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Fossil Fuels

Fossil Fuels


Since the beginning of the industrial revolution , fossil fuels have been important sources of energy. European industrialization began in the late 1700s in England, and coal soon became a major fuel. In 1850 wood was still the main energy source in the United States. During the latter half of the nineteenth century, the United States and other industrialized nations relied on coal (a fossil fuel) to provide the energy for industrialization. Coal remained the major fuel source for many years, and then, in the latter half of the twentieth century, oil and natural gas became the primary energy sources. The first oil well was drilled in Pennsylvania in 1859.

In 2000, fossil fuels accounted for almost 90 percent of the world's energy production (see Table 1). Nuclear power and hydroelectric plants supplied about 13 percent and geochemical, wind, and solar energy sources supplied only a fraction of 1 percent. Biomass , including the burning of wood, is not included in the table because it is so difficult to estimate.

Although coal combustion produces substantially greater air pollution problems than does oil or natural gas combustion, because of its great abundance in the United States and other countries (such as Russia), there has been renewed interest in developing technology to burn coal more cleanly. However, all fossil fuels consist mainly of hydrocarbons (compounds that contain only carbon and hydrogen), which, upon complete combustion, yield carbon dioxide, a major greenhouse gas.

It is widely accepted in the scientific community that fossil fuels (coal, oil, and gas) have a biological origin and are ultimately derived from the buried remains of plant and animal matter, although some still argue in favor of a nonbiological or inorganic source. It is believed that a small fraction (much less than 1%) of dead plant and animal matter accumulates as deposited matter, is removed from contact with atmospheric oxygen, is subject to elevated temperatures and pressures (inhibiting decomposition by bacteria), and over geological time, is transformed into fossil fuels.

WORLD ENERGY SOURCES IN 2000
Source Percent of Energy
Petroleum 39
Natural Gas 24
Coal 24
Hydroelectric 7
Nuclear 6

Coal

Coal is considered the remnant of plants that grew in swamps hundreds of millions of years ago, and thus its source is often characterized as terrestrial, signifying its association with continental land masses. Terrestrial plant material characteristically contains lignin, a carbon-based natural polymer that provides rigidity to nonaquatic plants and enables them to stand upright against the pull of gravity. Lignin is much more resistant to bacterial degradation than other botanical components, such as cellulose, and is considered a significant contributor to the chemical composition of coal.

The extent to which lignin and other plant matter has been metamorphosed by the high temperatures and pressures associated with the gradual burial of this material determines the grade of the coal produced. As the process of coal formation (coalification) proceeds, the product is increasingly characterized by lower moisture content, greater carbon and energy content, and a greater hardness. Lignite is the softest and least metamorphosed type of coal, with a relatively high moisture content, a low fixed carbon (nonvolatile carbon) content, and a low energy content. Subbituminous coal is the next highest grade, and upon further coalification it can be transformed to bituminous coal, or ultimately to anthracite. Anthracite is the hardest coal, possessing about 95 percent fixed carbon, the lowest moisture content, and the best energy content. Coals from different sources also contain differing amounts of inorganic mineral matter (ash), which remains as a residue upon burning and thus lowers the energy content of the coal. Table 2 compares the compositions of the various types of coal.

One mineral often associated with coal is pyrite, FeS2. The burning of coal contributes to pollution of the atmosphere, owing to the presence in coal of pyrite and organic sulfur-containing compounds. Coal is commonly burned in power plants that generate electricity, and both the inorganic (pyrite-containing) and organic forms of coal are oxidized to yield sulfur dioxide (SO2). Sulfur dioxide reacts in air to form sulfuric acid (H2SO4), which is a major cause of acid rain . Sulfuric acid and sulfur dioxide are also lung irritants, and thus health hazards, and contribute to the corrosion of structures by their acidification of all forms of precipitation (rain, snow, fog, sleet). The impact of the atmospheric precipitation of SO2 and H2SO4 has been minimized by chemical and physical processes that remove inorganic sulfur from coal (desulfurization), and by the use of coals with low sulfur content. One positive effect of higher H2SO4 levels in the atmosphere is the increase in cloud cover, due to the hygroscopic (water-absorbing) nature of this acid, and this may help to lower the average surface temperature of the planetalthough CO2 produced as a result of oxidization of the carbon in coal is a major contributor to global warming.

