This article takes an in depth look at electrostatic precipitators.
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An electrostatic precipitator is a device that uses electric charges to remove airborne impurities in solid, droplet, gaseous, or liquid forms. It functions as an air pollution control system, effectively capturing pollutants from the exhaust of factories, industrial processes, and power plants without the need for traditional filters.
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.
Many industries rely on fossil fuels for their manufacturing processes, resulting in smoke that contains soot, ash, and unburned carbon dioxide. Electrostatic precipitators (ESPs) use electric charges to remove these contaminants from the smoke, allowing only clean air to be released into the atmosphere. Removing these harmful particles is crucial, as they can damage buildings, harm the environment, and pose health risks to people.
Electrostatic precipitators are designed to remove particulate matter from polluted air, including dust, smoke, soot, ash, and fumes. These devices achieve a control efficiency of 99% for particulate matter that is 1.0 μm or larger in diameter. The efficiency of an electrostatic precipitator is generally unaffected by the size of the particulate matter. However, particles with very high or very low resistivity can be challenging for these devices to handle effectively; they perform best with particulate matter of moderate resistivity.
Air pollution refers to the presence of harmful particles in the air that degrade environmental quality and pose health risks to living organisms. These pollutants can be toxic and lead to serious health issues such as asthma, respiratory problems, and cancer. Key pollutants include nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), carbon dioxide (CO2), and other harmful particulates. Air pollutants contribute to environmental problems like smog, acid rain, and the 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.
Today, there is widespread awareness of the importance of clean air and a sustainable environment. The Environmental Protection Agency (EPA) has established the Clean Air Act (CAA) to regulate sources of air pollution and mitigate their effects on both the environment and human health. The CAA classifies pollutants into three main categories:
Air pollutants are generated at every stage of the industrial process, from raw material processing to distribution. Industries produce pollutants during activities such as sourcing, processing, and burning. To mitigate emissions of dust, vapors, chemicals, and other hazardous substances, many industries use control equipment. Fans or blowers are commonly employed to channel polluted air into filtration systems, which process and clean the air through the following methods:
Additional equipment used in the air pollutant removal process includes:
Flue gas is a byproduct of combustion processes in boilers, furnaces, or steam generators that exits through pipes. If not properly processed, flue gas can contaminate and pollute the environment and atmosphere. It is often referred to as exhaust or emissions.
Flue gas contains various pollutants, including particulate matter, carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2). These elements contribute to environmental issues such as smog, which forms when sunlight reacts with nitrogen oxides. Water vapor also constitutes a significant portion of flue gas, as it results from the combustion of fossil fuels and is a major component of the smoke emitted from smokestacks.
To protect the environment, flue gas must be treated before being released. Treatment methods include wet, dry, and semi-wet processing. Since flue gas composition varies by industry, treatment processes are tailored to address the specific types of flue gas produced in each sector.
Many industries require electrostatic precipitators due to their use of fossil fuels, which results in the emission of harmful chemicals and flue gases into the atmosphere. The effective operation of an electrostatic precipitator relies on several key components, including:
These are the fundamental components of an electrostatic precipitator:
The diagram illustrates the position and operation of each component in an electrostatic precipitator. An AC supply is provided to the control cabinet, where the voltage is increased by a high-voltage transformer and then rectified by a diode in the rectifier. This process converts AC current into DC current, which is then supplied to the discharge electrodes.
At this point, the flue gases pass through the discharge electrodes and become ionized. The ionized particles, which have an opposite charge and polarity to the collector electrodes, are attracted to and collected by these electrodes. The collected particulate matter is then transferred to hoppers, from which the dust is removed and discharged.
The discharge electrodes in this precipitator are composed of thin copper wires that are welded and annealed. These wires, which are hung vertically, generate a substantial amount of corona discharge. Their primary function is to create a strong electric field that ionizes the particles in the flue gas. The collecting electrodes, made of steel, attract these ionized particles for removal.
Rapper coils, also known as magnetic impulse gravity impact rapper coils (MIGI), apply a shearing force to the collecting plates to dislodge accumulated dust and smoke particles. The vibrations generated by the rapper coils are powerful enough to dislodge even the most stubborn and sticky particles. These vibrations help to loosen caked-on, hardened, and solidified particles, allowing them to fall from the collection plates or wires into the hopper.
