[Stoves] Fwd: Re: Improving cooking with charcoal

[ajheggie at gmail.com]

On Mon, 11 Nov 2019 at 17:36, Harris, Kirk <gkharris316 at comcast.net> wrote:

> Also, the char which falls to the ground between the rocks will produce
> CO, which has air to burn just below and directly inside the charcoal fuel
> stack, helping to keep it very hot.  There may be other advantageous
> chemical reactions which have to do with reduction, or the interaction of
> water vapor, which I know very little about.  Perhaps some or you, like
> maybe Crispin, could help with this question.
>
>
Crispin must be busy; I'm loathe to comment on Kevin's work with stones
without seeing a cook actually cooking an everyday meal with charcoal. My
experience with cooking with charcoal is on barbecues and I'm not very
competent at it.

When I look at char burning in my wood burning, space heating, stove once
all the volatiles have flamed off and I am just left with a bed of char I
note that when the bed is thin it glows red and there is no discernible
flame, if the bed is thick it still glows red but a blue flame dances on
the top.  This fits well with the perceived wisdom that oxygen molecules
dissociate on contact with hot char and combine to give out a lot of heat
and carbon dioxide, no flame is involved. My thoughts are below and I
welcome comments either way.

Spread thinly the char will radiate heat and the exhaust gases of CO2 and
N2 give rise  to convected heat.

As the bed gets thicker less heat can escape and the particles of char
mutually radiate thus less heat is lost to the outside and the bed becomes
hotter and in the absence of oxygen the endothermic reduction of CO2  to CO
occurs, this cools the bed but the CO given off can then ignite into the
blue CO flame once it contacts secondary air above the bed.

The late Dr. Tom Reed suggested that if the char bed were twenty char
particles thick mostly all the char would be converted to CO.

So we have conditions in a thin bed where the temperatures are low and
little CO is produced and in a thick bed contained  in a combustion chamber
where predominantly CO is produced. The reduction of CO2 to CO  largely
occurs when temperatures are above 800C

Most of the time with charcoal stoves we have a mixture of conditions
between these two extremes.

I'm not sure how the stones might modify conditions to push the balance
either way. If the char is distributed around the stones then they will
interfere with mutual radiation and possibly lower the temperature leading
to less CO, if they act as a grate, forming a plenum under the fire then
more air will be directed under the fire.

I can see that there will be a condition where the balance of Nitrogen, CO2
and CO is such that the CO will not ignite even when it meets air as it
will be too dilute in the nitrogen  and carbon dioxide mixture. Apart from
being potentially dangerous this represents a lost of heat from the stove.

Andrew
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20191113/2a5c1cda/attachment.html>