Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions (2012)

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77 This appendix discusses the probe sampling hardware used during the tests. It compares the results and documents the effect of the sampling method on hydrocarbon speciation. Background Material The SAE Aerospace Recommended Practice (ARP1256 revision B) describes the procedures for the continuous sam- pling and analysis of gaseous emissions from aircraft engines for carbon monoxide, carbon dioxide, nitric oxide, nitrogen dioxide and total hydrocarbons. At the Aircraft Particle Emis- sions Experiment (APEX—2004), a sampling system designed to preserve the magnitude and state of engine exhaust plane particulate emissions was developed (Wey et al. 2007). The fundamental premise of the sampling system was to use dry nitrogen gas to dilute and cool the extracted exhaust just after entry into the sampling probe tip. At that same experiment, measurements of speciated hydrocarbons were conducted (Knighton et al. 2007, Yelvington et al. 2007) using the probe with dilution gas at the probe tip and on a conventional gas- sampling probe using a heated transfer line without dilution. Differences in the hydrocarbon speciation profiles obtained with the two sampling methods were observed. A detailed comparison of the hydrocarbon profile measured from a sim- ilar but older engine using an undiluted, heated Teflon line (Spicer et al. 1994) with that determined at APEX (Knighton et al. 2007, Yelvington et al. 2007) showed good agreement in the relative abundance of hydrocarbon species. Subsequent experiments in 2008 using a new “chemical quick quench” (de la Rosa Blanco et al. 2010) probe and a traditional heated metal transfer line quantified differences in the hydrocarbon compounds and attributed them to the sampling line In order to ensure that the HAP species measured in this work are indicative of the true exit plane and were not modi- fied by the sampling method, the test protocol investigated the hydrocarbon profile using different sampling methods during the DAL 2009 test. This section describes the results of the comparison and makes some recommendations for preserving the hydrocarbon profile during sampling. The section is broken into four sections: • Summary, • Known issues in sampling, • Sampling methods tested, and • Comparison of sample methods. Summary The three field deployments (MDW 2009, DAL 2009, and MDW/ORD 2010) utilized two measuring distances as well as four sampling methodologies for measuring idle exhaust during controlled engine tests. Exhaust sampled from 1-meter behind the engine was sampled through either an undiluted “gas” probe, a dilution probe, or a chemical quick quench probe (CQQ). Sample was also collected using a “mobile probe” which utilized the ARI Mobile Laboratory (AML) as the “probe” as it moved in, out, and through the exhaust plume at distances between approximately 50 and 200 meters. Figure C-1 shows a quick overview of the sampling techniques used. Emission index measurements for CO and larger hydro- carbons generally agree using the four sampling methods. Discrepancies exist between the CQQ, dilution probes and the gas probes for the smaller hydrocarbons, specifically formaldehyde and ethylene. It appears that the most success- ful method of sampling at 1-meter uses a dilution probe with a non-reactive (Teflon) sample line. Known Issues in Sampling Highly accurate measurements are an absolute necessity for evaluating the emissions performance of an aircraft engine [JP1]. During all tests, it is critical that the exhaust be sam- pled and delivered to the measurement instruments without changes to the exhaust components of interest. Changes in A p p e n d i x C Exhaust Probe Sampling Techniques

78 CO and NOx concentrations have been observed using differ- ent types of probes, different probe distances from the engine, and different methods of transportation of the sample from the probe to the analysis instrumentation. For example, at the JETS/APEX2 and APEX3 field campaigns a discrepancy existed in the emission indices of NO2 when measured at a probe 1 m behind the engine compared to one that was 30 m from the engine. A lower emission index of NO2 for the one meter probe was most likely caused by catalytic oxidation in the probe due to the temperature of the exhaust flow so close to the engine. When the probe was water cooled (at JETS/APEX2), the differences between 1 meter and 30 meter NO2 emission indices were much Figure C-1. Depiction of the four sampling methods used. The “mobile probe” is shown on the left. The three other sampling methods use probes placed one meter behind the engine. From top to bottom at the right are the standard gas probe, the dilution probe, and the CQQ probe. Mobile Probe Staged Stationary 1-meter Probe Test or n engine Test or n Engine MDW’09 1 CFM56-3 3 CFM56-7 DAL '09 DAL '09 1 CFM56-7 CFM56-7 1 Particle Probe (4) Gas Probe (2) CQQ Probe (3) ORD-10 MDW '10 CFM56-7 1 V2527 4 Probe (1) ARI 1 PW4090 Probe (2) EPA Table C-1. Sampling Summary, tabulates the number and types of engines and sampling methods used during the emissions tests conducted at MDW, DAL, and ORD.

