Combustion is an exothermic chemical reaction in which a fuel reacts with an oxidizer, usually oxygen, releasing energy as heat and sometimes light. Fuels may be solids, liquids, or gases, and combustion may occur with a visible flame or without one. It underlies many heating and propulsion technologies, while also producing the heat and chemical products associated with fires. Its behavior depends on fuel composition, oxidizer availability, mixing, and heat transfer. (www1.grc.nasa.gov)
Chemical basis and products
Ordinary combustion is a reduction–oxidation reaction: the fuel is oxidized while the oxidizer is reduced. For fuels composed of hydrocarbons, ideal complete combustion produces carbon dioxide and water. For example, the balanced equation for complete combustion of methane is:
The equation expresses the overall material balance, rather than the individual molecular steps through which the reaction proceeds. The products depend on the fuel: burning hydrogen produces water, whereas burning magnesium in oxygen produces magnesium oxide. (openstax.org)
Stoichiometry specifies the proportions required by a balanced reaction. A fuel–oxidizer mixture is fuel-rich when it contains more fuel than can be completely oxidized by the available oxidizer, and fuel-lean when oxidizer is present in excess. Mixture proportions affect flame speed, stability, and the possibility of sustained burning. Incomplete combustion can leave partially oxidized gases, unburned organic compounds, or solid particles rather than converting all fuel to its ideal complete-combustion products. (ntrs.nasa.gov)
Energy release and ignition
The energetics of combustion are described through thermodynamics. At constant pressure, the heat exchanged by a reaction is represented by its enthalpy change. Combustion reactions have negative reaction enthalpies because the products have lower enthalpy than the reactants under the specified conditions. Standard molar enthalpy of combustion refers to the enthalpy change when one mole of a substance burns completely under standard-state conditions. The physical states of the products matter, particularly whether the water formed is liquid or vapor. (openstax.org)
An energetically favorable reaction does not necessarily proceed rapidly. Activation energy provides a barrier, so a spark, hot surface, or another initiating source may be required. Increasing temperature generally increases reaction rates by increasing the fraction of molecular encounters capable of overcoming that barrier. Ignition and burning therefore involve both thermodynamic energy differences and chemical kinetics. (openstax.org)
Once combustion begins, its own heat can sustain further reaction. Whether burning continues depends on the relationship between heat generation and heat loss, together with the continuing supply of reactants. The familiar “fire triangle”—fuel, oxygen, and heat—summarizes these requirements for ordinary oxygen-supported fires. Removing a necessary component can interrupt combustion, although the detailed extinction mechanism varies with the system. (www1.grc.nasa.gov)
Flames and modes of burning
In a premixed flame, fuel and oxidizer mix before entering the main reaction zone. The flame propagates through the unburned mixture; its burning velocity depends on composition and physical conditions. Mixtures outside their flammability limits cannot sustain ordinary flame propagation under the specified conditions. These limits are important properties of combustible mixtures, rather than universal constants independent of the experimental setting. (ntrs.nasa.gov)
In a diffusion flame, fuel and oxidizer initially occupy separate regions and react as they mix. Their transport toward the reaction zone, including diffusion, strongly influences the burning rate and flame structure. Consequently, flow patterns can be as important as reaction rates in determining flame behavior. Practical combustion systems may contain both premixed and nonpremixed regions rather than conforming to one idealized arrangement. (ntrs.nasa.gov)
Smoldering is a slower, flameless form of combustion involving reactions at the surface of a solid fuel. It is commonly accompanied by smoke and glowing, and may develop into flaming combustion. It differs from pyrolysis, which is thermal decomposition caused by heat rather than necessarily by reaction with oxygen. Heating a combustible solid may produce volatile gases that subsequently burn, while the remaining solid undergoes surface oxidation. (tsapps.nist.gov)
Applications and environmental effects
Combustion converts fuel energy into heat and, through suitable machinery, mechanical motion. An internal combustion engine uses combustion within the engine to generate power. Aircraft engines and rocket engines likewise depend on fuel burning; solid rockets incorporate fuel and oxidizer within their propellant, whereas air-breathing engines obtain oxygen from the atmosphere. Combustion also serves industrial fabrication processes. (openstax.org)
Combustion emissions depend on fuel composition and operating conditions. They can include carbon monoxide, nitrogen oxides, unburned organic gases, and particulate matter. Complete combustion of carbon-containing fuels still produces carbon dioxide; “complete” therefore describes chemical conversion, not an absence of environmental effects. Burning fossil fuels releases carbon dioxide, a greenhouse gas, while soot is another combustion product relevant to climate change. (epa.gov)
Historical interpretation and research
In eighteenth-century Europe, phlogiston theory explained burning as the release of an assumed substance from combustible materials. Antoine Lavoisier instead developed an oxygen-based explanation, proposing a theory excluding phlogiston by 1777. This reinterpretation connected combustion with measurable chemical combination and became an important part of the chemical revolution. Modern research examines reaction chemistry, flame propagation, solid-fuel burning, and extinction. Microgravity experiments help separate combustion behavior from buoyancy effects that complicate observations on Earth. (acs.org)