Coal is an abundant fossil resource that consists mostly of carbon. Energy content (Btu/pound) ranges from 5,000 to 15,000 depending on the type of coal. Coal reserves are located all over the world. Electric utilities consume about 87 percent of the total coal produced. In the United States, coal is used to generate more than half of all the electricity produced. It is also used as a basic energy source in many industries, and as a heating fuel. The U.S. is one of the top exporters of coal in the world. Most exported U.S. coal goes to Western Europe, Canada, and Japan.
Coal is recovered from the earth by surface mining or deep mining. Surface mining, or strip mining, is less expensive and usually occurs on flat land. Deep mining requires digging shafts and tunnels to get to the coal seam. Automation of deep mining has helped to counter its safety and health hazards. Coal can be gasified to form a synthetic fuel similar to natural gas. It can also be liquefied to make a synthetic crude oil. To date, it has not been economical to make synthetic fuels from coal on a large scale. As processes become more efficient, the use of synthetic fuels may become more economical.
Oil comes from crude oil, which is a mix of hydrocarbons with some oxygen, nitrogen, and sulfur impurities. One barrel of oil (42 U.S. gallons) can provide about 6 million Btu. Crude oil reserves are found all over the world, but the Middle East alone has about 63 percent of the known reserves. Of the oil consumed in the United States, most is used in transportation, and much of the rest goes to industrial, commercial, and residential uses. Crude oil is used to produce not only a range of fuels, but also petrochemical ingredients for plastics, inks, tires, pharmaceuticals, and a host of other products.
High-tech oil exploration technology and practices have led to the discovery of as many new reserves as have already been used. To make the most of this valuable resource, energy producers are developing more efficient refining methods, product makers are finding more efficient ways to use petrochemicals, and manufacturers are developing more efficient cars. New techniques of locating and extracting oil from the earth are also making it possible to recover oil that was once too expensive to produce.
Oil is usually recovered by drilling wells through the non-porous rock barrier that traps the oil. In general, about 30 percent of the oil trapped can be economically recovered by pumping. "Secondary" recovery can remove another 10 percent, by flooding the well with high-pressure water or gas. Another 10 percent can sometimes be recovered with "tertiary" methods that heat the oil to scrub it out. About half of the oil is left trapped in the rock. Oil producers are continually seeking economical ways to recover more of this oil.
The oil refining process separates crude oil into different hydrocarbons and removes impurities such as sulfur, nitrogen, and heavy metals. The first step is fractional distillation, a process that takes advantage of the fact that different hydrocarbons boil at different temperatures. In a tall tower called a fractionating column, crude oil is heated until it boils. Horizontal trays divide the column at intervals. As the oil boils, it vaporizes. Each hydrocarbon rises to a tray at a temperature just below its own boiling point. There, it cools and turns back into a liquid.
The lightest fractions are liquefied petroleum gases (propane and butane) and the petrochemicals used to make plastics, fabrics, and a wide array of consumer products. Next come gasoline, kerosene, and diesel fuel. Heavier fractions make home heating oil and fuel for ships and factories. Still heavier fractions are made into lubricants and waxes. The remains include asphalt.
The refining process then continues, with heavy fractions converted into lighter fractions. In most cases, "cracking" processes are used to transform large (heavy) hydrocarbon molecules and make the smaller, lighter molecules such as gasoline and jet fuel. Better refining technologies have made it possible to produce over 21 gallons of gasoline from a 42-gallon barrel of crude oil—a remarkable advance over the industry’s early days, when a barrel of oil yielded just 11 gallons of gasoline.
Oil shale was never buried deeply enough or heated enough to form crude oil. Its hydrogen content is between that of coal and crude oil. Concentrations of oil are low, so that, at most, one barrel of oil can be recovered from 2.4 tons of sand or 1.5 tons of rock. Huge amounts of oil shale are found all over the world. In fact, the total global resource is 1,000 times greater than crude oil reserves. But extracting the energy value of oil shale is not practical today. Scientists and engineers continue working on ways to recover oil shale for a reasonable cost.
Natural gas is the gas component of coal and oil formation. It is used in industrial and commercial heating and cooking, and, increasingly, to fuel electricity generation. In a compressed form, natural gas can also be used as a transportation fuel. Natural gas is either found mixed in oil or is released from coal. Energy in 6,000 cubic feet of natural gas is equivalent to one barrel of oil. World reserves of natural gas are greatest in Russian, Iran, Qatar, Saudi Arabia, United Arab Emirates, and the U.S. The U.S. consumed 19.7 million cubic feet of natural gas in 1999, nearly all of which came from domestic production. Five states—Texas, Louisiana, Alaska, New Mexico, and Oklahoma—hold more than 85 percent of U.S. natural gas reserves.
Wells for natural gas are drilled in underground reservoirs of porous rock. When it is removed from a reservoir, natural gas can either be pumped to the processing station for removal of liquid hydrocarbons, sulfur, carbon dioxide, and other components, or stored in large caverns underground until it is needed. Pipelines are the main method of transporting natural gas. Natural gas can also be liquefied and shipped overseas, but this process is complex and expensive.
Electrical generation by natural gas has been improved by the development of combined-cycle systems. These systems put together a natural-gas-fueled combustion turbine with a heat-recovery steam generator and steam turbine, to produce electricity in two ways rather than just one. The result: roughly 60 percent of the heat from the natural gas is harnessed to make electricity, creating a more energy-efficient system.