Wrong-Way Behavior of Soot Combustion in a Planar Diesel

Aug 19, 2009 - Infrared imaging was used to study the impact of a sudden decrease ... temperature by 100 °C can lead to sudden temperature rise (wron...
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Ind. Eng. Chem. Res. 2009, 48, 8451–8456

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Wrong-Way Behavior of Soot Combustion in a Planar Diesel Particulate Filter K. Chen, K. S. Martirosyan, and D. Luss* Department of Chemical and Biomolecular Engineering, UniVersity of Houston, Houston, Texas 77204

Infrared imaging was used to study the impact of a sudden decrease in the exhaust temperature on the spatiotemporal temperature of a single layer planar diesel particulate filter (DPF). The experiments revealed that a sudden decrease in the feed temperature by 100 °C can lead to sudden temperature rise (wrong-way behavior) of about 50 °C above that obtained with the constant original temperature. The transient temperature rise highly depended on the position where the temperature shift was initiated, that is, the time that the moving temperature front stayed in the DPF before exiting it. The temperature excursion near the end of DPF was much higher than the temperature rise in middle or near the entrance. The experiments reveal that the DPF temperature during dynamic operation can exceed in a counterintuitive fashion that obtained under stationary (constant) operating conditions. This suggests that the reported melting of cordierite DPF may have been caused by rapid changes in the feed conditions due to a change in the driving mode, such as a sudden vehicle deceleration. Introduction Diesel engines emit particulate matter (PM), which is a health hazard that has to be removed. A diesel particulate filter (DPF) is currently the most efficient device for PM removal from the engine effluents.1,2 This wall-flow monolith is a ceramic block consisting of thousands of square parallel channels, with the opposite ends of adjacent channels being plugged. The exhaust gas is forced to flow through the porous walls from the inlet to the outlet channels, while about 95% of the PM accumulates on the filter.3,4 The collected PM increases the backpressure of the diesel engine and the DPF needs to be periodically regenerated by combustion. The exothermic combustion sometimes leads to unexplained excessive local temperature excursions that melt the common Cordierite ceramic filter and destroy it.5–9 Thus, a safe and reliable regeneration is the most demanding technological challenge in the operation of the DPF. Following the pioneering work of Bissett10,11 there have been many theoretical and experimental studies of the maximum PM combustion temperature rise during the stationary (constant operating conditions) DPF regeneration.12–25 They revealed that the peak temperature depended on the diesel exhaust conditions, that is, oxygen concentration, temperature, and flow rate and on the DPF properties such as channel size, filter thickness, material, catalyst, initial DPF temperature, and the PM layer load.26–31,38,39 The maximum temperature rise under regular (periodic) PM regeneration and static conditions is usually not sufficiently high to melt the ceramic filter. Measurements by either a video or an IR camera have been used to measure the spatiotemporal temperature either on a half-cylindrical wallflow DPF,32–35 a single channel DPF,33 or single layer DPF.36,37 These experiments revealed that the PM combustion may occur either uniformly on the surface or as moving combustion front. The temperature rise is much higher in the later case. However, it is not sufficiently high to melt the DPF under stationary conditions. Simulations and experiments by Koltsakis et al.5,6,12 showed that a shift from regular to idle operation can generate a high temperature excursion under uncontrolled regeneration, at which the DPF temperature is initially rather high. A vehicle deceleration leads to several simultaneous effects. It decreases the exhaust * To whom correspondence should be addressed. E-mail: dluss@ uh.edu.

