Combustion is one of the primary energy release mechanisms in the universe. It occurs when a fuel and an oxidant combine and react, producing heat, light, mechanical work, and in some cases other chemical species. This rapid oxidation often requires an initial trigger – such as a spark or a hot surface – to bring molecules to a threshold energy level. Several types of combustion exist, depending either on the amount of reaction byproducts (as in complete and incomplete combustion), or on the conditions under which the reaction occurs.
This type of combustion, though characterized by the presence of incandescence and smoke, produces no flame. A relatively slow process, smoldering occurs between the oxygen in air and the surfaces of solid fuels such as coal, peat, wood, tobacco, and synthetic foams. These solid fuels glow when smoldering, indicating temperatures in excess of one thousand degrees Celsius. It may proceed even under oxygen-deficient conditions, provided the environment is hot enough. Smoldering, an incomplete combustion reaction, produces high levels of carbon monoxide.
Diffusion combustion results from the transfer of fuel vapors and oxygen across a concentration gradient into a reaction zone characterized by high temperatures and the correct proportion of reactants. Vapors may come initially from a solid fuel such as candle wax, a liquid fuel like alcohol, or a gaseous fuel like methane used in a typical Bunsen burner.
The reaction itself occurs some distance from the surface of the fuel itself, as the zone in between contains too much fuel vapor and too little oxygen for proper combustion. The flame produced from diffusion combustion begins as a smooth, laminar flame, increasing in turbulence as it grows and consumes more fuel and oxygen.
Rapid combustion releases massive amounts of energy in the form of heat and light, as is the case with fire. In some cases, combustion occurs so fast that large amounts of gas are released along with heat and light, causing a significant pressure shift in the surrounding atmosphere. This pressure shift, often accompanied by a very loud noise, is called an explosion. Internal combustion engines convert the energy produced by rapid combustion into usable kinetic energy.
Spontaneous Heating and Combustion
Spontaneous heating and combustion differs from most other types of combustion in that no external ignition source is required for it to proceed. An extremely slow process, spontaneous combustion can take up to several weeks. It consists of the gradual oxidation of certain materials when exposed to air, and is greatly dependent on the fuel’s heat-retaining capacity. As heat builds up, the rate of reaction increases, eventually causing smoldering or flaming combustion when the temperature rises above the fuel’s ignition point. Spontaneous combustion occurs in a variety of organic and inorganic materials, such as hay, coal, linseed oil, manure and cotton.