Power Plant Engineering

Formation.
 
Coal is composed mainly of carbon though it also contains hydrogen and oxygen
and varying small amounts of nitrogen, sulfur and other elements. It was formed by the decomposition
of the remains of vegetation growing in swamps or in large river deltas undergoing intermittent subsid-
ence. The decomposed material from plants and trees was transformed first by bacterial action into peat
which become buried by later sedimentary deposits. Later under the movement of the earth’s crust, the
layers of peat become more deeply buried, and under the influence of heat and biochemical reactions
they were transformed into various types of coal or lignite, during this coalification process, the carbon
content increased as oxygen and hydrogen were released. Methane (CH
4
) was formed and either es-
caped into atmosphere or migrated until it was captured in a geological trap so that it formed a natural
gas reservoir contained by an impervious layer similar to those that contain petroleum.
Properties. Coals are ranked according to their carbon content. Under mild conditions of heat
and pressure, the lowest rank coals were formed, consisting of brown coal and lignite. At higher tem-
peratures and pressures, sub-bituminous and bituminous coals were formed, and under very high pres-
sures, the highest rank coals, called anthracites, were formed. The anthracites contain more than 92%
carbon, 2–3% hydrogen together with oxygen, volatile matter and impurities. Bituminous coal contains
about 5% hydrogen and has a carbon content of 70–80%. The lowest ranks of lignite and brown coal
may have less than 50% carbon content. The rank by carbon content approximates to a ranking by heat
content though with some overlap between classes. Other classifications of importance include the
coking qualities for mechanical strength, ash content, and volatile matter content. Sulfur is an impor-
tant impurity as it appears in combustion products as oxides of sulfur (SO
2
), which pollutes the envi-
ronment.
Mining. Most hard coal (bituminous and anthracite) is obtained by deep mining though modern
technology has led to the increasing use of open-cast methods using large excavators capable of shift-
ing hundreds of tonnes per hour and the mines may reach depths of several hundreds of feet. Surface
mining is cheaper than deep mining and rapid expansion is possible. Deep mining requires minimum of
two shafts and expects to take 10 years to bring into operation. The mining shafts play a crucial role in
providing ventilation to the mine, to remove methane associated with coal and to reduce heat and
humidity. Two principal methods of mining in use are:
1. Longwall
2. Bord and Pillar or Room and Pillar
In Long wall method, coal is extracted in one operation from a face that may be of 600 m in
length. In the older Bord and Pillar method, the area is divided into rectangles by driving a series of
roadways at right angles to each other and then mining from each of these rectangles or pillars. In
modern mines, over 90% of the coal is mined, loaded and transported mechanically. Transport under-
ground is mainly by means of conveyor belts (replacing earlier tubs), which bring the coal to the main
shaft for rising to the surface. At the mine head, the coal is cleaned, sorted or screened and blended.

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NON-CONVENTIONAL ENERGY RESOURCES AND UTILISATION
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