Lubricant base oils can produce even more greenhouse gas emissions on a per-mass basis than petroleum-derived fuels if even a fraction of the lubricant is repurposed as fuel. The ability of the EBI process to yield jet fuel or lubricants could be a significant advantage for biorefineries.

Process flow diagram for alkane production options for jet/lubricant applications from Brazilian sugarcane. Source: Balakrishnan et al. Click to enlarge.

Aviation fuels pose a unique problem because stringent specifications require oxygen-free compounds, limiting the options available. Biofuel solutions such as farnesane have been proposed; however, these offer only modest GHG reduction benefits and the wide boiling range requirement for jet fuels sets a limit on the amount of single-component renewable fuels that may be blended. At the other end of the spectrum are automotive lubricant base oils where a narrow range of compounds is highly desirable. Poly-α-olefins (PAOs) containing 30 carbon atoms obtained from oligomerization of fossil-derived 1-decene are considered as the benchmark of superior performance for crankcase oils and have a high demand. Importantly, the GHG footprint associated with PAO base oils can be higher on a per-mass basis than petroleum-derived fuels if even a fraction of the lubricant is repurposed as fuel at its end of life.

The goal of our work was to develop a strategy for the flexible production of jet fuels and lubricant base oils in a Brazilian sugarcane refinery designed to achieve a meaningful reduction in life-cycle GHG emissions. Our approach involves conversion of sugars in sugarcane-derived sucrose and hemicellulose to ketones using a combination of chemical and biocatalytic processes. For example, 2-butanone, can be obtained by the dehydration of fermentation-derived 2,3-butanediol or via chemical/biochemical decarboxylation of levulinic acid.

The fermentation of various biomass-derived sugars using Clostridia strains produces a mixture of acetone, butanol, and ethanol (ABE), which can be used to synthesize a mixture of monoalkylated/dialkylated ketones, specifically 2-pentanone and 2-heptanone. Additional synthons may be produced from bioalcohol-derived olefins or biomass-derived furanic platform molecules, such as 2,5-dimethylfuran and 2-methylfuran, via hydrogenolysis to produce 2-hexanone and 2-pentanone, respectively, with as high as 98% selectivity. The biomass-derived methyl ketones are then catalytically self- and cross-condensed to produce C₁₂‒C₄₅ condensates, which serve as potential jet fuels (C₁₂‒C₂₁) and synthetic lubricants (C₃₃₊) after hydro-deoxygenation. We show that our strategy gives a new class of compounds that can be incorporated into production schemes that result in up to 81% reductions in GHG emissions, exceeding even those of conventional biofuels such as sugarcane ethanol.

—Balakrishnan et al.

Ketones—simple compounds that contain a carbonyl group (a carbon-oxygen double bond)—possess both electrophilic (attracted to electrons) and nucleophilic (a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction) functionality, which allows them to be used as building blocks similar to alkenes and aromatics in a petroleum refining complex.

The researchers first identified heterogeneous catalysts and appropriate reaction conditions for the self-condensation of ketones to produce dimer/trimer condensates with high overall yield. They then demonstrated the feasibility of producing jet fuel by cross-condensation of these methyl ketones to replicate the multicomponent mixtures of typical fuel blends. By adjusting the concentrations of ketones, they showed it possible to produce mixtures of alkanes that mimic conventional jet fuel.

They then modeled various pathways for producing drop-in jet fuel, lubricants, and ethanol using sucrose and bagasse-derived hemicellulose, with cellulose and lignin combusted for heat and power to quantify the carbon intensity of the lubricant and jet fuel production platform.

Using a combination of linear programming, process simulations, and life-cycle GHG modeling, they determined the optimal allocations of sucrose and hemicellulose subject to different objective functions. In each case, they constrained biorefinery acetone and hydrogen imports/exports to zero to prevent the facility from relying on fossil inputs and eliminated the need to find local markets for excess acetone and hydrogen coproducts. They found that:

• Maximizing the total reduction in annual GHG emissions achieved by the biorefinery (optimization A)—assuming ethanol displaces gasoline; bioderived jet fuel displaces petroleum jet fuel; and bioderived lubricants displace PAO base oil—resulted in a product mix comprising ∼40% lubricants, 40% ethanol, and 20% jet fuel by volume.

  In this case, 70% of sucrose is routed through a lubricant pathway via ABE fermentation, with a small jet fuel coproduct and the remaining 30% is used to produce ethanol via fermentation; the ethanol coproduct of ABE fermentation is also sold as fuel and the hydrogen coproduct is used on-site. All hemicellulose is converted to furfural, 78% of which is routed to jet fuel via 2-methylfuran and 22% of which is converted to lubricants with a minor jet fuel coproduct via 1-octanol.

• Maximizing jet fuel production (optimization B) was the only case in which on-site dedicated hydrogen production was required. They chose steam reforming of ethanol to model this step, although other options are also viable. The model suggested that 18% of sucrose is used to produce hydrogen via ethanol; the remaining 82% was converted to jet fuel via 2,3-butanediol; and all hemicellulose was converted to jet fuel via 2-methylfuran; the final output was 100% jet fuel.

• An attempt to maximize the lubricant base oil production (optimization C) resulted in an allocation similar to optimization A.

Life-cycle greenhouse gas results for selected optimization runs. Source: Balakrishnan et al. Click to enlarge.

By integrating various ketone synthons from biomass via self-/ cross-condensations, we have shown that a range of cyclic alkanes with desired composition, exceptional cold-flow properties, higher volumetric energy density, and appropriate boiling distributions can be produced for jet fuel applications. The condensation described here is catalyzed by inexpensive, heterogeneous mixed/pure oxides and is suitable for large-scale fuel production. In addition, ketone condensation can also produce a new class of biolubricants with properties comparable to fossil-derived lubricants.

Guided by life cycle analysis (LCA) combined with linear programming, integrated sugarcane biorefineries for producing jet fuels and lubricants could be built to minimize the overall GHG impact or maximize total energy output through novel combinations of furan and fermentation pathways. Increasing furan precursor yields and improving the ability to tune ABE fermentation product ratios will make such hybrid biorefineries even more efficient, offering dramatic GHG reductions relative to petroleum products. Needless to say, the commercial implementation of this technology would include financial implications that extend beyond GHG reductions; however, we hope that research such as that presented here will allow policymakers to create appropriate incentives to encourage optimal investments.

—Balakrishnan et al.

The new EBI process for making jet fuel and lubricants could also be used to make diesel and additives for gasoline, given some minimal modifications to both the catalysts and the reaction schemes, said co-author Amit Gokhale.

Although the goal of this study was to develop a strategy for the flexible production of jet fuels and lubricant base oils in a Brazilian sugarcane refinery, the strategy behind the process could also be applied to biomass from other non-food plants and agricultural waste that are fermented by genetically engineered microbes.

Although there are some additional technical challenges associated with using sugars derived entirely from biomass feedstocks like Miscanthus and switchgrass, there is no fundamental reason why we could not produce similar outputs, albeit in different proportions. We expect that further research will make this option increasingly attractive.

—Corinne Scown, one of three corresponding authors on the paper

Resources

• Madhesan Balakrishnan, Eric R. Sacia, Sanil Sreekumar, Gorkem Gunbas, Amit A. Gokhale, Corinne D. Scown, F. Dean Toste, and Alexis T. Bell (2015) “Novel pathways for fuels and lubricants from biomass optimized using life-cycle greenhouse gas assessment” PNAS doi: 10.1073/pnas.1508274112