FUELS

                Fuels are any materials that store potential energy in forms that can be practicably released and used as heat energy. The concept originally applied solely to those materials storing energy in the form of chemical energy that could be released through combustion, but the concept has since been also applied to other sources of heat energy such as nuclear energy (via nuclear fission or nuclear fusion), as well as releases of chemical energy released through non-combustion oxidation (such as in cellular biology or in fuel cells).

                

                 A substance that produces useful energy when it undergoes a chemical or nuclear reaction.

 

                  A material such as wood, coal, gas, or oil burned to produce heat or power.