Thermal pollution
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- The efficiency of coal-fired power plants is around 35%, meaning only 35% heat that is generated out of coal-burning is usable. Rest will go out to the atmosphere. In other words, for every 1000 MW of power produced 1500 MW of heat is wasted.
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Carbon Emissions
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- 1 gm of carbon burnt produces 3.5 g of CO2. A coal plant that produces 1GW of electricity emits 1 ton of carbon dioxide every 2 seconds.
- Solution: Carbon Capture and Storage
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Air pollutants
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- Three potential pollutants are formed when coal is burnt; sulphur oxides, nitrogen oxides and fly ash.
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Nitrogen
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- Normally nitrogen is very stable and burns only at high temperatures. At high temperatures when nitrogen reacts with oxygen it forms oxides of nitrogen. This is very harmful as it acts as nuclei for fine dust causing PM pollution.
- Solution: Remove nitrogen, reduce temperature of combustion
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Sulphur
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- Sulphur is of special interest again because it is a potential air pollutant. In addition, oxides of sulphur act as condensation nuclei during cloud formation leading to acid rain.
- Solution: desulphurization
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Ash, mainly fly ash
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- Left-over solid inorganic matter after burning of coal. (highest is bituminous)
- It can be collected at the bottom from where it can be removed.
- However, the problem is when the turbulent stream of gases in the boiler sweeps some of the ash out of the boiler along with flue gas. This is called fly ash.
- A 2016 report by IIT Kanpur says about 37% of PM 10 and 26% of PM 2.5 is caused by fly ash from coal plants.
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Coke (Carbon + Ash)
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- Solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C so that the fixed carbon & residual ash are fused together.
- Coke is used as a fuel & as a reducing agent in smelting iron ore in a blast furnace.
- Coke from coal is grey, hard, & porous & has a heating value of 29.6 MJ/kg
- Byproducts of this conversion of coal to coke include coal tar, ammonia, light oils, and “coal-gas“.
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