Energy & Fuels 2007, 21, 1543-1547
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Regulated and Non-Regulated Pollutants Emitted during the Regeneration of a Diesel Particulate Filter George Bikas†,§ and Efthimios Zervas*,‡ Institut fu¨r Technische Mechanik, Templergraben 64, 52056 Aachen, Germany, and Department of EnVironmental Engineering, Democritus UniVersity of Thrace, Vas. Sofias 12, 67100 Xanti, Greece ReceiVed January 18, 2007. ReVised Manuscript ReceiVed March 7, 2007
The emission of regulated and nonregulated pollutants [nanoparticle number and size distribution, soluble organic fraction (SOF), polycyclic aromatic hydrocarbons (PAHs), sulfates, cerium, individual hydrocarbons, and SO2] emitted during the regeneration of a diesel particulate filter (DPF) is studied on a Euro4 passenger car. The obtained results show that DPF regeneration increases significantly the emissions of HC and CO and slightly those of particulate matter. Regeneration increases also the number of nucleation nanoparticles; however, these nanoparticles are composed mainly from HC or sulfates. Cerium is effectively collected on the DPF, while SOF increases during regeneration. Total particulate PAHs increases during regeneration; however, PAH emission during regeneration is quite similar to PAH emissions upstream of the DPF. Methane is the major HC, and its percentage increases during regeneration. Particulate sulfates and SO2 emissions also increase during regeneration. Globally, even if DPF regeneration increases the emission of some pollutants, this increase is not very high compared to normal engine operation conditions.
Introduction Diesel particulate filters (DPF) are currently widely used to collect and eliminate the particulate matter (PM) emitted from diesel engines. This after-treatment device is very effective as it decreases more than 95-99% both the particulate mass and the particle number emitted from diesel engines.1-5 This device operates in two stages. In the first phase, soot mass is collected into the filter, and in the second phase, the collected particles are oxidized via CO toward CO2. The transition of the one phase to the second is controlled by the engine control unit. Once the pressure drop across the DPF increases to above a threshold value, the sensor which is observing this pressure drop sends the command to the control unit that the soot load exceeds the value for an efficient operation. The second phase initiates and is generally controlled by the injection strategy. Delayed or postinjection is utilized to increase the temperature in the exhaust gases, and both the timing and quantity of this injection are a challenge for the calibration strategy of the car.6 Catalytic DPFs are also widely used, as the catalytic phase assists the regenera* Corresponding author tel.: +30-24510 79383; e-mail: ezervas@ env.duth.gr. † Institut fu ¨ r Technische Mechanik. ‡ Democritus University of Thrace. § Present address: HMETC - Hyundai-Platz 1, 65428 Ru ¨ sselsheim, Germany. (1) ACEA Programme on Emissions from Passenger Cars (1); ACEA: Brussels, Belgium, 1999. (2) ACEA Programme on Emissions from Passenger Cars (2); ACEA: Brussels, 2002. (3) Ntziachristos, L.; Mamakos, A.; Samaras, Z.; Mathis, U.; Mohr, M.; Thompson, N.; Stradling, R.; Forti, L.; DeServes, C. SAE Tech. Pap. Ser. 2004, 2004-01-1985. (4) Zervas, E.; Dorlhe`ne, P.; Daviau, R.; Dionnet, B. SAE Tech. Pap. Ser. 2004, 2004-01-1983. (5) Geller, M. D.; Ntziachristos, L.; Mamakos, A.; Samaras, Z.; Schmitz, D. A.; Froines, J. R.; Sioutas, C. Atmos. EnViron. 2006, 40, 6988-7004. (6) Van Basshuysen, R.; Scha¨ffer, F. Internal Combustion Engine Handbook; SAE: Warrendale, PA, 2004.
