Within a few years, food waste could be fueling our airplanes.
Researchers have worked out a way to transform food scraps, used cooking oil, animal manure and wastewater sludge into jet fuel with a carbon footprint 165% lower than standard jet fuel, according to a new study published in the Proceedings of the National Academy of Sciences.
The emissions savings come from diverting the food waste from landfills, as well as from avoiding using fossil fuels. This “reduction in lifecycle greenhouse gas emissions would provide a path toward net zero jet fuel,” says the report.
The aviation sector has pledged to cut carbon emissions in half by 2050, but has found decarbonizing tough for the simple reason that fossil fuels have long been the cheapest and most efficient way to power planes. The U.S. uses more than 21 billion gallons of jet fuel every year, a figure that’s expected to double by 2050.
Aviation may make up a relatively small fraction of total global transport emissions — around 12% — and just 2.5% of all global greenhouse gas emissions. But air travel’s climate impact is projected to grow rapidly as flights become cheaper and more accessible to the growing global middle class, with emissions projected to triple by 2050, according to the United Nations’ International Civil Aviation Organization.
While the pandemic caused passenger numbers to plummet more than 60% in 2020, the industry is scrambling to get back on track as soon as possible.
The Quest To Replace Fossil Fuels
The idea of sustainable fuel that can be fairly easily produced from an existing (and free) waste stream has already attracted Southwest Airlines, which is participating in trials with the researchers involved in this new study. But some environmental campaigners are critical of the report’s analysis, saying that waste-to-jet-fuel technology is not a viable route to decarbonizing aviation and could be a distraction from the radical changes needed to tackle the industry’s significant emissions.
Airlines have taken some steps toward decarbonization. Lighter planes and better engines have increased fuel efficiency. Many airlines have also implemented voluntary carbon offset programs like reforestation and peatland restoration, although these programs are often criticized for providing tenuous climate benefits while allowing polluters to keep polluting.
To make the kind of deep emissions cuts needed to hit their climate pledges, many airlines are pinning their hopes on finding sustainable fuel alternatives.
Electric planes aren’t an option, at least in the short term. Solar-powered commercial planes exist, but the batteries required for long-haul flights using current technology would be far too heavy and take up too much space. And while biofuels made from crops like sugarcane and palm oil have been touted as another sustainable alternative, these can end up being environmentally damaging as farmers clear land to cultivate them.
By using food waste, the study’s researchers say, you have the double benefit of solving a waste management problem and creating a more sustainable fuel.
Food waste often ends up in landfills, where it emits methane, a greenhouse gas 84 times more potent than carbon dioxide in the first two decades after it’s released. The paper sets out a method of producing fuel with this waste in a way that avoids methane emissions and instead transforms it into “volatile fatty acids,” which can then be made into jet fuel.
“The ability to decarbonize aviation still needs very energy dense liquid fuels, and we wanted to present a technology where you didn’t have to reinvent a plane,” said Derek Vardon, one of the study’s authors and a senior research engineer at the National Renewable Energy Laboratory.
The fuel can just be used in planes’ engines without modification, according to the study, and it’s compatible with existing energy infrastructure. “You’re able to drop it into the infrastructure already so you wouldn’t have to create a whole new refinery,” said Nabila Huq, a researcher at NREL and a lead author on the study.
It could also reduce the amount of soot that planes produce by 34% compared to standard jet fuel, according to the report. Planes’ contrails form when water vapor condenses around soot to make cirrus clouds, thin clouds which can trap heat in a process that research suggests could have a greater climate change impact than planes’ carbon emissions.
The researchers plan to produce 300 gallons of the fuel for a flight demonstration with Southwest Airlines to show the technology can scale and to provide a blueprint of how quickly it can be implemented.
Vardon predicts that it could be possible to produce significant volumes of this fuel in three to four years.
“I’m a big proponent of multiple solutions to decarbonize aviation,” including electrification and other technologies, he said. But, he added, “a lot of those are beyond a 10-year time frame. I personally feel like climate change is a much more near-term problem.”
The researchers are not suggesting this fuel is a silver bullet, Vardon said. “I don’t think our goal is to say this solution is the magic solution, but it can show what science and technology can do as we’re trying to tackle these questions.”
We Need Radical Changes
Even with this caveat, not everyone is convinced. “On the surface, it makes complete sense,” said Cait Hewitt, deputy director of the U.K. nonprofit the Aviation Environment Federation. But, she added, “this kind of fuel is effectively another type of offset… What it does is to reduce emissions from landfill.”
There is so little the aviation industry can do to decarbonize in the near term, said Hewitt, that it’s desperate to jump on solutions that look good on paper. But it’s a risky move, she said: “We’ve got such a short time now between now… and when we need to have the whole economy at net zero emissions. And the aviation industry is so far from achieving that, that anything that sort of looks like a solution — and really isn’t in the long term — is a really dangerous thing to pursue.”
She’s concerned that people will be lulled into a false sense that the aviation industry has the answers, that “there’s this thing just around the corner that’s going to make flying fine.”
It could also distract from an honest debate about the really radical changes that need to be made, she said. These include the development of synthetic fuels, which take carbon from the atmosphere and turn it into a liquid fuel, and carbon capture and storage — technologies that promise large-scale emissions reductions but remain a long way off.
Ultimately, said Hewitt, “you can’t get away from the fact that at the moment, easily the best way to cut aviation emissions is to fly less.”
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