[Stoves] RE: Henson Center Fiure Burner System; Was: Re: improving charcoal stoves

[AJH]

On Wed, 31 May 2006 08:17:45 +0200, Crispin at newdawn.sz wrote:

>I am not convinced this is a new method of burning.  There are too many 
>examples of very similar stoves going back many years.  People have known 
>for a long time that having a hole through the fuelbed has certain benefits. 

Yes a number of examples have been posted now to which I will add the
combustible tube tlud stove that someone here built. I am also
reminded about a 2001 discussion here, between Reed and Boyt about
burning paper wrapped around a perforated metal core, by a subscriber
who has used this as inspiration for a waste paper burner fabricated
from a stainless steel waste bin, some flue pipe and a length of
perforated stainless steel exhaust pipe ( Thanks Robbie).

>
>As the supply of CO is highly variable, one cannot always rely on a flame 
>being present to light it.  Hence it is good to have something hot around to 
>light (increase the energy of) the gases as they leave the fuel bed.  For 
>example you can use hot secondary air, a flame from a reliable source (which 
>is how Dr Tom's camp stove works, if you think about it), a hot wire (as in 
>Lanny's example), a hot tube (as Dean mentioned)  or a hot plate against 
>which the gases are directed (as in the case of an FSP stove).
>
>The point is to provide reliable ignition for a variable gas source.  Hot 
>metal is a good one.  Their drawback is a short life in most cases.

The thing is there will be a number of effects working for and against
each other. We know that metals can catalyse reactions (lower the
threshold temperature at which the reaction starts) we see this in car
exhausts and the pre mixed cigarette lighters. To work against this is
the fact a metal grid conducts heat away from the combustion zone
(look for instance at the Davey safety lamp.

Now CO is an interesting gas in that it has a similar range of
combustibility in air to Hydrogen, this is quite unlike a "good" fuel
gas like methane that has a fairly narrow range of flammability. So
given that Crispin is allowing for a variable rate of production of CO
and that there are two major mechanisms for cleanly burning it we can
either establish a flame or maintain conditions above its autoigniton
temperature. This temperature is 690C in the absence of a catalyst.

I'm fascinated by flame holding and know little about it but CO may
well pose a problem here because of its low flame velocity in that an
increase of offgas velocity can blow the flame off the burner. Add a
little hydrogen and I imagine the flame will hold better.

Peter Verhaart considers the CO to be "difficult" to burn and I know
from results of stoking tests that CO goes up shortly before and
shortly after restoking a batch sequential stove. I think a lot of
this is due to the flaming secondary combustion zone being absent
under both these conditions. Yet I think in many ways CO is a good
gas, it's just that the conditions in which we are considering it, in
a charcoal stove, it is a by product of the char combustion rather
than the end gas of a char gasifier.

I think the nub is that a clean char fire under natural draught needs
a shallow, relatively cool (because it is radiating energy away) bed,
This ensures the chief product gas is CO2 which has no opportunity to
be reduced to CO (ADKarve reported low CO from the Sarai cooker). If
the bed deepens there is more opportunity for the equilibrium to
favour CO, especially if mutual radiation between char particles puts
the local temperature up above 700C. So the offgas becomes a mixture
of CO2 and CO, now if that falls below the autoigniton temperature it
is going to be very difficult to burn.

AJH