Over the past decades, significant technological developments have taken place in most areas of the aviation sector, except for the fossil-based fuel used by aircraft, which has remained relatively unchanged. Although alternative clean propulsion technologies are under development - such as electric-powered aircraft or cryogenic hydrogen fuel - these options are unlikely to be commercially ready before 2030. The last decade has seen considerable progress in developing Sustainable Aviation Fuels (SAFs) produced from bio-based feedstocks that have a lower carbon intensity, and which consequently could play an important role in mitigating the environmental impact of aviation.
Bio-based aviation fuels are obtained from sources other than petroleum, such as woody biomass, hydrogenated fats and oils, recycled waste or other renewable sources. In order for these fuels to be used in aircraft operations, they must have ‘drop-in’ characteristics, which means they have to meet strict fuel specifications and have comparable behaviour to fossil fuel during the combustion process. As such, the emissions reductions are achieved in their production process. These biobased aviation fuels can be mixed with conventional fossil-based aviation fuel at a blending ratio that is dependent on how the fuel is produced.
There is not a single internationally agreed definition of SAF. The definitions used can cover a wide set of criteria including not only a reduction in greenhouse gas (GHG) emissions, but also other environmental and social aspects such as biodiversity, land use (forests, wetlands, peatlands), water, labour standards applied in production processes and support to the social and economic development of communities involved in fuel production. For the purposes of this chapter, SAFs are defined as bio-based aviation fuels that reduce GHG emissions relative to conventional aviation fuel, while avoiding other adverse sustainability impacts.
Significant interest exists also for non-bio-based feedstocks, in particular the so-called drop-in Power-to-Liquids ‘electrofuels’. This pathway allows the production of a synthetic alternative fuel to fossil kerosene through the use of renewable electricity to produce hydrogen from water by electrolysis and a combination with carbon from CO2 (ideally captured from the air). The Power to-Liquid process can present a favourable greenhouse gas balance relative to conventional and bio-based aviation fuel streams with close to zero emissions . As of today, electrofuels are a technically viable solution to help decarbonise the aviation sector. However, few demonstrator projects are being brought forward due to the fact that electrofuels are 3 to 6 times more expensive than kerosene . According to one study, using electrofuels to meet the expected remaining fuel demand for aviation in 2050 would require 95% of the electricity currently generated using renewables in Europe .