Carbon particulate matter incineration in diesel engine emissions using indirect nonthermal plasma processing

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1 Carbon particulate matter incineration in diesel engine emissions using indirect nonthermal plasma processing Masaaki Okubo a,, Naoki Arita b, Tomoyuki Kuroki a, Toshiaki Yamamoto a a Department of Mechanical Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka , Japan b Denso Corporation, 1-1 Showa-cho, Kariya-city, Aichi , Japan Available online 6 March 2006 Abstract One of the promising applications of nonthermal plasma (NTP) for environmental cleanup technology is low-temperature oxidation or incineration of carbon particulate matter (PM) in diesel engine emissions. In this process, NO 2 and activated radical species induced by NTP can incinerate carbon PM trapped by a diesel particulate filter (DPF) at low temperature (<300 C). In the present study, an experiment was carried out on indirect NTP DPF regeneration for real diesel engine emissions comprising CO 2 of several per cent, hydrocarbons of several hundreds of ppm and moisture of several tens of percentages. It was confirmed that DPF regeneration is possible for a real diesel emission at a low temperature of 280 C. The removal energy efficiency was estimated to be 0.82g/kWh. This electric power range is sufficient to meet the recently proposed longterm national regulation for diesel automobiles in Japan Elsevier B.V. All rights reserved. Keywords: Nonthermal plasma; Particulate matter; Diesel engine; Aftertreatment 1. Introduction Corresponding author. Tel./fax: address: mokubo@me.osakafu-u.ac.jp (M. Okubo). For purifying automotive diesel engine emissions, various types of aftertreatment technologies for reducing nitrogen oxides (NOx) and particulate matter (PM) using catalysts [1 3] and nonthermal plasma processes [4 10] have been investigated. However, particulate matter, particularly carbon PM, requires a more effective aftertreatment technology. A ceramic diesel particulate filter (DPF) fabricated from SiC or cordierite has been widely used for collecting carbon PM [11 15]. The typical structure of a DPF is shown in Fig. 1[11]. The PM is deposited in the porous filter wall during the passage of the cross-flow. In order to develop a selfconsistent aftertreatment system, it is very important to consider DPF regeneration or the removal of deposited PM. Therefore, various methods have been proposed as shown in Fig. 2. Oxygen incineration with an afterburner or a heater shown in Fig. 2(a) is considered the most typical regeneration process [12,13]. The removal or incineration of carbon PM is carried out under an oxygen-rich environment using a high-temperature gas of T g >600 C. However, since the gas is at a high temperature, the DPF sometimes melts or cracks. Recently, the regeneration method of NO 2 catalytic incineration, which is shown in Fig. 2(b), has been attracting considerable attention [15 17]. In this method, the incineration of carbon PM begins at a low temperature of approximately 250 C if sufficient amount of NO 2 is available (NO 2 /C>8). NO 2 is obtained from NO oxidation using an oxidation catalyst selected from among the noble metals. However, the maximum efficiency of the oxidation catalyst is around 50% at high temperatures of C. Further, this efficiency is degraded by the influence of sulphur in light fuels. More recently, a new concept of regeneration using nonthermal plasma (NTP) was proposed [15,18 22]. In this method, the oxidation catalyst is replaced with a nonthermal plasma reactor that generally exhibits excellent performance in terms of NO oxidation or NO 2 production [6,7] even at atmospheric temperature. This regeneration method is considered to be effective, particularly when engines operate from a

2 Wall of porous filter Deposited PM Plug Inlet Outlet : Dusty gas : Clean gas Fig. 1. Structure of diesel particular filter (DPF). cold start or under light load. It is also effective in the engine brake operation during which the activity of the oxidization catalyst is suppressed due to the low temperature. Further, the Fig. 2. Methods of DPF regeneration. (a) Oxygen incineration with afterburner (T g >600 C). (b) NO 2 catalytic incineration with oxidation catalyst (T g >250 C). (c) NO 2 and ozone incineration with direct NTP reactor (T g >200 C). (d) NO 2 and ozone incineration with indirect NTP reactor (T g >200 C). problem of catalyst degradation under the influence of sulphur in light fuels does not occur. With the aim of developing a compact system, a new nonthermal plasma regeneration system was investigated in the previous study [23] by combining a DPF and a plasma reactor, as shown in Fig. 2(c). In this system, the nonthermal plasma generated inside the honeycomb path of the DPF oxidizes NO to NO 2, which along with an activated oxygen species simultaneously incinerates the deposited carbon PM. However, there are certain drawbacks such as the reduction in plasma performance and the production of additional NOx in higher temperature (300 C) NTP [23]. In the previous study, DPF regeneration is carried out by