Depending upon the evolutionary developmental phase of coal formation
Four kinds of coal
The first evolutionary phase is peat which is little more than wood
pulp that has been badly decomposed. There are large deposits of peat in
the Scandinavian countries and Greenland. One can strip mine it since it
is basically very close to the ground level. Strip mining is the
process of scaping the coal from the top surface of the ground. The
problem with peat is that it has a very low British Thermal Units (BTU)
production per pound of the fuel burned. In addition, strip mining is
ecologically very destructive unless the mining company makes a
conscious effort to restore the country side.
The second phase in the evolutionary development of coal is lignite.
Lignite is found in great quantities in the Western part of this country.
Again lignite is not particularly efficient in producing energy per mass
of fuel. There have been quite a bit of effort recently in the
liquification and gasification of lignite. Liquification converts lignite
into liquid crude petroleum. Gasification plants convert lignite into
natural gas products. The conversion process is quite expensive, and with
the present cost of other forms of fuel, it is economically infeasible.
However, if other fuels become too expensive, this could be a more
economical process. Other research has been conducted investigating other
uses of lignite such as a fertilizer in hydroponic plant growth.
Hydroponics is the use of nutrient containing water instead of soil in the
growth of plant life.
A third phase in this coal development is soft coal (Bitumenous)
which is one of the two stages used as a fuel in generating
The fourth and final phase results in the formation of hard coal
Two broad categories of coal
Coal is an organic sediment consisting of a complex mixture of substances.
More common and originates from peat deposits consisting mostly of organic
debris deposited in situ (autochthonous).
Derived from redeposited (allochthonous) resistant plant fragments such as
spores or aquatic plants.
The sapropelic coals can be further subdivided into:
Cannel coal is made up principally of uniformly sized plant fragments eg
spores boghead coal Consists mainly of alginite (a coal maceral derived
Peat is formed from the deposition of organic material with a
restricted supply of oxygen. Peat forming environments are known generally
Mires may be classified as limnic or paralic
Limnic coal deposits formed inland in freshwater basins, peat bogs,
or swamps. Peat forming environments isolated subsiding basins produce
limnic coal deposits.
Paralic deposit simply that there was a hydrological connection with
the sea at the time of peat deposition. Mires may be found along coastal
lowlands; as back barrier lagoons, estuaries and deltas.
The type of original plant input, the availability of nutrients, climatic
conditions, the level of the water table, the pH and Eh conditions all
help to determine the type of peat that is formed. Every part of the
ecosystem of the peatland or mire may be represented in the peat,
including the large trees, herbaceous shrubs, grasses, aquatic plants and
the micro-organisms that break down the organic material. For a coal to be
developed, the peat has to be buried and preserved. The process that
converts peat to coal is called coalification. The degree of coalification
which has taken place determines the rank of the coal.
The transformation of plant material into coal takes place in two stages,
biochemical degradation and physico-chemical degradation. Biochemical degradation involves chemical decomposition of
botanical matter assisted by organisms.
In tropical environments, this process may be faster, since the warm
moist conditions are ideal for the organisms that assist in this
process such as bacteria and fungi. However plant growth is also more
rapid and so the increased rate of decomposition may be balanced by
plant growth. In tropical conditions high rates of evaporation need to
be coupled with high precipitation to maintain plant growth and peat
In cooler climates the growth rate of vegetation may be cyclical in
nature and slower since the seasonal variation in conditions is
greater. The conditions are less ideal for fungi and bacteria so the
slower growth rate is matched by a slower rate of biochemical
Humification affects the soft contents of the plants cells before
the cell walls, which consist of cellulose, hemicellulose and lignin which
is the most resistant compound. Humification begins with the oxidation of
plant matter and attack by aerobic organisms such as fungi, insects and
aerobic bacteria. Hydrocarbons are extracted from the tissue and the
material left behind is relatively enriched in oxygen and carbon.
