A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but has since also been applied to other sources of heat energy, such as nuclear energy.
Firewood was one of the first fuels used by humans.
Wood as fuel for combustion
Coal is a solid fuel
A filling station
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result in fire, because a flame is only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction. While activation energy must be supplied to initiate combustion, the heat from a flame may provide enough energy to make the reaction self-sustaining. The study of combustion is known as combustion science.
The flames caused as a result of a fuel undergoing combustion (burning)
Air pollution abatement equipment provides combustion control for industrial processes.
The combustion of methane, a hydrocarbon
Colourized gray-scale composite image of the individual frames from a video of a backlit fuel droplet burning in microgravity