Unpolluted air contains 78% nitrogen, 21% oxygen, 1% argon, 0.04% carbon dioxide and trace amounts of other gases with up to 4% water vapour. An air pollutant is a substance normally absent in air, or a substance that is normally present but in excess amounts. Primary pollutants are carbon monoxide, oxides of nitrogen, sulphur dioxide, particulates and hydrocarbons. Secondary pollutants are compounds formed when primary pollutants react in the air.
CARBON MONOXIDE:
-natural source: incomplete oxidation of methane
-man-made source: incomplete combustion of fossil fuels
-effect on health: prevents haemoglobin from carrying oxygen by binding irreversibly to form carboxyhaemoglobin.
-methods of reduction: use lean burn engine, thermal exhaust reactor, or catalytic converter.
OXIDES OF NITROGEN:
-natural source: electrical storms and biological processes
-man-made source: at high temp.s inside internal combustion engines.
-effect on health: respiratory irritant that results in infections
-methods of reduction: use of lean burn engine, recirculation of exhaust gases or catalytic converter.
SULPHUR DIOXIDE [can be oxidised to form sulphur trioxide]:
-natural source: oxidation of H2S produced by volcanoes and decay of organic matter
-man-made source: combustion of sulphur-containing coal and smelting of sulphide ores
-effect on health: respiratory irritant that results in infections
-methods of reduction: removal of sulphur from fossil fuels before combustion, alkaline scrubbing, fluidised bed converter.
PARTICULATES:
-natural source: soot, ash, dust, asbestos, sand, smoke, pollen, bacterial and fungal spores
-man-made source: burning of fossil fuels (esp. coal and diesel)
-affects respiratory system, causing lung diseases (e.g. emphysema, bronchitis, cancer)
-methods of reduction: sedimentation chambers, electrostatic precipitation.
VOCs (CxHy or R-H):
-natural source: plants (like rice; emit unsaturated hydrocarbons called terpenes)
-man-made source: unburned or partially burned gasoline and other fuels, solvents.
-effect on health: carcinogens (e.g. benzene), can form toxic secondary pollutants (e.g. PANs)
-methods of reduction: catalytic converter
E.1.2 Evaluate current methods for the reduction of air pollution.
1) Thermal exhaust reactor: Exhaust from car engine is combined with more air and reacts (at high temperature from the heat of the exhaust gas). CO becomes CO2 and unburned hydrocarbons are combusted.
2) Lean burn engines: Carburettor is adjusted altering ratio of air:fuel. Higher ratios results in less CO emitted (more complete combustion). However, this results in higher temperatures that increase the formation of oxides of nitrogen. Conversely, lower ratios cause less oxides of nitrogen to form but more carbon monoxide.
3) Catalytic converter:
Hot exhaust gases are passed over catalysts (i.e. platinum/palladium/phodium) that complete combust CO and unburned VOCs, as well as catalysing the reaction between nitrous oxide and carbon monoxide to give carbon dioxide and nitrogen gas.
2CO + 2NO --> 2CO2 + N2
4) Removal of sulphur from fossil fuels (e.g. metal sulphides in coal) [pre-combustion]
i) Coal gasification removes most of the sulphur in high sulphur coal when converting it to synthetic natural gas (SNG). Any sulphur is converted to H2S (acidic and easily removed). However, this process requires a lot of energy.
ii) use low sulphur, cleaner burning coal (e.g. anthracite) with high heat content. However, such coal is scarcer and more expensive.
iii) remove sulphur in petroleum refining and natural gas processing with a catalyst to hydrogen sulphide.
iv) coal washing: sulphur removed from coal by crushing the coal and mixing it with water, so that the denser sulphides sink and the cleaned coal can be skimmed from the surface. However, a lot of sulphur is trapped below the surface of the particles and organic sulphur (chemically bonded) cannot be physically removed.
5) Alkaline scrubbing [post-combustion]: alkaline slurry of limestone and lime (large amounts required, and heating to convert limestone to lime requires large amounts of energy which is expensive) [or magnesium hydroxide] are used to remove sulphur dioxide from the exhausts of coal-burning plants, with the resulting sludge used to for landfills, or gypsum [CaSO4.2H2O] for plasterboards. A wet alkaline scrubber uses liquid (usually water-based) to remove contaminants by sprayin ghte alkaline liquid in downwards as the gas streams upwards).
CaCO3(s) + SO2(g) --> CaSO3(s) + CO2(g) (note CO2 is a greenhouse gas)
CaO(s) + SO2(g) --> CaSO3(s)
2CaSO3(s) + O2(g) + 4H2O(g) --> 2CaSO4.2H2O(s)
6)Fluidized bed combustion [post-combustion]: burning of coal on a bed of limestone removes sulphur as calcium sulphite or sulphate as the coal burns.
7) Electrostatic precipitation: Soild or liquid particles are suspended in the air, with larger particles allowed to settle in sedimentation chambers (gravity) while an electrostatic precipitation chamber can be used for smaller particles. Charged particles are attracted to oppositely charged electrodes which are shaken periodically to remove aggregated particulates from the bottom of the precipitator.
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