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41 APPENDIX D Additional Supporting Material for Chapter 5: Review of the Data from Measurement Campaigns D.1 PM Data Review Summary campaigns, and new instrumentation developments were in- corporated in the measurement suite as the campaigns evolved. D.1.1 ICAO Smoke Number Database Particle size distributions were measured with no fewer than The ICAO Smoke Number Database (ICAO 2008) does not three different instruments (DMS500, SMPS, and EEPS); par- provide adequate quantitative parameterization of PM char- ticle count was measured to determine EIn using several dif- acteristics suitable for local air quality applications. The ICAO ferent model condensation particle counter instruments (CPC); smoke number was developed as a metric for plume visibility EIm was measured using real-time instruments (DMS500, and is measured for all engines in the commercial fleet. It is SMPS, TEOM) and filter samples: black carbon soot mass was not a fundamental PM characterization parameter and can measured using two different types of instruments (MAAP only be applied to dispersion models if correlated with fun- and PSAP); particle composition was measured by two real- damental PM characterization parameters such as number, time instruments (AMS and PAS--for polycyclic aromatic size, and mass. Attempts at such correlations have met with hydrocarbon content) and complemented by filter samples. limited success (Paladino 1997; Whitefield et al. 2001; Soci- Additionally, the measurement suites deployed by the various ety of Automotive Engineers 2004; Wayson et al. 2006; FAA teams intentionally overlapped so that multiple measure- Jul 2007). ments of the same PM characteristics could be made in par- allel. The built-in redundancy helped ensure that operator errors could be identified and removed from the data set. D.1.2 Essential Fundamental PM Gas-phase data were used to build additional confidence in Characterization Parameters the measurement approach and in the maintenance state of The essential fundamental PM characterization parameters the gas turbines tested. Any given engine emission can be required for local air quality applications are the following: considered representative if its measured primary combus- tion gas profiles (NOX, CO, and HC) match those calculated · Number (number-based Emission Index, EIn); from certification data in the ICAO databank. The gaseous · Size distribution; emissions of the APEX engines exhibited the anticipated · Mass (mass-based Emission Index, EIm); trends with respect to engine power condition and the mea- · Composition. sured values were similar to the ICAO certification values (see Section D.2 for more details on the gas-phase measure- ments). Therefore, the extractive sampling dilution system D.1.3 Confidence in Fundamental was judged to be operating properly and the condition of the Parameter Measurement APEX engines was judged to be characteristic of typical in-use When the APEX series of campaigns was initiated, mea- engines representative of the fleet. suring fundamental properties of aircraft gas turbine PM was a new challenge. Since no comparable data existed on aircraft D.1.4 Data Reproducibility PM, the experiments contained two separate internal checks to ensure data accuracy: (1) measurement redundancy and During the campaigns, PM emissions data were collected (2) gas-phase measurements. State-of-the-art PM mea- at over 1,200 stable conditions (i.e., the power setting/fuel surement instruments were deployed during all of the APEX flow was stable at the desired set point). Table D1.1 lists all of
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42 Table D1.1. PM instruments deployed in APEX missions. Instrument Species Detected Detection Limit Sampling Campaign(s) Frequency Deployed Combustion Size distribution 5 nm 1 sec APEX1,2,3, DMS500 Delta-ATL TSI SMPS Size distribution 15nm 30 sec APEX1,2,3, Delta-ATL TSI CNC Total particle 7nm 1 sec APEX1,2,3, concentration Delta-ATL Aerodyne Aerosol Volatile PM >30 nm >3 sec APEX1, 2, 3 Mass Spectrometer composition and size >100 ng m-3 distribution (sulfate and organic) Thermo MAAP Black carbon soot >5 µg m-3 1 sec JETS mass APEX2, APEX3 the PM species measured during the APEX campaigns, along measurements made at 1 m [3 ft]. (For a given airframe/engine with the instruments used to make the measurements. State- combination, duplicate measurements are those made at the of-the-art instruments were used for the measurements, and same downstream distance and engine power condition.) Accu- the quality of the instruments is reflected by their fast time re- rate estimation of the measurement uncertainty is critical to sponses and low detection limits. the proper use of the APEX data set. Typically, each EI reported During an emissions test, the airplanes remained grounded is the average of between 3 and 6 replicate measurements. and chocked during all tests while the engine thrust was Experimental uncertainties were estimated by setting the un- varied to simulate operation at ground idle (4%), idle (7%), certainty in each EI equal to the standard deviation of all taxi (30%), climb-out (85%), take-off (93%), and intermediate available EI data points at a given set of conditions. These data power conditions including 15%, 45%, and 65% rated thrust. handling procedures ensure that reported errors accurately The power cycle during a typical experiment was as follows: represent experimental reproducibility. Sources of systematic (1) the engines were allowed to warm up for roughly 5 to 10 min; error are considered later in this section. (2) measurements commenced as the engines were operated at ground idle; (3) the test continued as the power was in- D.1.5 Measurement Reliability and Sources creased in a step-wise fashion (e.g., 4% to 7% to 15%, etc.) up of Systematic Error for PM to either take-off power or climb-out power; (4) the power was directly reduced to either idle or ground idle; (5) after In addition to reproducibility, absolute measurement accu- several minutes at idle, the power was increased directly to racy is also important. Two measurement uncertainties con- either take-off or climb-out; (6) the test concluded as the tribute to overall uncertainty in the EIs determined in these power was reduced step-wise back to ground idle. studies. They are related to the detection limits and the sys- PM samples were taken continuously throughout the entire tematic errors associated with the measurement of (1) the experiment and each stable point lasted for 2 to 5 min. Sam- CO2 concentration, and (2) the PM differential concentra- pling was performed both at 1 m [3 ft] from the engine exit tion. Systematic errors for PM EIs arise from particle line loss, plane and further downstream of the engine (15 m, 30 m, 45 m, sample dilution, flow rate, and particle density measurements. or 50 m [49 ft, 98 ft, 148 ft, or 164 ft] depending on the size A detailed discussion for PM error analysis, using electric of the engine). During each engine test, EI measurements mobility methodology, can be found in Schmid et al. 2004, were made at a given thrust rating both as the engine thrust where derived relative uncertainties (%) for EIn and EIm are was increased and as it was decreased to the set point. 20% and 30%, respectively. Table D1.2 lists EI number and EI mass data for all of the engines studied in the APEX series of campaigns. Idle/taxi D.1.6 Sample Sources (either 7% or 8% depending on the engine), approach (30%), and climb-out (85%) power conditions are emphasized in These studies focused on engine specific emissions and Table D1.2 since these are the set points used in ICAO certi- their downstream evolution. fication data. EIs for take-off power conditions are only available in certain · Dedicated aircraft cases due to the difficulty in operating stationary aircraft at full- Close to exit plane (1 m [3 ft]); rated thrust. Each EI reported in Table D1.2 is the average of all Near field plume (~10 m, ~15 m, ~30 m, and ~50 m available replicate points taken at a given set of conditions for [~33 ft, ~49 ft, ~98 ft, and ~164 ft]).
