Fire and CO2

[Dodom]

I have a silly question, for some reason I couldn't find a definitive answer on Google.

Does CO2 inhibit combustion aside from its ability to displace oxygen? If you had a room full of 20% oxygen and 80% CO2, would you be able to light a candle in it?

An atmosphere like that would inhibit cellular respiration and rapidly kill a person, but those processes are only refered to as combustion as a simplification, it involves multiple steps with lower energy differentials and is more sensitive to product inhibition.

[factotum]

Well, that kind of depends. In the 20% O2/80% CO2 example, CO2 is quite a bit denser than oxygen, so what will tend to form is a layer of oxygen at the top of the room with CO2 underneath it. If you had some sort of turbulence being generated to keep the gasses mixed throughout the room then I can't see any reason why the candle wouldn't light and stay burning, though. The reason CO2 fire extinguishers do a good job is because the cold, dense CO2 gas they contain tends to settle over the top of the fire and deprive it of oxygen, not because CO2 has any ability to put out fires by itself.

[Peelee]

Well, that kind of depends. In the 20% O2/80% CO2 example, CO2 is quite a bit denser than oxygen, so what will tend to form is a layer of oxygen at the top of the room with CO2 underneath it. If you had some sort of turbulence being generated to keep the gasses mixed throughout the room then I can't see any reason why the candle wouldn't light and stay burning, though.
Similarly, air is only ~18% Oxygen, IIRC, and is plenty agitated.

[Lord Torath]

Similarly, air is only ~18% Oxygen, IIRC, and is plenty agitated.True. But N2 and O2 have pretty similar densities. CO2 is about 50% denser than those two gasses: N2 is about 28 grams per mole, O2 is about 32 g/mol, and CO2 is 44 g/mol. And at the same temperature and pressure, a mole of any gas takes up the same volume as a mole of any other gas. So CO2 will tend to sink when mixed with N2 and O2.

Edit: Okay, 50% is an exaggeration. The overall point remains, though.

[Peelee]

True. But N2 and O2 have pretty similar densities. CO2 is about 50% denser than those two gasses: N2 is about 28 grams per mole, O2 is about 32 g/mol, and CO2 is 44 g/mol. And at the same temperature and pressure, a mole of any gas takes up the same volume as a mole of any other gas. So CO2 will tend to sink when mixed with N2 and O2.

Edit: Okay, 50% is an exaggeration. The overall point remains, though.

True, but largely irrelevant, which is why I ignored it. After all, were not exactly choking on all the CO2 down here at the bottom of the atmosphere. Like I said, plenty of agitation.

[Brother Oni]

That said, it depends on the fuel that's burning. A reactive metal fire (eg magnesium) can strip the oxygen out of the CO2 and keep on burning, although not as fiercely as before.

Nitrogen is also used to put out fires as part of fire suppression systems, so in these cases it's primarily the physical effects of the gas smothering and cutting off the oxygen from the fire that puts it out, rather than any innate property of nitrogen or carbon dioxide.

[wumpus]

If it does, it isn't significant. Halon is the only chemical I've heard of being used to inhibit fire by chemical action, and after it was more or less banned, CO2 certainly hasn't replaced it. I think CO2 simply has the advantages of being stored in liquid form at room temperature (low triple point), not being conductive, and boiling with or without direct contact with fire. I suspect it also works fine in grease fires.

[factotum]

I think CO2 simply has the advantages of being stored in liquid form at room temperature (low triple point), not being conductive, and boiling with or without direct contact with fire.

Plus the whole "denser than air so tends to sit on top of the flames rather than floating off into the atmosphere" thing.

[halfeye]

If it does, it isn't significant. Halon is the only chemical I've heard of being used to inhibit fire by chemical action, and after it was more or less banned, CO2 certainly hasn't replaced it. I think CO2 simply has the advantages of being stored in liquid form at room temperature (low triple point), not being conductive, and boiling with or without direct contact with fire. I suspect it also works fine in grease fires.

Liquid form? CO2? It's called dry ice for a reason.

It may be that it does something unexpected at high pressure, but I think CO2 is usually stored as the solid.

