Energy Fuels 2010, 24, 5883–5896 Published on Web 10/13/2010
: DOI:10.1021/ef100727t
Particulate Emissions of Gas Turbine Engine Combustion of a Fischer-Tropsch Synthetic Fuel )
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M. T. Timko,† Z. Yu,† T. B. Onasch,† H.-W. Wong,† R. C. Miake-Lye,† A. J. Beyersdorf,‡ B. E. Anderson,‡ K. L. Thornhill,§ E. L. Winstead,§ E. Corporan,^ M. J. DeWitt, C. D. Klingshirn, C. Wey,# K. Tacina,r D. S. Liscinsky,[ R. Howard,z and A. Bhargava*,£
Aerodyne Research Inc., Billerica, Massachusetts 01821, United States, ‡NASA Langley Research Center, Hampton, Virginia 23681, United States, §Science Systems and Applications, Inc., Hampton, Virginia 23681, United States, ^ Air Force Research Laboratory, Wright-Patterson AFB, Dayton, Ohio 45433, United States, University of Dayton Research Institute, Dayton, Ohio 45443, United States, #ASRC Aerospace Corp, Cleveland, Ohio 44135, United States, r NASA Glenn Research Center, Cleveland, Ohio 44135, United States, [United Technologies Research Center, East Hartford, Connecticut 06108, United States, zAEDC/ATA, Arnold AFB, Tennessee 37389, United States, and £Pratt & Whitney, East Hartford, Connecticut 06108, United States )
†
Received June 10, 2010. Revised Manuscript Received September 16, 2010
We have performed a comprehensive test of the effects of alternative fuels on the trace gas, nonvolatile particulate material (PM), and volatile PM emissions performance of a PW308 aircraft engine. The tests evaluated standard JP-8 jet fuel, a “zero sulfur” and “zero aromatic” synthetic fuel produced from a natural gas feedstock using the Fischer-Tropsch (FT) process, and a 50/50 blend of the FT fuel and JP-8. A Pratt & Whitney PW308 engine was operated under the same thrust and combustion conditions to ensure that the tests captured fuel differences, rather than engine operation differences. Emissions of trace gases, soot particles, and nucleation/growth PM were directly impacted by the sulfur and aromatic content of the fuel. FT fuel combustion greatly reduced SO2 (>90%), gaseous hydrocarbons (40%), and NO (6-11%) content compared to JP-8 combustion. In general, combustion of the JP-8/FT fuel blend resulted in emissions intermediate to the FT and JP-8 values. FT combustion dramatically reduces soot particle number, mass, and size relative to JP-8, but increases effective soot particle density. In all cases, the drag behavior of the soot particles indicates deviations from spherical shape and effective soot particle densities are consistent with the soot particles being aggregates of primary spherules. As expected, FT combustion plumes support negligible formation of nucleation/growth mode particles (the number of nucleation growth mode particles is 500% for sulfur containing JP-8). However, particle nucleation/growth for blended fuel combustion is enhanced relative to JP-8, despite the lower sulfur content of the FT/JP-8 fuel blend. A computational model explains the unexpected particle formation result primarily as the effect of much lower soot emissions present in blended fuel combustion exhaust compared to JP-8. Fuel composition, specifically aromatic and sulfur content, affect all aspects of emissions performance and the effect of simultaneously reducing aromatic and sulfur content can lead to surprising behavior.