79 smaller and partially caused by “real” conversion of NO to NO2 in the plume itself (rather than the probe or sampling lines) (Wood et al. 2008). Malte and Kramlich (Malte and Kramlich 1980) also reported a loss of CO and a shift between NO and NO2 when sampling along a low pressure sample line. This shift is due to reactive species that would react with NO2 and CO being eliminated from the mixture due to sur- face reactions rather than reaction in the gas flow (Malte and Kramlich 1980). Furthermore Kramlich and Malte (Kramlich and Malte 1978) reported that when the tip was hot, NO2 is most likely reduced to NO via combination with O or H. The most widely used method of quenching chemical reactions in the transfer lines has been to dilute the sample with inert gas at the probe tip. This dilution lowers both the concentration and temperature of the reactive components of the exhaust. Sampling Methods Tested Stationary 1-Meter Probe Two main types of testing were conducted during the three mobile lab deployments (MDW 2009, DAL 2009, MDW/ORD 2010)—stationary probe measurements and mobile lab measurements. The stationary probe measure- ments involved a probe “rake,” shown in Figure C-2, placed one meter directly behind engine 1 of the aircraft. Exhaust samples were then drawn along sample to the analytical instrumentation. The probe tip and rake system, shown on the right and below, were developed by AEDC and NASA and implemented in the stationary testing. The probe rake was integrated into the Missouri Science and Technology probe stand, which was placed directly behind engine 2 of the test aircraft and did not noticeably move when the engine was ramped up to 10% of the rated thrust. Minor modifications were required for the chemical quick quench probe tip, which previously had not been used for aircraft exhaust measure- ments. Sample lines, dilution lines, cooling hoses, and electrical control for the quick quench probe where then strung back via umbilical to the Aerodyne Mobile Lab, which was positioned just past the wingtip of the Aircraft being tested (Figure C-3). Descriptions of the sampling methods: i. Gas Probe The gas probe is a conventional probe following the SAE 1256b recommendations and was used in the stationary 1 meter probe tests. The sample line consisted of 50 feet of 3⁄8″ outer diameter (OD) stainless steel tubing heated to 317°F. For the gas probe testing at DAL 2009 the exhaust was not diluted until just before being sampled by the analytical instrumenta- tion inside the ARI mobile laboratory in order to bring the sample concentrations to within a range acceptable to the highly sensitive instruments. Figure C-2. The Probe Rake with various probe tips installed. The annotation to the right of the photo is the index, a description of the style of tip and the orifice diameter in inches. Figure C-3. Photograph depicting Staged 1-meter test setup at DAL 2009 test. Gas Probe

