Since the industrial revolution, large amounts of pollutants have been released into the atmosphere.

Fatema Jessa

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Since the industrial revolution, large amounts of pollutants have been released into the atmosphere. Consequently, photochemical oxidants are causing smog, acid rain is being formed, the greenhouse effect is increasing and ozone (O3) is being depleted from the stratosphere. Releasing large quantities of harmful pollutants, fuel-burning vehicles and coal-fired power stations are major sources of pollution.

A direct effect of pollutants is photochemical smog containing primary and secondary pollutants, which is usually seen as a haze in summer. ‘Primary pollutants are released directly into the atmosphere by various processes such as combustion of fuels in cars and power stations.’(1) Secondary pollutants form when primary pollutants react with each other or with other compounds in the air. The major component of photochemical smog is ozone, a secondary pollutant. The main primary and secondary pollutants are listed in Table 1.

Table 1: Primary & secondary pollutants formed as a result of motor vehicles (2)

Coal combustion occurring in coal-fired power plants also produces harmful pollutants including oxides of carbon, nitrogen and sulphur. These are primary pollutants contributing to the greenhouse effect. (Fig 1) (4)

Carbon dioxide is produced as a result of burning coal, composed primarily of carbon that combines with oxygen during combustion producing carbon dioxide. Many coals are contaminated with sulphur(5A) so when burnt to generate electricity, large amounts of sulphur dioxide are produced. During combustion*, sulphur compounds combine with oxygen to produce oxides of sulphur (SOx). Combustion of coal also produces oxides of nitrogen. Thermal NOx, the main component of emissions from coal-fired power stations, forms at high temperatures during combustion when atmospheric nitrogen and oxygen are combined (Reaction 1).

N2 (g) + O2 (g) 2NO (g)

Reaction 1: Illustrates formation of thermal NOx. The resulting NO is oxidised to NO2 in the cool atmosphere.

Photochemical smogs usually form in the summer because ozone (Equation 2) forms when sunlight shines on a mixture of primary pollutants (Equation 3).

O2 + O O3

Reaction 2: Illustrates formation of ozone in the troposphere.

NO2 hv NO + O

Reaction 3: Illustrates the action of sunlight on NO2 molecules. (Sunlight is represented by hv)

Oxygen atoms formed in Equation 3 react with O2 molecules in Equation 2 forming ozone. Ozone then reacts with NO to reform NO2 (Equation 4).

O3 + NO O2 + NO2

Reaction ...

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Reaction 1: Illustrates formation of thermal NOx. The resulting NO is oxidised to NO2 in the cool atmosphere.

Photochemical smogs usually form in the summer because ozone (Equation 2) forms when sunlight shines on a mixture of primary pollutants (Equation 3).

O2 + O O3

Reaction 2: Illustrates formation of ozone in the troposphere.

NO2 hv NO + O

Reaction 3: Illustrates the action of sunlight on NO2 molecules. (Sunlight is represented by hv)

Oxygen atoms formed in Equation 3 react with O2 molecules in Equation 2 forming ozone. Ozone then reacts with NO to reform NO2 (Equation 4).

O3 + NO O2 + NO2

Reaction 4: Illustrates the breakdown of ozone by NO.

All these reactions involve radicals. Radicals are ‘highly reactive atoms or groups of atoms with unpaired electrons’(5B) formed by homolytic fission in which one of the two shared electrons goes to each atom. When radicals react with other molecules a radical chain reaction is formed. Consequently, a steady state is reached and ozone concentration remains the same. However, when air is polluted with hydrocarbons and NOx more ozone forms resulting in photochemical smog. Equations 5, 6 & 7 show the reactions involved.

RCH3 + OH RCH2 + H2O

Reaction 5: RCH3 represents a hydrocarbon whose breakdown is initiated by a hydroxyl radical which acts as a scavenger.

RCH2 + O2 RCH2O2

Reaction 6: The resulting RCH2 then reacts with an oxygen molecule to form the peroxy radical RCH2O2.

RCH2 + NO RCH2O + NO2

Reaction 7: RCH2 reacts with NO to form NO2. Critical step in smog formation since NO2 is regenerated.

As shown by the reactions, NO2 is regenerated and unlike Reaction 4, no ozone is being destroyed thus causing ozone concentrations and therefore photochemical smog to increase. Hydroxyl radicals initiate many reactions because they are unstable hence removing hydrogen atoms from molecules to regain stability as H2O molecules (Reaction 5). However* they also originate from water molecules using up ozone (Reactions 8 & 9) but later doubling the amount of ozone since two OH radicals are formed causing the production of one ozone molecule each (Reactions 5, 6, 7, 3, 2). Thus it is evident that areas with high rates of pollution cause a higher concentration of photochemical smog and tropospheric ozone.

O3 hv O2 + O

Reaction 8: Ozone absorbs light & breaks down forming O2 and O radical.

