I'm not sure whether this is true, but I expect that electricity-from-coal for Bitcoins isn't a massive problem because doing so is simply unprofitable: the electricity costs more than the value of the Bitcoins, so it would be irrational to do.

I would expect that mining has become an arbitrage market that narrows in on the cheapest possible electricity anywhere in the world, and as such is dominated by excess "free" electricity from e.g. hydroelectric power that is generating more electricity than can be used (or transported).