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Suggested Citation:"Section VI - Additional Findings." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Suggested Citation:"Section VI - Additional Findings." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Suggested Citation:"Section VI - Additional Findings." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
×
Page 23
Page 24
Suggested Citation:"Section VI - Additional Findings." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Page 24

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21 VI.1 Engine Warm-Up Emissions During engine ground start there are three basic tempo- ral regimes to consider: pre-ignition, post-ignition/pre-idle acceleration, and post-ignition at ground idle. The pre-ignition VOC emissions will simply be the result of evaporated fuel. Around the moment of ignition there will be a mixture of fuel-like hydrocarbons and partially burned hydrocarbons. The post-ignition period will be characterized by increasing combustion efficiency, and VOC emissions will be accompa- nied by concomitant CO2 emissions. In the winter MDW 2009 test, an effort was made to char- acterize the post-ignition emissions to address the question of “warm-up.” Whenever possible, the mobile laboratory was positioned about 40 m downwind of the engine during engine start. This was only done when the engines were known not to have been operated for at least two hours prior to the start. The order of the test matrix initially precluded measurement of the engine starts for engines one and two. We found we had sufficient flexibility with the sampling scheme, however, to attempt two additional near-start observations. In Figure VI-1, a subset of the warm-up data offers insight into the time required to establish an equilibrium exhaust gas temperature. This result suggests that the exhaust gas temperature parameter requires three minutes to reach 90% of its steady-state value. The formaldehyde emission index as a function of time following ignition is depicted in Figure VI-2. The time offset has been computed from the flight data recorder information and the time-coded notes taken in the mobile laboratory. The chemical information in the emissions profile at other engine-state changes has been used to refine the estimate of the time offset between the flight data recorder and mobile labora- tory time. Conservatively, the absolute time since engine start is accurate within five seconds. Figure VI-2 suggests that the CFM56-7B24 engines at the temperatures of this test (-7°C, -2°C) have two characteristic times. The first, initial rapid emissions change takes place within 20 to 60 seconds, where the emission index shows a rapid decay. The second change in emission index is longer and is characterized by a time constant of approximately two to three minutes. The data suggest that following cold start, the VOC emission index is approximately doubled compared with that for warm operation for less than a minute. A simplistic approach to gauge the effect this has on inventory modeling of a 13-minute idle time (engine-on; taxi-out) is to add approximately one minute worth of additional emissions due to post-ignition warm-up. The argument’s weaknesses are that no study has system- atically looked at the reproducibility of this observation or the time needed to return to cold start conditions. Further- more, the initial ten seconds following ignition are not well represented in this dataset. The warm-up aspect of the study was secondary to the overall test goals, but these results could be used to further refine assessments of warm-up emissions, as well as to design better tests for future work. VI.2 Near-Idle VOC Scaling The relationships among exhaust concentrations of numer- ous VOCs (e.g., formaldehyde, benzene, acetaldehyde, ethene) have been discussed in several archival publications (Herndon et al. 2008, Herndon et al. 2009, Knighton et al. 2007, Yelvington et al. 2007). Briefly, it has been observed that VOC emissions all scale together (i.e., when the formaldehyde emission index doubles because of a decrease in temperature and/or fuel flow rate, so does the ethene emission index). These data and the initial detailed profile of Spicer and coworkers (Spicer et al. 1994, Spicer et al. 1992) have been used recently to refine the Speciate database (EPA 2008) VOC profile for commercial aircraft emissions (FAA Office of Environment and Energy and EPA Office of Transportation and Air Quality 2009). While this near-idle VOC scaling observation has proven S e c t i o n V i Additional Findings

22 Figure VI-1. Measured exhaust gas temperature as a function of the approximate elapsed time since engine start. Triangles, squares, and circles denote different aircraft tests. Dark blue points reflect lower N1 rotational speeds than the lighter blue points for two of the starts. Figure VI-2. Formaldehyde emission indices are plotted as a function of the post-ignition warm-up time. Although the relative times are precise, the absolute time following ignition for any of these curves is uncertain by five seconds. The solid lines represent fits of the data to an exponential decay. The approximate time constant associated with each curve is noted in the legend.

