Article pubs.acs.org/EF
Particulate Matter and Organic Vapor Emissions from a Helicopter Engine Operating on Petroleum and Fischer−Tropsch Fuels Greg T. Drozd,† Marissa A. Miracolo,† Albert A. Presto,† Eric M. Lipsky,‡ Daniel D. Riemer,§ Edwin Corporan,∥ and Allen L. Robinson*,† †
Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States Penn State Greater Allegheny, McKeesport, Pennsylvania 15132, United States § Rosenstiel School of Marine and Atmospheric Science (RSMAS)/Marine and Atmospheric Chemistry (MAC), University of Miami, Miami, Florida 33149, United States ∥ United States Air Force Research Laboratory, Wright Patterson Air Force Base, Dayton, Ohio 45460, United States ‡
S Supporting Information *
ABSTRACT: Particle and gaseous emissions from a T63 gas-turbine engine were characterized using three fuels: standard military jet fuel (JP-8), Fischer−Tropsch (FT) synthetic fuel, and a 50:50 blend of each. Primary emissions were sampled using a dilution tunnel and sampling trains with both filters and sorbent tubes. Primary particulate matter (PM) and gaseous emissions for the neat FT and blend fuels were reduced relative to emissions when using JP-8 fuel at both idle and cruise loads. At idle load, PM mass emissions are reduced by 65% with neat FT fuel and by 50% for the 50:50 blend compared to neat JP-8 fuel. The JP-8/ FT blend thus decreases emissions beyond the linear average of the emissions for the individual fuels. At idle load, FT fuel reduced total hydrocarbon emissions by 20%, while the blend showed no significant change compared to neat JP-8. At cruise load, neat FT fuel resulted in an 80% reduction in primary PM emissions and a 30% reduction in total hydrocarbon emissions compared to neat JP-8. Decreases in PM emissions at idle load come from lower elemental carbon (EC) and primary organic aerosol (POA), while at cruise load emissions, reductions are driven mainly by EC. Gas chromatography−mass spectrometry (GC−MS) and thermo-optical analysis of filter samples indicate that engine oil comprises a significant fraction of the POA emissions. When using FT fuel, POA emissions appear to be largely engine oil, but emissions with JP-8 fuel have a large fraction of partially oxidized organic material. The differences in POA composition may be due to both the presence of partially oxidized fuel as well as greater EC/soot levels when using JP-8 fuel. Thermodenuder and GC−MS measurements indicate that the POA emissions are semi-volatile; therefore, dynamic gas−particle partitioning will alter the contribution of primary emissions to ambient PM. Total gas-phase hydrocarbon emissions greatly outweigh POA emissions, and applying even moderate yields of secondary organic aerosol (SOA) will dominate over POA emissions. A high abundance of unsaturated volatile organic compounds (VOCs) in the gaseous emissions will enhance oxidation chemistry in the exhaust plume and promote the formation of SOA. exhaust plume.6 FT fuels can greatly reduce PM emissions, and this is generally attributed to the different chemical makeup of FT fuel engine emissions.4,5,7−9 Major differences between FTsynthetic and refinery stream jet fuels are decreased sulfur, decreased aromatic, and increased branched alkane contents in the synthetic fuels.10,11 The previous comparison studies for FT, JP-8, and FT/JP-8 blends have largely focused on fixedwing aircraft but less so on helicopter engines. Previous studies focused on the physical characteristics of the particulate emissions (e.g., size and number distributions); PM speciation has typically not gone further than to distinguish between organic material, soot, and sulfate. Much of the reductions of primary PM emissions, in particular the amount of elemental carbon (EC), have been attributed to the low aromatic content of FT fuel.3 Recently, PM emissions from jet engines have been characterized as having fuel and oil contributions, with the
1. INTRODUCTION Alternative fuels are a potential solution to problems of energy security and fuel cost instability. Currently, synthetic fuel made via the Fischer−Tropsch (FT) process is the most viable alternative for replacing petroleum-based aviation fuels, and FT fuels have recently been approved for use as blends with standard military jet fuel (JP-8) and commercial jet fuel.1−3 FT fuel can be produced from domestic feedstocks, including coal, natural gas, and biomass. Previous studies have shown that FT fuel can reduce engine emissions, thereby helping to mitigate the impacts of aircraft operation on air quality and the environment, although the detailed chemical composition of aircraft emissions with FT fuel is largely unknown.3−5 With the potential for large increases in FT usage, more detailed knowledge of the chemical characteristics and evolution of aircraft emissions with FT fuel is needed to accurately assess possible environmental impacts. Primary particulate matter (PM) includes directly emitted particles present at the engine exit plane plus any unreacted material that condenses into the particle phase in the evolving © 2012 American Chemical Society
Received: April 17, 2012 Revised: July 6, 2012 Published: July 10, 2012 4756
