Sustainable aviation fuel has the potential to slash emissions from aviation and create a market for waste that cannot be recycled. Ian Farrell asks what’s the catch?
Aviation is a highly carbon-intense activity, but only makes up 2.5% of the world’s CO2 emissions. That sounds like a contradiction, until we factor in an important statistic: it is estimated that only 10% of the world’s population fly.
As incomes and living standards rise, however, so will this figure. Social mobility in countries such as India and China will undoubtedly mean a rise in passenger aviation, while imports mean an increase in freight flights, too.
To add to the problem, while aviation only contributes 2.5% of CO2 emissions, it is responsible for 4% of anthropological global temperature change, thanks to emissions of soot and water vapour, sulphur aerosols and contrails. Some of these heat the planet; others cool it – but the net effect is one of warming.
The aviation industry’s goal of achieving net-zero emissions by 2050 looks like a difficult one to achieve. Other carbon-intensive activities are gradually being replaced or refined to make them less polluting – for example, the electrification of automobiles, and the rise in solar and wind farms. But aviation is much harder to decarbonise.
The energy density of hydrocarbon jet fuel is hard to match, especially with current approaches to electrification. There is a lot of research happening in the field of hydrogen-powered aircraft, but this will take time and require the replacement of perfectly serviceable aircraft that cost hundreds of millions of dollars each.
What’s needed in the meantime is a stop-gap solution that can be ‘dropped in’ to existing aviation-fuel infrastructures, enabling the industry to use existing equipment more sustainably while new technology is brought to market. This is where sustainable aviation fuel (SAF) comes in.
Blending fuels together
SAF is virtually identical to the JET-A1-grade kerosene fuel obtained by refining crude oil, except it comes from different sources that are part of a circular closed-loop cycle. At the moment, SAF is blended with JET-A1 up to a maximum 50% mix.
Both fuels are a mixture of hydrocarbon fuel molecules, but JET-A1 also contains aromatic (cyclic) molecules. These have a lubricating role in engines, but are costly to synthesise by themselves, so they are added by blending together SAF and JET-A1.
Eventually, 100% SAF flights will happen – test flights across the Atlantic have already taken place – but modifications to engines and fuel pumps will need to be made first.
SAF can be produced by a number of industrial processes, but the original starting materials are often organic waste. This could be waste oils and fats from food manufacturing, municipal food waste, agricultural by-products and waste, forestry residues, and animal slurries. Even the CO2 from carbon-capture processes can be used in SAF manufacture.
Currently, the most popular way of making SAF is from hydroprocessed esters and fatty acids – the so-called HEFA process. These waste materials are first treated with hydrogen to remove oxygen, which breaks them down into hydrocarbon chains. These are then chemically reacted to produce SAF. This process saves around three-quarters of the carbon emissions from fossil jet fuel and is already certified for use in a 50% blend on commercial flights.
The HEFA pathway is currently the only fully commercialised method, and will probably supply the majority of SAF until 2030. However, there is a naturally limited amount of available feedstock for it. Clearly, other processes are needed.
An alternative is to use the Fischer-Tropsch (FT) process – an old piece of organic chemistry that is used extensively in the chemical industry – to convert syngas into fuel. Syngas (which gets its name from synthesis gas) is formed from biomass, including municipal solid waste, agricultural wastes, forest wastes, wood, and energy crops. Johnson Matthey is one of the companies taking this forward, with its HyCOgen process, which uses bespoke catalysts to do the necessary chemistry. SAF made in this way can also be blended up to 50% with fossil-derived fuel.
A third option is alcohol-to-jet chemistry. This involves the conversion of cellulosic or starchy alcohols (usually ethanol and isobutanol) through a series of chemical reactions. The alcohols are derived from cellulosic or starchy feedstocks via fermentation or gasification reactions.
In total, there are eight certified processes for making SAF and more will surely emerge as research continues. It’s important to bear in mind, however, that SAF is only sustainable if it’s made from a sustainable feedstock.
The word sustainable means different things to different people – for example, is growing so-called ‘energy crops’ that are destined for SAF production, sustainable? Some would say ‘yes’, as they take CO2 from the air, which is then put back again when the fuel is burned – a closed system.
Others point out that there is more than climate change at play here: land use, biodiversity, water use and food security also need to be taken into account. Deforestation, to make space to grow energy crops, is obviously extremely unfavourable.
The waste industry, however, can provide exactly what SAF production needs, in the form of solid municipal waste.
Harder than it looks
It’s not time to enjoy guilt-free sustainable flying just yet, however. Getting the right waste to where it needs to be for SAF production is not as easy as you might think.
Although multiple biomass feedstocks can be used by gasification-based technologies, the supply chain for used cooking oil and animal fat, as well as forest and agricultural residues, is not well established.
Additionally, the low-energy density of forest residues makes it uneconomical to transport them long distances to a refinery. Unless supply chains are developed with intermediate densification (such as pellets or bio-oils), large-scale SAF facilities could be difficult to establish.
All this will require considerable investment and innovation, and support from governments and industry. The International Energy Agency’s Task 39 evaluated the current state of SAF production in its January 2024 report (bit.ly/3X37aXu). While numerous SAF production facilities have been announced, current production is less than 1% of the 400 billion litres required annually by 2050.
A 2023 Royal Society report (bit.ly/3EEarGq) estimates municipal waste arisings could contribute 12Mt per year of feedstock for bioenergy production. With a 10% conversion rate, this would contribute 1.2Mt per year of fuel, or 10% of the total amount of fuel required.
There is clearly potential for growth, and a market for those collecting municipal solid waste that cannot be recycled, but can SAF compete with current long-term contracts with energy from waste (EfW) facilities?
The inclusion of EfW in the Emissions Trading Scheme could affect things, as could legislation prohibiting the use of single-use plastics, but local authorities are unlikely to change contracts to an as-yet unproven SAF producer at the moment.
If they fail to dispose of the waste, then the local authority could fall into legal default and risk having to pay dearly to send it to landfill – defeating the purpose of the whole SAF exercise.
In summary, if SAF production from waste is to become a reality, there will need to be a wholesale overturning of the current UK waste management system that will take years to achieve.
Remember – SAF is only a stop-gap solution, favoured by the aircraft operators, who can use it within their existing fleets. The aircraft industry, however, is concentrating on its development into hydrogen. Rolls-Royce, General Electric, Airbus, Pratt & Whitney, and Boeing are all developing hydrogen-fuelled engines and aircraft.
It seems as if SAF, at the volumes currently being considered, could have a relatively short shelf life. l