Aviation’s quandary over ‘sustainable’ fuels

Although biomass-based fuel is far from the perfect long-term solution for an industry under intense pressure to reduce its carbon footprint, the alternatives have barely got off the ground yet
An illustration of a jet engine, where the propellers are green leaves

With the con trails of jet airliners painting the industry’s greenhouse gas emissions across the skies for all to see, aviation is an obvious target for those seeking to decarbonise the global economy. 

Its emergence from the Covid crisis is well under way, with global airline traffic last year recovering to more than two-thirds of 2019’s total. This has only added to the pressure on the sector to find greener fuels.

Much of the focus to date has been on biomass-derived sustainable aviation fuel (SAF), which is made by converting a natural, notionally sustainable, feedstock into a usable form using various industrial processes. But there’s a growing debate, in both the sector and the scientific community, about whether SAF is the best long-term option or whether the R&D effort should move on other fuels with more lasting potential.

Net-zero carbon dioxide emissions by 2050

Aviation knows it must continually reduce its carbon footprint if it’s to hit the 2050 net-zero target set by its industry body, the International Air Transport Association. It’s responsible for fewer than 3% of the world’s anthropogenic greenhouse gas emissions, but its visibility and likely growth in activity means that it holds a prominent position in people’s minds as a sector with room for improvement.

Although 60% of Heathrow Airport’s airlines by capacity have pledged to turn at least 10% of their fuel supplies to SAF by 2030, there is no guarantee that enough of it will be available.

“It represents only about 1% of consumption at present and we’re using all the SAF there is,” reports Claudia Galea, a former Boeing manager who is now global sustainability director at US consultancy Kearney. She adds that it’s “three times more expensive than conventional jet fuel, which makes it a hard pill for airlines to swallow”.

This means that ramping up the production of SAF will be crucial if it’s to stand any chance of mitigating the industry’s use of fossil fuels. But those seeking to do this will “need to think about the wider ecosystem and the whole value chain”, Galea says. “The case for using SAF has been proven. Airlines want to use it and governments want to make it affordable, so the pendulum has shifted towards the production side.”

Not a long-term fix, but a key interim measure

As with every type of fuel, the production of SAF has an impact on the environment and may draw on resources that could be used for other purposes. A policy briefing published by the Royal Society in February discusses the potential constraints of biomass-based fuel solutions, highlighting the fact that they exhibit “significant resourcing implications, particularly energy crops, which would require at least half of all UK agricultural land for their cultivation to supply the whole amount of jet fuel used in the UK”.

The document adds that this would incur “significant trade-offs with food production”, potentially adding to the nation’s carbon emissions via increased food imports and soil erosion.

SAF might not be the ideal solution, then, but few obvious alternatives to it have emerged so far.

“The development of SAF has gained significant attention as a short-term solution,” says Lee Sykes, commercial director at transport engineering firm Drive System Design. “But it’s widely recognised that it is a temporary measure while the industry transitions towards more sustainable alternatives.”

What are the alternatives to biomass fuels?

Broadly speaking, these alternatives include synthetic fuels (sometimes called electrofuels), hydrogen, ammonia and battery power.

Each one, predictably, has its pros and cons, with a common factor being the fact that virtually all are at an early, or even hypothetical, stage of development.

SAF will serve as a crucial stopgap solution, providing significant environmental benefits

Professor Nigel Scrutton is the co-founder and chief scientific officer of C3 Biotechnologies. He believes that the synthetic fuel his firm is developing has an advantage over SAF in that it relies on “readily available” waste material. He envisages that it could be manufactured on site by users, cutting the financial and environmental costs of transporting it.

“We want to move away from the central refinery model, because the carbon footprint and some of the costs of taking fuel around the world are enormous,” Scrutton says.

Battery-powered flights are “still subjects of extensive deliberation and research”, while the hydrogen-based systems require sophisticated tech for pumps, fans and thermal management, according to Sykes.

The Royal Society paper notes that the adoption of hydrogen and ammonia would require the “substantial modification of aircraft and infrastructure”, which could dampen demand for those fuels.

Ushering in electric aircraft

The Mission Possible Partnership (MPP) is a corporate coalition seeking decarbonisation throughout the value chains of the world’s most carbon-intensive sectors. In July 2022, it published an industry-backed strategy entitled Making Net-Zero Aviation Possible. This document sets out an optimistic scenario and a more prudent one for aviation’s path to carbon neutrality, with the former assuming “a faster cost decline of renewable electricity and hence, more favourable economic conditions for electricity-based technologies”. This would bring battery-powered aircraft and those powered by hydrogen fuel cells to market earlier and at a larger scale.

It’s notable that even the MPP’s “business as usual” scenario (in which no progress is made in these alternative technologies) predicts that continued improvements to the efficiency of existing aviation fuel at historical rates would lead to a 26% reduction in the industry’s global greenhouse gas emissions by 2050.

It should be noted that non-fuel-based innovations can play a key role in reducing the industry’s greenhouse gas emissions. Iris, the new air-traffic management system created by Inmarsat and the European Space Agency, is helping airlines to take more efficient routes, for instance. Inmarsat predicts that this could save 6.5 million tonnes of carbon dioxide from European flights each year by 2040.

The political aspects

Policy-makers will also be pivotal in shrinking aviation’s carbon footprint, as governments often hold the power to catalyse key industrial developments through taxation and regulation.

Airlines want to use SAF and governments want to make it affordable. The pendulum has shifted towards the production

Galea believes that the EU’s decision to scrap its free emissions permits for airlines could prove “counterproductive”, because this could take money away from aviation R&D. She argues that blended tax credits in the US, which offer tax breaks for jet fuel that’s a mix of SAF and conventional fuel, are a “very short-term fix” and that changes in how they are administered are needed to better support order visibility for SAF producers.

It’s also important that different jurisdictions don’t differ too dramatically in their approaches. Aviation is a global industry and aircraft must be able to refuel wherever they are. But some divergence is already happening, with most European investment targeting hydrogen solutions while those in the US tend to favour battery power.

Yet both technologies remain at an early experimental stage, whatever commercial potential they might show, Sykes notes.

“Designing and certifying new commercial aircraft incorporating these alternative propulsion methods is likely to take until 2030 at the earliest,” he predicts. “In the meantime, SAF will serve as a crucial stopgap solution, providing significant environmental benefits and enabling more immediate emissions reductions.”

With no easy take-off for any of the alternatives in development, industry-wide teamwork will be vital if it’s to stand a chance of meeting its commitments, according to Galea. 

It will take “an ecosystem effort”, she says, arguing that “it should not be falling on one or two players – airlines, for instance – to bear the cost burden. Public and private sector collaboration will be critical if we want to preserve the growth of this industry.”