Renewable diesel via hydrothermal liquefaction of oleaginous yeast and residual lignin from bioconversion of corn stover
A major barrier to profitable bioconversion of cellulosic biomass to hydrocarbon fuel precursors is the difficulty in breaking down feedstock lignin into carbon substrates that can be easily metabolized by industrial microbes. Hydrothermal liquefaction (HTL) of cellulosic biomass effectively breaks down and converts lignin into biocrude oil products, but provides relatively low yields of biocrude from feedstock carbohydrates. In the present study, bioconversion and HTL were integrated into a new hybrid system that employs the best of both processes to produce high quality fuel in high quantity. Bioreactor cultivation of the oleaginous yeast Lypomyces starkeyi produced dry cell mass yields up to 0.43 g/g of sugar and yields of intracellular triglyceride lipids (measured as fatty acid methyl esters) of up to 0.26 g/g of sugar in pretreated corn stover hydrolysate or simulated hydrolysate media. The lipid-rich bioreactor broth was mixed with pretreated corn stover lignin, and then co-fed to an HTL reactor to produce 0.40 g of biocrude oil per gram of feed on a dry weight basis. Optimizing the separation of HTL biocrude and the HTL aqueous phase could significantly increase the biocrude yield. Hydrotreating of the biocrude produced a high quality, renewable hydrocarbon fuel blendstock, with the majority of the product boiling in the distillate range. The estimated cost of production could be reduced to $3/gasoline gallon equivalent through addressing identified research gaps. Our results provide a proof-of-concept for a new hybrid biorefinery design that could enhance domestic production of renewable diesel, jet, and marine fuel in the United States.