Prepare for takeoff with plant-powered planes

What’s possible for sustainable aviation fuels?

A shot of a plane taken from an airport in Sweden. Philip Myrtorp, Unsplash
15 February 2022
15 February 2022

Two years ago, air travel was a ubiquitous part of life for much of the world. But COVID-19 changed that. Our global footprints diminished as international conferences ran remotely, work trips were done via Zoom and people began keeping their holidays domestic, in fear of being stranded abroad. 

And so our carbon footprint reduced in stride: emissions from aviation, which make up 2.1 percent of all human-induced carbon emissions, dropped by 48 percent in 2020 from their 2019 level. 

But as countries begin the staggering dance of re-opening and managing the risks that presents, air traffic is on the rise once again, with predictions it will return to pre-pandemic levels in 2024. That’s reassuring for long-distance relatives, tourism and the airline industry – but less so for the climate. 

So, what would it take to ‘green’ our flying habits? Electric and hydrogen-powered planes are still “decades away” from large-scale application, according to University of Bath chemical engineer and PhD student Stephen Doliente. But sustainable aviation fuels (SAFs) could make some decisive impacts in the shorter term – if we can manufacture them without causing other landscape challenges in the process.

What are SAFs?

SAFs are produced from sustainable feedstocks such as waste oils and agricultural residues. They can generate up to an 80-percent reduction in carbon emissions over their lifecycle when compared to regular jet fuel, because they burn cleaner and can be grown, extracted, developed and disposed of in more environmentally friendly ways.

These alternative fuels have already been in circulation for a while. The first test flight using blended biofuel (biofuel mixed with petroleum-derived fuel) took place in 2008, and fuels with up to 50 percent biofuel have been allowed on commercial flights since 2011. Leading aircraft manufacturer Boeing has said it will begin delivering airplanes capable of flying on 100 percent SAFs by 2030. 

According to International Air Travel Association (IATA) director general Willie Walsh, SAFs will be key to the global airline industry meeting its recent pledge to achieve net zero carbon emissions by 2050. He estimated in a press release that they could make up around 65 percent of the required emissions abatement.

Oilseed crops, such as palm oil, can be used to make SAFs. But are they sustainable? Nanang Sujana, CIFOR
Oilseed crops, such as palm oil, can be used to make SAFs. But are they sustainable? Nanang Sujana, CIFOR

Fossil-fuel mimicry, minus the footprint

That transition to ‘greener’ fuel will not be simple one. Regular aviation fuel is made from extremely refined kerosene, and creating SAFs that can function in the same way is difficult. “It’s a very specific formulation, because you don’t want it to freeze at higher altitudes,” says Doliente. “You need it to maintain its liquid state and not cause any problems with your turbines.” 

Scientists have found several ways to make SAFs that mimic these properties. Most commonly, they use vegetable oils, which can be derived from crops such as palm and rapeseed, and/or gathered as waste from deep-frying enthusiasts like restaurants, fast-food joints and commercial kitchens. Algae oil is another potential resource that could offer much higher emission savings, but the technology to cultivate, harvest and extract it at scale is not yet commercially viable. 

The Fischer-Tropsch method, a collection of chemical reactions that can extract energy from a range of source materials, is another pathway for creating SAFs. Using this method, the fuel can be generated from what’s known as ‘lignocellulosic biomass’ – abundant and renewable dry matter from plants, sourced from natural by-products, such as from agriculture (including straw, stover and husks) and forestry (such as sawdust, trimmings and shavings). U.S. company Fulcrum Bioenergy, for example, is using the Fischer-Tropsch method to make SAFs and other transportation fuels out of household rubbish.

SAFs can also be generated from bioethanol, which is made by fermenting crops such as sugarcane, and directly from sugars. In Brazil, where massive sugarcane plantations have already been established, there’s particular interest in these two methods. 

Biomass balancing acts

No matter which method of making SAF is used, raw material is required, which raises the question: can we grow and gather enough stuff to produce SAFs at a large scale – and in ways that are genuinely sustainable? 

At present, the answer is unclear. Demand for agricultural land is high across the globe, and if that space is used for SAFs, food production and standing forests may well pay the price. “We’re actually adding more pressure to the agricultural sector [by pushing for SAFs],” says Doliente, “because the aviation sector also wants a share of the feedstock that is already being used for food and road transport biofuels.” 

That has ethical implications, particularly given that most feedstocks used for SAFs are produced in large part in developing countries in the tropics. “Europe and North America will be the biggest users of aviation fuel,” says Doliente, “but I’m interested in how the developing countries can benefit from being the source of these raw materials – and how much this is going to impact their own food, water and energy systems, because it’s going to come from their lands.”

Pongamia pinnata seeds. Cerlin Ng, Flickr
Pongamia pinnata seeds. Cerlin Ng, Flickr

One potential solution is to focus on non-food oilseed crops that can be grown in areas where other agricultural practices are unlikely to thrive. For instance, researchers have been exploring the use of pongamia trees (Pongamia pinnata, a resilient Southeast-Asian native with oil-rich seeds) for bioenergy purposes, including jet fuel. Pongamia grows well in degraded, deforested environments, including ex-mining land and drained peatland – and because it’s a leguminous tree, it fixes nitrogen into the soil as it grows, improving fertility. However, now that U.S. company TerViva has worked out how to turn the seeds into an edible protein and oil, food production may make more sense for pongamia cultivators, as it offers higher economic returns and meets a more pressing need.

Making it count

While petroleum products remain artificially cheap, the cost of SAFs compared to standard jet-fuel remains a hurdle for building demand, developing value chains and facilitating larger-scale transition. As such, upping SAF use will require government intervention, for instance by investing in industry development, offering subsidies and requiring aircraft to use a certain percentage of the fuel when they travel. “It really is about a social, political, international movement,” says Doliente. 

“The actual split [in terms of the proportion of emission reduction achieved through SAFs and other avenues] and the trajectory to get there will depend on what solutions are the most cost-effective at any particular time,” says Walsh. “Whatever the ultimate path to net-zero will be, it is absolutely true that the only way to get there will be with the value chain and governments playing their role.”

In the meantime, those pandemic-induced habits of traveling less will continue to serve us well: the greenest flight is still the one we simply don’t take.

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