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This corresponds to a total PM emission reduction of about 70% and demonstrates the high oxidation potential for other partial load operating points o...
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Ind. Eng. Chem. Res. 2004, 43, 8001-8013

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Improvement on PM Reduction Using a Catalyst Based on V2O5/WO3/TiO2 E. Hums,*.† S. Liebsch,‡ and H. Zellbeck§ Argillon GmbH Redwitz, P.O. Box 60, 96254 Redwitz, Germany, IVA GmbH Chemnitz, Kauffahrtei 45, 09120 Chemnitz, Germany, and Department of Combustion Engines, TU Dresden, George Ba¨ hr Strasse 1c, 01069 Dresden, Germany

Studies on the oxidation of diesel particulates characteristically showed that the particle number concentration for a particle size of about 30 nm can be reduced by more than 90% when the characteristics of the particulates are adapted to the oxidation potential of the catalyst employed. This corresponds to a total PM emission reduction of about 70% and demonstrates the high oxidation potential for other partial load operating points of a light-duty truck EURO 3 engine. It was found that the impact of modified engine adjustments on the particle contribution of smaller particles meets the technical requirements for an optimized reduction using an SCR catalyst based on V2O5/WO3/TiO2. Although the number concentration of particles with a size of 70% NOx reduction when evaluated using the ETC (European transient cycle) and ESC (European steady-state cycle) test runs. These objectives were to be met in a joint program between DAF Trucks, Renault V. I. (Renault Ve´hicules Industriels), Engelhard Corp., and TNO Automotive.2 With experience from demonstration programs in Europe and the U.S., urea SCR technology for on-road applications developed by Siemens AG has proven itself to be a prototype, as reported in ref 3-7. Some of the vehicles employing such systems have reached more than 500,000 km. Europe will be the first market to introduce SCR into both heavy-duty and medium-duty truck applications. As emphasized by Scarnegie et al.,8 the development of * To whom correspondence should be addressed. E-mail: [email protected]. † Argillon GmbH Redwitz. ‡ IVA GmbH Chemnitz. § TU Dresden.

SCR systems for heavy-duty engines was started in early 1992 in the European Union by a consortium of four initial partners, namely, DaimlerChrysler, MAN, Iveco, and Siemens AG. The main focus of the present paper, therefore, is to address the total amount of particulate matter (PM) emitted and the particle size distribution. In addition, the PM analysis of diesel engine exhaust gas is expected to yield more detailed information on the soluble organic fraction (SOF) and the soluble inorganic fraction (SINOF), with a focus on sulfates, nitrates, ammonium, and the insoluble components, which will provide insight into the possible interaction between generated PM of the raw exhaust and the SCR catalyst. Representing the other operating conditions, two selected steady-state modes of the ESC test were investigated with a EURO 3 light-duty truck engine for varying urea doses and fuel sulfur contents. Depending on the engine design and operating conditions, diesel PM particle diameters usually range from 5 nm to 20 µm. Two distinct modes characterize the PM distribution: agglomeration mode and nucleation mode. Most of the particle mass is in agglomeration mode (50 nm to 20 µm). The agglomeration mode mainly consists of carboneous mass and adsorbed volatile compounds with higher molecular weights. These volatile compounds are known as the soluble organic fraction (SOF). Occasionally, the nucleation mode contains the majority of the particles but does not contribute significantly to the total PM mass. Particles in the nucleation mode mostly are composed of volatile organic and sulfur compounds. Within the scope of a program partially financed by the Dutch Ministry of Environment (VROM) involving Deutz AG, Siemens AG, and AVL List GmbH, a focus of interest has been investigating whether both the total

10.1021/ie040007f CCC: $27.50 © 2004 American Chemical Society Published on Web 11/10/2004

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Figure 1. Engine with EGR.

PM emission and the emission of the various components can be trained for even higher performance of the SCR catalyst using a particle-optimized engine. In that context, the test engine provided for this work (Deutz 1015 MV 12-L heavy-duty diesel engine) was modified by AVL to optimize combustion, fuel consumption, PM emission, and flue gas clouding independently of the used catalyst. The preset conditions of the engine setup (NOx, about 10 g/kWh; PM emission, 200 nm) in all operating modes. A slight shift of the maximum to greater number concentration of particles could be observed with the SMPS in the presence of the catalyst (about 66 nm) and urea dosing (about 68 nm) compared to the raw exhaust (about 63 nm). A decrease of both number and mass concentrations was clearly effected by the catalyst, as a reduction of up to 35% could be observed in the SMPS mass distribution. A tendency toward a greater reduction of the smaller particles was established, which was caused by the oxidation of SOF (Figure 4). When urea dosing was applied, an additional reduction of the particle number by approximately 10% was seen in the measurement range of the SMPS compared to catalyst operation without SCR. However, a slight increase in particle emission was recorded with the cascade impactor in the particle size range of 70028000 nm that was also reflected in the total amount

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Figure 12. Impact of the SINOx catalyst on the B25 particle size distribution (SMPS) for the fuel with low sulfur content.

