Planes Running on Thin Air? Researchers Make Jet Fuel from CO2

The airline industry is one of the largest contributors of greenhouse gas (GHG) emissions. According to the Air Transport Action Group (ATAG), worldwide flights in 2019 generated 915 million tons of carbon dioxide (CO2)—2 percent of all emissions. The Environmental and Energy Study Institute (EESI) estimates that commercial aircraft emissions could triple by 2050 based on the projected growth of passenger air travel and freight. In the U.S., which has the world’s largest commercial air traffic system, aircraft contribute 12 percent of transportation emissions and account for 3 percent of the nation’s total GHG production. These are significant numbers, and they need to be addressed in order to meet carbon-reduction goals.

There are many innovative solutions on the horizon. For example, Airbus has announced plans for the first zero-emission aircraft by 2035 that will use hydrogen fuel cells. Electric planes may also be a viable concept. For now, the potential to use hydrogen as a fuel or batteries to run planes is restricted by the range and power required. The size and weight of batteries or hydrogen fuel tanks make them impractical, as they would be much larger and heavier than current combustion engines.

Therefore, combustion engines are here to stay at least for the near future. In the meantime, the industry is working on offsetting its emissions in other ways, including purchasing carbon offsets or using sustainable aviation fuels. Another option is the promotion of the “circular economy.” 

Illustrating the difference between (a) an Aviation Jet Fuel Linear Economy and (b) an Aviation Jet Fuel Circular Economy. (Image courtesy of Nature Communications.)

Illustrating the difference between (a) an Aviation Jet Fuel Linear Economy and (b) an Aviation Jet Fuel Circular Economy. (Image courtesy of Nature Communications.)

The current approach to utilizing resources is what is known as a “linear economy,” where raw materials are extracted, processed into products, and disposed of. As the global population grows exponentially, this policy is becoming increasingly unsustainable considering that most resources are nonrenewable. In the circular economy, waste is recycled and reused into something more useful. Researchers at Oxford University have designed just such an experimental process, through which captured CO2 can be converted into jet fuel.

“Climate change is accelerating, and we have huge carbon dioxide emissions,” said Tiancun Xiao, a senior research fellow at Oxford’s Department of Chemistry and an author on the paper. “The infrastructure of hydrocarbon fuels is already there. This process could help relieve climate change and use the current carbon infrastructure for sustainable development.”

The Chemistry Behind the Experiment

Time for some high school chemistry. What happens when hydrocarbons are burned? They turn into CO2 and water while releasing energy.

Combustion of hydrocarbons: Methane is combined with oxygen to form carbon dioxide, water and energy. (Image courtesy of Energy Education/University of Calgary.)

Combustion of hydrocarbons: Methane is combined with oxygen to form carbon dioxide, water and energy. (Image courtesy of Energy Education/University of Calgary.)

The Oxford procedure essentially reverses this process. The engineers used a process called the Organic Combustion Method (OCM) to convert CO2 into jet fuel and other products. It involved using an iron-based catalyst (with added potassium and manganese) along with hydrogen gas, citric acid and CO2. The mixture was heated to 350 °C in a reaction chamber pressurized to 1 MPa (10 times the atmospheric pressure at sea level). The carbon atoms in CO2 molecules, forced apart from the oxygen atoms, were then bonded with hydrogen atoms in a process known as hydrogenation. This resulted in the hydrocarbon molecules that comprise liquid jet fuel. The leftover oxygen atoms from the CO2 joined up with other hydrogen atoms to form water.

Testing showed that over 20 hours, the catalyst was able to convert 38 percent of the CO2 into jet fuel and other products. The selectivity of the conversion to jet fuel was 48 percent, and other reaction products included ethylene, propylene and butenes.

A representation of the CO2 hydrogenation to jet fuel. (Image Courtesy of Nature Communications.)

A representation of the CO2 hydrogenation to jet fuel. (Image Courtesy of Nature Communications.)

Flying on Air or a Lot of Hot Air?

There have been many other studies about the conversion of CO2 into fuel. However, they relied on catalysts made of relatively expensive materials, like cobalt, and required multiple chemical processing steps. The resulting fuel is far more expensive than fossil fuels. The catalyst powder for this experiment is composed of the abundantly available minerals iron, manganese and potassium, and it transforms CO2 in a single step. Consequently, the catalyst is cheaper to prepare than many of the other candidates and will also produce a cheaper fuel. The researchers claim that their process is less expensive than other methods used to produce fuel for airplanes, such as the indirect method of converting CO2 to methanol to jet fuel—primarily because it uses less electricity.

