Plasma reactors can be operated as a particulate trap or as a NO{sub x} converter. The soluble organic fraction (SOF) of the trapped particulates can be utilized for the oxidation of NO to NO{sub 2}. The NO{sub 2} can then be used to non-thermally oxidize the carbon fraction of the particulates. This paper examines the energy density required for oxidation of the SOF hydrocarbons and the fate of NO{sub 2} during the oxidation of the particulate carbon. The energy density required for complete oxidation of the SOF hydrocarbons is shown to be unacceptably large. The reaction of NO{sub 2} with carbon is shown to lead mainly to backconversion of NO{sub 2} to NO. These results suggest that the use of a catalyst in combination with the plasma will be required to efficiently reduce the NO{sub x} and oxidize the SOF hydrocarbons

Brusasco, R.M.

Merritt, B.T.

Penetrante, B.M.

Pitz, W.J.

Vogtlin, G.E.

UNT Digital Library

U.S. Department of Energy Preprint UCRL-136283 Plasma Aftertreatment for Simultaneous Control of No, and Particulates B.M. Penetrante, R.M. Brusasco, B. T. Merritt, W.J. Pitz, G. E. Vogtlin This article was submitted to 1999 Diesel Engine Emissions Reduction Workshop, Castine, ME, July 5-8, 1999 October 28,1999 Approved for public release; further dissemination unlimited DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. This report has been reproduced directly from the best available copy. Available to DOE and DOE contractors from the Office of Scientific and Technical Information P.O. Box 62, Oak Ridge, TN 37831 Prices available from (423) 5768401 http://apollo.osti.gov/bridge/ Available to the public from the National Technical Information Service U.S. Department of Commerce 5285 Port Royal Rd., Springfield, VA 22161 http://www.ntis.gov/ OR Lawrence Livermore National Laboratory Technical Information Department’s Digital Library http://www.Ilnl.gov/tid/Library.html PLASMA AFTERTREATMENT FOR SIMULTANEOUS CONTROL OF NO, AND PARTICULATES B. M. Penetrante, R. M. Brusasco, B. T. Merritt, W. J. Pitz and G. E. Vogtlin Lawrence Livermore National Laboratory Abstract Plasma reactors can be operated as a particulate trap or as a NO, converter. The soluble organic fraction (SOF) of the trapped particulates can be utilized for the oxidation of NO to NO2. The NO2 can then be used to non-thermally oxidize the carbon fraction of the particulates. This paper examines the energy density required for oxidation of the SOF hydrocarbons and the fate of NO;! during the oxidation of the particulate carbon. The energy density required for complete oxidation of the SOF hydrocarbons is shown to be unacceptably large. The reaction of NO2 with carbon is shown to lead mainly to backconversion of NO2 to NO. These results suggest that the use of a catalyst in combination with the plasma will be required to efficiently reduce the NO, and oxidize the SOF hydrocarbons. Introduction Plasma reactors can be operated as a particulate trap [ref. l-31 or as a NO, converter [ref. 4-71. Particulate trapping in a plasma reactor can be accomplished by electrostatic precipitation. Corona-type reactors operating as electrostatic precipitators for diesel soot are reported in refs. [l-2]. A ferroelectric pellet bed reactor, also operating as an electrostatic precipitator of diesel soot, is reported in ref. [3]. Plasma-based traps need periodic regeneration just like any other particulate trap. Regeneration by the plasma can be achieved in a thermal mode or a non-thermal mode. The plasma reactor can be operated occasionally in the arc discharge mode to thermally oxidize the particulates. In ref. [2], this mode is referred to as the self-cleaning phase of the electrostatic muffler. Thermal oxidation of the particulates precipitated on the electrode surface is achieved in very localized regions of high temperature near the tip of the arc microdischarges. The plasma reactor can also be operated in a