Aviation and the Global Atmosphere


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7.8. Aviation Fuels


This section addresses the major fuel-related issues that have influenced and will continue to influence the development of aircraft into the foreseeable future. Almost all current civil and military aviation around the world uses a kerosene-type fuel. This class of fuel provides a good balance of properties currently required from an aviation fuel, in which energy density, operational issues, cost, and safety all need to be taken into account. This section examines some alternative fuels that will, no doubt, continue to be considered as the demand for air transport continues and its impact on the environment grows.

Figure 7-36: Historical trends of sulfur content in
U.S. and UK jet fuels.

Fuel specifications that define physical properties, chemical composition, and performance tests have evolved over several decades. These fuel specifications are designed to balance quality, cost, and availability, thus guarantee a product of worldwide consistency. Although many countries have their own fuel specification, by general agreement among governing bodies, fuel suppliers, and aircraft manufacturers, all civil aviation fuel must effectively meet the requirements of American Society of Testing and Materials (ASTM) D1655 (ASTM, 1997) and Defense Evaluation and Research Agency (DERA) DEF STAN 91-91 (DERA, 1998). The ASTM specification contains two relevant fuel designations (Jet A and Jet A-1), which differ only in their freezing points. The DEF STAN specification addresses only Jet A-1. Jet A fuel has a maximum freezing point of -40C and is used only in the United States, where moderate temperatures combined with short flight times justify a separate specification to increase availability. Jet A-1 has a maximum freezing-point requirement of -47C to meet the low-temperature requirements of long, high-altitude flights and is used everywhere in the world except the United States. Other differences between these fuel specifications are relatively minor and immaterial to this report.

The ICAO specification for fuel to be used in emissions testing of aircraft gas turbines is also a kerosene-type fuel (ICAO, 1993). Restrictions are placed only on the 10 properties that potentially affect emissions. Table 7-9 compares the ICAO specification with the ASTM and DEF STAN specifications for the relevant properties. The property limits of ICAO are somewhat more restrictive than the commercial specifications to limit testing concerns, but at the expense of cost and availability.

Also shown in Table 7-9 is the percentage of fuels sold in 1997 in the United Kingdom under DEF STAN 91-91 that meet the ICAO limits. Virtually all DEF STAN 91-91 fuels met the ICAO limits except for naphthalenes and hydrogen content; in both cases, the non-attainment fuels were to the side of lower emissions (i.e., lower naphthalenes and higher hydrogen). This comparison shows that the ICAO fuel specification for emissions testing is relevant to jet fuels being marketed.

The primary military fuels of North America and Western Europe are defined by the identical specifications F-34 (NATO) and JP-8 (United States). These specifications effectively define military fuels throughout much of the world because many countries buy their military aircraft from the same manufacturers. The only significant difference between these military fuels and Jet A-1 is the mandatory use of certain additives in the military fuels; however, some of these same additives may be found in some civilian fuels. For shipboard safety reasons, Navy aircraft use a high-flash-point kerosene fuel that is less volatile, but other relevant properties are similar; in Western Europe and North America, these fuels are designated as F-44 (NATO) and JP-5 (United States). For completeness, we mention here the existence of small volumes of special fuels used by military aircraft that fly at very high altitudes and/or require a higher thermal stability than conventional fuels provide. These fuels are also kerosene-type fuels but may have different volatility/freezing-point requirements and are more highly refined to improve thermal stability.


Table 7-10: Comparison of heats of combustion for candidate alternative aviation fuels.
Density
(kg m-3)
Specific
Energy
MJ kg-1)
Energy
Density
(103 MJ m-3)
Kerosene (typical) 783 43.2 33.8
Ethanol 785 21.8 17.1
Methanol 786 19.6 15.4
Methane (liquid) 421 50.0 21.0
Hydrogen (liquid) 70 119.7 8.4

All jet fuels are composed primarily of hydrocarbons as a blend of saturates, with no more than 25% aromatics. Olefins may be present, but they are effectively kept below about 1% by stability requirements. Additionally, a fuel may contain up to 0.3% sulfur by weight, although the level is generally less than 0.1%. Certain additives may also be present, as mentioned previously. Trace levels of oxygenated organics (e.g., organic acids) may be present but are effectively limited in concentration by the fuel specification to ensure product stability and materials compatibility. Metal contaminants such as iron, copper, and zinc can be picked up from plumbing and storage systems and can be present in the low ppb range. Halogens are not an issue because they are not used in refinery processes for kerosene. Additives currently used in jet fuels are all organic compounds that may also contain a small fraction of sulfur or nitrogen. The maximum allowable concentrations of these additives is controlled by relevant fuel specifications. These concentrations vary with the additive but are less than 6 mg L-1 (approximately 6 ppm), with the exception of two additives that contain no sulfur or nitrogen. Therefore, these additives presumably will have no measurable impact on emissions and are not an issue for this discussion (based on their constituents and very low concentrations); however, additional testing should be conducted to verify these conclusions.

In summary, differences between jet fuel specifications around the world are relatively minor and have little effect on fleet exhaust emissions. Thus, aviation fuel, in the context of this report, refers to all civilian and military jet fuels unless otherwise specified.


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