Energy & Fuels 2007, 21, 2615-2626
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Emissions Characteristics of a Turbine Engine and Research Combustor Burning a Fischer-Tropsch Jet Fuel Edwin Corporan,*,† Matthew J. DeWitt,‡ Vincent Belovich,† Robert Pawlik,† Amy C. Lynch,† James R. Gord,† and Terrence R. Meyer§ AFRL/PRTG, Loop Road N Bldg 490, Wright Patterson Air Force Base, Ohio 45433, UniVersity of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469, and Iowa State UniVersity, 2034 H.M. Black Engineering Bldg., Ames, Iowa 50011 ReceiVed January 10, 2007. ReVised Manuscript ReceiVed June 6, 2007
The emissions characteristics of two combustion platforms, a T63 turboshaft engine and an atmospheric swirl-stabilized research combustor, fueled with conventional military jet fuel (JP-8), a natural-gas-derived Fischer-Tropsch synthetic jet fuel (also referred herein as synjet or FT), and blends of the two were investigated. Nonvolatile particulate matter (PM) and gaseous emissions were analyzed to assess the impacts of the aromaticand sulfur-free synjet fuel on the combustion products of the two platforms. The engine was operated at two power settings, and the combustor at several equivalence ratios, to evaluate the emission production over a wide range of combustion temperatures. Conventional aerosol instrumentation was used to quantify particle number (PN), size, and PM mass emissions, while a Fourier Transform Infrared analyzer was used to quantify the gaseous species. Planar laser-induced fluorescence and laser-induced incandescence techniques were employed on the research combustor to study the effects of the FT fuel on the formation and oxidation of particles in the combustor primary zone. Test results show dramatic reductions in particle concentrations and mean size on both combustion platforms with the neat FT and synjet fuel blends relative to operation with JP-8. Reductions of over 90% in PN were observed on both platforms for several operating conditions with neat FT fuel. For the engine, over an 80% reduction in smoke number was observed with neat synjet relative to operation on JP-8. As expected, reductions in sulfur oxide emissions and slight increases in water vapor (measured only in the atmospheric combustor) resulted due to the sulfur-free nature and higher hydrogen-tocarbon ratio of the synthetic fuel. Minor impacts were observed for other gaseous emissions. American Society for Testing and Materials fuel specification tests showed that JP-8/synjet blends up to 50/50% by volume satisfied the JP-8 military fuel requirements and that only the minimum specific gravity requirement was not satisfied at higher synjet concentrations. Impacts of the synjet fuel on the emissions of the atmospheric combustor and the T63 engine, a comparison of emissions between the two platforms, and results of in situ laser-based measurements in the combustor reaction zone are discussed.
Introduction It is estimated that the U.S. imports approximately 63% of its oil and consumes over 25% of the oil produced worldwide.1 U.S. dependence on foreign oil is projected to increase gradually and reach imports of nearly 77% by the year 2025. Increases in the price of petroleum crude, currently over $60 a barrel, and uncertainties in the political and socio-economic status in oilrich countries have renewed interest into the production of liquid transportation fuels from alternative (domestic) sources. The primary benefits of domestic fuel production are the reduction in required oil imports and protection against future shortages or interruptions in supply, which adversely affects both the economy and national security of the U.S. In addition, increased domestic production could help control the rate of future OPEC price increases. Domestic resources considered for the production of liquid transportation fuels during the oil shortages of * Corresponding author. E-mail:
[email protected]. † Wright Patterson Air Force Base. ‡ University of Dayton Research Institute. § Iowa State University. (1) Energy Information Administration. www.eia.doe.gov/aer (accessed Jan 2005).
