Conversion efficiency and oil quality of low-lipid high-protein and high-lipid low-protein microalgae via hydrothermal liquefaction

Hydrothermal liquefaction (HTL) is a promising technology for converting algae into biocrude oil. Here, HTL of a low-lipid high-protein microalgae (Nannochloropsis sp.) and a high-lipid low-protein microalgae (Chlorella sp.) was studied. An orthogonal design was applied to investigate the effects of reaction temperature (220-300 °C), retention time (30-90 min), and total solid content (TS, 15-25% wt) of the feedstock. The highest biocrude yield for Nannochloropsis sp. was 55% at 260 °C, 60 min and 25% wt, and for Chlorella sp. was 82.9% at 220 °C, 90 min and 25% wt. The maximum higher heating values (HHV) of biocrude oil from both algae were ∼37 MJ/kg. GC-MS revealed a various distribution of chemical compounds in biocrude. In particular, the highest hydrocarbons content was 29.8% and 17.9% for Nannochloropsis and Chlorella sp., respectively. This study suggests that algae composition greatly influences oil yield and quality, but may not be in similar effects.