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How Coal Works

related links
 Coal Combustion
 DOE's Coal Information Page

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.

Thirty years ago, coal was seen as a fuel of the 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.


  • 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.

  • 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.

  • 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.

Photo of underground panel mining
Panel mining


  • 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.

  • 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.

  • 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.

  • 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:

  • 10,000 tons of sulfur dioxide. Sulfur dioxide (SOx) is the main cause of acid rain, which damages forests, lakes and buildings.

  • 10,200 tons of nitrogen oxide. Nitrogen oxide (NOx) is a major cause of smog, and also a cause of acid rain.

  • 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.

  • 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.

  • 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.

  • 720 tons of carbon monoxide. Carbon monoxide (CO) is a poisonous gas and contributor to global warming.

  • 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.

  • 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.

  • 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 mid-1970s, 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 well-known 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.