I. INTRODUCTION

There is increasing interest in the soot deposition that occurs in exhaust system components in particular for diesel based pollution control technologies, such as particulate filters and exhaust gas recirculation (EGR) systems with heat exchangers employed to reduce the NOx production [1]. Heat transfer rates are reduced as soot is deposited on the heat transfer tubes and fins and recirculation rates may also be adversely affected due to soot deposit flow blockage resulting in an increase in pressure drop in the EGR loop. The deposited soot may also be re-entrained in the exhaust gas stream and enter the combustion cylinder leading to an increase in emissions. Thus, there is interest in using plasma oxidation of deposited soot to reduce the amount of soot deposited and soot emissions [2-5]. Soot deposition is a consequence of a number of mechanisms including convection, diffusion, turbulent diffusion, thermophoresis, vapour condensation and electrical effects [6-9]. The deposition profile and thickness is also affected by soot re-entrainment due to removal by the exhaust gas flow and potential re-deposition [10, 11]. Entrance flow effects such as a large entrance contraction also appear to have significant effect on the soot deposition attributed to the separated flow

induced by the contraction [11]. Martin et al. [2] conducted an experimental and

analytical modeling study into the erosion of soot by oxidation using non-thermal plasma. In their study, the atmospheric pressure plasma/surface chemical interaction was modeled using a simplified, two layer, kinetic scheme for the surface reactions coupled with a gas-phase reaction mechanism appropriate for diesel exhaust gas. Brocilo et al. [12] examined plasma soot oxidation and proposed that other reactions can be initiated by metastables (*) and by negative and positive ions (-or+). Further, corona discharge in the plasma usually generates a strong electrophoretic-electrohydrodynamic (EHD) flow [13]. This EHD flow may also act to suppress the deposition of soot particles due to convection and re-entrainment. Soot particles have been found to be electrically charged during the combustion process [14] and these charged soot particles may be neutralized by the positive and negative ions with similar diffusion coefficients in the plasma, which may reduce the deposition of the soot. The objective of this work was to determine whether non-thermal plasma (NTP) oxidation is an effective method of soot removal during a period with the engine is not in operation. Experiments were performed in a cylindrical pipe using a diesel engine test facility. Real time neutron radiography was used to non-destructively measure the deposited soot thickness distribution [15].

Deposited Diesel Soot Oxidation and Removal Characteristics by Non-thermal Plasma

D. Brocilo1, J. S. Cotton2, E. dela Cruz1, G. D. Harvel1, 3, D. Ewing4, M. Bardeleben5, and J. S. Chang1

1McIARS, McMaster University, Canada 2Mechanical Engineering, McMaster University, Canada 3University of Ontario Institute of Technology, Canada

4Dan W. Ewing, Toronto, Canada 5Dana Holdings Corp., Canada

Abstract—Particulate matter (PM) emissions generated by vehicles and transports can reduce the effectiveness of environmental control devices such as exhaust gas recirculation (EGR) heat exchangers due to fouling of the heat transfer surface and fins. This experimental study examines a non-thermal plasma system to remove deposited soot on a 25.4 mm diameter exhaust pipe during periods when the engine is not in operation. The results suggest that Ox radicals, metastables, positive ions and negative ions in the flow stabized corona process can reduce the soot deposition downstream on the pipe wall. Both soot oxidation and ash reduction gaseous by-products were observed. The application of non-thermal plasma in the absence of exhaust gas flow resulted in minimal generation of NO and NO2 while the removal of sulphur content in the soot was observed through measured concentrations of H2S and SO2 in the range of 1-5 ppm. Transient concentration measurements of CO and hydrocarbons (HC) were observed downstream during the plasma cycles suggesting oxidation of the soot layer. This was demonstrated by soot thickness measurement comparisons of the test and control sections that show a modest reduction in thickness in the proximity to the corona discharge plasma stream.

Keywords—Soot removal, non-thermal plasma, flow stabilized corona, neutron radiography

Corresponding author: James S. Cotton e-mail address: cottonjs@mcmaster.ca Presented at the Seventh International Symposium on Non-Thermal/Thermal Plasma Pollution Control Technology & Sustainable Energy, ISNTP-7, in June 2010

Brocilo et al. 37

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II. EXPERIMENTAL APPARATUS

The experiments were performed in the McMaster -

Near Zero Emission Diesel Exhaust Test Facility (NZE-DTF) used previously by dela Cruz et al. [11]. A summary of the key components are outlined here. The diesel exhaust gas was supplied by a 5.5 kW single cylinder direct-injection diesel engine generator with a displacement volume of 350 cm3 that operated at a speed of 3600 rpm. The main exhaust gas pipe, 44.5 mm diameter, split into two branches, a test and control section. The physical geometry of both sections was the same to ensure the soot deposition would be similar in both branches. The branches rejoin and entered a Venturi meter, a diesel particulate filter followed by an electrical heater and catalytic converter. The exhaust gas, now filtered, is ejected into a fume hood.

