Coal

Introduction

Most of the world's electricity is produced by burning coal, a black, rocky substance consisting mostly of carbon. Coal is also used to fire cement-production kilns and to produce coke (a reduced form of coal) burned in steel manufacture. Coal is destructive to mine, and burning it releases pollutants that cause acid rain and lung cancer, including sulfur dioxide (SO₂) and mercury.

However, coal burning is also the cheapest source of baseline electric generation and its use is increasing around the world. Many proposals have been made for technologies to collect the carbon dioxide (CO₂) emitted from coal burning and inject it underground or into the ocean. As of 2007, a number of test projects for this technique, known as carbon capture and storage or sequestration, were under way, but no coal-fired power plant was actually sequestering its CO₂ output. Some liquid fuels were also being produced from coal, although without sequestration this technology actually increases the amount of CO₂ released per unit of energy released.

Proposals have been made to generate clean-burning hydrogen from coal by sequestering the CO₂ released by this process. Despite high hopes for such technologies, especially carbon capture and storage, as of early 2008 the burning of coal remained the largest single source of greenhouse-gas emissions and was a major source of air pollution as well, causing several hundred thousand deaths annually.

Historical Background and Scientific Foundations

Formation and History of Coal

Coal is a flammable black rock consisting mostly of carbon. It is composed of the compressed, chemically transformed remnants of plants that grew in vast swamps that flourished from about 286 to 300 million years ago, a period named the Carboniferous because of its association with coal. Because coal was formed in swamps, it tends to occur in extensive horizontal layers called seams or beds. In the millions of years after its formation, coal was covered by sediments that eventually hardened into sedimentary rock.

Today, to obtain coal humans must either tunnel under this rock or, if it is not too thick, strip it away— along with the overlying landscape—to expose the underlying coal seam. This practice, known as strip mining, includes the practice of mountaintop removal, in which the upper portions of entire mountains are removed and cast down into adjacent valleys. Strip mining destroys the original ecology and landscape of the mined area, but because of its cheapness is increasingly used worldwide.

The several grades or qualities of coal are classified according to their hardness and composition. Softer, lower-carbon coals burn more poorly and pollute more, but are in greater supply. The hardest, highest-carbon coal is anthracite, a black rock that is over 90% carbon by weight. Bituminous coal is from 70% to 90% carbon, and tends to be black or dark brown; lignite, the lowest grade, is 60% to 70% carbon. Although estimates of world coal resources are based partly on guesswork, most experts agree that many billions of tons of coal remain in underground deposits.

Small quantities of coal also appear on Earth's surface as outcroppings. This outcrop coal has always been accessible to human beings, and has been used as a fuel for several thousand years. However, for most of the last 800,000 years, wood has been humankind's primary fuel because it is more widely distributed. Coal mining began in earnest in the thirteenth century in Britain, with the rest of Europe soon following suit. Coal became an important fuel at this time because the forests of Europe had been mostly cut down for fuel and timber, creating the world's first energy crisis. Coal mining solved the crisis and began the large-scale emission by human beings of CO₂ to Earth's atmosphere.

Decaying or burning wood also releases CO₂, but does not increase total atmospheric CO₂ because the carbon in the wood was extracted by the growing tree from CO₂ in the air originally. Although the carbon found in coal and other fossil fuels was also originally extracted from the air by green plants, this was done so many millions of years ago that the reappearance of that carbon in today's atmosphere is equivalent to adding brand-new carbon to the environment.

From the Middle Ages to the early eighteenth century, coal burning remained small in scale by today's standards and did not release enough CO₂ to affect Earth's climate. Coal was burned in furnaces or fireplaces to heat buildings and to cook, brew, and the like, but machinery that could be powered by coal had not yet been invented. In 1712, British inventor Thomas Newcomen (1663–1729) demonstrated the world's first coal-powered steam engine for pumping water. The Newcomen engine proved a commercial success and served as a forerunner for more efficient coal-powered steam engines in factories, boats, and trains. It allowed the water in coal mines to be pumped out more cheaply than did the horse-powered pumps that had been used until that time. For the first time, a fossil fuel was both expanding demand for itself and making its own extraction more economical.

