[Stoves] Purple flame

Frank Shields frank at compostlab.com
Mon Sep 24 09:23:57 PDT 2012

Calcium also burns red.


From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin
Sent: Monday, September 24, 2012 8:24 AM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Purple flame


Dear Tom and Dean


It happens that the photos were of a stove burning a high potassium fuel and
I don't see much change in the flame, meaning that either it is there all
along or else it is something else. The only clear change is that the bright
centre ends and the blue from the edges continues, with some orange-red
flames in the centre. I agree with one comment that it comes from hydrogen.
They there is no H2 in a CO flame it is never reddish.


Dean, you have a gas chromatograph, right? Surely you could take a gas
sample with a syringe and inject it now and then into the GC to see how the
potassium concentration changes, if it does, during the burn?


That would help decide if it is a mix of rad and blue, or an actual purple
flame. How about posting some photos showing the colour evolution?







The mix of colors from both carbon and volatile combustion could be playing
optical tricks on us as we look down through the burn.  


If the carbon/char is burning then the local temperature at the carbon is
now about 2500 F (1370 C) instead of a gas combustion with high excess air
at 1500-1800 F (820-980C). This is enough to volatilize some of the ash
(potassium) in the char which would give you the purple hue. We see this
when burning crop residues, leaves and branches that have high
concentrations of potassium in the fuel. It takes some very sophisticated
instrumentation to measure the potassium concentrations so I guess we're
stuck with speculation.  



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