8.11 - How burning fuels produces sulfur dioxide

The sulfur source

A pure hydrocarbon contains carbon and hydrogen only. Real hydrocarbon fuels can also contain small amounts of impurities, including sulfur-containing compounds mixed with the hydrocarbons.

The fuel mixture can therefore contain sulfur even though its hydrocarbon molecules do not. A sulfur-containing organic compound is not itself a hydrocarbon. Some fuels contain sulfur impurities; this is not a property of every hydrocarbon fuel.

Oxidation during burning

During combustion, sulfur in the impurities reacts with oxygen from the air to form sulfur dioxide, SO₂. It is oxidised. The sulfur atom comes from the fuel impurity; the oxygen atoms come from oxygen in the air.

sulfur + oxygen → sulfur dioxide

For elemental sulfur, the simplified balanced equation is:

S + O₂ → SO₂

There is one sulfur atom and two oxygen atoms on each side. Sulfur compounds in real fuels also supply sulfur for SO₂; their full combustion equations depend on their other elements.