Hydrothermal liquefaction of microalgae: Effect on the product yields of the addition of an organic solvent to separate the aqueous phase and the biocrude oil

The microalgae species Nannochloropsis gaditana (marine) and Scenedesmus almeriensis (freshwater) were subjected to hydrothermal liquefaction (HTL) at 350 °C in small microautoclaves for 15 min to study the separation of the aqueous and biocrude oil products, either by gravity or assisted by an organic solvent (dichloromethane). The vast majority of the research available for microalgae HTL determines the product yields by separating the HTL phases with an organic solvent. This study shows that its utilization affects the product distribution, increasing the amount of biocrude oil produced and reducing the concentration of organic molecules in the aqueous phase. The increase in the biocrude oil yield comes at the expense of a higher nitrogen and oxygen content. This harms the quality of the biocrude oil in view of its application as biofuel, due to undesired emissions upon combustion. The results herewith presented indicate that the yields of the HTL products strongly depend on the separation method applied. As the use of large amounts of organic solvents for separating the products at industrial scales is unlikely, their use is also discouraged in laboratory experimentation in order to forestall creating false expectations about the biocrude oil yields obtained by means of microalgae HTL.