Combustion is the burning of a fuel and oxidant to produce heat and/or work.  The theory of combustion articulated by the French chemist Antoine Laurent Lavoisier in 1777 is a cornerstone of modern science.The harnessing of combustion in the steam engine, internal combustion engine, rocket engine, boilers, reactors and various turbines transformed human life and the environment.

Antoine Laurent Lavoisier

Combustion of hydrocarbons extracted from fuels such as natural gas, crude oil, and coal is the principal energy conversion process in civilization today.

Combustion includes thermal, hydrodynamic, and chemical processes. It starts with the mixing of fuel and oxidant, and sometimes in the presence of other species or catalysts. The fuel can be gaseous, liquid, or solid and the mixture may be ignited with a heat source. When ignited, chemical reactions of fuel and oxidant take place and the heat release from the reaction creates a self-sustained process. The combustion products include heat, light, chemical species, pollutants and mechanical work.

In the combustion of a hydrocarbon, molecules containing carbon are exposed to oxygen and heat, which creates a chain reaction, producing more heat to continue fueling the reaction.  The reaction ceases only when either the supply of oxygen or the supply of carbon runs out.  A simple, common example of this is the burning of methane, natural gas.  The process is shown below:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)      ΔH = -891kJ/mol

where ΔH is the enthalpy change. The minus sign for ΔH means the reaction is exothermic, i.e., it releases heat. The reaction also produces 0.330 moles of CO₂. 

The combustion of octane, a straight-chain alkane hydrocarbon in motor gasoline with the chemical formula CH₃(CH₂)₆CH₃, is given by:

2C₈H₁₈(l) +25O₂(g) → 16CO₂(g) +18H₂0(l)      ΔH = -11,020 kJ/mol

Note that in the combusion of methane and octane--and in the combustion of all hydrocarbons--the entire mass of carbon in the fuel is released as CO₂ to the atmosphere.  But fuels differ in the amount of carbon they release per unit mass.  Burning 1 mol^-1 of methane produces 1 mol^-1 of CO₂, whereas burning 1 mol^-1 of octane produces 8 mol^-1 of CO₂. This means that the energy mix of a nation--the fraction of energy use from various energy sources--has a strong influence on the amount of carbon dioxide released by human activity in that nation.

Respiration involves the combustion of glucose.

The combustion of glucose (C₆H₁₂O₆) in the process of respiration is another important example of combustion. Glucose is a monosaccharide (or simple sugar) that is a very important carbohydrate in biology because living cell uses it as a source of energy and metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration in both prokaryotes (bacteria and archaea) and eukaryotes (animals, plants, fungi, and protists). The combustion of glucose is given by:

C₆H₁₂O_{6 (aq)} + 6O_{2 (g)} → 6CO_{2 (g)} + 6H₂O _(l)      ΔH = -2808 kJ/mol

The energy released in respiration is used to synthesize adenosine triphosphate (ATP), which in turn stores some of that energy energy. The energy stored in ATP can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes.

The combustion of hydrocarbon fuels such as methane usually uses air is the source of oxygen (O₂). In air, each kg of oxygen is mixed with approximately 3.76 kg of nitrogen. The resultant flue gas from the combustion will thus contain nitrogen according to:

CH₄ + 2O₂ + 7.52N₂ → CO₂ + 2H₂O + 7.52N₂ + heat

When air is the source of the oxygen, nitrogen is by far the largest part of the resultant flue gas, and some nitrogen can be oxidized to various nitrogen oxides (NO_x). For example, the combustion of 1 kilogram of motor gasoline releases 27 grams of NO_x. Oxides of nitrogen are cause for concern because they are one of the main ingredients involved in the formation of ground-level ozone, which can trigger serious respiratory problems; they contribute to formation of acid rain, they contribute to nutrient overload that deteriorates water quality, they contribute to atmospheric particles that cause visibility impairment, and they contribute to global warming. 

Sources

• Ophardt, Charles E., Virtual Chembook, Department of Chemistry, University of Elmhurst, Accessed 7 May 2009.
• Wikipedia Contributors, Combustion, Wikipedia The Free Encyclopedia, Accessed 7 May 2009.
• Zhou, Chenn (Lead Author); Cutler J. Cleveland (Topic Editor). 2008. Combustion. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment).