Firewood was one of the first fuels used by humans.[1]
Fuel are anymaterials that can react with other substances to release energy asthermal energy or to be used forwork. The concept was originally applied solely to those materials capable of releasingchemical energy but has since also been applied to other sources of heat energy, such asnuclear energy (vianuclear fission andnuclear fusion).
The heat energy released by reactions of fuels can be converted intomechanical energy via aheat engine. Other times, the heat itself is valued for warmth,cooking, or industrial processes, as well as the illumination that accompaniescombustion. Fuels are also used in thecells oforganisms in a process known ascellular respiration, where organicmolecules are oxidized to release usable energy.Hydrocarbons and related organic molecules are by far the most common source of fuel used by humans, but other substances, including radioactive metals, are also utilized.
The first known use of fuel was thecombustion offirewood byHomo erectus nearly two million years ago.[2] Throughout most of human history only fuels derived from plants or animal fat were used by humans.Charcoal, a wood derivative, has been used since at least 6,000 BCE for melting metals. It was only supplanted bycoke, derived from coal, as European forests started to become depleted around the 18th century. Charcoal briquettes are now commonly used as a fuel forbarbecue cooking.[2]
Crude oil wasdistilled byPersian chemists, with clear descriptions given in Arabic handbooks such as those ofMuhammad ibn Zakarīya Rāzi.[3] He described the process of distilling crude oil/petroleum intokerosene, as well as other hydrocarbon compounds, in hisKitab al-Asrar (Book of Secrets). Kerosene was also produced during the same period fromoil shale andbitumen by heating the rock to extract the oil, which was then distilled. Rāzi also gave the first description of akerosene lamp using crude mineral oil, referring to it as the "naffatah".[4]
The streets ofBaghdad were paved withtar, derived from petroleum that became accessible from natural fields in the region. In the 9th century,oil fields were exploited in the area around modernBaku,Azerbaijan. These fields were described by theArab geographerAbu al-Hasan 'Alī al-Mas'ūdī in the 10th century, and byMarco Polo in the 13th century, who described the output of those wells as hundreds of shiploads.[5]
With the development of thesteam engine in the United Kingdom in 1769, coal came into more common use, the combustion of which releaseschemical energy that can be used to turn water into steam.[6] Coal was later used to drive ships andlocomotives. By the 19th century, gas extracted from coal was being used for street lighting in London. In the 20th and 21st centuries, the primary use of coal is to generateelectricity, providing 40% of the world's electrical power supply in 2005.[7]
Fossil fuels were rapidly adopted during theIndustrial Revolution, because they were more concentrated and flexible than traditional energy sources, such as water power. They have become a pivotal part of our contemporary society, with most countries in the world burning fossil fuels in order to produce power, but arefalling out of favor due to theglobal warming and related effects that are caused by burning them.[8]
Currently the trend has been towards renewable fuels, such asbiofuels like alcohols.
Chemical fuels are substances that release energy by reacting with substances around them, most notably by the process ofcombustion.
Chemical fuels are divided in two ways. First, by their physical properties, as a solid, liquid or gas. Secondly, on the basis of their occurrence:primary (natural fuel) andsecondary (artificial fuel). Thus, a general classification of chemical fuels is:
Solid fuel refers to various types ofsolid material that are used as fuel to produceenergy and provideheating, usually released through combustion. Solid fuels includewood,charcoal,peat,coal,hexamine fuel tablets, and pellets made from wood (seewood pellets),corn,wheat,rye and othergrains.Solid-fuel rocket technology also uses solid fuel (seesolid propellants). Solid fuels have been used by humanity for many years tocreate fire. Coal was the fuel source which enabled theIndustrial Revolution, from firingfurnaces, to runningsteam engines. Wood was also extensively used to runsteam locomotives. Both peat and coal are still used inelectricity generation today.The use of some solid fuels (e.g. coal) is restricted or prohibited in some urban areas, due to unsafe levels of toxic emissions. The use of other solid fuels as wood is decreasing as heating technology and the availability of good quality fuel improves. In some areas,smokeless coal is often the only solid fuel used. In Ireland, peatbriquettes are used as smokeless fuel. They are also used to start a coal fire.
Liquid fuels are combustible or energy-generating molecules that can be harnessed to createmechanical energy, usually producingkinetic energy. They must also take the shape of their container; the fumes of liquid fuels are flammable, not the fluids.
Most liquid fuels in widespread use are derived from thefossilized remains of dead plants and animals by exposure to heat and pressure inside the Earth's crust. However, there are several types, such ashydrogen fuel (forautomotive uses),ethanol,jet fuel andbio-diesel, which are all categorized as liquid fuels.Emulsified fuels of oil in water, such asorimulsion, have been developed as a way to make heavy oil fractions usable as liquid fuels. Many liquid fuels play a primary role in transportation and the economy.
