Be informed

Decarbonising aviation is very difficult. It is a good example of why oil-based fuels have been so important in the development of modern society. Nothing quite matches the energy intensity of oil, with the exception of nuclear. Sustainable Aviation Fuels (SAF) are trumpeted as the solution to aviation’s sustainability problem.

I am grateful here to Ben Purvis, Research Associate, Sustainability Assessment, University of Sheffield, for his contribution to the The Conversation for the content here. Purvis notes that there are so-called pathways for creating sustainable aviation fuel. These are:

• Oils or fats, including used cooking oil or tallow.
• Municipal solid waste, agricultural residues and sewage
• Hydrogen and captured carbon using renewable electricity.

It might seem that used cooking oil processing into aviation fuel is a win-win. But with the best will in the world, there is just not enough to go round. At the moment it is around 2 per cent of all aviation fuel. There is a UK mandate to increase this to 7 per cent by 2030 and 22 per cent by 2040. That is still only 1/5 or there about. To meet the current demand the UK imports 92 per cent of its used cooking oil from China and Malaysia (with its own carbon footprint). Currently the UK has one facility converting used-cooking oil to aviation fuel. That is the Phillips 66 Humber Refinery.

A recent report from the Royal Society notes that the 12.3 million tonnes of jet fuel per year needs 42.4 million tonnes of rapeseed biomass per year. In land terms, that is 68% of the UK’s agricultural land or 6.2 to 10.3 million hectares (see Innovate UK). The aviation industry’s own sustainability roadmap, CORSIA, precludes use of agricultural land for “fuel” crops.

The UK Chancellor of the Exchequer said on the 30 January 2025 (BBC radio 4, Today, c0815) that SAF could reduce aviation’s GHG emissions by 70 per cent. “Engines have become much more efficient. And, just at the beginning of this year, this government introduced the mandate for sustainable aviation fuel, which can reduce carbon emissions from flying by 70%. And of course, there’s going to be much more progress on that in the years to come.” (Quotes taken from the Guardian)

Equally there is global competition from both the EU and the USA (the latter now depends on the airlines rather than the state as burning fossil fuels now seems to be a US citizen imperative). Whilst it is clear that production could be increased with more investment, there is little confidence that it would be profitable; moreover, there is the small problem of cost – whichever pathway is taken, SAF costs more to manufacture than does aviation fuel (kerosene). That is £s on each ticket.

Surely there is enough municipal solid waste and sewage to go round? Well maybe, but the technology is in its infancy or not yet approved (see below) and no commercial facilities are producing it as yet. As for hydrogen, first it is packed with carbon if fossil fuels are the source of energy for the electrolysis necessary to produce it. Electrolysis by electricity from renewables remains distant. There is a long way to go before the UK grid is totally decarbonised. And now the British Government has added data centers to the mix, which means electricity supplies more generally are under pressure. Hence the Government’s latest endorsement of nuclear power and another un-tested technology, Small Modular Reactors (SMRs).

Not only manufacturing is a challenge, but the infrastructure to store hydrogen at airports or other facilities is just not in place. Hydrogen is also explosive and there are examples of denial of licences for users to store it, for example bus companies. It would require a major change to planning laws for widespread storage.

One UK company is undeterred: Logan Air. Logan Air has announced (12 February 2025) a plan to launch the world’s first hydrogen-fuelled commercial service by 2030. The company does not reveal the identity of the manufacturer of the aircraft that they will use for their point-to-point service (also as yet not stated). This against the backdrop of Airports Council International backtracking on its hydrogen ambitions in favour of scaling SAF, better air traffic management and improving aircraft engine efficiency.

There remains, therefore, the central question of actual aeroplanes. There was some succour in that Airbus was developing hydrogen planes (the A380 Airframer). This aeroplane was going to be a 100-seat 1850km range aircraft (right). But as recent as 6 February 2025 the Force Ouvriere trade union was informed that the launch date has been put off by between 5 and 10 years with an additional budget cut of 25 per cent. The company has identified the lack of available green hydrogen as one of the reasons for the delay. Another, less explicit but particularly troubling reason, is the company’s intention to replace its popular A320neo with a newer and more efficient conventionally-fuelled aircraft. The end of fossil-fuelled aircraft is nowhere in sight. The only option then, for sustainable aviation, is SAF from vegetable oils, tallow or waste.

There is a need, therefore, to clarify whether SAF is actually sustainable. In theory, because the things that it is made of already exist or are grown, burning it does not add to overall CO₂ levels. (Hydrogen, even more so, because it is derived from water and the emissions are just water.) So, aeroplanes still emit CO₂ when we really need to be capturing it and locking it into plants such as trees to generate negative emissions. Equally, it assumes that the crops and the waste had it not been for SAF would have degraded and decomposed releasing greenhouse gases in any case. The reality is that the area being used to cultivate crops to be turned into SAF would in actual fact be used either for food consumption or some form of rewilding. Essentially growing crops to fly planes (at least part of the way to their destinations) would displace food production. This would be a major distortion of land use.

The reason that SAF is so topical is because the British Government seeks to make the case that not only can we continue to fly at current levels, but that airport expansion is possible because the emission reductions from SAF rollout will offset increased flights (all in the name of growth). The above argument challenges that proposition. The calculation also needs to factor in the carbon emissions generated by constructing new runways. It is not trivial.

Here is Mike Berners-Lee discussing SAF.