Research Note pubs.acs.org/IECR
Experimental Validation of Particulate Matter (PM) Capture in Open Substrates Jonas Sjöblom,* Henrik Ström, Ananda Subramani Kannan, and Houman Ojagh† Department of Applied Mechanics, Chalmers University of Technology, SE 412 96 Gothenburg, Sweden ABSTRACT: The capture of engine-borne particulate matter (PM) in after-treatment systems is a complex process. Because of the intrinsic heterogenic nature of the PM, the particles undergo transformations that make it very difficult to isolate their motion and deposition in experiments. In a previous study, a model for hydrocarbons (HC) evaporation of the particles during the capture process was proposed to explain experimental results that showed a significant increase in the capture efficiency when compared to those predicted from theoretical models [J. Sjöblom and H. Ström, Ind. Eng. Chem. Res. 2013, 52, 8373]. In this work, inert NaCl particles were fed to an open substrate (cordierite monolith). It was demonstrated that the capture efficiency can be experimentally observed, isolated from other experimental phenomena and uncertainties, if the particles are truly inert. Consequently, the previously proposed model for HC evaporation is a valid starting point for development of comprehensive models for PM motion and transformations.
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INTRODUCTION The emission of particulate matter (PM) poses a severe threat to human health and the environment.1 In most countries, the legislation puts great demand on PM removal, especially for particulate number emissions. For diesel vehicles, this results in the implementation of diesel particulate filters (DPFs) designed as wall-flow filters. In the DPF, every second channel is plugged and the exhaust flow is forced through the permeable channel wall, which enables a very high capture efficiency (CE). However, the high CE is accompanied with a high-pressure drop and together with the need for periodic regeneration, the use of a DPF results in a fuel penalty in the order of 2%−3%.2 In order to enable optimization of DPF design and operation, a detailed understanding of the capture process is necessary. This also includes the processes taking place upstream to the DPF, such as those occurring in the diesel oxidation catalyst (DOC) commonly placed upstream to produce NO2 and to remove hydrocarbons (HC) and CO. The DOC is commonly designed as an open substrate (e.g., a cordierite monolith with square channels) that may have a profound effect on the PM capture in the downstream DPF, as it will change the PM characteristics. These changes include PM capture, creation of sulfate PM as well as transformation due to HC evaporation. To study the capture phenomena in open substrates, an experimental campaign was conducted using a passenger-car diesel engine connected to an exhaust after-treatment system (EATS).3 To reduce the inherent correlation (e.g., space velocity and temperature) in exhaust properties, the EATS is designed to deliver independent variation of flow and temperature, as well as enable the addition of air to the exhaust stream.4 This setup, together with the use of Design of Experiments (DoE), enabled the experimental study of isolated changes in flow parameters that could not be accomplished by changes to the engine operation alone, and thus improved the interpretation of the results. In the experiments, the flow (and thus the channel Reynolds number) was low in order to get a significant CE, despite the open channel structure, since the CE of a monolith substrate is otherwise much lower than that of a DPF, because © 2014 American Chemical Society
of the insufficient diffusive transport of PM toward the wall during the retention time in the channel.5 However, the CE was much higher than predicted from theory (Brownian motion/ diffusion) alone.6 Figure 1 shows an example of such poor agreement between the measured CE of automotive PM and the theoretically predicted CE of inert particles of identical size.
Figure 1. Example of experimental CE (solid lines), compared to CE predicted from theory (dashed lines).3
This deviation between experiments and theory was attributed to HC evaporation from the surface of the PM. Because of the short diffusion distance in the monolith channel and the rapid adsorption of the HC on the channel walls, the channel becomes a strong “HC sink” and thus drives off HCs from the PM. The evaporation process was incorporated into a tanks-in-series model and could be used to explain the experimental findings.3 The tanks-in-series model assumed a bulk concentration of HC equal to zero (i.e., assuming that the diffusion process of HC to the wall was faster than the Received: Revised: Accepted: Published: 3749
November 29, 2013 February 14, 2014 February 14, 2014 February 14, 2014 dx.doi.org/10.1021/ie404046y | Ind. Eng. Chem. Res. 2014, 53, 3749−3752
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Research Note
Figure 2. The Topas ATM230 and the design of the atomizer inside.
Figure 3. Experimental setup (left) and heating tape and insulation to obtain adiabatic conditions (right).