include stranded natural gas reserves, coal, and biomass. Unlike petroleum, the U.S. has large reserves of all three of these feedstocks and the potentially improved security and price stability of FT fuels has motivated the U.S. Military to investigate the feasibility of using them as an alternative to petroleum jet fuel. Potential environmental benefits are sometimes cited to motivate the switch from petroleum jet fuel to synthetic jet fuel. A primary environmental interest in considering alternatives to fossil fuel sources for jet fuel is on reducing the “carbon-footprint”, i.e. the CO2 emissions, from aviation. Due to their similar carbon/hydrogen ratios, the amounts of CO2 formed during combustion of petroleum and synthetic jet fuels will be similar. However, the combined environmental impact of FT fuel combustion must take into account both life cycle CO2 emissions and differences in emissions performance. Taking into account the entire life cycle, replacing petroleum with FT jet fuel derived from fossil fuel resources would likely result in equal or greater CO2 emissions than petroleum jet fuel
1. Introduction Alternatives to conventional petroleum-based fuels are receiving increasing attention because of concerns ranging from climate change to the security of fuel supplies. For aviation, the interest is particularly focused, since options other than hydrocarbon fuels are few, and any viable options will require very long lead times because of infrastructure constraints and design cycle times for aviation technology. The synthesis and combustion of synthetic fuels derived from the Fischer-Tropsch (FT) process1-3 has received considerable attention recently because the process can yield a hydrocarbon mixture with properties similar to petroleum based Jet-A or JP-8 jet fuels. Potential feedstocks for the FT process *To whom correspondence should be addressed. E-mail: anuj.bhargava@ pw.utc.com. (1) Dry, M. E. Appl. Catal., A 2004, 276, 1. (2) Dry, M. E. Catal. Today 2002, 71, 227. (3) Dry, M. E. J. Chem. Technol. Biotechnol. 2002, 77, 43. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 5883–5896
: DOI:10.1021/ef100727t
Timko et al.
Table 1. Instruments Deployed for Emissions Tests instrument aerosol mass spectrometer (AMS) scanning mobility particle sizer (SMPS) “nano” scanning mobility particle sizer (SMPS) condensation particle counter (CPC) multiangle absorption photometer (MAAP) engine exhaust particle sizer (EEPS) off-line chromatographic filter analysis smoke number apparatus NDIR CO2 analyzer flame ionization detector (FID) NOX analyzer SO2 analyzer CO/O2 analyzer multigas analyzer
model/manufacturer C-ToF-AMS/ARI
quantity measured
refs 11
3080/TSI, Inc. 3080N/Tsi, Inc.
particle size distribution 5-157 nm
3775/TSI, Inc.
particle number density >5 nm
Sem14
5012/Thermo Electron, Inc. 3090/TSI, Inc.
black carbon soot mass
Petzold and Sch onlinner15
particle size distribution (5.6-560 nm)
Hagen et al.16
n/a
polycyclic aromatic hydrocarbon (PAH) speciation and mass smoke number CO2 total unburned hydrocarbons
Corporan et al.5
n/a LI-COR/Applied Biosciences 30000 HM/Signal Instruments 600 HFID/California Analytic Instruments CLD 844 M hr ML9850/Monitor Laboratories Ultramat 23/Siemens and 602P/California Analytical Inc. 2030FTIR/MKS
Jayne et al.
Canagaratna et al.12
size resolved particle mass and composition (80-1000 nm) particle size distribution 11-289 nm
Wang and Flagan13
ICAO17 Corporan et al.5 SAE18
NO and NOX CO2, CO, and O2 CO and hydrocarbons
emissions from aircraft engines.7-10 Corporan et al.5 observed significant reductions in nonvolatile PM from an older military engine and from a research combustor using FT fuels, and these data raise the question of whether similar reductions can be obtained using commercial engines. FT fuels typically have lower sulfur and aromatic contents than petroleum fuels, and these fuel composition differences should be expected to change the quantities and characteristics of volatile and non-volatile PM emissions. Thus, much of the instrumentation and analysis was focused on quantifying the nonvolatile and volatile PM emissions in the PW308 exhaust. To complement, the PM measurements, standard trace gas measurements were also carried out for CO, unburned hydrocarbons (HC), NOx (NO, NO2), and SO2 emissions.
