SCR and SNCR
DeNOx
Also known as deNOx, NOx reduction, NOx control.
DeNOx is the collective term for post-combustion NOx-reduction technologies on industrial flue gas. The two dominant options are Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR). Both rely on a reagent — ammonia or urea — that reacts with NOx to produce nitrogen and water.
Why DeNOx is mandatory
NOx is a regulated pollutant under the Industrial Emissions Directive (IED), MATS, EPA NSPS, TA Luft 2021 and most national emission codes. Limits for coal-fired power stations and large WtE plants are usually 100–200 mg/Nm³ on a 30-day average, with stricter site-specific BAT-AEL values from BREF revisions.
Choice of technology
| Factor | Favours SCR | Favours SNCR |
|---|---|---|
| Reduction efficiency required | > 70% | 30–60% |
| Plant size | Large | Small / medium |
| Capital available | Higher | Lower |
| Space available | More | Less |
| Catalyst cost tolerance | Yes | Avoid |
| Fuel chemistry | Predictable | Variable |
Many plants run combined systems: SNCR provides bulk reduction, SCR polishes to meet permit limits.
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.
- NOx reduction efficiencyNOx reduction efficiency is the percentage of inlet NOx removed by the DeNOx system. The headline KPI for SCR (80–95%) and SNCR (30–60%) operation.
- 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.