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Separate experiments were performed with three different fuels: standard petroleum-based aviation fuel (JP-8), FT fuel derived from a coal feedstock, and a 50:50 (vol %) blend of the two fuels. Properties of the neat JP-8 and FT fuels are listed in Table 1. Two major
remainder classified as partially oxidized fuel, but the fraction of primary organic aerosol (POA) attributed to oil was not directly determined.4 A detailed study of PM composition is required to fully understand how fuel formulation impacts both EC and organic carbon (OC) primary PM emissions. While aircraft directly emit both particles and gases, the latter constitute the vast majority of emissions (>97%).12 A significant fraction of the organic emissions are low-volatility vapors. The gas−particle partitioning of semi-volatile organic compounds (SVOCs), those with an effective saturation concentration (C*) between 10−1 and 102 μg m−3, changes as the emissions are cooled and diluted in the atmosphere, complicating the definition of primary PM emission factors.6,13,14 The importance of SVOC partitioning therefore requires careful consideration of source sampling conditions, specifically the total concentration of organic aerosol, which is related to the dilution ratio and the total emission rate of a given source. Intermediate volatility compounds (IVOCs; 103 < C* < 106 μg m−3) may also undergo gas−particle conversion at the high concentrations present in many source tests but exist as vapors in the ambient atmosphere. The adsorption of IVOC and SVOC vapors to filter samples can result in sampling artifacts and overestimation of POA emissions.12,15−17 Once emitted to the atmosphere, IVOCs and other organic gases are photo-oxidized, forming secondary organic aerosol (SOA); IVOCs may be an important class of SOA precursors, especially from gas-turbine engines.18,19 Constraining the total contribution of gas-turbine engine emissions to ambient PM therefore requires quantification of three critical factors: (1) the direct emissions of soot, particulate sulfate, and low-volatility organic vapors, (2) the volatility distribution of the low-volatility organic vapors, which controls the gas−particle partitioning of SVOCs and, therefore, the amount of POA, and (3) the susceptibility to oxidation of SVOCs and IVOCs, which will largely govern SOA formation.20,21 This paper focuses on the first of these factors and discusses staged tests of a military helicopter engine operated on three different fuels: standard JP-8, FT fuel derived from coal, and a 50:50 blend of these two. We report the magnitude and detailed chemical composition of the emissions. The goal of this study is to understand how alternative fuels affect the primary PM emissions, the semi-volatile nature of the these emissions, and the potential for the emissions to form secondary PM. Gas−particle partitioning of the low-volatility organic emissions and the extent of SOA formation are described elsewhere.20,22
Table 1. Select Properties of the JP-8 and FT Fuels Used in This Study fuel property
JP-8
FT
hydrogen content (mass %) (D3343) aromatic content (vol %) (D1319) total sulfur content (wt %) (D4294) total hydrocarbon content (vol %) (D2425) paraffin (normal + iso) cycloparaffin alkylbenzene indan and tetralin indene and CnH2n−10 naphthalene acenaphthalene acenaptylene tricyclic aromatic
13.9 17.2 0.064
15.1 0.4 ≤0.001
50 31 12 5 0.6 1 ≤0.4 ≤0.4 ≤0.4
88 12 0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4
differences between the fuels are aromatic and sulfur contents. The sulfur content of the JP-8 fuel is 0.064% (640 ppmw) versus below the detection limit (106 μg m−3) emissions are primarily comprised of small, unsaturated hydrocarbons (see Tables S1−S3 of the Supporting Information for the complete listing of speciated emissions). Species with carbon numbers below 5 contributed 70−80% of the speciated VOC emissions by mass at idle load and more than 70% at cruise load. Therefore, the majority of the hydrocarbon emissions will not form SOA because their saturation vapor pressures will be too high. For all fuels, emissions of small, unsaturated (olefinic or aromatic) VOCs were in excess of 80 and 40% of the speciated VOCs at idle and cruise loads, respectively. Fuel type had a small effect on VOC composition at idle load, while changing from JP-8 fuel to FT fuel at cruise load shifted the speciated VOC emissions from mainly ≤C5 to exclusively ≤C5 organics. HCtot,gas values are greater than the sum of the speciated measurements from the canister analysis. The fractions of unspeciated VOCs at idle load (25−40%) and cruise load (2− 4%) are commensurate with previous studies.55 Important SOA precursors are aromatics and large hydrocarbons (C9 and bigger). Unidentified VOCs may also be important SOA precursors. Because most possible compounds smaller than C5 have been measured, unidentified VOCs will be biased toward larger carbon number and higher SOA formation potential. The remaining speciated VOC emissions include alkylated aromatics that are known SOA precursors. Emissions of singlering aromatics were significantly higher at idle load than cruise load for both JP-8 and FT fuels. At idle load, emissions of single-ring aromatics were higher with JP-8 fuel (1540 mg/kg of fuel) than FT fuel (270−510 mg/kg of fuel). The measured single-ring aromatics included benzene, toluene, xylenes, ethyl benzene, styrene, ethyl toluene, and trimethylbenzenes. Idle emissions for the 50:50 blend (145 mg/kg of fuel) were lower than for neat FT fuel. The reasons that aromatic emissions did not track aromatic content of the fuel in the case of the 50:50 blend are not clear. Although the FT fuel contains few aromatic
