To what extent can Sustainable Aviation Fuels (SAF) mitigate the environmental impact of flying?

To what extent can Sustainable Aviation Fuels (SAF) mitigate the environmental impact of flying?
By Mike McCurdy, P.E.
Mike McCurdy, P.E.
Director, Fuels and Power
Nov 4, 2021

Sustainable aviation fuels offer an opportunity to substantially reduce CO2 emissions from flying, but they are not yet the universal cure.

The nature of the decarbonization problem, and the opportunity for SAF

Aviation is a particularly difficult form of transportation to decarbonize. In light of this, the global air transport industry has adopted a long-term climate goal of net-zero carbon emissions by 2050, confirming the commitment of many of the world’s airlines, airports, air traffic management and the makers of aircraft and engines to reduce CO2 emissions in support of the Paris Agreement 1.5ºC goal. The aviation industry will need to leverage every possible advantage to achieve the goal, including developing aircraft technologies, improving operations and infrastructure, and increasing SAF use. IATA director general Willie Walsh commented the "actual split, and the trajectory to get there" for the carbon abatement effort, will "depend on what solutions are the most cost-effective at any particular time." The Air Transport Action Group’s (ATAG) second edition of Waypoint 2050 estimates a significant amount of SAF (330-445 million tons), alongside technological and operational improvements, will be required for the global aviation industry to achieve net-zero carbon emissions by 2050.

Since SAF is operationally identical and certified as “Jet-A1” fuel, no changes are required in aircraft or engine fuel systems, distribution infrastructure, or storage facilities. That allows SAF use as an interim—and long-term—solution to decarbonizing the aviation industry, while electric and clean hydrogen powertrains are developed, tested, and deployed and ground infrastructure is built.

Sustainable aviation fuels

Relative to fossil fuels, sustainably produced, unconventional, jet fuel results in a reduction in carbon dioxide (CO2) emissions across its life cycle. SAF is a non-conventional aviation fuel derived from a range of feedstocks—plant oils, used cooking oil, inedible fats, and municipal waste—converted through certified pathways then blended with fossil fuels and used as a synthetic equivalent to kerosene. The International Civil Aviation Organization (ICAO) recognizes sustainable aviation fuels as those that have “the potential to generate lower-carbon emissions than conventional kerosene on a life-cycle basis.” Emissions reductions from SAF, therefore, consider the carbon absorbed during the growth of the feedstocks, and offset the carbon emitted when the fuels are produced, transported, and combusted.

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Fossil fuels

SAF is also considered sustainable when compared with a fossil-fuel source because the latter is a finite commodity. The industry has been mindful of the need to identify globally acceptable standards for the sustainability of SAFs. To comply, SAFs must be capable of being repeatedly resourced and can deliver “the three pillars” of economic, social, and environmental objectives without disrupting ecological balance, reducing natural assets, or accelerating climate change compared to business as usual.

SAF feedstocks

While it is generally agreed that SAF feedstocks are less carbon-intensive than fossil feedstocks over their life cycle, there is a certain amount of CO2 released during the production and refinement of the SAF feedstocks. GHG studies of SAF feedstocks include emissions from farming practices and fertilizers, transport of the feedstock to the SAF production facility, and—in certain circumstances—land use changes. Palm oil, for example, has generally been excluded from use as a SAF feedstock due to concerns about deforestation in Southeast Asia to make way for new palm oil plantations.

Waste streams for SAF production

The use of waste streams to produce SAF has been particularly beneficial for the environment. Reusing used cooking oil, municipal solid waste, and other waste materials both reduces GHG emissions and keeps those waste products out of our natural environment. In fact, the aviation industry prefers SAFs to the term biofuels because although SAF can be derived from grown-for-purpose plant material, it can also be derived from waste products that include fossil-derived waste. Plastic, inorganic textiles, synthetic elastomers, and other fossil-derived products comprise a significant—and increasing—percentage of municipal solid waste worldwide.

New SAF products are subject to a rigorous, three-phase American Society of Testing and Materials (ASTM) International qualification process. That process includes detailed chemical analysis of the fuel properties versus Jet-A1, extensive testing in both test rigs and jet engines, and multistage reviews by OEMs, the FAA, and ASTM technical committees prior to its approval for use in commercial aircraft.

There are currently seven approved annexes for producing ASTM-compliant sustainable aviation fuels. Each of these annexes is distinct, including definition of the feedstock, production method, and final fuel composition. Each fuel has its own set of economic and social benefits and drawbacks, such as the availability and cost of feedstock, carbon and other pollutant reductions, labor requirements, and cost of processing. While some feedstocks may be more suitable than others in specific regions of the world—for various reasons—all seven certified SAF technology pathways have the potential to help the aviation sector reduce its carbon footprint, assuming all sustainability criteria are met.

Today, SAFs are typically blended with conventional fossil fuels for safety and technical reasons. With chemical characteristics and physical properties that are equivalent to that of conventional jet fuel, approved SAFs can be blended into Jet A-1 up to 50% and used within existing aircraft engines without any modifications. However, as blending techniques mature, future aircraft operators may be able to dispense with the fossil blend-stock to enable greater reductions in the carbon footprint and other pollutants.

