There are many initiatives underway to make aviation greener and lower overall emissions. Major changes such as electric or hydrogen power remain some way off, but others are already being adopted by airlines. More airlines are using Sustainable Aviation Fuel (SAF). We take a look here at how SAF is produced and used in aircraft engines.
What is Sustainable Aviation Fuel?
Firstly, a quick recap on standard jet fuel is helpful. Regular jet fuel is a high-density kerosene-based fuel. This has been developed to offer the high power needed for aircraft engines. It is ‘heavier’ than standard automotive gasoline, with longer strings of hydrocarbon atoms, and offers a higher flash point and lower freezing point.
As it is fossil fuel-based, though, it has high carbon emissions. Sustainable Aviation Fuels (SAF) are an alternative. They are based on renewable hydrocarbon sources that are not fossil fuel-based. This includes sources such as used cooking oil, municipal waste and forestry biomass. Sustainability is the driving force here – sources should be able to be repeatedly resourced. SAF was first used in 2008 (according to Shell).
Today, many airlines have made commitments to certain levels of SAF use, and this is expanding. ICAO predicted in 2019 that use would reach 8 billion litres by 2032 (from levels then of 6.45 million litres).
To produce SAF, the feedstock is first collected from one or multiple sources. This could be waste products, such as oils or household waste, or specifically grown plant material. Currently, fats, oils, and greases (FOGs) are a leading source.
Neste, for example, has a contract with McDonald’s to use cooking oil from its Dutch restaurants. Biomass and municipal solid waste (MSW) are expanding in use as sources. These have the potential to reduce overall greenhouse gas emissions even further (85% to 95% over their lifecycle compared to traditional fossil-based jet fuel). But with a lower energy density, more feedstock volume is needed.
Once collected and separated, these feedstocks are then refined to produce fuel. There are various technologies used for this conversion. For FOGs, the most widespread is HEFA (Hydrotreated Esters and Fatty Acids). In this process, purified liquids are exposed to a chemical reaction with hydrogen and catalysts and then distilled.
For solid biomass, FT-SPK (Fischer–Tropsch Synthetic Paraffinic Kerosene) is a leading process currently. This involves the gasification of solid biomass at a high temperature, producing a synthesis gas that is further converted into hydrocarbon chains.
Blending SAF with traditional fuel
SAF is not used directly as a fuel. But it can be used in existing jet engines after mixing with traditional jet fuel. This ensures similar performance and handling characteristics. Combined fuel must meet strict requirements.
SAF is blended with traditional fuel in ratios up to 50%. Currently, most fuels use much lower proportions of SAF, but it is expected that this will increase over time (and potentially up towards 100%). United Airlines has run test flights using 100% SAF for one engine- but without passengers.
The resulting fuel is re-certified as Jet A or Jet A-1 fuel and can then be handled, transported and used in the same way as traditional fuel. Critically, this means that (post-blending) no changes are needed to existing fuel infrastructure in the supply chain and at airports. This is a significant limitation faced when looking at new fuel sources, such as hydrogen.
Taking SAF use further
The current use of SAF is somewhat limited. Although some airlines have made high profile commitments to its use, overall adoption remains low compared to traditional jet fuel. Cost is a major factor here – this should reduce as collection and production availability expands.
Longer-term, though, it should offer more stable pricing than oil and will become ever more important as the industry works towards its target of halving overall emissions by 2025 (based on 2005 levels).
Embraer’s CEO, Arjan Meijer, spoke to Simple Flying about these challenges. He sees production and cost as major hurdles to get over as SAF use is scaled. He explained:
“Airports will have to invest in infrastructure and will for sure pass that bill onto airlines, and they will pass it onto the customers, so it is all going to find its way. But it is really about getting over the first hurdle as an industry, and then I think we’ll see the SAF production evolve over the years ahead. But there is a lot of SAF that is going to be needed, and there’s a big challenge ahead.”
Likewise, production technology continues to evolve. Power-to-liquid methods allow the production of synthetic kerosene, with the potential to reduce emissions by as much as 99%. Power-to-Liquid entails the conversion of renewable energy into fuels through electrolysis.
There is a long way to go for major adoption of this, but it could deliver a carbon-neutral circular system for aviation, using the carbon dioxide that aviation is responsible for and turning it back into power for the planes.