Operating Map for Regeneration of a Catalytic Diesel Particulate Filter

Article pubs.acs.org/IECR

Operating Map for Regeneration of a Catalytic Diesel Particulate Filter Valeria Di Sarli*,† and Almerinda Di Benedetto‡ †

Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche (CNR), Piazzale V. Tecchio 80, 80125 Napoli, Italy Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy

‡

ABSTRACT: In the work presented in this paper, transient computational fluid dynamics-based simulations of soot combustion in a single-channel catalytic diesel particulate filter were run to investigate the combined effects of inlet gas velocity and catalyst activity on the dynamics of regeneration. From numerical results, the operating map of the filter was built in the plane maximum temperature versus time of regeneration. It has been found that all points fall within two zones of the map: a first zone of low temperature and long time of regeneration or, alternatively, a second zone of high temperature and short time of regeneration. The first zone corresponds to a regime of slow combustion (i.e., slow dynamics of regeneration with substantially uniform combustion), whereas the second zone corresponds to a regime of intense combustion (i.e., fast dynamics of regeneration that proceeds by a reaction front moving along the filter). At high inlet velocity, when the regime of intense combustion is established (at high catalyst activity), high temperatures involve the whole filter. At low inlet velocity, regardless of the catalyst activity, regeneration occurs according to the regime of intense combustion. However, an abrupt temperature rise is found only during the final stage of combustion of the residual cake accumulated close to the exit section of the filter, whereas most of the regeneration process occurs at lower temperatures. Thus, strategies able to prevent or mitigate such a burn-up phenomenon, which is well localized in both time and space, would be useful to allow reasonably fast regeneration under controlled temperature conditions.

1. INTRODUCTION During thermal regeneration of diesel particulate filters (DPFs), local temperature excursions may arise which are sufficiently high to damage the filter. To overcome this problem, as well as other drawbacks associated with thermal regeneration (additional energy costs, complex means of control, incomplete soot combustion with CO emissions, etc.), catalytic regeneration has been proposed as an alternative approach.1 In principle, the catalyst may be used to achieve soot oxidation at temperatures lower (250−550 °C) than those required for thermal regeneration (>600 °C). However, there is still no general consensus regarding the ability of catalytic DPFs to oxidize soot at low temperatures and under conditions relevant to practical applications.2 Indeed, such ability is strictly dependent not only on the intrinsic catalyst activity, but also on the efficiency of the contact between the soot particles and the active sites of the catalyst particles.3,4 From the point of view of the efficiency of the solid−solid contact, a catalytic DPF may be conceptually divided into two zones, an internal zone and an external zone. The internal zone is the porous wall of the filter. The external zone is the soot cake layer. In the internal zone, good soot−catalyst contact may © 2016 American Chemical Society

be established, provided that the catalyst particles are dispersed inside the porosity of the filter walls, avoiding the accumulation of a catalytic layer on top of them. Conversely, for the external zone, which represents most of the soot retained by the filter, the contact with the catalyst is rather limited because of the low surface area available to the interaction between the two solid phases. Experiments of regeneration of catalytic DPFs have shown that, regardless of the presence of highly dispersed catalyst inside the filter macro-pores, the cake layer always burns via a thermal path, being substantially segregated from the catalytic wall.5 Indeed, the effect of the catalyst on the combustion process of the cake is essentially thermal (rather than chemical) in nature. This explains why the formation of hot zones remains a critical issue also for regeneration of catalytic DPFs. Combustion dynamics of the cake accumulated on planar catalytic single-layer DPFs has been deeply investigated by Received: Revised: Accepted: Published: 11052

July 1, 2016 September 7, 2016 October 7, 2016 October 7, 2016 DOI: 10.1021/acs.iecr.6b02521 Ind. Eng. Chem. Res. 2016, 55, 11052−11061

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic of the two-dimensional computational domain (not to scale).

