---
title: "Stack / breeching"
description: "The stack (chimney) is the vertical structure that releases flue gas to atmosphere at sufficient height to comply with dispersion-modelling regulations and prevent ground-level concentrations of pollutants. The breeching is the horizontal duct that connects the ID fan discharge (or boiler outlet on natural-draught units) to the stack inlet."
canonical_url: "https://sylio.co/glossary/stack-breeching"
last_updated: "2026-06-28T02:29:34.037Z"
---

The **stack** (chimney) is the vertical structure that releases flue gas to atmosphere at sufficient height to comply with dispersion-modelling regulations and prevent ground-level concentrations of pollutants. The **breeching** is the horizontal duct that connects the [ID fan](/glossary/id-fan) discharge (or boiler outlet on natural-draught units) to the stack inlet.

## Stack-related issues

- **Stack liner corrosion** from condensed sulphuric acid below the [acid dew point](/glossary/acid-dew-point)
- **Visible plume** caused by sulphate aerosol from [SO₂/SO₃ conversion](/glossary/so2-so3-conversion) or low-load operation
- **Opacity** exceedances from upstream particulate control failure
- **Breeching deposition** during low-load operation, occasionally falling as chunks during transients

## Sonic horns on breeching

Where breeching deposition is significant, [sonic horns](/glossary/sonic-horn) on the breeching wall prevent the accumulation that would otherwise risk falling debris during load changes.

## Related terms

- [ID fan / FD fan / PA fan](/glossary/id-fan)
- [Opacity](/glossary/opacity)
- [Boiler](/glossary/boiler)