Sep 14, 2021

What is SAF? Sustainable Aviation Fuel 101

Sustainable Aviation Fuel, or SAF, is a vital tool for making air travel more sustainable. Read on for a primer about how your next flight might be powered by sources ranging from food and forestry waste to carbon from the atmosphere.

SAF offers the key to more sustainable air travel

Sustainable Aviation Fuel, or SAF, is a vital tool for making air travel more sustainable. But what exactly is SAF — and how is it made? Read on for a primer about how your next flight might be powered by sources ranging from food and forestry waste to carbon from the atmosphere.

What is SAF?

SAF stands for sustainable aviation fuel. It’s jet fuel made from sustainable and renewable sources. It’s an alternative to fossil jet fuel (Jet A), which is made from crude oil — also called liquid petroleum — that is extracted from the ground.

The SAF available today is considered “drop-in” because it meets the same characteristics of Jet A. It can be used safely on existing aircraft and within current airport systems.

Do planes need special fuel compared with cars?

Yes, jet engines need Jet A. It’s a specially-formulated fuel for aircraft only.

→ Jet A has a lower freezing point than gasoline. Large aircraft operate at altitudes with very cold temperatures, so jet fuel must have a lower freezing point than the gasoline used in cars.

→ Jet A contains additives, such as anti-icing agents to further prevent icing, as well as corrosion inhibitors.

→ Jet A has lower viscosity (it’s more watery), which prevents it from gumming up and clogging an aircraft’s fuel system.

→ Jet A has a higher octane rating than gasoline, enabling higher compression ratios and fuel efficiency. A higher octane rating means the fuel is more stable.

How is SAF made?

Instead of pulling crude oil from the ground and refining it to make jet fuel, SAF producers use what’s already available — they aren’t extracting resources from the earth.

SAF is made from a wide range of “feedstocks.” Some of the many examples of feedstocks include waste oils like used cooking oil; forestry and agricultural waste; fast-growing plants such as algae; and carbon from the atmosphere (also called direct air capture — more on that below).

In addition, there’s work underway by companies such as Fulcrum to transform municipal solid waste (black bag garbage) into SAF. This source of waste includes items that aren’t recyclable and that go into landfills, such as discarded clothing.

Fulcrum’s first garbage-to-fuels facility, Sierra BioFuels, located near Reno, Nevada. Photo credit: Fulcrum BioEnergy

Is direct air capture a source for SAF production?

Beyond waste feedstocks, direct air capture technology promises to achieve nearly 100% carbon reduction. This technology is fast maturing. Companies, including Prometheus, are working to introduce economical, direct air capture SAF.

Direct air capture pulls carbon dioxide (CO2) from the air, removes the oxygen, then combines it with hydrogen to form hydrocarbon fuel. When produced with renewable energy, this is the future of sustainable aviation.

This next generation of SAF could come with additional benefits, including reduced soot emissions, increased energy density, and increased thermal stability — further improving the benefits of SAF.

Is SAF the same as ethanol?

No, SAF is a different type of fuel — and more sustainable than corn ethanol.

Ethanol is usually made from grains such as corn. It won’t work in jet engines because it has low energy density, meaning that there’s less power in each gallon of ethanol than there is in jet fuel. (In other words, you have to use much more ethanol to derive the same amount of power as jet fuel.) Ethanol can, however, work in some smaller aircraft.

And in case you’re wondering, grain alcohol is also a type of ethanol.

Do airlines currently use SAF?

Several airlines — including United and Japan Airlines — use SAF on a portion of commercial flights. Many airlines are also investing in SAF production facilities and suppliers, and promoting SAF through test flights and research. A number of major international airports already have a regular SAF supply, including Los Angeles International Airport (LAX), Frankfurt Airport (FRA) and Stockholm Arlanda Airport (ARN).

Is the SAF used today blended with Jet A?

Currently, commercial aircraft are only approved to operate passenger flights on a 50/50 blend of SAF and Jet A by regulators. Those percentages are expected to change within the coming years, with the goal of 100% SAF-powered commercial fights. (The 50/50 blend limit is approved by ASTM International, which works closely with the Federal Aviation Administration.)

What about electric aircraft?

Batteries are well suited to power smaller aircraft for shorter flights, and the technology is advancing rapidly. It’s anticipated that several electric aircraft and eVTOL (electric vertical takeoff and landing) programs will go into commercial service before the end of the decade.

But because current batteries can’t store as much energy as jet fuel, you would need an enormous battery to store enough power for a long-haul flight. Pound for pound, jet fuel — whether SAF or Jet A — has around 24 times more energy than current batteries. A battery with enough power to operate a long-haul flight would be so heavy that you may never get off the ground.

In the near future, it’s anticipated that travelers will have the lower-carbon option of flying electric aircraft for short-haul flights from their local airports to major hubs, where they will connect to SAF-powered long-haul international flights.

Does SAF require new equipment or infrastructure?

Airports and airlines won’t need to replace or purchase new equipment to operate flights on drop-in SAF. It can replace Jet A in existing infrastructure.

How much does SAF reduce emissions?

Currently, SAF can reduce carbon dioxide (CO2) lifecycle emissions by up to 80% compared to Jet A.

The lifecycle measurement includes quantifying carbon sequestered by the feedstock, as well as carbon emissions from production, refining, transportation, and ultimately, aircraft use of fuel. The most commonly used and widely accepted methodology for calculating the emissions reduction is ICAO’s (International Civil Aviation Organization) Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). CORSIA is ICAO’s program to limit international aviation CO2 emissions via offsetting as well as using SAF.

What other ways is the industry working toward more sustainable operations?

In addition to operating more flights on SAF, the industry is working toward more sustainable operations in many ways, including investing in newer, more fuel efficient aircraft technologies; developing better routing (fewer miles/kilometers traveled); investing in hybrid and electric technologies; and adopting carbon offsetting and reduction programs.

Click here to read more from Boom Supersonic and Japan Airlines about five initiatives empowering a more sustainable future of travel.



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