Can aviation be sustainable?

Air travel is one of the major contributors to global warming. Cambridge scientists are working with leading energy companies to help develop sustainable aviation fuels, which could reduce the industry’s carbon emissions by 80% or more.

Aviation was one of the miracles of the 20th century – it simultaneously made the world smaller and opened it up to new possibilities. We can visit family and friends living far from home, quickly transport goods between countries, and see faraway places.

However, air travel is also the most carbon-intensive form of transport: the aviation industry is responsible for about 2.4% of global carbon dioxide emissions. When including the gases and vapour trails produced by aircraft, the industry is responsible for around 5% of global warming.

Although just a small fraction of the world’s population regularly travels by air, that proportion is predicted to increase rapidly in the coming decades. The industry needs to decarbonise, and fast.

Sustainable aviation fuel

An airplane being refueled

Typical jet fuel has a very high energy density, far higher than batteries. “If we try to design a battery-powered passenger plane with a capacity and range of, for example, a Boeing 737, it would be too heavy to take off,” said Professor Andy Sederman from Cambridge’s Department of Chemical Engineering and Biotechnology. “Batteries just don’t have enough energy density for this type of aircraft – at least not yet.”

The fuel that powers aircraft is based on fossil fuels, but the aviation industry is working on ways to produce sustainable aviation fuels, whether they’re generated from plants, solid waste, or renewable electricity.

Sustainable aviation fuel, which goes by the acronym SAF in the industry, could reduce aviation’s emissions by 80% or more over its full lifecycle, according to the International Air Transport Association (IATA). More than 100 million litres of SAF were produced in 2021: an impressive number, but a tiny proportion of aviation’s total annual fuel use.

“As with almost any new technology, the challenges are cost and scale,” said Sederman. “We can make sustainable aviation fuel, but at present, it cannot compete commercially with fossil-based fuels.”

In research funded by Shell, Sederman and his colleagues at Cambridge are working on new ways to produce SAF and to improve the SAF we currently have.

Inside the black box

Plane in the night sky

The researchers use magnetic resonance (MR) methods – including imaging (MRI) and spectroscopy (NMR) – to further understand the Fischer-Tropsch reaction, which is a key technical process in one of the possible production routes for SAF. MR is a particular research strength at Cambridge – it’s also used to develop next-generation batteries, another vital technology in the energy transition.

For several years, the Cambridge researchers have been using NMR and MRI to better understand the chemistry and engineering processes inside a working Fischer-Tropsch reactor, where syngas – a combination of carbon monoxide and hydrogen – is converted into the hydrocarbon molecules which form the basis of liquid aviation fuel.

At their Energy Transition Campus in Amsterdam, Shell has lab-scale Fischer-Tropsch reactors to help them understand the overall reaction process. These reactors are somewhat like black boxes, and until now it has not been possible to probe the chemistry occurring inside the reactors, which makes optimising sustainable fuels in these reactors difficult. At Cambridge however, it’s possible to see inside the black box.

“At the end of the Fischer-Tropsch process, you have something that’s chemically similar to jet fuel,” said Professor Mick Mantle, also from the Department of Chemical Engineering and Biotechnology. “We use NMR and MRI to better understand what’s happening at a molecular level during the Fischer-Tropsch process.”

Using NMR techniques, the Cambridge researchers can monitor in real time the evolution of chemical product formation inside the Fischer-Tropsch reactor under realistic industrial conditions. For example, monitoring the evolution of hydrocarbon chain length growth enables researchers to better understand how to optimise the final product for use as an aviation fuel. These experiments provide valuable data to fuel companies, helping accelerate the transition to sustainable fuels.

A key advance of the work being done by the Cambridge group is the use of high-resolution MRI imaging, which provides reaction measurements inside individual catalyst pellets within the reactor. This provides unrivalled local information about the actual conditions under which the reaction is occurring.

“Our research is fundamental, in terms of providing the physical and chemical expertise to the companies who will ultimately make SAF at scale,” said Mantle.

“Here in Cambridge, we have a unique capability when it comes to imaging reactors at realistic conditions,” said Sederman. “We can observe reactions in real time and can then work out how to optimise them.”

“Companies in the traditional energy sector have realised that they have to be part of the energy transition themselves and be part of the solution – that’s what both academic researchers and industrial chemical engineers should be doing,” said Mantle.

“The collaboration with Shell brings together their industrial experts with our research expertise and enables us to make unique advances to these real-world problems facing us,” said Sederman.

“This is hopefully good for our research, but if we can help develop these processes it can also be good for the planet.”

Images via Getty Images

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