With the invention of commercially viable and popular steam engines, the switch from muscle and wind power to mechanical power based on fossil fuels had begun. By 1800 it was far advanced in the textile industry and other sectors, and CO₂ began to be released in quantities large enough to eventually change Earth's climate. The concentration of CO₂ in the atmosphere in 1750, about 280 parts per million, is used today as the pre-industrial standard against which subsequent anthropogenic (human-made) increases in greenhouse gases are measured.

By 2007, atmospheric CO₂ stood at about 383 parts per million, a 36.8% increase from 1750. Most of this increase had come from coal, with the second-largest share of total anthropogenic CO₂ coming from oil. As of 2007, humans had released a total of about 170 billion tons of carbon into the atmosphere from burning coal, about half of which had been absorbed by the oceans and other carbon sinks.

WORDS TO KNOW

ACID RAIN: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

BASELINE GENERATION: Generation of electricity for baseline demand, that is, demand that is steady around the clock. Demand for electricity rises above baseline during the daytime and during heat waves, when power demand for air conditioning peaks.

CARBON SEQUESTRATION: The uptake and storage of carbon. Trees and plants, for example, absorb carbon dioxide, release the oxygen, and store the carbon. Fossil fuels were at one time biomass and continue to store the carbon until burned.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

OUTCROPPING: Any rock formation that is accessible from the surface without digging: a protruding mass of rock, usually connected to a larger, buried mass of similar rock.

RENEWABLE ENERGY: Energy obtained from sources that are renewed at once, or fairly rapidly, by natural or managed processes that can be expected to continue indefinitely. Wind, sun, wood, crops, and waves can all be sources of renewable energy.

Coal Usage and Resources

Anthropogenic greenhouse-gas emissions have increased ever since the beginning of the Industrial Revolution, with the most rapid growth occurring most recently. Despite added dependence on oil, which became essential to transportation in the late nineteenth and early twentieth centuries, and on natural gas, which was first exploited on a mass scale in the 1950s and 1960s, coal has remained important. As of 2006, the world was using more coal than ever, namely about 6.3 billion tons (5.7 billion metric tons), 7.6% more than in 2005 and about twice as much as in 1990. As of 2005, 35% of world primary energy (that is, heat energy from fuel before it is transformed into electricity or other forms, as well as electricity produced directly, as by hydroelectric dams, solar cells, or windmills) was obtained from oil, 25% from coal, 21% from natural gas, 12% from renewables (including hydroelectricity), and 6% from nuclear power. About 90% of coal was used for electricity generation, with most of the rest being used to provide heat in the steel and concrete industries.

Coal's role was particularly important in making electricity. Heat from burning coal is used to make pressurized steam, which turns turbines that turn generators that produce electricity. In 2006, 40% of world electricity came from coal, 20% from natural gas, 16% from hydroelectric dams, 15% from nuclear power, 7% from oil, and 2% from miscellaneous sources, including non-hydro renewables. In specific countries, the mix varied: in Poland, for example, 93% of electricity was coal-generated in 2006, while in Germany only 47% was. In the United States, the world's largest energy consumer, 56% of electricity was coal-generated.

Although wind power had become the cheapest form of new electric generating capacity by 2007, it could not be used for baseline generation. Baseline generation is the making of electricity to meet steady, around-the-clock demand. Wind is not suitable for baseline because wind turbines only make electricity when the wind is blowing, so coal remains the cheapest source of baseline electricity. In 2007, coal cost about one sixth as much per unit of energy released as did oil or natural gas.

Estimates of how much coal remains in the ground worldwide vary widely. Energy experts distinguish between coal reserves and coal resources. Reserves are quantities of undug coal that are proven to exist and to be recoverable. Resources are coal deposits whose existence is assumed based on past patterns of discovery, but which have not been actually found. Estimates of world coal resources (undiscovered coal) are unreliable, and shrank from 1980 to 2005 by about 50%, to a little less than 1 trillion tons. World coal reserves are considerably smaller, about half a trillion tons.

