Other pollutants can also be formed. When incomplete combustion takes place, there is not enough oxygen to make CO2, so carbon monoxide (CO) is formed. Alkane gases are also produced, when the carbon in the coal reacts with hydrogen in the air. [2]
Describe and explain the most favourable conditions for forming photochemical smog, and how high concentrations of tropospheric ozone are produced (9)
Photochemical smog needs ‘reactants’ to form, which are Primary and Secondary pollutants. Like any other reaction, it needs the right conditions. Secondary pollutants form from primary pollutants undergoing further reactions. These reactions usually happen due to the photodissociation of primary pollutants, the products of which will react to make the secondary pollutants. For example the oxygen atom that reacts with a dioxygen molecule to form ozone comes from the photodissociation of NO2. [1]
Therefore sunlight is required, as the sun is the main source of ultraviolet radiation for photodissiociation reactions. Some chemical reactions also need a catalyst. This ‘reaction’ to form smog also needs a catalyst. It needs wind. The wind mixes all these chemicals, and increases the chance of primary pollutant particles colliding and reacting to form secondary pollutants. The reason why this helps is explained by the particle theory. [1]
Photochemical smog creates a higher concentration of ozone. Two reactions taking place to destroy and produce happen at the same time, and eventually, it reaches a steady state. [1]
(2) O2 + OO3 (Forms ozone)
(3) O3 + NOO2 + NO2 (Destroys ozone)
Therefore in photochemical smog, where there is more NO2 present, more oxygen atoms will be formed by photodissociation of NO2, therefore there will be more oxygen atoms to react with dioxygen molecules to form ozone in reaction (2). [1]
This works as a positive feedback, because when ozone is broken down by Nitrogen Monoxide (like in reaction 3) nitrogen dioxide is produced. So the original NO2 that helped form the ozone, is now recycled, so it joins the extra NO2 that is already there. [1]
Also when hydrocarbons are broken down, they form a peroxy radical, which reacts with NO to produce even more NO2. So now there is the NO2 from the exhaust fumes, the recycled NO2 from reaction (3) and now also from this reaction of the peroxy radical. [1]
Hydroxyl radicals initiate the break down of hydrocarbons, which leads to more NO2 and then Ozone. When ozone is broken down by photodissociation, an oxygen radical is formed that reacts with water to form two hydroxyl radicals, therefore even more help with ozone formation. [1]
Describe the chemistry of the processes chosen as BPEO’s at longannet for minimising sulphur dioxide and NOx emissions, and suggest why the longannet management made those choices.
The process for sulphur dioxide (SO2) removal at Longannet is sea water scrubbing, where the SO2 is dissolved in seawater (which is slightly alkaline, about pH 7.5). The SO2 dissolves to give sulphite ions. However sulphate ions are preferred over sulphite ions for disposal at sea, so the water is aerated to oxidise the sulphite ions to sulphate ions. At the end of this process, the seawater becomes slightly acidic (about pH6), but when it’s returned to the sea, the difference in pH is negligible. [2]
Longannet uses this process because it is near the sea, so sea water is readily available. There are no solid wastes that need to be disposed, and there are no by-products that need handling. [2]
The BPEO for NOx removal at Longannet is the gas reburn system. In this system, less NOx is formed, and the NOx that is formed is chemically removed. This system uses 3 burning zones in the boiler furnace. The diagram below shows what happens in each zone.
DIAGRAM
The coal dust is burnt in little air, so that the combustion rate is lower, so less NOx is produced. The bit that actually removes the NOx is in zone 2, where natural gas is added, like methane and ethane. These gases react with the NOx to produce nitrogen gas, carbon dioxide and water vapour. A reaction with methane is shown below: [2]
CH4(g) + 4NO(g) 2N2(g) + 2H2O(g)
Any natural gases left is oxidised in zone 3, and if it is an alkane, like methane, it produces carbon dioxide and water. Also the oxidation of these gases are exothermic, therefore it gives out energy, which can by used to generate electricity by Longannet. [2] & [3]
The advantages of this process are that all the NOx is removed, and that extra energy is generated by the combustion of the gases.
Outline the parts played by Chemists in the research on Photochemical smog
Chemists monitor pollutants to find out exactly what pollutants are involved in smog formation, and how they vary in concentration. These changes in concentration can show changes in the atmosphere, for example the presence of sunlight.
Chemists study reactions to see which pollutants react with which. Most importantly, to see which radicals are formed where, because they are very reactive, and cause a lot of atmospheric reactions. The speed of these reactions need to be measured to understand how fast substances are being made and destroyed.
Chemists can makes models of situations, to predict what will happen in the future. One such model is smog chamber simulations. These are huge plastic bags which are exposed to sunlight under controlled conditions. Analytical probes monitor the concentrations of different gases as the photochemical smog forms.