non-thermal corona discharge mode to provide continuous oxidation of the particulates. The non-thermal oxidation is presumably accomplished by the 0 and OH radicals resulting from electron-impact dissociation of oxygen and water vapor molecules, or by negative hydronium cluster ions, [(&O&02-], resulting from electron attachment. Ref. [ 11 observed a self-cleaning effect in the corona mode that correlates with the production of CO and CO2. Ref. [8] reports on the oxidation of soot in a dielectric-barrier discharge. It was conjectured that OH radicals oxidized the soot; the soot reduction did not occur in the presence of large amounts of CO, presumably because the OH radicals were consumed by CO. Diesel particulates are composed mainly of the carbon fraction and the soluble organic fraction (SOF). The SOF could possibly be utilized for the oxidation of NO to NO2 in a plasma. As a NO, converter, the plasma requires hydrocarbons to achieve high oxidation efficiency with low electrical energy consumption. The NO2 can then be used to oxidize the carbon fraction, similar to that in a Continuously Regenerated Trap (CRT) [ref. 121. The use of NO2 for the oxidation of trapped particulates in CRT devices is fairly well established. In CRT, a precious metal catalyst is used to oxidize NO to NO2 upstream of a particulate filter. The CRT method requires low sulfur fuel because the catalytic oxidation of NO to NO2 also leads to the oxidation of SO2 to SOa. The use of a plasma for the oxidation of NO to NO2 can make the process more tolerant to the sulfur content of the fuel. The fate of NO2 during the oxidation of carbon is important if one contemplates the use of plasma aftertreatment for the simultaneous removal of NO, and particulates. This paper examines the energy density required for oxidation of the SOF hydrocarbons and the fate of NO;! during the oxidation of the particulate carbon. Results Both chemical kinetics calculations and experimental measurements are presented in this section. We used propene as the hydrocarbon surrogate in the calculations because the chemical reaction database for propene is more established and facilitates comparison of the modeling to experiments. The plasma reactor used in the experiments is a pulsed corona discharge reactor consisting of a metal wire inside a metal cylinder. The plasma chemistry is not peculiar to this type of plasma processor; all electrical discharge plasma reactors accomplish essentially the same gas-phase plasma chemistry under the same gas conditions [ref. g-101. The important control parameter in the plasma reactor is the electrical energy density delivered to the plasma [ref. 9-l 11. Figure 1 shows the calculated concentration of aldehydes formed during plasma processing of 100 ppm NO in a simulated diesel exhaust at 200°C with a Cl/NO, of 6. Formaldehyde is the major product of the partial oxidation of propene in the plasma. Note that the electrical energy density required to convert NO to NO2 is much less than that required to fully oxidize the aldehydes. Whereas it is possible to convert 100 ppm of NO to NO2 with less than 10 J/L, conversion of the resulting 50 ppm of formaldehyde to Hz0 and CO, requires more than 150 J/L. This result shows that the electrical power required by the plasma to completely oxidize the hydrocarbons is unacceptably large. Some of the minor products shown in Figure 1 include CH3CHCO and C2H3CHO. Species CH3CHCO is methyl ketene and species C2HsCHO is acrolien. Acrolien is experimentally observed as an intermediate product in the low temperature oxidation of propene [ref. 131. The addition of 0 atom to propene to form a biradical and its decomposition forms methyl ketene. The rate of this process is based on the current understanding of the 0 + propene reaction as reviewed in Ref. [ 141. Reactions to consume acrolien and methyl ketene by radical attack are included in the detailed chemical kinetic mechanism. Figure 2 shows the calculated concentration of other species formed during plasma processing of 100 ppm NO in a simulated diesel exhaust at 200°C with a Ct/NO, of 6. Large amounts of CO are formed during the plasma oxidation of hydrocarbons. 