Aircraft alternative fuel potential - What are the options?
Several requirements need to be satisfied for fuels to be suitable in commercial aviation. Aviation fuels need to deliver a large amount of energy content per unit of mass and volume, in order to minimize fuel carried for a given range, the size of fuel reservoirs, and the drag related to the fuel storage. Aviation fuels also need to be thermally stable, to avoid freezing or gelling at low temperatures and to satisfy other requirements in terms of viscosity, surface tension, ignition properties and compatibility with the materials typically used in aviation.
A number of potential alternative fuels may be considered for aviation. These can be derived from coal, natural gas or biomass. Not all of them, however, would significantly reduce GHG emissions. The most likely alternative fuels for aviation are those with similar characteristics to conventional jet fuel. This can also be obtained by blending of different fuels.
The presently most mature technologies are presented below.
Biodiesel - Hydrotreated Vegetable Oils (HVOs)
Oils and fats constitute a first family of raw material that can be considered for aviation fuel.
Biodiesel-like fuels derived from vegetable oils are presently the most developed biofuels (with also ethanol for automotive industry). They are not generally suitable on their own for commercial aviation applications. Indeed conventional fatty-acid methyl esters (FAME) freeze at normal aircraft cruising temperatures; they are also not thermally stable at high temperatures in the engine.
However, vegetable oils can be hydro-treated to produce a HVO fuel that consists almost totally of hydrocarbons, and compared to FAME (fuel containing oxygen), HVO is much closer in properties to conventional jet fuel. Hydro-treating can be carried out at refineries. It's today a promising way to obtain "drop-in" fuel for aviation but at a higher cost than biodiesel.
For this last reason, further improvements of FAME are still explored considering both the production process and various kinds of oil.
Fischer-Tropsch (F-T) fuels from fossil feedstocks
Synthetic fuels are high-quality fuels that can be derived from natural gas, coal or biomass. These fuels are typically produced via a gasification step, through the formation of a synthesis gas (mainly CO and H2) and its conversion to liquid hydrocarbon fuels via the Fischer-Tropsch (F-T) process. The F-T process is technically mature, and synthetic jet fuels from coal, natural gas or other hydrocarbon feedstock are chemically similar to conventional kerosene jet fuels –and ideally suited to supplement or replace them. They have high energy density and exhibit excellent low-temperature and thermal stability. 
They can even provide an efficiency increase compared to conventional jet fuel [1]. Coal-derived fuel from Sasol was first approved in South Africa (as 50% blend with Jet-A1 and then as neat product) and, in September 2009, generic Fischer-Tropsch fuels were approved for 50% blend with Jet-A1 by the American Society for Testing and Materials (ASTM) in its new D7566 standards. Certification of 100% synthetic paraffinic kerosene (SPK) derived from the Fischer-Tropsch process is expected to be achieved in 2011. Apart from the high cost of production, the main drawback with synthetic fuels produced from fossil fuel is the CO2 emitted during the manufacturing process. If synthetic fuels are to contribute to GHG emission reductions, CO2 from the manufacturing process must be captured and stored (CCS). But even with this technology implemented the life cycle analysis of these fuels do not show significant reductions of CO2 emissions in comparison to conventional jet fuel.
Fischer-Tropsch (F-T) biomass-to-liquid (BTL) fuels
Biomass-to-liquids (BTL) processes using F-T technologies are progressing and are likely to be deployed within the next five to ten years. These fuels offer big advantages, as they come along with the production from completely renewable feedstocks and offer therefore a high CO2 reduction opportunity. Compared to conventional fossil fuel and according to some analyses focusing on road diesel BTL fuel, CO2 savings can exceed to more than 80% on a life-cycle basis [2]. Regarding their fuel characteristics and usability in a jet engine, Fischer-Tropsch fuels from biomass show almost the same performance as synthetic FT-fuels derived from coal (CTL) and natural gas (GTL) and are therefore highly applicable for the usage in aircrafts.
What about liquid hydrogen?
Hydrogen is a potential non-CO2 emitting fuel for aircraft, but its use poses a number of significant technical challenges. It would most likely be stored on board as a cryogenic liquid (LH2) to minimize volume. Nonetheless, a number of significant modifications would be required to both engine systems and airframe designs to accommodate liquid cryogenic fuels. Insulation requirements and pressurization issues make it impossible to store LH2 in aeroplane wings, as is done with kerosene jet fuels. In addition, though LH2 has a very high energy density per unit mass (weight), its volumetric energy density is only one-quarter that of current jet fuel. The storage tanks needed for the large volume of cryogenically cooled hydrogen would increase the weight of large commercial aircraft by over 10% [3]. Modifications would also be necessary to the fuel management system and temperature controls. In sum, use of LH2 would require a completely different aircraft design, and would pose significant challenges for the engine. It would also require substantial modifications to airport infrastructure. Being gaseous at ambient temperature, H2 would also be fundamentally different from jet fuel, requiring a completely different fuel distribution infrastructure. Overall, LH2 is not promising as an alternative fuel for aviation in the near future or the medium term. It could only be viable in the long term if there were significant technological developments, entirely new aircraft designs and substantial infrastructural change.
[1] Karagozian, et al., “Report on Technology Options for Improved Air Vehicle Fuel Efficiency: Executive Summary and Annotated Brief “, USAF-SAB, Washington DC, May 2006, Report Number SAB-TR-06-04
[2] Wang, M., Wu, M., Huo, H., “Life-cycle energy and greenhouse gas results of Fischer-Tropsch diesel produced from natural gas, coal, and biomass,” Center for Transportation Research, Argonne National laboratory, presented at 2007 SAE Government/Industry meeting, Washington DC, May 2007
[3] Daggett, D., Hadaller O., Hendricks, R., Walther, R., “Alternative Fuels and Their Potential Impact on Aviation”, Prepared for The 25th Congress of the International Council of the Aeronautical Sciences (ICAS), Hamburg, Germany, September 3–8, 2006, Report Number ICAS–2006–5.8.2 or NASA/TM—2006-214365