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43 Table D1.2. EI number and EI mass data for all of the engines studied in the APEX series of campaigns. Engine Model/Tail Engine EIn (10 15 particles/kg fuel burned)* EIm (g/kg fuel burned)* Number Location 7% 30% 85% 7% 30% 85% CFM56-2C1 / N817NA stbd 0.44±0.146 0.45±0.198 1.92±0.367 0.0048±0.0035 0.0067±0.00340 0.13±0.0370 CFM56-3B1 / N353SW stbd 1.08±.0.565 1.17±0.499 4.20 0.0043±0.00086 0.0060±0.00003 0.254 port 1.20±0.58 1.09±0.358 0.0045±0.00085 0.0060±0.00096 No Data CFM56-3B2 / N695SW stbd 1.15±0.784 1.64±0.323 2.57 0.0062±0.00211 0.016±0.00111 0.249 port 40.1±23.8 35.9±49.6 0.054±0.0186 0.021±0.0208 No Data CFM56-7B22 / N435WN stbd 0.50±0.104 0.50±0.173 1.12 0.0090±0.00298 0.0079±0.000923 0.0614 port 0.51±0.147 0.38±0.120 0.0083±0.0033 0.0055±0.000829 No Data CFM56-7B22 / N429WN stbd 0.28±0.184 0.26±0.249 1.09 0.0021±0.00171 0.0023±0.00180 0.073 port 0.065 0.098 0.00046 0.00098 No Data JT8D-219 / 908DL stbd 2.145±1.47 0.85±0.43 11.2±0.32 0.0042±0.00298 0.0014±0.000315 0.22±0.0395 JT8D-219 / 918DL stbd 8.81±1.78 0.58±0.02 10±0.69 0.042±0.0174 0.0013±0.000133 0.18±0.0106 CFM56-3B1 / N14324 stbd 0.18±0.0977 0.20±0.0919 1.44±0.120 0.0033±0.00099 0.0057±0.00172 0.13±0.00990 9 CFM56-3B1 / N70330 stbd 0.39 0.25 1.16 0.0063 0.0042 0.0837 RB211-535E4-B / stbd 0.34 1.26±0.145 1.48 0.013 0.072±0.0112 0.475 N75853 RB211-535E4-B / stbd 0.38±0.202 0.66 1.31±0.0665 0.013±0.0056 0.035 0.36±0.0177 N74856 PW4158 / N729FD stbd 10.5±21.89 6.79±16.234 1.89±0.316 0.27±0.487 0.048±0.102 0.16±0.0122 AE3007-A1E / N11193 stbd 3.39±3.23 0.62±0.0988 0.68±0.0299 0.059±0.0631 0.016±0.00170 0.043±0.00150 port 1.36±0.238 0.68±0.0436 1.03±0.0609 0.029±0.00206 0.016±0.000909 0.080±0.00655 AE3007-A / N16927 stbd 0.93±0.581 0.76±0.340 0.72±0.0125 0.022±0.00493 0.020±0.00223 0.057±0.00313 CJ6108A / N616NA stbd 0.49±0.162 3.07±1.11 8.53±0.660 0.0078±0.00259 0.071±0.00257 0.298±0.0541 Note: Idle/taxi (either 7% or 8% depending on the engine), approach (30%), and climb-out (85%) power conditions are emphasized since these are the set points used in ICAO certification data. *All available 1-m data. · Advected plumes from aircraft operating under normal D.1.7 General PM Emission Trends landing and take-off (LTO) conditions Taxi; When sampling at or close to the exit plane (within 1 m [3 ft]), Take off; emitted particles were log-normally distributed within a Approach. single size mode and ranged from a few nanometers (nm) to 300 nm in diameter (Figure D1.1). To address the impact of PM emission on local air quality it is necessary to obtain both exit plane and downstream PM D.1.7.1 PM Characteristics Change with Engine emission data. The emission products at the exit plane depend Operating Conditions for a Given exclusively on the engine design and operating conditions. Engine Type They evolve in the downstream plume. This evolution is greatly influenced by atmospheric conditions. The PM ob- D.22.214.171.124 All engines. At 1 m [3 ft], served in the downstream plume is a complex mixture of the emissions from the engine, the results of plume processing, i. Particle mass and black carbon emission indices (EIm and and the background ambient PM. EIm-soot respectively) were a minimum at low powers
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44 1.0E+08 Fuel flow rate (kg/s): 0.12 0.34 0.58 0.82 0.88 1.0E+07 1.0E+06 dN/dlogDp 1.0E+05 1.0E+04 1.0E+03 1.0E+02 Base Fuel, 1m 1.0E+01 10 100 1000 Dp (nm) Figure D1.1. Typical particle size distributions. Source: Lobo, Hagen, Whitefield, and Alofs, "Physical Characterization of Aerosol Emissions from a Commercial Gas Turbine Engine." Journal of Propulsion and Power, 2007, 23 (5), p. 922. and increased with power, reaching values more than and peaking at roughly the same diameters observed in 0.3 g-particle/kg-fuel at power levels higher than 65%. the 1-m [3-ft] samples, and the other occupied by freshly ii. The mean particle diameter increased linearly with power, nucleated sulfur and organic particles peaking at <12 nm. ranging from around 15 nm at idle to about 40 nm at ii. At high engine powers, particle mass emissions were maximum power (at engine exit). dominated by non-volatile PM. iii. Primary (non-volatile) particle EIn varied from 0.16 to iii. Volatile PM number and mass concentrations are de- 3 × 1015 particles/kg-fuel, and were greatest at idle and pendent on fuel sulfur concentrations. take-off thrust settings and a minimum at power levels iv. Non-volatile particle size as well as EIn and EIm were in- corresponding to approach. dependent of fuel properties or downstream sampling iv. EIm values were nonlinearly dependent on engine distance (plume age). power and typically less than 20 mg-particle/kg-fuel over v. From the advected plume data, on any given day the the 4% to 65% engine power range and greater than engine-engine variability within a given class is less than 5% 200 mg-particle/kg-fuel at and above 85% power level. for mass- and number-based emission indices (see Fig- v. The PM composition is primarily non-volatile: ures D1.2 and D1.3). Temperatures high enough to suppress formation of vi. From the advected plume data, the day-to-day variabil- volatile species. ity for a given engine class ranged from 10% to 30% for Validated by measurements at APEX1, JETS APEX2-- mass-based and 10% to 80% for number-based emission not APEX3. indices (see Figures D1.2 and D1.3). No dependence on fuel composition, specifically sul- vii. Changes in ambient atmospheric conditions are likely fur, and aromatics. Fuel composition was systemati- to impact PM emissions. Advected plume data indicate cally investigated in APEX1. JETS APEX2 and APEX3 that EIn is more sensitive to ambient conditions than provide further determinations of the dependence of EIm, consistent with EIn being dominated by volatile 1-m PM characteristics on fuel properties. nucleation/growth mode particles and EIm dominated by non-volatile soot particles. Table D1.3 summarizes the For downstream locations (>30 m [>98 ft]) and advected daily and day-to-day changes in atmospheric conditions plume data, during this study. i. Measured particle distributions typically exhibited two The observations discussed in items v to vii above provide distinct modes, one corresponding to non-volatile particles powerful tools for assessing aircraft operations on airport
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45 250 225 200 175 (1015 particles/kg fuel) 150 EIn 125 13 100 12 18 9 41 75 14 9 30 10 9 19 53 13 37 50 9 34 14 34 25 16 21 94 9 41 19 0 9/27/2004 9/28/2004 9/29/2004 COMPOSITE BR715 CF34 CF6-80 CFM56 JT8D PW 2037 Figure D1.2. Average number-based emission indices at take-off measured for six aircraft engine families during the Delta-Atlanta Hartsfield Study. 0.40 0.35 0.30 0.25 (g/kg fuel) EIm 0.20 34 0.15 19 94 18 19 41 9 13 41 0.10 53 12 13 21 9 14 10 37 14 9 30 34 0.05 16 9 9 0.00 9/27/2004 9/28/2004 9/29/2004 COMPOSITE BR715 CF34 CF6-80 CFM56 JT8D PW2037 Figure D1.3. Average mass-based emission indices at take-off measured for six aircraft engine families during the Delta-Atlanta Hartsfield Study.
