Development of a Urea Evaporation and Mixing System to Improve SCR Catalyst Performance, 03-R9891

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Principal Investigators
Reggie Zhan
Wei Li

Inclusive Dates:  10/01/08 – 09/30/09

Background -  Urea selective catalytic reduction (SCR) is the leading NOx aftertreatment technology to effectively reduce NOx emissions from diesel engines to meet more stringent heavy-duty diesel emissions regulations. The SCR technology requires NH3, typically decomposed from urea, as a reductant to selectively react with NOx under a lean (oxygen-rich) environment. The urea is injected into the diesel exhaust system as a urea-water solution, with 32.5 percent concentration. If not decomposed effectively, the injected urea may deposit on the SCR catalyst, leading to reduced catalyst efficiency or even catalyst plugging.

Approach - The primary objective was to develop a urea evaporation and mixing device (urea mixer). A computational fluid dynamics (CFD) modeling technique was used to analyze a complete aftertreatment system including exhaust pipes, catalyst inlet cone, urea mixer and SCR unit with a focus on this novel mixer. The analysis results show that the addition of a mixer increased the system pressure drop by a very small amount, while the mixing effectiveness was significantly improved.

The urea mixer was tested on a Cummins ISB 6.7L diesel engine equipped with an SCR aftertreatment system. Three different aftertreatment configurations, including "no mixer," "with mixer," and "with mixer and closer injector," were tested under both steady-state and transient engine operating conditions, with two different urea injection calibrations. The steady-state tests included 18 engine operating modes, and the standard heavy-duty Federal Test Procedure (FTP) and European transient cycle (ETC) cycles were used for the transient tests.

Accomplishments - A novel urea mixer was successfully developed with the following advantages:

  1. Low pressure drop. Under all 18 steady-state engine operating conditions tested, the maximum pressure drop of the mixer was only 0.25 kPa when the engine was operated at 2,248 rpm and 657 lb-ft torque.
     
  2. Improved low-temperature performance. Under certain engine operating conditions, e.g., 1,876 rpm and 145 lb-ft torque, the NOx conversion efficiency increased by 71.3 percent.
     
  3. Improved NOx conversion efficiency and reduced packaging space. With the "OEM calibration," the urea mixer generally improved the SCR NOx conversion efficiency and reduced NH3 slip under both steady-state and transient engine operating conditions. Compared to the "no mixer" configuration, the "with mixer and closer injector" configuration reduced tailpipe NOx by greater than 30 percent over the FTP cycles, while reducing the distance between the urea injection port to the SCR front face by approximately 50 percent. With the "revised calibration," the urea mixer significantly improved SCR NOx conversion efficiency and reduced NH3 slip under steady-state engine operating conditions.

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