Your Next Flight’s Fuel Could Be Made By Microbes

Over the next few months, the world’s first commercial-scale ethanol-to-jet fuel plant will begin producing it first gallons of aviation fuel. The plant, located in Soperton, Georgia will proceed producing the fuel developed by LanzaTech and its spin-out LanzaJet. The effort represents a major milestone in their nearly 20-year effort to turn waste industrial gases into ethanol, and then to jet fuel. LanzaTech’s aim is to use gas fermentation, in which microbes metabolize waste industrial gases as feedstock and convert them into ethanol and other useful chemical compounds. For now, the company’s aviation-focused spin-out, LanzaJet, produces ethanol-to-jet fuel using catalytic steps adopted from the petrochemical industry, but their ultimate goal is to leave much of the catalytic work to microbes through gas fermentation.

Organisms that ferment gas can use industrial carbon dioxide emissions as a feedstock. Such gas fermentation is not nearly as well developed as traditional fermentation, in which microbes are fed sugars or other biomass — including crops. But both biomass or gas-based approaches are amenable to genetically manipulating selected microbial strains and then growing them for fermentation in large tanks.

Carbon dioxide-based technology may have a long road ahead of it, but the opportunity to use this and other waste emissions as feedstocks makes it a technology worth pursuing. “A bioeconomy without gas fermentation is an incomplete bioeconomy,” says Fiona Mischel, director of international outreach at SynBioBeta, an industry organization and network for biological engineers. “Carbon is an amazing resource. Our whole world is built on it. But right now, it’s in the wrong form in the wrong spot — it’s atmospheric carbon — and we need to be utilizing it through biotech,” she says.

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LanzaTech is one of several companies globally exploring the use of gas fermentation to make specialty chemicals, fuel precursors and proteins. Visolis, for example, is developing different organisms that can utilize gas, biomass or municipal solid waste to produce mevalonic acid as an intermediate industrial chemical, which can be processed into rocket fuel, aviation fuel, synthetic rubber and specialty chemicals after exposure to further chemical catalysis.

In gas fermentation, carbon monoxide, carbon dioxide or gasified biomass are pumped into fermenters. Some organisms may require other gases, such as hydrogen, which can serve as an energy source. The gases are mixed into a liquid containing a microbe, which can be genetically engineered, that will metabolize the gases and produce the molecule of choice.

Some products require two-stage fermentation, in which an intermediate chemical such as acetic acid is produced through a round of gas fermentation. The acetic acid then becomes a feedstock for a round of traditional fermentation, in which more complex molecules are produced.

The microbes scientists have focused on for gas fermentation include anaerobic acetogens such as Clostridium, which natively use a chemical pathway involving acetyl-coenzyme A to synthesize acetate, and a hydrogen-oxidizing knallgas bacterium such as Cupriavidus necator, which can produce complex molecules for bioplastics and oils.

But developing microorganisms for gas fermentation has been slow to take off, in part because the microbes are not nearly as well characterized as the workhorses of traditional fermentation, such as yeast and Escherichia coli, says Hendrik Waegeman, head of business operations at Bio Base Europe in Ghent, Belgium, which serves as a pilot plant and accelerator for companies developing bio-based products. “It takes seconds to engineer E. coli or yeast, but with acetogens, it takes years to get simple mutations that improve functionality,” he says.

Since acetogens are strictly anaerobic, working with them in the lab can be tricky. “Even a small spoor of oxygen can kill them, which is not very convenient if you want to do manipulations with engineering,” Waegeman says. He estimates that there are fewer than 30 groups worldwide that have attempted to engineer Clostridium species for gas fermentation.

Gas fermentation can come with some expensive or energy-intensive technical challenges. The microbes, to do their jobs, need an energy source such as hydrogen pumped into their fermentation tanks. But conventional hydrogen production is carbon intensive, so some groups are creating renewable hydrogen on site by splitting water into oxygen and hydrogen using electrolysis — an expensive endeavor. Sufficiently mixing gas with microbes, which live in water, has also proved expensive and requires sophisticated engineering.

Adding to the challenges, microbe development and engineering has been laborious. LanzaTech put years into characterizing and developing Clostridium autoethanogenum for use in its gas fermentation system. This anaerobic bacterium can take in carbon monoxide and,metabolize it to produce acetate. Using a technique called directed evolution, LanzaTech steered the organism’s metabolism to convert carbon monoxide into ethanol.

LanzaTech now uses this species at six commercial-scale gas fermentation plants. Five of them — four in China and one in Belgium — are located at steel mills or ferroalloy production plants; they convert waste carbon monoxide from those facilities into ethanol. The sixth plant, in Panipat, India, draws carbon dioxide from an oil refinery and converts it to ethanol.

Most of the ethanol that LanzaTech produces from these plants goes into products that would otherwise use virgin fossil carbon, says John Holladay, vice president of government programs at LanzaTech. These products include textiles, shoe soles, packaging, cleaning products, surfactants and detergents. But LanzaTech is also looking to convert the ethanol it produces into jet fuel. “We’re focused on industries that are hard to decarbonize, and aviation is very hard to decarbonize,” says Holladay.

