Pilot plant testing of continuous hydrothermal liquefaction of microalgae

Abstract

We describe a pilot plant for continuous hydrothermal processing of biomass. Results were obtained for two microalgae strains, Chlorella and Spirulina, across a range of biomass loadings (1–10wt.%), temperatures (250–350°C), residence times (3–5min) and pressures (150–200bar). Overall, the bio-crude yields were found to increase with higher biomass loading, higher temperature and longer residence time. More severe reaction conditions also reduced the oxygen content of the bio-crude, while the nitrogen content was found to increase with higher temperatures, indicating an increase in the bio-crude production from the protein fraction of the algae. The maximum bio-crude yield obtained was 41.7wt.% for processing Chlorella with a solid loading of 10wt.% at 350°C and 3min residence time. The present results suggest that maximal yields may be obtained in much shorter residence times under continuous flow hydrothermal processing than batch studies have suggested. The maximal yield, however, may not be optimal in terms of properties.A substantial fraction of the feedstock carbon reported to the aqueous phase — this was up to 60% but decreased to 30% at the highest biomass loadings. Gas production (>90mol% CO2) increased with severity of processing, reaching up to 5% of the feedstock carbon. Finally, the solid yields consistently decreased with increasing temperatures and residence times.