Glossary
SCR and SNCR
Selective Non-Catalytic Reduction
Also known as SNCR, SNCR system.
Selective Non-Catalytic Reduction (SNCR) reduces NOx in flue gas by injecting ammonia or aqueous urea directly into the furnace at high temperature (850–1100 °C), where the reagent reacts homogeneously with NOx without needing a catalyst. SNCR is cheaper to install than SCR but achieves lower reduction (typically 30–60%) and produces higher ammonia slip.
Where SNCR is used
- Smaller industrial and utility boilers where SCR capital cost is unjustified
- Waste-to-energy and biomass plants — often as the primary DeNOx with optional SCR polish
- Cement preheater towers where the gas temperature window is naturally available
- As a retrofit on units where space prevents SCR installation
Fouling implications
SNCR does not have a catalyst to foul, but the reagent injection itself creates downstream deposit risks:
- Ammonia salt deposits — un-reacted ammonia combines with SO₃ and ash to form ammonium bisulphate on cold-end heat-transfer surfaces, particularly the air heater
- Urea / ammonia deposits on lance tips — injection lances can plug with urea solids or carbon deposits
Sonic horns on the cold-end air heater address ABS fouling that follows SNCR operation.
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.
- DeNOxDeNOx is the collective term for post-combustion NOx-reduction technologies. SCR and SNCR are the dominant options; both rely on reaction of NOx with ammonia or urea.
- Urea SNCR and aqueous-ammonia SNCRSNCR systems use either solid urea (dissolved on site) or aqueous-ammonia solution as the NOx-reducing reagent. Urea is safer to store; aqueous ammonia is more reactive.
- Ammonia slipAmmonia slip is unreacted ammonia leaving the DeNOx system in the flue gas. It is regulated, expensive in lost reagent, and causes ammonium-bisulphate fouling downstream.
- 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.