[Stoves] Purple flame

[Tom Miles]

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.  

 

Tom 

 

From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin
Pemberton-Pigott
Sent: Monday, September 24, 2012 6:59 AM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Purple flame

 

Dear Dean

 

I think it has something to so with the rate of the pyrolysis of the final
portion of the fuel. When the zone reaches the bottom, and there is metal
there, the pyrolysis rate of the final portion of fuel is at a higher rate
than any equal mass during the rest of the burn.

 

As a result, the composition of the gas changes because the reaction rate is
different. There is no fuel below the 'final bit' to absorb heat (esp from
drying the fuel below) and also there is no rising moisture from the fuel
below. Hot water vapour is highly reactive and changes the gas composition
by changing the reaction speed.

 

I have also notice the change in colour and have a few photos. Do you
suppose it can be used as an indicator to the cook that the 'end is near'?

 

Early fire, highest moisture level



Middle fire, constant moisture level

 



 

Late fire, low moisture level, highest pyrolysation rate, higher carbon
burning portion

 



 

Notice that the purple flame is always there, and it is always at the point
of secondary air entry. This indicates that is may be more related to the
carbon burn (CO) than any of the speculation above. The overall impression
is influenced by the bright white emissions from particles. As the volatiles
are burned away, there are very few particles and the white disappears (or
yellow if it is not a very hot fire). Thus the 'change in colour' might be
more because the PM disappeared along with the light, than the presence of a
new colour. The impression on the eyes would be the same in both cases.

 

Best regards

Crispin

 

Hi Stovers!

 

I've been noticing a shift from blue flames above the burning bio-char in a
TLUD to purple flames nearer the end of the burn when the bottom of the pile
is glowing bright orange. I suppose that the blue flame is caused by burning
CO but I wonder why the purple forms after the blue? The blue self
sustaining flame doesn't heat the water in the pot as well as the purple
flame phase although the pile of bio-char is fully ignited at that time and
may account for the more rapid rise in water temperature.

 

Any ideas?

 

Best,

 

Dean

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