Definition
Fly-ash resistivity
Fly-ash resistivity is the electrical resistance of a deposited dust layer. Very high or very low resistivity can both reduce ESP collection performance.
- Subject
- Electrostatic precipitators
- Also known as
- ash resistivity, fly ash resistivity, dust resistivity
Fly-ash resistivity is the electrical resistance of a deposited dust layer in an electrostatic precipitator. It controls how easily charge can leak from collected dust to the grounded collecting plate. ESP performance is best when resistivity sits in a workable middle range.
If resistivity is too low, collected dust loses charge quickly and can be re-entrained by gas flow or rapping. If resistivity is too high, charge cannot move through the dust layer fast enough. Voltage builds across the layer, the electric field inside the dust becomes unstable, and back-corona can occur. Back-corona reduces useful charging and can push particles away from the plate.
What affects resistivity
Resistivity depends on ash chemistry, gas temperature, moisture, sulphur trioxide, ammonia, unburned carbon, alkali metals, and particle surface condition. Coal changes can therefore shift ESP behaviour even when the mechanical plant is unchanged. Low-sulphur coal often produces higher-resistivity ash because less sulphur trioxide is available for natural conditioning.
Temperature is important because surface conduction and volume conduction behave differently. Many ashes have a high-resistivity window in the normal ESP temperature range. This is why gas conditioning, moisture, sulphur trioxide injection, or ammonia carryover can materially change performance.
Operating symptoms
High-resistivity ash may show as high secondary voltage, low current, sparking, back-corona, poor opacity, and dust that sticks tightly to collecting plates. Rapping may remove sheets unevenly or leave a hard insulating layer. Low-resistivity ash may show as easy dust release but higher re-entrainment.
Troubleshooting should compare electrical readings by field, opacity, fuel data, ash chemistry, gas temperature, and rapper behaviour. Mechanical cleaning changes alone cannot fully correct an ash that is electrically outside the ESP design window.
Acoustic cleaning context
Sonic horns help by preventing thick insulating dust layers and reducing the need for aggressive rapping. They do not change ash chemistry or intrinsic resistivity. If high resistivity is the controlling limit, acoustic cleaning should be paired with fuel review, gas conditioning, temperature control, or ESP electrical tuning.
Operating context
Ash resistivity changes with temperature, moisture, sulphur content, alkali content, unburned carbon, and flue-gas conditioning. In an ESP, very high resistivity can hold charge on the dust layer and cause back corona, while very low resistivity can let particles lose charge and re-enter the gas stream. The same ESP can therefore perform well at one fuel blend or load and poorly after a fuel, temperature, or conditioning change.
Operators infer resistivity problems from voltage-current behaviour, spark rate, opacity, field performance, and ash analysis. Maintenance still matters: warped plates, broken electrodes, poor rapping, hopper build-up, and sneakage can mimic an electrical dust problem. Acoustic cleaning may help with hopper and inlet deposits, but it does not change the electrical property of the ash. If resistivity is the root cause, the remedy is usually gas conditioning, temperature control, fuel management, or ESP electrical tuning.
Design and cleaning context
ESP design uses expected resistivity to choose specific collection area, field length, rapper arrangement, gas velocity, and whether flue-gas conditioning may be needed. The expected value should be tied to fuel range and temperature range, not a single ash sample. High-resistivity ash may need sulphur trioxide or moisture conditioning in some plants, while low-resistivity ash may need different rapping and gas distribution strategies.
Acoustic cleaning is relevant around deposits that reduce gas distribution or hopper evacuation, but it cannot compensate for dust that will not hold or release charge correctly. If an ESP shows poor performance after an acoustic system is added, engineers should still check electrical power input, rapper timing, ash chemistry, and sneakage. Cleaning hardware is a support system, not an electrical correction.
Related terms
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Related terms
4 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 pneumatic low-frequency sound emitter used to dislodge particulate fouling from boilers, ESPs, baghouses, ducts and silos while the plant stays online.
References