[David Cooper (OP)]

I were watching the Christmas Lectures this week, and during one of them I were a thinkin', if you can put out one o 'em flamin' things with a petrol soaked rag at the end o' it just by stickin' it in a room with 15% oxygen in it instead of the normal 21%, would that not be a fine way to put out a fire? You could flood the building with nitrogen with just enough oxygen added to make sure you aren't about to go killin' anyone in the building, but all the fires would stop in an instant.

So, I been a wonderin', do thems do that already when they is a fightin' fires, or be this a new idea?

[CliffordK]

I'm not sure if your calculations are correct.  An exhaled breath is about 15%, and one can certainly blow on a fire to make it burn better, at least making the coals hotter.  It may depend on what is burning.  Most gases have a LEL and UEL, with a concentration above or below which they don't burn, or at least don't explode.  So, changing the oxygen concentration in say a bottle full of methane would make a big difference.

CO₂ is often used in fire extinguishers, especially in rooms where water could significantly damage equipment.  The notes I'm seeing is that CO₂ is often chosen because it is heavier than air, and thus more likely low where the fire may be. 

I suppose if one is expecting the fire to be in equipment, and walls, then N₂ might be as effective.

Argon would also work, and would be non toxic, but it would be significantly more expensive.

[evan_au]

The opposite is also true - there are a surprising number of materials which we think of normally as non-flammable which will sustain a flame if the oxygen level is raised even 5% above the normal 21%.

This is sometimes described as the limiting oxygen index - some examples here for textiles: http://www.textilesfr.co.uk/FibresFlammability.html

There is a problem with the solid/liquid/gas phases of Nitrogen as a fire extinguisher - once the pipes cool down, very cold (but boiling) liquid nitrogen would spray out of the nozzles, causing frost burns to any humans in the area, and stressing any equipment. On the other hand, as soon as the pressure of CO₂ drops below about 5 atmospheres of pressure, it comes out as a (not nearly so cold) gas & solid flakes, which is much less dangerous than liquid N₂.

It is hard to ensure that you get a uniform concentration of a gas in a fire, so it is best to saturate it (with water, foam, CO2 or whatever).
However, Nitrogen N₂ is surprisingly deadly to humans - not because it is toxic, but because we have no protection against it. Unlike CO₂, which induces yawning and choking and an overwhelming urge to get out of there, N₂ allows you to keep breathing happily until you collapse unconscious and then die.

So, while N₂ would be excellent at extinguishing fires, it would also tend to extinguish human lives that would be saved if something like C0₂ were used instead.

[David Cooper (OP)]

Ah, but what thems clever folks did in the lecture was to try to set fire to things in a room with only 15% oxygen, and they wasn't able to do it. When they took things in that was already aflame, they all gets extinguished, thems do, but them scientists theyselves was breathing away like normal and wasn't droppin' dead at all. So, what I was a thinkin' is that if you was to pump enough nitrogen into a building to replace a bit more 'an about quarter of the air, that would reduce the oxygen to about 15% and kill the fire while leavin' any folks inside with enough oxygen to function fine. Now, maybees there be a problem with temperature drop freezin' things, but I reckon that would depend on how compressed it were to start with. I particularly likes the idea of having a smoke detector phone me when I is away on holiday so that I can takes a look through a webcam and click a button to fill the house with nitrogen if I spots a fire.

[CliffordK]

It might be an important part of fighting fires in large buildings, especially if the Nitrogen/Oxygen levels could be computer controlled.  Not too high, not too low. 

I would wonder if, however, there was a tendency to breach windows and doors and suck air from other parts of the building, thus making oxygen/nitrogen levels difficult to precisely regulate.  And, of course, infrastructure like computer systems often do poorly in fires.

It might depend a bit on how large and involved the fire is before the suppression system is engaged. 

[chris]

I would think the air currents created by a large fire would make maintaining and supplying sufficient concentrations of nitrogen almost impossible.

[David Cooper (OP)]

It would certainly be too late once the windows have started shattering, but if you sent in the nitrogen as soon as a smoke detector goes off, that could stop a fire in its tracks and do so without damaging anything. I was picturing this as an alternative to a sprinkler system where the water would likely wreck just about everything in the room/house/building. The supply of nitrogen could also be shared between many houses/buildings to keep the storage costs down, and it could contain just enough oxygen to ensure it doesn't kill people even if it completely floods the space they're in.

[Bored chemist]

"Could nitrogen be used to fight fires?"

It can: they do.
http://www.wi-ltd.com/fire/Suppression_and_Extinguishing_Systems/Gas_Extinguishing/Nitrogen_Inert_Gas_Extinguishing_System

Other systems are available.

[David Cooper (OP)]

Thanks for hunting that out - that's exactly what I was hoping could be done. I tried to find out the cost, but they aren't keen to reveal it, so I don't know how many houses would need to share this system to make it affordable for domestic use. I wonder if there's some other way you could reduce the oxygen levels in your house when you're away for a long time, perhaps with some device continually removing oxygen whenever the level goes over 15%.

[SeanB]

Not cheap, but the only thing left that does not damage computers since Halons were banned. Often it is a blend of high pressure nitrogen, a lot of argon and a small amount of helium as a leak detecting agent ( helium finds small leaks quite effectively and can be easily detected electronically. the volume is calculated to flood the room and drop oxygen to below 10% fast, while the extremely loud and close to hearing damaging level sirens and flashing lights that occur for 30-60 seconds before release are enough to alert even those deeply engrossed in a game to the forthcoming violent air displacement.

[Lab Rat]

There would be one main problem with replacing sprinklers with nitrogen jets-even if the oxygen level in the room is above 15%, if you are standing under that jet, you will probably lose consciousness.  It happens even at normal oxygen levels.  There was a case where a guy was leaning over a vat of liquid nitrogen at factory.  For some reason, a cloud of nitrogen gas shot up out of the vat, he breathed it in, passed out, and fell into the vat. There should be no need for further elaboration.

[Bored chemist]

Fortunately the designers of computer rooms realised this and so they didn't install vats of liquid nitrogen (or anything else) in the middles for people to land in if they fell unconscious.

It is true that nitrogen is a lot more dangerous than most people give it credit for.
On the other hand, so is fire.

[Lab Rat]

Ah, but there is an even greater danger in computer rooms than open vats of death...cords!!! (cue scary music)  Sure to wrap around your ankles like an anaconda and send you hurtling to the ground face first, no matter what you are carrying or how careful you are.  That, I would have to say is the real danger! []