Complete and incomplete combustion of fuels

Many fuels contain carbon and hydrogen. When these fuels burn, the products depend on both the elements in the fuel and the amount of oxygen available during combustion.

Complete combustion

When there is enough oxygen, the carbon in the fuel is fully oxidised to carbon dioxide, $$\ce{CO2}$$, and the hydrogen forms water vapour, $$\ce{H2O}$$. This is called complete combustion.

Incomplete combustion

When the oxygen supply is limited, combustion can be incomplete. This can also happen when the fuel and oxygen do not mix well, leaving some of the fuel without enough oxygen to burn completely.

In incomplete combustion, some carbon is only partly oxidised, forming carbon monoxide, $$\ce{CO}$$, rather than carbon dioxide. Some carbon may remain as solid particles called soot. Some hydrocarbons may also escape without burning and can contribute to particulates in the atmosphere. Carbon dioxide and water can still form during incomplete combustion.

To predict the products, consider both the fuel and the conditions: carbon and hydrogen in a well-oxygenated flame produce carbon dioxide and water, while limited oxygen or poor mixing can also lead to carbon monoxide, soot and unburned hydrocarbons.

Sulfur dioxide from sulfur-containing fuels

Some fuels contain sulfur, either as an impurity or as part of their composition. When such a fuel burns in oxygen, its sulfur reacts with oxygen to form sulfur dioxide, $$\ce{SO2}$$.

The reaction of sulfur with oxygen can be represented as:

This equation shows sulfur reacting with oxygen; it does not show the combustion of the rest of the fuel.

When predicting combustion products, include sulfur dioxide if the fuel contains sulfur and is burned in oxygen. A fuel that does not contain sulfur will not produce sulfur dioxide from the fuel itself.

Oxides of nitrogen from combustion in air

When a fuel burns in air, the high temperature of the flame or engine can cause nitrogen and oxygen from the air to react. This produces gases called oxides of nitrogen, often referred to collectively as $$\ce{NO_x}$$.

For example, nitrogen and oxygen can combine to form nitrogen monoxide:

These oxides can form because of the combustion conditions: they do not have to be present in the fuel beforehand. When predicting the products of fuel use, oxides of nitrogen may therefore be included if combustion occurs in air at a temperature high enough for nitrogen and oxygen to react.