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Fuel is any material that is capable of releasing energy when its chemical or physical structure is altered. Fuel releases its energy either through chemical means, such as burning, 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 solar power into a form that can be used to sustain life. ^[1] 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.

Extraction of energy from fuel and oxygen

Fuels do not store energy in their chemical structure, but rather the energy is stored in the fuel/oxygen system. It is during respiration of food or combustion of fuel that the fuel/food joins with the reactive gas oxygen. Oxygen has weakly bonded molecules, which can be broken apart with relatively small energy input. When the resultant atoms bond with the atoms from the fuel (mostly hydrogen and oxygen) more energy is released than was needed to break up the original molecules. This chemical energy is released and available to drive our technology or life itself. There are many ways to express and compare the amounts of energy stored in fuel/oxygen systems. For example, a fuel value is used to quantify fuel's potential energy. Because oxygen is freely available for respiration and combustion we carelessly think that the energy is stored in the fuel, and talk of food and fuels 'containing' energy. This is so commonplace that most scientists have come to believe that food and fuels 'contain' energy. We need to think of combustion and (aerobic) respiration as constructive processes that build up oxides. One of these, carbon dioxide, is released by the burning of fossil fuels and is being released into the atmosphere causing climate change. We need to keep separate accounts of the matter and energy when considering fuels: the matter in fuels joins with oxygen forming oxides; the energy, stored as chemical energy as the fuel and oxygen are kept apart, is released, usually as heat, but in respiration, as chemical energy again as the respiration process drives metabolism.

The method of conversion used for a particular type of fuel depends on the molecular structure of the fuel. For example, the most efficient method for obtaining energy from fossil fuels is by burning them in oxygen. In this process known as combustion, fuel reacts with oxygen and releases energy as heat. In chemistry and the field of fire safety, anything that is capable of burning is called fuel. Humans have also been able to extract energy from an atom by subjecting fissile materials to nuclear reaction.

Types of fuel

Theoretically any fuel can be used as a source of energy. Natural uses of fuel by organisms focus on conversion of food and oxygen into carbon dioxide and water. This chemical process releases energy. Other natural fuel use includes the nuclear fusion that occurs in stars. For use by humans it is economically practical to use only those fuels that, as they react, release large amounts of easily extractable energy. Different types of fuel are utilized, many of which can be obtained and prepared for consumption relatively inexpensively. Some types of fuels are focused on their ability to be transported easily from where they are produced to where they are used.

Fuel/oxygen systems storing chemical energy

Food

Carbohydrates, fats, and proteins, derived from food, are the fuels for biological systems. For instance, glucose (a simple carbohydrate) combines with oxygen to produce water and carbon dioxide. During this reaction weak bonds are replaced by strong ones and energy is released. In the bodies of most animals, the released energy is used by the muscles to allow them to move.

Fossil fuel

Solid fossil fuels include coal and perhaps peat. All these types of fuel are combustible (they create fire and heat). Coal was burnt by steam trains to heat water into steam to move parts and provide power. Peat and wood are mainly used for domestic and industrial heating, though peat has been used for power generation, and wood-burning steam locomotives were common in times past.

Non-solid fossil fuels include alkanes such as petroleum and gas (both fuel types have myriad varieties including petrol (gasoline) and natural gas). The former is widely used in the internal combustion engine while both are used in power generation. Note, once again, that energy is not stored in the fuels but is released only when they react with the reactive gas, oxygen, but because oxygen is 'freely' available we tend, falsely, to focus on the fuel/food as the source of the energy.

Other types of fuel

File:Hydrogengas.jpg

A flask containing hydrogen, which is naturally colorless and odorless.

Hydrogen also features as the essential component in the proposed hydrogen economy. As hydrogen fuel does not naturally occur in practical amounts, its production using other fuels or energy sources is necessitated. Because of this, it takes 4 units of energy to produce hydrogen for every 1 unit that it will yield. ^[2] Reacting cleanly with oxygen it yields only water (H₂O) as a byproduct.

Biofuels such as alcohol fuel and biodiesel are made from various kinds of biomass. Biodiesel and straight vegetable oil are the essential components of a vegetable oil economy. The biomass is plant matter from plants such as hemp, maize, sugarcane, etc, and byproducts from animals such as lard.

These biofuels are formed during photosynthesis where energy from the sun pulls oxygen from carbon dioxide and water. Whilst the oxygen and fuel remain apart, energy is stored. During combustion and respiration the oxygen 'springs back again' releasing the energy originally from the sunlight. Thus the use of bio-fuels is really a solar technology. Note also that the carbon dioxide released when they burn does not contribute to the enhanced greenhouse effect, because their carbon was only recently trapped, and is part of the active natural carbon cycle.

Nuclear fuel

In a nuclear reaction a radioactive fuel will undergo fission. This provides a useful source of energy without combustion. Also, in stars (and our sun), hydrogen (a gas) is the fuel for the nuclear fusion.

Environmentally-friendly fuels

Notes

^ "Metabolism," Encyclopaedia Britannica, retrieved August 17, 2006.
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