From microalgae to biofuels & biochars: promises and challenges of the hydrothermal pathways

Poster presented at AlgaEurope 2025, Riga, Latvia.

Since several decades, microalgae hold great promises for biofuel production as they have higher areal yields than terrestrial plants, and mostly do not compete with food uses. However, their production cost, environmental performances and energy balance still need to be improved.

Hydrothermal liquefaction (HTL) converts wet biomass into a biocrude at a temperature between 250 and 350◦C and a pressure of 50-100 bars. HTL accepts wet feedstock and has better energetic and mass yield than traditional lipid extraction pathway.

Two different microalgae strains were tested, a carbohydrate-rich (C. sorokiniana NIES2173) and a lipid-rich (C. vulgaris NIES227). 13 biomass batches of these two strains were obtained in pilot-scale photobioreactors (180 and 300L) with lipid contents as high as 53%.

HTL operating conditions were optimized to maximize biocrude production. Links were unveiled between composition of the biomass and characteristics of the HTL products, thereby leading to recommendations for feedstock specifications to reach optimal biocrude yield and quality.

In a circular economy approach, the aqueous phase produced during HTL was recycled to the microalgae culture. Originally, Chlorella vulgaris NIES227 was not able to grow on a diluted aqueous phase. Using Adaptive Laboratory Evolution, this strain was accommodated to grow on 100% recycled HTL aqueous phase with optimal biomass and lipid yields.

Biocrudes obtained from Chlorella vulgaris NIES227 biomass were hydrotreated using commercial presulfided NiWS/γ-Al2O3 catalyst, and resulted in upgraded fuel with low heteroatom content (0.6 wt.% of nitrogen; 0.7 wt.% of oxygen).

Life cycle analysis of the whole process revealed algal feedstock production to be the most impacting step. Prospective upscaling scenarios were studied, thereby enlightening that the GHG threshold required for transportation fuels by the RED II legislation (32.9 CO2 eq/MJ) could be eventually reached.