Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
7.8.2. Fuel Composition Effects on Emissions
For the most part, the design of the combustion chamber determines the gaseous and soot
emissions from a gas turbine; there are only limited opportunities for fuel properties to influence
emissions. Certainly there can be significant effects over the spectrum of hydrocarbon fuels from
methane or natural gas to heavy distillate and residual fuels. Within the narrow definition of
aviation kerosene, however, there is little opportunity for reducing emissions from current aircraft
by fuel modification, with the exception of steps taken to reduce sulfur.
Nitrogen for NOx comes from the air, not the fuel. Jet fuels contain only trace amounts of fuelbound
nitrogen, which cause storage stability problems and render the fuel unfit for use.
CO2 and H2O emissions are influenced by fuel composition. A fuel with a higher H/C ratio will
produce lower CO2 and correspondingly more water; however, only relatively small variations
are found in aviation fuel. The NIPER survey of U.S. fuels does not record hydrogen content;
however, the 1996 UK survey (Rickard and Fulker, 1997) and the Boeing survey (Hadaller and
Momenthy, 1990) are in very good agreement with regard to the range of hydrogen content in jet
fuels and the mean value. Figure 7-34 shows the distribution of fuel hydrogen content from the
UK survey. The bulk of the data fall between 13.5 and 14.1% with a mean value of 13.84%.
More than 90% of data in the 1989 Boeing worldwide survey also fall between these same
values, with a mean of 13.85%. Thus, typical emissions indices for CO2 and H2O are 3.15 ± 0.01
and 1.25 ± 0.03, respectively, where the variations allow for the range of hydrogen content found
in jet fuels.
Increasing the hydrogen content of jet fuel has been considered as a way of reducing CO2 emissions. Currently, the most efficient and effective means of generating hydrogen is by steam
reforming of natural gas; the thermal efficiency of this process is 78.5% (as compared to water
electrolysis, with a thermal efficiency of 27.2%) (Encyclopedia of Chemical Technology, 1995).
This process is expected to remain the most cost-effective means of producing hydrogen for the
foreseeable future (Tindall and King, 1994). Even if pure methane, which has the highest H/C
ratio of any fossil fuel, were used as the source, the inefficiencies of hydrogen production would
result in about three times as much CO2 being released as would be saved from combustion of
a fuel with higher hydrogen content. Even if hydrogen could be obtained from water without
using fossil energy, increasing the average hydrogen content by 0.05% (a significant amount)
would result in only a 0.6% reduction in the amount of CO 2 produced per pound of jet fuel
burned simply based on stoichiometric combustion calculations.
The most significant effect that changes in aviation kerosene can have on emissions is in
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Figure 7-37: Comparison of relative net greenhouse
effects for hydrogen and kerosene.