Are sustainable aviation fuels the future for commercial air travel?

This week, the U.K. government announced another $4.15 million worth of funding for the JetZero Council to continue its push for zero-emission flight infrastructure. According to its website, the JetZero Council is focusing on the development of net-zero and zero-emissions technology to fuel the first zero-emission passenger flight across the Atlantic.

As part of this plan, it is funding zero-emission technology for aviation and establishing facilities in the U.K. for the development and production of sustainable aviation fuels (SAF).

So what are sustainable aviation fuels and how do they fit into the future of commercial aviation?

What is current aviation fuel?

Before looking into sustainable aviation fuels and the role they may play in the future of commercial air travel, it is important to understand how the fuel we currently use works.

The fuel used to power a commercial aircraft must be volatile enough to be easily ignited in the combustion chamber of the engines, but not so volatile that it will burst into flames in the event of a fuel spill or accident. At the same time, it must be easily mass-produced, straightforward to transport but also able to withstand the high range of temperatures it will experience in the various stages of flight.

How is aviation fuel different to vehicle fuel?

While the fuel that you put in your car also comes from oil, the fuel that goes into an aircraft is somewhat different. You may remember from high school chemistry that raw oil is heated in a refinery. The vapors that are created condense into liquids at different temperatures, which then form the base of fuels such as petrol, diesel and kerosene. It is kerosene that is then used to make the Jet A-1 (or Jet A in the USA) fuel used in turbine engines, for a number of reasons.

Firstly, Jet A-1 has a much higher flash point than petrol, commonly around 464°F. Commercial aviation is all about safety, and this includes fuel. When you’re carrying 90 tons of fuel, you want it to be as stable as possible. This means that in the event of an accident, the fuel is less likely to ignite.

Secondly — and most importantly from a day-to-day aspect — it has a very low freezing point. While you’re seated enjoying a movie in a pleasant 70°F cabin, outside your window it’s bitterly cold — about minus 67°F. At higher latitudes it can get even colder, minus 97°F over Siberia is my personal record. When temperatures get this low, a conventional fuel would freeze. The Jet A-1 powering the engines has a freezing point of minus 52°F. Jet A, used only in the U.S.A., has a slightly warmer freezing point of minus 40°F.

If it’s minus -67°F outside, why doesn't the fuel freeze?

As the aircraft climbs, the outside air temperature decreases. Nominally by 35F, every 1,000 feet. This means that by the time it reaches 35,000 feet, the outside temperature will be around minus 67°F. This is the Static Air Temperature (SAT) and is the temperature you’d feel if you were just relaxing on a passing cloud.

However, the aircraft isn’t stationary. It is flying through this cold air mass at hundreds of miles per hour and this how we overcome the freezing fuel conundrum.

The speed of air over the wings creates friction, which heats the surfaces. This reduces the temperature experienced by the fuel in the wings and is known as the Total Air Temperature (TAT). At 38,000 feet, the TAT is normally around minus 5°F, sufficiently warm to stop the fuel from getting close to its minus 52° freezing point.

The carbon composite structure of the wings on the 787 helps reduce the cooling effect even more. As can be seen in the image of the fuel system below, at 38,000 feet the fuel temperature is just 26°F.

What happens if the fuel temperature gets close to minus 52F?

It is possible that, if flying for prolonged periods in extremely cold air masses, the fuel temperature could drop toward the freezing point. However, pilots are alert to this possibility and will take proactive steps to ensure that this doesn’t happen. Each aircraft type has a threshold at which the crew are alerted to a low fuel temperature.

On the 787, that threshold is minus 34°F, as can be seen in the image above. If this happens, the crew have options. They can either fly faster to increase the heating effect of the air, or descend into warmer air. Since aircraft tend to fly as fast as they are designed, normally the only viable option is to descend.

What is sustainable aviation fuel?

Sustainable aviation fuel (SAF) is a blanket term used to cover a variety of non-fossil fuel types which are being developed to move aviation into a greener and more environmentally friendly future. A key part of developing SAF is ensuring that the chemical and physical properties are almost identical to current aviation fuel.

This means that they can then be mixed into conventional aviation fuel, can use the same infrastructure and most importantly, do not require the adaptation of aircraft or their engines. These are known as "drop-in" fuels as they can be easily incorporated into existing airport fueling systems.

However, to be sustainable doesn't just mean that they are better for the environment when they are being used in the engines. There's no point in developing a fuel that is less polluting when it is burned if the carbon footprint created to actually produce it is even worse than conventional fuels.

Sustainable aviation fuel consists of three key elements: sustainability, an alternative to crude oil and fuel quality.

