headsUP: Catalyst inspection, maintenance

Cleaning catalyst properly takes know-how, experience

Clean catalyst is a prerequisite for efficient turbine operation within permit limits. The majority of gas-turbine-based powerplants installed in the last decade are equipped with an SCR for NOx control, most of those also have CO catalyst. Telltale indicators of plugged or fouled catalyst include an increase in pressure drop between the engine and stack exit, a decrease in power output, and/or an increase in ammonia slip.
   Jeff Bause of Groome Industrial Service Group suggests a thorough visual inspection of catalyst when the possibility of plugging or fouling exists. In most cases, you’ll see a blanket of insulation on the face of the catalyst; sometimes fines will collect in the pitch of the material. Look for a crack in the transition liner to locate the source of the insulation. Another foulant is the chemical byproducts of tube leaks.
   The cleaning technique Groome uses is to blow very clean, very dry air through the catalyst from the downstream face while vacuuming the upstream side. This is done until no foreign material is being collected by the vacuum. As cleaning proceeds, any catalyst blocks that have shifted during operation are returned to their proper position and the gasket material around the blocks is replaced as necessary.
   Not all jobs are the same, Bause said. Some catalysts have multiple layers that are so close together one layer must be removed to clean the second. A typical job is planned using boiler drawings so there is no delay when the cleaning crew shows up onsite. Groome does its work turn-key from the installation of scaffolding to its removal. 
   Groome has designed several different nozzles and developed multiple procedures to assure optimal cleaning no matter what the catalyst type or manufacturer. CO catalyst generally has a torturous path, Bause said, and debris often gets stuck in the catalyst turns. An SCR catalyst can have a torturous path but normally is straight-through. Groome recommends the plant owner/operators install instrumentation to monitor the pressure drop across individual catalyst beds.

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