218 POWER PLANT ENGINEERINGThere are many ways of classifying coal according to its chemical and physical properties The
most accepted system is the one used by the American Society for Testing and Materials (ASTM),
which classifies coals by grade or rank according to the degree of metamorphism (change in form and
structure under the influences of pressure, and water), ranging from the lowest state, lignite, to the
highest, anthrasite (ASTM D 388). These classifications are briefly described below in a de ..riding
order.
Anthracite. This is the highest grade of coal. It cotains a high content, 86 to 98 mass percent of
fixed carbon (the carbon content in the elemental state) on a dry, mineral matter-free basis and a low
content of volatile matter, less than 2 to 14 mass percent chiefly methane, CHn. Anthracite is a shiny
black, dense, hard, brittle coal that — borders on graphite at the upper end of fixed carbon. It is slow-
burning and has a heating value just below that of the highest for bituminous coal. Its use in steam
generators is largely confined to burning on stokers, and rarely in pulverized form. In the United States
it is mostly found in Pennsylvania.
The anthracite rank of coal is subdivided into three groups. In descending order of fixed-carbon
percent, they are meta-anthracite, greater than 98 percent anthracite, 92 to 98 percent; and semi-anthra-
cite, 86 to 92 percent.
Bituminous coal. The largest group, bituminous coal is a broad class of coals containing 46 to 86
mass percent of fixed carbon and 20 to 40 percent of volatile matter of more complex content than that
found in anthracite. It derives its name from bitumen, an asphaltic residue obtained in the distillation of
some fuels. Bituminous coals range in heating value from 11,000 to more than 14,000 Btu/lbm (about
25,600 to 32,600 kJ/kg). Bituminous coals usually burn easily, especially in pulverized form.
The bituminous rank is subdivided into five groups: low-volatile, medium-volatile, and high-
volatile A, B, and C. The lower the volatility, the higher the heating value. The low-volatility group is
grayish black and granular in structure, while the high volatility groups are homogeneous or laminar.
Subbituminous coal. This is a class of coal with generally lower heating values than bituminous
coal, between 8300 to 11,500 Btu/lbm (about 19,300 to 26,750 kJ/kg). It is relatively high in inherent
moisture content, as much as 15 to 30 percent, but often low in sulfur content. It is brownish black or
black and mostly homogeneous in structure. Subbituminous coals are usually burned in pulverized form.
The subbituminous rank is divided into three groups: A, B, and C.
Lignite. The lowest grade of coal, lignite derives its name from the Latin lignum, which means
‘‘wood.’’ It is brown and laminar in structure, and remnants of wood fiber are often visible in it. It
originates mostly from resin-rich plants and is therefore high in both inherent moisture, as high as 30
percent, and volatile matter. Its heating value ranges between less than 6300 to 8300 Btu/lb. (about
14,650 to 19,300 kJ/kg). Because of the high moisture content and low heating value, lignite it is not
economical to transport over long distances and it is usually burned by utilities at the mine site. The
lignite rank is subdivided into two groups: A and B.
Peat. Peat is not an ASTM rank of coal. It is, however, considered the first geological step in
coal’s formation. Peat is a heterogeneous material consisting of decomposed plant matter and inorganic
minerals. It contains up to 90 percent moisture. Although not attractive as a utility fuel, it is abundant in
many parts of the world. Several states in the United States have large deposits. Because of its abun-
dance, it is used in a few countries (Ireland, Finland, the USSR) in some electric generating plants and in
district heating.