temperature and flow rate and increases the oxygen concentration. The dynamic response to these simultaneous perturbations may be the cause of the encountered unexplained melting of DPF. The goal of this study is to study and explain the counterintuitive response to a sudden decrease in the exhaust temperature. The motivation of this study is the well-known occurrence of a “wrong-way behavior” in packed bed reactors and monolithic catalytic convertors in which a sudden decrease of the feed temperature may lead to large temporal temperature rise above the one attained when the feed temperature is constant. The cause of this counterintuitive dynamic response is the difference in the propagation speeds of the reactant concentrations and temperature perturbations caused by the large difference in the heat capacity of the gas and solid filter. This leads to a delay between the arrivals of these propagating perturbations to the downstream locations. This mechanism is rather different from the reaction-diffusion one. The wrong-way behavior was predicted to occur by Boreskov et al.40 and Crider and Foss.41 Early observations were presented by Hoiberg et al.,42 Van Doesburg and DeJong,43,44 and Sharma and Hughes.45 Oh and Cavendish46 and Sun et al.47 reported its impact on monolith automobile convertors. Mehta et. al48 developed criteria predicting this behavior using a pseudohomogeneous reactor model. Studies of the wrong-way behavior using a more realistic two-phase model of the reactor were presented by Pinjala et al.49 and Chen and Luss50 and Dudukovic and Kulkarni.51 We conducted an extensive set of experiments to determine the impact of a sudden decrease in the feed temperature to the DPF on its spatial-temporal temperature, while maintaining constant the feed flow rate and composition. Special attention was given to determine the impact of the direction of movement (up or downstream), temperature decrease amplitude, and the soot load. It is hoped that the understanding of the surprising, counterintuitive DPF temperature rise following a decrease in the exhaust temperature will enable development of operation and control strategies which circumvent the destruction of the DPF. Experimental System and Procedure The spatiotemporal temperature during soot combustion on a planar catalytic single layer DPF in a mixture of air and nitrogen was measured by a high speed (up to 60 frame/s) infrared camera (Merlin, MW18, Indigo Systems) held 50 cm

10.1021/ie900848d CCC: $40.75  2009 American Chemical Society Published on Web 08/19/2009

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Figure 1. (A) Schematic of the experimental system; (B) schematic of the reactor.

above the quartz window. The camera has a 256 × 256 indium antimonide detector array sensitive to 3-5 µm wavelength radiation. The IR signals were calibrated using K-type thermocouples readings before each experiment. They were recorded at the rate of 10 per second on a PC using the ImageDesk II software. The magnitude, shape, and motion of the thermal front on the DPF were determined by image analysis software (ThermaGRAM). Typical soot combustion lasted less than 1 min. The planar single layer DPF (90 mm long, 20 mm wide) was cut from a commercial DPF (NGK-6000YE). Both exterior sides and bottom of the single layer DPF were sealed by ceramic glue. The soot layer was deposited uniformly on the top of the planar DPF held in a stainless steel (316 L) insulated reactor (120 × 40 × 40 mm). The reactor top was an IR transparent quartz window, located about 3 mm above the planar DPF. It enabled visual and infrared imaging of the temperature field on the planar DPF. Owing to experimental setup limitations we captured images only for locations located between 30-90 mm from the DPF inlet. The preheated synthetic exhaust gas mixture passed through the soot layer and porous DPF wall to the outlet channel. Figure 1a is a schematic of the experimental system, while Figure 1b is of the reactor holding the filter. The PM regeneration in a DPF is rather sensitive to the PM deposition conditions and structure.25,3 We were unable to obtain a uniform reproducible PM deposition using the exhaust of a diesel engine.5 Thus, we simulated the PM by spraying a solution of alcohol containing carbon black nanoparticles (40 nm, Sigma Aldridge) on top of the planar DPF. The alcohol was later removed by a stream of nitrogen. As a check of the uniformity of the deposition each experiment was repeated at least three times. Preliminary experiments showed that the temperature profile and the location at which the ignition occurred were affected by heat loss from the reactor walls. To minimize this heat loss, the sides and bottom of the reactor were heated by an electrical ceramic heater (Watlow Instruments Co, Inc.) to a temperature set by a power controller (Staco Energy Products Co., 2PF1010). 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. Five K-type thermocouples (diameter ∼0.5 mm) placed at the bottom of the single layer DPF monitored its temperature. 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 was in the middle of DPF, and the fifth was 10 mm ahead of the outlet. The thermocouples signals were recorded and processed by an Omega data acquisition board connected to a PC. Compressed nitrogen purged the air in the reactor before each experiment. After that the reactor walls temperature was increased from room temperature to the preset temperature. The planar DPF was preheated to the desired experimental temperature by an electric heater (Hoskins furnace, FD303), while the reactor was fed by pure nitrogen. The reading of the first thermocouple in the reactor was used to control the feed temperature. After the desired temperature was reached the feed was switched to the air-nitrogen mixture. The gas flow rates were measured by rotameters (Fisher & Porter, 10A6100), calibrated periodically. The feed pressure was controlled by downstream pressure regulators. The experiments determined the impact of a rapid decrease of the feed temperature, the temperature front location, and direction of motion on the temporal temperature rise. Experimental Results Experiments were conducted to determine the dependence of the temporal temperature rise on the amplitude of the temperature decrease and the location of the temperature front when the change was implemented and its direction of motion. The experiments were conducted for soot loadings of 10 and 20 g/L, which corresponded to a thickness of about 120 and 240 µm, respectively. The feed conditions were a temperature of 620 °C (unless otherwise indicated), an oxygen concentration of 10 vol %, and a superficial filter velocity of 12 cm3/(cm2 · s). We conducted first a series of experiments under stationary conditions, that is, at constant feed conditions in order to

