Why is it said that  any substance which on burning in air produces heat energy is not a fuel ?? while any substance which is easily available in nature and burns in air at a moderate rate, producing a large amount of heat energy, without leaving behind any undesirable residue is called a fuel...

Dear Student, 

Actually, there is a difference in fuel and a common combustible substance. The fuel when burns, the energy released can be converted into useful mechanical works in a controlled manner and that is why they are termed as fuel, but some other substances though burn but they do not produce heat in controlled manner so that we could convert it into a mechanical work.

The fuel have a certain and high calorific value at per unit volume which gives the expected and calculated amount of energy and work done. So, every substance that burn is not necessary to be a fuel, the fuel may work without burning too.

I hope your confusion is clear now!