The test and control sections were identical (Fig. 1) consisting of a 25.4 mm diameter pipe of 20 cm in length. Two flow stabilized corona non-thermal plasma reactors were installed at the inlet and outlet of each branch [16]. The pipes extend into the non-thermal plasma reactor to the nozzle tip, thus the visible part of the pipe is only 15 cm long. Both sections were aluminum such that they could be readily analysed via real-time nuclear radiography (RTNR) without significant neutron attenuation [10, 15]. The non-thermal plasma corona radial injection consists of a stainless steel tube placed perpendicular to the test and control section pipes, two 1.58 mm diameter nozzles, 10 mm in length acted as the electrodes where the high voltage was applied. Soot deposition was achieved on initially clean branches at the following operating conditions: engine power 2.5 kW, exhaust gas temperature 250oC, exhaust gas flow rate 10 kg/h per branch for a period of 120 min. The exhaust gas used to foul the test section had time averaged component values of approximately 13.0-13.3% O2, 5.7-6.1% CO2, 432-512 ppm NOx, 630-800

ppm CO, 100-140 mg/m3 of particulate matter and 30-70 ppm HC [11]. The size distribution of the particulate matter had peaks near 10 nm and 100 nm [17].

The application of NTP occurred when the engine was not in operation, upon application of NTP air is injected in both the upstream and downstream electrodes and exits the nozzle tip in each branch. The electrode is insulated from the cross section by a Teflon part, and the cross-section is grounded. The injected air (with a flow of Qg-inj = 3.56 lpm/nozzle) is introduced in the four sections, but the high applied voltage is only applied to the electrodes in the test section. The injected air is measured via a 20 lpm air rotameter of accuracy 0.2% full scale. A dc voltage driven pulsed high voltage system is used to create the high voltage. The primary side is a variable dc power supply with charging automobile ignition coils; the trigger of the discharge is electronically controlled by a function generator and triggering circuit [12]. A typical pulse and current waveforms is presented in Brocilo et al. [12] the peak power investigated was 40 W and 200 W for peak applied voltages of 11.9 kV and 15.0 kV, respectively.

The soot oxidation and ash reduction gaseous by-products were measured using a GreenLine 8000 combustion gas analyzer (Eurotron Inc.) that can measure concentrations of O2, CO, CO2, NO, NO2, NOx, SO2, H2S, and CxHy. Non-thermal plasma corona radial injection was applied continuously for a period of 1 hour and repeated 3-times. The typical time period between NTP applications was approximately 10 hours.

A neutron imaging system in the McMaster Nuclear Reactor-Neutron Radiography Facility was used to measure the soot deposition thickness and profile after each NTP cycle. The technique uses the ability of the neutrons to be transmitted through metals and attenuated by carbon and hydrogen based materials (such as soot) [10, 15]. The imaging is done after the completion of each experimental test.

38 International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011

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III. RESULTS AND DISCUSSION

The experiments were initiated after the preliminary soot deposition was achieved, the test section was allowed to cool to ambient temperature and then the plasma reactors were operated. The gaseous byproduct measurements were started 10 minutes after the plasma reactors were operated to ensure a stable discharge and measurements continued for approximately 5 to 10 min after the non-thermal plasma was turned off. The NTP was applied for a period of 60 minutes once stabilized and cycled 3-times. Between cycles RTNR imaging was conducted to obtain the soot deposition.

A. Soot oxidation and ash reduction gaseous by-products The concentration of the exhaust gas components

during each 1 hour NTP cycle (at 200 W peak) are shown in Fig. 2 to 6. Fig. 2 shows the concentration of carbon monoxide (CO) measured downstream of the test section. The CO concentration was measured between 10 ~ 20 ppm during the first 10 min of byproduct measurement and the remained relatively constant at 2.5 ppm for remainder of the first cycle. Subsequent cycles produced minimal CO ranging between 1 to 2 ppm. The CO concentration detected in the downstream may be due to the oxidation of the soot by Ox (x = 1 to 3 for neutrals and 1 to 6 for ions) radicals, metastables, positive ions and

Fig. 2. Transient CO concentrations downstream of plasma reactor during three cycles of plasma removal/oxidation.

