Comparative studies of thermochemical liquefaction characteristics of microalgae using different organic solvents

The effect of different organic solvents, such as methanol, ethanol and 1,4-dioxane, on thermochemical liquefaction characteristics of Spirulina (a kind of high-protein microalgae) was systematically studied. The liquefaction experiments were conducted in a 1000 mL autoclave at different temperatures from 573 to 653 K with a fixed solid/liquid ratio. Liquefaction of Spirulina processed in methanol and ethanol favored the conversion rate and bio-oil yield compared with that in 1,4-dioxane solvent. The bio-oil generated in methanol contained higher C and H concentrations but a lower O content, resulting in a higher caloric value (39.83 MJ/kg). The results of FT-IR (Fourier Transform Infrared Spectroscopy) and GC–MS (Gas Chromatography–Mass Spectroscopy) analyses indicated that the compositions of bio-oil products were greatly affected by the type of solvent used for the liquefaction process. The major component of bio-oil produced with methanol was hexadecanoic acid methyl ester (C17H34O2, 35.53%). However, ethanol favored the formation of hexadecanoic acid ethyl ester (C18H36O2, 26.27%). When Spirulina were operated with 1,4-dioxane, the bio-oil was dominated by hexadecanenitrile (C16H31N, 22.7%). The presence of methanol and ethanol might promote the formation of esters. Low-boiling-points compounds with phenol ring structure or heterocyclics can be generated when 1,4-dioxane was employed as solvent.

► The effect of solvent type on liquefaction behavior of Spirulina was studied.
► Polar solvent promoted conversion of Spirulina better than dipolar solvent.
► Spirulina processed in methanol and ethanol resulted in large amounts of esters.
► 1,4-dioxane favored to form large amounts of phenols and nitrogen heterocycles. ▶ Ethanol may be a promising solvent for its high efficiency and renewability.