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Lignite mining, western North Dakota, US (c. 1945). Lignite is brownish-black in color and has a carbon content of 60–70 percent on a dry ash-free basis. However, its inherent moisture content is sometimes as high as 75 percent [1] and its ash content ranges from 6–19 percent, compared with 6–12 percent for bituminous coal. [5]
Coal rank is based on several characteristics of the coal. The fixed carbon content refers to the percentage of the coal that is neither moisture, nor ash, nor volatile matter. When evaluated on a dry, mineral-matter-free basis, the fixed carbon content is the fraction of the coal that is not volatile organic matter. [4]
Coal comes in four main types or ranks: lignite or brown coal, bituminous coal or black coal, anthracite and graphite. Each type of coal has a certain set of physical parameters which are mostly controlled by moisture, volatile content (in terms of aliphatic or aromatic hydrocarbons) and carbon content.
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.
Bituminous coal has a composition of about 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen, and 1.8% sulfur, on a weight basis. [40] The low oxygen content of coal shows that coalification removed most of the oxygen and much of the hydrogen a process called carbonization. [41]
For example: In case studies of the Black Sea, a modern anoxic environment, it has been shown that anoxia within the lower-levels of the water column alone do not produce significant amount of organic-rich sediments, even though sufficient organic material was supplied to the region in the Holocene.
Leonardite was first described from North Dakota and is found associated with virtually all the lignite deposits in the state. [6] Leonardite has also been described worldwide from deposits of lignite or sub-bituminous coals e.g. in Alberta, Canada, [4] in Achlada and Zeli, Greece, [7] in Turkey and in Bacchus Marsh, Australia.
Lignite or brown coal is even worse with a heat of combustion of only 15 MJ/kg, owing to the presence of non-combustible impurities. Bituminous coal has a value lower than anthracite, but neither lignite nor bituminous coal are smokeless owing to their volatiles content.