If so, the solution to the energy crisis is obvious; invade hell and take their oil.
Thing is, oil takes about a minimal of 10 million years to form naturally, though its more often around 100 million years, so unless hell has existed and had lots of biomass for much longer then humans have existed, it wont have oil. Also we now know it’s a ‘plane of existence’ rather then a world, so it probably doesn’t have the all the plate tectonics and major shifts in topography which are necessary to deeply bury biomass so it can turn into oil, even if it is old enough. Methane meanwhile starts forming pretty much as soon as something dies.
Course, as Wong pointed out, this is assuming hell works anything like earth. It might not be like that at all and magic might well be sustaining those flames. Just thinking about the oil issue has made me realize that even more fundamental question exist, like how does it have rock at all or how’s the atmospheres held in, given that sufficient gravity to do so would force the place to become a sphere. This might be linked to the lack of iron, another odd issue.
However as for the energy crisis, if earth liberate hell then we have what, 100+ billion plus extra people who don’t need food or sleep and the vast majority of whom had no useful skills in an industrial economy. They have to be kept busy somehow, and so we could have them all run on treadmills or turning hand cranks linked to generators… Heck who knows, maybe pure evil can be extracted from demon souls and turned into a energy source more potent then anti matter?
"This cult of special forces is as sensible as to form a Royal Corps of Tree Climbers and say that no soldier who does not wear its green hat with a bunch of oak leaves stuck in it should be expected to climb a tree"
— Field Marshal William Slim 1956