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How Coal Works
Coal is cheap, plentiful and dirty -- as cheap as
dirt, as plentiful as dirt, and as dirty as dirt -- since
after all, coal is little more than dirt that burns.
off-site
Coal Combustion
Thirty years ago, coal was seen as a fuel of the
DOE's Coal Information Page
past. Nuclear power and natural gas were going
to take us away from the Dickensian era of coal furnaces, steam-powered
locomotives, and grime. But King Coal recovered, and is now used in record
amounts. Forecasts of future energy use give a prominent role to coal. Some would
say that coal is back, and here to stay.
But coal is an unwelcome guest. Carbon emissions from burning coal are one of the
leading causes of global warming. Acid rain, from sulfur emissions, is almost entirely
due to coal burning. From mining to processing to transportation to burning to
disposal, coal has more environmental impacts than any other energy source. While
some of these can be lessened with effort, others, like carbon emissions, are an
inevitable product of coal use. Its time to send our dirty old King into retirement.
How Coal Forms
Coal is a sedimentary organic rock that contains a lot of carbon -- between 40 and
90 percent carbon by weight. Coal is formed by ancient plants and animals
accumulating in moist peat bogs. As plants die off in a wet area, they pile up into
peat. It takes between 4,000 and 100,000 years for one meter of peat to accumulate.
This process happens best in river deltas or coastal plains.
Over time, these peat seams are compressed by further deposits and the carbon
content of the coal is concentrated. The older the coal gets, generally, the harder
and blacker it gets. There are four "ranks" of coal: lignite, subbituminous,
bituminous, and anthracite, from lowest to highest. Rank is determined by energy
content and chemical composition. The ranks are really on a continuum from low
carbon/low energy content to high carbon/high energy content.
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The youngest coal is not even coal yet -- peat. Peat is a traditional fuel in
parts of the world, like Ireland, where it is cut from the earth, dried, and
burned for heat. The energy content of peat is quite low.
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Young coal is called lignite, and is soft and brown, not much different than
dried peat. Lignite has a low energy content, typically about 13 million Btu per
ton. The carbon content is low also, around 40 percent. Lignite is typically
used only when higher grades of coal are not available or affordable, such as
in Poland. In the US, only North Dakota and Texas use lignite.
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Subbituminous coal is common in the US. It has an energy content of about
18 million Btu per ton, and is used mostly in coal-fired power plants.
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Bituminous coal is the most widespread form in the US. It dates from the
carboniferous era, about 300 million years ago, and is high in energy content,
averaging 24 million Btu per ton. Bituminous and subbituminous account for
most coal use in America.
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The hardest coal, anthracite, is found mostly in Pennsylvania, but most
supplies of anthracite there have been exhausted. The energy content is high,
around 23 million Btu per ton, but it tends to have a high sulfur content. It is
more than 90 pecent carbon.
Coal can be formed in salt water or fresh water areas. High-sulfur coal was formed
in salt water swamps that were covered by sea water. Bacteria in the swamp
converted sulfate in the sea water into pyrite that became part of the coal. Low-sulfur
coal deposits were developed primarily under fresh-water conditions.
Coal deposits in the Eastern US date back mainly to the Pennsylvanian period of the
Earth's geologic history, about 300 million years ago, long before the age of
dinosaurs. By contrast, most of the coal in the West is younger, formed less than
140 million years ago in the Cretaceous period, when dinosaurs were alive, and in
the subsequent Tertiary period, when they became extinct.
Coal is present in 38 States, lying under 13 percent of the land area of the United
States. As can be seen on the map of coal fields in the U.S., bituminous coal comes
mostly from the Appalachian Basin and the Midwest, while the Western coals are
mostly subbituminous.
