---
title: "Back-corona"
description: "Back-corona (also reverse ionisation) is a destructive failure mode in an electrostatic precipitator in which the dust layer on the collecting electrodes accumulates so much charge that the gas trapped within it breaks down and emits ions of the opposite polarity. These positive ions discharge incoming negatively-charged dust particles before they reach the plate, and collection efficiency collapses."
canonical_url: "https://sylio.co/glossary/back-corona"
last_updated: "2026-06-28T02:29:30.067Z"
---

**Back-corona** (also *reverse ionisation*) is a destructive failure mode in an [electrostatic precipitator](/glossary/electrostatic-precipitator) in which the dust layer on the [collecting electrodes](/glossary/collecting-electrode) accumulates so much charge that the gas trapped within it breaks down and emits ions of the opposite polarity. These positive ions discharge incoming negatively-charged dust particles before they reach the plate, and collection efficiency collapses.

## When back-corona occurs

Back-corona is triggered by high-[resistivity](/glossary/resistivity) ash — typically above ~10¹¹ Ω·cm — combined with a thick, undisturbed dust layer. The conditions are common on:

- Low-sulphur Western US coals and sub-bituminous lignite
- Some [biomass](/glossary/waste-to-energy) and [WtE](/glossary/waste-to-energy) ashes
- ESPs that have slipped behind on rapper maintenance
- Cement-kiln ESPs after fuel switches or raw-mill stoppages

## Symptoms

- Falling secondary voltage at the [discharge electrode](/glossary/discharge-electrode)
- Rising secondary current with falling efficiency (the classic back-corona signature)
- Persistent stack opacity rise that does not respond to rapper intensification
- Sparking and arcing in the ESP power supply

## Sonic horns and back-corona

Because back-corona is fundamentally a dust-thickness problem, the strongest mitigation is to keep the plates thinner — continuously, not in periodic bursts. [Sonic horns](/glossary/sonic-horn) installed across the field deliver gentle, frequent dislodging that holds the plate dust layer below the critical thickness for back-corona, while reducing the re-entrainment penalty of aggressive rapping. Acoustic cleaning is therefore one of the most cost-effective retrofits on a back-corona-limited ESP.

## Related terms

- [Electrostatic precipitator](/glossary/electrostatic-precipitator)
- [Resistivity (fly-ash)](/glossary/resistivity)
- [Corona discharge](/glossary/corona-discharge)
- [Collecting electrode](/glossary/collecting-electrode)
- [Sonic horn](/glossary/sonic-horn)