Eighty-five percent of proven coal reserves are found in six countries, namely the United States, Russia, India, China, Australia, and South Africa. The United States holds about 30% of world reserves, some 120 billion tons. However, figures even for supposedly proven reserves can be inaccurate: in 2004, Germany decreased its official estimate of its hard coal reserves by 99%, from 23 billion tons (21 billion metric) to 183 million tons (166 million metric). The change, according to the World Energy Council, was due to the fact that earlier estimates of reserves—supposedly proven— were in fact mostly speculative.

In 2005, China was the world's largest coal producer and consumer, producing about 1.1 billion tons (998 million metric) a year, with reserves of 59 billion tons (54 billion metric), giving an annual depletion rate of 1.9% per year. The United States was second, producing 576 billion tons (523 billion metric) a year; Australia was third, producing 202 billion tons (183 billion metric) a year; and India was fourth, producing 200 billions tons (183 billion metric) a year. Although the United States was producing more coal by weight in 2005 than ever before, more of the coal being produced was low-carbon sub-bituminous coal. Thus, in terms of energy production, the country's coal industry had actually peaked in 2000. Although global coal production was rising, a 2007 study by the Energy Watch Group predicted that world coal production was likely to peak around 2030 at about 30% above 2007 levels, then decline slowly thereafter to 1990 levels by about 2100.

Impacts and Issues

Coal and the Environment

Coal is the most destructive form of obtaining primary energy in routine use. (The byproducts of nuclear power generation can be exploited to produce nuclear weapons, which have essentially unlimited destructive capacity, but this is not routine.) Coal's damage to the environment begins during mining. Strip mining, which supplies about 70% of coal from the United States, annihilates existing landscapes above the area mined for coal and destroys additional area that is buried under the removed overburden, as the layer of soil, rock, and forest covering the coal seam is termed. In both shaft and strip mines, sulfur associated with coal dissolves in water, forming sulfuric acid (H₂SO₄) that runs off into streams. Toxic heavy metals dissolve in the acidic water and accumulate in aquatic food chains. Solid mine wastes are bulky, toxic, and often flammable. Coal mines release methane (CH₄), another greenhouse gas.

When coal is burned, it releases scores of pollutants, including mercury, sulfur dioxide (which returns to Earth as acid rain), nitrogen oxides, soot particles, mercury, cadmium, uranium, and lead. According to a 2007 World Bank report, air pollution from coal causes on the order of 400,000 deaths annually in China. A 2004 study commissioned by environmental groups but carried out by a firm often employed by the U.S. Environmental Protection Agency (EPA) found that coal burning causes about 24,000 deaths a year in the United States, including over 8,000 from lung cancer. The accuracy of the latter study was disputed by representatives of the coal industry.

Coal and Climate Change

Coal is a major contributor to anthropogenic climate change. As of 2004, about 84% of global anthropogenic greenhouse-gas emissions were from energy production, with 95% of these emissions (that is, about 80% of all greenhouse-gas emissions) consisting of CO₂ released from burning fossil fuels. The other 5% of greenhouse-gas emissions from energy production consisted of methane from coal mining and hydroelectric dams and nitrogen-oxygen (NO_x) compounds from fuel burning. Because coal is more carbon-intensive than oil or natural gas, releasing almost all its energy from combustion of carbon rather than of hydrogen, it produces a larger share of CO₂ emissions than it does of primary energy. Coal produced only 25% of global primary energy, but 40% of global CO₂ emissions. Coal is about twice as carbon-intensive as natural gas.

In the early 2000s, global CO₂ emissions were rapidly increasing. From 2003 to 2004 alone, annual emissions increased by over 1.2 billion tons (1.1 billion metric) of CO₂, with 86% of this rise caused by increasing energy demand in developing countries. Increased coal usage accounted for 60% of the 2003 to 2004 increase in global CO₂ emissions.

Large underground coal fires, especially in China, are a significant source of greenhouse gases, although estimates of their magnitude vary widely. Estimates of CO₂ output from Chinese coal fires alone range from 150 to 450 million tons (136 to 408 million metric) of CO₂ per year (0.33-1% of annual global CO₂ emissions).

Alternative Coal Technologies

If global warming is to be stabilized at a 3.6°F(2°C) change, widely cited by scientists as the approximate limit for non-catastrophic climate change, low-carbon power sources will have to dominate by mid-century. A 2003 study published in Science projected that by about 2050, 75% to 100% of total power demand would have to be met by non-CO₂-releasing sources to stabilize warming at this level. At the same time, however, energy demand was projected to grow. Although many experts were urging a rapid expansion of nuclear power, and by 2007 renewable energy resources such as wind were already expanding rapidly (over 30% per year for wind power), most expansion in primary energy was expected to come from coal.

