Understanding fire

wegs

wegs

Matter and Pixie Dust
Valued Senior Member
So, I'm under the impression that fire can range in temperature from 900 to over 5000 degrees Fahrenheit. Is this because of the oxygen content? But, oxygen by itself, is not flammable, right? It just ''supports'' the process. What exactly determines the elevation in temperature?

And without gravity, there would be no fires at all? So, would there be no risk of fire if say we were aboard an aircraft traveling in space? If I were in said air craft, while traveling in space, and had one of those fire starter “sticks,” would it not produce a flame?

I was lighting candles earlier today, and these questions just popped into my mind.
 
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wegs said:
So, I'm under the impression that fire can range in temperature from 900 to over 7000 degrees Fahrenheit. Is this because of the oxygen content? But, oxygen by itself, is not flammable, right? It just ''supports'' the process. What exactly determines the elevation in temperature?

And without gravity, there would be no fires at all? So, would there be no risk of fire if say we were aboard an aircraft traveling in space?

I was lighting candles earlier today, and these questions just popped into my mind.
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Fire is really, really complicated. I remember having to write an essay on the chemistry of flames when I was at university and I spent hours in the science library reading papers on them, without really finding any good way to summarise some essential principles. Fire is of course an oxidation reaction that proceeds vigorously and emits light as well as heat. (You can also get so-called cool flames, which emit very little heat and have a temperature as low as 200C or so.) Flames involve hideously involved branched-chain free-radical reactions that are hard to describe simply. Some of the steps in the chain reaction generate fragments of molecules in electronically excited states, which emit light when they drop down to the ground state, hence creating the glow of the flame. Others simply release heat.

The temperature reached depends on two things: the amount of energy released per unit material reacting and the rate at which it reacts. Both contribute to the intensity of the heat release and hence the temperature. The rate can be greatly affected by how intimately the fuel and oxygen are mixed. For example in a Bunsen burner you can have a fairly cool yellow flame if the air holes are closed, because oxygen can only get to the gas from the edge of the flame, so it burns slowly from the outside inward. The yellow is the glowing of incompletely burnt particles of carbon. But if you open the air holes, the gas and air mix inside the tube, combustion is immediate and complete and the flame is then blue, intense and far hotter.

This idea that there would be no fires without gravity is interesting. It's true that fires rely on convection to carry upward the burning flame, leaving access to fresh air at the bottom to continue the process. In a weightless environment you would not get that, so I suppose the burning would not be so orderly and would be slower. But I've never really thought about it.
 
wegs said:
So, I'm under the impression that fire can range in temperature from 900 to over 5000 degrees Fahrenheit. Is this because of the oxygen content? But, oxygen by itself, is not flammable, right? It just ''supports'' the process. What exactly determines the elevation in temperature?

And without gravity, there would be no fires at all? So, would there be no risk of fire if say we were aboard an aircraft traveling in space? If I were in said air craft, while traveling in space, and had one of those fire starter “sticks,” would it not produce a flame?

I was lighting candles earlier today, and these questions just popped into my mind.
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The temperature of the fire is dependent on the molecular makeup of the fuel.

Without gravity a convection fire would quickly extinguish itself. You could still have a fire via radiation or conduction (I think).
 
wegs said:
But, oxygen by itself, is not flammable, right? It just ''supports'' the process. What exactly determines the elevation in temperature?
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'Oxygen is not flammable' in the same way 'water cannot become soaked'.
Water is the thing that does the soaking.
Oxygen is the thing that does the burning.

All fires, with a few exceptions, are the process of oxidation - the combining of oxygen with (almost any) other substance.

As to heat of fires:

The more oxygen you add to a fire (iow, the faster you add it), the faster it can oxidize the fuel - and that results in a rapid rise in heat.

Put another way, a low but unlimited supply of oxygen will eventually reduce all the fuel to oxidized by products - it will just take a lot longer. All told though, it will - in theory - produce the same total heat.

wegs said:
And without gravity, there would be no fires at all? So, would there be no risk of fire if say we were aboard an aircraft traveling in space? If I were in said air craft, while traveling in space,
and had one of those fire starter “sticks,” would it not produce a flame?
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This is what it does in microgravity (ISS). Without convection, which normally requires gravity, the flame can't burn very well.

fire-space.jpg
 
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exchemist said:
The temperature reached depends on two things: the amount of energy released per unit material reacting and the rate at which it reacts. Both contribute to the intensity of the heat release and hence the temperature. The rate can be greatly affected by how intimately the fuel and oxygen are mixed. For example in a Bunsen burner you can have a fairly cool yellow flame if the air holes are closed, because oxygen can only get to the gas from the edge of the flame, so it burns slowly from the outside inward. The yellow is the glowing of incompletely burnt particles of carbon. But if you open the air holes, the gas and air mix inside the tube, combustion is immediate and complete and the flame is then blue, intense and far hotter.
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Thanks for chiming in, exchemist. Okay, I'm going to offer a very elementary example as I'm approaching this as if I know nothing at all about fire.

So, when you talk about the 'rate' at which material 'reacts,' would that be similar to describing the heat we generate if we are running as fast as we can, as opposed to walking slowly? If we're walking slowly, the 'rate' would be slower as to generate heat in our bodies say to burn calories, but if we are running as fast as we possibly can, then the rate would be accelerated, burning more calories. Is this too simplistic?

This idea that there would be no fires without gravity is interesting. It's true that fires rely on convection to carry upward the burning flame, leaving access to fresh air at the bottom to continue the process. In a weightless environment you would not get that, so I suppose the burning would not be so orderly and would be slower. But I've never really thought about it.
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And strangely enough, the hottest part of a fire is at its base, even though heat rises...which is why we can wave our hand over a candle flame for example, and it doesn't burn us. So, the ''flame'' is really just the heat ''diffusing'' and the base is where the reaction is actually happening.
 
