Page 150 - Synthetic Fuels Handbook

136 CHAPTER FIVE

Thus, as geologic processes increase their effect over time, the coal precursors are trans-
formed over time into:

1. Lignite—also referred to as brown coal, is the lowest rank of coal and used almost exclu-
sively as fuel for steam-electric power generation. Jet is a compact form of lignite that is
sometimes polished and has been used as an ornamental stone since the Iron Age.
2. Subbituminous coal—whose properties range from those of lignite to those of bitumi-
nous coal and are used primarily as fuel for steam-electric power generation.
3. Bituminous coal—a dense coal, usually black, sometimes dark brown, often with well-
defined bands of bright and dull material, used primarily as fuel in steam-electric power
generation, with substantial quantities also used for heat and power applications in
manufacturing industry and to make coke.
4. Anthracite—the highest rank; a harder, glossy, black coal used primarily for residential-
and commercial-space heating.
Coal classification systems are based on the degree to which coals have undergone coal-
ification. Such varying degrees of coalification are generally called coal ranks (or classes).
The determination of coal rank has a number of practical applications such as the definition
of the coal properties. The properties include the amount of heat produced during combus-
tion, the amount of gaseous products released upon heating, and the suitability of the coals
for producing coke.
The rank of a coal indicates the progressive changes in carbon, volatile matter, and probably
ash and sulfur that take place as coalification progresses from the lower rank lignite through the
higher ranks of subbituminous, high-volatile bituminous, low-volatile bituminous, and anthra-
cite. The rank of a coal should not be confused with its grade. A high rank (e.g., anthracite)
represents coal from a deposit that has undergone the greatest degree of metamorphosis and
contains very little mineral matter, ash, and moisture. On the other hand, any rank of coal, when
cleaned of impurities through coal preparation will be of a higher grade.
The most commonly employed systems of classification are those based on analyses
that can be performed relatively easily in the laboratory, as described by the American
Society for Testing and Materials (ASTM) on the basis of fixed carbon content, volatile
matter content, and calorific value. In addition to the major ranks (lignite, subbituminous,
bituminous, and anthracite), each rank may be subdivided into coal groups, such as high-
volatile A bituminous coal. Other designations, such as coking coal and steam coal, have
been applied to coals, but they tend to differ from country to country.
The term coal type is also employed to distinguish between banded coals and non-
banded coals. Banded coals contain varying amounts of vitrinite and opaque material. They
include bright coal, which contains more than 80 percent vitrinite, and splint coal, which
contains more than 30 percent opaque matter. The nonbanded varieties include boghead
coal, which has a high percentage of algal remains, and cannel coal with a high percentage
of spores. The usage of all the above terms is quite subjective.
By analogy to the term mineral, which is applied to inorganic material, the term maceral
is used to describe organic constituents present in coals. Maceral nomenclature has been
applied differently by some European coal petrologists who studied polished blocks of
coal using reflected-light microscopy (their terminology is based on morphology, botanical
affinity, and mode of occurrence) and by some North American petrologists who studied
very thin slices (thin sections) of coal using transmitted-light microscopy.
Three major maceral groups are generally recognized: vitrinite, exinite, and inertinite. The
vitrinite group is the most abundant and is derived primarily from cell walls and woody tissues.
Several varieties are recognized, for example, telinite (the brighter parts of vitrinite that make up
cell walls) and collinite (clear vitrinite that occupies the spaces between cell walls).