3.2.3 Pollution from Combustion

• Alkanes are combusted (burnt) on a large scale for their use as fuels

Complete combustion

• When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively
  • For example, the complete combustion of octane to carbon dioxide and water

The complete combustion of alkanes

Incomplete combustion

• When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
• Some carbon is only partially oxidised to form carbon monoxide
  • For example, the incomplete combustion of octane to form carbon monoxide

 

The incomplete combustion of alkanes

• Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present
• With a reduced supply of oxygen, carbon will be produced in the form of soot:

• Car exhaust fumes include toxic gases such as carbon monoxide (CO), oxides of nitrogen (NO/NO₂) and volatile organic compounds (VOCs)
• When released into the atmosphere, these pollutants have serious environmental consequences damaging nature and health

Carbon monoxide

• CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death
  • The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide
  • Oxygen is transported to organs
  • Carbon dioxide is removed as waste material from organs

 

The high affinity of CO to haemoglobin prevents it from binding to O₂ and CO₂

Oxides of nitrogen

• Normally, nitrogen is too unreactive to react with oxygen in air
• However, in a car engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place:

N₂(g) + O₂(g) → 2NO(g)

N₂(g) + 2O₂(g) → 2NO₂(g)

• The oxides of nitrogen are then released in the exhaust fumes into the atmosphere
• Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)
• In air, the nitrogen oxides can react with these VOCs to form peroxyacetyl nitrate (PAN) which is the main pollutant found in photochemical smog
  • PAN is also harmful to the lungs, eyes and plant-life

• Nitrogen oxides can also dissolve and react in water with oxygen to form nitric acid which is a cause of acid rain
• Acid rain can cause corrosion of buildings, endangers plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health

Particulates 

• Solid carbon particles (or particulates) released from incomplete combustion clump together to form soot which gradually falls back to the ground
• Particulates can cause several problems:
• If they are inhaled they can damage the lungs and cause respiratory problems
• They can cover buildings and statues, making them look unclean and accelerating corrosion
• They can reflect sunlight back into space reducing the amount of light reaching the earth, this is called global dimming

Pollutants, their Effect & Removal Table