Coal Fire Pockets: Is Temperature Higher?

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Discussion Overview

The discussion centers around the phenomenon of brightness in coal fire pockets compared to exposed glowing coal, specifically questioning whether the temperature in these pockets is significantly higher. Participants explore various factors influencing brightness, temperature differences, and the role of volatiles in combustion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that while they have not observed the phenomenon, they suggest that brightness may not necessarily correlate with a significant temperature difference, possibly due to emissivity variations.
  • Another participant proposes that coal on the surface loses heat through convection and radiation, while pockets may retain heat due to their geometry and the presence of burning volatiles.
  • A different participant emphasizes that brightness does not always indicate higher temperature.
  • It is mentioned that burning wood also exhibits similar behavior, where the combustion of volatiles can produce a blue flame, indicating high temperatures despite lower total heat output compared to solid materials.
  • One participant agrees that volatiles are likely present and suggests that atomic transitions in carbon particles could contribute to brightness in pockets, while also noting a slight temperature difference.
  • Another participant suspects that a temperature difference is more significant, suggesting that even a few hundred Kelvin could account for the observed brightness differences.
  • One participant explains that in a well-managed coal fire, the interior is typically hotter and brighter due to various combustion dynamics, including the presence of coked coal and better air supply, while also noting that empty pockets are cooler.
  • Finally, a participant expresses confusion about the inclusion of this topic in a Quantum Physics text, questioning its relevance.

Areas of Agreement / Disagreement

Participants express a range of views on the relationship between brightness and temperature, with some suggesting that temperature differences are minimal while others argue for a more significant difference. The discussion remains unresolved, with multiple competing perspectives on the factors influencing brightness in coal fire pockets.

Contextual Notes

Participants acknowledge the complexity of the combustion process and the interplay of various factors, including emissivity, atomic transitions, and the behavior of volatiles, without reaching a consensus on the primary cause of brightness differences.

neelakash
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Coal is Glowing here!

I got this question in Resnick Eisberg:
Pockets formed in a coal fire seem brighter than the coals themslves.Is the temperature in such pockets appreciably higher than the temperature of an exposed glowing coal?

Strictly speaking,I have never observed this...however, it looks to me it is not very much essential to have appreciable difference in temperature.The factor amplifies the intensity to a higher value.Again, it may be for the difference in the value of emissivity of glowing coal and coal-pocket(I am not sure...and I do not know what is a coal-pocket).

Can anyone say something about it?
 
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Possibly.
Coal on the surface is losing heat to both convection and radiation - into the room and you, otherwise there wouldn't be much point.
In a cavity (either in a piece of coal or formed by a few pieces) the surface is mostly pointing at other burning coal at the same temperature so isn't losing much heat.

There may also be volatiles evaporating out of the coal that are burning giving a blue flame.
 
Also, 'brighter' doesn't always correlate with 'hotter'.
 
Coal is pretty much a black body!
 
It's not just coal. Burning wood releases volatiles that burn very vigorously and give a very blue color in our wood stove. The blue color is a sign that the volatiles are burning at a very high temperature, though the total heat output of those gases may be less than the cooler solids that are burning.
 
I agree that volatiles should be present.It is not very much essential to have appreciable difference in temperature. The reason may be that in the pockets, the free Carbon particles undergo atomic transitions. However, in the coal-interior, heat energy is dissipated to rupture the bonds, to make effective reactions etc;but here the atomic transition will be less. There is a slight difference in temperature but the factor T^4 amplifies the intensity to a higher value in the pockets.
 
I suspect it is more due to a temperature difference.
You would only need a few hundred K difference between black coal and glowing red cavities.
You are seeing brightness more as a function of wavelength as the peak emmission moves from the deep red to white (proportional to T) rather than absolute power ( proportional to T^4) your eye is very bad at measuring absolute power.
 
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In a properly tended coal fire, the interior is hotter and brighter than the surface. But, it's complicated. Resnick and Eisberg is a Quantum Physics text, I believe, and the higher temperature in the interior of a fire is due to several combustion effects: (1) the interior fuel is largely coked coal rather than coal; (2) the interior is receiving the most air, either by convection or blast; (3) the surrounding coal and ash are fairly good insulators for the interior; and (4) the radiation pattern of the interior looks somewhat like a black-body cavity.

Empty pockets (hollows) by the way are much cooler than actively burning pockets of fuel.

Anyway, the proof of the higher temperature may be seen by placing pieces of iron in the two locations and watching to see which heats up the most. You can tell that by the color of the iron.

Now that I've said all that, I'm baffled why Resnick and Eisberg had that in a Quantum book.
 
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