Functional modification of hydrothermal liquefaction products of microalgal biomass using CO₂

This study argues that CO₂ can be used to modify the composition of bio-oil generated from hydrothermal liquefaction of two different microalgae such as Microcystis aeruginosa (M. aeruginosa) and Nannochloropsis oceanica (N. oceanica). Prior to hydrothermal liquefaction, the lipid content of microalgae was quantified as ∼2 wt% of M. aeruginosa and ∼14.5 wt% of N. oceanica. In addition, the thermal degradation of M. aeruginosa and N. oceanica was characterized to evaluate the influence of CO₂ via quantifying major pyrolytic gases (i.e., H₂, CH₄, and CO). Hydrothermal liquefaction of these two microalgae was performed using a high-pressure batch reactor in N₂ and CO₂ atmospheres. The composition of chemical species containing N and O in bio-oil was significantly decreased under hydrothermal liquefaction in CO₂, which meant that the quality of microalgae-derived bio-oil was improved. In addition, the use of CO₂ allowed the production of more petro-diesel like bio-oil. This study suggests a practical means for utilizing CO₂ for the production of high-quality bio-oil from toxic and economic microalgal biomass.