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Impact of Biodiesel Blending on Diesel Soot and the Regeneration of Particulate Filters Andre´ L. Boehman,* Juhun Song, and Mahabubul Alam The Energy Institute, The Pennsylvania State University, 405 Academic Activities Building, University Park, Pennsylvania 16802 Received March 7, 2005. Revised Manuscript Received April 29, 2005
A critical requirement for the implementation of diesel particulate filters on diesel-powered vehicles is having a low “break-even temperature” (BET), which is defined as the temperature at which particulate deposition on the filter is balanced by particulate oxidation on the filter. This balance point needs to occur at sufficiently low temperatures, either to fit within the exhaust temperature range of the typical duty cycle for a diesel vehicle or to require a minimum of active regeneration. Catalytic coating on the diesel particulate filter, the use of a fuel-borne catalyst, and oxidation catalysts placed upstream of the particulate filter can all reduce the BET. Another important factor in reducing the BET is the sulfur content of the fuel, because the sulfur dioxide generated during combustion can poison catalyst activity. However, fuel formulation factors other than sulfur content can also have significant effects on the BET. Considered in this work were low sulfur diesel fuel (LSD, 325 ppm sulfur), ultralow sulfur fuel (ULSD, 15 ppm sulfur), and blends of both diesel fuels with 20 wt % biodiesel. The lowest observed BET was for the 325 ppm sulfur fuel that was blended with 20 wt % biodiesel, due, in part, to increased engine-out NOx emissions with the B20 blend, which shows that the engine-out exhaust composition can be as or more important than sulfur content. Furthermore, examination of the soot generated with these fuels shows a variation in the nanostructure and the oxidative reactivity for soots derived from the different fuels. There exists evidence of correlation between reactivity and structure in the case of carbon blacks or coal chars that are synthesized from different hydrocarbons and at different temperature conditions. Soot nanostructures of particulates produced from different fuels in a commercial direct injection (DI) diesel engine were compared by means of high-resolution electron microscopy imaging. The crystalline information, such as the graphene layer size and orientation, is used to interpret the quantitative reactivity differences measured in an idealized thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) oxidation experiment. Together, these results show the potential impact of biodiesel blending on the low-temperature oxidation characteristics of soot and the impact these soot characteristics can have on particulate filter regeneration.
Introduction Diesel particulate emissions pose a significant potential health hazard. Control of diesel particulate emissions is an issue requiring the attention of the fuels, engine, and after-treatment industries. To achieve the reductions in particulate emissions that have been mandated by the United States Environmental Protection Agency (USEPA) in 2007, the use of diesel particulate filters (DPFs) will be a necessity.1 To enable the implementation of particulate and NOx control technologies, the USEPA has mandated that ultralow sulfur fuels be available to enable advanced after-treatment strategies, which can be highly sulfur-sensitive.2 The negative impact of fuel sulfur on emission controls for diesel engines has been thoroughly documented in * Author to whom correspondence should be addressed. Telephone: 814-865-7839. Fax: 814-863-8892. E-mail address: boehman@ ems.psu.edu. (1) Allansson, R.; Blakeman, P. G.; Cooper, B. J.; Hess, H.; Silcock, P. J.; Walker, A. P. Optimising the Low Temperature Performance and Regeneration Efficiency of the Continuously Regenerating Diesel Particulate Filter (CR-DPF) System. SAE Tech. Pap. Ser. 2002, 200201-0428.
studies such as the Diesel Emission ControlssSulfur Effects (DECSE) program.3 A critical requirement for the implementation of DPFs on diesel-powered vehicles is having a low “break-even temperature” (BET), which is defined as the temperature at which particulate deposition on the filter is balanced by particulate oxidation on the filter. This balance point needs to occur at sufficiently low temperatures to fit within the exhaust temperature range of the typical duty cycle of a diesel vehicle. Catalytic coating on the DPF, the use of a fuelborne catalyst, and oxidation catalysts placed upstream of the particulate filter can all reduce this BET. Another important factor in reducing the BET is the sulfur content in the fuel, because the sulfur dioxide that is generated during combustion can poison catalyst activity.1,2 (2) Schmidt, D.; Wong, V. W.; Green, W. H.; Weiss, M. A.; Heywood, J. B. Review and Assessment of Fuel Effects and Research Needs in Clean Diesel Technology. Presented at the ASME Internal Combustion Engine Division Sping Technical Conference, April 29-May 3, 2001. (3) Diesel Emission ControlsSulfur Effects (DECSE) Program, Final Report: Diesel Oxidation Catalysts and Lean-NOX Catalysts, June 2001. (Sponsored by the U.S. Department of Energy, http:// www.ott.doe.gov/decse/pdfs/decserpt.pdf, accessed January 2005.)
