Brian Barton
Gaseous pollutants are harmful substances in the air that can have adverse effects on human health, property, and atmospheric visibility. These pollutants are caused by both natural sources and human activities, with the latter being subject to mitigation and control. Human activities that release gaseous pollutants include industrial processes such as those in tanneries, as well as transportation and power plants. To control these pollutants, various technologies and techniques are employed, including absorption, adsorption, incineration, and the use of control devices like scrubbers, electrostatic precipitators, and catalytic converters. These methods aim to remove or reduce the concentration of gaseous pollutants to minimize their harmful effects on the environment and human health.
| Characteristics | Values |
|---|---|
| Gaseous pollutants | Acid gases (hydrochloric acid, sulfuric acid, hydrogen sulfide), inorganic gases (sulfur oxides, nitrogen oxides, ammonia), organic gases (ethylene, benzene, ethanol) and volatile organic compounds (VOCs) |
| Control techniques | Absorption, adsorption, incineration (or combustion), condensation, cyclones, electrostatic precipitators, scrubbers, settling chambers, cyclone separators, fabric filters, baghouse filters |
| Absorption techniques | Wet scrubbers, packed scrubbers or towers, plate towers, spray towers, acid gas scrubbing |
| Adsorption techniques | Stationary bed units, moving bed units |
| Incineration techniques | Thermal incineration, catalytic combustion |
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What You'll Learn
Absorption
One common type of scrubber used in gas absorption is the wet scrubber, which is considered a control device that separates pollutants from a gas stream. Wet scrubbers can be designed in several ways, including cross-flow, bubble plate, and tray scrubbers. However, the most common type is the packed-bed counterflow scrubber. Wet scrubbers are often used for acid gas scrubbing, which controls emissions of oxidizing halogenated compounds such as HCl, H2S, and SO2. These compounds can form acid gases during the oxidation process in a thermal oxidizer. Wet scrubbers offer the advantages of low capital and installation costs and simple design. Nevertheless, it is important to consider the potential drawback of transferring air pollution to a water pollution problem.
Another type of scrubber used in gas absorption is the packed scrubber or tower, where the liquid is present on a wetted surface rather than as droplets. A common example is the countercurrent tower, where the polluted airstream flows upward through a wetted column of chemically inactive packing material while the liquid absorbent flows downward, maximizing contact between the gas and liquid.
In addition to wet scrubbing, absorption can also be achieved through dry scrubbing, specifically in the context of flue gas desulfurization (FGD) systems. In this process, flue gases come into contact with an absorbent, which can be a liquid or a slurry of solid material. The sulfur dioxide dissolves in or reacts with the absorbent and becomes trapped. Dry scrubbing offers cost and energy savings and easier operation compared to wet scrubbing, but it requires higher chemical consumption and is limited to specific types of flue gases.
Overall, absorption is a versatile technique that can be applied in various scrubber designs and tower configurations to effectively remove gaseous pollutants from the air by transferring them into a liquid phase.
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Adsorption
Activated carbon adsorption units can remove gases with an efficiency of over 95%. These systems can be designed as stationary bed units or moving bed units. In stationary bed adsorbers, the polluted air enters from the top, passes through a layer of activated carbon, and exits from the bottom. Moving bed adsorbers use gravity to slowly move the activated carbon downwards while the air passes through in a cross-flow current.
The adsorption process is influenced by the size of the molecules being captured. The adsorption capacity of an adsorbent is higher for larger molecules than for smaller ones. This makes physical adsorption particularly well-suited for the adsorption of organic compounds, as they tend to have larger molecules.
Saturation capacity is the maximum amount of pollutant that an adsorbent can hold. Before reaching saturation, the adsorbent reaches its breakthrough capacity, which is the amount of pollutant that can be adsorbed before a significant concentration of the pollutant exits the bed. The working capacity of an adsorption system is the difference between the breakthrough capacity and the heel capacity, which is the amount of pollutant remaining in the bed after regeneration.
Overall, adsorption is an important technique for controlling gaseous pollutants, especially when used in conjunction with other methods such as absorption and combustion. By utilising adsorption, we can effectively capture and remove a wide range of organic and inorganic contaminants from the air, helping to improve air quality and protect the environment.