Figure 13. Impact of the SINOx catalyst on the B25 particle size distribution (cascade impactor) for the fuel with low sulfur content.

of PM emission. This result reflects the facts that the SCR reaction and SOF reduction are competitive reactions and the effectiveness of SOF reduction is depressed if the catalyst design is not optimized. 3.1.2.2. Engine Operating Mode B100 (100% Load), Fuel with High Sulfur Content. The maximum of the number concentration for all operating modes lies at approximately 70 nm (Figure 14). An insignificant shift can be seen in detail from 68 nm for the raw exhaust to approximately 70 nm for R ) 0 and to 74 nm for R ) 1. In comparison with B100 at S < 10 ppm, high sulfur content clearly increases the particle number in the presence of the catalyst. Independent of the adjustment (raw exhaust, with catalyst for R ) 0, or with catalyst for R ) 1), a steady increase of the mass concentration to a plateau at approximately a particle size of 300 nm can be seen. In the context of the cascade impactor results, the plateau formation indicates that the model of the unit density, used for calculation of mass concentration based on number concentration, is not applicable over the total particle size range for this operating mode. This phenomenon can be explained only if one assumes that the

SOF loading is particle-size-dependent and that the different loading then has influence on the density of the particles as well. It can be seen by use of the cascade impactor that the emissions decrease strongly with increasing particle size (Figure 15). In comparison with B100 with S < 10 ppm, it is clear that the particle size distribution (PSD) does not depend on the adjustment (raw exhaust or use of the catalyst with R ) 0 or R ) 1). In contrast to the raw exhaust, the increase in particle number and particle mass is most clearly detected in the SMPS measurement range (16-626 nm). This trend also can be observed for the total PM emission for the adjustment R ) 1, but not for the adjustment R ) 0. 3.1.2.3. Engine Operating Mode B100 (100% Load), Fuel with Low Sulfur Content. For the fuel with S < 10 ppm, the maximum of the number concentration is shifted by the catalyst to slightly smaller particles (approximately 64 nm) compared to the raw exhaust (approximately 68 nm) (Figure 16). The SCR reaction, however, shifts the maximum of the distribution to larger particles (approximately 71 nm). A steady increase of the mass concentration to approximately 150 nm, which leads to a plateau above approximately 220 nm, is independent of the adjustment. Here again, the model of unit density is not applicable over the total particle size range. By examining the results from the cascade impactor, it can be seen that the particle size distribution (PSD) over the total size range 700-28000 nm follows a decreasing trend (Figure 17). With all three measuring tools (SMPS, cascade impactor, full-flow dilution tunnel filter loaded in accordance with Figure 9), it was found that the presence of the catalyst favored the increase of particle mass compared to the raw exhaust when high-sulfur fuel and urea dosing are used (Figure 7). 3.2. Test Runs Involving Changes in the Series Characteristics of the Engine Used. 3.2.1. Effects

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Figure 14. Impact of the SINOx catalyst on the B100 particle size distribution (SMPS) for the fuel with high sulfur content.

Figure 15. Impact of the SINOx catalyst on the B100 particle size distribution (cascade impactor) for the fuel with high sulfur content.

on the Particle Size Distribution (PSD) of the Raw Exhaust of Varying the Start of Fuel Delivery. Operating the engine with unchanged series characteristics at a partial load (B25) resulted in the PSD shown in Figure 18. The maximum of the particle number concentration appeared at 80 nm when the start of fuel delivery was adjusted to a crank angle (ca) of 8° before the top death center (TDC). In Figure 18, additional plots of the particle number concentration are presented for cases in which the start of fuel delivery was advanced by increasing the crank angle. With a fuel delivery start of 13°, it was found that the particle number concentration was shifted to a maximum of about 30 nm. This maximum was pushed to about 42 nm when the delivery start was moved to 18°. At this setting, the observed maximum showed a 4-times-higher particle number concentration at a particle size of 42 nm compared to the value for the raw exhaust at 8°. The decrease of larger particles with appropriate mass also became apparent for the total PM emissions when the start of fuel delivery was varied (Figure 19). It is worth mentioning that, generally, the increasing number of