80 ii. Dilution/Particle Probe The dilution probe used in the stationary 1 meter probe tests consisted of 50 feet of either 3⁄8″ OD stainless steel tubing at ambient temperature (Particle probe at DAL 2009) or 50 feet of PFA (perfluoroalkoxy) Teflon tubing heated to 40°C (MDW 2010). For both of these tests, dry gaseous nitrogen (from liquid nitrogen dewars) was used as a diluent and mixed with the engine exhaust at the probe tip. The dilution probe used in MDW 2010 was heated to 40°C to prevent condensa- tion of water on the walls of the transfer line due to the low ambient temperatures. iii. Chemical Quick Quench Probe The chemical quick quench involves nitrogen dilution at the probe tip as well as a seven fold pressure drop effected in order to minimize probe chemistry and to expedite the transfer of exhaust from the engine exhaust to the sampling instrumentation (the volumetric flow rate increased by a factor of seven following the pressure drop). The probe sample line consisted of 50 feet of ½″ OD PFA Teflon tubing. iv. Mobile Probe In all three field deployments (MDW 2009, DAL 2009, and ORD 2010), for a portion of the emissions tests the Aerodyne mobile laboratory was driven behind stationary aircraft oper- ating at varying idle engine conditions. Exhaust samples were naturally mixed with ambient air before being sampled by Dilution/Particle Probe Chemical Quick Quench Probe Mobile Probe the inlet on the mobile lab. These tests proved to be valuable because not only could the fully evolved plume be sampled in a controlled scenario, but also did not require extended amounts of setup time that the stationary 1-meter probes need. There was also minimal contact between the exhaust and sampling tubing, since the only tubing involved was that inside the truck. Comparison of Sample Methods At DAL 2010 the stationary probe tests were conducted consecutively using the same aircraft (Boeing 737-700) and all tests were conducted on engine 2 (starboard). The same test matrix of near-idle engine conditions was used for each probe type. Engine conditions included the N1=25% fan speed, which corresponds to the ICAO 7% ground idle throttle setting, as well as ground idle throttle settings with varying amounts of bleed air demand which elevates the fuel flow. For all four probe types tested in the DAL 2009 field mission, there was overall agreement for CO and most large hydro carbons. Figure C-4 (top) depicts decreasing emission indices for CO as the fuel flow increases, using data from all probes. Figure C-4 (bottom) shows the relationship of CO emissions (measured by QCL spectroscopy) and benzene emissions (measured using the proton transfer mass spec- trometer [PTR-MS]). Points in both plots marked with blue circles note points where the engine was still warming up and therefore had lower combustion efficiency. All these “warm-up” points were measured with the chemical quick quench probe. Note that as the test progressed the data became very similar to the other probe measurements. This same phenomena also occurred with the mobile probe test- ing where the first point recorded (ground idle with nominal bleed air) was significantly higher than the same fuel flow point at the end of the test. It is important to note that the high “warm-up” CO and benzene EI data points early in the tests are in fact variation in the engine exhaust, not instrument uncertainty because it is consistent through different analytical techniques. While CO and the larger hydrocarbons show consistency through the three types of sample probes, formaldehyde and ethene emissions are slightly different when sampled through the gas probe (Figure-C5) compared to the other probes. Emis- sion indices of HCHO were as much as 75% lower for the gas probe than other probes at low fuel flow rates, which is when the total emissions are greatest. The emission indices of form- aldehyde are more or less constant for all gas probe points, suggesting chemistry occurring at some point in the sample process. The dilution probe points are also consistently lower than those of the chemical quick quench probe. In these tests, the dilution probe transfer line was stainless steel which might

81 Figure C-4. CO versus Benzene Staged emission indices. have been interacting with the sample flow similarly to the gas probe, but to a lesser extent because the sample was diluted and the walls were not being heated. CO2, CO, and C2H4 con- centrations are similar to those from the CQQ probe; however, the HCHO and NOx numbers are lower. The emission indices are actually approximately 1⁄3 of the values of those with nominal engine bleed during the CQQ test and are relatively static when compared to those of HCHO during the other tests. An independent measurement of HCHO provided by the PTR-MS corroborates the HCHO results shown in Figure C-5. Figure C-6 shows three infrared spectra measured with the QC-TILDAS for a rotation-vibration set of lines for formaldehyde. Each spectra are 1 second averages collected

82 on three different probes. The data and fit line show the selectivity of this approach. The anomalous behavior of HCHO on the gas probe cannot be attributed to the pres- ence of another carbonyl absorber. Reliable sampling of semi-volatile material through long lengths of metal tubing is not achieved by either dilution on the particle probe or by heating as on the gas probe. Adsorption of the low vapor pressure compounds onto the sample line surface leads to retention of the sample within the lines. This retention leads to both a time delay and poor temporal response to rap- idly changing events. The reduction in pressure employed via the quick quench probe more faithfully transports low vapor pressure compounds through the sampling lines. Assessment of reliable sampling is evaluated by examining the integrity of Figure C-5. Emission Indices vs. fuel flows for ethylene (top) and formaldehyde (bottom).