Some common properties of liquid fuels are that they are easy to transport and can be handled easily. They are also relatively easy to use for all engineering applications and in home use. Fuels likekerosene are rationed in some countries, for example in government-subsidized shops in India for home use.
Conventionaldiesel is similar togasoline in that it is a mixture ofaliphatic hydrocarbons extracted frompetroleum. Kerosene is used inkerosene lamps and as a fuel for cooking, heating, and small engines.Natural gas, composed chiefly ofmethane, can only exist as a liquid at very low temperatures (regardless of pressure), which limits its direct use as a liquid fuel in most applications.LP gas is a mixture ofpropane andbutane, both of which are easily compressible gases under standard atmospheric conditions. It offers many of the advantages ofcompressed natural gas (CNG) but is denser than air, does not burn as cleanly, and is much more easily compressed. Commonly used for cooking and space heating, LP gas and compressed propane are seeing increased use in motorized vehicles. Propane is the third most commonly used motor fuel globally.
Fuel gas is any one of a number of fuels that aregaseous under ordinary conditions. Many fuel gases are composed ofhydrocarbons (such asmethane orpropane),hydrogen,carbon monoxide, or mixtures thereof. Such gases are sources of potentialheat energy orlight energy that can be readily transmitted and distributed through pipes from the point of origin directly to the place of consumption. Fuel gas is contrasted with liquid fuels and from solid fuels, though some fuel gases areliquefied for storage or transport. While their gaseous nature can be advantageous, avoiding the difficulty of transporting solid fuel and the dangers of spillage inherent in liquid fuels, it can also be dangerous. It is possible for a fuel gas to be undetected and collect in certain areas, leading to the risk of agas explosion. For this reason,odorizers are added to most fuel gases so that they may be detected by a distinct smell. The most common type of fuel gas in current use isnatural gas.
Biofuel can be broadly defined as solid, liquid, or gas fuel consisting of, or derived frombiomass. Biomass can also be used directly for heating or power—known asbiomass fuel. Biofuel can be produced from any carbon source that can be replenished rapidly e.g. plants. Many different plants and plant-derived materials are used for biofuel manufacture.
Perhaps the earliest fuel employed by humans is wood. Evidence shows controlled fire was used up to 1.5 million years ago atSwartkrans, South Africa. It is unknown which hominid species first used fire, as bothAustralopithecus and an early species ofHomo were present at the sites.[9] 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 anenergy density of 10–20MJ/kg.[10]
Recently biofuels have been developed for use in automotive transport (for examplebioethanol andbiodiesel), but there is widespread public debate about howcarbon neutral these fuels are.[citation needed]
Fossil fuels arehydrocarbons, primarilycoal andpetroleum (liquid petroleum ornatural gas), formed from thefossilized remains of ancient plants and animals[11] by exposure to high heat and pressure in the absence of oxygen in theEarth's crust over hundreds of millions of years.[12] Commonly, the term fossil fuel also includes hydrocarbon-containingnatural resources that are not derived entirely from biological sources, such astar sands. These latter sources are properly known asmineral fuels.
Fossil fuels contain high percentages ofcarbon and include coal, petroleum, and natural gas.[13]They range fromvolatile materials with low carbon:hydrogen ratios likemethane, to liquid petroleum to nonvolatile materials composed of almost pure carbon, likeanthracite coal. Methane can be found in hydrocarbon fields, alone, associated with oil, or in the form ofmethane clathrates. Fossil fuels formed from the fossilized remains of dead plants[11] by exposure to heat and pressure in the Earth's crust over millions of years.[14] Thisbiogenic theory was first introduced by German scholarGeorg Agricola in 1556 and later byMikhail Lomonosov in the 18th century.
Fossil fuels arenon-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being made. So we must conserve these fuels and use them judiciously. The production and use of fossil fuels raise environmental concerns. A global movement toward the generation ofrenewable energy is therefore under way to help meet increased energy needs. The burning of fossil fuels produces around 21.3 billiontonnes (21.3gigatonnes) ofcarbon dioxide (CO2) per year, but it is estimated that natural processes can only absorb about half of that amount, so there is a net increase of 10.65 billion tonnes of atmospheric carbon dioxide per year (one tonne of atmospheric carbon is equivalent to44⁄12 (this is the ratio of the molecular/atomic weights) or 3.7 tonnes of CO2.[17] Carbon dioxide is one of thegreenhouse gases that enhancesradiative forcing and contributes toglobal warming, causing theaverage surface temperature of the Earth to rise in response, whichthe vast majority of climate scientists agree will cause majoradverse effects.Fuels are a source of energy.
