Air Pollution


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Unit-I & VIII

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Air Pollution

  1. 1. Air PollutionUnit- I & VIII
  2. 2. Syllabus Air Quality Management Air pollution fundamentals, Air quality management, control technologies.
  3. 3. Air Pollution • Definition:- Air pollution is define as “Excessive concentration of foreign matter in air, which adversely affects the well being of individuals or causes damage to the property”.
  4. 4. Composition of Air
  5. 5. Ambient Air Quality Standards Pollutant Time Weighted Average Concentration in Ambient Air Industrial Area Residential, Rural and other Sensitive Area Sulphur Dioxide (SO2) Annual 24 hours 80 µg/m3 120 µg/m3 60 µg/m3 80 µg/m3 15 µg/m3 30 µg/m3 Oxides of Nitrogen (NO2) Annual 24 hours 80 µg/m3 120 µg/m3 60 µg/m3 80 µg/m3 15 µg/m3 30 µg/m3 Suspended Particulate Matter (SPM) Annual 24 hours 360 µg/m3 500 µg/m3 140 µg/m3 200 µg/m3 70 µg/m3 100 µg/m3 Respirable ** Particulate Matter (RPM) Annual 24 hours 120 µg/m3 150 µg/m3 60 µg/m3 100 µg/m3 50 µg/m3 75 µg/m3 Lead (pb) Annual 24 hours 1.0 µg/m3 1.5 µg/m3 0.75 µg/m3 1.00 µg/m3 0.50 µg/m3 0.75 µg/m3 Carbon Monoxide(CO) 8 hours 1 hour 5.0 µg/m3 10.0 µg/m3 2.0 µg/m3 4.0 µg/m3 1.0 µg/m3 2.0 µg/m3 * Ministry of Environment and Forests, Government of India notification ** Particle size less than 10 µm
  6. 6. Major Sources of Air Pollution
  7. 7. Major Sources of Air Pollution • Natural Sources:- The natural sources of air pollution are volcanic eruptions, releasing of poisonous gases like SO2 ( Sulfur dioxide), Hydrogen sulphide (H2S), Carbon Monoxide(CO), Forest fires, natural organic and inorganic decays, Pollen grains, wind blown dust, among them pollen grain are important because it causes allergic reactions in individuals.
  8. 8. Major Sources of Air Pollution
  9. 9. Anthropogenic Sources • Rapid Industrialization:- The Industries such as pulp and paper, chemical, metallurgical plants, refineries are responsible for air pollution, the most common pollutants are CO2, SO2, CO, NO, H2S etc.. • Transportation:- Automobile exhausts release smoke. The smoke is a result of incomplete combustion of carbonaceous matter. The common pollutants are CO(Carbon Monoxide), CO2, and Hydrocarbons. Rapid Industrialization Transportation
  10. 10. • Burning of Fossil fuels and fires:- The conventional sources of energy are wood, coal, and fossil fuels. The byproducts of fossil fuels is nothing but poisonous gases such as CO, CH4 (methane), SO2, and NOX. • Deforestation:- The balance of O2 and CO2 is maintained in the nature by the vegetation. The deforestation by man for his own needs has disturbed the balance of CO2 and O2, Concentration. Burning of Fossil fuels and fires Deforestation
  11. 11. • Increase in Population:- an increase in population leads to global warming and emission of green house gases. It also contributes to losses in forest cover and loss of wild life. • Agriculture activities:- Various pesticides and insecticides are used for agricultural purposes. Thus they cause air pollution. • Radioactive fallout:- Nuclear reaction, nuclear weapon testing contributes towards Nuclear pollution. Nuclear Pollutants are very harmful to man, animals and vegetations.
  12. 12. Anthropogenic sources Increase in Population Agriculture activities Radioactive fallout
  13. 13. Classification of Air Pollutants • Air pollutants are broadly classified as • (a) According to origin • (b) According to chemical composition • (c) According to State of matter.
  14. 14. According to Origin • Air Pollutants can be classified as • (a) Primary Pollutants • (b) Secondary Pollutants • Primary Pollutants:- Primary pollutants are those which are directly emitted from identifiable source • These pollutants are emitted directly to the atmosphere.
