EXPERIENCE

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. 

The SCR projects FERCo has performed encompass the following areas:

• Cold flow modeling of SCR systems (View Full Project List)
• SCR process calculation
• Full-Scale AIG Tuning and Guarentee Testing (View Full Project List)
• Designed and operate SCR pilot facilities (see below)
• Laboratory studies of advanced SCR and hybrid technologies and testing of catalyst activity (see below)

Pilot-Scale Studies:

• Designed/fabricated gas analysis systems for EPRI's SCR pilot plants (TVA, NYSEG, Ni-Mo, PG&E)
• Participated in the design of the EPRI pilot plants
• Test contractor at the Ni-Mo pilot plant
• Test contractor at the PG&E Morro Bay pilot plant
• Operated the EPRI SCR pilot at PSCC Arapahoe
  
• Performed EPRI-funded testing of InterLayer Mixing for improved SCR performance at the SCR pilot at Southern Company's Plant Crist
• Designed/built control and analysis system for TXU Sandow slipstream/pilot

Laboratory Activities:

• FERCo's laboratory has 2 bench-scale test rigs
• Catalyst activity test for users and vendors
• Catalyst evaluation for hybrid (SNCR+SCR) process
• Use of urea for SCR processes
• Designed/fabricated bench-scale SCR facility for EPRI
• Designed/fabricated an in-situ catalyst aging device
• Determined effect of NO2 on SCR processes

THE IMPORTANCE OF AIG TUNING

One of the most important SCR operating parameters is the NH3/NOx distribution across the catalyst surface.  The impact of NH3/NOx ratio maldistributions on SCR performance is illustrated in Figure 1.  The figure was derived using FERCo’s process model for SCR performance prediction based on a typical SCR system.

The figure to the right shows the importance of the NH3/NOx distribution on performance. In this figure, NH3 slip is plotted as a function of NOx reduction for varying levels of NH3/NOx non-uniformity, or maldistribution.  For overall NOx reductions of 90% or more, the NH3/NOx distribution becomes extremely important.  The maldistribution is characterized by the so-called RMS.  The RMS is the standard deviation of the NH3/NOx ratio across the surface, divided by the average NH3/NOx ratio.  If lower NOx reductions are required, a much larger NH3/NOx maldistribution can be tolerated. 

The emphasis on achieving uniform NH3/NOx ratios across the catalyst points to the importance of the ammonia injection grid (AIG) as part of the SCR system.  The AIG is used to adjust the distribution of the ammonia flow across the duct commensurate with the inlet NOx distribution to achieve the required NH3/NOx distribution.  For this reason, 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.  Thus, AIG tuning is one of the key startup activities of an SCR installation.

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. 

Please contact us for additional information.