Page 151 - Synthetic Fuels Handbook

FUELS FROM COAL 137

Coal is also classified on the basis of its macroscopic appearance (generally referred
to as coal rock type, lithotype, or kohlentype) and there are four main types: (a) vitamin
(Glanzkohle or charbon brillant), which is dominated by vitrinite group macerals and appears
glassy to the unaided eye, (b) clarain (Glanzstreifenkohle or charbon semi-brillant), which is
composed of both vitrinite and exinite and has an appearance between that of vitrain and
durain, (c) durain (Mattkohle or charbon mat), which is generally composed of fine-grained
inertinites and exinites and has a dull, mat-like luster, and (d) fusion (Faserkohle or charbon
fibreux), which is composed mainly of fusinite and resembles wood charcoal (because it
soils the hands just as charcoal would).
Coal analysis may be presented in the form of proximate and ultimate analyses, whose
analytic conditions are prescribed by organizations such as the ASTM. A typical proximate
analysis includes the moisture content, ash yield (that can be converted to mineral matter
content), volatile matter content, and fixed carbon content.
It is important to know the moisture and ash contents of a coal because they do not
contribute to the heating value of a coal. In most cases ash becomes an undesirable residue
and a source of pollution, but for some purpose (e.g., use as a chemical feedstock or for
liquefaction) the presence of mineral matter may be desirable. Most of the heat value of a
coal comes from its volatile matter, excluding moisture, and fixed carbon content. For most
coals, it is necessary to measure the actual amount of heat released upon combustion, which
is expressed in British thermal units (Btu) per pound.
Fixed carbon is the material, other than ash, that does not vaporize when heated in the
absence of air. It is determined by subtracting the weight percent sum of the moisture, ash,
and volatile matter—in weight percent from 100 percent.
Ultimate analyses are used to determine the carbon, hydrogen, sulfur, nitrogen, ash,
oxygen, and moisture contents of a coal. For specific applications, other chemical analyses
may be employed. These may involve, for example, identifying the forms of sulfur present;
sulfur may occur in the form of sulfide minerals (pyrite and marcasite, FeS ), sulfate miner-
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als (gypsum, Na SO ), or organically bound sulfur. In other cases the analyses may involve
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determining the trace elements present (e.g., mercury, chlorine), which may influence the
suitability of a coal for a particular purpose or help to establish methods for reducing envi-
ronmental pollution.
5.3 MINING AND PREPARATION

Early coal mining (i.e., the extraction of coal from the seam) was small-scale, the coal
lying either on the surface, or very close to it. Typical methods for extraction included drift
mining and bell pits. In Britain, some of the earliest drift mines (in the Forest of Dean)
date from the medieval period. As well as drift mines, small scale shaft mining was used.
This took the form of a bell pit, the extraction working outward from a central shaft, or
a technique called room and pillar in which rooms of coal were extracted with pillars left
to support the roofs. Both of these techniques however left considerable amount of usable
coal behind.
Deep shaft mining started to develop in England in the late eighteenth century, although
rapid expansion occurred throughout the nineteenth and early twentieth century. The coun-
ties of Durham and Northumberland were the leading coal producers and they were the
sites of the first deep pits. Before 1800 a great deal of coal was left in places as support
pillars and, as a result in the deep pits (300–1000 ft deep) of these two northern counties
only about 40 percent of the coal could be extracted. The use of wood props to support the
roof was an innovation first introduced around 1800. The critical factor was circulation of
air and control of explosive gases.