Co-Solvent Assisted Hydrothermal Liquefaction of Algal Biomass and Biocrude Upgrading

Abstract

This study reports the hydrothermal liquefaction (HTL) of microalgae Spirulina platenesis in the presence of alcohol or formic acid co-solvents. HTL runs are performed in a 1.8-L batch reactor at 300 °C using an alcohol (methanol and ethanol) or formic acid co-solvent. Consequently, hydrodeoxygenation (HDO) of resultant algal biocrude is performed at 350 °C for 2 h under high hydrogen pressure (~725 psi) using the Ru/C catalyst. The HTL results are compared with the control HTL run performed in water only. The results of the study show that the addition of co-solvents leads to a 30–63% increased biocrude yield over the control HTL run. Formic acid results in a 59.0% yield of biocrude, the highest amongst all co-solvents tested. Resultant biocrudes from formic acid-assisted and ethanol-assisted HTL runs have 21.6% and 3.8–11.0% higher energy content, respectively, than that of the control run. However, that of the methanol-assisted HTL results in biocrude with 4.2–9.0% lower energy density. Viscosity of biocrude from methanol- or ethanol-assisted HTL is higher than the control HTL but formic acid-assisted HTL results in a less viscous biocrude product. In addition, the HDO study leads to a 40.6% yield of upgraded oil, which is characterized by a higher net energy content and lower O/C and N/C ratios when compared to the initial HTL biocrude.