Combustion of Fuels

The fire triangle is a model used to explain the three requirements for combustion to occur.

The three components are: fuel (the substance that burns), oxygen (from the air), and heat (the activation energy needed to start the reaction).

Removing any one of these three stops combustion. The correct answer is fuel, oxygen, and heat.

The choice “Fuel, nitrogen, and heat” includes nitrogen. Nitrogen makes up 78% of air but does NOT support combustion — it is unreactive under normal conditions. Eliminate the choice “Fuel, nitrogen, and heat”.

The choice “Fuel, carbon dioxide, and heat” includes carbon dioxide, which is a product of combustion, not a reactant required to start it. Eliminate the choice “Fuel, carbon dioxide, and heat”.

The choice “Carbon, oxygen, and water” replaces fuel with carbon, and includes water. Water is a product of combustion, not a requirement. Eliminate the choice “Carbon, oxygen, and water”.

The choice “Fuel, oxygen, and heat” — fuel, oxygen, and heat — correctly identifies all three components of the fire triangle.

Complete combustion occurs when a hydrocarbon fuel burns in a plentiful (excess) supply of oxygen.

In complete combustion, all carbon atoms are fully oxidised to ${\text{CO}}_2$ and all hydrogen atoms are oxidised to ${\text{H}}_2\text{O}$.

The only products are carbon dioxide (${\text{CO}}_2$) and water (${\text{H}}_2\text{O}$).

Carbon monoxide (CO) is produced during incomplete combustion (limited oxygen), not complete combustion. Eliminate the choices “Carbon monoxide and water” and “Carbon dioxide and carbon monoxide”.

Soot (carbon particles) is also a product of incomplete combustion only. Eliminate the choice “Soot and water”.

The choice “Carbon dioxide and water” — carbon dioxide and water — correctly describes the products of complete combustion.