How to maintain SCR catalyst at peak effectiveness

The testing, cleaning, maintenance, and replacement of SCR and oxidation catalysts is a popular topic at gas-turbine user-group meetings—WTUI in particular—for a very simple reason: You can’t operate your generating units if they don’t meet the permitted emissions levels for NOx and CO. The Western Turbine leadership assures ongoing coverage of emissions control systems at its annual meetings. At the 25th anniversary conference in Long Beach, Cormetech’s Elizabeth Govey and Karolyn Hagan, updated attendees on SCR catalyst technology.

Team Cormetech packed 10 pounds of information into a five-pound bag, using 57 slides in the hour available to cover the science of emissions control, performance monitoring, maintenance, and catalyst replacement. Recall that the SCR process is relatively simple: Nitrogen oxides present in the gas-turbine exhaust stream are contacted by ammonia sprayed into the flow path—ideally from both sides of the SCR—in the presence of a catalyst; the ensuing chemical reaction produces nitrogen (an inert gas) and water vapor, which are vented to the atmosphere.

Good distribution of ammonia across the entire exhaust stream is critical to system performance the speakers told the group. The quality of aqueous ammonia is important, too; it should be prepared using deionized water. Require that a chemical analysis certification accompany every shipment to your plant.

Inspect the SCR regularly, during planned outages. Check the condition of the perforated plate; also the condition of seals within the SCR modules and between the modules and reactor frame. Make repairs as necessary. Keep in mind that some seal loss is acceptable, but repacking is needed periodically. In cases where catalyst elements have shifted, it may be necessary to remove SCR modules to reposition properly. Consider installing retention plates as a mechanical barrier to hold seal packing in place.

Inspect lances, vaporizers, the piping network, air blowers, etc—and the catalyst, of course. Regarding the last, pay attention to catalyst appearance, particularly any discoloration and the uniformity of that discoloration. Check for fouling of the catalyst bed by insulation or particulate matter and clean as required. Important: Keep the catalyst dry when the SCR is out of service. Every other year, pull catalyst samples from the unit for testing.

SCR catalyst deactivates over time, but the process is slower for gas-fired plants than for coal-fired ones. Remaining activity is determined in the laboratory. There also may be a change in the physical structure of the catalyst, but it may not be visible to the untrained eye; lab tests will provide a proper assessment. When catalyst testing reveals a significant loss of performance, and ammonia use has increased, replacement generally is recommended by a rigorous economic evaluation. Regeneration, common in coal applications, is not suggested for gas plants. The deactivation mechanisms differ with the fuel.

Team Cormetech advised that the typical lead time for replacement catalyst is nine to 12 months and installation time ranges from three to five days depending on whether one or two shifts are used.

Other recent presentations at WTUI on emissions control and catalyst testing, care, and performance, etc:

2014, Dan Ott, Ted Heron, and Joe Otto, Environex Inc. “Recent Developments in SCR and CO Catalyst Systems.”  

2013, Nathan White, Haldor Topsoe Inc. “Integrating SCR and CO oxidation catalysts improves plant performance, reduces cost.”

2012, L J Muzio, T D Martz, and R A Smith, Fossil Energy Research Corp. “SCR PERFORMANCE MANAGEMENT: AIG tuning, catalyst for life forecasting.”

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