the correct term for it is not coal. but CHARcoal. in that its coal generated by charring the wood. it is very low efficiency compared to actual mined coal (as its a burnt wood there is less "fuel") but it is a well used way to generate burnable fuel.. charcoal is still used today, in the form of BBQ fuel and some places still use it for smoking fish/meat to keep traditional methods alive.

but ya.. charcoal compared to actual coal is, pretty much like comparing dirt to rock.. both can build a house but only one is better building material...