VOLUME 18, NUMBER 2
MARCH/APRIL 2004
© Copyright 2004 American Chemical Society
Articles Particulate Emission from Combustion of Diesel and Fischer-Tropsch Fuels: A Shock Tube Study Moshan S. P. Kahandawala,† John L. Graham,‡ and Sukh S. Sidhu*,†,‡ Mechanical and Aerospace Engineering, and Environmental Science and Engineering, University of Dayton, 300 College Park, Dayton, Ohio 45469-0132 Received May 21, 2003. Revised Manuscript Received November 6, 2003
Motor vehicle emissions have been identified as a major source of particulates, and have been associated with adverse health effects and decreased ambient air quality. Recent published studies have shown that Fischer-Tropsch fuels can reduce particulate emissions. The fuel composition, temperature, and pressure affect the kinetics of a chemical reaction and ultimately the yield of end products. To see how the above factors impact the particulate yields, D-2 diesel and two Fischer-Tropsch fuels, Shell MDS and Mossgas COD, were investigated in this study over a pressure range of 5 to 24 atm and temperature range of 1000 to 2300 °C. All experiments were conducted in a modified single-pulse reflected shock tube. Fuels were injected using a highpressure liquid injector. The results from Leco carbon analysis indicated that for most test conditions, Shell MDS had the lowest particulate yield compared to D-2 diesel followed by Mossgas COD. At relatively low temperatures (∼1150 °C) and high temperatures (∼2250 °C) the particulate yields decreased for all fuels tested. At higher pressures an increase in particulate yield was observed. An attempt was made to correlate the observed difference in soot yields to physical properties and chemical composition of the investigated fuels. The results are well in agreement with previous studies that relate lower sulfur and aromatic contents to lower particulate yields for the test conditions studied. No direct benefit was seen from a high cetane number on particulate emissions.
Introduction Particles emitted to the atmosphere from various combustion sources have long been associated with adverse health effects, deterioration of visibility, and ambient air quality.1,2 Among these, motor vehicle * Author to whom correspondence should be addressed: Telephone: (937) 229-3605. Fax: (937) 229-2503. E-mail: sidhu@ udri.udayton.edu. † Mechanical and Aerospace Engineering. ‡ Environmental Science and Engineering. (1) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. J. Air Waste Manage. Assoc. 2000, 50, 1565-1618. (2) Dockery, D. W.; Pope, A.; Xu, X.; Spengler, J. D.; Ware, J. H.; Fay, M. E.; Ferris, B. G.; Speizer, F. E. N. Engl. J. Med. 1993, 329, 1753-1759.
emissions have been identified as a major anthropogenic source of atmospheric particulates. As vehicle-miles driven begin to outweigh the emission reductions that have been achieved with technologically advanced engines, environmental protection agencies are imposing more stringent regulations to reduce emissions. Due to its high efficiency, diesel engines have found increased application in the majority of heavy-duty vehicles and increasingly in light-duty vehicles and for power generation. Field studies have confirmed that dieselpowered vehicles in urban areas are a major source of suspended particulate matter (PM).3,4 As observed in emissions tests, diesel exhaust particles are mostly carbonaceous particles that range from
10.1021/ef0340108 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/15/2004
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Figure 1. General schematic of the UDRI shock tube.
10 to 80 nm.5 These particles, which are less than 2.5 µm in diameter, are most likely to be deposited deep in the respiratory tract. The organic and inorganic compounds associated with combustion-generated soot have carcinogenic, mutagenic, and irritant properties. Before they are removed from the atmosphere, these ambient particles can also react with various atmospheric pollutants or can absorb them onto their surface and thereby expose people to other hazardous air pollutants as well. Published reviews and proceedings from healthrelated-research6,7 discuss toxicological and epidemiological evidence of health risks from internal combustion engines,8,9 the toxicology of ultrafine anthropogenic atmospheric particles,10 and their relation to respiratory as well as other health conditions.11 To increase the environmental compatibility of diesel engines, the U.S. Department of Energy (US DOE) has funded various studies to investigate the impact of fuel composition on the mitigation of particulate emissions by the use of alternate fuels. Recent developments in combustion fuel studies have cited benefits from Fischer-Tropsch (F-T) fuels and its blends as an alternative to reducing emissions resulting from diesel combustion.12-22 The characteristics of F-T fuels make (3) Venkataraman, C.; Friedlander, S. K. Environ. Sci. Technol. 1994, 28, 563-572. (4) Harrison, R. M.; Deacon, A. R.; Jones, M. R. Atmos. Environ. 1997, 31, 4103-4117. (5) Morawska, L.; Bofinger, N. D.; Kocis, L.; Nwankwoala, A. Environ. Sci. Technol. 1998, 32, 2033. (6) Phalen, R.; Bell, Y. M. Proceedings of the 3rd Colloquium on Particulate Air Pollution and Health; Air Pollution Health Effects Laboratory, University of California: Irvine, CA, 1999. (7) PM 2000; Particulate Matter and Health-The Scientific Basis for Regulatory Decision-Making, Charleston, South Carolina; Extended Abstract Book; Air & Waste Management Association: Pittsburgh, PA, 2000. (8) Diesel Exhaust: A Critical Analysis of Emissions, Exposure, and Health Effects; Health Effects Institute: Cambridge, MA, 1995. (9) Mauderly, J. L. Environ. Health Perspect. 1994, 102 (4), 165171. (10) Spurny, K. R. Toxicol. Lett. 1998, 96-97, 253-261. (11) Rantanen, L.; Mikkonen, S.; Nylund, L.; Kociba, P.; Lappi, M.; Nylund, N. O. SAE Technical Paper No. 932686, 1993. (12) Clark, R. H.; Unsworth J. F. Proceedings of the 2nd International Colloquium on Fuels; Technol. Akad. Esslingen: Ostfildern, Germany, 1999. (13) Gardner, T. P.; Low, S. S.; Kenney, T. E. SAE Technical Paper No. 2001-01-0149, 2001. (14) Johnson, J. W.; Berlowitz, P. J.; Ryan, D. F.; Wittenbrink, R. J.; Genetti, W. B.; Ansell, L. L.; Kwon, Y.; Rickeard, D. J. SAE Technical Paper No. 2001-01-3518, 2001.
