Glossary
SCR and SNCR
Catalyst poisoning
Also known as SCR catalyst poisoning, catalyst deactivation.
Catalyst poisoning is the chemical deactivation of SCR catalyst active sites by trace species in the flue gas. Unlike masking (physical blanket) or pluggage (channel blockage), poisoning is a chemical process that binds molecules to the catalyst's vanadium, tungsten or titanium active centres. Cleaning cannot reverse it; the affected layer must be regenerated off-site or replaced.
Common poisons
| Poison | Source |
|---|---|
| Arsenic | Coal-fired flue gas, especially sub-bituminous |
| Alkali metals (K, Na) | Biomass, agricultural-residue and waste-fuel ash |
| Phosphorus | Animal-fat biofuels, sewage-sludge co-firing |
| Calcium | Wet limestone scrubbers upstream, biomass |
| Sulphur trioxide (high concentration) | SO₂ + V₂O₅ oxidation at high SCR temperature |
| Lead and zinc | Waste-to-energy, some industrial off-gas streams |
Mitigation
- Fuel selection / blending to control fuel-bound poison content
- Guard layers (sacrificial top catalyst layers protecting layers below)
- Catalyst formulation tuned to expected poisons (e.g. alkali-resistant for biomass)
- Catalyst regeneration vs replacement campaigns to extend catalyst life
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.
- 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 regeneration vs replacementRegeneration removes accumulated masking and partial poisoning from used SCR catalyst, restoring activity to 90% of fresh and saving 60–70% of replacement cost.