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

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78 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. CO and NOx concentrations have been observed using differ- behind the engine compared to one that was 30 m from the ent types of probes, different probe distances from the engine, engine. A lower emission index of NO2 for the one meter probe and different methods of transportation of the sample from was most likely caused by catalytic oxidation in the probe due to the probe to the analysis instrumentation. For example, at the the temperature of the exhaust flow so close to the engine. When JETS/APEX2 and APEX3 field campaigns a discrepancy existed the probe was water cooled (at JETS/APEX2), the differences in the emission indices of NO2 when measured at a probe 1 m between 1 meter and 30 meter NO2 emission indices were much 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. 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

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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. Figure C-3. Photograph depicting Staged 1-meter test setup at DAL 2009 test. Sampling Methods Tested Stationary 1-Meter Probe samples were then drawn along sample to the analytical Two main types of testing were conducted during the instrumentation. The probe tip and rake system, shown on three mobile lab deployments (MDW 2009, DAL 2009, the right and below, were developed by AEDC and NASA MDW/ORD 2010)--stationary probe measurements and and implemented in the stationary testing. The probe rake mobile lab measurements. The stationary probe measure- was integrated into the Missouri Science and Technology ments involved a probe "rake," shown in Figure C-2, placed probe stand, which was placed directly behind engine 2 of the one meter directly behind engine 1 of the aircraft. Exhaust 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 317F. 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. Gas Probe

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80 ii. Dilution/Particle Probe the inlet on the mobile lab. These tests proved to be valuable The dilution probe used in the stationary 1 meter probe because not only could the fully evolved plume be sampled tests consisted of 50 feet of either 3/8 OD stainless steel tubing in a controlled scenario, but also did not require extended at ambient temperature (Particle probe at DAL 2009) or amounts of setup time that the stationary 1-meter probes 50 feet of PFA (perfluoroalkoxy) Teflon tubing heated to 40C need. There was also minimal contact between the exhaust (MDW 2010). For both of these tests, dry gaseous nitrogen and sampling tubing, since the only tubing involved was that (from liquid nitrogen dewars) was used as a diluent and mixed inside the truck. with the engine exhaust at the probe tip. The dilution probe used in MDW 2010 was heated to 40C to prevent condensa- Comparison of Sample Methods tion of water on the walls of the transfer line due to the low ambient temperatures. 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 Dilution/Particle Probe throttle setting, as well as ground idle throttle settings with varying amounts of bleed air demand which elevates the fuel flow. iii. Chemical Quick Quench Probe For all four probe types tested in the DAL 2009 field The chemical quick quench involves nitrogen dilution at mission, there was overall agreement for CO and most large the probe tip as well as a seven fold pressure drop effected hydro carbons. Figure C-4 (top) depicts decreasing emission in order to minimize probe chemistry and to expedite the indices for CO as the fuel flow increases, using data from all transfer of exhaust from the engine exhaust to the sampling probes. Figure C-4 (bottom) shows the relationship of CO instrumentation (the volumetric flow rate increased by a factor emissions (measured by QCL spectroscopy) and benzene of seven following the pressure drop). The probe sample line emissions (measured using the proton transfer mass spec- consisted of 50 feet of OD PFA Teflon tubing. 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 Chemical Quick Quench Probe 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 iv. Mobile Probe point at the end of the test. It is important to note that the high In all three field deployments (MDW 2009, DAL 2009, and "warm-up" CO and benzene EI data points early in the tests ORD 2010), for a portion of the emissions tests the Aerodyne are in fact variation in the engine exhaust, not instrument mobile laboratory was driven behind stationary aircraft oper- uncertainty because it is consistent through different analytical ating at varying idle engine conditions. Exhaust samples were techniques. naturally mixed with ambient air before being sampled by 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, Mobile Probe the dilution probe transfer line was stainless steel which might

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

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

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

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84 The dilution probe and quick quench probes utilize dilu- References Cited in Appendix C tion and dilution plus pressure reduction respectively to de la Rosa Blanco, E., J. Peck, R.C. Miake-Lye, F.B. Hills, E.C. Wood, quench any chemical transformation from occurring within S.C. Herndon, P.E. Yelvington, and T. Leach. 2010. Minimizing the sample lines. Both of these probes appear to limit these sampling loss in trace gas emissions measurements for aircraft engines chemical transformations, as their results are similar. The by using a chemical quick-quench probe, Journal of Engineering for temporal characteristics of the particle probe are similar to Gas Turbines and Power 133 (7). that observed for the heated gas probe. The quick quench 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 probe provides much improved temporal response as can Aircraft Engine Hydrocarbon Emissions Using Proton Transfer be seen in the accompanying figure. This figure shows that Reaction Mass Spectrometry. Journal of Propulsion and Power. 23 the naphthalene signal tracks the ethene response even dur- (5): 949958. ing the rapid transitions in engine operation and sample line Kramlich, J.C., and P.C. Malte. 1978. Modeling and Measurement of purges. The reduction in pressure employed via the quick Sample Probe Effects on Pollutant Gases Drawn from Flame Zones. Combust. Sci. Technol. 18: 91104. quench probe more faithfully transports low vapor pressure Malte, P.C., and J.C. Kramlich. 1980. Further Observations of the Effect compounds through the sampling lines. of Sample Probes on Pollutant Gases Drawn from Flame Zones. Of the three stationary 1-meter probes tested during the Combust. Sci. Technol. 22: 263269. DAL 2009 field campaign the chemical quick quench probe Spicer, C.W., M.W. Holdren, R.M. Riggin, and T.F. Lyon. 1994. seemed to provide the best results for all the species of interest. Chemical composition and photochemical reactivity of exhaust It had the best time response for transferring sample from from aircraft turbine engines, Ann. Geophysicae 12: 944955. Wey, C.C., B.E. Anderson, C. Wey, R.C. Miake-Lye, P.D. Whitefield, the probe to the instrumentation due to the seven fold pres- and R. Howard. 2007. Overview of the Aircraft Particle Emissions sure drop and appeared to maintain sample chemistry. The Experiment, Journal of Propulsion and Power 23: 898905. chemical quick quench probe might be somewhat excessive, Wood, E.C., S.C. Herndon, M.T. Timko, P.E. Yelvington, and R.C. however, for what is required. While doing staged testing at Miake-Lye. 2008. Speciation and Chemical Evolution of Nitrogen Chicago Midway airport in the winter of 2010, a dilution probe Oxides in Aircraft Exhaust near Airports. Environ Sci. Technol. 42: 18841891. with a Teflon transfer line was implemented and seemed to Yelvington, P.E., S.C. Herndon, J.C. Wormhoudt, J.T. Jayne, R.C. minimize chemical reactions occurring in the sample line. Miake-Lye, W.B. Knighton, and C.C. Wey. 2007. Chemical It is the suggestion of this study not to rely on undiluted heated Speciation of Hydrocarbon Emissions from a Commercial Aircraft metal sampling for speciated hydrocarbons. Engine. Journal of Propulsion and Power 23: 912918.