Coal | National Geographic Society (2022)

Coal is a black or brownish-black sedimentary rock that can be burned for fuel and used to generate electricity. It is composed mostly of carbon and hydrocarbons, which contain energy that can be released through combustion (burning).

Coal

is the largest source of energy for generating

electricity

in the world, and the most abundant fossil fuel in the United States.

Fossil fuels

are formed from the remains of ancient organisms. Because

coal

takes millions of years to develop and there is a limited amount of it, it is a nonrenewable resource.

The conditions that would eventually create

coal

began to develop about 300 million years ago, during the Carboniferous period. During this time, the Earth was covered in wide, shallow seas and dense forests. The seas occasionally flooded the f

orested

areas, trapping plants and algae at the bottom of a swampy wetland. Over time, the plants (mostly mosses) and

algae

were buried and compressed under the weight of overlying mud and vegetation.

As the plant debris sifted deeper under Earth’s surface, it encountered increased temperatures and higher pressure. Mud and acidic water prevented the plant matter from coming into contact with oxygen. Due to this, the plant matter decomposed at a very slow rate and retained most of its carbon (source of energy).

These areas of buried plant matter are called peat bogs.

Peat

bogs

st

ore

massive amounts of carbon many meters underground.

Peat

itself can be burned for fuel, and is a major source of heat energy in countries such as Scotland, Ireland, and Russia.

Under the right conditions,

peat

transforms into

coal

through a process called carbonization.

Carbonization

takes place under incredible heat and pressure. About 3 meters (10 feet) of layered

vegetation

eventually

compresses

into a third of a meter (1 foot) of

coal

!

Coal

exists in underground formations called “coal seams” or “

coal

beds.” A

coal

seam

can be as thick as 30 meters (90 feet) and stretch 1,500 kilometers (920 miles).

Coal

seams

exist on every continent. The largest

coal

reserves are in the United States, Russia, China, Australia, and India.

In the United States,

coal

is mined in 25 states and three major regions. In the Western

Coal

Region, Wyoming is the top producer—about 40% of the

coal

mined in the country is extracted in the state. M

ore

than one-third of the nation’s

coal

comes from the Appalachian

Coal

Region, which includes West Virginia, Virginia, Tennessee, and Kentucky.

Coal

extracted from Texas in the Interior

Coal

Region supplies mostly local markets.

Types of Coal

Coal

is very different from mineral rocks, which are made of inorganic material.

Coal

is made of fragile plant matter, and undergoes many changes bef

ore

it becomes the familiar black and shiny substance burned as fuel.

Coal

goes through different phases of

carbonization

over millions of years, and can be found at all stages of development in different parts of the world.

Coal

is ranked according to how much it has changed over time. Hilt's Law states that the deeper the

coal

seam

, the higher its rank. At deeper depths, the material encounters greater temperatures and pressure, and m

ore

plant debris is transformed into carbon.

Peat
Peat

is not

coal

, but can eventually transform into

coal

under the right circumstances.

Peat

is an accumulation of partly decayed

vegetation

that has gone through a small amount of

carbonization

.

However,

peat

is still considered part of the

coal

“family” because it contains energy that its original plants contained. It also contains high amounts of volatile matter and gases such as methane and mercury, which are environmentally hazardous when burned.

Peat

retains enough moisture to be spongy. It can absorb water and expand the

bog

to form m

ore

peat

. This makes it a valuable environmental defense against flooding.

Peat

can also be integrated into soil to help it retain and slowly release water and nutrients. For this reason,

peat

and so-called “

peat

moss” are valuable to gardeners.

Peat

is an important source of energy in many countries, including Ireland, Scotland, and Finland, where it is dehydrated and burned for heat.

Lignite
Lignite

coal

is the lowest rank of

coal

. It has carbonized past the point of being

peat

, but contains low amounts of energy—its carbon content is about 25-35%. It comes from relatively young

coal

deposits, about 250 million years old.