employing an indirect or remote nonthermal plasma method (Fig. 2(d)) [24 26]. The nonthermal-plasma-treated air including several activated species (O, O 2, O 3, N, OH, etc.) is injected into an exhaust pipe, thereby oxidizing NO to NO 2. The induced NO 2 and activated oxygen species burn the carbon PM deposited in the DPF at low temperature. In this method, highly efficient regeneration can be achieved by injecting a small amount of activated gas (injection flow rate/ main flow rate =1% 10%) into the main exhaust gas stream. Moreover, the problem of plasma performance reduction at higher temperatures can be avoided because the plasma is generated at atmospheric temperature. Similar remote plasma or indirect plasma techniques have been reported in many studies [27 30]. In each system shown in Fig. 2, the oxides of nitrogen including NOx, N 2 O 5 and HNO 3 should be reduced by another method such as the urea-scr (selective catalytic reduction) method or the proposed plasma-hybrid [6,7] or plasma desorption method [31,32]. Previous studies [21 26] have reported experimental results obtained by laboratory tests to confirm the principle of NTP PM incineration using a mixture of dry NOx and air in cylinders (simulated exhaust gas). However, in practice, diesel engine emissions comprise CO 2 of several per cent, hydrocarbons of several hundreds of ppm and moisture of several tens of percentages. Since the effect of these components on PM incineration has not been clarified, an experiment was performed to examine this effect and confirm the regeneration

3 Acrylic barrier Gas flow Needle electrode 110 of DPF in a real diesel emission by employing the indirect NTP method shown in Fig. 2(d). 2. Experimental apparatus and methods 2.1. Nonthermal plasma reactor Fig. 3 shows a needle-to-plane barrier-type plasma reactor used for indirect plasma processing. A similar nonthermal plasma reactor was used by Takaki and Fujiwara [33] for the exhaust gas aftertreatment of diesel engines. The channel was made of an acrylic board. The dimensions of the square grounded electrode are mm. The distance between the tip of the stainless needle, which serves as the positive electrode, and the acrylic barrier plate is 12 mm. The thickness of the barrier is 2 mm. The total number of needles is = 100. The dimensions of the square region of needles are mm 2. The residence time of the gas in the plasma reactor is 10.2s for an airflow rate of Q=1L/min Pulse high-voltage power supply Positive pulse high voltage Gas flow Grounded electrode Fig. 3. Needle-to-plane barrier-type nonthermal plasma reactor used for indirect plasma processing. A pulse high-voltage power supply (Masuda Research Inc., PPCP Pulsar SMC-30/1000) with an IGBT element was 12 used to energize the plasma reactor. The electrical circuit is shown in Fig. 4. In this system, a pulse having a fast rise time of 400ns and a short width of 300ns can be produced by applying a high DC voltage of kV and increasing its peak by means of a pulse transformer (1: 20) and magnetic pulse compression circuit. The maximum peak voltage obtained is greater than 35kV for a rated load (C=170pF). The pulse frequency can be varied in the range Hz. The applied voltage and current waveforms of the reactor were measured using a digital oscilloscope (Yokogawa Electric Corp., DL1740-1GS/s) through the voltage and the current dividers (Sony Tektronix P6015A and P6021) Experimental setup The layout of the experimental apparatus for DPF regeneration is shown in Fig. 5. Emissions from a small diesel engine generator (Yammer Co., Ltd., YDG200SS-6E, direct injection type for a single cylinder, displacement volume=200cm 3, maximum electric power output=2.0kw) are sampled using a pump, and a large proportion of PM and moisture is removed using a DPF and dryers. The flow rate is controlled by a mass flow controller (MFC). In this process, exhaust gas at a rate of 9L/min and dry air at a rate of 1L/min activated by the indirect NTP reactor (Fig. 3), which is energized by the pulse high-voltage power supply (Fig. 4), are mixed just before the cordierite DPF. The inner diameter of the gas tube is 3.5mm, and the distance between the plasma reactor and the gas mixer is 0.5 m. The mixed gas flows into a stainless steel tube (inner diameter =50mm) in which the cordierite ceramic DPF with deposited carbon PM is inserted and sealed with a mat material. The specifications of the DPF are as follows: diameter=46mm; length=50mm; electrical resistivity =10 12 Ωcm; cell number density=100cpsi (=15.5cells/cm 2 ); and thickness of porous filter wall = 17 mil (=0.43 mm). The amount of deposited carbon PM is 36mg (0.43g/L [unit DPF volume]). The DPF is heated at a constant temperature (280 C) using a tubular electric furnace (ISUZU Co., AT-58). In this experiment, low-temperature regeneration is performed. RC C0 Magnetic pulse compression SR + IGBT 1.5 ~ 1.9kV C1 0.7nF RD1 LD1 Pulse high voltage output 9Hz ~ 1kHz 35kV AC200V, Max 7A DC power supply Pulse transformer 1:20 High voltage tank Gate driven circuit Fig. 4. Electric circuit for the high-voltage pulse power supply using IGBT switching.

