Underground fires have been burning under Centralia, PA, for decades. the carbon monoxide generated has forced the abandonment of the town. My question: how does the oxygen get underground to keep the reaction going? One would think such a fire would be self-extinguishing.
So how does the fire get oxygen?
Coincidentally I was wondering the other day, why they don’t use halon to put them out. http://www.h3rcleanagents.com/support_faq_2.htm
It can’t be used in extinguishers here because it would be lethal in confined spaces as it sucks all the oxygen out of the air. I would have thought that these underground fires (we have some too) would be a good place for it though.
Centralia is one of many, many coal seam fires around the world. For coal seams near the surface, the oxygen comes directly from the atmosphere. For fires deeper underground, it arrives via mine shafts. The oxygen can move through the cracks and porosity in the coal structures, allowing it to penetrate into depths that make firefighting efforts extremely difficult. It doesn’t take a whole lot of oxygen to keep the fire going because the heat is retained so well (due to insulation by many, many feet of surrounding soil); that’s why it smolders along slowly for decades instead of burning up in just a few years.
My understanding is that coal seam fires get oxygen from mine shafts and ventilation shafts.
The explanations I find say that “sucks the oxygen out of the air” is wrong, but they don’t exactly explain how halon works in a sensible way, instead offering meaningless statements like this (from the link you supplied) “its essential extinguishing technique lies in its capacity to chemically react with the fire’s components. It actually interrupts the chain reaction of fire”
I think it just displaces the air. It is a very heavy gas that sinks down and smothers the fire, so keeping the oxygen in the much lighter air away from it. To put out a fire, it does not need to react with the burning stuff, it needs to not react with it. After a while the halon will just mix into the air and diffuse away, but that takes time. Before that it is heavy enough to sink and smother.
It might work in principle with underground coal fires, but you would probably need a heck of a lot of it, and I imagine it is quite expensive stuff.
Short version (according to Wikipedia): halon consumes the available hydrogen atoms, interrupting the combustion reaction even when oxygen is still available. This is part of why a room can be “flooded” with halon to a level that extinguishes fires, but enough oxygen remains available to prevent occupants from being asphyxiated. OTOH, a fire suppression system that floods a room with CO2 requires occupants to leave the room ASAP lest they be asphyxiated, because it relies on lowering the oxygen concentration enough to snuff the fire reaction.
The fact that halon consumes hydrogen atoms suggests one possible reason it might not be suitable for coal fires, since there isn’t much hydrogen present: coal is mostly carbon.
The other reason is that in order to kill a fire, you have to interrupt the combustion reaction for long enough to allow the heat to dissipate. If you don’t do that, the fire will start again as soon as the extinguishing agent is dissipated. As mentioned above, underground coal seam fires are really good at retaining heat. In order to extinguish one, you need to interrupt the gas exchange process for extended periods of time, i.e. years. That being the case, blanketing it with a short-lived layer of halon or CO2 isn’t enough. You need to bury it with impermeable soil and/or inject water/mud into the seam. Fires that have been left alone for years are likely to be substantial in breadth and depth, so this can be a massive undertaking, and apparently the track record is kind of spotty.
For the coal seam fire near the surface, the the burning coal disappears as CO2, leaving the cavity, which collapses and causes subsidence… cracks…
So the volume of hole to plug is quite large… it would be quite an effort to try to fill it .
For a deep coal seam fire, the CO2 eventually cools ,and CO2 collects at the lower end… so the deep coal seam fire is self limiting.
The long running “burning mountain” occurs where the coal is near the surface.
No one has mentioned coal dust explosion which kills like a fuel air bomb (sucks in the oxygen) and is a real doozy. One such explosion killed 1,500 in a Chinese coal mine. Another in France killed 1,000.
“Sucks in the oxygen” is not a helpful description of how a fuel-air explosive mix works.
Any fuel-air blend offers the same group of destructive effects:
-blast/overpressure wave
-heat
-asphyxiation (the hot combustion products can expand/convect to kill those beyond the initial blast/heat radius)
The oxygen inside the (premixed) cloud of fuel and air is consumed, yes, but it does not “suck in oxygen” from outside of that reactant cloud.