|Aviation and the Global Atmosphere|
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7.8.1. Databases on Fuel Properties
Only two comprehensive, annual surveys conducted on jet fuel properties are publiCly available. Both of these surveys have been carried out annually since the early 1970s-one by the National Institute for Petroleum and Energy Research (NIPER) for U.S. fuels and the other by the Defence Evaluation and Research Agency (DERA) for UK fuels. The 1996 DERA survey covered 1467 batches of fuel representing 14.85 million m3 (Rickard and Fulker,1997). The UK survey is thought to be fairly representative of fuel used in the rest of Western Europe as well, because much of the UK fuel comes from refineries located there. These two surveys are useful in defining trends in fuel properties, although they are both sample averaged, not volume averaged.
Three snapshot surveys have been reported. Bowden et al. (1988) reported a survey by the U.S. Army Fuels and Lubricants Research Laboratory of 90 JP-8 fuels from Western Europe, plus one from Singapore and two from Korea. In the late 1980s, Boeing conducted a survey of 39 civil aviation fuels from around the world (Hadaller and Momenthy, 1990). For the two properties that will be discussed here-sulfur and hydrogen contents-the averages and distributions for these two worldwide surveys were similar to the UK results. In 1996, a snapshot survey of U.S. jet fuel properties was conducted jointly by the American Petroleum Institute (API) and the National Petroleum Refiners Association (NPRA); this survey included Jet A fuels from 105 refineries representing 2.47 million m3. Average sulfur levels in this survey agreed very closely with the average NIPER sampling for that year, lending some credence to the NIPER results on sulfur (API/NRPA, 1997).
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 fuel-bound 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 CO2 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 reduction of SOxO; all of the sulfur in the fuel is converted to sulfur oxides in the exhaust. The sulfur content in aviation fuels is limited to 0.3%, although most aviation kerosene has a sulfur content significantly below this limit. Figure 7-35 shows the distribution of sulfur content for jet fuel in 1996 in the UK. The average sulfur concentration was reported as 0.047% (Rickard and Fulker, 1997). The 1996 average reported in the NIPER survey was slightly higher, at 0.062%. The average value in the Boeing survey was 0.038% (Hadaller and Momenthy, 1990); the U.S. Army survey (Bowden, 1988) had an average value of 0.07%. However, most of the larger values came from two small refineries in southern Europe, whereas larger refineries had much lower values. Thus, average sulfur content around the world is probably in the range of 0.04-0.06%; this results in a typical EI(SO2) of 0.8-1.2.
Most jet fuel contains a sulfur content much lower than the specifications allow because of the use of low-sulfur crude oils and/or the use of hydro-processing to meet other parts of the jet fuel specification. Sulfur is removed as a part of hydro-processing. Purely from equilibrium calculations (without considering inefficiencies), 1 mole of hydrogen, as H2, is needed to remove 1 mole of sulfur, reacting to form H2S. At 78.5% efficiency, steam reforming of methane is currently the most efficient means to produce hydrogen; it produces about 5.1 moles of CO2 per mole of H2, hence per mole of sulfur. Assuming 0.06% sulfur in the fuel, this figure translates to an average of 0.0033 kg of CO2 released to produce 1 kg of sulfur-free fuel. Because burning 1 kg of fuel results in the release of about 3.15 kg of CO2, mandating zero-sulfur fuel would increase the amount of CO2 attributable to aircraft by about 0.1%.
Reducing the sulfur level would have no direct impact on engine performance or durability. Although low-sulfur fuels tend to have low lubricity-which can lead to accelerated wear in fuel pumps and fuel controls-it is not the sulfur that provides the lubricity but organic acids that are removed during the sulfur-removal process (Wei and Spikes, 1986). Military fuels use a lubricity additive to guard against the possibility of low-lubricity fuels. Civilian airlines have not found a need to use such additives, except in a few isolated localities where pump failures have occurred and there was little flexibility in fuel selection.
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