Glossary
SCR and SNCR
NOx reduction efficiency
Also known as DeNOx efficiency, SCR efficiency, NOx conversion.
NOx reduction efficiency is the percentage of NOx removed from the flue gas by a DeNOx system, calculated as (NOx_in − NOx_out) / NOx_in. It is the headline KPI for any SCR or SNCR installation and the figure permit compliance is measured against.
Typical performance
| System | Reduction range | Typical ammonia slip |
|---|---|---|
| SCR (high-dust) | 80–95% | 2–5 ppm |
| SCR (tail-end) | 90–98% | 1–3 ppm |
| SNCR | 30–60% | 5–10 ppm |
| Combined SNCR + SCR | up to 99% | 2–5 ppm |
What erodes efficiency over time
- Catalyst masking — fine ash blanket reducing active surface area
- Catalyst poisoning — chemical de-activation
- Catalyst pluggage — channel blockage and gas channelling
- AIG distribution drift — uneven NH₃/NOx mixing
- Operating outside the temperature window — too cool or too hot for the catalyst
How cleaning preserves efficiency
Sonic horns and steam sootblowers attack masking and pluggage directly. A well-cleaned catalyst maintains 85–90% of its initial efficiency for 30,000 operating hours, against 60–70% for a poorly cleaned catalyst of the same age. The economic case for active cleaning is therefore measured in deferred catalyst replacement and avoided ammonia-over-injection cost.
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.
- 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.