Photochemical smog can be characterised as a faint brown colour, this is due to nitrogen oxide and they are also responsible for the production of ground level ozone. Most NO produced by power stations is thermal NOx, produced when atmospheric nitrogen and oxygen react at high temperatures; N2(g) + O2(g) → 2NO(g).
VOC's are hydrocarbons that have not fully combusted. They are released along with other gasses and react to create highly toxic gasses.
Ideal conditions for photochemical smog are obviously areas of high vehicle emissions or other dense concentrations of primary pollutants.
Where there is plentiful sunlight then a number of reactions will take place;
NO2(g) hv NO(g) + O• (g)
The oxygen radical formed will react with an oxygen molecule to form tropospheric (the lowest region of the atmosphere between the earth's surface and the tropopause, characterized by decreasing temperature with increasing altitude) 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. Later in the day the NO2 is broken down by the sunlight and its by-products (oxygen radicals) begin forming tropospheric ozone. At the same time, some of the NO2 will react with VOC's and produce toxic chemicals such as PAN.
Factors which limit photochemical smog's are:
- Precipitation
- Winds
- Geography (smog's more common in valleys)
- Sunlight
- Temperature
The temperature must be at least 18 degrees Celsius for the necessary reactions to take place, also the lower the temperature, the closer the harmful chemicals will be to the surface.
The best practical environmental option (BPEO) is the most effective way of reducing harmful outputs in relation to the input and energy produced. The BPEO for a coal fired power plant involves limiting the amounts of Sulphur dioxide and Nitrogen oxides.
The seawater scrubbing process exploits the natural alkalinity of seawater to absorb acidic sulphur dioxide gases. 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.
References
- The Salter Advanced Chemistry course material
- Google pictures, name: Photochemical smog
