AIG Tuning
One of the most important SCR operating parameters is the NH3/NOx distribution across the catalyst surface. For overall NOx reductions of 90% or more, the NH3/NOx distribution becomes extremely important. This maldistribution is characterized by the RMS. If lower NOx reductions are required, a much larger NH3/NOx maldistribution (RMS) can be tolerated.
This emphasis on achieving uniform NH3/NOx ratios across the catalyst highlights the importance of the ammonia injection grid (AIG) as part of the SCR system. AIGs should be designed with the flexibility to bias ammonia flow in multiple zones across both dimensions of the duct even when static mixers are utilized. Although ductwork design is optimized to achieve a uniform velocity distribution at the AIG inlet, the actual NOx distribution may not be uniform due to the unit’s boiler and combustion system design and overall operating characteristics.