By 2007, concern was mounting about the impact of coal on climate. In October of that year, the state of Kansas became the first governmental body in the United States to cite CO₂ emissions as a reason for refusing a construction permit for a new coal-fired electric generation plant. A legal basis for the action had been supplied in April 2007, by a U.S. Supreme Court decision ruling that greenhouse gases, including CO₂, are pollutants under the terms of the federal Clean Air Act.

The only way to use steady or increasing amounts of coal while decreasing CO₂ emissions is to employ carbon capture and storage (CCS) technologies, also known as carbon sequestration or clean coal technology. There are a number of CCS technology concepts, but all involve pumping CO₂ from power plants into ocean waters or deep underground reservoirs rather than allowing it to escape into the air. As of 2007, several industrial-scale demonstration projects were in operation. For example, in Ketzin, Germany, a pilot project was injecting CO₂ into sandstone 2,600 ft (800 m) underground. Over two years, the project planned to sequester 66,000 tons (60,000 metric tons) of CO₂, about the same amount emitted in a year by 40,000 cars. The European Union was considering requiring all new coal plants in Europe to include CCS technology from 2020 onward, with some experts urging a moratorium on all new coal-plant construction until CCS technology could be applied.

Concerns about CCS involved its cost, whether marine sequestration would enhance acidification of the oceans, and whether gas pumped into underground reservoirs would remain there. Also, some critics of the technology were concerned that making coal less harmful in greenhouse terms will encourage reliance on the fuel, whose extraction is environmentally destructive regardless of how it is burned.

See Also Carbon Sequestration Issues; Energy Contributions; Nuclear Power; Renewable Energy; Wind Power.

BIBLIOGRAPHY

Books

Miller, Bruce G. Coal Energy Systems. San Diego, CA: Academic Press, 2004.

Periodicals

Berstein, Lenny, et al. “Carbon Dioxide Capture and Storage: A Status Report” Climate Policy 6 (2006): 241-246.

Caldeira, Ken. “Climate Sensitivity Uncertainty and the Need for Energy Without CO₂ Emission.” Science 299 (2003): 2052-2054.

“Coal Use Grows Despite Warming Worries.” The New York Times (October 28, 2007).

Holloway, Sam. “Storage of Fossil Fuel-Derived Carbon Dioxide Beneath the Surface of the Earth.” Annual Review of Energy and the Environment 26 (2001): 145-166.

Kintisch, Eli. “Report Backs More Projects to Sequester CO₂ from Coal.” Science 315 (2007): 1481.

Mufson, Steven. “Democrats Push Coal-to-Liquids Energy Plan.” The Washington Post (June 13, 2007).

Mufson, Steven. “Power Plant Rejected Over Carbon Dioxide for First Time.” The Washington Post (October 19, 2007).

Quadrelli, Roberta. “The Energy-Climate Challenge: Recent Trends in CO₂ Emissions from Fuel Combustion.” Energy Policy 35 (2007): 5938-5952.

Sanderson, Katharine. “King Coal Constrained.” Nature 3449 (2007): 14-15.

Stauffer, Hoff. “New Sources Will Drive Global Emissions.” Energy Policy 35 (2007): 5433-5435.

Web Sites

“Emissions of Greenhouse Gases in the United States 2005.” U.S. Energy Information Administration, November 2006. < http://www.eia.doe.gov/oiaf/1605/ggrpt/carbon.html> (accessed November 5, 2007).

Metz, Bert H., et al. “IPCC Special Report on Carbon Dioxide Capture and Storage.” Intergovernmental Panel on Climate Change, 2005. < http://www.ipcc.ch/activity/srccs/SRCCS.pdf> (accessed November 5, 2007).

Sharp, Philip, et al. “The Future of Coal.” Massachusetts Institute of Technology, 2007. < http://web.mit.edu/coal/The_Future_of_Coal.pdf> (accessed November 5, 2007).

Larry Gilman