coal mining

Table of Contents

 Introduction
 History
 Coal deposits
 Prospecting and exploration
 Choosing a mining method
 Surface mining
 Underground mining
 Coal preparation
 Coal transportation

coal mine Schematic diagram of an underground coal mine, showing surface

facilities, access shafts, and room-and-pillar and longwall mining methods.(more)

Coal mining, extraction of coal deposits from the surface of Earth and
from underground.

Mongolia: coal mineTavantolgoi coal mine, southern Gobi, Mongolia.

Coal is the most abundant fossil fuel on Earth. Its predominant use has
always been for producing heat energy. It was the basic energy source
that fueled the Industrial Revolution of the 18th and 19th centuries, and
the industrial growth of that era in turn supported the large-scale
exploitation of coal deposits. Since the mid-20th century, coal has yielded
its place to petroleum and natural gas as the principal energy supplier of
the world. The mining of coal from surface and underground deposits
today is a highly productive, mechanized operation.

History
Ancient use of outcropping coal
There is archaeological evidence that coal was burned in funeral pyres
during the Bronze Age, 3,000 to 4,000 years ago, in Wales. Aristotle
mentions coal (“combustible bodies”) in his Meteorologica, and his pupil
Theophrastus also records its use. The Romans in Britain burned coal
before 400CE; cinders have been found among the ruins of Roman villas
and towns and along the Roman wall, especially in Northumberland, near
the outcrop of coal seams. The Hopi Indians of what is now the
southwestern United States mined coal by picking and scraping and used
it for heating, cooking, and in ceremonial chambers as early as the 12th
century CE; in the 14th century they used it industrially in pottery
making. Marco Polo reports its use as widespread in 13th-century China.
The Domesday Book (1086), which recorded everything of economic
value in England, does not mention coal. London’s first coal arrived by
sea in 1228, from the areas of Fife and Northumberland, where lumps
broken from submarine outcroppings and washed ashore by wave action
were gathered by women and children. Thereafter, the name sea
coal was applied to all bituminous coal in England. Later in the century,
monks began to mine outcroppings in the north of England.

Developments in mine entry

Shafts
Except for the Chinese, who may have mined coal underground, all the
early coal seams were worked from the surface, in fully exposed
outcroppings. In the later Middle Ages, however, exhaustion of outcrop
coal in many places forced a change from surface to underground,
or shaft, mining. Early shaft mines were little more than wells widened as
much as miners dared in the face of danger of collapse. Shafts were sunk
on high ground, with adits—near-horizontal tunnels—for drainage driven
into the side of the hill. In England some shallow mine shafts were
exhausted as early as the 14th century, making it necessary to go deeper
and expand mining at the shaft bottoms. These remained small
operations; a record of 1684 shows 70 mines near Bristol, employing 123
workers. Greater depth created many problems. First, water could no
longer simply be drained away. Crude methods were devised to lift it to
the surface. A bucket-and-chain device was first powered by men and
later by horses; a continuous belt of circular plates was drawn up
through a pipe. Windmills were used for pumps. But shafts had to be
restricted to depths of 90 to 105 metres (300 to 350 feet) and a mining
radius of 180 metres. It was not until 1710 that the water problem was
eased by Thomas Newcomen’s steam atmospheric engine, which supplied
a cheap and reliable power source for a vertical reciprocating lift pump.
Hoisting
Raising the coal itself was another problem. Manpower, operating a
windlass, was replaced by horsepower; and, as the shafts went deeper,
more horses were added. At Whitehaven in 1801, coal was hoisted 180
metres by four horses at the rate of 42–44 metric tons (46–48 tons) in
nine hours. The introduction of the steam engine to hoist coal was a
major turning point for the industry. Small steam-powered windlasses
were successfully tried out about 1770. About 1840 the first cage was
used to hoist the loaded car; and from 1840 onward advances in coal-
mining techniques were rapid.

Ventilation
The presence of noxious and flammable gases caused miners to recognize
the critical importance of ventilation in coal mines from the earliest days.
Natural ventilation was afforded by level drainage tunnels driven from
the sloping surface to connect with the shaft. Surface stacks above the
shaft increased the efficiency of ventilation; their use continued in small
mines until the early 20th century. The most reliable method, before the
introduction of fans, was the use of a furnace at the shaft bottom or on
the surface. Despite the hazard of fire and explosion, there were still a
large number of furnaces operating, at least in nongassy mines, in the
early 20th century.

Open-flame illumination, however, was a much more common cause of

explosions until the introduction of the Davy safety lamp (about 1815), in
which the flame is enclosed in a double layer of wire gauze that prevents
ignition of flammable gases in the air of the mine. Presence of strong air
currents, however, made even the Davy lamp unsafe.

Rotary ventilating fans were introduced in mines in the 18th century.

Originally of wood and powered by steam, they were improved
throughout the 19th and 20th centuries by the introduction
of steel blades, electric power, and aerodynamically efficient shapes for
the blades.

From manual to mechanized extraction

Conventional mining
Early European miners wedged coal out of the seam or broke it loose
with a pick. After explosives were introduced, it was still necessary to
undercut the coal seam with hand tools. The advent of
steam, compressed air, and electricity brought relief from this hard,
dangerous work. In 1868, after almost 100 years of trial and error, a
commercially successful revolving-wheel cutter for undercutting the coal
seam was introduced in England. This first powered cutting tool was soon
improved by introduction of compressed air as a power source in place of
steam. Later, electricity was used. The longwall cutter was introduced in
1891. Originally driven by compressed air and later electrified, it could
begin at one end of a long face (the vertical, exposed cross section of a
seam of coal) and cut continuously to the other.

Development of continuous mining

The conventional mining techniques described above, made up of the
cyclic operations of cutting, drilling, blasting, and loading, developed in
association with room-and-pillar mining. The oldest of the basic
underground methods, room-and-pillar mining grew naturally out of the
need to recover more coal as mining operations became deeper and more
expensive. During the late 1940s, conventional techniques began to be
replaced by single machines, known as continuous miners, that broke off
the coal from the seam and transferred it back to the haulage system.
The Joy Ripper (1948) was the first continuous miner applicable to the
room-and-pillar method.

Origins of longwall mining

The other principal method of modern mining, longwall mining, had been
introduced as early as the 17th century and had found general use by the
19th century, but it had long been less productive than room-and-pillar
mining. This began to change in the 1940s, when a continuous system
involving the “plow” was developed by Wilhelm Loebbe of Germany.
Pulled across the face of the coal and guided by a pipe on the face side of
a segmented conveyor, the plow carved a gash off the bottom of the
seam. The conveyor snaked against the face behind the advancing plow
to catch the coal that chipped off from above the gash. Substantially
reducing the labour required at the coal face (except that needed to
install roof support), the Loebbe system quickly became popular in
Germany, France, and the Low Countries.

