Combustion or burning is an exothermic reaction between a substance (the fuel) and a gas (the oxidizer) to release heat. Combustion normally occurs in oxygen (often in the form of gaseous O₂) to form oxides. However, combustion can also take place in other gases like chlorine.

Since the fuel is most commonly of a hydrocarbon origin, the products of such reactions usually include water (H₂O) as well as carbon monoxide (CO) or carbon dioxide (CO₂), or both. Other byproducts, such as partially reacted fuel and elemental carbon (C), may generate visible smoke and soot.

The process of combustion involves four stages.

1. The mixing of the oxidant and fuel.

2. Ignition, where the activation energy of the reactants is exceeded, and the reaction can proceed spontaneously.

3. Reaction.

4. Dispersion of products

Combustion is arguably the most important physical phenomenon in industrial applications as the majority of industrial processes involve combustion. Examples of this are:

• manufacture, such as kilns for cement or furnaces for glass .
• separation, such as ore smelting furnaces or drying kilns.
• utilities supply, such as steam raising boilers.

In industrial flames, the mixing step is rate-limiting, since ignition and reaction occur in a time scale of milliseconds, and mixing occurs in 1 to 5 seconds. Therefore, understanding the mixing step and designing industrial burners to achieve excellent mixing characteristics is critically important for industry. This design is informed by both the aerodynamic aspects of the burner system as well as the regulation of fuel/oxidant ratios.

Rapid combustion

Rapid combustion is a form of combustion in which large amounts of heat and light energy are released. This is used in forms of machinery, such as internal combustion engines, and in fuel-air explosives and spud guns.

Slow combustion

Slow combustion is a form of combustion which takes place at low temperatures. Respiration is slow combustion.

Chemical equation

Generally, the chemical equation for combusting a hydrocarbon in oxygen (such as octane) is as follows:

C_xH_y + (x + y/4)O₂ → xCO₂ + (y/2)H₂O

For example, the burning of propane is:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

See also

• Heat of combustion
• spontaneous combustion
• fire
• deflagration
• detonation
• pyrolysis
• phlogiston
• pyrophoric
• internal combustion engine
• measurement techniques
  • Calorimeter
  • Particle Image Velocimetry
  • Laser Doppler Velocimetry
  • Laser Induced Fluorescence
  • Coherent Anti-Stokes Raman Spectroscopy (CARS)