  15. 15. • e.g. (a) Particulate matter (b) Oxides of Sulphur (c) Oxides of Nitrogen (d) Carbon Monoxide (e) Radioactive Compounds
  16. 16. Particulate Matter
  17. 17. Oxides of Sulphur
  18. 18. Oxides of Nitrogen
  19. 19. Carbon Monoxide
  20. 20. Radioactive Compounds
  21. 21. • Secondary Air Pollutants:- Secondary air pollutants are those which are produced in the air by the reaction of two or more primary pollutants. • e.g. • Ozone • Formaldehyde • Photochemical smog • Peroxy acetyl nitrate (PAN)
  22. 22. Secondary Air Pollutants Ozone Formaldehyde
  23. 23. Secondary Air Pollutants Photochemical smog
  24. 24. Secondary Air Pollutants Peroxy acetyl nitrate (PAN)
  25. 25. According to Chemical Composition All the air pollutants are classified as (a) Organic Pollutants (b) In Organic Pollutants (a) Organic Pollutants:- Organic Pollutants contain carbon and hydrogen. e.g. • Hydro Carbon • Aldehyde and Ketones • Alcohols • Organic Sulphur compounds
  26. 26. Inorganic Pollutants:- Inorganic compounds mainly includes (a) Carbon monoxide (CO) (b) Carbon dioxide (CO2) (c)Carbonates (d) Oxides of sulphur, nitrogen (e) Ozone
  27. 27. Organic Pollutants Hydro Carbon
  28. 28. Inorganic Pollutants Carbon dioxide (CO2)
  29. 29. According to State of Matter Air pollutants according to state of matter are classified as (a) Natural Contaminants (b) Particulate Matter (c) Gases and Vapours
  30. 30. Natural Contaminants • The air contaminants which are produced from natural sources are called natural contaminants • Eg. Pollen grains are emitted from weeds, grasses and trees • This pollen grains are irritating to some individuals • Size of pollen grains = 10 to 50 (µ) Pollen Grains
  31. 31. Particulate matter These may be liquid or solid. The particulate matter are identified as aggregates which are larger than 0.002 µ but smaller than 500 µ Dust (1 To 10,000 µ) : Small solid particles resulting from break up of large masses through processes such as crushing, grinding or blasting. Smoke ( 0.5 To 1 µ) Consist of finely divided solid particles produced by incomplete combustion of organic particles such as coal, wood, or tobacco.
  32. 32. • Mist:- These are liquid particles formed by condensation of vapor • Fog (1 to 40 µ ):- High concentration of mist is called as fog. It is dispersion of liquid particles in air. • Fumes ( 0.03 to 0.3 µ ):- Fine solid particles formed by the condensation of vapors of solid material. The fumes are generally emit from melting operations. • Ash ( 1 to 1000 µ ):- fine, noncombustible particles are known as fly ash. • Spray (10 to 1000 µ):- Liquid particles formed by automation
  33. 33. Particulate Matter Mist Fog Smoke Ash
  34. 34. Gases And Vapors • Carbon Monoxide:- (CO) This is an odorless, tasteless and colorless gas formed due to incomplete combustion of carbonaceous matter. The sources of carbon monoxide are motor vehicles, burning of fossil fuel and industrial processes. • Oxides of Sulphur:- They Include 4 different gaseous compounds such as Sulfur monoxide ( SO), Sulfur dioxide ( SO2), Sulphur trioxide ( SO3) and sulphur tetra oxide (SO4). • It is a major air pollutant gas produced by the combustion of fuels like coal. The main source of electricity production is by burning of fossil fuels in India and the whole world. The sulphur content of the coal varies from 1 to 4% and fortunately the Indian coal is low in sulphur content. SO is also produced in the metallurgical operations.
  35. 35. Gases And Vapors Carbon Monoxide
  36. 36. Oxides of Nitrogen Oxides of Nitrogen • Oxides of nitrogen are produced either in the production of nitric acid or in the automobile exhausts and as the effluent of power plants. • Out of the seven oxides of Nitrogen (N2O, NO, NO2, NO3, N2O) • only nitric oxide and nitrogen dioxide are classified as the main pollutants. • All the oxides of nitrogen are collectively known as NOX.
  37. 37. Hydrocarbon • Organic compounds containing only carbon and hydrogen are classified as Hydrocarbons. • They are of 2 types. • (a) Aliphatic hydrocarbon • (b) Aromatic hydrocarbon • Hydrocarbon present in the atmosphere in the atmosphere from both natural and man made sources. • The major sources of hydrocarbon is burning of gasoline and industrial processes.
