8.3.3 Reducing Sulfur Oxide Emissions

• The removal of sulfur from fossil fuels can either take place pre-combustion or post-combustion
• The oxides of sulfur, SO₂ and SO₃, are both acidic and toxic gases 
• Sulfur dioxide is produced naturally during volcanic eruptions, but large quantities have been and continue to be emitted by burning coal, oil and natural gas

Pre-combustion

• Pre-combustion of sulfur takes place for coal and petroleum, although it is expensive to remove all the sulfur, so a small percentage often remains
  • For example, the average sulfur content of gasoline is 347ppm (this is the same as 347 mg per litre)
• It is essential to remove most of the sulfur as it damages the workings of internal combustion engines
• The sulfur is removed by reacting it with hydrogen in a process called hydrodesulfurization
• The sulfur is recovered and used in the manufacture of sulfuric acid

Post-combustion

•  Post-combustion is carried out on in coal-fired power stations
• The waste gases from burning the coal contain sulfur dioxide
• The waste gases are passed through a wet slurry of calcium oxide and calcium carbonate which react with the SO₂ and produce calcium sulfate

CaO (s) + SO₂ (g) + ½O₂ (g) →  CaSO₄ (s)

CaCO₃ (s) + SO₂ (g) + ½O₂ (g) →  CaSO₄ (s)  + CO₂ (g)

• The calcium sulfate is also known as gypsum and is used to make plasterboard and other useful building materials

What does the future hold?

• Global policies working towards combating global warming will have the additional benefit in reducing acid deposition
• As we switch away from burning fossil fuels for energy there will be a fall in the emission of oxides of sulfur and nitrogen, which can only be a good thing for the environment
• Ultimately reducing the emission of primary pollutants is achieved by greater use of renewable energy sources, greater use of public transport and more efficient energy transfer systems