Lignite

, a crumbly brown rock also called brown

coal

or rosebud

coal

, retains m

ore

moisture than other types of

coal

. This makes it expensive and dangerous to mine, st

ore

, and transport. It is susceptible to accidential

combustion

and has very high carbon emissions when burned. Most

lignite

coal

is used in power stations very close to where it was mined.

Lignite

is mainly

combusted

and used to generate

electricity

. In Germany and Greece,

lignite

provides 25-50% of

electricity

generated by

coal

. In the U.S.,

lignite

deposits generate

electricity

mostly in the states of North Dakota and Texas.

Sub-Bituminous Coal
Sub-bituminous

coal

is about 100 million years old. It contains m

ore

carbon than

lignite

, about 35-45%. In many parts of the world,

sub-

bituminous

coal

is considered “brown

coal

,” along with

lignite

. Like

lignite

,

sub-

bituminous

coal

is mainly used as fuel for generating

electricity

.

Most

sub-

bituminous

coal

in the U.S. is mined in the state of Wyoming, and makes up about 47% of all of the

coal

produced in the United States. Outside the U.S., China is a leading producer of

sub-

bituminous

coal

.

Bituminous Coal
Bituminous

coal

is formed under m

ore

heat and pressure, and is 100 million to 300 million years old. It is named after the sticky, tar-like substance called bitumen that is also found in petroleum. It contains about 45-86% carbon.

Coal

is a

sedimentary rock

, and

bituminous

coal

frequently contains “bands,” or strips, of different consistency that mark the layers of plant material that were

compressed

.

Bituminous

coal

is divided into three major types: smithing

coal

, cannel

coal

, and coking

coal

. Smithing

coal

has very low ash content, and is ideal for forges, where metals are heated and shaped.

Cannel

coal

was extensively used as a source of coal oil in the 19th century.

Coal

oil

is made by heating cannel

coal

with a controlled amount of oxygen, a process called pyrolysis.

Coal

oil

was used primarily as fuel for streetlights and other illumination. The widespread use of kerosene reduced the use of

coal

oil

in the 20th century.

Coking

coal

is used in large-scale industrial processes. The

coal

is coked, a process of heating the rock in the absense of oxygen. This reduces the moisture content and makes it a m

ore

stable product. The steel industry relies on coking

coal

.

Bituminous

coal

accounts for almost half of all the

coal

that is used for energy in the United States. It is mainly mined in Kentucky, Pennsylvania, and West Virginia. Outside the U.S., nations such as Russia and Colombia rely on

bituminous

coal

for energy and industrial fuel.

Anthracite
Anthracite

is the highest rank of

coal

. It has the most amount of carbon, up to 97%, and theref

ore

contains the most energy. It is harder, m

ore

dense

, and m

ore

lustrous than other types of

coal

. Almost all the water and carbon dioxide have been expelled, and it does not contain the soft or fibrous sections found in

bituminous

coal

or

lignite

.

Because

anthracite

is a high-quality

coal

, it burns cleanly, with very little soot. It is m

ore

expensive than other

coals

, and is rarely used in power plants. Instead,

anthracite

is mainly used in stoves and furnaces.

Anthracite

is also used in water-filtration systems. It has tinier p

ores

than sand, so m

ore

harmful particles are trapped. This makes water safer for drinking, sanitation, and

industry

.

Anthracite

can typically be found in geographical areas that have undergone particularly stressful geologic activity. For example, the

coal

reserves on the Allegheny Plateau in Kentucky and West Virginia stretch to the base of the Appalachian Mountains. Here, the process of orogeny, or mountain formation, contributed to temperatures and pressures high enough to create

anthracite

.

China dominates the mining of

anthracite

, accounting for almost three-quarters of

anthracite

coal

production. Other

anthracite

-mining countries include Russia, Ukraine, Vietnam, and the United States (mostly Pennsylvania).

Graphite
Graphite

is an allotrope of carbon, meaning it is a substance made up only of carbon atoms. (Diamond is another

allotrope

of carbon.)