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Figure 3. Thermal images of motion of hot region during DPF regeneration. Feed temperature was decreased from 620 to 520 °C. O2 ) 10 vol % and V ) 12 cm3/(cm2 · s); PM loading of 10 g/L.

Figure 2. Moving temperature zone during stationary DPF regeneration. Feed temperature of 620 °C, O2 ) 10 vol % and V ) 12 cm3/ (cm2 · s). PM loading: (a) 10 g/L; (b) 20 g/L.

determine the temperature rise due to the decrease in the feed temperature. Under these operating conditions a DPF loaded with 10 g/L of soot ignited at the DPF end (Figure 2a). The hot region was separated from the surrounding colder zone by a sharp temperature gradient and moved from the downstream to the upstream of the DPF, eventually conquering the whole surface. As the temperature front moved toward the upstream the amplitude of the peak temperature decreased from 762 to 745 °C. Montierth52 observed a similar upstream moving front in experiments using fuel with regeneration assisted additives. This motion was also observed in numerical simulations.53 Usually for low soot loading or low O2 concentration, the reaction rate at the upstream section was not sufficiently high to ignite the upstream soot. The heat generated by the reaction heated the downstream flowing gas, enabling ignition at the downstream. For high soot loading or high oxygen concentration the upstream reaction rate was sufficiently high to cause ignition there. When the filtering flow rate was decreased to 5 cm3/ (cm2 · s) the ignition occurred at the upstream section of the DPF and the combustion front moved in the downstream direction. For a soot loading of 20 g/L, ignition occurred at the upstream section of the DPF under normal operation (Figure 2b). However, when the oxygen concentration was increased to 12.5 vol % ignition occurred in the downstream section of the DPF. The arrows in Figure 2, 3, and 7 denote gas flow direction. The time marked in the figures is of time after the reactants mixture was fed to the reactor. Times marked with a plus sign (Figures 3 and 7) denote time after the feed temperature was changed.

Figure 4. Moving ignited region originating from downstream of DPF. Images shown in Figure 3.

Figure 3 shows IR images of a case in which the feed temperature was decreased rapidly from 620 to 520 °C when the ignited zone was still at the downstream section of the DPF and the soot loading was 10 g/L. The upstream moving temperature front increased in about one second after the decrease in the feed temperature. The maximum transient temperature of 813 °C was 51 °C higher than the maximum of 762 °C, obtained when the feed temperature was kept constant at 620 °C. The amplitude of the upstream moving front decreased with time. It faded for times larger than 10 s, so that the soot at the upstream section did not burn and get consumed. This extinction occurred as under these operating conditions the DPF was quenched at a constant feed temperature of 520 °C. The dynamic features of a wrong-way behavior can be better discerned from a sequence of temporal temperature profiles than from thermal images, such as those shown in Figure 3. Figure 4 is such a sequence for the case shown in Figure 3. The figure shows that the reaction front moved smoothly in the upstream direction from the end of the reactor, with an expected slight decrease in the amplitude with time. As the reaction front moves from the end of the reactor in the upstream direction, the

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Figure 6. Temporal temperature measured by thermocouples in the reactor: thermocouples 2-5 are attached to DPF; no. 1 is ahead of DPF.

