TL;DR
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.
What you might see
- NOx removal efficiency declining over multiple months
- required ammonia injection rate increasing for same NOx target
- ammonia slip rising at normal AIG demand
- catalyst activity factor trending down on performance model
Likely causes
Thermal sintering from sustained operation above catalyst temperature limits
Arsenic poisoning from fuel with elevated arsenic content masking active sites
Alkali metal (potassium, calcium) deposition reducing catalyst porosity
Cumulative fly-ash erosion reducing catalyst wall thickness and active surface area
Required tools
- Catalyst sampling equipment (core drill, sample bags, gloves)
- Performance trend data from DCS historian
- Catalyst activity test laboratory contact
Safety first
- Catalyst contains vanadium pentoxide, a hazardous compound. Wear nitrile gloves and a dust mask when handling catalyst samples. Dispose of waste catalyst per your hazardous waste plan.
- Perform catalyst sampling only during scheduled outages with the SCR flue-gas path isolated and cooled.
Procedure
- 1
Pull monthly performance data and plot NOx removal efficiency and ammonia slip vs. time to confirm a declining trend and separate it from load-dependent variation.[1]
- 2
Pull a representative catalyst sample from each catalyst layer per the manufacturer's sampling plan during a planned outage.
- 3
Send samples to a catalyst testing laboratory for BET surface area, activity, and poison (arsenic, alkali) analysis.
Warning: Catalyst material contains vanadium compounds. Follow your hazardous material handling procedure when sampling, bagging, and shipping catalyst cores. - 4
Review fuel analysis records for arsenic content. Fuel with arsenic above 1-2 mg/kg is a known catalyst poison in most SCR applications.
- 5
If the lab reports arsenic or alkali poisoning below the threshold for regeneration, plan for catalyst layer addition or replacement at the next outage.
- 6
If the catalyst shows only low-level deactivation, schedule a catalyst wash to remove surface deposits and re-test activity.
- 7
Update your catalyst life model with the lab results and revise the replacement schedule accordingly.
Sources
Babcock & Wilcox B&W SCR SCR / Emission Control general technical documentation, Babcock & Wilcox
SCR catalyst deactivation mechanisms and management, general power-plant emission control references (general)
More guides for Babcock & Wilcox B&W SCR
How to diagnose ammonia injection malfunction on a Babcock and Wilcox B&W SCR
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 clear catalyst plugging and high backpressure on a Babcock and Wilcox B&W SCR
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.
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