Graphite

is the final stage of the

carbonization

process.

Graphite

conducts

electricity

well, and is commonly used in lithium ion batteries.

Graphite

can also resist temperatures of up to 3,000 °Celsius (5,400 °Fahrenheit). It can be used in products such as fire-resistant doors, and missile parts such as nose cones. The most familiar use for

graphite

, however, is probably as pencil “leads.”

China, India, and Brazil are the world’s leading producers of

graphite

.

Coal Mining

Coal can be extracted from the earth either by surface mining or underground mining. Once coal has been extracted, it can be used directly (for heating and industrial processes) or to fuel power plants for electricity.

Surface Mining
If

coal

is less than 61 meters (200 feet) underground, it can be extracted through surface mining.

In surface mining, workers simply remove any overlying sediment,

vegetation

, and rock, called overburden. Economically, surface mining is a cheaper option for extracting

coal

than underground mining. About two and a half times as much

coal

can be extracted per worker, per hour, than is possible with underground mining.

The environmental impacts of surface mining are dramatic. The landscape is literally torn apart, destroying habitats and entire ecosystems. Surface mining can also cause landslides and subsidence (when the ground begins to sink or cave in). Toxic substances leaching into the air, aquifers, and water tables may endanger the health of local residents.

In the United States, the Surface Mining Control and Reclamation Act of 1977 regulates the process of

coal

mining, and is an effort to limit the harmful effects on the environment. The act provides funds to help fix these problems and clean up abandoned mining sites.

The three main types of surface

coal

mining are strip mining, open-pit mining, and mountaintop removal (MTR) mining.

Surface Mining: Strip Mining
Strip mining is used where coal seams are located very near the surface and can be removed in massive layers, or strips. Overburden is usually removed with explosives and towed away with some of the largest vehicles ever made. Dump trucks used at strip mines often weigh more than 300 tons and have more than 3,000 horsepower.

Strip mining can be used in both flat and hilly landscapes. Strip mining in a mountainous area is called contour mining. Contour mining follows the ridges, or contours, around a hill.

Surface Mining: Open-Pit Mining
Open-pit mining is used when

coal

is located deeper underground. A pit, sometimes called a borrow, is dug in an area. This pit becomes the open-pit mine, sometimes called a quarry.

Open-pit mines

can expand to huge dimensions, until the

coal

deposit has been mined or the cost of transporting the

overburden

is greater than the investment in the mine.

Open-pit mining is usually restricted to flat

landscapes

. After the mine has been exhausted, the pit is sometimes converted into a landfill.

Surface Mining: MTR
During mountaintop removal mining (MTR), the entire summit of a mountain is stripped of its

overburden

: rocks, trees, and topsoil.

Overburden

is often hauled to nearby valleys, earning the process the nickname “valley fill” mining. After the

summit

is cleared of

vegetation

, explosives are used to expose the

coal

seam

.

After the

coal

is extracted, the

summit

is sculpted with

overburden

from the next mountaintop to be mined. By law, valuable topsoil is supposed to be saved and replaced after mining is done. Barren land can be replanted with trees and other

vegetation

.

Mountaintop removal began in the 1970s as a cheap alternative to underground mining. It is now used for extracting

coal

mainly in the Appalachian Mountains of the U.S., in states including Virginia, West Virginia, Tennessee, and Kentucky.

MTR is probaby the most controversial

coal

mining technique. The environmental consequences are radical and severe. Waterways are cut off or contaminated by

valley fill

.

Habitats

are destroyed. Toxic byproducts of the mining and explosive processes can drain into local waterways and pollute the air.

Underground Mining
Most of the world’s

coal

reserves are buried deep underground. Underground mining, sometimes called deep mining, is a process that retrieves

coal

from deep below the Earth’s surface—sometimes as far as 300 meters (1,000 feet). Miners travel by elevator down a mine shaft to reach the depths of the mine, and operate heavy machinery that extracts the

coal

and moves it above ground.

