Background: Oleaginous microorganisms are attractive feedstock for production of liquid biofuels. Direct hydrothermal liquefaction (HTL) is an efficient route that converts whole, wet biomass into an energy-dense liquid fuel precursor, called 'biocrude'. HTL represents a promising alternative to conventional lipid extraction methods as it does not require a dry feedstock or additional steps for lipid extraction. However, high operating pressure in HTL can pose challenges in reactor sizing and overall operating costs. Through the use of co-solvents the HTL operating pressure can be reduced. The present study investigates low-temperature co-solvent HTL of oleaginous yeast, Cryptococcus curvatus, using laboratory batch reactors.
Results: In this study, we report the co-solvent HTL of microbial yeast biomass in an isopropanol-water binary system in the presence or absence of Na2CO3 catalyst. This novel approach proved to be effective and resulted in significantly higher yield of biocrude (56.4 ± 0.1 %) than that of HTL performed without a co-solvent (49.1 ± 0.4 %)(p = 0.001). Addition of Na2CO3 as a catalyst marginally improved the biocrude yield. The energy content of the resulting biocrude (~37 MJ kg(-1)) was only slightly lower than that of petroleum crude (42 MJ kg(-1)). The HTL process was successful in removing carboxyl groups from fatty acids and creating their associated straight-chain alkanes (C17-C21). Experimental results were leveraged to inform techno-economic analysis (TEA) of the baseline HTL conversion pathway to evaluate the commercial feasibility of this process. TEA results showed a renewable diesel fuel price of $5.09 per gallon, with the HTL-processing step accounting for approximately 23 % of the total cost for the baseline pathway.
Conclusions: This study shows the feasibility of co-solvent HTL of oleaginous yeast biomass in producing an energy-dense biocrude, and hence provides a platform for adding value to the current dairy industry. Co-solvents can be used to lower the HTL temperature and hence the operating pressure. This process results in a higher biocrude yield at a lower HTL temperature. A conceptual yeast HTL biofuel platform suggests the use of a dairy waste stream for increasing the productivity and sustainability of rural areas while providing a new feedstock (yeast) for generating biofuels.
Keywords: Biocrude; Biofuel; Co-solvent; Hydrothermal liquefaction (HTL); Techno-economic analysis; Yeast.