Kiera Jefferson
Air pollution control devices are designed to remove particles and pollutant gases from industrial exhaust streams. These devices are used to prevent harmful substances from entering the atmosphere, primarily from industrial smokestacks. There are two broad categories of air pollution control devices: devices that control the amount of particulate matter leaking into the environment and devices that control the emission of acid gases. Particulates include the suspension of fine solid or liquid particles in the air, such as dust, fog, fumes, mist, smoke, or sprays. Common air-cleaners for particulates include bag houses, which trap dust by passing air through many layers of fabric. Other devices include electrostatic precipitators, which remove fine particles from the air by placing an electrical charge on them, and centrifugal collectors, which separate particulates from the air by forcing the air to spin. To control the emission of acid gases, devices such as scrubbers are used. Scrubbers are a diverse group of air pollution control devices that can be used to remove particulates and/or gases from industrial exhaust streams.
| Characteristics | Values |
|---|---|
| Name | Scrubber, Electrostatic Precipitator, Cyclone Collector, Baghouse, Mist Collector, Incinerator |
| Mechanism | Use of water, electrical charge, fabric filters, combustion, etc. |
| Effectiveness | Up to 99% efficiency for particles as small as 0.3 micrometers |
| Uses | Removal of dust, gas, vapours, fumes, fine particles, etc. |
| Industries | Power plants, metallurgical plants, oil and gas refineries, paper mills, waste combustion facilities |
| Regulations | Must meet local and national environmental pollution control regulations |
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What You'll Learn
Scrubbers are used to remove gases and particles
Scrubbers are a group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. They are one of the primary devices that control gaseous emissions, especially acid gases.
Wet scrubbers use water or other solvents to remove particles and gases from the air. Water is the most common solvent used to remove inorganic contaminants, especially dust. The wet scrubber's packing height and type are adjustable, with various heights and types working best against different pollutants. This improves mass transfer and removes more particles from the air. The wet scrubber's particulate collection efficiency is directly related to the amount of energy expended in contacting the gas stream with the scrubber liquid.
Dry scrubbers, on the other hand, do not saturate the flue gas stream with moisture. They are used to remove acid gases, such as SO2 and HCl, primarily from combustion sources. Dry scrubbing systems can be categorized as dry sorbent injectors (DSIs) or spray dryer absorbers (SDAs).
Wet scrubbers can also be used for heat recovery from hot gases by flue-gas condensation. In this process, water from the scrubber drain is circulated through a cooler to the nozzles at the top of the scrubber. If the gas temperature is above the water dew point, it is initially cooled by evaporation of water drops.
Scrubbers are used in a variety of settings, including power plants, waste-to-energy facilities, and industrial and utility boilers. They are an important tool in the effort to reduce air pollution and improve air quality.
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Electrostatic precipitators use electrical charges to remove particles
Electrostatic precipitators are devices that use electrical charges to remove particles from the air. They are commonly used in fossil-fuel power-generating stations and other industrial settings to reduce air pollution. The basic principle behind their operation is that gas-borne particles are given an electric charge and then attracted to a plate with the opposite charge, where they are collected and removed.
The first commercially successful electrostatic precipitator was patented in 1908, following experiments by American chemist Frederick Gardner Cottrell at the University of California, Berkeley. Early units were used to protect vineyards in northern California from lead emissions during smelting activities.
Electrostatic precipitators consist of two sets of electrodes: positive and negative. The negative electrodes are in the form of a wire mesh, and the positive electrodes are plates. These electrodes are vertically placed and alternate with each other. The gas stream flows horizontally through the spaces between the wires and then passes through the stack of plates.
To ionize the particles in the gas stream, a negative voltage of several thousand volts is applied between the wire and the plate. If the applied voltage is high enough, an electric corona discharge ionizes the air around the electrodes, which then ionizes the particles in the air stream. These ionized particles are then attracted to the positively charged collection plates due to the electrostatic force.
The collected particles are usually removed by rapping or vibrating the plates, causing them to fall into bins or hoppers at the base of the precipitator. This can be done without interrupting the airflow, and modern automatic plate-rapping systems allow electrostatic precipitators to stay in continuous operation for extended periods.
Electrostatic precipitators are available in various sizes and types, including dry and wet precipitators. Dry electrostatic precipitators operate above the dew point of the gas stream to remove impurities from smoke and dust. Wet electrostatic precipitators, on the other hand, operate with saturated airstreams that have 100% relative humidity and are used to remove liquid droplets or sticky, wet contaminants.
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Cyclones separate particles from air by spinning the air
Air pollution control devices are used to remove particles and pollutant gases from industrial exhaust streams. One such device is a cyclone collector, which separates particles from air by spinning the air.
Cyclones are cylindrical or conical containers that establish a high-speed rotating airflow. Air moves in a helical pattern, beginning at the top (wide end) and ending at the bottom (narrow end) before exiting the cyclone. This spinning motion forces the particles to the outside of the air stream, causing them to fall or settle out of the air. The geometry of the cyclone, along with the volumetric flow rate, determines the size of the particle that will be removed with a 50% efficiency.
