Bronte Wells
Smokestacks emit harmful pollutants, including soot, dust, smoke, carbon monoxide, and sulfur dioxide, which contribute to air pollution and adverse health effects. To combat this, various technologies are employed to remove pollutants from smokestack emissions. These technologies aim to capture pollutants before their release into the atmosphere. The choice of technology depends on factors such as the size of the pollutants, emission flow rate, temperature, and chemical properties. Common methods include electrostatic precipitators, fabric filters, venturi scrubbers, cyclones, settling chambers, and adsorbers. While these technologies aid in removing pollutants, the installation and maintenance of such equipment can be costly, presenting a challenge for many facilities.
| Characteristics | Values |
|---|---|
| Devices used to remove pollutants from smokestacks | Electrostatic precipitators, fabric filters, venturi scrubbers, cyclones, settling chambers, scrubbers, incinerators, activated charcoal |
| How electrostatic precipitators work | Use magnetic attraction to draw smaller-sized pollutants out of the emissions stream |
| How fabric filters work | Remove pollutants as the emission stream passes through porous fabric designed to remove fine particles; fabric must withstand high temperatures and corrosive chemicals |
| How venturi scrubbers work | Mix water into the emission gas in specially designed tubes; velocity and pressure are increased to combine pollutant particulates with water |
| How cyclones work | Mimic a natural cyclone's motion to force larger-sized pollution particles to fall to a hopper at the bottom and clean emission gases to exit from the top |
| How settling chambers work | Remove large particles of pollutants from emissions by slowing down the velocity of gaseous emissions as it moves through the chamber, causing larger-sized particles to drop out into a hopper |
| How scrubbers work | Wet scrubbers combine misters with additional water filters to remove particles from gas; contaminated droplets wash out of a lower pipe while purified gas continues to expand and travels through a high pipe in the scrubbing chamber |
| How incinerators work | Moderate the emission of waste gases into the air |
| How activated charcoal works | Absorb gases |
| Pollutants removed by smokestacks | Particulates (soot, dust, smoke), gas emissions (carbon monoxide, sulfur dioxide), mercury, acid gases, hydrochloric acid, sulfur compounds, toxic chemicals, odorous substances |
| Reasons for removing pollutants from smokestacks | To reduce global warming, decrease carbon dioxide emissions, improve air quality, reduce hazardous air pollutants, reduce acid rain, protect the environment and human health |
| Considerations when choosing technology to remove pollutants | Size of pollutants, emission flow rate, temperature, moisture, chemical properties like flammability and acidity, cost |
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What You'll Learn
Electrostatic precipitators
The basic design of a precipitator consists of a row of thin vertical wires and a stack of large flat vertical metal plates. The plates are spaced anywhere from 0.5 inches to 7 inches apart, depending on the application. The electrodes can take the shape of metal wires, bars, or plates inside a pipe or the smokestack itself. One of the electrodes is charged with a high negative voltage, which causes particulates inside the smoke to obtain a negative charge as they pass by. Further along the pipe, the second electrode carries a high positive voltage, attracting the negatively charged particles, which then stick to it.
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Fabric filters
The filter's ability to collect small micrometer and sub-micrometer particles is due to the accumulated dust cake, not the fabric itself. As dust builds up on the filter surface, the pressure drop across the filter increases, and to avoid excessively high pressure drops, the filter material is cleaned periodically. The cleaning phase of the filter bags is particularly important to address the continuous dust accumulation that impedes airflow through the pores, impacting equipment performance and operational efficiency.
The cost of fabric filters is addressed in the EPA Air Pollution Control Cost Manual, and specific tools have been developed to estimate fabric filter costs when used to control particulate matter from coal-fired power plants and coal-fired utility boilers.
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Venturi scrubbers
A venturi scrubber consists of three sections: a converging section, a throat section, and a diverging section. The inlet gas stream enters the converging section, and as the area decreases, gas velocity increases. Liquid is introduced either at the throat or at the entrance to the converging section. The inlet gas, forced to move at extremely high velocities in the small throat section, turbulently mixes with the liquid, producing a large number of tiny droplets. The entrainment separator is a crucial component of Venturi scrubbers, as it prevents the high-velocity gas from carrying the droplets out with the outlet clean gas stream.
There are different types of Venturi scrubbers, including the adjustable-throat venturi scrubber, which can maintain efficiency over a larger range of flow rates by changing the size of the throat according to the gas flow rate. Another type is the ejector or jet venturi scrubber, which derives its energy from a high-pressure spray of liquid from a nozzle, allowing it to act as a vacuum ejector and draw process gas through the device without external assistance.
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Cyclones
Overall, cyclones are a useful technology for removing larger-sized pollutants from smokestacks, but they should be used in conjunction with other methods to ensure smaller particles are also captured.
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Settling chambers
The simplest settling chamber design is a horizontal duct where large particles settle on the floor. More advanced chambers use baffles or deflectors to change the direction of the gas flow. The primary section of the chamber is characterized by its cross-sectional area (width x height) and length. The cross-sectional area is designed to be larger than the inlet and exit ducts to substantially reduce the gas stream's inlet linear velocity. The length of the chamber determines the amount of time the particles remain at the reduced rate, allowing them sufficient time to settle out into the hoppers.
The collection efficiency of settling chambers varies depending on particle size and chamber design. They are most effective for large and/or dense particles. Settling chambers are also referred to as gravity settling chambers, gravity collectors, expansion chambers, and outfall chambers. Multiple-tray settling chambers are also known as Howard settling chambers.
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Frequently asked questions
Some methods to remove pollutants from smokestacks include electrostatic precipitators, fabric filters, venturi scrubbers, cyclones, and settling chambers.
Electrostatic precipitators use magnetic attraction to draw smaller-sized pollutants out of the emissions stream. The emission gases pass through a specially designed chamber that first charges the pollutants, which then causes them to be drawn to specially charged plates where they are collected.
Fabric filters, also referred to as baghouses, use porous fabric to remove fine particles from the emission stream as it passes through. The fabric must be able to withstand high temperatures and corrosive chemicals.
Venturi scrubbers are a type of wet scrubber that mixes water into the emission gas in specially designed tubes. First, velocity and pressure are increased to combine pollutant particles with water, then the mixing process is stopped and the pollutant particles are removed.
Cyclones are machines that mimic a natural cyclone's motion to force larger-sized pollution particles to fall to a hopper at the bottom while clean emission gases exit from the top. Cyclones are a cost-efficient and low-maintenance method but are only suitable for larger-sized particles.
