Soot Removal Mechanism in an Electric Diesel Particulate Trap

Transcription

1 F2008-SC-017 Soot Removal Mechanism in an Electric Diesel Particulate Trap 1 Hiroyuki Hoshino*, 2 Youhei Mitsuyama, 1 Masahiro Saito, 1 Tomohiko Furuhata, and 1 Masataka Arai 1 Gunma University, Japan, 2 Fuji Heavy Industries Ltd., Japan KEYWORDS Particulate Trap, Soot Trapping, Soot Re-burning, Electric Field, Soot Bridge ABSTRACT - In order to improve the performance of electric diesel particulate trap (E-DPT) that has been proposed by the authors, behavior of trapping and re-burning of diesel soot was investigated using an electric diesel particulate trap with a pair of electro-plates (1-channel E-DPT). The E-DPT was capable to trap soot by applying DC electric field between electro-plates. Experimental results showed that soot was accumulated on the electro-plates and some of soot deposits were like whiskers. Soot trapped on the plates formed bridge structures in narrow space between electro-plates and was ignited with Joule heat of electrification. When a soot bridge was re-burned, soot deposits on the plates near the bridge were also re-burned. After the re-burning, a new soot bridge was re-formed and above behaviors were automatically repeated. INTRODUCTION Diesel engine has high thermal efficiency and it is very economical. Therefore, diesel vehicles are widely used in transportation. However, exhaust gas from diesel engine contained much PM (Particulate Matter) and NO x those have negative influences on human body and global environment.then, reduction of these pollutions is an important issue for diesel engine development. There are two ways of countermeasures for reduce of these pollutions. One is a modification in engine combustion (EGR, Lean combustion engine, etc.). The other is an after-treatment (DPF, Urea-SCR, etc). As for the after-treatment systems of PM, a continuous regeneration DPF (CR-DPF) is the mainstream at present development. This system uses catalysis and soot is re-burned with a help of high temperature exhaust gas. However it has a problem that pressure loss increases during long time use and exhaust gas temperature should be kept high for regeneration. (1)~(4) Therefore, various DPF materials were investigated such as wall-flow ceramic. (5)~(7) Authors have developed a new type diesel particulate trap (E-DPT) using electric field (8)~(11) with 50 electro-plates. The developed device shown in Fig.1 was capable to trap diesel soot Figure 1 E-DPT device

2 with an electric field between electro-plates. Diesel soot was trapped on the plates and was re-burned continuously with Joule heat of electrification through soot bridges formed in a narrow space between the plates. In this study, in order to investigate the soot removal mechanism of E-DPT, soot trapping and soot re-burning behavior were observed with a one channel diesel particulate trap (1-channel E-DPT). Re-burning Soot bridge + - Soot flow Electro-plate E E-DPT DEVICE Figure 2 Soot trapping and re-burning mechanism Soot trapping and re-burning mechanism of E-DPT is schematically shown in Fig.2. DC (direct current) field was applied to the electro-plates. The positive and negative electroplates were arranged alternately. It was considered that particles of diesel soot were adhered to the electro-plates with particle diffusion or electro dynamic force caused by its electric polarity. When the deposit amount of diesel soot on the electro-plates increased, soot bridges were formed between electro-plates. In other words, soot bridges were formed when the deposits of the soot exceeded over a critical amount. Joule heat was generated by electrification through the soot bridge, and it resulted self re-burning of the soot bridge. Soot re-burning on a local spot was automatically stopped when the soot bridge was completely burned out, and again soot was deposited there. Then this E-DPF device could create a field of continuous regeneration because trapping and re-burning of diesel soot were repeated. In the previous report (11) Analysis of Soot Removal Mechanism using an Electric Diesel Particulate Trap by the authors, the experiment was performed with an apparatus shown in Fig.3. As the result of the previous studies, it was clear that trapping and re-burning of the diesel soot were performed continuously, and soot removal efficiency of around 50% was attained at the steady state condition shown in Fig.4. However, the detail mechanism of soot re-burning in the E-DPT was not clear because of the difficulty of observation. To observe the inside behavior of trapped diesel soot inside of the E-DPT, more simple experimental device was needed. Load Heater Engine In T Electro-plate VR A R V E g T Thermocouple Out 3-way valve Orifice Glass fiber filter Manometer Soot removal efficiency η % Ms-in = 25mg/min V = 0.3m/s L = 1.2mm Steady soot re-burning Eg=200V Eg=250V DC power supply Elapsed time t min Figure 3 Schematic diagram of experimental apparatus (E-DPT) Figure 4 Soot removal efficiency of E-DPT

