Rank of Coal

The degree of coal maturation is known as the rank of coal and is an indication of the extent of metamorphism the coal has undergone. Rank is also a measure of carbon content as the percentage of fixed carbon increases with extent of metamorphism. In the United States, lignites and subbitu-minous coals are referred to as being low in rank, while bituminous coals and anthracites are classified as high-rank coals. Figure 1-4 illustrates the relationship between rank and fixed-carbon content [14]. The fixed-carbon content shown in Figure 1-4 is calculated on a dry, mineral-matter-free basis. Figure 1-4 also shows the comparison between heating value and rank; the heating value is calculated on a moist, mineral matter-free basis. Note that the heating value increases with increasing rank but begins to decrease with semi-anthracitic and higher rank coals. This decrease in heating value is due to the significant decrease in volatile matter, which is shown in Figure 1-4 [14].

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100 80 i 60

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