coal technology

Transcription

coal technology

EBS 425/3 -Mineral Perindustrian
Coal Technology – Part I
PART I : ARANG BATU (COAL)
What are fossil fuels?
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Derived from the remains of once living organisms.
Formed some 500-200 million years ago.
The three major fossil fuels are oil, coal, and natural gas.
Currently being consumed at a rate faster than they are being produced.
After food, fossil fuel is humanity's most important source of energy. There are
three major fuels -- coal, oil and natural gas. Coal is used primarily to
produce electricity. It therefore provides us with light, motive power from
electric motors, and our many electronic devices. Oil gives us our mobility, our
cars, planes, trains, trucks and boats. Natural gas is used primarily to produce
heat, for our buildings, hot water, and industrial processes.
Natural gas, oil and coal are the three (fossil) fuels that are abundantly used.
These fuels are remains (fossils) of life forms such as marine organisms and
plant life, that flourished on our planet millions of years ago. This energy is thus
a stored form of solar energy that accumulated over millions of years, and
at the current and projected rates of consumption, fossil fuels will be used up in
a fraction of time compared to the time it took to collect the energy from the
sun.
It is a pretty safe bet that fossil fuels will be depleted. The question is when?
Fossil Fuels
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Coal, Petroleum, Oils Shale, Tar Sands
Concentrated Organic Matter (> 2% TOC)
Humic and sapropelic components
Unevenly distributed in space & time
What is Coal?
Coal is a generic term referring to a family of solid fossil fuels with a wide range
of physical and chemical compositions. Coal is formed from large accumulations
of plant materials that have been preserved from complete decay and later
altered by chemical and physical conditions in the accumulation.
Coal is a readily combustible rock containing more than 50 percent by weight of
carbonaceous material, formed from compaction and indurations of variously
altered plant remains similar to those in peat [adapted from AGI's "Glossary of
Geology"]. Most coal is fossil peat. Peat is an unconsolidated deposit of plant
remains from a water-saturated environment such as a bog or mire; structures
of the vegetal matter can be seen, and, when dried, peat burns freely [adapted
from AGI's "Glossary of Geology"].
Coal is actually a heterogeneous rock composed of different kinds of organic
matter which vary in their proportions in different coals, and no two coals are
Disedia oleh : Dr. Kamar Shah Ariffin ( 6/30/2003)
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absolutely identical in nature, composition or origin. A definition of coal
has been proposed:
"Coal is a compact stratified mass of metamorphosed plants which
have, in part, suffered arrested decay to varying degrees of
completeness."
The organic material in coal was formed from plant debris which has been layed
down in a peat swamp, over many millions of years. These swamps were
extremely large, and it has been calculated that the thick brown coal seams in
Victoria (Australia), now about 200 m thick, were formed from debris with a
total thickness of five kilometres. With time, the plant debris was covered with
sediments, and undergone various changes of temperature and pressure which
produced a sequence of coals beginning with peat and terminating with
anthracite.
The definition of the term "coal" in steel industrial point of view
The term "coal" is defined in the following way: hard coal, hard coal briquettes,
coke and semi-coke derived from hard coal, lignite, lignite briquettes and coke
and semi-coke derived from lignite.
The term "hard coal" includes the high and medium-ranking "A" coals (subbituminous coals) as defined in the "International codification system of coal" of
the UN Economic Commission for Europe. The term "lignite" includes the lowranking "C" coals (or ortho-lignites) and the low-ranking "B" coals (or metalignite) of the same classification. With regard to lignite, the programme shall
apply to lignite used for electricity production or for combined head/electricity
production, or for the manufacture of briquettes or semi-coke.
Formation of Coal
Coal Creation
Between 200 and 300 million years ago, long before there were any reptiles,
birds or mammals, in many parts of the world there existed warm and human
climatic conditions. These conditions favored the growth of huge tropical ferns
and giant trees, which grew and died in vast swamp areas. The dead plants fell
into the boggy waters, which tended to exclude oxygen and bacteria, so that
they only partially decomposed to produce a peat-like material. This fossilized
plant debris was the beginning of coal, but only the beginning.
