Hydrothermal liquefaction of pretreated low-lipid microalgae for the production of bio-oil with low heteroatom content

The conversion of microalgae to bio-oil by hydrothermal liquefaction (HTL) is considered an effective method to obtain environmentally friendly energy. However, the heteroatomic organic species, mainly nitrogen and oxygen-containing chemicals, in bio-oil hamper its practical utilization. A low-temperature liquefaction pretreatment process (150–225 °C, 10–50 min) was performed to improve the quality of bio-oil from low-lipid microalgae. After the pretreatment, 11–61% of the nitrogen content was removed from the microalgae. Then, the algae were converted via HTL at 340 °C for 120 min into bio-oil with lower heteroatom content and higher conversion yield compared to those obtained by direct HTL. Specifically, when algae were pretreated at 225 °C for 10–50 min, the yield and higher heating value of the bio-oil reached 26.5–34.3 wt% and 34.9–37.1 MJ/kg, respectively, while the nitrogen and oxygen contents were reduced by 37% and 36%, respectively. Furthermore, GC–MS, FT-IR and elemental analyses revealed that the bio-oil derived from pretreated algae contained a lower level of N-containing compounds and that the content of desired long-chain hydrocarbons had increased to 30%. These findings demonstrate that great improvements in the quality of bio-oil can be achieved by pretreatment of the algae.