ARTICLE pubs.acs.org/IECR
Characterization of Particulate Matter Emissions from a Common-Rail Diesel Engine D. Fino and N. Russo* Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy ABSTRACT: Particulate matter (PM) emissions from a common-rail automotive diesel engine were characterized as a function of the engine operating regime in terms of mass, particulate number, size distributions, and composition (soot and volatile fraction), as well as by means of the optical measurements currently used for engine calibration. Exhaust gas samples were taken both upstream and downstream of the diesel oxidation catalyst (DOC), so to evaluate its effect on PM modification. Moreover, the effects of the engine calibration were also investigated at two different exhaust gas recirculation (EGR) rates. Fundamental information on the PM characteristics, which is essential for the knowledge-based design of a new generation of diesel particulate filters (DPFs), was gathered. The loading of a DPF entails the need of trap regeneration by particulate combustion, whose efficiency and frequency are somehow affected by the way soot is deposited along the channels. Small lab-scale 300 cpsi DPF samples were loaded downstream the DOC in an ad hoc designed reactor capable of hosting five samples with part of the entire flow produced by an automotive diesel engine at the 2000 5 μBEP operating condition. Soot layer thickness was estimated by means of FESEM observations after sample sectioning at progressive locations, obtained through a procedure defined not to affect the distribution of the soot inside the filter and to enable estimation of the actual soot thickness along the channel length.
1. INTRODUCTION Particulate matter (PM) is the only pollutant regulated by the United States Environmental Protection Agency (US EPA) that is not chemically well-defined. Diesel combustion is widely used in both stationary and mobile applications, especially where high power output is needed.1 The new PM emission limits are below what can be achieved solely by engine design and, therefore, require exhaust aftertreatment. The preferred approach to control the emissions of diesel engines is the adoption of an exhaust gas recirculation (EGR) system followed by a diesel oxidation catalyst (DOC) in front of a diesel particulate filter (DPF). EGR systems were introduced in the early 1970s to reduce an exhaust emission that was not being cleaned by the other smog controls. EGR systems work by means of an EGR valve that recirculates exhaust into the intake stream. Exhaust gases have already combusted, so they do not burn again when they are recirculated. These gases displace some of the normal intake charge. This chemically slow and cools the combustion process by several hundred degrees, thus reducing NOx formation. A diesel oxidation catalyst (DOC) is a modern catalytic converter that consists of a monolith honeycomb substrate coated with platinum group metal catalyst, packaged in a stainless steel container. The honeycomb structure with many small parallel channels presents a high catalytic contact area to exhaust gases. As the hot gases contact the catalyst, several exhaust pollutants are converted into harmless substances: carbon dioxide and water. The diesel oxidation catalyst is designed to oxidize carbon monoxide, gas phase hydrocarbons, and the organic fraction (SOF) fraction of diesel particulate matter to CO2 and H2O. Diesel exhaust contains sufficient amounts of oxygen necessary for the above reactions. The concentration of O2 in the exhaust gases from a diesel engine varies between 3 and 17%, depending on the engine load. The catalyst activity increases with temperature. A minimum exhaust r 2011 American Chemical Society
temperature of about 200 °C is necessary for the catalyst to “light off”. At elevated temperatures, conversions depend on the catalyst size and design and can be higher than 90%. Conversion of diesel particulate matter is an important function of the modern diesel oxidation catalyst. The catalyst exhibits a very high activity in the oxidation of the SOF of diesel particulates. Conversion of SOF may reach and exceed 80%. The diesel oxidation catalyst, depending on its formulation, may also exhibit some limited activity toward the reduction of nitrogen oxides in diesel exhaust. NOx conversions of 10-20% are usually observed. The NOx conversion exhibits a maximum at medium temperatures of about 300 °C. Although diesel filter applications date back to 1980s, the diesel engine was subjected to revolutionary design advances during the 1990s (HSDI, common rail, etc.). This also changed the characteristics of diesel particulate emissions.2 An extensive research in the specialized publications and patent literature shows a lack in reliable estimation of specific particulate properties (morphology, density, permeability, etc.). The effects of engine type and operation conditions are important and not yet well investigated and understood. Significant work has been done internationally in filter pressure drop characteristics (due to the importance in the vehicle application) but the results are not fully satisfactory, mainly due to unrealistic assumptions regarding the homogeneity of soot and flow distribution across the different channels of DPF. Therefore, collecting information on the particulate matter (PM) characteristics and soot thickness along the channel are essential for a knowledge-based design of a new Received: October 15, 2010 Accepted: January 10, 2011 Revised: December 20, 2010 Published: February 2, 2011 3004
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Figure 2. Scheme of the sampling procedure for TGA tests.
