a closed-loop biorefinery. The HTL process utilizes water at sub-/near-critical conditions (200–374°C, 5–28 MPa) as both the reaction medium and solvent for a host of simultaneous reactions, converting algal biomass into a bio-crude oil, alongside a nutrient-rich aqueous phase, a solid char, and a number of gaseous products. The bio-crude oil produced is acidic, highly viscous, and contains high proportions of N and O. While the HTL oil contains fewer heteroatoms than pyrolysis oils, the bio-crude must still be catalytically upgraded to produce various fractions of hydrocarbon fuels, including aviation kerosene. However, the nitrogen content is substantially higher in algal bio-oils than alternative terrestrial oils, and as such, removal of the N-compounds in the bio-crude is one of the most significant challenges that must be overcome in order to advance HTL technology for biofuel production from algae.