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46 Table D1.3. Summary of the daily and day-to-day changes in atmospheric conditions during the Delta Atlanta-Hartsfield Advected Plume Study. Date Ambient condition min max avg std dev % Deviation 9/27/2004 Temperature (°C) 19.1 20.4 19.5 0.2 1.1% Rel. Humidity (%) 91.7 100.0 99.0 2.0 2.0% 9/28/2004 Temperature (°C) 21.5 27.5 24.8 1.3 5.2% Rel. Humidity (%) 53.0 76.9 64.8 6.3 9.7% 9/29/2004 Temperature (°C) 19.9 28.5 24.2 2.2 8.9% Rel. Humidity (%) 34.6 78.1 54.7 12.9 23.6% local air quality but were drawn from only one study (Lobo particles. In the 30-m [98-ft] data, non-lognormal size dis- et al. 2008). Their potential value warrants further advected tributions were often observed. The 30-m number-based plume studies of this type. emission indices were 10 to 100 times greater than those measured at 1 m [3 ft] due to the appearance of a large quan- D.126.96.36.199 The CFM56 studies. The engine type most ex- tity of particles smaller than 15 nm in diameter. The <15 nm tensively studied in these campaigns is the CFM56, which is particles present in the 30-m [98-ft] samples were attributed the most prevalent engine type operating in the commercial to nucleation of condensable materials as the hot exhaust fleet. The CFM56 common to all four campaigns and the gases cooled. Since the <15 nm particles do not contribute CFM56 data provide the most insight into the engine-to- significant mass, EIm and related parameters did not vary engine variability issue. Furthermore, the data provide a unique strongly with sampling position. Because the coating layer opportunity to examine the variability within sub-classes of was thin (<1 nm), the volatile coatings on the soot particle did the engine, specifically: -2C1, -3B1, and -7B22. not alter the soot particle size distribution. At 1 m [3 ft] (see Figure D1.4): · For all CFM56 sub-classes, mean particle diameter increases D.1.7.2 PM Characteristics Also Change with increasing engine power. No statistically significant between Engine Types difference between the particle number diameter of the PM At both the ~1-m and 30-m (~3-ft and 98-ft) locations: emitted by the various CFM56 technologies is discernable The mean particle diameter (Figure D1.5), number-based due to measurement variability (i.e., real engine-to-engine emission index (Figure D1.6), and mass-based emission index variability) and measurement uncertainty. (Figure D1.7), depend on engine type for all PM parameters · Both the -3B1 and -7B22 engine sub-classes demonstrate measured as follows: a minimum number-based emission index (EIn) at ~20% power. The newer technology -7B22 engines produced fewer · All engines have comparable minima and particles per kilogram of fuel burned than did the older -3B1 · Significant variation is observed between the maxima. engines. Averaged across all powers, the -7B22 engines ex- hibited a 79 ± 12% reduction in number-based emissions normalized to fuel flow relative to the -3B1. EIn for the -2C1 D.1.7.3 PM Characteristics Change during engine fall between those of the -3B1 and -7B22 series. Engine Warm Up · The mass-based emission index (EIm) increased with in- The number-based emission indices, EIn, for a given operat- creasing power. The trend is stronger for the older engine ing power were observed to decrease with engine on-time technology (-3B1). At 85% power, EIm for the 7B22 engines following a cold start. For example, in the case of a CFM56- is 72% less than the -3B1 engines. A statistically significant 7B22 engine sampled at 50 m [164 ft], EIn decreased by about decrease is also observed for the CFM56-2C1 engine com- 60% after the engine warmed up. The effect was observed for pared to the -3B1. other engine types where cold starts were studied. At 30 m [98 ft]: D.1.8 Chemical Composition The onset of gas-to-particle conversion was apparent at of Aviation Particles downstream locations for low to medium powers as the formation of new particles. At high powers, gas-to-particle As a complement to aviation particle physical characteri- conversion resulted in formation of a coating on the soot zation, the APEX studies included measurements of particle
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47 a. 50 At the end of their travel, the particles strike a heater which -7B22 -3B1 -2C1 is usually held at 600°C. Volatile components become 40 vaporized and are ionized by collision with high energy Dgeom (nm) electrons. The resulting ions are extracted into a mass spec- 30 trometer which separates them based on the ratio of the 20 molecular mass to ionic charge. Individual ions give rise to an electronic signal which is converted via a series of 10 internal amplification stages and external calibration standards 0 to a mass concentration. The experimental measurements b. 6 provide size resolved chemical information for a collection of -7B22 -3B1 -2C1 particles. 5 The AMS can detect and quantify any species that is vapor- Eln (1e15/kg_fu) 4 ized at 600°C. The AMS identifies particle-bound ammonium, chloride, nitrate, sulfate, water (though only partially due 3 to its high vapor pressure), and various organic species in 2 ambient air samples. On some occasions, it has detected cer- 1 tain metal species including lead, lead oxides, zinc, and zinc oxides. A wide range of organic materials have been identi- 0 fied in ambient air (Dzepina et al. 2007) and vehicle exhaust c. 0.30 -7B22 -3B1 -2C1 (Canagaratna et al. 2004). Aviation PM contains appreciable 0.25 amounts of organic material and sulfate. The sulfate origi- nates directly from sulfur compounds present in the jet fuel. Elm (g/kg_fu) 0.20 The combustion process converts fuel sulfur compounds into 0.15 SO2 quantitatively. Post-combustion, about 0.5% to 5% of the 0.10 SO2 becomes oxidized to SO3 (Lukachko et al. 2008) within a fraction of a second. In the presence of water, SO3 condenses 0.05 as H2SO4 on the time scale of seconds to minutes for ground 0.00 level emissions. The composition of the organic material is 0 20 40 60 80 100 more complex. Detailed studies (Timko, Onasch et al. 2008) Power (%) have identified engine oil and partially burned hydrocarbons Figure D1.4. Mean particle diameter, as the two primary types of organic materials in aviation PM. number-based emission index, and All engines studied during the APEX missions have emitted mass-based emission index as a function engine oil. Engine oil is a larger fraction of the total organic of engine power for the -2C1, -3B1, PM at high powers, when efficient combustion drives the and -7B22 models of the CFM56 class emission of unburned hydrocarbons (UHCs) to near zero. of engines. Engine oil constitutes about 50% of the organic PM at high power, though for some engines the figure is closer to 90%. chemical composition. Just like particle mass, number, and At low power, engine oil constitutes roughly 20% of the size, the chemical composition is likely to play an important organic PM. Understanding that the organic content of avi- role in potential human health and environmental impacts. ation PM is divided between engine oil and partially burned The aerosol mass spectrometer (AMS) was one of the instru- hydrocarbons should aid future engine design efforts to ments used to measure particle chemical composition during reduce PM. Likewise, the potential human health and envi- APEX. The AMS is a powerful instrument that obtains size- ronmental impacts of partially burned hydrocarbon parti- resolved particle chemical data. The operation of the AMS cle contributions are likely to be different from those of has been described elsewhere in detail (Jayne et al. 2000; engine oil. Canagaratna et al. 2007; Drewnick et al. 2005; DeCarlo et al. Circumstantial evidence suggests that the organic and 2006). Briefly, a specially designed inlet focuses particles in a sulfate components are internally mixed in aviation PM-- sample gas at the expense of the gas-phase molecules. The in other words, each individual particle contains roughly the focused particles enter a high vacuum chamber and are accel- same fraction of sulfate and organic material as the next. erated to their vacuum terminal velocity. Since larger particles The evidence for internally mixed particles comes in the travel more slowly than smaller particles, they become size form of size-resolved chemical data which show that the size separated during their travel through the vacuum chamber. distributions for organic and sulfate materials occur over
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48 Figure D1.5. Ranges of measured mean particle diameter for engines emissions sampled in the near field plume. Figure D1.6. Ranges of measured number-based emission index for engines emissions sampled in the near field plume.