In some electrostatic precipitator designs, a water spray is used alongside the rapper coils to enhance the cleaning of the collection plates.
The rectification unit converts the AC voltage supplied to the electrostatic precipitator into DC voltage, which is essential for creating the electrical field needed to ionize particles in the flue gases. The DC voltage is applied to the discharge unit, generating a negative electrical field that imparts a negative charge to the particles. This negative charge is crucial for the collection process, as the positively charged plates or wires of the precipitator attract and capture the negatively charged particles.
The hopper is an inverted pyramidal container designed to collect particulate matter. Dust accumulated on the collecting electrodes is dislodged by the rapper coils and falls into the hopper. Once the hopper has collected a sufficient amount of dust, the particles are expelled through its bottom. Vibrators mounted on the outer walls of the hopper assist in the release of the particulate matter.
An electrostatic precipitator, also known as an electrostatic air cleaner, is designed to purify air emitted from industrial chimneys. Flue gases generated by the combustion of solids in industrial processes contain harmful pollutants that are dangerous to both human health and the environment. It is essential to remove these pollutants from the air released by industrial chimneys.
To achieve this, an electrostatic precipitator is installed between the furnace and the chimneys. It captures particulate matter from the flue gases, effectively removing pollutants and releasing cleaner, less harmful gases into the atmosphere.
Electrostatic precipitation operates by generating an electrostatic force that ionizes the particles in the flue gas. It involves two types of electrodes: a positive electrode, typically in the form of plates, and a negative electrode, usually consisting of mesh wire. These electrodes are arranged vertically and alternately within the precipitator.
The negative electrode is connected to the negative terminal of a DC voltage supply, while the positive electrode is connected to the positive terminal. To enhance the negative charge on the negative electrode, the positive terminal may be occasionally grounded. The spacing between the DC voltage supply, the negative electrode, and the positive plate is carefully adjusted to maintain a high voltage gradient. This setup ionizes the air between the negative electrodes and the adjacent positive plates, creating a corona discharge that surrounds the electrodes and establishes a negatively charged environment.
The area between the electrodes becomes fully ionized, resulting in a high concentration of free electrons and ions. The electrostatic precipitation process occurs within a metallic container, which has an inlet for flue gas and an outlet for clean, pollutant-free gas. As the flue gas enters the precipitator, dust particles are captured by the free electrons and ions. These particles become negatively charged and are attracted to the positively charged plates through electrostatic forces.
Once the negatively charged dust particles adhere to the positive plates, the excess free electrons are removed from the plates, and the particles fall off due to gravity. The precipitator includes hoppers that collect the dust particles from the collecting plates. Clean air is then discharged from the outlet into the atmosphere. Additionally, water sprays are used at the top of the precipitator to facilitate the rapid removal of dust particles from the plates.
In some systems, electrostatic precipitators are paired with denitrification units to remove harmful air pollutants like nitrogen dioxide and sulfur dioxide. These precipitators are often marketed as air purifiers or cleaners, serving as alternatives to traditional furnaces in various industries. While they prevent bacterial growth, the plates can be challenging to clean and may generate ozone or nitrogen oxides. Special filters are used to address these issues, and soak-off cleaners are available to ensure thorough removal of dust particles.
Regular and constant use of an electrostatic precipitator can lead to performance issues, which may arise from operational errors or quality problems. Proper maintenance is crucial for maintaining the efficiency and effectiveness of an electrostatic precipitator (ESP). Many performance issues can be prevented with careful planning and regular inspections.
If flue gas is introduced into the chamber at an excessively high rate, dust particles can accumulate in the electric field, leading to the formation of dust masses. This accumulation can make it difficult to remove particulate matter from the gas effectively.
The distribution of air flow significantly impacts the performance of the precipitator. Efficiency is higher in areas with low air flow, where dust removal occurs more rapidly. In contrast, areas with high air flow experience reduced efficiency due to the effects of high-speed smoke compared to low-speed smoke.