The International Energy Agency (IEA) predicts that fossil fuel prices will decline, with oil stabilizing around $75 to $80 per barrel as electric vehicle adoption surges and renewable energy expands. Additionally, the IEA anticipates a notable increase in liquefied natural gas capacity, enhancing Europe's energy diversification.[18]
The amount of energy from different types of fuel depends on thestoichiometric ratio, the chemically correct air and fuel ratio to ensure complete combustion of fuel, and itsspecific energy, the energy per unit mass.
(The fuel-air ratio (FAR) is the reciprocal of the air-fuel ratio (AFR).)
λ is the air-fuel equivalence ratio, andλ=1 means that it is assumed that the fuel and the oxidising agent (oxygen in air) are present in exactly the correct proportions so that they are both fully consumed in the reaction.
TwoCANDU ("CANada Deuterium Uranium") fuel bundles, each about 50cm long and 10cm in diameter
Nuclear fuel is any material that is consumed to derivenuclear energy. In theory, a wide variety of substances could be a nuclear fuel, as they can be made to release nuclear energy under the right conditions. However, the materials commonly referred to as nuclear fuels are those that will produce energy without being placed under extreme duress. Nuclear fuel can be "burned" bynuclear fission (splitting nuclei apart) orfusion (combining nuclei together) to derive nuclear energy. "Nuclear fuel" can refer to the fuel itself, or to physical objects (for example bundles composed offuel rods) composed of the fuel material, mixed with structural,neutron moderating, or neutron-reflecting materials.
Nuclear fuel has the highestenergy density of all practical fuel sources.
Nuclear fuel pellets are used to release nuclear energy.
The most common type of nuclear fuel used by humans is heavyfissile elements that can be made to undergonuclear fission chain reactions in anuclear fission reactor;nuclear fuel can refer to the material or to physical objects (for example fuel bundles composed offuel rods) composed of the fuel material, perhaps mixed with structural,neutron moderating, or neutron reflecting materials.
When some of these fuels are struck by neutrons, they are in turn capable of emitting neutrons when they break apart. This makes possible a self-sustainingchain reaction that releases energy at a controlled rate in anuclear reactor, or at a very rapid uncontrolled rate in anuclear weapon.
In contrast to fission, some lightnuclides such astritium (3H) can be used as fuel fornuclear fusion. This involves two or more nuclei combining into larger nuclei.Fuels that produce energy by this method are currently not utilized by humans, but they are the main source of fuel forstars. Fusion fuels are light elements such ashydrogen whose nucleii will combine easily. Energy is required to start fusion by raising the temperature so high that nuclei can collide together with enough energy that they stick together before repelling due to electric charge. This process is called fusion and it can give out energy.
In stars that undergo nuclear fusion, fuel consists ofatomic nuclei that can release energy by the absorption of aproton orneutron. In most stars the fuel is provided by hydrogen, which can combine to formhelium through theproton-proton chain reaction or by theCNO 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. Onceiron-56 ornickel-56 nuclei are produced, no further energy can be obtained by nuclear fusion as these have the highest nuclear binding energies.[19] Any nucleii heavier than56Fe and56Ni would thus absorb energy instead of giving it off when fused. Therefore, fusion stops and the star dies. In attempts by humans, fusion is only carried out with hydrogen (2H (deuterium) or3H (tritium)) to form helium-4 as this reaction gives out the most net energy. Electric confinement (ITER),inertial confinement (heating by laser) and heating by strong electric currents are the popular methods.
Most transportation fuels are liquids, because vehicles usually require highenergy density. This occurs naturally in liquids and solids. High energy density can also be provided by aninternal combustion engine. These engines require clean-burning fuels. The fuels that are easiest to burn cleanly are typically liquids and gases. Thus, liquids meet the requirements of being both energy-dense and clean-burning. In addition, liquids (and gases) can be pumped, which means handling is easily mechanized, and thus less laborious. As there is a general movement towards a low carbon economy, the use of liquid fuels such as hydrocarbons is coming under scrutiny.
^Salim Al-Hassani (2008). "1000 Years of Missing Industrial History". In Emilia Calvo Labarta; Mercè Comes Maymo; Roser Puig Aguilar; Mònica Rius Pinies (eds.).A shared legacy: Islamic science East and West.Edicions Universitat Barcelona. pp. 57–82 [63].ISBN978-84-475-3285-8.
^One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Fuel".Encyclopædia Britannica. Vol. 11 (11th ed.). Cambridge University Press. pp. 274–286.
^IPCC AR5 WG1 Summary for Policymakers 2013, p. 4: Warming of the climate system is unequivocal, and since the 1950s many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased;IPCC SR15 Ch1 2018, p. 54: Abundant empirical evidence of the unprecedented rate and global scale of impact of human influence on the Earth System (Steffen et al., 2016; Waters et al., 2016) has led many scientists to call for an acknowledgment that the Earth has entered a new geological epoch: theAnthropocene.
