Onboard Measurements of Nanoparticles from a SCR-Equipped

Article pubs.acs.org/est

Onboard Measurements of Nanoparticles from a SCR-Equipped Marine Diesel Engine Åsa M. Hallquist,*,

†,#

Erik Fridell,

†

Jonathan Westerlund,‡ and Mattias Hallquist‡

†

IVL Swedish Environmental Research Institute Ltd., Box 5302, SE-400 14 Göteborg, Sweden Department of Chemistry and Molecular Biology, Atmospheric Science, University of Gothenburg, SE- 412 96 Göteborg, Sweden

‡

S Supporting Information *

ABSTRACT: In this study nanoparticle emissions have been characterized onboard a ship with focus on number, size, and volatility. Measurements were conducted on one of the ship’s four main 12 600 kW medium−speed diesel engines which use low sulfur marine residual fuel and have a Selective Catalytic Reduction (SCR) system for NOX abatement. The particles were measured after the SCR with an engine exhaust particle sizer spectrometer (EEPS), giving particle number and mass distributions in the size range of 5.6−560 nm. The thermal characteristics of the particles were analyzed using a volatility tandem DMA system (VTDMA). A dilution ratio of 450−520 was used which is similar to the initial real-world dilution. At a stable engine load of 75% of the maximum rated power, and after dilution and cooling of the exhaust gas, there was a bimodal number size distribution, with a major peak at ∼10 nm and a smaller peak at around 30−40 nm. The mass distribution peaked around 20 nm and at 50−60 nm. The emission factor for particle number, EFPN, for an engine load of 75% in the open-sea was found to be 10.4 ± 1.6 × 1016 (kg fuel)−1 and about 50% of the particles by number were found to have a nonvolatile core at 250 °C. Additionally, 20 nm particles consist of ∼40% of nonvolatile material by volume (evaporative temperature 250 °C), while the particles with a particle diameter 10 nm, 85−110% load plume, total; n.v. Dp > 10 nm plume, CPC 3022 (Dp 7 nm 50%) 3076 (Dp 11 nm 50%) plume, Dp > 5 nm plume, distillate fuel- residual fuel, CPC 3025 (Dp 3 nm 50%) test bed, 2-stroke engine, range dependent on load plume, medium-speed diesel engine plume, 2-stroke, HFO, Dp > 10 nm plume, average of 735 ships, total; n.v. Dp 5.6−560 nm

10.4 ± 1.6 1.90 ; 0.97 3.43 ± 1.26 1.36 ± 0.24 ; 0.88 ± 0.10 0.4−2 4.65 4 ± 0.4 - 6.2 ± 0.6 0.5−4 1.08 ± 0.68 1.3 ± 0.2 2.46 ± 0.11 ; 1.16 ± 0.19

this study this study Petzold et al.19 Petzold et al.19 Hobbs et al.23 Chen et al.24 Sinha et al.25 Kasper et al.20 Lack et al.27 Murphy et al.26 Jonsson et al.28

a

One standard deviation.

passing the plume, or stationary measurements.28 For the data from the literature presented in Table 3 there is a variation in, e.g. fuel used, type of engine, operating condition, and instruments used for particle characterization. Still, the EFs for number of particles for the listed studies are in the same

passages. For an engine load of 75% the average EFPN was 10.4 ± 1.6 × 1016 (kg fuel)−1 and 2.05 ± 0.27 × 1016 (kWh)−1. In the literature there is not much data on EFPN. Most data are from plume measurements, where the measurements have been conducted onboard a plane19,23−26 or onboard a ship27 while 776

dx.doi.org/10.1021/es302712a | Environ. Sci. Technol. 2013, 47, 773−780

Environmental Science & Technology

Article

Figure 3. Particle number size distributions in the transition between phase 2 and phase 3 (left) and shut down (right) during Passage 3. Time series are in the order red (start time) and black and blue (end time) (DR = 457).