Finding coal is typically a simple matter. In the West, where coal seams are not far
underground, rocks called "clinker" are found on the surface. Clinker is made when
exposed coal seams are ignited by prairie fires, which turns rocks and minerals into
a sort of slag. If clinker is found on the ground, a coal seam is bound to be
underneath. Sometimes, as in the photo below, the coal seam itself is visible. In
truth, there is so much coal already known about that exploration is unnecessary.
How Coal Is Mined
Seams of coal may be close to the surface or buried deep underground. Removing it
is simple in principle: just expose the coal, break it up, and cart it off to be burned. In
practice, coal mining is an energy intensive, labor intensive and money intensive
undertaking. Underground mining is one of the most hazardous of occupations,
killing and injuring many in accidents, and causing chronic health problems. Most
underground mining occurs in the East, while surface mining dominates in the West.
Underground
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Panel mining -- Panel mining, as seen in the photo below, uses a moveable
roof support to hold up the "overburden" while coal is removed. Then the
support is moved allowing the overburden to collapse. In longwall mining, two
long tunnels are cut, up to a mile long and 600 feet apart. The seam of coal
between is cut away and loaded onto conveyor belts. This is the most
common and productive approach in Eastern mining. Shortwall mining is
similar but uses shorter tunnels. Shortwall costs less, but is less productive.
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Room and pillar -- When the coal is cut and removed, underground rooms
are built. Pillars made of coal are left in place, in order to support the roof.
This reduces the overall recovery of the mine.
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Continuous -- With either panel or room and pillar mining, there are three
different ways to cut the mine out. In continuous mining, a cutter chops into a
coal seam, and loose coal is picked up by conveyer and loaded into carts.
This has become more common than the second approach, using dynamite
to loosen coal. The third approach uses water to break up the coal and form a
slurry that is pumped to the surface. Hydraulic mining is not as common as
the other two, and can cause groundwater contamination problems.
Panel mining
Surface
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Area mining -- also known as "strip mining," this approach is common in the
West where coal seams are flat and lie close to the surface. The top soil is
removed, and the seams of coal are simply loaded into huge trucks and
carried away. Area mining accounts for two-thirds of surface mining
production. A recent variation on this in the East is "mountaintop removal,"
where the tops of mountains are literally taken off, exposing the coal beneath.
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Contour mining -- is done when coal seams are exposed in hillsides,
typically in Appalachia. Contour mines are smaller than area mines, but still
account for most of the rest of surface production.
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Pit mining -- deep pits are dug to get a steeply dipping coal seams. One
example can be found in the Iron Range of northern Minnesota.
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Auger mining -- rarely used, this involves drilling large holes with augers,
seven feet in diameter by 200 feet deep. It is also used for underground
mining when poor roof conditions are present.
Where Our Coal Comes From
Historically, underground mines in the Appalachian region have provided most of
America's coal. In 1950, underground mines produced 382 million metric tons, 75
percent of the coal produced. Surface mining has grown steadily since then,
however, and now accounts for 60 percent of production.
Underground mines declined in the 1970s, to as low as 220 million tons in 1978. But
while they now produce as much as they ever have, almost 400 million metric tons
per year, surface mining has risen from 126 million in 1950 to 536 million in 1991. In
1995, Wyoming was the top coal state, producing one-third of the U.S. total.
Kentucky and West Virginia were next, with 15 and 11 percent each.
Although production costs have been an important factor in this transition, another
has been the sulfur content of the coal. Subbituminous Wyoming coal is only 0.35
percent sulfur by weight, while Kentucky coal is 1.59 percent sulfur. Since the Clean
Air Act Amendments of 1977, sulfur dioxide emissions have been heavily regulated
for coal-burning power plants.
While Western coal may have less sulfur, it also has fewer btu's of energy, or a
lower "heat rate." Wyoming coal has, on average, 8600 Btu's of energy per pound.
Eastern coal has heat rates of well over 12,000 Btu's per pound. The result is that
power plants need to burn 50 percent more Western coal to match the power output
from Eastern coal.
