Microalgae have tremendous potential as a feedstock for production of liquid biofuels, particularly biodiesel fuel via transesterification of algal lipids. However, biodiesel production results in significant amounts of algal residues, or “lipid extracted algae” (LEA). Suitable utilization of the LEA residue will improve the economics of algal biodiesel. In the present study, we evaluate the hydrothermal carbonization (HTC) of whole and lipid extracted algal (Spirulina maxima) feedstocks in order to produce a solid biofuel (hydrochar) and value-added co-products in the aqueous phase. HTC experiments were performed using a 2-L Parr reactor (batch type) at 175–215 °C with a 30-min holding time. Solid, aqueous and gaseous products were analyzed using various laboratory methods to evaluate the mass and carbon balances, and investigate the existence of high value chemicals in the aqueous phase. The HTC method is effective in creating an energy dense, solid hydrochar from both whole algae and LEA at lower temperatures as compared to lignocellulosic feedstocks, and is effective at reducing the ash content in the resulting hydrochar. However, under the treatment temperatures investigated, less than 1% of the starting dry algae mass was recovered as an identified high-value chemical in the aqueous phase.

Increasing food waste generation (1.6 billion tons per year globally) due to urban and industrial development has prompted researchers to pursue alternative waste management methods. Energy valorization of food waste is a method that can reduce the environmental impacts of landfills and the global reliance on crude oil for liquid fuels. In this study, food waste was converted to bio-crude oilviahydrothermal liquefaction (HTL) in a batch reactor at moderate temperatures (240–295°C), reaction times (0–60 min), and 15 wt.% solids loading. The maximum HTL bio-crude oil yield (27.5 wt.%), and energy recovery (49%) were obtained at 240°C and 30 min, while the highest bio-crude oil energy content (40.2 MJ/kg) was observed at 295°C. The properties of the bio-crude oil were determined using thermogravimetric analysis, fatty acid methyl ester (FAME) analysis by gas chromatography with flame ionization detection, CHNS elemental analysis, and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS). FT-ICR MS results indicated that the majority of the detected compounds in the bio-crude oil were oxygen-containing species. The O₄class was the most abundant class of heteroatom-containing compounds in all HTL bio-crude oil samples produced at 240°C; the O₂class was the most abundant class obtained at 265 and 295°C. The total FAME content of the bio-crude oil was 15–37 wt.%, of which the most abundant were palmitic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), and polyunsaturated fatty acids (C18:3N:3, C18:3N:6).

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. 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. 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.