10.1021/ef0500585 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/01/2005
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Diesel fuel production from renewable sources such as vegetable oils and animal fats offers the potential of both reducing fossil carbon emissions and producing alternative ultraclean transportation fuels. Blending with oxygenated or zero sulfur fuels can lead to particulate emissions reductions by interfering with the soot formation process and by decreasing the formation of sulfates. However, in the case of biodiesel fueling (e.g., “B20”, a blend of 20 vol % methyl soyate in diesel fuel), there is a well-documented increase of 2%-4% in NOx emissions.4 As shown by Van Gerpen and co-workers5,6 and Szybist et al.,7 the NOx increase with biodiesel fueling is attributable to an inadvertent advance of fuel injection timing. The advance in injection timing is due to the higher bulk modulus of compressibility, or speed of sound, in the fuel blend, which leads to a more rapid transferal of the pressure wave from the fuel pump to the injector needle and an earlier needle lift. The bulk modulus is shown to be dependent on the molecular structure of the fuel and correlates with the density of the fuel.8 In addition, for biodiesel blends, this impact of bulk modulus on NOx emissions can be overcome by addition of cetane improver9 and by modification of the distribution of methyl esters in the biodiesel fuel.10 It is well-known that biodiesel, neat or in blends, can provide reductions in particulate matter (PM) mass emission through either oxygen content or enhanced air entrainment, because of the higher boiling range of biodiesel.4 However, observations from the present work indicate that biodiesel may provide other benefits, with regard to particulate emissions. Here, we will show that a variation in the oxidation reactivity exists with soots derived from different fuels, in the same manner as was demonstrated for soots derived from ethanol, benzene, and acetylene by Vander Wal and Tomasek.11 They demonstrated that acetylene-derived soot was significantly less reactive than soot derived from ethanol and benzene, and that this variation in reactivity correlated directly with the degree of ordering of the soot primary particles. As shown by Hurt and co-workers, the “equilibrium” soot structure consists of a shell of graphene layers surrounding an amorphous core.12 Vander Wal and Tomasek showed transmission electron microscopy (TEM) images of soot particles that revealed a greater (4) Graboski, M. S.; McCormick, R. L. Combustion of Fat and Vegetable Oil Derived Fuels in Diesel Engines. Prog. Energy Combust. Sci. 1998, 24 (2), 125-164. (5) Monyem, A.; Van Gerpen, J. H.; Canakci, M. The Effect of Timing and Oxidation on Emissions from Biodiesel-Fueled Engines. Trans. ASAE 2001, 44 (1), 35-42. (6) Tat, M. E.; Van Gerpen, J. H.; Soylu, S.; Canakci, M.; Monyem, A.; Wormley, S. The Speed of Sound and Isentropic Bulk Modulus of Biodiesel at 21 Degrees C from Atmospheric Pressure to 35 MPa. J. Am. Oil Chem. Soc. 2000, 77 (3), 285-289. (7) Szybist, J. P.; Morris, D.; Boehman, A. L.; Esen, E. Diesel Fuel Formulation Effects on Injection Timing and Emissions. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 2003, 48 (1), 428-429. (8) Boehman, A. L.; Morris, D.; Szybist, J.; Esen, E. The Impact of Bulk Modulus of Diesel Fuels on Fuel Injection Timing. Energy Fuels 2004, 18, 1877-1882. (9) McCormick, R. L.; Alvarez, J. R.; Graboski, M. S.; Tyson, K. S.; Vertin, K. Fuel Additive and Blending Approaches to Reducing NOx Emissions from Biodiesel. SAE Tech. Pap. Ser. 2002, 2002-01-1658. (10) Szybist, J. P.; Boehman, A. L.; Taylor, J. D.; McCormick, R. L. Evaluation of Formulation Strategies to Eliminate the Biodiesel NOx Effect. Fuel Process. Technol. 2005, 86, 1109-1126. (11) Vander Wal, R. L.; Tomasek, A. J. Soot Oxidation: Dependence upon Initial Nanostructure. Combust. Flame 2003, 134, 1-9. (12) Hurt, R. H.; Crawford, G. P.; Shim, H.-S. Equilibrium Nanostructure of Primary Soot Particles. Proc. Combust. Inst. 2000, 28, 2539-2546.