numerical results, the operating map of the filter was built in the plane maximum temperature versus time of regeneration. The effect of the coupling between combustion of the soot in the catalytic wall and combustion of the cake on the spatiotemporal evolution of the filter temperature was investigated also in terms of ignition location of the cake.

means of infrared measurements of the spatiotemporal temperature (see, e.g., refs 6−10). A transition has been observed from a regime of uniform combustion, characterized by slow regeneration and moderate temperature rise, to a regime of front propagation, characterized by fast regeneration and high temperature rise, with increasing soot loading,6,10 oxygen concentration,6 and exhaust gas temperature.8 This transition is accompanied by a decrease in the number of ignition points.10 Indeed, in the case of slow regeneration, many ignition points coalescence with each other so that combustion occurs all over the surface. Conversely, in the case of fast regeneration, moving hot zones emanate from a few (distinct) ignition points, the number and locations of which are strictly dependent on the operating conditions. In particular, as the inlet velocity is decreased, the location of single-point ignition shifts from downstream to upstream.6 Ignition and combustion of the cake within DPFs have also been studied by means of mathematical models of different complexity (see, e.g., refs 11−14). Yu et al. developed a onedimensional two-phase single (inlet/out) channel model to derive explicit criteria predicting the inlet gas temperature and time needed to get ignition in an initially cold filter,11 as well as the maximum temperature attained during regeneration.12 Bensaid et al. used a three-dimensional computational fluid dynamics (CFD)-based model of thermal regeneration to simulate the transition from slow to fast regeneration with increasing inlet gas temperature.13 Recently, we developed a two-dimensional CFD-based model of soot combustion in a single-channel catalytic DPF.14 Numerical results have shown that an abrupt transition from slow regeneration to fast regeneration may occur because of increased catalyst activity. In particular, under the simulated conditions of fast regeneration, it has been found that combustion of the soot in the catalytic wall provides downstream ignition for the cake. Following ignition, a reaction front moves from downstream to upstream and, once the inlet section of the filter has been reached, from upstream to downstream. During this up-and-down motion, high temperatures are generated involving the whole filter. However, the maximum temperature is associated with the phase of downstream propagation, when the catalytic wall is almost completely regenerated and, thus, the residual cake burns substantially alone. From these results, it has been concluded that, in order to prevent the formation of hot zones in catalytic DPFs, (thermal) combustion of the cake has to be driven by (catalytic) combustion of the soot trapped inside the filter wall. The aim of the work presented in this paper was identifying operating conditions under which the catalytic wall of the filter is able to sustain combustion of the cake in a uniform and gradual manner, thus allowing reasonably fast regeneration under controlled temperature conditions. To this end, simulations of filter regeneration were run with the previously developed model14 by varying the inlet gas velocity and, for each value of velocity, by increasing the catalyst activity. From

2. MATHEMATICAL MODEL A CFD-based model of soot combustion in a single-channel catalytic DPF was developed. Figure 1 shows a schematic of the Table 1. Geometrical Details of the Computational Domain length of the filter [mm] length of the inlet and outlet channels [mm] height of the inlet and outlet channels [mm] thickness of the porous wall of the filter [mm] initial thickness of the cake layera [mm] a

30 45 1.4 0.38 0.12

Assumed to be constant along the axial coordinate.

Table 2. Kinetic Parameters for Equations 2 and 315 eq 2, slow oxidation A Ea a b

eq 3, fast oxidation

6.05 × 107 s−1 161 kJ/mol 0.7 0.8

A′ (= ADarcy ′ ) E′a a′ b′

1.19 × 105 s−1 114 kJ/mol 0.3 0.8

Table 3. Simulation Conditions Inlet Conditions 1−5a 15 813

velocity [m/s] O2 concentration [% mol] temperature [K] Initial Conditions velocity [m/s] O2 concentration [% mol] soot concentration [kg/m3] temperature [K]

0 15 15 (wall); 200 (cake)b 523

Superficial filtration velocity equal to 0.047−0.233 m3/m2 s. bSpecific soot loading equal to around 10 kg/m3 (of filter). a

