Photochemical smog.

O2 + O                   O3

The oxygen radical needed for the formation of ozone is produced when nitrogen dioxide ( a primary pollutant) absorbs energy from the sunlight:

                                                                           hv

NO2                      NO + O

The ozone formed in the reaction is however, used up by Nitrogen oxide to form nitrogen dioxide, this is ozone depletion:                                                                             (202)

O3 + NO                  O2 + NO2

The rate in which ozone is produced is balanced by the rate at which it is used; therefore the concentration of ozone in the troposphere remains constant.

However when primary pollutants are formed, and the hydrocarbons react with the oxides of nitrogen, an oxidation reaction occurs which breaks down the hydrocarbons into carbon dioxide and water in the troposphere.

RCH2O2 + NO                  RCH2O + NO2

This reaction converts NO back to NO2 . This means that ozone is no longer required to form nitrogen dioxide because of the addition of the primary pollutants, and so ozone is not depleted and the concentration of ozone in the troposphere increases. Ozone breaks down due to the absorption of light energy to form an oxygen radical:

                                          hv

O3                O2 + O

This then reacts with water:

O + H2O                    2OH

One molecule of O3 produces two OH radicals, which in turn produces one O3 molecule. Therefore a chain reaction occurs in which O3 is being readily used up, however two molecules of O3 are produced due to this and so the concentration of ozone in the troposphere increases.                 (170)

Figure 2 shows formation of photochemical smog and its effects

The effects on the environment as a result of sulphur and nitrogen emissions have been recognised, and at Longannet best practical environmental options (BPEO’s) have been developed. One way of minimising sulphur emissions is the BPEO, sea water scrubbing. This involves, the flue gasses passing through sea water at an alkaline pH of 7.5. This causes the sulphur to dissolve froming sulphite ions. By aerating the water the sulphite ions are oxidised, to form more harmless sulphate, which can be deposited at sea. After the sulphur dioxide has been removed by this process, the flue gasses are reheated and released into the environment, however causing less environmental damage.

Another way that is used at Longannet is to react the sulphur dioxide with a solution of limestone to form calcium sulphate. The Longannet management have decided to use sea water scrubbing as their BPEO as it produces no solid waste matter, no by products needed to be marketed and sea water is in vast supply and it can be returned because of Longannet’s location. Also, the limestone process requires huge amounts of limestone and releases great quantities of carbon dioxide.                 (758)

One way of minimising nitrogen oxide emissions is to reduce the temperature of combustion then less thermal NOx will be produced. However at lower temperatures the conversion of heat energy to electrical energy is lower. Therefore the BPEO is gas reburn, which removes NOx. Firstly, less NOx is produced due to a reduced combustion rate because the coal is oxidised in less air. Inside the boiler natural gases are added. These react with the NOx to produce nitrogen, carbon dioxide and water vapour:

CH4 + 4NO                2N2 + CO2 + 2H2O

Any excess alkane and carbon monoxide is removed by oxidation. Longannet management decided to use gas reburn as their BPEO because it produces a cleaner waste gas which has minimal effect on the environment and as the oxidation of natural gas is exothermic, more electricity is produced.        ***

Research is being undertaken by chemists to understand which pollutants are present in the troposphere and how their concentrations vary. Monitoring stations have been set up to develop their research further. Chemists are researching reactions of photochemical smog and how quickly they occur in order to make predictions about pollution. This is achieved by measuring reaction rates under a variety of carefully controlled conditions. Further technological advances are being looked into to develop more accurate information on computers to predict the behaviour of pollutants. Practical research in laboratories using cutting edge technology such as probes that monitor concentrations are helping to further develop the knowledge of chemists about photochemical smog.

Bibliography