An air scrubber is an air purification system that removes particulate matter from the air through the use of moisture or by cooling or filtering the airstream as it enters the scrubber...
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This article takes an in depth look at electrostatic precipitators.
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An electrostatic precipitator is a filterless device that uses an electric charge to remove impurities from the air that are in solid, droplet, gaseous, or liquid form. It is an air pollution control device used for removing pollutants from smokestacks of factories, industrial manufacturing, and power plants.
As smoke or gas leaves a burner or furnace, it passes over wires or plates in the electrostatic precipitator that gives the smoke or gas a static charge that is collected on a second plate that has an opposite charge where the pollutant particles are trapped. Electrostatic precipitators can be adjusted to fit the exact needs of the pollutant conditions using a minimal amount of electrical energy.
Most industries use fossil fuels for their manufacturing processes, the result of which is the emission of smoke, which contains soot, ashes, and unburned Co2. Electrostatic precipitators (ESPs) take the soot, ashes, and unburned carbon dioxide from the smoke using an electric charge and release clean air or smoke into the atmosphere. Extraction of these harmful particles is important since they can cause harm to buildings, the environment, and people.
Electrostatic precipitators are used for the removal of particulate matter from polluted air. The Different types of particulate matter include dust, smoke, soot, ashes, and fumes. Precipitators give a control efficiency of 99% for particulate matter that is ≥1.0 μm in diameter. The size of particulate matter does not affect the efficiency of an electrostatic precipitator. However, high or low resistivity particles are difficult to handle in these devices and can only deal with a moderate resistivity of particulate matter
Air pollution is any unwanted particles present in the air that pollutes the environment and adversely affects the health of living organisms. These particles can be poisonous and may lead to serious health issues such as asthma, respiratory problems, and cancer. The various pollutants are NO2, SO2, CO, CO2, and other harmful particulate matter that have negative effects on the quality of air. Air pollutants are responsible for many environmental issues such as smog, acidic rain, and greenhouse effect.
Air pollution can be generated from two sources: human sources or from natural ones. Human sources are gases emitted from automobiles, air conditioners, refrigerators, industries, factories, or power plants. Natural sources are wildfires, volcanic eruptions, and other gases erupting from mines or gas reserves. This air pollution if not treated well can cause adverse effects on human health and on the protective ozone layer of the earth. There are many different types of air pollution control equipment to minimize the air pollution.
In modern times people are now well aware of the importance of clean air and a sustainable environment. The environmental protection agency (EPA) has implemented the Clean Air Act (CAA) which helps in regulating all the sources of air pollution and also plays a key role in mitigating its effects on the environment and human beings. This act has divided all the pollutants into three main categories:
It consists of six different types of gases that are harmful for humans and the environment. These gases are sulphur oxide, nitrogen oxide, ozone, particulate matter, carbon monoxide and lead.
It contains almost 180 air pollutants such as organic chemicals, volatile organic compounds, metals and its compounds, fuels, solvents, mercury and many more.
This category contains chlorofluorocarbons, carbon dioxide, methane, and ozone gases. All these are dangerous for the environment and living organisms.
Air pollutants are produced at every step of the industrial process from raw material processing to distribution. Air pollutants are created in industries during every single process such as sourcing, processing, and burning. Control equipment installed in most of the industries prevent the emission of dust, vapors, chemicals, and other hazardous substances into the air. Fans or blowers are present in the industries for this purpose. These fans direct the hazardous air into equipment that filters the polluted air through the following processes:
Other equipment used in the air pollutant removal process include:
Flue gas is a byproduct of combustion from boilers, furnaces, or steam generators that escapes through pipes. If released into the air without processing, it can contaminate and pollute the environment and atmosphere. There are several words used to describe flue gas including exhaust or emissions.
