Combustion of hydrocarbon fuels

Hydrocarbon fuels can undergo complete combustion or incomplete combustion, depending on the amount of oxygen available.

Complete combustion happens when there is a good supply of air. Carbon and hydrogen atoms in the hydrocarbon fuel react with oxygen in an exothermic reaction:

Carbon dioxide is an atmospheric pollutant. Incomplete combustion produces other pollutants.

Incomplete combustion

Incomplete combustion happens when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced. Less energy is released than during complete combustion.

For example, here is one possible equation for the incomplete combustion of propane:

propane + oxygen → carbon + carbon monoxide + water

C3H8 + 3O2 → C + 2CO + 4H2O


The carbon is released as fine black particles. We see this in smoky flames, and it is deposited as soot. Soot can cause breathing problems and it blackens buildings. It may block boilers and other appliances, or cause a fire.

Incomplete combustion produces smoke and invisible carbon monoxide
Incomplete combustion produces smoke and invisible carbon monoxide

Carbon monoxide

Carbon monoxide is a toxic gas. It is absorbed in the lungs and binds with the haemoglobin in the red blood cells. This reduces the capacity of the blood to carry oxygen. Carbon monoxide causes drowsiness, and affected people may fall unconscious or even die.