[Stoves] Limiting factor for secondary burn?

[AJH]

On Thu, 10 May 2007 13:36:35 -0700, frank wrote:

>Can't we make a gasifier by igniting the fuel at the bottom and starving 
>it of O2 resulting in the pyrolysis gases moving up through the fuel to 
>the secondary oxidation step?

Well that's how a standard bonfire works but it's hardly a gasifier
because though the exhaust is all gas and aerosol  its all been
diluted and degraded as it passes through the fuel and gets mixed up. 

You can turn this into an updraught gasifier but you need to meet a
few conditions, the first is to have a sufficient depth of burning
char at the bottom to convert all the incoming oxygen in the primary
air to CO and not a mixture of CO2 and CO. The second is few heat
losses. If you don't get the temperature in this char burning zone hot
enough then you don't produce CO and as the offgas rises through the
fuel it cools as it heats and dries the fuel, resulting in an off gas
that's cool, diluted by steam and CO2 which won't support a flame,
hence a bonfire initially just gives off dense white smoke.

>Is there an advantage to using the TLUD configuration over the other 
>configurations

Yes, it produces a high calorific value offgas at a steady rate from a
batch loaded device.

> (BLUD, TLDD, BLDD)?  It would seem to me having the 
>primary pyrolysis moving up with the fuel in front and the char below 
>would help dry out the fuel and prepare it. 

It would except the offgas then gets cooled and diluted by the water
from the drying.


>The char would be left out 
>of the way.

No it wouldn't because it's hot and the first thing the incoming air
contacts, so it burns preferentially to any offgas.

>For now I just want to know the best configuration to pyrolysis of the 
>fuel and provide clean gases for the secondary burn.

That's a $64k question, there are lots of possibilities.
AJH