Natural Gas

I

Introduction

Natural Gas, flammable gaseous mixture consisting mostly of hydrocarbons (chemical compounds that contain carbon and hydrogen). Along with coal and petroleum, natural gas is a fossil fuel. Natural gas may contain as much as 85 percent methane (CH₄) and about 10 percent ethane (C₂H₆), and also contains smaller amounts of propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂), and other alkanes. Natural gas, which is usually found together with petroleum deposits in Earth’s crust, is extracted and refined into fuels that provide approximately 25 percent of the world energy supply.

Natural gas contains small amounts of impurities, including carbon dioxide (CO₂), hydrogen sulfide (H₂S), and nitrogen (N₂). Because these impurities can detract from the heating value and properties of natural gas, they are often removed during the refining process and used as commercial by-products.

II

Uses

Natural gas is used both as a fuel and as a raw material in the manufacture of chemicals. As a residential fuel, it is burned in furnaces, water heaters, cooking stoves, and clothes dryers. As an industrial fuel, it is burned in kilns (special furnaces) used to bake bricks and ceramic tiles and to produce cement. Natural gas is also used for generating steam in water boilers and as a source of heat in glass making and food processing.

Natural gas serves as a raw material for creating petrochemicals, which are chemicals that are specifically derived from natural gas or petroleum. In turn, petrochemicals are used as a base product for making fertilizers, detergents, pharmaceuticals, plastics, and numerous other goods.

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III

History

Humans have used natural gas for centuries. Historical records show that natural gas was burned as fuel in China as early as ad 250. In the 17th century, natural gas was used for heating and lighting in northern Italy. In the United States, it was first discovered in Fredonia, New York, in 1821.

As its name implies, natural gas emerges from the ground in gas form, which is difficult to transport and store. As a result, natural gas has historically been used in areas close to the gas reservoirs. As the petroleum industry grew in the 19th and 20th centuries, natural gas that was recovered with petroleum from underground reservoirs was typically treated as a waste by-product and was often burned off at the well. Today, natural gas is transported through extensive networks of pipelines or is liquefied and transported by ship.

IV

Formation

Natural gas is formed from plankton, water-dwelling microorganisms that include algae and protozoans. As these microorganisms died and accumulated on the ocean floors, they were slowly buried and the remains were compressed under layers of sediment. Over millions of years, the pressure and heat exerted by the overlying sediments chemically transformed this organic material into natural gas.

Because petroleum and natural gas are formed by similar natural processes, these two hydrocarbons are often found together in underground reservoirs. After gradually forming in Earth’s crust, petroleum and natural gas slowly flow into the tiny holes of nearby porous rocks that serve as reservoirs. Because this porous reservoir rock is often filled with water, the petroleum and natural gas, both of which are lighter than water and less dense than the surrounding rock, migrate upward through the crust, sometimes for long distances. Eventually, some of these upward-moving hydrocarbons become trapped by an impermeable (nonporous) layer of rock, known as the cap rock. Natural gas is lighter than petroleum, so it forms a layer over the petroleum. This layer is called a gas cap.

Coal beds contain appreciable quantities of methane, the principal component of natural gas. In coal deposits, methane is often dispersed throughout the pores and fractures of the coal bed. This type of natural gas is often referred to as coal-bed methane.

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