The plow itself had limited application in British mines, but the power-
advanced segmented conveyor became a fundamental part of equipment
there, and in 1952 a simple continuous machine called the shearer was
introduced. Pulled along the face astride the conveyor, the
shearer bore a series of disks fitted with picks on their perimeters and
mounted on a shaft perpendicular to the face. The revolving disks cut a
slice from the coal face as the machine was pulled along, and a plow
behind the machine cleaned up any coal that dropped between the face
and the conveyor.

Roof support
The technique of supporting the roof by rock bolting became common in
the late 1940s and did much to provide an unobstructed working area for
room-and-pillar mining, but it was a laborious and slow operation that
prevented longwall mining from realizing its potential. In the late 1950s,
however, powered, self-advancing roof supports were introduced by the
British. Individually or in groups, these supports, attached to the
conveyor, could be hydraulically lowered, advanced, and reset against
the roof, thus providing a prop-free area for equipment (between the coal
face and the first row of jacks) and a canopied pathway for miners
(between the first and second rows of jacks).

Haulage
Manual labour to electric power
In the first shaft mines, coal was loaded into baskets that were carried on
the backs of men or women or loaded on wooden sledges or trams that
were then pushed or hauled through the main haulage roadway to the
shaft bottom to be hung on hoisting ropes or chains. In drift and slope
mines, the coal was brought directly to the surface by these and similar
methods. Sledges were pulled first by men and later by animals,
including mules, horses, oxen, and even dogs and goats.

Steam locomotives designed by Richard Trevithick were used in the fields

of South Wales and Tyne and later in Pennsylvania and West Virginia, but
they created too much smoke. Compressed-air locomotives, which
appeared in the 1880s, proved expensive to operate. Electric
locomotives, introduced in 1887, rapidly became popular, but mules and
horses were still working in some mines as late as the 1940s.

Mechanized loading
The loading by hand of broken coal into railcars was made obsolete early
in the 20th century by mobile loaders. The Stanley Header, the first coal-
loading machine used in the United States, was developed in England
and tested in Colorado in 1888. Others were developed, but few
progressed beyond the prototype stage until the Joy machine was
introduced in 1914. Employing the gathering-arm principle, the Joy
machine provided the pattern for future successful mobile loaders. After
the introduction in 1938 of electric-powered, rubber-tired shuttle cars
designed to carry coal from the loading machine to the elevator, mobile
loading and haulage rapidly supplanted track haulage at the face of
room-and-pillar mines.

Conveyors
In 1924 a conveyor belt was successfully used in an anthracite mine in
central Pennsylvania to carry coal from a group of room conveyors to a
string of cars at the mine entry. By the 1960s belts had almost
completely replaced railcars for intermediate haulage.

Preparation
The history of coal preparation begins in the 19th century, with
the adaptation of mineral-processing methods used for enriching metallic
ores from their associated impurities. In the early years, larger pieces of
coal were simply handpicked from pieces composed predominantly of
mineral matter. Washing with mechanical devices to separate the coal
from associated rocks on the basis of their density differences began
during the 1840s.

At first, coal preparation was necessitated by the demand for higher

heating values; another demand was for such special purposes as
metallurgical coke for steelmaking. In recent years, as concern has
grown over the emission of sulfur dioxide in the flue gases of power
plants, coal preparation has taken on greater importance as a measure to
remove atmospheric pollutants.

Coal deposits
Formation
Coalification
In geologic terms, coal is a sedimentary rock containing a mixture
of constituents, mostly of vegetal origin. Vegetal matter is composed
mainly of carbon, hydrogen, oxygen, nitrogen, sulfur, and some inorganic
mineral elements. When this material decays under water, in the absence
of oxygen, the carbon content increases. The initial product of this
decomposition process is known as peat. Peat can be formed in bogs,
marshes, or freshwater swamps, and in fact huge freshwater swamps of
the geologic past provided favourable conditions for the formation of
thick peat deposits that over time became coal deposits.
The transformation of peat to lignite is the result of pressure exerted by
sedimentary materials that accumulate over the peat deposits. Even
greater pressures and heat from movements of Earth’s crust (as occurs
during mountain building), and occasionally from igneous intrusion,
cause the transformation of lignite to bituminous and anthracite coal.

Major coal eras

Coal deposits are known to have formed more than 400 million years ago.
Most anthracite and bituminous coals occur within the 299- to 359.2-
million-year-old strata of the Carboniferous Period, the so-called first coal
age. The formation of coal deposits continued through the Permian,
Triassic, and Jurassic periods into the “second coal age,” which includes
the Cretaceous, Paleogene, and Neogene periods. Coals of
the Cretaceous Period (145.5 million to 65.5 million years ago) are
generally in the high-volatile to medium-volatile bituminous ranks.
Cenozoic coals, formed less than 65.5 million years ago, are
predominantly of the subbituminous and lignitic ranks.

Rank and grade

The rank of a coal indicates the progressive changes in
carbon, volatile matter, and probably ash and sulfur that take place as
coalification progresses from the lower-rank lignite through the higher
ranks of subbituminous, high-volatile bituminous, low-volatile
bituminous, and anthracite. The rank of a coal should not be confused
with its grade. A high rank (e.g., anthracite) represents coal from a
deposit that has undergone the greatest degree of devolatilization and
contains very little mineral matter, ash, and moisture. On the other hand,
any rank of coal, when cleaned of impurities through coal preparation,
will be of a higher grade.

Resources and reserves

Distribution worldwide
Coal deposits are found in sedimentary rock basins, where they appear
as successive layers, or seams, sandwiched between strata of sandstone
and shale. There are more than 2,000 coal-bearing sedimentary basins
distributed around the world. World coal resources—that is, the total
amount of coal available in the world—are approximately 11 trillion tons.
The distribution of the estimated coal resources of the world is
approximately as follows: Europe (including Russia and the former Soviet
republics) 49 percent; North America 29 percent; Asia 14
percent; Australia 6 percent; and Africa and South America 1 percent
each. Distinct from coal resources are coal reserves, which are only those
resources that are technically and economically minable at a particular
time. The current recoverable coal reserves of the world are estimated at
760 billion tons. Their distribution by continent is: Europe 44 percent;
North America 28 percent; Asia 17 percent; Australia 5 percent; Africa 5
percent; and South America 1 percent.

Economic factors
Among the most important factors that influence the movement of a coal
deposit from a resource to a reserve or vice versa are the price of coal in
the energy market and the costs of producing the coal for that market.
Currently, seams less than 30 centimetres (1 foot) in thickness are not
considered economically recoverable. Furthermore, extraction from
seams at great depth—i.e., over 1,000 metres (3,300 feet)—presents
great difficulties. Other geologic features, such as excessively steep
seams, extensive faulting and folding, washouts created by erosion and
sedimentation, and burnout of the coal seams by igneous intrusion, all
affect the amount and quality of coal that can be recovered from a seam.