I remember seeing the stuff used by ice-cream vans back in the '50s or early '60s, my chemistry knowledge is almost non-existant, as I understood things the CO2 wasn't kept under pressure then, just insulated to keep the heat out.

[Peelee]

Liquid form? CO2? It's called dry ice for a reason.

It may be that it does something unexpected at high pressure, but I think CO2 is usually stored as the solid.

I remember seeing the stuff used by ice-cream vans back in the '50s or early '60s, my chemistry knowledge is almost non-existant, as I understood things the CO2 wasn't kept under pressure then, just insulated to keep the heat out.

I don't mean this condescendingly, but do you know what a triple point is? A phase diagram would probably make everything clear.

[gomipile]

Halon is the only chemical I've heard of being used to inhibit fire by chemical action,[...]
No, Halon works the same as CO2 in fire suppression. It was chosen because it is heavy and relatively unreactive.


Liquid form? CO2? It's called dry ice for a reason.

It may be that it does something unexpected at high pressure, but I think CO2 is usually stored as the solid.

I remember seeing the stuff used by ice-cream vans back in the '50s or early '60s, my chemistry knowledge is almost non-existant, as I understood things the CO2 wasn't kept under pressure then, just insulated to keep the heat out.

I guess you've never played paintball with CO2-powered markers, or used a CO2-powered airgun of any kind, or used CO2 in welding or on the back side of a soft drink fountain or beer tap system.

CO2 is liquid at normal human living temperatures if you confine it in a pressure vessel that can handle its liquid vapor pressure. Which is useful, as that pressure isn't terribly high. It's ess than 80 atmospheres for most livable temperatures. Which is a lot less than the 200 to 300 atmospheres used in high pressure air or nitrogen tanks. That's why CO2 tanks are relatively cheap.

Also, since it's stored in liquid form, one gets a lot more volume of output gas from a CO2 tank than from a similar size 300 bar HPA tank.

Liquid CO2 is also pretty inexpensive to buy, since the production process is cheap and simple.

So, cheap containment vessels, lots of output per tank volume, and cheap refills, plus some other qualities make CO2 a very useful propellant gas, when stored in liquid form.

[halfeye]

I don't mean this condescendingly, but do you know what a triple point is? A phase diagram would probably make everything clear.

Yeah, sort of, vaguely.

I had a look, Wikipedia had one. That was after I posted.


I guess you've never played paintball with CO2-powered markers, or used a CO2-powered airgun of any kind, or used CO2 in welding or on the back side of a soft drink fountain or beer tap system.

Yep, never done any of that. Did use propane/butane (was never very clear which if not a mix) for spraypainting.


CO2 is liquid at normal human living temperatures if you confine it in a pressure vessel that can handle its liquid vapor pressure. Which is useful, as that pressure isn't terribly high. It's ess than 80 atmospheres for most livable temperatures. Which is a lot less than the 200 to 300 atmospheres used in high pressure air or nitrogen tanks. That's why CO2 tanks are relatively cheap.

Also, since it's stored in liquid form, one gets a lot more volume of output gas from a CO2 tank than from a similar size 300 bar HPA tank.

It's the gas that has a huge volume, the tanks would work more or less the same if the contents were solid.


Liquid CO2 is also pretty inexpensive to buy, since the production process is cheap and simple.

So, cheap containment vessels, lots of output per tank volume, and cheap refills, plus some other qualities make CO2 a very useful propellant gas, when stored in liquid form.

[gomipile]

It's the gas that has a huge volume, the tanks would work more or less the same if the contents were solid.

But they're not, so they work they way I said they do, in practice.

[factotum]

It's the gas that has a huge volume, the tanks would work more or less the same if the contents were solid.

The tank might work the same, but actually getting the gas out would be a bit problematic--too much chance of still-frozen chunks of dry ice blocking the output, for a start. Liquids are better because they will always settle into the bottom of the tank and thus have virtually no chance of being sucked into the outflow, and even if they do they won't block it.

[DavidSh]

For those of you who regularly use pressurized CO2 tanks, are there any issues involved if they get above the critical temperature? It's only about 88 degrees F, or 31.1 degrees C. Is the step from liquid to supercritical fluid noticeable from outside the tank?