the 1970s and 1980s included coal, oil shale, and tar sands.2-6 It has been estimated that the equivalent of 2.5 × 1012 barrels of oil lie in shale deposits, 1.5 × 1012 barrels in tar sands, 53 × 1012 barrels in coal, and 3.0 × 1012 barrels in natural gas.1 Crude oil from shale and tar sands may be refined into transportation fuels using techniques similar to those used for petroleum; however, the final product is relatively high in sulfur, nitrogen, and aromatic compounds, and the initial handling and processing of these feedstocks is difficult. Natural gas and coal (2) Sikonia, J. G.; Hilfman, L.; Wilcox, J. R.; Board, T. G.; Gembicki, V. A.; Yu, E.; Gatsis, J. G.; Latos, E. J. USAF Shale Oil to Fuels; U.S. Air Force Wright-Aeronautical Labs., AFWAL-TR-81-2116, Vol. II, Dayton, OH, July 1982. (3) Reif, H. E.; Schwedock, J. P.; Schneider, A. An Exploratory Research and DeVelopment Program Leading to Specifications for AViation Turbine Fuel From Whole Crude Shale Oil; U.S. Air ForceWright-Aeronautical Labs., AFWAL-TR-81-2087, Part V, Dayton, OH, Mar. 1982. (4) Moore, H. R.; Henton, L. M.; Johnson, C. A.; Fabry, D. A. Refining of Military Jet Fuels from Shale Oil; U.S. Air Force Wright-Aeronautical Labs., AFWAL-TR-81-2056, Dayton, OH, 1982. (5) Smith, E. B.; Guffey, F. D.; Nickerson, L. G. Production of Jet Fuels from Coal DeriVed Liquids, Vol. III - Jet Fuels Potential of Liquid ByProducts from the Great Plains Gasification Project; U.S. Air Force WrightAeronautical Labs., AFWAL-TR-87-2042, Vol. III, Dayton, OH, May 1988. (6) Talbot, A. F.; Swesey, J. R.; Magill, L. G. Turbine Fuels from Tar Sands Bitumen and HeaVy Oil; U.S. Air Force Wright-Aeronautical Labs., AFWAL-TR-2043, Vol. II, Dayton, OH, Sept 1987.
10.1021/ef070015j CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
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Figure 1. Swirl-stabilized atmospheric pressure research combustor.
can be converted into a highly refined transportation fuel via indirect liquefaction processes, such as a gas-to-liquid (GTL) or coal-to-liquid (CTL) processes. GTL and CTL technologies were discovered in Germany in the mid-1910s and further developed in 1923 by German scientists Drs. Franz Fischer and Hans Tropsch. In the GTL/CTL processes, the feedstock (e.g., natural gas or coal) is partially oxidized in the presence of steam, oxygen, and a catalyst to produce synthesis gas (carbon monoxide and hydrogen). The Fischer-Tropsch (FT) reaction converts the synthesis gas (syngas) into paraffinic hydrocarbons, which are then refined to the desired liquid products via product upgrading (e.g., cracking, fractionation, and isomerization). During World War II, FT technology was used by the Germans to produce liquid fuels from coal. The FT product is nearly free of heteroatoms and aromatics, thus making it attractive for diesel and jet fuel applications. Blends of FT-processed jet fuels produced from coal and natural gas with conventional petroleumderived jet fuel have been studied recently to assess their suitability for aircraft use.7-8 A specific fuel blend, namely, Sasol semisynthetic jet fuel (SSJF), has undergone extensive evaluation. Studies conducted by the Southwest Research Institute demonstrated that the properties of this “semisynthetic” fuel were well within the Jet A-1 fuel specifications and that the fuel was compatible with aircraft and engine components.7 As a result, the United Kingdom Ministry of Defence DEF STAN 91-91 Turbine Fuel Standard currently allows the use of up to 50% by volume of the iso-paraffinic FT fuel with petroleum-derived Jet A-1 provided the mixture has a minimum aromatic content of 8% and conforms to all Jet A-1 specification requirements. SSJF is currently being used in commercial aircraft in South Africa. The Department of Energy (DOE) National Energy Technology Laboratory and the Fuels Branch of the Air Force Research Laboratory (AFRL/PRTG) established a collaborative research and development program in 2000 to study and demonstrate clean aviation fuels as part of the DOE Ultra Clean Transportation Fuels Initiative. More recently (2006), the Department of Defense (DOD) established an Assured Fuels Initiative to develop, test, certify, and use jet fuels produced from alternative sources.9 Various studies have been performed to investigate the potential use of fuels produced via the FT process for aviation applications.9-17 Some have demonstrated that the FT (7) Moses, C. A.; Stavinoha, L. L.; Roets, P. Qualification of Sasol SemiSynthetic Jet A-1 as Commercial Jet Fuel: 1997; SwRI-8531, San Antonio, TX, Nov. 1997. (8) Roets, P.; Botha, J.; Moses, C.A.; Stavinoha, L. Stability and Handling of Sasol Semi-Synthetic Jet Fuel, Proceed. 6th Int’l Conf. on Stab. and Hand. of Liq. Fuels; Department of Energy, Rept. DOE/CONF 971014, Vol. II, 1998. (9) Harrison, W. E.; Zabarnick, S. The OSD Assured Fuels InitiatiVeMilitary Fuels Produced From Coal, Proceed. 2006 Clearwater Coal Conf.; Clearwater, FL, May 2006. (10) Edwards, J. T.; Minus, D.; Harrison, W.; Corporan, E.; DeWitt, M.; Zabarnick, S.; Balster, L. AIAA 2004-3885, 2004.