3 In Out Orifice 1-channel E-DPT Heater V E g A Engine Load Digital Camera DC power supply Figure 5 1-channel E-DPT device Table 1 Exhaust gas composition O 2 (%) 4.9 CO 2 (%) 8.5 CO (%) 2.0 NO X (ppm) Excess air ratio Total PM (mg/kwh) Engine output (kw) Fuel JIS 2 Diesel High speed video camera Figure 6 Schematic diagram of experimental apparatus (1-channel E-DPT) Table 2 Experimental condition Gas flow rate Q l/min 10 Mass flow rate of supplied soot M s-in mg/min 2.5 DC applied voltage E g V 0, 200, 360 Space between electro-space L mm 1.0 Gas mean velocity V m/s 3.6 EXPERIMENTAL APPARATUS AND METHOD 1-channel E-DPT to observe the trapping and re-burning behavior in a narrow space between electro-plates is photographically shown in Fig.5. A pair of stainless steel plates (300mm 46mm, thickness 15mm) was set in the device with narrow space. DC field was applied to the electro-plates. In order to observe the behavior of soot particles in the 1- channel E-DPF device, outer package was made of acrylic boards and PYREX glasses. A schematic diagram of the experimental apparatus is shown in Fig.6. A part of exhaust gas from 4 stroke diesel engine (400cc displacement, KUBOTA Co., Ltd, Z-402E) was introduced into the device. Concentration of diesel soot in the exhaust gas was very low at normal operation. Therefore, exhaust gas with high diesel soot concentration was generated by throttling intake air to the engine. The exhaust gas composition for E-DPT test is shown in Table 1. A part of exhaust gas was introduced into the 1-channel E-DPT while keeping the gas temperature at about 100 with heater. The gas flow rate introduced in the device was monitored by an orifice and a manometer. Table 2 shows that the experimental condition. The exhaust gas flow rate Q was fixed at 10 l/min. It was 2% of total exhaust gas of the engine. The mass flow rate of soot M s-in supplied to the device was 2.5mg/min. DC voltage E g applied to the electro-plates was ranged from 0V to 360V with a constant voltage DC power supplying device (Matsusada Precision Inc, PQ350-2). The space between electro-plates L was fixed on 1.0mm. Mean velocity V of sample gas in narrow space between electro-plates was 3.6m/s. Space velocity was 44,000h -1 at this experimental condition. Appearances of soot trapping and reburning were taken with a digital camera (Nikon, D50) and a high-speed video camera (PHOTRON Ltd, FASTCAM-512PCI, 2000frames/sec).

