---
title: "Catalyst pluggage"
description: "Catalyst pluggage is the physical blockage of SCR catalyst channels by particulate material. Unlike catalyst masking (a thin surface blanket), pluggage fills the catalyst channels themselves, stopping gas flow through affected cells. The result is ΔP rise across the SCR, gas-flow maldistribution into the remaining open cells, and channelling effects that reduce overall NOx reduction."
canonical_url: "https://sylio.co/glossary/catalyst-pluggage"
last_updated: "2026-06-28T02:29:36.999Z"
---

**Catalyst pluggage** is the physical blockage of [SCR catalyst](/glossary/selective-catalytic-reduction) channels by particulate material. Unlike [catalyst masking](/glossary/catalyst-masking) (a thin surface blanket), pluggage fills the catalyst channels themselves, stopping gas flow through affected cells. The result is ΔP rise across the SCR, gas-flow maldistribution into the remaining open cells, and channelling effects that reduce overall NOx reduction.

## Sources of pluggage material

- **Large-particle ash (LPA)** — slag fragments and agglomerated ash carried over from the boiler
- **Popcorn ash** — porous low-density ash particles that wedge into honeycomb cells
- **Ammonium-salt deposits** — [ammonium bisulphate](/glossary/ammonium-bisulphate) on tail-end SCRs at lower temperatures
- **Refractory debris** — fragments from upstream furnace or duct repairs

## Prevention

- **LPA screens** — coarse mesh screens upstream of the catalyst trap large particles
- **Guard layers** — sacrificial top catalyst layer with larger pitch absorbs the initial particulate
- **Larger pitch on the top layer** — wider cell openings on the first catalyst layer pass LPA through to a removable screen below
- **Periodic sonic-horn cleaning** — dislodges accumulating ash before it cements
- **Steam sootblowing** — for harder deposits

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Large-particle ash](/glossary/large-particle-ash)
- [Popcorn ash](/glossary/popcorn-ash)
- [Catalyst masking](/glossary/catalyst-masking)
- [Honeycomb catalyst](/glossary/honeycomb-catalyst)
- [Sonic horn](/glossary/sonic-horn)