They are used, in general, two methods to determine the composition of the coals: the "elementary analysis", establishes the total percentages of the elements gifts (, hydrogen, oxygen, e nitrogen); e the "approach analysis" supplies an empirical estimate of the amounts of humidity, volatile ash and materials, and of fixed carbon. The coals are classified or in accordance with commanded its fixed carbon content, whose ratio increases to the measure that the ore if form. Orderly ascendant, the main types are: linhito, that if it consumes quickly, can be set on fire espontaneamente and has low calorific value; he is used over all in and in ; sub-bituminous coal, used mainly in generating stations; bituminous coal, the type most common and that, transformed frequent into coke it has ample industrial job; the antracito, a bright coal, of slow, excellent combustion for domestic use.<P> Although used as combustible, in Wales, in , since as the milênio one.C., the coal alone started to be minerado of more or less systematic form in for return of séc. XIII, time where already he was known of the North American indians. The first commercial mine of coal of was opened in Richmond. U.S.A. (1745), and the antracito one was extracted in the Pensilvânia for 1970 return. The industrial revolution extended the demand of the miério, that only reduced in century XX, with the diffusion of the job of as combustible. The world-wide reserves of coal are esteem in about 7 trillions of tons, the sufficient to take care of the demand during some centuries, in the current taxes of consumption.
Formation of the húmicos Coals:
Deposition of organic substance in a basin is given to it (in the formation place or not). Of followed the external incarbonização to a small distance of the surface starts, in which if it decomposes the organic substance in Carbon and Volatile, by means of anaerobic bacteria that if they feed of carbohydrates. The internal incarbonização comes lands after to suffer to a discouragement or subsidência, being thus in conditions of bigger pressure and temperature... In these condiçõs it goes during wide periods of time: to diminish the size and to increase the density; to increase the carbon percentage for loss of other components; to diminish the concentration of volatile.