In the combustion of coal the nitrogen compounds are oxidised to form the fuel NO×, which is a primary pollutant.
Another way this primary pollutant is formed is when at the high temperatures of combustion, atmospheric nitrogen combines with oxygen to from thermal NO×. Most NO× emissions are thermal so it is this way of Nitrogen combing with oxygen at high temperatures that result in the largest quantities of NO×.
SO× emissions happen in the same sort of way. As all living organisms contain sulphur in some form when the coal burns in the power station, sulphur compounds are converted to oxides of sulphur (SO×).
There are some factors that can seriously affect air pollution. These are; Wind as it can disperse pollutants from there sources (cities), Sunlight, as the majority of secondary pollutants require a lot of sunlight for their formation, Atmospheric pressure is another, the presence of an inversion (very stable air) results in the air being trapped in a region and the worst pollution episodes can occur. Finally, the mixing layer is a factor that will affect air pollution, the larger the layer the weaker the concentration of pollution.
There are certain conditions, which make it most favourable for photochemical smog to from. One of these is large sources of pollution very close to each other. Such as lots of power stations or a big city with many cars.
Another is a stationary high-pressure system, this results in clear skies and lots of sunshine with only light winds. This means secondary pollutants will be formed easily due to the sunshine and there won’t be to much wind to disperse the smog which means it will be of a heavy concentration in one place.
A further condition is a strong inversion as it caps the atmosphere that will result in a very small mixing layer.
Finally, valleys as they exaggerate the small mixing region. (2)
These conditions are outlined in the diagram below.
(3)
Tropospheric ozone can act both as a direct greenhouse gas and as an indirect controller of greenhouse gas lifetimes. As a direct greenhouse gas, it is thought to have caused around one third of all the direct greenhouse gas induced warming seen since the industrial revolution. High concentrations of it is largely due to an influx from the stratosphere, however a large source is produced by the photochemical reactions, the amounts increasing with high levels of air pollution. This is shown in the reactions below:
As we know a main source of primary pollutants are power stations however, one power station, Longannet has methods of taking ‘the best environmental option’ (BEPO) when it comes to pollutant emissions.
To control its SO× emissions Longannet uses a process called ‘seawater scrubbing’. This is where SO× is put into contact with seawater and there is a fast and efficient reaction between the SOx and Calcium Carbonate (CaCO3) in the seawater, to form Calcium Sulphate (gypsum) and CO2. The reaction neutralises the acidity of SOx, and consumes some of the buffering capacity of the seawater. The reaction is complete in a very short time.
“The key environmental benefit of scrubbing is that it short circuits the whole SOx cycle, and returns the sulphur to the sea in the safest, quickest and least harmful way.”(4)
As you can imagine for a Power station to use this method of controlling SO× emissions it must be relatively close to the sea. Longannet uses this method as its BPEO seawater is readily available as the station sits on a bank called ‘Firth of Fourth’ and the water can be easily returned to where it came from.
At Longannet for controlling their NO× emissions they have the choice of using one of two methods. One is to use low NO× burners. These allow the temperature of combustion to be lowered, which results in less thermal NO× being produced.
Another is known as Gas reburn. In this process the NO× gases are still produced but are chemically removed later by reacting them with alkanes to produce Carbon Dioxide, water vapour and Nitrogen. This is shown in the reaction below:
Longannet like most power stations used low NO× burners to begin with as that’s what the power station was originally fitted with. However, now after increased testing the gas reburn method is used mostly at Longannet as that is now seen as the best BPEO.
Chemists play a huge part today’s research on photochemical smog and have done for the past few years. Chemists have strong research programmes in atmospheric chemistry ranging from laboratory and field measurements to global numeric modelling. Chemists today now monitor troposheric pollutants to get an accurate idea of way the pollutants react and their concentrations in the troposphere.
They also study individual reactions, which they simulate in the laboratory. This allows them to make a record of the types and times of reactions that take place. This means chemists can predict the rate at which a reaction will happen in given conditions. A major experiment done by chemists is the monitoring the behaviour of photochemical smog in a smog chamber. This is where “Primary pollutants are mixed in a huge plastic bag (smog chamber) and exposed to sunlight under carefully controlled conditions” (5)
All these things are being done by chemists on a wide scale to increase our knowledge on photochemical smog and its behaviour.
Source Index
- Assisting source Storylines, DF5 pg 33-35
- Green House Gases article. Times
- Assisting source www.bbc.co.uk/searchlogannetinfo
- Storylines, DF6