| In an effort to reduce emissions of nitrous oxides, many power plants are installing selective catalytic reduction (SCR) systems. SCRs work by injecting ammonia into the flue gas upstream of a fixed catalyst. The NOx and ammonia react in the presence of the catalyst to produce nitrogen gas and water. In order for an SCR to work efficiently, it is important to achieve a uniform velocity profile, a uniform temperature profile, and a uniform ammonia concentration at the upstream face of the catalyst. Both physical modeling and computational fluid dynamics (CFD) can be utilized in order to achieve these goals. Figure 1 shows a 1/12th scale physical model of an SCR and its associated ductwork. A fan draws air through the model, and a tracer gas is used to simulate the injection of ammonia. |
Velocity measurements are then taken at the catalyst face with a hot mandrel probe, while a gas sampling probe is used at the same location to determine the tracer gas concentration. Figure 2 presents a CFD model of the ductwork upstream of the SCR. For coal-fired plants, it is critical that no larger pieces of ash residue enter the SCR or air preheater. A detailed design study can determine what modifications are required in the economizer outlet region to collect all large-sized particulate in the economizer hoppers. ASC has the skills, experience, and capabilities to optimize the flow to an SCR and to help minimize any catalyst pluggage caused by large "popcorn" ash. |
 Figure 1: Physical Model of an SCR System |
 Figure 2: Particulate Tracking Before Modification |
| SCR Flow Modeling |
| ASC often uses physical modeling and CFD to complement each other. See here an SCR which was modeled both ways |
 
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| ASC also uses physical modeling to track ammonia into an SCR |
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