refining and combustion. While CO2 emissions are likely important for estimating global climate change impacts, a recent report on the climate change potential of aviation4 suggests that emissions other than CO2, including soot and NOX, need to be included in forecasts of potential aviation induced global climate change. Because of the compositional differences, non-CO2 emissions from aviation engines may be also be significantly affected when alternative fuels are consumed,5,6 and the non-CO2 emissions of FT fuel combustion in gas turbine combustors need to be characterized more fully. We report the emissions performance of a business jet class gas turbine engine burning petroleum JP-8 jet fuel, FT synthetic fuel, and a 50/50 volume blend of the two. A PW308 business jet class gas turbine engine was operated over a range of engine power conditions to provide exhaust gas for analysis. We performed comprehensive measurements of the effects of fuel composition on trace gas, nonvolatile soot, and volatile particulate matter (PM) emissions performance. Some of the observed differences were attributable to multiple fuel composition changes, for example, simultaneous changes in fuel sulfur and fuel aromatic levels. We used a microphysical model to interpret our data and evaluate different potential mechanisms for the observed trends. A primary focus of this work is on understanding the effects of fuel composition on volatile and nonvolatile PM
2. Experimental Section The emissions test was performed at the Pratt & Whitney Engine Test Facility in West Palm Beach, FL (March 17-30, 2008). We provide a schematic diagram of the experimental layout in the Supporting Information. A PW308 gas turbine engine (21 kN rated thrust, internally ducted core/bypass and with lobed mixers) served as the test engine. Pratt and Whitney had modified the PW308 test engine for previous experiments and (10) Mazaheri, M.; Johnson, G. R.; Morawska, L. Environ. Sci. Technol. 2009, 43, 441. (11) Jayne, J. T.; Leard, D. C.; Zhang, X. F.; Davidovits, P.; Smith, K. A.; Kolb, C. E.; Worsnop, D. R. Aerosol Sci. Technol. 2000, 33, 49. (12) Canagaratna, M. R.; Jayne, J. T.; Jimenez, J. L.; Allan, J. D.; Alfarra, M. R.; Zhang, Q.; Onasch, T. B.; Drewnick, F.; Coe, H.; Middlebrook, A.; Delia, A.; Williams, L. R.; Trimborn, A. M.; Northway, M. J.; DeCarlo, P. F.; Kolb, C. E.; Davidovits, P.; Worsnop, D. R. Mass Spectrom. Rev. 2007, 26, 185. (13) Wang, S. C.; Flagan, R. C. Aerosol Sci. Technol. 1990, 13, 230. (14) Sem, G. J. Atmos. Res. 2002, 62, 267. (15) Petzold, A.; Schonlinner, M. J. Aerosol Sci. 2004, 35, 421. (16) Hagen, D. E.; Lobo, P.; Whitefield, P. D.; Trueblood, M. B.; Alofs, D. J.; Schmid, O. J. Propul. Power 2009, 25, 628. (17) (ICAO), I. C. A. O. 1995. (18) Engineers, S. o. A. Aircraft Gas Turbine Engine Exhaust Smoke Measurement, ARP1179, 1997.
(4) Lee, D. S.; Fahey, D. W.; Forster, P. M.; Newton, P. J.; Wit, R. C. N.; Lim, L. L.; Owen, B.; Sausen, R. Atmos. Environ. 2009, 43, 3520. (5) Corporan, E.; DeWitt, M. J.; Belovich, V.; Pawlik, R.; Lynch, A. C.; Gord, J. R.; Meyer, T. R. Energy Fuels 2007, 21, 2615. (6) DeWitt, M. J.; Corporan, E.; Graham, J.; Minus, D. Energy Fuels 2008, 22, 2411. (7) Onasch, T. B.; Jayne, J. T.; Herndon, S.; Worsnop, D. R.; Miake-Lye, R. C.; Mortimer, I. P.; Anderson, B. E. J. Propul. Power 2009, 25, 1121. (8) Timko, M. T.; Herndon, S. C.; Wood, E. C.; Onasch, T. B.; Northway, M. J.; Jayne, J. T.; Canagaratna, M., R.; Miake-Lye, R. C.; Knighton, W. B. J. Eng. Gas Turbines Power 2010, 132, 061504. (9) Timko, M. T.; Onasch, T. B.; Northway, M. J.; Jayne, J. T.; Canagaratna, M.; Herndon, S. C.; Wood, E. C.; Miake-Lye, R. C.; Knighton, W. B. J. Eng. Gas Turbines Power 2010, 132, 061505.