size distribution is characteristic for diesel engine emissions.30 However, the peak mode diameter for the soot/accumulation mode is larger for road diesel engines (∼60 nm).30 In comparison with the literature, Lyyränen et al.31 obtained one number peak at 40−45 nm after having heated the particle sample to 150 °C. In a test rig study of a four-stroke marine diesel engine with heavy fuel oil a bimodal number size distribution with peak modes of 15 and 50 nm was obtained.19 Kasper et al.20 obtained a number peak at 30−40 nm when running a two-stroke marine diesel engine on marine diesel oil, and the mean particle size decreased to around 25 nm when running on heavy fuel oil (HFO). Ship plume measurements (age: 30−500 s) in Jonsson et al.28 generally showed unimodal size distributions with an average geometric mean diameter (GMD) for passenger ships of 38 nm. This suggests that for those conditions (initial real-world dilution + plume processes) there is either enough condensational sinks available suppressing nucleation by adsorption/condensation or that coagulation are taking place leading to a decrease in number but preserved mass. As can be seen in Figure 2a,b, at start-up, maneuvering, and acceleration of the ship (Phases 1 and 2) particle mass showed a positive relationship with the CO concentration, i.e. high mass numbers were obtained at high CO concentrations. When looking at the number size distribution at the point where the mass concentration was peaking larger particles were more dominant, presumably soot particles. In comparison, when the particle number concentration was peaking small particles ∼10 nm were more dominant. At the start-up of the engine there was initially a bimodal size distribution with one peak at around 100 nm and another at around 40 nm. The peak at around 100 nm was decreasing with time, whereas the peak at ∼40 nm was increasing and further another mode at around 10 nm was evolving with time (Figure 3). During the start-up, maneuvering, and acceleration of the ship (Phases 1 and 2) also high NOX concentrations were measured. The decrease in NOX occurred somewhat later than for particles and CO. The observed decrease in NOX concentration is related to the start of injection of urea into the SCR system. There is also a need

order of magnitude. The lower EFPN observed for ship plume data compared to the onboard data may be due to coagulation and higher surface area in real-world dilution, favoring adsorption/condensation over nucleation. Additionally, this study showed that particle number emission ratios (ERPN), expressed as number of particles per ppm CO2, during stable operating conditions in phase 4 can be about a factor of 7−64 higher than during phases 1 and 2 (Figure 2). ERPN peaked in phase 3, being about 40% higher than in phase 4 (Figure 2). The average emission factor at 75% engine load for particle mass (EFPM) in this study was 0.65 ± 0.12 g (kg fuel)−1 and 0.13 ± 0.02 g (kWh)−1 (Table 2). Fridell et al.16 reported an average EFPM of 0.13 g (kg fuel)−1 for PM1 for a ship with 0.49 wt % sulfur (S); however, the load was 41% and for hot exhaust gases. Other available data for HFO in the literature are mostly for higher fuel sulfur content and for a larger particle size range, e.g. Agrawal et al.29 EFPM2.5 1.757 g kWh−1 (load 70% and S 2.05%); Moldanova et al.18 EFTSP 1.03 g kWh−1 (load 84% and S 1.9%); and Murphy et al.26 EFPM2.5 2.62 g kWh−1 (load not stated and S 3.01%). Compared to most ship plume data in Jonsson et al.28 the average EFPM in the present onboard study is lower, and possible reasons for this discrepancy are higher temperature used when sampling onboard, typically 30−40 °C vs 25 °C and higher temperature may suppress nucleation/ condensation of semivolatile material. Additionally, the onboard EFPM were derived for stable operating conditions in the open-sea (75% engine load), whereas the plume measurements were conducted in the harbor area where EFPM can be higher due to maneuvering. As is seen in Figure 2, the emission ratio for particle mass (ERPM) can be up to a factor of 20 higher during phase 1 and about a factor of 6 higher during phase 2 compared to the stable 75% engine load in phase 4. 3.3. Size Distributions (Number and Mass). The number size distributions obtained for an engine load of 75% in the open-sea (Phase 4) after dilution and cooling of the exhaust gas were bimodal with a large peak at around 12 nm (nucleation mode) and a smaller peak at 30−40 nm (soot/ accumulation mode). The mass size distribution was also bimodal, peaking at 20 nm and 50−60 nm. A bimodal number 777



