Coal is a sedimentary rock formed by the accumulation and decay of organic substances derived from plant tissues and exudates that have been buried over periods of geological time along with various mineral inclusions. Coal is classified by type and rank. Coal type classifies coal by the plant sources from which it was derived.

Coal rank classifies coal by its degree of metamorphosis from the original plant sources and is therefore a measure of the age of the coal. The process of metamorphosis or aging is termed coalification.

The study of coal by type is known as coal petrography. Coal type is determined from the examination of polished sections of a coal sample using a reflected-light microscope.

The degree of reflectance and color of a sample are identified with specific residues of the original plant tissues. These various residues are referred to as macerals. Macerals are collected into three main groups: vitrinite, inertinite, and exinite (sometimes referred to as liptinite).

Coal rank is the most important property of coal, since it is rank which initiates the classification of coal for use. Rank is a measure of the age or degree of coalification of coal. Coalification describes the process which the buried organic matter goes through to become coal.

When first buried, the organic matter has a certain elemental composition and organic structure. However, as the material becomes subjected to heat and pressure, the composition and structure slowly change.

Certain structures are broken down, and others are formed. Some elements are lost through volatilization while others are concentrated through a number of processes, including being exposed to underground flows which carry away some elements and deposit others. Coalification changes the values of various properties of coal.

Thus, coal can be classified by rank through the measurement of one or more of these changing properties. In the United States and Canada, the rank classification scheme defined by the American Society of Testing and Materials (ASTM) has become the standard. In this scheme, the properties of gross calorific value and fixed carbon or volatile matter content are used to classify a coal by rank.

Gross calorific value is a measure of the energy content of the coal and is usually expressed m units of energy per unit mass. Calorific value increases as the coal proceeds through coalification. Fixed carbon content is a measure of the mass remaining after heating a dry coal sample under conditions specified by the ASTM.

Fixed carbon content increases with coalification. The conditions specified for the measurement of fixed carbon content result in being able alternatively to use the volatile matter content of the coal measured under dry, ash-free conditions as a rank parameter.

The rank of a coal proceeds from lignite, the “youngest” coal, through subbituminous, bituminous, and semibituminous, to anthracite, the “oldest” coal. Others prefer to classify such deposits as graphite.

Graphite is a minimal resource and is valuable primarily for uses other than as a fuel.) According to the ASTM scheme, coals are ranked by calorific value up to the high volatile A bituminous rank, which includes coals with calorific values (measured on a moist, mineral matterfree basis) greater than 14,000 Btu/lb (32,564 kJ/kg). At this point, fixed carbon content (measured on a dry, mineral matter-free basis) takes over as the rank parameter.

Thus, a high volatile A bituminous coal is defined as having a calorific value greater than 14,000 Btu/lb, but a fixed carbon content less than 69 wt%. The requirement for having two different properties with which to define rank arises because calorific value increases significantly through the lower-rank coals, but very little (in a relative sense) in the higher-ranks, whereas fixed carbon content has a wider range in higher-rank coals, but little (relative) change in the lower-ranks. The most widely used classification scheme outside of North America is that developed under the jurisdiction of the International Standards Organization, Technical Committee 27, Solid Mineral Fuels.

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