Temperature Excursions during Soot Combustion in a Diesel

Apr 20, 2010 - The central part of the experimental system (Figure 1a) was a ..... M. Regeneration Capability of Diesel Particulate Filter System Usin...
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Temperature Excursions during Soot Combustion in a Diesel Particulate Filter (DPF) K. Chen, K. S. Martirosyan, and D. Luss* Department of Chemical and Biomolecular Engineering, UniVersity of Houston, Houston, Texas, 77204

A major technological challenge in the operation of diesel particulate filters (DPFs) is prevention of the occasional melting of the ceramic filters during regeneration (combustion of accumulated particulate matter). The cause of this melting is still an established question. Experiments and simulations indicate that during stationary regeneration (fixed feed conditions) the temperature rise is not sufficiently high to cause this melting. We conjecture that the melting is due to a transient temperature excursion caused by a rapid shift of a car from normal driving to idle during regeneration. Infrared imaging was used to follow the response of the spatiotemporal temperature on a planar DPF to sudden changes in the feed conditions. As conjectured, the peak transient temperature was higher than that corresponding to stationary operation under either the initial or final conditions. The amplitude of the temperature excursion was sensitive to the direction in which the temperature front moved and to the period between the initiation of the temperature front and the change in the feed conditions. Introduction Strict limits exist on the emissions of diesel engines. The diesel particulate filter (DPF) is the best existing technology for removal of particulate matter (PM). It is a wall-flow monolith consisting of many parallel extruded square porous ceramic cells. Every second channel is plugged at alternate ends. The exhaust gases enter the inlet channels and pass through the porous walls to adjacent outlet channels. The PM accumulated in the inlet channels is periodically removed by controlled combustion.1-4 The regeneration may be conducted in two different modes. In the first mode, the PM is slowly combusted all over the surface. In the second, which is much faster and thus commonly used, local ignition leads to formation of a sharp temperature front that propagates on the surface combusting the PM. The ignition is accomplished by temporarily heating of the exhaust gases (to about 550 °C) either by an external heater or by diesel injection to the exhaust gases.5-9 This regeneration is the most demanding technological challenge in the operation of the DPF, as the exothermic combustion sometimes leads to excessive local temperature excursions that melt the Cordierite ceramic filter (melting temperature ∼1250 °C).10-14 Unfortunately, the knowledge and understanding of what causes the formation of the hot regions is still an open question. Following the pioneering work of Bissett,15,16 there have many theoretical and experimental studies of the maximum temperature rise during DPF regeneration under stationary (constant) feed conditions.17-31 This maximum temperature generated is not sufficiently high to melt the cordierite support. We have recently conjectured that the temperature excursion that melts the DPF is a dynamic response to a sudden change in the engine load. Our experiments revealed that a sudden decrease in the feed temperature or velocity can lead to a counterintuitive transient temperature rise above that which would have existed if the feed temperature would not have been lowered.32 This dynamic feature cannot be predicted or explained from knowledge of the initial and final steady states. This dynamic response is similar to the wrong-way behavior which has been observed and studied in fixed bed and monolith reactors.33-42 Intuition and simulations * To whom correspondence should be addressed. E-mail: dluss@ uh.edu.

suggest that the largest temperature excursion will occur following a rapid shift in the driving mode from regular driving to idle. This leads to a simultaneous rapid decrease of the exhaust gas temperature and flow rate and an increase of the oxygen concentration.3,7 A sudden increase in the oxygen concentration leads as expected to a higher DPF temperature rise. A sudden decrease in the feed flow rate usually increases temperature rise, similar to the wrong way behavior caused by the impact of sudden decrease of feed temperature. This is another way of wrong way behavior.27 Development of a DPF operation that circumvents its melting depends on the ability to predict the magnitude of the temperature rise upon simultaneous changes of three feed state variables (temperature, oxygen concentration, and flow rate). Thus, it is important to gain an understanding of the impact of the dynamic interaction among the three perturbations that move at different velocities along the DPF on the temperature rise. We report here a study of the dynamic responses to rapid changing of two feed state variables. In order to gain insight about the interactions between the various perturbations, we compare these responses to the sum of the responses by changing only one feed state variable. The dynamic response to a change of the three variables and a comparison with the sum of the responses by changing only one will be reported in the future. Experimental System and Procedure The central part of the experimental system (Figure 1a) was a stainless steel (316 L) insulated reactor (120 × 40 × 40 mm) with an IR transparent quartz window on its top. Its temperature was controlled by an electrical heater. Inside the reactor was a planar catalytic single layer DPF (90 mm long, 20 mm wide), cut from a commercial DPF (NGK-6000YE). Both its exterior sides and bottom were sealed by ceramic glue. Carbon black nanoparticles (40 nm, Sigma Aldridge) were deposited on top of the planar DPF by spraying a well-mixed solution of alcohol containing the nanoparticles. The alcohol was later removed by a stream of nitrogen. A schematic of the reactor is shown in Figure 1b.