4 DPF Pump Tank MFC Thermocouple DPF in stainless tube (200~300 C) Dryer Dryer Exhaust = 9 L/min Heater & controller Diesel engine Δ P sensor IGBT high voltage pulse power supply Oscilloscope Air compressor Air filter & dryer MFC + V probe I probe Needle to barrier plate type indirect nonthermal plasma reactor Data recorder Radical gas = 1 L/min CO 2 & N 2 O Gas analyzers THC NO, NO 2 CO & O 2 Exhaust Fig. 5. Layout of the experimental apparatus for low temperature (280 C) DPF regeneration using the indirect nonthermal plasma processing. The pressure difference at the inlet and outlet of the DPF is monitored and recorded using a small semiconductor-type pressure difference transducer (Kyowa Electronic Instruments Co., Ltd., PDV-25GA) and digital recorder (Yokogawa Electric Corp., OR100E). The time-dependent carbon PM incineration can be identified from the decrease in the pressure difference by maintaining a constant flow rate and gas temperature. The gas temperature is monitored with a thermocouple placed near the DPF. Downstream, NOx (=NO +NO 2 ), total hydrocarbon (THC) and O 2 and reaction by-products such as CO, CO 2 and N 2 O are measured by a set of gas analyzers (Horiba Ltd., PG-235, VIA-510 and FIA-510). 3. Experimental results and discussions 3.1. Performance of the plasma reactor Prior to the regeneration experiment, the NO oxidization performance of the remote nonthermal plasma method was investigated at atmospheric temperature (25 C) using the apparatus shown in Fig. 5. The test was carried out with the sampled gas from the diesel engine under 50% load (electric power output = 1.0kW). The concentrations of NOx and NO were 230ppm and 220ppm, respectively. The relative humidity was 30%, and the total flow rate was Q = 10.0 L/ min. The following results were obtained: NO = 34 ppm, 19:16:18 Stopped 359 CH V << Main: 1k >> CH V Voltage 1k Normal 500MS/s 200ns/ div Ch1 1: V/div DC Full 10 kv Current Ch3 1: V/div DC Full 10 A 0.600us Max (C1) Rise (C1) +Wd (C1) Max (C3) 3.00 V 42.0 V 200 ns V 408ns 294ns mV V peak =34.7kV, I peak =9.6A P=10.7W (840Hz) Rise(C3) 194ns +Wd (C3) 78.0ns Edge CH1 AutoLevel 2.6 V Fig. 6. Waveforms of voltage and current for the nonthermal plasma reactor.

5 NOx=230ppm, CO=534ppm, O 2 =15.5%, CO 2 5%, THC 140ppm and N 2 O 5ppm. 85% NO could be oxidized into NO 2. The concentration of ozone was measured as 1800 ppm at the outlet of the plasma reactor using a gas detection tube. In this test, waveforms of the voltage V and current I in the pulse high-voltage supply were recorded using a digital oscilloscope, as shown in Fig. 6. The applied peak voltage V peak and peak current I peak are 34.7kV and 9.6A, respectively. The time-averaged discharge power is calculated as 10.7W from the product of the positive area bounded by the V I curve and the horizontal axis, a conversion factor related to the voltage and current probe and the pulse frequency, f=840hz. Gas concentrations (ppm) (CO 2 ~5%, ΔCO 2 ~200 ppm with NTP on) On Off Plasma on Off 600 CO O NOx NO 100 THC Elapsed time (hr) O 2 concentrations (%) 3.2. DPF regeneration experiment The DPF regeneration experiment was performed under the same NTP condition as that in Section 3.1. The experimental result of pressure difference vs. elapsed time is shown in Fig. 7 (50% engine load). The temperature of the DPF is maintained almost constant at 280 C. From the graph, it is confirmed that the pressure difference decreases significantly over time only when the plasma is turned on and the plasma regeneration occurs. The pressure difference is almost constant when the plasma is turned off because the main flow is already purified by the DPF adjacent to the diesel engine; a small amount of PM is being deposited in the DPF. The PM concentration in the main flow was 1.1mg/Nm 3. Fig. 8 shows the relation between the gas concentration and the elapsed time during the experiment. The performance of the plasma reactor at room temperature is shown at the point t=0 when the plasma is turned on. The temperature of DPF reaches 280 C at t=0.5h. When the plasma is turned on at t=2.75h, the concentration of CO increases due to the incineration of carbon PM. Although CO 2 concentration has not been plotted in this figure, it was approximately 200 ppm. In previous papers [24,25], it has been reported that the reductions in NO 2 and some types of oxygen species (mainly ozone) contribute to the incineration of carbon PM Fig. 8. Relation between the gas concentration and the elapsed time during the experiment. deposited inside the DPF based on the following chemical reactions: C þ 2NO 2 CO 2 þ 2NO ð1þ C þ NO 2 CO þ NO C þ 2O 3 CO 2 þ 2O 2 C þ O 3 CO þ O 2 In Fig. 8, NOx and THC are also reduced when the plasma is turned on. Although these reductions are necessary for purifying the diesel emissions, the detailed mechanism has not been clarified. The concentration of ozone was approximately zero at the DPF outlet because it was almost completely ð2þ ð3þ ð4þ Temperature (deg. C) Pressure difference Plasma off Temperature Indirect NTP method Q = 10 L/min Inlet NO = 260 ppm Plasma on Plasma off Pressure difference (kpa) Elapsed time (hr) Fig. 7. Changes in pressure difference with elapsed time when the nonthermal plasma is turned on and off. Fig. 9. Photographs of the lateral cross section of the DPFs obtained before and after regeneration: (a) before regeneration; (b) after regeneration for 2.5h.