Every type of flue gas contains different pollutants such as particulate matter, carbon monoxide (CO), nitrogen oxide (NO), and sulfur oxide (SO2), which are the elements found in smog that comes from the sun’s rays reacting with nitrogen oxide. Water vapor is a large portion of flue gas since flue gas comes from the ignition of fossil fuels with water being the main part of the smoke from smokestacks.
For the protection of the environment, flue gas has to be treated as it leaves its place of origin. The three methods used to treat flue gas include wet and dry processing and semi-wet processing. Since the types of flue gas vary by industry, treatment processes are designed to specifically treat the variant of flue gas in a particular industry.
Many industries need electrostatic precipitators, since they work with fossil fuels, which results in emission of harmful chemicals and flue gases in the atmosphere. The precipitator has the following components in its design that helps it work and function properly. These components are:
These are the basic components of a precipitator.
The figure shows the position and working of each component in a precipitator. AC supply is given to the control cabinet where the voltage is stepped up by a high voltage transformer and is then rectified by the diode in the rectifier. Next, AC current is converted into DC current, which is passed to the discharge electrodes.
At this stage, flue gases enter the discharge electrode and get ionized. The ionized particles have an opposite charge and polarity to the collector electrodes, which attracts and collects them. Passed from collector electrodes, the particulate matter are then passed down into hoppers and the dust is discharged out.
The discharge electrodes that are present in this precipitator are made up of tubes that are welded and annealed. These are copper wires that are smaller in diameter. The wires are hung vertically in the system and can produce a very high amount of corona discharge. The main function of these electrodes is to create a strong electric field that sends a flux to ionize the particles present in the flue gas. The collecting electrodes are made of steel, which helps in attracting the ionized particles.
Rapper coils, known as magnetic impulse gravity impact rapper coils (MIGI), send a shearing force to the collecting plates to release collected dust and smoke particles. The strength and force produced by the rapper coils creates vibrations that are capable of removing the most resistant, sticky types of particles. The vibrations loosen the caked on, hardened, and solidified particles. The constant action of the rapper coils breaks up the particles and causes them to fall from the collection plates or wires into the hopper.
In some designs of electrostatic precipitators, a water spray is used in addition to the rapper coils to further clean the collection plates.
The rectification unit changes the AC voltage supplied to the electrostatic precipitator into DC voltage, which is necessary to create the electrical field that will ionize the particles from the flue gases. The DC voltage is fed to the discharge unit to create a negative electrical field that produces a negative charge in the particles. This aspect of the process is the key to the collection of the pollutants in the flue stream. The plates or wires of the electrostatic precipitator are positively charged and will attract the negative charged particles.
The hopper is a long inverted pyramidal shaped container used for collecting particulate matter. The dust collected on the collecting electrodes is released by the rapper coils and falls into the hopper. When the hopper has collected enough dust particles, the particles are expelled through the bottom of the hopper. Vibrators are present on the outer walls of the hopper to help release the particulate matter.
Electrostatic precipitator, also known as electrostatic air cleaner, is used to clean impure air emitted from industrial chimneys. Flue gases, produced by industrial combustion of solids, have harmful and dangerous pollutants, which are poisonous to humans and the environment. It is necessary to eliminate these pollutants from air emitted by industrial chimneys.
For this purpose, an electrostatic precipitator is placed between the furnace and chimneys to extract particulate matter from flue gases. Flue gas enters the electrostatic precipitator that removes the pollutants from the gas and dissipates less harmful gas into the atmosphere.
Electrostatic precipitation works by creating an electrostatic force and ionizing the particles. It also works in the presence of two electrodes. One is a positive electrode and the other is a negative electrode. The positive electrode is in the form of plates and the negative one is in the form of mesh wire. Both are placed vertically and alternatively to each other in the precipitator.
The negative electrode is connected to the negative terminal of the DC voltage and the positive is connected to the positive DC voltage terminal. To have stronger negativity in the negative electrode the positive terminal may be grounded occasionally. The distance between the DC voltage that is supplied, the negative electrode, and the positive plate is all well adjusted and keeps the voltage gradient high. This ionizes the medium between the negative electrodes and the adjacent positive plates. The medium that is present between these negative electrodes is air. By the negativity of these negative electrodes there will be a discharge of corona. This corona discharge will surround the electrodes and create a negatively charged environment.