  38. 38. Aliphatic hydrocarbon
  39. 39. Aromatic Hydrocarbon
  40. 40. Other Gases • Hydrogen Sulphide: Hydrogen Suphide is an obnoxious (bad smelling) gas. It is produced mainly by the anaerobic (in absence of air) decomposition of organic matter. Other air polluting sulfur compounds are methyl mercaptan (CH3 SH) and dimethyle Sulphide (CH3 S CH ) etc.. • Hydrogen fluoride: It is an important pollutant even in very low concentrations. It is produced in the manufacturing of phosphate fertilizers. • Chlorine and hydrogen chloride: It is mixed in the air either from the leakages from water treatment plants or other industries where it is produced or used. Hydrogen chloride is also evolved in various industrial chemical processes. The main effect of chlorine is respiratory irritation which may be fatal. • Ozone: It is a desirable gas in the upper layers of atmosphere as it absorbs the UV radiation of sunlight. But near the earth surface it is a poisonous gas. It makes poisonous chemicals by photochemical reactions.
  41. 41. Other Gases • Aldehydes: They are produced by the incomplete oxidation of motor fuels and lubricating oil. They may also be formed because of photochemical reactions. Formaldehydes are irritating to the eyes.
  42. 42. Other Gases Hydrogen Sulphide Hydrogen fluoride Hydrogen Chloride Ozone
  43. 43. Classification Based On Position • Another way of classification of air pollutants is the source being stationary or mobile. • This way they can be classified as:- • (i) Point source (large stationary source as power plants) • (ii) Area source (small stationary sources like residential heating) • (iii) Mobile source (line source like highway vehicles or area source like Aircraft at airports)
  44. 44. Point Source
  45. 45. Area Source
  46. 46. Mobile Source
  47. 47. Air Pollutants And Their Major Sources.
  48. 48. Effects of Some Common Air Pollutants Particulate Matter • Effects on human:- • The pollutants of size less than 1 µ causes lung damage • Asbestos fibers causes cancer • Lead from automobile exhaust affects brain • It causes several respiratory diseases • Silica , arsenic, dust causes cancer
  49. 49. Effects of Air Pollution on Humans
  50. 50. Effects of Air Pollution on Humans
  51. 51. Effects on Plants • Deposition of particulate matter containing toxic metals affects the growth of plants • The particulate matter after deposition on plant leaves block stomata opening of plants and reduce plant growth. • The particulate matter with acid rain, reduces pH of the soil which makes the infertile.
  52. 52. Effects on Plants
  53. 53. • Effect on Material • The particulate matter causes damage to building • The corrosion is enhanced • It causes cracks and fading of pointed surface • Particulates accumulate on the soil and reduces fertility of soil.
  54. 54. Effect on Material
  55. 55. Carbon Monoxide • Effects on humans • The high concentration of CO can cause death • The combination of CO and hemoglobin leads to the formation of carboxylhaemoglobin (COHb) reduces the oxygen carrying capacity of blood. • At concentration of 100 ppm people experience dizziness and headache. • The cigarette smoke contains 400 to 450 ppm CO the percentage of COHb in blood of cigarette smokers increases with increase in smoking. • At concentration of 750 ppm of CO it will cause death.
  56. 56. Carbon Monoxide (COHb)
  57. 57. • Effects on plants:- • CO reduces nitrogen fixing capacity of bacteria. Which affects the plant growth. • High concentration of causes leaf drops, reduces the size of leaf and ageing • Effects on materials:- • Carbon monoxide appears to have no detrimental effect on materials.
  58. 58. Oxides of Sulphur • Effects on human health:- • It causes irritation of eyes and respiratory tracts. • Increase in SO2 concentration in the atmosphere, may lead to lung cancer • SO2 may obstruct breathing • SO2 leads to the formation of H2 SO4 which is 20 times more irritant than SO2
  59. 59. • Effects on Plants:- • The low concentration for long period may cause discoloration of leaves • SO2 affects the growth of plants • At high concentration the leaf tissues gets damaged. • H2SO4 is extremely toxic to plants and soil fertility.
  60. 60. Effects on Plants
  61. 61. • Effects on Materials:- • The sulphuric acid will attack building materials containing carbonates. This will form CaSO4 the CaSO4 gets easily washed away leaving discolored surface • Paper absorb SO2 causing the paper to become brittle • Leather looses the strength and flexibility.
  62. 62. Acid Rain
  63. 63. Oxides of Nitrogen • Effects on Human Health:- Nitric Oxide reduces the Oxygen carrying capacity of blood. • Nitrogen dioxide causes irritation of lungs • High level of NOx causes pneumonia, lung cancer, oxygen deficiency. • NOx causes irritation of respiratory system, nervous system and digestive tracts. • NOx is extremely dangerous to human health.
