---
title: "Catalyst layer / module"
description: "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."
canonical_url: "https://sylio.co/glossary/catalyst-layer-module"
last_updated: "2026-06-28T02:29:36.904Z"
---

A **catalyst module** is a steel-framed cassette that holds multiple individual catalyst elements ([honeycomb](/glossary/honeycomb-catalyst) blocks or [plate](/glossary/plate-catalyst) packs). A **catalyst layer** is a horizontal stack of modules covering the full cross-section of the [SCR](/glossary/selective-catalytic-reduction) 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](/glossary/large-particle-ash) and [popcorn ash](/glossary/popcorn-ash)
- **Middle layers** — main NOx-reduction work
- **Bottom layer** — polishes residual NOx before flue gas exits

## Service cycle

Layers are replaced or [regenerated](/glossary/catalyst-regeneration-vs-replacement) 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](/glossary/sonic-horn) extends this materially.

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Honeycomb catalyst](/glossary/honeycomb-catalyst)
- [Plate catalyst](/glossary/plate-catalyst)
- [Catalyst regeneration vs replacement](/glossary/catalyst-regeneration-vs-replacement)