Fig. 3. Transient HC concentrations downstream of plasma reactor during three cycles of plasma removal/oxidation.

Fig. 4. Transient SO2 concentrations downstream of plasma reactor during three cycles of plasma removal/oxidation.

Fig. 5. Transient NO concentrations downstream of plasma reactor during three cycles of plasma removal/oxidation.

Fig. 6. Transient NO2 concentrations downstream of plasma reactor during three cycles of plasma removal/oxidation.

Brocilo et al. 39

TABLE I SUMMARY OF GASEOUS COMPOSITION DOWNSTREAM OF TEST SECTION AT DIFFERENT PEAK POWERS

SpeciesDiesel off phase

Downstream (% or ppm) at 200 W Downstream (% or ppm) at 40 W

NO 0-13 0

NO2 18 0

CO 0-20 -

CO2 - -

H2S 0-2 0-5

SO2 0-5 0

HC 2-72 2-62

O2 21% 21%

PM 0-5 [mg/m3] -

a)Test section after 1st,2nd and 3rd NTP cycle

b)Control section after 1st,2nd and 3rd NTP cycle

a)Test section after 1st,2nd and 3rd NTP cycle

b)Control section after 1st,2nd and 3rd NTP cycle

a)Test section after 1st,2nd and 3rd NTP cycle

b)Control section after 1st,2nd and 3rd NTP cycle

Fig. 7. Soot thickness obtained from RTNR images of the exhaust pipe: (a) with plasma and (b) without plasma after each cycle of plasma removal/oxidation.

negative ions as proposed by Brocilo et al. [12]. The concentration of CO2 is expected to increase due to soot oxidation however it was not observable within the measurement accuracy of the gas analyser.

Fig. 3 shows transients of total hydrocarbon, where a significant 10 to 70 ppm hydrocarbon concentration was observed downstream of the plasma reactor. The hydrocarbons may be generated from the plasma soot surface reactions or evaporation. The adhesive properties of soot layer may be reduced by the removal of hydrocarbon from the soot surface. This may explain the small quantity of particulate matter (PM) 0-5 mg/m3 (Table 1) measured in the gas flow upon application of 200 W (peak) and the increase in CO at 50 min as the dried soot is oxidized.

Fig. 4 shows the transient sulphur dioxide (SO2) concentration during the experiment measured

downstream of the test section. The SO2 concentration fluctuated between 1 and 5 ppm during each cycle of the experimental period. Since the present diesel fuel contains 30 ppm of fuel sulphur, sulphates in the ashes are expected during combustion. The presence of SO2 is likely due to the oxidation of sulphates in the soot ash components.

Figs. 5-6 presents the measured transient concentrations of NO and NO2 respectively. As shown the application of non-thermal plasma in the absence of exhaust gas flow resulted in minimal generation of NO and NO2. Further, based on periodic gas sampling and analysis by with FTIR there was no evidence of O3, HNO3 and N2Ox within the detection limit of the device. A summary of the gaseous composition downstream of the test section is presented in Table 1; the composition suggests some oxidation of the soot layer.

40 International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011

B. RTNR Images of Soot Deposition The RTNR imaging was used for the soot thickness measurement. RTNR images are taken after the experiment, which corresponds to the NTP operation for 1 hour. The RTNR images with and without plasmas are shown in Figs. 7a and 7b respectively. Fig. 7b, the controlled section with no NTP shows that the soot deposition is noticeable. A significant amount of soot deposition appears on the sides of the image as the test section is a cylinder and the integral along the neutron beam path is greatest at the edge. Comparison of the RTNR image for the control side, Fig 7b with the test section, Fig 7a, show that the plasma treatment was modestly effective at removing the soot build-up with the most significant amount removed near the inlet and outlet.

IV. CONCLUDING REMARKS

An experiment was conducted to remove deposited

soot from the pipe wall by non-thermal plasma generated radicals, metastables and ions. The results show that the oxidation and reduction that occur downstream of the flow stabized plasma process may remove deposited soot on the pipe wall since some soot oxidation and ash reduction gaseous by-products were observed. Non-thermal plasma also contributed to the removal of hydrocarbons adsorbed in soot. The physical removal of deposited soot is also observed. This was demonstrated by soot thickness measurement comparisons of the test and control sections that show a modest reduction in thickness in the proximity to the corona discharge plasma stream.

