6N – Formation of fossil fuels

Supporting article N: An article on the formation of Fossil Fuels

http://library.thinkquest.org/3646/earth/resources/fosfuels.html

The formation of Fossil Fuels

Fossil fuels are made out of dead animals and plants that have been compressed in the earth for many years. Coal, petroleum, and natural gas are three types of fossil fuels, which we use to power our electric plants. However, fossil fuels are not a replensible source of energy. We will completely use up our fossil fuel supplies within the next few centuries.

Coal
Coal is made from dead animals and leaves. It takes millions of years to form. Coal is formed when the dead plants and animals have been put under a lot of pressure. Because it takes so long to form, the plants and animals were probably from the prehistoric times. If we run out of coal, it would take millions of years to form.
Coal is a solid fuel of plant origin. In remote geological times, and particularly in the Carboniferous period, between 345 and 280 million years ago, much of the world was covered with luxuriant vegetation growing in swamps. Many of these plants were types of ferns, some as large as trees. This vegetation died and subsided into water, where it gradually decomposed. As decomposition took place, the vegetable matter lost oxygen and hydrogen atoms, leaving a deposit with a high percentage of carbon. This formed peat bogs. As time passed, layers of sand and mud settled from the water over some of the peat deposits. The pressure of these overlying layers, as well as movements of the earth’s crust and sometimes volcanic heat, acted to compress and harden the deposits, thus producing coal.

Various types of coal are classified according to fixed carbon content, which is greatest for types of coal that have undergone the greatest changes from the original vegetable matter. Peat, the first stage in the formation of coal, has a low fixed carbon content and a high moisture content. The carbon content is greater in lignite , the lowest rank of coal. Bituminous coal has even more carbon and a correspondingly higher heating value. Anthracite coal has the highest carbon content and heating value. Coal may be transformed by further pressure and heat into graphite that is almost pure carbon.

The sulfur and nitrogen in coal form oxides during combustion, a process that can contribute to acid rain formation. According to the federal National Acid Precipitation Assessment Program, acid rain is the result of a series of complex reactions involving chemicals and compounds from many industrial, transportation, and natural sources. Sulfur dioxide (SO2) emissions from new coal-fired facilities have been controlled since the 1970s by the Clean Air Act. As a result, SO2 emissions have dropped even though coal use has increased. All ranks of coal have some value or use. For centuries peat has been used as a fuel for open fires, and more recently peat and lignite have been made into briquettes for burning in furnaces. Electric utilities use about 86 percent of all the bituminous coal consumed in the U.S.

General industry is the second largest consumer. Steel producers use metallurgical coal, or coke, a distilled fuel that is almost pure carbon. The process of producing coke yields a number of chemical by-products, including coal tar (q.v.) , which are used in the manufacture of many other products. Coal was also used, from the early 19th century to the World War II era, for the production of fuel gas ( see Gases, Fuel ), just as coal liquefaction techniques were used to produce liquid oil products. Manufacture of fuel gas and other products from coal diminished as natural gas became widely available. In the 1980s, however, the U.S. and other industrialized nations again became interested in gasification and new clean coal technology (CCT). Coal liquefaction supplies all of the Republic of South Africa’s oil needs.

Petroleum
Petroleum is formed under the earth’s surface by the decomposition of marine organisms. The remains of tiny organisms that live in the sea-and, to a lesser extent, those of land organisms that are carried down to the sea in rivers and of plants that grow on the ocean bottoms-are enmeshed with the fine sands and silts that settle to the bottom in quiet sea basins. Such deposits, which are rich in organic materials, become the source rocks for the generation of crude oil. The process began many millions of years ago with the development of abundant life, and it continues to this day. The sediments grow thicker and sink into the seafloor under their own weight. As additional deposits pile up, the pressure on the ones below increases several thousand times, and the temperature rises by several hundred degrees. The mud and sand harden into shale and sandstone; carbonate precipitates and skeletal shells harden into limestone; and the remains of the dead organisms are transformed into crude oil and natural gas.

