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Air Pollution Control Strategies
Transcript of Air Pollution Control Strategies
Control of particulates
AIR QUALITY CONTROL
The easiest way to control air pollution is to eliminate the source of the pollution. Surprisingly, this is often also the most economical solution to an air pollution problem. In other cases as a modification of the process, such as switching to natural gas instead of coal in an electrical power plant, will eliminate the immediate air pollution problem. Most often, however, control is achieved by some from air treatment similar in concept to water treatment. In this chapter some of the alternatives available for treating emissions are discussed, followed by a review of the last control strategy- dispersion.
The spray tower or scrubber, is an effective method for removing large particulates. More efficient scrubbers promote the contact between air and water bby violent action in a narrow throat section into which the water is introduced. Generally, the more violent the encounter, the smaller the gas bubbles or water droplets and the more effective the scrubbing. Wet scrubbers efficient devices but have two major drawbacks.
• They produce a visible plume, albeit only water vapor. The lay public seldom differentiates between a water vapor plume and any other visible plume, and hence, public relations often dictate no visible plume.
• The waste is now in liquid form, and some manner of water treatment is necessary.
Electrostatic precipitators are widely used in power plants, mainly because power is readily available. The particulate matter is removed by being charged by electrons jumping from one high-voltage electrode to the other and then migrating to the positively charged collecting electrode.
Control of Gaseous Pollutants
CYCLONE used for dust collection
The simplest devices for controlling particulates are settling chambers consisting of nothing more than wide places in the exhaust fuel where larger particles can settle, usually with a baffle to slow the emission stream. The most popular, economical, and effective means of controlling particulates is the CYCLONE. The dirty air is blasted into a conical cylinder, but off centerline. This creates a violent swirl within the cone, much like a centrifuge. The heavy solids migrate to the wall of the cylinder, where they slow down due to friction, slide down the cone, and finally exit at the bottom. The clean airis in the middle of the cylinder and exits out the top.
BAG FILTER used for control of particulate air pollutants
Bag filters used for controlling particulates operate like
the common vacuum cleaner. Fabric bags are used to collect dust, which must be periodically shaken out of the bags. The fabric will remove nearly all particulates, including submicron sizes. Bag filters are widely used in many
industrial applications but are sensitive to high temperatures and humidity.
The basic mechanism of dust removal in fabric filters is
thought to be similar to the action of sand filters in water
treatment. The dust particles adhere to the fabric due to
entrapment and surface forces. They are brought into
contact by impingement and/or Brownian diffusion.
can removed gaseous pollutants by simply dissolving them in water. A chemical (such as lime) may be injected into the scrubber water that then reacts with the pollutants.
is a useful method when it is possible to bring the pollutant into contact with an efficient adsorber, such as activated carbon.
Incineration or flaring
is used when an organic pollutant can be oxidized to CO2 and water. A variation of incineration is catalytic combustion in which the temperature of the reaction is lowered by the use of a catalyst that mediates the reaction.
The control of gases involves the removal of the pollutant from the gaseous emissions, a chemical change in the pollutant, or a change in the process producing the pollutant
As noted earlier, sulfur oxides are serious and yet ubiquitous air pollutants. The major source of sulfur oxides is coal-fired power plants. The increasingly strict standards for sulfur oxides control have prompted the development of a number of options and techniques for reducing their emission.
Mitra, Feby Anne
Change to low-sulfur fuel.
Natural gas and oil are considerably lower in sulfur than coal. However uncertain and expensive supplies make this option risky.
Desulfurize the coal
. Sulfur in coal can be organic or inorganic. The inorganic form is iron pyrite, which can be removed by washing because it occurs in discrete particles.
The removal of the organic sulfur requires chemical reactions and is most
economically accomplished if the
coal is gasified (changed into a gas
resembling natural gas).
Build tall stacks
. A short-sighted, albeit locally economical, method of sulfur oxides control is to build incredibly tall smokestacks and disperse the sulfur oxides. This option was employed in Great Britain and was in part responsible for the acid rain problem in Scandinavia.
Desulfurize the flue gas
. The last option is to reduce the sulfur oxides emitted by cleaning the gases coming from the combustion, the so-called flue gases.
Dispersion is the process of
spreading the emission over a
large area, thereby reducing the
concentration of the specific
pollutants. The plume spread or
dispersion is in two dimensions:
horizontal and vertical.
Control of Moving Sources
1. Tuning the engine to burn fuel efficiently.
2. Installation of catalytic reactors
3. engine modifications