So maybe this whole thread should be talking about mining coal as more the limit. The survivors wont have enough energy.
The required level is achievable with a furnace made from clay, with an air pump made from sticks and leaves and mud, heated by charcoal made from heating logs in the absence of oxygen: https://youtube.com/playlist?list=PLGnWLXjIDnpBVRqu5lz5JGaQx...
(And again, this guy is starting basically from scratch each time; he's more interested in that than in practical re-industrialisation).
Coal is nice to have, not mandatory.
So maybe this whole thread should be talking about mining coal as more the limit. The survivors wont have enough energy.