[Peelee]

Phase diagram for CO2, for anyone interested or for quick reference:

http://d2vlcm61l7u1fs.cloudfront.net/media%2F654%2F654abc63-5ebf-4f33-9507-c38acd719f38%2FphpAsNMSr.png

[Grey_Wolf_c]

That said, it depends on the fuel that's burning. A reactive metal fire (eg magnesium) can strip the oxygen out of the CO2 and keep on burning, although not as fiercely as before.

Yes, and if the fuel is Chlorine Trifluoride, it'll burn even if you cover it in ash, water or asbestos, but I feel is a bit like cheating to not assume a regular candle (yes, I know magnesium-infused wicks are a thing. I hated them when I was a kid).

Edit: bonus thing I just learnt: candle wax does NOT react with Chlorine Trifluoride, so you could technically make a candle with it as the fuel. Shortly followed by your permanent and irrevocable imprisonment, I'd hope.

GW

[monomer]

The tank might work the same, but actually getting the gas out would be a bit problematic--too much chance of still-frozen chunks of dry ice blocking the output, for a start. Liquids are better because they will always settle into the bottom of the tank and thus have virtually no chance of being sucked into the outflow, and even if they do they won't block it.

It wouldn't even work the same. To store the CO2 in it's solid form, you would need a cryogenic tank to keep it below the sublimation temperature. You could then bleed off small amounts of CO2 as it slowly heats up, but this probably wouldn't be good for putting out a fire.

You'd be better off with an insulated box of dry-ice, some gloves, and a good throwing arm.

[Peelee]

Yes, and if the fuel is Chlorine Trifluoride, it'll burn even if you cover it in ash, water or asbestos, but I feel is a bit like cheating to not assume a regular candle (yes, I know magnesium-infused wicks are a thing. I hated them when I was a kid).

Edit: bonus thing I just learnt: candle wax does NOT react with Chlorine Trifluoride, so you could technically make a candle with it as the fuel. Shortly followed by your permanent and irrevocable imprisonment, I'd hope.

GW

Hmmm.
[Searches wiki]
"Chlorine trifluoride*is an*interhalogen compound*with the formula ClF3. This colourless, poisonous, corrosive, and extremely reactive*gas*condenses to a pale-greenish yellow liquid, the form in which it is most often sold (pressurized at room temperature). The compound is primarily of interest as a component in*rocket fuels, in plasmaless cleaning and*etching*operations in the*semiconductor industry,[7][8][9]*in*nuclear reactor fuel processing,[10]and other industrial operations.[11]"

So what you're saying is this stuff is fun and I absolutely need it.

[Grey_Wolf_c]

So what you're saying is this stuff is fun and I absolutely need it.

You've never heard of Chlorine Trifluoride? Oh, what fun you shall have (http://blogs.sciencemag.org/pipeline/archives/2008/02/26/sand_wont_save_you_this_time).

GW

[Peelee]

You've never heard of Chlorine Trifluoride? Oh, what fun you shall have (http://blogs.sciencemag.org/pipeline/archives/2008/02/26/sand_wont_save_you_this_time).

GW

Note to self: find a copy of Ignition! I like that guy's writing style.

ETA: Huh. A few other books I have wishlisted are popping up in relation to that. Kind of surprised I never saw that one before.

[monomer]

Note to self: find a copy of Ignition! I like that guy's writing style.

ETA: Huh. A few other books I have wishlisted are popping up in relation to that. Kind of surprised I never saw that one before.

His similar article on FOOF (http://blogs.sciencemag.org/pipeline/archives/2010/02/23/things_i_wont_work_with_dioxygen_difluoride) is really good too.

[Peelee]

His similar article on FOOF (http://blogs.sciencemag.org/pipeline/archives/2010/02/23/things_i_wont_work_with_dioxygen_difluoride) is really good too.

I meant John Clark (author of Ignition! which is excerpted in the article). No slight against Derek Lowe, of course, but Clark's book is on the history of spaceflight, which I like reading about.

[factotum]

Ignition is a superb book, definitely worth a read. Long out of print, though, so finding a physical copy might be difficult.

[Peelee]

Ignition is a superb book, definitely worth a read. Long out of print, though, so finding a physical copy might be difficult.

22 bucks on Amazon, though I hear I can get a free PDF legally.

