SOx emissions
All living organisms contain compounds of sulphur which are the origin of the sulphur found in coal. When coal burns, the sulphur compounds are converted to oxides of sulphur.
NOx Emissions
The flue gases in the power station contain oxides of nitrogen (NOx). This is because fuels contain compounds of nitrogen formed from the proteins contained in organisms. When the fuel is burnt, these nitrogen compounds are oxidised to form fuel NOx .At the high temperature of combustion, atmospheric nitrogen and oxygen combine to form thermal NOx .
Favourable conditions for photochemical smog
Certain conditions are required for the formation of photochemical smog. These conditions include:
1. A source of nitrogen oxides and volatile organic compounds. Industrialization and transportation create these pollutants through fossil fuel combustion. [1]
2. The time of day is a very important factor in the amount of photochemical smog present. [1]
3. Several meteorological factors can influence the formation of photochemical smog.[1]
4. Topography is another important factor influencing how severe a smog event can become. [1]
Formation of OZONE
Ozone is present in the air that surrounds us. It is formed when hydrocarbons and nitrogen oxides (NOx) from forests, industries, and automobile exhaust react with heat and sunlight. This tropospheric ozone is often called "bad" ozone because it damages living tissue. Some tropospheric ozone is natural. Lightning and static discharges are one natural source of tropospheric ozone. Some ozone is also produced when natural hydrocarbons formed by trees and other vegetation react with nitrogen oxides in the atmosphere and sunlight.
O2+O → O3
Ozone reacts with NO to form NO2
O3+NO → O2+ NO2
Processes to control SO2 and NOx
A power station manager should make some important decisions which are concerned with minimising the effects of the waste, from the power station, on the environment. This is known as Best Practical Environmental Option(BPEO). The BPEO may vary in different power stations.
The sulphur dioxide emissions from the power station can be removed by reacting it in the power stations flue gas with limestone, eventually forming calcium sulphate (CaSO4). It involves passing the flue gas though slurry of ground limestone(CaCO3). The calcium sulphate formed is then aerated to oxidise the sulphite ions to sulphate ions.
2 CaCO3(s) + 2SO2(g) + H2O
(CaSO3) 2 . H2O(s) + 2CO2(g)
2(CaSO3) 2.H2O(s) + O2 +H2O
2(CaSO4) 2.2H2O(s)
Another way of removing sulphur dioxide emissions is by the seawater scrubbing process . This method however depends on whether the station is near enough to the sea to make seawater available.
However at the Longannet power station they have chosen a method known as gas reburn which is the best to remove NOx emissions. In this process, not only less NOx is produced but is also chemically removed. There are three burning zones in the boiler furnace.
In the primary combustion zone, powdered coal is oxidised in less air than normal and therefore less NOx is formed. Natural gas is injected into the boiler above the flame in the reburn zone. The NOx reacts with the alkanes to produce nitrogen, CO2 and water vapour
CH4(g) + 4NO(g) → 2N2 (g)+ CO2(g)+ 2H2O (g)
Any excess alkane and any CO produced are oxidised in the cooler third burning zone, away from the flame. In addition the heat produced contributes to the generation of electricity. This method is adapted mostly because it delivers low cost power and causes minimum spoiling of the environment.
Research on photochemical smog formation
Research work is taking place on several fronts to discover the chemistry of the formation of photochemical smog
1. Monitoring of tropospheric pollutants
An important step is to know which pollutants are present in the atmosphere. Several monitoring stations which record pollutant concentrations are now plenty in the country.
2. Studying individual reactions in the laboratory
To make predictions about pollutions, chemists need to know what reactions can take place and how quickly they occur. They have to measure the rate at which each reaction takes place under a variety of conditions from which they can predict how the rate of a reaction will proceed for different conditions
3. Modelling studies
The information on rate of reactions is used in computer simulation to reproduce and predict the behaviour of pollutants in a photochemical episode. Accurate information is required for close simulation if the observed behaviour.
4. Smog chamber simulations
These are laboratory experiments in which primary pollutants are mixed in a huge plastic smog chamber and exposed to sunlight under carefully controlled conditions. The concentration of various species is monitored as the photochemical smog builds up.
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Bibliography
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