Table 4. Solid Properties intrinsic density [kg/m3] specific heat capacity [J/kg K] thermal conductivity [W/m K]

SiC

soot

3240 1120 18

2500 900 10

two-dimensional computational domain consisting of inlet channel, outlet channel, and porous regions (wall of the filter and cake layer). Table 1 summarizes the geometrical details of the computational domain. 11053

DOI: 10.1021/acs.iecr.6b02521 Ind. Eng. Chem. Res. 2016, 55, 11052−11061

Article


Figure 2. Position of the monitor points.

model.16 To couple the soot conservation equation with the fluid flow equations, the local soot concentration was specified as a user-defined scalar. The regeneration kinetics was implemented through user-defined-subroutines. The model equations were discretized using a finite volume formulation on a uniform grid with 219 600 square cells (cell size equal to 0.025 mm). The spatial discretization used second-order schemes for all terms. The time integration was performed by using the second-order implicit Crank− Nicholson scheme. The time step was set equal to 0.1 s. The operating pressure was set to 1 atm. At the inlet section of the computational domain, fixed flat profiles were assumed for velocity, species concentration, and temperature. The total gauge pressure was allowed to rise, in response to the computed static gauge pressure, to whatever value necessary to provide the prescribed velocity distribution. At the exit section, the static gauge pressure was imposed as equal to 0 atm. At the remaining boundaries, adiabatic wall conditions were applied. Simulations were run by varying the inlet gas velocity, Vin, and for each value of velocity, by increasing the catalyst activity, i.e., the pre-exponential factor, A′, in eq 3. In particular, Vin was varied from 1 to 5 m/s, whereas the ratio, k, between the preexponential factor, A′, and the pre-exponential factor of the kinetics by Darcy et al.,15 A′Darcy, was increased from 0 (thermal regeneration) to 10. The simulation (inlet and initial) conditions are listed in Table 3. The computational domain was initially filled with a quiescent O2/N2 mixture (with the same composition as the inlet mixture) at lower temperature, 523 K, than the inlet mixture temperature, 813 K (before regeneration, a preheating phase takes place). Uniform soot concentration was assumed for both porous wall of the filter (15 kg/m3) and cake layer (200 kg/m3). The values of the solid properties used for SiC and soot are listed in Table 4. The values of porosity (defined in the model as void fraction) and viscous resistance coefficient (i.e., inverse permeability) for the (clean) wall of the filter were set equal to 0.5 and 2 × 1012 m−2, respectively. For the cake layer, the (initial) value of viscous resistance coefficient was set equal to 4 × 1013 m−2. Parallel computations were performed by means of the segregated solver of the code ANSYS Fluent 15.0.16 The SIMPLE method was adopted to treat the pressure−velocity coupling. The solution for each time step required around 100

The model was described in detail in ref 14. Briefly, the governing fluid flow equations are the mass, momentum, species, and energy conservation equations. In the porous zones, these equations are coupled to the soot conservation equation. It was assumed that the fluid and solid (i.e., silicon carbide, SiC, for the wall and soot for the cake layer) phases are in local thermal equilibrium (i.e., in any control volume, the fluid and solid have the same temperature). The kinetics of catalytic oxidation of diesel soot with oxygen proposed by Darcy et al.15 was implemented in the model. This kinetics does not account for the impact of NO. The soot oxidation rate is expressed as the sum of two contributions, a first contribution associated with “slow oxidation”, which involves all the soot present in the system, and a second contribution associated with “fast oxidation”, which involves only the soot directly in contact with the catalyst (0.4% Pt/ CeZrO2): rtotal = rslow_oxidation + rfast_oxidation

(1)

with rslow_oxidation mtotal, t = 0

⎛ mtotal, t ⎞b ⎟⎟ (xoxygen) ⎜⎜ ⎝ mtotal, t = 0 ⎠

(−Ea / RT )

