What is sustainable aviation fuel?

Sustainable fuels

With the global growth in air travel, the aviation industry is investing in alternatives to fossil jet fuels to reduce flight emissions. One innovation is sustainable aviation fuels, which are jet fuels created from waste products and other sustainable feedstocks that have the potential to reduce emissions by 80%. The development of alternative aviation fuels could be the key to sustainable air travel, contributing hugely to the industry's emissions-reduction strategy. Here’s how.

The use of SAFs provides significant reductions in overall CO2 lifecycle emissions compared to fossil fuels – beyond 80% in some cases – even when emissions created in the production of SAFs are accounted for.

Aircraft today are powered by liquid aviation fuel, made mostly from fossil fuel sources. Yet new fuels have been developed that have the potential to dramatically reduce aviation’s net CO2 emissions. Although supply is currently limited (0.01% of global jet fuel use), sustainable aviation fuels (SAFs), often known as ‘next-generation biofuels’ or ‘advanced biofuels’, are already in use today and are poised for growth.

  •  SAFs can be mixed with traditional kerosene and are already in use in many commercial flights. Since SAFs were certified for commercial aviation in 2011, more than 225,000 commercial flights have been powered by SAFs and six billion litres of SAFs are in current forward purchase agreements by airlines.
  • CO2 absorbed by plants during the growth of biomass is roughly equivalent to the amount of carbon dioxide produced when the fuel is burned in a combustion engine and returned to the atmosphere. When made from waste sources, it reduces the amount of CO2 or methane entering the atmosphere and recycles this valuable resource.
  • Currently, SAFs are more expensive than traditional jet fuel. Estimates range from 2x for some waste-based sources to 6-10x for synthetic fuels using carbon capture. This is mainly due to the small production runs. However, further production facilities are being constructed or planned, and a number of leading airlines are signing significant forward purchase agreements which help provide offtake certainty to new energy suppliers, allowing them to finance new plants. This will help bring down the cost of SAFs in the mid- to long-term.

Are aviation biofuels and sustainable aviation fuels the same thing?

SAFs are different to standard biofuels. ‘Biofuels’ generally refer to fuels produced from biological resources (plant or animal material), and their production isn’t always sustainable. For example, some biofuels, if produced from non-sustainable feedstocks, like unsustainably-produced palm oil or crops that require deforestation, can cause additional environmental damage.

The aviation industry has a clear vision for its use of SAFs, and will adopt only fuels made from feedstocks that can be grown or produced without the risk of unintended environmental and social consequences, such as competition with food production or deforestation.

The aviation industry is working together through groups such as the Sustainable Aviation Fuel Users Group (SAFUG) and sustainability certification schemes such as the Roundtable on Sustainable Biomaterials (RSB) to make sure that any fuels used by the industry are, in fact, sustainable.

What are sustainable aviation fuels made from?

SAFs are made using a variety of feedstocks and waste products.

Municipal solid waste: waste that comes from households and businesses. This includes product packaging, grass clippings, furniture, clothing, bottles, food scraps and newspapers. Rather than simply dumping municipal waste in a landfill site, where it will gradually emit CO2 and other gases, it can be used to create jet fuel instead.

Cellulosic waste: the excess wood, agricultural, and forestry residues. These residues can be processed into synthetic fuel.

Used cooking oil: this typically comes from plant or animal fat that has been used for cooking and is no longer usable.

Camelina: an energy crop, with high lipid oil content. The primary market for camelina oil is as a feedstock to produce renewable fuels. Camelina is often grown as a fast-growing rotational crop with wheat and other cereal crops within the same year, when the land would otherwise be left fallow as part of the normal crop rotation programme.

Jatropha: a plant that produces seeds containing inedible lipid oil that can be used to produce fuel. Each seed produces 30 to 40% of its mass in oil. Jatropha can be grown in a range of difficult soil conditions, including arid and otherwise non-arable areas, leaving prime land available for food crops.

Halophytes: salt marsh grasses and other saline habitat species that can grow either in salt water or in areas affected by sea spray where plants would not normally be able to grow.

Algae: these microscopic plants can be grown in polluted or salt water, deserts and other inhospitable places. They thrive off carbon dioxide, which makes them ideal for carbon sequestration (absorbing CO2) from sources like power plants. To date, we have not seen algae fulfil its early promise due to challenges surrounding commercialisation. However, continued R&D may result in wider application in the future.

Non-biological alternative fuels: these include ‘power-to-liquid’, which typically involves creating jet fuel through a process involving electric energy, water and CO2. This fuel can be sustainable if the inputs are recovered as by-products of manufacturing already taking place, and/or if renewable electric energy is used in its production. For example, using the waste gases produced as a by-product in steel manufacturing to produce sustainable aviation fuel is showing great promise. While direct power-to-liquid options are based on technically proven steps, the process is currently expensive and needs further development.

Other, more advanced, technologies are in early stages of development, such as solar jet fuel (or sun-to-liquid), which uses highly concentrated sunlight to break up water and CO2 molecules.

How do sustainable aviation fuels work?

The chemical and physical characteristics of sustainable aviation fuels are almost identical to those of conventional jet fuel, meaning the two can be safely blended together. For this reason, SAFs are known as ‘drop-in fuels’ because they can be used without any technical modifications to existing aircraft, engines or airport fuel systems.

