Preparation of bio-oil from Scenedesmus acutus using thermochemical liquefaction in a 1 L reactor

Biomass from microalgae is a potential feedstock for biofuels production. It poses no threat to food security as it does not compete with agricultural crops for arable land. Scenedesmus acutus was used as feedstock to produce bio-oil in a large liquefaction reactor. The influence of reaction temperature (280–360ºC), reaction atmosphere (N2 or CO2) and solvent on bio-oil yield, C-16 fatty acid yield and oil properties were investigated. Oils were characterised using gas chromatography, Fourier transform infrared (FTIR) spectroscopy and ultimate analysis. Higher bio-oil yields were obtained in a CO2 atmosphere (250 g.kg-1 dry microalgae) than in a N2 atmosphere (210 g.kg-1 dry microalgae) whilst higher C16 fatty acid concentrations (600 g.kg-1 bio-oil) were recorded in N2 atmosphere compared to oil prepared in a CO2 atmosphere (500 g.kg-1 bio-oil). The oil yield increased to a maximum at 320°C, after which there were no significant changes. Highest bio-oil yields (425 g.kg-1 dry microalgae) were obtained in ethanol as solvent. FTIR spectroscopy and ultimate analysis showed that proteins present in the feedstock were degraded by breakage of peptide linkages, and nitrogen present in the oils is peptide fragments from protein degradation. The carbon content of all produced oils was high, but the hydrogen content was low, leading to low hydrogen/carbon ratios. Energy consumption and energy efficiency calculations showed that liquefaction in both reaction atmospheres results in a net energy gain, and a CO2 atmosphere is best for high energy efficiency.