Prospecting and exploration

The fundamental objective of coal prospecting is to discover coal
resources through a search. In areas where coal mining has not been
previously practiced, the search process should result in obtaining coal
samples that give reasonable evidence of the existence of a coal seam.
Once a seam has been discovered, considerable further work is
necessary in order to advance knowledge of the particular geologic
aspects and the extent of the coal deposit. The term coal exploration is
used to describe these activities. Coal exploration includes activities and
evaluations necessary to gather data for making decisions on such issues
as the desirability of further exploration, the technical feasibility of
mining (including favourable and unfavourable factors), and economic
feasibility (including size of mine, coal quality assessment, marketability,
and preparation of mined coal for market requirements).

Mapping
Geologic mapping is an important task in exploration. Mapping involves
compiling detailed field notes on coal seams, strata above and below the
seam, rock types, geologic structures, stream data, and man-made
structures. Good maps and mapping techniques provide a means for
planning and accomplishing exploration, development, reclamation, day-
to-day operations, and equipment moves. Calculation of material
volumes, location of physical elements, and determination of mining
conditions are expedited by the use of maps. Maps also provide a method
for recording data so that they can be organized and analyzed for ready
reference.

Aerial photography and mapping methods (photogrammetry) are

increasing in usefulness, particularly in the exploration and mining of
surface deposits. Photogrammetric methods are relatively easy and
inexpensive, can be adjusted to any scale, and are highly accurate in
any terrain. Aerial photography can be conducted at an altitude designed
to produce maps that show drainage configuration, roads, buildings,
lakes, streams, timber, power lines, railroads, and fences or other
features that may be missed by a ground survey.

Drilling
Drilling is the most reliable method of gathering information about a coal
deposit and the mining conditions. It provides physical samples of the
coal and overlying strata for chemical and physical analysis.

Spatial patterns
Numerous factors are associated with a drilling program. One is the
spatial pattern of the holes in an exploration area. When very large areas
are being studied, hole spacings vary greatly and generally are not in any
set pattern. When the program is narrowed to a specific target area, a
grid pattern is most common. In areas where coal is known to exist,
closely spaced drill-hole patterns are required.

Core drilling and rotary drilling

A second factor associated with a drilling program is the choice between
core drilling and rotary drilling. In core drilling, a hollow drill bit is
attached to a core barrel so that cylindrical samples of the strata can be
obtained. (Since the drill bit is faceted with diamonds for cutting the
strata, this method is also called diamond core drilling.) Photographing
the cores as they come out of the hole can provide data of great
reliability. In rotary drilling, the samples obtained are the chips and
pulverized rock produced by the abrasive and chipping action of the drill
bit. Rotary drilling is faster and comparatively less expensive than core
drilling. In fact, it is not uncommon to drill down to the top of the coal
seam by rotary drilling and then replace the drill tools for core drilling.
In most programs, only 10 to 25 percent of the holes are actually cored
for detailed information on overlying strata and coal. Coring of the coal
seam itself, however, should closely approach 100 percent; if it does not,
the analytical information obtained should be considered suspect.

Dozer cutting
Exploration of coal outcrops may be accomplished with dozer cuts at
regular intervals. Dozer cutting provides information on the attitude of
the coal and on the nature of the overburden—important factors with
regard to machine operation.
Geophysical exploration
In geophysical exploration, the seismic, electric, magnetic, radiometric,
and gravitational properties of earth materials are measured in order to
detect anomalies that may be caused by the presence of mineral
deposits. Their form of exploration may begin with airborne methods in
regional and target-area investigations and continue with on-ground
methods during detailed investigations. The most widely utilized airborne
methods are, in increasing order of use, magnetic, magnetic plus
radiometric, magnetic plus electromagnetic, and electromagnetic. These
methods are almost always accompanied by aerial photography.

Ground geophysical methods have a major advantage over the airborne

methods in that they are in direct contact with the earth. The principal
methods are electrical, magnetic, electromagnetic, radiometric,
gravimetric, and refraction-seismic. The drill-hole geophysical survey,
called logging, is an important method of extending data acquisition
beyond the drill hole. A combination of logging methods is advantageous:
gamma-ray and density logging for identifying the type of coal present;
gamma-ray (radiometric), resistivity (electric), and calliper logs for
determining the thickness of the seam; and sonic and density logs for
determining the condition of the roof and floor strata.

Choosing a mining method

The various methods of mining a coal seam can be classified under two
headings, surface mining and underground mining. Surface and
underground coal mining are broad activities that incorporate numerous
variations in equipment and methods, and the choice of which method to
use in extracting a coal seam depends on many technological, economic,
and social factors. The technological factors include, at a minimum, the
number of seams, the thickness and steepness of each seam, the nature
and thickness of the strata overlying the seams, the quality of the coal
seams, the surface topography, the surface features, and the
transportation networks available. Economic factors include energy
demand and its growth, the supply and cost of alternative sources of
energy, coal quality and the cost of coal preparation, the selling price of
coal, advancements in technology that affect costs of production, and
environmental legislation. Social factors include prior history of mining
in the area, ownership patterns, availability of labour, and local or
regional government support.

It is a general rule that technological factors dictate a clear choice

between surface and underground mining, whereas economic and social
factors determine whether a coal reserve will be mined at all. Some coal
reserves, however, are surface-mined first and then deep-mined when the
coal seam extends to such great depths that it becomes uneconomical to
continue with surface mining. The point where it becomes
economically necessary to switch from one method to the other can be
calculated with the aid of stripping ratios, which represent the amount of
waste material that must be removed to extract a given amount of coal.
Stripping ratios can also consider the selling price of coal, and a certain
minimum profit can be added to the total cost of producing and
marketing the coal for a more thorough cost-benefit analysis.

Analysis of world coal production indicates that contributions from

surface and underground production are approximately equal. Anthracite
seams (less than 10 percent of world coal production) are generally
mined by underground methods, whereas lignite seams (25 percent) are
most often surface-mined. Bituminous seams (approximately 65 percent)
are mined in roughly equal proportions by both methods.

Surface mining
Surface coal mining generally involves the following sequence of unit
operations: (1) clearing the land of trees and vegetation, (2) removing
and storing the top layers of the unconsolidated soil (topsoil), (3) drilling
the hard strata over the coal seam, (4) fragmenting or blasting the hard
strata with explosives, (5) removing the blasted material, exposing the
coal seam, and cleaning the top of the coal seam, (6) fragmenting the
coal seam, as required, by drilling and blasting, (7) loading the loose coal
onto haulage conveyances, (8) transporting the coal from the mine to the
plant, and (9) reclaiming lands affected by the mining activity.

