How "cold" can fire be?

[BrunoPuntzJones]

I mean yeah fire can pretty hot...

But is there any sort of minimum temperature where combustion won't occur? I'm sure it depends on the fuel and oxidizer.

But say would a match light fine in a room -200F ?

Ethyl alchohol burns almost clear and is relatively cool to the touch compared to say the surface of the sun (assuming the surface of the sun is hot, I haven't touched it).

There any sort of combo where you could have a flame cool enough to touch?

 

[IEC]

Originally posted by: BrunoPuntzJones
I mean yeah fire can pretty hot...

But is there any sort of minimum temperature where combustion won't occur? I'm sure it depends on the fuel and oxidizer.

But say would a match light fine in a room -200F ?

Ethyl alchohol burns almost clear and is relatively cool to the touch compared to say the surface of the sun (assuming the surface of the sun is hot, I haven't touched it).

There any sort of combo where you could have a flame cool enough to touch?

I suppose you could make a fire in the Arctic circle in mid-winter and find out... you can throw boiling water into the air and have it turn into ice before it hits the ground.

As for the sun - plasma != fire. The sun is basically a huge fusion reaction. The thermal energy given off by the nuclear reaction is much more powerful than the chemical action of burning. You wouldn't be able to get near the sun without being fried or irradiated to death.

 

[f95toli]

Fire is essentially just a very "violent" chemical redox reaction and the heat of the flame essentially just depends on the amount of energy released; it has nothing to do with the ambient temperature.
You could easily light a match at -200F which is after all only about 140K, well above the temperature were e.g. oxygen becomes a liquid.

 

[SuperFungus]

First off "I mean yea fire can pretty hot...", you might want to work on that.

Now that that?s out of the way, I found a useful definition of fire here http://www.straightdope.com/columns/021122.html. If that site is accurate, i would guess that to answer the question about the minimum temperature for "flame" you would need to know whether liquid or solid oxygen can combust, and if it does if it still gives of the "incandesant gas" we know as flame (maybe it would just asplode?).

As for combustion, he says that rusting is combustion (Google defines combustion lots of different ways, this one seems to support him: "Chemical oxidation accompanied by the generation of light and heat") So, I kind of doubt there is a minimum temperature for combustion (above 0 K), although it would be an interesting experiment. You could put several pieces of iron in water, one in the fridge, one in the freezer, one inside and one outside and see which rusts faster. If the colder ones rusted slower I guess that might indicate a minimum necessary temperature for combustion. It'd also be interesting to know if an object really emits light as it rusts. But I digress. As for a relatively cool flame, well all the flames I?ve come into contact with are relatively cool compared to the sun (which isn't a big ball of gasoline burning by the way, you can tell because we don't have any troops there) but I would guess that in order for the gas to be combusting fast enough to emit visible light it would be releasing enough energy to be too hot to touch but maybe there's an infrared flame that?s cool enough to touch? Well I guess it should be apparent that I?m no expert but I thought it was an interesting question. I count on and welcome the fact that someone much more intelligent than myself will come along to point out all the errors in my understanding.

 

[Kyanzes]

Originally posted by: SuperFungus
compared to the sun (which isn't a big ball of gasoline burning by the way, you can tell because we don't have any troops there)

 

[Mark R]

I'd guess that the lowest temperature that could support combustion is that where oxygen ceases to be a fluid.

Things can burn quite nicely in liquid oxygen - in fact, liquid oxygen supports combusion extremely strongly, and the actual temperature of the flame is often exceedingly high.

You may not have done this experiment at school, but if you get a diamond heat it up to red heat and drop it in a bowl of liquid oxygen it will burn very nicely - with a bright yellow flame.

 

[OrganizedChaos]

Originally posted by: Mark R
I'd guess that the lowest temperature that could support combustion is that where oxygen ceases to be a fluid.

