Photochemical smog, which can form from a mixture of ozone and other chemical substances, needs specific conditions to develop. It forms when emissions from power stations such as NOX enter sunlight and form ground level ozone. The smog forms best in anticyclones which are areas of high pressure where there are large amounts of sunlight (used to form the 03) due to little cloud cover and very light prevailing winds which take the smog to rural areas causing more damage (reference 2). The anticyclones can cause a rise to still air where there is little mixing with higher air meaning the smog is trapped lower in the atmosphere. The map bellow shows the air quality in the USA:
(ref 3)
Large amounts of tropospheric ozone have to be produced to cause health problems (more than 80 ppb averaged over an eight-hour period is thought to cause harm). Ozone is formed from UV radiation breaking O2 molecules into atomic Oxygen (a single atom) which has large amounts of energy and so is able to join with the O2:
O2 + sunlight O + O
This now means that when one of these single oxygen’s come into contact with an O2 at high energy they form O3:
O + O2 O3
This formation of ozone can be reversed with the reactions:
O3 + Sunlight = O2 + O
And
O3 + O O2 + O2
This means there is a constant level of ozone being formed and destroyed. To gain large amounts different reactions occur. This can include the splitting of NO2 to form NO + O which leaves a single oxygen free to form O3 as in the above reaction. Bellow shows the reaction using NO2 to form O:
NO2 + sunlight NO + O
Ozone can also be formed through VOC’s (Volatile organic compounds) such as hydrocarbons, which are released from things such as car exhausts:
NOx + VOC + sunlight O3 (and other products)
(Ref 4)
At Longannet coal firepower station, things are done to try to reduce the emissions of sulphur and nitrogen oxides. To reduce sulphur emissions they use a process called seawater scrubbing. This is done by passing gases emitted from the power station through seawater. As the water is slightly alkaline the sulphur dioxide dissolves in the solution forming sulphur ions. The water has air passed through it, which oxidises the sulphite to more harmless sulphide:
S03 + O SO4
This used sea water is then returned back to the sea. After this the remaining flue gasses are reheated so they rise up and out into the atmosphere. I think this process was the BPEO at Longannet because it means there are no solid wastes, and no products to be handled and marketed like in the alternative limestone process, which produces calcium sulphate, which is known as gypsum (ref 6). Also there is nearby seawater to use in the process.
Nitrogen oxide emissions at Longannet were first reduced by Low NOX burners. This worked by reducing the temperature of combustion meaning the rate of thermal NOx would be lessened due to the fact that reaction rate increases with higher temperature. This was done by using finely ground coal meaning temperatures wouldn’t have to be as high for combustion, this is due to a smaller surface area.
However this produced problems, as it had to be made sure that the control of the air supply did not reduce the steam temperature in the boilers and reduce the amounts of completely oxidised coal. This meant another way was developed at Longannet called gas reburn.
This involved 3 burning zones, a primary combustion zone, a reburning zone and burning zone. In the primary combustion zone powdered coal was oxidised in less air than normal meaning the combustion rate is lowered so less NOx is formed. In the next area there is an injection of natural gas meaning the NOX reacts to form nitrogen, carbon dioxide and water vapour as shown bellow:
CH4 (g) + 4NO(g) 2N2 (g) + CO2 (g) +2H20(g)
In the next and final zone air is added meaning any alkanes and CO are oxidised. This is shown in the diagram bellow:
In addition this also helps the generation of electricity, as the oxidation of natural gas is exothermic meaning heat is given out which contributes to the furnace temperature.
I think this process was chosen due to the fact it is considerably better at reducing emissions as it produces less NOX and any that is formed is chemically removed. It also doesn’t cause any other large amounts of damage to the environment, which again makes it the BPEO for NOX emission reduction.
Chemists have played a large part in the reduction of photochemical smog formation. They have researched into measuring rates of reactions under different conditions meaning they can make predictions about pollution. They have also done large scale simulations involving producing huge smog chambers up to 1500 m3 in size to see the effects of pollution. This is a very important area of research as it can help us see what we need to do to reduce pollution on the earth.
