Glossary
SCR and SNCR
Honeycomb catalyst
Also known as honeycomb SCR catalyst, extruded catalyst.
A honeycomb catalyst is a monolithic extruded ceramic block containing a dense grid of parallel square channels through which flue gas flows. The active catalytic material — typically vanadium pentoxide and tungsten trioxide on a titanium-dioxide carrier — is incorporated into the bulk ceramic. Honeycomb is the most common form of SCR catalyst.
Strengths and weaknesses
| Strength | Weakness |
|---|---|
| Very high geometric surface area per unit volume | Channels susceptible to pluggage by ash |
| Low pressure drop in clean condition | Brittle — handle with care during install / replacement |
| Mature, large supplier base | Channels are harder to clean than open structures |
| Wide range of pitch options (3.5–7.4 mm typical) | Smaller pitch = more risk of pluggage |
Pitch selection
Pitch (centre-to-centre channel spacing) trades surface area against pluggage risk:
- Smaller pitch (3.5–4.5 mm) — high surface area, used on clean gas streams (NGCC HRSGs, gas-fired duty)
- Larger pitch (6–7.4 mm) — used on dusty coal, biomass and WtE duty where pluggage risk dominates
Layer assembly
Individual honeycomb blocks are loaded into a catalyst layer / module and stacked 2–4 layers deep inside the SCR reactor. Sonic horns and steam sootblowers are positioned between layers to keep channels clear.
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.
- Plate catalystA plate catalyst uses parallel coated steel plates instead of extruded honeycomb. Wider gas channels make it preferred for high-dust SCR duty where pluggage risk is significant.
- Corrugated catalystA corrugated catalyst uses corrugated fibre-reinforced sheets coated with active material. Lighter than honeycomb, particularly common on tail-end SCR and marine duty.
- Catalyst layer and moduleAn SCR catalyst module is a steel-framed cassette holding multiple catalyst elements. Modules are stacked into layers; layers are stacked into the SCR reactor.
- 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.