83 delivered sample from a composition and temporal perspec- tive. To be considered reliable the sampling system must not alter the composition of the exhaust gas or perturb its temporal relationship with respect the CO2, which serves as the dilu- tion tracer or in the case of these figures, whose emission rate responds to the changing engine conditions. The heated gas probe fails on both criteria set forth for reli- able sampling. As discussed previously, the formaldehyde con- centrations measured using this probe are altered substantially from that obtained by other sampling methods. Additionally, the temporal characteristics of semi-volatile compounds like naphthalene are not well preserved as seen in Figure-C7a. Figure C-6. Infrared absorption spectra from Aerodyne quantum cascade laser infrared spectrometer of the formaldehyde absorption line. Figure C-7a. Time series of ethene (black) and naphthalene (tan) for the heated probe inlet. Figure C-7b. Time series of ethene (black) and naphthalene (tan) for the Quick quench probe.

84 The dilution probe and quick quench probes utilize dilu- tion and dilution plus pressure reduction respectively to quench any chemical transformation from occurring within the sample lines. Both of these probes appear to limit these chemical transformations, as their results are similar. The temporal characteristics of the particle probe are similar to that observed for the heated gas probe. The quick quench probe provides much improved temporal response as can be seen in the accompanying figure. This figure shows that the naphthalene signal tracks the ethene response even dur- ing the rapid transitions in engine operation and sample line purges. The reduction in pressure employed via the quick quench probe more faithfully transports low vapor pressure compounds through the sampling lines. Of the three stationary 1-meter probes tested during the DAL 2009 field campaign the chemical quick quench probe seemed to provide the best results for all the species of interest. It had the best time response for transferring sample from the probe to the instrumentation due to the seven fold pres- sure drop and appeared to maintain sample chemistry. The chemical quick quench probe might be somewhat excessive, however, for what is required. While doing staged testing at Chicago Midway airport in the winter of 2010, a dilution probe with a Teflon transfer line was implemented and seemed to minimize chemical reactions occurring in the sample line. It is the suggestion of this study not to rely on undiluted heated metal sampling for speciated hydrocarbons. References Cited in Appendix C de la Rosa Blanco, E., J. Peck, R. C. Miake-Lye, F. B. Hills, E. C. Wood, S. C. Herndon, P. E. Yelvington, and T. Leach. 2010. Minimizing sampling loss in trace gas emissions measurements for aircraft engines by using a chemical quick-quench probe, Journal of Engineering for Gas Turbines and Power 133 (7). Knighton, W. B., T. Rogers, C. C. Wey, B. E. Anderson, S. C. Herndon, P. E. Yelvington, and R. C. Miake-Lye. 2007. Quantification of Aircraft Engine Hydrocarbon Emissions Using Proton Transfer Reaction Mass Spectrometry. Journal of Propulsion and Power. 23 (5): 949–958. Kramlich, J. C., and P. C. Malte. 1978. Modeling and Measurement of Sample Probe Effects on Pollutant Gases Drawn from Flame Zones. Combust. Sci. Technol. 18: 91–104. Malte, P. C., and J. C. Kramlich. 1980. Further Observations of the Effect of Sample Probes on Pollutant Gases Drawn from Flame Zones. Combust. Sci. Technol. 22: 263–269. Spicer, C. W., M. W. Holdren, R. M. Riggin, and T. F. Lyon. 1994. Chemical composition and photochemical reactivity of exhaust from aircraft turbine engines, Ann. Geophysicae 12: 944–955. Wey, C. C., B. E. Anderson, C. Wey, R. C. Miake-Lye, P. D. Whitefield, and R. Howard. 2007. Overview of the Aircraft Particle Emissions Experiment, Journal of Propulsion and Power 23: 898–905. Wood, E. C., S. C. Herndon, M. T. Timko, P. E. Yelvington, and R. C. Miake-Lye. 2008. Speciation and Chemical Evolution of Nitrogen Oxides in Aircraft Exhaust near Airports. Environ Sci. Technol. 42: 1884–1891. Yelvington, P. E., S. C. Herndon, J. C. Wormhoudt, J. T. Jayne, R. C. Miake-Lye, W. B. Knighton, and C. C. Wey. 2007. Chemical Speciation of Hydrocarbon Emissions from a Commercial Aircraft Engine. Journal of Propulsion and Power 23: 912–918.