Types
There are several types of alternative fuels, usually classified depending on the original raw material and the process of transformation:
Synthetic fuels
Fischer-Tropsch (FT) fuels. Its use in aviation has been recently approved for commercial aviation with blends of up to 50% (ASTM D-7566 & DEF-STAN 91-91). FT process can use three groups of raw materials:
- Alternative fossil sources. These fuels contribute to the security of supply and allow improving local air quality, but do not reduce carbon dioxide emissions. Obtained from two main raw materials:
- Coal, known as CTL (Coal-To-Liquid). This kind of fuel has been used for many years in South Africa by SASOL.
- Natural gas, , known as GTL (Gas-To-Liquid), currently in production in Qatar, already tested (2009) in a commercial flight -with passengers- by Qatar Airways, using a 50% blend.
- Biomass, known as BTL (Biomas-To-Liquid). BTL has the advantages of improving improve security of supply and the local air quality as well as of mitigating the global emissions. BTL can be obtained from a flexible range of feedstocks: crops, urban waste, industrial waste, etc.
Fermentation of sugars (and starch)
Ethanol can be obtained by fermentation of biomass from crops rich in sugars (sugar cane, corn ...) also from ligno-cellulosic waste (like straw) or crops (like switchgrass). In the first case (sugars) those crops are considered food competitors. In the case of the lingo-cellulosic material food competency depends on the type of feedstock. Due to its characteristics of energy density, bioethanol is not very suitable as fuel for the commercial aviation (jet fuel). Its use is accepted in Brazil for an Embraer’s agricultural aircraft.
Currently another pathway for transformation of sugars is being developed, called sugars to hydrocarbons, that allows to obtain fuels more similar to jet fuel from sugars.
Hidrogenated oils (HVO)
A high-energy fuel from a vegetable oil (sometimes from animal fats) can be obtained. The procedure has been enhanced so that the fuel is stable in situations of change in pressure and temperature during the flights. The transformation process can be biochemical (using microorganisms and/or enzymes) or thermochemical.
The vegetable oils can be obtained from:
- First generation crops: as soy, rapeseed, coconut or palm, often used for human food and cosmetics. Aviation industry rejects this kind of crops for their implications in sustainability and food prices
- Second generation crops: jatropha, camelina, halophytas, are non-food feedstocks, with low requirements of water or soil. Halophytas can be irrigated with salt water. Generally more sustainable, this feedstock are a short-medium term alternative for aviation.
- Third generation crops: microalgae, can be cultivated in bioreactors or directly on the sea, so this crops do not compete for water or land with other crops. Very promising at the medium-long term.
Pyrolisis (HPO)
Pyrolisis oils can be obtained, by fast pyroslisis, from lignocellulosic biomass, usually wastes or residues like straw. Pyrolisis oils can be hydrotreated for obtained jetfuel. This pathway is close to demonstration level. Its main advantages is that it contents a certain quantity of aromatic compounds that are needed for guarantee a good performance of engine seals and, in HVO or FT cases, requires blending biojet fuel with conventional fuel.
Other alternatives
Liquefied hydrogen or methane are being studied as alternative fuels for aviation. However, their physical or chemical stability, as well as their energy content (related to weight and volume), together with other technical issues requires great modifications in order to be used in aircrafts (not drop-in). So these are only long-term alternatives.