Question

All combustible substances are not fuels. Why?

Answer 1

Fuel is a substance which on burning releases a large amount of energy as heat at a reasonable cost and leaves behind little ash content. For example, coal, natural gas, petroleum nearly meet all these conditions.
Combustible substance is the one which liberates heat on burning. The amount of heat liberated is different for different substances. This determines whether or not a substance is a good fuel.
All the combustible substances are not fuels because many of them produce less amount of heat for a particular amount of energy input on burning and leave behind high ash content; for example, paper, cloth, etc.