Electrostatic precipitators
Fly-ash resistivity
Also known as ash resistivity, fly ash resistivity, dust resistivity.
Fly-ash resistivity is the electrical resistance of the dust layer deposited on the collecting electrodes of an ESP. It is the single most important fuel-dependent variable in ESP performance, because it controls whether the collected dust can discharge its acquired charge to ground or instead accumulates trapped charge that triggers back-corona.
The resistivity window
| Resistivity (Ω·cm) | ESP behaviour |
|---|---|
| Below 10⁸ | Dust discharges too quickly; re-entrainment dominates |
| 10⁸–10¹¹ | Ideal range; standard ESP operation |
| Above 10¹¹ | High risk of back-corona; collection efficiency collapses |
What raises resistivity
- Low sulphur content in coal (less SO₃ to condition the ash)
- Low gas temperature near the acid dew point
- High-alkali biomass ash
- Certain cement-kiln dust compositions
Mitigation
The classic remedy is flue-gas conditioning — injecting SO₃ or ammonia ahead of the ESP to lower ash resistivity. A complementary remedy is to keep the plate dust layer thin enough that back-corona cannot establish, which is where sonic horns earn their keep on high-resistivity ESPs: continuous gentle dislodging prevents the critical thickness from developing.
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.
- Back-coronaBack-corona is reverse ionisation through a high-resistivity dust layer on ESP collecting plates. It collapses collection efficiency and is mitigated by keeping plates clean.
- Corona dischargeCorona discharge is the electrical breakdown around an ESP's discharge electrode that ionises gas molecules and charges dust particles for collection.
- 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.