The cyclone's shape plays a crucial role in particle separation. As the rotating flow moves towards the narrow end of the cone, the rotational radius decreases, allowing for the separation of smaller particles. Cyclones can be designed with a secondary airflow to prevent collected particles from striking the walls and protect them from abrasion. This secondary airflow also enables horizontal mounting of the collector.
Cyclone collectors are commonly used for removing coarse dust and are often employed as pre-cleaners before more efficient dust collectors or as product separators. They are used in various industries, including woodworking, rubber grinding, and in combination with fabric filters. Large-scale cyclones are used in sawmills, oil refineries, and the cement industry, while smaller cyclones are used for airborne particle separation and analysis.
In addition to cyclone collectors, other devices such as electrostatic precipitators, scrubbers, and filters are also used for particle removal in pollution control.
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Fabric filters trap dust by passing air through fabric layers
Devices designed to remove particles and pollutant gases are known as scrubbers. These are a group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. Scrubbers can be wet, dry, or a combination of the two. Wet scrubbers use water to force dust, gas or vapour contaminants from the air.
One type of scrubber is the baghouse, also known as a baghouse filter, bag filter, or fabric filter. This is an air pollution control device and dust collector that removes particulates entrained in gas released from commercial processes. Baghouses are used to control emissions of air pollutants from power plants, steel mills, pharmaceutical producers, food manufacturers, and chemical producers. They are cylindrical bags or tubes made of woven or felted fabric as a filter medium.
There are several types of fabric filters, including pulse-jet fabric filters and reverse air fabric filters. Pulse-jet fabric filters have bags supported by metal cages, and the dirty air enters the fabric filter, with the dust collecting on the outside of the bags as the air passes from the outside to the inside. The now-clean air then exits the fabric filter. The bags are cleaned by short bursts of compressed air at regular intervals.
Reverse air fabric filters collect dust on the inside of the bags, and there are no cages supporting the bags. As the dust cake cuts off the inflow of dirty air, it uses the reverse flow of clean air to remove the dust, with the bags partially collapsing to remove the dust.
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Incinerators use combustion to break down pollutants
Incineration is a waste treatment process that involves the combustion of substances contained in waste materials. The process converts waste into ash, flue gas and heat. The flue gases must be cleaned of gaseous and particulate pollutants before they are dispersed into the atmosphere.
Incinerators are furnaces for burning waste. Modern incinerators include pollution mitigation equipment such as flue gas cleaning. There are various types of incinerator plant designs: moving grate, fixed grate, rotary-kiln, and fluidised bed. The burn pile or burn pit is one of the earliest forms of waste disposal, consisting of a mound of combustible materials piled on the open ground and set on fire, which often leads to pollution. Burn barrels are a more controlled form of private waste incineration, containing the burning material inside a metal barrel with a metal grating over the exhaust. However, the very high heat of incineration causes the metal to oxidize and rust over time, and the barrel must eventually be replaced.
The combustion process in incinerators involves a reaction between a fuel (predominately waste, although fossil fuels may be co-fired) and oxygen. This reaction produces carbon dioxide, water vapour, and, in the case of incomplete combustion, carbon monoxide and carbon-containing particles. To ensure the proper breakdown of toxic organic substances, the flue gases must reach a temperature of at least 850 °C (1,560 °F) for 2 seconds. In some cases, backup auxiliary burners are required to reach this temperature.
To reduce emissions, incineration facilities employ air pollution control devices such as electrostatic precipitators, fabric filters, and wet inertial scrubbers. These devices help to control particulate matter, hydrochloric acid, sulfur dioxide, and NOx emissions. For example, electrostatic precipitators remove fine particles from the air by placing an electrical charge on the particles, which are then attracted to an oppositely charged collection plate. Fabric filters, such as baghouses, can achieve high collection efficiencies, even with the finest particles. Wet scrubbers use water to force dust, gas, or vapour contaminants from the air.
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Frequently asked questions
A device designed to remove particles and pollutant gases is called an air-cleaning device or air pollution control equipment.
Some examples of air-cleaning devices include scrubbers, electrostatic precipitators, baghouses, cyclones, mist collectors, and incinerators.
Scrubbers are available in wet, dry, and combined wet-dry configurations. Wet scrubbers use water to force dust, gas, or vapour contaminants from the air. The principle mechanism is the impaction of dust particulates on water droplets.
Electrostatic precipitators remove fine particles from the air by placing an electrical charge on the particles. The charged particles are then attracted to an oppositely charged collection plate.
Baghouses capture particulate contaminants as they pass through a specialized fabric filter. Gradually, a dust layer accumulates on the fabric, which acts as a filter. After the continued buildup of dust, the fabric must be replaced or cleaned using an automatic system.