23 to be a valuable guide, it is not universal. The tests under- taken by this project have, in conjunction with observations from other aircraft emission studies, revealed some subtle but important deviations of the near-idle VOC proportional scaling behavior. Two important observations derived from these tests are discussed below. The emission of benzene in aircraft exhaust arises from several sources: unburned benzene in the fuel, dealkylation of higher molecular weight aromatics present in the fuel, and formation through radical-radical recombination reac- tions occurring within the combustor. The near-idle scaling observation permits these fuel effects to be extracted because it focuses on changes in the exhaust composition. Deducing compositional information from individual emission indices requires that one correct for the dominating influences of engine power and ambient temperature from the measurements. The scaling process naturally accomplishes this because the temperature and fuel flow corrections are applied equally to both compounds. The results presented in Sections III and IV demonstrated that different compounds were equally affected by fuel flow and ambient temperature. The relationship of benzene and formaldehyde is inves tigated in Figure VI-3. The slopes of these plots provide information regarding the influence of fuel structure. While the variability in the plots appears to be scatter, a more complete analysis of all of the existing data reveals that the benzene emissions are in fact nonlinearly related to the aromatic fuel content. This result is discussed further in the fuel effects section. A second example that demonstrates a deviation from the near-idle VOC scaling behavior is the emission of 1,3-butadiene, an important HAP. The development of the NO+ reagent ion mode for the PTR-MS has permitted the first real-time measurement of 1,3-butadiene (Knighton et al. 2009). This technique was employed during this project. A plot of the 1,3-butadiene emission index versus ethene emission index is shown in Figure VI-4. While the data are highly correlated, linear fits always suggest a negative intercept. Note that plots versus formaldehyde show similar results. This observation results from the fact that the emission of 1,3-butadiene has a stronger dependence on engine power than do other VOCs, producing a violation of the near-idle VOC proportional scaling rule. The emission of 1,3-butadiene scales more rapidly with changes in engine power, which demonstrates that errors (either positive or negative) can be made by applying a single-scaling variable drawn from EPA’s Speciate database (EPA 2008). VI.3 Effect of Fuel Composition on Emissions Aromatic fuel content influences benzene emissions. The lower panel of Figure VI-5 depicts the scaled benzene emis- sion index (normalized by the formaldehyde emission index) plotted versus the fuel aromatics content. In the upper panel, the shaded gray distribution of fuel aromatics was computed from the US military fuel stocks for 2007. Although the fuel stock used in commercial aviation is different from the military sources, it is likely that commercial aviation fuel typically contains 15%–22% aromatics. The larger blue points were collected at the MDW 2009 test and represent some of the most precise measurements of benzene performed to date. Additional data from other testing campaigns have been included for comparison purposes. The alternative fuels exhaust measurements in orange circles and red diamonds show that benzene emissions flatten out (i.e., do not decrease to zero for 0% aromatic content). It is plausible that at the Figure VI-3. Correlation between emission indices of benzene and formaldehyde. Figure VI-4. Correlation of 1,3-butadiene and ethane emissions.

24 modest combustion temperatures at near-idle, there are two general pathways for the formation of benzene: one from assembly of small radical precursors and a second from the pyrolysis of larger aromatic compounds. The second pathway would have a dependence on the content of larger aromatic precursors, while the first would be less dependent on fuel aromatic content. These data imply that if benzene emissions are targeted for regulation, the fuel aromatic content could be reduced to cut benzene emissions. The data also suggest that there is a point of diminishing value from reducing fuel aromatics, with little benzene reduction below 12% aromatic content. Figure VI-5. Benzene emission index and the aromatics fuel content. The upper panel shows the distribution of aromatics in the fuel analysis for JP8 (assumed to be a good proxy for JetA). The lower panel contains the benzene/ formaldehyde emission fraction.

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TRB’s Airport Cooperative Research Program (ACRP) Report 63: Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions is designed to help improve the assessment of hazardous air pollutants (HAP) emissions at airports based on specific aircraft operating parameters and changes in ambient conditions.

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