Figure 1. Scheme of the experimental test rig.
generation of diesel particulate traps in the EURO VI regulation perspective. The aim of this study is to provide fundamental information regarding the PM composition as well as the different amount and number of particles emitted at different engine conditions and to correlate these information with the DPF filtration behavior in terms of soot layer thickness, compactness, morphology and homogeneity deposition along the filter. This knowledge should be useful to better address the development of new DPF substrates as well as to better design the catalyst formulation and microstructure inside the filter.
2. EXPERIMENTAL SECTION 2.1. Experimental Setup. Experimental tests were carried out at the ICE Advanced Laboratory of Politecnico di Torino on the test rig shown in Figure 1, where the turbocharged Common Rail DI Diesel engine (four cylinders in-line, total displacement 2.0 L) is connected to a Borghi&Saveri FE260 eddy-current brake dynamometer. An AVL 733S gravimetric fuel meter provided the measurement of the fuel consumption, while the analysis of the raw gaseous emissions (CO, CO2, SO2, NO, NO2, O2, H2O, HCs), sampled both upstream and downstream of the DOC, was carried out by means of a Fisher-Rosemount NGA 2000 gas analyzer, and smoke emissions were measured by means of an AVL 415s smoke meter. During tests, inlet air temperature and humidity were controlled at 20 °C, 50% relative humidity (HR), respectively, while several K-type thermocouples and piezoresistive pressure transducers in the exhaust and intake systems allowed the measurement of the gas temperatures and pressures in the most significant locations (i.e., upstream and downstream of the VGT, DOC, DPF, etc.). To evaluate the exhaust gas recirculation (EGR) rate the O2 concentration was measured upstream of the aftertreatment system and in the engine intake manifold. Tests were performed at engine speed corresponding to rated torque speed and at 25% of maximum load; this operating point (at relatively low engine load and speed) was chosen as representative of the urban segment of the New European Driving Cycle (NEDC). Moreover, two different EGR ratios were analyzed, 0% and 24% EGR ratio, the latter being the value corresponding to the best trade-off between NOx reduction and soot penalty.
2.2. TPM Measurements. Total particulate matter (TPM) was collected by means of the partial flow dilution system (Control Sistem PSS-20); the system is designed to draw a constant sample from the exhaust flow and to dilute it with a constant flow of conditioned dilution air in order to obtain, for steady-state operating conditions, a constant dilution ratio (DR) within the microtunnel (DR = 10). The particulate sampling system probes were installed upstream and downstream of the DOC (replacing the DPF with a dummy component). Tissuquartz fiber filters (70 mm, Pallflex) were used for TPM measurements; prior to each experiment, the filters were preconditioned at 20 ( 1 °C and 50 ( 5% RH for a period of 24 h. A Mettler Toledo microbalance, with full scale of 2.1 g and an accuracy of 0.1 μg, was used to determine filter weights. Contemporary TPM measurements were carried out by means of optical measurements (AVL 415s smoke meter).3 2.3. TGA analyses. A TA Mettler Toledo Instruments STDA 851 was used to analyze the PM composition. Because of the small size of the TGA pans, only a portion of the filter can be analyzed each time. This circular portion, having a radius of 3.5 mm (see Figure 2), is first cut with scissors, then folded with tweezers, and finally placed in the pan. Direct contact with the particulate matter was avoided throughout this procedure. The sample is heated in the TGA according to a heating program where temperature, time, sample weight, and other variables were continuously recorded during the whole program. The first step in the program was an isothermal step for 30 min at 25 °C in argon atmosphere. When a new test has been started, the TGA automatically captures the pan and the samples with its hook wire, causing an undesirable swing movement. The heating ramp should not be started before the end of this movement because in that case the weight data recorded would not be accurate. After 30 min it was considered that this movement was completely vanished and the heating ramp (5 °C/min) was then started, increasing the temperature up to 600 °C, followed by an isothermal stage for 90 min. This temperature was selected because it is high enough to ensure the volatilization of the organic compounds.4,5 The percentage mass loss was determined as the volatile fraction (VF). This fraction includes water and volatile organic compounds. Then the atmosphere was changed into air. This final condition (600 °C in air) was held for 60 min to make sure that all the soot has been oxidized. 2.4. Particle Size and Number Measurements. The distribution of the ultrafine particles was measured both upstream and downstream of DOC via a TSI Scanning Mobility Particle Sizer (SMPS 3080) equipped with two differential mobility analyzers (Long DMA 3085 and Nano DMA 3081) combined with a ultrafine condensation particle counter (UCPC 3025A) 3005
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Figure 3. Scheme of the reactor hosting five small DPF samples (DPF sampler).
