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?"
<><><>
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.
<><><>
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.
<><><>
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 <[email protected]> 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
>
_______________________________________________
Stoves mailing list
to Send a Message to the list, use the email address
[email protected]
to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
for more Biomass Cooking Stoves, News and Information see our web site:
http://stoves.bioenergylists.org/