The efficiency of a precipitator is influenced by the specific resistance of the dust particles. For optimal collection, the specific resistance should be between 10,000 and 10,000 Ω·cm. If the specific resistance is too low, particles may move and escape through the outlet with the gas stream, reducing efficiency. Conversely, if particles are too large, they may not escape. To address this, it is necessary to vibrate the dust particles effectively, ensuring they are retained by the device for proper 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 efficiency of dust removal increases when the temperature of the flue gas is between 110-130°C. At higher temperatures, the specific resistance of the dust particles decreases, the viscosity reduces, and the gas flow speed increases, which can lead to lower dust removal efficiency. Conversely, if the temperature is too low, humidity rises, ionization weakens, corona discharge may close, and dust removal efficiency decreases. Generally, the optimal flue gas flow rate is between 0.8 and 1.2 m/s.
High soot concentration increases the amount of dust particles in the electric field. Excessive soot concentration can lead to corona discharge closure, preventing current from passing through the electric field and reducing the precipitator's efficiency. If the flue gas has high soot concentration, it should be pretreated. The soot concentration in the flue gas should be kept below 5 g/m³.
Dust accumulation can lead to the complete failure of an ESP system. It is crucial to keep the charge plates, hopper, and rectifier clean and regularly checked. Regular maintenance is essential to avoid system failures or operational shutdowns. Neglecting the cleanliness of the ESP can undermine its effectiveness and expose companies to fines for inadequate pollution control.
Rapper coils play a critical role in the cleaning process by providing vibrations to remove buildup on collection plates or wires. Failure of the rapper coils can result in excessive buildup on the collection plates, reducing their efficiency and ability to capture charged particles. If not addressed, this buildup can eventually cause the system to stop functioning.
Electrostatic precipitators come in various types to meet specific functional requirements and economic considerations. Below, we explore the different types of electrostatic precipitators and their respective applications in detail.
Electrostatic precipitators come in various types to meet specific functional requirements and economic considerations. Below, we explore the different types of electrostatic precipitators and their respective applications in detail.
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 feature tubes arranged in parallel, with high voltage electrodes running along their axes. These tubes can be configured in various shapes, such as circular, square, or honeycomb, and the gas can flow either upward or downward through them. Tubular precipitators are single-stage units, meaning that all the gas passes through the tubes without any leakage.
Plate electrostatic precipitators, on the other hand, are more commonly used. They are particularly effective in handling wet or sticky particulate matter because they can be tightly sealed to prevent leakage. While tubular precipitators are ideal for controlling hazardous gases, plate precipitators offer versatility and reliability for a wider range of applications.
Dry electrostatic precipitators are designed to collect particulate matter from dry environments. Similar to rapper coils, these precipitators periodically vibrate to dislodge dust particles from the collector plates and discharge electrodes, directing the dust into a collection hopper. However, these vibrations can occasionally release small particles back into the air stream. Dry precipitators are generally not effective at capturing submicron particles and are primarily used for collecting ash or cement.
Wet electrostatic precipitators are designed to remove pollutants from wet gas streams, such as resin, oil, tar, and paint. They utilize a continuous water spray to collect dust particles from the gas. At the inlet of the precipitator, a fine mist of water and air conditions the particulate matter for collection. High voltage corona electrodes and collection tubes charge and separate the particles. The collected solid materials and liquid from the spray are then drained out of the precipitator.
Wet electrostatic precipitators are particularly effective for capturing wet, sticky, flammable, and high-resistivity solids and can capture smaller particles compared to dry precipitators. They typically have a tubular shape and are more costly than dry precipitators. Given their reliance on water, these precipitators are constructed from corrosion-resistant materials. The collected particulate matter is removed as a slurry, which is flushed from the system.
Most electrostatic precipitators are single-stage devices. These precipitators operate at very high voltages to charge and collect particles. In a single-stage system, the electrodes and collecting plates are arranged parallel to each other and function in the same direction. Both the charging and collection processes occur within the same section of the precipitator.
Single-stage electrostatic precipitators can have either tubular or plate configurations and typically use rapping coils or a liquid film to remove the collected particles. The charging wires and collection plates are positioned close together in the same area of the precipitator. This contrasts with two-stage electrostatic precipitators, where the charging and collection processes are separated into different sections.
Two-stage electrostatic precipitators are arranged in series rather than in parallel. This configuration allows particles to spend more time being charged and reduces the likelihood of back corona. They are designed to handle smaller particles and are often used as air purifiers in conjunction with air conditioning systems. Two-stage precipitators are typically used for low-volume and smaller-scale applications.
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