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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