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

[AJH]

On Fri, 2 Jun 2006 12:46:34 +0200, Crispin Pemberton-Pigott wrote:

>
>That is my point today.  CO could be ignited in the hot space between
>burning coals, 

The CO in between the burning coals can only be ignited if there is
sufficient oxidant available to oxidise it to CO2 even if the
temperature is high enough to cause spontaneous combustion.

Consider a very shallow layer of charcoal sitting in air, nothing
happens until it gets hot enough to "ignite", why? 

It's because the charcoal is an agglomeration of carbon atoms which
will only react with the O2 in air once the O=O bond is broken to give
two O atoms which are then free to combine with the C atoms. So the
initial hurdle (or auto ignition temperature) which has to be overcome
is to supply enough energy to the double oxygen molecule bond for the
oxygen to dissociate on the hot carbon surface and react to form a CO
molecule, which rapidly itself is oxidised to CO2, releasing far more
energy than was required to break the O=O bond, hence everything gets
hotter, both the remaining carbon and the escaping CO2. Heat then
leaves this system in the CO2+diluent nitrogen and by the charcoal
radiating heat (to the food or pan). Now the temperature at which this
is self sustaining is in the order of 300-400C, allow sufficient heat
to radiate away and the char goes out, try this by striking a match
and burning all the char to nothing.

If you keep all the heat in by allowing adjacent coals to mutually
radiate to each other the temperature can rise. this rising
temperature gives the CO2 sufficient energy to react with the hot
carbon and be reduced back to CO. This uses up energy so the effluent
gas temperature goes down, it self limits at about 1100C if there are
no other losses from the system. so we have an equilibrium that
favours production of CO2 if the temperature is low and there is
excess air and favours CO if the temperature is high and there is
excess carbon. There are many stages in between so most fires produce
CO and CO2 plus diluent nitrogen.

I think a shallow charcoal fire is a special case where if there is
sufficient excess air and the heat losses keep the temperature low
there could be little CO.

Most burning of char doesn't achieve this condition and so the offgas
is a mixture of nitrogen, CO2 and CO. You will realise that the CO is
the only remaining fuel gas in this mixture but it is now diluted by
CO2 and N2 so its chances of encountering an oxygen molecule and still
having sufficient energy to react with it get lower as its dilution
increases. As a rule of thumb I was told that to sustain an open flame
the fuel gas should contain at least 2MJ/m^3 of gas, I think an air
blown gasifier runs ~5MJ/m^3 but I'm not sure what the significance of
sensible heat in the offgas would be (i.e. whether the 2MJ figure
relates to total enthalpy of the offgas and air mixture). In a highly
insulated turbulent burner clean combustion can be sustained at lower
CVs.

This charcoal in a bucket updraught stove falls somewhere between the
tin charcoal layer and a full CO generator.







>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.

As above, very little heat is needed for ignition, just enough to
break the bond in the oxygen molecule after that it is just the
conditions within the stove that determine what is produced.

>  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'm not sure about this, remember in a charcoal fire you no longer
have the luxury of a flame sustained by volatiles with a range of
flame speeds to keep the flame seated and force all the reacted gases
to pass through a high temperature zone. You either need to keep the
CV of the offgas up and sustain a flame or prevent the formation of CO
in the first place.
>
>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.

Yes 700+ seems to be the region where most of the carbon dioxide can
be reduced to CO. The gasifier people aim for the gas to leave the
reduction zone at ~850C and then be rapidly quenched to prevent
reformation of CO2.
>
>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. 

This is of course the sort of thing we expect to see in a coal burner,
just sufficient air entering the bottom to keep the grate cool, this
then reacts with a bed of coal which is deep enough to maintain a
temperature above 800C so all the offgas leaves the bed where it
encounters blasts of secondary air in a very hot, turbulent combustion
zone of sufficient size for the time taken for the gases passing
through to guarantee their encountering sufficient oxygen to burn out.

>
>>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. 

No, it is a necessary mechanism to keep the coals cool enough, in
exactly the same way it is necessary for the space shuttle to radiate
heat as a consequence of it using the atmosphere as a brake.


> 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. 

OK don't mistake my meaning, I was not advocating this as a means of
getting heat into the food but as a means of cutting CO emissions from
a charcoal bed. I was pointing out I saw two means of cutting CO from
a charcoal fire, one to cut the bed temperature and ensure excess air,
the other to force the production of CO by increasing bed depth and
maintaining temperatures above 800C (difficult in a small stove) and
subsequently sustaining a CO flame.
>
>>...(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.  

Dr.Karve reported a figure of 6ppm with the attribution that it was
measured by Dr.Karabi Dutta, of ARTI. I was amazed at the time.

Andrew Heggie