ACKNOWLEDGMENT

The authors thank I. E. Gerges, J. Burgers, and K. Urashima for valuable discussion and comments. The financial support of NSERC of Canada, Dana Holdings Corp. and CRESTech, OCE Inc. is acknowledged.

REFERENCES [1] M. Duernholz and H. Endres, "Exhaust-Gas Recirculation. A

Measure to Reduce Exhaust Emissions of DI Diesel Engines," Transactions of SAE, Fuel Systems and General Emissions, vol. 10, pp. 151-158, 1992

[2] A. R. Martin, J. T. Shawcross, and J. C. Whitehead, "The oxidation of carbon soot in a non-thermal, atmospheric pressure plasma: Experiment and modeling," Journal of Advanced Oxidation Technologies, vol. 8, pp. 126-132, 2005.

[3] M. Okubo, T. Kuroki, Y. Miyairi, and T. Yamamoto, "Low-temperature soot incineration of diesel particulate filter using remote non-thermal plasma induced by a pulsed barrier discharge," IEEE Transactions on Industry Applications, vol. 40, pp. 1504-1512, 2004.

[4] S. Yao, M. Okumoto, and T. Yashima, "Diesel particulate matter and NOx removals using a pulsed corona surface discharge," AIChE Journal, vol. 50, pp. 715-721, 2004.

[5] M. Higashi, S. Uchida, N. Suzuki, and K. -I. Fujii, "Soot elimination and NO, and SO, reduction in diesel-engine exhaust

by a combination of discharge plasma and oil dynamics," IEEE Transactions on Plasma Science, vol. 20, pp. 1-12, 1992.

[6] W. C. Hinds, Aerosol Technology, Addison-Weslet, New York, 1982.

[7] D. B. Taulbee and C. P. Yu, "Simultaneous diffusion and sedimentation of aerosols in channel flows," Journal of Aerosol Science, vol .6, pp. 433-441, 1975.

[8] A. Messerer, R. Niessner, and U. Poschl. "Thermophoretic deposition of soot aerosol particles under experimental conditions relevant for modern diesel engine exhaust gas systems," Journal of Aerosol Science, vol. 34, pp. 1009-1021, 2003.

[9] T. G. Beuthe and J. S. Chang, "Gas discharge phenomena," in Handbook of Electrostatic Processes, Chapter 9, Eds. J. S. Chang, A.J. Kelly, J.M. Crowley, Marcel Dekker Inc., New York, pp. 147-194, 1995.

[10] B. Ismail, D. Ewing, J. S. Cotton and J. S. Chang, "Characterization of the Soot Deposition Profiles in Diesel Exhaust Gas Recirculation Cooling Devices Using a Digital Neutron Radiography Imaging Technique," SAE Transactions, Journal of Fuel and Lubricants, SAE 2004-01-1433, pp. 791-800, 2004.

[11] E. dela Cruz, J. S. Chang, D. Ewing, A. A. Berezin, J. S. Cotton, E. Gerges and M. Bardeleben, "Neutron Radiography of Convective and Thermophoretic Diesel Engine Exhaust Soot Depositions in a Cooled Rectangular Chamber," 2008 SAE World Congress & Exhibition, 2008-01-1174, SP-2152, pp. 181-190, 2009.

[12] D. Brocilo, J. S. Chang, D. Ewing, K. Urashima, and J. S Cotton, "Plasma removal of wall deposited diesel soot," in Proc. 17th International Symposium on Plasma Chemistry, ISPC-17, Toronto, Canada, pp. 1187-1188, 2005.

[13] J. S. Chang and A. Watson, "Electromagnetic hydrodynamics," IEEE Transactions on Electrical Insulation, vol. 1, pp. 871-890, 1995.

[14] M. M. Mariq, "On the electrical charge of motor vehicle exhaust particles," Journal of Aerosol Science, vol. 37, pp. 858-874, 2006.

[15] B. Ismail, D. Ewing, J. S. Chang and J. S. Cotton, "Development of a non-destructive radiography technique to measure the three-dimensional soot deposition profiles in diesel engine exhaust systems," Journal of Aerosol Science, vol. 35, pp. 1275-1288, 2004.

[16] I. Maezono and J. S. Chang, "Flow enhanced corona discharge-corona torch," Journal of Applied Physics, vol. 59, pp. 2322-2324, 1988.

[17] B. Ismail, "The heat transfer and the soot deposition characteristics in diesel engine exhaust recirculation system cooling devices," Ph.D. Thesis, McMaster University, Hamilton, Ontario, Canada, 2004.

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