low inertia to reach the plate and instead they follow the air stream. The droplet size distribution will cover a size range of ∼0.1−1 μm. By feeding this droplet aerosol into a heated pipe, the salt-containing droplets will be transformed into NaCl nanoparticles of sizes in the same range as automotive PM (10−200 nm). The NaCl nanoparticles are cubic-shaped crystallites (see, e.g., ref 8) that closely resemble spheres. Note that even though PM from diesel exhaust often has a fractal shape, the measured diameter of a particle in the DMS500 system (Cambustion) is always the diameter of the corresponding sphere of the same aerodynamic drag. Different operating parameters were investigated in order to have a good control of the atomizing process and the resulting particle size distribution (PSD). These parameters include salt concentration, upstream pressure and back pressure for the atomizer as well as heater conditions (temperature and residence time within the heated tube) and sampling dilution conditions. The particle aerosol was then fed into the EATS, together with dilution air, to obtain the desired flow and temperatures matching the conditions in the previous study3 (see Figure 3). The substrate used was an uncoated cordierite substrate (5.66 in. × 6 in., 400 cpsi). In order to eliminate any thermal gradients within the substrate, a heating tape was used to control the radial temperature profile and obtain adiabatic conditions, despite the low flow rates (see Figure 3). Temperature measurements at different radial and axial positions were used in conjunction with CFD to analyze and to verify the adiabatic conditions. The effect of nonadiabatic conditions (as in ref 3) on the PM capture was also investigated and confirmed to be much smaller than that observed, for example, in Figure 1, effectively ruling out thermophoresis as a significant phenomenon in these experiments.9 Also, the effect of varying residence times and diffusivities due to radial
evaporation process). This assumption was verified when the flow field and concentration gradients were resolved in detailed CFD simulations.7 However, when proposing the HC evaporation as the main explanation to the experimental observations, one important assumption had to be made: That when HC evaporation was compensated for, the main capture process is predominantly the Brownian diffusion, and that the CE thus could be predicted using the existing theoretical models together with the new HC evaporation model. In other words, the HC evaporation mechanism was assumed to dominate all other possible particle deposition phenomena (e.g., inertial impaction, thermophoresis, lift forces, and interception mechanisms) not included in the theoretical model.6b The objective of this study is to verify this assumption and prove that the motion and deposition of automotive PM in these experiments on an open substrate can be described by Brownian diffusion alone, and that this mechanism can be decoupled from simultaneous HC evaporation effects in numerical predictions of PM deposition. Thereby, the experimental setup and procedure used in our previous work will also be validated.
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EXPERIMENTAL SECTION Solid nanoparticles were generated by atomizing a salt (NaCl) solution followed by drying the droplets in a heated tube. The atomizer was the Model ATM230 system from Topas, and the atomization principle is illustrated in Figure 2. In the ATM230 system, a venturi nozzle creates a low pressure locally, which will suck the solution into the jet stream. When leaving the nozzle, a spray of droplets is formed at a high velocity. By placing a plate/baffle in front of the spray, most droplets will hit this wall and flow down to the solution reservoir again. However, the smallest droplets will have too 3750
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Research Note
The experimental capture efficiencies of the inert NaCl particles were compared to CFD simulations using the methodology described in the work of Ström et al.6b that has been proposed for inert automotive PM. In this approach, the number-based CE is obtained numerically by relating the massweighted average of a passive tracer signal at the outlet of the computational domain (X̅ i,outlet) to its uniform (arbitrary) value on the inlet (Xi.inlet). The local tracer signal in the monolith channel is solved for via the transport equation:
temperature gradients was confirmed to be minor, compared to the observed deviations. The PSD was measured with a fast particle analyzer (Model DMS500, Cambustion). The DMS500 system has a primary dilutor, whose main function is to lower the concentration to prevent condensation of water inside the instrument. Since this was never an issue, the primary dilution was kept constant to unity. There is also a rotary secondary diluter, which is used to get good signal intensity. The secondary dilution factor was set to 12−25. The capture efficiency was measured for varying temperatures (150−250 °C) and flows (150 − 473 dm3/min), corresponding to different channel Reynolds numbers (Re = 6−24). All trends reported in the current work were confirmed under both adiabatic and nonadiabatic conditions in experiments as well as CFD calculations.
∂Xi + u ·∇Xi = Di∇2 Xi ∂t
where Xi is the passive tracer signal mass fraction, u the gasphase velocity vector (obtained from the Navier−Stokes equations with a continuity equation for incompressible flow), and Di the Brownian diffusivity of the particle size represented by Xi. The boundary condition at the channel walls corresponding to the particles that are captured (if they reach there) is Xi = 0. By choosing an appropriate number of tracer signals, a complete PSD of choice can be evaluated. In this paper, the CE is only displayed at 150 °C and at 150 dm3/min and 473 dm3/min (see Figure 5). Figure 5 clearly shows that the Brownian diffusion, as predicted from the CFD simulations, is the dominating process for determining the CE under these conditions. It is also clear that the experimental setup and measurement technique employed is capable of verifying this. The results obtained in this study verify that a theoretical model for inert PM deposition based on Brownian diffusion is applicable for open substrates under the conditions of our experiments. It should be stressed here that, in a typical lightduty diesel application, the channel Reynolds numbers would be significantly higher than in this work. Therefore, the data obtained here cannot help settle whether there is still a possibility that the inertial impact of PM on the leading edge of the monolith substrate could become a relevant PM deposition mechanism in cases of very high exhaust flow rates and PSDs dominated by very large particles. As demonstrated in our previous study,3 the monolith channels act as strong HC sinks. HC removal has also been demonstrated using (reactive) catalytic strippers, often with emphasis on regulatory measurements (e.g., ref 10). Our prime objective was to investigate the PM nature as it is reaching the after-treatment components and, hence, we did not measure the solid part of the PM using a catalytic stripper. However, a monolithic reactor (even being catalytically inert) can serve as a
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RESULTS AND DISCUSSION The resulting PSD for different salt concentrations are displayed in Figure 4. In this figure, three PSDs are shown
Figure 4. Example of a particle size distribution (PSD) from the ATM230 system, showing different PSD for different salt concentrations.
