Electrostatic Precipitators (ESPs) are relatively large, low velocity dust collection devices. ESPs remove particles similar to that of static electricity in cloth picking up small pieces of lint. Here the Transformers are used to create extremely high voltage drops between charging and collecting electrode plates generating the electrical field. The gas stream is then made to pass through this high voltage electric field which introduces a charge on the particles (generally negative charge), which is then attracted to the collecting plates. Periodically the collected dust over the collecting plates is removed by a hammer device by striking the plates (rapping) dislodging the particulate, which falls to a bottom hopper from which the collected particulates are removed periodically.
All ESPs contain six main components: discharge electrodes, collection electrodes, electrical systems, rappers, hoppers, and a shell.
Discharge electrodes (DEs)
Discharge electrodes impart an electrical charge (usually negative) to particulates in the gas stream. The electrodes are usually small-diameter wires that hang vertically in the ESP or are attached to rigid frames. They can also be rigid plates with needle strips. As the flue gas passes through the electric field, the particulate takes on a negative charge which, depending on particle size, is accomplished by field charging or diffusion.
Collection electrodes (CEs)
Collection electrodes collect the charged particles. They can be either tubes or flat plates, and they have a charge opposite to that of the discharge electrodes. The negatively charged particles are attracted toward the CEs and migrate across the gas flow finally collected at collection electrode. Some particles are difficult to charge, requiring a strong electric field. Some particles are charged easily and moved toward the plates, but particles also may lose the charge easily, requiring recharging and recollection. Gas velocity is an important factor in the collection process, lower velocities permit more time for the charged particles to migrate to the CEs and avoid the re-entrainment.
Electrical systems such as transformer-rectifier, or T-R, sets are used to control the strength of the electric field between the discharge and collection electrodes.
Rappers are mechanisms that create vibration or shock at both the collection and discharge electrodes. This causes the attached particles to fall into hoppers from electrodes. While most of the particles are attached to the collecting electrodes, some positively charged particles attach to the discharge electrodes, so a separate rapping system is therefore used to remove deposits from both of these electrodes.
Hoppers are bins used to collect and temporarily store the particles that are removed during rapping. The dislodged particulate (ash) falls from the collection surface into hoppers. Once the particulate has reached the hopper it is important to ensure, that it remains there in bulk form with minimal re-entrainment until the hopper is emptied. They are located at the bottom of an ESP.
The shell encloses the electrodes, supports and all other components of precipitator. It helps to maintain proper electrode alignment and configuration. The shell is covered with insulation to conserve heat and prevent corrosion. The outer shell wall is usually made of steel.
The ESPs can handle large volumes of exhaust gas at low pressure drops. It is the most commonly used collector when coal is used as fuel. To achieve high collection efficiency, the system should be designed properly with respect to precipitator sizing, rapping mechanism and flow distribution. The collection efficiency can be more than 99%. ESPs are also widely installed on utility boilers that fire oil as their principal fuel.
Other common non coal-fired industrial units where ESPs are successfully being applied include municipal refuse incinerators and wood, bark, and oil-fired boilers.