Abstract
BackgroundOleaginous 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.ResultsIn 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.ConclusionsThis 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0345-5) contains supplementary material, which is available to authorized users.
Highlights
Oleaginous microorganisms are attractive feedstock for production of liquid biofuels
Hydrothermal liquefaction (HTL) is one of the most attractive thermochemical conversion options, in which wet biomass is transformed into a liquid crude in a single-step process conducted in hot, compressed water [7]
Lower ash content, and higher energy content make it an excellent feedstock for biofuel conversion compared to other biomass feedstocks such as lignocellulosic feedstocks, sewage sludge, animal manure, and municipal solid wastes [14, 31, 32]
Summary
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’. Hydrothermal liquefaction (HTL) is one of the most attractive thermochemical conversion options, in which wet biomass is transformed into a liquid crude (called biocrude) in a single-step process conducted in hot, compressed water [7]. Overall economics in the HTL pathway are more strongly influenced by improvements in biomass productivity rather than extractable lipid content (unlike the solvent-based lipid extraction processes) for production of hydrocarbon fuels [10]. HTL promises higher performance than fast pyrolysis and other thermochemical processes because it can work directly with wet biomass (10–20 % total suspended solids) without the need for additional energy expended for evaporative drying, unlike the other thermochemical processes such as pyrolysis. Some of the major disadvantages of HTL include the high operating pressure associated with the process (can lead to high capital investments and operational issues), lack of information on standard product separation and purification, and insufficient knowledge about large-scale process development
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