Mining methods
Surface techniques can be broadly classified into (1) contour strip
mining, (2) area strip mining, (3) open-pit mining, and (4) auger mining.

Contour strip mining

Contour mining is commonly practiced where a coal seam outcrops in
rolling or hilly

terrain. Basically, the method consists of removing the overburden above

the coal seam and then, starting at the outcrop and proceeding along the
hillside, creating a bench around the hill. In the past, the blasted
overburden spoil was simply shoved down the hill; currently, soil is either
carried down the mountain to fill a chosen valley in horizontal layers or is
replaced on the working bench itself in places where coal has been
removed. If the break-even stripping ratio remains favourable, further
cuts into the hillside will be made. Otherwise, if there
are sufficient reserves under the knob of the hill, the coal may be
recovered by underground mining or by augering.
Area strip mining
Area mining, applied where the terrain is flat, commences with
a trench or “box cut” made through the overburden to expose a portion
of the coal seam. This trench is extended to the limits of the property in
the strike direction. After coal removal, a second cut is made parallel to
the first one, and the overburden material from this cut is placed in the
void of the first cut. The process is repeated in successive parallel cuts
until the stripping ratio indicates that continued surface mining is
uneconomical.

Open-pit mining
In open-pit mining of the coal seam, several benches are established in
both the overburden strata and the coal seam. The open-pit method is
generally practiced where thick coal seams are overlain by thick or thin
overburden; it is also used for mining steeply pitching coal seams. In the
beginning stages of mining, considerable volumes of overburden
materials must be accumulated in large dump areas outside the mine.

Auger mining
Auger mining is usually associated with contour strip mining. With this
method, the coal is removed by drilling auger holes from the last contour
cut and extracting it in the same manner that shavings are produced by a
carpenter’s bit. Coal recovery rates approach 60 percent with this
method. The cutting heads of some augers are as high as 2.5 metres. As
each stem works its way into the coal seam, additional auger stems are
added, so that hole depths of more than 60 to 100 metres are not
uncommon. Problems of subsidence, water pollution, and potential fires
are associated with augering.

Highwall mining is an adaptation of auger mining. Instead of an auger

hole, an entry into the coal seam is made by a continuous miner,
remotely operated from a cabin at the surface. The cut coal is
transported by conveyors behind the miner to the outside. Using a
television camera, the operator can see and control the miner’s progress.
The entry can be advanced 300 to 400 metres into the coal seam, after
which the miner is retreated to the surface and repositioned to drive an
entry adjacent to the previous one. Advantages over augering include
higher productivity, greater safety, and lower cost.

Equipment
Dozers and scrapers
A variety of equipment is used in a surface mining operation. In land
clearing, topsoil removal, and preparation of the mining area for
subsequent unit operations, bulldozers and scrapers have extensive
applications. These pieces of equipment have grown bigger and better
over the years. Currently, scrapers for rock have bucket capacities of 33
cubic metres (1,165 cubic feet; about 47 tons of material), and scrapers
for coal have capacities of 43 cubic metres (37 tons). Bulldozers have
blade capacities up to 30 cubic metres.

Drilling and blasting

Where strata are hard, drilling and blasting are necessary. Blastholes are
generally drilled from the surface, are vertical, and vary in diameter from
25 to 100 centimetres. In some mines, horizontal holes are drilled into
the overburden with the drill sitting on the coal surface. The holes are
charged with explosives that are based on a mix of ammonium
nitrate and fuel oil (ANFO) in dry mix, slurry, or emulsion form. It is
common to have a bulk-explosive truck drive into the area where holes
have been drilled to fill holes with custom-designed explosive mixtures.

Shovels and trucks

Overburden removal is the most important operation in the system. When
the haul distances are small (for example, 500 to 1,000 metres) and the
overburden material soft, a fleet of scrapers can load, haul, and dump the
overburden. Where distances are very small (for example, 30 to 40
metres), mobile front-end loaders, or wheel loaders, may be used to load,
haul, and dump. At greater haul distances, a fleet of trucks may be
necessary, the trucks being loaded by front-end loaders.

Three types of shovel are currently used in mines: the stripping shovel,
the loading (or quarry-mine) shovel, and the hydraulic shovel.
The hydraulic mining shovel has been widely used for coal and rock
loading since the 1970s. The hydraulic system of power transmission
greatly simplifies the power train, eliminates a number of mechanical
components that are present in the loading shovel, and provides good
crowding and breakout forces. Hydraulic and loading shovels are
available with capacities up to and over 30 cubic metres. The capacity of
the loading shovel is carefully matched with the haul unit into which the
load will be dumped. In open-pit coal mines, the haul units for
overburden material are usually large, off-highway, end-dumping trucks;
their capacities range from 35 to 250 tons. The stripping shovel has a
large bucket, usually sits in the pit on the top of the coal seam, digs into
the overburden material, and deposits it in the adjacent mined-out area.

Draglines
Draglines are by far the most commonly used overburden-removal
equipment in surface coal mining. A dragline sits on the top of the
overburden, digs the overburden material directly in front of it,
and disperses the material over greater distances than a shovel.
Compared with shovels, draglines provide greater flexibility, work on
higher benches, and move more material per hour. The largest dragline
in operation has a bucket capacity of 170 cubic metres.

Wheel excavators
The bucket-wheel excavator (BWE) is a continuous excavation machine
capable of removing up to 12,000 cubic metres per hour. The most
favourable soil and strata conditions for BWE operation are soft,
unconsolidated overburden materials without large boulders. BWEs are
widely employed in lignite mining in Europe, Australia, and India. In
these mines, the wheel excavators deposit the overburden and coal
materials onto high-speed, high-capacity belt conveyors for transport to
the mined-out areas of the pit and the coal stockpile, respectively. In
the United States, wheel excavators have been used in combination with
shovels or draglines, with a wheel handling soft topsoil and clay layers
and a shovel or dragline removing hard strata.

Coal removal
Coal is usually loaded by front-end loaders, loading shovels, or wheel
excavators into off-highway, bottom-dump trucks for transport to the
stockpile. In small operations, it can be loaded into on-highway trucks for
direct shipment to customers. In some open-pit operations with BWEs,
rail haulage is practiced in the benches themselves, coal and overburden
being loaded directly into railcars by the wheel excavator. Nevertheless,
in BWE operations belt haulage is preferable, as it facilitates continuous
mining.

Reclamation equipment
Equipment used in reclaiming mined lands includes bulldozers, scrapers,
graders, seeders, and other equipment used extensively in agriculture.
Reclamation operations, which include backfilling the last cut after coal
removal, regrading the final surface, and revegetating and restoring the
land for future use, are integrated with the mining operation in a timely
manner in order to reduce erosion and sediment discharge, slope
instability, and water-quality problems.

