You may be aware that occasionally a fire starts in a coal seam that gets out of control and burns for years or even decades. Several are currently burning in the U.S.A. and some have been burning since the 1960s.
They are tough to put out because they are underground and any tunnels are filled with toxic fumes. They manage to keep themselves supplied with oxygen by opening up cracks in the earth leading to the surface.
Still, some recent successes have been reported -- see link -- and yet I still wonder why this particular Idea has never been implemented.
Pour Liquid Nitrogen down those cracks! LN2 is pretty cheap to make, and of course it automatically brings to a fire two of its enemies: It displaces oxygen and it is C-O-L-L-L-L-D!
Well?-- Vernon, Apr 13 2005 Fire fighting foam (nitrogen based) http://www.aquafoam.com/MineFires.htmlA couple successes are described. [Vernon, Apr 13 2005] or cement over the cracks.-- po, Apr 13 2005 You have to cover a wide area and also go deep. There is no real fire here, just a high temperature like in your barbeque. This idea is like throwing an ice cube on the bbq.-- zeno, Apr 13 2005 I'm with [po]. Cement over the hige square mileage of ground above the mine to make sure all cracks are covered, and then wait. Eventually the fire will run short of oxygen, and convert all the coal it can find into charcoal. Extract, and take to [zeno]'s barbeque.-- david_scothern, Apr 13 2005 Is this really a [Vernon] idea? I can fit it all on a single screen.-- AbsintheWithoutLeave, Apr 13 2005 Thanks for the typo tip. Fixed.-- Vernon, Apr 13 2005 [zeno], in order for the high temp. to be maintained there must be some combustion going on. If you could get the nitrogent to hang around for long enough then I don't see why it couldn't work in principle.-- moomintroll, Apr 13 2005 These areas are really, really huge. I read 20 square miles for one ohio fire. That is a lot of nitrogen. Perhaps a coal power nitrogen extractor could be perched atop to run night and day, pouring cold nitrogen down the hole.-- bungston, Apr 13 2005 What if these fires are the result of higherto unknown fissures that conduct heat from deep in the Earth's crust? Attempts to extinguish these would be futile.-- reensure, Apr 13 2005 On reading about them, it seems that there is fossil evidence of ancient coal fires that predate humans. Also anccient evidence of spontaneous thermonuclear reactions, deep in the earth.-- bungston, Apr 14 2005 Will liquid nitrogen displace oxygen? I thought the specific gravity of nitrogen was lower than oxygen.
Maybe the nitrogen doesn't even need to be liquid?
Maybe I misunderstand the whole specific gravity thing.-- bristolz, Apr 14 2005 [bungston], for a large fire, it can probably be put out in sections.
[reensure] and [UnaBubba], most of these fires were accidently started by people. Plain ordinary coal is burning.
[bristolz] Liquid nitrogen is heavier than air, so obviouly it can be poured down a hole to where a fire burns. The heat will boil the liquid and create over a thousand times the volume of gas, all of it nitrogen. This expanding gas will push aside all the other gases present, including oxygen. After a time, though, the various gases will start to mix again. The key here is that the COLD liquid nitrogen will extinquish a big chunk of fire, that can only re-start slowly, as heat from neighboring regions and oxygen GRADUALLY gets back to the extinguished region. By the time that happens, though, another batch of LN2 should have been poured down upon at least one of those neighboring regions....-- Vernon, Apr 14 2005 [moomintroll], there is not combustion all the time all over the "fire" area. The hot patches are extremely well isolated. I don't know if that's the right word for it I mean like in those things you keep your coffee warm in all day long you know? So after a considerable amount of time, when new air comes through, the fire is rekindled.-- zeno, Apr 14 2005 Thermal shock may be an issue. when you rapidly cool the rock, it may shatter, resulting in collapse and possily sinkholes to open up on the surface.-- oneoffdave, Apr 14 2005 Couldn't you get away with water instead? That *must* be cheaper than liquid nitrogen, and is generally available in larger quantities. Or you could try covering the whole area with clingfilm.-- Loris, Sep 30 2005 [Loris], water and hot coal can react chemically. producing carbon monoxide, adding MORE toxic fumes to what already comes out of the ground. I think that if water worked well, some of those fires would already have been extinguished decades ago.-- Vernon, Oct 01 2005 I have been thinking about that hot coal. Some of these fires have been going for years. I bet there is enormous amounts of thermal energy stored down there - not just the coal, but thew rocks, dirt etc. The nitrogen may well cut off O2 for combustion but I cannot imagine it will be enough to cool off cubic miles of hot rocks and coal.
I bet that coal, like oily rags, will spontaneously begin to burn when hot enough. That means that when the nitrogen goes away, hot beds will reignite.
Water has more promise for displacing O2 as well as cooling. As regards carbon monoxide, I imagine that these fires, choked for oxygen as they are, already generate loads of CO.-- bungston, Oct 02 2005 Putting out a coal mine fire would require adding a huge of liquid nitrogen over a long period of time. I don't really see that as practical.
On the other hand, I've sometimes wondered whether one might instead try a different tack: build a Sterling-cycle engine which would capture air coming out of the mine, burn any carbon monoxide in it, harness the heat (from the original and secondary combustion), and then pump the O2-free air back into the mine. I don't know if that would extinguish the fire in any useful time frame, but if the air going into the mine was free of oxygen it shouldn't hurt, but it should supply a significant amount of energy for other purposes.
Merely trying to suffocate a coal mine fire wouldn't be very effective because the super-heated coal will remain hot essentially indefinitely if there isn't something to cool it down. Forced circulation of oxygen-free air, however, should be able to cool things much more quickly.-- supercat, Nov 18 2007 Why not sink pipes and use it as a sort of geothermal energy source?-- MaxwellBuchanan, Nov 18 2007 Good idea, [MaxB]. Geothermal energy extraction would cool the fires, and maybe put them out someday. Or, just use the geothermal holes to introduce and control oxygen input, and keep it all going.
Why are we digging up coal anyhow? Why don't we just burn it underground?
Back on topic: Liquid CO2 has less cold in it than liquid nitrogen, so to speak, but it's easier to make and to handle. Gaseous CO2 is heavier than air, so it'd stay down there better. It might convert to carbon monoxide, though--gotta look that up.-- baconbrain, Nov 18 2007 Are there any estimates of the rate of burning (in tons-per-year) of these fires? It seems like it ought to be putoutable, but on the other hand we may be out by several orders of reality. The "20 square miles" quoted above is a bit intimidating. A woman might not piss it out.-- MaxwellBuchanan, Nov 18 2007 From what I understand, a major problem with extinguishing coal mine fires is that they generate a huge amount of heat; restricting air flow won't do anything to cool down the endless tons of rock that have already been heated to ignition temperature. The amount of inert gas required to extinguish a coal mine fire would be way too big to be trucked in. On the other hand, such such fires generate huge quantities of carbon dioxide, I would think one could use that. It would have to be cooled first, but as noted that would provide a useful energy source.-- supercat, Nov 19 2007 I think that Augean stables solution is the only one for these coal fires.-- bungston, Nov 19 2007 random, halfbakery