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Plasma Assisted Catalysis for Heavy Duty Diesel Engines C. Habeger,(a) C. Aardahl,(b) and M. L. Balmer Supported by DOE Energy Efficiency Office of Advanced Automotive Technology and CRADA. (a) Environmental and Health Sciences Division. (b) Environmental Technology Division. The objective of this program is to develop an exhaust aftertreatment system for heavy-duty diesel engines that will achieve 90% NOx reduction using 3-5% of the engine power on a heavy-duty diesel engine. An aftertreatment system involving a non-thermal plasma in conjunction with a catalyst is being developed to reduce NOx emissions. A partnership between PNNL and Caterpillar Incorporated has been established under a Cooperative Research and Development Agreement (CRADA). FY99 Accomplishments A test bench has been designed and built that is capable of operating at temperatures exceeding 600°C. Catalyst materials capable of withstanding high operating temperatures have been synthesized and tested up to 600°C. Catalytic activity up to 50% for NOx reduction has been observed for simulated diesel exhaust. Plasma reactors employing single dielectric barriers have been designed that operate at temperatures up to 400°C. It has been demonstrated that hydrocarbon is necessary for NOx reduction, and that a 3:1 ratio of C1 to NOx is needed for most efficient power conversion. Analysis of gas phase components has shown that methyl nitrate and acetaldehyde are the predominant partial oxidation products when propene is used as the reductant. Non-thermal plasma-assisted catalysis has been demonstrated to be an effective method for reducing NOx emissions in simulated diesel exhaust; however, further advances in plasma system efficiency and catalysts are needed for vehicle applications. Research in FY99 was focused on developing a reactor test bench that could routinely operate at temperatures exceeding 600°C and developing a catalyst that could also withstand the elevated temperatures and remain active. FY99 research was concentrated on high-temperature reactor design, catalyst development, and mechanistic understanding. Currently, over 40 catalysts have been synthesized and another 9 commercially available materials have been acquired. Select physical properties of all of the materials have been characterized. A number of the catalyst materials have also been further modified. Many of these materials are being tested for NOx catalytic activity. The current plasma reactor can operate up to 350°C with simulated exhaust streams and at 500°C with dry nitrogen. Simulated exhausts are composed of N2, O2, H2O, NO, NO2, CO, CO2, and SO2. A variety of commercially available and laboratory-synthesized catalysts have been tested at temperatures up to 600°C with the plasma-processed gas feed. Thus far, conversions up to 50% have been obtained for our plasma catalysis configuration. This is slightly lower than data reported on similar materials, but the difference in activity could be attributed to the presence of SO2 in our gas mix and the increased flow rate (i.e., 20,000 hr-1). Figure 7.29 shows the results of the most promising off-the-shelf catalyst. The activity of the catalyst increases with temperature, which is expected for this material. Noteworthy is the substantial thermal activity (32%) at higher temperatures. Figure 7.29. Non-thermal plasma catalysis performance data for a commercially available catalyst. Another important aspect of our work in FY99 is the data presented in Figure 7.30 where the ability of the plasma to convert NO to NO2 is presented for a variety of hydrocarbon levels (£600 ppm NOx). 50 ppm is a typical hydrocarbon level in the exhaust from a heavy-duty diesel engine. Conversion of NO to NO2 is required for the plasma-based catalysts to perform. We therefore expect no substantial catalyst activity at 50 ppm hydrocarbon. As the hydrocarbon level rises, the conversion to NO2 becomes increasingly efficient. We found that there is no benefit in adding hydrocarbon above a 3:1 ratio of hydrocarbon to NOx on a C1 basis, which is consistent with the findings of other groups working in this area. Figure 7.30. Effect of hydrocarbon loading on NO to NO2 conversion in the plasma. In conclusion, a test stand for plasma-based aftertreatment of NOx in diesel exhaust has been constructed and plasmas and catalysts have been tested up to 350°C and 600°C, respectively. Thus far, 10 commercially available and 8 laboratory synthesized materials have been tested in the apparatus for NOx reduction activity. The highest activity observed in the first set of tests was 55% with the best power efficiency at a ratio of 3:1 hydrocarbon to NOx on a C1 basis. William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: April 17, 2000 Security & Privacy PNNL-13147