A primary goal of reclamation is to restore or enhance the land-use

capability of disturbed land. Various reclamation programs aim at
restoring the ground for farming and livestock raising, reforestation,
recreation, and housing and industrial sites. Even spoil banks that can be
revegetated present only minor problems and have great potential for
development. There are, however, marginal and problem spoils (such as
those containing acids or toxic wastes) that require special attention and
additional planning.

Underground mining
In underground coal mining, the working environment is completely
enclosed by the geologic medium, which consists of the coal seam and
the overlying and underlying strata. Access to the coal seam is gained by
suitable openings from the surface, and a network of roadways driven in
the seam then facilitates the installation of service facilities for such
essential activities as human and material transport, ventilation, water
handling and drainage, and power. This phase of an underground mining
operation is termed “mine development.” Often the extraction of coal
from the seam during mine development is called “first mining”; the
extraction of the remaining seam is called “second mining.”

Mining methods
Modern underground coal-mining methods can be classified into four
distinct categories: room-and-pillar, longwall, shortwall, and thick-seam.

Room-and-pillar mining
In this method, a number of parallel entries are driven into the coal
seam. The entries are connected at intervals by wider entries, called
rooms, that are cut through the seam at right angles to the entries. The
resulting grid formation creates thick pillars of coal that support the
overhead strata of earth and rock. There are two main room-and-pillar
systems, the conventional and the continuous. In the conventional
system, the unit operations of undercutting, drilling, blasting, and
loading are performed by separate machines and work crews. In a
continuous operation, one machine—the continuous miner—rips coal
from the face and loads it directly into a hauling unit. In both methods,
the exposed roof is supported after loading, usually by rock bolts.

Under favourable conditions, between 30 and 50 percent of the coal in an

area can be recovered during development of the pillars. For recovering
coal from the pillars themselves, many methods are practiced, depending
on the roof and floor conditions. The increased pressure created by pillar
removal must be transferred in an orderly manner to the remaining
pillars, so that there is no excessive accumulation of stress on them.
Otherwise, the unrecovered pillars may start to fail, endangering the
miners and mining equipment. The general procedure is to extract one
row of pillars at a time, leaving the mined-out portion, or gob, free to
subside. While extraction of all the coal in a pillar is a desirable objective,
partial pillar extraction schemes are more common.

At depths greater than 400 to 500 metres, room-and-pillar methods

become very difficult to practice, owing to excessive roof pressure and
the larger pillar sizes that are required.

Longwall mining

A longwall miner shearing coal at the face of a coal seam; from an underground mine
in southern Ohio, U.S.

In the longwall mining method, mine development is carried out in such a

manner that large blocks of coal, usually 100 to 300 metres wide and
1,000 to 3,000 metres long, are available for complete extraction
(see photograph ). A block of coal is extracted in slices, the dimensions of
which are fixed by the height of coal extracted, the width of the longwall
face, and the thickness of the slice (ranging from 0.6 to 1.2 metres). In
manual or semimechanized operations, the coal is undercut along the
width of the panel to the depth of the intended slice. It is then drilled and
blasted, and the broken coal is loaded onto a conveyor at the face. The
sequence of operations continues with support of the roof at the face and
shifting of the conveyor forward. The cycle of cutting, drilling, blasting,
loading, roof supporting, and conveyor shifting is repeated until the
entire block is mined out.

In modern mechanized longwall operations, the coal is cut and loaded

onto a face conveyor by continuous longwall miners called shearers or
plows (see photograph). The roof is supported by mechanized, self-
advancing supports called longwall shields, which form a
protective steel canopy under which the face conveyor, workers, and
shearer operate. In combination with shields and conveyors, longwall
shearers or plows create a truly continuous mining system with a huge
production capacity. Record productions exceeding 20,000 tons per day,
400,000 tons per month, and 3.5 million tons per year have been
reported from a single U.S. longwall shearer face.

Two main longwall systems are widely practiced. The system described
above, known as the retreating method, is the most commonly used in
the United States. In this method the block is developed to its boundary
first, and then the block is mined back toward the main haulage tunnel.
In the advancing longwall method, which is more common in Europe,
development of the block takes place only 30 to 40 metres ahead of the
mining of the block, and the two operations proceed together to the
boundary.

In longwall mining, as in the room-and-pillar system, the safe transfer of

roof pressures to the solid coal ahead of the face and to the caved roof
behind the face is necessary. Caving of the overlying strata generally
extends to the surface, causing surface subsidence. The subsidence over
a longwall face is generally more uniform than it is over room-and-pillar
workings. If conditions are such that the roof will not cave or subsidence
to the surface is not allowable, it will be necessary to backfill the void
with materials such as sand, waste from coal-preparation plants, or fly
ash. Owing to technical and environmental reasons, backfilling is
practiced in many mining countries (e.g., Poland, India), but the cost of
production is much higher with backfilling than it is without.

Shortwall mining
In the shortwall mining method, the layout is similar to the longwall
method except that the block of coal is not more than 100 metres wide.
Furthermore, the slices are as much as three metres thick and are taken
by a continuous miner. The mined coal is dumped onto a face conveyor or
other face haulage equipment. The roof is supported by specially
designed shields, which operate in the same manner as longwall shields.
Although a great future was envisioned for shortwall mining, it has not
lived up to expectations.

Thick-seam mining
Coal seams as much as five metres thick can be mined in a single “lift” by
the longwall method, and seams up to seven metres thick have been
extracted by conventional mining systems in one pass. However, when a
seam exceeds these thicknesses, its extraction usually involves dividing
the seam into a number of slices and mining each slice with longwall,
continuous, or conventional mining methods. The thickness of each slice
may vary from three to four metres. Many variations exist in the manner
in which the complete seam is extracted. The slices may be taken
in ascending or descending order. If the roof conditions or spontaneous-
combustion liability of the seam requires that there be no caving, the
void created by mining will be backfilled. The backfill material then acts
as an artificial floor or roof for the next slice. Caving is the preferred
practice, however.

Thick coal seams containing soft coal or friable bands and overlain by a
medium-to-strong roof that parts easily from the coal can be fragmented
by a high-pressure water jet. For successful operation, the floor must not
deteriorate through contact with water, and the seam gradient must be
steep enough to allow the water to flush the broken coal from the mined
areas. Under favourable conditions, hydraulic mining of coal is
productive, safe, and economical. It has been employed experimentally
within the United States and Canada, but it is practiced extensively in
the Kuznetsk Basin of Siberia for the extraction of multiseam, steeply
pitching deposits. Here the water is also used to transport the coal from
the working faces to a common point through open channels and from
the common point to the surface through high-pressure hydraulic
transportation systems.

