Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
and availability.
U.S. and UK jet fuels.
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 )
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
Energy
Density
(10 3 MJ m -3 )
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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