  64. 64. Oxides of Nitrogen Irritation of lungs lung cancer
  65. 65. • Effects on Vegetation:- • NO2 and primary pollutants can damage plant tissues • High concentration of NO Causes Damage to leaves. • Secondary Pollutants such as smog, O3 may damage the vegetation • Effects on Materials:- • Nitric acid causes corrosion to metal surface • NO2 fades the color of clothes • NO2 causes cracking of rubber damaged plant tissues Corrosion
  66. 66. Hydrocarbon • Effects on human health:- • Aromatic hydrocarbon may lead to cancer • Inhalation of hydrocarbon causes irritation of respiratory tract. • Methane creates narcotic effects on human beings • Most of the hydrocarbon are carcinogenic to lungs irritation of respiratory tract
  67. 67. • Effects on Vegetation:- • HC affects plant growth • Discoloration of leaves • Effects on Materials:- • It causes discoloration of materials • Material becomes less elastic and more brittle. Discoloration of leaves
  68. 68. Harmful Impacts of Air Pollution on Plants
  69. 69. Harmful Impacts of Air Pollution on Human beings
  70. 70. Harmful Impacts of Air Pollution on Human beings
  71. 71. Harmful Effects of Air Pollutants on Human Health
  72. 72. Harmful Effect of Air Pollution on Material
  73. 73. Control of Air Pollution • Air pollution cannot be fully abated but can be controlled if certain measures are taken. • Controlling measures for air pollution can be mainly divided into • Preventive measures • Control measures • i.e. Control of gaseous/ particulate pollutants • From industry/ automobiles using equipments
  74. 74. Preventive Measures • These measures are mainly aimed at correction right at the source so there will be lesser amount of pollutant emitted from the industry/ automobiles. • Some of the important preventive measures are • Using low sulphur coal in power plants • Using natural gas in place of coal for power generation. • Using LPG/ CNG instead of diesel or petrol in automobiles.
  75. 75. Preventive Measures • “Modifications” in industrial processes and/ or equipments to reduce the emissions • Section of suitable site and zoning for industrial unit: Zoning means setting aside separate areas for industries so that they are far from residential areas. This will help in proper dispersion of pollutants resulting in lower concentration of pollutants in the air of residential area. • By using proper environment impact studies before setting the industry.
  76. 76. Control Measures • The most common method of eliminating or reducing pollutants to an acceptable level includes: • Collecting the pollutants by using equipments to prevent its escape into atmosphere. • Destroying the pollutants by thermal or catalytic combustion • Changing the pollution to less toxic form. • By releasing the pollutants through tall chimneys for greater dispersion. Tall chimneys will liberate the pollutants beyond the inversion layer there by reducing the concentration of pollutants near the source. But the liberated pollutants will settle somewhere and cause of the air pollutants even far away from the source.
  77. 77. Control Of Air Pollution Prevention is always better than cure. • one can prevent the air pollution. For this purpose some of the examples are as follows:- • (i) Controlling the spread of coal dust by sprinkling water on it before handling in a thermal power plant. • (ii) Preventive maintenance by repairing leaky valves in advance so as to prevent the leakages of the harmful gases in air. • (iii) Applying zoning to distribute the impact of air pollution in a community. • (iv) Selection of proper material. For example using low sulfur coal reduces the problem.
  78. 78. Control Of Air Pollution • In addition to the preventive measures one has to have control technologies as follows. • Depending upon the situation different control technologies are adopted. For dealing with the particulate matter the following control technologies are used, • Settling chambers • Cyclone separators • Fabric filters • Electrostatic precipitators • Wet collector (scrubbers) • The first two of the above are used for coarse particulates where as the other three are used for fine particulates.
  79. 79. Air pollution Control • The atmosphere like a river, do possesses self- cleansing properties, which continuously clean and remove the pollutants from the atmosphere under natural processes. So long as the pollutants discharged by man into the environment, is lower than the natural cleansing capacity of the environment, we live happily without any air pollution. But as when the discharged pollutants exceed the natural cleansing capacity, our environment becomes polluted, Attempts are then made to reduce the emission of pollutants from the automobiles of factories by adopting mechanical means, or by using high rise chimneys for better dispersion and dilution of pollutants over a longer range of environment..
