---
title: "Selective Catalytic Reduction (SCR)"
description: "Selective Catalytic Reduction (SCR) is the dominant flue-gas NOx-control technology on coal-fired and gas-fired utility boilers, HRSGs in combined-cycle plants, waste-to-energy and biomass boilers, cement plants and major refining furnaces. Ammonia or aqueous urea is injected upstream of a catalyst bed; the catalyst lowers the activation energy for the reaction NOx + NH₃ → N₂ + H₂O, achieving 80–95% NOx reduction across the reactor."
canonical_url: "https://sylio.co/glossary/selective-catalytic-reduction"
last_updated: "2026-06-28T02:29:37.785Z"
---

**Selective Catalytic Reduction (SCR)** is the dominant flue-gas NOx-control technology on coal-fired and gas-fired utility boilers, [HRSGs](/glossary/heat-recovery-steam-generator) in combined-cycle plants, [waste-to-energy](/glossary/waste-to-energy) and [biomass](/glossary/waste-to-energy) boilers, cement plants and major refining furnaces. Ammonia or aqueous urea is injected upstream of a catalyst bed; the catalyst lowers the activation energy for the reaction NOx + NH₃ → N₂ + H₂O, achieving 80–95% NOx reduction across the reactor.

## Reactor layout

A typical SCR reactor is a vertical or horizontal duct containing 2–4 layers of catalyst modules. Upstream of the catalyst sits the [ammonia injection grid (AIG)](/glossary/ammonia-injection-grid) that distributes the ammonia evenly into the flue gas. Most installations operate in the **high-dust** position (between economiser and air heater) where catalyst temperature is around 300–400 °C; **tail-end** SCRs sit downstream of particulate control at lower temperatures, with the trade-off of needing flue-gas reheating.

## Fouling and cleaning

SCR catalysts foul in two ways:

- **Pluggage** — fly ash, [popcorn ash](/glossary/popcorn-ash) and [large-particle ash](/glossary/large-particle-ash) wedge into the catalyst cells, blocking the gas path
- **Masking** — a thin layer of deposit covers the active sites; gas flow continues but catalytic activity falls

Both reduce NOx-reduction efficiency, raise [ammonia slip](/glossary/ammonia-slip), and shorten catalyst life. Cleaning options include steam [sootblowers](/glossary/steam-sootblower), [sonic horns](/glossary/sonic-horn) and offline campaigns (vacuum / water wash / regeneration). Sonic horns are increasingly favoured because they continuously dislodge ash before it cements onto the catalyst face, without the steam erosion of mechanical sootblowing.

## Related terms

- [Selective Non-Catalytic Reduction (SNCR)](/glossary/selective-non-catalytic-reduction)
- [Ammonia injection grid](/glossary/ammonia-injection-grid)
- [Ammonia slip](/glossary/ammonia-slip)
- [Catalyst masking](/glossary/catalyst-masking)
- [Catalyst pluggage](/glossary/catalyst-pluggage)
- [Honeycomb catalyst](/glossary/honeycomb-catalyst)
- [Sonic horn](/glossary/sonic-horn)