Techniques For Reducing Air Pollution From Smoke Stacks

Smokestacks, or chimneys, are tall vertical pipes used by power plants to exhaust combustion gases into the air. These gases, known as flue gases, contain various harmful pollutants such as nitrogen oxides, sulfur oxides, carbon dioxide, carbon monoxide, and particulate matter. To reduce air pollution, various technologies can be employed to remove these pollutants before they are emitted through the smokestack. These technologies include electrostatic precipitators, fabric filters, venturi scrubbers, cyclones, and settling chambers. While taller smokestacks can help disperse pollutants over a wider area, they do not actually reduce total emissions and can negatively impact air quality in downwind communities. Regulatory controls and technologies, such as scrubbers and electrostatic precipitators, have been developed to remove combustion products and reduce the impact of air pollution on human health and the environment.

Electrostatic precipitators

The basic idea of electrostatic precipitation is attributed to Dr Frederick Cottrell, who carried out experiments with the technology in 1906. The first commercially successful process was patented in 1908 following experiments by American chemist Frederick Gardner Cottrell at the University of California, Berkeley.

Fabric filters

The filter is usually in the form of cylindrical fabric bags, although it may also be in the form of cartridges constructed of fabric, sintered metal, or porous ceramic. The fabric must be able to withstand high temperatures and corrosive chemical characteristics. The emission stream exiting the chamber is about 99% clean of small pollutants.

There are three types of fabric filters, categorised by the method used to clean the filter material: shaking, reverse air, and reverse pulse or pulse jet.

Shaker fabric filters clean the bags by gently shaking them. The open bottoms of cylindrical bags are attached to holes in a tube sheet, and the closed tops of the bags are attached to the shaking mechanism in the top of the casing. The contaminated gas stream enters the hopper, flows through the holes in the tube sheet and into the inside of the vertical bags. The gas stream then flows through the bags, leaving a dust cake on the inside. Periodically, the gas flow is stopped, and the bags are shaken to dislodge the dust cake, which falls into the hopper and is removed from the collector.

Reverse air fabric filters are similar to shaker filters, with the bags attached to holes in the tube sheet and the closed tops attached to a support structure in the top of the casing. The contaminated gas stream enters the hopper and flows into and through the bags, again leaving a dust cake on the inside. To clean the bags, the gas flow is stopped and another gas flow is introduced, flowing in the reverse direction. This pressurises the compartment, causing the bags to collapse partially and the dust cake to crack and fall into the hopper.

Reverse pulse or pulse jet fabric filters clean the bags using short-duration pulses of compressed air. The bags hang from holes in a tube sheet located near the top of the vertical casing, and a wire-mesh cage inside the bags prevents them from collapsing. The contaminated gas stream enters through the side or the hopper of the collector and flows into the bags and up through the tube sheet, leaving the dust cake on the outside of the bags. Cleaning is accomplished by directing the compressed air pulse to one or a few rows of bags, while the others continue to provide filtration. The dust cake dislodged by the compressed air pulse falls into the hopper and is removed from the collector.

Venturi scrubbers

One of the key advantages of Venturi scrubbers is their ability to handle large inlet volumes at high temperatures, making them desirable for use in various industrial applications. They are commonly used to control emissions from cement kilns and basic oxygen furnaces in the steel industry, where inlet gas temperatures can exceed 350 °C (660 °F). Venturi scrubbers are also used to control fly ash and sulfur dioxide emissions from industrial and utility boilers.

Cyclones

The design of a cyclone machine is simple: it forces dirty gases and air to move in a circular motion, causing the larger particles of pollution to fall to the bottom of a hopper, while the cleaned gases exit from the top. This process is known as centrifugal separation. Cyclones are a cost-effective and low-maintenance method of removing pollutants from smoke stack emissions. However, they are only suitable for larger-sized particles.

The choice of technology to remove pollutants depends on the size of the pollutants, the emission flow rate, temperature, moisture, and chemical properties of the emissions such as flammability and acidity. Other technologies used to remove pollutants from smoke stacks include electrostatic precipitators, fabric filters, venturi scrubbers, 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 causes them to be magnetically drawn to specially charged plates where they are then collected. The emission stream exiting the chamber is about 99% clean of small pollutants.

Fabric filters, also known 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 mix water into the emission gas in specially designed tubes. First, velocity and pressure are increased to combine the pollutant particles with the water. Then, the mixing processes are stopped, and the pollutant particles and water droplets settle out of the gas stream as it exits the tube. This technology uses a lot of water and creates wastewater that must be treated.

Settling chambers slow down the velocity of gaseous emissions as they move through the chamber, causing larger-sized particles to drop out into a hopper. Settling chambers are often used in conjunction with other technologies because smaller-sized pollutants likely remain in the emissions that exit the chamber.

Settling chambers

The design of settling chambers involves maintaining gas flow velocities below the reentrainment velocity of deposited particulate matter. High-efficiency settling chambers may be fitted with baffles or deflectors to change the direction of gas flow. Settling chambers are dry-type primary collectors and are among the most common and important devices used by environmental engineers.

Overall, settling chambers play a crucial role in removing large particles of pollutants from smoke stack emissions. While they may not capture all pollutants, their simplicity, cost-effectiveness, and ease of construction make them a valuable tool in the effort to improve air quality.

Frequently asked questions