them attractive as alternate diesel fuels, and has prompted the US DOE to initiate a rule to determine if F-T fuels should be designated as alternative fuels under the Energy Policy Act of 1992, which requires that the fuels provide “substantial environmental benefits”.22 However, all published studies on F-T fuels have been based on engine and in-use vehicle tests. It is difficult to compare results from some of these studies with those obtained from diesel fuel because different engines were used for different fuels. This can lead to different fuels experiencing different combustion conditions, thus making direct comparison difficult. Another problem is that several fuels are designated as F-T fuels because different companies use different variations of the F-T process to generate fuel. Thus, different F-T fuels and their blends could have different chemical compositions. Therefore, many F-T blends and types are worth investigating in order to develop a fuel that provides optimum engine performance and satisfies the necessary emission control regulations. The fuel composition, temperature, and pressure are known to affect the kinetics of a chemical reaction and ultimately the yield of end products. The purpose of this study was to use a simple procedure to compare how these factors affect the soot yield from fuels with varying composition by evaluating the emissions that will be specific to the properties and composition of fuels that have claimed or shown significant emissions benefits in previous studies, thereby increasing the fundamental knowledge base that facilitates sustainable clean fuel development. (15) Korn, S. J. SAE Technical Paper No. 2001-01-0150, 2001. (16) Clark, N. N.; Atkinson, C. M.; Thompson, G. J.; Nine, R. D. SAE Technical Paper No. 1999-01-1117, 1999. (17) May, M. P.; Vertin, K.; Ren, S.; Gui, X.; Myburgh, I.; Schaberg, P. SAE Technical Paper No. 2001-01-3520, 1999. (18) Norton, P.; Vertin, K.; Bailey, B.; Clark, N.; Lyons, D.; Goguen, S.; Eberhardt, J. SAE Technical Paper No. 982526, 1998. (19) Norton, P.; Vertin, K.; Clark, N.; Lyons, D.; Gautam, M.; Goguen, S.; Eberhardt, J. SAE Technical Paper No. 1999-01-1512, 1999. (20) Bertoli, C.; Migliaccio, M.; Beatrice, C.; Del Giacomo, N. SAE Technical Paper No. 982492, 1998. (21) Schmidt, D.; Wong, V. W.; Green, W. H.; Weiss, M. A.; Heywood, J. B.; Proceedings of ASME-ICE Spring Technical Conference; Philadelphia, PA, 2001. (22) Alleman, T. L.; McCormick, R. L.; Vertin, K. Assessment of Criteria Pollutant Emissions from Liquid Fuels Derived From Natural Gas, Report No. NREL/TP-540-31873, National Renewable Energy Laboratories, 2002.
Particulate Emission from Combustion of Fuels
Figure 2. UDRI shock tube test section configured for particulate sampling and liquid fuel injection.