Vegetation continued to grow for many generations and centuries, forming vast,
thick peat beds which were later to turn into coal. After a time the areas of
swamp gradually became submerged by shallow seas, where they were covered
by sediment. These sediments would later become sedimentary rock. This cycle
of swamp followed by submersion was often repeated a number of times, so
that a sequence of horizontal bands of peat and inorganic, sedimentary rock was
built up. This formed the first stage, called the biochemical stage. Coal
formation occurred in other geologic periods as well.
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Subsequently, the bands of peat were altered by the action of pressure and
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temperature during the second, or geochemical stage, to form the various
kinds of coal found today. As much as a 20-fold reduction in the thickness of the
original plant deposits sometimes occurred. During the course of time these
horizontal coal seams were further altered as they became folded, tilted and
eroded. Much of this action was due to the motion of the continents, as
mountains formed and were then worn away. The study of fossils is called
paleontology. The creation of coal from fossils is part of geology.
Coal is formed by the physical and chemical alteration of peat (coalification) by
processes involving bacterial decay, compaction, heat, and time. Coal is an
agglomeration of many different complex hydrocarbon compounds, some of
which owe their origin to the original constituents in the peat. Peat deposits are
actually quite varied and contain everything from pristine plant parts (roots,
bark, spores, etc.) to decayed plants, decay products, and even to charcoal if
the peat caught fire. Peat deposits typically form in a waterlogged environment
where plant debris is accumulated; peat bogs and peat swamps are examples.
In such an environment, the accumulation of plant debris exceeds the rate of
bacterial decay of the debris. The bacterial decay rate is reduced because the
available oxygen in organic-rich water is completely used up by the decay
process. Anaerobic (without oxygen) decay is much slower than aerobic decay.
In order for the peat to become coal, it must be buried by sediment. Burial
causes compaction of the peat and, consequently, much water is squeezed out
during the first stages of burial. Continued burial and the addition of heat and
time, cause the complex hydrocarbon compounds in the deposit to start to break
down and alter in a variety of ways. The gaseous alteration products (methane
is one) are typically expelled from the deposit and the deposit becomes more
and more carbon-rich (the other elements drop out). The stages of this trend
proceed from plant debris, peat, lignite, sub-bituminous coal, bituminous coal,
anthracite coal, to graphite (a pure carbon mineral).
Because of the amount of squeezing and water loss that accompanies the
compaction of peat after burial, it is estimated that it took vertical 10 feet of
original peat material to produce one vertical foot of bituminous coal. The peat
to coal ratio is variable and dependent on the original type of peat the coal came
from and the rank of the coal.
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In other point of view, Coal was formed from plant life under the action of
immense pressures and temperatures prevailing within the earth's crust over a
period encompassing millions of years. The major elements present in the
"organic portion" of coal are carbon, hydrogen, oxygen, nitrogen and sulphur.
Sulphur (mostly as iron pyrite) is also present as part of the "inorganic
portion" or ash in the coal along with oxides of alumina, silica, iron,
alkaline earths and alkalis. Coal also contains some chlorine. Coal is
classified into the following four types according to the degree of
metamorphism:
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anthracite which is low in volatile matter (which
forms tars, oils and gasses when coal is heated) and
consists of mostly carbon (fixed carbon)
bituminous which contains significant amounts of
the volatile matter and typically exhibit swelling or
caking properties when heated
sub-bituminous is a younger coal and contains in
addition to the volatile matter, significant amounts of
moisture
lignite is the youngest form of coal (when peat is
not included in the broader definition of coal types)
and is very high in moisture content resulting in a
much lower heating value than the other types of
coal.
First of the fossil fuels – drove the industrial revolution
Known since ancient times, but only really important since 18th century.