10.1021/ie1004465  2010 American Chemical Society Published on Web 04/20/2010

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Figure 1. (a) Schematic of the experimental system. (b) Schematic of the reactor.

Preliminary experiments showed heat loss from the reactor walls affected the temperature profile and the location at which the ignition occurred. To minimize this heat loss, all sides of reactor except the one with the window were heated by an electrical ceramic fiber heater (Watlow Instruments Co, Inc.) to a temperature set by a power controller (Staco energy product Co, 2PF1010). Five K-type thermocouples (diameter ∼0.5 mm) were installed at the bottom of the single layer DPF to help monitor the DPF temperature during the regeneration. The first one placed near the entrance measured the inlet gas temperature. The second was located 10 mm downstream from the inlet; the third was located 30 mm away from the inlet, the fourth in the middle of the DPF, and the fifth 10 mm ahead of the outlet. The thermocouples’ readings were recorded and processed by an Omega data acquisition board connected to a PC. A 5 mm thick mineral wool insulation was placed between the planar DPF and the reactor walls to minimize the conduction, convective, and radiation heat exchange with the reactor walls. The spatiotemporal temperature profile was measured by a high speed (up to 60 frame/s) infrared camera (Merlin, MW18, Indigo Systems) held -50 cm above the quartz window. The camera has a 256 × 256 indium antimonide detector array sensitive to 3-5 µm radiations. Due to experimental setup limitations, we could capture images only for locations located between 3090 mm from the DPF inlet. The IR signals were periodically calibrated using the K-type thermocouples readings. The images were recorded at the rate of 10 per second on a PC with an ImageDesk II software. Compressed nitrogen purged the air in the reactor before each experiment. The reactor walls temperature was then heated up from room temperature to a preset temperature. While the reactor was fed by preheated nitrogen, the planar DPF was preheated to the desired temperature by an electric heater (Hoskins furnace, FD303). After the DPF attained the desired temperature, a mixture of pressurized nitrogen and air was fed to the reactor. The gas flow rates were measured by rotameters (Fisher & Porter, 10A6100). The feed pressure was controlled by downstream pressure regulators. After ignition occurred and the reaction front propagated, two of the three controlled feed variables (temperature, oxygen concentration, and flow rate) were rapidly changed. The loaded carbon layer was usually

Figure 2. Impact of filtration velocity on temporal temperature profiles following a rapid shift in Ti ) 620-520 °C and O2 from 10 to 15 vol % (a) V ) 5 cm3/cm2 · s; (b) V ) 12 cm3/cm2 · s. PM loading of 10 g/L.

combusted within one minute. Each experiment was repeated at least three times to check for reproducibility. Experimental Results Experiments were conducted to determine the response to simultaneous sudden changes of two input variables and to compare these to the responses generated by the step change of a single feed condition. The soot loadings were either 10 or 20 g/L, which corresponded to a layer thickness of about 120 and 240 µm, respectively. The initial feed temperature was 620 °C and (unless otherwise indicated) oxygen concentration was 10 vol % and the superficial filter velocity 12 cm3/cm2 · s. Preliminary experiments showed that under stationary feed conditions increasing the oxygen concentration or the feed temperature led as expected to higher DPF temperatures. The impact of the filtration velocity was more intricate.43 At low or high filtration velocities, a slow flow rate increase led to a higher DPF temperature as the increase in the heat release due to the increased reactant supply more than compensated for the higher rate of heat removal due to the increased heat transfer coefficient. However, at intermediate filtration velocities the increase in the rate of heat removal rate following an increase of the velocity is larger than the increased rate of heat generation. A rapid decrease of the velocity led to a transient temperature rise.27 Experiments under stationary feed conditions revealed that the ignition position and hence the direction of the moving reaction zone strongly depended on the operating conditions. Usually for low O2 concentration and low filtration velocity, most of the incoming oxygen was consumed in the upstream of the DPF. The heat generated by the reaction in that region was not efficiently removed by the low filtration velocity, leading to ignition in the upstream section. The temperature front moved in the same direction as the gas flow. An example of such a case is shown in Figure 2a. At high filtration velocities,