  80. 80. The Natural Self-Cleansing Properties of the Environment • The Various natural properties, which continuously clean the environment, automatically, are: • Dispersion • Gravitational Settling with or without flocculation of particles • Absorption including washout and scavenging; • Rainout and • Adsorption • All these processes, naturally occur periodically in the ambient air, thereby removing or reducing the various pollutants entering the air. These processes are briefly discussed below:
  81. 81. Dispersion • Dispersion of pollutants by winds reduces the concentration of air pollutants at one place, although in the strict sense, it does not remove them from the environment as a whole. This mechanism is therefore a diluting mechanism only. • The dispersion through long chimneys, dilutes the air pollutants near their source only, and does not reduce their long term undesirable effect on the community as a whole.
  82. 82. Dispersion
  83. 83. Gravitational Settling • Gravitational setting is the most important natural mechanism, under which large heavy particles from ambient air settles down on buildings, trees and other objects. This generally happens for the particles which are larger than 50 µm in size. • This process also helps in removing flocculated particles formed by uniting of smaller particles over larger particles, till a floc particle, large and heavy enough to settle out under gravity, is formed.
  84. 84. Gravitational Settling
  85. 85. Absorption • In the natural absorption process, the gaseous as well as particulate pollutants from the air get collected in the rain or mist, and may settle out with that moisture. This phenomenon takes place below the cloud level, when falling raindrops absorb pollutants, and is also known as washouts or scavenging. The process, however, does not help in removing particles smaller than 1 µm in size. • The gaseous pollutants are removed in dissolved state with moisture, either with or without chemical changes.
  86. 86. Absorption
  87. 87. Rainout • Rainout is the process involving precipitation above the cloud level, where submicron particles present in the atmosphere in the cloud, serve as condensation nuclei. Around which drops of water may form, and fall out as raindrop. This phenomenon helps in increased rainfall and fog formation in urban areas, containing huge quantity of such particles, raising high above the cloud level.
  88. 88. Adsorption • Adsorption is the phenomenon in which the gaseous, liquid or solid pollutants present in the ambient air are kept attached, generally electrostatically, by a surface, where they are concentrated and retained. Natural surfaces, such as soils, rocks, leaves, blades of grass, buildings, and other objects can adsorb and retain pollutants. The particles may come in contact with such surfaces either by gravitational setting or by inertial impaction, under which the pollutants are carried to such surfaces by winds. Impaction is particularly effective for particles of size 10 to 15 µm.
  89. 89. Dilution Method for Controlling Air Pollution from Stationary Sources • Transporting the pollutants over larger distances, and thus, reducing the pollution near the emission source. • This method is largely adopted in developing countries, because here the pollution is generally confined over smaller environment near cities and industrial towns only. The neighbouring environment, which is free from emission, is thus made to share some of the pollutants burden, thereby causing somewhat equitable distribution of the pollutants. Nevertheless, this method only reduces the concentration of pollutants at particular place (s), rather than reducing or removing the pollution load from the total environment, as a whole.
  90. 90. Controlling Air Pollution from Stationary Sources by Installing Engineering Devices • In order to reduce the pollution load entering the environment from stationary sources, several measures may be taken; out of them, replacement of burning fuel by electricity or solar energy is by far, the best method, as it will eliminate the vary production of pollutants in the combustion process. Besides this we can make use of better quality of fuels and efficient engines, for reducing pollution loads from emissions. Say for example, LPG and LNG may be used in industries in place of coal, as they will produce much less pollutants in emission.
  91. 91. Controlling Air Pollution from Stationary Sources by Installing Engineering Devices • Replacement of old obsolete processes in industries with the new efficient processes may also lead to reduce pollution emissions. Besides such innovations and precautions, certain mechanical devices may be installed in the industrial processes, which may help in reducing the emission of pollutants. • Such mechanical devices, are generally divided into two categories; i.e. • (i) those devices which help in reducing particulate matter; and • (ii) those devices which help in reducing gaseous pollutants.
  92. 92. Control of Particulate Matter in Industry • The important devices, which are used to control particulate matter are; • (a) Gravitational Settling chambers • (b) Centrifugal collectors including cyclone collectors and dynamic precipitators. • (c) Wet Scrubber including spray towers, wet cyclone scrubbers and venturi-scrubbers. • (d) Electronic Precipitators; • (e) Fabric Filters.
  93. 93. Gravitational Settling Chamber • It is used to remove particles with size greater than 50µ m. Velocity of the flue gas is reduced in large chamber thereby inducing settling the particles under gravitational force. Settling is governed by stokes law. • The usual velocity of the flue gas through settling chambers is between 0.5 to 2.5 m/s. • The largest sized particles (d) that can be removed with 100 % efficiency in such chamber of length L and height H is given by equation.