Abraham Darby, 1710, used to smelt iron. Later use spread across Europe.
Use aided by transportation, first canals and later railroads.
In United State first coal production in Virginia (1750) and Pennsylvania (1759).
In Malaysia, first coal production came from Batu Arang Selangor and Labuan
from Tanjung Kabung (1847-1911). Batu Arang field development initially was
started in 1904, by J.A. Russell & Co (BOH tea plantation founder). The story
behind Batu Arang fields is rather interesting. A Malay man had picked up pieces
of coal in the jungle and brought them to Archie Russell. Currently actively mine
at Nanga Merit (Lombong Merit Pila) and Beradai, Kapit division Serawak. This
coal is mine to be supplied for power generation by SESCO
All coal some type of carbon compound or pure carbon formed from fossilized
plant material. Will only be formed when the plant growth is abundant and
when environment conditions will result in rapid burial. Higher plants (needed to
form humic coals) did not evolve until Devonian (400 Ma) so no large coal
deposits rocks formed before that time.
Most coals formed in tropical, swampy areas. River delta’s ideal because also
carry sediment to bury accumulated vegetation.
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Coalification
As vegetation-rich layers are buried under sediments, pressure and increasing
temperature change the nature of the material.
Coalification of coal
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Wide variety of terrestrial environments accumulate peat
Coalification related to time and pressure, expressed as rank
Lignite (<70% TOC; >31% volatiles), Bituminous coal (80-90% TOC;
22-31% volatiles), Anthracite (>90% TOC; 2-14% volatiles)
Macerals indicate coal constituent origin
Vitrinite - degraded structured cellular remains, glassy
Inertiniate - non-degraded cellular remains of bark & wood; fungal
remains
Liptinite - resistant biopolymers such as cutan, resins, & sporopollenin
vegetation -> peat -> lignite (brown coal) -> bituminous -> anthracite
[low rank]
[high rank]
At each stage volatiles lost, so is a consequent increase in carbon content. Thus
high rank coals provide more energy. Layers of coal called seams.
Coal Formation sequence
Peat ------->
Lignite
------->
Bituminous -------> Anthracite
Energy
Content
Moisture
Content
Sulfur
Content
Subbituminous
------->
Low ------------------------------------------------------------------------------------> High
High ------------------------------------------------------------------------------------> Low
High ------------------------------------------------------------------------------------> Low
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What are the components of coal?
Coal contains two major constituents:
THE USEFUL BITS: These are the parts of the coal which are of direct benefit
to a process because they produce heat, or used as a source of carbon. This
part is usually termed the "organic" part of the coal substance, and originated
mainly from the carbon in the original plant material.
THE USELESS BITS: These are those parts of the coal which have no value to
the utilisation of the coal. Two materials are present in
this category:
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The "inorganic" part of the coal substance, and is present in the form of
"minerals" which remains in the coal substance by virtue of inert
substances contaminating the plant material as it was being laid down in
the peat swamp, and present in the form of minerals (which produce
ash).
The coal "moisture" which is retained within the porous coal structure,
and on the coal surface.
Some organic constituents including nitrogen, and organic sulphur.
These components of the coal are illustrated in the simple box diagram, and it
should be noted that some of the carbon, hydrogen and oxygen are the only
useful bits if the coal. Some of the carbon and all the hydrogen and oxygen
report to the volatiles, and the balance of the carbon reports to the fixed
carbon.
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Rocks are made up of grains of one or more minerals. Similarly, coal, an organic
deposit, is made up of organic grains called macerals. Coal petrographers,
people who study coal under the microscope, separate the macerals into three
maceral groups, each of which are composed of several maceral types. These
groups are liptinite, vitrinite, and inertinite and are defined according to
their grayness in reflected light: liptinites are dark gray, vitrinites are medium
to light gray, and inertinites are white and can be very bright. Liptinites were
made up of hydrogen-rich hydrocarbons derived from spores, pollens, cuticles,
and resins in the original plant material. Vitrinites were made up of wood, bark,
and roots and contained less hydrogen than the liptinites. Inertinites are
mainly oxidation products of the other macerals and are consequently richer in
carbon. The inertinite group includes fusinite, most of which is fossil charcoal,
derived from ancient peat fires.