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amount of ordering into the “shell-core” structure for acetylene-derived soot and more-amorphous character for the benzene- and ethanol-derived soots, leading to greater oxidative reactivity. Identifying the dominant mechanism during oxidation, if any, may have practical implications for reducing the temperature required to regenerate a catalyzed DPF.1,13 There exists evidence of correlation between reactivity and structure in the case of carbon blacks or coal chars that are synthesized from different hydrocarbons and under different temperature conditions.11,14 However, the manner in which crystallinity or pore structure affects soot oxidation rates has not been clarified for diesel soot, regardless of whether that soot is derived from conventional or alternative fuel sources. In this paper, we present a comparative study of the impact of sulfur content and biodiesel blending on particulate trap operation. The means of comparison is analysis of the impact of fuel composition on the BET. To determine the mechanisms by which biodiesel fueling alters the operation of the particulate trap, we also present a comparison of soot nanostructures of particulates produced from four different fuels (a low sulfur diesel fuel (LSD), an ultralow sulfur diesel fuel (ULSD), and their B20 blends) on a commercial direct injection (DI) diesel engine by means of high-resolution electron microscopy imaging. The crystalline information, such as the graphene layer size and orientation, is used to interpret the quantitative reactivity differences measured in an idealized thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) oxidation experiment. Together, these results show the potential impact of biodiesel blending on the low-temperature oxidation characteristics of soot and the impact these soot characteristics can have on particulate filter regeneration. Experimental Section Fuels. Four test fuels were considered, a low sulfur diesel fuel with 325 ppm sulfur content obtained from BP (LSD), an ultralow sulfur diesel with 15 ppm sulfur content obtained from BP (ULSD), and B20 blends of each (i.e., a blend of 20 wt % methyl esters in ultralow sulfur diesel, ULSD/B20, or in low sulfur diesel, LSD/B20 fuel). Some fuel properties are provided in Table 1. Engine and Particulate Filter Testing. A six-cylinder Cummins ISB 5.9L direct injection (DI) turbodiesel engine, connected to a 250 HP eddy current dynamometer, was used to produce different particulate samples at fixed engine operating conditions (2400 rpm and 156 N m, equivalent to 25% of peak load). The engine has been heavily instrumented with a 0.1 crank angle (CA) resolution crank shaft encoder, a cylinder pressure sensor, and a needle lift sensor. The engine and dynamometer are operated through an automated control system. To isolate any effect of cylinder temperature history from possible changes in soot nanostructure due to differences in the fuels, the time evolution of in-cylinder mean temperature was obtained through cylinder pressure trace analysis, along with consideration of injection timing and cylinder geometry. (13) Stanmore, B. R.; Brilhac, J. F.; Gilot, P. The Oxidation of Soot: A Review of Experiments, Mechanisms and Models. Carbon 2001, 39, 2247-2268. (14) Marsh, H. Kinetics and Catalysis of Carbon Gasification. In Introduction to Carbon Science; Butterworths: London, 1989; pp 107152.
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Table 1. Properties of Test Fuels Value test cetane number (engine) corrosion, copper strip distillation IBP (°C) T10 (°C) T20 (°C) T30 (°C) T40 (°C) T50 (°C) T60 (°C) T70 (°C) T80 (°C) T90 (°C) T95 (°C) FBP (°C) % recovered % residue density @ 60 °C (kg/m3) flash point (°C) viscosity @ 40 °C (cSt) pour point (°C) cloud point (°C) sulfur, total (ppm, by wt ) lubricity (SLBOCLE) (gm) aromatic content/PNA (mass %) lower heating value (MJ/kg) a
method ASTM D 613 ASTM D 130 ASTM D 86
ASTM D 4052 ASTM D 93 ASTM D 445 ASTM D 97 ASTM D 5773 ASTM D 2622/D 5453 ASTM D 6078 ASTM D 5186-99 ASTM D 3338
ULSDa
LSDb
ULSD/B20
LSD/B20
50.5 1a
46.8 1a
52.5 1a
49.2 1a
167.4 203.4 219.5 234.1 247.2 260.4 273.4 287.2 303.1 322.3 337.7 347.7 98.3 1.2 0.837 63 2.48 -33 -10 15 3800 20.4/5.2 43.1
180 205.6 214.8 222.2 249.8 258.9 267.9 277.9 290.1 307.3 323.5 330.5 97.9 0.9 0.843 64 2.5 -27 -19 322 4600 22.8/4.8 43
171.9 198 209.8 229.6 248.4 265.6 282 279.9 311.9 323.6 334 342.9 97.8 0.9 0.846 66 2.73 -30 -9 13 5850 N/A 42.2
183.3 220.7 236.5 250.6 262.7 274.8 287.8 301.5 315.8 325.6 336.6 342.7 98.7 0.5 0.851 68 2.71 -30 -13 252 6100 N/A 42.1
Ultralow-sulfur diesel. b Low-sulfur diesel.
