Air pollution and control

Industrial methods of controlling SO₂ emission

• SO₂ is a primary pollutant (ie directly released into the atmosphere) emitted into the atmosphere mainly from combustion of fossil fuels eg coal.
• SO₂ emissions is controlled by several methods so that it can be used to make useful products mainly sulphuric acid:

 

i) flue gas desulphurization

• These are set of technologies that control the emission of sulphur dioxide into atm.
• The disulphurization process include these following methods:

 

ii) Wet H₂SO₄ process

• Includes the conversion of S₈ containing pollutants eg H₂S and S0₂ into commercially and industially useful H₂SO₄.
• The process involves the following steps:

 

(a) Combustion

Aerobic combustion of H₂S to form SO₂. H₂S + O₂→ SO₂ +H₂

(b) Oxidation:

SO₂ is cleaned and reacted with oxygen in the presence of vanadium (v)oxide catalyst at a temperature of 400-500 degrees celcius to form SO₃

(c) Reaction of SO₃ and H₂O (hydration)

SO₃ + H₂O → H₂SO₄(g) ie the reaction produces a mist of sulphuric acid

(d) Condensation reaction

this involves the removal of heat(cooling) of the mist of H₂SO₄(g) to form H₂SO₄(l)

 

iii) Wet alkaline scrubbing

It involves the scrubbing of sulphur dioxide with an alkaline reactant eg limestone thereby removing atm SO₂ eg Ca(OH)₂ + SO₂ → CaSO₄/CaCO₃ + water
In this reaction the acidic sulphur dioxide reacts with the lime to produce calcium sulphate/sulphite solid which can be removed.CaSO₄/CaCO₃ can be used in making sulphuric acid as shown: CaSO₄ + H₂O → H₂SO4 + CaO.

 

iv) Use of limestone based fluidized beds

Flue gases are passed over limestone fluidized beds to reach the emissions of SO₂ into the atm:

CaCO₃(s) + SO₂ → CaSO₄/CaCO₃ + CO₂.
CaSO₄/CaCO₃ face the same fate as met alkaline scrubbing

Calcium carbonate and calcium oxide absorbs sulphur dioxide to produce calcium sulphate which can be oxidized by air to form calcium sulphate

CaCO₃ + SO₂(g) → CaS0₃(s) + CO₂(g)
CaO(s) + SO₂(g)→ CaS0₃(s)
2CaCO₃(s) + O₂(g) +4H₂O(l) → 2CaSO₄.2H₂O(s)

The sludge of hydrated calcium sulphate can be used in cement manufacture, plaster-board etc..

Use of lean-burn engines

in reducing pollutant emissions from petrol-driven cars

• They have a high air: petrol ratio hence efficient combustors of petrol.
• The high air : petrol ratio improve the combustion of petrol and decreases the amount of CO and CxHy from vehicles,
• However it can not control the emission of NOx from vehicles engines hence they still require catalytic converters to act synergestically.

 

Catalytic converters

in reducing pollutant emissions from petrol-driven cars

• Vehicles emissions form primary pollutants of the atm eg NOx, CO and CxHy (unburnt hydrocarbon).
• The reactions invovled are favoured at typical exhaust temperatures.
• They are de-activated by lead i.e it is preferentially adsorbed on the catalyst surface thereby occupying their active sites.
• Cars with catalytic converters use lead-free petrol.
• Catalytic converters are vehicle emission control devices which are of two types;

i) A two way catalytic converter(oxidation catalytic conversion)

• Palladium and platinum promote the oxidation of CO and CxHy to less harmful substances.
• They only catalyze CO an CxHy as shown:
• 2CO(g) + O₂(g) → 2CO₂(g)
• C(s) + O₂(g) → CO₂(g)
• CxHy +O₂ → CO₂(g) + H₂O

ii) A three way catalytic converter( reduction and oxidation catalytic conversion)

• Rhodium promotes the reduction of nitrogen monoxide to nitrogen.
• Redox reactions in conversion of NOx, CO and CxHy into less harmful substances.
• These are effective as vehicle controlling emissions.
• 2NO(g) + 2CO(g) → N₂(g) + 2CO₂(g)

 

Environmental considerations

of the usage and generation of power

• fossil fuels eg coal liberate high amounts of CO , sulphur dioxide which earlier resulting in formation of tropospheric ozone which damages the environment and later causing adverse effects
• use of fossil fuels eg wood leads to the damage of environment by erosion and land degradation
• usage of fossil fuels eg oil leads to ground water contamination and spoilage of oil in oceans and seas which affect the marine environment
• spoilage of oil also affect ecological areas or tourist attraction eg in beaches
• use of nuclear energy to generate power may result in accidents which release radio-active atoms into the environment (soil and water) were they eventually affect the health of animals and human beings
• fossil fuels also emit a high degree of oxides which cause environmental damages

Potential sources of power

• wind
• water-hydroelectricity
• sun-solar energy

Hazards

Associated with random emission from uranium-bearing rocks and with nuclear accidents

• radon is a naturally occurring radioactive noble gas produced from the decay of radioactive intermediates eg thorium and uranium
• radon is colourless and tasteless and is difficult to detect in the environment
• uranium is naturally occurring radioactive in minerals eg the metamorphic rocks
• uranium undergoes radioactive decay and radio-ionization radiation which is dangerous to the environment.
• Ra has serious consequences to health of human and animals where it causes mutations of DNA resulting in lethal effects.
• problems of safe radioactive waste disposal