Joseph Otto | July 28th, 2022
Join Environex at the 2022 Combined Cycle Users Group Conference in San Antonio on August 30th for “Is Your SCR/CO System Ready for Turndown? How New VOC Emissions Requirements are Increasing NO2/NOx and Complicating SCR Operation.” Andy Toback will be presenting.
We will review how higher NO2/NOx ratios in exhaust gas increase SCR system performance requirements. NO and NO2 are the primary components of NOx in turbine exhaust. NO2 requires significantly more SCR catalyst activity than NO to achieve the same level of reduction, so higher NO2/NOx increases the minimum activity requirement to achieve the required NOx conversion. Field data shows that turndown operation is causing higher NO2/NOx out of gas turbines, and systems with CO catalysts can have even higher NO2/NOx at the SCR catalyst inlet due to NO to NO2 oxidation across the CO catalyst.
New field data collected by Environex is showing another phenomenon that is causing even higher NO2/NOx at the SCR catalyst inlet: lower VOC emissions requirements. Newer systems, along with some retrofits, are being required to reduce VOC emissions to the lowest levels that we have ever seen. VOCs are reduced by being converted to CO2 across the same oxidation catalysts that are used to reduce CO emissions. However, VOCs are much more difficult to oxidize to CO2 than CO is. As a result, oxidation catalysts for these systems are being designed with much more oxidation potential than a standard CO catalyst. While these new oxidation catalyst designs are more effective at reducing VOCs (and highly effective at CO reduction), there is a major unintended consequence: even higher levels of NO to NO2 oxidation, particularly in the temperature range for combined cycle applications.
The higher NO2/NOx at the SCR inlet caused by these highly active oxidation catalysts is creating significant problems for SCR systems. SCR catalyst designs that would normally have sufficient excess activity to provide a normal catalyst lifecycle are failing upon start-up due to the increased minimum activity levels that are imposed by the elevated NO2/NOx levels. This presentation will include field data, case studies, and lab test data demonstrating these issues. With this information in hand, plant operators will know what to look out for when specifying SCR and CO catalysts for these systems, as well as ways to mitigate the impact of oxidation catalyst design/selection on SCR system performance.