TL;DR
Rising backpressure on an SCR catalyst bed is almost always ash or soot accumulation. Trigger soot-blowing, inspect inlet baffles, and schedule a catalyst wash if the differential pressure stays high.
What you might see
- backpressure across catalyst bed above normal band
- NOx removal efficiency declining
- ammonia slip rising on analyzer
- soot or fly-ash visible at inlet face
Likely causes
Fly-ash accumulation on catalyst leading edges blocking flow channels
Soot blower failure or reduced steam supply leaving lanes un-cleaned
Popcorn ash bridging across catalyst cell openings
AIG (ammonia injection grid) ammonium bisulfate deposition at low load
Required tools
- Differential pressure gauge or DCS trend data
- Flashlight
- Personal protective equipment for flue-gas atmosphere (half-face respirator, safety glasses, heat-resistant gloves)
Safety first
- Never open a flue-gas side access door without confirming the unit is offline or at minimum safe load and the opening is upstream-isolated. Flue gas contains SO2 and particulates.
- High-voltage systems power soot blowers and AIG actuators. Follow lockout/tagout before performing mechanical work on blowers or nozzles.
Procedure
- 1
Review the differential-pressure trend across the catalyst bed and log the current value against the design baseline.
- 2
Confirm all soot blowers associated with the SCR section are in service and completing their travel cycles without fault alarms.
- 3
Initiate a manual soot-blowing sequence on the SCR catalyst layers if your controls allow; document the pre- and post-differential-pressure readings.[1]
- 4
Inspect inlet turning vanes and distribution baffles from the access door for visible ash bridging or large agglomerated particles. Use a strong flashlight.
Warning: Ensure unit is at minimum load or offline before opening any access door to the flue gas path. The gas stream contains particulates, SO2, and is at elevated temperature. - 5
Check the AIG for plugged nozzles. Low-load operation below the minimum catalyst temperature window increases ammonium bisulfate formation; raise unit load if possible.
- 6
If differential pressure remains elevated after soot blowing, schedule an offline catalyst wash per the catalyst manufacturer's water-wash procedure.
- 7
Restore unit to normal operation, re-log differential pressure, and verify NOx efficiency returns to permit levels.
Sources
Babcock & Wilcox B&W SCR SCR / Emission Control general technical documentation, Babcock & Wilcox
SCR catalyst maintenance and soot blower operation, general industry guidance for power-plant emission control systems (general)
More guides for Babcock & Wilcox B&W SCR
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Zero or erratic AIG flow almost always means a stuck control valve, plugged nozzle, or cold vaporizer. Verify vaporizer temperature, check valve positioner output, and inspect the injection grid nozzles.
How to diagnose catalyst deactivation and declining NOx efficiency on a Babcock and Wilcox B&W SCR
Gradual NOx efficiency loss that requires increasing ammonia to compensate is a sign of catalyst deactivation. Pull a catalyst sample for activity testing and check for arsenic or alkali poisoning.
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