Formation inside combustion chamber:
N2(g) + O2(g) → 2NO(g)
(Nitric Acid)
When the NO is released from the combustion chamber, is readily oxidised by O2 molecules:
2NO(g) + O2(g) → 2NO2(g)
The Photochemical smog is the outcome of both primary and secondary pollution. The production of secondary pollutants mainly varies according to the intensity of the sunlight and the concentration of primary pollutants. A high concentration of primary pollutants eg NOx and a high intensity of sunlight would be favourable for photochemical smog because NO2 dissociates into NO and O* Radicals at wavelengths lower than 425nm (Ultraviolet Spectrum).
NO2 + hv → NO + O*
A high concentration of NO2 would be favourable because more NO2 could be dissociated at a given time and so more O* would be produced more quickly. A high intensity of sunlight would be favourable because more energy would be available to dissociate NO2, and so the rate of reaction would be faster. The O* radical released would be a diradical because it would have 2 unpaired electrons. O* is highly reactive and is so a very strong oxidising agent, and will therefore oxidise the primary pollutants to form secondary pollutants. The O* radical produced in the dissociation of NO2 would readily oxidise O2 molecules to form ozone in the lower troposphere:
O* + O2 → O3
As a higher concentration of NO2 is available to dissociate, more O3 will be formed.
The flow chart below shows how NO2 is dissociated and O3 is produced.
A high concentration of unburnt hydrocarbons is also favourable in the production of photochemical smog. An OH* radical is previously produced as shown:
O* + H20 → 2OH*
This is a very significant reaction because one O3 molecule can dissociate to produce one O*, which will in turn produce 2 OH* radicals. These OH* radicals can then react with the high concentration of hydrocarbons to produce peroxy radicals:
RCH3 + OH* → RCH2 + H2O
(Hydrocarbon) (Hydrocarbon)
This RCH2 hydrocarbon then reacts with an O2 molecule to form the peroxy radical:
RCH2 + O2 → RCH2O2*
(Hydrocarbon) (Peroxy Radical)
These peroxy radicals can either form peroxyacetyle nitrates (PAN) or can react with NO to form NO2, which will then in turn dissociate and so on. The peroxy radicals are highly reactive and can react with water to for Hydrogen peroxide.
A high intensity of sunlight is favourable for the production of photochemical smog because O3 molecules dissociate at wavelengths <310nm, which is in the ultraviolet spectrum. A higher intensity would be able to dissociate more O3 molecules at the same time to increase the rate of the production of O* radicals. These O* would then be able to produce the compounds already mentioned.
The sea scrubbing process is used at Longanette. It changes the sulphur into sulphite ions, which are then oxidised into sulphate ions. This process is ideal for Longanette mainly because Longanette is situated on the banks of the Firth of Forth in Scotland where sea water is readily available for the sea scrubbing process. There are also no solid products produced the process to be handles and marketed.
Gas Reburn is used to remove the NOx formed. The NOx given off in the lower parts of the furnace react with methane and ethane to produce safe N2, CO2 and H20 molecules.
CH4(g) + 4NO(g) → 2N2(g) + CO2(g) + 2H2O(g)
Because there may be some remaining alkanes which are not used, the burning zone allows the excess unburnt alkanes to be oxidised to H2O and CO2.
Longanette have chosen the gas reburn method of controlling NOx production because the maximum energy can be released from the coal without incomplete combustion, which produces CO. The NOx is also converted into N2 which is safe and is a major constituent in the air. The alkanes used in the gas reburn are also converted to safe CO2 and H2O.
The chemists have played a major role in the research on photochemical smog. They have developed new ways to reduce NOx emissions from power stations, such as the gas reburn process used at Longanette. Monitoring stations across the country also allow pollutant concentrations to be recorded, and so can warn the population if there is a dangerous level if photochemical smog. The monitoring stations can also allow any current technology to be improved. Chemists have also produced smog chamber simulations. The chemists can use these to monitor the photochemical smog build up and find ways to prevent or reduce the effects of radicals on the atmosphere.
Smog Chamber in Valencia, Spain
