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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 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.
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]
The value of bituminite increases with grade. At high grade, i.e. high maturity, bituminite has high hydrogen to carbon content [citation needed]. A high hydrogen/carbon ratio bituminite indicates a good hydrocarbon source. However, low grades of bituminite vary depending on type, meaning that there is variable hydrogen/carbon ratios. [1]
[1] [2] The most common types include coal, lignite, oil shale, or black shale. [2] The organic material may be disseminated throughout the rock giving it a uniform dark color, and/or it may be present as discrete occurrences of tar, bitumen, asphalt, petroleum, coal or carbonaceous material.
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]
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.
It is essential that the oxidizer supplied is insufficient for complete oxidizing (combustion) of the fuel. During the reactions mentioned, oxygen and water molecules oxidize the coal and produce a gaseous mixture of carbon dioxide (CO 2), carbon monoxide (CO), water vapour (H 2 O), and molecular hydrogen (H 2). (Some by-products like tar ...
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