Sustainable Aviation Fuel
Air transport sector is an integral part of economic growth & development, the only available means of transporting passengers & goods across the globe within a single day [WB, 2023].
Prior to the COVID-19 pandemic, aviation accounted for over 2,5% of global energy-related CO2 emissions (reaching higher levels - 3.5% - when including other GHG emissions and aviation-induced cirrus cloudiness), and 12% of total emissions from transportation in 2019, the others being road at 70%, shipping at 9%, and rail at 1%, having grown faster in recent decades than road, rail, or shipping, and expected to double its emission by 2050. Incidentally, transport continues to rely on oil products for nearly 91% of its final energy [WB, 2023] [IEA, 2022].
Sustainable aviation fuels (SAF) are drop-in aviation fuels made from renewable or waste resources such as biomass, solid, liquid, and gaseous wastes (MSW, CO, CO2 waste streams), and atmospheric CO2 that can reduce life cycle CO2 emissions by up to 80% compared to conventional fuel [IPB, 2021] [BSR, 2023].
Sustainability criteria & pathways
SAF have to meet stringent sustainability standards with respect to land, water, and energy use, avoiding direct & indirect land use change impacts, and not displace or compete with food crops. There are many different pathways to produce SAF, transforming a wide range of biomass & waste feedstocks into jet fuel. There are now SEVEN technology pathways approved by ASTM (American Society for Testing and Materials) to produce SAF for use in commercial aviation, among others FT, HEFA, and AtJ [SK, 2022].
1 - Fischer-Tropsch (FT) - FT-SPK, 2009
In this process, carbon containing materials (MSW, forest, and agriculture residues) are broken into individual building blocks in a gas form (syngas), and then FT combines these building blocks into SAF. Two different FT processes have been certified by ASTM [SK, 2021]: straight paraffinic jet fuel (FT-SPK) and synthesized aromatic kerosene (FT-SAK) (FT-SPK/A). Maximum blend ratio for both is 50% (ASTM for SPK: D7566-Annex 1; for SAK: D7566-Annex 4).
2 - Hydrotreated esters & fatty acids (HEFA) - HEFA-SPK, 2011 - by far the most used
HEFA, a high maturity level & commercially available conversion tech, refines vegetable oils, waste oils, or fats into SAF thru hydrogenation. In the first step of the process, the oxygen is removed by hydrodeoxygenation. Next, the straight paraffinic molecules are cracked & isomerized to jet fuel chain length. Maximum blend ratio is 50% (D7566-Annex 2). HEFA-jet is co-produced with HEFA-diesel (green diesel or HVO) for the road sector, and the relative share of the products can be adjusted to meet market needs [ICAO, 2022].
3 - Alcohol to Jet (AtJ) - ATJ-SPK, 2016
Atj produces jet fuel from sugary, starchy, and lignocellulosic biomass, such as sugarcane, corn grain, and switch-grass, via fermentation of sugars to ethanol or other alcohols [BMC, 2017]. It converts alcohols into SAF by removing the oxygen and linking the molecules together to get the desired carbon chain length, a process historically utilized by the oil refining & petrochemical industry. Two feedstocks approved for use in the AtJ technology: ethanol & iso-butanol. The maximum blend ratio is 50% (ASTM spec: D7566-Annex 5).
In total there are SEVEN processes, which include Hydro-processed fermented sugars to synthetic isoparaffins (HFS-SIP, 2014) (D7566-Annex 3), catalytic hydrothermolysis synthesized kerosene (CH-SK or CHJ, 2020) (D7566-Annex 6), and synthesized paraffinic kerosene from hydrocarbon-hydroprocessed esters and fatty acids (HC-HEFASPK) (D7566-Annex 7) [GCC, 2023].
International Civil Aviation Organization (ICAO) defines SAF as a renewable or waste-derived aviation fuel that meets a set of Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA).
Chemical & physical characteristics of SAF i) are almost identical to those of conventional jet fuel and can be safely mixed with the latter to varying degrees, ii) use the same supply infrastructure, and iii) do not require the adaptation of aircraft or engines [ISCC, 2023].
SAF currently accounts for only 0.1% of global aviation fuel demand and costs 2 to 5 times more than conventional jet fuel [WB, 2023].
Higher production & costs are the main challenges compared to conventional jet fuels.
The hydrocarbon fuel is the only option for aviation for now, and HEFA-based biofuels are the only product that is commercially available today and powered over 95% of all SAF flights [SK, 2022]
Figure 1 - SPK (Synthetic Paraffinic Kerosene) types.
Figure 2 - HEFA production process.
Figure 3 - SAF production costs by different tech platforms (routes): PtL (Power-to-Liquid), which uses renewable electricity, has the greatest long-term potential.
Figure 4 - World aviation subsetor energy demand by carrier.
Figure 5 - SAF main players.
Figure 6 - Feedstock potential vs cost of production.
Figure 7 - All seven approved pathways (five of which can be blended at rates up to 50%).
Figure 1: SPK types
Figure 2: HEFA production process
Figure 3: SAF production costs
Figure 4: World aviation subsector energy demand by carrier
Figure 5: SAF main players
Figure 6: Feedstock potential vs cost of production
Figure 7: All seven approved pathways