Air Pollution Control: What's Not Equipment?

Air pollution control equipment is used to regulate and remove hazardous emissions, such as particulate matter and gases, which are produced by manufacturing, process systems, and research activities. These emissions, if left uncontrolled, can enter the air, atmosphere, and surrounding environment. There are various types of air pollution control equipment, including scrubbers, filters, electrostatic precipitators, incinerators, and biofilters. This equipment is designed to prevent pollutants from entering the atmosphere, primarily from industrial sources. The selection of air pollution control equipment depends on various factors, such as the inlet acid gas loading, regulatory requirements, and the type of pollutant.

Dust traps collect dust and other impurities from air or gas

Dust traps, also known as dust collectors, are systems used to improve the quality of air released from industrial and commercial processes by collecting dust and other impurities from the air or gas. They are designed to handle high-volume dust loads and are commonly used in factories and processing centres where large volumes of dust are produced. Dust traps are particularly important when the dust is explosive or toxic to workers or products.

Dust collection systems work on the basic formula of capture, convey and collect. Firstly, the dust must be captured or extracted, which is achieved using devices such as capture hoods to catch dust at its source of origin. The machine producing the dust may have a port to which a duct can be directly attached. Secondly, the dust must be conveyed via a ducting system, which must be properly sized to maintain a consistent minimum air velocity to keep the dust in suspension for conveyance to the collection device. Finally, the dust is collected in a dust trap, which consists of a blower, dust filter, a filter-cleaning system, and a dust receptacle or dust removal system.

Dust traps are distinct from air purifiers, which use disposable filters to remove dust. Dust traps are often part of a larger air quality management program that includes large airborne particle filtration units and mask systems for workers. Masks are available in a variety of forms, from simple cotton face masks to elaborate respirators with tanked air, depending on the environment in which the worker is operating.

There are several types of dust collection systems, including baghouses, cyclone separators, electrostatic precipitators, and impinger systems. Baghouses use large fabric bags to filter gas streams, while cyclone separators use centrifugal force to separate fine particles from air or gas. Electrostatic precipitators charge particles of dust by passing dust-laden air through a strong electrostatic field, causing the particles to be attracted to oppositely charged plates so they can be removed from the airstream. Impinger systems, on the other hand, remove particles by impacting them into a liquid.

Wet scrubbers, dry scrubbers, and combinations of wet-dry scrubbers

Dry scrubbers, on the other hand, use a three-step process to remove acids from exhaust gases, which helps reduce the incidence of acid rain and lower harmful air pollution levels. This process involves cooling the exhaust gases, exposing them to alkaline reagents, and then filtering out the pollutant-containing powder. Dry scrubbers are often used in energy production, manufacturing, and waste treatment facilities, as they do not require a separate wastewater treatment system to handle water waste like wet scrubbers.

Wet-dry scrubbers combine the technologies of wet and dry scrubbers to achieve effective air pollution control. Venturi scrubbers, for example, are a type of wet-dry scrubber that uses a constriction in the flow path to achieve high relative velocities and efficient particle removal. The selection of a specific type of scrubber depends on various factors, including inlet acid gas loading, potential acid removal capability, particulate loading, particle size distribution, and regulatory requirements.

Electrostatic precipitators capture, treat, and remove sulfuric acid

Wet electrostatic precipitators (ESPs) are effective air pollution control equipment used to capture, treat, and remove sulfuric acid. ESPs are particularly useful in industrial and utility applications to eliminate sulfuric acid aerosols from flue gas, thereby improving air quality.

The first wet electrostatic precipitator was developed by Dr. Frederick Cottrell in 1907 to control sulfuric acid mist from copper smelting. Since then, ESP technology has evolved and become widely adopted in various sectors. ESPs are now commonly used in coal-fired power plants, boilers, and air pollution control systems to address sulfuric acid emissions.

Wet ESPs utilize a combination of electrostatic forces and water to capture and remove sulfuric acid from gas streams. The technology exploits the principle of electrostatic attraction, where charged particles are drawn toward oppositely charged collection surfaces. ESPs can effectively remove sub-micron particulate matter, condensables, and sulfuric acid mist.

The removal efficiency of sulfuric acid is influenced by several factors, including residence time, gas temperature, and SO3 concentration. Prolonged residence time and lower gas temperatures generally result in higher removal efficiency. Additionally, the use of novel pre-chargers and perforated designs has been shown to enhance the performance of ESPs in sulfuric acid removal.

ESPs are often used in conjunction with other air pollution control technologies, such as ultra-low emission (ULE) control devices and fabric filters, to achieve optimal pollutant capture and treatment. The selection of an appropriate air pollution control system involves analyzing chemicals, evaluating existing exhaust systems, and considering pre-cooling and APC equipment compatibility.

In summary, electrostatic precipitators are indeed air pollution control equipment specifically designed to capture, treat, and remove sulfuric acid aerosols from industrial processes and improve overall air quality.

Biofilters use microorganisms to remove water-soluble chemicals

Biofilters are commonly used in wastewater treatment to remove organic matter and control levels of biochemical oxygen, demand, chemical oxygen demand, and suspended solids. They are also used in drinking water treatment to improve water quality by breaking down material in the water using microorganisms.

Biofiltration techniques can be biological, physical, or chemical. The primary biological method is nitrification, while physical methods include mechanical techniques and sedimentation. Chemical methods are often used in tandem with one of the other methods. For example, in drinking water treatment, slow sand filters or carbon filters are used to provide a surface for microorganisms to grow. These biological treatment systems reduce water-borne diseases, dissolved organic carbon, turbidity, and colour in surface water, improving overall water quality.

The challenge in operating biofilters is maintaining proper moisture throughout the system. The air is normally humidified before entering the bed using various methods, and the type of packing media used can impact the longevity of the system. Natural, organic packing media, such as peat, vegetable mulch, bark, or wood chips, may last for several years, while engineered and synthetic component packing materials can last up to 10 years.

Biofiltration techniques are also used to remove waterborne pathogens, including bacteria, viruses, and protozoa, which pose a serious threat to public health. Stormwater biofilters, for example, have multiple ecosystem functions, including aesthetics, pollinator habitat, and water supply, in addition to removing heavy metals and other pollutants.

Inertial separators use centrifugal, gravitational, and inertial forces to separate dust from gas streams

Inertial separators are a type of air pollution control equipment that separates dust from gas streams. They use a combination of centrifugal, gravitational, and inertial forces to achieve this separation.

The basic principle behind inertial separators is to change the direction of the flowing gas, causing particle trajectories to cross over the gas streamlines. This results in the concentration of particles in a small part of the gas flow or their separation by impingement onto a surface. The most widely used type of inertial separator is the cyclone, in which the gas follows a vortex motion, with gas acceleration directed centripetally. As a result, particles move centrifugally towards the outside of the cyclone.

Inertial separators come in three primary types: settling chambers, baffle chambers, and centrifugal collectors. Settling chambers, the oldest and simplest type of inertial separator, use gravity to remove particles larger than 50 μm. They are simple to design and operate, but they require a large space and have low collection efficiency. Baffle chambers, on the other hand, use a fixed baffle plate to force the gas stream to change direction suddenly. Large-diameter particles do not follow this new direction but continue into a dead air space where they settle. Louver dust separators are another type of inertial separator characterised by a stream of dust-laden air incident upon a row of blades or vanes.

Dust traps are another type of inertial separator. They are designed to handle high-volume dust loads and consist of a blower, dust filter, a filter-cleaning system, and a dust removal system. They are commonly used in industrial and commercial processes to enhance the quality of air released.

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