Editing Combustion (section)

{{Short description|Chemical reaction between a fuel and oxygen}}
{{redirect|Burning|the type of injury|Burn|combustion without external ignition|spontaneous combustion|the vehicle engine|internal combustion engine|other   uses|Burning (disambiguation)|and|Combustion (disambiguation)|and|Firing (disambiguation)}}
[[File:Fire, Fireplace, Rostov-on-Don, Russia.jpg|thumb|The [[flame]]s caused as a result of a [[fuel]] undergoing combustion (burning)]]
[[File:Regenerative thermal oxidizer.jpg|thumb|[[Regenerative thermal oxidiser|Air pollution abatement equipment]] provides combustion control for [[industrial processes]].]]

'''Combustion''', or '''burning''',<ref>colloquial meaning of burning is combustion accompanied by flames</ref> is a high-temperature [[exothermic]] [[redox]] [[chemical reaction]] between a [[fuel]] (the reductant) and an [[oxidant]], usually atmospheric [[oxygen]], that produces oxidized, often gaseous products, in a mixture termed as [[smoke]]. Combustion does not always result in [[fire]], because a [[flame]] is only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction. While [[activation energy]] must be supplied to initiate combustion (e.g., using a lit [[match]] to light a fire), the heat from a flame may provide enough energy to make the reaction self-sustaining. The study of combustion is known as '''combustion science'''.

Combustion is often a complicated sequence of [[elementary reaction|elementary]] [[Radical (chemistry)|radical reactions]]. [[Solid fuel]]s, such as [[wood]] and [[coal]], first undergo [[endothermic]] [[pyrolysis]] to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that [[incandescence|incandescent]] [[light]] in the form of either [[Smouldering|glowing]] or a [[flame]] is produced. A simple example can be seen in the combustion of [[hydrogen]] and [[oxygen]] into [[water]] [[vapor]], a reaction which is commonly used to fuel [[rocket engine]]s. This reaction releases 242{{nbsp}}[[Joule per mole|kJ/mol]] of heat and reduces the [[enthalpy]] accordingly (at constant temperature and pressure):

: <chem>2H_2(g){+}O_2(g)\rightarrow 2H_2O\uparrow</chem>

Uncatalyzed combustion in air requires relatively high temperatures. Complete combustion is [[stoichiometric]] concerning the fuel, where there is no remaining fuel, and ideally, no residual oxidant. Thermodynamically, the [[chemical equilibrium]] of combustion in air is overwhelmingly on the side of the products. However, complete combustion is almost impossible to achieve, since the chemical equilibrium is not necessarily reached, or may contain unburnt products such as [[carbon monoxide]], [[hydrogen]] and even [[carbon]] ([[soot]] or ash). Thus, the produced [[smoke]] is usually toxic and contains unburned or partially oxidized products. Any combustion at high temperatures in [[Atmosphere|atmospheric]] [[air]], which is 78 percent [[nitrogen]], will also create small amounts of several [[nitrogen oxides]], commonly referred to as [[NOx]], since the combustion of nitrogen is thermodynamically favored at high, but not low temperatures. Since burning is rarely clean, fuel gas cleaning or [[catalytic converter]]s may be required by law.

[[Fire]]s occur naturally, ignited by [[lightning]] strikes or by [[volcanic]] products. Combustion ([[fire]]) was the first controlled chemical reaction discovered by humans, in the form of [[campfire]]s and [[bonfire]]s, and continues to be the main method to produce energy for humanity. Usually, the fuel is [[carbon]], [[hydrocarbon]]s, or more complicated mixtures such as [[wood]] that contain partially oxidized hydrocarbons. The thermal energy produced from the combustion of either [[fossil fuel]]s such as [[coal]] or [[oil]], or from [[renewable fuel]]s such as [[firewood]], is harvested for diverse uses such as [[cooking]], production of [[electricity]] or industrial or domestic heating. Combustion is also currently the only reaction used to power [[rocket]]s. Combustion is also used to destroy ([[incineration|incinerate]]) waste, both nonhazardous and hazardous.

Oxidants for combustion have high oxidation potential and include atmospheric or pure [[oxygen]], [[chlorine]], [[fluorine]], [[chlorine trifluoride]], [[nitrous oxide]] and [[nitric acid]]. For instance, [[hydrogen]] burns in [[chlorine]] to form [[hydrogen chloride]] with the liberation of heat and light characteristic of combustion. Although usually not catalyzed, combustion can be catalyzed by [[platinum]] or [[vanadium]], as in the [[contact process]].