A catalyzed DPF is connected to the engine exhaust. Exhaust samples are pulled upstream and downstream of the DPF, while the exhaust temperature, filter temperature, and pressure drop across the DPF are monitored. Gaseous emissions were measured using analyzers integrated into an AVL CEB II emissions bench. Exhaust gases were kept at a constant temperature of 190 °C with a heated sample line. NOx and hydrocarbon emissions were measured without exhaust cooling, using an EcoPhysics chemiluminescence analyzer and an ABB hot flame ionization detector (FID), respectively. A portion of the sample gas was chilled to strip the water before being analyzed with Rosemount CO (IR), CO2 (IR), and O2 (paramagnetic) detectors. All gaseous emissions were sampled continuously throughout the testing and measurements were automatically logged by the data acquisition system every 15 seconds via serial communication. Prior to a BET test, the filter is cleaned of particulate via operation at elevated temperature. The exhaust temperature then is reduced while the filter is filled with particulate matter until the pressure drop across the filter reaches 49.5 mbar (20 in. of H2O). To initiate regeneration, the exhaust temperature is increased in steps by increasing the load on the engine. Soot Characterization. For temperature-programmed oxidation (TPO) tests, bulk samples were collected in quartz filters from diluted exhaust gas via a mini-dilution tunnel (Sierra Instruments BG-1). The filter then was crushed to a powder, and 10 mg of the powder was evenly deposited into the sample pan in the furnace. For high-resolution transmission electron microscopy (HRTEM) imaging, thermophoretic sampling was used to capture PM from the raw exhaust. In addition to PM mass measurement, particulate samples were extracted directly from the exhaust stream using a thermophoretic sampling approach patterned after the work of Megaridis15 and Koylu,16 and described previously by Song et (15) Megaridis, C. M. Thermophoretic Sampling and Soot Aerosol Dynamics of an Ethene Diffusion Flame, Ph.D. Thesis, Division of Engineering, Brown University, Providence, RI, 1987. (16) Koylu, U. O. Emission, Structure and Optical Properties of Overfire Soot from Buoyant Turbulent Diffusion Flames, Ph.D. Thesis, Aerospace Engineering, University of Michigan, Ann Arbor, MI, 1992.
al.17 The thermophoretic samples were collected on 3-mmdiameter grids, constructed of 1-µm-thick copper and coated on one side with silicon oxide, using a pneumatically controlled probe to inject the sampling grid into the undiluted exhaust near the location where the other emissions samples were obtained in the exhaust pipe. Two different laboratory reactors provided the means of evaluating the differences in reactivity of the soot samples via TPO. All particulate samples were treated for 30 min by heating at 500 °C under inert gas (i.e., argon) in the TGA, to eliminate the soluble organic fraction (SOF). The samples then were subjected to slow heating to obtain the burning rate of each sample on both differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). This pretreatment for SOF removal has been reported to yield the same effect as post-extraction with dichloromethane (CH2Cl2).18 The ignition temperature was used to determine the oxidation reactivity of the samples. From the mass-loss curve from the TGA, the ignition temperature is determined as the temperature at which soot begins to oxidize in air at an appreciable rate. The detection of heat release by DSC was used as a supplement, to gauge the ignition temperature of different particulate samples.19 In this DSC configuration, the ignition temperature is determined by thermal runaway, which is controlled by competition between heat from combustion and heat loss to the gas flow. Structural properties of the diesel particulates were obtained by electron-beam probes. Among several characterization techniques to detect the degree of crystallinity of graphene layer, a bright-field imaging method via HRTEM was used on a field-emission JEOL 2010F microscope that was operated at 200 kV, with a point-to-point resolution of 0.23 nm. In bright-field imaging mode, graphene layer segments are (17) Song, J.; Cheenkakorn, K.; Wang, J.; Perez, J.; Boehman, A. L.; Young, P. J.; Waller, F. J. Effect of Oxygenated Fuel on Combustion and Emissions in a Light-Duty Turbo Diesel Engine. Energy Fuels 2002, 16, 294-301. (18) Stratakis, G. A.; Stamatelos, A. M. Thermogravimetirc Analysis of Soot Emitted by a Modern Diesel Engine Run on Catalyst-Doped Fuel. Combust. Flame 2003, 132, 157-169. (19) Neeft, J. P. A.; Nijhuis, X.; Smakman, E.; Makkee, M.; Moulijn, J. A. Kinetics of the Oxidation of Diesel Soot. Fuel 1997, 76, 11291136.
