[Stoves] Crispin´s kiln-was Re: Traditional Charcoal Making

[AJH]

On Tue, 10 Jul 2007 12:13:14 +0800, Crispin Pemberton-Pigott wrote:
Tom Miles wrote
>>These same reasons are probably why
>>we hear that most kilns in developing areas do not reach temperatures
>higher
>>than about 750C or 950 C.  
>
>The main reasons seem to be poor combustion and then high excess air.  Damp
>fuel (sawdust) I have noted in some places is a significant limiting factor
>reducing the flame temperature (CIMOC in Boane, Mozambique being one
>example).

Poor combustion is really wasted fuel, Products of Incomplete
Combustion are vented, yet they still have chemical energy. Damp fuel
needs more air plus it robs the fire of all that latent heat. We did
have a post that demonstrated the maximum temperature that wood could
attain verse moisture content and from my experience burning 50% mc
wood requires at least 50% excess air and I seldom see flue exit
temperatures of much better than 800C. Dry wood  will quickly show
1100C on my thermocouple.


>  Poor insulation is a factor but only because the total power
>rating is too low.  Increasing the power level more than compensates for
>poor insulation. 

All at a high cost in fuel, I would have thought even a simple brick
kiln would benefit from a roll of 12mm cerablanket.

> Unless there is a real concentrated effort to limit the
>excess air, and like a good stove, preheat the secondary air, there is
>little one can do about the low temps in rural kilns.

I agree than preheating the secondary air is probably the first use
that should be made of the flue waste heat, simply in order to boost
the combustion temperatures the kiln contents get exposed to. As you
say it will be an iterative process as the heat losses from the kiln
will go up with the change in absolute temperature.

AJH