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material that can be burned or otherwise consumed to produce heat. The common fuels used in industry, transportation, and the home are burned in air. The carbon and hydrogen in fuel rapidly combine with oxygen in the air in an exothermal reaction—one that liberates heat. Most of the fuels used by industrialized nations are in the form of incompletely oxidized and decayed animal and vegetable materials, or fossil fuels, specifically coalcoal,
fuel substance of plant origin, largely or almost entirely composed of carbon with varying amounts of mineral matter. Types

There is a complete series of carbonaceous fuels, which differ from each other in the relative amounts of moisture, volatile matter,
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, peatpeat,
soil material consisting of partially decomposed organic matter, found mainly in swamps and bogs in various parts of the northern temperate zone but also in some semitropical and tropical regions.
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, lignitelignite
or brown coal,
carbonaceous fuel intermediate between coal and peat, brown or yellowish in color and woody in texture. It contains more moisture than coal and tends to dry and crumble when exposed to the air; the flame is long and smoky and the heating power
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, petroleumpetroleum,
oily, flammable liquid that occurs naturally in deposits, usually beneath the surface of the earth; it is also called crude oil. It consists principally of a mixture of hydrocarbons, with traces of various nitrogenous and sulfurous compounds.
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, and natural gasnatural gas,
natural mixture of gaseous hydrocarbons found issuing from the ground or obtained from specially driven wells. The composition of natural gas varies in different localities.
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. From these natural fuels other artificial ones can be derived. Coal gascoal gas,
gas obtained in the destructive distillation of soft coal, as a byproduct in the preparation of coke. Its composition varies, but in general it is made up largely of hydrogen and methane with small amounts of other hydrocarbons, carbon monoxide (a poisonous gas),
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, cokecoke,
substance obtained by the destructive distillation of bituminous coal. Coke bears the same relation to coal as does charcoal to wood. A hard, gray, massive, porous fuel, coke is the solid residue remaining after bituminous coal is heated to a high temperature out of
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, water gaswater gas,
colorless poisonous gas that burns with an intensely hot, bluish (nearly colorless) flame. The gas is a mixture of carbon monoxide and hydrogen with very small amounts of other gases, e.g., carbon dioxide, and is almost entirely combustible as a result.
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, and producer gasproducer gas,
fuel gas consisting chiefly of carbon monoxide and nitrogen. It is prepared in a furnace or generator in which air is forced upward through a burning fuel of coal or coke. Although the fuel is introduced through the top, no air is admitted there.
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 can be made using coal as the principal ingredient. Gasolinegasoline
or petrol,
light, volatile mixture of hydrocarbons for use in the internal-combustion engine and as an organic solvent, obtained primarily by fractional distillation and "cracking" of petroleum, but also obtained from natural gas, by destructive distillation
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, kerosenekerosene
or kerosine,
colorless, thin mineral oil whose density is between 0.75 and 0.85 grams per cubic centimeter. A mixture of hydrocarbons, it is commonly obtained in the fractional distillation of petroleum as the portion boiling off between 150°C; and
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, and fuel oil are made from petroleum. For most transportation, fuel must be in a liquid form.

There is a growing concern about the environmental contamination caused by the burning of great amounts of fossil fuels and about the increasing expense of finding them and processing them into easily usable forms (see energy, sources ofenergy, sources of,
origins of the power used for transportation, for heat and light in dwelling and working areas, and for the manufacture of goods of all kinds, among other applications.
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). During the last 100 years the amount of carbon dioxide in the atmosphere has increased, and there is evidence that this phenomenon may be due to the burning of fossil fuel. Use of biomass, which consists of plants or plant waste, would not produce excess carbon dioxide because the plants absorb the gas for their growth. Wood is not as concentrated a form of energy as fossil fuels, but it can be converted into a more energy-rich fuel called charcoalcharcoal,
substance obtained by partial burning or carbonization (destructive distillation) of organic material. It is largely pure carbon. The entry of air during the carbonization process is controlled so that the organic material does not turn to ash, as in a conventional
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. Burning fossil fuel also releases acidic oxides of sulfur and nitrogen, which are deposited on the earth in rainwater (see acid rainacid rain
or acid deposition,
form of precipitation (rain, snow, sleet, or hail) containing high levels of sulfuric or nitric acids (pH below 5.5–5.6).
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). The clearing of forestsforest,
a dense growth of trees, together with other plants, covering a large area of land. The science concerned with the study, preservation, and management of forests is forestry.
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, particularly in the tropical regions, also threatens to increase the amount of carbon dioxide in the atmosphere because the forests utilize carbon dioxide for growth.