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for a high enough exhaust gas temperature, i.e. engine load, for the SCR system to work efficiently.32 3.4. Effect of Engine Load and Urea Injection. When going from a stable operating condition the number size distribution was shifted toward larger particle modes when maneuvering from higher to lower engine load, and the opposite was true when going from lower to higher engine load, where there was a shift toward smaller particles. Similar results were obtained for all passages. During phase 4 in Passage 2, the engine load was rapidly decreased by about 20%, and this resulted in increased particle mass and number concentrations. This decrease in engine load was followed by an unstable period regarding the engine load (∼55−65%) before returning to stable conditions of 75% about 20 min later (Figure 1). At this time the mass and number concentrations were back to a similar level as before the change in engine load. During Passage 2 the effect of adding urea for the SCR system on the particle emissions was studied, and as is shown in Figure 1 and Table 2 urea had no significant effect on number or mass of particles emitted. It should be noted that the SCR system was not bypassed, and the catalyst was still at a high temperature for the test without urea injection. Thus the catalyst may still be active as an oxidative catalyst to a certain extent. The NOX concentrations increased significantly when no urea was added, as expected. 3.5. Thermal Characteristics. The volatility of the particles was analyzed for particles with an initial diameter, Dpi, in the range 20−100 nm. The thermal characteristics of the particles for the same operating conditions for different passages were, in analogue to the other parameters, very similar (Figure 4, Table 2). As is seen in Figure 4, the volatility

Figure 5. Residual particle number size distribution at evaporative temperatures between 301 and 523 K: 301 K (black line), 343 K (gray line), 394 K (circles unfilled), 403 K (circles filled), 423 K (diamonds unfilled), and 523 K (blue line). DR = 457.

organic compounds.14 The particles peaking around 40 nm were less volatile. These results resemble the observations done by Lyyränen et al.31 as discussed in section 3.2, where only one number peak at 40−45 nm was observed after heating the particle sample to 150 °C. Generally, about 50% of the total particles (by number) contained a nonvolatile core, which is in line with plume measurements,19,28 and about 30% of the total mass of particles evaporated at 250 °C, which also is similar to plume measurement results (24%).28 However, the particle size range during these onboard measurements was 12.6−300 nm which means that some of the smaller particles were not accounted for. According to Rönkkö et al.34 tests on a heavy duty diesel vehicle showed that a nonvolatile core is formed before the dilution process and that these core particles grow because of condensation of semivolatile material (mainly hydrocarbons) during the dilution. This is in agreement with the volatility results in this study. However, in the case with marine diesel the contribution from sulfuric acid condensing onto the particles as well as undergoing nucleation is significant as there is much higher sulfur content in marine fuels than in fuels used in heavy duty vehicles (HDVs) (a factor of about 1000).

4. ATMOSPHERIC IMPLICATIONS This study enabled a detailed analysis of particle emissions from a passenger ship using modern pollution reduction measures such as SCR and low fuel sulfur content. The operating conditions did influence the emissions, and it was demonstrated that significantly higher ERPM were measured before stable conditions in the open-sea (i.e., during phases 1, 2, and 3). The average ERPM for phases 1−3 was a factor of 3 higher than during phase 4. The average ERPN for phases 1−3 was on the other hand lower, a factor of 1.8 compared to stable conditions in the open-sea. However, ERPN peaked at unstable conditions when there were high enough exhaust temperatures during phase 3. This has a potential to be optimized in the future, enabling a reduction in emissions close to shore, i.e. close to where a large number of people may be exposed to the emissions. The NOX emission reduction technique applied (SCR) is depending on urea injection, but this injection was not found to influence the number or mass of particles emitted.