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Figure 1. Particulate composition during the filter loading process (2400 rpm and 156 N m torque) for the various test fuels.
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Figure 2. Variation in engine-out NOx emissions during the filter regeneration process (2400 rpm and 156 N m torque) for the various test fuels.
observed as the dark lines blocking/scattering the incident electron beam, thereby creating a dark image on the screen. A thin isolated particle deposited on a perforated carbon film is used to obtain the sharp phase contrast while minimizing the interference with the condensable fraction and the grid substrate, because the existence of adsorbed hydrocarbon may block the high contrast imaging of the carbon-rich dry soot.
Results and Discussion Particulate Emissions and Particulate Filter Regeneration. As shown in Table 1, four fuel formulations were considered. The emissions and performance of the test engine during operation on these four fuels over the AVL 8 mode test protocol has been described in detail elsewhere.20 Here, the emissions relevant to the operation of the particulate filter are presented as well, but in ref 20, a broader range of operating conditions were considered. As shown by Boehman and co-workers,7,21 fuel injection timing advances of 0.3 CA degrees and 1 CA degree are observed in “pump-line-nozzle” configuration fuel systems, for B20 and B100 (neat biodiesel), respectively. This injection timing advance in a purely mechanical fuel injection system may not be as likely in an electronically controlled fuel injection system. In the Cummins ISB engine, the Bosch fuel system is electronically controlled, which potentially complicates the interpretation of fuel effects on injection timing. The engine controller itself may shift the injection timing, due to differences in throttle position that are required to meet the required load, because of differences in the calorific value of the test fuels. Figure 1 shows the particulate composition during the filter loading period and indicates that there are some modest differences in the emission rate and composition of the particulate that is depositing on the DPF. Figure 2 shows the variation of engine-out NOx emissions during the filter regeneration process, caused by the differences in fuel composition and the presence of biodiesel fuel. Figure 3 shows the extent of conversion of engine-out NO to NO2 across the catalyzed DPF due to oxidation. Figure 4 indicates the variation in BET with test fuel composition. The BET is determined by following the slope in the variation of pressure drop with (20) Alam, M.; Song, J.; Acharya, R.; Boehman, A.; Miller, K. Combustion and Emissions Performance of Low Sulfur, Ultralow Sulfur and Biodiesel Blends in a DI Diesel Engine. SAE Tech. Pap. Ser. 2004, 2004-01-3024. (21) Szybist, J. P.; Boehman, A. L. Behavior of a Diesel Injection System with Biodiesel Fuel. SAE Tech. Pap. Ser. 2003, 2003-01-1039.
Figure 3. Variation in NO conversion to NO2 across the diesel particulate filter (DPF) during the filter regeneration process (2400 rpm and 156 N m torque) for the various test fuels.
Figure 4. Filter temperature versus slope of the pressure drop, indicating the break-even temperature for the various test fuels.
time (expressed in units of mbar/min) as exhaust temperature increases. The temperature at which the slope of the variation of pressure drop with time changes sign from positive to negative is the BET. The results in Figure 4 are representative of BET measurements from repeated trials. Although the absolute values of the BET results varied from test to test by 10 °C, the trends with fuel composition were repeatable. The results in Figures 1-4 show that, although there is modest variation in the composition and mass emissions rate of particulate matter from the engine between the test fuels, there are differences in engine-out NOx and, as a consequence, NO conversion across the catalyzed DPF. For the BP325 base fuel, there is an increase in engine-out NOx with addition of biodiesel, which is comprised mostly of NO, whereas, for the BP15 base fuel, there is a decrease in engine-out NOx with the addition of biodiesel. Subsequently, the engine-out NO is oxidized to NO2 over the catalyst that is impregnated into the DPF. The NO2 assists in the oxidation of the particulate matter on the DPF, because NO2 is a more aggressive oxidizer of diesel particulate matter than O2.1 However, the conversion of NO to NO2 is inhibited by the presence of SO2 in the exhaust, which
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Figure 5. Injection timing shifts in the Cummins ISB engine with the various test fuels at 2400 rpm and 156 N m torque. Despite the increase in the bulk modulus of compressibility with each B20 blend, the ultralow sulfur diesel (ULSD) shows a retardation of injection timing with biodiesel addition.