The University of Dayton Research Institute (UDRI) Shock Tube Facility allowed us to generate and collect particulates from the combustion of hydrocarbon fuels under conditions representative of spark ignition (SI) or compression ignition (CI) engines.23 Shock tubes have also been used by other researchers successfully to study soot formation.24,25 The significance of the shock tube facility is that organic particulate matter can be reproducibly generated under carefully controlled laboratory conditions and examined in detail as opposed to particulate generated from engine tests where the results are specific to the engine and fuel combination, and the particulates are formed from a broad range of incylinder combustion conditions. This process does not involve the complications of modifying the test engines to accommodate each fuel, because some of these modifications may not necessarily represent the optimal performance for the specific fuel. Experimental Setup A single-pulse reflected shock tube, shown in Figure 1, was used for all experiments. The shock tube is comprised of a 7.6 cm ID × 2.74 m long driver section, a 5.08 cm ID × 2.75 m long driven section, and a 5.08 cm ID × 0.9 m long test section. It also includes a 30.5 cm ID × 61 cm long dump tank and an evacuation subsystem. The driven and test sections are connected through a pneumatic ball valve controlled through the system’s automatic digital firing system. A dump tank is connected to the system through a manually operated ball valve. The entire structure is made from 1.27 cm thick, 304 stainless steel. As illustrated in Figure 1, the UDRI shock tube test section consists of six general-purpose access ports, which can be fitted with a variety of devices such as injectors, samplers, sensors, and purging systems. It also consists of an interchangeable endplate that provides a further avenue for customizing the tube. For this study, as shown in Figure 2, the test section was fitted with high-speed pressure transducers, fuel injector, optical sensor, helium purge, and an exhaust particle sampler. While the nominal test conditions are set by the initial state of the system, the actual test conditions are determined from the reflected shock velocity through the test gas. The incident velocity was measured by dividing the distance by the arrival time of the shock past two piezoelectric pressure transducers mounted on the sidewall and endplate. The conditions in the test section were then calculated using a model based on an algorithm developed by Gardiner et al. and data from JANAF tables.26 The pressure events related to fuel injection were measured using a pressure transducer connected to the injector mounted on the end plate of the test section. The combustion (23) Armstrong, L. V.; Hartman, J. B. The Diesel Engine, Its Theory, Basic Design, and Economics; Macmillan: New York, 1959. (24) Wang, R.; Cadman, P. Combust. Flame 1998, 112 (3), 359370. (25) Alexiou, A.; Williams, A. Fuel 1995, 74 (2), 153-158.
Energy & Fuels, Vol. 18, No. 2, 2004 291 was monitored with a silicon optical pressure sensor (responsivity of 190-1100 nm) that detects the radiation emitted during combustion. The ignition delay times were determined with the optical sensor data and the data from the pressure transducer connected to the injector. All pressure events were monitored using a single-sweep digital oscilloscope. The test fuels were introduced using a Bosch diesel fuel injector fitted with a single-stroke positive displacement fuel pump. The pump delivers a small volume (50 µL) of fuel at a pressure of ∼150 atm in a precisely timed manner and in a narrow spray pattern that does not impinge on the tube walls. The fuel is introduced 600 µs behind the reflected shock, which allows for vibration relaxation of the test gas prior to injecting the fuel. The high pressure, high-temperature shock-heated gas behind the reflected shock provides the fuels an exposure similar to that experienced in an engine combustion chamber. The test gas in the driven section was comprised of 20 Torr of oxygen and, depending on the required final exposure conditions, the remainder was filled with argon to the desired driven section pressures. The gas used in the driver section was helium. The fire control system closes the pneumatic test section isolation valve approximately 0.5 s after the diaphragm is ruptured, sealing the combustion products in the test section. For each test, the exhaust valve to the sampler was opened and the test section was purged with 20 L (∼10 change volumes) of dry helium immediately after the isolation valve closed. As the gas was swept from the system, the particulate fraction from the combustion products was captured on pretreated quartz filters using a high-volume sampler and the amount of fuel injected was measured from a gauge on the injector as shown in Figure 2. The filters were pre-cleaned by baking them at 420 °C to remove any organics from the surface of the filters that could contribute to a bias reading from oxidation of surface contaminants during carbon analysis. The mass of the collected particle samples was then determined using a Leco Model RC 412 Multiphase Carbon Determinator (MCD) which thermally desorbs the samples from ambient temperature to 1000 °C. The desorption is done in an oxygen atmosphere and the exhaust is passed through a small catalytic reactor where all the carbon is converted to carbon dioxide. A quantitative measurement of the evolution of carbon dioxide as a function of sample temperature is obtained. This Leco test also provides information on the temperature range in which organic species evolve and the relative molecular weight range of these species.
Results and Discussion The test fuels used in this study were Shell MDS (Middle Distillate Synthesis) and Mossgas COD (Conversion of Olefins to Distillate) provided by National Renewable Energy Labs (NREL), and D-2 diesel that was obtained from a local commercial vendor. The Shell MDS F-T diesel fuel had been obtained from Shell’s middle distillate plant. This plant re-forms natural gas with pure oxygen to generate synthesis gas. The synthesis gas is then converted through a catalysis process into liquid hydrocarbon fuels. The Mossgas COD synthetic diesel was produced using the Mossgas conversion of olefins to distillate process. Mossgas produces a range of automotive fuel products and chemicals using natural gas feedstock. This fuel is a byproduct of the F-T reaction, where light olefins are branched together over a zeolite catalyst. The properties of the fuels studied are shown in Table 1.27 (26) Gardiner, W. C., Jr.; Walker, B. F.; Wakefield, C. B. In Shock Waves in Chemistry; Lifshitzs, A., Eds.; Marcel-Dekker: New York, 1981.
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Table 1. Fuel Properties property
SI units
No. 2 Diesel
Mossgas COD
Shell MDS
density kinematic viscosity cetane number sulfur total Aromatics
kg/m3 mm2/s unitless ppm mass %
838.96 2.41 46.1 350 21.6
804.2 2.98 48.9 74a