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49 Figure D1.7. Ranges of measured mass-based emission index for engines emissions sampled in the near field plume. the same size range and have similar shapes (Onasch et al. D.1.9 PM Measurement Methodology 2008). Development Engine operating condition (Timko, Onasch et al. 2008) and fuel sulfur content (Timko, Onasch et al. 2008; Onasch The campaigns provided the opportunity to improve air- et al. 2008) influence the chemical composition of aviation PM. craft PM characterization methods, resulting in a more accu- At idle, aviation PM seems to be predominantly organic rate PM emissions data. The APEX campaigns occurred over material, a result of the relatively lower combustor efficiencies a 2-year period and afforded much insight into sampling at low power conditions. At climb-out and take-off, combustor methodology and diagnostic techniques for aircraft PM char- efficiency is greater than 99%, very little UHC exits the engine, acterization. The general approach had been defined in such and the particles contain roughly equal amounts of sulfate previous studies as Howard et al. 1996; SUCCESS (Hagen et al. and organic material. As stated previously, much of the organic 1997); SONEX/POLINAT (Hagen et al. 1999; Schlager et al. material emitted at climb out/take-off appears to be due to 1997); EXCAVATE (Anderson et al. 2005); and NASA engine oil, which has nothing to do with the combustor. QinetiQ (Whitefield et al. 2002). A detailed description of the APEX1 provided an excellent opportunity to understand the basic methodology can be found in Schmid et al. 2004. The effects of fuel sulfur content on aviation PM chemical com- same methods were employed throughout the campaigns but position. As might be expected, increasing fuel sulfur content were continuously improved as a result of lessons learned increases the mass of sulfate particles emitted (Onasch et al. from each campaign. The lessons learned have led to subse- 2008). The effect of fuel sulfur content on sulfate emissions is quent focused studies sponsored by the FAA, NASA, and also seen in the JETS APEX2 and APEX3 data (Timko, Onasch DoD to refine further the methodology employed in the et al. 2008). More surprisingly, increasing fuel sulfur content APEX studies. These studies are being used extensively by the also increases the amount of organic PM emitted. The organic SAE E-31 committee (aircraft exhaust emissions measure- material contained in nucleation/growth mode particles is ments) to develop standards for measuring PM emissions more sensitive to fuel sulfur content than that coated onto from aircraft gas turbine engines. SAE E-31 also interacts soot. It seems likely that condensed sulfate acts as nucleation strongly with the ICAO Committee on Aviation Environ- sites for conversion of organic materials to the particle phase. mental Protection (CAEP) to ensure that all standards for
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50 aircraft emission measurements are developed for interna- In addition to greater chemical detail, the APEX studies tional application. provide data on the effects of two important operational vari- ables which impact aviation emissions: (1) the ambient tem- perature, and (2) emissions at power conditions other than D.2 Gas-Phase Data Review ICAO idle (7%), approach (30%), climb-out (85%), and take- Summary off (100%). To remove variability and provide a common In addition to the PM measurements described in Section 6.1, baseline, the ICAO emissions databank tabulates data at stan- the APEX studies included a variety of gas-phase emissions dard day conditions (15°C, 760 torr, 60% RH). Deviations measurements. The purpose of the gas-phase measurements from standard day conditions are common during normal was to complement the existing ICAO emissions databank. airport operations, and the APEX data can help link actual Whereas ICAO certification requires reporting of total UHCs ambient conditions to emissions performance. Likewise, air- and total NOX emissions, the APEX measurements provide craft engines operate at power conditions not tabulated in the much more in-depth chemical-level detail. Potential health ICAO databank. In fact, the runway studies conducted at and environmental implications depend critically on the prop- Atlanta-Hartsfield airport (Herndon et al. 2008) suggest in- erties of the emitted compounds, and the APEX measurements frequent use of ICAO power--especially 7% idle. The APEX contain more chemical information than the ICAO databank. studies measured emissions at a number of additional power Using the APEX data should therefore improve the complete- conditions, including a low-power idle (4%) and several in- ness of local air quality models used for airport emissions, and termediate power conditions (15%, 40%, and 65%). Use of may potentially improve understanding health effects. the more complete data set should enable generation of more The APEX gas-phase measurements can be divided broadly accurate airport emissions inventories. Moreover, the emissions into two classes: nitrogen oxides and hydrocarbons. Other gas- effects of potential operational changes (e.g., reduced-thrust phase emissions are carbon dioxide, water vapor, and sulfur take-off, minimized idling times) can be calculated using the dioxide. Hydrocarbon emissions decrease with increasing en- APEX data so that alternative scenarios can be explored before gine thrust (and are most important at low power, e.g., idle/ policy changes are instituted. taxi), whereas nitrogen oxide emissions are highest at high This section begins with a description of the reproducibility power (take-off), though are important both at low power of the gas-phase APEX measurements and--when possible-- and high power. Nitrogen oxides (NOX) are emitted prima- direct comparisons to ICAO certification data. Experimental rily as NO and NO2. Unlike many other combustion-based details are provided to explain how the measurements were engines, jet engines emit a substantial fraction of total NOX as made. Representative gas-phase data are presented to provide NO2, at least at idle. A third nitrogen oxide species, HONO, a better understanding of the range of data available. Follow- is also emitted in smaller quantities (Wormhoudt et al. 2007). ing the general discussion of the data, several potential sources Aviation engines emit a wide range of hydrocarbons, the most of systematic error are presented. The next two sections de- prevalent being ethylene (C2H4) and formaldehyde (HCHO). scribe the hydrocarbon and nitrogen oxide in greater detail. Benzene, toluene, styrene, acetaldehyde, methanol, and The final section provides two examples of how the APEX naphthalene are some of the other UHCs that are emitted by data can be used: (1) effects of reduced thrust take-off and aircraft engines and were measured during the APEX meas- extended idle time on NOX emissions, and (2) estimating urement campaigns. Figure D2.1 shows representative gas- uncertainties in hydrocarbon emissions due to ambient tem- phase EIs measured for a CFM56-3B1 engine during APEX3. perature effects and uncertain power condition. Some features of EIs are well known: at idle, EI CO dominates, while EI NOX (defined as the sum of EI NO and EI NO2) dom- D.2.1 Gas-Phase Data Reproducibility inates at take-off. The exceptional aspect of Figure D2.1 is the and Comparability to ICAO degree of chemical detail which far exceeds that available in ICAO certification data. During the APEX campaigns (i.e., the combined APEX1, Separate measurement of all hydrocarbon species emitted JETS APEX2 and APEX3 campaigns) emissions data were col- by gas turbines to certify commercial aviation engine models lected at over 1,200 stable conditions (i.e., the power setting/ would be a daunting task. Fortunately, one of the useful find- fuel flow was stable at the desired set point) for 21 gas-phase ings of the APEX studies is that emission of the various hydro- species. Table D2.1 lists all of the gas-phase species measured carbon species scale with one another, regardless of engine during the APEX campaigns, along with the instruments used type or thrust setting. Even when the absolute magnitudes to make the measurements. State-of-the-art instruments change by an order of magnitude or more (due to changes in were used for the measurements, and the quality of the instru- engine power condition, for instance), the ratio of one hydro- ments is reflected by their fast time responses and low detection carbon to the next remains constant. limits.