= Ae

a

(2)

and rfast_oxidation mcatalyzed, t = 0

⎛ mcatalyzed, t ⎞b ′ ⎟⎟ (xoxygen) ⎜ ⎝ mcatalyzed, t = 0 ⎠

(−Ea′ / RT )

= A′e

a ′⎜

(3)

In eqs 2 and 3, xoxygen is the oxygen mole fraction in the gas phase. In eq 2, mtotal is the local concentration of soot present in the system. In eq 3, mcatalyzed is the local concentration of soot directly in contact with the catalyst. Both mtotal and mcatalyzed are expressed in kilograms per cubic meter. Table 2 summarizes the values of the kinetic parameters for eqs 2 and 3. In the porous wall of the filter, both contributions of slow (eq 2) and fast (eq 3) oxidation were implemented. Furthermore, all the soot present was assumed to be in contact with the catalyst (mcatalyzed = mtotal). In the cake, only the slow contribution was implemented (the contact between the cake and the catalyst was fully neglected). Indeed, recent experiments of catalytic filter regeneration have confirmed that the cake burns via a thermal path, being substantially segregated from the catalytic wall.5 The model was developed by using the platform of the CFD code ANSYS Fluent 15.0 and, in particular, the porous medium 11054


Article

Industrial & Engineering Chemistry Research iterations to converge with the residual of each equation smaller than 1 × 10−8. During the computations, the time histories of temperature and soot concentration were registered at different points. Figure 2 shows the position of the monitor points.

3. RESULTS AND DISCUSSION Figure 3 shows the maximum filter temperature attained during the regeneration process, Tmax, as a function of k (i.e., the catalyst activity) at different inlet gas velocities.

Figure 3. Maximum filter temperature, Tmax, as a function of k at different inlet gas velocities. Results at Vin = 3 m/s are from ref 14. Vertical lines separate different regimes of regeneration (black lines, Vin = 3 m/s; gray lines, Vin = 5 m/s).

In Figure 4, the time at which the volume-averaged soot conversion overcomes 85% is plotted versus k at different inlet velocities, for both porous wall of the filter (tfilter wall) and cake (tcake). In the case of thermal regeneration (k = 0), the values of tfilter wall and tcake overlap. In the case of catalytic regeneration, the time of filter regeneration is controlled by the time of cake consumption. As found at Vin = 3 m/s,14 even at Vin = 5 m/s, three regimes can be identified as a function of k: (1) a regime characterized by low values of temperature and high values of tcake (i.e., slow dynamics of regeneration with substantially uniform combustion); (2) a regime characterized by intermediate values of temperature and high values of tcake; (3) a regime characterized by high values of temperature and low values of tcake (i.e., fast dynamics of regeneration). These regimes have been referred to as (1) “regime of slow combustion”, (2) “regime of transition”, and (3) “regime of intense combustion”.14 In the regime of slow combustion (except for k = 0), tcake is much higher than tfilter wall: the catalyst activity is too low and, thus, combustion of the soot in the porous wall of the filter is not able to appreciably accelerate combustion of the cake. In the regime of intense combustion, the catalyst activity is high enough to make the porous wall of the filter an effective pilot for the cake. Therefore, tcake becomes comparable to tfilter wall. When increasing the inlet velocity from 3 to 5 m/s, the transition from the regime of slow combustion to the regime of intense combustion occurs at higher values of k. Furthermore, it covers a wider range of k values. Conversely, at Vin = 1 m/s, there is no regime transition: regardless of the catalyst activity, regeneration occurs according to the regime of intense combustion (this is true even in the case of thermal

Figure 4. Time at which the volume-averaged soot conversion overcomes 85% plotted versus k at different inlet velocities, for both porous wall of the filter and cake. Results at Vin = 3 m/s are from ref 14. Vertical lines separate different regimes of regeneration.

