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Fossil Fuels


Fossil fuels or mineral fuels are fossil source fuels, that is, hydrocarbons found within the top layer of the earth’s crust. 

It is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over hundreds of millions of years. This is known as the biogenic theory and was first introduced by Mikhail Lomonosov in 1757.


Mikhail Lomonosov Russian writer, chemist, and astronomer, who made important contributions to both literature and science 

Mikhail Lomonosov

Russian writer, chemist, and astronomer, who made important contributions to both literature and science 

Fossil fuels are non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being formed.

When coal, natural gas or oil are burned, they release gases into the atmosphere:

  • Carbon dioxide (CO2) is a "greenhouse gas," trapping heat in the lowest part of the earth's atmosphere. This contributes to "global warming" - the average temperature of the earth slowly increases, affecting ecosystems across the globe.

  • Sulfur dioxide (SO2) is a key contributor to acid rain, primarily in the northeast U.S.

  • Nitrogen oxide (NOx) contributes to acid rain and smog, as well as health issues such as lung inflammation, immune system changes and eye irritation.


There are three major forms of fossil fuels: 

    • coal

      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 

      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

      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.


 All three were formed many hundreds of millions of years ago before the time of the dinosaurs - hence the name fossil fuels. The age they were formed is called the Carboniferous Period. It was part of the Paleozoic Era. "Carboniferous" gets its name from carbon, the basic element in coal and other fossil fuels. The Carboniferous Period occurred from about 360 to 286 million years ago. At the time, the land was covered with swamps filled with huge trees, ferns and other large leafy plants. The water and seas were filled with algae - the green stuff that forms on a stagnant pool of water. Algae is actually millions of very small plants. 


Petroleum and Natural Gas Formation


All fossil fuels, whether solid, liquid, or gas, are the result of organic material being covered by successive layers of sediment over the course of millions of years. Some deposits of coal can be found during the time of the dinosaurs. For example, thin carbon layers can be found during the late Cretaceous Period (65 million years ago) - the time of Tyrannosaurus Rex. But the main deposits of fossil fuels are from the Carboniferous Period.  Fossil fuels supply over 80% of the world’s energy needs.


Coal Power Plant

Coal is derived from the accumulation of partially decayed land plants. As the sediment solidifies into rock, the organic material decomposes under the influence of great pressure and high temperature. 

credit: Energy Information Administration, U.S. Department of Energy, World Coal institute, OPEC, Shell Oil company