[Stoves] Blue Flame -- Natural Draft -- Rice Husk

[Kobus Venter]

Hi Jaakko,

Interesting observations.  It is always difficult to have a stove to 
burn both wood and the resultant charcoal.  I know Crispin has had much 
success with this.  Not wanting to get into that discussion but more on 
how to achieve clean combustion "blue flames" with a small charcoal 
gasifying stove.  There are 2 ways:

1) build up a red hot mass of coals inside a totally sealed off chamber 
allowing in only small amount of primary air.  Ambient secondary air to 
diffuse with exit gases, manually ignited with a spark or catalyst 
(close coupled combustion), or piped away to an adjacent burner 
(something I only see on the charcoal gasifier groups).  Thickness of 
bed important because most of the naturally occurring volatiles have 
already been consumed.  Batch loaded pretty much.  Gas temperature 
remain surprisingly high due to sealing of the chamber.

  2) allow pre-heated secondary air to mix with CO coming off a top lit 
burn whereby insulated space above the bed allows for the exit gas 
temperature not to diminish below the spontaneous combustion point of 
CO, which is around the 600°C mark.  On 600 grams of charcoal you should 
get 45 minutes on volatile gas and 45 minutes on carbon/gas combination, 
with flame color becoming increasingly more yellow.  Thickness of bed is 
an interesting phenomenon.  Thin bed = higher temperatures and thick bed 
= lower temperatures. Interesting thing is that with a fully charged 
chamber (±600 grams) however is that you double the volatile component 
which compensates for the expected lower temperatures and as the "blue 
flame" starts diminishing, the low bed height results in almost a white 
hot burn, which allows meals to cook for the full 1.5 hours at a 
constant temp, well almost.  This setup also allows for continues 
topping up with charcoal to maintain the blue flame.

Highly insulative fire bricks being the key along with constant air flow 
though the stove.  A litle shake of the stove will allow the bed to 
settle and open channels to fill and alllow for stoichionetric 
conditions to form once again.

Regards

Kobus


-- 
Please vote for VermiChar: http://bit.ly/FNBVermiChar


> Message: 1
> Date: Tue, 3 Sep 2013 13:56:32 +0000
> From: Saastamoinen Jaakko <Jaakko.Saastamoinen at vtt.fi>
> To: Discussion of biomass cooking stoves
> 	<stoves at lists.bioenergylists.org>
> Subject: Re: [Stoves] Blue Flame -- Natural Draft -- Rice Husk
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> 	<447D2D437C205A45BB6E36E116A40D9A59BE5243 at VTTMAIL3.ad.vtt.fi>
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>
> Dear all,
>
> I have seen blue flames in my heat storing stove under natural draft using wood logs in char combustion stage, when the logs have been broken into pieces. It is CO burning as has been discussed here by others. Later the blue flame disappears due to lower bed temperature so that CO cannot burn but goes to chimney.
>
> I have also calculated this phenomena. One can see blue flames  (CO is burning) if the thickness of bed of char particles and air rate are suitable.  If the bed is too thick, blue flames are inside the bed, increasing gas temperature and gasification takes place above producing CO from the bed. (This CO could be burned if somehow oxygen could be mixed and temperature of the gas is high enough). If the bed is too thin, excess air (due to lower flow resistance) cools the gas so that CO is not burning or it burning rate is very low.  So it needs suitable bed thickness and air rate which are difficult to maintain with natural draft.
>
> As Richard Stanley has experienced, blue flame is very sensitive to air rate.  I noticed this when calculating the burning with a model. The calculation was based on iteration and on an initial guess. I was astonished that depending on this initial guess I got, not a chaotic solution, but two different stable (quasi)steady state solutions for the temperature and species distributions in the char bed, a high temperature solution and a low temperature solution.  Conclusion is that both solutions could be right (CO is either burning or it is not). It depends on the burning history of the stove which solution is the right one. It the fire is disturbed and cooled down a little, CO does not burn but if it remains hot, CO can burn.
>
> I throw small twigs in the end stage to get some CO escaping to chimney to burn in the volatiles flames, increase the draft and speed up the burning of residual char at the end of the heating.
>
> Jaakko