4 Electric current Im A M s-in = 2.5 mg/min V = 3.6 m/s F = 200 kv/m L = 1.0 mm Soot accumulation Soot re-burning Steady soot re-burning Elapsed time t min Figure 7 Relation between electric current and elapsed time(e g =200kV/m, L=1.0mm) RESULTS AND DISCUSSION Electric current Relation between electric current and elapsed time at condition of E g =200V is shown in Fig.7. Electric current increased linearly with the elapsed time. It was considered that electric current increased proportionally with an increase of soot bridges in 1-channel E-DPT. The electric current increased up to I m =0.25A at t=15min. Then, rapid soot re-burning was observed. As a result, electric current decreased down to I m =0.15A. After this, electric current increased again. At t=23min, the second rapid soot re-burning was observed, and electric current decreased to I m =0.07A. After then, quiet behaviors of re-burning of soot were repeated with low electric current, and soot trapping and re-burning were balanced. Soot trapping Figure 8 shows soot trapping process in narrow space between electro-plates at E g =200V and E g =360V. Elapsed time from the beginning of exhaust gas loading was described by t min. In both conditions, positive voltage was applied to the upper electrode. The observation area was indicated in Fig.8 In 1-chnnel E-DPT device, diesel soot were adhered to electro-plates by effect of electric intensity. At E g =200V, a whisker type soot accumulation appeared clearly at t=2min. It was considered that diesel soot particles deposited on the electro-plates and formed soot whiskers. Since a whisker type accumulation could not observed in the condition of E g =0V, it was considered that the appearance of soot whisker was caused by applying DC field to electro-plates. After t =2min, soot whisker started to grow from both electro-plate sides to the center of narrow space between the plates. The soot whiskers growing from negative electro-plate (lower plate) were hardly different from that of growing from positive one (upper plate). At t=10min, soot whiskers growing from both electrodes were connected each other and formed soot bridges between electro-plates. Further, small soot branch whiskers grew around the soot bridges. After this, soot adhered to the soot bridges(t=15min, 20min). As a result, it was observed that soot bridges grew up thickly (t=30min, 35min). Moreover, small soot branch whisker was hardly observed. It considered that small soot branch whiskers were

5 100mm View point In + - E g =200V E g =360V Out 1mm 1mm 0 t=20 min 0 t=15 min t=2 min t=30 min t=5 min t=16 min t=10 min t=35 min t=8 min t=20 min t=15 min t=40 min t=13 min t=40 min Figure 8 Soot trapping (E g =200V, 360V) involved into soot bridges or were moved downstream by exhaust gas flow. As for the case that higher voltage (E g =360V) was applied, soot whiskers grew and formed soot bridges. Compared to the case of E g =200V, it was observed that a lot of soot whiskers and soot bridges were formed thinly in the case of E g =360V. It was considered that the electric intensity was strong in the space between electro-plates, therefore, diesel spot particles were directly deposited on the plates and soot whiskers and soot bridges were less formed. It seemed that the amount of soot whisker on positive electrode was as similar as that on negative one. It was considered that a population of positively charged particles was as similar as that of negatively charged ones in exhaust gas from diesel engine. Soot bridge re-burning In order to investigate the re-burning behavior of diesel soot in E-DPT, the behavior of soot bridge re-burning at E g =360V was observed using a high-speed video camera. Photographs of soot bridge re-burning were shown in Fig.9. In this figure, time t means elapsed time from the observation start. At t=0, a soot bridge was already formed between electro-plates. The soot bridge hardly moved (t=1.0ms), and a luminescence of light was observed on it at t=2.0ms. It means the beginning of the soot bridge re-burning. After this, the luminescence spot moved upward

6 1mm t =0 t =6.0ms t =1.0ms t =6.5ms t =2.0ms t =7.0ms t =2.5ms t =7.5ms t =4.0ms t =8.0ms t =5.5ms t =8.5ms Figure 9 Soot bridge re-burning (E g =360V, L=1.0mm) plate direction (t=2.5ms). It was considered that soot particles deposited on the upper electroplate were ignited by the soot bridge re-burning. At t=4.0ms, the luminescence disappeared. Next re-burning of a soot bridge was started (t=5.5ms) with intense luminescence at the different position from the previous re-burning. The luminescence intensity (t=5.5ms) was changed (t=6.0ms ~ t=7.5ms). At t=8.0ms, two other re-burning spots appeared. It means that the re-burning was propagated toward both electro-plates through the soot bridge. The luminescence disappeared (t=8.5ms). After all, the soot re-burning was finished and the soot bridge structure disappeared completely from the space between electro-plates. It was revealed that soot bridge was re-burned in the narrow space between electro-plates at first, after then, the soot deposited on the plates was ignited by soot bridge re-burning.