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Energy Fuels 2010, 24, 5883–5896
: DOI:10.1021/ef100727t
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its exact configuration is proprietary. A typical PW308 engine is a high bypass ratio turbofan (BPR = 4.5), primarily used on private aircraft for business travel. The engine diameter was roughly 64 cm. The engine was operated under identical conditions to ensure that observed differences could be attributed directly to variations in fuel composition. More specifically, the engine fan speed (N2) was set to within 6% for all three fuels, thus ensuring that the engine thrust was identical. The fuel/air ratio (4%), thrust specific fuel flow (4%), and combustor temperature (5%) were held constant throughout the test. On the basis of the identical engine operating conditions, we assert that observed emissions differences can be attributed solely to fuel composition differences. During a typical test, the engine power was increased stepwise from 7%, 30%, 65%, 75%, 85% (where the percent power refers to percentage of maximum rated thrust), and then decreased in a stepwise fashion through the same power settings back to 7% idle. Each test lasted between about 2 and 4 h total. The engine was mounted to a test stand and remained stationary during the entire test program. Ambient temperature ranged from 18.9 to 23.9 C and relative humidity from 42% to 53% during the entire test period. 2.1. Instrumentation. The emissions team deployed a comprehensive suite of particle and trace gas (including CO2) measurement instruments. Table 1 lists the primary instruments used during this test and provides references for more information where appropriate. 2.2. Sample Extraction and Handling. Exhaust gas samples were extracted from two locations: (1) roughly 1 m behind the engine exit plane and (2) 50 m downwind of the engine. Consistent with previous experience, we found that soot was the dominant PM form at the engine exit plane (>90% on a number basis, >95% on a mass basis).7,9 Samples collected at 50 m contained nucleation/growth mode particles, lubrication oil, soot, and ambient PM. A sample rake/dilution probe system used in previous tests19,20 was used to extract samples from directly (∼1 m) behind the engine exit plane. As has been standard procedure in similar measurement efforts,19,20 exhaust gas was drawn into the dilution probe using engine ram pressure for operation >45% power and using pumps for power dimer curves for different plume trajectories and (2) the “kinks” present in the blended fuel traces at roughly 5-20 s, but absent from the JP-8 curves. To address the order of the EIn-sulfate > dimer curves, the data in Figure 4 indicate that EIn-sulfate > dimer are greater for slightly off-axis dilution (θ = 4 or 6) than either on-axis dilution or peripheral (12) dilution. The likely explanation for this behavior is the coupling of temperature and exhaust gas dilution fraction. On the plume center line, both temperature and exhaust gas fraction are maximized and both decrease monotonically as the distance from the center line increases. High temperatures tend to reduce the microphysical rates while high exhaust gas fractions tend to increase them. At the engine center line, the temperature remains sufficiently high for long enough that nucleation is suppressed relative to the colder slightly off-axis portions of the plume. For large departures from the center line (e.g., θ = 12) increased dilution and reduced exhaust gas fraction tends to decrease time evolution of EIn-sulfate > dimers, leading to an overall decrease relative to the 4/6 departures. The nuanced dependence of EIn-sulfate > dimers on plume trajectory (and on cross winds, by extension) signals that interpreting field particle number data must take into account subtle differences in wind conditions and plume temperature/ dilution history. The “kinks” present in the blended fuel curves correspond to extraction of the plume into the sampling line. Markers in
Figure 4. Number emission index of sulfate nucleation mode particles that contain at least two sulfuric acid monomers calculated as function of time for the various fuels and dilution histories.
