[Stoves] Purple flame

Alex English english at kingston.net
Sun Sep 23 18:29:45 PDT 2012

Hi Dean,
I'm a bit confused by your first sentence. You seem to be suggesting  
that there is a blue flame before the bottom is bright orange.

Tell me if this makes any  sense.

In a TLUD the bio-char doesn't start to burn until the pyrolysis front 
reaches the bottom. At that time the bottom is glowing bright orange and 
char oxidation is taking place. The hot CO2 formed at the bottom then 
may reduce to CO as it passes hot carbon above the bottom layer. This CO 
can then burn with additional air in a blue flame above.

This is somewhat stove dependent but there can be some tar condensation 
on the fuel chamber walls during pyrolysis. During the char burning 
phase and perhaps in this case at the end when the top of the char pile 
is radiant orange, this layer of tar could re-volatize and mix with the 
CO to form a higher energy flame with a reddish component. It would also 
be helpful if we knew what the excess air values were during these phases.

My two sense,


On 23/09/2012 8:10 PM, Dean Still 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. 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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