Meghan Hodges
Air pollution control devices are used to prevent gaseous and solid pollutants from entering the atmosphere, primarily from industrial smokestacks. These devices can be divided into two categories: those that control the amount of particulate matter escaping and those that control acidic gas emissions. Specific machinery is used to remove particulate matter from flue gases, such as scrubbers, which use water or reagents to react with pollutants to form stable compounds. Cyclones and electrostatic precipitators are also used to remove gaseous pollutants, with the latter being the most efficient option.
| Characteristics | Values |
|---|---|
| Device Type | Scrubbers, Cyclones, Electrostatic Precipitators, Fabric Filters |
| Mechanism | Use of scrubbing liquids like water or reagents to react with pollutants; use of filters and demisters to prevent release of particulate matter; adsorption columns with media like Granular Activated Carbon (GAC); thermal techniques for removing hydrocarbons and volatile oxidizable pollutants |
| Pollutants Removed | Gaseous releases, particulate matter, acidic gases, VOC emissions, sulphur oxides, mercury |
| Industries Used In | Power plants, ceramics, woodworking |
| Effectiveness | Electrostatic precipitators have an efficiency of 99% |
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Scrubbers
Air pollution control devices are used to prevent gaseous and solid pollutants from entering the atmosphere, primarily from industrial smokestacks. Scrubbers are one such system, used to remove harmful materials from industrial exhaust gases before they are released into the environment. There are two main types of scrubbers: wet scrubbers and dry scrubbers.
Wet scrubbers use a liquid, usually water, to remove pollutants from an exhaust stream. The liquid absorbs the pollutant, with other liquids used for gases with low water solubility, such as hydrocarbons or hydrogen sulfide. Wet scrubbers are often used to remove acid gases, water-soluble inorganic contaminants, and volatile organic compounds. They are highly effective, with removal efficiencies exceeding 90%. Wet scrubbers are also used in wastewater treatment facilities, keeping pollutants from contaminating the outside air.
Wet scrubbers are designed to maximise the surface area for liquid/gas interaction. This is achieved by passing the liquid over various media, such as packing, meshing, grids, or trays, or by creating a spray of droplets. Wet scrubber designs include spray tower, tray-type, and packed-bed scrubbers, which are generally low-energy scrubbers. Packed bed scrubbers are classified according to the relative direction of the gas and liquid flows. In a counter-current design, the liquid is introduced at the top of the tower and flows downward, while the contaminated gas enters at the bottom and flows upward.
Dry scrubbers, on the other hand, do not saturate the flue gas stream with moisture. They can be categorised as dry sorbent injectors (DSIs) or spray dryer absorbers (SDAs). Spray dryer absorbers are also called semi-dry scrubbers or spray dryers. Dry scrubbers are often used for the removal of odorous and corrosive gases from wastewater treatment plant operations. The medium used is typically an activated alumina compound impregnated with materials to handle specific gases. For example, hydrogen sulfide is treated with an activated alumina compound impregnated with specific materials. Dry sorbent injection involves adding an alkaline material, such as hydrated lime, soda ash, or sodium bicarbonate, directly into the gas stream to react with the acid gases.
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Cyclones
The design of a cyclone typically includes an inlet cylindrical section, a conical section, an outlet gas duct, an outlet dust tube, and a collection hopper. Cyclones can vary significantly in size, depending on the volume of flue gas that needs to be filtered. They are often used as pre-cleaners or pre-treatments to remove larger, abrasive particles before the gas enters more effective pollution control devices. This helps to reduce the inlet loading of particulate matter and improve the efficiency of downstream collection devices. Cyclones have relatively simple construction and generally have no moving parts, which keeps maintenance and operating costs low.
There are four main types of cyclones, classified based on how the gas stream is introduced and how the collected dust is discharged:
- Tangential inlet, axial dust outlet
- Tangential inlet, peripheral dust outlet
- Axial inlet, axial dust outlet
- Axial inlet, peripheral dust outlet
While cyclones are effective in removing larger particulate matter, they have limitations when dealing with smaller particles, especially those below 10 micrometres in size. Additionally, cyclones may struggle with sticky or tacky materials. Cyclones are also susceptible to operational problems, such as erosion of components, plugging of the dust outlet or gas inlet vanes, and air inleakage, which can affect their efficiency.
Overall, cyclones play an important role in air pollution control by removing particulate matter from flue gases, particularly when used in conjunction with other pollution control devices. Their simplicity, low cost, and ability to handle large volumes of gas make them a valuable tool in reducing the release of harmful gases into the environment.
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Electrostatic precipitators
Air pollution control devices are designed to prevent gaseous and solid pollutants from entering the atmosphere, primarily from industrial sources. These devices can be broadly categorized into two types: devices that control particulate matter leakage and devices that control acidic gas emissions.
The basic mechanism of an electrostatic precipitator involves imparting an electrostatic charge to the exhaust gases as they traverse through wires or plates within the device. This charged gas is then collected on an oppositely charged plate, effectively trapping the pollutants. The majority of electrostatic precipitators are of the plate type, with flat, parallel surfaces that collect the particles. The contaminated gases pass through the spaces between the plates, and the particles become charged and adhere to the collection plates. The collected particles are then removed by rapping the plates, and they are deposited in bins or hoppers at the base of the precipitator.
Wet electrostatic precipitators (WESPs) are a variation of ESPs that have been used in small, industrial-type settings and the metallurgical industry for nearly 100 years. They are effective in collecting finer particulates and aerosols, as well as controlling acid mists, submicron particulates, mercury, metals, and dioxins/furans.
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Fabric filters
Air pollution control devices are designed to prevent gaseous and solid pollutants from entering the atmosphere, particularly from industrial smoke stacks. Fabric filters, also known as baghouses, are a type of air pollution control device that removes particulate matter from a process gas stream before it is emitted into the atmosphere.
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Thermal techniques
Pyrolysis is another thermal technique where organic material, such as biomass, is heated in the absence of oxygen. In this process, the material does not combust but thermally decomposes into combustible gases and charcoal. The combustible gases can be condensed into a flammable liquid called pyrolysis oil, which is used in various industrial processes. Gasification is a similar process that converts biomass or fossil fuel-based materials into gases by reacting them at high temperatures without combustion, controlling the amount of oxygen and steam present.
Thermal oxidizers, including regenerative thermal oxidizers (RTOs), are advanced technologies used in modern industry to address gaseous emissions. These systems operate at extremely high temperatures, ranging from 800°C to 1000°C, and utilize residence time and turbulence to ensure complete oxidation of pollutant gases. Thermal oxidizers are highly effective in destroying VOCs, with reported rates of up to 99%, making them crucial for meeting regulatory standards and improving industrial sustainability.
The choice of thermal oxidation system depends on several factors, including the composition of the pollutants, the volume of gaseous flow, and the specific thermal efficiency objectives of the plant or process. Additionally, the implementation of remote and satellite monitoring programs helps ensure continuous control, emission traceability, and compliance with environmental regulations.
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