Policy actions and the efforts of industry have led to improvements in fuel efficiency over recent years. For instance, the amount of fuel burned per passenger dropped by 24% between 2005 and 2017. However, these environmental benefits have been outpaced by a sustained growth in air traffic, with passengers in 2017 flying on average 60% further than in 2005.
In the EU in 2017, direct emissions from aviation accounted for 3.8% of total CO2 emissions. The aviation sector creates 13.9% of the emissions from transport, making it the second biggest source of transport GHG emissions after road transport. Before the COVID-19 crisis, the International Civil Aviation Organization (ICAO) forecasted that by 2050 international aviation emissions could triple compared with 2015.
A market-ready, zero-emission aircraft by 2035, carbon-neutral scheduled collective transport for journeys under 500km by 2030, and possible quotas on low-carbon fuels are among the goals set by the European Commission in its Sustainable and Smart Mobility Strategy. The strategy, which was published on 9 December 2020, is aimed at delivering a 90% reduction in emissions from the European Union’s transport sector by 2050. Targets outlined in the Green Deal that relate to aviation include a “zero-emission large aircraft” that “will become ready for market” by 2035. The goal for 2030 is that “scheduled collective travel of under 500km should be carbon neutral within the EU”.
Alternative aviation fuels
Currently, aviation mainly uses jet fuels including Jet A-1, Jet A (only US), JP-5, and JP-8 (military aircraft), although Jet B and JP-4 are also used as blends of kerosene-naphtha and kerosene-gasoline. Aviation also uses a small amount of aviation gasoline (Avgas), a residual lead-blended aviation fuel, mainly used in reciprocating-engine or piston-engine aircraft.
- Bio-jet fuels
Similarly, low blending of bio-jet fuels with conventional jet fuel reduces exhaust toxicity. The energy content (by weight) and other fuel properties of bio-jet fuels are rather like those of conventional jet fuel, which aids adoption in existing engines.
- Electro-jet fuels
Electrofuels are primarily produced from electricity via electrolysis of water with the use of captured carbon (or nitrogen), forming, for example, Fischer-Tropsch kerosene, methane, methanol, hydrogen, ammonia, and n-octane.
- Liquefied methane
The studies and experimental tests have shown that LNG is a viable option as an alternative aviation fuel; however, it is not used in normal service and operations. The main energy carrier in LNG is methane, which can also be produced from biomass pathways (e.g., liquefied biogas) and electrofuels pathways. However, several challenges remain in operating LCH4 aircraft, where design and construction of the LCH4 storage tanks and supply chain infrastructure are the biggest challenges. Cryogenic fuel tanks are required to operate LCH4 in an aircraft; these are larger and heavier than other fuel tanks.
- Hydrogen
H2 is perceived as an attractive alternative aviation fuel both in recent and past research as it has a great supply potential, contains three times the energy content per weight of traditional jet kerosene (43.2 MJ/kg vs 120 MJ/kg respectively) and does not produce CO2 from combustion. It is flammable, has a very short ignition time in comparison to conventional jet fuel, and provides a wider stability range. It has the highest thermal conductivity among all fuels, and high heat capacity and low dynamic viscosity, which provide superior cooling properties for operation at high speeds and high combustor temperatures.
- Ammonia
Ammonia (NH3) is perceived as a potential fuel for gas turbines as it has a high H2 content but not any carbon atoms. Ammonia, mixed with H2 or LCH4, can be used as aviation fuel in low blending or as a dual fuel solution in modified aircraft engines and fuel cells.