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Photochemical smog.

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Introduction

Photochemical smog Photochemical smog is a type of air pollution produced when sunlight acts upon exhaust gases (primary pollutants) to form harmful substances (secondary pollutants) such as ozone (O3), aldehydes and peroxyacetylnitrate (PAN). The exhaust gasses, or primary pollutants, are produced by combustion of fossil fuels; they include NOx, volatile organic compounds (VOC's) and CO. When the right frequency of sunlight acts on NO2 it can break apart and form highly oxidising radicals, these react to produce the secondary pollutants.1 NO2 + hv NO + O The main primary pollutants created by a coal fired power station are NOx, SOx and VOC's. Sulphur oxides are created from the burning of the coal. Coal naturally contains sulphur, the amount of which varies depending on which organisms created the coal. When the coal is burnt, so also is the sulphur. ...read more.

Middle

ozone; O�(g) + O2(g) --> O3(g). When the ration of NO2 to NO is greater than 3 the formation of ozone is the dominant reaction. If the ratio is less than 0.3, then the nitric oxide reaction destroys the ozone at about the same rate as it is formed, keeping the ozone concentration below harmful levels. Ozone (O3) absorbs UV light and because of photolysis (chemical decomposition induced by light or other radiant energy) produces OH radicals. OH radicals react with VOC's to produce PAN and aldehydes. O3 + hv O2 + O� O* + H2O --> 2OH� The time of day is a very important factor in the amount of photochemical smog present, morning traffic increases the amount of nitrogen oxides and VOC's in the local atmosphere. Later in the morning the VOC's and nitrogen oxides begin to react, forming NO2 increasing its concentration. ...read more.

Conclusion

When the flue gasses are mixed with the sea water, there is a fast reaction between the SO2 and the CaCO3 in the water. SO2(g) + CaCO3(s) --> CaSO3(s) + CO2(g) The resulting solution is then aerated to oxidise sulphite into sulphate before it is returned to the sea. This method would have been chosen as the BPEO for Longannet because it is up to 99% effective and the power station is close enough to seawater for it to be viable. Gas reburn has recently become the most recommended method of reducing NOx emissions in power stations. It is relatively cheap and effective on a large scale. It works by burning natural gas (methane or ethane) in a chamber just above the primary combustion zone. This reduces the NOx to N2 which can be released safely into the atmosphere. CH4(g) + 4NO(g) --> 2N2(g) + CO2(g) + 2H2O(g) The reason for Longannet adopting this method of reducing NOx emissions is due to the more effective than the low NOx boilers that they had used before. ...read more.

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