Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
It is very unlikely that military aircraft will ever use any fuel except current kerosene-type fuels, for reasons of logistics and the desire to have one fuel for all aircraft and
ground equipment.
7.8.5. Summary
On balance, it appears that current types of aviation fuel will continue to be the preferred option for gas turbine powered aircraft. This situation could change if liquid
hydrogen could be produced by an environmentally acceptable and economically competitive method or if the need to reduce CO 2 emissions from aviation becomes
overwhelming. Aircraft now consume about 2.5% of all fossil fuels burned; therefore, they are not major contributors to anthropogenic CO 2 discharged into the
atmosphere. Future aviation demand growth rates are discussed in Chapter 8. The only emissions that are directly influenced by fuel type are CO 2 , H 2 O, and SO x O.
The properties of aviation fuels are controlled within fairly narrow limits, and allowable variations can have very little impact on exhaust emissions, with one exceptionremoval
of sulfur. The sulfur content in jet fuel currently averages about 0.05% worldwide, well below the specification allowance of 0.3%. In the United States, sulfur
content has been increasing slightly since 1990, but it appears relatively stable in Western Europe and probably the rest of the world. Sulfur will probably remain at this
level for the foreseeable future unless it is legislated downward. The penalty for removing sulfur from petroleum-derived jet fuel would be about a 0.1% increase in CO2 emissions attributable to the aircraft sector as a result of the need to manufacture additional hydrogen; there would be no CO2 penalty if the hydrogen came from a
renewable energy source or if nuclear power were used to extract it from water.
Sulfur-free kerosene can be produced by F-T processes from synthesis gas that could be produced from natural gas, coal, or biomass. The economics of these
processes are improving. F-T jet fuel produced from biomass-derived synthesis gas would be essentially CO 2 -neutral.
Alternative fuels to kerosene that appear to be environmentally friendly have been identified; even if these benefits can be verified, however, introduction of such fuels
will be hindered by significant technical problems in adapting these fuels to current aircraft designs and airport infrastructures. Using current technology, such changes
would increase CO 2 released to the atmosphere.
Alternative liquid fuels appear to offer little promise. Alcohols are not compatible with the fuel systems of current aircraft and will suffer significant range penalties as a
result of lower heats of combustion. The use of esters of vegetable oils is limited to 2% at most before fuel blends fail freezing-point requirements. Burning alcohols
and esters would increase emissions of CO, hydrocarbons, organic acids, and aldehydes.
The introduction of any cryogenic fuel would require the design and development of a new fleet of aircraft, as well as a new supporting infrastructure for the storage
and handling of such fuel at airports. Cryogenic fuels are not compatible with the fueling systems of current aircraft, and their lower energy density would require much
larger fuel tanks than on current aircraft. Studies have shown that cryogenic hydrogen could be a viable alternative to kerosene with significant reductions in
greenhouse effects for long-range commercial aircraft if design and infrastructure problems can be solved.
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