VPE 511: Environment and Environmental Hygiene (2+1)

AIR POLLUTION

Definition

• Engineers Joint Council of USA defines air pollution as “ the presence in the outdoor atmosphere of one or more contaminants, such as dust, fumes, gas, mist, odour, smoke or vapour in quantities, with characteristics, and of duration which as to be injurious to human, plant and animal life or to property, or which unreasonably interfere with the comfortable enjoyment of life and property”.
• According to the Bureau of Indian Standards. IS-4167 air pollution is “ the presence in ambient atmospheres of substances, generally resulting from the activity of man, in sufficient concentration, present for a sufficient time and under circumstances such as to interfere with comfort, health or welfare of persons or with reasonable use or enjoyment of property”.

Pollutants

• Thermal power amounts to more than 70 per cent of power generation and 1000 tons of coal is burnt per year to produce.
• 1 MW power, producing the following pollutants ( in tons/ MW/ year)
• Ash 750, carbon dioxide 7000, Sulfur dioxide 12, Oxides of Nitrogen 20. In addition, carbon monoxide, and cancer causing heavy metals , present as impurities in coal, is also released.
• Thermal power stations are responsible for about 14% of the global air pollution, while transport systems and industry contribute 60% and 16% respectively.

Impacts

• The impacts from the air pollutants of thermal power plants are as follows
  • Green house effect from CO₂ release
  • Corrosion of structures by SO₂ and NO₂
  • Acid rains and death of aquatic life.
  • Irritation of eyes and mucous membranes and respiratory distresses
  • Anemia due to CO which destroys red blood cells
  • Cancers due to some metal particulates
  • Reduced yield of crops and dwarfed growth of plant life.

Control Measures

• Proper Sitting: 25 km away from outer boundaries of cities, and sensitive ecosystems, and sufficient distance from highways, coastline, agricultural land etc.
• EIA: Detailed assessment of impacts on the land, water, air and socioeconomic environments.
• SPM Removal : Particulate removal from stack gases using settling chambers, cyclone separators , bag glitters and electrostatic precipitators.
• Removal of Gases: Removal of SO₂ from exhaust gases by absorption, adsorption etc.
• Subjecting the polluting body or company to rigorous control using Air (Prevention and Protection of Pollution) Act.

Air pollution from refineries

• Pollution due to petroleum refineries encompases three stages , viz crude oil production, transportation and refining operations. Maximum pollution occurs during refining, where crude oil is subjected to distillation, cracking and hydro cracking operations to give naphtha , kerosene, diesel oil, gasoline, fuel oil, lubricants, LPG, asphalt, coke, grease and waxes. Air pollutants comprise of H₂S, NH₃, SO₂, NOX, HF, HCL, CHX, aldehydes, mercaptans, smoke, CO, CO₂, fumes, mists and dusts. The major pollutants are SO₂, H₂S, CHX and SPM. They are controlled as follows.
• SO₂: De sulfurisation of effluent gases, besides pollution control can meet the sulphur shortage. The desulfurization processes employed are: Cat- OX, Cal-OX, NOs- OX and ammos- OX. In the cat- OX, SO₂ is catalytically oxidized to SO₃ and converted to sulfuric acid ; with and efficiency of 93%. In the cal- OX & NOs- OX processes, SO₂ is scrubbed with and alkaline solution. In the Ammos – OX process ammonium sulfite and sulfate solutions are used for scrubbing SO₂.
• H₂S: Ethyl amines are most commonly used to remove H₂S along with CO₂, carbon disulfide and carbonyl sulfide. Sulfur is recovered in the process and the absorbent amine is recycled, H₂S can also be removed by adsorption using activated carbon, alumina, zeolites, bauxite or iron oxide. The adsorbent can be regenerated and reused, by a suitable method.
• NH₃: Ammonia is scrubbed by dilute sulfuric acid to form ammonium sulfate. Alternatively NH₃ is oxidized to NO₂^, which can be absorbed in water to give HNO₃.
• CHX: Hydrocarbons can be processed for recovery by cooling and adsorption, if economically feasible . Otherwise they are disposed off by catalytic or non catalytic combustion. Flares are provided for combustion of waste hydrocarbons.
• Particulates: SPM (Suspended Particulate matter) in the tail gases are controlled by cyclone separators, , scrubbers, filters and electrostatic precipitators.