7 Re-burning Fig.10 Behavior of soot bridges after re-burning (E g =360V) Behavior of after soot bridge re-burning In order to investigate appearance between electro-plates after soot bridge re-burning, remained soot bridges were observed. Figure 10 shows behavior of soot bridges after reburning at E g =360V. At first, a part of soot bridge disappeared by soot re-burning. Then, soot adhered to the broken soot bridge and soot bridges were re-formed. However, broken soot bridges were not always re-formed. It seems that some of them were remained on the electro-plates as soot clusters, or peeled off and exhausted from 1-channel E-DPT device. CONCLUSIONS From the observation of 1-channel E-DPT, following conclusions were derived. (1) In soot accumulation state, electric current between electro-plates increased linearly due to formation of soot bridges. Rapid soot re-burning occurred when electric current exceeded a certain value. Then, electric current decreased. After that, quiet re-burning of soot repeated with low electric current, and soot trapping and re-burning were balanced. (2) Soot whiskers grew up and bridge structures were formed. There were no effects of polarity of electro-plate. (3) Soot re-burning was propagated through the soot bridge toward both electro-plates, so soot deposited on electro-plates was also re-burned. (4) After soot bridge re-burning, a new soot bridge was formed from broken soot bridge and soot particles.

8 REFERENCES (1) Y.Tsushino, K.Harada and A.Takami, Development of Catalyzed Diesel Particulate, Proceedings of 2005 JSAE Annual Congress in Spring, 2005, No.31-05, pp (2) J.Gieshoff, M.Pfeifer, A.Schafer-sindlinger, U.Hackbarth and O.Teysset, Regeneration of Catalytic Diesel Particulate Filters, SAE Paper No (3) P.Richards, B.Terry and M.W.Vincent, Combining Fuel Borne Catalyst, Catalytic Wash Coat and Diesel Particulate Filter, SAE Paper No (4) M.Masoudi, A.G.Konstandopoulos, M.S.Nikitidis, E.Skapedas, D.Zarvalis, E.Kladopoulos and C. Altiparmakis, Validation of a Model and Development of a simulator for Predicting the Pressure Drop of Diesel Particulate Filters, SAE Paper No (5) Y.Miyairi, S.Miwa, F.Abe, Z.Xu and Y.Nakasuji, Numerical Study on Forced Regeneration of Wall-Flow Diesel Particulate Filters, SAE paper No (6) G.A.Merkel, W.A.Cutler and C.J.Warren, Thermal Durability of Wall-Flow Ceramic Diesel Particulate Filters, SAE Paper No (7) J.Kitagawa, Current and Future Tasks of Ceramic Honeycomb Diesel Particulate Filter JSAE, SYMPOSIUM, Challenge for Future Clean Diesel, 1996, pp (8) Z.Hang, M.Saito, K.Amagai and M.Arai, Soot Trapping and Re-burning by Means of an Electric Field, JSME, Vol.34, No.2, 2003, pp (9) Y.Mitsuyama, M.Saito,T.Furuhata and M.Arai, Soot removal characteristics of an electric diesel particulate filter, JSAE-KSAE 18th Internal Combustion Symposium, 2007,pp (10) M.Arai, M.Saito, and Y.Mitsuyama Continuous regeneration of electrically heated diesel particulate trap, J.Engine Res. Vol.8, 2007, pp (11) H.Hoshino, Y.Mitsuyama, M.Saito, T.Furuhata and M.Arai, Analysis of Soot Removal Mechanism using an Electric Diesel Particulate Trap, Transactions of JSAE, Vol.38 No.6, November 2007, pp