O + H2O 2OH

Reaction 9: The O radical reacts with H2O to form 2 hydroxyl radicals.

Pollution is not the only cause of high rates of photochemical smog and tropospheric ozone (Table 2), weather is also an important factor. ‘High temperatures, low winds, intense radiation and low precipitation’ (6) all cause levels to rise.

Table 2: Main factors encouraging smog formation & high levels of tropospheric ozone. (6)

Since coal-fired power stations are major contributors to pollution, stations such as Longannet try to minimise harmful emissions, such as SO2 and NOx to lessen environmental damage.

Although coal used by Longannet contains less than 1% sulphur, a considerable quantity of SO2 gases will be produced since a lot of coal is being used. SO2 gas is harmful since it leads to acid rain, is dangerous to our health, and causes visibility degradation. Various methods can be used to minimise SOx emissions, seawater scrubbing being the one used by Longannet. This process takes advantage of the alkaline nature of seawater to absorb acidic gases. In this process, flue gases, (Fig 2) contained in an absorption tower and flowing counter current to seawater*, are passed through seawater.

Heat from the flue gases causes seawater to heat up and gases to cool down. Being naturally alkaline, seawater causes sulphur to dissolve forming sulphite ions (Reaction 10 & 11).

SO2(g) + H2O(l) SO32-(aq) + 2H+(aq)

Reaction 10: Sulphur dioxide dissolves in water to form sulphite ions and H+ ions.

HCO3-(aq) + H+(aq) CO2(g) + H2O(l)

Reaction 11: HCO3- ions present in seawater react with H+ ions formed in Reaction 10 to produce carbon dioxide and water.

Prior to disposal, these ions need to be oxidized to less harmful sulphate ions. It passes through a water treatment plant and seawater is added increasing the pH. Air is supplied to oxidise sulphite ions.

2SO3-(aq) + O2(g) 2SO42-(aq)

Reaction 12: Sulphite ions are oxidised to sulphate ions.

Seawater is discharged to the sea and CO2 is released into the atmosphere. This process is very reliable removing up to 99% of SO2.

The reason Longannet chose seawater scrubbing as their BPEO is not only because of its efficiency but also because it’s much better than other processes such as the limestone process in which flue gas is passed through slurry of ground limestone (CaCO3). This results in the formation of calcium sulphate (CaSO4), i.e. gypsum, in large quantities (Reaction 13).
CaCO3(s) + SO2(g) +1/2O2(g) +2H2O(l) CaSO42H2O(s) + CO2(g)

Reaction 13: Overall Reaction. Principle: SO2 gas is absorbed by limestone and is oxidised by reacting with air in an Absorber Recirculation Tank. (7)

Although gypsum is widely used it’s difficult to compete with prices and quality from other sources. Additionally, large quantities of CO2 would be produced (40 tonnes for every 100 tonnes of limestone). Furthermore, environmental damage will occur since limestone needs to be quarried. (8)

To control formation of thermal NOx Longannet uses gas reburn technique lowering NOx emissions by 70% (9). It consists of 3 stages (Fig. 3). Initially, powdered coal is oxidised in less* than normal air resulting in a lowered combustion rate thus producing less NOx. Natural gas is injected at Stage 2 (methane/ethane) causing NOx to react with alkanes (Reaction 14).

CH4(g) + 4NO(g) 2N2(g) + CO2(g) + 2H2O(g)

Reaction 14: NOx reacts with methane producing nitrogen, carbon dioxide and water.

Excess alkane and CO (result of partial oxidation of hydrocarbons) are oxidised in Stage 3 which is cooler since it’s furthest from the flame.

Gas reburn has been chosen as the BPEO at Longannet because it’s more efficient at reducing NOx emissions than previous methods, e.g. low NOx burners in which air was controlled to give lower temperature flames resulting in a significant reduction of thermal and fuel NOx. Gas reburn is better since not only less NOx is produced but any NOx that is formed is chemically removed making it the BPEO for Longannet. Additionally, extra heat is produced (oxidation of gas is exothermic) which can generate electricity.

To prevent harmful photochemical smogs chemists need to study the reactions involved in the formation of photochemical smog and their speeds. Chemical reactions are investigated in laboratories and their speeds measured under different conditions. These rates are inputted into computers which reproduce and predict behaviour of pollutants in smog. Experiments on a much larger scale, such as smog chamber simulations, are also being conducted in which primary pollutants are mixed in a smog chamber (a huge plastic bag) and exposed to sunlight under controlled conditions. (5C) As* photochemical smog develops, concentrations of various species are monitored by analytical probes. More recently though, ‘bench-top gas chromatography and mass spectrometry systems coupled to smog chambers provide realistic photochemical experiments’ (7) enabling chemists to carry out their studies in conditions more like the atmosphere thus acquiring more reliable data.

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