SCR and SNCR
Ammonia slip
Also known as NH3 slip, ammonia breakthrough.
Ammonia slip is the concentration of unreacted ammonia (NH₃) in the flue gas leaving an SCR or SNCR system. It is the single most important operational KPI after NOx reduction itself: slip is regulated (typically capped at 2–10 ppm in permits), represents wasted reagent, and drives downstream fouling.
Causes of high ammonia slip
- Poor NH₃/NOx mixing at the AIG
- Catalyst masking or pluggage reducing active surface area
- Catalyst age and de-activation towards end of life
- Operating temperature outside the catalyst window
- Over-injection of ammonia to compensate for falling NOx-reduction efficiency
Downstream consequences
Slipped ammonia combines with SO₃ in cooling flue gas to form ammonium bisulphate (ABS), a sticky low-melting deposit that fouls air heaters, economisers and downstream catalysts and filters. Excessive slip can therefore destroy the cold end of a boiler within months.
Sonic horns and slip reduction
Sonic horns reduce slip indirectly by keeping the catalyst face clear of masking deposits, which preserves active surface area, which lets the catalyst convert ammonia to nitrogen instead of letting it slip. They also keep the AIG decks clean, preserving the designed spray pattern.
Related terms
Related terms
- Selective Catalytic ReductionSCR is the dominant NOx-control technology on industrial combustion plant. Ammonia is injected upstream of a catalyst that converts NOx to nitrogen and water.
- Selective Non-Catalytic ReductionSNCR injects ammonia or urea directly into the furnace at 850–1100 °C to reduce NOx without a catalyst. Cheaper than SCR but lower efficiency and higher slip.
- Ammonia injection gridAn AIG is the array of nozzles that distributes ammonia evenly into flue gas upstream of an SCR catalyst bed. Poor AIG performance is the leading cause of high ammonia slip.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.
- Catalyst maskingCatalyst masking is the deposition of a thin ash layer on the SCR catalyst face that blocks ammonia and NOx from reaching the active sites. Distinct from pluggage and poisoning.