The fuels used in industry are either solid, liquid or gaseous, the predominant class in the Soviet Union being the first named. Liquid and gaseous fuels are employed to a lesser, but still important extent. In speaking of "fuel" it is still the universal practice to mean the combustible material which in conjunction with atmospheric oxygen forms the source of heat. Actually the fuel in the ordinary sense is only one of two components needed for the generation of heat by combustion. Air being the source of the oxygen required to produce the combustible mixture is, therefore, equally a part of the fuel, as are the other materials already mentioned. Thus, an air jet to be introduced into an atmosphere of, say, coal, and gas will burn in an identical manner to a gas jet in air. Excess of either of these components beyond the correct proportions is expected to lead to waste of heat.

A feature common to all industrial fuels is that the basic combustible is carbon and, with the exception of blast­furnace gas, this is combined in various proportions with hydrogen together with small portions of oxygen and sulphur. Non-combustible components are practically only met with in the case of solid fuels, and go to form the ash residue after the combustible matter has been consu­med. There will also be a widely varying percentage of moisture associated with solid fuels when delivered and fired.

Solid fuels are composed, from the practical point of view, of "fixed" carbon, volatile matter, sulphur and moisture. The fixed carbon, the volatile matter and, to a mi­nor extent, the sulphur compose the combustible matter, the classification of solid fuels being usually based upon the ratio of volatile matter to the fixed carbon.

The volatile matter is that proportion of the fuel which is distilled and gasified on heating to a temperature of 900° C, without access of air, leaving a residue of solid coke consisting of carbon and the incombustible ash and a portion of the sulphur. The fixed carbon is that remain­ing in the coke after distillation of the volatile matter. It is determined by complete combustion of the coke in a stream of oxygen so that only the ash remains.

Moisture occurs in fuel as free or acquired moisture which is largely or mainly influenced as to percentage by external conditions such as wheather encountered in transit and storage or by deliberate wetting. It also occurs as inherent or hygroscopic moisture, the former being removed by exposure to an air current at ordinary room temperature (say, 60° F) for about twenty-four hours, when it is said to be air-dried. The inherent moisture is that remaining after air drying. It is removed only by exposure to a temperature of about 212° F, when it will be fully evapo­rated. If the fuel is allowed to cool down and to be exposed to the atmosphere, it will once more take up inherent moisture. The line of demarcation between free and inherent moisture is somewhat vague and arbitrary, since the ratio must be to a large extent influenced by atmospheric humidity.

Natural fuels like coal, gas and oil are being rapidly exhausted. The application of atomic power is becoming increasingly broader because at present it is the only known potential capable of meeting mankind's power requirements. The power obtained from atomic reactions has been applied to various fields of man's peaceful activity.