Electrostatic precipitators
Back-corona
Also known as reverse ionisation, back ionisation, back corona.
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.
When back-corona occurs
Back-corona is triggered by high-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 and WtE 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
- 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 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
Related terms
- Electrostatic precipitatorAn ESP removes particulate from flue gas by charging dust and collecting it on plate electrodes. Sonic horns are widely used to dislodge ash from plates and to keep hoppers from bridging.
- Fly-ash resistivityFly-ash resistivity is the electrical resistance of a deposited dust layer. Resistivity above ~10¹¹ Ω·cm triggers back-corona and degrades ESP performance.
- Corona dischargeCorona discharge is the electrical breakdown around an ESP's discharge electrode that ionises gas molecules and charges dust particles for collection.
- Collecting electrodeThe collecting electrode is the grounded plate or tube on which charged particulate accumulates inside an ESP. Dust must be released to hoppers without re-entraining into the gas stream.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.