As far as I can gather carbon forms covalent bonds with nitogen
So the way to put out a coal fire, is to flood the coal reserve with nitogen – as the deep produces oxygen gas.
No oxygen, and the fire is extinguished. Liquid nitrogen is produced as a waste product in the gasification of air – so virtually free. And non-toxic.
So we can extinguish coal fires that have been burning underground for 30 years.