Auxiliary and unit operations

Those activities which are essential to maintain safe and productive
operating conditions both at the working faces and in all parts of the
mine are known as auxiliary operations. These include ground control,
ventilation, haulage, drainage, power supply, lighting, and
communications. Those activities which are conducted sequentially in a
production cycle—i.e., cutting and hauling the coal and supporting the
immediate exposed roof after coal removal—are called unit operations.
Unit operations are planned and conducted so as to use the auxiliary
services most effectively for maintaining health and safety as well as
productivity at the locations where coal is actually being mined.

Access
Accesses to a coal seam, called portals, are the first to be completed and
generally the last to be sealed. A large coal mine will have several
portals. Their locations and the types of facilities installed in them
depend on their principal use, whether for worker and material
transport, ventilation, drainage and power lines, or emergency services.
In many cases, the surface facilities near a portal include bathhouses and
a lamp room; coal handling, storage, preparation, and load-out facilities;
a fan house; water- and waste-handling systems; maintenance
warehouses; office buildings; and parking lots.

There are three types of portal: drift, slope, and shaft. Where a coal seam
outcrops to the surface, it is common to drive horizontal entries, called
drifts, into the coal seam from the outcrop. Where the coal seam does not
outcrop but is not far below the surface, it is accessed by driving sloping
tunnels through the intervening ground. Slopes are driven at as steep an
angle as is practicable for transporting coal by belt. Commonly, a pair of
slopes is driven (or a slope is divided into two separate airtight
compartments) or ventilation and material transport. Where the
minimum coal-seam depth exceeds 250 to 300 metres, it is common to
drive vertical shafts. (Poor ground conditions are another factor in
selecting a shaft over a slope.) Shafts, too, may be split into separate
compartments for fresh air, return air, worker and supply transport, and
coal haulage.

Capital and operating costs for coal haulage are lowest in a drift access.
Capital investment for coal haulage in a shaft or a slope is somewhat
similar, but operating costs are generally higher in a shaft, owing to the
noncontinuous nature of shaft coal-handling facilities. It has been
estimated that shafts and slopes, drifts, and permanent equipment in
these access openings may account for more than 30 percent of the
capital investment in a large mine.

Ground control and roof support

Overall ground control—i.e., long-term stability of mine accesses and
entries and subsidence control—can be regarded as an auxiliary
operation, whereas supporting the roof at production faces (roof control)
is a unit operation. Ground control is concerned with the design of
underground entries, their widths, the distance between the entries, and
the number of entries that can be driven as a set. A hierarchy of entries
exists in underground coal mines. Main entries are driven so as to divide
the property into major areas; they usually serve the life of the mine for
ventilation and for worker and material transport. Submain entries can
be regarded as feeders from the mains that subdivide each major area.
From the submains, panel entries take off to subdivide further a block of
coal into panels for orderly coal extraction.

In some cases, complete collapse of the overlying strata during

extraction eventually travels to the surface, resulting in surface
depressions. This effect is called subsidence. Clearly, the wider and more
numerous the entries, the more effective they will be for
ventilation, materials handling, and first-mining extraction percentage.
However, with increased width may come problems in entry and
pillar stability. Often, by limiting the first mining to a small fraction of the
coal seam and by laying out large undisturbed blocks of coal, subsidence
may be reduced. The science of rock mechanics is well advanced and is
useful for understanding such stability problems and for the design of
mine openings, pillar sizing, extraction techniques, and planned
subsidence.
Inserting steel bolts to support the roof of an underground mine in West Virginia, U.S.

Roof support at the face (the area where coal is actively mined) is
intended to hold the immediate roof above the coal face. In modern
mechanized mines, roof bolting is the most common method employed.
Steel bolts, usually 1.2 to 2 metres long and 15 to 25 millimetres
in diameter, are inserted in holes drilled into the roof by an electric
rotary drill and are secured by either friction or resin. The bolts are set in
rows across the entry, 1.2 to 1.8 metres apart. Several theories explain
how roof bolts hold the roof. These include the beam theory (roof bolts
tie together several weak strata into one), the suspension theory (weak
members of the strata are suspended from a strong anchor horizon), and
the keying-effect theory (roof bolts act much like the keystone in an
arch).

Additional supporting systems for entries (mains, submains, and panels)

include temporary or permanent hydraulic or friction props, cribs (made
of timber or reinforced concrete block), yieldable steel arches, and roof
trusses.

Haulage
Coal haulage, the transport of mined coal from working faces to the
surface, is a major factor in underground-mine efficiency. It can be
considered in three stages: face or section haulage, which transfers the
coal from the active working faces; intermediate or panel haulage, which
transfers the coal onto the primary or main haulage; and the main
haulage system, which removes the coal from the mine. The fundamental
difference between face, intermediate, and main haulages is that the last
two are essentially auxiliary operations in support of the first. Face
haulage systems must be designed to handle large, instantaneous
production from the cutting machines, whereas the outer haulage
systems must be designed to accommodate such surges from several
operating faces. Use of higher-capacity equipment in combination with
bins or bunkers is common. In addition, face haulage systems generally
discharge onto ratio-feeders or feeder-breakers in order to even out the
flow of material onto the intermediate systems and to break very large
lumps of coal or rock to below a maximum size.
In room-and-pillar systems, electric-powered, rubber-tired vehicles
called shuttle cars haul coal from the face to the intermediate haulage
system. In some semimechanized or manual longwall operations, chain
haulage is used, while the face haulage equipment of choice in modern
mechanized longwall systems is an armoured face conveyor (AFC). In
addition to carrying coal from the face, the AFC serves as the guide for
the longwall shearer, which rides on it (see above, Mining methods:
Longwall mining).

Intermediate haulage in coal mines is provided by panel belts or by mine

cars driven by locomotives. Panel belts have widths ranging from 90 to
150 centimetres, the wider belts being used with longwall panels. The
use of mine cars and locomotives requires detailed considerations of
shuttle-car dumping ramps, locomotive switching requirements, the
inventory of mine cars, and track layout for empties and loads.
Locomotives are electric- or diesel-powered. Mainline haulage is also
provided by belt or railcar. The major differences are only in the size,
scope, and permanence of installations. For example, mainline belts are
laid for the life of the mine and are much wider and faster than
intermediate belts. Mainline locomotives are also much larger than
intermediate locomotives, and mainline tracks are built to more exacting
standards of speed and reliability.

For the transport of maintenance and operating supplies to the working

sections, advantage is taken of the mainline, intermediate, and face
haulage systems. Monorail systems or endless-rope haulage systems,
which are much like ski lifts, are commonly used in intermediate and face
systems to transport supplies to the working faces. In all-belt mines, it is
not unusual to have trolley rail haulage for carrying workers and
materials to and from the working face. Other supply haulage equipment
includes scoops and battery- or diesel-powered trucks.

