Photochemical smog is a result of photochemical reaction involving primary pollutants. The time of day also can affect the production of photochemical smog
Adapted from:
http://royal.okanagan.bc.ca/mpidwirn/atmosphere.html
The graph above illustrates the diurnal variations of NO, NO2 , and O3 typically detected in a photochemical smog situation. The morning rush hour peak in the NO emissions is followed by the gradual conversion to NO2 and the subsequent rise of O 3 which decays as the sun goes down in late afternoon.
For photochemical smog to occur there must be abundance of atmospheric pollutants, previously mentioned, and certain environmental conditions. To begin the chemical process of photochemical smog development of the following conditions must occur:
Sunlight.
The production of oxide of nitrogen NOx.
The production of volatile organic compounds (VOCs).
Temperatures greater than 18 oC.
If those criteria are met, several reactions will occur, producing toxic chemical constituents of photochemical smog. 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. The introduction of a volatile organic compound result in an alternative pathway for the nitrogen oxide, still forming nitrogen dioxide but not consuming the ozone, and therefore tropospheric ozone concentrations can be elevated to toxic levels.
At the Longannet power station in Scotland measures have been taken to reduce Sulphur dioxide and NOx emissions as a result of burning coal. At this power station the best practical environmental option (BPEO) they have chosen for minimising sulphur dioxide emissions is the sea water scrubbing of flue gasses, and for minimising NOx emissions, gas reburn.
The seawater scrubbing process exploits the natural alkalinity of seawater to absorb acidic gases. Flue gases are contained in an absorption tower where they flow counter current to seawater. The heat of the flue gas causes the seawater to be heated and the gases cooled. During this process SO2 is absorbed by the seawater, before passing to a water treatment plant where further seawater is added to increase the pH. Air is supplied to oxidise the absorbed SO2 to sulphate and to saturate the seawater with oxygen. The seawater is then discharged to the sea. This system is a simple and inherently reliable, which can remove up to 99% of SO2,. However, heavy metals and chlorides are present in the water released to the sea. This option was most likely chosen by the management team at Longannet because the power station is near the sea and therefore sea water scrubbing would be the most feasible option.
Gas reburning involves injecting natural gas above the main coal combustion zone in a boiler. This upper level injection and partial combustion, by limiting available oxygen, creates a fuel-rich zone. 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.
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References:
Articles 1 &2 from open book paper
Longannet: clean coal power? + Photochemical smog: the killer on a summer day
Salters advanced chemistry, ‘Chemical Storylines’, Heinemann, Pages:33-35
http://www.bartleby.com/65/ai/airpollu.html
http://www.energy.org.uk/EFCoal.htm
http://ncf.davintech.ca/freeport/social.services/eco/info/primer/smog/menu
http://www.lalung.org/cleanair/whatispollution.htm
http://www.kingston.ac.uk/~ku05346/atmospheric/smogpic2.htm