Coal and other fossil fuels are primarily made up of Carbon and when they are burned those Carbon molecules attach themselves to Oxygen molecules to form CO2. They do this in a two to one ratio hence CO2, so for every Carbon molecule they are sequestering for eternity they also sequestering 2 Oxygen molecules. Now you may not know this but Oxygen is a good and we only have a finite number of Oxygen molecules. We only get more oxygen when plants take CO2 from the atmosphere capture the Carbon molecule and release it into the atmosphere for us to breath the more CO2 in the atmosphere the more plant growth there is and then we get more Oxygen as well. It’s the symbiotic relation between man and plants called photosynthesis and it’s been the driving force of life on this planet for millenia and I think it works pretty good and we shouldn’t mess with it.