All living organisms contain sulphur compounds and these are the origin of sulphur found in coal. When coal burns, the sulphur compounds are converted to sulphur oxides (SOx). This is one primary pollutant.
Another primary pollutant is nitrogen oxide (NOx) contained in flue gases. Fuels contain compounds of nitrogen, formed from proteins contained in organisms. When the fuel is burnt, these nitrogen compounds are oxidised to form what is called fuel NOx. Also, at high temperature of combustion, atmospheric nitrogen and oxygen combine to form what is called thermal NOx. This is another primary pollutant.
- Describe and explain the most favourable conditions for forming photochemical smog, and how high concentrations of tropospheric ozone are produced.
The reactions leading to the formation of photochemical smog are initiated by sunlight and involve hydrocarbons and nitrogen oxides emitted from vehicles and power stations. A photochemical reaction occurs when a molecule absorbs light energy and then undergoes a chemical reaction. Photochemical smogs usually occur in the summer during high pressure conditions in the lower troposphere (the part of the atmosphere closest to the ground). The still air means that there is much less mixing with high altitude air and the pollutants are trapped near ground level. If there is a light prevailing wind (shown in the diagram below), the polluted air will be transported from the built up urban area it was generated and will move along rural areas.
This diagram shows the formation
of photochemical smog
The troposphere acts as a huge reaction vessel, with a vast array of chemical reactions taking place all the time. Most of these reactions involve radicals. Some reactions take place when molecules absorb energy from sunlight (this has to be of a specific frequency) and undergo a photochemical reaction. This is indicated by the symbol hv where v is t
O that creates ozone therefore more ozone is produced.
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Describe the chemistry of the processes chosen as BPEOs at Longannet for minimising sulphur dioxide and NOx emissions, and suggest why the Longannet management made those choices.
The more efficient a power station is, the more electricity it produces for a fixed amount of one of the inputs or outputs. Some of the most important decisions a power station manager has to make are concerned with minimising the effects of waste produced from the power station on the environment.
This is known as choosing the best practical environmental option (BPEO). The BPEO for minimising the output of a particular waste material will vary from each power station and Longannet are at the forefront of the development and application of new technologies due to their research into minimising emissions of SOx and NOx.
As Longannet burns such a vast majority of coal the management would have made the choice to try and control SOx emmisions as these were very high and would cause serious environmental damage.
One way to reduce the problem is to react sulphur dioxide in the power stations flue gas with limestone which will form calcium sulphate (CaSO4).This is called the limestone process. Another option is to dissolve sulphur dioxide into seawater.
There are two methods for lowering NOx emissions as well.
- Outline the part played by chemists in the research on photochemical smog formation.
Research is taking place to discover and explain the complex chemistry involved in photochemical smog formation.
- Monitoring of tropospheric pollutants
An important step is to know which pollutants are present in the troposphere and how their concentrations vary. For example, ozone concentrations in urban areas show regular fluctuations reflecting changes in sunlight and vehicle emissions during the day. The diagram below is one way of representing this data.
This diagram shows the variation of the
concentrations of ozone and nitrogen
oxides on a summer weekday in
London
- Studying individual reactions in the laboratory
To make predictions about pollution, chemists need to know what reactions take place and how quickly they occur. Many of these reactions involve broken down fragments of molecules called radicals. Reactions with radicals happen very quickly but other reactions happen very slowly. Chemists measure the length of time of these reactions to predict the rate at which a reaction will proceed for any set of conditions.
- Modelling Studies
The information on rates of reactions is used in computer simulation studies to reproduce and predict the behaviour of pollutants during a smog episode. The more accurate the information used, the more closely the model simulates the observed behaviour.
- Smog Chamber Solutions
These are laboratory experiments on a large scale. Primary pollutants are mixed in a huge clear plastic bag called a smog chamber and exposed to sunlight under carefully controlled conditions. Probes monitor the concentrations of various species as the photochemical smog builds up. The chamber has to be big to minimise any ‘surface effects’ where the reactions take place on the walls of the container instead of the gas phase.