Still, the sulfur content per Btu is favorable to Western coal, so strip mines in
Wyoming, especially in the Powder River Basin, are king. The Wyodak coalbed, the
nation's leading source of coal, covers 10,000 square miles in the Powder River
Basin of Wyoming and Montana. It has seams of coal averaging 70 feet thick and
exceeding 100 feet in places. With further restrictions from the 1990 Amendments to
the Clean Air Act, Western coal is expected to continue to dominate.
Coal Facts. In 1993, the Nation consumed more than 2 million
tons of coal per day -- about 20 pounds for each person every
day. Coal production averaged around 30 tons per second,
enough to fill a railroad car every 3 seconds. To produce that
much coal, valued at about $19 billion, more than 100,000 miners
worked in some 2,500 mines.
How Coal Is Transported And Refined
Coal is shipped primarily by train and by barge. Hundreds of coal trains and barges
run day and night, delivering coal at a rate of 2.5 million metric tons per day to power
plants and factories around the country. Coal from Wyoming is shipped as far away
as Georgia.
Shipping is a significant cost of coal production. Wyoming coal sent to Georgia, for
example, was sold at $29 per metric ton in 1995, while it sold in Wyoming went for
only $13. With improvements in long-distance power transmission in recent years,
"mine mouth" coal plants have been built. In mine mouth plants, the coal is dug out
of the ground and put on a conveyor belt that runs directly into the power plant. Coal
plants near the Four Corners region of Arizona, New Mexico, Colorado and Utah,
ship their power to Southern California.
An important constraint to mine mouth plants is that these coal-rich western areas
are often water-poor. A 500 megawatt coal plant uses 2.2 billion gallons of water a
year, for cooling and for steam production. Much of this can be recycled, but in dry
areas it is still a major use of water.
Coal refining is nowhere near as complex as oil refining. Coal is washed with a water
or chemical bath to remove some impurities. As much as 30 percent of the sulfur
can be removed by washing. At the power plant, coal is pulverized to a heavy
powder just before being burned.
How Coal is Burned
In the most common type of coal plant, pulverized coal is blown into the furnace
where it burns while airborne. Water flows through tubes that run through the
furnace. The water is heated to boiling while under pressure. This pressurized steam
blasts through a turbine, which turns a generator to produce electricity. After the
steam has passed through the turbine, it is condensed into water and cooled, and
sent back into the furnace. This cycle is known to engineers as the Rankine Cycle,
and is used in nuclear power plants as well.
When the coal burns, it gives off sulfur dioxide, nitrogen oxide and carbon dioxide,
among other gases. The sulfur particulates are partly removed with scrubbers or
filters. Scrubbers use a wet limestone slurry to absorb sulfur as it passes though.
Filters are large cloth bags that catch particles as they go through the cloth.
Scrubbers are more common, and can reduce sulfur emissions by up to 90 percent,
when working properly. Still, smaller particulates are less likely to be absorbed by
the limestone, and can pass out the smokestack into the air.
Another type of coal plant uses "fluidized bed combustion" instead of a standard
furnace. A fluidized bed is made up of small particles of ash, limestone and other
non-flammable materials, which are partially suspended in an upward flow of hot air.
Powderized coal and limestone are blown into the bed at high temperature. They
burn in the bed, and the limestone binds with sulfur released from the coal. The heat
then boils water in pipes which completes the Rankine Cycle. The advantage of
fluidized bed combustion is that sulfur emissions are lower than in standard coal
plants. The down side is that the plants are more complex and require more
maintenance.
Sulfur control methods like scrubbers, fluidized bed combustors and switching to
low-sulfur coal reduced sulfur emissions by 33 percent between 1975 and 1990,
even while coal use increased by 50 percent. Nitrogen oxide emissions have stayed
pretty much the same over this period. Carbon dioxide emissions, which can't be
removed from the plant's exhaust, have risen with coal use however.
