The Influence of Ammonia to NOX Ratio on SCR Performance

It is likely that use of urea-based selective catalytic reduction (SCR) will be needed to meet U.S. Tier 2 diesel emission standards for oxides of nitrogen (NO_x). The ideal ratio of ammonia (NH₃) molecules to NO_x molecules (known as alpha) is 1:1 based on urea consumption and having NH₃ available for reaction of all of the exhaust NO_x. However, SCR efficiency can be less than 100% at low temperatures in general, and at higher temperatures with high exhaust SCR catalyst space velocities. At the low temperatures where NO_x conversion efficiency is low, it may be advantageous to reduce the alpha ratio to values less than one (less NH₃ than is needed to convert 100% of the NO_x emissions) to avoid NH₃ slip. At higher space velocities and high temperatures, the NO_x conversion efficiency may be higher with alpha ratios greater than 1. There is however concern that the additional NH₃ will be slipped under these conditions. This paper reviews the influence of the alpha ratio on NO_x conversion efficiency for state of the art High-Temperature and Low-Temperature SCR formulations. Based on the experimental data obtained, certain conditions are highlighted where the alpha ratio can be less than or greater than 1 to optimize NO_x conversion and NH₃ slip. NH₃ can oxidize over an SCR catalyst at higher temperatures, so that overdosing of urea can lead to improved NO_x conversion efficiency without NH₃ slip. In the low temperature region, it is shown that for a range of alpha slightly less than one to slightly greater than one, the NO_x conversion efficiency is nearly independent of alpha, so that the NH₃ slip can be reduced without impacting NO_x conversion efficiency. The work shown in this paper can lead to improved overall NO_x conversion and reduced NH₃ slip.