The immediate environmental impact of underground mining appears less dramatic than surface mining. There is little

overburden

, but underground mining operations leave significant tailings.

Tailings

are the often-toxic residue left over from the process of separating

coal

from gangue, or economically unimportant

minerals

. Toxic

coal

tailings

can

pollute

local water supplies.

To miners, the dangers of underground mining are serious. Underground explosions, suffocation from lack of oxygen, or exposure to toxic gases are very real threats.

To prevent the buildup of gases, methane must be constantly ventilated out of underground mines to keep miners safe. In 2009, about 10% of the U.S. methane emissions came from ventilating underground mines; 2% resulted from surface mining.

There are three major types of underground

coal

mining: longwall mining, room-and-pillar mining, and retreat mining.

Underground Mining: Longwall Mining
During

longwall mining

, miners slice off enormous panels of

coal

that are about 1 meter (3 feet) thick, 3-4 kilometers (2-2.5 miles) long, and 250-400 meters (800-1,300 feet) wide. The panels are moved by conveyor belt back to the surface.

The roof of the mine is maintained by hydraulic supports known as chocks. As the mine advances, the

chocks

also advance. The area behind the

chocks

collapses.

Longwall mining

is one of the oldest methods of mining

coal

. Bef

ore

the widespread use of conveyor belts, ponies would descend to the deep, narrow channels and haul the

coal

back to the surface.

Today, almost a third of American

coal

mines use

longwall mining

. Outside the U.S., that number is even higher. In China, the world’s largest

coal

producer, m

ore

than 85% of

coal

is exracted using the longwall method.

Underground Mining: Room and Pillar
In the room-and-pillar mining method, miners carve a “room” out of coal. Columns (pillars) of coal support the ceiling and overburden. The rooms are about 9 meters (30 feet) wide, and the support pillars can be 30 meters (100 feet) wide.

There are two types of room-and-pillar mining: conventional and continuous. In conventional mining, explosives and cutting tools are used. In continuous mining, a sophisticated machine called a continuous miner extracts the coal.

In the U.S., most room-and-pillar mining uses a continuous miner. In developing countries, room-and-pillar coal mines use the conventional method.

Underground Mining: Retreat Mining
Retreat mining is a variation of room-and-pillar. When all available coal has been extracted from a room, miners abandon the room, carefully destroy the pillars, and let the ceiling cave in. Remains of the giant pillars supply even more coal.

Retreat mining may be the most dangerous method of mining. A great amount of stress is put on the remaining pillars, and if they are not pulled out in a precise order, they can collapse and trap miners underground.

How We Use Coal

People all over the world have been using

coal

to heat their homes and cook their food for thousands of years.

Coal

was used in the Roman Empire to heat public baths. In the Aztec Empire, the

lustrous

rock was used for ornaments as well as fuel.

The Industrial Revolution was powered by

coal

. It was a cheaper alternative than wood fuel, and produced m

ore

energy when burned.

Coal

provided the steam and power needed to mass-produce items, generate

electricity

, and fuel steamships and trains that were necessary to transport items for trade. Most of the collieries, or

coal

mines, of the

Industrial Revolution

were in northern England, where m

ore

than 80% of

coal

was mined in the early 18th century.

Today,

coal

continues to be used directly (heating) and indirectly (producing

electricity

).

Coal

is also essential to the

steel

industry

.

Fuel
Around the world, coal is primarily used to produce heat. It is the leading energy choice for most developing countries, and worldwide consumption increased by more than 30% in 2011.

Coal can be burned by individual households or in enormous industrial furnaces. It produces heat for comfort and stability, as well as heating water for sanitation and health.

Electricity
Coal

-fired

power plants

are one of the most popular ways to produce and distribute

electricity

. In

coal

-fired

power plants

,

coal

is

combusted

and heats water in enormous boilers. The boiling water creates steam, which turns a turbine and activates a generator to produce

electricity

.

