Introduction to Gas (natural)
Gas, a fuel. Natural gas is taken from underground deposits where it has been trapped for centuries. Manufactured gases are produced by any of several methods discussed later in this article.
Gas provides the world, and especially the United States, with a large part of its total fuel energy. In many homes it is burned for heating and cooking. It can also be used for incinerating waste, for powering motor vehicles, and for operating air conditioners, clothes driers, water heaters, and refrigerators. In industry, gas is burned to provide heat for hundreds of different processes. Gas was formerly widely used to light streets and buildings.
Gas has many advantages as a fuel. It can easily be made to flow from one place to another, and it is easily stored by the utility companies that supply it. Most consumers do not need to store gas, but may rely on a steady, uninterrupted flow of the fuel from the utility company. When ignited, gas burns evenly and completely.
Gas may be an explosion hazard if allowed to concentrate in unventilated rooms. Natural gas is not toxic (poisonous), but can cause death by suffocation if allowed to replace the air in a room. Improvements in the design of gas equipment have greatly reduced the chances of gas leaks. Modern gas-burning equipment has many safety features, including automatic cutoff devices that stop the flow of fuel. Gas properly handled, in equipment installed and checked by a utility company, is safe.
Natural Gas
Natural gas is found in reservoirs deep in the earth. Natural gas, like coal and petroleum, is believed to have been formed over a period of millions of years from decayed and compressed organic matter. Natural gas is composed chiefly of a mixture of gaseous hydrocarbons (compounds of hydrogen and carbon). The exact makeup of natural gas varies from region to region. The major ingredient in most natural gas is a hydrocarbon called methane. Other substances found in natural gas include hydrogen sulfide, nitrogen, carbon dioxide, water, and helium. Natural gas containing hydrogen sulfide generally has a strong, disagreeable odor and is known as sour gas.
Before natural gas is supplied to consumers, any hydrogen sulfide and carbon dioxide it contains is removed. The resulting gas is odorless. To enable the consumer to detect gas leaks, strong-smelling substances, called thiols, or mercaptans, are added to the gas.
OccurrenceDeposits of natural gas occur in many parts of the world. A certain amount of natural gas is found in all areas where petroleum occurs. Some major natural gas deposits are located under the ocean floor.
The world's leading producers of natural gas are Russia and the United States; together they account for more than half of the total production. Other major producers include Canada, the Netherlands, Great Britain, Turkmenistan, Algeria, and Indonesia. Almost two-thirds of the United States' production comes from Texas, Louisiana, and Oklahoma.
Natural gas may occur by itself; at the top of a petroleum deposit; or dissolved in petroleum. Gas-bearing beds are usually porous rock, such as limestone or coarse-grained sandstone. Nonporous rock, such as shale, forms a covering that keeps the gas in and air out. Various underground rock formations that can trap natural gas are shown in the article PETROLEUM, illustration titled Oil Traps.
ProductionGas wells are sunk to depths of 300 to 20,000 feet (90 to 6,100 m) or more. The diameter of a gas well ranges from two to eight inches (5 to 20 cm).
With a new well, the gas is typically under so much pressure that it flows freely from the well. When the pressure is low, the gas is removed by pumping it out or by forcing water down into the deposit to increase the pressure.
Most of the natural gas obtained from petroleum deposits is wet gas—natural gas that contains a relatively large proportion of heavy hydrocarbons that condense into liquid form under ordinary conditions of pressure and temperature. These hydrocarbons are usually removed from the natural gas near the well site. Most natural gas that occurs in deposits by itself is dry gas—gas relatively free of these hydrocarbons.
TransportationGas is brought from the well to the consumer through a system of welded steel pipes. In the United States, pipelines more than 1,000 miles (1,600 km) long connect major gas fields to some distant cities. Pipelines are also used to send natural gas from one country to another and to carry gas from offshore wells to land.
Gas pressure in the pipelines is maintained by compressor stations spaced 50 to 100 miles (80 to 160 km) apart. Gas is distributed locally through underground pipes called mains. Smaller pipes called service pipes lead from the mains to individual buildings. The amount of gas consumed at any one place is measured by a gas meter.
For shipment by sea, natural gas is turned into a liquid, which greatly reduces its bulk. Liquefied natural gas (LNG) can present a serious explosive hazard, but in the early 1960's methods were developed for producing and handling LNG safely. LNG is produced by cooling natural gas to appromately –260 F. (–162 C.). To maintain it at this temperature during shipment on a tanker, the LNG is carried in insulated compartments. Although a small portion of the LNG evaporates en route, it usually is not wasted; most LNG tankers are designed to burn this gas as fuel.
