Carbon dioxide, CO2. Just separate the oxygen, which has plenty of uses. That leaves the carbon to make diamonds, pencils for third world schools, or throw it back in the firebox to burn again.
Burned again carbon, what could be more righteous than that?
OK, I've provided the solution, you handle the implementation.
Can anyone envision one of these gigantic CO2 stashes finding a path to the surface, and suffocating whole towns like the volcano gases have done?