Figure 5. Upstream moving temperature profiles with the highest peak for 10 g/L; feed temperature was decreased when temperature front was at (1) downstream, (2) middle, (3) upstream section of DPF.

incoming gas flows in a direction opposite to that of the moving hot zone. The temperature of the incoming gas is much lower than that of the ignited hot zone. The sudden decrease in the feed temperature at t ) 41 s led to a sharp increase in the peak temperature to 813 °C. When the feed temperature was decreased just to 570 °C, while the ignited region was still at the end of the DPF, the maximum transient temperature was 794 °C, that is, 19 °C lower than when the feed temperature was decreased to 520 °C. The experiment was repeated decreasing the feed temperature as the front was at the middle of the DPF. The corresponding maximum transient temperature was 791 °C, that is, a wrong-way temperature rise of 29 °C. Decreasing the feed temperature when the front was close to the reactor inlet led to a wrong-way of only 10 °C, that is, the maximum temperature rise was 772 °C. Figure 5 describes temporal profiles with the highest temperature peak for three cases. In case 1, the feed temperature was decreased to 520 °C when the ignited region was at the DPF downstream. Graphs 2 and 3 are of cases in which the temperature was decreased when the moving temperature front was in the middle and upstream section of the DPF, respectively. The figure points out that the peak transient temperature was a monotonic decreasing function of the duration of the temperature front in the DPF before the feed temperature was decreased. Ignition at the upstream section of a DPF occurs at a soot loading of 10 g/L for a filtering velocity of 5 cm3/ (cm2 · s). When the feed temperature in such a case was rapidly decreased from 620 to 520 °C while the reaction front was in the middle of the DPF, the maximum transient temperature was 770 °C, which exceeds by 30 °C the maximum obtained for a constant feed temperature of 620 °C. Figure 6 describes the temporal temperature measured by five thermocouples, the positions of which are defined in the experimental section. The temperatures measured by thermocouples 3-5 were the same until the temperature front reached thermocouple 3. (At t ) 40 s). When the temperature front was close to thermocouple no. 4, the inlet temperature (measured by thermocouple 1) was decreased. This caused the maximum temperature at thermocouple no. 5 to reach 770 °C. The temperature at thermocouples 2-5 became equal after the temperature wave exited the DPF. When the feed temperature was decreased to 570 °C the maximum transient temperature was 763 °C, almost the same as when the feed temperature was decreased to 520 °C.

Figure 7. Thermal images of motion of the hot region during DPF regeneration. Feed temperature was decreased from 620 to 520 °C. O2 ) 10 vol % and V ) 12 cm3/(cm2 · s); PM loading of 20 g/L.

Experiments similar to those conducted with a loading of 10 g/L were conducted with 20 g/L loading. The ignition in these cases occurred at the reactor inlet and the temperature front moved downstream. Figure 7 shows IR images for a case in which the feed temperature was rapidly decreased from 620 to 520 °C while the ignited zone was still at the upstream section of the DPF. The rapidly moving front reached a maximum temperature of 813 °C, which exceeded by 38 °C that attained under constant feed temperature of 620 °C. When the feed temperature was decreased as the temperature front was in the middle (downstream section) of the DPF, the peak temperature exceeded only by 30 °C (12 °C) that attained under constant feed temperature. Figure 8 describes the dependence of the maximal local temperature during the regeneration. Graph 1 is of a case in which the feed temperature was decreased while the ignited zone was in the upstream section of the DPF. Graphs 2 and 3 are of cases in which the temperature was decreased as the reaction front was in the middle and downstream section of the DPF, respectively. The figure points out that the peak transient temperature was a monotonic decreasing function of the duration of the temperature front in the DPF before the feed temperature was decreased. Moreover, the maximum local temperature occurred at the downstream section of the DPF. The experiments revealed a rather moderate impact of the magnitude of the temperature decrease on the amplitude of the wrong-way temperature rise. For example, when the temperature front was in the middle of the DPF, a decrease of the feed temperature to 520 °C led to a transient peak temperature of

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Figure 8. Moving temperature profiles with the highest peak for 20 g/L; feed temperature was decreased when the temperature front was at (1) upstream, (2) middle, (3) downstream section of DPF.

