Computational Fluid Dynamics (CFD) study of gaseous ammonia mixing in an exhaust pipe using static mixers.

Paper #:
  • 2017-01-1018

Published:
  • 2017-03-28
Abstract:
The growth of ground vehicle traffic has a detrimental effect on health and environment. NOx are at the origin of respiratory diseases. Consequently, the emission of NOx, among other pollutants, are more and more limited by stringent emission standards. The Selective Catalytic Reduction (SCR) is one consolidated after-treatment technique to reduce the emissions of NOx. The system currently used consists in the injection of an urea water solution (UWS) upstream a catalytic converter. The use of such liquid ammonia precursors presents different problems, pointed out in several studies. Indeed, The temperature required to release NH3 is high, causing problems especially during cold operations, with a consequent undesired wall-film formation, due to the lack of evaporation. The cycles of heating and cooling cause a solid deposit formation, that affects the performance and the durability of the system. Solid ammonia precursors (e.g. ammonium salts and metal ammines) offer the ability to release NH3 with a solid to gas transition at lower temperatures and consequently, to directly introduce gaseous ammonia upstream the catalytic converter, avoiding solid deposit problems. A key point to increase the global performance of the system is to insure a uniform distribution of ammonia at the catalyst surface, that can be achieved using a static mixer between the injector and the catalyst. The aim of this study is to compare numerically the effects of static mixers on the ammonia distribution at catalyst entrance and to evaluate the pressure loss using a compact design of the pipe, injecting gaseous ammonia into air. A first series of simulations were performed to evaluate the effects of the air mass flow rate and of the ammonia mass flow rate, with different temperature conditions. Then a family of mixers has been designed to evaluate the impact of the mixers’ geometry on pressure drop and on ammonia distribution.
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