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the high rate of heat removal prevented ignition of the upstream soot. The heat generated by the reaction heated the downstream flowing gas, causing ignition at the downstream. The corresponding reaction front moved in a direction opposite to that of the gas flow (Figure 2b). In general the front temperature increased as the temperature front propagated in the downstream direction while the peak temperature decreased as the temperature front moved toward the upstream. The direction of the moving temperature front can usually be manipulated by a change in the flow rate. To determine the impact of the temperature front propagation direction on the maximum transient temperature, we conducted experiments at various flow rates at which the front moved in either one of these two directions. Figure 2 shows two cases in which the feed temperature was rapidly decreased from 620 to 520 °C and the oxygen concentration increased from 10 to 15 vol % 42 s after the hot reactive feed was introduced to the reactor. The soot loading was 10 g/L, and the initial DPF temperature was 620 °C. In all figures, the time is reported from the introduction of the reactants mixture to the reactor. Figure 2a describes a case that the filtration velocity was 5 cm3/cm2 · s for which the reaction front propagated downstream. Under stationary feed conditions, the peak temperature became higher as the temperature wave moved downstream. The dashed line with solid diamonds in all figures is the peak local temperature under stationary feed conditions. The sudden change in the feed caused the highest transient temperature (in the downstream) to exceed that attained under stationary feed conditions. The maximum temperature of 800 °C reached at the downstream of the inlet channel exceeded by 60 °C that attained under constant feed conditions. If only the oxygen concentration or temperature would have been step changed, the corresponding temperature rise would have been 40 and 30 °C, respectively. The sum of both these changes (70 °C) is slightly higher than the 60 °C when both step changes occurred simultaneously. Under stationary feed conditions, a moving temperature front could not form using a feed temperature of 520 °C for any of the oxygen concentration and flow rates used in our experiments. Figure 2b describes a case in which the filtering flow rate of 12 cm3/cm2 · s caused ignition at the downstream section of the DPF (Figure 2b) and the combustion front moved in the upstream direction. The same step changes in the feed temperature and oxygen concentration as in Figure 2a were done at t ) 44 s. This transient peak temperature of 849 °C exceeded by 91 °C the maximum of obtained under stationary feed conditions (758 °C). Extinction occurred within 8 s of the step changes as an ignited state cannot exist under the new operating conditions. A step change of only the temperature or oxygen concentration would lead to a transient temperature rise of 43 and 53 °C, respectively. The sum of both changes was slightly larger than that when both changes occurred simultaneously. In our experiments, the amplitude of the temperature rise over that attained under stationary feed conditions decreased monotonically with the time the front stayed in the reactor before the step changes were conducted. For example Figure 3 shows the temporal profiles with the highest temperature peak for three cases following a temperature decrease from 620 to 520 °C and a filtration velocity decrease from 12 to 8.3 cm3/cm2 · s. The initial conditions led to a downstream ignition so that the temperature front moved opposite to the direction of the flow. Cases 1, 2, and 3 describe cases for which the step changes occurred when the temperature front was in the downstream, middle, and upstream section of the DPF. The corresponding

Figure 3. Upstream moving temperature profiles with the highest peak for 10 g/L. Feed temperature and filtration velocity were decreased when the temperature front was at the (1) downstream, (2) middle, and (3) upstream section of the DPF.

Figure 4. Moving temperature front profiles following a rapid shift in Ti ) 620-520 °C and O2 from 10-15 vol %; V ) 12 cm3/cm2 · s; PM loading of 20 g/L.