Figure 4. Volume fraction remaining (VFR) of 100 nm (crosses), 65 nm (black circles), 33 nm (gray circles), and 20 nm (diamonds) particles as a function of evaporative temperature. Filled symbols and crosses Passage 3 and unfilled symbols Passage 1.

was increasing with decreasing initial particle diameter; hence more material is evaporating off a 20 nm particle compared to a 100 nm particle. Still, about 40% of the 20 nm particles (by volume) consisted of nonvolatile material at 250 °C. The thermal characteristics of the particles were also analyzed by using the VTDMA system in a thermodenuder mode. In this mode the whole size range was passed through the oven unit. These measurements showed that particles of an initial diameter of about 10 nm evaporated at a temperature between 130 and 150 °C (Figure 5). This corresponds well with sulfuric acid particles, having an evaporative temperature of 139 °C.33 However, these particles may also consist of semivolatile 778



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(2) Corbett, J. J.; Fischbeck, P. Emissions from ships. Science 1997, 278 (5339), 823−824. (3) Eyring, V.; Isaksen, I. S. A.; Berntsen, T.; Collins, W. J.; Corbett, J. J.; Endresen, Ø.; Grainger, R. G.; Moldanova, J.; Schlager, H.; Stevenson, D. S. Transport impacts on atmosphere and climate: Shipping. Atmos. Environ. 2010, 44, 4735−4771. (4) Hjelle, H. M.; Fridell, E. When is short sea shipping environmentally competitive? In Environmental Health - Emerging Issues and Practice; Oosthuizen, J., Ed.; 2012; pp 1−18. (5) Eyring, V.; Kohler, H. W.; van Aardenne, J.; Lauer, A. Emissions from international shipping: 1. The last 50 years. J. Geophys. Res., [Atmos.] 2005, 110, D17. (6) Winnes, H.; Fridell, E. Particle emissions from ships; dependence on fuel type. J. Air Waste Manage. Assoc. 2009, 59 (12), 1391−1398. (7) Lack, D. A.; Corbett, J. J. Black carbon from ships: a review of the effects of ship speed, fuel quality and exhaust gas scrubbing. Atmos. Chem. Phys. 2012, 12, 3985−4000. (8) WHO, Air quality and health, Fact Sheet N313, August 2008. 2008. (9) Delfino, R. J.; Sioutas, C.; Malik, S. Potential role of ultrafine particles in associations between airborne particle mass and cardiovascular health. Environ. Health Perspect. 2005, 113 (8), 934− 946. (10) Heal, M. R.; Kumar, P.; Harrison, R. M. Particles, air quality, policy and health. Chem. Soc. Rev. 2012, 41 (19), 6606−6630. (11) Poschl, U. Atmospheric aerosols: Composition, transformation, climate and health effects. Angew. Chem., Int. Ed. 2005, 44 (46), 7520− 7540. (12) Salo, K.; Hallquist, M.; Jonsson, Å. M.; Saathoff, H.; Naumann, K.-H.; Spindler, C.; Tillmann, R.; Fuchs, H.; Bohn, B.; Rubach, F.; Mentel, T. F.; Müller, L.; Reinnig, M.; Hoffmann, T.; Donahue, N. M. Volatility of secondary organic aerosol during OH radical induced ageing. Atmos. Chem. Phys. 2011, 11, 11055−11067. (13) Rader, D. J.; McMurry, P. H. Application of the tandem differential mobility analyzer to studies of droplet growth or evaporation. J. Aerosol Sci. 1986, 17 (5), 771−787. (14) Jonsson, Å. M.; Hallquist, M.; Saathoff, H. Volatility of secondary organic aerosols from the ozone initiated oxidation of apinene and limonene. J. Aerosol Sci. 2007, 38 (8), 843−852. (15) Cooper, D. A. Exhaust emissions from high speed passenger ferries. Atmos. Environ. 2001, 35 (24), 4189−4200. (16) Fridell, E.; Steen, E.; Peterson, K. Primary particles in ship emissions. Atmos. Environ. 2008, 42 (6), 1160−1168. (17) Agrawal, H.; Welch, W. A.; Miller, J. W.; Cocker, D. R. Emission measurements from a crude oil tanker at sea. Environ. Sci. Technol. 2008, 42 (19), 7098−7103. (18) Moldanova, J.; Fridell, E.; Popovicheva, O.; Demirdjian, B.; Tishkova, V.; Faccinetto, A.; Focsa, C. Characterisation of particulate matter and gaseous emissions from a large ship diesel engine. Atmos. Environ. 2009, 43 (16), 2632−2641. (19) Petzold, A.; Hasselbach, J.; Lauer, P.; Baumann, R.; Franke, K.; Gurk, C.; Schlager, H.; Weingartner, E. Experimental studies on particle emissions from cruising ship, their characteristic properties, transformation and atmospheric lifetime in the marine boundary layer. Atmos. Chem. Phys. 2008, 8 (9), 2387−2403. (20) Kasper, A.; Aufdenblatten, S.; Forss, A.; Mohr, M.; Burtscher, H. Particulate emissions from a low-speed marine diesel engine. Aerosol Sci. Technol. 2007, 41 (1), 24−32. (21) Abdul-Khalek, I.; Kittelson, D. The influence of dilution conditions on diesel exhaust particle size distribution measurements. SAE Tech. Pap. Ser. 1999, 01−1142. (22) Mathis, U.; Ristimäki, J.; Mohr, M.; Keskinen, J.; Ntziachristos, L.; Samaras, Z.; Mikkanen, P. Samling conditions for the measurement of nucleation mode particles in the exhasut of a diesel vehicle. Aerosol Sci. Technol. 2004, 38 (12), 1149−1160. (23) Hobbs, P. V.; Garrett, T. J.; Ferek, R. J.; Strader, S. R.; Hegg, D. A.; Frick, G. M.; Hoppel, W. A.; Gasparovic, R. F.; Russell, L. M.; Johnson, D. W.; O’Dowd, C.; Durkee, P. A.; Nielsen, K. E.; Innis, G.

