Using the solution of self-settling microalgae Scenedesmus obliquus as processing medium for hydrothermal liquefaction of biomass

Abstract

Microalgae has gathered significant interest as a promising feedstock for biocrude production through hydrothermal liquefaction (HTL). However, the challenge of microalgae harvesting presents a significant obstacle to its economic viability and technical feasibility. This study aimed to overcome the complexities of microalgae harvesting processes by exploiting the self-settling characteristic of microalgae Scenedesmus obliquus (S. obliquus). The supernatant was discarded, and the S. obliquus solution was utilized as the processing medium for HTL of various types of biomass. HTL of Chlorella pyrenoidosa (C. pyrenoidosa) in S. obliquus solution at 300 °C resulted in a synergistic increase in biocrude yield (approximately 10 wt%, daf); however, increasing the temperature to 350 °C diminished the observed synergy. HTL of spent coffee grounds in S. obliquus solution generated 39.0 wt% (daf) and 45 wt% (daf) biocrude at 300 °C and 350 °C, respectively, with no synergistic effect observed. Conversely, macroalgae Enteromorpha prolifera (E. prolifera) and sawdust, which had a high carbohydrate content, showed antagonistic effects on biocrude yield when S. obliquus solution was used at high temperatures (350 °C). These observations were further verified by carrying out HTL of carbohydrate model component in S. obliquus solution. Principal component analysis of product yields was performed in this study, and it revealed that HTL of C. pyrenoidosa, protein, and lipid were significantly different from spent coffee grounds, E. prolifera, sawdust, and carbohydrate. Overall, depending on the feedstock used, the self-settling S. obliquus solution could serve as a promising HTL processing medium for biocrude production, eliminating the need for microalgae dewatering processes and the consumption of additional freshwater as an HTL processing medium.