COAL COMPOSITION
Type of Coal %C %H %O %N %Moisture Heating Value (kcal/kg)
*Peat is a dark, woody soil that has not yet been coalified to lignite.
Anthracite 9295 34 23 02 14 75008000
Bituminous 7592 46 320 12 530 50008000
Lignite 6075 46 1735 12 3050 30004500
Peat* 4560 37 2045 13 7090 <3000

Coke

Coal can be transformed into coke and other fuels by various industrial and experimental processes. Coke is produced by the pyrolysis (heating in the absence of air) of coal and is used in the production of iron and steel. The coking procedure removes moisture and other volatile components from coal, yielding an extremely carbon-rich material. Coal can also be transformed (via intrafuel conversion) into relatively clean liquid and gaseous fuels (liquifaction and gasification). However, this is accomplished at high costin money and energy.

Petroleum

Petroleum is an extremely complex mixture of hydrocarbons, which can be separated into liquid (oil) and gas fractions. Compared to coal, petroleum being a liquid is easier to transport. It probably originated in marine sediments, in contrast to the terrestrial origins of coal.

Because petroleum varies greatly in composition and distribution throughout the world, elaborate systems of refining and transport have been developed. Major oil fields or giant petroleum fields ("giant" indicating oil fields capable of producing at least 500 million barrels of oil) are found primarily in the Middle East, North and South America, and countries that made up the former Soviet Union. The uneven natural distribution of oil, and the consequent need to transport oil across vast distances, has led to instances of contamination due to oil spills. Coastal waters are particularly vulnerable, not only to oil spills, but also to contamination by bilge water and tank-washing water from commercial oil tankers. Even though it is a major producer of oil, the United States has found it necessary to import significant additional amounts of oil in order to meet ever-increasing industrial and home-related energy demands. Most plastics and other petrochemicals are made from petroleum, along with almost all gasoline, diesel fuel, jet fuel, heating oil, and lubricants. However, Earth's supply of petroleum is limited. Some experts estimate that world production of oil could climax as early as 2004. Although most, if not all, of the major oil-producing fields associated with continental masses have been discovered, and many offshore wells have been drilled, there still may be other major oil discoveries in less accessible areas such as under the oceana largely unexplored territory.

Natural Gas

The history of natural gas dates back to 900 b.c.e., when its use was mentioned in China. It was apparently unknown in Europe until 1659, when it was discovered in England. It was not discovered in the United States until 1815 in West Virginia. In the early twenty-first century, natural gas has become the favorite fuel of industrial nations. The United States is the largest producer as well as the largest consumer of natural gas. The largest natural gas reserves are located in Russia, Kazakhstan, and Iran.

Natural gas, which consists mainly of methane (CH4), can contain up to 20 percent of other gasesmainly ethane (C2H6), and possibly propane (C3H8), butane (C4H10), pentane (C5H12), carbon dioxide (CO2), and nitrogen (N2). Some natural gases contain small amounts of hydrogen, argon, carbon monoxide, or even hydrogen sulfide. Certain gas wells in Oklahoma also contain helium. In fact, they are a major source of helium in the United States. Natural gas is also colorless, odorless, and nontoxic but very flammable. (The odor we associate with natural gas is because of a mercaptan added to make gas leaks detectable.) Most natural gas is burned as fuel; however, ethane and the higher alkanes can be separated out and cracked to ethylene and propylene for making plastics. Although it is considered a "clean" and environmentally friendly fuel, compared to oil and coal, it is itself a major greenhouse gas and upon combustion yields carbon dioxide, the other major greenhouse gas. Like carbon dioxide, methane is also a greenhouse gas. However, natural gas fuel is thought to be only a minor contributor to methane in the atmosphere. Methane is constantly being generated by marsh and swamp terrain and by certain animals. Some experts believe that animals are the main source of atmospheric methane.

Other Sources of Fossil Fuels

Oil shales and tar sands also contain significant amounts of hydrocarbon materials that might eventually prove to be important energy sources. Oil shales are fine-grained sedimentary rocks (shales) that contain hydrocarbons that are dispersed within the matrix of the rock. A ton of shale contains from 10 to 100 gallons of kerogen, a waxy material that breaks down to oils when heated in the absence of air. It is estimated that three states (Utah, Colorado, and Wyoming) contain shale bearing more oil than exists in all the proven reserves in the world. Tar sands are the extremely viscous petroleum deposits associated with sedimentary rocks. They are mixtures of clay, sand, and extremely viscous oils called bitumens. The utility of oil shales and tar sands is currently limited, because of problems having to do with hydrocarbon recovery and the disposal of large amounts of inorganic residues.

see also Chemistry and Energy; Coal; Energy Sources and Production; Gasoline; Industrial Chemistry, Organic; Petroleum.

