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Large-scale efforts are underway to develop fuels that will be cheaper and cleaner than the fuels currently in use in aviation. These include both petroleum-based fuels that may be cleaner burning and more efficient than current petroleum-based fuels, and also fuels that are derived from non-petroleum sources: - Gas To Liquid (GTL) and Coal To Liquid (CTL): Fuels that are made from coal or from natural gas offer an alternative to the current aviation fuels. Coal can be used to produce a synthetic gasoline, as Germany did on a large scale during World War II. With improvements in the technology of converting coal to a liquid fuel, coal may one day help to reduce the dependence upon petroleum-based fuels. Converting natural gas to a liquid fuel is attractive because natural gas is the cleanest burning of all the fossil fuels. Airbus recently demonstrated the viability of using GTL fuel in modern power plants by flying an Airbus 380 on a three-hour flight with one of the three engines burning GTL fuel.
- Biofuels: It will be many years, if ever, before biofuels fully replace petroleum-based fuels in aviation, but the potential does exist, and much effort is being put into developing biofuels that could be produced relatively cheaply and on a very large scale. In 2008, Virgin Atlantic flew a Boeing 747 from London to Amsterdam with one of the engines operating on a 20% biofuel mix of babassu oil and coconut oil. The long-term research effort at developing viable biofuels is focused on using sustainable, non-food sources which could be produced on a very large scale. Some of the possible biofuel sources being considered are algae and cellulose-based sources such as sawdust and switchgrass.
- Hydrogen: The use of hydrogen as an aviation fuel offers huge benefits as well as extreme difficulties. Hydrogen is the cleanest burning of fuels, with the exhaust from a hydrogen-burning engine being primarily water (though, when used by high-flying jets, there is some concern that the water vapor emissions could increase cloud cover beyond natural levels). Hydrogen is also available in virtually unlimited quantities. However, the process of isolating hydrogen from other elements is currently very expensive and energy intensive. Additionally, since liquid hydrogen must be stored at a temperature of minus 424 degrees, the on-board storage of the fuel presents great difficulties. And the extreme volatility of the fuel poses potential hazards (remember the hydrogen-filled Hindenburg?). The use of hydrogen as a primary fuel is probably less likely - or at least much farther away - than any of the other alternative fuels currently being researched.
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