Figure 5. Temperature of the porous wall of the filter versus time as registered in the monitor points of Figure 2. Regeneration stage: Vin = 5 m/s; k = 10.

regeneration, meaning that, under the simulated conditions, there is no need for catalytic pilot to get ignition of the cake). However, lower values of temperature are found along with higher values of tcake (and tfilter wall). This means that, as the inlet 11055


Article


Figure 6. Temperature maps computed at different time instances during the propagation of (A) u wave and (B) d wave: Vin = 5 m/s; k = 10. The computational domain was scaled along the transverse direction using a scaling factor equal to 4.

more interesting than the regime of slow combustion. However, high temperatures characterize this regime. 3.1. Regime of Intense Combustion. In order to investigate the opportunity for reducing the temperature rise, while keeping short times of regeneration, the attention is here focused on the regime of intense combustion and, in particular,

velocity is decreased, the regeneration process becomes somewhat slower and more uniform. In particular, more uniform combustion is attained with more active catalyst (i.e., k > 2). From an applicative point of view, the short time of filter regeneration makes the regime of intense combustion much 11056


Article


Figure 7. Soot concentration versus time as registered in the monitor points of Figure 2: Vin = 5 m/s; k = 10. Solid lines, wall of the filter; dashed lines, cake.


on the spatiotemporal evolution of temperature and soot concentration at high velocity (Vin = 5 m/s) and low velocity (Vin = 1 m/s). 3.1.1. High Inlet Gas Velocity (Vin = 5 m/s). The regeneration dynamics for the regime of intense combustion at high inlet velocity (Vin = 5 m/s) is substantially insensitive to variations in the catalyst activity (i.e., k). Figures 5−7 illustrate the regeneration dynamics for k = 10. In Figure 5, the temperature of the porous wall of the filter is plotted versus time as registered in the monitor points of Figure 2. All the temperature time histories exhibit two dominant peaks. The first peak is associated with a temperature wave propagating from downstream to upstream (u wave). Conversely, the second peak is associated with a temperature wave propagating from upstream to downstream (d wave). The up-and-down wave motion can be better followed by looking at the time sequence of temperature maps shown in Figure 6 (A, u wave; B, d wave). High temperatures involve the whole filter. Indeed, the same behavior has also been found at Vin = 3 m/s.14 In Figure 7, the soot concentration is plotted versus time as registered in the monitor points of Figure 2. Ignition occurs downstream at t ∼ 20 s. During the phase of upstream propagation of the reaction front (u wave), a strong coupling is established between combustion of the soot in the

Figure 9. Temperature maps computed at different time instances during the propagation of d wave: Vin = 1 m/s; k = 1. The computational domain was scaled along the transverse direction using a scaling factor equal to 4.

catalytic wall and combustion of the cake. Conversely, during the phase of downstream propagation (d wave), the cake burns alone. Thus, this is a partially catalyst-assisted regeneration mode, which does not allow the cake to burn according to a uniform combustion process. 3.1.2. Low Inlet Gas Velocity (Vin = 1 m/s). The regeneration dynamics for the regime of intense combustion at low inlet velocity (Vin = 1 m/s) is here described through 11057


Article




two cases representative of low catalyst activity (k ≤ 2) and high catalyst activity (k > 2). 3.1.2.1. Low Catalyst Activity (k = 1). In Figure 8, the temperature of the porous wall of the filter is plotted versus time as registered in the monitor points of Figure 2. Once again, two temperature waves propagate along the filter. The first wave propagates from downstream to upstream (u wave), whereas the second wave propagates from upstream to downstream (d wave). The phase of propagation of the d wave lasts much longer than the phase of propagation of the u wave. Differently from the high velocity case (Vin = 5 m/s), at low velocity, most of the regeneration process takes place at low temperatures, whereas an abrupt temperature rise is found only during the final stage of the downstream propagation. This is confirmed in Figure 9 showing the time sequence of temperature maps as calculated during the propagation of the d wave. The hottest zone is found close the exit section of the filter, whereas much lower temperatures are found elsewhere. Figure 10 shows the soot concentration versus time as registered in the monitor points of Figure 2. After (downstream) ignition (t ∼ 85 s), combustion of the cake and combustion of the soot in the catalytic wall proceed almost simultaneously during both the phases of upstream and downstream propagation of the reaction front. Thus, this is a