operating at room temperature with a 10 times DR.6 This analyzer can measure a wide range of diameters (3-1000 nm). The SMPS capability of extracting from a heterogeneous stream a narrow range of particles was extremely useful to study the phenomena in depth. 2.5. Soot Layer Thickness Evaluation. A sampling device (see Figure 3, which will be from now on referred to as DPF sampler) was specifically designed in order to load small lab-size DPF samples diverting part of the exhaust flow produced by the engine; the sampling probe of the device was installed between the DOC and the DPF, so that the sampled gases passing through the DPF lab-size samples have the same characteristics of those crossing the full-scale DPF. The DPF sampler was designed in order to allow the simultaneous loading of five DPF lab-size samples (four in the radial and one in the central zone): the hosting module is contained inside an oven at constant temperature of 250 °C; the oven temperature, as well as the temperature of the sample line, is automatically controlled by a PID controller.7 The sampled gas is then forced to cross a heat exchanger and a condensate separator in order to prevent damages to the sampling pump. To ensure an exhaust gas velocity passing through the DPF sampler equal to that of passing across the full scale DPF (isokinetic sampling), the flow crossing through the DPF sampler was automatically controlled by varying the pump speed so to maintain a pressure drop across the lab-size samples equal to the pressure drop across the full scale DPF. This is a key condition that will allow the same soot distribution within the DPF samples and the full scale DPF. Before each test, each lab-size sample (SiC 300 cpsi wall-flow filters, length = 177.8 mm; diameter = 25.4 mm, wall thickness = 0.25 mm, channel width = 1.2 mm, volume percent porosity = 43 ( 3%, mean pore size = 11þ/22 μm) was radially partially cut (at different locations along its length in order to predivide the entire sample in eight subcomponents so as to create a breaking point onto the reinforced surface of DPF by preserving the inside channel organized structure; after the loading test the eight portions are separated and analyzed through a field emission scanning electron microscope (FESEM-Leo 50/50VP with Gemini column), so that the soot deposition in the same channel at eight different axial locations can be observed.
The soot loading of the DPF samples was performed at the following steady-state engine operating condition (2000 rpm, 5 bar BMEP) at the two different EGR rates (0 and 24%). Labsize 300 cpsi DPF samples were loaded with the following soot loading values: 4 and 8 g/L downstream of the diesel oxidation catalyst (DOC) in the ad hoc designed reactor. After the loading phase every small subcomponent was analyzed through a FESEM aiming at the evaluation of the soot penetration into the DPF porous wall and the deposition characteristics, in terms of thickness and roughness. The FESEM observations were carried out following the same channel along the eight subcomponents, and for each channel 10 FESEM pictures with about the same magnification level (1000) were taken along the same side of the channel so as to follow the soot layer accumulation behavior of the same wall of a single channel. The 10 FESEM micrographs were then analyzed by means of a specifically designed software tool for image acquisition and analysis so to evaluate the average soot layer thickness for each subcomponents. To verify an equal soot distribution within the DPF samples and the full scale DPF, the same testing procedure was adopted. The full-scale DPF 8 g/L loaded was cut with a diamond blade in appropriate portions and analyzed via FESEM microscope.
