By Nancy W. Stauffer, MIT Energy Initiative
September 20, 2023
Many groups have actually targeted a 100 percent sustainable hydrocarbon fuel for aircraft, but up until now there hasnt been much success. Part of the obstacle is that aviation fuels are so securely regulated. “This is a subclass of fuels that has very specific requirements in regards to the chemistry and the physical residential or commercial properties of the fuel, due to the fact that you cant risk something failing in a plane engine,” says Yuriy Román-Leshkov, the Robert T. Haslam Professor of Chemical Engineering. “If youre flying at 30,000 feet, its really cold outside, and you dont want the fuel to thicken or freeze. Thats why the solution is extremely specific.”
The 2 vials revealed contain products from essential steps in the treatment. The brown lignin oil in the left-hand vial formed when the biomass was exposed to the very first catalyst. When the lignin oil passed though the trickle-bed reactor for the second time, the clear liquid in the right-hand vial formed. Now without oxygen molecules, it is the aromatic fraction needed to make todays air travel fuel 100 percent sustainable. Credit: Gretchen Ertl
The Composition of Aviation Fuel
Air travel fuel is a combination of 2 large classes of chemical substances. “This is comparable to what we would find in diesel fuels, so its a classic hydrocarbon that is out there,” explains Román-Leshkov.
In a lot of transport fuels, aromatic hydrocarbons are seen as a source of pollution, so theyre gotten rid of as much as possible. In air travel fuels, some aromatic particles should remain because they set the needed physical and combustion properties of the overall mix.
As a result, aromatics are a needed component– but theyre likewise a stumbling block in the move to create sustainable air travel fuels, or SAFs. Business understand how to make the aliphatic portion from inedible parts of plants and other renewables, however they have not yet developed an authorized technique of producing the fragrant fraction from sustainable sources. As a result, theres a “mixing wall,” discusses Román-Leshkov.
Postdoc Jamison Watson joined the Román Lab in January 2021 to concentrate on the catalytic upgrading and conversion of lignin into the aromatic portion needed to make air travel fuel totally sustainable. Here, he changes the trickle-bed reactor to optimize its performance. Credit: Gretchen Ertl
No Shortage of Renewable Source Material– Or Attempts To Convert It
For the previous 5 years, understanding and resolving the SAF issue has been the goal of research by Román-Leshkov and his MIT group– Michael L. Stone PhD 21, Matthew S. Webber, and others– as well as their collaborators at Washington State University, the National Renewable Energy Laboratory (NREL), and the Pacific Northwest National Laboratory. Their work has actually concentrated on lignin, a tough material that gives plants structural support and defense against fungi and microorganisms. About 30 percent of the carbon in biomass is in lignin, yet when ethanol is produced from biomass, the lignin is left as a waste item.
Despite worthy efforts, nobody has discovered a financially practical, scalable method to turn lignin into beneficial items, including the fragrant molecules needed to make jet fuel 100 percent sustainable. Why not? As Román-Leshkov says, “Its because of its chemical recalcitrance.” Its hard to make it chemically respond in helpful methods. As a result, every year countless lots of waste lignin are burned as a low-grade fuel, used as fertilizer, or simply gotten rid of.
Understanding the issue requires understanding whats happening at the atomic level. A single lignin particle– the starting point of the challenge– is a huge “macromolecule” comprised of a network of lots of fragrant rings linked by oxygen and hydrogen atoms. Simply put, the key to transforming lignin into the fragrant fraction of SAF is to break that macromolecule into smaller pieces while in the process eliminating all of the oxygen atoms.
In basic, a lot of industrial procedures begin with a chain reaction that prevents the subsequent upgrading of lignin: As the lignin is extracted from the biomass, the fragrant molecules in it react with one another, connecting together to form strong networks that will not respond further. As an outcome, the lignin is no longer beneficial for making air travel fuels.
To avoid that result, Román-Leshkov and his team make use of another method: They utilize a catalyst to cause a chemical reaction that wouldnt usually take place during extraction. By responding the biomass in the existence of a ruthenium-based driver, they have the ability to remove the lignin from the biomass and produce a black liquid called lignin oil. That item is chemically steady, indicating that the fragrant particles in it will no longer respond with one another.
