Natural Gas Formation

Most natural gas is formed from plankton—tiny water-dwelling organisms, including algae and protozoans—that accumulated on the ocean floor as they died. These organisms were slowly buried and compressed under layers of sediment. Over millions of years, the pressure and heat generated by overlying sediments converted this organic material into natural gas. Natural gas is composed primarily of methane and other light hydrocarbons. As discussed previously, natural gas frequently migrates through porous and fractured reservoir rock with petroleum and subsequently accumulates in underground reservoirs. Because of its light density relative to petroleum, natural gas forms a layer over the petroleum. Natural gas may also form in coal deposits, where it is often found dispersed throughout the pores and fractures of the coal bed.

Other Fossil Fuels

Geologists have identified immense deposits of other hydrocarbons, including gas hydrates (methane and water), tar sands, and oil shale. Vast deposits of gas hydrates are contained in ocean sediments and in shallow polar soils. In these marine and polar environments, methane molecules are encased in a crystalline structure with water molecules. This crystalline solid is known as gas hydrate. Because technology for the commercial extraction of gas hydrates has not yet been developed, this type of fossil fuel is not included in most world energy resource estimates.

Tar sands are heavy, asphaltlike hydrocarbons found in sandstone. Tar sands form where petroleum migrates upward into deposits of sand or consolidated sandstone. When the petroleum is exposed to water and bacteria present in the sandstone, the hydrocarbons often degrade over time into heavier, asphaltlike bitumen. Oil shale is a fine-grained rock containing high concentrations of a waxy organic material known as kerogen. Oil shale forms on lake and ocean bottoms where dead algae, spores, and other microorganisms died millions of years ago and accumulated in mud and silt. The increasing pressure and temperature from the buildup of overlying sediments transformed the organic material into kerogen and compacted the mud and silt into oil shale. However, this pressure and heat was insufficient to chemically break down the kerogen into petroleum. Because the hydrocarbons contained in tar sand and oil shale are not fluids, these hydrocarbons are more difficult and costly to recover than liquid petroleum.

Removing and Refining Fossil Fuels

Geologists use a variety of sophisticated instruments to locate underground petroleum, natural gas, and coal deposits. These instruments allow scientists to interpret the geologic composition, history, and structure of sedimentary basins in the earth’s crust. Once located, petroleum and natural gas deposits are removed by wells drilled down into the deposit, while coal is removed by excavation.

Petroleum and Natural Gas
To locate deposits of petroleum and natural gas, exploration geologists search for geologic regions containing the ingredients necessary for petroleum formation: organic-rich source rock, burial temperatures sufficiently high to generate petroleum from organic material, and petroleum-trapping rock formations.

When potentially petroleum-rich geologic formations are identified, wells are drilled into the sedimentary basin. If a well intersects porous reservoir rock containing significant petroleum and natural gas deposits, pressure inside the trap may force the liquid hydrocarbons spontaneously to the surface. However, pressure inside the trap typically declines to the point where the petroleum must be pumped to the surface.

Once petroleum has been extracted from the ground, it is transported by pipeline, truck, or tanker to a refinery to be separated into liquid and gas components. Raw petroleum is heated to distill hydrocarbons by molecular weight. Lighter molecules are separated and refined into gasoline and other fuels, while heavier molecules are processed into engine lubricants, asphalt, waxes, and other products. Because demand for fuel far exceeds demand for the products made from the heavier hydrocarbons, refiners often break apart the heavy molecules into lighter ones that can be refined into gasoline. They do so by means of processes called thermal cracking and catalytic cracking.

Coal
Because of their enormity, the world’s most extensive coal beds have already been identified. Modern underground mining commonly employs machines called longwall miners to remove coal. These machines use rotating drums studded with picks to rip coal from seams in large chunks.

Surface-mine operators use mammoth earth-moving shovels to mine coal. These shovels first remove overlying soil and rock so the coal beds can be blasted apart. The blasted coal is scooped up and loaded into the beds of huge trucks for transport.

Fossil Fuels: Commercial Uses

Once fossil fuel has been extracted and processed, it can be burned for direct uses, such as to power cars or heat homes, or it can be combusted for the generation of electrical power.

Direct Combustion
Fossil fuels are primarily burned to produce energy. This energy is used to power automobiles, trucks, airplanes, trains, and ships around the world; to fuel industrial manufacturing processes; and to provide heat, light, air conditioning, and energy for homes and businesses.

To provide fuel for transportation, petroleum is refined into gasoline, diesel fuel, jet fuel, and other derivatives used in most of the world’s automobiles, trucks, trains, aircraft, and ships.

Demand for natural gas, historically considered a waste by-product of petroleum and coal mining, is growing in business and industry because it is a cleaner-burning fuel than petroleum or coal. Natural gas, which can be piped directly to commercial plants or individual residences and used on demand, is used for heating and for air conditioning. Residential uses of natural gas also include fuel for stoves and other heating appliances.

Electricity Generation
In addition to direct combustion for commercial uses, fossil fuels are also burned to generate most of the world’s electric power. In 2001 fossil fuel fired power plants produced 64 percent of the world’s electrical power, down from 71 percent in the late 1970s. In 2001 the world’s remaining electricity supply was generated primarily by hydroelectric power (17 percent) and nuclear fission (17 percent), with solar, geothermal, and other sources accounting for a relatively small amount.