The area or field between the electrodes is completely ionized which results in the presence of plenty of free electrons and ions in that field. The whole process of electrostatic precipitation is done within a metallic container with an inlet for the entrance of flue gas and an outlet on the opposite side for moving pollutant free gas into the air. When the flue gas is put into the precipitator the dust particles get attached and trapped by the free electrons and ions that are present. These dust particles become negatively charged and are attracted by the electrostatic force of positive plates.
After the attachment of these negatively charged dust particles on positive plates the extra free electrons are removed from the plates and the particles dropped due to gravitational force. There are containers called hoppers present in the precipitator that collect the dust particles from the collecting plates. From the outlet, clean air is then discharged into the air. Water sprays are also introduced onto the top of the precipitator for quick removal of dust particles from the plates.
Sometimes the electrostatic precipitators also work with denitrification units where there is a removal of harmful air pollutants nitrogen dioxide and sulphur dioxide. Now these precipitators are usually marketed as air purifiers or cleaners for many industries as a replacement for furnaces. These precipitators do not allow the bacteria to breed, though the plates are difficult to remove and may produce ozone or nitrogen oxides. Special filters are applied to this removal and are sold with soak off cleaners that cause the complete removal of dust particles.
The daily and constant use of an electrostatic precipitator can lead to issues with its performance, issues that can be man made or quality problems. A key factor regarding the performance and efficiency of an electrostatic precipitator is proper maintenance. Many of the problems leading to the poor performance of an ESP can easily be avoided through planning and regular inspections.
If the flue gas is introduced into the chamber at a very excessive rate then the dust particles will accumulate in the electric field leading to the formation of dust masses. This will cause an incubation of dust particles, making it difficult to remove the particulate matter from the gas.
The distribution of air flow also affects its performance. The efficiency of the precipitator is high when there is low air flow. The removal of dust happens more quickly at the area of low air flow and it is reduced in areas of high air flow. It is due to the effects caused by high speed smoke over the low speed smoke.
The efficiency of a precipitator depends on the resistance of dust particles. The specific resistance of particles should be 10,000 to 10 times Ω.cm for best collection. If the specific resistance is too low the particles will move and escape from the outlet by gas stream, which decreases the efficiency of the device. Whereas, if the particles are too large they won’t escape. It is necessary to vibrate the dust particles at a higher rate so that they can vibrate twice, and be retained by the device for purification.
In the operation of negative pressure of dust collection air leakage will be a cause to produce secondary dust flying. Due to air leakage the smoke speed of the gas increases which results in the short residence time of the flue gas in the device. Air leakage also decreases the temperature of the flue gas that may lead to condensation and even corrosion.
The dust removal efficiency increases when the temperature of the flue gas is between 110-130°C. If the temperature of the flue gas is too high, specific resistance is reduced, viscosity shrinks, and the drive in speed of the gas is high. This will result in low efficiency of dust removal. Whereas, if the temperature is low then humidity increases, ionization weakens, corona closure happens, and the efficiency of dust removal also decreases. The general sampling of flue gas rate is 0.8~1.2m/s.
The soot concentration will increase the dust particles in the electric field. If the soot concentration is too high it will cause the corona closure. This means no current will get passed through the electric field. This non generation of corona will reduce the efficiency of the precipitator. If the flue gas has a lot of soot concentration it should be pretreated. The flue gas content should be kept below 5g/m3.
Dust accumulation leads to complete failure of an ESP system. It is essential that the charge plates, hopper, and rectifier be kept clean and checked regularly. Due to the importance of an ESP system, it is essential that it has a regular maintenance schedule to avoid system failures or operational shutdowns. Unclean ESPs defeats their purpose and opens companies to fines for poor pollution control.