The amount of fossil fuel available is limited and new methods of recovery are being developed. One proposed alternative fuel is hydrogen, which is now employed as a fuel only for a few special purposes because of its high cost. Hydrogen can be produced by electrolysis of water for which nonfossil fuels would supply the energy. Solar energy could be utilized either by direct conversion to electricity using photovoltaic cells or by trapping solar heat. Fuels are rated according to the amount of heat (in calories or Btu) they can produce. Nuclear fuels are also possible substitutes for fossil fuels. Nuclear fuels are not burned; they undergo reactions in which the nuclei of their atoms either split apart, i.e., undergo fission, or combine with other nuclei, i.e., undergo fusion. In either case, a small part of the nuclear mass is converted to heat energy. All nuclear fuels currently employed in practical, nonweapons applications react by fission.

High-energy fuels for jet engines and rockets are rated by their specific impulse in thrust per pound of propellant per second. Hydrogen, which is the lightest element, is usually used in the form of compounds, because the density of liquid hydrogen is low and therefore a large volume is required. Addition of aluminum powder or lithium increases the efficiency. Rockets usually have a self-contained supply of oxygen or some other oxidizer, such as ammonium, lithium, or potassium perchlorate. Fuels such as turpentine, alcohol, aniline, and ammonia use nitric acid, hydrogen peroxide, and liquid oxygen as oxidizers. More power can be obtained by oxidizing hydrazine, diborane, or hydrogen with oxygen, ozone, or fluorine.

See oil gasoil gas,
any of a group of fuel gases produced from oil by exposing it to high temperatures. High-Btu oil gas is so called because of its high heating value; it is often used to supplement natural gas during periods of high demand.
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; liquefied petroleum gasliquefied petroleum gas
or LPG,
mixture of gases, chiefly propane and butane, produced commercially from petroleum and stored under pressure to keep it in a liquid state.
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; gas, fuelgas, fuel,
gaseous substance that burns in air and releases enough heat to be useful as a fuel, while also remaining sufficiently stable at ordinary temperatures to permit long-term storage without deterioration or undue hazard.
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; nuclear energynuclear energy,
the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m
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a combustible substance that, upon burning, liberates a significant amount of heat, which may be used directly in a production process or transformed into other forms of energy. Various devices are used in the combustion of fuels, including burners, furnaces, and combustion chambers. Although there are many combustible substances, only those substances that are abundant and easily obtainable and that do not produce harmful combustion products are considered fuels. Substances that are largely composed of carbon meet these requirements; they include organic minerals—brown coal, fuel gases, oil shales, hard coal, petroleum, and peat—as well as wood and plant wastes, such as straw and husks. Fuels for rocket engines form a separate class (seeROCKET PROPELLANT and METAL PROPELLANT).

In nuclear power engineering, atomic fuel is any substance the nuclei of which decompose when bombarded by neutrons and liberate energy, principally the kinetic energy of nuclear fission fragments and neutrons. Thus, ordinary chemical fuels are called organic fuels, in contradistinction to nuclear fuels. Natural organic fuels are the prime source of heat used by man. The petrochemical industry and the production of lubricants are based almost entirely on raw materials obtained from natural fuels (seeINDUSTRIAL ORGANIC SYNTHESIS and PETROLEUM PRODUCTS).

Originally plant fuels, such as wood, were mainly used to produce heat from fire. The fossil fuels coal and petroleum have been known since antiquity, but they began replacing plant fuels with relatively lower calorific values only in the middle of the 19th century, thus producing a great effect on the conservation of forests.

The properties of a fuel are largely determined by the fuel’s chemical composition (in percent by weight). The chemical elements in fuels are designated by their chemical symbols: C, H, O, N, and S; ash and water components are designated by A and W, respectively. The moisture and ash content of any single fuel is subject to significant variation; therefore, in order to provide a more precise characterization, fuel compositions may be given for the weight of the fuel upon introduction into a furnace (superscript f), dry weight (d), combustible weight (c), or organic matter weight (o). For example, Cc = 91 indicates that the combustible weight of a given fuel contains 91 percent carbon by weight.

