Coal, biomass, cement kilns, steel mills. Anywhere solid and some liquid fuels are being combusted, you're generating particulate matter (PM).
At many sites, the electrostatic precipitator (ESP) is a critical system to maintain air permit compliance, and good relations with the community.
An ESP removes particulate from flue gas using high-voltage electric fields. No filters to replace, pressure drop 5-10x lower than a baghouse, and collection efficiencies above 99% when properly sized and maintained.
How it works
- Gas distribution: Dirty flue gas enters through the inlet nozzle and slows from duct velocity of 12-24 m/s (49-79 ft/s) to about 1.5-1.8 m/s (4.9-5.9 ft/s). Slow enough for electrical attraction forces to overcome gas carrying velocity and turbulence.
- Particle charging: Transformer/rectifiers (T/Rs) create a secondary voltage of between 30kV and 130kV DC, creating a corona discharge that generates negative ions. These ions attach to passing particles, imparting a negative charge.
- Collection: Charged particles migrate toward grounded collector plates, where they accumulate.
- Rapping: Mechanical rappers periodically strike the collector plates and discharge electrodes, dislodging accumulated dust that falls into the hoppers below.
- Removal: Hoppers collect the ash and feed it to the plant's ash handling system. Cleaned gas exits through the outlet nozzle.
Common failure modes
Rapper malfunction - Failed or mistimed rappers allow dust buildup that reduces plate spacing, increases sparking, and can cause electrical shorts and spikes in discharge opacity.
Back corona - High-resistivity ash creates a voltage drop across the dust layer, generating positive ions that travel backward and neutralize incoming negative ions, severely degrading collection. Calcium is a common element that increases the electrical resistivity of flue gas and ash, thus lowering potential collection efficiency.
Electrode misalignment - Warped or broken discharge electrodes reduce the electric field strength and create dead zones where particles pass uncollected.
Hopper plugging - Accumulated ash in hoppers backs up into the gas passage, creating re-entrainment and reducing effective collection area.
Insulator failure - Cracked or contaminated support insulators cause electrical tracking, reducing voltage to the affected field.
Beyond particulate control
An ESP's primary function is reducing particulate concentration for air permit compliance. However, ESPs also have critical system objectives that depend on the application and configuration. For systems with downstream equipment, ESPs protect ID fans from erosion (especially backward curved or airfoil designs), and they prevent poisoning of NOₓ catalysts by removing potassium and phosphorus carried in fly ash.
Inspection recommendations
Regular internal inspections during outages - corrosion checking, collector plate conditions, checking electrode alignment, cleaning insulators, verifying rapper operation - should be part of every maintenance program.