Air pollutants are the results of both mobile and stationary sources across a wide range of sectors. There are various types of control devices for particulate contaminants and air pollution management systems available.
In an industrial setting, control devices for particulate contaminants refer to the equipment and systems used to regulate and eliminate the emission of potentially hazardous substances into the atmosphere, like particulate matter and gases, from manufacturing systems and research applications.
Control devices for particulate contaminants are used in various sectors to prevent the discharge of chemicals, gases, and dust, as well as to filter and clean the air in the workplace. Industrial exhaust and emission usually direct into air pollution control equipment and systems, which remove or decrease air pollutants using one or more of the following processes:
Combustion – Destroying the harmful pollutant
Conversion – Converting the chemical nature of the pollutant to a less harmful compound
Collection – Removing the particulates from the waste air before its release at the point source.
Some examples of air pollution management devices used in industrial settings that employ one or more of the methods of air pollutant removal or reduction listed above are: Scrubbers, Electrostatic Precipitators, Biofilters, Mist Collectors, Cyclones, Air Filters, Catalytic Reactors, Incinerators.
Gravitational Settling Chambers
This straightforward particle collecting system relies on gravity to settle suspended particles in a gas stream flowing through its lengthy chamber. The most important feature of a device will be a chamber where the carrier gas velocity slows down to allow particulate debris to settle out of the flowing gas stream due to gravity. This particle stuff later gathers at the chamber’s bottom. Chamber is manually cleaned to dispose of the water.
The settling chamber’s gas velocities must be low enough for the particles to settle owing to gravitational force. According to the literature, a gas velocity of less than 3 m/s is required to prevent re-entrainment of the settling particles.
The findings will be good if the gas velocity is less than 0.5 m/s. Curtains, rods, baffles, and wire mesh screens can be suspended in the chamber to reduce turbulence and maintain uniform flow. Due to the entrance and exit losses, the pressure drop through the chamber is generally minimal. Stokes’ law may be used to calculate the particle velocity in the settling chamber:
Vs = (g (ρp -ρ ) D2 ) /18 µ
Where, D = Diameter of the particle, g = acceleration due to gravity, ρ = density of the particle, r = density of the gas, µ = viscosity of the gas.
Centrifugal collectors remove tiny particles by using centrifugal forces. They are sometimes called Cyclone separators or inertial separators.
A vertically arranged cylinder with an inverted cone connected to its base makes up the cyclone. At the cylinder’s input point, the particulate-laden gas stream enters tangentially. The incoming gas stream’s velocity is subsequently converted into a confined vortex, from which centrifugal forces tend to propel suspended particles to the cyclone’s walls.
In a smaller inner spiral, the vortex turns upward after reaching the bottom of the cylinder. The dust particles are gathered at the bottom in a storage hopper by gravity, while the clean gas is evacuated from a central cylindrical aperture at the top.
The diameter of a cyclone determines its effectiveness. Because centrifugal force increases with decreasing radius of rotation, the smaller the diameter, the better the efficiency for a given pressure decrease. Depending on the diameter of the cyclone, centrifugal forces used in current systems range from 5 to 2500 times gravity. For particles with a diameter of ten microns, cyclone efficiencies are more than 90%. Efficiency is over 95% for particles with a diameter greater than 20.
The efficiency of a cyclone can be boosted by using many cyclones in series or parallel. The following is a brief description of both arrangements:
1. Multiple Cyclones: A paralleled battery of smaller cyclones designed to maintain a steady pressure drop in each chamber. The setup is small, with simple input and output locations. They can capture tiny particles in a big gas flow.
2. Cyclones in series: A series of two cyclones is employed. The second cyclone collects particles that were not collected in the first cyclone owing to statistical dispersion throughout the intake or inadvertent re-entrainment due to eddy currents and re-entrainment in the vortex core, resulting in increased efficiency.
Wet scrubbers, also referred to as wet adsorption scrubbers or wet collectors, collect and remove water-soluble gas and particle pollutants from industrial emissions using liquid solutions—usually water. A gas stream is then passed through a liquid solution, or a liquid solution is injected into a gas stream in the wet scrubbing process. This solution absorbs the design of wet scrubbers, or any other air pollution control equipment, is determined by the circumstances of the industrial process and the type of air pollutants involved. The qualities of the inlet gas and the dust properties (if particles are present) are critical. Scrubbers can be built to gather both particulate and gaseous contaminants. Wet scrubbers may be created in various designs, all of which are designed to ensure proper contact between the liquid stream and dirty gas stream.
Dust particles are captured in liquid droplets by wet scrubbers. The liquid collects the droplets, dissolving or absorbing the polluting gases. The effectiveness of a wet scrubber to gather microscopic particles is frequently directly related to the power input. Spray towers and other low-energy devices capture particles bigger than 5 micrometers.
High-energy devices, such as venturi scrubbers, or enhanced devices, such as condensation scrubbers, are often required to achieve high-efficiency particle removal of 1 micrometer (or fewer) particles. An entrainment separator or mist eliminator must be correctly constructed and operated to obtain high removal efficiency. The higher the potential emission levels, the more liquid droplets missed by the mist eliminator.
Absorbers are wet scrubbers that remove gaseous contaminants from the air. To achieve high removal efficiency in absorbers, good gas-to-liquid contact is required.
Fabric Filters (Bag-house filter)
Air filters are pollution-control systems that use a specific type of filtering media—for example, cloth, sintered metal, ceramic, etc.—to capture and remove dry particles and pollutants from the air flowing through them, such as dust, pollen, microorganisms, and chemicals.
Fabric filters, also known as baghouses, are a type of air filter that employs cylindrical fabric bags to capture and remove dust and other pollutants in the air. Particulates from and aggregates on the filter’s surface as air travels through a baghouse. The buildup improves the filter’s effectiveness by allowing smaller particles to be caught and causing pressure to build up across the filter fabric. Certain baghouses may achieve a 99.9% efficiency rate even for little particle matter.
These filters can be used to filter contaminants from the air in a range of industrial operations, such as power plants, metal processing centers, foundries, and multi-stage cleaning systems. Periodic cleaning is required due to particle collection and the associated pressure difference. Baghouses utilize a variety of ways to remove the buildup from the filter bags, including:
Introducing an airflow into the filter bags that is the polar opposite of the filtering process
Compressed air is pumped into the filter bags in the opposite direction of the filtering process.
Particulates fall from the filter cloth to the bottom of the baghouse enclosure into a collecting hopper for processing and disposal, as mentioned above.
Electrostatic precipitators (ESPs) collect and remove dust particles from pollutant sources like air filters and cyclones. ESPs employ transformers to provide a significant static difference in electrical potential between charged electrodes and collecting plates. When gas streams move between the two components, an electrical charge is applied to the particles, attracting the particulate matter to the collecting plates.
PM accumulation in air filters is regularly removed from the collecting plates and deposited in a collection hopper below, either physically or by injecting water to rinse the particles. Wet ESPs are those that employ the latter method. Because ESPs have many collecting plates, their efficiency typically exceeds 99 percent.
The basic idea of all electrostatic precipitators is to charge particles in a gas stream with electrostatic charge before passing them through an electrostatic field that pushes them to a collecting electrode.
The electrostatic precipitators require a high potential difference between the two electrodes, one of which is a discharging electrode and the other a collecting electrode, to function properly. A significant ionizing field is created due to the large potential difference between the two electrodes. Extremely high potentials, up to 100 kV, are employed. The typical voltage range is 40-60 kV. The ionization causes the ‘corona’ or ‘corona glow,’ an active glow zone (blue electric discharge). The breakdown of gas molecules into free ions is known as gas ionization.
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