Coal>charcoal

Indeed.
But if you use a charcoal to burn it, you’re effectively getting 8 charcoal for 1/8 of coal

But charcoal burns as long as coal does

Yes but if you can get 8 charcoal from 1 coal
And you get 8 charcoal from 1 charcoal
You get 64 charcoal from 1 coal

Exactly!