Reading this morning that wildfires break out in Montana from time to time when coal seam fires break onto the surface.
How do fires manage to burn underground? Seems like there wouldn’t be a lot of oxygen and there would be water.
So, how does a fire manage to burn in the coal?
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Because these coal seams are relatively close to the surface (close enough to break through) they are not ever really sufficiently submerged by ground water.
And yes they don’t get much oxygen. Not nearly the oxygen exposure you get on the surface, but enough still makes it’s way down to keep it alive.
Because don’t think of these as raging underground fires. They are the coal effectively smoldering for days, weeks, years, decades, until they one day get exposed again/burn to the surface.
You’re right that there’s not a lot of oxygen underground, that’s why coal seam fires mostly smolder for ages, using up what oxygen is available, or just slowly decomposing under heat without oxygen. The point is that it’s hot enough that it CAN ignite when the time comes. It’s a very insidious process, because the people on the surface can be completely oblivious until disaster strikes.
Then, whe a fissure opens, or a mine shaft is reached, a rush of oxygen can flare up that smoldering in the coal into an actual fire, which can weaken the ground, opening more fissures, delivering more oxygen, and so on in a self sustaining action.
Not all the ground is filled with ground water. It depends on the terrain. A lot of the coal in these mines are above the ground water table so the coal will be dry and allowed to burn. Oxygen is indeed a problem. The heat does create some convection currents as it rises up through cracks acting as natural vents sucking more oxygen into the fire from the sides. But this is a fairly slow process. This means that the coal will be burning very slowly as there is not a lot of oxygen available. But for the same reasons the heat will take a long time to escape from the fire. So even without oxygen the coal seam does not cool down. That means that the fire will still burn with the little oxygen it gets. It is kind of like how you can get a fire to flair up by blowing air into it but if you blow too much air into it you end up blowing it out. In the case of the underground coal fires there just is not enough air to blow it out and it will continue to burn.
Because the burned out areas often provide the path for oxygen to get to the fire. And once a coal fire has followed the seam underground the ground around it is already hot. And to extinguish the fire the oxygen and the heat have to be removed.
And sometimes poorly planned human intervention provides sources of oxygen (drilling test bores to find and isolate the fire).
The Centrailia Fire is a good example of a fire that has broken through the surface a couple times, and every time it has broken through all of the waste gas is expelled and the oxygen is pulled in.
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