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

 Uses of Coal
 Coal Formation
 World coal reserves
 Global Temperature over time
 Age of coal deposits
 Global Carbon Dioxide emissions
  Peat
Atmospheric Carbon Dioxide levels
 Peat and coal
 Greenhouse effect


surface mine

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).


Two broad categories of coal

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.


Coalification

  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 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.


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). 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.
Rank Stages
                                                 %carbon
                                                                %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.

room and pillar mine

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.
longwall mining
 

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 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

underground mine


Surface Coal Mining

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.



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