Any conversation around sustainability in aviation has revolved around the concept of sustainable aviation fuel (SAF). But SAF can be a broad category that includes fuels from many different feedstocks and production pathways; in addition, it can be certified as JetA with different blend maximums, all of which effects how much carbon it actually reduces. Technology and feedstocks are already evolving so it is helpful to get a comprehensive look at the current production landscape.

The Different Methods of Making SAF

Like most things in aviation, SAF must meet strict certification requirements to ensure it is safe for aircraft use. All fuel needs to be certified under ATSM D1655, the same standard as JetA, but before SAF can reach that step, it must also be certified to ATSM D7566, a standard specific for synthetic fuels. ATSM D7566 evaluates which technologies, under specific circumstances and characteristics, can be used for producing on-specification neat SAF. Today, there are several manufacturing processes used to produce SAF that meet ATSM D7566, and with each comes its own advantages and disadvantages.

Today

Hydroprocessed Esters and Fatty Acids (HEFA)

HEFA is currently the most widely used method for producing SAF. The process involves refining vegetable oils, animal fats (tallow) or used cooking oil into a mixture of hydrocarbons that can be blended with traditional jet fuel. The HEFA process is very similar to the process used to produce biodiesel.

The advantages of the HEFA process are that it is a mature technology, at a commercial scale, and can use a variety of feedstocks. However, the process requires large amounts of hydrogen, consumes a significant amount of energy, and today predominantly uses waste feedstocks in limited supply.

Fischer-Tropsch (FT)

The FT process is a well-established technology used to produce synthetic fuels from coal or natural gas. In recent years, it has been adapted to produce SAF by using renewable feedstocks such as biomass—organic matter such as plant straws and stocks—or waste materials. The process involves converting the feedstock into a gas, which is then converted into liquid hydrocarbons using a series of chemical reactions.

The advantages of the FT process are that it can use a variety of feedstocks, which produces a high-quality fuel suitable for use in aviation. However, the process is complex, expensive, and energy intensive.

Alcohol to Jet (ATJ)

The ATJ process involves converting bio-based alcohols, such as ethanol or butanol, into jet fuel using a series of chemical reactions. The resulting fuel can be blended with traditional jet fuel or used on its own.

The advantages of the ATJ process are that it can use a variety of feedstocks, including waste materials, and it produces a high-quality fuel that is suitable for use in aviation. However, the process is relatively new and requires further development before being commercially ready.

Catalytic Hydrothermolysis Jet (CHJ)

The CH process involves heating up waste materials, such as agricultural waste or sewage sludge, in the presence of water and a catalyst. The resulting fuel can be blended with traditional jet fuel or used on its own.

The advantages of the CH process are that it can use a variety of waste materials and does not require any hydrogen or external energy. However, the process is still in the early stages of development and requires further optimization.

On the Horizon

Power to Liquid

Direct air capture (DAC) is a promising technology that can help to mitigate climate change by removing carbon dioxide directly from the atmosphere. This captured CO2 can be a feedstock to produce sustainable aviation fuel (SAF) through the use of renewable energy. By capturing CO2 from the air and converting it into SAF, this approach offers a sustainable alternative to fossil fuels that 100% recycles CO2 in the atmosphere.

While direct air capture is still a relatively new technology, there is a large amount of investment going into the technology outside of aviation. Direct air-captured CO2 as a feedstock does require a large amount of energy and is costly, so other sources of captured CO2 are being explored – particularly from other production or industrial processes.

Algae

Algae-based biofuel has been touted as a promising feedstock. Algae are rich in oils, which can be extracted and processed into a renewable fuel that can be used to power aircraft. One of the major advantages of using algae as a feedstock for aviation biofuels is that it does not compete with food crops for land or water resources, which can be a concern with other biofuels. Additionally, algae can be grown in a variety of environments, including deserts, and can be grown using wastewater or other nutrient-rich sources, making it a low-cost and environmentally friendly option.

However, there are still challenges to be addressed in terms of scaling up production and ensuring the sustainability of the process, but the potential benefits of algae-based aviation biofuels could be significant.

It’s Important To Know Where Your Fuel Came From

SAF is a powerful tool in a comprehensive sustainability program. However, not all SAF is created equal, and knowing where your SAF came from is an important task for accurate voluntary or regulatory reporting. Even with the best intentions, replacing fossil-based fuels with bio-based sources could contribute to issues like deforestation, loss of biodiversity, global hunger, and water pollution. Tracing SAF back to the source can often be challenging and time-consuming, but it becomes necessary to prove a fuel is actually sustainable, especially when using sustainable fuels as part of your Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) or Emission Trading Scheme (ETS) reporting.

SAF and Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA)

Special consideration will need to be taken by international operators that fall under ICAO’s CORSIA. To claim the SAF use as part of a strategy to comply with CORSIA, additional criteria must be met by the fuel refiners. This additional requirement takes into consideration the source of the feedstock to ensure that it does not contribute to issues like deforestation, loss of biodiversity, global hunger, and water pollution. Currently, Roundtable for Sustainable Biomaterials (RSB) and International Sustainability and Carbon Certification (ISCC) are two accredited pathways for certifying a fuel as eligible for CORSIA. Knowing what went into your SAF can save you a headache and possibly money.

Sustainable aviation fuel has the potential to significantly reduce the carbon footprint of aviation through in-sector means. The different types of production processes have their advantages and disadvantages, and each process and feedstock have different carbon intensity implications. As the demand for SAF increases, it is likely that more manufacturing processes will be developed, and existing processes will continue to be optimized.

No matter the size of your operation, 4AIR can help you find, document, and manage your SAF reporting. Learn how at 4AIR.aero.