tion phase and thus decreases the necessary temperature for DPF regeneration.6 Diesel passenger cars emit CO, HC, NOx, and PM, which are regulated. Many authors study also the number and size distribution of emitted particles.1-4,7,8 However, other pollutants with lower concentrations are also emitted from these engines. Some authors study, among other things, the emission of individual hydrocarbons,9,10 polycyclic aromatic hydrocarbons (PAHs),5,11 and carbonyl compounds.9,12 These studies are performed on normal engine operation conditions. Even if some results on the emission of regulated pollutants and particle number and size distribution are reported during DPF regeneration,8 the emissions of other nonregulated pollutants are not reported, and there is no proof that DPF increases or decreases their exhaust concentration. This work studies the emission of regulated and nonregulated pollutants, such as nanoparticle number and size distribution, individual HC, cerium, the soluble organic fraction (SOF), PAHs, and SO2 in the New European Driving Cycle (NEDC) and at a steady speed, during normal engine operation and during DPF regeneration. The two modes are compared in order to find out if DPF regeneration increases or decreases and how much the exhaust concentration of each pollutant. Experimental Section A passenger car equipped with a 1.9 L diesel Euro4 engine is used for this study. The after-treatment system is composed of a commercial oxidation catalyst of 1.6 L with 50 /ft3 of Pt and a (7) Kittelson, D. B. J. Aerosol Sci. Technol. 1998, 29, 575-588. (8) Mohr, M.; Forss, A. M.; Lehman, A. EnViron. Sci. Technol. 2006, 40, 2375-2383. (9) Zervas, E.; Montagne, X.; Lahaye, J. Atmos. EnViron. 2001, 35, 1301-1306. (10) Nam, E. K.; Jensen, T. E.; Wallington, T. J. EViron. Sci. Technol. 2004, 38, 2005-2010. (11) Miguel, A. H.; Kirchestetter, T. W.; Harley, R. A.; Hering S. V. EnViron. Sci. Technol. 1998, 32, 450-455.
10.1021/ef070024s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/25/2007
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Figure 3. Evolution of particles number at 30 nm and 90 nm at the EUDC and HC/CO emissions. Effect of delayed injection, cold and hot NEDC, DPF of 20 g of soot charge. DI: delayed injection.
Figure 1. Emissions of regulated pollutants. Lower bars: emissions on the entire NEDC. Middle bars: emissions on the ECE. Upper bars: emissions on the EUDC. Euro4: limits Euro4. Filtr.A: cold NEDC, particles collection, without delayed injection. Reg: hot NEDC, regeneration, with delayed injection. Filtr.B: hot NEDC, particles collection, without delayed injection. PM emissions are analyzed only on the entire NEDC.
Figure 2. Evolution of particles number at 30 nm and 90 nm. Cold and hot NEDC, DPF of 20 g of soot charge, ECE. DI: delayed injection.
noncatalyzed DPF of 2.5 L made of SiC. The fuel is a commercial Euro4 diesel fuel with 50 ppm of S, also containing 30 ppm of CeO2 to assist DPF regeneration. It must be noted that the DPF is retrofitted to this vehicle, which was initially not equipped with DPF, and thus not optimized to treat eventually increased emissions occurring during DPF regeneration. Nanoparticle size distribution is performed with a scanning mobility particle sizer (SMPS). The chemical composition of particulate matter is also analyzed, including determination of the SOF, PAHs (using high-performance liquid chromatography), and particulate sulfates (using ionic chromatography). Determination of exhaust cerium (using X fluorescence), individual hydrocarbons (12) Graham, L. Atmos. EnViron. 2005, 39, 2385-2398.