Once the petroleum forms, it flows upward in the earth’s crust because it has a lower density than the water that is saturated with salt. The crude oil and natural gas rise into the microscopic pores of the coarser sediments lying above. Frequently, the rising material encounters a dense layer of rock that prevents further migration; the oil has become trapped, and a reservoir of petroleum is formed. A significant amount of the upward-migrating oil, however, does not encounter impermeable rock but instead flows out at the surface of the earth or onto the ocean floor.

As gas and oil are produced from a petroleum field, the pressure in the reservoir that forces the material to the surface gradually declines. Eventually, the pressure will decline so much that the remaining oil or gas will not migrate through the porous rock to the well. When this point is reached, most of the gas in a gas field will have been produced, but less than one-third of the oil in an oil field will have been extracted. Part of the remaining oil can be recovered by using gas or water to push the oil to the well, but even then, one-fourth to one-half the oil is usually left in the reservoir. In an effort to extract this remaining oil, oil companies are now beginning to use chemicals to push the oil to the well, or to use fire or steam in the reservoir to make the oil flow easier.

Crude oil is transported to refineries by pipelines, barges, or giant oceangoing tankers. Refineries contain a series of processing units that separate the different constituents of the crude oil by heating them to different temperatures, chemically modifying them, and then blending them to make final products, principally gasoline, kerosene, diesel oil, jet fuel, home heating oil, heavy fuel oil, lubricants, and “feedstocks” fed to petrochemical plants. Natural gas is transported, usually by pipelines, to customers who burn it for fuel or, in some cases, make petrochemicals from it. Natural gas can be liquefied at very low temperatures and transported in special ships. This method is much more costly than transporting oil by tanker. Oil and natural gas compete in a number of markets, especially in generating heat for homes, offices, factories, and industrial processes.

Because of the great need for oil in modern times, and the long time for oil to be made, oil is being used up too quickly. What all of us can do is go to a recycling center and have them recycle the used oil..

Natural Gas
Natural gas contains valuable organic elements that are important raw materials of the natural-gasoline and chemical industries. Before natural gas is used as fuel, heavy hydrocarbons such as butane, propane, and natural gasoline are extracted as liquids. The remaining gas constitutes so-called dry gas, which is piped to domestic and industrial consumers for use as fuels; dry gas, devoid of butane and propane, also occurs in nature. Composed of the lighter hydrocarbons methane and ethane, dry gas is used also in the manufacture of plastics, drugs, and dyes.

Methane, or marsh gas, is the main component (about 85%) of natural gas. Widely distributed in nature, it is a by-product of the natural fermentation, or decay, of vegetable matter. It is also a major constituent of the atmospheres of the outer, gaseous planets. The gas is nonpoisonous but inflammable, burning with a pale blue flame. In addition to its familiar use for cooking and heating, methane is used to manufacture many organic chemicals, including acetylene, ammonia, formaldehyde, and methyl alcohol. Atmospheric methane has been implicated as a contributor to the greenhouse effect of global warming.

Propane is a colorless, odorless gas of the alkane series of hydrocarbons. It occurs in crude oil, in natural gas, and as a by-product of petroleum refining. Propane does not react strongly at room temperature. It does react, however, with chlorine at room temperature if the mixture is exposed to light. At higher temperatures, propane burns in air, producing carbon dioxide and water as final products, and is valuable as a fuel.

About half the propane produced annually in the U.S. is used as a domestic and industrial fuel. When it is used as a fuel, propane is not separated from the related compounds, butane, ethane, and propylene. Propane forms a solid hydrate, or a compound formed by the union of water with some other substance at low temperatures, and this causes great inconvenience when a blockage occurs in a natural-gas line. Propane is used also as so-called bottled gas, as a motor fuel, as a refrigerant, as a low temperature solvent, and as a source of propylene and ethylene.