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51 2-ring aromatics (a) idle 1-ring aromatics propene CO acetaldehyde UHC NO C2H4 NO2 HCHO total gas-phase EI = 42 ± 5 g kg-1 2-ring aromatics (b) take-off 1-ring aromatics NO2 propene acetaldehyde NO CO UHC C2H4 HCHO total gas-phase EI = 20 ±4 g kg-1 Figure D2.1. Gas-phase emissions measurements for a CFM56-3B1 (N14324, APEX3) at (a) idle and (b) take-off powers. Area corresponds to the mass-based EI. All data meaured at 30 m [98 ft]. Total gas-phase pollutant EIs provided in figure. Benzene, toluene, styrene, and phenol are the 1-ring aromatics included in this figure. Naphthalene, methylnaphthalene, and dimethylnaphthalene are the 2-ring aromatics included. The level of chemical detail provided by the APEX measurements far exceeds that available from ICAO certification data, which should lead to improved understanding of the potential impacts of airport operations on air quality. During an emissions test, the airplanes remained grounded gas samples were taken continuously throughout the entire and chocked during all tests while the engine thrust was experiment and each stable point lasted for 2 to 5 min. Sampling varied to simulate operation at ground idle (4%), idle (7%), was performed both at 1 m [3 ft] from the engine exit plane taxi (30%), climb-out (85%), take-off (93%), and intermedi- and further downstream of the engine (15 m, 30 m, 45 m, or ate power conditions including 15%, 45%, and 65% rated 50 m [49 ft, 98 ft, 148 ft, or 164 ft] depending on the size of the thrust. The power cycle during a typical experiment was as engine). During each engine test, EI measurements were made follows: (1) the engines were allowed to warm up for roughly at a given thrust rating both as the engine thrust was increased 5 to 10 min; (2) measurements commenced as the engines and as it was decreased to the set point. With a notable ex- were operated at ground idle; (3) the test continued as the ception, no systematic biases or hysteresis effects were found; power was increased in a step-wise fashion (e.g., 4% to 7% EI CO for the RB211-535E4-B engines was 18 g kg-1 when the to 15%, etc.) up to either take-off power or climb-out power; power was rapidly decreased from climb-out or take-off to (4) the power was directly reduced to either idle or ground idle; ground idle, as compared to 35 g kg-1 when ground idle power (5) after several minutes at idle, the power was increased di- was approached gradually in step-wise fashion. EI CO for rectly to either take-off or climb-out; (6) the test concluded as the RB211-535E4-B did not exhibit hysteresis at power set- the power was reduced step-wise back to ground idle. Exhaust tings other than 4%, though the power was never rapidly
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52 Table D2.1. Gas-phase instruments deployed in APEX missions. Instrument Species Detected Detection Limit Time Campaign(s) Resolution Deployed LICOR IR Gas CO2 3 ppm 1 sec APEX1,2,3 Analyzer (models 6262 and 820) Siemens (Ultramat CO <25 ppm 1 sec APEX1,2,3 23) CO2 <500 ppm O2 <2,500 ppm Multi-Gas Analyzer N2O, CH4, HCHO, species 1 sec APEX1,2,3 (MGA) C2H4, CH3OH, dependent (AFR-2010) HCOOH, Jet Fuel, ranging from 0.1 SO2, H2O, CO2, CO, (HCOOH, C2H4) NO to 85 (H2O) QCL-TILDASa NO2 0.5 ppb 1 sec APEX2,3 C2H4 2 ppb 1 sec APEX2,3 CO 5 ppb 1 sec APEX2,3 HCHO 1 ppb 1 sec APEX3 TILDASb HCHO 0.5 ppb 1 sec APEX1,2,3 NO 1 ppb 1 sec APEX3 HONO 1 ppb 1 sec APEX1 Eco-Physics (CLD NO >50 ppm APEX1,2,3 700 EL ht) NOX >50 ppm NOx Chemi- NO 0.2 ppb 1 sec/20 APEX2,3 luminescence sec APEX2,3 Analyzer ThermoElectron NOYc 0.2 ppb 20 sec model 42C Signal (300 HM) UHC >40 ppm 1 sec APEX1,2,3 PTR-MSd acetaldehyde, propene, 5 ppb 8 sec APEX1,2,3 benzene, toluene, styrene, C2- benzenee , phenol, naphthalene, methylnaphthalene, dimethylnaphthalene a quantum-cascade tunable infrared laser differential absorption spectrometer (Nelson et al. 1998) (Aerodyne Research, Inc.). b tunable infrared laser absorption spectrometer (Nelson et al. 2002) (Aerodyne Research, Inc.). c NOY implies NO + NO2 + RNO2 + RONO. In practice, NOY = NOX = NO + NO2 for these experiments. d proton-transfer reaction mass spectrometer (Hansel et al. 1995). e C2-benzene implies o-xylene,m-xylene, p-xylene, and ethylbenzene. dropped from take-off or climb-out to any powers except have been corrected for this effect (Wey et al. 2006). Each EI ground idle. The RB211-535E4-B EI HCHO and some of the reported in Table D2.2 is the average of all available replicate particulate measurements (e.g., total number count) followed points taken at a given set of conditions. (For a given airframe/ similar trends as EI CO. None of the other APEX engines engine combination, duplicate measurements are those made exhibited the engine warm-up/cool-down hysteresis effect at the same downstream distance and engine power condition.) and the extent of the phenomenon is unknown. Accurate estimation of the measurement uncertainty is criti- Table D2.2 lists EI CO, EI NOX, and EI HCHO (a hydro- cal to the proper use of the APEX data set. Typically, each EI carbon shown to be representative for most hydrocarbon reported is the average of between 3 and 6 replicate measure- emissions) for all of the engines studied in JETS APEX2 and ments. Experimental uncertainties were estimated in two APEX3. The EI data presented in Table D2.2 are reproduced ways. In the first method, the uncertainty was set equal to the from a similar table originally provided by Timko, Onasch et al. standard deviation of all available data points at a given set of (2008). Idle/taxi (either 7% or 8% depending on the engine), conditions. In the second, the uncertainties in each of the ex- approach (30%), and climb-out (85%) power conditions are perimental quantities which define an EI (e.g., uncertainty in emphasized in Table D2.2 to facilitate comparison to ICAO the CO2 and pollutant concentrations in the ambient air and certification data. EIs for take-off power conditions are only exhaust gas) were propagated as a Taylor series expansion fol- available in certain cases due to the difficulty in operating lowing a standard method. The larger of these two estimates stationary aircraft at full-rated thrust. Relative humidity can in- of the uncertainty are reported here. These data handling fluence NOX emissions, and the EI NOX reported in Table D2.2 procedures ensure that reported errors accurately represent
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Table D2.2. Summary of EIs measured for some gas-phase species during APEX. Engine Model/ Engine a F00b EI COc (g kg-1) EI NOXd (g kg-1) EI HCHOe (mg kg-1) Tail Number Locationj 00 (kN) 7% 30% 85% 7% 30% 85% 7% 30% 85% CFM56-2C1f/N817NA stbdm 36±5 5±0.5 1.6±0.2 3.8±0.6 8.2±0.6 16.0±0.6 380±140 80±20 15±10 CFM56-3B1 (ICAO) 22.44 89.41 34.4 3.8 0.95 3.9 8.3 15.5 2,280 80 50 CFM56-3B1/N353SW stbdm 30±1 3.6±0.5 1.0±0.3 2.8±0.6 7.0±0.4 17±1 164±6 1.7±0.5 port 28±2h 3.7±0.1h 3.4±0.1h 7.8±0.3h 283±17h NDg CFM56-3B1/N14324 stbdm 34±1 4±1 1.4±0.1 3.3±0.2 6.8±0.4 14±1 540±170 22±3 CFM56-3B1/ N70330 stbdm 40.0±0.8 5.1±0.3 1.6±0.1 2.99±0.07 13.2±0.7 NDg NDg CFM56-3B1/N695SW stbdm 27±1 4.1±0.5 1.5±0.3 2.9±0.4 6.5±0.6 17±2 528±39 11.5±0.8 0.5±0.4 port 28±1h 3.8±0.3h 3.2±0.1h 7.0±0.7 h 410±30 NDg CFM56-7B22 (ICAO) 24.41 100.97 22.8 2.50 0.60 4.50 10.00 19.00 2,500 100 100 CFM56-7B22/N435WN stbdm 24±7 1.9±0.7 0.40±0.03 4.3±0.3 9.5±0.6 19±4 270±40 15±8 7.3±0.5 port 23.3±3.1h 1.71±0.05h 4.2±0.8h 11±1h 380±60h 12±7h CFM56-7B22/N429WN stbdm 19±4 1.2±0.1 0.6±0.2 4.2±0.3 10.1±0.5 24±5 280±50 2.5±0.8 1.8±0.2 port 1.5±0.05h 9.1±0.3 4.3±0.7 RB211-535E4-Bl (ICAO) 27.9 191.7 18.24 2.43 0.26 4.58 8.65 19.3 140 5 0 RB211-535E4-Bl/N75853 stbdm 18±8 2.9±1.4 0.22±0.08 5±1 9.3±0.8 24±5 80±2 NDg 11±3 RB211-535E4-Bl/N74856 stbdm 19±1 2.1±0.2 0.20±0.03 5.0±0.6 10±1 23.9±0.7 219±9 NDg NDg PW4158 (ICAO) 30.7 258.0 20.99 1.88 0.54 4.8 11.8 23.7 1,780 140 2 PW4158k/ N729FD stbdm 39±3 2.2±0.5 3.5±0.3 9.6±0.8 22.4±2 1,010±70 NDg AE3007-A1E (ICAO) 19.06 37.16 37.97 5.63 0.64 4.26 7.42 14.91 AE3007-A1Ei/N11193 stbdm 29±12 3±1 0.267±0.004 3.7±0.7 7.7±0.5 13.2±0.6 400±100 12±1 NDg port 35±1 4.4±0.1 0.30±0.01 3.43±0.09 7.3±0.2 12.1±0.7 660±20 NDg NDg AE3007-A (ICAO) 18.08 33.73 33.73 17.35 3.28 3.83 