[Stoves] Flame color and Boudouard Reaction

[Tom Reed]

Dear Richard and All combustors, gasifiers and Pyrolysers:

Good question, "Why is the match flame blue near the base, but turns yellow in the outer region?"

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As the wood in the match heats, the cellulose in the wood breaks down to form CO and H2. (The lignin in the wood turns to charcoal.) 

I could write a long equation using cellulose (C6H10O5)n, but the chemistry is easier if I approximate it for Carbo-Hydrates,  "CH2O", an approximate formula for all sugars, starch and cellulose.  

     CH2O ===> CO + H2 

Solids emit black body radiation mostly proportional to their surface temperature.  But burning gases emit arbitrary colors due to the flame reactions.  So the flame close to the match shows these blue flame colors.  

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So why does the flame give off black body radiation at a greater distance from the burning match?  

Carbon monoxide is a queer duck.  It is THE MOST STABLE molecule at high temperatures.  However, at low temperatures  (below ~700C as the flame gas cools) it disproportionates to carbon and CO2.    

      2CO ===> C(soot) + CO2

This is known as the Reverse Boudouard reaction.  However, it takes a while for the soot to form.  

The same thing is true if you look very closely at a candle flame.
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I started to answer your question this morning, but did not have a good answer.  When I returned to the question in the afternoon my subconscious must have accessed "Reverse Boudouard".  Funny how the mind works.  

Your flame chemist, 

Tom Reed  

Thomas B Reed 
280 Hardwick Rd
Barre, MA 01005
508 353 7841

> On Oct 19, 2013, at 2:52 PM, Richard Stanley <rstanley at legacyfound.org> wrote:
> 
> Hi Tom, 
> Thanks much for that but what about the bluish color at base of the flame, both immediately above and beneath stick  ?
> 
> Richard Stanley
> 
> ===========
> On Oct 19, 2013, at 7:02 AM, Thomas Reed wrote:
> 
> Dear Stephen, Dean and All
> 
> I recently warned that "temperature" is only really defined when solids and gases are in thermal equilibrium.  
> 
> During flaming pyrolysis of wood they are not in equilibrium.  The gases could be over 1000C while the surface of the solid, cooled by the reaction, is below 500C.  However, the subjective color is a good indication of approximate temperature as indicated in the following table from Wikipedia.  
> 
> "Subjective color to the eye of a black body thermal radiator[edit]
> 
> °C (°F)	Subjective color[5]
> 480 °C (896 °F)	faint red glow
> 580 °C (1,076 °F)	dark red
> 730 °C (1,350 °F)	bright red, slightly orange
> 930 °C (1,710 °F)	bright orange
> 1,100 °C (2,010 °F)	pale yellowish orange
> 1,300 °C (2,370 °F)	yellowish white
> During flaming pyrolysis, the flames appear to me to be a bright orange, (due to incandescence of small particles) and so are 930 or higher.  At the same time, the pyrolysing material appears black, and so could be less than 480C.  Here's a picture of a burning match to ponder...
> 
> <image.jpeg>
> Thomas B Reed 
> 280 Hardwick Rd
> Barre, Ma 01005
> 508-353-7841
> 
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