Things can burn quite nicely in liquid oxygen - in fact, liquid oxygen supports combusion extremely strongly, and the actual temperature of the flame is often exceedingly high.

You may not have done this experiment at school, but if you get a diamond heat it up to red heat and drop it in a bowl of liquid oxygen it will burn very nicely - with a bright yellow flame.

an open flame and a bowl of LOX? i must be missing something here because this sounds like a Bad IdeaTM.

 

[Jeff7]

Depends what's burning. I assume that there could be something with an extremely low ignition temperature, but it doesn't exist on Earth in its combustible form. I guess any reaction that is exothermic could produce fire in a cold enough environment.
Go out to Saturn's moon Titan. Really cold there. I'm sure you could find something that'd be quite thoroughly exothermic there, that'd seem like fire when compared to the frigid surroundings.

 

[oynaz]

Originally posted by: OrganizedChaos


an open flame and a bowl of LOX? i must be missing something here because this sounds like a Bad IdeaTM.

Boooooommmm!!!

 

[Kyanzes]

So what's the lowest possible temperature for an open flame (at sea level here on Earth assuming you could use any kind of material)? I'm asking the temperature of the flame itself.

 

[Gibsons]

Originally posted by: Mark R
I'd guess that the lowest temperature that could support combustion is that where oxygen ceases to be a fluid.

Things can burn quite nicely in liquid oxygen - in fact, liquid oxygen supports combusion extremely strongly, and the actual temperature of the flame is often exceedingly high.

You may not have done this experiment at school, but if you get a diamond heat it up to red heat and drop it in a bowl of liquid oxygen it will burn very nicely - with a bright yellow flame.

Doesn't the hot diamond evaporate the liquid so that it's really gaseous oxygen that's supporting the combustion? Never done the experiment myself.

 

[BrunoPuntzJones]

Originally posted by: Kyanzes
So what's the lowest possible temperature for an open flame (at sea level here on Earth assuming you could use any kind of material)? I'm asking the temperature of the flame itself.

Yeah this is more of what I was leaning towards.

I know the sun isn't "on fire" like I said

But would be neat if there was some fuel that burned at a very low temp. Could douse yourself in it and run around on fire, but not actually burn...which I know isn't a realistic idea, but was what piqued my curiosity, since rubbing alcohol burns so cool compared to say nitro methane.

 

[Paperdoc]

First of all, to a Chemist, Oxidation covers a huge area and it does NOT always genereate heat and light. In fact, most oxidations (like rusting) do not. (Well, actually, most do generate heat, but some so small we don't notice). And for all those reactions, there really is no practical lower limit in temperature (above absolute zero, that is) that will stop it. The reaction rate, though depends on the temperature, so that it is much slower when colder - just does not stop altogether.

Now, the specific classes of oxidations that we call flames are just those that proceed so rapidly and emit so much energy in a broad range that some of it is in the heat frequencies (infrared) and some in the visible light fequencies. In virtually all such situations, if there is enough energy output in the visible light region for us to see a flame, there us bound to be much more energy output in the adjacent lower-frequency range known as infrared - HEAT! So if you see a flame, you are virtually guaranteed a lot of heat. That is because the rapid free-radical reactions in these systems produce huge numbers of atoms and molecules at high and unstable energy states - and many such states - which release that excess energy by radiating it out at many frequencies (because there are so many energy states in total). The higher energy emissions are what we see as visible light. But that usually leaves a molecule at a lower but still unstable energy state, and it emits more energy again, only this time at lower frequency in the infrared region. So, bottom line, anything we can see a flame on is going to produce enough heat we would call the flame "hot", certainly to our skin!

