Effects of solvent phase recycling on microalgae liquefaction in ethanol: Bio-oil production and nitrogen transformation

Liquefaction of microalgae in ethanol offers an eco-friendly bio-oil alternative, but solvent recycling is crucial for sustainability due to extra costs. In this work, Chlorella vulgaris was liquefied in supercritical ethanol at 260 °C, and the solvent phase (SP) separated from bio-oil was recovered and reused. Five liquefaction cycles were performed at identical temperature and pressure conditions to investigate the effects on oil production and nitrogen transformation. The findings demonstrated a gradual increase in water content in recycled SP. Ethanol-water co-solvent as the reaction medium promoted the decomposition and re-polymerization of protein in raw material, thus increasing the bio-oil yield (76.84 %) and higher heating value (33.53 MJ/kg) to some extent. Simultaneously, the relative nitrogen content of bio-oil rose from 8.03 % to 8.52 %, predominantly in the form of nitrogen heterocycles. The potential pathway for nitrogen conversion was revealed, which establishes a theoretical basis for the subsequent denitrification of bio-oil.