Figure 12. Temperature maps computed at different time instances during the propagation of d wave: Vin = 1 m/s; k = 10. The computational domain was scaled along the transverse direction using a scaling factor equal to 4.

fully catalyst-assisted regeneration mode, which allows a more uniform combustion process of the cake to take place. Global assessment of Figures 8−10 shows that the maximum temperature attained during the regeneration process corresponds to a burn-up phenomenon, i.e., rapid combustion of the residual cake accumulated close to the exit section of the filter. This phenomenon is well localized in both time and space. 11058


Article


temperature decreases in going from low to high catalyst activity. 3.2. Operating Map. In Figure 14, the operating map of the filter is shown as built in the plane maximum temperature versus time of regeneration (i.e., time of cake consumption), Tmax versus tcake. All points fall within two zones of the plane (bounded by dashed lines): a first zone of low temperature and long time of regeneration or, alternatively, a second zone of high temperature and short time of regeneration. The first zone corresponds to the regime of slow (and substantially uniform) combustion, which is not found at Vin = 1 m/s (the points corresponding to the regime of transition were also included in the first zone). The second zone corresponds to the regime of intense combustion (with front propagation), which is found at all the values of velocity investigated. Figure 14 demonstrates that the management of the regeneration process is a difficult task: it is not possible to perform fast regeneration while still avoiding high temperature rise. Even in the case of the regime of intense combustion at Vin = 1 m/s, slightly lower temperatures are somewhat counterbalanced by slightly longer times of regeneration. On the other hand, when looking at the spatiotemporal evolution of temperature and soot concentration, it turns out that, during regeneration at Vin = 3 and 5 m/s, high temperatures involve the whole filter. Conversely, at Vin = 1 m/s, an abrupt temperature rise is found only during the final stage of combustion of the residual cake accumulated close to the exit section of the filter, whereas most of the regeneration process occurs at lower temperatures. Thus, strategies able to prevent or mitigate such a burn-up phenomenon, which is well localized in both time and space, would be useful to allow reasonably fast regeneration under controlled temperature conditions. This concept is better highlighted by the square symbols in Figure 14, which correspond to the results obtained at Vin = 1 m/s, when ideally stopping the regeneration process before the burnup phenomenon can occur.


Figure 14. Operating map of the filter as built in the plane Tmax vs tcake, starting from results of simulations run by varying k at different inlet velocities. Results at Vin = 3 m/s are from ref 14.

4. CONCLUSIONS Transient CFD-based simulations of soot combustion in a single-channel catalytic diesel particulate filter were run to investigate the combined effects of inlet gas velocity and catalyst activity on the dynamics of regeneration. From numerical results, the operating map of the filter was built in the plane maximum temperature versus time of regeneration. It has been found that all points fall within two zones of the map: a first zone of low temperature and long time of regeneration or, alternatively, a second zone of high temperature and short time of regeneration. The first zone corresponds to a regime of slow combustion (i.e., slow dynamics of regeneration with substantially uniform combustion), whereas the second zone corresponds to a regime of intense combustion (i.e., fast dynamics of regeneration that proceeds by a reaction front moving along the filter). At high inlet velocity, when the regime of intense combustion is established (at high catalyst activity), high temperatures involve the whole filter. Indeed, combustion of the cake takes place according to a partially catalyst-assisted regeneration mode: catalytic combustion of the soot in the porous wall of the filter just provides local violent ignition for the cake, which thus burns substantially alone. At low inlet velocity, regardless of the catalyst activity, regeneration occurs according to the regime of intense

