coal
Coal Types, Formation and Methods of Mining
Four
kinds of coal
Two broad
categories of coal
Coalification
Concept
of Coal Rank
Types
of underground mines
Surface
Coal Mining
Types
of surface coal mines
Coal - Uses, formation affects
Depending upon the evolutionary developmental
phase of coal formation
there are:
Four
kinds of coal.
The first evolutionary phase ispeat
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 electrical power.
The fourth and final phase results in the formation of hard coal (anthracite).
Coal is an organic sediment consisting of a complex mixture of substances.
humic More common and originates
from peat deposits consisting mostly of organic debris deposited in
situ
(autochthonous).
sapropelicDerived from redeposited
(allochthonous) resistant plant fragments such as spores or aquatic
plants.
The sapropelic coals can be further subdivided
into:
cannel coalCannel coal is made
up principally of uniformly sized plant fragments eg spores boghead
coalConsists mainly of alginite (a coal maceral derived from algae).
Peat is formed from the deposition of organic
material with a restricted supply of oxygen. Peat forming environments
are known generally as 'mires'.
Mires may be classified as limnic or paralic
Paralic depositsimply 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.
Peat forming environments isolated subsiding basins produce limnic coal deposits.
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.
CoalificationThe 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 accumulation.
In cooler climatesthe 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
degradation.
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 macerals.
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 begins.
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). See Table 1 for details of the different rank stages.
Table 1. From Diessel (1992) indicates the difference in rank parameter with increase in rank.
%volatile matter
specific energy
% in situ moisture
% vitrinite
reflectance random max
wood 50 >65
peat 60 >60 14.7 75 0. 20 0. 20
brown coal 71 52 23 30 0.40 0.42
sub-bituminous 80 40 33.5 5 0. 60 0.63
high volatile bituminous coal 86 31 35.6 3 0.97 1.03
medium volatile bituminous coal 90 22 36 <1 1.47 1.58
low volatile bituminous coal 91 14 36.4 1 1.85 1.97
semi-anthracite 92 8 36 1 2.65 2.83
anthracite 95 2 35.2 2 6.55 7
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 continuous mining.
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.
The decision of what type of mine to construct depends on the depth of the coal seam and the surrounding terrain.
Drift mineshave 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 mine.
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 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 remove overburden.