Particle Removal

Dust that has accumulated to a certain thickness on the collection electrode is removed by one of two processes, depending on the type of collection electrode. As described in greater detail in the next section, collection electrodes in precipitators can be either plates or tubes, with plates being more common. Tubes are usually cleaned by water sprays, while plates can be cleaned either by water sprays or a process called rapping. Rapping is a process whereby deposited, dry particles are dislodged from the collection plates by sending mechanical impulses, or vibrations, to the plates. Precipitator plates are rapped periodically while maintaining the continuous flue-gas cleaning process. In other words, the plates are rapped while the ESP is on-line; the gas flow continues through the precipitator and the applied voltage remains constant. Plates are rapped when the accumulated dust layer is relatively thick (0.08 to 1.27 cm or 0.03 to 0.5 in.). This allows the dust layer to fall off the plates as large aggregate sheets and helps eliminate dust reentrainment. Most precipitators have adjustable rappers so that rapper intensity and frequency can be changed according to the dust concentration in the flue gas. Installations where the dust concentration is heavy require more frequent rapping.

Dislodged dust falls from the plates into the hopper. The hopper is a single collection bin with sides sloping approximately 50 to 70° to allow dust to flow freely from the top of the hopper to the discharge opening. Dust should be removed as soon as possible to avoid (dust) packing. Packed dust is very difficult to remove. Most hoppers are emptied by some type of discharge device and then transported by a conveyor. In a precipitator using liquid sprays to remove accumulated liquid or dust, the sludge collects in a holding basin at the bottom of the vessel. The sludge is then sent to settling ponds or lined landfills for proper ultimate disposal.

Spraying occurs while the ESP is on-line and is done intermittently to remove the collected particles. Water is generally used as the spraying liquid although other liquids could be used if absorption of gaseous pollutants is also being accomplished