(using gas chromatography/flame ionization detection), and SO2 (using mass spectrometry) is also performed. The measurement protocol is the following: The vehicle runs a cold NEDC with 20 g of soot charge on the DPF. Regulated pollutants are analyzed, and the nanoparticles of 30 nm are monitored using SPMS. At the end of this cycle, the soot charge remains approximately at 20 g as a very small quantity of soot is added. During this cycle, there is only soot collection without DPF regeneration. After this first, cold NEDC, a second, hot NEDC with a regeneration starting at 980 s from the beginning of the cycle, at the extra-urban part of the cycle (extra-urban driving cycle, EUDC), is performed. The DPF regeneration is achieved using delayed injection, which increases the exhaust temperature and thus regenerates the DPF, over 130 s. The regeneration stops when 80% of the accumulated soot mass is oxidized, leaving 4 g of soot on the DPF. At this cycle, the nanoparticles of 30 and 90 nm are monitored online using SMPS. SOF, PAHs, SO2, and individual hydrocarbons are also analyzed on the entire NEDC, and sulfates on the EUDC. A third, hot NEDC, using the DPF with 4 g of soot charge, where there is only collection of PM and not regeneration, is performed after the second NEDC. Measurements include monitoring of the nanoparticles of 30 nm using SMPS and measurement of SOF, PAHs, SO2, and individual hydrocarbons during the entire cycle and sulfates during EUDC. A last measurement of PAHs is performed on a hot NEDC upstream of the DPF, which corresponds to the emissions of this vehicle without DPF. As SMPS has a low-time resolution and is not adapted for transient measurements, another measurement is performed at a steady speed of 100 km/h, fifth gear, to examine the evolution of nanoparticle total number and size distribution during regeneration. DPF contains 20 g of soot charge at the beginning of this test. A delayed injection is applied from 300 to 830 s, and nanoparticle emissions were monitored using multiple SMPS scans from 0 to 3000 s. At the end of this regeneration (at 3000 s), 65% of the initial soot charge is oxidized.
Results and Discussion Emission of Regulated Pollutants. Figure 1 shows the emission of regulated pollutants of this vehicle, which are within the regulatory limits for the first cold NEDC cycle with filtration.
Nonregulated Pollutants
Figure 4. Evolution of the total particle number and of the median diameter of the particles emitted at 100 km/h, fifth gear, before, during, and after regeneration. 0: Tunnel blanc. 1: Before regeneration. 2: Start of delayed injection and regeneration. 3: Regeneration. 4: Regeneration and stop of delayed injection. 5-10: After regeneration. Each test is performed every 3 min.
Figure 5. Evolution of particle size distribution during the delayed injection at 100 km/h, fifth gear with DPF at 20 g. Upper curves: from before the beginning of delayed injection to the end of delayed injection. Bottom curves: from the end of delayed injection to the end of the test.
As expected, the hot NEDC cycle with filtration has lower emissions of HC and CO,10 because the catalytic converter is already active, and lower NOx, because the higher temperature of the engine oil leads to decreased friction, and thus the engine operates at lower loads. It must be noted that, for both cycles, the PM emissions are almost zero due to high DPF efficiency.1,2,4
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Comparing the two hot cycles, a significant increase on HC and CO emissions is observed in the case of the regeneration cycle due to delayed injection. NOx emissions are somewhat lower. Even if HC emissions increase, the NOx + HC emissions are within the regulatory limits. CO emissions are higher than Euro4 limits; however, this does not constitute a major problem because excess CO can be easily oxidized, increasing the volume of the oxidation catalyst. This bigger oxidation catalyst could also treat the increased HC emissions. Another solution would be the use of a catalytic DPF, which will oxidize a part of the increased CO and HC emissions. Figure 1 shows that the major part of the HC and CO emissions increase of the regeneration cycle comes from its extra-urban part (EUDC), where regeneration takes place. No PM emissions are observed during the filtration cycle; however, the regeneration cycle produces 0.015 g/km of particulate matter. This will be analyzed in the following sections. Particle Size Distribution on NEDC. Figures 2 and 3 show the emissions of nanoparticles of 30 and 90 nm during the urban part of the NEDC (ECE) and the extra-urban one (EUDC) for the two types of NEDC used (cold with filtration, hot with regeneration). At the ECE part of the cycle, there is no significant difference between the cold cycle with filtration and the hot cycle with regeneration. In the case of the hot cycle with regeneration, the number of 90 nm nanoparticles is slightly lower than the number of the 30 nm ones. The number of nanoparticles is very close to the number of tunnel backgrounds,13 and for this reason, there is practically no particulate mass found on the filters. However, Figure 3 shows that, at the extra-urban part of the cycle, where regeneration occurs, the number of particles of the hot cycle with regeneration is higher than the