Electrostatic Precipitator

Overview

An electrostatic precipitator (ESP) removes solid particles from a flue gas stream by electrically charging the particles and then collecting them on grounded plates. It is the dominant particulate control device on coal-fired boilers because it handles very high gas volumes with low pressure drop and reaches mass collection efficiencies of 99.0 to 99.9 percent. A utility ESP can treat 100 to 1,500 cubic metres per second of gas at 120 to 180 °C while adding only 100 to 250 Pa of draft loss, far less than a fabric filter of comparable capacity.

The device works in three steps that happen continuously as gas flows through it: charging, collection, and removal. High-voltage Discharge Electrode wires generate a corona that charges the dust particles. The charged particles drift across the gas passage and stick to grounded Collecting Plate surfaces. Periodic rapping shakes the accumulated dust cake loose, and it falls into Ash Hopper sections below for disposal.

How corona charging works

Each gas passage is bounded by two grounded collecting plates with a row of discharge electrodes suspended on the centreline. A Transformer-Rectifier Set applies 40 to 100 kV of negative DC between the electrodes and the plates. The electric field is most intense at the small radius of the discharge electrode, and where the field exceeds the breakdown strength of the gas a corona forms. The corona is a thin glowing region in which gas molecules are ionised, producing free electrons and negative gas ions.

Outside the corona region the electrons attach to oxygen and other gas molecules to form negative ions. These ions fill the passage and collide with the suspended dust particles. Larger particles (above about 1 micron) charge mainly by field charging, where ions follow field lines onto the particle until its accumulated charge repels further ions. Smaller particles charge by diffusion charging, driven by the random thermal motion of ions. Most fly ash particles reach their saturation charge within a fraction of a second of entering the field.

Collection and migration velocity

A charged particle in the electric field feels a force toward the grounded plate. It accelerates until electrical force balances aerodynamic drag, after which it moves toward the plate at a steady migration velocity, usually written as w. For fly ash, w is typically 5 to 15 cm/s. The gas itself moves through the passage at 0.9 to 1.5 m/s, so each particle has to migrate only a few hundred millimetres sideways while it travels several metres downstream.

The Deutsch-Anderson equation relates collection efficiency to migration velocity and to the specific collection area, the ratio of total collecting plate area to volumetric gas flow. Efficiency rises as 1 minus the exponential of the negative product of w and the specific collection area. Because the relationship is exponential, reaching 99.9 percent needs a far larger plate area than reaching 99 percent. Utility ESPs are therefore sized with a specific collection area of 60 to 120 square metres per cubic metre per second, divided among several series fields. The Control and Instrumentation cabinet runs an Automatic Voltage Controller that holds each field just below its spark-over voltage, which is the operating point that gives the highest collection for that field.

Fields and gas distribution

A precipitator is split into three to six fields in the gas-flow direction, each with its own T/R set. Splitting the unit lets the inlet fields, which see heavy dust loading, run at a different voltage and rapping schedule from the clean outlet fields. If one T/R set trips, the remaining fields keep operating, so the unit degrades gracefully instead of failing.

Even gas distribution is essential, since the Deutsch relationship penalises any region of high local velocity. The Gas Distribution System system uses an Inlet Nozzle that diverges from the duct to the full field cross-section, followed by perforated Gas Distribution Screen plates and Turning Vane sets that flatten the velocity profile to within a few percent across the face. Anti-Sneak Baffle plates block gas from sneaking around the active field or down through the hoppers, which would carry uncleaned gas straight to the stack.

Rapping and dust removal

Collected dust forms an insulating cake on the plates. If the cake grows too thick its surface charge opposes the corona and collection falls; if a high-resistivity cake is left in place it can trigger back corona, a reverse discharge from the dust layer that wastes current and re-entrains particles. To prevent this, rapping systems strike the electrodes on a timed cycle.

The Collecting Plate Rapping System system uses Tumbling Hammer rappers mounted on a Rapper Shaft. A Rapper Drive Motor turns the shaft slowly through a Helical Gear Pair in a Gearbox Housing, and the staggered hammers fall in sequence onto the Plate Rapping Anvil of each plate row. The shock travels along the plate and shears the dust cake free as coherent sheets that drop into the hopper with little re-entrainment. The Discharge Electrode Rapping System system cleans the discharge wires, commonly with MIGI Rapper magnetic-impulse units that lift and drop a Rapper Drop Rod against the live frame through a Rapper Insulator. The Rapper Sequence Controller tunes the rapping interval and intensity per field, since over-rapping puffs dust back into the gas while under-rapping lets resistivity problems build.

Ash handling and resistivity control

Dust shaken from the plates collects in pyramidal Ash Hopper sections, where a Hopper Discharge Valve feeds it to the plant ash conveyor. Each hopper carries a Hopper Heater built from Heating Element strips that hold the ash above the acid dew point so it stays dry and flows freely, and a Hopper Level Detector that signals before the ash backs up into the field.

Fly ash resistivity governs how well an ESP performs. Low-sulphur coal produces high-resistivity ash, which is hard to charge and prone to back corona, so cold-side units running at 120 to 180 °C may need flue gas conditioning with a few parts per million of SO3 to lower resistivity. Very low resistivity ash gives up its charge to the plate too quickly and re-entrains during rapping. The Stack Opacity Monitor on the stack closes the loop, letting the controls confirm that voltage, rapping, and hopper performance together keep emissions within permit limits.