For many people, especially in rich countries, flying in planes amounts to the largest share of their personal carbon footprint. When I calculated my own footprint, flying amounted to about half of my emissions. A round trip from New York City to London emits around 1000 kilograms of carbon dioxide, more than many people in the world emit in a year.
Overall, global aviation makes up a much smaller proportion of total greenhouse gas emissions — just over 2 per cent in 2022. Yet cutting emissions from aviation is one of the most challenging aspects of the path to net zero. As demand for aviation fuel increases and other sectors become cleaner, flying is expected to make up an ever-increasing share of emissions.
What’s to be done? In this month’s Fix the Planet, I’ll look at the promise and perils of the so-called “sustainable aviation fuels” in which the aviation industry has placed its green hopes. I’ll focus in particular on a type of fuel made with captured CO₂ and hydrogen, which I saw firsthand during a reporting trip to a plant in Canada. Such “electro fuels” have so far been made only in trifling amounts but may soon be produced by the hundreds of millions of litres in plants around the world.
Sustainable fuels
You may have seen the billboards. Airlines everywhere are touting their use of “sustainable aviation fuel”, or SAF, as a more climate-friendly way of flying. Later this year, Virgin Atlantic plans to operate the first transatlantic flight powered entirely by SAF, made mainly from used cooking oil.
These alternative fuels can help to reduce emissions compared with conventional jet fuel; Virgin Atlantic says its greasy intercontinental hop emit 70 per cent less CO₂ than a normal flight. But the story isn’t as simple as all the publicity would have it.
The major advantage of all types of SAF is that they are “drop in” fuels that can power regular jet engines. That means airlines can use SAF to power their existing fleets. Hydrogen may eventually prove a better option, but there are no jets currently able to run on the clean-burning gas. Electric propeller planes powered by batteries or hydrogen fuel cells are also emerging, but are limited to smaller planes travelling short distances. Improving the fuel efficiency of engines also helps but can only go so far. Aside from flying less, using more SAF is the main option now available to cut emissions from aviation.
SAF currently makes up a tiny fraction of all aviation fuel. According to the International Air Transport Association, 300 million litres of SAF of all types were produced in 2022, less than 0.2 per cent of all aviation fuel used. However, that was triple the amount produced the previous year, and the group estimates that annual production could see a 100-fold increase to 30 billion litres by 2030 based on expanding production and commitments from airlines, as well as new interest from big oil companies.
Governments have also pushed to use more SAF. The UK is finalising a rule that would require SAF make up 10 per cent of the aviation fuel blend by 2030. In the US, the Biden administration has an initiative to produce more than 11 billion litres of SAF a year by 2030, and enough to supply all of US aviation by 2050. In April, the European Union went one further, mandating that at least 2 per cent of jet fuel be SAF by 2025 and 70 per cent by 2050, a policy it estimates will reduce CO₂ emissions from Europe’s aviation by two-thirds.
That all sounds good, but much depends on how all that fuel gets made. “Not all SAF is created equal,” says Matteo Mirolo at European environmental group Transport & Environment. “There are circumstances where SAF can be a cure that is worse than the disease.”
Nearly all the SAF produced today is made using biomass, like vegetable oils, animal fat or waste. Another kind of SAF can also be made using ethanol or other alcohols. Over their life cycle, these can, in theory, substantially reduce emissions compared with conventional jet fuels. However, in practice, they may actually increase emissions by competing with other industries for biomass or requiring more land to grow crops. Even if this problem could be avoided, there isn’t enough biomass to go around to meet the aviation industry’s expected demand.
“There simply are not the scale of feedstocks available to produce the amount of biofuel they are asking for,” says Guy Gratton at Cranfield University in the UK, who co-authored a recent report for the Royal Society detailing the pros and cons of different low-emissions aviation fuels.
Power to liquid
With biofuels in short supply, many are looking to what are known as “electrofuels” or “synthetic fuels” to meet future demand. These are mainly produced using captured CO₂ and hydrogen made by splitting water molecules with clean electricity. They are also sometimes called power-to-liquid fuels because they transform electricity into liquid fuel.
