Fuel is any material that is capable of releasing energy when its chemical structure is changed or converted.
In chemistry and the field of fire safety, anything that is capable of combustion is called fuel.
Extraction of energy from fuel
Fuel stores energy in its chemical structure. When the chemical structure of the fuel changes, this chemical energy is converted into other forms and released. There are many ways to express and compare the amounts of energy stored in fuel. For example, a fuel value is used to quantify fuel's potential energy.
All carbon-based life forms—from microrganisms to animals and humans—depend on and use fuels as their source of energy. Their bodies posess a natural mechanism called metabolism to convert energy stored in food. Additionally, humans employ a variety of techniques to convert one form of energy into another, producing usable energy for purposes that go far beyond energy needs of a human body. The application of energy released from fuels ranges from heat to cooking and from powering weapons to propulsion and generation of electricity. 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 releasing 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. Humans have also mastered 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. However, it is economically practical to use only those fuels that contain large amounts of easily extractable energy. Luckily, there are many different types of fuel that can be obtained and prepared for consumption relatively inexpensively.
Solid fuels include coal, wood and peat. All these types of fuel are combustible (they create fire and heat). Coal was burnt by steam railtrains 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. Steam power is becoming more and more desirable as oil and gas supplies begin to run out, given the wide number of possible things that can burn to heat water.
Liquid and gaseous fuels
Non-solid fuels include alkanes such as petroleum and gasoline (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.
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.
Other types of fuel
Hydrogen also features as an upcoming fuel for automobiles with Oxygen in the Fuel Cell. This involves a reaction where the hydrogen and oxygen react to produce water (H2O) and electrical energy, however most of the energy comes from turning hydrogen into electrons which is electricity and protons using platinum as a catalyst, which, then can supply an electrical motor in order to run a car (or a variety of other uses). In this reaction the chemical energy of the chemicals is converted into electrical energy due to redox.