3.1.2.2. High Catalyst Activity (k = 10). Figure 11 shows the temperature of the porous wall of the filter versus time as registered in the monitor points of Figure 2. At high catalyst activity, a single temperature wave propagates from upstream to downstream (d wave). This wave motion is also detailed in the time sequence of temperature maps shown in Figure 12. Once again, the hottest zone is found close to the exit section of the filter. In Figure 13, the soot concentration is plotted versus time as registered in the monitor points of Figure 2. Combustion of the soot in the catalytic wall and combustion of the cake are synchronized with each other. Indeed, at low velocity, regardless of the catalyst activity, a fully catalystassisted regeneration mode is established. At low velocity, the increase in catalyst activity allows the location of the ignition point to move from downstream to upstream. Thus, differently from the case of k = 1, at k = 10, ignition takes place upstream (instead of downstream) and the reaction front moves one way (i.e., from upstream to downstream instead of up-and-down). Because of the absence of downstream ignition, the burn-up phenomenon of the residual cake, which is responsible for the maximum temperature attained during the regeneration process, occurs starting from lower temperatures. This explains why the maximum 11059


Article

Industrial & Engineering Chemistry Research combustion. However, combustion of the soot in the catalytic wall and combustion of the cake proceed almost simultaneously. This allows a fully catalyst-assisted regeneration mode to be established, which leads to more uniform, over space, and gradual, over time, combustion process of the cake. Indeed, an abrupt temperature rise is found only during the final stage of combustion of the residual cake accumulated close to the exit section of the filter, whereas most of the regeneration process occurs at lower temperatures. This burn-up phenomenon is less severe at high catalyst activity, when ignition of the cake occurs upstream (instead of downstream) and the reaction front moves one way along the filter (i.e., from upstream to downstream instead of up-and-down). Because of the absence of downstream ignition, combustion of the residual cake occurs starting from lower temperatures, thus resulting in lower temperature rise. It is worth highlighting that strategies able to prevent or mitigate the burn-up phenomenon, which is well localized in both time and space, would be useful to get reasonably fast regeneration under controlled temperature conditions. Future work will be devoted to the definition of strategies to circumvent the temperature rise associated with the burn-up phenomenon.

■

■

REFERENCES

(1) Fino, D.; Bensaid, S.; Piumetti, M.; Russo, N. A Review on the Catalytic Combustion of Soot in Diesel Particulate Filters for Automotive Applications: From Powder Catalysts to Structured Reactors. Appl. Catal., A 2016, 509, 75. (2) Southward, B. W. L.; Basso, S.; Pfeifer, M. On the Development of Low PGM Content Direct Soot Combustion Catalysts for Diesel Particulate Filters. SAE Tech. Pap. Ser., 2010; 2010-01-0558. DOI: 10.4271/2010-01-0558 (3) van Setten, B. A. A. L.; Schouten, J. M.; Makkee, M.; Moulijn, J. A. Realistic Contact for Soot with an Oxidation Catalyst for Laboratory Studies. Appl. Catal., B 2000, 28, 253. (4) Hinot, K.; Burtscher, H.; Weber, A. P.; Kasper, G. The Effect of the Contact between Platinum and Soot Particles on the Catalytic Oxidation of Soot Deposits on a Diesel Particle Filter. Appl. Catal., B 2007, 71, 271. (5) Di Sarli, V.; Landi, G.; Lisi, L.; Saliva, A.; Di Benedetto, A. Catalytic Diesel Particulate Filters with Highly Dispersed Ceria: Effect of the Soot-Catalyst Contact on the Regeneration Performance. Appl. Catal., B 2016, 197, 116. (6) Chen, K.; Martirosyan, K. S.; Luss, D. Soot Combustion Dynamics in a Planar Diesel Particulate Filter. Ind. Eng. Chem. Res. 2009, 48, 3323. (7) Chen, K.; Martirosyan, K. S.; Luss, D. Temperature Excursions during Soot Combustion in a Diesel Particulate Filter (DPF). Ind. Eng. Chem. Res. 2010, 49, 10358. (8) Chen, K.; Martirosyan, K. S.; Luss, D. Transient Temperature Rise during Regeneration of Diesel Particulate Filters. Chem. Eng. J. 2011, 176−177, 144. (9) Chen, K.; Martirosyan, K. S.; Luss, D. Temperature Gradients within a Soot Layer during DPF Regeneration. Chem. Eng. Sci. 2011, 66, 2968. (10) Martirosyan, K. S.; Chen, K.; Luss, D. Behavior Features of Soot Combustion in Diesel Particulate Filter. Chem. Eng. Sci. 2010, 65, 42. (11) Yu, M.; Luss, D.; Balakotaiah, V. Analysis of Ignition in a Diesel Particulate Filter. Catal. Today 2013, 216, 158. (12) Yu, M.; Luss, D.; Balakotaiah, V. Regeneration Modes and Peak Temperatures in a Diesel Particulate Filter. Chem. Eng. J. 2013, 232, 541. (13) Bensaid, S.; Marchisio, D. L.; Fino, D. Numerical Simulation of Soot Filtration and Combustion within Diesel Particulate Filters. Chem. Eng. Sci. 2010, 65, 357. (14) Di Sarli, V.; Di Benedetto, A. Modeling and Simulation of Soot Combustion Dynamics in a Catalytic Diesel Particulate Filter. Chem. Eng. Sci. 2015, 137, 69.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 0817622673. Fax: +39 0817622915. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research activity was performed within the framework of the SOLYST project funded by the Italian Ministry for Education, University and Research (MIUR) through the FIRB - Futuro in Ricerca 2012 initiative (Grant: RBFR12LS6M 002). Vincenzo Smiglio and Luigi Muriello are gratefully acknowledged for their technical assistance in the computing activity.