7.79 17.47 AE3007-Ai /N16927 stbdm 32.8±0.7 4.0±0.4 0.33±0.03 3.4±0.3 6.7±0.5 10.4±0.8 520±20 NDg 17.1±1.2 CJ6108Ai,j/N616NA stbdm 140±7 45±7 21±2 2.4±1.6 3.2±0.5 4.6±0.3 2,500±500 400±130 47±11 a 00 = pressure ratio. b F00 = rated thrust (kN). c EI-CO measurements averaged over all available downstream sampling locations. d EI-NOX measurements averaged over all available sampling locations. EI-NOX equals the sum of EI-NO2 and EI-NO, both in units of mass of NO2. e EI HCHO reflects average value of all available 1-m data. ICAO values are for UHCs--EI-UHC. ICAO does not speciate hydrocarbon data. f The CFM56-2C1 engine was mounted on a NASA airframe not used in the commercial fleet. No ICAO data are available for this engine. g ND indicates that the species concentration was less than the detection limits of the instrument. Blank spaces indicate missing data. h Measurements made at 1-m sampling location used for this data point-- no downstream data available. i The 7% thrust setting was not studied for this engine--data measured at 8% rated thrust. j The CJ6108A thrust is lower than the ICAO certification limit. No ICAO data are available for this engine. k The 85% thrust condition was not studied for this engine--data measured at 80% thrust. l The RB211-535E4-B engines studied during the APEX missions were equipped with the Phase V combustor. m The abbreviation "stbd" refers to the "starboard" engine. Likewise, "port" is the port engine. Data excerpted from Timko, Herndon et al. 2008.
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54 experimental reproducibility. Sources of systematic error are the APEX engines are provided in Table D2.2 for comparison considered later in this section. with the data collected during the measurement campaigns. A number of internal and external consistency checks were The APEX data set generally agrees with ICAO EIs. The good used to help build confidence in the gas-phase data. Specifi- overall comparison between APEX data and ICAO certifica- cally, when multiple instruments were used to measure the tion data further underlines that the experimental data are same quantity (i.e., NOX), the various measurements typically high quality. Some important exceptions and qualifiers are agreed to within the specified limits of instruments uncer- required, as follows: tainty. Moreover, data measured for multiple examples of the same engine technology agree within the limits of experimen- · Field measurements of EI CO agree with the ICAO values tal reproducibility. When such comparisons are possible, the within the limits of experimental uncertainty for most of the APEX data are in good general agreement with ICAO certifi- engines, though the variability in the data is large enough cation data. These two comparisons establish the quality of (generally about 25%) to obscure small differences. the APEX data. More details on the two comparisons are pro- · Discrepancies between APEX EI CO and ICAO EIs generally vided in the following two paragraphs. occurred only when the actual fuel flow rate deviated from Qualitatively, the EIs behave as expected. As engine power the ICAO value. Measurements of EI CO for the AE3007 increases, so too do combustor efficiency and EI NOx, whereas engines were higher than ICAO EI CO, but the experimen- EI CO and EI HCHO decrease. Quantitatively, EI CO and EI tal fuel flow rate was lower than the ICAO value. NOX data for the 4 CFM56-3B1 engines agree nearly to the · Measured EI NOX is generally lower than the ICAO value limits of experimental uncertainty. EI CO for the CFM56- by about 10%. Given known experimental uncertainties 3B1/N70330 engine is an apparent outlier. Similarly, duplicate and biases, 10% can be considered to be good agreement. measurements of EI CO and EI NOX for the other engines · EI NOX increases monotonically for all engines in the agree reasonably well when such data are available. EI HCHO APEX data set. measured for different examples of a given engine type, how- · Discrepancies between APEX EI NOX and ICAO EIs gen- ever, is much more variable than EI CO or EI NOX. For ex- erally occurred only when the actual fuel flow rate deviated ample, EI HCHO takes values ranging from 164±6 (N353SW, from the ICAO value. starboard) to 540±170 mg kg-1 (N14324, starboard) for CFM56-3B1 engines at 7% rated thrust. Ambient tempera- D.2.2 Gas-Phase Measurement Reliability ture is the likely source of much of the EI HCHO variability, and Sources of Systematic Error and this topic is discussed in the section on hydrocarbon emissions. At high powers, the concentration of HCHO was In addition to reproducibility, absolute measurement frequently lower than the detection limits of the instrument accuracy is also important. Instrument detection limits are (about 0.5 ppb). The abbreviation "ND," short for "not typically on the order of 1 ppb to 10 ppb. Typical measured detected," indicates that the concentration was below the de- concentrations are on the order of 10 ppb to 1,000 ppb. tection limits. (Table D2.1 lists detection limits of the various Therefore, the uncertainties of EIs less than about 0.05 g kg-1 instruments.) EI NOX is generally highest for the larger engines are greater than experimental variability. The uncertainty is (i.e., PW4158 and RB211-535E4-B), which is expected as the imposed by detection limits, such as hydrocarbons (especially certification process makes allowances for maximum rated at high thrust conditions) and HONO. Additionally, we have thrust. EI CO does not follow the engine size trend and ranges identified several other sources of systematic errors, as follows: from about 20 g kg-1 (RB211-535E4-B) to about 30 to 40 g kg-1 (CFM56) for the turbofan engines at 7% thrust. The single 1. EI CO is lower when measured at the engine exit plane turbojet engine that was tested (CJ6108A/N616NA) has the than when measured "downstream" by much as 20% at lowest EI NOX and the highest EI CO, by a factor of about 2. "ground" idle conditions (4% power) and by as much as It has the highest EI HCHO by about a factor of 5. The low 10% at "ICAO" idle (7%). Two likely mechanisms have fuel efficiency and inlet combustor temperature of the turbojet been identified: (1) errors in the fitting procedure used to CJ6108A accounts for the high EI CO and EI HCHO and low convert absorbance signals into concentrations for highly EI NOX. The good agreement for EIs measured for different concentrated samples (>5,000 ppb). The fitting error is examples of the same engine technology (e.g., CFM56-3B1) estimated to be no more than 5%, which is not sufficient indicates that the quality of the data is good and can be trusted to explain the observed discrepancies by itself; (2) oxidation for calculations. The CJ6108A is an older engine technology; of CO into CO2 in the 1-m probe used to sample engine newer turbojets may perform differently. exit plane gases. NO2, OH, O, and HO2 are likely CO oxi- Comparison of measured EIs to ICAO certification data dants either in the gas-phase or on the heated metal surfaces helps understand the quality of the APEX data. ICAO EIs for in the 1-m probe. The apparent loss of CO at the engine exit