not FT fuel. In other words, partially burned fuel contributes significantly to the idle load POA emissions when the engine is operated on JP-8 fuel but not FT fuel. This is likely due to differences in fuel composition, particularly the lower volatility of the JP-8 fuel (Table 1). 3.3. POA Volatility. Figure 7 presents the TD data, which directly demonstrate the semi-volatile nature of the primary PM
Figure 7. Results from TD analysis for fuels at idle load. The fraction of organic aerosol mass fraction remaining (OA MFR) of POA is plotted versus the maximum temperature in the heated section of the TD.
emissions. For example, more than half of the POA at idle load evaporates when the aerosol is heated to 40 °C for all three fuels. This is consistent with the TD−GC−MS data (Figures 3−6), which indicates that SVOCs contribute roughly 90% of the total, artifact-corrected POA (i.e., 90% of m/z 57 of the Q− QBT signal elutes between 34.5 and 48.5 min). At cruise load, POA emissions appear to be nearly completely SVOCs (≥94%); however, POA contributes to a much smaller fraction of the total PM emissions at cruise load than at idle load (Figure 2). Finally, the large amount of adsorbed organic gases on the QBT filter provides a final piece of evidence that a large fraction of the POA emissions measured on a bare quartz filter are IVOCs (Figures 3−6). The gas−particle partitioning of semi-volatile material depends upon both the temperature and organic aerosol concentration of the exhaust.14 Therefore, if semi-volatile organics contribute to a large fraction of the POA, one must be careful interpreting PM emissions data because the gas−particle partitioning of semi-volatile material can be biased in source tests relative to the more dilute ambient atmosphere.14 The organic aerosol concentrations in the dilution sampler were 100−400 μg m−3 at idle load and 3−14 μg m−3 at cruise load. For the idle load tests, this is at least an order of magnitude higher than typical ambient concentrations. At such high concentrations, the partitioning of SVOCs is biased toward the particle phase, increasing the fraction of SVOCs that contribute to the POA and total PM emission factors.14 Accounting for the dynamic gas−particle partitioning of the POA and, thus, obtaining a more accurate representation of POA emission factors require quantification of the volatility distribution of the POA emissions. We address this fully in a separate paper,22 and in this paper, we focus the discussion to “traditional” measurements of OC, EC, and PM from a dilution sampler and provide evidence that the POA is semi-volatile. In a separate publication,22 we map the organic emissions from this engine collected on a bare quartz filter onto the volatility basis set.52 These data can be used to extrapolate the POA emissions measured at high concentrations inside the dilution sampler to more atmospherically relevant conditions. 4762
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(SOA). A recent emissions study has shown typical methane emissions to be moderate be at idle load (25−40%) and low at cruise load (2−4%);55 therefore, unidentified VOCs (VOC-UI) will be mainly non-methane, particularly at cruise load. Important SOA precursors include single-ring aromatics and C10 and larger vapors (VOCs, IVOCs, and SVOCs).60 To quantify the potential importance of SOA formation, Figure 8 shows the contributions of different classes of SOA
species, aromatics can form from smaller unsaturated organics.56 Naphthalene and alkylated naphthalene emissions trended with the aromatic content of the fuels; their emissions were highest for JP-8 fuel and lowest for FT fuel. Tenax TA sorbent samples were collected and then analyzed by TD−GC−MS to characterize the emissions of intermediate volatile organic compounds (IVOCs). Similar to the organics collected on a quartz filter, the majority of the IVOCs could not be speciated. Following the approach by Presto et al.,22 the total mass of IVOCs captured in the sorbent tubes was estimated using the various fuels as calibration standards. Briefly, calibration curves for IVOC-UCM mass versus the m/z 57 signal from the TD−GC−MS analysis before 34.5 min were developed for each fuel sample and then applied to the exhaust samples.22 Table 2 indicates that the emissions of gas-phase IVOC were both load- and fuel-dependent. At idle load, IVOC emissions varied from 3067 to 5258 mg/kg of fuel for FT and JP-8 fuels, respectively. The blend fuel IVOC emissions were intermediate at 3954 mg/kg of fuel. In comparison, IVOC emissions detected by the TD−GC−MS analysis of the Tenax sorbent tubes were negligible at cruise load at 6 and 23 mg/kg of fuel for JP-8 and FT fuels, respectively. The trend with