Ventilation
The primary purpose of underground-mine ventilation is to
provide oxygen to the miners and to dilute, render harmless, and carry
away dangerous accumulations of gases and dust. In some of the gassiest
mines, more than six tons of air are circulated through the mine for every
ton of coal mined. Air circulation is achieved by creating a pressure
difference between the mine workings and the surface through the use of
fans. Fresh air is conducted through a set of mine entries (called intakes)
to all places where miners may be working. After passing through the
workings, this air (now termed return air) is conducted back to the
surface through another set of entries (called returns). The intake and
return airstreams are kept separate. Miners generally work in the intake
airstream, although occasionally work must be done in the return
airways.
The task of bringing fresh air near the production faces is an important
auxiliary operation, while the task of carrying this air up to the working
faces—the locations of which may change several times in a shift—is the
unit operation known as face ventilation. The major difference between
main ventilation and face ventilation is the number and nature of the
ventilation control devices (fans, stoppings, doors, regulators, and air-
crossings). In face ventilation, plastic or plastic-coated nylon cloth is
generally used to construct stoppings and to divide the air along a face
into the two streams of intake and return air. Furthermore, the
stoppings, which are generally hung from the roof, are not secured at the
bottom, in case machinery and coal must be transported from one side to
the other. Main ventilation stoppings and air crossings, on the other
hand, are constructed of brick or blocks and coated with mortar; the
fans, regulators, and doors are also of substantial construction.

Monitoring and control

Advancements in sensor technology and in computer hardware and
software capabilities are finding increasing application in underground
coal mines, especially in the monitoring and control of ventilation,
haulage, and machine condition. Longwall shearers and shields can be
remotely operated, and continuous miners have also been equipped with
automatic controls. The atmospheric environment is remotely monitored
for air velocity, concentrations of various gases, and airborne dust; fans
and pumps are also monitored continuously for their operational status
and characteristics.

Health, safety, and environment

In coal mining—particularly underground coal mining—there are
numerous conditions that can threaten the health and safety of the
miners. For this reason, coal mining worldwide is heavily regulated
through health and safety laws. Through the development of new
equipment for personnel protection, new approaches to mine design,
more effective emergency preparedness plans and procedures, and major
changes in legislation, regulation, and enforcement, higher standards of
health and safety are now achieved. For example, the self-contained self-
rescuer (SCSR) represents a significant development in raising a miner’s
chances of survival and escape after an explosion, fire, or similar
emergency contaminates the mine atmosphere with toxic gases. This
lightweight, belt-wearable device is available worldwide and
is mandated in several countries to be carried on the person whenever
underground.
The never-ending underground fire of Centralia, PennsylvaniaLearn about the
underground coal mine fire burning in Centralia, Pennsylvania.(more)

The effects of mining on the water, air, and land outside the mine are as
important as those that occur in the mine. These effects may be felt both
on- and off-site; in addition, they may vary in severity from simple
annoyance and property damage to possibly tragic illness and death.
Even abandoned lands from past mining activities present such problems
as mine fires, precipitous slopes, waste piles, subsidence, water
pollution, derelict land, and other hazards endangering general
welfare and public health. Growing environmental consciousness has
brought about a greater consideration of environmental factors in the
planning, designing, and operating of mines.

Coal preparation
As explained above, during the formation of coal and subsequent
geologic activities, a coal seam may acquire mineral matter, veins of clay,
bands of rock, and igneous intrusions. In addition, during the process
of mining, a portion of the roof and floor material may be taken along
with the coal seam in order to create adequate working height for the
equipment and miners. Therefore, run-of-mine (ROM) coal—the coal that
comes directly from a mine—has impurities associated with it. The buyer,
on the other hand, may demand certain specifications depending on the
intended use of the coal, whether for utility combustion, carbonization,
liquefaction, or gasification. In very simple terms, the process of
converting ROM coal into marketable products is called coal preparation.

Levels of cleaning
Coal preparation results in at least two product streams, the clean coal
product and the reject. Generally, five levels of preparation can be
identified, each being an incremental level of cleaning over the previous
one:

Level 0:

At this level, no coal cleaning is done; ROM coal is shipped directly to the
customer.

Level 1:

ROM coal is crushed to below a maximum size;

undesirable constituents such as tramp iron, timber, and perhaps strong
rocks are removed; the product is commonly called raw coal.

Level 2:

The product from level 1 is sized into two products: coarse coal (larger
than 12.5 millimetres) and fine coal (less than 12.5 millimetres); the
coarse coal is cleaned to remove impurities; the fine coal is added to the
cleaned coarse coal or marketed as a separate product.

Level 3:

Raw coal of less than 12.5 millimetres is sized into two products: an
intermediate product (larger than 0.5 millimetre) and a product smaller
than 0.5 millimetre; the intermediate product is cleaned to remove
impurities; the smaller product is added to the cleaned intermediate
product or marketed separately.

Level 4:

Cleaning is extended to material less than 0.5 millimetre in size.

Preparation steps
In the early days of coal preparation, the objective was to provide a
product of uniform size and to reduce the content of inert rock materials
in ROM coal. Reduction of impurities increased the heating value of the
cleaned product, reduced deposits left on the furnace, reduced the load
on the particle-removal system, and increased the overall operating
performance of the furnace. Today, air-pollution regulations require that
ROM coal be cleaned not only of ash and rocks but of sulfur as well. The
processing of raw coals at levels 2, 3, and 4 therefore requires a
maximized recovery of several characteristics (e.g., ash content, heating
value, and sulfur content) in the respective product streams (i.e., clean
coal and the reject). Four steps need to be considered: characterization,
liberation, separation, and disposition.

Characterization
Characterization is the systematic examination of ROM coal in order to
understand fully the characteristics of the feed to the preparation plant.
Washability studies are performed to determine how much coal can be
produced at a given size and specific gravity and at a particular level of
cleaning. The studies provide a basis for selecting the washing
equipment and preparation-plant circuitry.

Liberation
Liberation is the creation of individual particles that are
more homogeneous in their composition as either coal or impurities. (In
practice, middlings, or particles containing both coal and impurities, are
also produced.) Liberation is achieved by size reduction of the ROM coal.
It is a level-1 process, the product of which is the input to a level-2 plant.
In general, the finer the ROM coal is crushed, the greater the liberation
of impurities. However, the costs of preparation increase nonlinearly
with decreasing desired size.

Separation
Schematic diagram of a flotation separation cell.

In the separation step, the liberated particles are classified into the
appropriate groups of coal, impurities, and middlings. Since impurities
are generally heavier than middlings and middlings heavier than coal,
the methods most commonly used to separate the input stream into the
three product streams are based on gravity concentration. Relying on
differences in the two physical properties of size and specific gravity,
equipment such as jigs, heavy-media baths, washing tables, spirals, and
cyclones separate the heterogeneous feed into clean, homogeneous coal
and waste products. For extremely fine coal, a process
called flotation achieves this purpose. (A schematic diagram of a flotation
separation cell is shown in thefigure.)

