FUEL, Comparative Heating Values of. The effective and economical utilization of the inherent heat energy of fuel in the production of light and power is perhaps the most import ant industrial question of the times. It is one that plays such a necessary part of modern life and the cost of the fuel used oftentimes is so great a factor in the conduct of an industrial enterprise on an economical basis, that con stant efforts are being directed to improve the present methods of fuel utilization and ap proach more nearly the possibilities of theoreti cal efficiency.
Kinds of FueL— Nature furnishes a great variety of fuel in three general forms—the solid, such as wood and coal; the liquid, such as petroleum; and the gaseous, such as natural gas. The solid forms of fuel are the most common; the liquid contain the largest propor tion of heat energy; and the gaseous are the most convenient for use and economical of labor. The fact that any kind of gaseous fuel is the most convenient and, in a great many cases, the most effective for use, has been demonstrated principally through the utiliza tion of natural gas; but as the supply of natural gas is gradually failing and the cost of oil-firing is much higher, the tendency of the times is to perfect methods for the conversion of solid and liquid fuel into gaseous form. These conditions render the methods of firing employed very important.
Methods of Firing.— In general, the eco nomical and effective utilization of a fuel de pends upon the completeness of the combustion thereof. In the combustion of solid facts by ordinary grate or "direct firing" methods the greater part of the heat liberated is lost in the form of gases which arc not combustible at the temperatures attained in grate-fired fur naces. On the other hand, when solid fuel is first converted into gas and the gas thus ob tained utilized by "gas firine• methods, the re sults accomplished more than compensate the 15 or 20 per cent loss of heat energy sustained by the process of conversion. As generally applied, gas firing results in more complete combustion and the attainment of higher com bustion temperature and thereby make possible metallurgical operations which are impracti cable with direct firing. There is also less loss
of heat through the waste products of com bustion and greater efficiency in transfer of heat. Furthermore, by suitable methods, the heat from the hot waste gases can be recovered and returned to the combustion chamber in preheated air, so that the gas and air supply, and therefore the combustion of the fuel, are placed under easy and complete control.
In practice, the theoretical amount of air necessary for complete combustion is always exceeded. Direct firing requires at least twice the theoretical amount to even approximate complete combustion, especially in the use of soft coals. With the progress of combustion, the fuel bed becomes more compact, so that with a given draught the amount of air pene trating into the bed decreases with the increase of the depth and compactness of the fuel. Under these conditions, a fresh charge of coal requires a greater amount of air to consume its volatile components and demands it at a time when its passage is most retarded and the combustion rendered more incomplete by the reduction of temperature accompanying volatili zation. As the final result is an irregular air demand, the grates have to be arranged to ad mit the greater excess of air at all times, otherwise large heat losses will occur from the escape of unconsumed gases.
On the other hand, in gas firing, the air sup ply is always under control and therefore can be made to approximate closely the theoretical amount, thus reducing the bulk of the prod ucts of combustion and ensuring more com plete combustion. The greater amount of heat then liberated is concentrated in smaller volume, and raises the temperature of combustion, so that the oxygen of the air combines more read ily with the combustible constituents of the gas. Furthermore, the smaller the excess of air the less the dilution of the gaseous mixture by nitrogen and other inert gases which retard combustion.