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 forested 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 store 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. More 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 before 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 more plant debris is transformed into carbon.
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 more 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 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 more moisture than other types of coal. This makes it expensive and dangerous to mine, store, 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 is about 100 million years old. It contains more 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 is formed under more 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 more 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 is the highest rank of coal. It has the most amount of carbon, up to 97%, and therefore contains the most energy. It is harder, more dense, and more 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 more 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 pores than sand, so more 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 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 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.
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.
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. Before 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, more 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 more 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 more 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.
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.
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.
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 before 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.
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 stored 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 stored 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.
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 before 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
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.
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 forever.
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.
Photograph by James P. Blair
“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.
Carbon fiber, used in everything from lightweight bicycles to bullet-protecting Kevlar vests, is a type of graphite, the highest rank of coal.
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.
Top Coal Producers
2. United States
4. European Union (leading producer: Germany)
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.