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We collected and analyzed the gas, crude bio-oil, dissolved aqueous solids, and insoluble residual solids product fractions arising from hydrothermal liquefaction of Nannochloropsis sp. at 350 °C for 60 min. Most of the carbon and hydrogen in the algal biomass appears in the crude bio-oil product, as desired. A majority of the original nitrogen appears as ammonia in the aqueous phase. We also determined how the solvent used to recover the crude bio-oil affected the yields and compositions of the product fractions. We used both nonpolar solvents (hexadecane, decane, hexane, and cyclohexane) and polar solvents (methoxycyclopentane, dichloromethane, and chloroform). Hexadecane and decane provided the highest gravimetric yields of bio-oil (39 ± 3 and 39 ± 1 wt %, respectively), but these crude bio-oils had a lower carbon content (69 wt % for decane) than did those recovered with polar solvents such as chloroform (74 wt %) and dichloromethane (76 wt %). We quantified the amount of 19 different individual molecular components in the crude bio-oil for the first time. Fatty acids were the most abundant components, but some aromatic and sulfur- and nitrogen-containing compounds were also quantified. The amount of free fatty acids in the crude bio-oil significantly depended on the solvent used, with polar solvents recovering more fatty acids than nonpolar solvents. The bio-oil recovered with chloroform, for example, had a fatty acid content equal to 9.0 wt % of the initial dry algal biomass.

We investigated the fast hydrothermal liquefaction of green marine alga Nannochloropsis sp. at batch reaction times of 1, 3, and 5 min and set-point temperatures of 300–600 °C. We also performed conventional liquefaction for 60 min at the same temperatures. These experiments cover the broadest range of reaction conditions yet reported for algae liquefaction. The biocrude yield obtained for 1 min reaction times, which was only long enough to heat the reactor from room temperature to about half of the set-point temperature (in °C), increased with an increasing set-point temperature to 66 ± 11 wt % (dry and ash-free basis) at a set-point temperature of 600 °C. The biocrude obtained at this condition contains 84% of the carbon and 91 ± 14% of the heating value present in the dry algae feedstock. This biocrude yield and corresponding energy recovery are the highest reported for liquefaction of this alga. For a reaction time of 1 min, as the set-point temperature increases, light biocrude (e.g., hexane solubles) makes up less of the total biocrude. The biocrudes produced by fast liquefaction have carbon contents and higher heating values similar to biocrudes from the traditional isothermal liquefaction process, which involves treatment for tens of minutes. These results indicate that biocrudes of similar quality may be produced in higher yields and in a fraction of the time previously thought necessary. Such a decrease in the reaction time would greatly reduce the reactor volume required for continuous biocrude production, subsequently reducing the capital costs of such a process. We also show that the reaction ordinate is a useful parameter for interpreting results from algae liquefaction performed at different temperatures and reaction times.

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