The scientists have now effectively broken the initial lignin macromolecule into pieces that contain simply one or two aromatic rings each. While the separated pieces do not chemically react, they still contain oxygen atoms. One job remains: finding a way to eliminate the oxygen atoms.
In truth, says Román-Leshkov, obtaining from the particles in the lignin oil to the targeted fragrant molecules required them to achieve 3 things in a single step: They required to selectively break the carbon-oxygen bonds to free the oxygen atoms; they required to avoid incorporating noncarbon atoms into the fragrant rings (for example, atoms from the hydrogen gas that must be present for all of the chemical improvements to occur); and they required to preserve the carbon backbone of the molecule– that is, the series of connected carbon atoms that connect the aromatic rings that stay.
Ultimately, Román-Leshkov and his group found a special component that would suffice: a molybdenum carbide driver. “Its in fact a truly amazing driver due to the fact that it can carry out those three actions extremely well,” says Román-Leshkov. “In addition to that, its extremely resistant to toxins. Plants can contain a lot of parts like proteins, salts, and sulfur, which frequently poison catalysts so they dont work anymore. Molybdenum carbide is very robust and isnt highly influenced by such pollutants.”
Trying It Out on Lignin From Poplar Trees
To evaluate their technique in the lab, the scientists initially created and developed a specialized “trickle-bed” reactor, a kind of chemical reactor in which both gases and liquids circulation downward through a jam-packed bed of catalyst particles. They then acquired biomass from a poplar, a type of tree referred to as an “energy crop” since it grows quickly and does not require a great deal of fertilizer.
To start, they reacted the poplar biomass in the presence of their ruthenium-based driver to draw out the lignin and produce the lignin oil. They then flowed the oil through their trickle-bed reactor consisting of the molybdenum carbide driver. The mix that formed included a few of the targeted product however also a great deal of others that still contained oxygen atoms.
Román-Leshkov notes that in a trickle-bed reactor, the time throughout which the lignin oil is exposed to the catalyst depends entirely on how quickly it leaks down through the packed bed. To increase the direct exposure time, they attempted passing the oil through the exact same driver twice. Nevertheless, the circulation of items that formed in the 2nd pass wasnt as they had actually forecasted based upon the outcome of the very first pass.
The very first time the lignin oil leaks through the reactor, it deposits oxygen onto the catalyst. “The temperature and oxygen material set the condition of the driver in the very first pass,” says Román-Leshkov. The process can hence run constantly: Two different reactors containing independent catalyst beds would be connected in series, with the very first pretreating the lignin oil and the second getting rid of any oxygen that remains.
Based on a series of experiments involving lignin oil from poplar biomass, the scientists identified the operating conditions yielding the very best result: 350 degrees Celsius in the initial step and 375 C in the 2nd action. Under those optimized conditions, the mixture that forms is dominated by the targeted aromatic items, with the rest consisting of little quantities of other jet-fuel aliphatic particles and some staying oxygen-containing molecules. The catalyst stays stable while generating more than 87 percent (by weight) of fragrant molecules.
” When we do our chemistry with the molybdenum carbide driver, our overall carbon yields are nearly 85 percent of the theoretical carbon yield,” says Román-Leshkov. “In the majority of lignin-conversion procedures, the carbon yields are really low, on the order of 10 percent. Thats why the catalysis community got very delighted about our outcomes– due to the fact that individuals had actually not seen carbon yields as high as the ones we generated with this driver.”
There remains one key question: Does the mixture of components that kinds have the homes needed for air travel fuel? “When we work with these brand-new substrates to make new fuels, the mix that we develop is various from standard jet fuel,” says Román-Leshkov. “Unless it has the specific homes required, it will not certify for certification as jet fuel.”
To examine their items, Román-Leshkov and his team send out samples to Washington State University, where a group operates a combustion lab dedicated to screening fuels. Outcomes from initial testing of the composition and properties of the samples have been encouraging. Based on the structure and released prescreening tools and treatments, the researchers have actually made preliminary residential or commercial property predictions for their samples, and they looked great. The freezing threshold, viscosity, and point sooting index are forecasted to be lower than the worths for conventional aviation aromatics. (In other words, their material should flow more quickly and be less most likely to freeze than conventional aromatics while also creating less soot in the atmosphere when they burn.) Overall, the forecasted homes are near to or more favorable than those of conventional fuel aromatics.