Mary L. Sohn

Bibliography

Spiro, Thomas G., and Stigliani, William M. (1996). Chemistry of the Environment. Upper Saddle River, NJ: Prentice-Hall.

Yeh, The Fu (1999). Environmental Chemistry: Essentials of Chemistry for Engineering Practice, Vol. 4A. Upper Saddle River, NJ: Prentice-Hall.

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Fossil Fuels

Fossil Fuels

Fossil fuels are buried deposits of plants and animals that have been converted to coal, petroleum, natural gas, or tar by exposure to heat and pressure in the Earth's crust over hundreds of millions of years. The energy in fossil fuels comes from sunlight, either directly or indirectly.

Coal comes primarily from the remains of plants that were buried in anaerobic conditions (without oxygen). Coal ranges from 55 percent to 90 percent carbon mixed with water and other substances including compounds of nitrogen and oxygen. Coal is graded according to hardness and carbon content. The lowest grade, lignite, is soft and brown in color. The hardest, anthracite, is nearly pure carbon. It is so hard it can be polished like a gemstone.

Petroleum (crude oil) is a liquid containing primarily hydrocarbon compounds along with small amounts of compounds containing oxygen, sulfur, and nitrogen. Crude oil originates with the buried remains of various types of plankton, primarily diatoms. It varies in consistency from a thin liquid the color of port wine to a thick, black, tarlike substance that must be heated before it will flow. Crude oil is the source of gasoline, jet fuel, diesel, heating oil, bunker oil, plastics, and other compounds.

Natural gas is usually found in varying amounts along with crude oil and with coal. It is also found by itself. Natural gas consists of a mixture of methane (CH4) and other hydrocarbons such as ethane (C2H6), propane (C3H8), and butane (C4H10). Methane is a natural byproduct of the anaerobic decomposition of organic remains.

Oil and Natural Gas

Petroleum ranges in quality from a relatively thin, free-flowing liquid called light or sweet crude to a thick, gooey black liquid with high sulfur content called heavy or sour crude. Because sweet crude is cleaner to burn and easier to transport, it is more valuable. Some oil flows naturally to the surface but most must be pumped. After primary recovery, hot water, steam, or high-pressure carbon dioxide can be injected into adjacent wells to force out some additional wells. Only about one-third of the oil can be extracted in primary and secondary recovery.

Oil, a useful fuel that can power automobiles, trucks, and airplanes, is a relatively inexpensive energy source. It has a high energy content and is easily transported. However, most experts expect little of the world's original oil reserves to remain by the middle of the twenty-first century. If world oil consumption increases at a rate of 2 percent per year, 80 percent of the world's supply will be used up by 2037.

When natural gas deposits are tapped, the gas is pressurized, which causes the propane and heavier hydrocarbons to liquefy. This liquefied petroleum gas (LPG) is stored in pressurized tanks and used in rural areas where natural gas supplies are not available. The remaining gas, mostly methane, is dried, cleaned of hydrogen sulfide, and distributed through pressurized pipelines.

Natural gas is a very clean-burning substance. If the gas is properly treated to remove sulfur and other contaminants, combustion products consist of water and carbon dioxide. Natural gas burns hotter and produces less pollution than any other fossil fuel.

The U.S. Department of Energy has estimated that all known and unknown reserves of natural gas will last until 2045 at present levels of consumption. If consumption rises by 2 percent per year, natural gas reserves will be depleted by 2022.

Coal

Coal is the world's most abundant fossil fuel. The United States, China, and Russia contain about two-thirds of known and estimated undiscovered coal reserves. Much of that coal contains large amounts of sulfur. When coal containing sulfur is burned, sulfur dioxide is created. Sulfur dioxide is one of the primary components of acid rain, so it is a pollutant. It is very difficult to remove the sulfur before the coal is burned, so the sulfur must be removed from the stack gases. Removing the sulfur is costly, although part of the cost can be recovered by selling the byproduct, sulfuric acid. World reserves of coal will last 220 years at current consumption rates and 65 years if consumption rates increase by 2 percent per year. Identified coal reserves in the United States will last about 300 years at current consumption rates.

