Firstly, cars could use cleaner fuels. In Singapore, oil fuels used in factories are forbidden to contain more than 2% of sulfur. Also, the amount of sulfur allowed in diesel fuel for vehicles has been steadily reduced in recent years. In 1996, diesel fuel could contain up to 0.5% sulfur by weight but in 1999, the allowed amount of sulfur was reduced to 0.05%. In Singapore, power stations are now burning more natural gas while buses and taxis are increasingly using compressed natural gas (CNG) instead of diesel. Natural gas is mainly methane and contains no sulfur. The products of combustion are carbon dioxide and water which are non-polluting.
Furthermore, powdered limestone (calcium carbonate) is also added to the hot gases produced from the burning of fuels at a coal or oil burning power station.
The heat decomposes the limestone to give calcium oxide, as represented in the equation below:
CaCO3 --> CaO (s) + CO2 (g)
The calcium oxide then removes the sulfur dioxide, tuning it into calcium sulfite:
CaO (s) + SO2 (s) --> CaSO3 (s)
Limestone is used because of its inexpensive price. About 95% of the sulfur dioxide is removed by these reactions. The calcium sulfite, CaSO3, is reacted with air to convert it into unreactive calcium CaSO4, and then it is dumped.
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