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 combustion | Heat of combustion (megajoules/kg) | |||||||
| Wf | Af | Cf | Hf | Sf | Nf | Of | ||||
| 1 Megajoules/m3 | ||||||||||
| Wood | 40 | 0.6 | 30.3 | 3.6 | – | 0.4 | 25.1 | 85 | 1600 | 10.2 |
| Chopped peat | 50 | 6.3 | 24.7 | 2.6 | 0.1 | 1.1 | 15.2 | 70 | 1500 | 8.1 |
| Brown coal (Kansk-Achinsk type) | 33 | 6 | 43.7 | 3 | 0.2 | 0.6 | 13.5 | 48 | 1800 | 15.7 |
| Hard coal | 8 | 23 | 55.2 | 3.8 | 3.2 | 1.0 | 5.8 | 40 | 2050 | 22 |
| Anthracite dust | 0.5 | 23 | 63.8 | 1.2 | 1.6 | 0.6 | 1.3 | 3.5 | 2150 | 22.6 |
| Mazut (high-sulfur) | 3 | 0.1 | 83 | 10.4 | 2.8 | – | 0.7 | – | 2100 | 39.2 |
| Gasoline | – | – | 85 | 14.9 | 0.05 | – | 0.05 | – | 2100 | 44 |
| Natural gas | – | – | 74 | 25 | – | 1.0 | – | – | 2000 | 35.61 |
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.
I. N. ROZENGAUZ