Coal provides just over half of the electricity produced in the US.
A Case Study: The Side Effects of a Coal Plant
A 500 megawatt coal plant produces 3.5 billion kilowatt-hours per year, enough to
power a city of about 140,000 people. It burns 1,430,000 tons of coal, uses 2.2
billion gallons of water and 146,000 tons of limestone.
It also puts out, each year:
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10,000 tons of sulfur dioxide. Sulfur dioxide (SOx) is the main cause of acid
rain, which damages forests, lakes and buildings.
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10,200 tons of nitrogen oxide. Nitrogen oxide (NOx) is a major cause of
smog, and also a cause of acid rain.
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3.7 million tons of carbon dioxide. Carbon dioxide (CO2) is the main
greenhouse gas, and is the leading cause of global warming. There are no
regulations limiting carbon dioxide emissions in the U.S.
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500 tons of small particles. Small particulates are a health hazard, causing
lung damage. Particulates smaller than 10 microns are not regulated, but may
be soon.
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220 tons of hydrocarbons. Fossil fuels are made of hydrocarbons; when they
don't burn completely, they are released into the air. They are a cause of
smog.
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720 tons of carbon monoxide. Carbon monoxide (CO) is a poisonous gas and
contributor to global warming.
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125,000 tons of ash and 193,000 tons of sludge from the smokestack
scrubber. A scrubber uses powdered limestone and water to remove pollution
from the plant's exhaust. Instead of going into the air, the pollution goes into a
landfill or into products like concrete and drywall. This ash and sludge
consists of coal ash, limestone, and many pollutants, such as toxic metals like
lead and mercury.
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225 pounds of arsenic, 114 pounds of lead, 4 pounds of cadmium, and many
other toxic heavy metals. Mercury emissions from coal plants are suspected
of contaminating lakes and rivers in northern and northeast states and
Canada. In Wisconsin alone, more than 200 lakes and rivers are
contaminated with mercury. Health officials warn against eating fish caught in
these waters, since mercury can cause birth defects, brain damage and other
ailments. Acid rain also causes mercury poisoning by leaching mercury from
rocks and making it available in a form that can be taken up by organisms.
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Trace elements of uranium. All but 16 of the 92 naturally occurring elements
have been detected in coal, mostly as trace elements below 0.1 percent
(1,000 parts per million, or ppm). A study by DOE's Oak Ridge National Lab
found that radioactive emissions from coal combustion are greater than those
from nuclear power production.
The 2.2 billion gallons of water it uses for cooling is raised 16 degrees F on average
before being discharged into a lake or river. By warming the water year-round it
changes the habitat of that body of water.
Coal mining creates tons of hazardous and acidic waste which can contaminate
ground water. Strip mining also destroys habitat and can affect water tables.
Underground mining is a hazard to water quality and to coal miners. In the mid1970s, the fatality rate for underground miners was 0.4 per million tons of coal -- one
miner would be killed every two years to supply our 500 MW plant. The disabling
injury rate was 38 people per million tons -- 106 miners would be disabled every two
years to supply this plant. Since coal mining is much more automated now, there
are many fewer coal miners, and thus many fewer deaths and injuries.
Transportation of coal is typically by rail and barge; much coal now comes from the
coal basins of Wyoming and the West. Injuries from coal transportation (such as at
train crossing accidents) are estimated to cause 450 deaths and 6800 injuries per
year. Transporting enough coal to supply just this one 500 MW plant requires 14,300
train cars. That's 40 cars of coal per day.
The Future of Coal
Coal is abundant in America, and in many countries around the world. The amount
of coal that can be mined at a competitive price in the U.S. is currently estimated at
about 265 billion short tons. This is evenly divided between low-sulfur coal in the
West (100 billion tons), medium-sulfur coal in the West and Appalachia (80 billion)
and high-sulfur coal in the Midwest and Appalachia. Underground mining is required
for about two-thirds of U.S. coal reserves; the rest can be surface mined.