fuels can provide significant improvements in thermal oxidative stability and emission production with adequate low-temperature properties. However, as synthesized, the FT-processed paraffinic fuels will not satisfy required density specifications and possibly other fit-for-purpose properties. Therefore, compatibility with aircraft and engine materials, minimum aromatic content to ensure proper elastomer swell for seals to prevent fuel leaks, and lubrication properties of the FT fuel and blends are being addressed by the Air Force.16-17 In order to further assess the potential benefits of FT fuels on the emissions of legacy and more current combustor technologies, the present effort investigated the effects of a natural-gas-derived FT fuel on the emissions of a T63 turboshaft engine and of a CFM56-based low-emissions atmospheric combustor. Testing was performed on these platforms using neat FT and JP-8 fuels, and blends of these at varying volumetric ratios. Experimental Section Combustion Systems. The turboshaft engine and the atmospheric pressure swirl-stabilized combustor are located in the Propulsion Directorate at Wright-Patterson Air Force Base. The T63 engine is used primarily for helicopter applications and is employed inhouse to evaluate turbine engine lubricants, fuels, fuel additives, and sensors. A detailed description of the engine has been provided in an earlier publication.18 JP-8 fuel was supplied to the engine from an underground facility tank. Neat FT fuel and JP-8/FT blends were supplied from an external tank pressurized with nitrogen to feed the engine pump. The engine was initially operated on JP-8 and then transitioned to operation with the fuel blends or the neat FT fuel. Each test condition was run for approximately 30 min. For a given engine setting, the fuel flow rate was controlled to obtain a constant turbine exit temperature (T5). This approach ensured the best run-to-run repeatability with respect to engine power output and combustor temperature for the conditions considered. For these tests, the engine was operated at idle and cruise power. The swirl-stabilized combustor is used for fundamental studies of complex combustion phenomena and to assess the impact of (11) Bauldreay, J. M.; Heins, R. J.; Smith, J. Synthetic Jet Fuels and Their Role in the Future, Proceed. 8th Int’l Conf. on Stab. and Hand. of Liq. Fuels; Steamboat Springs, CO, 2003. (12) Freerks, R. L.; Muzzell, P. A. Preprint ACS DiV. Petr. Chem. 2004, 49 (4), 407. (13) Muzzell, P. A.; Freerks, R. L.; Baltrus, J. P.; Link, D. D. Preprint ACS DiV. Petr. Chem. 2004, 49 (4),411. (14) Chang, P. H.; Colbert, J. E.; Hardy, D. R.; Leanoard, J. T. Preprint ACS DiV. Petr. Chem. 2004, 49 (4), 414. (15) Lamprecht, D. Energy Fuels 2007, 21 (3), 1448-1453. (16) DeWitt, M. J.; Striebich, R.; Shafer, L.; Zabrnick, S.; Harrison, W. E.; Minus, D. E.; Edwards, T. EValuation of Fuel Produced Via the FischerTropsch Process for Use in AViation Applications, Proceed. AIChE Spring Meeting, Houston, TX, 2007. (17) Graham, J. L.; Striebich, R. C.; Myers, K. J.; Minus, D. K.; Harrison, W. E., III. Energy Fuels 2006, 20 (2), 759. (18) Corporan, E.; DeWitt, M. J.; Wagner, M. Fuel Process. Technol. 2004, 85, 727-742.