is 20 times higher for the LSD than for the ULSD. Thus, the NO conversion to NO2 is lower and requires an exhaust temperature that is more than 50 °C higher to become significant for the LSD, because of its higher sulfur content. Figure 5 provides the explanation for the differences in engine-out NOx emissions with the addition of biodiesel that are seen in Figure 2. Although a higher bulk modulus of compressibility leads to an advance of injection timing in a conventional pump-line-nozzle fuel injector, in the Cummins ISB engine with electronic controls, the situation is more complicated. For an electronically controlled engine, shifts in the throttle position, which arise from differences in calorific value and cetane number of the fuel, may lead to shifts in injection timing, because of variations in the programming of injection timing with location in the speed-load map for the engine. In the present case, biodiesel addition to the LSD base fuel yields a 0.2 CA degree advance of injection timing, which increases NOx emissions, whereas biodiesel addition to the ULSD base fuel yields a 0.3 CA degree retardation of the injection timing, which decreases NOx emissions. The ULSD base fuel has a higher cetane number than that of the LSD base fuel, as shown in Table 1, accounting for some of the difference in engine-out NOx emissions between the two base fuels. The fact that the net effect of the higher engine-out NOx emissions for the LSD/B20 fuel leads to the lowest BET, 308 °C, is a point of great interest. Consistent with one’s expectations, the LSD has the highest BET. However, the ULSD and ULSD/B20 fuels yield the same BET. This difference in BET may be explained by the generally lower NOx emissions for the ULSD and ULSD/ B20 fuels, relative to the LSD and LSD/B20 fuels. There is more NOx with the LSD and LSD/B20 fuels to serve to oxidize the PM on the DPF after conversion of NO to NO2; however, for the LSD, the NO conversion is inhibited and the particulate oxidation is inhibited by the higher sulfur content. The results in Figures 1-5 show that there are distinct differences in the response of the engine and the DPF to the different fuels. The apparent increase in ease of regeneration of the DPF when biodiesel is present (shown in Figure 4) raises the question of whether engine-out NOx and the rate of NO2 formation in the DPF alone can account for the differences in regeneration for the various fuels. To address this issue,
Figure 6. High-temperature regeneration test showing, as a function of time, (a) the enhanced rate of recovery of pressure drop across the DPF with a 20 wt % biodiesel blend fuel and (b) differences in NO2 production across the DPF.
Figure 7. Burning-rate differential scanning calorimetry (DSC) curve for soot samples obtained for the various test fuels from the Cummins ISB engine. Soot samples were heat-treated in inert gas to remove the soluble fraction before thermal analysis.
Figure 8. Thermogravimetric analysis (TGA) of weight loss for soot samples obtained for the various test fuels from the Cummins ISB engine. Soot samples were heat-treated in inert gas to remove the soluble fraction before thermal analysis.
one must consider the composition of the particulate, the soluble fractions versus insoluble fractions (as shown in Figure 1), and the question of whether the primary soot particles are affected by the fuel (as observed by Vander Wal and Tomasek11). To eliminate the temperature dependency of the catalytic steps in the regeneration process and thereby remove the dependence of the NO2 production process on the sulfur content of the fuel, the DPF was loaded with particulate, as in the BET test procedure, and the load was increased to achieve an exhaust temperature of 480 °C, at which NO to NO2 conversion is likely to reach equilibrium, as indicated in Figure 6. The pressure drop across the DPF was monitored from the moment the load was increased, to determine if the rate of removal of the particulate
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Table 2. Analysis of Soluble Organic Fraction (SOF) and Volatile Organic Fraction (VOF) Content of the Diesel Particulate Matter from the Cummins ISB Engine Operating on the Four Test Fuels fuel
particulate matter (PM) emission (g/h)
SOF content (%)
dry soot reduction, relative to LSD (%)
organic carbon (150-300 °C)
organic carbon (300-450 °C)
elemental carbon (450-750 °C)
LSD LSD/B20 ULSD ULSD/B20
29.4 25.1 26.6 27.8
52.4 57.6 57.8 61.1
24 20 23
23 32 32 27
42 48 48 53
35 20 20 20
