Equipment used to lessen, regulate, or completely remove air pollutants generated by mobile and industrial sources is known as an air pollution control device (APCD). Pollutants are captured, gathered, or transformed by these technologies prior to their discharge into the atmosphere.
A wide range of contaminants are targeted by APCDs, including:
- Particulate Matter (PM): minute airborne particles that can aggravate cardiovascular and respiratory conditions.
- Gaseous pollutants include harmful gases such as volatile organic compounds (VOCs), carbon monoxide (CO), nitrogen oxides (NOx), and sulphur dioxide (SO2).
- A broad class of substances known or believed to be carcinogenic or to have other detrimental impacts on health are known as hazardous air pollutants, or HAPs.
Particulate Control Devices:
The purpose of particulate control devices (PCDs) is to extract particulate matter (PM) from various air streams and industrial exhaust gases. PM is made up of microscopic solid or liquid particles that are suspended in the air and have the potential to harm both the environment and human health. Depending on the size, density, and other characteristics of these particles, PCDs use a variety of techniques to trap them.
A complete overview of various particle control equipment is provided below:
Cyclones:
Mechanism: To extract particles from the gas stream, cyclones use centrifugal force. The gas forms a whirling vortex as it enters the cyclone tangentially. Cleaner gas escapes from the top, while heavier particles are thrown outward towards the cyclone walls and settle to the bottom for collection.
Advantages: Low initial cost, easy to maintain, capable of handling large amounts of dust.
Drawbacks: Not appropriate for sticky or solid particles; lower effectiveness for tiny particles (usually >10 microns).
Electrostatic Precipitators (ESPs):
Mechanism: To charge the particles in the gas stream, ESPs employ high-voltage electrical fields. After that, the charged particles are drawn to collection plates with opposing charges, where they stick and are eliminated.
Advantages: Low pressure drop, high gas volume handling, excellent efficiency for a wide variety of particle sizes (including submicron).
Drawbacks: Expensive upfront costs, susceptible to fluctuating gas conditions, necessitates a power supply.
Baghouses (Fabric Filters):
Baghouses use fabric filter bags as a mechanism for collecting particulates. Particles become stuck on the fabric’s surface and in the filter media as the gas passes through it. The bags are frequently shaken or pulsed to remove the accumulated dust.
Benefits: Excellent for high-temperature applications, versatile in handling different types of dust, and highly efficient for extremely fine particles (even submicron).
Drawbacks: Needs to clean the bags on a regular basis; can be damaged by dust that is abrasive or corrosive; has a greater pressure drop than some other devices.
Wet Scrubbers:
Mechanism: To remove particles from the gas stream, wet scrubbers employ a liquid, commonly water. Through a variety of interactions (spraying, bubbling, etc.), the gas and liquid make contact, trapping and washing away particulates.
Benefits: Suitable for high-temperature and high-humidity applications, it can remove gaseous and particulate pollutants concurrently and manage corrosive or sticky particles.
Cons: Needs more upkeep and complexity than some other devices; produces wastewater that must be treated; may cause a visible plume if improperly engineered.
Selection of the Proper Device:
The best PCD choice is determined by a number of criteria, such as:
- Particle Size Distribution: Certain particle size ranges are better captured by particular PCDs.
- Temperature and Gas Flow Rate: The PCD must be made for the particular operating circumstances.
- Particle Characteristics: Specific PCDs may be needed for sticky, corrosive, or abrasive particles.
- Regulation Requirements: Standards and emission limitations may specify the PCD’s minimum level of efficiency.
- Cost considerations: When making decisions, capital, operational, and maintenance costs are crucial variables to take into account.
Efficiency:
| PCD Type | Particle Size Range (microns) | Typical Efficiency (%) | Notes |
| Cyclones | > 10 | 50-90 | Efficiency increases with particle size and decrease with cyclone diameter. Best for coarse particles. |
| Electrostatic Precipitators (ESPs) | 0.01-100 | 90-99.9+ | High efficiency for a wide range of particle sizes. Efficiency increases with increasing collection plate area and applied voltage. |
| Baghouses (Fabric Filters) | 0.01-100 | 99-99.99+ | High efficiency for fine particles. Efficiency depends on fabric type, cleaning mechanism, and dust cake properties. |
| Wet Scrubbers | 0.1-100 | 50-99 | Efficiency varies depending on scrubber type, liquid-to-gas ratio, and particle characteristics. Can remove both particulate and gaseous pollutants. |
Gaseous Pollutant Control Devices
The purpose of gaseous pollutant control devices (GPCDs) is to extract and eliminate toxic gases and vapours from various air sources, including industrial exhaust streams. Sulphur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), volatile organic compounds (VOCs), and hazardous air pollutants (HAPs) are a few examples of these pollutants. GPCDs eliminate or neutralise toxic gases using a variety of chemical and physical methods, safeguarding both the environment and public health.