At present, SAFs must be blended with conventional jet fuel (up to 50%). This is because some less environmentally favourable components of conventional jet fuel, like sulphur, allow seals to swell in engines and prevent fuel leaks. Newer engines do not have this problem, and SAFs have been performance tested at 100% in some new aircraft.

To ensure technical and safety compliance, SAFs must undergo strict laboratory, ground, and flight tests under an internationally-recognised standard. Tests look at specific fuel consumption at several power settings from ground idle to take-off speed, which is then compared to performance with conventional jet fuel.

Tests are also undertaken to measure the amount of time it takes for the engine to start, how well the fuel stays ignited in the engine and how the fuel performs in acceleration and deceleration. Tests are also carried out to ensure that the fuels don’t have a negative impact on the materials used in building aircraft and components. Finally, an emissions test determines the exhaust emissions and smoke levels for the SAF.

How are sustainable aviation fuels made?

There are currently five internationally-approved processes through which SAFs can be produced. Each of these pathways has its benefits, such as the availability of feedstock, cost of the feedstock, carbon reduction or cost of processing. Some may be more suitable than others in certain areas of the world. But all of them have the potential to help the aviation sector reduce its carbon footprint significantly, assuming all sustainability criteria are met.

Fischer-Tropsch Synthetic Paraffinic Kerosene with aromatics (FT-SPK/A) – made from renewable biomass such as municipal solid waste, agricultural wastes and forestry residues, wood and energy crops. The blending limit is up to 50%.

Hydroprocessed Esters and Fatty Acids (HEFA-SPK) – made from oil-bearing biomass, such as algae, jatropha and camelina. The blending limit is also up to 50%.

Hydroprocessed Fermented Sugars to Synthetic Isoparaffins (HFS-SIP) – made through the microbial conversion of sugars to hydrocarbon. The blending limit is up to 10%.

FT-SPK with aromatics (FT-SPK/A) – made from renewable biomass such as municipal solid waste, agricultural wastes and forestry residues, wood and energy crops. The blend limit is 50%.

Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) [isobutanol, respectively ethanol] – made from agricultural waste products, such as stover, grasses, forestry slash, crop straws. The blending limit is 30% for isobutanol and 50% for ethanol.

Catalytic hydrothermolysis synthetic jet fuel (CHJ) – made from triglyceride-based feedstocks such as plant oils, waste oils, algal oils, soybean oil, jatropha oil, camelina oil, carinata oil and tung oil. The blending limit is 50%.

Can sustainable aviation fuels make flying carbon neutral?

Relative to fossil fuels, SAFs result in a reduction in CO2 emissions across their lifecycle. CO2 absorbed by plants during the growth of biomass is roughly equivalent to the amount of carbon dioxide produced when the fuel is burned in a combustion engine, which is simply returned to the atmosphere, meaning SAFs can be classed as carbon neutral over their lifecycle.

But, this doesn’t take into account the emissions produced during the production of SAFs, like the equipment needed to grow crops, transport raw goods, refine the fuel and so on. Nor does it take into account SAFs made from non-biological sources, such as municipal waste. However, even when emissions created in the production of SAFs are accounted for, the use of SAFs is still shown to provide significant reductions in overall CO2 lifecycle emissions compared to fossil fuels – beyond 80% in some cases. Some sources of fuel could provide reductions of more than 100% when sequestration of carbon and other harmful gases are included. In the future, we maybe be able to rely on SAFs as a driver of carbon neutral air transport.

Can today’s passenger aircraft run on sustainable aviation fuel?

Today, commercial aircraft already run on SAFs on a daily basis. If SAFs are mixed with conventional jet fuel, they can be added to existing aircraft without the need for adaptation to the aircraft or engine or the fuel distribution network. Since the certification of SAFs in 2011, more than 225,000 commercial flights have been powered by SAFs and six billion litres of SAFs are in current forward purchase agreements by airlines.

Why isn’t the entire aviation industry using sustainable aviation fuels already?

Many airlines have shown real leadership in advancing the production and deployment of SAFs and have participated in its development from the outset.

A number of airlines are driving forward the use of SAFs by signing multi-million dollar forward purchasing agreements. Others have invested in start-up support for SAF deployment, and some have promoted SAFs through test flights, research, and investigation of local opportunities. Five airports also have a regular SAF supply: San Francisco, Los Angeles, Oslo, Bergen and Stockholm.

However, scaling up the use of SAFs to a global market is challenging and requires substantial investment. The industry has called on governments to assist potential SAF suppliers to develop the necessary feedstock and refining systems – at least until the fledgling industry has achieved the necessary critical mass and prices drop thanks to economies of scale.

Recognising the benefits of using SAF to reduce global carbon emissions means this additional cost can be shared between society (economic support backed by the Treasury) and airline passengers (charged in the ticket price).

Governments can also support the aviation industry by introducing environment and fuel quality legislation to promote the development of SAF markets. Policy options include:

  • Commercial risk reduction policies
  • Ensuring that aviation has access to the same alternative fuel policies as other transport modes
  • Prioritising aviation as a user of liquid alternative fuels (as other transport modes have better options), potentially by using multipliers for credits
  • Supporting research and technology into new production processes and feedstocks
  • Providing access to more cost-effective debt, debt guarantees and capital grants for production facility construction
  • Supporting the technical fuels approvals process at ASTM
  • Diverting economic support from fossil fuels towards renewable and sustainable aviation fuels
  • Ensuring that any government support is contingent on global sustainability standards being adhered to