matter was affected by the fuel. As seen in Figure 6, the presence of biodiesel substantially increases the rate of pressure recovery by oxidation of the filtered PM. Thus, there are inherent differences in the reactivity of the particulate. Particulate Composition, Structure, and Reactivity. Given that the literature shows that the soluble fraction of diesel particulate can increase substantially with biodiesel blending,4 as is seen in Figure 1 as well, there is a need to factor out the effects of the SOF. To accomplish this objective, bulk thermal analysis of the reactivity of the soot samples was performed. Prior to the thermal analysis of reactivity, the particulate samples were treated for 30 min by heating at 500 °C under inert gas (i.e., argon) in the TGA, to eliminate the SOF. Figure 7 compares the mass-loss curve during TGA of the soot samples from the four fuels, whereas Figure 8 compares the heat-release curves from DSC of the soot samples from the four fuels. The results in both Figures 7 and 8 indicate the differences in the ignition temperature of the soot samples. For either base diesel fuel, the biodiesel blend yields soot that exhibits a lower ignition temperature (by 40-50 °C) in both the burning-rate curve by DSC and mass reduction by TGA. This result indicates that at least a portion of the differences in particulate regeneration between the base diesel fuels and the biodiesel blends is attributable to differences in the oxidation reactivity of the primary soot particles. Some of the difference in particulate regeneration observed in the DPF is likely to be due to the composition and quantity of the SOF in the particulate matter. Although the proportions of SOF and insoluble fractions are affected by the presence of biodiesel, as seen in Figure 1, a clear indication of the differences in the particulate composition and morphology with biodiesel blending are provided in Table 2 and Figure 9. Table 2 provides results from SOF and volatile organic fraction (VOF) measurements of the PM emitted by the Cummins ISB engine during the DPF filling process. The VOF measurement was obtained using a Series 5100 Diesel Particulate Measurement System from R&P Co. (Albany, NY) that was attached to the exhaust stack by means of a heated sample tube. The mass evolution from a quartz filter is measured as a function of the temperature of the quartz filter and provides an indication of the volatility of the PM. The measurements reported in Table 2 sort the mass in three temperature ranges, which give a general indication of the particulate composition. Details on the VOF measurement procedure are provided elsewhere.22 Consistent with the increases of SOF and VOF with biodiesel addition (see in Table 2), Figure 9a-d show that biodiesel addition (22) Vittal, M.; Borek, J.; Boehman, A. L.; Okrent, D. The Influence of Thermal Barrier Coatings on the Composition of Diesel Particulate Emissions. SAE Tech. Pap. Ser. 1997, 972958.
yields a visible increase in condensed hydrocarbons surrounding the aggregated particle (see Figure 9b and 9d). Results from HRTEM imaging are shown in Figure 10a-d. The images in Figure 10a-d compare the soot nanostructure for the diesel and B20 soot and show that a more amorphous and disordered arrangement of shortrange graphene segments is apparent for the B20 soots (Figure 10b and 10d). Within this primary particle from the B20 soot, wrinkled or curved crystallites with many misalignments, relative to each other, and structural defects are more pronounced. In contrast, the diesel soots possess the typical shell-core structure in which graphene layers are oriented parallel to the external outer surface but are randomly oriented in a central core (this feature is particularly clear in the case of the ULSD-derived soot, see Figure 10c). Hurt et al. have shown that this arrangement is the lowest energy state for the soot particle, with the aromatic segments organized by edge and face alignment, provided there is sufficient mass addition. The more reactive edge-site carbon, which is more prevalent with short-range amorphous arrangement of the crystallites, is known to be more vulnerable to oxidative attack, because of greater accessibility and electronic affinity for O2 chemisorption.11,14 Consistent with this understanding of the relationship between structure and oxidative activity, B20 soot indeed leads to higher reactivity, compared to
Figure 9. Transmission electron microscopy (TEM) images of particulate morphology for the four test fuels sampled from the exhaust of the Cummins ISB engine at 2400 rpm and 156 N m torque. Images shown at a magnification of 195 k×.
Impact of Biodiesel Blending on Diesel Soot
Figure 10. High-resolution transmission electron microscopy (HRTEM) images of soot nanostructure showing the effect of biodiesel blending for the four test fuels. Particles were sampled from the exhaust of the Cummins ISB engine at 2400 rpm and 156 N m torque. Images shown at a magnification of 500 k×. Particularly for B20 blending in ULSD, the soot particle has a less-ordered structure.
