The total economic loss is (economic gains from burning coal to generate electricity) - (economic losses from the pollution and injuries caused by burning coal to generate electricity).
If the gain from burning coal to generate electricity is more than enough to counter the loss to do so, then leaving the coal in the ground is an economic loss.
If the gain from burning coal to generate electricity is more than enough to counter the loss to do so, then leaving the coal in the ground is an economic loss.