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Carbon Monoxide - CO
CO is a poisonous gas and is harmful to life. Carbon monoxide is formed due to the incomplete combustion fuels such as coal. Modern coal fired boilers have excess air levels that are 15 to 20 % greater than stoichiometric requirements. Furnace aerodynamics ensure the proper mixing of fuel and air so under normal conditions, the CO formed is negligible or almost nil.
High levels of CO, 2000 -3000 ppm or higher are possible if the combustion is not good. This is mainly due to operator fault or improper maintenance of pulverizers, burners, and air registers
Apart from the poisonous nature of the CO, CO formation is an indication of inefficient heat release from the coal. This in turn requires more coal to produce the same amount of power, increasing CO2 emissions and associated problems.
Almost all power plants have continuous monitoring of CO. This is an emission that can be eliminated.
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Sulphur Dioxide - SOx
Sulphur is present in coal in varying quantities and can range from 0.1% to as high as 4.5%. Sulphur in coal is due to the formation of the coal millions of years ago and varies widely from mine to mine. Sulphur dioxide is again an inevitable result of combustion. A small amount of SO3 or sulphur-trioxide is also formed. Collectively they are known as SOX.
The effect of SOX in the atmosphere is to form sulphuric acid droplets. This forms acid rain and is very detrimental to health and vegetation. This also results in the formation of ground level ozone, which contributes to global warming.
Power plant operators who get low sulphur coal are lucky. But others with logistic access only for higher sulphur coal have to make arrangements to remove the SOX formed, increasing the cost of the power. Almost all countries have very strict limitations on SOX emissions. National Air Quality Standards in the US have limitations restricting SOX emissions to 0.03 ppm on an annual arithmetic mean, with once-only 24-hour allowable emissions of 0.14 ppm. The rest have to be captured. Indian air quality norms cap SOX at 80 μg/m3.
Almost all coal fired plants today are equipped with flue gas desulphurization, or FGD. Dry or wet flue gas desulphurization plants use mostly limestone, CaCO3, to absorb the SO2 as CaSO4. Even though the technology is available, the cost of operating a chemical plant within a power plant deters the use of the FGD units.
The other alternative is to use a CFBC boiler where limestone or other sorbents can be mixed with the fuel to absorb the SO2 formed during the combustion process itself.
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Nitrogen Dioxide - Nox
Nitrogen oxides are formed at high temperatures in the range of 1500 °C during the combustion of coal. This is called "thermal NOx." The nitrogen itself is from the combustion air- 76.8%. The major portion is NO2 with small amounts of NO and is collectively called NOX .
Even though NOX formation is an inherent combustion related activity, this can be avoided by proper flame patterns and air fuel mixing. Due to the very complex flow patterns during combustion, there may be certain parts of the flame that are higher than the 1500 °C.
The low NOX burning systems are designed to streamline the flame to eliminate these hot spots. The air distribution systems and the aerodynamic design of the burner components are the two main items that are considered in low NOX burners.
The NOx itself contributes to acid rain, smog formation, visibility, and eye irritation.
National Air Quality Standards of the US restrict NOx emissions to 100 μg/m3 on an annual arithmetic mean. The Indian cap is 80 μg/m3.
"Selective Catalytic Reduction" is the method to eliminate NOX. NOX is absorbed by urea liquid in the presence of a catalyst to form ammonia which disintegrates to form nitrogen and water vapor.
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