ELECTROSTATIC PRECIPITATOR for Restaurant & Industrial Plants



home | profile

Basic Principles
Electrostatic precipitation removes particles from the exhaust gas stream of an industrial process. Often the process involves combustion, but it can be any industrial process that would otherwise emit particles to the atmosphere. Six activities typically take place:
• Ionization - Charging of particles
• Migration - Transporting the charged particles to the collecting surfaces
• Collection - Precipitation of the charged particles onto the collecting surfaces
• Charge Dissipation - Neutralizing the charged particles on the collecting surfaces
• Particle Dislodging - Removing the particles from the collecting surface to the hopper
• Particle Removal - Conveying the particles from the hopper to a disposal point

The major precipitator components that accomplish these activities are as follows:
• Discharge Electrodes
• Power Components
• Precipitator Controls
• Rapping Systems
• Purge Air Systems
• Flue Gas Conditioning

Design & Performance Requirements
Designing a precipitator for optimum performance requires proper sizing of the precipitator in addition to optimizing precipitator efficiency. While some users rely on the precipitator manufacturer to determine proper sizing and design parameters, others choose to either take a more active role in this process or hire outside engineering firms.

Precipitator performance depends on its size and collecting efficiency. Important parameters include the collecting area and the gas volume to be treated. Other key factors in precipitator performance include the electrical power input and dust chemistry.

Precipitator sizing : The sizing process is complex as each precipitator manufacturer has a unique method of sizing, often involving the use of computer models and always involving a good dose of judgment. No computer model on its own can assess all the variables that affect precipitator performance.
Collecting Efficiency : Based on specific gas volume and dust load, calculations are used to predict the required size of a precipitator to achieve a desired collecting efficiency.
Power Input : Power input is comprised of the voltage and current in an electrical field. Increasing the power input improves precipitator collecting efficiency under normal conditions.

Process Variables
Gas characteristics and particle properties define how well a precipitator will work in a given application. The main process variables to consider are:

Gas flow rate : The gas flow rate in a power plant is defined by coal quality, boiler load, excess air rate and boiler design. Where there is no combustion, the gas flow rate will have process-specific determinants.
Particle size and size distribution : The size distribution in a power plant is defined by coal quality, the coal mill settings and burner design. Particle size for non-combustion processes will have similar determinants.
Particle resistivity : The resistivity of fly ash or other particles is influenced by the chemical composition and the gas temperature.
Gas temperature

Following are details of these process variables :

Gas Flow Rate : A precipitator operates best with a gas velocity of 3.5 - 5.5 ft/sec. At higher velocity, particle re-entrainment increases rapidly. If velocity is too low, performance may suffer from poor gas flow distribution or from particle dropout in the ductwork.
Particle Size : A precipitator collects particles most easily when the particle size is coarse. The generation of the charging corona in the inlet field may be suppressed if the gas stream has too many small particles (less than 1 m).
Very small particles (0.2 - 0.4m) are the most difficult to collect because the fundamental field-charging mechanism is overwhelmed by diffusion charging due to random collisions with free ions.
Particle Resistivity : Resistivity is resistance to electrical conduction. The higher the resistivity, the harder it is for a particle to transfer its electrical charge. Resistivity is influenced by the chemical composition of the gas stream, particle temperature and gas temperature. Resistivity should be kept in the range of 108 - 1010 ohm-cm.
High resistivity can reduce precipitator performance. For example, in combustion processes, burning reduced-sulphur coal increases resistivity and reduces the collecting efficiency of the precipitator. Sodium and iron oxides in the fly ash can reduce resistivity and improve performance, especially at higher operating temperatures.
On the other hand, low resistivity can also be a problem. For example (in combustion processes), unburned carbon reduces precipitator performance because it is so conductive and loses its electrical charge so quickly that it is easily re-entrained from the collecting plate.
Gas temperature : The effect of gas temperature on precipitator collecting efficiency, given its influence on particle resistivity, can be significant.
Interactions to Consider : Particle size distribution and particle resistivity affect the cohesiveness of the layer of precipitated material on the collecting plates and the ability of the rapping system to dislodge this layer for transport into the precipitator hopper without excessive re-entrainment.