3. RESULTS AND DISCUSSION Figure 4 shows the TPM results obtained both via gravimetric analysis and via optical measurements at 0 and 24% EGR rates. If engine-out PM emissions (i.e., samples taken upstream of DOC) are considered, a remarkable increase in PM emissions measured by filter weights can be observed (from 1.1 g/h @ 0% EGR to 1.83 g/h @ 24% EGR), although the optically estimated PM (which is commonly used during engine calibration activities at test rig) shows remarkably higher increases (from 0.68 g/h @ 0% EGR to 1.67 g/h @ 24% EGR). Besides, optically estimated soot emissions rise almost 4 times (from 0.24 g/h to 1.18 g/h @ 24% EGR). If the number and size of particles are considered, as shown in Figure 5, a considerable increase (about 6 times) in particle number can be observed, with a growth of particles diameter mode from 46 nm @ (0% EGR) to 76 nm (24% EGR). By a 3006
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Figure 4. Comparison between TPM measurements obtained via gravimetric analysis and via optical estimation.
Figure 5. Comparison between number size distribution measured upstream of the DOC for two different EGR rates.
comparison of these results with those obtained through the smoke meter measurement, it can be observed that the increase in soot emissions is well related to the increase in particles number emitted measured by means of SMPS, while the increase in PM optical estimated is mostly related to the gravimetrically measured TPM increase. If data sampled downstream of DOC are considered, similar remarks can be made. Furthermore, by a comparison of samples taken at same EGR rates upstream and downstream of DOC, it can be observed that both weight and optical measurements show modest variations (see Figure 4), as confirmed also by SMPS measurements (see Figure 6), thus highlighting a modest overall effect on PM reduction by the DOC at the considered low load/low speed operating condition, with a temperature of the exhaust ranging between 200 and 250 °C.
Figure 7 shows the thermogravimetric experimental results regarding the PM composition for two different EGR conditions (0% and 24%) for PM samples collected in both cases upstream and downstream of a diesel oxidation catalyst (DOC). In the same figure the percentage values concerning the carbonaceous fraction are also reported. The volatile fraction (VF) of the sample produced by the engine working with 0% EGR is 87%. This VF fraction value decreases, as expected after the previous results, down to 80% when the EGR value is fixed at 24%. Shifting to the downstream, results come into view of the oxidative affect of the DOC. In fact, the VF of the PM produced with no EGR crossing through the DOC is reduced from 87% to 70%, whereas the decrease of the VF of the PM produced with an higher EGR value is less evident, from 80% to 75%. This difference could be probably due to the different VF chemical composition out 3007
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Figure 6. Comparison between number size distributions measured upstream and downstream of the DOC; EGR rate = 24%.
Figure 7. TGA curves of soot sampled at the engine operating condition 2000 5 μBEP with two different EGR rates, upstream and downstream of the DOC.
(hydrocarbons easier to oxidize) coming from the EGR effect.8 The effect of the DOC on the VF fraction is very low, 20% and 7%, respectively, confirming the experimental data obtained by weight and optical measurements. This reduced DOC performance is probably due to the very low exhaust gas temperatures typical of the engine condition analyzed (2000 rpm, 5 bar). The observed discrepancy among the results of the different methodologies (weight, opacity, number and size distribution, thermogravimetric analyses) adopted lies most likely in the different physical working principals. However, the same trend is observed with all measurement techniques applied. Shifting to the evaluation of the soot layer thickness, lab-size 300 cpsi DPF samples loaded with both the highest soot loading and EGR values (8 g/L and 24%, respectively) showed the shortest loading phase time compared to both the lowest soot loading and EGR values (4 g/L and 0%), which showed the longest loading phase time. A maximum deviation of (4% was perceived
on all the lab-scale samples analyzed. Figure 8 shows two micrographs of a section of a subcomponent of one of the five samples loaded with 4 g/L of soot at the engine operating condition 2000 5 μBEP at two different magnification levels: (a) 80 and (b) 1000. When observing the image at higher magnification level a very compact soot layer with a thickness ranging between 15.00 and 20.12 μm can be noticed. Figure 9 depicts two similar pictures of a DPF sample loaded with 8 g/L of soot at the same engine operating condition acquired again at two different magnification levels: (a) 80 and (b) 1000. The morphology of the soot deposit appears similar to that of the previous sample. In this case the soot layer thickness ranges between 23.81 and 25.40 μm. It is worthwhile to underline that in both cases and in all the observation carried out and not reported for the sake of briefness, the diesel soot particles did not penetrate inside the porosity of the DPF wall. This is quite surprising because it is well-known that wall-flow filters act by deep 3008
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Figure 8. FESEM view of the section of the DPF loaded at the engine operating condition 2000 5 μBEP; 4 g/L: (A) magnification 80; (B) magnification 1000.