So, is there is minimum temperature below which combustion can occur? No. Hypothetically, for some particular systems the combusion might begin and then stop. You see, combustion is a chain reaction - to keep going it needs some of the energy being emitted by the burning molecules to get the next ones going. And if you can remove that emitted energy (much of it in the form of excited free radicals) very fast, you might stop it. That is exactly how fires are put out with water. The water added absorbs huge amounts of heat energy and traps free radicals without producing any replacement free radicals to keep the chain going, and the reaction is stopped. But for most practical systems just having the initial flame in a cold chamber won't do that. The cold chamber is a place for infrared emitted energy to flow, but that still does not stop the free radicals that are the key propogators of the chain reactions.

 

[BrunoPuntzJones]

Thanks Paperdoc, was exactly what I was looking for. And makes sense that if you can see it, it's gonna be hot!

 

[Smilin]

Originally posted by: oynaz

Originally posted by: OrganizedChaos


an open flame and a bowl of LOX? i must be missing something here because this sounds like a Bad IdeaTM.

Boooooommmm!!!

mmm, doesn't quite work that way. That "bad idea" part is quite correct though.

An open flame an bowl of LOX would not explode by themselves. You would still need something to burn....given it's holding LOX the bowl itself might make a candidate..depends on what it's made of

As an FYI: Try exposing a petroleum product to pure oxygen... There is no flame or spark required. It will spontaneously ignite.


When I was in the Navy we almost had a fight break out on the flightline one time because an AOE came out and swapped a LOX bottle while we were in the middle of a refueling operation. The concensus was a fat lip may have helped him where his brain was lacking.

 

[Smilin]

Originally posted by: Gibsons

Originally posted by: Mark R
I'd guess that the lowest temperature that could support combustion is that where oxygen ceases to be a fluid.

Things can burn quite nicely in liquid oxygen - in fact, liquid oxygen supports combusion extremely strongly, and the actual temperature of the flame is often exceedingly high.

You may not have done this experiment at school, but if you get a diamond heat it up to red heat and drop it in a bowl of liquid oxygen it will burn very nicely - with a bright yellow flame.

Doesn't the hot diamond evaporate the liquid so that it's really gaseous oxygen that's supporting the combustion? Never done the experiment myself.

Heh, I can't say I have any "spare" diamonds laying around either.

It probably is gaseous when it reacts though... not sure if anyone else has done this but you can actually stick your hand into liquid nitrogen (briefly! In-and-out) and won't suffer any side effects. The heat of your hand is enough to cause the nitrogen to instantly evaporate leaving your hand protected by a thin "glove" of gaseous nitrogen. The gas is a poor enough conductor that you are safe from the cold for a very short period.

It feels really wierd. You can feel the added weight of having your hand in a liquid but it otherwise just feels like sticking your hand in and out of the cold air of a home freezer.

 

[Gibsons]

Originally posted by: Smilin

Originally posted by: Gibsons

Originally posted by: Mark R
I'd guess that the lowest temperature that could support combustion is that where oxygen ceases to be a fluid.

Things can burn quite nicely in liquid oxygen - in fact, liquid oxygen supports combusion extremely strongly, and the actual temperature of the flame is often exceedingly high.

You may not have done this experiment at school, but if you get a diamond heat it up to red heat and drop it in a bowl of liquid oxygen it will burn very nicely - with a bright yellow flame.

Doesn't the hot diamond evaporate the liquid so that it's really gaseous oxygen that's supporting the combustion? Never done the experiment myself.

Heh, I can't say I have any "spare" diamonds laying around either.

It probably is gaseous when it reacts though... not sure if anyone else has done this but you can actually stick your hand into liquid nitrogen (briefly! In-and-out) and won't suffer any side effects. The heat of your hand is enough to cause the nitrogen to instantly evaporate leaving your hand protected by a thin "glove" of gaseous nitrogen. The gas is a poor enough conductor that you are safe from the cold for a very short period.

It feels really wierd. You can feel the added weight of having your hand in a liquid but it otherwise just feels like sticking your hand in and out of the cold air of a home freezer.

And then you can throw at peoples feet and watch them jump. Always freaks the newbies out.