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55 plane is most notable in the APEX3 data, consistent with the fact that the 1-m sample rake was not cooled during this 30 mission, unlike the others. Because of the uncertainties in (a) the engine exit plane CO measurements, only downstream 25 RB211-535E4-B CO measurements are included in Table D2.2. (N=2) 2. Certain organic acids, namely formic and acetic acids, have 20 NO been observed at downstream sample rakes. The presence EI (g kg-1) of organic acids is due to chemical reactions which occur in 15 NO2 the downstream probe and are not indicative of actual en- gine emissions. EIs for organic acids are not reported here. 10 3. NO2 may be converted to NO in the 1-m probe, especially at idle conditions. The NO2/NO transformation in the 5 1-m probe may be chemically coupled to the CO/CO2 con- version mentioned previously. Thus, NO2 may indirectly 0 serve as the oxidant for conversion of CO into CO2. Fur- 0 20 40 60 80 100 thermore, evidence suggests that NO may be converted Power % into NO2 in the exhaust plume with a reaction time scale on the order of seconds (Wood et al. 2008). The two con- 30 version effects may partially balance one another making (b) quantitative speciation of NOX into NO and NO2 challeng- 25 CJ6108A (N=1) ing when measured at the engine exit plane. Just as with EI CO, the deviation between engine-exit-plane and down- 20 EI (g kg-1) stream EI NO2 data is most pronounced for measurements made with the uncooled 1-m probe used during APEX3. 15 Also, HONO may either be formed or destroyed in the 1-m NO2 NO sampling probe and this chemistry is poorly understood. 10 4. Butanol is used for certain particle size measurements. During transitions from one power to the next, butanol 5 can be ingested into the internal transfer lines and carried from the particle instruments to the gaseous instruments. 0 0 20 40 60 80 100 Butanol in the internal transfer lines gives rise to a false Power % signal in the hydrocarbon measurement instrumentation (specifically a m/z = 57 signal in the PTR-MS) that prevents Figure D2.2. NOx speciation for quantification of the important pollutants, butadiene and (a) RB211-535E4-B and (b) CJ6108A acrolein. Efforts are underway (Knighton et al. 2007a) to engines. ICAO data points ( ) are shown for reference, when available. discriminate between butanol, butadiene, and acrolein. The CJ6108A thrust is below the ICAO threshold for certification, and no ICAO data are available for this D.2.3 NO2, NO, and Total NOX Emissions engine. Both EI NO and EI NO2 are plotted in units of NO2 mass Most combustion sources emit NOX primarily in the form of equivalents for direct comparison NO (Heywood 1988). Gas turbine engines operating at low with ICAO EI-NOX. The data shown power conditions are an exception, as they emit a substantial in this figure are representative amount of NOX as NO2. Figure D2.2 plots the nitrogen oxide of the entire data set. The experimental EIs for a RB211-535E4-B (a turbofan) and a CJ6108A (a turbo- data agree with ICAO EI NOX for all jet). Figure D2.2a is representative of all of the turbofan engines engines except the AE3007s at high considered in the APEX missions. Figure D2.2b is shown for thrust. The data are not corrected for ambient humidity, though this contrast and the data may be relevant for general aviation effect was found to be small (<6%). engines. Some general features are apparent in the NOX data: N denotes the number of engines studied. · For the turbofan, NO2 is a significant fraction of the total NOX at low thrust (idle) conditions. In fact, 99% of the
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56 NOX emitted from certain CFM56 engines was in the form The final two points are especially important and are de- of NO2 at ground idle (4% rated thrust). scribed in more detail in the following sections. · EI NO2 steadily decreases and EI NO steadily increases as power is increased for the turbofan engines. Less than 10% Mutual scaling of the hydrocarbon EIs. One of the of the total NOX is emitted as NO2 at high power condi- major findings of the APEX series of experiments is the tions for the turbofan engines. mutual scaling of the various hydrocarbon EIs. As power · The turbojet engine, which is shown for contrast, emits conditions and/or ambient conditions change, all of the a substantial fraction of the NOX as NO2 at all power hydrocarbon EIs change in concert so that the ratios of the conditions. hydrocarbon EIs with respect to one another are constant. Typically, HCHO is used as a reference, a selection originally Wormhoudt et al. (2007) provide more details on APEX1 made because HCHO is measured by a separate instrument NOX data, including HONO data. Wood et al. (2008) de- than the majority of the hydrocarbons (see Table D2.1). Fig- scribe APEX2 and APEX3 NOX data, both as measured dur- ure D2.3 contains plots of APEX1 measurements of EI ing dedicated engine tests and from advected plume studies. HCHO and EI benzene as functions of percent rated thrust for several engine conditions and a correlation plot of EI benzene versus EI formaldehyde for all available engine con- D.2.4 Speciated Hydrocarbon Emissions ditions (Knighton et al. 2007b). Spicer et al. (1994) performed The APEX data set includes EIs for a number of hydrocar- measurements of hydrocarbon EIs and their data is shown bons including formaldehyde, ethylene, acetaldehyde, ben- for comparison. The highly correlated benzene-formaldehyde zene, and styrene (see Table D2.1 for a complete list). The plot shows that, even though the individual measurements major conclusions that can be drawn from the hydrocarbon have substantial variability (e.g., due to changes in ambient data are described below: temperature), the ratio of benzene to formaldehyde remains constant. · Hydrocarbon EIs are highest at low thrust conditions and The relationships depicted in Figure D2.3 suggest that each individual EI falls to values below 0.1 g kg-1 at thrusts measurement of one hydrocarbon EI might be used to de- above 15%. termine the EIs of other unmeasured hydrocarbons. The · Qualitatively, EI HCHO follows the same trends with re- scaling law depicted in Figure D2.3 applies to all hydrocarbons spect to engine thrust as does EI CO. which are measured during a standard APEX experiment · Formaldehyde and ethylene are the most prevalent hydro- (see Table D2.1). Due to the sensitivities of the hydrocarbon carbons emitted from gas turbine engines on an EI basis. instruments deployed during the APEX missions, alkane · The sum of all measured hydrocarbon EIs is within about EIs have not yet been measured in gas turbine exhaust and 10% to 20% of the value of UHC EIs measured by the we cannot confirm if alkane EIs obey the "universal" scaling ICAO flame-ionization detection method (FID). The quan- law. Measurements made by Spicer et al. (1994) suggest that titative agreement between the speciated measurements the overall contribution of alkanes to the total hydrocarbon and the FID indicate that many of the UHCs have been in- EI is less than roughly 10%. Likewise, an analysis performed cluded in the speciated analysis, yet the direct comparison by Yelvington et al. (2007) suggests that the alkane contri- is difficult since the FID is not equally sensitive to all HCs bution to the total UHC EI is minor. In that treatment, the (aldehydes are under quantified) and not all HC species individual gas-phase EIs were summed on a per carbon were separately quantified. atom basis to yield a total hydrocarbon EI which was within · Hydrocarbon EIs decrease by a factor of 100 or more as the about 10% to 20% of that measured using the standard power condition is adjusted from idle to take-off. Over ICAO FID method (Yelvington et al. 2007). Therefore, almost the entire range, hydrocarbon EIs vary in proportion the alkane contribution to the total hydrocarbon EI is to one another. Therefore, accurate measurement of one likely to be less than about 10%; however, precise numbers hydrocarbon EI may allow quantification of all other hy- are not available because the FID UHC number may not drocarbon EI, provided a consistent speciation profile. represent a total HC measurement due to its non-uniform The mutual scaling may not hold when the EIs fall below sensitivity. 0.1 g kg-1, though this distinction has little bearing on emissions inventories. The effect of ambient temperature on hydrocarbon EIs. · Ambient temperature strongly influences hydrocarbon Ambient temperature strongly influences hydrocarbon EIs EIs. A 20°C decrease in ambient temperature resulted in a (but much less so NO, NO2, or CO). During APEX1, a 20°C 10-fold HCHO increase (when power setting is used as the decrease in ambient temperature (from 35°C to 15°C) resulted scaling variable) or a 3-fold HCHO increase (when fuel in a 3-fold increase in EI HCHO. (The same change in ambient flow rate is used as the scaling variable). temperature results in a 10-fold increase in EI HCHO when