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Figure 2. Simplified flow diagram of emissions sampling and instrumentation system.
research fuels and additives on soot formation/oxidation processes. The combustor consists primarily of a fuel injector, a square crosssectional flame tube (combustion section), and an exhaust nozzle (Figure 1). The injector is a generic swirl-cup liquid-fuel nozzle consisting of a commercial pressure-swirl atomizer (Delavan Model 27710-8) with a nominal flow number of 1.6. The 4-cm-exitdiameter fuel injector is centrally located in the 15.25 cm × 15.25 cm square cross-sectional dome. Most of the air to the combustor enters through the swirl-cup injector, while a small percentage enters through aspiration holes along the dome wall. The combustion products from the primary flame zone are allowed to mix thoroughly along the 48-cm-long flame tube before entering a 43-cm-long, 5.7cm-exit-diameter converging exhaust nozzle designed to generate a uniform exhaust gas temperature and particle concentration profile. The combustor is optically accessible for in situ laser-based diagnostics and video images via 75-mm-wide quartz windows along the top and sides. For these tests, the combustor overall equivalence ratio (Φ) was varied from Φ ) 0.60 to 1.10 by changing the pressure drop across the fuel injector from approximately 1.5 to 10 atm, resulting in fuel mass flow rates of 1.02.2 g/s with constant mass air flow. The fuel flow rate was measured with a positive-displacement flow meter and the air flow with a sonic nozzle. The inlet air was heated with an electric heater and the flow rate kept constant at approximately 0.028 kg/s throughout the study. The air pressure drop across the combustor dome was approximately 5.0% of the main supply. Emissions Instrumentation. Nonvolatile particulate matter (PM) and gaseous emissions were extracted from the research combustor and engine with oil-cooled probes (maintained at 150 °C) and transported to the analytical instruments via heated lines. Details on the particle probe configuration and design are provided in a previous publication.18 The probes used for the gaseous emissions and smoke sample were undiluted and consisted of a simple sampling port nominally 0.457 cm in diameter. The probes were installed facing the flow near the center and at the exit plane of the engine or combustor. A simplified flow diagram of the emissions sampling system is shown in Figure 2. In order to minimize particle loss by diffusion and impaction, the probes and sample lines were stainless steel, and sharp bends were avoided. Also, the samples were immediately diluted with dry air at the probe tip, and the sample lines were maintained at 75 °C to minimize water condensation and particle loss to agglomeration and thermophoresis. Dilution also prevented the saturation (over-range) of the analytical equipment. Dilution ratios (air-to-sample) of nominally 10-16:1 were used in these studies. The diluted sample was drawn into the instruments via a vacuum pump, and the dilution air and sample flows were controlled and measured with high-precision flow controllers. All PM measurements presented herein were corrected for dilution. On-line analysis
of the mostly nonvolatile PM emissions was performed using a TSI Model 3022A condensation particle counter (CPC) to provide a count of the total particles per unit volume (particle numbe, PN) and a TSI Model 3936 scanning mobility particle sizer (SMPS) to obtain particle size distributions (PSDs). Details on the theory of operation of the CPC and SMPS can be found in Cheng19 and Flagan.20 For the SMPS, a nano differential mobility analyzer TSI model 3085 with a model 3025 CPC were used to classify and quantify the particles in the 4-160 nm mobility diameter range. A tapered element oscillating microbalance (TEOM) R&P model 1105 diesel particle monitor was employed for the real-time measurement of the engine PM mass concentration (mg/m3). The TEOM quantifies the mass on the basis of the change in the oscillating frequency of a tapered element as PM is collected on an attached filter. Due to its principle of operation, the TEOM can be prone to generating highly unsteady data in high-noise and -vibration environments. However, for these tests, the instrument (located in the control room and isolated from noise) was unaffected by engine vibration, resulting in stable data for all test conditions. An inhouse-designed smoke sampler was used to collect soot samples to determine smoke numbers (SNs) following the SAE Aerospace Recommended Practice (ARP) 1179.21 Gaseous emissions were quantified using an MKS MultiGas 2030 Fourier-transform infrared based gas analyzer and a flame ionization detector based total hydrocarbon analyzer. The sample lines to the gaseous emissions systems and the smoke sampler were maintained at 150 °C to prevent the condensation of water or volatile organic species. Laser-Based Diagnostics Systems. Effects of the fuel composition on soot formation in the combustor reaction zone were assessed using droplet and OH planar laser-induced fluorescence (PLIF) and soot laser-induced incandescence (LII). Details of the PLIF and LII systems have been provided previously,22 and only a brief description is included here. The PLIF system consisted of a frequency-doubled, Q-switched Nd:YAG laser operated at 50% power to pump a dye laser which was then frequency-doubled to the ultraviolet for droplet and OH PLIF. A 1.5-m-focal-length spherical plano-convex lens and a 75-mm-focal-length planoconcave lens were used to form a laser sheet of 330 µm at fullthickness-half-max (FTHM) that entered the combustor through the top window. Fluorescence was collected using an intensified charge(19) Cheng, Y. S. Condensation Detection and Diffusion Size Separation Techniques. In Aerosol Measurement, 2nd ed.; Wiley-Interscience: New York, 2001; Chapter 19. (20) Flagan, R. C. Electrical Techniques. In Aerosol Measurement, 2nd ed.; Wiley-Interscience: New York, 2001; Chapter 18. (21) Aircraft Gas Turbine Exhaust Smoke Measurement; SAE Aerospace Recommended Practice ARP 1179; SAE: Warrendale, PA, 1970. (22) Meyer, T. R.; Roy, S.; Gogineni, S. P.; Belovich, V. M.; Corporan, E.; Gord, J. R. Appl. Opt. 2005, 44 (3), 445-454.