small particles does not seriously affect the total PM emissions. Concerning the low sulfur content of the fuel used, it is difficult to understand whether measured particles are similar to those described by Kittelson.14 Therefore, added importance was placed on the formulation of gaseous soot precursors in the gas phase and the homogeneous nucleation of precursors of ultrafine particles formed in the combustion chamber; this work still in progress.15 The coagulation and coalescence of the ultrafine particles due to surface reactions and particle aggregation into fractal-like objects were of less interest because of the lack of analytical detection. Although most of the fuel is converted to CO2 and H2O by combustion, small portions of the fuel remain as unburned components. These materials either are transported into the homogeneous gas phase if they have no chance to generate particulates or become adsorbed on the particulates as SOF. Therefore, it is expected that the amount of SOF adsorbed on the particulates will reach saturation depending on the attainable particle surface and the SOF level. As a consequence, a decrease of the particle size followed by an increase of the specific surface area, provides an excellent potential for the adsorption of SOF. This was clearly evidenced by the determination of the SOF amounts for particle sizes of 700, 1400, and 2800 nm using the cascade impactor. The results of the PM analysis are shown in Figure 20. These findings are not at variance with the total PM emissions measured as a function of particle size, as shown in Figure 21. Unlike operating mode B25, operating mode B100 showed a different effect of varying the start of fuel delivery on the PSD. The impacts caused by advancing the start of fuel delivery from 12.8° to 19° for both the raw exhaust and the exhaust in the presence of the catalyst are shown in Figure 22. In this figure, it is notable that, on one hand, the advanced start of fuel delivery decreased the particle number concentration, but on the other hand, the maxima were not shifted

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Figure 16. Impact of the SINOx catalyst on the B100 particle size distribution (SMPS) for the fuel with low sulfur content.

Figure 17. Impact of the SINOx catalyst on the B100 particle size distribution (cascade impactor) for the fuel with low sulfur content. Figure 19. Total PM emissions in the raw exhaust upon variation of the start of fuel delivery for the B25 operating point and use of catalyst.

Figure 18. Particle size distributions in the raw exhaust upon variation of the start of fuel delivery for the B25 operating point and use of catalyst.

toward smaller particles as obtained for the operating mode B25. 3.2.2. Effects on the Particle Size Distribution (PSD) of the Raw Exhaust of Varying the Start of

Fuel Delivery in the Presence of the SCR Catalyst. The impact of the catalyst on the total PM emission is shown in Figure 19. Whereas the particle number concentration in the range of 40-100 nm is reduced by only about 65% at a crank angle (ca) of 8°, it is reduced by about 90% in the total SMPS measurement range when the start of fuel delivery is adjusted to a crank angle of 18° (Figure 23). Because no information was available on the behavior of SOF adsorption in that particle size range, SOF adsorption was studied in the particle size range of the cascade impactor. By extrapolation, a rough idea was obtained correlating the amount of adsorbed SOF to the particle surface area of the corresponding particle size. As shown before, in the range of particle sizes between 700 and 2800 nm, it could be demonstrated that smaller particles indeed are able to adsorb much more SOF than the larger ones (Figure 23). Obviously, particles with a large fraction of soluble organic components can be easily oxidized by the catalyst employed.

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Figure 20. SOF as a function of particle size, determined using the cascade impactor for the B25 operating point.

Figure 21. Total PM emissions as a function of particle size, determined using the cascade impactor for the B25 operating point.

Figure 22. Particle size distribution in the raw exhaust upon variation of the start of fuel delivery for the B100 operating point and use of catalyst.

On the other hand, the amount of SOF was not be the only distinguishing characteristic for particles of operating mode B100 compared in terms of adsorbed SOF after they had passed the catalyst. For all measured particle sizes, they showed nearly the same amount of adsorbed SOF. In contrast to operating conditions B25, the variation of start of fuel delivery at crank angles between 12.8° and 19° did not shift the

Figure 23. Particle oxidation by the catalyst relative to that of the raw exhaust with the basic particle size distribution.

maximum of the particle number concentration (Figure 22). Although sufficient oxygen was still present in the exhaust, no influence on the reduction of the particle number concentration or the total PM emission was observed when the catalyst was used. Moreover, it is obvious that the shift of the particle size distribution (PSD) toward smaller particles was not fulfilled at operating conditions B100 to meet the requirements for an optimized reduction using the SCR catalyst based on V2O5/WO3/TiO2. Varying the start of fuel delivery showed that the particle number concentration increased in the presence of the SCR catalyst. It is an open question whether additional injection of fuel or oxygen before exhaust gas is passing the catalyst might show an influence in reducing particulate emission for critical operating points such as B100. 3.2.3. Effects of EGR on the Particle Size Distribution (PSD) of the Raw Exhaust of Varying the Start of Fuel Delivery. Although it is generally reported that EGR results in an increase of particulates (soot promotion) and smoke, a number of studies have shown that EGR also can provide for opposite effects, i.e., suppression of particulate generation. At delayed injection timing, the application of EGR usually decreased NOx but resulted in an increase of particulate emission. This particulate increase is mostly caused by an increase in the dry soot component. Using a constantvolume sampling (CVS) minidilution tunnel system, Machacon et al.16 showed that, at higher loads in the range of 20% of the exhaust gas recirculating rate, a slight decrease of particulate emissions caused by a decrease of SOF can be obtained. They pointed out that, at medium loadings, EGR showed less or no impact on the particulate, dry soot, and SOF emissions. On the other hand, at low loadings with EGR up to 30% exhaust gas recirculation rate, they found that the dry soot component did not change whereas the SOF decreased. They stated that exhaust gas recirculation showed an impact on suppressing particulate emission at lower loads with advanced injection timing. In summary, lengthening of the ignition delay with EGR means that the effect of CO2 and N2 dilution predominates over the effect of the charge temperature increase. Thus, overall, this indicates that most of the emission results of EGR might be caused by its dilution effects and not by the slight temperature decrease in the flame front. A rough plot of the impact of the EGR ratio on the particle size distribution (PSD) at partial loading (B25)