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the two fuels in terms of temperature-time history during ignition and the premixed burn period, during which time changes in temperature could have altered soot inception and growth. Therefore, other factors, which are most likely related to fuel composition and its decomposition chemistry, must be responsible for the more-amorphous structure in the B20 soot. Based on calculations of the evolution of equilibrium soot structures,12 when insufficient polycyclic aromatic hydrocarbons (PAHs) are available during the soot inception period to attain the threshold mass requirement, a soot particle will not transition into the ordered morphology of the typical shell-core structure. Differences in the soot growth mechanism, caused by different soot growth species formed during the fuel decomposition period, may also lead to differences in structure, as hypothesized and verified in recent work by Vander Wal and Tomasek.24 Although the formation of heavy PAH species leads to an amorphous structure through the coalescence growth mechanism, the addition of light acetylene (C2H2) on the radical sites of PAH species leads to the formation of graphitic structures through the HACA (hydrogen abstraction carbon addition) mechanism. The coalescence growth mechanism may be more pronounced for combustion and sootgrowth conditions with the B20 fuel, which leads to a more amorphous soot nanostructure. Conclusions
Figure 11. Time evolution of in-cylinder mean gas temperature and heat-release rate during ignition and premixed burn period of diesel combustion process, leading to soot inception and growth as a function of crank angle degrees after “top dead center” (TDC): (- - -) ULSD fuel and (;) ULSD/B20 fuel.
the diesel soot, partly because of differences in soot nanostructure. Although identifying the dependence of soot nanostructure on the conditions during soot formation is not the main objective of the present work, a preliminary hypothesis can be offered to explain the amorphous soot structure observed with biodiesel addition, related to effects of biodiesel on the combustion process and affecting soot formation. Because of the effect of temperature and its time variation on the extent of carbonization of soot, variation in soot nanostructure and soot oxidation rate is known to exist between mature soot and soot precursor particles that are not yet fully carbonized.23 As seen in Figure 11, a comparison of temperature-time history (regarding the length of time that soot precursor particles experience various temperatures after soot inception and through the carbonization period), for the case of BP15 and BP15-B20, shows that there is no significant difference between (23) Chen, H. X.; Dobbins, R. A. Crystallogenesis of Particles Formed in Hydrocarbon Combustion. Combust. Sci. Technol. 2000, 159, 109128.
The results presented here show that biodiesel blending alters the composition of the exhaust gas species and particulate matter (PM), as well as the nanostructure of the primary soot particles and thereby alters the regeneration behavior of a diesel particulate filter (DPF). The inclusion of biodiesel in the fuel reduces the temperature required to initiate regeneration of the DPF, and this effect is attributable to a combination of factors. Biodiesel fueling leads to shifts in injection timing that can increase the amount of NO emitted by the engine and, consequently, the amount of NO2 available to oxidize the filtered PM in the DPF. Biodiesel fueling increases the SOF content of the particulate, providing additional reactive hydrocarbons to oxidize catalytically in the DPF. Biodiesel fueling alters the nanostructure and oxidation reactivity of the primary soot particles, yielding a more amorphous soot structure, which enhances the rate of soot oxidation. The results presented here provide additional evidence, supporting observations by Vander Wal and Tomasek and other researchers that there is a structureproperty relationship between soot nanostructure and oxidation reactivity. The results presented here also provide additional justification for the use of biodiesel as a blendstock in diesel fuels, because, in addition to providing renewable energy content to diesel fuel and reducing some engine-out emissions, biodiesel blending can enhance the ease of regeneration of DPFs. Acknowledgment. The authors wish to thank Etop Esen, Kirk Miller, Doug Smith, Keith Lawson, Ed Casey, Rafael Espinoza, and Jim Rockwell (all of (24) Vander Wal, R. L.; Tomasek, A. J. Soot Nanostructure: Dependence upon Synthesis Conditions. Combust. Flame 2004, 136, 129140.
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ConocoPhillips) and John Wright and Edward LyfordPike (of Cummins Engine Company) for their support of this work. This work is a part of ongoing Ultra Clean Fuels project entitled “Ultra Clean Fuels from Natural Gas,” sponsored by U.S. Department of Energy under Cooperative Agreement No. DE-FC26-01NT41098. The Government reserves for itself, and others acting on its behalf, a royalty-free, nonexclusive, irrevocable, worldwide license for governmental purposes to publish, distribute, translate, duplicate, exhibit, and perform this copyrighted paper. High-resolution transmission electron microscopy was performed using the facilities of the Materials Research Institute at The Pennsylvania
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State University, and the authors are grateful for access to these facilities. This material was also prepared with the support of the Pennsylvania Department of Environmental Protection (DEP). Any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the DEP. Note Added after ASAP Publication. This article was published on the Web on 06/01/05 with an incomplete ref 11. The version posted 09/12/05 and the print version are correct. EF0500585
