Put simplistically, we dig up coal, burn it, turn the waste product into coal and put it back into the mine. Put simplistically, the energy wasted in neutralizing the carbon dioxide balances out the energy gained by burning the coal in the first place.
Now, I know it must be more complicated than that. That there is an appropriate set of chemical reactions that will lock up the carbon dioxide as effectively as it was locked up in coal in the first place, where producing the chemicals needed won’t eat up more energy than was gained by burning the coal in the initially, where producing the chemicals doesn’t release equivalent amounts of carbon dioxide into the atmosphere. But it doesn’t really seem intuitively obvious that this must be the case.
I keep thinking that it’s far more likely that this is an elegant smoke-and-mirrors game which will allow coal fired power stations to keep belching their way into the 21st century while being less obvious about what the environmental consequences are. I’m probably just being cynical. Aren’t I?