SCR and SNCR
Catalyst masking
Also known as SCR catalyst masking, catalyst fouling, face plugging.
Catalyst masking is the deposition of a thin blanket of fine ash on the face of an SCR catalyst that physically blocks ammonia and NOx molecules from reaching the underlying active sites. Gas continues to flow through the catalyst cells, but the active surface area is shadowed and reaction efficiency falls.
How masking differs from related failure modes
| Failure mode | Mechanism | Reversible? |
|---|---|---|
| Masking | Ash blanket on the active surface | Yes — cleaning restores activity |
| Pluggage | Particles physically block catalyst channels | Sometimes (depends on hardness) |
| Poisoning | Chemical species bind to active sites | Usually no — catalyst replacement |
Masking is the most operationally manageable of the three because it responds to cleaning.
What deposits cause masking
- Calcium-rich fly ash (Western US sub-bituminous, biomass)
- Ammonium-salt films on tail-end SCRs
- Sub-micron silica from biomass fuels
- Iron-oxide carry-over from blast-furnace or sinter-plant SCR applications
Sonic horns and masking control
Sonic horns positioned upstream of each catalyst layer continuously dislodge the developing ash blanket before it consolidates. Combined with periodic steam sootblowing, this two-tier cleaning typically restores catalyst activity by 10–30% within months of installation.
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 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.
- Catalyst poisoningCatalyst poisoning is the chemical binding of trace species (arsenic, alkali metals, phosphorus, sulphur) to SCR active sites. Usually irreversible — the catalyst layer must be replaced.
- 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.