load agrees with previous studies and is expected as combustion efficiency increases with engine load.12 Lower total gas-phase emissions with FT fuel have also been correlated to higher combustion efficiency, agreeing with the measured trend in IVOC emissions.7 3.5. Secondary PM Formation Potential. Upon entering the atmosphere, the emissions are photo-oxidized, which can produce secondary PM (PM formed from the oxidation products of gaseous emissions). For example, SO2 emissions are oxidized in the atmosphere to form particulate sulfate, and some of the organic vapor emissions are oxidized to form SOA. Miracolo et al.21 reported that photo-oxidation of the exhaust from a CFM56 engine formed significantly more secondary PM than the direct, primary PM emissions. Therefore, secondary PM production should be accounted for when assessing the impact of gas-turbine engine emissions on ambient PM levels. In this section, we examine the gas-phase emissions data summarized in Table 2 from the perspective of secondary PM production. As expected, SO2 emissions varied systematically with fuel sulfur content. Table 2 indicates that the SO2 emissions were highest for JP-8 fuel (∼1.0 g/kg of fuel), below detection for FT fuel, and intermediate for the FT/JP-8 blend (0.63 g/kg of fuel). Complete conversion of fuel sulfur in the JP-8 fuel (0.064 wt %) to SO2 would give 1.28 g of SO2/kg of fuel; therefore, the measured SO2 emissions of 1 g/kg of fuel are reasonable. The SO2 emissions when the engine is operating on JP-8 are 2−5 times higher than the primary PM emissions. Under typical summertime, clear sky conditions, 1−3% of the SO2 emissions will be converted to sulfate per hour.57−60 Therefore, in about 1 day, secondary sulfate formed from oxidation of SO2 will likely exceed the primary PM emissions. This problem is avoided by using FT fuel because it does not contain any sulfur. SOA formation depends upon the concentration, volatility, and molecular structure of organic vapors. The composition of the organic emissions was very different at idle and cruise engine loads, particularly on a volatility basis. Although the emissions of non-methane organic gases (speciated VOCs, IVOCs, and SVOCs) greatly exceed the primary PM emissions, not all organic gases can form secondary organic aerosol
Figure 8. Mass balance for the organic emissions based on volatility. Solid areas correspond to speciated (Spec) material, and hashed areas correspond to unspeciated (UCM)/unidentified (UI) material. VOCs, IVOCs, and SVOCs are shown in blue, red, and yellow, respectively. VOC-UI is unidentified VOC material that may form SOA, and VOCPSOA includes VOCs known to form SOA (e.g., toluene). Bars on the left side of the figure correspond to idle-load emissions, and bars on the right side of the figure correspond to cruise-load emissions. The black points show artifact-corrected quartz filter measurements of POA. It is clear that even small yields of SOA from VOCs, IVOCs, and SVOCs will greatly outweigh POA.
precursors to the total organic emissions. VOC-UI is unidentified VOCs; this is the difference between the measured HCtot,gas and the speciated VOCs from the canister measurements. P-SOA is the sum of the speciated VOCs that are known to be SOA-forming (e.g., benzene, toluene, etc.) from the canister measurements. IVOC-UCM is the unresolved complex mixture of IVOCs, as measured from the TD−GC− MS analysis of the Tenax tubes with the fuel calibration curve. IVOC-Spec is speciated IVOCs, as measured from the TD− GC−MS analysis of the Tenax tubes with authentic standards. Analogous definitions apply to the SVOC-UCM and SVOCSpec categories. Also plotted in Figure 8 are the POA emissions. The ratio of the SOA precursor emissions to the POA emissions provides a measure of the potential importance of SOA formation. Relative to JP-8, total SOA precursor emissions decrease by roughly 20% for neat FT fuel (idle and cruise loads) and 10% lower for the blended fuel (idle load). At idle load, both VOC and IVOC emissions are important SOA precursors. At cruise load, emissions of potential SOA precursors are dominated by unidentified VOCs. At both engine loads, POA emissions (black points) are just a few percent of the total SOA precursor emissions (total height of bar). Given that typical first-generation (one lifetime for oxidation reactions) yields are on the order of 10%,22,61,62 we therefore expect that, within a few hours of leaving the engine, concentrations of SOA formed from photo-oxidizing the emissions will be roughly 5 times more PM than the direct 4763
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levels, secondary PM will likely exceed the direct primary PM emissions within 1 day. This conclusion is demonstrated in a companion paper that presents the results of secondary PM production in the smog chamber.20 Therefore, secondary PM production must be taken into account in determining the effects of aircraft emissions on air quality and climate. Switching to FT fuel reduces both the SO2 and SOA precursor emissions. Therefore, increasing FT fuel use, by either blending or substitution, can reduce the contribution of aircraft emissions to levels of both primary and secondary PM.