ETA: In any event, it'll wait until after I get Moon Lander: How We Developed the Apollo Lunar Module. I love the LEM, and that's next on my list to get and read (not counting Star Wars books and books I already own, of course).

[Brother Oni]

22 bucks on Amazon, though I hear I can get a free PDF legally.

I looked into this and obtained a copy. It's got a foreword by Isaac Asimov!

Definitely going to be reading this after laughing my arse off at the before and after picture of a jet engine test. :smallbiggrin:

[gomipile]

For those of you who regularly use pressurized CO2 tanks, are there any issues involved if they get above the critical temperature? It's only about 88 degrees F, or 31.1 degrees C. Is the step from liquid to supercritical fluid noticeable from outside the tank?

Not really noticeable, no. It can still be felt to slosh in the tank. I've never noticed a discontinuity of any type there. CO2 does work progressively "better" as a propellant as the temperature increases through that area, but I never noticed a break point.

[Knaight]

It's the gas that has a huge volume, the tanks would work more or less the same if the contents were solid.
There's two main ways to get the volume down though - you can freeze it, where long term storage involves long term refrigeration at very cold temperatures, or you can pressurize it, where long term storage involves the pressure vessel continuing to apply pressure. The former produces solid phase CO2, the latter liquid phase. The former also involves constant energy input and probably some pretty specialized refrigerants and large areas, while the latter involves making a pressure tank that can take the pressure, stuffing it, sealing it, and calling it a day.


For those of you who regularly use pressurized CO2 tanks, are there any issues involved if they get above the critical temperature? It's only about 88 degrees F, or 31.1 degrees C. Is the step from liquid to supercritical fluid noticeable from outside the tank?
No. The different properties of supercritical fluids here really don't matter - the tanks can hold gas phase material just fine, the tanks aren't soluble in CO2 even in supercritical fluid phase (which is generally better as a solvent), etc.


Not really noticeable, no. It can still be felt to slosh in the tank. I've never noticed a discontinuity of any type there. CO2 does work progressively "better" as a propellant as the temperature increases through that area, but I never noticed a break point.
That's usually true of supercritical fluids - even a dense gas can be felt to slosh, and the phase is basically the gas and liquid phases slowly merging into each other in a lot of ways, instead of something radically different.

[Peelee]

There's two main ways to get the volume down though - you can freeze it, where long term storage involves long term refrigeration at very cold temperatures, or you can pressurize it, where long term storage involves the pressure vessel continuing to apply pressure. The former produces solid phase CO2, the latter liquid phase. The former also involves constant energy input

Only if you're in a place that if already getting rather large constant energy input. Conversely, you could ship it off to Antarctica and not have to worry too much about energy requirements.

Of course, that might be a bit inconvenient for most applications. On an unrelated note, why aren't server farms built in the Yukon?

[factotum]

Of course, that might be a bit inconvenient for most applications. On an unrelated note, why aren't server farms built in the Yukon?

They are:

https://www.theglobeandmail.com/report-on-business/economy/canada-competes/why-cold-canada-is-becoming-a-hot-spot-for-data-centres/article6598555/

I believe Google has a major data centre in Finland for the same reasons. However, the further away from the people actually using the data the server farm is, the higher the ping times are, which limits performance--also, data centres require *stable* conditions, so you still have to control the internal climate even if it's 10 below outside.

[Knaight]

Only if you're in a place that if already getting rather large constant energy input. Conversely, you could ship it off to Antarctica and not have to worry too much about energy requirements.

Antarctica isn't nearly cold enough here - we need continual storage at -78.5 C or colder. Even the south pole only averages -49 C.

[Peelee]

Antarctica isn't nearly cold enough here - we need continual storage at -78.5 C or colder. Even the south pole only averages -49 C.

Cooling it by 30K is easier than cooling it by 90K. Ignoring all other inherent problems, of course.

[Knaight]

Cooling it by 30K is easier than cooling it by 90K. Ignoring all other inherent problems, of course.

Sure, but it's still a process that requires constant energy expenditure. Meanwhile pressure vessels are a one and done affair - it's just easier storage.

There's a reason that dry ice is generally used to cool things, and not just as a storage system for carbon dioxide.