The Effects of Algae Growth Stage on Biocrude Characterization from Hydrothermal Liquefaction Using Chlorella Kessleri Algae

Hydrothermal liquefaction (HTL) is a process that converts biomass into a biocrude oil in subcritical water. High temperatures and pressures are required to achieve the subcritical stage creating a highly reactive environment. This environment produces a higher biocrude yield, as compared to other conversion processes such as lipid extraction, because it breaks down carbohydrates, proteins, and lipids to produce the biocrude oil. A large advantage of the HTL process is that it eliminates the high energy and thus costly step of dewatering in pyrolysis because HTL is run in an aqueous environment. The products of high ash algae are an aqueous product, a solid product, and biocrude oil. Chlorella kessleri algae was grown in a controlled environment and harvested during the exponential, transition, and stationary growth phases of the algal growth curve. The different growth stages resulted in varying protein, carbohydrate, and lipid composition. The collected algae was loaded into an unmixed tubular reactor at 10 wt% concentration of algae in water. During HTL the reactor was maintained at 350 °C for one hour. The resulting biocrude was extracted and analyzed for its properties. The techniques used for this analysis include GC-MS, simulated distillation based on thermogravimetric analysis, and CHN-O analysis. This study analyzed the differences in biocrude oil properties for algae harvested during each of the different growth phases to see how this affected the resulting oil compositionâ??s molecular, elemental, and fractional composition. The amount of unsaturated hydrocarbons increased by a total of approximately 6% moving along the growth phase and the amount of saturated hydrocarbons decreased by approximately 5%. The weight percentages of the lighter fractions of Heavy Naphta and Gasoline varied by approximately 5% among the different growth phases. The observed differences in biocrude composition suggest that to promote desirable oil characteristics algae for HTL processing should be harvested during the exponential or transition growth phases.