Fuels and Combustion

Fuels can be separated into two categories:

1. Fossil fuels – Includes coal, oil and gas

2. Renewable fuels – Includes wood and biofuels


Combustion is a more scientific word for burning. When fuels burn, energy transfers to the surroundings. The two types of combustion are complete combustion and incomplete combustion.

Complete combustion

In a complete combustion reaction, a fuel reacts with oxygen to produce carbon dioxide and water. For example:

Methane + OxygenCarbon dioxide + Water

Or more specifically

CH4 + 2O2CO2 + 2H2O

We use many different fuels in our every day life, such when we use vehicles. To move, vehicles burn fuels such as petrol and diesel. These fuels contain hydrocarbons (molecules that are made up of hydrogen and carbon). The product formed when hydrocarbons burn are carbon dioxide and water.

Sulphur dioxide can be produced during combustion, when fuels contain sulphur, The equation for this reaction is:

Sulphur + OxygenSulphur dioxide

Nitrogen oxides can also be produced in a hot car engine. The equation for this reaction is.

Nitrogen + OxygenNitrogen oxide

Incomplete combustion

Incomplete combustion occurs when there is not enough oxygen for complete combustion to take place, so the fuel does not fully burn. For example:

Hydrogen + OxygenCarbon monoxide + Water + Carbon

Other products are also formed, such as carbon monoxide and carbon particles (smoke and soot).

The problem with producing carbon monoxide is that…

  • It is a poisonous gas – Which means it is toxic and can kill people

The problem with producing carbon is that…

  • It comes in the form of small particles (which we call particulates) – Which can cause global dimming, – As when it builds up in the atmosphere, less sunlight will penetrate through to the Earth’s surface

Hydrogen Fuels

An alternative fuel that we can burn is hydrogen, which is a much cleaner alternative. As you can see in the chemical equation below, when hydrogen reacts with oxygen in a combustion reaction, the only product is water.

Hydrogen + OxygenWater

H2 + O22H2O

In this case, there is no release of carbon dioxide, which contributes to global warming or any other harmful gases that lead to environmental problems.

Therefore, in the future, hydrogen fuel may be used more for combustion compared to hydrocarbons.