StorageThe demand for gas fuel depends a great deal upon the weather and is only partly predictable. In many areas of the United States, a portion of the gas delivered during the summer, when demand is low, is diverted to storage. The stored gas is used in the winter to supplement pipeline gas during periods of peak demand. The gas is usually stored in underground reservoirs of porous rock, such as abandoned gas or oil fields. Some natural gas is stored as LNG in large insulated tanks built below ground level.
Manufactured Gases
Manufactured gas fuels were widely used in the 19th and early 20th centuries. Today, however, manufactured gases are mainly used to supplement natural gas when demand for gas fuel is unusually great. Gas fuels that are by-products of an industrial process are often used as fuel for the industrial plant where they are produced.
There are many kinds of manufactured gases. They vary in composition and thus in heating value—that is, the amount of heat that is obtained by burning a given amount of gas. Manufactured gases include the following:
Blast-furnace Gasa by-product in the manufacture of pig iron. Blast-furnace gas consists mainly of carbon monoxide, carbon dioxide, and nitrogen. It has a very low heating value.
Coal Gas, or Coke-oven Gasobtained by heating coal in the absence of air. In the process, the coal is changed into coke. Coal gas is a mixture of hydrogen, carbon monoxide, methane, and small quantities of other gases. It has a medium heating value.
Liquefied Petroleum (LP) Gasobtained during petroleum refining and from natural gas. It consists primarily of butane and propane and has a very high heating value.
Oil Gasmade by heating gas oil, a petroleum derivative, to a high temperature or by treating the oil with steam. Oil gas is mainly hydrogen and methane and has a medium heating value.
Producer Gasmade by forcing air or a mixture of air and steam through a bed of very hot coke or other solid fuel. It consists mainly of hydrogen, carbon dioxide, and nitrogen and has a low heating value.
Refinery Gasobtained as a by-product in the processing of petroleum in a refinery. It is a mixture of ethane, propane, butane, hydrogen, methane, and small quantities of other gases. It has a high heating value.
Substitute, or Synthetic, Natural Gas (SNG)any gas made to be similar to natural gas. It thus is composed largely of methane and has a high heating value. SNG can be made from petroleum derivatives, coal, peat, or other fuels. Most processes for making SNG involve producing methane from hydrogen and carbon monoxide.
Water Gas, or Blue Gasproduced by passing steam over very hot coal or coke. It is composed chiefly of hydrogen and carbon dioxide in equal portions and has a low heating value. Water gas whose heating value has been increased with oil gas is called carburetted water gas.
History
Ancient Greek writers mentioned gas—natural gas—that seeped from rocks and could be burned. The Chinese are reported to have burned natural gas at least 2,000 years ago to evaporate seawater in shallow pans to produce salt.
The modern use of gas fuels began in the late 18th century, primarily with the development of gas lighting using coal gas. William Murdock, a Scottish engineer, contributed to the development with experiments in the production of coal gas and in its use as an illuminating gas. Around 1810, coal gas began to be used for lighting streets in London.
In 1816, Baltimore became the first city in the United States to install gas street lights. The gas was manufactured in a central plant and distributed by small pipes. Gas lighting for streets, public buildings, and factories came into wide use during the next few decades, and gas lighting in homes came into general use after the Civil War.
In 1885, Carl Auer von Welsbach, an Austrian physicist, invented the gas mantle. The mantle, consisting of a fabric hood coated with materials that glow a brilliant white when heated in a flame, greatly increased the illumination that gas lights could provide. However, electric lighting—chiefly electric-arc lights and incandescent lightbulbs—came to supplant gas lighting during the early 20th century. Faced with the loss of its lighting market, the gas industry turned toward promoting the use of manufactured gases for cooking and heating. The demand for manufactured gases grew as gas stoves and gas-fired furnaces were installed in many homes.
In the United States, some natural gas was being produced by the mid-19th century. Although natural gas was superior to manufactured gases as a source of heat, its use was largely limited through the 19th century to areas near oil and gas fields. In the early 20th century, the introduction of seamless pipes and welded pipeline joints—innovations that made pipes strong enough to withstand high gas pressure—made it possible to transport natural gas over greater distances. By the late 1930's, natural gas had become the major gas fuel in the United States.
The building of an extensive system of natural gas pipelines was begun following World War II. Natural gas became available in virtually every part of the country, and consumption increased steadily through the 1960's. In the mid-1970's there was a shortage of natural gas and uncertainty regarding future supplies. This situation sparked interest in developing large-scale projects for producing substitute natural gas—manufactured gases with a high heating value—from coal or oil shale. However, by the early 1980's, supplies of natural gas were again plentiful following a reduction in demand and an increase in exploration and drilling spurred by the deregulation of natural gas prices in the United States.
The American Gas Association, founded in 1918, is an organization of gas distributors and transporters that compiles statistics and provides information on the industry. Headquarters are in Arlington, Virginia.