Microscopic view of coal; yellow area is vitrinite; white grains are
inertinite; gray shapes are liptinites (plant spores); dark brown areas
are tiny grains of vitrinite.
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Coals can also be divided into two types based on their macroscopic (not
microscopic) appearance: banded and non-banded. Non-banded coals include
cannel and boghead coals, both of which are dull and blocky. Cannel is derived
from the word "candle," beca use pencil-shaped pieces were used as candles in
the past. Banded coals grade from dull banded ("splint coal") to bright banded
coals, depending upon whether dull bands or bright bands are dominant. The
bands are divided into lithotypes. Dull bands are called durain; satiny bands are
clarain; charcoal bands are fusain; and black, glassy bands are vitrain. Bright
coals have lots of vitrain and clarain; dull coals are rich in durain bands. Fusain
generally occurs only in thin and sporadic bands.
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Splint coals are durain-rich and can be massive (non-banded) or banded. Most
vitrain- and clarain-rich banded coals break into small blocky pieces along joints
called cleats. Vitrain and clarain are brittle and break easily. "Block coals" are
dull coals that break into large blocks because they have fewer vitrain and
clarain bands, but have a composition higher in liptinite macerals, which are
tough. "Bone" and "bone coals" have a high ash content in the form of clays and
silts; they form part of a continuum between dark shale and dull (banded or
non-banded) coal in the following sequence: dark shale, bone (greater than 50
percent ash), boney coal (less than 50 percent ash), dull coal (cannel, boghead,
or splint).
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Kinds and Types of Coal
The kinds of coal, in increasing order of alteration (or rank), are lignite (brown
coal), sub-bituminous, bituminous, and anthracite. These classes are further
divided into subclasses based on their degree of alteration (measured by
volatile-matter content, Btu's, or by petrographic means). The bituminous coals
are also subdivided into types of coal as well: banded and non-banded. Nonbanded massive coals are cannel, boghead, and some types of "splint" coal.
Banded coals are divided into subtypes based on the nature of the bands and
are either bright-banded or dull-banded. The bands are classified into four major
lithotypes: vitrain (bright, black, glassy, brittle), clarain (bright, satiny texture,
brittle), durain (dull, grainy texture, tough), and fusain (dull black, charcoal
texture, gets hands dirty).
How did the coal-bearing rocks form?
The numerous layers of coal beds in many coal fields are intermixed with shale
and sandstone (and rarely with thin limestone ). Most of the major coal beds in
many part of the world were formed as widespread peat swamps or mires during
the Pennsylvanian Period. During the Pennsylvanian Period the area that is now
Kentucky, US was near the equator and had a climate much like that of modern
Indonesia. Tropical climates allowed lush vegetation to accumulate into
widespread peats.
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The alternation of coals with shales, sandstones, and thin limestones records
alternating periods of sea-level cha nges. As sea level rose, swamps spread
across many low area. Ultimately, the seas covered the swamps. Shallow seas
covered parts of Kentucky more than 50 times during the Pennsylvanian. When
sea level fell, the seas withdrew to the edges of the continent and large rivers
snaked across low area. In this kind of area, the peat swamps formed on
extensive coastal plains. When sea level rose, the peat deposits were covered by
muddy sediments. When sea level started to lower, coastal plains and small
deltas built back over the muddy sea sediments. During the next low sea level,
coastal peat (in swamps) was again deposited over the coastal plain sediments.
The peats, mud, and sand, buried by increasing layers of sediments, slowly
became compacted. Eventually the peat transformed into coal and the grains in
the muds and sands became cemented, transforming them into shales and
sandstones. Remains of plants and animals buried by the sediments, were
preserved and became fossils, if conditions were right. A variety of plant and
animal fossils are found in the coal-bearing rocks.
