9.3 COAL
427
9.3 COAL
9.3.1 Introduction
There is no doubt that coal is of vegetable origin because coals not only contain recognizable plant remains, but transitions can be found between obvious accumulations of
vegetable matter, for example, peat, through lignites or brown coals, into true coals and
on into anthracite. This series, from peat, through lignite, into the humic coals and finally anthracite is called the "coal series." The position of a coal in the series is termed
its "rank." Thus lignite is a very low-rank coal, while at the other extreme, anthracite
is a very high-rank coal. Physical appearance, physical properties, and chemical composition of coals change with rank as do their utilization characteristics, such as calorific
value, coking properties, and gas generation potential (Gayer and Harris, 1996).
Thus knowledge of the rank of a coal can be a guide to its utilization. The changes
that vegetable matter undergo in the course of alteration to coal are termed "maturation" or "coalification." Maturation takes place in two stages: the peat stage and the
burial stage. In the peat stage the plant material suffers a measure of biochemical degradation, and when it is buried, progressive increase in both overburden load and temperature bring about dynamothermal maturation that slowly turns the peat into coal.
The peat stage is an essential prerequisite for the formation of coal. Under normal circumstances when plants die, they are exposed to air and are broken down primarily
by oxidation and also by various organisms, particularly the fungi and aerobic bacteria.
Where plant remains accumulate in swamp or bog environments, however, they become
water saturated. Aerobic decay soon depletes the water of oxygen, the aerobic organisms die off and anaerobic bacteria take over. The anaerobic bacteria operate without
oxygen but they are equally as capable of breaking down organic matter as the aerobic
forms. Because of the stagnant nature of swamps and bogs, however, the waste products of the bacteria are not flushed away, but build up in interstitial waters and ultimately render the environment sterile. Bacterial activity is thus curtailed and the partially decomposed plant material remains in a state of arrested decay. In this state the
material is peat. If the peat is drained the toxic materials are flushed out, decomposition sets in again, and the peat may ultimately be destroyed. If the peat is not drained,
however, but is buried under relatively impermeable sediments, its geological preservation becomes possible.
9.3.2 Coal Petrography
Coal is a mature Type III variety of kerogen (see Section 7.3.2). Chemically, coals comprise the three elements carbon, hydrogen, and oxygen with minor proportions of sulfur
and nitrogen and mineral impurities. The latter remain as ash after the coal has been
burned and, clearly, high ash content is undesirable. Inherent sulfur is normally present
in a very small amount and was probably derived from sulfur proteins in the original
plants. Some coals have a high sulfur content due to the presence of disseminated pyrites: this is deleterious because it generates sulfurous fumes on combustion of the coal.
Although the nitrogen content of coal is small, it was economically important in the
production of ammonia as a by-product of the coal-gas industry.
427
9.3 COAL
9.3.1 Introduction
There is no doubt that coal is of vegetable origin because coals not only contain recognizable plant remains, but transitions can be found between obvious accumulations of
vegetable matter, for example, peat, through lignites or brown coals, into true coals and
on into anthracite. This series, from peat, through lignite, into the humic coals and finally anthracite is called the "coal series." The position of a coal in the series is termed
its "rank." Thus lignite is a very low-rank coal, while at the other extreme, anthracite
is a very high-rank coal. Physical appearance, physical properties, and chemical composition of coals change with rank as do their utilization characteristics, such as calorific
value, coking properties, and gas generation potential (Gayer and Harris, 1996).
Thus knowledge of the rank of a coal can be a guide to its utilization. The changes
that vegetable matter undergo in the course of alteration to coal are termed "maturation" or "coalification." Maturation takes place in two stages: the peat stage and the
burial stage. In the peat stage the plant material suffers a measure of biochemical degradation, and when it is buried, progressive increase in both overburden load and temperature bring about dynamothermal maturation that slowly turns the peat into coal.
The peat stage is an essential prerequisite for the formation of coal. Under normal circumstances when plants die, they are exposed to air and are broken down primarily
by oxidation and also by various organisms, particularly the fungi and aerobic bacteria.
Where plant remains accumulate in swamp or bog environments, however, they become
water saturated. Aerobic decay soon depletes the water of oxygen, the aerobic organisms die off and anaerobic bacteria take over. The anaerobic bacteria operate without
oxygen but they are equally as capable of breaking down organic matter as the aerobic
forms. Because of the stagnant nature of swamps and bogs, however, the waste products of the bacteria are not flushed away, but build up in interstitial waters and ultimately render the environment sterile. Bacterial activity is thus curtailed and the partially decomposed plant material remains in a state of arrested decay. In this state the
material is peat. If the peat is drained the toxic materials are flushed out, decomposition sets in again, and the peat may ultimately be destroyed. If the peat is not drained,
however, but is buried under relatively impermeable sediments, its geological preservation becomes possible.
9.3.2 Coal Petrography
Coal is a mature Type III variety of kerogen (see Section 7.3.2). Chemically, coals comprise the three elements carbon, hydrogen, and oxygen with minor proportions of sulfur
and nitrogen and mineral impurities. The latter remain as ash after the coal has been
burned and, clearly, high ash content is undesirable. Inherent sulfur is normally present
in a very small amount and was probably derived from sulfur proteins in the original
plants. Some coals have a high sulfur content due to the presence of disseminated pyrites: this is deleterious because it generates sulfurous fumes on combustion of the coal.
Although the nitrogen content of coal is small, it was economically important in the
production of ammonia as a by-product of the coal-gas industry.