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Production of Photochemical Smog.

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Introduction

Atmospheric Pollution Caused by Coal-Fired Power Stations and Motor Vehicle Engines Production of Photochemical Smog Photochemical smog comprises of 'primary' and 'secondary' pollutants, which are extremely oxidising. Primary pollutants are pollutants created as by-products of chemical processes such as combustion in car engines and industrial factories, and released directly into the atmosphere. These pollutants produced from motor vehicles include nitrous oxides, (NOx) carbon monoxide, (CO) and volatile organic compounds (VOCs) such as hydrocarbons. Secondary pollutants are formed when primary pollutants undergo further reactions. Secondary pollutants from motor vehicles include ozone, nitrogen dioxide (NO2), Hydrogen Peroxide (H202), Peroxylacetyl nitrate (PAN), incompletely oxidised VOCs and Nitric Acid (HNO3). Ozone is formed when the sun shines on a mixture of primary pollutants. (This indicates the origin of the name 'photochemical' smog.) Hydrocarbons are expelled from car exhausts as unburnt fuel, and can be released by evaporation from petrol and diesel fuel. Formation of Primary Pollutants Primary pollutants are often formed by oxidation under high heat during combustion in coal-fired power stations and in car engines. ...read more.

Middle

Other pollutants are produced in the following processes: (Ref 1) NO2 can also react with radicals produced from VOCs in a series of reactions to form toxic products such as PAN. NO2 + R* ? PAN (Ref 5) "The World Health Organisation advise that people's lungs and breathing system can be damaged if the ozone concentration rises above 50 parts per billion (ppb)." (Ref 4) "On 3rd May 1995, this concentration reached 71 ppb over 8 hours in Yorkshire, UK." Many health experts believe this concentration of ozone to be carcinogenic, and periods of smog lasting for 5 days, such as in 1991, can have long lasting negative effects on health. Although its composition varies greatly due to the nature of the primary and secondary pollutants, local geography, time of day, and weather conditions, a photochemical smog can still have devastating effects for those who breathe in the air. Healthy humans can suffer breathing difficulties and soreness in the eyes and nose; if vulnerable people such as the elderly, small children or people with respiratory problems such as asthmatics breathe in smoggy air, the effects can be much more dangerous. ...read more.

Conclusion

Flue gases also contain NOx, (mainly thermal NOx.) Many power stations including Logannet use 'low NOx burners,' which burn at a lower temperature, decreasing the production of NOx. Also, finely-ground coal can be controlled to give lower temperature burning, which gives significantly lower NOx emissions. Using 'gas reburn' techniques, NOx formed are chemically turned into nitrogen with the addition of natural gas into the boilers, above the flame. NOx react to form nitrogen, carbon dioxide and water. The Logannet management chose to implement these procedures to improve efficiency in the power station, to cut waste products, e.g. Wasted Sox can now be turned into useful building products and reduce pollutant release. Research Into Photochemical Smog Scientists have set up monitoring stations in over 20 locations nationwide, to monitor the concentration of ozone and NOx in the troposphere. Chemists measure the rate of formation of photochemical smog, and make predictions about pollution in the future. Using computer modelling, chemists can simulate the behaviour of pollutants. Huge smog chambers (volume 1500m3 to minimise any surface effects) contain primary pollutants which are mixed and exposed to sunlight to measure the formation and concentration of photochemical smog. ...read more.

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