Almost all the

electricity

in South Africa (about 93%) is generated by

coal

. Poland, China, Australia, and Kazakhstan are other nations that rely on

coal

for

electricity

. In the United States, about 45% of the nation’s

electricity

is driven by

coal

.

Coke
Coal

plays a vital role in the

steel

industry

. In order to produce

steel

, iron

ore

must be heated to separate the iron from other

minerals

in the rock. In the past,

coal

itself was used to heat and separate the

ore

. However,

coal

releases impurities such as sulfur when it is heated, which can make the resulting metal weak.

As early as the 9th century, chemists and engineers discovered a way to remove these impurities from

coal

bef

ore

it was burned.

Coal

is baked in an oven for about 12-36 hours at about 1,000-1,100 °C (1,800-2,000 °F). This drives off impurities such as

coal

gas, carbon monoxide, methane, tars, and oil. The resulting material—

coal

with few impurities and high carbon content—is

coke

. The method is called coking.

Coke

is burned in a blast

furnace

with iron

ore

and air that is about 1,200 °C (2,200 °F). The hot air ignites the

coke

, and the

coke

melts the iron and separates out the impurities. The resulting material is

steel

.

Coke

provides heat and chemical properties that gives

steel

the strength and flexibility needed to build bridges, skyscrapers, airports, and cars.

Many of the biggest

coal

producers in the world (the United States, China, Russia, India) are also among the biggest

steel

producers. Japan, another leader in the

steel

industry

, does not have significant

coal

reserves. It is one of the world’s largest

coal

importers.

Synthetic Products
The gases that are released during the coking process can be used as a source of power.

Coal

gas can be used for heat and light.

Coal

can also be used to produce syngas, a combination of hydrogen and carbon monoxide.

Syngas

can be used as a transportation fuel similar to

petroleum

or diesel.

In addition,

coal

and

coke

byproducts

can be used to make synthetic materials such as tar, fertilizers, and plastics.

Coal and Carbon Emissions

Burning

coal

releases gases and particulates that are harmful to the environment. Carbon dioxide is the primary emission.

Carbon dioxide is an essential part of our planet’s atmosphere. It is called a greenhouse gas because it absorbs and retains heat in the atmosphere, and keeps our planet at a livable temperature. In the natural carbon cycle, carbon and carbon dioxide are constantly cycled between the land, ocean, atmosphere, and all living and decomposing organisms. Carbon is also sequestered, or st

ored

underground. This keeps the

carbon cycle

in balance.

However, when

coal

and other

fossil fuels

are extracted and burned, they release

sequestered

carbon into the atmosphere, which leads to a build-up of

greenhouse gases

and adversely affects climates and

ecosystems

.

In 2011, about 43% of the

electricity

in the U.S. was generated from burning

coal

. However,

coal

production was responsible for 79% of the country’s

carbon emissions

.

Other Toxic Emissions
Sulfur dioxide and nitrogen oxides are also released when

coal

is burned. These contribute to acid rain, smog, and respiratory illnesses.

Mercury is emitted when

coal

is burned. In the atmosphere, mercury is usually not a hazard. In water, however, mercury transforms into methylmercury, which is toxic and can accumulate in fish and organisms that consume fish, including people.

Fly ash (which floats away with other gases during

coal

combustion

) and bottom ash (which does not float away) are also released when

coal

is

combusted

. Depending on the composition of the

coal

, these

particulates

can contain toxic elements and irritants such as cadmium, silicon dioxide, arsenic, and calcium oxide.

In the U.S.,

fly ash

must be captured with industrial “scrubbers” to prevent it from polluting the atmosphere. Unfortunately,

fly ash

is often st

ored

in

landfills

or

power plants

, and can drain into groundwater. As a response to this environmental hazard,

fly ash

is being used as a component of concrete, thereby isolating it from the natural environment.

