4 Air Pollution Control Methods

April 11, 2023 JNTUH 1

Air Pollution Control Methods


Control Methods

• The most effective means of dealing with the problem of air pollution is to prevent the formation of the pollutants or minimize their emission at the source itself.

• In the case of industrial pollutants, this can often be achieved by investigating various approaches at an early stage of process design and development, and selecting those methods which do not contribute to air pollution or have the minimum air pollution potential. These are known as “Source Correction Methods”.


• Application of these methods to existing plants is difficult, but still some of these correction methods could be applied without severely upsetting the economy of the operation.

• Control of the pollutant at the source can be accomplished in several ways through raw material changes, operational changes, modification or replacement of processes equipment, and by more effective operation of existing euipment.


Methods of air pollution control can be divided into two categories: the control of particulate emissions and the control of gaseous emissions.

The term particulate refers to tiny particles of matter such as smoke, soot, and dust that are released during industrial, agricultural, or other activities.

Gaseous emissions are industrial products such as sulfur dioxide, carbon monoxide, and oxides of nitrogen also released during various manufacturing operations.


Source Control Technology • Air quality management sets the tools to control air

pollutant emissions.

• Control measurements describes the equipment, processes or actions used to reduce air pollution.

• The extent of pollution reduction varies among technologies and measures.

• The selection of control technologies depends on environmental, engineering, economic factors and pollutant type.


Gravitational Settling Chamber• Like settling basins in water and

waste water treatment, settling chambers provide enlarged areas to minimize horizontal velocities of air flow thus allow time for gravity to carry the particle to the floor.

• These are simple in design and operation but they require large spaces for installation and have relatively low efficiency especially for removal of small particles.


• Settling chambers use the force of gravity to remove solid particles.

• The gas stream enters a chamber where the velocity of the gas is reduced.

• Large particles drop out of the gas and are recollected in hoppers.

• Because settling chambers are effective in removing only larger particles, they are used in conjunction with a more efficient control device.


Settling chambers


Howard Settling Chamber• In the settling chamber

the gas stream, with its entrained particles, is allowed to flow at a low velocity, allowing sufficient time for the particles to settle down.

• Howard type settling chamber is a more elaborate .


• By inserting several trays, the collection efficiency of the device is improved since the gas flow velocity in the chamber remains substantially the same and yet each particle has a much shorter distance to fall before reaching the bottom of the passage between trays.


Centrifugal Collectors• Centrifugal collectors employ a centrifugal

force instead of gravity to separate particles from the gas stream.

• Because the centrifugal forces than can be generated are several times greater than gravitational forces, particles can be removed in centrifugal collectors that are much smaller than those that can be removed in gravity settling chambers. i.e particles of 10 µm size.


Cyclone Separator• Cyclone separators utilise a

centrifugal force generated by a spinning gas stream to separate the particulate matter from the carrier gas.

• The centrifugal force on the particles in a spinning gas stream is much greater than gravity, therefore, cyclones are effective in the removal of much smaller particles than gravitational settling chambers, and require much less space to handle the same gas volumes.


• The general principle of inertia separation is that the particulate-laden gas is forced to change direction. As gas changes direction, the inertia of the particles causes them to continue in the original direction and be separated from the gas stream.

• The walls of the cyclone narrow toward the bottom of the unit, allowing the particles to be collected in a hopper.


• The cleaner air leaves the cyclone through the top of the chamber, flowing upward in a spiral vortex, formed within a downward moving spiral.

Cyclones are efficient in removing large particles but are not as efficient with smaller particles. For this reason, they are used with other particulate control devices.


Venturi Scrubbers • Venturi scrubbers use a liquid stream to remove

solid particles.

• In the venturi scrubber, gas laden with particulate matter passes through a short tube with flared ends and a constricted middle.

• This constriction causes the gas stream to speed up when the pressure is increased.


• The difference in velocity and pressure resulting from the constriction causes the particles and water to mix and combine.

• The reduced velocity at the expanded section of the throat allows the droplets of water containing the particles to drop out of the gas stream.

• Venturi scrubbers are effective in removing small particles, with removal efficiencies of up to 99 percent.