Aviation accounts for about 2 percent of global energy-related carbon dioxide emissions, according to the International Energy Agency, and it has proven difficult to reduce that figure. Battery- and hydrogen-powered aircraft have both been proposed as solutions, but both come with major technical limitations and would require the expensive endeavors of redesigning aircraft and retrofitting airports.

For those reasons, trade groups such as the International Air Transport Association have pushed for sustainable aviation fuel as the most viable option. The International Civil Aviation Organization, a United Nations agency, defines sustainable aviation fuel as renewable or waste-derived aviation fuel that meets sustainability criteria.

Airlines seem to be open to the idea. Delta, Air France, International Airlines Group and Oneworld Alliance have all agreed to a target of 10 percent sustainable aviation fuel by 2030. In November, Virgin Atlantic and Gulfstream Aerospace separately flew test flights across the Atlantic using 100 percent sustainable aviation fuel.

Governments are mandating and incentivizing use of sustainable aviation fuel. Europe in October finalized the adoption of the ReFuelEU aviation regulation program, which requires fuel suppliers to blend increasing amounts of sustainable aviation fuel with conventional jet fuel, starting with a 2 percent minimum blend in 2025 and rising to 70 percent in 2050. The United States offers tax incentives for renewable fuels and has since 2005 maintained a renewable fuel standard program that requires that a certain volume of renewable transportation fuel replace petroleum-based fuel.

Most sustainable aviation fuel currently on the market is made from cooking oil, vegetable oil, edible animal fats and industrial grease — a product called hydroprocessed esters and fatty acids. But the supply of these fats and oils is limited and fragmented. As a result, trade groups have urged sustainable fuel developers to look more broadly into diverse feedstocks, including biomass and municipal waste.

Converting such feedstocks to jet fuel is likely to require combining different technologies. One option is to pair traditional fermentation, to convert crops or other biomass to an alcohol such as isobutanol or ethanol, with a series of chemical reactions called alcohol-to-jet,to convert the alcohol to jet fuel. Another route is to gasify biomass into syngas, which is a mixture of hydrogen and carbon monoxide, and then convert it to jet fuel in the well-established Fischer–Tropsch chemical process.

But these routes require large quantities of biomass and come with land-use concerns, particularly when crops are the starting material. This has led some groups to turn to waste gas such as carbon dioxide or carbon monoxide as a feedstock. Gas fermentation is a newer route to converting the gas; thermochemical and electrochemical processes are more established.

It is also possible to capture carbon with enzymes. Novonesis (previously Novozymes) in Bagsværd, Denmark, and its partner Saipem in Milan are developing carbonic anhydrase enzymes that capture carbon dioxide and convert it to bicarbonate. The genes encoding the enzyme came from a thermophilic, deep-sea organism. Novozymes transferred the genes to a proprietary host organism that produces the enzyme at large scale.

Saipem built a demonstration plant that captures 30 tons of carbon dioxide per day. Enzymatic carbon capture is more efficient than traditional, solvent-based capture approaches because it requires less energy to operate and less purification of flue gases, says Klaus Skaalum Lassen, Novozymes’ head of carbon capture and storage.

Microbial gas fermentation comes with several advantages over chemical approaches to gas conversion, says Heleen De Wever, biotechnology project manager at VITO in Mol, Belgium. For instance, microbes are better at handling certain impurities and fluctuations in gas composition, she says. And microbes can produce more complex molecules than chemical catalytic approaches, with the production occurring at a higher specificity for the target molecule.

“If you want to make simpler, one-carbon chemicals, like formic acid or methanol, there is no need to go with gas fermentation. There are good chemical catalysts for that,” says De Wever. “But if you want to make more complex molecules with five carbons or more, or even polymers like polyesters, that is exactly what biotechnology is capable of,” she says.

LanzaTech is pressing on with decarbonizing jet fuel, primarily through its partnership with LanzaJet, which converts ethanol to jet fuel using a chemical catalyst. The new ethanol-to-jet fuel facility in Georgia, built by LanzaJet, will be the largest alcohol-to-jet plant in the world, with a capacity of 10 million gallons of sustainable aviation fuel per year. Production at this plant alone will account for two-thirds of sustainable aviation fuel in the U.S., the company estimates.

But here’s the rub: LanzaJet’s plant doesn’t source its ethanol from LanzaTech’s gas fermentation process. For now, LanzaJet’s ethanol comes from Brazilian sugar cane. That’s because ethanol derived from the waste gas of steel or ferroalloy plants is not eligible for incentives under the U.S. Renewable Fuel Standard program. “Policy is lagging innovation,” says Holladay. So LanzaJet is buying its ethanol from a source that qualifies.

While it awaits policy change, LanzaTech is pursuing a flurry of projects that source different kinds of feedstock from non-metal alloy industrial sites. These include syngas, derived from reforming biogas or by gasifying forestry or agricultural residues, and municipal solid waste, which the company will convert to ethanol using gas fermentation and then to jet fuel using LanzaJet’s catalytic alcohol-to-jet process. The hope is that these feedstocks will qualify the company’s sustainable aviation fuel for incentives, but the projects are still in early stages.

Such are the challenges for a biotech company on the front wave of a new technology.

This article is reproduced with permission and was first published on February 15, 2024.


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