As mentioned above, surveys show that the sulfur content of most fuels is well below specified limits. All of the surveys show that about 90% of fuels have sulfur content less than 0.1%. Figure 7-36 shows the historical trends for the U.S. and UK surveys. In the UK, average sulfur level has remained relatively constant since 1988 (Rickard and Fulker, 1997); in the United States, however, the average sulfur content in the NIPER survey has been increasing. This trend conflicts with reports (Hadaller and Momenthy, 1993) based on projections of increased hydro-treatment to reduce sulfur in gasoline and diesel fuel. However, changes in gasoline production have not significantly affected jet fuel because there is very little overlap in the boiling range.
The impact of the trend to use low-sulfur diesel fuels is not clear. Many refineries worldwide do not have the hydro-treating capability to make low-sulfur fuels. The API/NPRA survey for 1996 reported that 46% of the jet fuel blendstock in the United States was straight-run material that was not hydro-treated (API/NPRA, 1997). For many of these refineries with limited hydro-treating capability, the most economical approach may be to shift blending stocks with higher sulfur content to jet fuel, saving streams with lower sulfur for diesel fuel.
Without legislation, it is unlikely that average sulfur worldwide will change much from current levels of 0.04-0.06%. The fuel specification certainly could be tightened to allow no more than 0.1% without any apparent increase in cost or availability, and closer to 0.05% might be possible. There will be areas around the world where the sulfur will come down on its own, but there will also be pockets where it will stay relatively high. Specific details on future trends of sulfur content do not exist in the literature; a special survey of worldwide refinery plans would be required to develop a better picture.
Gas-to-liquid conversion processes to produce kerosene (e.g., Fisher-Tropsch processes) yield jet fuel that is almost sulfur-free. Although this approach is attractive in a few regions where there is an abundance of unused natural gas, it is unlikely that this resource will be significant until well into the next century, as the process becomes more economically viable (Singleton, 1997). Biomass gasification could also be used to produce the synthesis-gas feedstock for Fisher-Tropsch conversion.
Current aircraft-along with the airport infrastructure for supply, delivery, and storage of fuel-are specifically optimized for the use of current kerosene fuels; any 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.
Figure 7-39: Idle efficiency trend for small engines.
Figure 7-40: Engine pressure ratio versus EI(NOx)
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, respectively-compared 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 NOx 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 the energy density of jet fuel, fuselages would have to be considerably larger than current designs-increasing drag and fuel consumption. For long-range flights, this penalty would be offset by a reduction in take-off weight because hydrogen and, to a small extent, methane have higher specific energies than kerosene. Design studies for hydrogen-fueled, long-range (10,000 km) aircraft have shown that the lighter fuel weight results in almost a 20% reduction in energy consumption compared to kerosene-fueled aircraft even accounting for losses (Momenthy, 1996). The same study showed, however, that for medium- and short-range (5500 km and 3200 km) aircraft, there is an energy penalty of 17-38%. For methane, there was only a small benefit for long-range aircraft and penalties of 10-28% for medium- and short-range aircraft.
Of these two cryogenic fuels, hydrogen may be more attractive from an emissions standpoint. CO2 and SOxO emissions would be eliminated. However, water vapor would increase significantly despite the reduction in energy consumption. For the same energy consumption, burning methane would yield about 25% less CO2 and about 60% more H<span class="subscript">2</span>O than burning jet fuel. Burning hydrogen would result in 2.6 times as much water vapor as burning jet fuel, but no CO2. Table 7-11 compares the energy-specific emissions indices of CO2 and H2O at constant payload. (Effects of weight savings/penalties are not included in Table 7-11 because they depend heavily on the range of the aircraft.)
This basis of comparison could yield a quantitative "greenhouse" comparison of these three fuels if the greenhouse equivalency were known. Figure 7-37 presents the results from such a study for a hydrogen-fueled aircraft derived from the Airbus A310; the analysis takes into account the relative greenhouse effects of H2O, CO2, and NOx at different altitudes and shows that hydrogen offers a significant reduction in greenhouse effect over kerosene at all altitudes for this aircraft (Klug et al, 1996). Inefficiencies of production would alter these results somewhat in a comprehensive energy comparison, although water emissions are of environmental concern only at cruise altitudes. Figure 7-38 compares relative CO2 emissions from the manufacture and use of alternative aviation fuels from different resources (Hadaller et al., 1993). The potential benefits of hydrogen can be realized only if hydrogen can be obtained from water without the use of fossil fuels to provide the energy. Nuclear power is the best method identified in Figure 7-38. The Kvaemer process is being developed as a method for converting hydrocarbons into hydrogen, with carbon as a byproduct (as opposed to CO2 with the steam reforming process); if the energy requirements are sufficient, this fuel would appear on Figure 7-38 with a value less than 1.0. Kerosene from biomass [via a Fischer-Tropsch (F-T) synthesis process] would also have relative CO2 emissions less than 1.0 if included in Figure 7-38.
Liquid hydrogen offers an environmental advantage only if this fuel were produced on a renewable energy basis, as explained above. The necessary technology exists, but such liquid hydrogen is not economically competitive with kerosene at current price levels. On the other hand, liquid hydrogen based on renewable energy is the only candidate aviation fuel known today that would completely eliminate CO2 emission by aviation. Safety issues in the siting of storage and handling systems at airports pose significant challenges, however.
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.
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 CO2 emissions from aviation becomes overwhelming. Aircraft now consume about 2.5% of all fossil fuels burned; therefore, they are not major contributors to anthropogenic CO2 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 CO2, H2O, and SOxO.
The properties of aviation fuels are controlled within fairly narrow limits, and allowable variations can have very little impact on exhaust emissions, with one exception-removal 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 CO2-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 CO2 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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