Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
significant changes in fuel type or specification would require major modifications to all of these elements. These are non-trivial matters involving major perturbations in
the existing system, with significant efforts and costs. With this in mind, several alternative fuels have been considered in terms of their environmental impact. These
alternatives include alcohols, methane, and hydrogen; more recently, some consideration has been given to using methylated esters of vegetable oils as kerosene
extenders. Such fuels must be compatible with the basic capabilities and requirements of existing aircraft. They must have sufficient energy density, for example, to
meet payload and range requirements. They must also be compatible with all materials (metallic and non-metallic) used in the engine's fuel system and have adequate
lubricity to ensure that current margins and standards of safety-critical items such as fuel pumps are not compromised.
Introduction of an alternative fuel that does not meet the requirements of current aircraft would
imply the use of a two-fuel system at all airports until all current aircraft are replaced by new,
alternatively fueled aircraft. The prospect of limiting the availability of a new fuel to a few airports
does not appear to be viable because of the need to retain full services for aircraft diverted by
weather or mechanical problems.Although alternative fuels may offer some emissions benefits,
the major disadvantage is significantly lower energy density compared with kerosene. This
density deficit means that the aircraft would have to be designed with larger fuel tanks. Table 7-
10 compares the net heats of combustion for several alternative fuels on the basis of mass and
volume. For cryogenic fuels, there must also be consideration for the mass and volume of
insulation.
Ethanol and methanol are liquid fuels that can be pumped and metered in conventional fuel
systems, and they can be made from renewable energy sources. They are impractical fuels for
aviation, however, because of their very low heat content, in mass and volume terms. From a
safety standpoint, these alcohols have very low flash points-only 12 and 18°C, respectivelycompared
with the minimum allowed of 38°C. There are also chemical incompatibilities
associated with fuel system materials, although these problems could be remedied with relatively
minor changes. Furthermore, the combustion of alcohols produces organic acids and aldehydes
in the exhaust at idle conditions on the ground, with attendant health hazards to ground support
personnel (Eiff et al., 1992).There have not been any definitive engine studies using methyl
esters of vegetable oils, such as soybean or rapeseed oils, although some evaluations are
underway (e.g., Scholes et al., 1998). Adding such a material to jet fuel would not be allowed
under any current fuel specifications for jet fuel because of compositional considerations.
Furthermore, studies with methyl esters of soybean oil suggest that more than about 2% will
raise the freezing point above the specification maximum. Based on the results of Eiff et al.,
(1992) using ethanol blends with jet fuel, adding methyl esters of vegetable oils to jet fuel would
result in lower exhaust smoke/particulates at high-power conditions but increased CO and
hydrocarbons at idle conditions, along with the presence of acids and aldehydes. The effect on
NO x is uncertain, but would be directly related to any changes in flame temperature.
Aircraft gas turbines can be designed to operate on cryogenic fuels such as methane or
hydrogen; conventional fuel systems, however, cannot handle these fuels. Such fuels would
require new aircraft fuel system designs, as well as new ground handling and storage systems.
Moreover, cryogenic fuels would have to be stored in the fuselage rather than the wings to
reduce heat transfer. Because methane and hydrogen have only 65 and 25%, respectively, of
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Figure 7-39: Idle efficiency trend for small engines.