Combustion

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The process of burning fuels. Traditionally biomass was used as fuel, but now fossil fuels are the major source of energy for human activities. Combustion is essentially an oxidation process that yields heat and light. Most fuels are carbon and hydrogen which use oxygen in the air as an oxidant. More exotic fuels are used in some combustion processes, particularly in rockets where metals such as aluminum or beryllium or hydrazine (a nitrogen containing compound) are well known as effective fuels. As rockets operate beyond the atmosphere they carry their own oxidants, which may also be quite exotic.

Combustion involves a mixture of fuel and air, which is thermodynamically unstable. The fuel is then converted to stable products, usually water and carbon dioxide, with the release of a large amount of energy as heat. At normal temperatures fuels such as coal and oil are quite stable and have to be ignited by raising the temperature. Combustion is said to be spontaneous when the ignition appears to take place without obvious reasons. Large piles of organic material, such as hay, can undergo slow oxidation, perhaps biologically mediated, and increase in temperature. If the amount of material is very large and the heat cannot escape, the whole pile can suddenly burst into flame. Will-o'-the-wisps or jack-o'-lanterns (known scientifically as ignis fatuus) are sometimes observed over swamps where methane is likely to be produced. The reason these small pockets of gas ignite is not certain, but it has been suggested that small traces of gases such as phosphine that react rapidly with air could ignite the methane.

Typical solid fuels like coal and wood begin to burn with a bright turbulent flame. This forms as volatile materials are driven off and ignited. These vapors burn so rapidly that oxygen can be depleted, creating a smoky flame. After a time the volatile substances in the fuel are depleted. At this point a glowing coal is evident and combustion takes place without a significant flame. Combustion on the surface of the glowing coal is controlled by the diffusion of oxygen towards the hot surface. If the piece of fuel is too small, such as a spark from a fire, it is likely to lose temperature rapidly and combustion will stop. By contrast a bed of coals can maintain combustion because of heat storage and the exchange of radiative heat between the pieces. The most intense combustion takes place between the crevices of a bed of coal. In these regions oxygen may be in limited supply which leads to the production of carbon monoxide. This is subsequently oxidized to carbon at the surface of the bed of coals with a faint blue flame. The production of toxic carbon monoxide from indoor fires can occasionally represent a hazard if subsequent oxidation to carbon dioxide is not complete.

Liquid fuels usually need to be evaporated before they burn effectively. This means that it is possible to see liquid combustion and gaseous combustion as similar processes. Combustion can readily be initiated with a flame or spark. Simply heating a fuel-air mixture can cause it to ignite, but temperatures have to be high before reactions occur. A much better way is to initiate combustion with a small number of molecular fragments of radicals. These can initiate chain reactions at much lower temperatures than molecular reactions. In a propane-air flame at about 2000° K, hydrogen and oxygen atoms and hydroxyl radicals account for about 0.3% of a gas mixture. It is these radicals that support combustion. They react with molecules and split them up into more radicals. These radicals can rapidly enter into the exothermic (heat releasing) oxidative processes that lie at the heart of combustion. The reactions also give rise to further radicals that support continued combustion. Under some situations the radicals reaction branch, such that the reaction of each radical produces two new radicals. These can enter further reactions, producing yet further increases in the number of reactions and very soon the system explodes. However the production of radicals can be terminated in a number of ways such as contact with a solid surface. In some systems, such as the internal combustion engine, an explosion is desired, but in others, such as a gas cooker flame, maintaining a stable combustion process is desirable.

In terms of air pollution the reaction of oxygen and nitrogen atoms with molecules in air leads to the formation of the pollutant nitric oxide through a set of reactions known as the Zeldovich cycle. It is this process that makes combustion such an important contributor of nitrogen oxides to the atmosphere.

Campbell, I. M. Energy and the Atmosphere. New York: Wiley, 1986.