Characterization of biocrudes recovered with and without solvent after hydrothermal liquefaction of algae

Bench-scale, batch experiments exploring algae hydrothermal liquefaction typically use organic solvents such as dichloromethane (DCM) to recover biocrude from the reactor contents. Commercial-scale, continuous processes, however, may separate biocrude and the aqueous phase co-product without solvent use. Herein, we provide the first thorough examination of the influence of DCM extraction on biocrude yield and composition from microalgae liquefaction in hot compressed water (350 °C, 20 min). We provide gravimetric biocrude yields and biocrude characterization via elemental analysis, GC-MS, NMR, and IR spectroscopies. The application of DCM increases the biocrude yield because it extracts water-soluble compounds from the aqueous phase co-product. Indeed, the biocrude recovered from the aqueous phase accounted for about 8 wt% of the total amount of biocrude. The addition of these molecules diminishes the biocrude quality, however, by decreasing the C and H content, increasing the O and N content, and thereby decreasing the higher heating value of the biocrude. The biocrude extracted from the aqueous phase had a remarkably different elemental composition than did the biocrude recovered without solvent. The O and N contents were more than twice as high, and the higher heating value was 20% lower. The biocrudes recovered directly from hydrothermal liquefaction (e.g., without water and DCM coming into contact) were 87%-88% C and H, and possessed a higher heating value of > 39 MJ/kg.