Investigation on the Formation of Dioxins by Heavy-Duty Diesel Engines with
Selective Catalytic Reduction Technology, 08-R8080

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Principal Investigators
Joseph C. Pan
Reggie R. Zhan
Christopher A. Sharp

Inclusive Dates:  07/07/09 – 01/07/11

Background - Starting January 1, 2010, new diesel engines have been required by the U.S. Environmental Protection Agency to emit no more than 0.2 grams per brake horsepower hour (g/bhp-hr) of nitrogen oxide (NOx). To meet such a stringent NOx emission requirement, a SCR (selective catalytic reduction) technology has been developed in which NOx is reduced to N2 and water in the presence of NH3 (from the decomposition of urea) and a metal-based catalyst. Among several catalysts, copper-zeolite (Cu-Z) has been proposed for such a purpose. However, it is known that based on prior research, dioxins formation is enhanced during combustions when certain metals, especially copper, are present as catalysts. The EPA is extremely concerned about the potential dioxins emission by heavy-duty diesel engines equipped with SCR. SwRI carried out an investigation into this matter.

Approach - This project accomplished the following objectives:

  • Analyzed fuel, lubricant, water and intake air for chlorine content.

  • Investigated dioxin formation in HD diesel engines under steady-state conditions with DPF only and with DPF-CuZ SCR at temperature between 250°C and 450°C with approximately 50°C increment.

  • Tested engines with various configurations of CuZ SCR aftertreatment technologies under steady-state conditions with chlorine-doped fuel at 860 ppb.

  • Tested three aftertreatment technologies (DOC-SCR, SCR only, and SCR-AMOX) under transient conditions with chlorine-doped fuel.

  • Performed emission testing using a production 2008 Caterpillar C13 engine modified to meet the 2010 emission standards.

  • Collected dioxin samples on 4-inch PUF/XAD-2/PUF TRAPs for the "volatile" emission and 20-inch by 20-inch fiberglass filters for the particulates. Samples were extracted with appropriate solvents, and sample extracts were cleaned and analyzed for 17 2,3,7,8-substituted PCDDs/PCDFs on a HRGC/HRMS at 10,000 resolution.

Accomplishments - Results were as follows:

  • Chlorine was found in cooling tower water (17 ppm) and new (144 ppm) and used lube (174 ppm). Trace levels of chlorinated organic compounds were found in the intake air of the test cell.

  • Low levels of dioxins were also found in the tunnel blanks at two temperatures, 25°C and 75°C. The concentrations were independent of the dilution tunnel temperature, suggesting the primary source was from the dilution air itself.

  • The production 2007 engine with DPF on standard fuel produced dioxin levels similar to the tunnel blanks on steady-state tests between 250 and 450°C, and also on transient tests.

  • The addition of a Cu-based SCR system produced no change in dioxin levels compared to the DPF-out results.

  • When the fuel was doped with additional chlorine, engine-out measurements (no aftertreatment) indicated the formation of small amounts of dioxins at roughly two times to three times the level of the tunnel blanks, a net TE increase of 5 to 9 pg/hp-hr on steady-state tests and 19 pg/hp-hr on transient tests. The coefficient of variation on these measurements was roughly 20 percent.

  • None of the aftertreatment configurations tested (SCR Only, DOC+SCR, SCR+AMOX) with the chlorine doped fuel caused any significant change in dioxin levels compared to the engine-out measurements on steady-state measurements. There was a decrease in transient net TE levels with all of the aftertreatment systems to roughly12 pg/hp-hr, although that change is only slightly larger than the measurement coefficient of variation.

Based on these results it can be concluded that no significant increase in dioxins was detected from any of the CuZ-based SCR systems in any of the test configurations with either standard fuel or chlorine doped fuel.

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