Extraction of Coal
Originally very labor intensive, mainly underground mining, but helped by
fact that occurs in layers. In early years many horrors including child labor, 60
hour work weeks etc. (see Emile Zola’s “Germinal” for account of coal mining in
late 19th France). Also very dangerous, cave-ins, explosions (due to methane
gas a.k.a “fire damp” etc. Made mining hotbed of union activity.
Coal Mining Methods
There are two primary methods of mining coal, surface mining and
underground mining. There are over 1,000 surface mines and more than
1,000 underground mines in the US. Underground mining is more difficult and
requires more miners, but much of our best coal is underground. Mining has
become much safer and more efficient over the years. In 1980 there were over
220,000 coal miners in the country. Today there are fewer than 100,000. But
while 1980 production was about 800,000 tons, today we produce over 1 billion
tons with fewer than half the number of miners.
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In many countries, there are three main mining methods employed by the
majority of the mines:
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Opencast, using dragline, truck and shovel
Underground Bord and Pillar, using continuous mining machinery
Underground Longwall, using coal shearers and conveyor systems
Surface Opencast
Underground-mining methods include drift, slope, and shaft mining. Drift
mines are mines that enter into the side of a hill and mine the coal within the
hill. Slope mines usually begin in a valley bottom, and a tunnel is built that
slopes down to the coal to be mined. Shaft mines are the deepest mines; a
vertical shaft with an elevator is made from the surface to the coal. In western
Kentucky, US one shaft mine reaches 1,200 feet below the surface. Surface mining methods include area, contour, mountaintop removal, and augur
mining. Area mines are surface mines that remove shallow coal over a broad
area where the land is fairly flat. Rocks overlying the coal (called overburden)
are commonly removed by huge dragline shovels. After the coal has been
removed, the rock is placed back into the pit.
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Underground - Bord and Pillar
Bord and pillar mining methods are used in flat tabular deposits (seams) from
1.5 to 7 metres in thickness where it is required to prevent subsidence
(collapse) of mined-out areas from affecting the surface.
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The seam is mined in a ‘streets’ and ‘avenues’ fashion (‘bords’) advancing in one
direction. ‘Pillars’ of coal are left behind to support the roof and prevent
collapse. Additional support is provided through the use of roofbolts.
Coal is cut by continuous miners and loaded onto haulers which tip the coal onto
a conveyor system. From there, it is transported out of the mine.
Contour mines are surface mines
that mine coal in steep, hilly, or
mountainous terrain. A wedge of
overburden is removed along the
coal outcrop on the side of a hill,
forming a bench at the level of the
coal. After the coal is removed, the
overburden is placed back on the
bench to return the hill to its
natural slope. Mountaintop removal
mines are special area mines
where several thick coal seams
occur near the top of a mountain.
Large quantities of overburden are
removed from the top of the
mountains, and this material is
used to fill in valleys next to the
mine. Large areas of elevated flat
land (where none existed before)
are produced by this method.
Augur mines use surface-mine
benches (before they are covered
up) and drill out the coal in the
side of the hill that can't be reached by contour mining.
Underground Longwall
Longwall mining is classified as a total extraction methodology and can recover
over 75% of the minable coal. The mining process takes place over a face length
of between 100m (shortwall) and 250m (longwall). The coal is cut from the face
by a coal shearer which traverses backwards and forwards across the exposed
coal face. The coal produced by the shearer is transported along the face by an
armored face conveyor. At the end of the face, the coal is loaded by a stage
loader onto conventional toughed conveyor belts and transported out of the
mine. The roof over the shearer and armoured face conveyors is supported by
hydraulically operated shields. As the coal face is cut and removed, the shields
advance in the direction of mining, allowing the unsupported roof to collapse
behind the mining operation.
Courtesy : Kentucky Geological Survey (KGS)
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coal technology

Transcription

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