Coal must be extracted by mining, the most environmentally destructive and expensive form of extraction. The least expensive form of mining is strip mining. The layer of rock and soil over the coal is removed by heavy machines, the coal is extracted by other heavy machines, and the rock and soil are replaced. Existing laws require that strip-mined land be returned to its original contour and replanted in suitable ground cover. When properly done, this restoration leaves the land in good condition.

Unfortunately, much of the land that was strip-mined before the laws were passed has not been restored. This results in erosion and pollution. Much of the coal that can be strip-mined in the United States is in the arid west, where restoration is more difficult and expensive. These requirements have made coal more expensive. Since natural gas is cheaper and burns cleaner, most new electric power plants being built are gas-fired.

Carbon Dioxide

All fossil fuels represent millions of years of stored solar energy. Plants remove carbon dioxide from the air and use the energy of sunlight to separate the carbon from the oxygen. The oxygen is released and the carbon is used to build carbohydrates, fats, and proteins. The carbon stored in fossil fuels is the result of this process. When fossil fuels are burned, this process is reversed. Oxygen from the air combines with carbon in the fossil fuels to form carbon dioxide, which is released into the atmosphere. Carbon dioxide levels in the atmosphere have been increasing steadily since the beginning of the industrial revolution. Most scientists now think that these increasing levels of carbon dioxide in the atmosphere are contributing to global warming through a process known as the greenhouse effect.

see also Global Warming.

Elliot Richmond

Bibliography

Miller G. Tyler, Jr. Living in the Environment. 6th ed. Belmont, CA: Wadsworth, 1990.

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Fossil Fuels

Fossil Fuels


Coal, petroleum, and natural gas are referred to as fossil fuels. Their common origin is as living matter, plants, and, in particular, microorganisms that have accumulated in large quantities under favorable conditions during the earth's long history. They have been preserved (fossilized) through burial under younger sediments, to great depths and over many millions of years. The "organic" elements hydrogen (H) and carbon (C) are the primary source of their heat content (hence the derivation of the word hydrocarbons ). Coal has a relatively high carbon content; petroleum and natural gas have much higher hydrogen contents. The burning of fossil fuels releases large quantities of the powerful greenhouse gas carbon dioxide (CO2) into the atmosphere, where it remains for a long time and contributes to global warming.

Fossil fuels have powered the industrialization of the world for several centuries. During the nineteenth and early twentieth centuries, coal was the primary source of energy. Then, after World War I, petroleum and later natural gas became increasingly important and together they contribute approximately 62 percent of the primary energy sources in the United States. Coal nevertheless still provides about 23 percent, mostly by conversion into electricity at large power plants.

see also Coal; Electric Power; Energy; Petroleum.

Internet Resource

energy information administration. "monthly energy review." available from http://www.eia.doe.gov/emeu/mer.

Heinz H. Damberger

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fossil fuel

fossil fuel Coal, oil, and natural gas, the fuels used by humans as a source of energy. They are formed from the remains of living organisms and all have a high carbon or hydrogen content. Their value as fuels relies on the exothermic oxidation of carbon to form carbon dioxide (C + O2 → CO2) and the oxidation of hydrogen to form water (H2 + @2O2 → H2O).

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fossil fuel

fossil fuel All deposits of organic material that are dug from the ground (from the Latin fossilis, meaning ‘dug up’) and are capable of being burnt for fuel; chiefly coal, oil, and gas. These are formed under pressure by alteration or decomposition of plant or animal remains.

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fossil fuels

fossil fuels Term to describe coal, oil, and natural gasfuels that were formed millions of years ago from the fossilized remains of plants or animals. By their very nature, fossil fuels are a non-renewable energy source.

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fossil fuel

fossil fuel All deposits of organic material capable of being burnt for fuel; chiefly coal, oil, and gas. These are formed under pressure by alteration or decomposition of plant or animal remains.

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fossil fuel

fos·sil fu·el • n. a natural fuel such as coal or gas, formed in the geological past from the remains of living organisms.

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fossil fuel

fossil fuel: see energy, sources of; fuel.

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