#2 | Kerosine from CO2
Boarding a plane without guilt
Door Jelka Pospig
By Jelka Pospig
Jelka Pospig
Jingxiu Xie scribbles letters on a piece of paper. A C for carbon, an O for oxygen, an H for hydrogen. Then she adds the correct coefficients to balance the equation. She points at one of the Cs and says: ‘It is just like with Lego – you can make anything out of it as long as you know how to build it.’
Carbon is one of life’s essential building blocks, found in nearly everything on Earth. What she, as a chemical engineer, can create from it with these other elements is fuel – sustainable kerosene, to be exact. And if she succeeds in finding a way to simplify the process so that this fuel can be used in commercial aviation, it would be revolutionary.
Fossil fuels are a major cause of climate change, with aviation responsible for approximately 2 percent of global CO2 emissions. That might not seem like much, but for ordinary citizens, their flying habits are one of the key ways they can impact their carbon footprint—cleaner flying would, therefore, be incredibly valuable to them.
Fischer-Tropsch synthesis
To understand how kerosene could be produced sustainably, it helps to first look at how it’s made today. Kerosene traditionally comes from crude oil that is pumped from the earth. This oil is a complex mixture of various chemicals, called hydrocarbons. During a refining process, the mix is separated into various fractions, each serving a specific purpose, including gasoline, diesel, and jet fuel.
Carbon-neutral isn’t the same as carbon-negative, which is what we need
Scientists have developed a method, however, to produce kerosene using something readily available in our atmosphere: carbon dioxide (CO2). In this process, CO2 is first converted into carbon monoxide (CO), which can then be combined with hydrogen to enter a process called Fischer-Tropsch synthesis, producing hydrocarbons. As the third step, kerosene is formed from the mix of hydrocarbons.
These steps have already been tested in practice. In 2019, Shell produced 500 liters of green kerosene by combining CO2 collected at an airport with green hydrogen. This sustainable fuel successfully powered a flight from Amsterdam to Madrid.
It was only that one flight though; the stunt was never repeated. ‘This was a nice showpiece, but you have to ask yourself why it hasn’t been commercialised yet’, Xie stresses.
Fewer steps
One of the reasons for that is that the production process is still too complicated. That’s what Xie hopes to change with her research. She’s already partly succeeded, because she and her team managed to reduce the three steps to two. ‘Our idea was: wouldn’t it be nice if we started directly from CO2?’ So that’s what they did. Now, CO2 is turned into hydrocarbons in the same step. That sounds like a small difference, but it makes the process much more energy efficient and with it, cheaper and more sustainable.
Ultimately, she hopes to go straight from CO2 to kerosene in a single step. Her face lights up as she discusses this vision. ‘It’s a huge challenge − no one has managed it yet − but that’s what I’m working toward.’
The proof of concept that those first two steps of the process can be combined earned her an ERC Starting Grant, a 1.5 million euro grant that is awarded to outstanding early career researchers.
Production costs
Does that mean guilt-free air travel is on the horizon, then?
Well, no, says Xie, not anytime soon. ‘It is true that we can create a fuel that is carbon-neutral’, she explains. CO2 is captured from the atmosphere, turned into fuel and released again when that fuel burns during flight, theoretically resulting in net-zero emissions. ‘But carbon-neutral isn’t the same as carbon-negative, which is what we need to truly combat climate change.’
There simply isn’t enough green hydrogen available yet
Additionally, current engine designs limit planes to using only up to 50 percent sustainable fuel, as it isn’t chemically identical to conventional kerosene. ‘That means flying on “sustainable” kerosene isn’t fully carbon-neutral yet.’
And then there’s the high cost of producing the fuel. ‘It requires green hydrogen, which is very, very expensive. It accounts for 70 to 80 percent of the cost.’ Purifying CO2 from waste gasses is also expensive, she says, but pales in comparison to the price of green hydrogen. ‘There simply isn’t enough green hydrogen available yet, and creating more requires more green energy, which we also lack.’
Reducing energy consumption
Increasing the production of nuclear energy might be one potential solution, she says, but that will take many years to realise. ‘Flying is just one way we use energy, and right now, there’s simply not enough green energy to support our current consumption levels’, she says. Her personal conviction: ‘The only way to truly combat climate change is for everyone to consume less energy. ’
That’s why Xie limits herself to one long-haul trip a year, to visit her family in Singapore. ‘For shorter distances, I always take the train. And I do wish that more people would do that and try to avoid unnecessary flying.’
For now, however, her research offers a hopeful prospect for eco-conscious travelers. Because it might only reduce global emissions by 2 percent, but it’s still a step closer to solving the complex puzzle of climate change.