I INTRODUCTION Gases, Fuel, any combustible gaseous mixture used as fuel to provide energy for domestic or industrial use (see Combustion; Fuel).
Fuel gases consist principally of hydrocarbons, that is, of molecular compounds of carbon and hydrogen. The properties of the various gases depend on the number and arrangement of the carbon and hydrogen atoms within their molecules. All these gases are odorless in the pure state, and carbon monoxide is toxic. It is therefore common practice to add sulfur compounds to manufactured gas; such sulfur compounds, which are sometimes normally present in the gas, have an unpleasant smell and serve to give warning of a leak in the supply lines or gas appliance. In addition to their combustible components most gases have varying amounts of noncombustible nitrogen and water as their end products.
The devices used to burn gas for either heat or illumination consist of a burner nozzle and some means of mixing air with the gas before it reaches the nozzle, as, for example, in the Bunsen burner invented by the German chemist Robert Wilhelm Bunsen. See Lamp.
Fuel gases still in use are coal gas, made by the destructive distillation of coal (see Coal; Coke); producer gas and blast-furnace gas, made by the interaction of steam, air, and carbon; natural gas, drawn from gas deposits in the earth; and bottled gases, made from the lighter hydrocarbons.
II COAL GAS
The most important coal-gasification processes aim chiefly at production of so-called pipeline quality gas, which is reasonably interchangeable with natural gas. Gas from coal, besides having pumping and heating specifications, must meet strict limits on content of carbon monoxide, sulfur, inert gases, and water. To meet these standards, most coal-gasification processes culminate with gas cleanup and methanation operations. Various hydrogasification processes, in which hydrogen reacts directly with coal to form methane, are used today; these processes bypass the indirect step of producing synthesis gas, hydrogen and carbon monoxide, before an upgrading yields methane. Other coal-gas processes include the carbon dioxide acceptor process, employing the lime-bearing material dolomite, and the molten salt process. These processes work indirectly to produce synthesis gas first. Other gases manufactured formerly from coal and coke, such as illumination gas and coke-oven gas, are of little or no importance today.
III PRODUCER AND BLAST-FURNACE GASES
Producer gas is a form of water gas, a term applied to steam-process gases. It is made by burning low-grade fuel (such as lignite or bituminous coal) in a closed vessel, called a producer, while passing a continuous stream of steam and air through the producer. Because of the air present in the producer, the resulting gas is approximately 50 percent incombustible nitrogen and is low in fuel value, having only about 28 percent the heating value of coke-oven gas.
Blast-furnace gas, which results from the interaction of limestone, iron ore, and carbon in blast furnaces, has some heating value because of its carbon monoxide content but contains about 60 percent nitrogen. Enormous quantities are produced during the operation of furnaces. Most of this gas is consumed in heating the air blast and driving the compressors for the blast. The heating value of blast-furnace gas is about 16 percent of that of coke-oven gas. For a discussion of oil gas, made by the pyrolysis of petroleum hydrocarbons, See Cracking.
IV NATURAL GAS
A certain amount of natural gas almost always occurs in connection with oil deposits and is brought to the surface together with the oil when a well is drilled. Such gas is called casing-head gas. Certain wells, however, yield only 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, 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.
V BOTTLED GAS
Several of the lighter hydrocarbons, such as propane, butane, pentane, and mixtures of these gases are liquefied and employed as fuels. These so-called bottled gases, which are usually stored in steel cylinders, make possible the use of appliances such as cooking stoves and heaters in localities where a centralized gas supply is not available. Such bottled gases are produced from natural gas and petroleum.