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57 Figure D2.3. Formaldehyde and benzene EI data measured during APEX1 (CFM56-2C1). (a) EI HCHO as a function of power condition; (b) EI benzene as a function of power condition; (c) EI benzene as a function of EI HCHO. The individual hydrocarbon EIs vary by about a factor of 10 at a given power condition (either 4% or 7%), and by more than that between the two power conditions shown here. The apparent variability in individual EIs is captured as a strong correlation in the scatter plot. All hydrocarbons measured thus far vary in proportion to one another--benzene/HCHO provides a representative example. Comparison data provided by Spicer et al. (1994). power setting, rather than fuel flow rate, is held constant. more challenging, though some data are consistent with the In other words, lower ambient temperatures require lower APEX1 results. For instance, the effect of ambient temperature fuel flow rates to achieve a desired power setting. EI HCHO may be reflected in the EI HCHO comparison between increases with decreasing fuel flow rate. The relationship be- N14324/CFM56-3B1 (540 mg kg-1 at 7%, 8°C) and N353SW/ tween fuel flow rate and EI HCHO accounts for about one- CFM56-3B1 (160 mg kg-1 to 280 mg kg-1 at 7%, 13°C). Like- third of the observed dependence of EI HCHO on ambient wise, the two RB211-535E4-B engines studied during APEX3 temperature.) Figure D2.4 is a plot of EI HCHO measured may show a temperature effect: EI HCHO for N75853 equals during APEX1. By virtue of the hydrocarbon scaling law, Fig- 80 mg kg-1 (17°C) while that of N74856 (10°C) is 219 mg kg-1. ure D2.4 is representative of all hydrocarbon emissions. The Yelvington et al. (2007) show that temperature variability of variability in the EI versus thrust plots depicted in Figure D2.3 fuel flow rate accounts for about 1/3 of the observed variability is due to the temperature sensitivity. APEX1 was unique among in hydrocarbon EIs and suggest that relative humidity effects the APEX series of missions as a single test engine that was and/or instrument/sampling variability account for the rest. studied over a wide range of ambient temperature. Quantify- The likely emissions ramifications of the ambient temperature ing the effect of ambient temperature in the APEX data set is effect are clear: failure to account for the ambient temperature
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58 strate the use of APEX data for estimating emissions during 2 4% LTO cycles, and the Wood approach is adopted here. Table 5.5% 1 20 25 30 35 D2.3 contains the results of a sample calculation for the nitro- 8 ambient temperature (°C) 6 gen oxide emissions of a CFM56-3B1 engine calculated over the 7% 4 standard ICAO LTO cycle. Several observations can be made: EI HCHO (g kg-1) 2 · APEX data and ICAO data yield similar estimates for total 15% NOX emissions; 0.1 8 · Almost 20% of the total NOX is emitted as NO2; 30% 6 · Most of the NO2 is emitted during idle; 4 · About half of the total NOX is emitted during climb-out. 2 APEX-1 CFM56-2C1 In addition to applying the APEX data set to standard LTO 0.01 cycles, the emissions of various pollutants in hypothetical sce- 2 3 0.1 narios can be calculated. In Table D2.4, the total NOX, NO2, Fuel Flow Rate (kg sec-1) CO, and HCHO emitted during several hypothetical LTO Figure D2.4. Formaldehyde emission index cycles are listed for a CFM56-3B1 engine. The first two rows (EI) as a function of fuel flow rate measured of Table D2.4 present data for the standard ICAO LTO cycle, during APEX1 for a CFM56-2C1 engine. Power using either ICAO or APEX EIs. The final NOX/NO2 figures condition is indicated directly on the graph. presented in Table D2.3 can be compared directly to the first two rows of Table D2.4. The difference in APEX and ICAO effect may lead to estimated EIs which are inaccurate by a estimates of CO emissions is due to a discrepancy in the EIs factor of 10 or more. at 7% (28.1 g/kg for APEX compared to 34.4 g/kg for ICAO). Each row subsequent to the second lists emission estimates The effect of ambient temperature is clear--decreasing with one LTO parameter changed from the default. ambient temperature by 20°C results in a 10-fold increase in EI-HCHO. Data for power conditions >30% are omitted as · Row 3: Reduced power idle. HCHO emissions increase by the EIs are small (<0.01 g kg-1) and noise in the measurment about 40% and CO by nearly 20%. sometimes exceeds the absolute value. · Row 4: Prolonged idle. Doubling the idle time to 52 min increases NOX emissions by only 20% but increases NO2 emissions by 67%. HCHO and CO emissions roughly D.2.5 Potential Use of APEX Data double. and the ICAO LTO Cycle to Generate Emissions Inventories APEX data also capture variability in emissions data. The APEX data can be used in conjunction with airport op- Depending on the application and on the analysis technique erations data to generate airportwide emissions inventories. used to interpret the data, real emissions variability data can be The depth of chemical information and the wider range of op- very useful for understanding the range of emissions that can erational conditions included in the APEX data set allow it be realistically be expected during the course of normal operations. used to generate more comprehensive emissions inventories Primary sources of variability may include experimental errors, than is possible with ICAO data. Wood et al. (2008) demon- engine age and maintenance history, and ambient conditions. Table D2.3. Emission indices and engine parameters used to calculate the total NO2 and NOx emissions from a CFM56-3B1 engine during a standard landing take-off cycle. LTO Time in Fuel Flow NOX EI NO2 EI Total Total Phase Mode (min) Rate (kg/s) (g/kg) (g/kg) NOX (kg) NO2 (kg) Approach 4 0.306 6.9 0.98 0.51 0.072 Idle 26 0.114 2.98 2.98 0.53 0.39 Take-off 0.7 0.953 16.8 1.2 0.67 0.048 Climb-out 2.2 0.886 15.1 1.15 1.77 0.13 Totals APEX 3.5 0.64 ICAO 3.6 n/a
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59 Table D2.4. CFM56-3B1 NOX, CO, and HCHO emissions for different operating scenarios. Row # Total Total NOx Total NO2 Total CO Scenario HCHO (kg) (kg) (kg) (kg) ICAO base case (ICAO times, 1 thrust, and EIs) 3.6 n/a 6.5 n/a Base case (ICAO times and 2 thrust levels, APEX EIs) 3.5 0.64 5.5 0.082 3 4% idle, 26 min 3.6 0.59 6.4 0.13 4 7% idle, 52 min idle time 4.3 1.1 10.6 0.16 For the current stage of measurement development and for the in ambient temperature (here, a 20°C range has been consid- number of engines tested, discerning one potential source of ered) and to account for reduced power idle (4% compared variability from the next is not always possible. As APEX style to 7%). Readily apparent is that aircraft engines emit most-- measurements become more routine, experimental variability almost 90%--of the hydrocarbons during idle, a consequence will be reduced so that engine-to-engine variability can be iso- of both the EI and the time in mode. The APU hydrocarbon lated. In the meantime, the effects of ambient conditions, emissions are shown for comparison and are clearly negligible especially ambient temperature, and power conditions are compared to those of the aircraft engines. Also apparent is the clear. Figure D2.5 captures data variability graphically for total high degree of variability for the estimated idling emissions. UHC emissions. In Figure D2.5, the UHC emission rate (i.e., The errors bars in the diagram represent an estimate of the mass of hydrocarbons emitted per second) is plotted over the uncertainty in the true UHC emissions rate which is greatly course of a standard LTO (where the idle time has been re- impacted by ambient temperature and the actual thrusts used. duced from 26 min to 22 min). The area under the curve is pro- Detailed analyses which take into account this real emissions portional to the total quantity of emitted UHC. The variability variability will provide more realistic emissions inventories for in the UHC emissions has been calculated based on variability use in chemical dispersion models. Figure D2.5. Total UHC emission rates plotted as a function of time in mode during LTO cycle. The area under the curve (i.e., the area of the "boxes") is proportional to the total amount of UHC emitted during a portion of the cycle. About 90% of the UHCs are emitted during idling. The error bars are an estimate of the range of the emission rate and account for uncertainties in the true thrust values used (e.g., 4% idle vs. ICAO 7%) and uncertainties in the influence of ambient temperature on hydrocarbon EIs (see Figure D2.4).