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Figure 3. Gas chromatogram of JP-8 fuel.
Figure 4. Gas chromatogram of synthetic jet fuel.
coupled device (ICCD) camera. Two 1 mm WG-295 filters were used to eliminate scattering from droplets (verified in a separate sample cell), and a UG11 filter was used to eliminate flame emission and scattering from the LII laser. A 105-mm-focal-length f/4.5 UV lens and an intensifier gate width of 20 ns were used to capture the droplet and OH PLIF signals. The two PLIF signals could be distinguished because the droplet PLIF signal intensity was approximately an order of magnitude higher than that for OH PLIF. In addition, the signal occurred primarily near the injector or at isolated points. OH signals, visible at low equivalence ratios or with low aromatic fuels, were verified by tuning the PLIF laser source on and off of the Q1(9) transition of OH at 283.922 nm (in air). The LII system used 50% of the energy from a frequencydoubled Nd:YAG laser, formed into a laser sheet using a 2 m planoconvex spherical lens and a 50 mm plano-concave cylindrical lens. The FTHM of the laser sheet was about 700 µm. The LII signal, consisting of incandescence from laser-heated particles, was detected using an ICCD camera and an f/1.2, 58-mm-focal-length glass lens. The PLIF and LII cameras were synchronized for simultaneous imaging using an external delay generator driven by the Q-switch TTL output of the Nd:YAG laser.
Fuels. JP-8 fuel is petroleum-based commercial Jet A-1 with a military additive package that includes an icing inhibitor (∼10001500 ppm), corrosion inhibitor/lubricity enhancer (∼20 ppm), and a static dissipater additive (∼5 ppm). JP-8 is the standard fuel used for all USAF aviation applications. The synthetic jet fuel (herein called synjet) used in this study was produced from natural gas by Syntroleum Corporation via a low-temperature FT process. The synjet fuel was not treated with the JP-8 additives; however, these are not expected to impact emissions due to their low concentration and mostly organic nature. The JP-8 and synjet fuels were analyzed using gas chromatography/mass spectrometry (GC/MS) and highperformance liquid chromatography techniques to determine their chemical composition. Their respective gas chromatograms are shown in Figures 3 and 4. JP-8 is composed of various hydrocarbon types, with branched (iso-) and normal (n-) paraffins being the primary classes. The n-paraffins typically range from n-octane (n-C8) to n-hexadecane (n-C16), with maximum concentrations from n-decane (n-C10) to n-dodecane (n-C12). The molecular weight distribution is determined by the required volatility (e.g., distillation) range for the upgraded fuel. JP-8 also contains appreciable concentrations of cycloparaffins, olefins, and aromatics. During low-temperature FT synthesis, the
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Table 1. Summary of GC/MS Results for the Synthetic Fuel and an Average JP-823 component
synthetic fuel
average jp-8 (jet-a 1)
n-paraffins iso-paraffins monocycloparaffins dicycloparaffins tricycloparaffins alkyl benzenes indans + tetralins naphthalene substituted naphthalenes
18.0 82.0
