The Influence of Ammonia to NOX Ratio on SCR Performance 2007-01-1581
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 (NOx). The ideal ratio of ammonia (NH3) molecules to NOx molecules (known as alpha) is 1:1 based on urea consumption and having NH3 available for reaction of all of the exhaust NOx. 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 NOx 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 NOx emissions) to avoid NH3 slip. At higher space velocities and high temperatures, the NOx 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 NOx 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 NOx conversion and NH3 slip. NH3 can oxidize over an SCR catalyst at higher temperatures, so that overdosing of urea can lead to improved NOx 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 NOx conversion efficiency is nearly independent of alpha, so that the NH3 slip can be reduced without impacting NOx conversion efficiency. The work shown in this paper can lead to improved overall NOx conversion and reduced NH3 slip.