[Stoves] Charcoal & Stoves

[AJH]

On Sun, 8 Apr 2007 18:47:55 -0400, Crispin Pemberton-Pigott wrote:

>Dear Michael
>
>In experiments done with Tsotso stoves in 1999 I encountered the same 
>problem of smouldering coconut husks.  They are 'burning' but not with a 
>flame.  The attached picture shows the Tsotso boiling a cast iron pot of 
>water with limited smoke.

I take it this was a reply to an offlist post as I cannot see the
original?
>
>The solution I found at the time that made an impact (i.e. created a flame) 
>was to get the primary air preheated to the point where the addition of the 
>smouldering heat was sufficient to maintain a flame.

This seems a sensible approach where either the cv of the gas you are
trying to burn is low or below it's spontaneous ignition temperature.
There must be many reasons why a combustible gas won't support a
flame, low enthalpy and wrong gas velocity would be two.


>  I was wondering it 
>your TLUD stove might be able to burn the smoke.  It might be quite damp 
>smoke which is a huge drawback.

This would depend on the secondary burner, generally the hot tarry gas
from a tlud is a very high cv, principally because the thing only
works with relatively dry fuels.
>
>I also found that the burn rate was also important because it raises the 
>fuel temperature more rapidly. 

This would be because the increased heat flux outpaced local heat
losses, the same happens when blowing on a smouldering fire, the
overall increase in excess air is offset by the local high velocity
air gasifying char, thus increasing temperature above the spontaneous
ignition temperature of the offgas and initiating a flame.

> You can get the burn rate up by increasing 
>the draft with a chimney, increasing the internal stove height (internal 
>chimney), blowing on it (as with a fan), or mixing other things into the 
>fuel (like oily nuts).

Oily nuts would both increase the cv and produce an offgas with a low
spontaneous ignition temperature compared with pyrolysis offgas, the
same is true of old resinous wood.

>and most of the fuel goes off as dense smoke.

Which of course is wasted energy, so this is a negative feedback.

>
>I am optimistic that your TLUD will be able to burn at least parts of the 
>husk well.

I'd like to see the attempt made but in general the poor heat feedback
of the tlud men as I'm not optimistic, a simple updraught, well
insulated, serially batch fed stove with some heat re circulation
looks the way to go.

If it is a wet fuel then the char route may offer a way of refining it
to an acceptable fuel and the briquetting will increase the bulk
density.

AJH