primary emissions. In addition, the prevalence of small, unsaturated VOC emissions will increase the rate of oxidant formation by propagation of radical chemistry. This will in turn increase SOA formation through oxidation of larger compounds and potentially decrease volatility of existing POA. Given the load dependence of the emissions, we also expect there to be much more SOA formation at idle load than cruise load.
4. CONCLUSION Gas and particle emissions were measured from a T63 gasturbine engine operating on three different fuels: neat JP-8, a 50:50 blend of JP-8 and FT, and neat FT fuels. Both the blend and neat FT fuels significantly reduced primary PM emissions compared to the neat JP-8 fuel at both idle and cruise loads. The blend fuel yields PM emissions below the linear average of the neat fuels at idle load. The use of FT fuel nearly eliminates EC emissions at idle load, as seen in previous studies, which has been attributed to the lack of aromatic content of FT fuel.4,5,7,8,23,24 Switching to FT fuel also reduces both the total hydrocarbon and SO2 emissions, which reduces the potential to form secondary PM. In comparison to other gas-turbine engines, such as the CFM56 engine, the emissions from the T63 engine are much higher; primary PM emissions are a factor of 10−20 higher for the T-63 engine, and total hydrocarbon emissions are nearly doubled.12,30,42 However, the trends with load and fuel composition are similar. For example, as seen for other gasturbine engines, OC dominates PM at idle engine load, while EC dominates at cruise load.29,30,40,41 Our measurements of fractional EC reduction compare well to the data by Timko et al.4 for a CFM56 engine, who measured 41 and 91% reductions in EC (black carbon soot) for 50:50 JP-8/FT blend and neat FT fuels, respectively. In the current study, EC reductions of 63 and 96% were measured, showing similar effects of FT fuel across these two engines. Corporan et al. report smaller decreases in total PM emissions with FT fuel from T-701C, a newer, larger helicopter engine.5 In that case, there were decreases of roughly 30 and 20% at idle and cruise loads, respectively. Although variable, the decreases in total PM mass emissions because of the use of FT fuel are significant for all engines at any engine load, and this study adds to that body of evidence. Both the TD and TD−GC−MS data demonstrate that a large fraction of POA emissions are semi-volatile. Therefore, dynamic gas−particle partitioning will alter the contribution of primary emissions to ambient PM. Furthermore, these emissions cannot be represented using a single emission factor. Instead, they must be represented using a volatility distribution. The volatility distribution of the emissions from this engine is reported by Presto et al., and it can be used to predict the effects of partitioning.22 The PM emission factors reported here, especially at idle load, are biased high relative to much more dilute ambient conditions. Therefore, one must be careful when extrapolating emissions data to ambient conditions. The potential biases of emissions sampling and gas−particle partitioning must be taken into account for realistic estimates of the ultimate contribution of engine exhaust to atmospheric PM. For this engine, both the SO2 emissions (from JP-8 fuel) and hydrocarbon emissions greatly exceed the primary PM emissions. Photo-oxidation of these gases produces secondary PM in the atmosphere. Given typical atmospheric oxidant
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ASSOCIATED CONTENT
S Supporting Information *
GC−MS data for speciated VOC measurements and TD− GC−MS data for filter samples (Tables S1−S3 and Figure S1− S4). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The views, opinions, and/or findings contained in this paper are those of the authors and should not be construed as an official position of any of the funding agencies. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding was provided by the U.S. Department of Defense Strategic Environmental Research and Development Program (SERDP) under Project WP-1626. We thank Chris Klingshirn, David Anneken, Matt DeWitt, and Joe Mantz from University of Dayton Research Institute and Matt Wagner and Dean Brigalli from the Air Force Research Laboratory for help with the testing. We also thank Amy Sullivan of Colorado State University for IC analysis of the Teflon filter samples.
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REFERENCES
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