■

mcatalyzed = Local concentration of soot directly in contact with the catalyst [kg/m3] mtotal = Local concentration of soot present in the system [kg/m3] rfast_oxidation = Consumption rate of soot associated with fast (catalytic) oxidation [kg/m3 s] rslow_oxidation = Consumption rate of soot associated with slow (thermal) oxidation [kg/m3 s] rtotal = Total consumption rate of soot [kg/m3 s] R = Universal gas constant [kJ/mol K] t = Time [s] tcake = Time of cake consumption [s] tfilter wall = Time of regeneration of the porous wall of the filter [s] tsoot conversion 85% = Time at which the volume-averaged soot conversion overcomes 85% [s] T = Temperature [K] Tmax = Maximum temperature [K] Vin = Inlet gas velocity [m/s] xoxygen = Oxygen mole fraction in the gas phase [−]

NOTATION

a = Exponent to the oxygen mole fraction in the kinetic equation for slow (thermal) soot oxidation [−] a′ = Exponent to the oxygen mole fraction in the kinetic equation for fast (catalytic) soot oxidation [−] A = Pre-exponential factor in the kinetic equation for slow (thermal) soot oxidation [s−1] A′ = Pre-exponential factor in the kinetic equation for fast (catalytic) soot oxidation [s−1] A′Darcy = Pre-exponential factor in the kinetic equation for fast (catalytic) soot oxidation [s−1] (value from ref 15) b = Exponent to the soot mass concentration in the kinetic equation for slow (thermal) soot oxidation [−] b′ = Exponent to the soot mass concentration in the kinetic equation for fast (catalytic) soot oxidation [−] Ea = Activation energy in the kinetic equation for slow (thermal) soot oxidation [kJ/mol] E′a = Activation energy in the kinetic equation for fast (catalytic) soot oxidation [kJ/mol] k = Ratio between A′ and ADarcy ′ [−] L = Length of the filter [mm] 11060


Article

Industrial & Engineering Chemistry Research (15) Darcy, P.; Da Costa, P.; Mellottée, H.; Trichard, J.-M.; DjégaMariadassou, G. Kinetics of Catalyzed and Non-Catalyzed Oxidation of Soot from a Diesel Engine. Catal. Today 2007, 119, 252. (16) ANSYS Fluent Theory Guide (Release 15.0); ANSYS, Inc.: Canonsburg, PA, 2013; http://www.ansys.com.

11061


Operating Map for Regeneration of a Catalytic Diesel Particulate Filter

Recommend Documents