Interstellar Matter

Interstellar Matter, gas and dust between the stars in a galaxy. In our own galaxy, the Milky Way, we can see glowing gas and dark, obscuring dust between the galaxy’s many visible stars. This gas and dust makes up interstellar matter. Galaxies differ in the density of interstellar matter that they contain. Spiral galaxies, such as the Milky Way, have much more interstellar matter than elliptical galaxies, which have almost none. About 3 percent of the mass of the Milky Way Galaxy is interstellar gas, and 1 percent is interstellar dust. Stars make up the rest of the ordinary matter in the galaxy. Dark matter—a material that does not reflect or emit light or other forms of electromagnetic radiation—also makes up some of the mass of the galaxy. Astronomers consider interstellar matter separately from intergalactic matter, or matter between galaxies.

Hydrogen gas makes up most of the interstellar matter, but essentially all of the chemical elements occur in interstellar matter. About 90 percent of the atoms in space are hydrogen, about 9 percent helium, and less than 1 percent consists of all the other chemical elements. The interstellar matter is so spread out that the space it occupies would be considered a vacuum in laboratories on Earth.

Ultraviolet Radiation

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Ultraviolet Radiation, electromagnetic radiation that has wavelengths in the range between 4000 angstrom units (Å), the wavelength of violet light, and 150 Å, the length of X rays. Natural ultraviolet radiation is produced principally by the sun. Ultraviolet radiation is produced artificially by electric-arc lamps.

Ultraviolet radiation is often divided into three categories based on wavelength, UV-A, UV-B, and UV-C. In general shorter wavelengths of ultraviolet radiation are more dangerous to living organisms. UV-A has a wavelength from 4000 Å to about 3150 Å. UV-B occurs at wavelengths from about 3150 Å to about 2800 Å and causes sunburn; prolonged exposure to UV-B over many years can cause skin cancer. UV-C has wavelengths of about 2800 Å to 150 Å and is used to sterilize surfaces because it kills bacteria and viruses.

The earth's atmosphere protects living organisms from the sun's ultraviolet radiation. If all the ultraviolet radiation produced by the sun were allowed to reach the surface of the earth, most life on earth would probably be destroyed. Fortunately, the ozone layer of the atmosphere absorbs almost all of the short-wavelength ultraviolet radiation, and much of the long-wavelength ultraviolet radiation. However, ultraviolet radiation is not entirely harmful; a large portion of the vitamin D that humans and animals need for good health is produced when the human's or animal's skin is irradiated by ultraviolet rays.

When exposed to ultraviolet light, many substances behave differently than when exposed to visible light. For example, when exposed to ultraviolet radiation, certain minerals, dyes, vitamins, natural oils, and other products become fluorescent—that is, they appear to glow. Molecules in the substances absorb the invisible ultraviolet light, become energetic, then shed their excess energy by emitting visible light. As another example, ordinary window glass, transparent to visible light, is opaque to a large portion of ultraviolet rays, particularly ultraviolet rays with short wavelengths. Special-formula glass is transparent to the longer ultraviolet wavelengths, and quartz is transparent to the entire naturally occurring range.

In astronomy, ultraviolet-radiation detectors have been used since the early 1960s on artificial satellites, providing data on stellar objects that cannot be obtained from the earth's surface. An example of such a satellite is the International Ultraviolet Explorer, launched in 1978.

Air

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Air, mixture of gases that composes the atmosphere surrounding Earth. These gases consist primarily of the elements nitrogen, oxygen, argon, and smaller amounts of hydrogen, carbon dioxide, water vapor, helium, neon, krypton, xenon, and others. The most important attribute of air is its life-sustaining property. Human and animal life would not be possible without oxygen in the atmosphere. In addition to providing life-sustaining properties, the various atmospheric gases can be isolated from air and used in industrial and scientific applications, ranging from steelmaking to the manufacture of semiconductors. This article discusses how atmospheric gases are isolated and used for industrial and scientific purposes.

GASES IN THE ATMOSPHERE

The atmosphere begins at sea level, and its first layer, the troposphere, extends from 8 to 16 km (5 and 10 mi) from Earth’s surface. The air in the troposphere consists of the following proportions of gases: 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, 0.03 percent carbon dioxide, and the remaining 0.07 percent is a mixture of hydrogen, water, ozone, neon, helium, krypton, xenon, and other trace components. Companies that isolate gases from air use air from the troposphere, so they produce gases in these same proportions.


PURIFYING AIR

Most larger air-separation plants continue to use cryogenic distillation to separate air gases. Before pure gases can be isolated from air, unwanted components such as water vapor, dust, and carbon dioxide must be removed. First, the air is filtered to remove dust and other particles. Next, the air is compressed as the first step in liquefying the air. However, as the air is compressed, the molecules begin striking each other more frequently, raising the air’s temperature. To offset the higher temperatures, water heat exchangers cool the air both during and after compression. As the air cools, most of its water vapor content condenses into liquid and is removed.

After being compressed, the air passes through beds of adsorption beads that remove carbon dioxide, the remaining water vapor, and molecules of heavy hydrocarbons, such as acetylene, butane, and propylene. These compounds all freeze at a higher temperature than do the other air gases. They must be removed before the air is liquefied or they will freeze in the column where distillation occurs.

COMPRESSED AIR

Not all industrial uses of air require it to be separated into its component gases. Compressed air—plain air that has been pressurized by squeezing it into a smaller-than-normal volume—is used in many industrial applications. When air is compressed, the gas molecules collide with each other more frequently and with more force, producing higher kinetic energy. The kinetic energy in compressed air can be converted into mechanical energy or it can be used to produce a powerful air flow or an air cushion. Compressed air is easily transmitted through pipes and hoses with little loss of energy, so it can be utilized at a considerable distance from the compressor or pressure tank.