6 decomposed in the DPF heated at 280 C in the present configuration. Photographs of the lateral cross section of the DPFs obtained before and after regeneration are shown in Fig. 9.InFig. 9(b), a triangular region of black carbon PM remains near the DPF outlet after regeneration. It is considered that this triangular unburned PM region may be caused by the nonuniform distributions of NO 2 and O 3 in the cross-section of the DPF. A measurement performed with an electric balance (Shimadzu Corp., AX-120) revealed that the mass of the initially loaded carbon was 36mg while that of the decreased carbon was 22mg. Therefore, 61% of black carbon PM is burnt for 2.5h and regeneration is confirmed at the temperature of 280 C Energy efficiency The ratio of the removed PM mass to plasma energy is an important index for evaluating the energy efficiency of the proposed method; it is 0.82g/kW h in the present result. This value is slightly smaller than 2.9g/kWh obtained in the previous paper [24]. According to the post, new long-term national regulation on PM which is planned in Japan for the year 2009: the ratio for heavyweight automobiles (more than 3.5 tons) should be 0.01g/kWh. This value can be achieved by a 63% reduction in the present regulation value (new long-term regulation for the year 2005) of 0.027g/kWh. In this case, if it is assumed that the averaged efficiency of automobile alternator is 90%, it is necessary to reduce =0.017g/ kw h of PM by using the NTP at 0.017/ 0.82/ = 2.3% of the engine power. Although a further improvement is required, this electric power value is sufficient to meet this regulation. The specific energy density (SED) is often used as another index of the energy efficiency. In the present study, it exhibits an improvement as compared with the SED of the previous study [22,23]. The SED is defined as the ratio of the plasma power to the exhaust gas flow rate. In the present experiment, the SED is estimated as 18Wh/m 3 (=64J/L). Further improvement in the energy efficiency or decrease in SED is required. If an SED of 2.8Wh/m 3 (=10J/L) is achieved, the electric power consumption for a 3-L diesel engine is estimated as 670W [24]. In the automobile test cycle of the United States Federal Test Procedure (US FTP), this value is approximately 3% of the automotive diesel engine power output (fuel penalty) [34], which is within a practical level. Based on these results, it can be concluded that lowtemperature PM incineration using a nonthermal plasma is one of the most promising technologies in nonthermal plasma processing of exhaust gas. 4. Conclusions An experiment on DPF regeneration was carried out successfully by employing an indirect nonthermal plasma (NTP) method for diesel emissions based on the principle of NO 2 and O radical incineration. This regeneration method is considered to be effective for low-temperature emissions. It was experimentally confirmed that the pressure drops and the DPF mass decreases considerably when the plasma is turned on, and that the DPF regeneration is in fact possible even at the low temperature of 280 C. It was also confirmed that DPF regeneration is possible for diesel emissions that contain CO 2 and hydrocarbons at the low temperature of 280 C. When DPF regeneration begins, the concentrations of CO 2 and CO increase suddenly due to the incineration of carbon PM. As a result, approximately 61% of NO 2 is reduced to NO. When the plasma is turned on, NOx and total hydrocarbon are reduced. The removal energy efficiency was estimated as 0.82g/kW h, and the specific energy efficiency (SED) was 18Wh/m 3 (=64J/L). Although a further improvement is required, this electric power value is sufficient to meet the present regulation. Acknowledgements The authors are grateful to Dr. Y. Miyairi and Mr. H. 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