Next Steps and Potential Impact
“These particles are not the normal aromatic particles that you use in jet fuel,” says Román-Leshkov. “Preliminary tests with sample seals show that theres no difference in how our lignin-derived aromatics swell the seals, however we require to validate that.
In addition, he and his team are working with their NREL collaborators to scale up their approaches. NREL has much bigger reactors and other facilities needed to produce large amounts of the brand-new sustainable blend. Based on the promising outcomes so far, the group wants to be prepared for the additional screening needed for the accreditation of jet fuels. In addition to testing samples of the fuel, the full certification treatment requires showing its behavior in an operating engine– “not while flying, however in a lab,” clarifies Román-Leshkov. In addition to needing big samples, that demonstration is both lengthy and costly– which is why its the extremely last action in the stringent screening needed for a brand-new sustainable aviation fuel to be approved.
If further testing confirms that their aromatic items can replace the aromatics now in jet fuel, “the mixing wall could vanish,” says Román-Leshkov. “Well have a means of producing all the components in air travel fuel from sustainable material, potentially leading to airplane fuel thats 100 percent sustainable.”
Recommendation: “Continuous hydrodeoxygenation of lignin to jet-range fragrant hydrocarbons” by Michael L. Stone, Matthew S. Webber, William P. Mounfield III, David C. Bell, Earl Christensen, Ana R.C. Morais, Yanding Li, Eric M. Anderson, Joshua S. Heyne, Gregg T. Beckham and Yuriy Román-Leshkov, 22 September 2022, Joule.DOI: 10.1016/ j.joule.2022.08.005.
This research was at first moneyed by the Center for Bioenergy Innovation, a U.S. Department of Energy (DOE) Research Center supported by the Office of Biological and Environmental Research in the DOE Office of Science. More recent funding came from the DOE Bioenergy Technologies Office and from Eni S.p.A. through the MIT Energy Initiative. Michael L. Stone PhD 21 is now a postdoc in chemical engineering at Stanford University. Matthew S. Webber is a graduate trainee in the Román-Leshkov group, now on leave for an internship at the National Renewable Energy Laboratory.
“This is a subclass of fuels that has really specific requirements in terms of the chemistry and the physical properties of the fuel, since you cant risk something going incorrect in an aircraft engine,” states Yuriy Román-Leshkov, the Robert T. Haslam Professor of Chemical Engineering. Postdoc Jamison Watson signed up with the Román Lab in January 2021 to focus on the catalytic updating and conversion of lignin into the fragrant portion needed to make aviation fuel completely sustainable. Regardless of valiant efforts, no one has actually found a financially practical, scalable method to turn lignin into helpful items, consisting of the aromatic particles required to make jet fuel 100 percent sustainable. “When we work with these new substrates to make new fuels, the mix that we produce is different from basic jet fuel,” states Román-Leshkov. “Well have a way of producing all the parts in aviation fuel from sustainable material, potentially leading to aircraft fuel thats 100 percent sustainable.”
Teacher Yuriy Román-Leshkov and collaborators have actually shown a brand-new way to produce an important component of air travel fuel from lignin, a plant material thats normally discarded as waste throughout biomass processing.Credit: Gretchen Ertl
MIT researchers are transforming the plant product lignin into hydrocarbon particles that might help make jet fuel 100 percent sustainable.
An MIT research team is dealing with transforming lignin, a plant waste item, into 100% sustainable air travel fuel utilizing an unique driver. This breakthrough could reinvent the aviation market by providing a sustainable fuel alternative.
Nearly a quarter of the worlds carbon dioxide emissions in 2021 originated from the transport sector, with air travel being a substantial contributor. While the growing usage of electrical automobiles is helping to clean up ground transportation, todays batteries cant complete with fossil fuel-derived liquid hydrocarbons in terms of energy provided per pound of weight– a major concern when it comes to flying. On the other hand, based on predicted growth in travel need, usage of jet fuel is forecasted to double in between now and 2050– the year by which the worldwide air travel industry has vowed to be carbon neutral.