| The classifications
in which the different coals are sorted depend mainly on the proportion
of carbon that they , respectively, contain. Thus, the fuels of this
class are placed by their carbon richness in the following ascending
scale: firewood, 50 percents of carbon; peat , 52 to 60 percents;
lignite and brown coal, 55 to 65 percents; dry soft coal, 75 to 80
percents; soft or bituminous coal or soft coal, 65 to 85 percents;
anthracite, 75 to 95 percents. On this last substance is the graphite,
that is almost pure carbon and that is fireproof. These different
coal classes, whose carbon richness varies from 60 to 95 percent,
form a series of imperceptible gradations, by the conditions under
which they have formed, constituting the central group of this series
the bituminous coal also called soft coal, which is represented by
the Pittsburgh coal type in North-America.
The substances of the terrestrial
crust that enter in the formation of the mining coal: in addition
to carbon, the mineral coal contains hydrogen, oxygen , nitrogen,
sulfur and other components in small proportions. The hydrogen,
oxygen and nitrogen decrease as it increases the proportion of carbon.
The other ingredients form the ashes that the coal leaves after
burning itself. When these last elements, called fixed elements
, form a mixture of easily fusible materials, they form dreg when
burning the coal.
Of course, a deposit or carboniferous
bed does not show all these different fuel classes immediately,
such as the pit , the lignite, the soft coal, the anthracite; but
the gradation can be noticed considering extensive regions. In Montana
( United States ), for example, the coal of the eastern part of
that State, where the rocks are flat, belongs to the lignite group
; but in the western portion, where the rocks form rugged layers
that constitute the Rocky Mountains, the mineral fuel is of bituminous
nature, that is to say, it forms greasy soft coals. In the western
part of Pennsylvania, the mineral coals been have not been exposed
to a sufficient degree of heat and the pressure that is developed
when the terrestrial crust is wrinkled to form coals of greater
degree than the bituminous ones. But, in the northeast of Pennsylvania
, the rocks have been folded of a very marked way, and the resulting
heat and the pressure have eliminated part of the gases that enter
in the composition of these greasy soft coals, and the result has
been that the same rocks that contain greasy soft coal in the west
of Pennsylvania , to the northwest of the same State, these ones
contain anthracite.
It is necessary to make a geologic
investigation of the existing carboniferous deposits in the world,
and to consider the several types of vegetation that have produced
them and that were buried under different conditions, subject to
greater or smaller pressures and to chemical and local reactions
rather incidental than of general character, if we intend to properly
classify the several coal types .
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