[Stoves] Purple flame

ajheggie at gmail.com ajheggie at gmail.com
Mon Sep 24 07:56:36 PDT 2012

[Default] On Sun, 23 Sep 2012 17:10:45 -0700,Dean Still
<deankstill at gmail.com> wrote:

>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.

Frans, Alex and Kevin have suggested this may be potassium in the
flame and I'd offer the thought that in a traditional wood burning
stove the wood offgas and char burn at the same time, so any colour is
masked by the yellow incandescence of the carbon particles. Ash
largely falls out of the bottom of the fire.

The TLUD first pyrolyses the wood and the char is preserved above the
descending pyrolysis front in an inert atmosphere rich in CO2, I don't
think oxides of potassium are stable so it's likely potassium
carbonate is formed as the mineral ash in the char is exposed.

Once the pyrolysis front reaches the bottom then  the TLUD simply
changes into an updraught charcoal burner, if there is sufficient
oxygen and depth of char then instead of the char turning into CO2 it
generates CO, as Alex said. This CO then burns with a pale blue
secondary  flame when it encounters more air.

As you note the char glows orange as it gasifies char and exposes this
potassium carbonate, my guess is that this char bed will be around
1100C and potassium carbonate decomposes below this temperature
(around 900C) so the purple could  be glowing potassium ions.

> 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.

I don't quite follow this last bit but it may be that your self
sustaining blue flame is a diffuse CO flame that is not burning out
below the pot.

I wonder why you need to qualify the char with the term bio-char, I
thought this was reserved for char that had uses in the soil rather
than charcoal used in a cooking process.


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