---
title: "Selective Non-Catalytic Reduction (SNCR)"
description: "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."
canonical_url: "https://sylio.co/glossary/selective-non-catalytic-reduction"
last_updated: "2026-06-28T02:29:37.945Z"
---

**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](/glossary/selective-catalytic-reduction) but achieves lower reduction (typically 30–60%) and produces higher [ammonia slip](/glossary/ammonia-slip).

## Where SNCR is used

- Smaller industrial and utility boilers where SCR capital cost is unjustified
- [Waste-to-energy](/glossary/waste-to-energy) and [biomass](/glossary/waste-to-energy) plants — often as the primary DeNOx with optional SCR polish
- [Cement preheater towers](/glossary/preheater-tower) 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](/glossary/ammonium-bisulphate) on cold-end heat-transfer surfaces, particularly the [air heater](/glossary/air-heater)
- **Urea / ammonia deposits on lance tips** — injection lances can plug with urea solids or carbon deposits

[Sonic horns](/glossary/sonic-horn) on the cold-end air heater address ABS fouling that follows SNCR operation.

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [DeNOx](/glossary/denox)
- [Urea SNCR / aqueous-ammonia SNCR](/glossary/urea-sncr-aqueous-ammonia-sncr)
- [Ammonia slip](/glossary/ammonia-slip)
- [Ammonium bisulphate](/glossary/ammonium-bisulphate)