How does electrostatic precipitator

Additionally, some low- sulfur coals that are burned have a higher electrical resistivity , which makes it more difficult to remove the ash produced by this coal using electrostatic precipitators. Since , emissions of particulate matter smaller than 10 micrometers have been reduced by a factor of 5. The effectiveness of a certain precipitator is determined by how well the specific device deals with the unique features and problems of the plant it is used at.

Additionally, precipitator effectiveness is also determined by the temperature and moisture content of the flue gas. Fossil Fuels. Nuclear Fuels. Acid Rain. Climate Change. Climate Feedback. Ocean Acidification.

Rising Sea Level. The collecting electrodes are typically electrically grounded and connected to the positive polarity of the high-voltage power supply. The discharge electrodes are suspended in the flue gas stream and are connected to the output negative polarity of a high-voltage power source.

An electric field is established between the discharge and collecting electrodes, and the discharge electrodes will exhibit an active glow, or corona. As the flue gas passes through the electric field, the particulate takes on a negative charge.

The negatively charged particles are attracted toward the grounded collecting electrodes and migrate across the gas flow. Some particles are difficult to charge, requiring a longer residence time.

Other particles are charged easily and driven toward the plates, but also may lose the charge easily after contacting the grounded CE, requiring recharging and recollection.

Lower resistivity indicates improved ability to accept a charge and be collected in an ESP. Gas velocity between the plates is also an important factor in the collection process since lower velocities permit more time for the charged particles to move to the CEs and reduce the likelihood of migrating back into the gas stream re-entrainment.

A series of CE and DE sections is generally necessary to achieve overall particulate collection requirements. The ash particles form an ash layer as they accumulate on the collection plates.

The particles remain on the collection surface due to the forces from the electric field as well as the cohesive forces between particles.

These forces also tend to make the individual particles agglomerate, or cling together. The ash layer must be periodically removed. The most common removal method is rapping which involves mechanically striking the collection surface to dislodge the ash. It is important that the rapping frequency allows an adequate thickness of dust to collect on the plates so that the accumulated ash can be removed in sheets.

This sheeting is important to prevent the re-entrainment of individual particles into the flue gas stream, requiring additional recharging and recollection downstream. While most of the particles are driven to the CEs, particles in close proximity to the DEs receive a positive charge and are therefore attracted to the DEs.

The gas distribution plates consist of several perforated plates which help maintain proper flow distribution of the entering gas stream.

The discharge electrodes are divided into fields. Most ESPs have three or four fields in series; however, very large units may have as many as fourteen fields in series.

Discharge electrodes are energized by a single transformer-rectifier T-R set power supply. The energized electrodes create ions that collide with the particles and apply the electrical charge to the particles contained in the incoming gas stream. The collection plates or pipes provide the collection surfaces for the charged particulate matter.

The rapping system is responsible for removing the collected particulate matter from the collection surfaces. The primary difference between the two classifications is the method by which the collector plates are cleaned. In dry ESPs, the collector plates are cleaned by applying mechanical impulses or vibration to the plates, which knocks loose the collected particulate matter referred to as rapping. In wet ESPs, the collector plates are cleaned by rinsing with water. Wet ESPs are typically employed when gas streams contain sticky particles with low resistivity.

ESP performance can be affected by particle resistivity.