for varying salt concentrations. The particle number concentrations are seen to differ, but the size range covered remains approximately the same. In this example, the upstream pressure was 4 bar, the backpressure was atmospheric, the heated line was set to 200 °C, and the aerosol stream was fed directly to the system without further dilution. (The sample dilution factor of the secondary diluter was 25, and the primary dilution was set to unity.)
Figure 5. Comparison of CEs obtained from experiments and simulations at 150 °C, 150 dm3/min, Re = 7.8 (left) and 150 °C, 473 dm3/min, Re = 24 (right). (Adiabatic conditions.) 3751
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Research Note
useful tool to assess the HC content in exhaust flows, which is described more in ref 7. By applying this methodology, further validation of the conceptual model (or any analogue model) for HC content can be performed; such work is also ongoing at our department.
(4) Sjöblom, J. Bridging the Gap between Lab Scale and Full Scale Catalysis Experimentation. Top. Catal. 2013, 56, 287. (5) Knoth, J. F.; Drochner, A.; Vogel, H.; Gieshoff, J.; Kogel, M.; Pfeifer, M.; Votsmeier, M. Transport and Reaction in Catalytic WallFlow Filters. Catal. Today 2005, 105, 598. (6) (a) Ström, H.; Andersson, B. Simulations of Trapping of Diesel and Gasoline Particulate Matter in Flow-through Devices. Top. Catal. 2009, 52, 2047. (b) Ström, H.; Sasic, S.; Andersson, B. Design of Automotive Flow-through Catalysts with Optimized Soot Trapping Capability. Chem. Eng. J. 2010, 165, 934. (7) Sjöblom, J.; Ström, H.; Kannan, A. S.; Ojagh, H. Modeling of Particulate Matter Transformations and Capture Efficiency. Can. J. Chem. Eng. 2014, in press. (8) Tumolva, L.; Park, J.-Y.; Kim, J.-s.; Miller, A. L.; Chow, J. C.; Watson, J. G.; Park, K. Morphological and Elemental Classification of Freshly Emitted Soot Particles and Atmospheric Ultrafine Particles Using the TEM/EDS. Aerosol Sci. Technol. 2010, 44, 202. (9) Ström, H.; Sasic, S. The Role of Thermophoresis in Trapping of Diesel and Gasoline Particulate Matter. Catal. Today 2012, 188, 14. (10) Swanson, J.; Kittelson, D. Evaluation of Thermal Denuder and Catalytic Stripper Methods for Solid Particle Measurements. J. Aerosol Sci. 2010, 41, 1113.
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CONCLUSIONS In our previous study,3 large discrepancies between experiments and theory were observed for the capture of automotive particulate matter (PM) in an open substrate. By developing a conceptual model of hydrocarbons (HC) evaporation, a plausible explanation of these discrepancies could be made. This conceptual model is potentially a very useful tool to obtain PM characteristics in situ.7 However, it relies on the assumption that the theoretical models describing Brownian diffusion as the main phenomenon determining the automotive PM motion and deposition in the absence of HC evaporation are reliable. In this study, solid nanoparticles of NaCl were produced to obtain an inert feed of PM into the exhaust gas after-treatment system. It was shown that the Brownian diffusion mechanism indeed is the dominating deposition mechanism under the conditions of this investigation. These results present important experimental evidence that the transformations of automotive PM inside the catalyst substrates need to be taken into account in detailed modeling of the filter performance, and that the previously proposed conceptual model for HC evaporation from automotive PM can serve as a valid starting point for the development of a comprehensive model of this type.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +46 (0) 31 772 1389. E-mail: Jonas.sjoblom@chalmers. se. Present Address †
Department of Chemical and Biological Engineering, Chalmers University of Technology, SE 412 96 Gothenburg, Sweden.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ABBREVIATIONS CE = capture efficiency CFD = computational fluid dynamics EATS = exhaust gas after-treatment system HC = hydrocarbons PM = particulate matter PN = particulate number PSD = particle size distribution
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REFERENCES
(1) European Environment Agency (EEA). The Contribution of Transport to Air QualityTerm 2012: Transport Indicators Tracking Progress Towards Environmental Targets in Europe; Report No. 10/ 2012; 2012. (2) Johnson, T. V. Diesel Emissions in Review. SAE Tech. Pap. Ser. 2011, Paper No. 2011-01-0304. (3) Sjöblom, J.; Ström, H. Capture of Automotive Particulate Matter in Open Substrates. Ind. Eng. Chem. Res. 2013, 52, 8373. 3752
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