Question

why can hydrogen used as a fuel

Answer 1

Hiiii guys..
This is because hydrogen gas is lighter than air and rises into the atmosphere as a result. Natural hydrogen is always associated with other elements in compound form such as water, coal and petroleum. Hydrogen has the highest energy content of any common fuel by weight.
Hope it helps u

Answer 2

As it is flammable gas and it produce only blue flame it does not produce any residue