  94. 94. Gravitational Settling Chamber • The above equation applies to quiescent condition but quiescent condition cannot be maintained in a flow through settling chamber, so a correction factor is applied and eqn becomes.
  95. 95. Gravitational Settling Chamber
  96. 96. Gravitational Settling Chamber
  97. 97. Advantages of Settling Chamber • Low Initial Cost. • Simple to design • Low pressure drop • Low Maintenance cost • Dry and continuous disposal of solid particulates.
  98. 98. Disadvantages of Settling Chamber • The collection efficiency can be increased by providing the baffles and horizontal baffles/ tray in the chamber to reduce the settling path. • Settling chambers are used widely for removal of large solid particulates from natural draft furnace, kiln, etc.. They are also, sometimes used in the process industries, particularly the food and metallurgical industries, as first step in dust control.
  99. 99. Cyclone Separator • Centrifugal force is utilized to separate the particulate matter from the gas. As Centrifugal force is much greater than gravitational force smaller particles can be removed. (10 to 50 µm) • A cyclone is a specially designed closed chamber, in which the velocity of the inlet gas is transformed into spinning vortex, and the particles from the gas are thrown out under the centrifugal force. The particles thrown out on the walls of the chamber, slides down to the hopper and thus removed. Its efficiency depends on the centrifugal force generated, which in turn, depends on the mass of a particular(Mp) , inlet gas velocity Vi and radius of cyclone (R) and is given by the eqn • Its collection efficiency is in between 50 to 90 %
  100. 100. Cyclone Separator
  101. 101. Cyclone Separator
  102. 102. Cyclone Separator
  103. 103. Advantages of Cyclone Separators • Low Initial Cost • Requires less floor area • Simple construction and maintenance • Low to moderate pressure loss (2.5 to 20 cm) • Can handle large volume of gas at high temperature. • Dry continuous disposal of collected dust.
  104. 104. Dis-advantages of Cyclone Separators • Requires large head room • Less efficiency for smaller particles (< 10 µ) • It is subjected to severe abrasive deterioration. • Quite Sensitive to variable dust load and flow rates. • Cyclones are widely used in industries producing larger quantities of gas containing larger sized particles like, cement and fertilizer industries, petroleum refineries, grain mills and textile industries. • Cyclone can be arranged in series and in parallel to increase the efficiency of collection compare to a single large sized cyclone.
  105. 105. Electrostatic Precipitators • It works on the principle of electrical charging of the particulate matter(-ve Charge) and collecting it on a differently charged (+ve charged) collecting surface. It has a very high efficiency of about 99 % and can remove particles in the size range of 0.1 µm to 1 µm efficiently.
  106. 106. Electrostatic Precipitators
  107. 107. Electrostatic Precipitators
  108. 108. Advantages of Electrostatic Precipitators • High Collection Efficiency • Particles as small as 0.1 µm can be removed • Particles may be collected dry or wet. • They can be operated at high temperature up to 300- 450 0C • Maintenance is nominal, unless corrosive and adhesive materials are present in the flue gases. • Few moving parts. • Treatment time is negligible (0.1 – 10 S).
  109. 109. Disadvantages of Electrostatic Precipitators • High Initial Cost. • They use high voltage, and hence may pose risk to personal safety of the staff. • Possible explosion hazards during collection of combustible gases or particulates. • Collection efficiency reduces with time. • Space requirement is more because of the large size equipments. • They are widely used in thermal power plants, cement factories, paper and pulp industries, mining and metallurgical industries, iron and steel industries, chemical industries etc.
  110. 110. Fabric Filters • In this device, fabric filter’s out the particulate matter from the gas stream and allow clear gas to flow. In such a system, the flue gas is allowed to pass through a woven or felted fabric, which filters out the particulate matter and allows the gas to pass. Small particles are retained on the fabric, initially through interception and electro-static attraction; and later on, when a dust mat is formed, the fabric starts collecting particles more efficiently it can remove particles up to 1 µm. Its efficiency is around 99 %.
  111. 111. Fabric Filters • A bag house filter provided in an ordinary room of the factory, contains several vertically hanging fabric cylindrical bags (1.8 to 9m long) ; the upper ends of the bags are closed, and lower ends are attached to a hopper, where the inlet of the flue gas is also located. The upwards moving gas drops out particulate matter in these bags, which settles down in to the hopper, and cleaner gas goes out through the fabric filters. The framework, housing the hanging bags, is provided with a automatic shaking device for removing the collected dust.
