Urea Deposit Predictions on a Practical Mid/Heavy Duty Vehicle After treatment System

Paper #:
  • 2018-01-0960

  • 2018-04-03
Urea/SCR systems (Urea-SCR systems or Urea based SCR systems) have become an essential equipment on mid/heavy duty diesel for both on and off road vehicles to reduce NOx emissions. These systems have been proven effective at reducing NOx over a wide range of operating conditions. However, design changes due to reduction in the size of modern compact Urea/SCR systems and lower exhaust temperatures have increased the possibility of urea deposit formation. Urea deposits result when urea in films and droplets decomposes in a range of temperatures (undergoes undesirable secondary reactions). This results in the formation of by-products such as Ammelide, Biuret and Cyanuric Acid (CYA). Biuret is easy to decompose at a lower temperature below 200℃, but, Ammelide and CYA are difficult to decompose and lead to the formation of solid deposits on the surface that can degrade the performance of the after treatment system by decreasing overall mixing efficiency, lowering de-NOx efficiency and increasing pressure drop. Avoiding or mitigating urea deposits is a primary design goal of modern diesel after treatment systems. The purpose of current study is to introduce the CFD simulation approaches to predict urea deposit formation using detailed urea decomposition scheme with 12 reactions to identify actual urea deposit formations in the Isuzu exhaust system. Conjugate Heat Transfer (CHT) is used along with the advanced splashing and film evaporation models to correctly predict the film temperature. A modified multi component evaporation model was used with the detailed decomposition mechanism to capture the proper evaporation rate of water for film predictions. The results will be compared with simple urea decomposition containing only thermolysis and hydrolysis for deposit risk analysis. The results of introduced approaches are also compared against engine dyno test data for mass accumulation prediction and gas-chromatograph (GC) – QTOFMS results for urea deposit component prediction.
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