Fuel is a material that provides useful energy. Fuels are used to heat and cool buildings, cook food, power engines, and produce electric power. Some fuels occur naturally and others are artificially created. Such natural fuels as coal, petroleum, and natural gas are obtained from underground deposits that were formed millions of years ago from the remains of plants and animals. These fuels, called fossil fuels, account for about 90 percent of the energy people use today.
Synthetic fuels can be made from fossil fuels, certain types of rock and sand, and biomass. Biomass is the name given for such replaceable organic matter as garbage, wood, and animal manure that can be used to produce energy. Biofuels are made from biological material, such as corn, switchgrass, and other plants. Biofuels, unlike fossil fuels, are a renewable form of fuel, because they are regrown each year.
Most fuels release energy by burning with oxygen in the air. But some—especially chemical fuels used in rockets—need special oxidizers to burn. Oxidizers are compounds that contain oxygen. Nuclear fuels do not burn but release energy through the fission (splitting) or fusion (joining together) of atoms.
Since the 1970’s, shortages of some fuels and concerns about the environmental effects resulting from the burning of fuels have led people to explore other sources of energy. This article discusses five groups of fuels—(1) solid fuels, (2) liquid fuels, (3) gas fuels, (4) chemical fuels, and (5) nuclear fuels—and their uses. For information on the availability of fuels, their effect on the environment, and alternative energy sources, see Energy supply and Environmental pollution .
Solid fuels
Coal
is used chiefly to produce electric power. It is burned to create heat to turn water into steam. The steam is then used to rotate turbines, machines that generate electric power (see Turbine ). Some coal is made into coke, a charcoallike solid that is an essential raw material in the production of iron and steel. Coal is also used to heat buildings and to provide energy for industrial machinery.
There are four types of coal: (1) lignite, or brown coal, (2) subbituminous coal, (3) bituminous coal, and (4) anthracite. Bituminous coal is the most plentiful coal used by industry. It contains more carbon and produces more heat than either lignite or subbituminous coal. It is also the coal best suited for making coke. Anthracite is the least plentiful and hardest coal. It contains more carbon and produces more heat than other coals. However, anthracite is difficult to ignite and burns slowly.
Peat
is partially decayed plant matter found in swamps called bogs. It is used as a fuel chiefly in areas where coal and oil are scarce. Peat can be cut, formed into blocks, and dried. The dried blocks are then burned to heat homes.
Biomass.
Wood has been used as a fuel since prehistoric times—longer than any other material. Today, it is an important fuel chiefly in less developed countries, where it is used for cooking and heating. In the United States and other industrialized nations, it is not a major source of energy. But some paper and pulp factories, which make wood products, obtain the energy for their manufacturing processes by burning bark, sawdust, and other wood waste. Wood is also used to make charcoal.
Biomass materials other than wood are also used as fuel. For example, heat produced by burning nutshells, rice and oat hulls, and other by-products of food processing is often used to operate plant equipment.
Liquid fuels
Liquid fuels are made mainly from petroleum, but some synthetic liquid fuels are also produced. Liquid fuels are easy to store and transport. They are the major source of energy for automobiles, airplanes, and other vehicles. Liquid fuels are also used to heat buildings.
Petroleum,
also called crude oil, ranges from clear yellow-brown oils to thick, black tars. Some crude oil can be burned as fuel in stoves and boilers without processing. However, most petroleum is refined to produce such fuels as gasoline, diesel oil, and kerosene. Gasoline is used to provide energy for most motor vehicles and piston-engine airplanes. Diesel oil powers most trains, ships, and large trucks. Kerosene provides energy for jet airplanes.
Other fuel oils obtained by refining petroleum include distillate oils and residual oils. Distillate oils are light oils, which are used chiefly to heat homes and small buildings. Residual oils are heavy, thick oils. They provide energy to power utilities, factories, and large ships. They also are used to heat large buildings.
Synthetic liquid fuels
include fuels made from coal, natural gas, biomass, oil shale, and bituminous sands. Oil shale is a rock that contains kerogen, a substance that yields oil when heated. Bituminous sands contain bitumen, a substance from which oil can be obtained. Synthetic liquid fuels are processed mainly in areas where one type of fuel is abundant, but other vital fuels are scarce. For example, South Africa has several large plants that make gasoline from coal. In this way, South Africa—with its abundance of coal and scarcity of petroleum—can provide its own motor fuel.
In the Canadian province of Alberta, plentiful bituminous sands are processed to yield oil. In Brazil, biomass in the form of sugar cane pulp is used to produce biofuel for automobiles.
Gas fuels
Gas fuels include natural and manufactured gases. Such fuels flow easily through pipes and are used to provide energy for homes, businesses, and industries. In many countries, vast networks of pipelines bring gas fuels to millions of consumers.
Natural gas
is used to heat buildings, cook food, and provide energy for industries. It consists chiefly of methane, a colorless and odorless gas. A small amount of a foul-smelling chemical compound called mercaptan is usually mixed with natural gas so that gas leaks can be promptly detected.
Butane and propane, which make up a small proportion of natural gas, become liquids when placed under large amounts of pressure. When pressure is released, they change back into gas. Such fuels, often called liquefied petroleum gas (LPG), are easily stored and shipped as liquids. They provide energy for motor homes and can serve as fuel for people who live far from natural gas pipelines.
Manufactured gas,
like synthetic liquid fuels, is used chiefly where certain fuels are abundant and others are scarce. Coal, petroleum, and biomass can all be converted to gas through various engineering processes. Gas can also be produced by treating such biomass as animal manure with bacteria called anaerobes, which expel methane as they digest the waste.
Chemical fuels
Chemical fuels, which are produced in solid and liquid form, create great amounts of heat and power. They are used chiefly in rocket engines. Chemical rocket propellants consist of both a fuel and an oxidizer. A common rocket fuel is a chemical compound called hydrazine. The oxidizer is a substance, such as nitrogen tetroxide, that contains oxygen. When the propellant is ignited, the oxidizer provides the oxygen the fuel needs to burn. Chemical fuels are also used in some race cars.
Nuclear fuels
Nuclear fuels provide energy through the fission or fusion of their atoms’ nuclei (cores). Uranium is the most commonly used nuclear fuel, though plutonium also provides nuclear energy. When the nuclei of these elements undergo fission, they release tremendous amounts of heat. Nuclear fuels are used mainly to generate electric power. They also power some submarines and ships. The fusion of hydrogen nuclei can release even more heat than nuclear fission. But scientists have not yet developed the technology needed to harness fusion energy for electric power.
