(635g) Hydrothermal Carbonization Pretreatment to Extract Lipids from Microalgae

Microalgae are gaining significant attention as a promising feedstock for the production of biofuels, such as biodiesel, as well as nutritional oils, such as ω-3 concentrates, due to their extremely fast growth rate, high oil content, and ability to grow on marginal land. One of the major challenges of producing algal oil at large scale is to extract lipids from the biomass in an environmental and energy-efficient way. The extraction methods investigated in previous works are mainly supercritical fluid extraction and cell disruption followed by solvent extraction. Most of these methods are either too energy intensive or involve the use of toxic chemicals, which limits their potential for commercial implementation.

In this work, we explored the possibility of extracting algal lipids by using the hydrothermal carbonization (HTC) as the pretreatment method. HTC can process the wet algal biomass directly and has shown to be a net energy positive process. Nannochloropsis slurry (15% solids) was first carbonized in subcritical water for 15-30 mins. This carbonization process concentrated about 90% of the algal lipids into a carbonized solid, or hydrochar, while dissolving about half of the cell material into the aqueous phase, forming a nutrient-rich co-product. The hydrochar was then extracted using hexane or ethanol to obtain crude oils. Different experimental conditions, such as extraction temperature, extraction time, and number of extraction passes, were studied and optimized.  The results showed that more than 80% of the total lipids can be extracted from the hydrochar in two hours using these solvents. Overall, this work demonstrates that HTC followed by solvent extraction is likely to produce algal oil in a net energy positive process that is suitable for refining into both biofuels and nutritional oils.