Selective Catalytic Reduction (SCR) systems are used for NOx control in stationary combustion systems utilized for power and process steam generation including gas turbine engine systems. Over the years, FERCo has performed numerous projects optimizing SCR systems and diagnosing performance issues. These have covered a range of applications from small (3 MW) natural gas-fired turbines to large (1300 MW) coal-fired utility boilers.
FERCo has developed tools for optimization and diagnostic purposes, including instrumentation and methodologies for 1) tuning the ammonia injection grid (AIG), 2) measuring the NH3/NOx and velocity distribution at the catalyst face, 3) identifying flue gas bypass, and 4) monitoring catalyst activity in situ. These tools are applicable to SCR systems ranging from gas turbines to coal fired power plants.
FERCo has been actively involved in tuning SCR systems and has developed an approach using instrumentation and techniques that allow the NH3/NOx distribution to be measured by making only NOx measurements at the catalyst exit.
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.