Many countries do not

regulate

their

coal

industries as strictly as the U.S., and emissions

pollute

air and water supplies.

Coal Fires
Under the right conditions of heat, pressure, and ventilation,

coal

seams

can self-ignite and burn underground. Lightning and wildfires can also ignite an exposed section of the

coal

seam

, and smoldering fire can spread along the seam.

Coal

fires emit tons of

greenhouse gases

into the atmosphere. Even if the surface fire is extinguished, the

coal

can smolder for years bef

ore

flaring up and potentially starting a wildfire again.

Coal fires can also begin in mines as a result of an explosion. Coal fires in China, many ignited by explosions used in the extraction process, may account for 1% of the world’s carbon emissions. In the U.S., it is more common for abandoned mines to catch fire if trash is burned in nearby landfills.

Once coal catches fire and begins smoldering, it is extremely difficult to extinguish. In Australia, the coal fire at “Burning Mountain” has been burning for 5,500 years!

Advantages and Disadvantages

Advantages
Coal

is an important part of the world energy budget. It is relatively inexpensive to locate and extract, and can be found all over the world. Unlike many renewable resources (such as solar or wind),

coal

production is not dependent on the weather. It is a baseload fuel, meaning it can be produced 24 hours a day, 7 days a week, 365 days a year.

We use and depend on many things that

coal

provides, such as heat and

electricity

to power our homes, schools, hospitals, and industries.

Steel

, vital for constructing bridges and other buildings, relies on

coke

for almost all production.

Coal

byproducts

, such as

syngas

, can be used to make transportation fuels.

Coal

mining also provides economic stability for millions of people worldwide. The

coal

industry

relies on people with a wide range of knowledge, skills, and abilities. Jobs associated with

coal

include geologists, miners,

engineers

, chemists, geographers, and executives.

Coal

is an

industry

that is critical to countries in both the developed and developing world.

Disadvantages
Coal

is a nonrenewable source of energy. It took millions of years to form, and a finite amount of it exists on our planet. Although it is a consistent and reliable source of energy at this point in time, it will not be available f

orever

.

Mining is one of the most dangerous jobs in the world. The health hazards to underground miners include

respiratory illnesses

, such as “black lung,” in which

coal

dust builds up in the lungs. In addition to disease, thousands of miners die every year in mine explosions, collapses, and other accidents.

Burning

coal

for energy releases toxins and

greenhouse gases

, such as carbon dioxide. These have an immediate impact on the local air quality, and contribute to global warming, the current period of

climate

change.

Surface mining permanently alters the

landscape

. In mountaintop removal, the

landscape

itself is obliterated and

ecosystems

are destroyed. This increases erosion in the area. Floods and other natural hazards put these areas at great risk.

Coal

mining can impact local water supplies in several ways. Streams may be blocked, increasing the chances for flooding. Toxins often

leach

into

groundwater

, streams, and

aquifers

.

Coal

is one of the most

controversial

energy sources in the world. The advantages of

coal

mining are economically and socially significant. However, mining devastates the environment: air, land, and water.

Fast Fact

Carbon Fiber
Carbon fiber, used in everything from lightweight bicycles to bullet-protecting Kevlar vests, is a type of graphite, the highest rank of coal.

Fast Fact

Clean Coal
“Clean coal” is a term used for any technology that reduces the carbon emissions of coal combustion. Clean coal usually refers to the process of carbon capture, where emissions are trapped and stored underground.

Fast Fact

Coal Fossils
Coal puts the “fossil” in “fossil fuel.” Paleontologists have discovered brilliantly preserved fossils of some of the world’s oldest tropical rain forests in coal seams.

Fast Fact

Top Coal Producers
1. China
2. United States
3. India
4. European Union (leading producer: Germany)
5. Australia

Fast Fact

It’s the Pits
The North Antelope Rochelle Complex in the U.S. state of Wyoming is the world’s largest coal mine. The open-pit mine has shipped more than 1.4 billion tons of coal since opening in 1983.

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