  112. 112. Fabric Filters
  113. 113. Advantages of Fabric Filters • High collection efficiencies for all particle sizes, especially for particles smaller than 10 µm in diameter. • Performance decrease becomes visible, giving prewarning. • Nominal power consumption • Dry disposal of collected particles.
  114. 114. Dis-advantages of Fabric Filters • High temperature gases need to be cooled to the range within which filter are stable. • High maintenance and fabric replacement cost. • Large size equipments. • The fabric is liable to chemical attack.
  115. 115. Fabric Filters
  116. 116. Scrubbers • In scrubbers the particulate matter are incorporated into liquid droplets and thus are removed from the gas stream. Different types of Scrubbers includes. • (a) Spray towers • (b) Venturi Scrubbers • (c) Cyclone Scrubber • In these devices the flue gas is made to push up against a down falling water current. The particulate matter mixes up with water droplets and thus, falls down and get removed. • Water solution, when mixed with aqueous chemical solutions, like lime, potassium carbonate, slurry of MnO and MgO, etc., do help in removing gaseous pollutants also from the flue gases.
  117. 117. Cyclone Scrubber
  118. 118. Spray Towers
  119. 119. Venturi Scrubbers
  120. 120. Advantages of Scrubbers • They can simultaneously remove particulate and gaseous pollutants. • Hot gases can be cooled down • There is no particles re-entrainment • Corrosive gases can be recovered and neutralize. • The separated gases through contact with aqueous chemicals may produce useful byproducts, as chemicals and fertilizers.
  121. 121. Disadvantages of Scrubbers • A lot of wastewater needing disposal may be produced. • Wet outlet gases cannot rise high from the stack. • Poses freezing problems in cold countries. • Maintenance cost is high, when corrosive materials are collected.
  122. 122. Control of Gaseous Pollutants • For gaseous pollutants the following control technologies are used. • Absorption • Adsorption • Combustion • For the control of gases such as NO2 and SO produced in combustion, wet and dry scrubbers are used.
  123. 123. Control of Gaseous Pollutants • The gaseous pollutants can be controlled through the techniques of combustion, absorption and adsorption. • (a) In combustion process oxidizable gaseous pollutants are completely burnt at a high temperature. This processes is used to control gaseous pollutants in petro-chemical, fertilizers, paint and varnish industry. • (b) In absorption techniques effluent gases are passed through absorbers containing liquid absorbents that remove, treat or modify one or more gaseous pollutants. Liquid adsorbent may utilize either chemical or physical change to remove pollutants. • In Adsorption, the air pollutants are adsorbed on a solid surface. Commonly used adsorbents are activated carbon, activated alumina, silica gel etc..
  124. 124. Control of Gaseous Pollutants Combustion or Incineration • Combustion or Incineration Equipments may be used to purify polluted gases, when the pollutants in the gas stream are oxidisable to an inert gas. Pollutants, like hydrocarbons and carbon monoxide (CO) can be easily burnt, oxidized, and removed in such equipments. • Both Direct flame combustion by afterburners and catalytic combustion have been used in commercial applications.
  125. 125. Control of Gaseous Pollutants Absorption Units • The absorption units work on the principle of transfer of the pollutants from the gas phase to the liquid phase. In other words, the pollutants from the dirty gas, get absorbed in the liquid, through which the gas is made to pass, in these units. Such absorption takes place by diffusion as well as dissolution. • The effectiveness of these devices, naturally depends upon the solvent, through which the gas is made to pass. • Absorption Units Like Spray towers, plate towers, packed towers, and venturi scrubbers. Out of these devices, spray towers and venturi-scrubbers can simultaneously be used for removing particulate pollutants. Scrubbers are generally less effective in removing gaseous pollutants than towers, but then, towers get frequently clogged by particulate matter. • Other effective devices for removing gaseous pollutants, i.e. plate towers and packed towers.
  126. 126. Control of Gaseous Pollutants Adsorption Units • Using adsorbents like activated carbon, molecular sieves, activated alumina, silica gel, etc.. • In adsorption units, the dirty gas are made to pass through the beds of such adsorbent materials, wherein the pollutants are effectively caught and removed. Activated carbon beds can very effectively catch hydrocarbons, H2S and SO 2 • The above adsorbents, except activated carbon, have the drawback of preferably capturing water, before catching any of the gaseous pollutants; and hence have to be removed from the gas, before it is treated in such units. • However, almost all the adsorbents are subjected to destruction at moderately high temperature (150 0 C to 600 0 C) Hence they prove to be very inefficient for purifying industrial gases at such high temperature.
