10% gets eaten by organisms in the soil, but at least 90% turns into CO2.
Of course, Natural Gas thermal combined-cycle electricity generation is MUCH more efficient than Coal, no matter which way you look at it. (Unless you have an old already-paid-for coal plant next to a coal mine and only care about dollars)
There's also potential to get natural gas plants up to 60-80% efficient, which means still less CO2 production compared to a 30-40% for coal.
For Combined cycle low capital costs means the financial risk involved is much lower. Much more likely it'll pay off the capital costs. After that if the economics get dodgy it can be run intermittently or mothballed. IE, the utility isn't stuck paying off bonds on a plant that is shut down.
Flip side is coal fired plants have higher capital costs, longer payback and high financial risk. If carbon taxes make your plant uneconomic to run before the capital costs are paid off, the owners take a bath. (Nuclear and solar plants have the same issues, very high initial capital costs)