1 ton of oxygen + 1 ton of coal, equals:
a) 1 ton of unburnt coal + 1 ton of air for you to breathe
or
b) 2 tons of CO2 + zero air for you to breathe. Not to mention such pleasantries as sulphuric gases, acid rain, or any other cute side-effects of burning all that coal just because you can.
(in short: C + O2 = CO2 + zero oxygen)
Ok, so they'll try stuffing that CO2 down into the ground, to minimise the greenhouse effect. It still equals zero breathable air.
Remember, the air is also finite. We've got 1 -one- planet's worth of fresh air available. When that's spent, it's spent.
Also, coal isn't just carbon, there's all kinds of shit there too, sulphur just being one. And that shit will phuxx0r most (if not all) of the plantlife that's supposed to take care of the carbon.