The use of lower fuel sulfur content should reduce particle emissions, but the ship did still emit considerable amounts of number of particles, comparable to ships running on higher fuel sulfur content (Table 3). In addition, the number fraction of the nonvolatile particles were also in-line with these studies, and a clear connection with the sulfur content could not be established. The significant fraction of the nonvolatile particles is most probably consisting of soot but could also contain ash or metals.35 These particles will persist in the atmosphere and will slowly be removed by coagulation/deposition or by growing, acting as cloud condensation nuclei. The nonvolatile fraction has been attributed with increased health risks, and soot is also known to absorb radiation, thus contributing to global warming. Compared to using road traffic diesel where the fuel sulfur content is much lower, the ship EFs for both particle number and mass are significantly higher. For comparison, the EFPN for a diesel passenger car is 1−5 × 1015 (kg fuel)−1 36 which is a factor of 10−100 less than EFPN from ships. Correspondingly, for HDVs the average EFPN is 2−20 × 1015 (kg fuel)−1 37 when assuming an average fuel consumption of 0.45 l km−1 (www. vtech.se) and a density of 0.815 kg dm−3.38 This demonstrates that there is a potential for improvement regarding particle ship emissions even if the direct link to the actual fuel sulfur content needs to be established further. It should be noted that the investigated ship may not be representative for the global fleet where a majority today are running on fuels with even higher sulfur content and are not equipped with SCR. However, the information will, in addition to providing fundamental understanding, be valuable for assessment on future development and potential means for reducing particle ship emissions both reading mass and number.

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ASSOCIATED CONTENT

S Supporting Information *

A schematic of the experimental setup is presented in Figure S1. Table 1S gives a description on the experimental conditions and instruments used during the analyzed ship passages. Table 2S gives gas concentrations and emission factors for an engine load of 75%. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. # Maiden name: Jonsson.

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ACKNOWLEDGMENTS This work was financed by The Swedish Association of Graduate Engineers, SI, and The Foundation for the Swedish Environmental Research Institute, SIVL. Kjell Pettersson and Henrik Fallgren are gratefully acknowledged for technical support, and we also gratefully acknowledge the crew onboard the measurement ship for their assistance and hospitality.

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REFERENCES

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Onboard Measurements of Nanoparticles from a SCR-Equipped

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