Disposition
Disposition is the handling of the products of a preparation plant. The
entire plant process includes ROM storage, raw coal storage, crusher
house, screening plants, various slurries (coal-water mixtures),
dewatering system, thickeners, thermal dryer, process-water systems,
clean-coal storage, clean-coal load-out system, monitoring and process-
control system, and refuse-disposal system. Occupational health and
safety hazards as well as environmental problems are associated with
each of these processes. Detailed planning and designing can eliminate
the worst problems of noise, dust, and visual blight and can also
significantly reduce adverse impacts on air, water, and land.

Coal transportation
There are several methods for moving prepared coal from the mine to the
markets. The cost of transport can be substantial and can account for a
large fraction of the total cost to the consumer.

Railroads
Rail transportation is by far the most common mode of hauling coal over
long distances. Roadbed and track requirements and large fixed
investment in railcars make rail transport capital-intensive. However, the
long life of the permanent assets, relatively trouble-free operation with
minimum maintenance, the large-volume shipments that are possible, the
high mechanical efficiencies that are obtained with low rolling
resistances, and the dedicated nature of the origin and destination of the
runs are some of the factors that make rail transport most attractive for
long-term, long-distance, high-volume movements of coal.

In the United States, about half of the coal carried by rail is transported
by unit trains, groupings of 100 or more cars of 100- to 110-ton capacity
each. Unit trains generally carry 10,000 to 15,000 tons of coal in a single
shipment. A “dedicated unit train” is made up especially for movement
between one point of origin and one destination. In order to attain
high efficiency, carefully matched loading and unloading terminals are
necessary. In one example, a unit train transporting 17,400 tons per
1,200-kilometre round trip from mine to plant has a turnaround time of
72 hours—including a 4-hour loading and 10-hour unloading and
servicing time per train.

On-highway trucks
If haul distances and shipment sizes are small, it may be advantageous to
transport coal by truck through a network of public roads. Whereas off-
highway trucks have exceeded 250 tons in capacity, on-highway trucks
are usually much smaller, not exceeding 25-ton payloads. Advantages
over railroads are that trucks can negotiate more severe grades and
curves, roads can be resurfaced or constructed more readily and with far
lower capital investments than can railways, and the coal flow can be
made continuous by adding new trucks and replacing failing trucks.

Barges
Rivers and lakes have long played a major role in the transport of bulk
commodities like coal in Germany, The Netherlands, France, Belgium,
Canada, and the United States. The costs of barge transport depend on
the number of barges being towed by a single towboat; this in turn
depends on the dimensions of the waterway. For example, the
Cumberland, Ohio, Tennessee, and upper Mississippi rivers in the United
States can take up to 20- to 25-barge tows, and the lower Mississippi can
take 25- to 35-barge tows. Each barge has a capacity of up to 1,500 tons.
Waterways are usually circuitous, resulting in slow delivery times.
However, transport of coal on barges is highly cost-efficient.

Conveyors
While use of conveyors for carrying coal over long distances from
producing to consuming centres is uncommon, it is not uncommon to find
conveyors transporting coal from mines to barge-loading stations. In
addition, where a power plant is in close proximity to a mine, conveyors
are generally used to transport coal to the power plant stockpile.
Conveyors can traverse difficult terrain with greater ease than trucks or
rail systems, and they can also be extended easily and have the
advantage of continuous transport. Conveyors with wide belts and high
operating speeds can have enormous capacities, varying from 2,000 to
5,000 tons per hour.

Slurry pipelines
Coal slurry is a mixture of crushed coal and a liquid such as water or oil.
The traditional mixture, first patented in England in 1891, consists of 50
percent coal and 50 percent water by weight. So-called heavy coal
slurries or slurry fuels consist of 65 to 75 percent coal, with the
remainder being water, methanol, or oil. Unlike traditional slurry—which
is transported by pipeline to the user, who separates the water from the
coal before burning—slurry fuels can be fired directly into boilers.

Coal slurry pipelines currently in operation in the United States and

Europe cover distances ranging from a few kilometres to several hundred
kilometres. They have several advantages. A large portion—
approximately 70 percent—of the costs involved in a slurry pipeline
are invested in the initial construction of the line and pumping stations
and are fixed for the life of the pipeline. Therefore, the total costs of
moving slurry during the life of the line do not increase in proportion to
inflation. The advantage over rail and truck transport is clear, as the
costs of these latter modes escalate with inflation. Furthermore,
pipelines require less right-of-way, much less labour, and about half of
the steel and other supplies required for other transport methods.

On the other hand, slurry pipelines involve potential environmental

problems. Water requirements are substantial: almost one ton of water is
needed to move one ton of coal—an important issue in Australia and the
western United States, where water supplies are scarce and its
availability cannot be guaranteed. Other concerns focus on water
pollution at the mouth of the pipeline as well as along its length. For this
reason, efforts to obtain right-of-way to lay a pipeline have often faced
legal and environmental challenges.

Electric wire
In the early 1960s, dedication of large coal reserves to mine-mouth
power plants resulted in the development of huge complexes
involving mining, preparation, and utility plants. Transportation
of electricity from coal-fired power plants to distant consuming centres is
still attractive for several reasons. Coal is generally available in
abundance and is the lowest-cost fuel in many instances. In addition, the
search for inherently cleaner and more efficient ways to burn coal in
electric utilities has intensified. The world’s highest-voltage transmission
line (1,150 kilovolts) transports electricity from Siberia to consumers in
the western republics of the former Soviet Union—a distance of more
than 3,000 kilometres. In the United States, coal-fired plants account for
50 percent of electricity generation. The U.S. electrical grid consists of
three networks—one in the east, one in the west, and one in Texas.
Although there are only small transfers between networks, the ability to
transmit power from one network to another reveals the potential for
greater use of electrical wire for coal power transport.

Ships
It is predicted that coal exports and, therefore, the importance of ocean
transport will increase. Ocean transport of coal requires
detailed considerations of (1) transportation from the mine to the port,
(2) coal-handling facilities at the export port, (3) ocean carrier decisions
such as number and size of ships, contractual obligations, management
of the fleet, and route decisions, (4) coal-handling facilities at the
importing port, and (5) transportation from the port to the customer.

Transportation costs have an important impact on coal exports. Mining,

rail, port, and shipping costs may vary greatly for different overseas
buyers, and the combined cost may represent more than one-half of the
delivered price of coal to overseas ports. In addition, substantial capital
costs are involved in developing the necessary facilities and in
maintaining sizable stockpiles at the exporting ports. Since all these
costs differ considerably among suppliers, they are important in
determining the competitiveness of various coals in world markets.