An important part of the cleaning process are the rapper coils that provide the vibrations to remove build up on the collection plates or wires. A failure of the rapper system creates buildup on the collection plates reducing their efficiency and their ability to accumulate the charged particles. If the problem is not handled properly, the buildup will continue until the system ceases to function.
There are different types of electrostatic precipitators because of specific function requirements and economic reasons. There are several types of electrostatic precipitators which are discussed in depth below.
There are two types of plate precipitators, plate wire and flat plate. The most commonly used plate precipitator is plate wire.
Plate Wire Precipitator - Plate wire electrostatic precipitators are used with boilers, paper mill incinerators, and basic oxygen furnaces. In this type of precipitator, the flue gas flows between the metal sheets that are placed parallel to each other and have high voltage electrodes.
The electrodes are hung in between the plates or supported by a rigid frame. Gas flows horizontally from vertically placed plates. As it flows, it passes through each wire in the arranged order. Plate wire electrostatic precipitators are used for large volumes of gas.
Flat Plate Precipitator - Flat plate precipitators are used to increase the surface area for particle collection and provide a strong and increased electrical field. Since corona cannot be produced on its own in flat plate electrostatic precipitators, corona producing electrodes are placed behind and ahead of the collecting electrodes. Flat plate precipitators can be operated with little or no corona current flowing from ionized particles. They can be used for particles that have high resistivity but are small in diameter. Fly ash is captured and collected by flat plate electrostatic precipitators.
Tubular electrostatic precipitators have tubes arranged in parallel fashion with high voltage electrodes running along their axis. Tube arrangements can take an assortment of shapes such as circular, square, or honeycomb with gas flowing upward or downward. Tubular precipitators are one stage units in which all the gas passes through the tubes without any leakage.
Plate electrostatic precipitators are more common than tubular electrostatic precipitators. They are used in applications that have wet or sticky particulate matter since they can be sealed tightly to prevent the leakage. Tubular electrostatic precipitators are ideal for controlling the flow of hazardous gases.
Dry electrostatic precipitators are used to collect particulate matter from a dry environment. Like rapper coils, they vibrate periodically to dislodge dust particles from the collector plates and discharge electrodes, sending the dust layer into the collection hopper. On occasion, the vibrations let small particles back into the air stream. Dry precipitators are not well suited for extracting submicron particles and are mainly used for collection of ash or cement.
Wet electrostatic precipitators remove pollutants from wet gas streams such as resin, oil, tar, and paint. They use a continuous water spray to collect the dust particles from the gas. At the inlet of the precipitator, a fine mist of water and air preconditions the particulate matter for collection. A high voltage corona, electrodes, and collection tubes charge and separate the particles. The solid collected materials and liquid from the spray, drain out of the precipitator.
Wet, sticky, flammable, and high resistivity solids are collected by wet electrostatic precipitators and are capable of capturing smaller particles than dry electrostatic precipitators. Wet precipitators have a tubular shape and are more expensive than dry precipitators. Since water is an essential part of their operation, wet electrostatic precipitators are made of corrosion resistant materials. Collected particulate matter takes the form of a slurry that is flushed from the precipitator.
Most of the electrostatic precipitators are single stage. These precipitators require very high voltages to charge the particles and collect them. The set of electrodes and collecting plates are parallel to each other and operate in the same direction. The charging and collection steps take place in the same part of the precipitator.
Single stage electrostatic precipitators have tubular or plate configurations and use rapping coils or a liquid film to remove collected particles. The charging wires and collection plates are located close together in the same section of the precipitator, which is unlike a two stage electrostatic precipitators where the charging and collection are in different sections.
Two stage precipitators are positioned in a series rather than parallel. The stages are separated, which gives particles more charging time and less chance of back corona. They are constructed for smaller size particles and are used as air purifiers with other air conditioning systems. Two stage electrostatic precipitators are designed for low volume and small applications.
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