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In urban areas like London motor vehicles, amongst other sources, are the main cause of air pollution. Photochemical reactions produce a cloud of toxic chemicals including ozone and a variety of caustic agents.

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Introduction

SALTERS CHEMISTRY OPEN-BOOK PAPER In urban areas like London motor vehicles, amongst other sources, are the main cause of air pollution. Photochemical reactions produce a cloud of toxic chemicals including ozone and a variety of caustic agents. This is often covering whole cities a photochemical smog. This photochemical smog consists of two criteria of pollutants: primary and secondary pollutants. Primary pollutants are emitted directly from their sources into the atmosphere. Examples of primary pollutants include: carbon monoxide (CO), NOx, sulphur dioxide (SO2), and various hydrocarbons (HxCy), also known as volatile organic compounds (VOC). The prime source of air emissions generated by motor vehicles such as truck diesel emissions is CO, NOx and HxCy Secondary pollutants can be produced over a period of time when photochemical reactions take place in the atmosphere. An example of a secondary pollutant is ozone (O3), which is one of the products formed when NOx react with hydrocarbons (from motor vehicles), in the presence of sunlight. ...read more.

Middle

Below are the outlines of the processes required for the formation of the two most dominant toxic components: ozone and peroxyacetyl nitrate. R represents a hydrocarbon which is primarily created from volatile organic compounds. Nitrogen dioxide can be formed by one of the following reactions. Nitrogen oxide acts to remove ozone from the atmosphere. This mechanism occurs naturally in an unpolluted atmosphere. Formulae adapted from 'Photochemical smog: the killer on a summer's day' O3 + NO O2 + O2 NO2 + RO2 NO2 + other products Sunlight can break down nitrogen dioxide back to nitrogen oxide. NO2 + sunlight NO + O The atomic oxygen formed in the above reaction then reacts with one of the abundant oxygen molecules producing ozone. O + O2 O3 Nitrogen dioxide can also react with radicals produced from the volatile organic compounds in a series of reactions to form toxic products such as peroxyacetyl nitrates (P.A.Ns). NO2 + R products such as P.A.Ns Ozone can be produced naturally in an unpolluted atmosphere. However, it is consumed by nitrogen oxide as illustrated in the first reaction. ...read more.

Conclusion

NOx moving upward from coal combustion in the lower furnace is stripped of oxygen as the reburn fuel is partially combusted in the reburn zone and converted to molecular nitrogen. Over fire air ports above the reburn zone provide for complete combustion in a relatively cooler region of the boiler. Reburning allows the low-NOx burners to operate at excess air levels far below that needed for complete combustion, thus enhancing their effectiveness. The synergistic effect of adding a reburning stage to wall-fired boilers equipped with low-NOx burners was intended to lower NOx emissions by up to 70%. This was probably chosen because it produces heat that can be used to generate even more electricity. The end product of this process is summarised below. Formula from 'Longannet: clean coal power?' CH4(g) + 4NO(g) 2N2(g) + CO2(g) + 2H2O(g) Adapted from http://www.lanl.gov/projects/cctc/factsheets/eerco/images/eerco_schematic.jpg Diagram of gas reburn The part played by chemists in their research is that they implement experiments to predict rate of reactions in different conditions. They do this though modelling studies and smog chamber simulations. Ultimately they are researching to combat pollution through burning fossil fuels. ...read more.

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