Researchers have report success in using lignin as a path toward a drop-in 100% sustainable aviation fuel


An underutilized natural resource could be just what the airline industry needs to curb carbon emissions.

Researchers at three institutions – the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), the Massachusetts Institute of Technology (MIT), and Washington State University – report success in using lignin as a path toward a drop-in 100% sustainable aviation fuel.

Lignin makes up the rigid parts of the cell walls of plants. Other parts of plants are used for biofuels, but lignin has been largely overlooked because of the difficulties in breaking it down chemically and converting it into useful products.

The newly published research demonstrated a process the researchers developed to remove the oxygen from lignin, such that the resulting hydrocarbons could be used as a jet fuel blendstock. The research, “Continuous Hydrodeoxygenation of Lignin to Jet-Range Aromatic Hydrocarbons,” appears in the journal Joule.

Gregg Beckham and Earl Christensen are the researchers involved from NREL.

The paper points to the need to use sustainable sources for jet fuel as the airline industry has pledged to dramatically reduce carbon emissions. Airlines consumed 106 billion gallons of jet fuel globally during 2019, and that number is expected to more than double by 2050. Accomplishing the industry’s goal of achieving net carbon neutrality during that same period will require a massive deployment of sustainable aviation fuel (SAF) with high blend limits with conventional fuel.

Jet fuel is a blended mixture of different hydrocarbon molecules, including aromatics and cycloalkanes. Current commercialized technologies do not produce those components to qualify for a 100% SAF. Instead, SAF blendstocks are combined with conventional hydrocarbon fuels.

As the largest source of renewable aromatics in nature, lignin could hold the answer to achieving a complete bio-based jet fuel. This newly published work illustrates the ability of a lignin pathway to complement existing and other developing pathways. Specifically, the lignin pathway described in this new work allows the SAF to have fuel system compatibility at higher blend ratios.

Because of its recalcitrance, lignin is typically burned for heat and power or used only in low-value applications. Previous research has yielded lignin oils with high oxygen contents ranging from 27% to 34%, but to be used as a jet fuel that amount must be reduced to less than a half-percent.

Other processes have been tried to reduce the oxygen content, but the catalysts involved require expensive noble metals and proved to be low yielding. Researchers at the trio of institutions demonstrated an efficient method that used earth-abundant molybdenum carbide as the catalyst in a continuous process, achieving an oxygen content of about 1%.

NREL, MIT, Washington State University Collaboration Provides Pathway to Sustainable Jet Fuel

Containers are of poplar biomass (left), the extracted lignin oil, and the resulting sustainable aviation fuel.

Containers are of poplar biomass (left), the extracted lignin oil, and the resulting sustainable aviation fuel.

Here, we report a continuous, two-stage catalytic process using molybdenum carbide to deoxygenate lignin from poplar into aromatic hydrocarbons with 87.5% selectivity toward aromatic hydrocarbons at 86% of the theoretical carbon recovery.

Tier α fuel property testing indicates that the SAF-range lignin-derived aromatic compounds are likely performance-advantaged across multiple properties relative to conventional jet fuel aromatic compounds. This work demonstrates an effective approach to convert lignin into aromatic SAF blendstocks.

The Center for Bioenergy Innovation (CBI) vision is to accelerate domestication of bioenergy- relevant, non-model plants and microbes to enable high-impact innovations at multiple points in the bioenergy supply chain. CBI addresses strategic barriers to the current bioeconomy in the areas of 1) high-yielding, robust feedstocks, 2) lower capital and processing costs via consolidated bioprocessing (CBP) to specialty biofuels, and 3) methods to create valuable byproducts from the lignin. CBI will identify and utilize key plant genes for growth, composition and sustainability phenotypes as a means of achieving lower feedstock costs, focusing on poplar and switchgrass. We will convert these feedstocks to specialty biofuels (C4 alcohols, C6 esters and hydrocarbons) using CBP at high rates, titers and yield in combination with cotreatment, pretreatment or catalytic upgrading. CBI will maximize product value by in planta modifications and biological funneling of lignin to value-added chemicals.

The aviation industry requires sustainable aviation fuels (SAF) capable of reducing greenhouse gas emissions while satisfying strict safety and quality standards. Lignin is a promising renewable feedstock for the production of aromatic hydrocarbons, the missing fraction needed to achieve 100% SAF. The use of lignin in SAF hinges on reducing oxygen content while limiting ring hydrogenation and maximizing yields of C8-C20 hydrocarbons. Herein, we utilize molybdenum carbide (Mo2C) catalysts to hydrodeoxygenate lignin oil produced via reductive catalytic fractionation (RCF) of untreated poplar.

We designed a 3-phase trickle-bed reactor that generates steady-state partial-conversion kinetic data to analyze catalyst activity while deoxygenating complex lignin feeds, concluding that surface oxidation is the key catalyst limitation while processing neat lignin oil. At 350°C, complete deoxygenation is achieved in a single-pass at steady-state, with 94.2% selectivity of monomeric products to propylbenzene and methylpropylbenzene and 70.8 C-mol% recovery of whole oil.

While achieving high monomer recovery, single-pass reactions at 350°C have low recovery of dimers and larger oligomers. We hypothesize this is due to high reactivity of oxygenated oligomers toward unwanted side reactions at 350°C.

To recover larger oligomers, a multi-pass reaction was performed in which 50% of oxygen was removed at 300°C, an additional 25% oxygen was removed at 325°C and the remaining oxygen was removed in a third pass at 350°C. This multi-pass experiment resulted in an oil containing 49.5 wt.% 1-ring aromatics, 25.6 wt.% 2-ring aromatics, and 14.1 wt.% cycloalkanes. Deoxygenation of neat RCF oil corresponded to an increase in carbon content from 65.4 to 88.7 mass %, a decrease in oxygen content from 26.8 to 0.7 mass %, an increase of lower heating value from 21.73 to 39.99 MJ/kg, and a decrease in viscosity at 40°C from 231 to 1.04 cP.

The Center for Bioenergy Innovation is a U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological and Environmental Research in the DOE Office of Science.

More information: Michael L. Stone et al, Continuous hydrodeoxygenation of lignin to jet-range aromatic hydrocarbons, Joule (2022). DOI: 10.1016/j.joule.2022.08.005


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