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Lignite is often found in thick beds located near the surface, making it inexpensive to mine. However, because of its low energy density, tendency to crumble, and typically high moisture content, brown coal is inefficient to transport and is not traded extensively on the world market compared with higher coal grades.
It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the highest ranking of coals. The Coal Region of Northeastern Pennsylvania in the United States has the largest known deposits of anthracite coal in the world with an estimated reserve of seven billion short tons . [ 2 ]
Relative density or specific gravity of the coal depends on the rank of the coal and degree of mineral impurity. Knowledge of the density of each coal play is necessary to determine the properties of composites and blends. The density of the coal seam is necessary for conversion of resources into reserves.
The low oxygen content of coal shows that coalification removed most of the oxygen and much of the hydrogen a process called carbonization. [41] Carbonization proceeds primarily by dehydration, decarboxylation, and demethanation. Dehydration removes water molecules from the maturing coal via reactions such as [42] 2 R–OH → R–O–R + H 2 O
Its bank density (the density of a coal seam prior to breaking up during mining) is about 1346 kg/m 3 (84 lb/ft 3) while the bulk density of extracted coal is up to 833 kg/m 3 (52 lb/ft 3). [12] Bituminous coal characteristically burns with a smoky flame and softens and swells during combustion. [13]
Sub-bituminous coal is a lower grade of coal that contains 35–45% carbon. The properties of this type are between those of lignite, the lowest grade of coal, and those of bituminous coal, the second-highest grade of coal. [1] Sub-bituminous coal is primarily used as a fuel for steam-electric power generation.
— "Values ranging from 21.3 to 21.5 gm/cm 3 at 20 °C have been reported for the density of annealed platinum; the best value being about 21.45 gm/cm 3 at 20 °C." 21.46 g/cm 3 — Rose, T. Kirke. The Precious Metals, Comprising Gold, Silver and Platinum .
The Pittsburgh coal seam is the thickest and most extensive coal bed in the Appalachian Basin; [1] hence, it is the most economically important coal bed in the eastern United States. The Upper Pennsylvanian Pittsburgh coal bed of the Monongahela Group is extensive and continuous, extending over 11,000 mi 2 through 53 counties.