temperature rise above that of stationary feed conditions are 72, 61, and 45 °C, respectively. If only the temperature or filtration velocity would have been step changed at downstream the corresponding temperature rise would have been 43 and 31 °C, respectively. In all three cases, temperature rise upon a simultaneous change of the two inputs was about equal to the sum of those caused by the individual step changes, which were started at the same front position. The experiments described in Figures 2 and 3 were repeated with higher soot loading of 20 g/L to check its impact. In both of these cases described in Figures 4 and 5, the ignition occurred at the upstream of the DPF. Figure 4 describes a case in which the feed conditions and changes were identical to those in Figure 2b. The step changes led to a maximum temperature exceeding by 94 °C that under stationary operation. If only the temperature or oxygen concentration would have been step changed, the corresponding temperature rise would have been 40 and 50 °C, respectively. The sum of both changes of 90 °C was slightly smaller than that when both changes occurred simultaneously. Figure 5 describes a case similar to that shown in Figure 3 but with a higher PM loading of 20 g/L. Thus, the ignition occurred in the upstream section of the DPF and the temperature front moved in the flow direction. The simultaneous step changes led to a downstream temperature rise of 72 °C above that under stationary feed conditions. A step change of only the temperature or filtration velocity led to a temperature rise of 40 and 28 °C above that under stationary conditions. The

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Figure 5. Moving temperature front profiles following a rapid shift in Ti ) 620-520 °C and filtration velocity from 12-8.3 cm3/cm2 · s; O2 ) 10 vol %; PM loading of 20 g/L.

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Figure 7. Downstream moving temperature wave profiles following a sudden shift in O2 ) 10-15 vol % and filtration velocity from 12 to 8.3 cm3/(cm2 · s); Ti ) 620 °C; PM loading of 20 g/L.

The dashed line in this figure is the maximum temperature that was attained under stationary feed conditions equal to those after the step changes were made. The maximum temperature at the end of the bed (857 °C) was higher than that under the stationary conditions (832 °C). Thus, the transient was not bounded between the temperatures corresponding to stationary operation under the initial or final feed conditions Discussion and Concluding Remarks

Figure 6. Moving temperature profiles with the highest peak for 20 g/L. The feed temperature and filtration velocity were decreased when the temperature front was at the (1) upstream, (2) middle, and (3) downstream section of the DPF.

observed transient temperature rise was slightly higher than the sum of changes of the two individual changes input. Figure 6 describes a case under feed conditions and step changes identical to those in Figure 5 so that the temperature moved in the direction of the flow. The figure shows that the location of the reaction front when the step changes were done strongly affected the peak transient temperature. Curves 1, 2, and 3 correspond to the cases in which the step changes were done when the reaction front was in the upstream, middle, and downstream section of the reactor, respectively. The corresponding peak temperatures exceeded by 72, 55, and 42 °C those attained under stationary feed conditions. Clearly, in our experiments, the peak temperature was a monotonic decreasing function of the sojourn of the temperature front in the DPF before the step changes were done. Figures 2-6 were of cases which included a decrease in the feed temperature, which is known to lead to a counterintuitive response. Figure 7 describes a case in which the soot loading was 20 g/L and the feed temperature was not perturbed. The feed velocity was step changed from 12 to 8.3 cm3/cm2 · s while the oxygen concentration was increased from 10-15 vol %. When only the oxygen concentration or the filtration velocity was step changed, the corresponding temperature rise would have been 50 and 28 °C, respectively. When both changes were done simultaneously, the peak downstream temperature exceeded by 82 °C that attained under stationary feed conditions. That value is slightly higher than the sum of these two changes.

The chemical reaction engineering literature includes many experimental examples and theoretical predictions of the wrongway behavior, i.e, the response to a sudden decrease of the feed temperature to a packed bed reactor.33-42 A similar counterintuitive temperature rise occurs upon a sudden decrease of the feed temperature to a DPF which is undergoing regeneration.32 A sudden shift of a car from normal driving to idle leads to a simultaneous decrease of the exhaust gas temperature and flowrate and an increase in the oxygen concentration. The occasional melting of DPFs during regeneration points out the practical importance of being able to predict the impact of a sudden change of several input variables to a DPF on the transient temperature rise. In particular, it is of interest to relate the overall temperature rise to those caused by a sudden change of each of the individual variables in cases that at least one of them leads to a counterintuitive response. The nonlinear features of chemical reacting systems probably prevent getting a general answer. An important lesson from the study is that determination of the temperature rise on a DPF under stationary operation is not sufficient for predicting the transient one. This information can be obtained only from studies of the transient features of the DPF. This conclusion is obvious when one of the step changes, such as the feed temperature leads to a counterintuitive temperature rise. However, the case described in Figure 7 shows that an excursion exceeding the bounds corresponding to the two limiting stationary operations may occur even when the feed temperature is kept stationary. The experiments were conducted under conditions that a transient temperature rise occurred upon either a step decrease in the feed temperature, a decrease in the filtration velocity, or an increase in the oxygen concentration. The experiments revealed that the temperature rise upon a simultaneous step decrease of the temperature and a step change in either the oxygen concentration or filtration velocity was about equal to the sum of the temperature rise caused by the change of only