The following is a thorough explanation of typical gaseous pollutant control devices:
Absorbers:
Mechanism: To dissolve and extract gaseous contaminants from the exhaust stream, absorbers use a liquid solvent. In a crowded tower or spray chamber, the gas and solvent come into contact and the pollutants are absorbed into the liquid. As the solvent, which now contains the contaminants, is cleansed or regenerated for reuse, the cleaned gas leaves the absorber. READ in Detail
Benefits: Capable of handling high gas flow rates, efficient in eliminating soluble gases like SO2 and HCl, and capable of recovering important compounds from the gas stream.
Drawbacks: May not work with all types of gases; involves careful handling and selection of solvents; may produce effluent that needs to be treated.
Adsorbers:
Mechanism: Adsorbers draw and retain gaseous contaminants on their surface by using a solid substance with a high surface area, such as zeolites or activated carbon. The contaminants are captured and extracted from the gas as it passes through an adsorbent material bed. Regenerating the adsorbent through heating or other techniques allows for reuse. FURTHER READING.
Benefits include low pressure drop, the ability to be used at room temperature, and effectiveness in eliminating a variety of gases and vapours, including HAPs and VOCs.
Drawbacks: Adsorbent material is not appropriate for high-concentration gas streams and can get saturated, requiring regeneration.
Thermal Oxidizers:
Mechanism: Gaseous pollutants are broken down into less hazardous chemicals by high temperatures in thermal oxidizers. In a combustion chamber, the gas stream is heated to a high temperature, causing the contaminants to oxidise. Water vapour and carbon dioxide are usually the combustion products that result.
Benefits: high destruction efficiency for a variety of organic contaminants, versatility in handling different gas concentrations, and ease of use and dependability.
Drawbacks: Excessive energy usage, potential for high temperature production of nitrogen oxides (NOx), and need for regular maintenance to guarantee full combustion. Further Reading.
Catalytic Oxidizers:
Mechanism: They work by using a catalyst to reduce the necessary operating temperature for combustion, much like thermal oxidizers do. By encouraging the oxidation of pollutants at lower temperatures, the catalyst lowers energy consumption and the likelihood that NOx will occur.
Benefits include the ability to recycle heat from the exhaust stream, high destruction efficiency, and lower operating temperatures than thermal oxidizers.
Cons: More expensive initially than thermal oxidizers, catalysts are not appropriate for all gas types, and some contaminants have the potential to poison them. Further Reading.
Biofilters:
Mechanism: Gaseous pollutants are broken down by microorganisms in biofilters. The gas stream travels through an organic material bed (such as wood chips or compost), where microorganisms convert the contaminants into less dangerous forms.
Benefits include low running costs, environmental friendliness, and the ability to remove a variety of pollutants.
Drawbacks: Limited to biodegradable contaminants, requires careful control over operating parameters (temperature and moisture), and can take some time to create a stable microbial population. Further Reading.
Efficiency:
| GPCD Type | Typical Efficiency (%) | Notes | |
| Absorbers | Soluble gases (SO2, HCl) | 90-99+ | Efficiency depends on solvent type, contact time, and pollutant concentration. |
| Adsorbers | VOCs, HAPs | 90-99.9+ | Efficiency depends on adsorbent type, contact time, and pollutant concentration. Requires periodic regeneration of the adsorbent. |
| Thermal Oxidizers | VOCs, HAPs | 95-99.99+ | Efficiency depends on operating temperature, residence time, and pollutant type. |
| Catalytic Oxidizers | VOCs, HAPs | 95-99.99+ | Efficiency depends on catalyst type, operating temperature, and pollutant type. Lower operating temperatures than thermal oxidizers. |
| Biofilters | Biodegradable VOCs, H2S | 70-95 | Efficiency depends on operating conditions (moisture, temperature, pH), pollutant type, and microbial population. |
| Wet Scrubbers | Acidic gases (SO2, HCl), some VOCs | 90-99 | Efficiency depends on scrubber type, liquid-to-gas ratio, and pollutant solubility. Can simultaneously remove particulate matter. |
| Selective Catalytic Reduction (SCR) | Nitrogen oxides (NOx) | 80-95 | Uses ammonia or urea as a reducing agent to convert NOx to nitrogen and water. Requires a catalyst and careful control of operating conditions. |
| Selective Non-Catalytic Reduction (SNCR) | Nitrogen oxides (NOx) | 50-80 | Similar to SCR but does not require a catalyst. Lower capital cost but lower efficiency than SCR. |
| Flue Gas Desulfurization (FGD) | Sulfur dioxide (SO2) | 90-99 | Typically uses a wet scrubber to remove SO2 using a limestone or lime slurry. Produces gypsum as a byproduct. |
All Done!!!!!!
Now, take this QUIZ……….
Which of the following is not a common cleaning mechanism used in baghouses?
Iske, answer me doubt h
please clarify
Baghouse filters can be cleaned using several mechanisms, including shaking, rapping, reverse air flow, and sonic cleaning. Shaking involves vibrating the filter, while rapping utilizes a device to strike the bags, both aiming to dislodge particulate matter and dust. Reverse air flow temporarily changes the airflow direction to remove dust. Sonic cleaning employs sound waves for dust removal but it is less popular and expensive than other methods.