and blended fuels, 1000 for JP-8). The plume was assumed to travel in a straight line connecting the sampling inlet and the center of the engine exit plane for the purposes of this calculation. By doing so, we have neglected many higher order effects, including the effects of plume/ground interactions. Additionally, we may have introduced an artifact. Our treatment assumes that plume temperature and velocity both decrease in concert with increasing plume dilution. Therefore, rapid plume dilution at the periphery also leads to slower moving plumes. For a fixed travel distance, the slower moving periphery of the plume has a greater processing time than the center line of the plume would have. To remove this processing time artifact, we analyze our data using processing time rather than travel distance as the independent variable and provide simulation results plotted as functions of plume travel distance in the Supporting Information. To first order, we anticipate that the simple extension of the Wong approach69 will properly capture qualitative trends. To cover the actual range of conditions encountered during the tests, we used plume dilution trajectories corresponding to θ = 0, 4, 6, and 12 as input to a 1-D microphysical model for each of the fuels. On the basis of the observed CO2 concentrations, we equate the actual JP-8 dilution trajectory with the 12 vector and the FT and blended fuel trajectories with the 4 vector. In all cases, we calculated microphysical processing both in the diluting plume as well as in the sampling line system which was used to bring the exhaust plume to the instruments. The Supporting Information provides (a) a schematic drawing of the sampled plume dilution trajectories and (b) plots of the exhaust gas mass fraction, velocity, and temperature as functions of distance traveled. We performed a series of calculations using the dilution histories described in the Supporting Information and the initial conditions (EIm-soot, particle size, fuel sulfur content) measured during the test. Figure 4 shows the predicted number emission index of sulfate nucleation mode particles (EIn-sulfate >dimers, defined as any assembly of sulfuric acid containing at least two monomers) with different fuels and different dilution trajectories to simulate the effects of cross winds. The selection of the sulfate dimers cutoff is based on several considerations: (1) Gas turbine engine nucleation PM has been reported to be >90% organic.9 5894
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: DOI:10.1021/ef100727t
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microphysics dominates time evolution in the JP-8 plume for processing times greater than about 3 s. Figure 5a shows that the mass of nucleation/growth mode particles initially is greater for the JP-8 plume than the blended fuel plume, but after 20 s, the mass of nucleation mode particles is instead greater in the blended fuel plume than the JP-8 plume. If that were the case, the mass of nucleation/growth mode particles would remain greater in the JP-8 plume than the blended fuel plume, even as particle number density decreased in the JP-8 plume. In contrast, Figure 5b shows that the mass of the soot coatings is at least 3-times greater in the JP-8 plume than in the blended fuel plume for all conditions. Figure 5b indicates that the rapid decline in EIn-sulfate > dimers predicted to occur after about 2 s in the JP-8 plume is due to coagulation of nucleation/growth mode particles with soot surfaces. Figure 4 and 5 suggest that particle nucleation occurs prior to accumulation onto the soot, resulting in a peak in EIn-total. The interaction of fuel sulfur content (high in JP-8, low in the blended fuel) and EIm-soot (high for JP-8 combustion, low for the blended fuel) results in complex EIn-total behavior. Additionally, dilution history makes an important contribution to particle microphysics. Evaluating the potential human health and climate effects of FT fuel/JP-8 combustion therefore must account for interactions between the full range of variables. Our work provides a starting point for performing this type of analysis. The sulfate PM mass data provided in Table 6 provide a check on the microphysical simulations. By adding the calculated sulfate PM mass obtained for nucleation mode (Figure 5a) and soot mode (Figure 5b) for the estimated dilution vectors (4 for FT and blended fuel, 12 for JP-8), we can determine the total sulfate PM mass EI. Quantitatively, EIm-sulfate obtained from the simulations is 6 mg kg-1 for the blended fuel and 22 mg kg-1 for JP-8, in poor agreement with the observed data reported in Table 6. The quantitative difference is likely due in large part to the inability of the aerosol mass spectrometer to measure quantitatively particles smaller than 70 nm.9 Therefore, the discrepancy is consistent with sulfate inhabiting a soot mode, which the mass spectrometer detects nearly quantitatively, a nucleation mode, which is detected imperfectly, and a vapor mode, which is not detected at all. Despite the poor absolute comparison, normalizing EIm-sulfate measured for 50/50 blended fuel by the corresponding JP-8 value, which should remove some of the systematic biases, brings measurement and simulation into good agreement: 0.27 for simulation and 0.4 ( 0.2 experimentally.
Figure 5. (a) Mass emission index of sulfate nucleation mode particles that contain at least two sulfuric acid monomers calculated as a function of time. (b) Mass emission index of sulfate on soot particles calculated as a function of time. The vertical scale in a is 1000-fold less than in b.
