SCR and SNCR
Selective Catalytic Reduction
Also known as SCR, SCR system, SCR reactor.
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.
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) 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 and 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, and shorten catalyst life. Cleaning options include steam sootblowers, sonic horns 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
Related terms
- 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.
- 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.
- 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.
- 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.
- 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.
- Catalyst pluggageCatalyst pluggage is the physical blockage of SCR catalyst channels by large-particle ash, popcorn ash or ammonium-salt deposits. It causes ΔP rise and gas-flow maldistribution.
- Honeycomb catalystA honeycomb catalyst is an extruded ceramic block with parallel square channels, the most common SCR catalyst form. High surface area but susceptible to channel pluggage.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.