Figure 9. FESEM view of the section of the DPF loaded at the engine operating condition 2000 5 μBEP; 8 g/L: (A) magnification 80; (B) magnification 1000.
bed filtration at the beginning of the soot loading process.9 Conversely the formation of a soot cake acting as filter seems to take place almost immediately. The soot layer thickness was measured by means of a specific FESEM tool (tool capable to measure the distance between two ad hoc points selected by the operator). All 10 pictures taken for each subcomponent were then elaborated in order to obtain the average values ((4% of maximum deviation) reported in Figure 10 so as to follow the soot layer thickness profile along the DPF channel for both soot loading values. The two soot layer thickness profiles are almost the same (except from a central point in the profile at 4 g/L probably due to to a defect in the wall-flow cell structure of one of the samples analyzed) with a thickness difference between the two different soot loading values ranging between about 1 and 9 μm. In both cases the soot layer thickness decreased along the filter from the inlet to the center region and started to increase at the end of the channel.9 This is related to the presence of concentrated pressure losses at the entrance and the end of the channel resulting in a higher localized flow through the channel wall. This is in line with previous investigations by G. A. Stratakis,10 and this nonhomogeneous pattern should be taken into account when designing the trap.
Figure 10. Soot layer thickness profile along the DPF channel.
The pressure drop of the 8 g/L is much less than 2-fold higher than that of the 4 g/L. This is a sign that soot layer is compressible,11 that is, the soot layer gets denser as long as the filtration process goes on. This should influence the contact conditions between soot and a catalyst eventually lined over the channel walls. 3009
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Industrial & Engineering Chemistry Research For the full scale DPF 8 g/L loaded an almost equal profile of the small 8 g/L DPF sample was found ((5% of maximum deviation), which strengthens the potential of the devised method. The gathered data are currently employed to validate a model capable of predicting the exact location of the filtered soot along the channel wall and to optimize the location of a catalyst over the channel walls so as to minimize the pressure drops across the filter and maximize the contact points between catalyst and soot. In a number of previous papers by the group (e.g., refs 12 and 13) the catalyst-to-soot contact conditions have been shown to constitute the controlling step of this peculiar solid-solid catalysis. This could also minimize the detrimental effect of the high EGR values used with the aim of reducing NOx formation.
4. CONCLUSIONS Some fundamental information on the particulate matter (PM) characteristics emitted by an automotive diesel engine was gathered in order to provide a precious tool for the knowledge-based design of a new generation of diesel particulate traps in the EURO VI regulation perspective. PM emissions were characterized in terms of mass (by means of a partial dilution tunnel), in terms of particulate number and size distributions (by means of Scanning mobility particle sizer) and in terms of chemical composition (via thermogravimetric measurements); moreover, optical measurement techniques which are currently used for engine calibration were also employed, sampling exhaust gases both upstream and downstream of the diesel oxidation catalyst (DOC), so to evaluate its effect on soot dimension under steady state engine operating conditions (2000 5 μBEP). Finally, the effects of the engine calibration were also investigated, evaluating the effects of different exhaust gas recirculation (EGR) rates also on the loading of small DPF samples in an ad hoc designed reactor capable of hosting five samples with part of the entire flow produced by an automotive diesel engine. Soot emission trends evaluated by means of optical techniques were found to be in good agreement with particle number evaluated through SMPS, while optically evaluated TPM was found to satisfactorily correlate with gravimetric measurements, thus confirming a good reliability of in situ measurement techniques which are currently used on test rigs for engine calibration also for low emission levels. The results obtained were matched up to the full-scale DPF. A uneven soot distribution was found, and these data are now currently employed for DPF model validation purposes. Great efforts are thus spent to improve the understanding of the filtration process of DPFs, aimed at obtaining a deeper insight into the relationship between engine performance and filter loading so as to take advantage of this insight for DPF design and optimization purposes. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ39-011-0904710. Fax: þ39-011-5644699. E-mail:
[email protected].