Sustainability

To be sustainable, a fuel has to be something that can be repeatedly made in an environmentally, socially and economically sustainable way. It needs to maintain an ecological balance by avoiding the depletion of natural resources. A fuel that requires mass deforestation to grow the required crops to create the fuel can neither be considered as sustainable nor environmentally friendly.

Crude oil alternative

The aim of a SAF is to use a raw material that is not a fossil fuel. This is known as the "feedstock." These are varied, ranging from cooking oil to municipal waste and from waste gases to agricultural residues.

Fuel quality

Simply put, any SAF created for commercial aircraft must meet certain technical and certification requirements. As mentioned above, Jet A-1 is used because of its high flash point and low freezing point. Any SAF must be able to perform to the same standards, ensuring that safety is not compromised.

What are the benefits of SAF?

Compared to fossil fuels, the use of SAFs can result in a huge reduction in carbon emissions across the fuel's life cycle. In the case of biomass fuels, those created from plants, the carbon dioxide created from burning the fuel is roughly the same that is absorbed by the plants grown to make the fuel.

As a result, there is almost a net-zero creation of carbon dioxide with the use of this type of fuel and an 80% reduction in carbon emissions compared to current fuel when elements such as transport and fuel refining are taken into consideration. In addition to this, SAFs contain fewer other impurities such as sulphur, reducing the emissions of sulphur dioxide.

Related: What your favorite airlines and hotels are doing to fight climate change

For SAF produced from municipal waste, the benefits come from using matter that would ordinarily be left to decompose in landfill sites, creating environmentally damaging gases such as methane.

The use of SAF would also reduce the environmental impact of the production of aviation fuels. In its current format, fuel must be originally sourced from one of few oil-producing countries. Before it can be converted to aviation fuel, it must be transported by ship, a process that creates huge carbon emissions in itself.

If a country is able to grow its own biomass to create SAF, it's no longer reliant on importing oil to create aviation fuel. Depending on the local environment, a variety of SAF feedstocks can be grown around the world, wherever the aviation industry needs it.

How widely is it used?

According to IATA, SAFs are being produced and used every day on commercial flights. It was reported this week that charter cargo flights from Stuttgart to Atlanta were powered using SAF, with the operator claiming that it was the first time charter flights were operated on net-zero emissions. To complete the flights, 307 thousand gallons of SAF were sourced and mixed with fossil jet fuel to comply with aviation fuel standards.

In Japan, ANA conducted a domestic flight on a Boeing 787 using a SAF developed from microalgae and in France, an Air France flight to Montreal avoided the emission of 20 tons of carbon dioxide by using a blend of SAF sourced from used cooking oil.

On the ground, airports are also developing their infrastructure to enable the use of SAFs. Heathrow Airport recently tested its ability to add SAF to the fuel supply. Even though the test was only enough to supply SAF for five to 10 short-haul flights, it proved that "drop-in" fuels like this can work well on a bigger scale.

What are the problems with SAF?

So far, we've seen the obvious benefits of SAF so it seems bizarre that we're not using more of it. Unfortunately, like most things in aviation, it all comes down to cost. For now, SAF remains pricey — around two to five times more expensive than conventional aviation fuel. With the costs so high, no airline will be able to afford to fly exclusively on SAF.

Part of this problem is scaling up production. At the moment, SAFs contribute just 0.1% of global fuel and even optimistic estimates only see this rising to 8% by 2035. The issue comes from a lack of funding to ramp up the production. Like with many new projects, the costs only start to reduce once large scale production increases.

In addition, not all airlines believe that SAF is the way forward. According to EasyJet CEO Johan Lundgren, “SAFs and, in particular, power-to-liquid (PtL) will play a role for long-haul, but it is definitely not something we as a short-haul operator would look at as part of our end game [of zero-emissions flying across Europe] at all.”

He suggested that short-haul operators “better go for electric or hydrogen solutions, or a combination of those two,” because he was sure that aircraft similar in size to today's short-haul aircraft using those fuel types will soon become available.

If many airlines take this approach to SAF, agencies may find it more difficult to raise the funding needed to ramp up SAF production and reduce the cost to airlines.

Bottom Line

The aviation industry is well aware that it needs to evolve and commit to a cleaner and more environmentally friendly future. The benefits of SAF are clear to be seen, but still, there is a long way to go. Without investment in production infrastructure, the current high cost of using SAFs will not fall.

Airlines are keen to use SAFs, as they are a safe and environmentally friendly alternative to the burning of fossil fuels. However, with the cost of SAFs being two to five times more expensive, no one can really expect airlines to make the shift to 100% usage of SAFs.

There is definitely serious potential in the usage of SAFs but until the production and transportation infrastructure is upgraded, the mass usage of these new fuels is still some way off.