[Stoves] RE Central channel combustion stoves. Was RE: Henson CenterFiure Burner System

[AJH]

On Fri, 9 Jun 2006 18:02:36 +0200, Crispin at newdawn.sz wrote:

>Well...let me approach the low CO this way:  if burning charcoal at a low 
>temperature in some way avoids creating the CO in the first place because 
>excess air is present and the carbon burns to CO2 immediately, wow we have a 
>winner!

Yes if it can be done!
>
>I wish I could replicate this feat.

I'm not aware it has been done yet, I'm suggesting possible mechanisms
for what happens and seeing if something can be achieved.
Unfortunately I have no success with a cheap CO meter and still cannot
justify a posh one.
>
>There is a small contribution that came from Dean sometime last year where 
>he wrote to say that they put a fan on a charcoal fire and the CO went up, 
>which surprised everyone.

Well the fire needs two elements, fuel and air, if you raise the
velocity of the air then you increase the mass of air being supplied
in a given time, so even though the global effect is to introduce
excess air the local effect is to create a hot spot, it's raising the
local temperature of a smouldering piece of wood that increases the
production of pyrolysis gas and raises it above its auto ignition
temperature, causing a flame and thence the positive feedback from the
heat released by the burning gases that gets the fire going.
>
>That was a valuable test because everyone expects the increased 'heat' to 
>produce less when in fact it was chilling the fire by adding excess air 
>while simultaneously increasing the burn rate.

My conclusion differs from yours, I think the increase of velocity
through the carbon increased production of CO, not subsequently
burning it out in a flame is why it continued to exist in the flue
gas. I posted the flammability limits of CO in air, now whilst I
suspect that this referred to gas and air at ambient temperature and I
don't know how much elevated temperatures will effect the range it is
plain that any CO produced is already diluted by Nitrogen and CO2 so
it will nearly always be outside the range at which it will sustain a
flame on its own. So once produced, unintentionally, it will be
difficult to clean up.


>  If you blow cold air onto 
>the whole charcoal fire, the CO level goes up because it heats the charcoal 
>(this is my thesis) produce far more CO and there isn't enough heat or air 
>around to burn it.

See above but I agree that the local heating of charcoal  will favour
production of CO and then this is difficult to burn out.
>
>Thus we have two completely different understandings or possibilities (both 
>could be right) and they are summarised as:
>
>Carbon burns preferentially to CO2 immediately when it has a chance as long 
>as there is enough air around.

Actually I think it always burns to CO2, the CO is produced
subsequently but I stand to be corrected.
>
>Or
>
>Choking the air supply raises the CO level (low excess air), as does blowing 
>cold air onto burning coals (high excess air).


>
>It is hard to believe the former proposal because it is were so, blowing 
>even more air would reduce the CO and it doesn't.

Again see my view above 

>When Dr Karve reported a 'low CO level' was it a low COr or low ppm? 

It was low ppm that was stated and as you say excess air would dilute
that.

> It is the ratio that is 
>important and I haven't seen the SARAI Stove figure for that.

Yes I think CO:CO2 ratio is a good indicator of completeness of
combustion, I have no means of knowing how the Sarai performs in this
respect.

You miss out a major point in my earlier post, that is there needs to
be a mechanism to remove heat from the C+O2 => CO2 reaction because it
is highly exothermic, if you cannot remove the heat then the
temperature of the remaining char goes up, as it goes up it shifts the
equilibrium to favour reduction of the, now hot, CO2 on the surface of
the hot char. As we know the heat produced by burning C in air is
sufficient to melt iron (1600C??) we know the mass flow of the
reagents is unable to carry the heat with it, as the temperature goes
up the endothermy of the CO2 to CO reduction controls the temperature
of the offgas. The only mechanism (other than increasing the mass
flow) I could see for limiting the temperature was if the coals were
radiating it away. This effect is difficult to make use of as it
varies with the 4th power of the absolute temperature of the surface.


>
>If it is true that charcoal can be burned with low CO (say, below 400 C) 
>thenI propose we have another type of reaction: low temperature oxidation 
>rather than 'burning' in the conventional sense.

Rusting is low temperature oxidation and in the presence of a
catalyst, salt, makes a handy low temperature warmer for body
extremities.

AJH