number of particles of the cold cycle with filtration.2,8 This increase occurs just at the moment of the delayed injection. For this reason, the regeneration cycle emits 0.015 g/km of PM (which is lower than Euro4 limits). The emission of 30 nm particles is higher than the emission of the 90 nm ones, indicating that the emitted particles are mostly nucleation particles created from the condensation of HC or other volatiles, and are not composed from solid carbonaceous matter. This figure shows that HC and CO emissions also increase during delayed injection. Particle Size Distribution on the Steady Speed of 100 km/ h. Figure 4 shows the total number of nanoparticles emitted during the different stages at a steady speed of 100 km/h. Each bar is obtained after an interval of 3 min from the previous one. The total number increases after the beginning of the delayed injection (bar 2)8 and reaches a maximum value of 3.5 × 1012 L/km at the middle of the delayed injection period (bar 3). This number is more than 10 times less than the number of the particles emitted from a diesel engine without DPF.1,2,4 After this point, the total number decreases to 2.5 × 1012 L/km at the end of delayed injection (bar 4). However, some minutes after the end of the delayed injection, the total number increases again, to reach a value of 3.7 × 1012 L/km (bar 6) and then decreases to the value of tunnel backgrounds. As the corresponding median diameter is low (Figure 4), this increase must be due to volatile condensation rather than that of solid matter. The median diameter of nanoparticles emitted during the different stages at a steady speed of 100 km/h is also shown in Figure 4. Just before the beginning of delayed injection (bar 1), the median diameter is only 14 nm, indicating that the (13) Zervas, E.; Dorlhe`ne, P.; Forti, L.; Perrin, C.; Momique, J. C.; Monier, R.; Ing, H.; Lopez, B. Aerosol Sci. Technol. 2005, 39, 333-346.
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Figure 6. Emission of polycyclic aromatic compounds, during filtration and regeneration, and comparison with the same emissions upstream of DPF.
emitted particles are formed by a nucleation process caused by the condensation of volatile compounds. After the beginning of delayed injection (bar 2), the median diameter increases, to reach a value of about 40 nm at the end of delayed injection (bar 4). This value is lower than typical values of the median diameter of nanoparticles emitted from a diesel engine without DPF.1,4 After this point, the median diameter decreases regularly, and some minutes after the end of the delayed injection, the distribution curve is similar to the curve of tunnel backgrounds. The particle size distribution of the different stages of regeneration is shown in Figure 5. It is clearly shown that the distribution just before the beginning of delayed injection (curve 1) is close to tunnel backgrounds, with a small peak at the nucleation particles. This peak increases when delayed injection begins (curve 2), while the rest of the curve remains as it previously was. The form of this curve indicates that these particles are mainly formed from HC condensation due to increased HC emissions caused by delayed injection, while the accumulated soot does not begin burning. As regeneration continues (curves 3 and 4), the curve is shifted to bigger particles, which must correspond to emission of sulfates due to the increased temperature of exhaust gas. The total number of particles emitted at this point is higher than the number corresponding to filtration (curve 1); however, it remains lower than the corresponding number of a diesel passenger car without DPF.1,4 After the end of delayed injection (curve 4), the curves progressively approach the form of curve 1.
Chemical Analysis of Particulate Matter Emitted on the NEDC. No cerium is detected on the exhaust gas, confirming that it is effectively collected on the DPF. Due to low mass of the particles collected on the filter, the SOF and PAH analysis is performed only on the entire NEDC. The chemical analysis of the filters shows that the SOF is 0.007 g/km in the case of the regeneration cycle compared to 0.0001 g/km of the filtration one. The SOF fraction of PM is higher in the case of delayed injection (about 50%) compared to the filtration cycle (almost 0%) and upstream DPF (which is 10%, while ACEA2 reports a SOF fraction of 10-15% using Euro3 passenger cars without DPF). In the case of the regeneration cycle, the totality of the SOF is composed of hydrocarbons. A chemical analysis of these hydrocarbons shows that 77% of these hydrocarbons come from fuel and the remaining 23% from engine oil. The SOF analysis indicates that the higher emission of nanoparticles during regeneration is mainly caused by the condensation of volatiles rather than by the emission of solid matter. The emission of sulfate compounds is very low during filtration as they correspond to only 0.0001 g/km or 3% of the total particulate mass. However, these emissions increase in the case of the regeneration cycle, where sulfates correspond to 0.002 g/km or 12% of the total particulate mass. This increase is linked to the increased fuel consumption and to higher temperatures of DPF during regeneration, which apparently increases the emission of the sulfates collected on the filter.
