Hi Group
Re The above in coal mines here in NSW Australia
In shallow mines there will generally alway be some ingress of atmospheric air (esp oxygen) thru surface subsidence or unsealed entries. Barometric variations will induce the tidal flow.
This oxygen will be taken up by the coal (and to a lesser extent by timber) producing carbon dioxide. If any combustion is present there will also be some carbon monoxide.
Carbon dioxide will eventually dissolve into (esp running ) mine water. Over time, carbon monoxide will also dissipate, possibly back into 'dioxide...) In formerly gassy mines a diminishing quantity of methane will continue to exude from the coal. It will also vanish in time perhaps due to bacteria.
Eventually a nitrogen atmosphere will prevail.
In deep mines pretty much the same takes place though more rapidly. I have reopened one deep mine sealed after a fire fifty years ago and the atmpsphere was 99% nitrogen with a trace of methane(0.5%) and very hot...in fact one week after the admission of air it was on fire again.
Regards
John
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