Annual coal production is projected to remain around 1 billion tons into the next
century. At a steady rate of use, our coal won't be depleted for 265 years. At a rate
of growth of only two percent per year, however, this depletion occurs after 93 years.
At a growth rate of 3 percent, it happens at 73 years.
But while physical supplies of coal may be substantial, and production costs are low,
other factors may limit coal use. Pollution controls can remove a significant part of
the sulfur and particulate emissions, if properly monitored and maintained. Even so,
the environmental impacts of coal are enormous.
And despite the many innovative coal combustion technologies being developed, the
only practical way to reduce carbon dioxide emissions from coal is to get more
energy out of each pound of coal -- to increase the efficiency. But the efficiency of
typical coal plants has peaked at about 33 percent, limited mostly by their steam
turbines. What doesn't become electricity becomes waste heat.
The first way to increase the efficiency of turning coal into electricity is to capture the
waste heat. "Cogeneration," the generation of heat and power together, is a wellknown technology, but is not always applied. One method of cogeneration is to use
the waste heat to warm nearby buildings. Such "district heating" systems are
common in northern Europe, but are rarely used in the US.
Utilities in New York and Wisconsin are experimenting with ways to burn to biomass
along with coal in power plants. In New York, fast-growing willow trees are chopped
up and mixed with coal; in Wisconsin, switchgrass is being used. Sometimes when
biomass is burned alone in a conventional furnace, the temperatures are too low to
clean out all the residue, and a slag builds up in the furnace. By burning the biomass
with coal, slagging problems are minimized and carbon and sulfur emissions are
reduced.
Another technology under development is the coal gasification combustion turbine
(CGCT). In this approach, coal is heated until it gives off volatile gases, such as
methane, which are burned in a gas turbine. After this hot air passes though a gas
turbine, it is used to heat water which drives a steam turbine. This combined cycle is
more efficient than steam turbines alone, with efficiencies approaching 50 percent.
By gasifying the coal first, emissions are reduced as well. This approach is also
being applied to biomass.
An approach with even lower carbon emissions is to run the coal gas through a fuel
cell. Fuel cells are battery-like devices that convert hydrogen-rich gases, such as
methane, into electricity without combustion. Using pure hydrogen, fuel cells are
almost 80 percent efficient. Since gasified coal would contain a number of impurities,
notably carbon, the gas would have to be cleaned up significantly. Cost effective
cleaning techniques are still under development.
A final approach, still in the research stage, is magnetohydrodynamics, or MHD.
With MHD, superheated gases from coal combustion blast through a magnetic field
created by superconducting magnets, producing an electric charge as they pass.
The gases then power a conventional gas turbine, extracting as much energy as
possible from the heat. In this combined-cycle approach, efficiency can get up to 50
or 60 percent. Interest in MHD may be waning though, due to some fundamental
technical difficulties. In an MHD plant, gases at 2000 degrees celsius pass through a
duct at supersonic speeds, just centimeters away from magnets that must be kept a
few degrees above absolute zero (-273 degrees celsius). Since gasified coal run
through combined-cycle plants can be nearly as efficient, and offer many fewer
engineering problems, MHD is unlikely to be developed commercially.
Despite all of these advanced techniques, it may never be possible to produce
energy from coal without carbon emissions. Most of the heat produced from coal is
generated from carbon, which provides more than 70 percent of the energy content.
Since there is so much coal in the world, and the cost of extracting it is so low, it will
take a concerted effort to avoid massive carbon emissions. More efficient use is a
start, but replacing coal with renewables is the ultimate solution to the environmental
impacts of coal.
Further Reading
Scientific American, Energy for Planet Earth, Chapter 8, "Energy from Fossil Fuels," 1991.
"The Future for Coal," New Scientist, January 23, 1993, pp. 20-41.
© 2000 Union of Concerned Scientists