Enve 411 - Advanced Air Pollution Control Controlling Particulate Emissions with Electrostatic Precipitators
Electrostatic Precipitators (ESPs) can be designed for many types of particulate control processes and systems. The following web pages are designed to introduce to the viewer to the principals of ESP usage and design for particulate control.
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What is an Electrostatic Precipitator?
An electrostatic precipitator is air pollution control device used to separate solid particulate matter from a contaminated air stream. Contaminated air flows into an ESP chamber and is ionized by electron emitting electrodes; also known as the corona chamber. The suspended particles are charged by the electron field and migrate to a collection plate. Accumulate particulate matter is removed from the collection plates at periodic intervals by rapping or hitting the plates with rappers (mallets type hammers). Heavy particles fall to the base of the ESP where hoppers hold the removed particles for disposal.
There are typically three types of ESP units: dry negative corona units, wet negative corona units and wet positive corona units. Dry negative corona units have inherently better voltage/current characteristics, are utilized more frequently and will be the main focus of this website; however, wet negative corona units will be discussed for their applicative differences. The following is a small list of typical industrial applications for ESPs.
ESP Operation and Basic Design
A dry negative corona ESP, is designed to generate and disperse negative electrons through suspended electrodes (wires). Excess electrons migrate from the corona toward a positve (grounded) collection plate. Electrons are readily adsorbed onto passing electronegative gas molecules and particals. As the electrons are accumulated on the dust particles they are transported and deposited on to the collection plate. Below is a typical dry gas flow schematic of an ESP. For more detailed and thourough design instruction, consult the ESP design videos on the Video Instruction page.
As dust particles collect on the grounded plate, they transfer their charge thus completing the electrical circuit. Particles are retained on the plate by friction and the constant collection and transfer of particle electrons. As the dust layer increases, electron conduction is dampened by the resistance. The measure of resistance is known as resistivity. Resistivity has a strong influence on particle collection efficiency. View video #5 on the Video Instruction page for more information.
To improve collection efficiency and ensure proper functional use of the precipitator, a rapping system is applied to the collection plates and electrodes to dislodge the collected dust layer. A falling weight or fixed rotating hammer raps the collection plates, causing a vibration that knocks off the dust layer. The dust drops into steeply sloped hoppers, which are periodically emptied for disposal. The collection plates should be smooth enough to prevent frictional resistance during rapping removal and have sufficient oscillation behavior to ensure particle dislocation across the length of the plate. Each plate is rapped individually to minimize the escape of dust particles from the system. Rapping intervals are dependent upon gas flow composition, corona voltage, and precipitator size.
The volumetric flow rate and gas stream composition are the two important empirical factors for determining a precipitator design. The velocity component, other wise known as the migration velocity, is the dominate factor which helps to determine the dust removal efficiency. The following parameters can also effect the migration velocity component:
The inlet gas stream typically has a high temperature and may require pretreatment. Flue gas conditioning should be considered to facilitate particle collection. By spraying water into the flue gas, the fly ash is cooled to an efficient precipitator operating temperature. In addition, this increases the gas humidity which lowers the dust resistivity. Particle resistivity is material, temperature and humidity dependent and should be thoroughly understood for proper ESP design.
For further instruction please view the US EPA training course videos taught by John Richards Ph.D., of the Air Pollution Training Institute.