SCR and SNCR
Catalyst layer and module
Also known as SCR catalyst module, catalyst layer, catalyst element.
A catalyst module is a steel-framed cassette that holds multiple individual catalyst elements (honeycomb blocks or plate packs). A catalyst layer is a horizontal stack of modules covering the full cross-section of the SCR reactor. SCR reactors typically contain 2–4 layers, with a fourth or fifth layer space sometimes left empty for future installation if regulatory limits tighten.
Module dimensions
A typical module measures about 1 m × 1 m in plan and 1 m in height. A medium-size coal-fired SCR reactor might hold 60–100 modules per layer; large utility-scale reactors hold 200+.
Layer assignment
- Top layer (guard layer) — sometimes a sacrificial larger-pitch design protecting layers below from LPA and popcorn ash
- Middle layers — main NOx-reduction work
- Bottom layer — polishes residual NOx before flue gas exits
Service cycle
Layers are replaced or regenerated on a rolling schedule based on catalyst activity testing. Typical economic life is 24,000–32,000 operating hours before service; cleaning with sonic horns extends this materially.
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.
- 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.
- 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.
- 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.