05-04-2012, 04:05 PM
ELECTROSTATIC PRECIPITATOR
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CHAPTER-1 INTRODUCTION Nowadays, the environment protection has become a crucial problem and the authorities are requested to set increasingly more stringent limits , one of which is the emissions from the industrial plants of solid particulate and other gaseous pollutants. Dusts and mists (aerosols) each have their own individual properties. It is important to select an electrostatic precipitator that is suitable for the properties of the gas and the particles to be collected. We use our know-how and abundant data accumulated over 80 years to propose highly reliable Electrostatic precipitator with the appropriate performance to suit the customers' needs. Electrostatic (ESPs) are the most commonly used, effective, and reliable particulate control devices .They are employed mostly in power plants and other process industries. The particle-laden flue gas from the boiler flows through the ESP before it enters the environment .It is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators 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 contrast to wet scrubbers which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy (in the form of electricity). Precipitators function by electro-statically charging the dust particles in the gas stream. The charged particles are then attracted to and deposited on plates or other collection devices. When enough dust has accumulated, the collectors are shaken to dislodge the dust, causing it to fall with the force of gravity to hoppers below. The dust is then removed by a conveyor system for disposal or recycling. Depending upon dust characteristics and the gas volume to be treated, there are many different sizes, types and designs of electrostatic precipitators. Very large power plants may actually have multiple precipitators for each unit.
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CHAPTER-2
SYSTEM COMPONENENT OF ESP All electrostatic precipitators, regardless of their particular designs, contain the following essential components: • Discharge electrodes • Collection electrodes • High voltage electrical systems • Rappers • Hoppers • Shell
2.1 Discharge electrodes
These are either small-diameter metal wires that hang vertically (in the electrostatic precipitator), a number of wires attached together in rigid frames, or a rigid electrode made from a single piece of fabricated metal. The most common size diameter for wires is approximately 0.25 cm (0.1 in.). Discharge electrodes create a strong electrical field that ionizes flue gas, and this ionization charges particles in the gas. The size and shape of the electrodes are governed by the mechanical requirements for the system, such as the industrial process on which ESPs are installed and the amount and properties of the flue gas being treated.
2.2 Collection electrodes
These electrodes are either flat plates or tubes with a charge opposite that of the discharge electrodes which collect charged particles. The plates are generally made of carbon steel. However, plates are occasionally made of stainless steel or an alloy steel for special flue-gas stream conditions where corrosion of carbon steel plates would occur. The plates range from 0.05 to 0.2 cm (0.02 to 0.08 in.) in thickness.
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2.3 High voltage equipment
It provides the electric field between the discharge and collection electrodes used to charge particles in the ESP. This is accomplished by using power supply sets consisting of three components: a step-up transformer, a high-voltage rectifier, and control metering and protection circuitry (automatic circuitry). The power system maintains voltage at the highest level without causing excess spark over between the discharge electrode and collection plate. The transformer steps up the voltage from 400 volts to approximately 50,000Volts.
2.4 Rappers
Rappers remove dust that has accumulated on both collection electrodes and discharge electrodes. Occasionally, water sprays are used to remove dust from collection electrodes.
2.5 Hoppers-
These are located at the bottom of the precipitator. Hoppers are used to collect and temporarily store the dust removed during the rapping process. Hopper vibrators are electrically operated devices that cause the side walls of the hopper to vibrate, thereby removing the dust from the hopper walls.
2.6 Shell
The shell structure encloses the electrodes and supports the precipitator components in a rigid frame to maintain proper electrode alignment and configuration.
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Fig 2.1 ESP Components
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CHAPTER-3
TYPES OF ELECTROSTATIC PRECIPITATOR ESPs are configured in several ways. Some of these configurations have been developed for special control action, and others have evolved for economic reasons. Some of the popular types are following:-
plate-wire precipitator
flat plate precipitator
the tubular precipitator
two-stage precipitator
wet type precipitator
dry type precipitator
3.1 Plate-Wire Precipitators
In this type of ESP, gas flows between parallel plates of sheet metal and high-voltage electrodes. These electrodes are long wires weighted and hanging between the plates or are supported there by mast-like structures (rigid frames). Within each flow path, gas flow must pass each wire in sequence as flows through the unit. Plate-wire ESPs are used in a wide variety of industrial applications, including coal-fired boilers, cement kilns, solid waste incinerators, paper mill recovery boilers, petroleum refining catalytic cracking units, sinter plants, basic oxygen furnaces, open hearth furnaces, electric arc furnaces, coke oven batteries, and glass furnaces.
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Fig. 3.1 PLATE TYPE PRECIPITATOR
3.2 Flat Plate Precipitators
The flat plates increase the average electric field that can be used to collect the particles, and they provide an increased surface area for the collection of particles.it is corona free ESP. Flat plate ESPs seem to have wide application for high-resistivity particles with small (1 to 2 μm) mass median diameters (MMDs). Fly ash has been successfully collected with this type of ESP, but low-flow velocity appears to be critical for avoiding high rapping losses.
Fig.3.2: Flat-plate and Plate-wire ESP Configurations
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3.3 Tubular Precipitators
Tubular precipitators consist of cylindrical collection electrodes with discharge electrodes located on the axis of the cylinder. The contaminated gases flow around the discharge electrode and up through the inside of the cylinders. The charged particles are collected on the grounded walls of the cylinder. The collected dust is removed from the bottom of the cylinder. Tubular precipitators have typical applications in sulphuric add plants, coke oven by-product gas cleaning (tar removal), and, recently, iron and steel sinter plants.
3.4 Wet type precipitator
In this type of systems, water is passed over the collecting electrode surface of an electrostatic precipitator to form a water film, and the water film collects the aerosol (dust, mist), which is washed away along with the water film, or an aerosol consisting of liquid droplets in a mist, etc., is collected and washed down off the collecting electrode surface. Wet type Electrostatic precipitator can be a duct-type that uses parallel plates as the collecting electrodes, or a tubular-type that uses a cylindrical electrode as the collecting electrode. In wet type Electrostatic precipitator the collecting electrode is covered with a water film i.e. no back corona due to high-resistivity dust, no re-entrainment due to low-resistivity dust, and no effect due to the electrical resistivity of the dust. Also, the dust collection efficiency is very high.
3.5 Dry type precipitator
When the dust is a dry solid, such as dust or fumes, and it is collected on a collecting plate in the dry state, the precipitator is referred to as dry type. The dust particle collecting on the surface of the plates are periodically removed by rapping or shaking of the plates and drop into hopper below. Dry type Electrostatic precipitator can be a fixed-electrode type or a moving-electrode type, depending on the method for removing the collected dust.