’ NOMENCLATURE BMEP = brake mean effective pressure CPSI = cells per square inch DMA = differential mobility analyzers DOC = diesel oxidation catalyst
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DPF = diesel particulate filter DR = dilution ratio EGR = exhaust gas recirculation FESEM = field emission scanning electron microscope HSDI = high speed direct injection ICE = internal combustion engine NEDC = new european driving cycle NOx = nitrogen oxides PID = proportional integral derivative PM = particulate matter RPM = revolutions per minute SMPS = scanning mobility particle sizer SOF = soluble organic fraction TGA = thermal gravimetric analysis TPM = total particulate matter UCPC = ultrafine condensation particle counter US EPA = united states environmental protection agency VF = volatile fraction VGT = variable geometry turbocharger VP = variable pressure
’ REFERENCES (1) Weiss, M. A.; Heywood, J. B.; Drake, E.; Schafer, A.; Au Yeung, F. On the road in 2020: A life-cycle analysis of new automobile technologies. Energy Laboratory Report #MIT EL-00-003; Massachusetts Institute of Technology: Cambridge, MA, 2000. (2) Maricq, M. Chemical characterization of particulate emissions from diesel engines: A review. J. Aerosol Sci. 2007, 38, 1079. (3) Maricq, M.; Chase, R. E.; Podsiadlik, D. H.; Vogt, R. Vehicle exhaust particle size distributions: A comparison of tailpipe and dilution tunnel measurements. SAE Tech. Pap. 1999, 1999-01–1461. (4) Stratakis, G. A.; Stamatelos, A. M. Thermogravimetric analysis of soot emitted by a modern diesel engine run on catalyst-doped fuel. Combust. Flame 2003, 132, 157. (5) Van Gerpen, J. The effect of biodiesel fuel composition on diesel combustion and emissions. SAE Tech. Paper 1996, 961086. (6) Christian, R.; Knopf, F.; Jaschek, A.; Schindler, W. Eine neue messmethodik der Boschzahl mit erh€ohter empfindlichkeit. MTZ 54 1993, 54, 16–22. (7) Fino, D.; Russo, N.; Millo, F.; Vezza, D. S.; Ferrero, F.; Chianale, A. New tool for experimental analysis of diesel particulate filter loading. Top. Catal 2009, 52, 2083–2087. (8) Al-Qurashi, K.; Boehman, A. L. Impact of exhaust gas recirculation (EGR) on the oxidative reactivity of diesel engine soot. Combust. Flame 2008, 155, 675. (9) Koltsakis, G. C.; Konstantinou, A.; Haralampous, O. A.; Samaras, Z. C. Measurement and intra-layer modeling of soot density and permeability in wall-flow filters. SAE Tech. Pap. 2006, 2006-01–0261. (10) Stratakis, G. A. Ph.D. Thesis, Experimental investigation of catalytic soot oxidation and pressure drop characteristics in wall-flow diesel particulate filters. University of Thessaly, 2004. (11) Sanguedolce, A.; Ranalli, M.; Yamamura, N.; Punke, A. FiatGM powertrain development of exhaust system with catalysed particulate filter for diesel passenger car application, Proceedings of 2004 ATA Congress, Bari, Italy, 2004. (12) Russo, N.; Furfori, S.; Fino, D.; Saracco, G.; Specchia, V. Lanthanum cobaltite catalysts for diesel soot combustion. Appl. Catal. B 2008, 83, 85. (13) Fino, D.; Russo, N.; Badini, C.; Saracco, G.; Specchia, V. Effect of active species mobility on soot-combustion over Cs-V catalysts. AIChE J. 2003, 49, 2173.
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