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$114 $150
Incineration
To control gaseous pollutants from incineration, air pollution control devices are necessary. The flue gases produced must be cleaned of gaseous and particulate pollutants before being released into the atmosphere. Gas cooling techniques are an important aspect of incineration system design, with lower temperatures aiding in the reduction of mercury, dioxins, furans, and acid gases. Dry air pollution control devices (APCDs), such as scrubbers and particulate control devices, are effective in reducing these pollutants.
Particulate matter can be controlled using electrostatic precipitators, fabric filters, or wet inertial scrubbers. Hydrochloric acid (HCl) and sulfur dioxide (SO2) can be controlled with wet scrubbers, spray dryer absorbers, or dry-sorbent injection. NOx emissions can be partially controlled through combustion-process modification and ammonia or urea injection.
Additionally, the concentration of dioxins and mercury can be substantially reduced by injecting activated carbon into the flue gas or using a carbon sorbent bed. Newer incineration plants have improved control over odours and dust, utilizing negative pressure and airflow routing through boilers to prevent odour escape.
While incineration can reduce waste volume and generate energy, it is essential to carefully consider the health and environmental risks associated with the release of harmful chemicals and pollutants.
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Scrubbers
Wet scrubbers use a liquid, usually water, to remove pollutants from an exhaust stream. The liquid serves as a solvent for the pollutant or captures it through a chemical reaction. Wet scrubbers are highly effective at removing pollutants, with removal efficiencies exceeding 90%. They are commonly used to remove acid gases, water-soluble inorganic contaminants, and volatile organic compounds. The design of wet scrubbers can vary, including spray tower, tray-type, and packed-bed scrubbers. Packed bed scrubbers use packing material to provide a large surface area for liquid-gas interaction, enhancing the removal of pollutants.
Dry scrubbers, on the other hand, do not saturate the flue gas stream with moisture. They are often used for removing odorous and corrosive gases from wastewater treatment operations. Dry scrubbers can be categorized as dry sorbent injectors (DSIs) or spray dryer absorbers (SDAs). Spray dryer absorbers introduce flue gases into an absorbing tower where they come into contact with an alkaline slurry, which absorbs and neutralizes acid gases.
Ionizing wet scrubbers are another type of scrubber that removes gases and reduces smoke opacity. Crossflow scrubbers are designed for low-profile applications and are highly effective at absorbing gaseous pollutants.
Overall, scrubbers play a crucial role in controlling gaseous pollutants by utilizing various scrubbing agents, solutions, and technologies to remove and neutralize harmful pollutants, gases, and particulate matter from industrial processes.
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Electrostatic precipitators
The basic design of an electrostatic precipitator consists of a row of thin vertical wires and a stack of large flat vertical metal plates. The wires impart a static charge to the exhaust gas or smoke as it passes through, causing the charged particles to be attracted to and deposited on the oppositely charged plates. The treated air then passes out of the precipitator through a stack to the atmosphere. When enough particles have accumulated on the plates, they are shaken off by mechanical rappers and fall into a hopper at the bottom of the unit, where they are transported away for disposal or recycling.
Regular maintenance of electrostatic precipitators is essential to avoid system failures. Dust accumulation can lead to complete failure, so it is important to keep the charge plates, hopper, and rectifier clean and regularly checked. Rapper coils play a critical role in the cleaning process by providing vibrations to remove buildup on collection plates or wires. ESPs can be designed to work with gas streams with specific temperature and moisture characteristics, allowing for flexibility in their application.
Overall, electrostatic precipitators are an important tool for air pollution control, particularly in industrial settings where large volumes of gas need to be treated. They help to reduce harmful emissions, improve air quality, and protect public health.
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Frequently asked questions
Some common gaseous pollutants are acid gases, inorganic gases, and organic gases. Examples of these include hydrochloric acid, sulfur oxides, nitrogen oxides, ammonia, ethylene, benzene, and ethanol.
Three basic techniques to control gaseous pollutants are absorption, adsorption, and incineration. Absorption involves passing polluted gases through liquid absorbents, adsorption uses solid adsorbents like activated carbon to concentrate pollutants on surfaces, and incineration destroys pollutants through oxidation.
Some devices used to control gaseous pollutants are packed towers, wet scrubbers, cyclones, electrostatic precipitators, and baghouse filters.
Gaseous pollutants can have adverse effects on human health, property, and atmospheric visibility. They can also contribute to climate change and air quality issues. Therefore, it is important to control gaseous pollutants to protect the environment and human health.
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