phlogiston

Phlogiston was the term used by Georg Ernst Stahl,in 18 century which means "burned".It was a hypothetical principle of fire .It was a hypothetical  concept according to which phlogiston was a volatile substance that was present in every compound that burns . A compound that burns on combustion has some calorific value which makes it considerable as a fuel.

FIrstly phlogiston is hypothetical and otherwise also it cannot be considered as a fuel as it don't burn in combustion instead it is released during combustion.