• One drawback of this device, however, is the production of wastewater.

April 11, 2023 JNTUH 17Venturi scrubber

http://en.wikipedia.org/wiki/File:Venturiscrubber.jpg


Wetted throat venturi scrubber

http://en.wikipedia.org/wiki/File:Wettedthroat.jpg


Non-wetted throat venturi scrubber

http://en.wikipedia.org/wiki/File:Throatsprays.jpg


Rectangular throat venturi scrubber

http://en.wikipedia.org/wiki/File:Rectthroat.jpg


Table 1. Operating characteristics of venturi scrubbers

PollutantPressure drop (Δp)

Liquid-to-gas ratio

(L/G)

Liquid-inlet pressure

(pL)

Removal efficiency

Gases

13-250 cm of water (5-

100 in of water)

2.7-5.3 l/m3 (20-40

gal/1,000 ft3)

< 7-100 kPa (< 1-15 psig)

30-60% per venturi,

depending on pollutant

solubility

Particles

50-250 cm of water (50-

150 cm of water is

common)20-100 in of water (20-60 in. of water is common)

0.67-1.34 l/m3(5-10

gal/1,000 ft3)

90-99% is typical


Filters• Filters consist of porous structure composed of

granular or fibrous material which tends to retain the particulate matter as the carrier gas passes through the voids of the filter.

• Filtration may be by the fabrics or by mats i.e. fibrous media, felted media, porous media.

• They stop the particles due to impaction, interception, diffusion, settling, sieving action and electrostatic attraction forces.


• These filters may have single layer where dust particles are collected in the interstitial spaces.

• They may be packed beds where air passes through the length of a filtering medium.

• Fabric filters require frequent cleaning. Filters have high collection efficiency, flexibility of operation and continuous removal is possible.

• These filters may be Bag house filters or Electrostatic filters.


Fabric filters, or baghouses • Fabric filters, or baghouses, remove dust from a gas

stream by passing the stream through a porous fabric. The fabric filter is efficient at removing fine particles and can exceed efficiencies of 99 percent in most applications.

• The selection of the fiber material and fabric construction is important to baghouse performance.


• The fiber material from which the fabric is made must have adequate strength characteristics at the maximum gas temperature expected and adequate chemical compatibility with both the gas and the collected dust.

• One disadvantage of the fabric filter is that high-temperature gases often have to be cooled before contacting the filter medium.


• Fig. shows a simple bag house filter. Dirty air passes through the felt pad and the solid particles are collected.

• A shaker mechanism helps in collecting the dust through the hopper outlet at the bottom of the filter.


Fabric filter (baghouse) components


Electrostatic Precipitators• Electrostatic precipitators provide mutual

attraction between particles of one electric charge and a collecting electrode of opposite polarity.

• They may be used to handle large gas volume or high temperature gases effectively.

• Deposition of material takes place on a grounded collection surface.


• Adhesive, cohesive and electrical forces should prevent reentering of particles into the gas stream.

• Efficiency of collection depends on dust concentration, collector area, gas flow rate, electrical field intensity and other reasons.

• 0.05 µm to 200 µm particles sizes also can be removed.

• These esp devises have the advantage of range of size of particles collected, complete collection of both solid and liquid particles and high efficiency even with small size particle collection.


• First an electrostatic charge is imparted to the particles and an electric field is generated in the flow region.

• An attractive force is exerted on the particles and causes them to migrate towards the opposite charged electrodes.

• Particles are collected by gravity, shaking the electrodes (rapping) helps in the collection of particles at the bottom. Dust or slurry is removed by flushing.


• For industrial applications, vertical plates exposed to horizontal gas flow are normally used.

• In this type of collector the gas flows between two vertical parallel plates between which are suspended a number of vertical wires held in place by weights attached at the bottom.

• These wires form the discharge electrode, while the vertical plates form the collection electrode.

• A unique feature of electrostatic precipitators is that the separating force is applied directly to the particles without necessity of accelerating the gas as is done for all other particulate collection devices.


Electrostatic precipitator components


Equipment for the complete recovery and control of acid and

oxide emissions

4 Air Pollution Control Methods

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