 

[BrunoPuntzJones]

Originally posted by: Smilin

Originally posted by: oynaz

Originally posted by: OrganizedChaos


an open flame and a bowl of LOX? i must be missing something here because this sounds like a Bad IdeaTM.

Boooooommmm!!!

mmm, doesn't quite work that way. That "bad idea" part is quite correct though.

An open flame an bowl of LOX would not explode by themselves. You would still need something to burn....given it's holding LOX the bowl itself might make a candidate..depends on what it's made of

As an FYI: Try exposing a petroleum product to pure oxygen... There is no flame or spark required. It will spontaneously ignite.


When I was in the Navy we almost had a fight break out on the flightline one time because an AOE came out and swapped a LOX bottle while we were in the middle of a refueling operation. The concensus was a fat lip may have helped him where his brain was lacking.

Doesn't phosphorus (or some derivative of) ignite spontaneously at about 80F or so? White phosphorus? Made lots of smoke and fire in Vietname without the need of an ignition system IIRC

 

[homonculus]

Phosphorous will simply burn in the presence of air, this is why you store it submerged in oil. Gets really hot too, much higher than 80 F!

 

[glugglug]

Binaca breath spray burns endothermically.

We used to coat the walls of our dorm room with it and light it for a cool looking fascade in videos.

And a normal candle burns cool enough you can hold your fingers in the flame without getting burnt as long as you have good circulation to carry the heat away.

 

[SuperFungus]

Originally posted by: Paperdoc
Now, the specific classes of oxidations that we call flames are just those that proceed so rapidly and emit so much energy in a broad range that some of it is in the heat frequencies (infrared) and some in the visible light fequencies.

Is it not also necessary to say that in order for visible flames to form the oxidation must take place in a gas? As i understood it, the flame is a clould of oxidizing gas. I know that some metals (like sodium) oxidize very quickly but no flames form. Are these oxidatons just too slow or is it related to the metal being a solid as it is oxidized and not a gas?

Originally posted by: Paperdoc
In virtually all such situations, if there is enough energy output in the visible light region for us to see a flame, there us bound to be much more energy output in the adjacent lower-frequency range known as infrared - HEAT! So if you see a flame, you are virtually guaranteed a lot of heat.

Wow and I was just guessing! Thanks for the confirmation.

 

[glugglug]

Originally posted by: Paperdoc
In virtually all such situations, if there is enough energy output in the visible light region for us to see a flame, there us bound to be much more energy output in the adjacent lower-frequency range known as infrared - HEAT! So if you see a flame, you are virtually guaranteed a lot of heat.

Lighting Binaca while spraying it produces a visible flame, like a blowtorch, yet it is COOL to the touch.

 

[Smilin]

Originally posted by: glugglug

Originally posted by: Paperdoc
In virtually all such situations, if there is enough energy output in the visible light region for us to see a flame, there us bound to be much more energy output in the adjacent lower-frequency range known as infrared - HEAT! So if you see a flame, you are virtually guaranteed a lot of heat.

Lighting Binaca while spraying it produces a visible flame, like a blowtorch, yet it is COOL to the touch.

What is actually in the spray that is burning? I thought it just had a fair amount of alcohol (which would burn)

 

[Mark R]

Originally posted by: Smilin
What is actually in the spray that is burning? I thought it just had a fair amount of alcohol (which would burn)

It's mainly alcohol - that's what's burning.

The burning is certainly not endothermic - although, the heat transfer to the surface is limited because the alcohol itself evaporates, so you lose the latent heat. Same thing with the reaction - a lot of water is produced in combustion of alcohol, so a lot of heat gets carried off in the latent heat of the steam. (This is why acetylene is used for cutting torches - maximum carbon, minimum hydrogen, and why hydrogen is useless for torches).

 

[silverpig]

If by 'fire' you mean flame, then you need to be hot enough to cause the air to glow. And that's hot. The air glows because it's roughly a blackbody and radiates thermal energy.