Nonregulated Pollutants
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Figure 7. SO2 emissions during filtration and regeneration, hot NEDC.
This statement explains the emissions of nanoparticles of a median diameter of 40 nm during the regeneration. Particulate PAH emissions are lower than the emissions reported in previous studies,5,11,14,15 where vehicles of older technology are used, indicating that vehicles of modern technology emit lower emissions. Particulate PAHs emitted during the filtration cycle are very low (0.086 µg/km or 29 ppm of the total particulate mass), almost at the detection limits. Total particulate PAHs during the regeneration cycle (2.03µg/km or 135 ppm of the total particulate mass) are quite similar to the emissions upstream of the DPF (Figure 6), which correspond to the emissions of the same engine without DPF. However, there is a shift toward lighter PAHs, with the quasi-disappearance of heavier PAHs. B(a)pyrene practically does not exist during regeneration. It must be noted that the measurement uncertainties of particulate PAHs is about 40%. The PAH emissions are expected to be lower in the case of a catalytic DPF, as DPF will oxidize a part of them. Emission of Individual HC. Methane corresponds to about 15% of the total HC emitted on NEDC during the filtration cycle; however, this percentage increases to more than 30% during the regeneration cycle. This is explained by the higher temperature of exhaust gas during the regeneration cycle and thus the increased oxidation of higher hydrocarbons on the oxidation catalyst, leading to an increased percentage of methane which is oxidized with more difficulty.16 Naturally, the percentages of the other exhaust HCs change too. Benzene and 1,3butadiene exhaust concentrations are about 4 and 6 times higher in the case of the regeneration cycle, following the trends of total HC. It must be noticed that a bigger oxidation catalyst could oxidize these increased emissions. Emission of SO2. The emission of SO2 increases from 1.1 mg/km at the filtration cycle to 15 mg/km at the regeneration cycle (Figure 7). This increase is due to the higher fuel consumption due to delayed injection and takes place only during delayed injection. However, as regeneration occurs only (14) Polycyclic Aromatic Hydrocarbons in AutomotiVe Exhaust Emissions and Fuel; CONCAWE: Brussels, Belgium, 1998. (15) Marr, L. C.; Kirchstetter, T. W.; Harley, R. A.; Miguel A. H.; Hering, S. V.; Hammond, S. K. EnViron. Sci. Technol. 1999, 33, 3091-3099. (16) Meziere, I.; Castagna, F.; Prigent, M.; Pentenero, A. SAE Tech. Pap. Ser. 1995, 950932.
for a very limited time, this increase is not significant for the entire life of the vehicle. Conclusions The emission of regulated and nonregulated pollutants (nanoparticle number and size distribution, individual HC, cerium, SOF, PAHs, and SO2) emitted during the regeneration of a DPF is studied using a Euro4 passenger car. Measurements were performed on NEDC and at a steady speed of 100 km/h, fifth gear. Regeneration of DPF is achieved using delayed injection. Results show that DPF regeneration increases significantly the HC and CO emissions (however, a bigger oxidation catalyst could oxidize them) and slightly the particulate mass emissions. Postinjection increases nucleation nanoparticles. However, a chemical analysis of SOF shows that these nanoparticles are formed from the condensation of HC or sulfates. No emission of cerium is observed, as it is effectively collected on DFP. Regeneration increases the SOF percentage from 0% in the case of filtration to about 50% in the case of regeneration. The emission of sulfates increases from 3% of the total particulate mass in the case of filtration to 12% in the case of regeneration. Total particulate PAHs are quite similar in the case of the regeneration cycle and a NEDC upstream of the DPF; however, regeneration shifts emissions toward lower PAHs. Compared to filtration and regeneration cycles, total PAH is increased in the case of regeneration. Methane is the major HC, as it corresponds to about 15% of the total HC emitted on NEDC during NEDC with filtration and reaches to more than 30% during the NEDC with regeneration. SO2 emissions increase during regeneration due to increased fuel consumption. Globally, DPF regeneration increases the emissions of some pollutants; however, this increase is not so significant compared to normal engine operation conditions due to the very small duration of DPF regeneration. EF070024S