200°C 100 ppm NO C,/NOx = 6 c 0 n z c;n J NO / CH,O Figure 1. Chemical kinetics calculation of the concentration of aldehydes formed during plasma processing of 100 ppm NO in a simulated diesel exhaust with 10% 02, 10% CO2,5% H20, balance N2. Propene additive, Cl/NO, = 6. Gas temperature = 200°C. Figure 2. Chemical kinetics calculation of the concentration of various species during plasma processing of 100 ppm NO in a simulated diesel exhaust with 10% 02, 10% CO2,5% H20, balance N2. Propene additive, Ct/N9, = 6. Gas temperature = 200°C. Figure 3 shows the comparison between the model predictions and experimental measurements of the concentration of formaldehyde and CO formed during plasma processing of 500 ppm NO in a gas mixture with 10% 02, balance N2, at 100°C with a Cl/NO, of 6. This level of NO is typical of that in heavy-duty diesel engine exhaust. There is fairly good agreement between the modeling and the experiment. With a Cl/NO, of 6, about 25 J/L is required to get maximum oxidation of NO to NOz. At this energy density, about 150 ppm of formaldehyde has already been formed. The amount of aldehydes increases further as the energy density is increased. Oxidation of these aldehydes will require very large electrical energy density input to the plasma. Energy Density (J/L) Figure 3. Experimental measurements (points) and modeling predictions (lines) of the concentration of formaldehyde and CO formed during plasma processing of 500 ppm NO in a gas mixture with 10% 02, balance N2. Propene additive, Cl/NO, = 6. Gas temperature = 100°C. The oxidation of the carbon by NO2 can lead to reduction of NO, or backconversion of NO2 to NO. The following experiments measure the NO, reduction efficiency and the amount of backconversion of NO;! to NO. Figure 4 shows the fate of NO;! during its reaction with carbon pellets at 250°C in a gas stream containing only N2. There is about 20% NO, reduction. The NO, remaining after the reaction is composed mostly of NO. Figure 5 shows the same type of experiment conducted in a gas stream containing 10% 02, balance N2. The oxygen seems to have promoted the NO, reduction efficiency to about 30%. Again the NO, remaining after the reaction is composed mostly of NO. inlet Outlet Figure 4. Oxidation of carbon by NO2 in N2. Gas temperature = 250°C. 300 g 250 2 - 200 5 -5 150 z g 100 5 0 50 0 250°C 1 1 J Inlet Outlet Figure 5. Oxidation of carbon by NO2 in 10% 02, balance Nz. Gas temperature = 250°C. ::-.. : Figure 6 shows the result of another experiment to simulate the effect of the volatile organic fraction of particulates on the NO2 reduction. The gas stream in this experiment contains 10% 02 and 1500 ppm Cl kerosene. The NO, reduction efficiency is about 35%, with the remaining NO, being composed mostly of NO. NO X Inlet NO Outlet Figure 6. Oxidation of carbon by NO2 in 10% 02 + 1500 ppm Ct kerosene. Gas temperature = 200°C. The results in Figures 4-6 show that even though the NO2 could be utilized for the oxidation of the carbon fraction of the particulates, the reaction with the carbon fraction cannot provide a high level of NOx reduction. Another means will have to be provided to achieve a high NO, reduction efficiency. If one is going to use plasma-assisted SCR for the NO, reduction, the regeneration of the particulate trap will have to be done without the NO2. A way to do this would be to operate the plasma reactor occasionally in the arc mode to thermally oxidize the carbon fraction of the particulates. Ref. [2] describes an example of the effectiveness of this technique and how it can be implemented. Conclusions The electrical energy density required for complete oxidation of