There are many different features that we can use to decide if a fuel is a good one. Natural gas may be the best fuel for heating, but it is of no use at all when trying to cook or on a camping expedition in the mountains.
The heat evolved during a combustion can be used economically for industrial or other use. All combustible substances which contain carbon as the main constituent are called fuels. Thus, a fuel may be defined as any combustible substance, chemical or reactant containing carbon as the main constituent which on proper burning produces heat that can be used economically for domestic and industrial purposes and in generation of power.