Carbon and hydrogen are the only elementary fuels; sulphur and traces of other elements do burn and give off heat, but these are collectively so small as to be considered negligible and their constituents as nuisance impurities. Oxygen from the air is the only elementary burning agent necessary to every heat producing reaction.

Coal is a complex substance containing mainly carbon, hydrogen, sulphur, oxygen and nitrogen. Typical approximate percentages of these constituents are shown in Table 2-1. It must be recognized, however, that there are different kinds of coals whose characteristics may differ from the typical values shown.

Coals are classifi ed with the aid of the various characteristics by which they may be distinguished. Among the more important are:

Volatile Matter - This term describes the mixture of gases and hy- drocarbon vapors that may be given off when coal is heated at very high temperatures; these may include acetylene, ethylene, ethane, methane, and others. The more the volatile matter, the more liable the coal will produce smoke. This is an indicator of the property of the coal.

Fixed Carbon - This term applies to that portion of the carbon left in the coal after the volatile matter is subtracted. It is usually combined with the percentages of moisture and ash in the coal when classifying coals.

Coals may have different properties depending on where they are mined and require different ways of firing for best results. In addition to heating values, the amounts of ash and smoke produced when the coal is fired are different for the several kinds of coal. Their classification is indicated in Table 2-3.

Ash - Ash is the residue after all the carbon, hydrogen and the small quantities of sulphur are burned away; coal ash varies considerably in the different kinds of coal. A large amount of ash in coal not only lowers its heating value but causes ash removal to become more difficult.

The clinkering of coal depends largely on its composition. Ash may consist of alumina and silica derived from clay, shale and slate, iron oxide from pyrites or iron sulfide, and small quantities of lime and magnesia.

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