1) W h a t is c o m b u s t i o n ?

Combustion is the act or process of burning. For combustion to occur, fuel, oxygen (air), and heat must be present together.

Per definition combustion is the chemical reaction of a particular substance with an oxidant. Generally this will mean atmospheric oxygen and will be treated as such in the rest of this site.

The combustion process is started by heating the fuel above its ignition temperature in the presence of oxygen. Under the influence of heat, the chemical bonds of the fuel are split. If complete combustion takes place, the elements carbon (C), hydrogen (H) and sulphur (S) react with the oxygen content of the air to form carbon dioxide CO₂, water vapour H₂O and sulphur dioxide SO₂ and, to a lesser degree, sulphur trioxide SO₃.

If not enough oxygen is present or the fuel / air mixture is insufficient then the burning gases are partially cooled below the ignition temperature (too much air or cold burner walls), and the combustion process stays incomplete. The flue gases then still contain burnable components, mainly carbon monoxide CO, carbon C (soot) and various hydrocarbons C_xH_y. Since these components are, along with NO_x, pollutants which harm our environment, measures have to be taken to prevent the formation of them.

To ensure complete combustion, it is essential to provide a certain amount of excess air. Combustion optimisation saves money!

The quality of a combustion system is determined by a maximum percentage of complete combustion, along with a minimum of excess air (commonly 5 to 20% above the necessary level for ideal combustion )

Flue gas will generally contain a certain amount of CH₄ (methane) if the combustion was not complete. Other hydrocarbons will not occur under normal conditions. higher hydrocarbons are only produced under conditions of high pressure and high temperature such as occur in an internal combustion engine.