Is Fire a Plasma? | Physics Debate

[gtsimpedes]

Can electricity be harnessed directly from fire in an efficient manner therefore doing away with the old standard of conversion of fire to mechanical energy which then is used to generate electricity?

 

[bigfooted]

Can electricity be harnessed directly from fire: yes.
in an efficient manner: yes, you can potentially capture all of the charged particles.
Use it as an alternative to generate electricity: no.

Premixed methane-air flames have charged particle mass fractions of the order of 10^{-10}. That will not generate a lot of electric current (order of a couple of mA), considering that flames have very high electrical resistance (order of MOhms).
It is more efficient to use the released heat to generate electricity.

 

[stedwards]

I don't believe there is proof or disproof of the statement: "a flame is a plasma". All material above absolute zero temperature will have a fraction of free electrons. At what degree of ionization one wishes to define something as a plasma, due to heat, is a matter of convenience to a particular application.

 

[Hercuflea]

Yes, fire is a low-temperature plasma that is not in thermal equilibrium. This is a chart from Lawrence Livermore National Laboratory.

 

[stedwards]

read some of the 63 previous posts. everybody has an opinion, where there is no strict dividing line.

 

[OmCheeto]

read some of the 63 previous posts. everybody has an opinion, where there is no strict dividing line.

Yah! And I'm still of the opinion that I don't know.

All I can do is show experiments, that demonstrate, that the Veritasium video is not proof that a flame is a plasma:



Now, if you were to take the ionized tape up to the ISS, chop them into bits, and place them in between two charged plates, I can imagine that they would demonstrate the same properties as the flame. So then, is ionized cellophane tape a plasma?

I'm still leaning towards "no". I just got out my multimeter, did the experiment referred to in post #2, and measured: resistance = ∞

...
Before writing to you, just to make sure, I took an electric meter and measured the resistance between two metal contacts separated by a small distance, putting both in the flame of a gas oven, which gets pretty hot. No electric current could be detected, both inside the flame and away from it, meaning the flame did not conduct any observable electric current.

You can get electrons to flow in a vacuum. Is the vacuum a plasma? I don't think so.

 

[stedwards]

Well, this thread is about plasma due to heat; the net charge should be zero. I'd challenge all to show me that there is a hard division line between plasma and non-plasma of the heat generated variety. A logarithmic scale might serve better, such as pH, or earthquake intensity, Perhaps log(free electrons over total atoms). Of course, this wouldn't do for a solid metal.

 

[bigfooted]

I'm still leaning towards "no". I just got out my multimeter, did the experiment referred to in post #2, and measured: resistance = ∞

We did a series of experiments in the lab where we created a quasi-onedimensional setup using a flat flame burner and we placed it between a cathode and an anode. Depending on the applied potential difference we can measure a current and estimate that in our situation, the diodic resistance of the flame (lean methane-air flame) is between 5-25 MOhm.

A detailed chemical mechanism describing a methane-air flame can have more than 300 reactions. There are only 1-4 reactions involving charged species like electrons and H3O+. The concentration of these species is very low and they don't affect the other species much so they are usually neglected. But when you ignite a flame, the charged species become very important and if you want to study flame ignition you cannot ignore this.

 

[OmCheeto]

... the diodic resistance of the flame (lean methane-air flame) is between 5-25 MOhm.

...

This would seem to confirm that a flame is not a plasma.

I just realized that this thread is ancient.
hmmmm...
Let me synopsisize the opinions over the last 8 years:
username(mm/yy)

OP: Oblef; "Is fire a plasma?"

No                 Yes            Partially?
Doc Al(12/07)                     lantresman(12/07)
Astronuc(12/07)                   Sojourner01(12/07)
MaWM(01/08)        chefcrsh(07/08)
russ_waters(05/10)
Drakkith(05/11)
jetwaterluffy(10/11)              juanrga(10/11)      
BadBrain(10/11)    Abstractness(10/13)
OmCheeto(10/13)
Bandersnatch(10/13)
meBigGuy(10/13)
lunaloca(10/13)
bigfooted(06/15)   Hercuflea(06/15)

It appears that most people believe that fire does not fit the definition of plasma.

ps. In my forays into the quest for an answer this morning, I had some weird questions, which generated more questions:

Is the interior of a fluorescent bulb an example of a plasma?

Fluorescent lamps are negative differential resistance devices, so as more current flows through them, the electrical resistance of the fluorescent lamp drops, allowing for even more current to flow. Connected directly to a constant-voltage power supply, a fluorescent lamp would rapidly self-destruct due to the uncontrolled current flow. To prevent this, fluorescent lamps must use an auxiliary device, a ballast, to regulate the current flow through the lamp.

+

Usually very high: For many purposes, the conductivity of a plasma may be treated as infinite.

Or is it a borderline, coincidentally similar effect?

Touching a fluorescent bulb, the surface temperature is obviously not a bazillion degrees, as seems to be required for a "proper" plasma.

 

[russ_watters]

Can electricity be harnessed directly from fire in an efficient manner therefore doing away with the old standard of conversion of fire to mechanical energy which then is used to generate electricity?

If you loosen your definition of "fire" to cover the same chemical reaction at any temperature, what you are asking for is a fuel cell:
https://en.m.wikipedia.org/wiki/Fuel_cell

Their current efficiency is not typically good enough to prefer them over the old fashioned way for most applications.