The efficiency and long-term performance of an SCR System is largely dependent on several key design considerations of the various equipment within the SCR System. When complex NOx reductions on Boilers are required, there are critical aspects to the system which must be addressed to ensure the success of the overall emissions control system.

While the SCR Catalyst often gets all the credit, and all the blame when performance declines, careful consideration of the design of the entire system can alleviate and prevent many of the top factors that contribute to SCR issues in your boiler.

The SCR Catalyst design itself is crucial to the effectiveness of the emissions control system. The Catalyst volume, formulation and pressure drop must consider the totality of operating scenarios the Boiler will encounter. This evaluation should include all load levels (MECL) that the Boiler will operate within. All ambient conditions should also be considered. For Boilers firing multiple fuels, or blended fuel streams, this is especially important. This evaluation must include all operating cases from the Boiler OEM, but also interpolation of existing plant DCS data trends, if evaluating an existing Boiler design for Catalyst performance. 

Additional testing to ascertain the NO/NO2 speciation, as high NO2 speciation can be a major issue to SCR efficiency and require specific SCR Catalyst Formulations to achieve NOx reductions. 

Turndown of the Boiler must also be considered, as NOx produced is typically higher, and the decrease in exhaust temperature will limit SCR Catalyst efficiency. On existing Boilers, the SCR Catalyst should be inspected every outage to ensure the catalyst face is not blocked due to rust or insulation. This can majorly affect the SCR Catalyst performance by masking the active pore sites of the SCR. 

Bypass can greatly affect NOx and Ammonia Slip. The catalyst support structure frame, as well as the perimeter seals, should also be inspected for the any areas that may have exhaust gas and ammonia bypass. The catalyst modules should be well packed to ensure that there is not ammoniated exhaust gas passing unreacted thru gaps around the catalyst bed. On a high-performance SCR, small amounts of bypass can drastically add up to the inability to meet. 

The Ammonia distribution within the exhaust gas cross-section is a huge factor in the Catalyst’s ability to properly reduce the NOx levels. This important part of the system falls on the design of the Ammonia Injection Grid (AIG). The AIG design must factor the amount of ammonia and diluent that is required for the reaction, as well as the ability to inject and mix the ammonia with the NOx present in the exhaust gas. Careful consideration of injection pressures, mass flow, and density change along the length of each AIG lance. Ammonia Injection Grid design for advanced NOx control will carefully vary injection orifice sizing, spacing, and angle of injection necessary to ensure proper distributions. This is critical to ensuring the NH3:NOx distribution is matched to the volume of SCR Catalyst. 

The ammonia injection grid should also be inspected every outage to ensure there is not significant plugging. This can have a huge effect on the SCR Catalyst performance by impacting the Ammonia distribution of the SCR. The AIG should be cleaned if plugging is found, as well as a root cause should be investigated. Often the design of the AIG can be improved for better performance and can offer some resilience against frequent plugging issues. 

Avoiding many SCR performance issues begin with the Ammonia Supply. It can be valuable to work with a reputable and accountable chemical supplier to ensure you are getting the ammonia purity that is necessary for your system. Avoiding contamination of the Ammonia by requiring dedicated trucks for each haul. 

Also requiring certificates and test reports before offload is important to keeping the system free of things like chlorides and calcium. These impurities can damage and plug the various components of the Ammonia system.

Specifying correct purity grade of ammonia is critical! For Aqueous Ammonia systems, reagent grade ammonia is the best option. The key differentiation is the purity of the water content of the solution that may contain soluble minerals that can plug, foul, erode damage the SCR equipment. 

These impurities in the reagent solution can lead to vaporizer fouling, Ammonia Injection Grid plugging, and potential catalyst performance problems. It only takes one bad load of ammonia to experience the headaches associated with ammonia impurity. 

Ensuring these key design factors are considered in SCR and Ammonia system design, and addressing problems when they are discovered, are essential to an efficient SCR system capable of advanced NOx reduction.