Semifusinite, an inertinite maceral may be formed in this manner. Various
humic substances are formed at this time, these are acidic in nature. If
this continues the plant material will be completely degraded into carbon
dioxide and water. When the plant material or degraded plant material is
buried below the ground water table aerobic organisms and oxidation can no
longer attack the material. Anaerobic bacteria may still decompose the
plant matter until it reaches a depth or conditions unsuitable for these
organisms. Anaerobic bacteria utilise the oxygen in the plant matter, so
all molecules may be attacked even the more resistant compounds. However
the softer tissue may be more rapidly affected. Biochemical coalification ends at the rank of sub-bituminous coal,
when humic substances have polymerised. Physico-chemical coalification which follows is caused by
conditions of burial.
The overburden which is deposited, the heat flows in the earth's crust and
tectonic heat and pressure change the chemistry and structure of the
altered organic material. The same conditions are applied to all the
Water is squeezed out and pore size is reduced as pressure increases and
oxygen and hydrogen are released during thermal cracking. Water and carbon
dioxide are the first products released.
When rank reaches medium volatile bituminous coal demethanation
Concept of Coal Rank
The rank of a coal refers to the degree of coalification endured by
the organic matter. It is estimated by measuring the moisture content,
specific energy, reflectance of vitrinite or volatile matter (these are
known as rank parameters). stages.
Coal uses based on rank
Underground Coal Mining
This drawing depicts the room and pillar method of underground mining.
Most underground coal is mined by the room and pillar method,
whereby rooms are cut into the coal bed leaving a series of pillars, or
columns of coal, to help support the mine roof and control the flow of
air. Generally, rooms are 630 metres wide and the pillars up to 100 metres
wide. As mining advances, a grid-like pattern of rooms and pillars is
formed. When mining advances to the end of a panel or the property line,
retreat mining begins. In retreat mining, the workers mine as much coal as
possible from the remaining pillars until the roof falls in. When retreat
mining is completed, the mined area is abandoned.
There are two types of room and pillar mining--conventional mining and
. Conventional miningis the oldest method and accounts for only about
12% of underground coal output. In conventional mining, the coal seam is
cut, drilled, blasted and then loaded into cars. Continuous miningis the most prevalent form of underground mining,
accounting for 56% of total underground production. In continuous mining,
a machine known as a continuous miner cuts the coal from the mining face,
obviating the need for drilling and blasting.
Types of underground mines--shaft mines,
slope mines and drift mines.
The decision of what type of mine to construct depends on the depth of the
coal seam and the surrounding terrain. Drift mines have horizontal entries into the coal seam from a
hillside. Slope mines, which usually are not very deep, are inclined from the
surface to the coal seam. Shaft mines, generally the deepest mines, have vertical access to
the coal seam via elevators that carry workers and equipment into the
Almost all underground mines are less than 300 metres deep, but some
mines reach depths of about 600 metres. miners in Nova Scotia actually
mine coal beneath the ocean
Surface Coal Mining
open pit mine
open pit terms
Surface mining is accomplished by removing overburden from the coal seam
and then blasting and removing the coal. The ratio of overburden excavated
to the amount of coal removed is called the overburden ratio. The lower
the ratio, the more productive the mine. The lowest overburden ratios are
found in western surface mines. , often more than one coal seam is mined.
There are several types of surface coal mines. Area surface mines , usually found in flat terrain, consist of a
series of cuts 30 to 60 metres wide. The overburden from one cut is used
to fill in the mined out area of the preceding cut. Contour mining , occurring in mountainous terrain, follows a coal
seam along the side of the hill. When contour mining becomes too
expensive, additional coal can often be produced from the mine's highwall
by the use of augers or highwall miners. Open pit mining is usually found where coal seams are thick. Open
pit mines can reach depths of a hundred metres.
Equipment used in surface mines include draglines, shovels, bulldozers,
front-end loaders, bucket wheel excavators and trucks. In large mines,
draglines remove the overburden while shovels are used to load the coal.
In smaller mines, bulldozers and front-end loaders are often used to