The electrostatic precipitator is one of the most widely used collection devices for particulates. An electrostatic precipitator (ESP) is a particulate collection device that removes particles from a flowing gaseous stream (such as air) using the force of an induced electrostatic charge. ESP can be operated at high temperature and pressures, and its power requirement is low. For these reasons the electrostatic precipitation is often the preferred method of collection where high efficiency is required with small particles. ESP are highly efficient filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream.
In the electrostatic precipitation process the basic force which acts to separate the particles from the gas is electrostatic attraction. The particles are given an electrical charge by forcing them to pass through a corona, a region in which gaseous ions flow. The electrical field that forces the charged particles to the walls comes from electrodes maintained at high voltage in the center of the flow lane .
Control of emissions from the industrial sources has served the threefold purpose of
1. Recovery of the for economic reason
2. Removal of abrasive dusts to reduce wear of fan component
3. Removal of objectionable natter from gases being discharged into the atmosphere
APPLICATION OF ELECTROSTATIC PRECIPITATORS:
Table 2.5.1. Advantages and Disadvantages of ESP.
|High collection efficiency.||High initial cost.|
|Low maintenance and operating costs.||More space requ ireme nt.|
|Handles large volume of high temperature gas.||Possible explosion hazards.|
|Negligible treatment time.||Production of poisonous gas.|
REQUIREMENT OF ELECTROSTATIC PRECIPITATION PROCESS
STEPS IN ELECTROSTATIC PRECIPITATION
Figure 2.5.1. Electrical field generation
Figure 2.5.2. Movement of dust and air in ESP
PRINCIPLE OF ESP
Principle of ESP has four distinct phases as follows:
(I) Ionization or corona generation: When the potential difference between the wire and electrode increases, a voltage is reached where an electrical breakdown of the gas occurs near the wire. This electrical break down or ion discharge is known as corona formation and thereby gas is transformed from insulating to conducting state.
Two types of corona discharge can be generated which are:
(a) Negative corona: In negative corona, discharge electrode is of negative polarity and the process of electron generation occurs at narrow region
(b) Positive corona: When positive voltage is applied to discharge electrodes in the same way as negative corona, large number of free electron and positive ions are generated. Or large number of positive ions produced move towards collecting electrode and thus transfer charge to dust particles upon collision.
Figure 2.5.3.Variation of field strength between wire and plate electrodes
Negative coronas are more commonly used in industrial application, while for cleaning air in inhabited space positive coronas are used. Due to ozone generation in negative corona its application for air cleaning in inhabited area is avoided.
(II) Charging of Particles: Particle charging takes place in region between the boundary of corona glow and the collection electrode, where particles are subjected to the rain of negative ions from the corona process. Mainly two mechanisms are responsible for particle charging. Each mechanism becomes significant according to particle size ranges. For particles having diameter greater than 1µm, field charging is dominant force; and for particle size less than 0.2 µm diffusion charging predominates.
(III) Migration and precipitation of particle:
(IV) Removal of deposited dust: Once collected, particle can be removed by coalescing and draining, in the case of liquid aerosols and by periodic impact or rapping, in case of solid material. In case of solid material, a sufficiently thick layer of dust must be collected so that it falls into the hopper or bin in coherent masses to prevent excessive re-entrainment of the material into the gas system .
TYPES OF ELECTROSTATIC PRECIPITATORS
ESPs are configured in several ways. Some of these configurations have been developed for special control action, and others have evolved for economic reasons.
[A] SINGLE STAGE PRECIPITATORS
Flat Plate Precipitators