Aviation Fuel

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AVIATION FUEL

Aviation fuel is the fuel used to power aircraft in flight. It must satisfy the unique requirements of both the engine and the airframe of the aircraft. Currently the great majority (more than 99%) of aviation fuel used in both civil and military aircraft is jet fuel. A small quantity of aviation gasoline is still used in small aircraft. Early aircraft used motor gasoline to power their spark ignition engines because the aviation and auto worlds shared the same early engines. In recognition of aviation's more stringent requirements compared to ground transportation, separate specifications for aviation gasoline were developed after World War I. Subsequent aircraft spark ignition engine developments as World War II approached identified the need for high octane in aviation fuel for improved performance. This resulted in the development of 100-octane aviation gasoline and the unique refinery processes necessary to produce it. Beginning in the mid-1930s, research was initiated in both Great Britain and Germany on the development of a gas turbine aircraft engine, which was radically different from the spark-ignition, reciprocating engines used since the days of the Wright brothers. The new jet engine was capable of markedly improved high-speed performance. During this development, illuminating kerosene used as a fuel for lamps, was chosen as the liquid fuel for the jet engine because it did not conflict with the very strong military demand for high-octane aviation gasoline. This use for jet engines of distillate-based fuels different in composition from high-octane gasoline has continued to this day. The first operational use of jet-engine-powered aircraft occurred in a military aircraft (the German Me-262) late in World War II, and its performance proved so superior to propeller-powered, piston-engine aircraft that subsequently all air forces changed to the use of jet aircraft. The development and rapid growth of higher-speed commercial transport aircraft using jet engines began in the late 1950s. As a result of the switch of both military and commercial aircraft to jet engines from spark ignition engines, jet fuel demand rose rapidly, and jet fuel over time displaced aviation gasoline as the dominant fuel for aviation use.

FUEL TYPES

Jet fuels in use today are essentially all kerosene-based but differ somewhat in their compositions. For civil fuels, Jet A is used primarily in the United States and Jet A-1 throughout most of the rest of the world. Jet A and Jet A-1 differ principally in their freezing point, which is the temperature at which solid wax crystals form in the liquid fuel as it cools. Commercial aircraft store their fuel primarily in wing tanks, and there is a concern that during long international flights through cold-weather conditions the formation of wax could interfere with the flow of fuel from the wing tanks into the engines. Thus all jet fuels specify a freezing point suitable for its intended flight use. The military fuel used by both the U.S. Air Force and NATO air forces is JP-8, which is similar in composition to commercial Jet A-1, but employs military-designated additives. The U.S. Navy uses JP-5, a jet fuel with a higher flash point (a measure of the fire hazard associated with the fuel) than Jet A, Jet A-1, or JP-8 because of concern about fire safety aboard aircraft carriers, particularly in combat operations. In the past the U.S. Air Force used a very low flash point fuel called JP-4, composed of a mixture of kerosene and lighter-boiling refinery streams, but switched to the higher-flash-point kerosene-based JP-8 fuel to reduce combat losses and post-crash fire and handling incidents. A commercial low-flash-point fuel designated Jet B, similar to military JP-4, is used only in very cold Arctic areas because of difficulties in starting engines with the more viscous kerosene-type fuels.

SPECIFICATIONS

Jet fuel requirements are defined by engine and airframe technical needs, which are balanced against the need for a widely available and low-cost fuel. These technical and economic requirements are translated into fuel specifications that define physical properties and chemical compositional limits and that also require the fuel to pass a number of unique performance tests designed to predict satisfactory use. Jet fuel is a tightly specified, high-technology commodity. A number of commercial and military jet fuel specifications are used throughout the world. Commercial specifications include ASTM D 1655, which is an industry consensus specification; Defense Standard 91/91, issued by U.K. Ministry of Defense for their Civil Aviation Authority; and the International Air Transport Association (IATA) Guidance Material. At major airports, where fueling systems are operated by a number of different companies rather than a single company, a combination of the most stringent requirements of ASTM D 1655, Defense Standard 91/91, and the IATA Guidance Material called the "Check List" is often used. Attempts are under way to harmonize the major commercial Western specifications and to get non-Western countries to join in using common worldwide specification and test methods. In addition, the U.S. military as well as other governments write specifications for jet fuel.

PROPERTY REQUIREMENTS

Because the jet engine was free of the demanding need for high-octane fuel, in the early days of the jet-engine development it was thought that it could use practically any liquid fuel. However, subsequent experience proved this to be untrue, as a number of potential problem areas indicated that control of fuel properties, reflecting both bulk and trace components, were important for satisfactory use. Over the years these important property requirements were translated into specification requirements that put restrictions on what is acceptable as jet fuel.