hydrocarbons in a plasma is much greater than that required to achieve maximum NO, conversion. Aldehydes and CO will be formed. The energy density required to oxidize the aldehydes will be unacceptably large even for light-duty applications. A catalyst in combination with the plasma will be required to take care of the aldehydes and CO. The NO2 from the plasma can be used to non-thermally oxidize the carbon fraction of trapped particulates. However, the reaction of NO2 with carbon cannot provide a high level of NO, reduction and leads mostly to the backconversion of NO2 to NO. If one is going to use plasma-assisted SCR for the NO, reduction, the regeneration of the particulate trap will have to be done without the NOz. A way to do this would be to operate the plasma reactor occasionally in the arc mode to thermally oxidize the carbon fraction of the particulates. Acknowledgments This work was performed at Lawrence Livermore National Laboratory under the auspices of the U.S. Department of Energy under Contract Number W-7405-ENG-48, with support from the Chemical Sciences Division of the DOE Office of Basic Energy Sciences, the DOE Office of Fossil Energy, the DOE Office of Transportation Technologies, the Strategic Environmental Research and Development Program, and a Cooperative Research and Development Agreement with Cummins Engine Company. References [l] Masuda, S. and Moon, J.-D., “Electrostatic Precipitation of Carbon Soot from Diesel Engine Exhaust”, ConJ: Rec. 1982 IEEE Industry Applications Society Annual Meeting, pp. 1086- 1093. [2] Colletta, A., Costa, G., Pinti, M. and Scorsone, V., “Self-Cleaning Electrostatic Muffler for Diesel Vehicles”, SAE Paper 952389 (1995). [3] Mizuno, A. and Hiroshi, I., “An Electrostratic Precipitator Using a Ferroelectric Pellet Layer for Particle Collection”, Co@ Rec. 1986 IEEE Indusby Applications Society Annual Meeting, Part II, pp. 1106-1112. [4] McLarnon, CR. and Penetrante, B.M., “Effect of Gas Composition on the NO, Conversion Chemistry in a Plasma”, SAE Paper 982433 (1998). [SJ Penetrante, B.M., Brusasco, R.M., Merritt, B.T., Pitz, W.J., Vogtlin, G.E., Kung, M.C., Kung, H.H., Wan, C.Z., and Voss, K.E., “Plasma-Assisted Catalytic Reduction of NOx”, SAE Paper 982508 (1998). [6] Hammer, T. and Broer, S., “Plasma Enhanced Selective Catalytic Reduction of NO, for Diesel Cars”, SAE Paper 982428 (1998). [7] Hoard, J. and Balmer, M.L., “Analysis of Plasma-Catalysis for Diesel NO, Remediation”, SAE Paper 982429 (1998). [8] Harano, A., Sadakata, M. and Sato, M., “Soot Oxidation in a Silent Discharge”, J. Chem. Eng. Japan 24, 100 (1991). [9] McLarnon, C.R. and Penetrante, B.M., “Effect of Reactor Design on the Plasma Treatment of NO,“, SAE Paper 982434 (1998). [lo] Penetrante, B.M., Hsiao, M.C., Merritt, B.T., Vogtlin, G-E., Wallman, P.H., Neiger, M., Wolf, O., Hammer, T. and Broer, S., “Pulsed Corona and Dielectric-Barrier Discharge Processing of NO in Nz”, Appl. Phys. Lett. 68,3719 (1996). [ll] Penetrante, B.M., Hsiao, M.C., Bardsley, J.N., Merritt, B.T., Vogtlin, G.E., Kuthi, A., Burkhart, C.P., and Bayless, J.R., “Identification of Mechanisms for Decomposition of Air Pollutants by Non- Thermal Plasma Processing”, Plasma Sources Sci. Tech. 6,25 1 (1997). [12] Cooper, B.J., Jung, H.J. and Thoss, J.E., “Treatment of Diesel Exhaust Gases”, US Patent 4,902,487 (February 20, 1990). [ 131 Wilk, R.D., Cernansky, N.P., Pitz, W.J. and Westbrook, C.K., “Propene Oxidation at Low and Intermediate Temperatures: A Detailed Chemical Kinetic Study”, Combust. Flame 77, 145 (1989). [ 141 Tsang, W., “Chemical Kinetic Database for Combustion Chemistry Part V. Propene”, J. P&s. Chem. Ref: Data 20,221 (1991). 

Plasma Aftertreatment for Simultaneous Control of NOx and Particulates

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Plasma Aftertreatment for Simultaneous Control of NOx and Particulates

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