Oxidation of a fire may seem different then oxidation of rust, fruit rotting, and burning fat, all of which don't require extremely high temperatures. Oxygen appears to release the energy from what's burning. Without Oxygen an object would just get really hot if you applied energy to it. So oxygen eats away at whatever is burning the same way it eats away at iron and fruit and fat. It just does it at a higher temperature and a lot faster.

Oxygen is attracted to or easily bonds with energy in fire, fruit and fat, but also bonds easily with iron which is close to a cube number of 3.
 
An example of increasing the rate would be when you use a blower for a fire. That could be just as simple as using your breath to blow on a smoldering ember or it could be an alcohol burner in a simple chemistry set and you blow on it through a glass tube.

You are increasing the oxidation rate and you see the flame turn from yellow (perhaps) to blue or from blue to white.

Think about explosives it's similar. The explosion of a firecracker is "slow" by comparison to "high explosives" like nitroglycerin which creates an instantaneous shock wave where, if you are standing too close, even the sudden change in pressure could kill you.

As a kid we would light a firecracker and it would go "boom". If you took a firecracker and split it open and then lit it, it wouldn't explode but would turn into a sparkler, in effect. If you stomped on the "sparkler" before all the gunpowder was used up, it would explode again. You just changed the rate (and compression) of the reaction.

(I'm not a scientist so take my explanations with a grain of salt):)
 
Does more carbons mean more energy?
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The amount of energy released is dependent on the oxidation state of the carbons in the hydrocarbon which is related to the hydrogen/carbon ratio. The more hydrogen per carbon, the lower the oxidation state and the more energy that will be released during the oxidation reaction.
 
Please do not post pseudosciece in the Science subforums.
Carbon has an unusually high boiling and melting point for its atomic mass of 6. I bet a melting point so high would make the carbon atoms extremely hot when they bond with the cooler oxygen from the air, and the cooler Oxygen binds with the carbon to release some of its heat.
 
Just in case, dear readers, it is not already obvious that Trevor can't tell the diff between his ass and a hole in the ground:


That's not how any of this works.
 
wegs said:
Thanks for chiming in, exchemist. Okay, I'm going to offer a very elementary example as I'm approaching this as if I know nothing at all about fire.

So, when you talk about the 'rate' at which material 'reacts,' would that be similar to describing the heat we generate if we are running as fast as we can, as opposed to walking slowly? If we're walking slowly, the 'rate' would be slower as to generate heat in our bodies say to burn calories, but if we are running as fast as we possibly can, then the rate would be accelerated, burning more calories. Is this too simplistic?


And strangely enough, the hottest part of a fire is at its base, even though heat rises...which is why we can wave our hand over a candle flame for example, and it doesn't burn us. So, the ''flame'' is really just the heat ''diffusing'' and the base is where the reaction is actually happening.
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More or less. Take a simple example: hydrogen burning. 2H2 + O2 -> 2H2O. Both hydrogen and oxygen come as diatomic molecules, i.e. with 2 atoms joined together by a single bond, A-A, which is written "A2" . So this reaction says 2 molecules of hydrogen react with 1 molecule of oxygen to give you 2 molecules of water. The water molecules have stronger bonds than the hydrogen and oxygen molecules do, which means water molecules are in a lower energy state than hydrogen and oxygen molecules. That means energy is released when the reaction takes place. The amount released is a fixed amount per set of molecules.

Therefore, the faster you bring hydrogen and oxygen molecules together, the greater the rate at which heat is released. You can do that in various ways. One is to pre-mix hydrogen and oxygen. Another is to increase the pressure of the gases, so that there are more molecules per unit volume. Another is to increase the temperature, so the molecules are moving faster and so the rate at which they encounter one another goes up. With solids, finely divided powders react much faster with oxygen than big lumps, because the surface area for contact with oxygen is greater - in effect a sort of pre-mixing. This is why there is a risk of explosions in flour mills, for example.

The hottest part of the flame is not quite at the base but close to the reacting materials. A flame has a complex structure. Often there is a blue part, close to the base, which is the hottest. Yellow flames usually indicate incomplete combustion, as in the example I gave of the Bunsen burner.
 
trevor borocz johnson said:
Does more carbons mean more energy?
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The amount of energy released is dependent on the oxidation state of the carbons in the hydrocarbon which is related to the hydrogen/carbon ratio. The more hydrogen per carbon, the lower the oxidation state and the more energy that will be released during the oxidation reaction.
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Utter garbage. Reported.
 
DaveC426913 said:
Just in case, dear readers, it is not already obvious that Trevor can't tell the diff between his ass and a hole in the ground:


That's not how any of this works.
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I'll talk to you later, the medicine you lay down as writing is below par even for a nasty comment. You'll have to actually think which I don't know you, you clearly don't like me for thinking, but less drowning in the smell of your eye candy to attract exchemist as it were since none of the 'readers' chimed in.
 
trevor borocz johnson said:
I'll talk to you later, the medicine you lay down as writing is below par even for a nasty comment. You'll have to actually think which I don't know you, you clearly don't like me for thinking, but less drowning in the smell of your eye candy to attract exchemist as it were since none of the 'readers' chimed in.
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There's two possibilities here:

You're posting nonsense even though you know better about a given topic - that's trolling.
You're posting nonsense because you don't know better about a given topic - that's straight up ignorance.

The outcome, however, is the same: don't post nonsense.
 
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