Back-corona

Back-corona is reverse ionisation within a high-resistivity dust layer on the collecting plates of an electrostatic precipitator. Instead of the normal corona charging particles and driving them to the plate, electrical breakdown occurs through the dust cake and injects ions of the wrong polarity back into the gas stream.

The result is poor particle charging, unstable electrical fields, sparking and a sharp drop in collection efficiency. Operators may see lower usable voltage, higher spark rate, opacity excursions and ash re-entrainment even though the ESP appears energised.

Conditions that cause it

Back-corona is associated with high dust resistivity, which is influenced by ash chemistry, gas temperature, moisture, sulphur trioxide conditioning, unburned carbon and the thickness of the dust layer. Low-sulphur coal ash and some biomass or industrial dusts can be particularly difficult. A thick layer makes the voltage gradient through the dust cake worse.

Controls

Controls include flue-gas conditioning, temperature adjustment, improved rapping, better gas distribution, sectional electrical tuning and keeping collecting plates from carrying excessive dust. Rappers must remove ash without causing too much re-entrainment. Hopper evacuation must also work, or removed ash simply builds up again.

Acoustic-cleaning relevance

Sonic horns can support ESP performance by keeping plates and internals cleaner between rapper cycles and by reducing thick dust layers that encourage back-corona. They do not change dust resistivity directly. Their value is strongest where ash is dry and can be kept mobile before it compacts into an insulating cake.

Operating variables

Back corona is strongly linked to ash resistivity, which is affected by fuel chemistry, sulphur content, moisture, gas temperature, sodium content, unburned carbon and flue-gas conditioning. High-resistivity ash holds charge on the dust layer instead of allowing it to leak to the collecting plate. The electric field inside the dust layer can then become high enough to create reverse ionisation, reducing the useful charging field in the gas space.

Operators may see unstable secondary voltage, suppressed current, sparking at lower-than-expected power, poor opacity performance and dust layers that resist normal rapping. The problem can vary by field because gas temperature and ash properties change through the ESP. It can also appear after fuel switching, low-sulphur operation, sorbent injection or changes in upstream combustion.

Diagnosis and controls

Diagnosis combines electrical data, opacity trends, rapper response, ash analysis, gas temperature and field-by-field inspection. A field with high voltage but poor collection may not be healthy if back corona is limiting particle charging. Controls can include flue-gas conditioning with SO3 or ammonia where permitted, temperature adjustment, power-control optimisation, rapper tuning, gas distribution improvements and fuel or additive changes.

Maintenance checks look for heavy dust layers, warped plates, damaged discharge electrodes, rapper failures, hopper evacuation problems and air in-leakage that changes gas temperature. If ash is left to accumulate because hoppers bridge, electrical symptoms can be misread as only a power-supply problem.

Acoustic-cleaning relevance

Acoustic cleaning cannot change ash resistivity, but it can help keep collecting surfaces, inlet zones and hoppers from carrying excessive loose dust. Removing or weakening layers before they become thick reduces one condition that favours back corona. The cleaning effect must be coordinated with ESP rapping so released dust does not cause re-entrainment or overload downstream fields.

Where high-resistivity ash is the root cause, acoustic horns are a support tool rather than the main correction. The main engineering response remains electrical tuning, ash chemistry control, gas-temperature management and reliable dust removal from hoppers.

Operating evidence

Back corona should be confirmed with several indicators because its symptoms can resemble poor gas distribution, rapper failure or power-supply faults. Useful evidence includes field electrical curves, spark rate, opacity response, ash resistivity testing, deposit thickness and whether performance changes with gas temperature. A field that improves after rapping or hopper cleanout may have had a dust-layer problem layered on top of a resistivity problem.

Acoustic cleaning trials should therefore be judged field by field. If horns reduce dust-layer thickness but opacity remains limited by ash resistivity, the result is still useful but incomplete. The remaining work belongs to conditioning, temperature control or ESP electrical tuning.

Cleaning evidence

When acoustic horns are added near an ESP, outage photographs should record plate cleanliness, hopper inventory and rapper marks before and after the trial. Those records help separate improved dust removal from electrical effects. If plate layers are thinner but current remains limited, the operating team can focus on resistivity and power controls rather than adding more cleaning force.

Diagnostic evidence

Back corona is usually diagnosed from electrical behaviour and performance together. Operators may see high secondary voltage with limited current, unstable sparking, poor power input, elevated outlet dust or opacity that does not respond to normal rapper adjustment. Ash resistivity, gas temperature, moisture, sulphur content and conditioning all influence the risk. Inspection can show hard ash layers on plates that insulate the collecting surface. Acoustic cleaning is relevant only if it helps keep those layers thinner and less consolidated; it cannot change the electrical properties of the ash. If resistivity is the controlling issue, gas conditioning, temperature control or process chemistry may be more important than additional cleaning force.

Related terms

• Collecting electrode
• Electrostatic precipitator
• Resistivity