During the early jet-engine development work, it was recognized that the combustion system of the engine would be a critical component. Fuel-combustion-related properties are controlled via limits on the total concentration of aromatic-type compounds as well as the concentration of condensed ring aromatic compounds (i.e., naphthalenes). Since these types of hydrocarbon compounds tend to burn with higher levels of flame radiation than do other hydrocarbon compound types, it makes combustor wall cooling more difficult. In addition, specific combustion performance prediction tests such, as the smoke point test and the luminometer number test, were developed and added to specification requirements. Other concerns are energy content, density, and volatility. The minimum energy content of the fuel is specified for range considerations. The density of the fuel controls the weight of fuel that can be carried in a given volume. The volatility or boiling range of the fuel is controlled because it impacts on a number of properties. Lower boiling fuels are easier to use, either when starting a cold engine or when attempting to relight an engine at altitude, but early attempts to use very light

Net Heat of Combustion, MJ/kg 42.8 Min
Boiling Range, 10% Recovered, °C 205 Max
Boiling Range, Final Boiling Point, °C 300 Max
Flash Point, °C 38 Min
Density, kg/m3 775 to 840
Freezing (wax appearance) Point, °C -40 Max

fuels encountered problems with fuel boiling off from the vented wing tanks at the low pressure at higher altitudes. Subsequent experience also demonstrated the increased safety risks inherent in low-flash-point fuels in either civil or combat military use. Specifications include controls on the boiling range of the fuel as well as a flash point test measurement. The potential for harmful wax crystal formation in aircraft wing tanks at low temperatures in flight is controlled via the inclusion of a freeze point test requirement in all specifications. The viscosity of the fuel at low temperatures is limited to ensure the proper operation of fuel injection nozzles during low-temperature startup. Problems with the stability of the fuel in storage leading to unwanted gums and deposit formation were anticipated in early fuel development work and led to restrictions on the olefin (unsaturated hydrocarbon) content of the fuel in specifications. Subsequent operational experience also discovered stability problems in flight caused by the exposure of the jet fuel to hot metal surfaces where reactions of the fuel with the dissolved oxygen in the fuel led to deposit formation in critical components such as the fuel nozzle, heat exchanger surfaces, and narrow-tolerance moving components in fuel control units. These high-temperature-thermal stability problems led to the development of tests designed to simulate the high-temperature exposure of the fuel, and all specifications require the fuel to pass such a test.

The use of high-sulfur-content fuels could enhance undesirable carbon-forming tendencies in the engine combustion chamber as well as result in higher amounts of corrosive sulfur oxides in the combustion gases. Mercaptans (a type of sulfur compound) cause odor problems and can attack some fuel system elastomers. Both the concentration of total sulfur compounds as well as the concentration of mercaptan sulfur compounds are controlled in specifications. The corrosivity of the fuel toward metals caused by the presence of elemental sulfur or hydrogen sulfide is controlled by the use of tests such as the copper strip corrosive test. Acidic compounds present in the fuel, such as organic acids or phenols, are controlled by a total acidity test.

Another area of importance is contamination. Jet fuels are tested for the presence of heavier fuel contamination by use of an existent gum test, which detects the presence of heavier hydrocarbons from other products. Testing also is carried out to detect the presence of excessive levels of undissolved water and solids, as well as for surfactants that can adversely affect the ability of filters and coalescers to remove dirt and water from the fuel.

Additives also are used to enhance jet fuel quality in a manner similar to that of gasoline, but unlike gasoline, are tightly controlled. Only additives specifically cited in a specification can be used within allowed limits. The mandated or permitted use of additives varies somewhat in different specifications, with military fuels tending to the greater use of additives compared to civil fuels. A static dissipater additive is used in many fuels to enhance the rapid dissipation of any electrostatic charge in the fuel created by the microfiltration used for dirt removal. To prevent the formation of deleterious hydroperoxides during prolonged fuel storage, many specifications require that an antioxidant (a compound that slows down or prevents oxidation) additive be added to fuels that have been hydrotreated. This must be done because the natural antioxidants present in the fuel that were unavoidably removed.

Another additive used is a metal deactivator to chemically deactivate any catalytic metals such as copper accidentally dissolved in the fuel from metal surfaces. Uless they are chemically deactivated, dissolved metals cause the loss of good stability quality.

Corrosion inhibitor/lubricity improvement additives are used particularly in military fuel for the dual purpose of passivating metal surfaces and improving the lubricating properties of the fuel in equipment such as fuel pumps. The military also specifies the use of a fuel system icing inhibitor as an additive to prevent filter blocking by ice crystal formation, because military aircraft tend not to use fuel line filter heaters, which are standard equipment on civil aircraft.

