When you see a fire burning, you have wood (fuel) interacting with oxygen which (through a chemical reaction) gives off heat - and light.

But the light appears to emit at some point well-away from the actual fuel - i.e. the flames appear to rise inches (or more) above the actual fuel source where (I assume) the actual combustion/reaction is taking place.

I assume this obviously has something to do with the fact that heat rises - and there is oxygen up there, but there is no fuel up there.

I might conclude that the light is artifact of the elevated temperature of the oxygen, alone up there - but this wouldn't explain the fact that different elements burn different colors - i.e. color is related to the fuel.

So it would appear that the only logical explanations would be either:

a. Fuel becomes "detached" from it's (solid) fuel-source, rises with the heat and emits light after rising.

b. Light is emanated near the fuel-oxygen reaction but appears higher due to some sort of bending it it's travel path due to high heating of nearby air - like how light squiggles over a hot BBQ grill.

There is also the fact that in a very hot "coal" fire the light does emit from the fuel - but in a "roaring" fire the flames appear higher. (Is this because a "roaring" file has higher heat - which correlates to the difference in flame color?) Why would these flames appear "above" the fuel?

Okay - I hope I've illustrated my line of questioning sufficiently. So how does it work?

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    $\begingroup$ With wood, it is purely option 'a'. $\endgroup$ – Ivan Neretin Sep 29 '18 at 5:56
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    $\begingroup$ The final reaction steps of combustion (formation of water and CO2) have the major energy yield, and they largely occur in the gas phase. Wood fire additionally produces gaseous pyrolysis products, which obviously rise before burning up. $\endgroup$ – Karl Sep 29 '18 at 6:46

Light is emitted from flames by two primary mechanisms: one is small particles glowing incandescently because they are hot (the same mechanism that drives an incandescent light bulb); the other is from electronic transitions from specific energy levels in excited atoms in the flame produces as a by product of the combustion process (this is partly why flames can coloured specifically by elements present in the burning material).

But why do flames and their emissions appear so far from the primary materials that are burning? Clearly solids themselves can burn (eg charcoal in a barbecue is glowing from surface combustion of the carbon with atmospheric oxygen and the emission you see is because this makes the carbon hot enough to glow with heat).

But when things are that hot, plenty of other reactions can occur and this is particularly noticeable if the thing that is burning is less pure than charcoal (which is mostly carbon). Raw coal, for example, contains a lot of volatile impurities (smokeless coal is deliberately treated to reduce the volatile impurities). When the primary reaction of burning carbon happens in raw coal, those volatiles are often turned to gases and driven off the coal to combust as gases far from the solid coal. Of course this is happening at the same time as solid carbon in the coal is burning so the two effects are mixed up to give both glowing coal and gases burning some distance from the glowing coal. Old home chemistry sets used to contain experiments that allowed these effects to be separated. Raw coal is placed in a vessel connected to a tube allowing any emerging gas to be directed far away from the coal. The coal is heated with an external source (eg a Bunsen burner). The emerging volatile gases can then be lit without setting the coal on fire giving a flame a long distance away from the heated coal.

The same can be done with wood (which contains even more volatile and flammable components than coal).

So, when wood or coal burn in an uncontrolled way both the burning of the solid and the burning of the volatiles contained with the solid occur at the same time but the volatiles (as gases) can move far from the solids before they burn. This is why the position of the fame can be far from the burning solid.

Also worth noting

In addition to the volatiles being driven from the burning solid, partial combustion can also create gases like carbon monoxide which can then travel some distance away from the solids while burning (though CO tends to have not very bright blue flame). In fact a controlled version of this reaction (which sometimes also used steam to create hydrogen) was once the primary way of creating town gas (which was widely distributed to support gas lighting and cookers in cities.

If the primary goal is to create flames designed to illuminate rather than heat then burning can be designed to create a lot of small hydrocarbon particles in the flame. This is intentional for things like candles. Here the heat may come from burning the was but the light comes because that burning creates small incandescent soot particles which are heated enough to glow turning a lot of the energy into light.

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  • $\begingroup$ Thanks for the explanation! So would it be correct to say that when fires "burn down to coals" and are still emmitting heat and "red glow" - that most of the gassious volatiles have been released and burned - and we're mostly down to the rawer carbon/charcoal? $\endgroup$ – Brad Oct 1 '18 at 19:43
  • $\begingroup$ @matt_black So would it be correct to say that the heat released from the reaction temporarily excites the electrons in the products to higher energy levels, which then restabilize by releasing light and returning to their ground state? $\endgroup$ – David Reed Apr 13 at 21:54
  • $\begingroup$ @DavidReed basically, yes. $\endgroup$ – matt_black Apr 13 at 21:58

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