805 °C. When the feed temperature was decreased to 570 °C the peak temperature was 800 °C. For a soot loading of 20 g/L the ignition location can be shifted to the end of the DPF by a sufficient increase in oxygen concentration. We conducted such experiments using an oxygen concentration of 12.5 vol % for cases in which the temperature front was in the middle of the DPF. When the feed was cooled to 520 and 570 °C, the transient peak temperatures were 815 and 807 °C, respectively. Thus, the amplitude of the temperature decrease had a rather mild impact on that of the transient temperature rise irrespective of the direction of motion of the temperature front.

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unexpected damage of DPFs may have been caused by some dynamic behavior and this explains why it occurs only sometimes. An uncontrolled regeneration of a DPF occurs only in a small fraction of the diesel engine operation time. Thus, most changes in the exhaust properties upon vehicle deceleration will not lead to a DPF temperature rise that may cause its destruction. Our experiments were of cases in which only the feed temperature was suddenly decreased. In practice, a vehicle deceleration leads to a simultaneous change of the exhaust temperature, oxygen concentration, and flow rate. For example, the exhaust gas of a typical 1998 passenger car diesel engine with 2000 rpm operating at full loading has a temperature of 530 °C, contains 5.1 vol % oxygen and a mass flow rate of 0.26 kg/s. Under idle operation the exhaust has a temperature of 190 °C, contains 14.4 vol % oxygen and its flow rate is 0.08 kg/s.2 Under stationary operation an increase in the oxygen concentration and/or the flow velocity increases the DPF temperature. The knowledge and ability to predict the impact of the simultaneous changes of the feed temperature, oxygen concentration, and flow rate are essential for the proper design of DPF control and operation strategies and this is currently studied by us. The PM deposition in a DPF is usually nonuniform. Moreover, the filtration velocity depends on the axial position in the DPF. It is of interest to determine the impact of such nonuniformities on the maximal local temperature rise. In particular, under what conditions can these lead to an excessive temperature sufficient to cause local melting of a cordierite filter. Acknowledgment We wish to thank the NSF for financial support of this research and the NGK Company for providing the DPF monolith.

Discussion and Conclusions The experiments showed that a sudden decrease in the feed temperature to DPF, while maintaining constant exhaust flow rate and oxygen concentration, can lead to a counterintuitive transient temperature rise (wrong-way behavior). A surprising finding was the fact that the peak transient temperature depended on the time that the moving temperature front stayed in the system (Figure 5). The observation that when the temperature front was moving in the downstream the maximum local temperature occurred at the downstream section of the DPF (Figure 8) may explain the why most of the damage to commercial DPFs is encountered in their downstream section. The heat loss to the surrounding in our experimental system was much larger than in a commercial one because of the IR transparent window above the single layer DPF. Moreover, the impact of the heat losses normal to the flow direction in our system differs from the radial heat loss in a commercial DPF. Thus, the observed temperature rise due to the decrease of the feed temperature in our experiments is smaller than that that would be encountered under the same operating conditions in a commercial DPF. The experiments provide a qualitative description of the behavioral feature of the wrong-way in a DPF, but not a quantitative one. It would be very useful to develop some criteria predicting the magnitude of the wrong-way behavior and its dependence on the operating conditions, amplitude of the temperature decrease, and when it occurs. The present study indicates that the temperature rise during the dynamic operation of the DPF can exceed that under stationary operating conditions. This suggests that reported

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ReceiVed for reView May 23, 2009 ReVised manuscript receiVed July 18, 2009 Accepted August 3, 2009 IE900848D