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Figure 24. Impact of the EGR ratio on the particle size distribution for a fuel delivery start at an 18° ca before the TDC, for the B25 operating point without catalyst.

Figure 25. Particle size distribution in the raw exhaust as a function of fuel delivery start variation, for the B25 operating point with 22% EGR and with catalyst.

is shown in Figure 24, when the delivery was started at a crank angle of 18° before the TDC. Quite impressively, the maximum of the particle size concentration was shifted from high to low exhaust recirculation rate by an amount equivalent to the shift resulting from advanced delivery start (Figure 18). With a constant EGR ratio of 22%, a variation in the start of delivery shows, by SMPS, a constant size distribution maximum for 8° and 18° at a particle size of about 90 nm (Figure 25). Advanced injection timing at 21° and 23° results in a clear shift to smaller particles with maxima in the range of 40-50 nm for the raw exhaust. Furthermore, the particle size concentration increased significantly for crank angles of 21-23° compared to the former adjustment. 3.2.4. Effects of EGR on the Particle Size Distribution (PSD) of the Raw Exhaust of in the Presence of the SCR Catalyst. Plots of the particle number concentration for the case in which exhaust has passed the catalyst are summarized in Figure 25. It can be seen that the catalyst used resulted in a decrease of the particle number concentration. This effect was much more developed for particles with a size of about 40 nm. Small particles without and with EGR seem to meet the same requirements for optimized catalytic oxidation. 4. Summary and Conclusions The present work studies the impact of the urea SCR system on the particle emission of a light-duty diesel engine (EURO 3) in operating modes B25 (partial load) and B100 (full load) of the ESC test.

Test runs with fuel of high sulfur content (S ) 320 ppm) caused an increase of the total PM emission of the raw exhaust for partial-load operation, whereas for fullload operation (B100), lower sulfur content (S < 10 ppm) did not significantly affect the reduction of the total PM emission. In contrast to the case of the partial-flow dilution tunnel, the lower amount of soluble organic components (SOF) measured in the full-flow dilution tunnel can be explained by the longer residence times of the exhaust gas at higher temperatures in the fullflow dilution tunnel. For fuel of either quality, at a partial load, the total PM emission was reduced by catalyst compared to that of the raw exhaust. A slight increase of the total PM emission could be detected with increasing urea dosage. Nevertheless, the emission clearly remained below that of the raw exhaust. The catalyst caused a clear reduction of SOF that partly led to the formation of insoluble components (carbonization). Whereas a significant increase of total PM emission can be detected for full-load operation with increasing urea dosage when sulfur-rich fuel (S ) 320 ppm) is used, the total PM emission increases only marginally when sulfur-poor fuel (S < 10 ppm) is used. This effect can be seen primarily with the partial-flow dilution tunnel because the residual time is not long enough to oxidize or desorb SOF. Because of the SCR reaction, an uneven increase of the total PM emission can be seen independently of the operating point and sulfur content in the fuel, increasing with the dosing of the reducing agent. The main reason for this result is the suppressed abatement of SOF, which could be offset by additional catalyst volume. No generation of smaller particulates was found when the SCR system was used. The characteristics of the particulate size distribution of the raw exhaust were unchanged for the exhaust exposed to the catalyst. Moreover, a shift of the particle sizes to larger rather than smaller particles in the presence of the catalyst could be observed by SMPS for both sulfur contents. Studies of the dependence of the catalytic oxidation of diesel particulate on the particle size distribution clearly showed that the particle number concentration for a particle size of about 30 nm could be reduced by >90% when the characteristics of particles were adjusted to the oxidation potential of the catalyst employed. This corresponds to a total PM emission reduction of about 70% and also demonstrates the high oxidation potential of the B25 operating point (ESC) of a light-duty EURO 3 engine. Shifting the particle size distribution (PSD) toward smaller particles meets the requirements for an optimized reduction using an SCR catalyst based on V2O5/WO3/TiO2. Although the particle number concentration at particle sizes of