Because this process isn’t limited by biomass, electrofuels are widely seen as a better option than biofuels for the very large scales needed to power aviation. “We see a lot of potential for it, says Mirolo. He adds that another benefit of electrofuels is that they produce less air pollution, which could help reduce the significant climate warming effect of contrails produced by high-flying plane
“In the long run, it may prove to be the only game in town,” says Gratton. If it is made using CO₂ captured directly from the atmosphere and some of this carbon is stored in the process, “it is the only fuel really capable of being a true net-zero fuel,” he says.
In August, I visited a plant in Montreal’s industrial East End that recently made some of the first litres of such synthetic jet fuel. Built by a company called the SAF+ Consortium, the demonstration plant was a squat building full of metal piping and valves set in the middle of a large petrochemical facility that smelled strongly of tar.
A poster attached to some pipes explained how the process works: CO₂ is first converted to carbon monoxide (CO) under heat and pressure. This CO is mixed with hydrogen to produce something called “syngas”. Combining the syngas with more hydrogen and a catalyst, in what is known as the Fischer-Tropsch process, results in hydrocarbons that can then be refined to make jet fuel and other products, such as paraffinic wax.
“We re used to burning fuel to make power. Now, we are using power to make fuel,” Nasser Seraj, one of the company’s engineers, told me during the visit. He showed me some of the final product, a clear liquid sloshing in a big glass jug.
The humble jug held some of the only litres of synthetic jet fuel that have been made so far anywhere. “You’ve touched the future, because there’s nothing like it in North America,” says Jean Paquin, the company’s CEO.
He says the firm’s full-scale plant, which he hopes will start production in Quebec by 2029, will be able to produce 100 million litres of the fuel each year. It will rely on clean electricity from the region’s substantial hydropower and use 280,000 tonnes of CO₂ each year from a still undisclosed industrial source. Paquin says the fuel will reduce emissions compared with conventional jet fuel by up to 90 per cent.
From Germany and Denmark to the US, Australia and elsewhere in Canada, a slew of other companies are also preparing to ramp up production. In Washington state, a company called Twelve (named for the atomic mass of carbon) just broke ground on its first commercial electrofuel plant. The plant will use electricity from hydropower and CO₂ trucked in from a nearby ethanol plant, relying on an efficient process to convert CO₂ to CO, says Andrew Stevenson at the firm. Twelve aims for the plant to be producing about 750 litres a day by 2024, and eventually millions of litres a year.
“It’s a very small contribution at the moment,” says Stevenson. “But over time we see it as being one of the, if not the primary, ways to decarbonise aviation.”
Clean energy crunch
But electrofuels face a big problem as they ramp up production: making all that hydrogen, converting CO₂ to CO and powering the rest of the process requires vast amounts of clean electricity that’s already in limited supply.
It takes around 3 to 4.5 times as much energy to make a litre of electrofuel as the energy contained in the fuel itself, according to the Royal Society report. That makes producing the fuel a very inefficient way to use energy. The report found producing enough of the fuel to supply all of UK aviation would require 5 to 8 times the total renewable energy capacity of the UK in 2020.
A similar analysis for the US by Robert Weber at the Pacific Northwest National Laboratory and his colleagues found that sufficient production in the country would require nearly as much new clean energy as all the solar, wind, hydropower, and nuclear energy the US currently generates.
“The technology is there. Whether we can do it at scale isn’t so clear,” says Weber. Access to enough CO₂ could also be an issue in the long term. Capturing CO₂ directly from the atmosphere would require even more energy.
“The technology is there. Whether we can do it at scale isn’t so clear,” says Weber. Access to enough CO₂ could also be an issue in the long term. Capturing CO₂ directly from the atmosphere would require even more energy.
“In some sense, aviation needs to be privileged, because there’s no real alternative,” says Weber. But he argues it would initially make a bigger impact on emissions to use any new clean energy to replace fossil fuel generation on the grid, rather than to make cleaner jet fuel. “If you’re using electricity to make that fuel, you’re not using electricity to power cars or houses,” he says.
Mirolo, however, says that electrofuel production has no time to waste in starting to scale up if there is any hope of making enough fuel to meet climate targets. “Are the [e-fuel] projects going to be able to start production before 2030?” he says. “That is going to determine whether or not the aviation sector is going to decarbonise by 2050.”
In the meantime, he says, the best way to avoid emissions from flying is simple: fly less.
James Dinneen
Environment reporter, New Scientist