Though it is a promising experiment, many unanswered questions remain. “This does look different, and it looks like it could work,” said Joshua Heyne, associate professor of mechanical and chemical engineering at the University of Dayton. “Scale-up is always an issue, and there are new surprises when you go to larger scales. But in terms of a longer-term solution, the idea of a circular carbon economy is definitely something that could be the future.”

As reported by the BBC, there is already a similar initiative at Rotterdam Airport in the Netherlands to build a pilot plant. It will capture CO2 using Climeworks’ direct air capture technology running on energy from solar panels and convert it into jet fuel. The aim is to produce a thousand liters of jet fuel a day starting in 2021. Oskar Meijerink’s company is developing the plant in partnership with Climeworks and the airport’s owners. “The main element is the cost,” he conceded. “Fossil jet fuel is relatively inexpensive. Capturing CO2 from the air is still a nascent technology and expensive.”

Direct air capture machines by Climeworks, which extract CO2 from the atmosphere (Image courtesy of Climeworks.)

Direct air capture machines by Climeworks, which extract CO2 from the atmosphere (Image courtesy of Climeworks.)

Which is why the uptake of carbon capture technology has been slow. To capture CO2 at a power plant or another point source, costs range from approximately $50 to more than $100 per metric ton. But CO2 in the atmosphere is many orders of magnitude less concentrated than emissions from a power plant’s smokestack. Using the same technology to extract CO2 from the atmosphere would lead to estimated costs from $600 to $1,000 per metric ton.

The operating principle behind point-source capture and atmospheric extraction of CO2 are similar, according to Christopher Jones, a chemical engineering professor at the Georgia Institute of Technology. A stream of air is sent through a liquid or solid sorbent that absorbs CO2 from the stream and leaves behind clean air. The sorbent is then heated to release CO2 in a concentrated form. The energy used to heat the sorbent to release the captured CO2 is the biggest cost driver. That is why extracting CO2 from the air is pricier, as it will require a higher volume of captured air and much more heat to extract the same amount of CO2. The Oxford University researchers claim that their conversion system could be installed directly at the point source of carbon dioxide, making it more feasible.

As Friends of the Earth campaigner Jorien de Lege told the BBC about the upcoming technology at Rotterdam airport, “If you think about it, this demonstration plant can produce a thousand litres a day based on renewable energy. That’s about five minutes of flying in a Boeing 747.” Similarly, the OCM process was only able to produce a miniscule amount of jet fuel, and that too at a selectivity of only 48 percent. Reaction yield is definitely one of the major hurdles, and the process needs to be optimized. However, the by-products slightly balance this, as the other hydrocarbons are important raw materials for the petrochemical industry. Hence, some of cost of capturing and converting CO2 can be offset.

What’s in Store for the Future?

Fundamentally, the fuel produced is the same as what is currently used in the aviation industry. So how does it compare in terms of cost? “This is the critical question that now occupies us,” researcher Benzhen Yao told Forbes. Though the team is collaborating with the aviation industry on cost comparisons, Yao revealed that “the capital cost of our process is half of the indirect route”—which is the process of converting CO2 to methanol and then to jet fuel. A direct comparison with jet fuel from crude oil is not yet available.

The team is aiming to collaborate with industry to build the world’s first net zero-emission demonstration plant. “The main challenges will be the cost of CO2 and H2,” Yao said, “but we believe that, with the development of CO2 capture technology and cheaper H2 generated via renewable energy, the cost will be significantly reduced.”

Ultimately, it all depends on how quickly the technology can be scaled up. The researchers want to connect their new system to established carbon emitters, such as coal-burning power plants, which would of course require continued fossil fuel production. The cost is also prohibitive and might not be appealing to businesses even if it did work, due to the low yields.

Regardless, with climate change accelerating and aviation only set to increase in the foreseeable future, the team argues that CO2 conversion and utilization is “an integral and important part of greenhouse gas control and sustainable development.”

“This, then, is the vision for the route to achieving net-zero carbon emissions from aviation,” the study concludes, “a fulcrum of a future global zero-carbon aviation sector.”

Developing a cheap, high-yield catalyst is a major step toward making the idea of a circular economy more feasible. Even though getting planes to fly using nothing but air sounds like a pipe dream, it just became a little bit closer to reality.