The most important index characterizing the practical value of a fuel is the heat of combustion. For comparative analysis, the concept of a standard fuel with a heat of combustion of 7,000 Calories per kg (29,308 kilojoules per kg) is used. The quality of hard coals is characterized by the volume of volatile components Vv that enter into a gaseous or vapor state when the coal is heated without contact with air. This process causes a nonvolatile residue to be formed, whose properties determine the coal’s tendency to cake, which characterizes its suitability for coking.

The oxidizability of a fuel at normal temperatures determines how the fuel can be stored and for how long. Fuels with high oxidizability may undergo spontaneous combustion. The tendency of a fuel toward spontaneous combustion is determined by the ignition temperature. Liquid fuels are also characterized by the flash point—the tendency of a mixture of air and fuel vapors to ignite without ignition of the liquid itself. This characteristic is crucial in the combustion of fuels in internal-combustion engines. The possibility of obtaining high temperatures from the combustion of a fuel depends on the maximum temperature Ta theoretically attainable upon complete combustion of a fuel in the air with the liberated heat going entirely to heat the products of combustion.

The mechanical strength of a solid fuel is important in the transportation of the fuel over long distances and repeated re-loadings. In the combustion of fuels in the form of dusts, the expenditures of energy for the preparation of the pulverized fuel is characterized by the fuel’s pulverizability. The granulometric composition of a fuel, that is, the content of particles of varying size within a fuel, is important in the laminar combustion of the fuel. The principal characteristics of several fuels are given in Table 1.

Fuels are divided into solid, liquid, and gaseous fuels depending on their aggregate state. A distinction is made between natural fuels, such as coal and petroleum, and chemical fuels obtained from the processing of natural fuels. For example, the quality of a

Table 1. Principal characteristics of some fuels
 Composition (percent, by weight)Volume of volatile matter released percent, by weight)Maximum temperature of combustionHeat of combustion (megajoules/kg)
1 Megajoules/m3
Chopped peat506.324.
Brown coal (Kansk-Achinsk type)33643.730.20.613.548180015.7
Hard coal82355.
Anthracite dust0.52363.
Mazut (high-sulfur)30.18310.42.80.7210039.2
Natural gas74251.0200035.61

solid fuel may be improved, without changing the fuel’s chemical composition, by briquetting, enrichment, or pulverization. Coke used in blast furnaces is prepared by heating a fuel, usually hard coal, to 950°-1050°C without contact with air (seeCOKING and COKE CHEMISTRY). Petroleum products are obtained by means of distillation, cracking, and pyrolysis; pyrolysis is one of the most important industrial methods used to obtain raw material for petrochemical synthesis. Gaseous chemical fuel is obtained by the gasification of solid and liquid fuels. (SeeHYDROLYSIS OF VEGETABLE MATTER for information on the biochemical conversion of plant fuels.)

At the 1975 level of extraction, the known reserves of coal will suffice for thousands of years. However, the prospective reserves of petroleum and natural gas at the present level of extraction will suffice for only 100–150 years; if the increasing rate of extraction is taken into account, these reserves may be exhausted in 50–60 years. The limited reserves of natural gas and petroleum and the significant increase in their prices have resulted in a tendency to conserve fossil fuels and to use other sources for the generation of power.

Since nearly all the fuels extracted from the earth are burned (only about 10 percent of petroleum and natural gas is used as raw material), the yearly introduction of pollutants released into the earth’s atmosphere from the combustion of fuels has reached enormous proportions: approximately 150 million tons of ash, 100 million tons of oxides of sulfur, 60 million tons of oxides of nitrogen, and 20 billion tons of carbon dioxide. In order to protect the environment, various methods are being developed for trapping harmful substances in combustion products, as well as methods of combustion in which such substances, for example, nitrogen oxides and carbon monoxide, are not produced.


See references under the articles on specific types of fuels.



A material that is burnt to release heat energy, for example, coal, oil, or uranium.


1. any substance burned as a source of heat or power, such as coal or petrol
a. the material, containing a fissile substance, such as uranium-235, that produces energy in a nuclear reactor
b. a substance that releases energy in a fusion reactor
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