**6. Alternative or synthetic fuels**

The aviation community uses huge amounts of fuel and requires a reliable source of high quality fuel. The uncertain markets for petroleum and the differences in fuel composition due to different crude oil feed stocks has led the industry to investigate alternate sources of fuel. In the United States, several alternative energy sources are possible, including natural gas, coal and biomass. Natural gas and coal are in abundant supply in the United States but are still non-renewable and have a large environmental footprint [39]. Biomass based sources, are potentially more environmentally friendly, although the life cycle analysis must certainly be taken into account [40, 41]. The United States military has estimated that at least 50% of its energy use would be from renewable sources by the year 2020 [42]. Several processes are under investigation, but considering current market conditions of low crude oil prices, prospects for widespread development are not promising [43].

Primary attention has been paid to processes which produce fuels that can serve as drop in replacements for petroleum based fuels requiring no modifications to the aircraft, and can be interchanged with conventional fuel depending upon local availability [44]. It is also important that the feedstock can be produced sustainably and are not either used in foods or raised on land that can also be used to raise food crops [45]. Two processes that are the most developed and can produce a synthetic fuel from a variety of feedstocks to provide an alternative fuel source are synthetic fuels through the Fisher-Tropsch process and hydroprocessed esters and fatty acids fuels.

The synthesis of liquid fuels from natural gas, coal or biological sources has been an important goal for many years. The primary industrial process that is used is the Fischer-Tropsch process that was initially developed in Germany in the early 1920s. The process takes carbon monoxide and hydrogen and converts them in the presence of a catalyst to paraffinic and branched chain hydrocarbons. The carbon monoxide and hydrogen can come either from coal, natural gas or renewable sources. Depending on the catalyst and reaction temperature different types of fuel can be prepared [46]. Fischer-Tropsch fuels have the advantage of containing no aromatic content, no metals and no heteroatom containing impurities. These fuels have been shown to be cleaner burning with reduced particulate emission than conventional fuels. Current specifications for aviation use state that synthetic fuels must either be mixed 50–50 by volume with petroleum based fuels for semi-synthetic jet fuel or be tested to ensure at least 8% aromatic content for a fully-synthetic jet fuel.

The aviation industry has shown an intense interest in developing fuels from renewable resources. Initial evaluation of fatty acid methyl esters similar to biodiesel showed the fuel did not have low temperature properties needed for aviation [47]. The need however for a renewable source of jet fuel, preferably from a non-food source led to the development of a hydrotreated renewable jet fuel (HRJ) from camelina. The hydrotreating process for jet fuel converts the typical esters into paraffins and isoparaffins by reaction with hydrogen in the presence of a catalyst. Camelina based HRJ and other isoparaffin-rich bioderived fuels are similar in composition to ultralow sulfur hydrocarbon fuels [48]. Isoparaffin rich fuels also have the advantage of lower exhaust emission of nitrogen oxides and particulate matter.

The storage stability of Fischer Tropsch fuels and hydrotreated renewable jet fuel will be likely enhanced by the reduced solubility of water in fuels that have no aromatic content. Since the heteroatoms are not present, many of the deposit formation reactions found in normal fuels should be absent. The primary concern would be the ability of microorganisms to metabolize the hydrocarbons that comprise the majority of the fuel. Microorganisms have been shown to metabolize synthetic paraffinic kerosene readily, since aromatics and sulfur containing compounds in conventional fuels are toxic to many of the microorganisms. The use of synthetic jet fuel has been limited to a partially synthetic 50:50 blend of the synthetic fuel with a petroleum jet fuel; with the exception of a fully-synthetic fuel produced by Sasol. The primary limitation is due to the lack of aromatics which can result in seal cracking. The added petroleum based jet fuel or additional treating in the SASOL product increases the aromatic content to 8% which is adequate to avoid seal shrinkage [49]. The increased aromatic content, however leads to an increase in oxidative addition reactions which are closely associated with the formation of deposits [50].