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one input variable. We currently study the temperature rise caused by a simultaneous change of the three feed inputs to a DPF, temperature, composition, and flow rate. It will be useful to determine by additional experiments and simulations if it is possible to relate or bound the maximum transient temperature rise upon changing the three feed variables to those generated by individual perturbations. When the ignition started at the DPF upstream the temperature front propagated in the flow direction the maximum peak temperature was obtained at the downstream of the DPF. The reason is that the temperature rise is determined by a balance between the heat generated by the combustion and the cooling of the PM layer by the gas flowing through it. As the reaction front moves downstream the increased flow rate through the inlet region in which the PM has been combusted decreases the flow rate downstream. Hence, the convective cooling in the downstream is less effective than that in the upstream of the inlet channel. This causes the temperature rise to increase as the front moves downstream. In our experiments, the maximum temperature rise was a monotonic decreasing function of the sojourn of the temperature front in the DPF before the step change was conducted. On the other hand, when the ignition was at the downstream and the temperature front propagated against the flow direction, the maximum temperature excursion occurred shortly after the step changed occurred and its amplitude decreased as the front moved upstream. Thus, predictions of the transient temperature excursion require in addition to knowledge of the initial operating conditions and the step change knowledge of when it occurred after the combustion front formed. Our experiments were of very rapid, essentially step changes. In practice the changes happen not that fast and it is important to determine the impact of the period in which the change occurred on the amplitude of the temperature rise. Acknowledgment We wish to thank the NSF for financial support of this research and the NGK Company for providing the DPF monolith. Literature Cited (1) Adler, J. Ceramic Diesel Particulate Filters. Int. J. Appl. Ceram. Technol. 2005, 2 (6), 429. (2) Konstandopoulos, A. G.; Kostoglou, M.; Skaperdas, E.; Papaioannou, E.; Zarvalis, D.; Kladopoulou, E. Fundamental Studies of Diesel Particulate Filters: Transient Loading, Regeneration and Aging; SAE Tech. Pap., 2000; 2000-01-1016. (3) Johnson, V. T. Diesel Emission Control in ReViews the Last 12 Months; SAE Tech. Pap., 2003; 2003-01-0039. (4) Setten, V. B.; Makkee, M.; Moulijn, J. A. Science and Technology of Catalytic Diesel Particulate Filters. Catal. ReV. 2001, 43 (4), 489. (5) Schmidt, N.; Root, T.; Wirojsakunchai, T.; Schroeder, E.; Kolodziej, C.; Foster, D. E.; Suga, T.; Kawai, T. Detailed Diesel Exhaust Particulate Characterization and DPF Regeneration behaVior measurements for Two Different Regeneration Systems; SAE Tech. Pap., 2007; 2007-01-1063. (6) Kobashi, K.; Hayashi, K.; Aoki, H.; Kurazono, K.; Fujimoto, M. Regeneration Capability of Diesel Particulate Filter System Using Electric Heater; SAE Tech Pap. 1993; 930365. (7) Koltsakis, G. C.; Stamatelos, A. M. Modes of Catalytic Regeneration in Diesel Particulate Filters. Ind. Eng. Chem. Res. 1997, 36, 4155. (8) Stratakis, G. A.; Pontikakis, G. N.; Stamatelos, A. M. Experimental Validation of a Fuel Additive Assisted Regeneration Model in Silicon Carbide Diesel Filters. Proc. Inst. Mech. Engr. (Part D) 2004, 218, 729. (9) Bogdanic, M.; Behrendt, F.; Mertins, F. The Influence of a 2-component Model on the Computed Regeneration Behavior of an Uncoated Diesel Particulate Filter. Chem. Eng. Sci. 2008, 63, 2601. (10) Cutler, W. A. Overview of Ceramic Materials for Diesel Particulate Filter Applications. Ceram. Eng. Sci. Proc. 2004, 25 (3), 421.

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ReceiVed for reView February 28, 2010 ReVised manuscript receiVed April 4, 2010 Accepted April 9, 2010 IE1004465