---
title: "Large-particle ash (LPA)"
description: "Large-particle ash (LPA) is fly ash significantly larger than typical particulate (above ~1 mm and sometimes up to 25 mm), produced by fragmentation of waterwall and superheater slag, agglomeration of finer ash, or thermal break-up of refractory. LPA is the dominant cause of SCR catalyst channel pluggage on coal-fired utility boilers."
canonical_url: "https://sylio.co/glossary/large-particle-ash"
last_updated: "2026-06-28T02:29:37.462Z"
---

**Large-particle ash (LPA)** is fly ash significantly larger than typical particulate (above ~1 mm and sometimes up to 25 mm), produced by fragmentation of waterwall and superheater slag, agglomeration of finer ash, or thermal break-up of refractory. LPA is the dominant cause of [SCR catalyst](/glossary/selective-catalytic-reduction) channel [pluggage](/glossary/catalyst-pluggage) on coal-fired utility boilers.

## Why LPA causes pluggage

Normal fly-ash particles are smaller than typical honeycomb [catalyst pitch](/glossary/honeycomb-catalyst) (3.5–7.4 mm) and pass through. LPA particles match or exceed the pitch dimension, wedge into a channel mouth, and progressively block the cell. A single LPA particle can block one channel; clusters of LPA across the top of the catalyst face can block tens of percent of the open area.

## Mitigation

- **LPA screens** — coarse mesh installed upstream of the catalyst, ahead of the [AIG](/glossary/ammonia-injection-grid) or just below it, trapping particles above a set size
- **Pop-up grids** in the economiser hopper trap LPA before it reaches the SCR inlet
- **Larger-pitch top guard layer** — first catalyst layer with wider channels admits LPA which then drops through to a screen below
- **Sonic horns and sootblowers** — dislodge accumulating LPA-driven deposits between maintenance windows

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [Popcorn ash](/glossary/popcorn-ash)
- [Catalyst pluggage](/glossary/catalyst-pluggage)
- [Economiser](/glossary/economiser)