The critical work of SAFs in meeting the industry's GHG emissions

An ICF report for ATAG Waypoint 2050 estimates the possible contributions of SAF in three scenarios—as outlined in the Waypoint 2050 report—each considering a different future:

  • Scenario 1: Pushing technology and operations
  • Scenario 2: Aggressive sustainable fuel development
  • Scenario 3: Aspirational and aggressive technology perspective

These scenarios provide a range of sustainable fuel volumes (330-445Mt SAF) required to meet the aviation industry’s climate ambitions, given the expected level of air travel growth. According to ICF’s Fueling Net Zero report, achieving these volumes of SAF is ambitious but possible, as ICF observe aviation can fairly use 20 EJ sustainable bio-feedstock, enough for ~200 Mt SAF. The remainder of SAF will come from renewable electricity (Power-to-Liquid), which will require policies to ensure efficient feedstock allocation, and rapid deployment of renewable electricity generation.

Deploying sufficient sustainable aviation fuel to meet climate ambitions

Fueling net zero: How the aviation industry can deploy sufficient sustainable aviation fuel to meet climate ambitions

Benefits of using sustainable aviation fuels to reduce emissions

SAFs provide a far greater array of value compared to conventional fuels, including environmental benefits, economic benefits, social value, and increased potential for energy security. In the short-to-medium term, the four types of gas turbine engines used in aircraft are well-known, tried, and tested after 50 years of development and refinement. These engines have proven to be dependable, viable, and have an enviable power-to-weight ratio. Therefore, coupling this mature technology with SAFs presents a pragmatic and realistic choice for reducing the sector’s CO2 emissions where global aviation is likely to be reliant on liquid fuels for many years to come.

In addition to the environmental benefits outlined above, SAFs usually contain fewer impurities—most notably sulfur and aromatic carbons—such that existing engines produce significantly less sulfur dioxide and particulate emissions compared to operation with conventional jet fuel. Reductions in these contaminants allow land-based power plants using aero-derivative turbines to significantly increase their maintenance intervals. While additional testing is required, preliminary testing indicated that neat SAFs fuels have the potential to notably reduce engine operating temperatures, increasing power while reducing maintenance requirements.

SAFs constitute a more diverse, global geographic fuel supply. ICF’s Fueling Net Zero report  estimates between 5,000-7,000 global refineries are needed to produce the required volume of SAF to improve energy security and resilience for many nations. That renewable technology presents economic and social opportunities, including rural jobs, associated economic growth, and the potential for poverty and inequality reduction—particularly within developing countries. ICF’s report estimates that the SAF requirements will create or sustain up to 13.7 million jobs and will require a total investment of 1.1-1.4 trillion USD, which per year represents 6% of annual oil and gas investment.

Risks and challenges to overcome to increase uptake levels

Lack of commercial facilities: Only one of the seven approved technology pathways, Hydroprocessed Esters and Fatty acids (HEFA), has been used to produce SAF for commercial sale. While technical challenges can be substantial, commercial facilities face challenges with feedstock availability, price volatility of fossil jet fuel, and other traditional business trials. Operating commercial analogs are critical to attract equity investment in the space as well as secure debt financing for the construction and operation of new facilities, particularly in the absence of loan guarantees or other government support for development.

Difficulties in monetizing the benefits of SAF: Feedstock costs and the processing costs of waste feedstocks such as municipal solid waste are generally higher than the cost of crude oil at current market prices. These increased costs result in a product that is generally considered to be between two and seven times more than that of jet fuel derived from conventional fossil fuels. Global blending standards and energy security policies have enabled the construction of several SAF production facilities to date, but construction has been slow given limited commercial opportunity. Recent GHG reduction policies—such as the California Low Carbon Fuel Standard that allow producers to monetize GHG reductions—have been critical in incentivizing the private sector to construct new SAF facilities that are coming online in 2021 and beyond.

Fuel is generally the largest single operating cost for airlines—around a third of their costs—so fuel-saving measures are typically a key focus for airlines. Although SAFs’ life-cycle carbon emissions are much lower than conventional fossil-based sources, SAF is currently more expensive and hence the tradeoff airlines face. By 2050, however, ICF estimates the average industry cost will be $760-$900 per ton SAF, which is comfortably within the range of historical fossil fuel prices. Furthermore, although SAFs account for a very minor share of the commercial aviation fuel market to date, the number and size of SAF offtake agreements between airlines and fuel producers is growing. Partnerships include JetBlue and SG Preston, Finnair and Neste, KLM and SkyNRG, United Airlines and World Fuels.

A call for policy support

Strong support is needed to shift emphasis from carbon-based fuels to sustainable, low-carbon options as soon as possible. An International Council on Clean Transportation (ICCT) paper suggests that a precedent for anticipating some of the immediate challenges might be drawn from the journey taken in switching to advanced alternative road fuels. Potential barriers can be overcome through policies and incentives such as mandates, fiscal incentives, sovereign guarantees, decarbonization programs, and grants. Procurement contracts and associated measures can and have helped improve the feasibility of SAF projects and mitigate some of the risks associated with SAF production. Tax regimes and specific financing should also be considered to help reduce operational costs and boost investment in SAF projects to accelerate projection and deployment.

Special consideration

The incremental gains in technical performance that SAF can offer on their own are unlikely to increase uptake to the level necessary to address global climate change. However, smart policy, regulatory, and economic support for SAF production and use has the potential to provide significant benefits in combating climate change, creating rural jobs, securing our energy supply, and reducing the impact of waste on our natural environment. In all future scenarios, ICF predicts that SAF will be central to the industry’s commitment to reach net-zero carbon emissions by 2050.

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Meet the author
  1. Mike McCurdy, P.E., Director, Fuels and Power