There is a growing concern about the environmental contamination caused by the burning of great amounts of fossil fuels and about the increasing expense of finding them and processing them into easily usable forms (see energy, sources of). During the last 100 years the amount of carbon dioxide in the atmosphere has increased, and there is evidence that this phenomenon may be due to the burning of fossil fuel. Use of biomass, which consists of plants or plant waste, would not produce excess carbon dioxide because the plants absorb the gas for their growth. Wood is not as concentrated a form of energy as fossil fuels, but it can be converted into a more energy-rich fuel called charcoal. Burning fossil fuel also releases acidic oxides of sulfur and nitrogen, which are deposited on the earth in rainwater (see acid rain). The clearing of forests, particularly in the tropical regions, also threatens to increase the amount of carbon dioxide in the atmosphere because the forests utilize carbon dioxide for growth.
The amount of fossil fuel available is limited and new methods of recovery are being developed. One proposed alternative fuel is hydrogen, which is now employed as a fuel only for a few special purposes because of its high cost. Hydrogen can be produced by electrolysis of water for which nonfossil fuels would supply the energy. Solar energy could be utilized either by direct conversion to electricity using photovoltaic cells or by trapping solar heat. Fuels are rated according to the amount of heat (in calories or Btu) they can produce. Nuclear fuels are also possible substitutes for fossil fuels. Nuclear fuels are not burned; they undergo reactions in which the nuclei of their atoms either split apart, i.e., undergo fission, or combine with other nuclei, i.e., undergo fusion. In either case, a small part of the nuclear mass is converted to heat energy. All nuclear fuels currently employed in practical, nonweapons applications react by fission.
High-energy fuels for jet engines and rockets are rated by their specific impulse in thrust per pound of propellant per second. Hydrogen, which is the lightest element, is usually used in the form of compounds, because the density of liquid hydrogen is low and therefore a large volume is required. Addition of aluminum powder or lithium increases the efficiency. Rockets usually have a self-contained supply of oxygen or some other oxidizer, such as ammonium, lithium, or potassium perchlorate. Fuels such as turpentine, alcohol, aniline, and ammonia use nitric acid, hydrogen peroxide, and liquid oxygen as oxidizers. More power can be obtained by oxidizing hydrazine, diborane, or hydrogen with oxygen, ozone, or fluorine.
Fuel is any material that is burned or altered in order to obtain energy. Fuel releases its energy either through a chemical reaction means, such as combustion, or nuclear means, such as nuclear fission or nuclear fusion. An important property of a useful fuel is that its energy can be stored to be released only when needed, and that the release is controlled in such a way that the energy can be harnessed to produce work.
All carbon-based life forms—from microorganisms to animals and humans—depend on and use fuels as their source of energy. Their cells engage in an enzyme-mediated chemical process called metabolism that converts energy from food or light into a form that can be used to sustain life. Additionally, humans employ a variety of techniques to convert one form of energy into another, producing usable energy for purposes that go far beyond the energy needs of a human body. The application of energy released from fuels ranges from heat to cooking and from powering weapons to combustion and generation of electricity.
Perhaps the earliest fuel that was employed by humans is wood. Evidence shows controlled fire was used up to 1.5 million years ago at Swartkrans, South Africa. It is unknown which hominid species first used fire, as both Australopithecus and an early species of Homo were present at the sites. As a fuel, wood has remained in use up until the present day, although it has been superseded for many purposes by other sources. Wood has an energy density of 10–20 MJ/kg.
Recently biofuels have been developed for use in automotive transport (for example E10 fuel), but there is widespread public debate about how carbon efficient these fuels are.
Modern large-scale industrial development is based on fossil fuel use, which has largely supplanted water-driven mills, as well as the combustion of wood or peat for heat. With global modernization in the 20th and 21st centuries, the growth in energy production from fossil fuels, especially gasoline derived from oil, is one of the causes of major regional and global conflicts and environmental issues. A global movement toward the generation of renewable energy is therefore under way to help meet the increased global energy needs.
The burning of fossil fuels by humans is the largest source of emissions of carbon dioxide, which is one of the greenhouse gases that enhances radiative forcing and contributes to global warming. The atmospheric concentration of CO2, a greenhouse gas, is increasing, raising concerns that solar heat will be trapped and the average surface temperature of the Earth will rise in response.
Nuclear fuel is any material that is consumed to derive nuclear energy. Technically speaking this definition includes all matter because any element will under the right conditions release nuclear energy, the only materials that are commonly referred to as nuclear fuels though are those that will produce energy without being placed under extreme duress.
The most common type of nuclear fuel used by humans is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor; nuclear fuel can refer to the material or to physical objects (for example fuel bundles composed of fuel rods) composed of the fuel material, perhaps mixed with structural, neutron moderating, or neutron reflecting materials. The most common fissile nuclear fuels are 235U and 239Pu, and the actions of mining, refining, purifying, using, and ultimately disposing of these elements together make up the nuclear fuel cycle, which is important for its relevance to nuclear power generation and nuclear weapons.
In stars that undergo nuclear fusion, fuel consists of atomic nuclei that can release energy by the absorption of a proton or neutron. In most stars the fuel is provided by hydrogen, which can combine together to form helium through the proton-proton chain reaction or by the CNO cycle. When the hydrogen fuel is exhausted, nuclear fusion can continue with progressively heavier elements, although the net energy released is lower because of the smaller difference in nuclear binding energy. Once iron-56 or nickel-56 nuclei are produced, no further energy can be obtained by nuclear fusion as these have the highest nuclear binding energies.
World Bank reported that the USA was the top fuel importer in 2005 followed by the EU and Japan.
Coal was first used as a fuel around 1000 BCE in China. With the development of the steam engine in 1769, coal came into more common use as a power source. Coal was later used to drive ships and locomotives. By the 19th century, gas extracted from coal was being used for street lighting in London. In the 20th century, the primary use of coal is for the generation of electricity, providing 40% of the world's electrical power supply in 2005.