COMPOSITION AND MANUFACTURING

Aviation turbine fuels are produced in refineries primarily using petroleum crude oil as the sole starting material. The exceptions are Canada, which uses some liquids produced from tar sands, and South Africa, which uses some liquids produced from coal. The overwhelming percentage of chemical compounds—present in jet fuel are hydrocarbon compounds, that is, compounds composed of carbon and hydrogen. These hydrocarbon compounds include branched and normal paraffins; single-ring and multiring cycloparaffins, which are also called naphthenes; and single-ring and multiring aromatics, hydroaromatics, and olefins. The distribution of hydrocarbon compound types varies considerably, primarily depending on crude source. Heteroatom compounds, which are hydrocarbon compounds that also contain sulfur, nitrogen, or oxygen, are present at trace levels, and are important because heteroatoms can have a disproportionate effect on fuel properties.

Much jet fuel is produced by simply distilling a kerosene fraction from the crude oil followed by some form of additional processing. The initial boiling points for the distillation are generally set to produce a jet fuel that meets the flash-point requirement, and the final boiling points are set to meet requirements such as freeze-point, smoke-point or naphthalene content. Jet fuel often is blended from a number of streams. In addition to simply distilled kerosene fractions, blend stocks are produced from heavier crude oil fractions or refinery product streams by breaking them down into lower-boiling fractions. The processing steps used to prepare blend stocks after distillation vary considerably, depending on factors such as crude oil type, refinery capabilities, and specification requirements. Crude oils, whose kerosene fractions are low in total sulfur content, can be chemically processed to reduce mercaptan sulfur or organic acid content. For example, a kerosene with a high organic acid content but a low total and mercaptan sulfur content can be simply treated with caustic (sodium hydroxide) to lower the acid level. Similarly, a jet fuel blend stock low in total sulfur but too high in mercaptan sulfur can be chemically treated in a so-called sweetening process, which converts the odorous mercaptan sulfur compounds into odor-free disulfide compounds. Chemical treatment is often followed by passage through both a salt drier to lower water levels, and a clay adsorption bed to remove any trace impurities still present.

Another type of processing employed is treatment in a catalytic unit with hydrogen at elevated temperatures and pressures. Catalytic processing is used, for example, when higher total sulfur levels require total sulfur removal, which cannot be achieved by chemical treatment. In addition, catalytic treatment with hydrogen, depending on the process conditions, can be used to break down heavier fractions into the kerosene range. More severe processing conditions such as higher pressures used to affect a boiling point reduction, will also generally extensively remove heteroatoms and markedly lower the level of olefins. Jet fuel normally sells at a premium compared to other distillates, and reflects the cost of crude oil.

Jet fuel is shipped in a highly complex system designed to prevent or eliminate excess water, particulates such as dirt and rust, microbial growths, and contamination from other products in the fuel being delivered into aircraft. Transportation may involve shipment in pipelines, railcars, barges, tankers, and/or trucks. Techniques employed include dedicated storage and transportation, the use of filters to remove particulates, and the use of coalescers and water-adsorbing media to remove water. The elimination of water will prevent microbial growth.

ENVIRONMENTAL ISSUES

As for all hydrocarbon fuels, the combustion of jet fuel produces carbon dioxide and water. Turbine engines are designed to be highly efficient and to produce low levels of unburned hydrocarbons and carbon monoxide. Nitrogen oxides and sulfur oxides also are emitted from the turbine engine. There is little organic nitrogen in the fuel, and oxides of nitrogen are produced from the nitrogen and oxygen in the air during the combustion process. As a result, control of nitrogen oxides emissions is essentially an engine combustor design issue. Sulfur oxides are produced from the low levels of sulfur compounds present in jet fuel during the combustion process, and thus control of sulfur oxides is essentially a fuel-related issue. Only a small fraction of all sulfur oxides emissions are produced by the combustion of jet fuel because of the relatively low use of jet fuel compared to total fossil-fuel combustion. However, jet-fuel-produced sulfur oxides emissions are unique because aircraft engines are the only source emitting these species directly into the upper troposphere and lower stratosphere an issue of growing interest to atmospheric and climate change researchers.

William F. Taylor

See also: Aircraft; Air Pollution; Air Travel; Climatic Effects; Gasoline and Additives; Gasoline Engines; Kerosene; Military Energy Use, Modern Aspects of; Transportation, Evaluation of Energy Use and.

BIBLIOGRAPHY

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Dyroff, G. V., ed. (1993). "Aviation Fuels." In Manual on Significance of Tests for Petroleum Products, 6th ed. Philadelphia ASTM.

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Waite, R., ed. (1995). Manual of Aviation Fuel Quality Control Procedures. West Conshohocken, PA: ASTM.