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

[Crispin Pemberton-Pigott]

Dear Andrew

I want to disagree with some of your analysis so the conversation continues
a little on this most valuable topic.

>>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. [snip]

Yeah... I am not sure there is something unusual here but let's not take any
wind out of the sails of the students!  For them it is new.  That's enough.

>The thing is there will be a number of effects working for and against
>each other.

That is my point today.  CO could be ignited in the hot space between
burning coals, however that heat is not really used for ignition, it is used
for making more and more CO because that heat is next to the charcoal.  From
what I see it is important to get a hot zone away from the charcoal in order
to light the CO without further increasing the production of gas.

I have not read anything about this so I am going strictly on what I measure
and understand.  The temperature at which the charcoal starts to 'burn'
(evolve gases) is as low as 300 to 400 C.  Running the temperature up to
700+ doesn't really lower the CO by lighting it, the heat just produces more
CO.

When there is some volatile material in the charcoal it runs out and carries
a flame which ignites the CO _above_ the charcoal.  This means that the key
to burning charcoal cleanly is to have a two stage process, a cool one in
which the CO is generated at a variable rate according to the cooking
demand, and a high temperature stage in which is it burned with or without
volatiles.  The flames seen at the beginning of a BBQ are misleading.  It is
when they disappear (volatiles now gone) that the poor combustion of an open
system is evident.

>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.

Let us please give up on this 'radiating' energy business.  I will write
something on the subject at little later on.  It does not matter how the
heat gets into the pot as long as it gets there.  Do not mess up the
necessary stove architecture to chase a broad radiating bed.  The wider the
bed, the more likely the heat will escape from the sides around the pot.
The shallower it is the less space for the CO flame to finish burning.  With
both issues the JIKO fails and that is why its CO production is so high and
its side heat losses unconstrained, unless the pot us huge.

>...(ADKarve reported low CO from the Sarai cooker)...

I recall that he had very high CO levels in ppm but did not have a CO/CO2
ratio.  As it only burns 100 gm at a time I expect the total CO produced to
cook to be low, but it is not a low CO device as the combustion is cool and
slow with little flame.  It is efficient, of course!  I rate the device
'good' because of the low total CO, but the rate of CO produced per Kg of
fuel burned is quite high, it appears.

In relation to Lannys central air gap, it is not yet clear to me whether it
is better to go with a conical pile burning on the outside with hot
secondary drafted over it, or a cylinder with secondary air drafted up the
middle.  Both seem meritorious.

Regards
Crispin