Figure 4 indicate the time at which the plume has traveled 50 m; for 12 off-axis plumes the 50-m markers occur at 230 s of processing time and are not shown in Figure 4. The physical significance of the sampling line is that when the plume enters the sampling line, it ceases to dilute. As a result, nucleation of new particles (which had slowed in the “free” plume) is promoted in the initial portion of the sampling line. The promotion of nucleation in the initial portion of the sampling line leads to a bump in the EIn-sulfate > dimer curve. The kinks are absent from the JP-8 curves because nucleation has effectively ceased by the time the JP-8 plume enters the sampling line. By itself, Figure 4 does not explain why nucleation persists longer in the blended fuel plume than in the JP-8 plume, but by analyzing the simulation data a step further we can understand the underlying mechanism. Figure 5 provides a partial interpretation for the EIn-total results presented in Figure 4; EIn-total for the blended fuel plume is greater than for JP-8 plume because nucleation of new particles persists for a longer time in the blended fuel combustion plume. Specifically, Figure 5 contains plots of the sulfate PM mass indices for nucleation/growth mode particles and sulfur soot coatings, respectively. We can infer from Figure 5 that the particle number density in the JP-8 plume decreases due to coagulation of nucleation/growth with soot and that soot
5. Conclusions We report comprehensive emissions performance data for a business class aircraft gas turbine engine burning fossil JP-8 fuel, synthetic FT fuel, and a 50/50 blended fuel. In summary, we find that fuel composition affects the trace gas, nonvolatile, and volatile PM emissions performance. At idle, we find that EIm-CO and EIm-HC are reduced for FT fuel combustion relative to JP-8. EIm-CO and EIm-HC for the 50/50 blend are intermediate to the JP-8 and FT fuel extremes. EIm-SO2 is drastically reduced for the FT fuel compared to JP-8; as expected, EIm-SO2 for the blended fuel is intermediate to the JP-8 and FT fuel values. EIm-NOX decreases by between 6% and 11% for operation at power g30% for FT fuel combustion relative to JP-8. The NOX reduction is greater than can be explained by experimental uncertainty and is therefore statistically significant. Dramatic, statistically significant 5895
Energy Fuels 2010, 24, 5883–5896
: DOI:10.1021/ef100727t
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reductions in PM soot mass and number emissions are observed when FT fuel replaces JP-8. The PM emissions reductions are most substantial at 7% idle, but persist at all powers up to 85% thrust. PM soot mass and number emissions reductions are observed with the blended fuel, though the values are not equal to the arithmetic average of the FT fuel and JP-8 extremes. We make three additional important observations: (1) FT combustion soot particles are smaller, have greater effective densities, and are more nearly spherical than JP-8 combustion soot particles; (2) in strong contrast with exhaust plumes from JP-8 and JP-8 blends, nucleation/growth mode particles are nearly absent from the FT fuel combustion plumes; and 3) nucleation/growth particles in the combustion plume are more pronounced for the 50/50 JP-8/FT blend than for JP-8, despite the fact that the JP-8 fuel has twice the sulfur content as the blended fuel. We used a microphysical simulation algorithm to verify that the absence of nucleation/growth mode particles in the FT fuel combustion plume is due to the negligible sulfur content of the FT fuel. Using the same algorithm, we determined that the enhanced nucleation/ growth mode observed for the blended fuel relative to pure JP-8 combustion is because of the reduced EIm-soot observed for blended fuel combustion, a reduction that is directly attributable to the reduced aromatic content of the blended fuel. The presence of more soot surface area in the JP-8 plume promotes greater nucleation/growth mode particle coagulation onto soot and reduces particle number densities relative to the blended fuel plume. Differences in dilution history and microphysical regimes (e.g., a shift from particle nucleation
to particle growth) may play secondary roles and their importance should be included in the analysis of other data sets or in predictions of environmental impacts of aircraft engine combustion. Acknowledgment. The U.S. Air Force sponsored this measurement effort (F33615-03-D-2354). The entire measurement team thanks the Pratt & Whitney test stand crew for use of engine facilities and kind support during the emissions tests. Aerodyne personnel thank NASA (NRA grants NNC07CB57C and #NNC07CB58C) for supporting their involvement in this measurement and analysis activity. Scott Herndon and Ezra Wood (Aerodyne) offered helpful advice before and during the engine tests, during data analysis, and throughout the manuscript preparation process. Charlie Hudgins (NASA Langley Research Center), Dan Bulzan (NASA Glenn Research Center), John Jayne (Aerodyne), Joel Kimmel (Aerodyne) and Bill Brooks (Aerodyne) assisted during instrument preparation activities and provided logistical support during field measurement activities. Supporting Information Available: Schematic drawing of the test layout and sample extraction rake, representative time series data obtained by interception of plumes at 50 m, GC/MS data of the test fuels, representative aerosol mass spectra of the aircraft PM emissions, schematic of the configuration used for the microphysical model, temperature, velocity, and concentration profiles used as inputs for the microphysical model, simulation results of the EIn-dimers plotted as a function of distance traveled. This material is available free of charge via the Internet at http://pubs.acs.org.
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