  127. 127. Control of Automobile Exhaust Automobile Exhaust can be controlled by: • (a) The un-burnt hydrocarbon in automobile emission can be reduced by use of efficient engines (MPFI, CRDI, TDI technologies). • (b) Catalytic converter can convert NO to Nitrogen gas and reduce potential hazards. • By enforcing stringent emission standards for the vehicles like Bharat Stage 4 or Euro 4 right at the manufacturing stage. • Using lead free petrol. • By using cleaner fuel like CNG, hybrid car, electric car, etc.. • By proper maintenance of the vehicles • Developing good mass transportation facilities.
  128. 128. Control of Automobile Exhaust
  129. 129. Control of Automobile Exhaust
  130. 130. Notations • parts-per-million (ppm) 10–6, parts-per-billion (ppb) 10–9, and parts-per-trillion (ppt) 10–12 level
  131. 131. Case Study: Air Pollution Episode Bhopal Gas Tragedy • The Story of Bhopal Gas Tragedy: The Lucky Ones Died That Night • It was five minutes midnight in Bhopal on December 2 1984. The congested city was asleep and the winter air was heavy. Many had gone sleep. It was all quite, but the city was to change forever. • Suddenly, 27 tons of lethal gases including methyl Isocyanate (MIC) started leaking from Union Carbide’s Pesticide factory. The cloud of gases rapidly blanked the city. • Grossly under designed safety systems were either malfunctioning, under repair, or had been switched off as part of a cost cutting exercise. The warning siren at the factory had reportedly been turned off. It was the world’s worst Industrial Disaster.
  132. 132. Case Study: Air Pollution Episode Bhopal Gas Tragedy • Here is a survivor’s account of what happened on that ill fated night: • “ The poison cloud was so dense and searing that the people were reduced to near blindness. As they gasped for breath its effects grew more suffocating. The gases burned the tissues of their eyes and lungs, and attacked their nervous systems. People lost control of their bodies Those who escaped with their lives are the unlucky ones; the lucky ones are those who died that night.”
  133. 133. Case Study: Air Pollution Episode Bhopal Gas Tragedy • 70,000 people were evacuated from the area after the accident and 2,00,000 more fled in panic. The effect of the gas was felt up to 100 sq km from the factory. More than 8000 people died in the few days following the disaster. Within few months death toll raised to 20000 due to exposure related illness. • The gases affected more than 5,00,000 people, leaving them with the life time of ill health and trauma.
  134. 134. Bhopal Gas Tragedy
  135. 135. How did the accident occur? • How did the accident occur? • Most probably water entered the storage tank and caused a runway chemical reaction that led to an increase in temperature, which converted the liquid MIC into gas. Investigations reveled that there had been six early accidents in the plant and that workers had complained off exposure to dangerous substances. Yet proper safety mechanisms were allegedly not put in place.
  136. 136. Bhopal Gas Plant
  137. 137. Bhopal Gas Plant
  138. 138. Case Study: Air Pollution Episode Bhopal Gas Tragedy • Union Carbide accepted only moral responsibility for the disaster and not any liability. The Government of India filed a case against the company for US $ 3 billion, but strangely accepted US $470 million as settlement in 1989. Nearly 95 % of the survivors had received just Rs 25000 for lifelong injury and loss of livelihood. That works out to be 9 US cents a day for more than 20 years of Unimaginable suffering. • A criminal case was filed in the court of Bhopal against Union Carbide, and its then chairmen Warren Anderson for negligence,. They did not attend the court proceedings and Anderson has been proclaimed a fugitive from justice by the court.
  139. 139. Bhopal Gas Tragedy
  140. 140. What does the story of the Bhopal Tragedy Tell us ? • The tragedy shows that poor communities are disproportionately affected by toxic materials discharged into air, land and water. When a crisis occurs, or an accident occurs these people cannot get justice from the polluters, or the governments. Source: Basics of Environmental Studies By R. Rajgopalan
  141. 141. Exam Questions • What is Air Pollution? Explain in detail its effects on property and materials. • Explain primary and secondary air pollutions. State the effects of air pollution on plant and material. • Explain the effect of air pollution on men, material and animals. • Describe different primary and secondary air pollutants. • Write a S.N. on Electrostatic precipitator. • Describe the devices used for control of particulate matter.
  142. 142. References Sewage Disposal and Air Pollution Engineering : By Prof S.K. Garg Khanna Publishers Internet Websites
  143. 143. Thanks… 