What are Sustainable Aviation Fuels?

In order to decrease its emissions significantly, the aviation sector needs to reduce its current exclusive reliance on fossil-based jet fuel and accelerate its transition to innovative and sustainable types of fuels and technologies.

A Sustainable Aviation Fuel (SAF) is a sustainable, non-conventional, alternative to fossil-based jet fuel. Several definitions and terminology may apply, depending on regulatory context, feedstock basis, and production technology.

According to the ReFuelEU Aviation regulatory proposal [13] , SAF are defined as drop-in aviation fuels that are either biofuels produced from feedstocks listed in Annex IX of the Renewable Energy Directive (RED II) [1]  or synthetic aviation fuels, and which comply with the sustainability and greenhouse gas (GHG) emissions reductions criteria in Article 29 of the RED II. A variety of terminologies are used for synthetic fuels, such as Renewable liquid transport Fuels of Non-Biological Origin (RFNBO), Electrofuels, e-Fuels and Power-to-Liquid (PtL).

In order to be used in commercial aircraft, drop-in SAF have to go through an exhaustive approval process [2] [3]  to fulfil strict certification criteria and prove that their physical and chemical characteristics are almost identical to fossil-based jet fuel [4]  and can therefore be safely blended together. This enables SAF to be used within the existing global fleet and does not require any adaptation to the aircraft or fuel supply infrastructure.

As of January 2022, seven SAF production processes have been approved [3] . In addition, two pathways for the co-processing of renewable feedstocks in petroleum refineries are approved [4]  with a blending limit of 5%.

The technological maturity of each production pathway can be defined through a Technology Readiness Level (TRL) [5] , which ranges from 1 for basic ideas, to 9 for an actual system proven in an operational environment (see Table 4.1 ).

30The listed feedstocks are technologically feasible for the specific production pathway, but not necessarily applicable under certain regulations (e.g. ReFuelEU Aviation)
31FT-SPK: Fischer-Tropsch synthesised paraffinic kerosene.
32HFS-SIP: hydroprocessed fermented sugars to synthetic iso-paraffins.
33TRL 7-8 for conventional sugar feedstock; TRL 5 for lignocellulosic sugar feedstock.
34FT-SPK/A: Fischer-Tropsch synthesised paraffinic kerosene with Aromatics.
35CH-SK: catalytic hydrothermolysis synthesised kerosene.
36HC-HEFA-SPK: Synthesised paraffinic kerosene from hydrocarbon-hydroprocessed esters and fatty acids.

The following four production pathways are expected to play a major role in the near future.

Hydroprocessed Esters and Fatty Acids (HEFA)

Potential feedstocks include waste and residue fats (e.g., vegetable oil, used cooking oil, animal fats) and purposely grown plants (e.g., jatropha, camelina). Feedstock is converted using hydrogen to remove oxygen and produce hydrocarbon fuel components. HEFA is currently the only commercially used SAF with a TRL of 8-9. However, the availability of sustainable feedstock, and competition with other sectors, e.g. road, is a limitation to the supply capacity.

Alcohols to Jet (AtJ)

Currently at TRL 7-8, AtJ SAF can be produced from the fermentation of processed lignocellulosic feedstocks (agricultural and forest residues) as well as sugar or starch crops (e.g. corn, sugarcane, wheat). Some AtJ pathways have the possibility to produce SAF that contain aromatics. While reducing the aromatic content of fuels is beneficial for air quality and the environment, fuel that does not contain any aromatics may have airworthiness consequences for parts of the aircraft engine (e.g. rubber seals). This makes AtJ fuels an option for future 100% SAF certification, exceeding today’s blending limits (see text box on next page).

Biomass Gasification + Fischer-Tropsch (Gas+FT)

Biogas, or syngas, is obtained from the gasification of feedstock and subsequently processed within a Fischer- Tropsch reactor. The Gas+FT pathway can process similar feedstocks as the AtJ, as well as municipal solid waste. Both Gas+FT and AtJ are considered advanced biofuels if produced from feedstock listed in Annex IX Part A of the RED II, and have significant emissions reduction and supply potential, but are not yet available on a commercial scale within the EU.

Power-to-Liquid (PtL)

Water and electricity are used in an electrolyser to produce hydrogen, which is subsequently synthesised with CO2 into syngas. The resulting syngas is then further processed into fuel by the Fischer-Tropsch (FT) reactor or alternatively by methanol synthesis. The CO2 needed for the PtL process can be sourced from industrial waste gases, biomass or captured directly from the atmosphere. 

The production of the electricity and the sourcing of CO2 are the determining factors in the sustainability as well as the overall costs of PtL. As with other pathways, several by-products (e.g. synthetic road fuel or materials for the chemical industry) from the process offer resilience and potential additional income from PtL production. PtL fuels are already approved if produced through the FT production pathway. ( Figure 4.1. )

Non-drop-in fuels (e.g., hydrogen) would not necessarily be compatible with the existing global fleet and thus would potentially require aircraft redesign and certification, as well as new supply infrastructure.

Achieving 100% SAF

Approved SAF currently have associated maximum blending ratios ( Table 4.1 ) that may limit the ability to use larger amounts of SAF in the future. As such, dedicated task groups within fuel standard committees are assessing options to facilitate the use of 100% SAF in aircraft engines, with an initial timeline of having approved fuels ready by 2030.

One drop-in option is to blend two or more SAFs to produce a fuel with characteristics that are fit for purpose in terms of 100% use. Another option is the adaptation of currently used raw materials and production processes to produce a fully formulated 100% SAF in a single process stream (e.g. AtJ, FT- SPK/A and CHJ) or the use of new raw materials and processes yet to be developed and approved.

The aviation industry is already performing the needed research and test flights to evaluate the effects of 100% SAF on emissions and the performance of aircraft, with promising early results [6] [7] [8] . For example, in October 2021 the first in-flight study of a single-aisle aircraft running on unblended SAF was launched. An Airbus A319neo aircraft operated on 100% fuel made from cooking oil and other waste fat (HEFA). In March 2022, a ground engine test campaign was completed with the same fuel, to correlate with the flight tests emissions and to evaluate the benefit of SAF on airport air quality [6] .

Rolls-Royce Trent-1000 engine operating with 100% SAF on a Boeing 747-200 Flying Test Bed.
Rolls-Royce Trent-1000 engine operating with 100% SAF on a Boeing 747-200 Flying Test Bed.