Corona discharge

A corona discharge is a self-sustaining electrical discharge that occurs when the field gradient around a sharp electrode exceeds the breakdown threshold of the surrounding gas. In an ESP the corona forms around the discharge electrode, ionises flue-gas molecules, and the resulting ions attach to dust particles. The charged particles then drift to the collecting electrodes under the electric field.

Negative corona dominates

Industrial ESPs almost always run on negative corona because it sustains a higher voltage before sparking - but it also produces some ozone, which is one of the reasons WESPs in confined ventilation paths sometimes use positive corona instead.

What disrupts the corona

Excessive dust on the collecting plate - raises plate-face voltage, narrows the working gap
High ash resistivity - traps charge in the dust layer, leading to back-corona
Bent or broken discharge electrodes - local field collapse, sparking, eventual short
Fouled discharge electrode tips - suppressed corona, reduced ion current

Acoustic cleaning addresses two of these (plate dust thickness and discharge-electrode fouling) without the broken-electrode risk of aggressive mechanical rapping.

Role inside an ESP

In an electrostatic precipitator, corona forms near the high-voltage discharge electrode where the electric field is strong enough to ionise gas molecules. The ions move through the gas and charge dust particles. Charged particles then migrate toward grounded collecting electrodes, where they form a dust layer for later removal.

Most industrial ESPs use negative corona because it is stable and efficient for many flue-gas applications. The transformer-rectifier set raises voltage until useful corona current flows, then controls sparks and arcs so the field remains productive rather than destructive.

Operating limits

Corona performance depends on electrode geometry, gas composition, particulate loading, moisture, temperature and ash resistivity. Too little voltage gives poor particle charging. Too much voltage causes sparking, wasted power and possible equipment damage. High-resistivity ash can hold charge on the collecting plate and create back-corona, which sends opposite ions into the gas and reduces collection.

Operators watch secondary voltage, secondary current, spark rate, opacity, rapper behaviour and field trips. A dirty discharge electrode can suppress corona because the sharp points become rounded by dust. A misaligned electrode can create local sparking rather than uniform charging.

Cleaning context

Sonic horns support corona indirectly by keeping discharge electrodes and collecting plates cleaner. Cleaner electrodes maintain sharper electric-field gradients, while cleaner plates reduce resistive dust layers and re-entrainment. Acoustic cleaning does not replace voltage control or rapper maintenance, but it helps the electrical system operate closer to its intended field strength for longer periods.

Field checks

Corona quality is inferred from ESP voltage-current curves, spark rate, secondary power, opacity and field-by-field trends. A healthy field accepts voltage and current without excessive sparking, while a fouled or misaligned field may spark early, draw unstable current or show low power even when the transformer-rectifier set is healthy. Gas temperature, moisture, sulphur chemistry and dust resistivity strongly influence the result.

Maintenance inspections look for discharge-electrode build-up, broken wires, misaligned frames, cracked insulators, dirty insulator housings and collecting-plate distortion. Deposits on discharge electrodes can blunt the corona points and reduce charging intensity. Wet or conductive dust can create flashover paths. Acoustic cleaning is relevant where dry deposits around electrodes, gas distribution devices or hoppers interfere with stable energisation. It cannot correct a failed transformer-rectifier, poor alignment or an ash chemistry that requires conditioning, but it can reduce one mechanical cause of weak corona.

Operators also use corona behaviour to tune rapping and gas conditioning. If more power only creates sparks, the field may need cleaner electrodes, different conditioning, or a lower setpoint until dust properties change. Treating corona as a process signal prevents unnecessary electrical stress.