[Stoves] Limiting factor for secondary burn?

[AJH]

On Fri, 11 May 2007 12:36:11 -0700, frank wrote:

>So, if I understand this, the char is VERY important in a gasifier  
>because we need to convert the pyrolysis gases to CO before it goes to 
>the secondary oxidation.

First there are no fixed lines here, some CO will be from pyrolysis
offgas but most will come from the gasification of the residual char.
The char is formed higher in the bed by heat from the reactions below,
as it forms pyrolysis offgas is driven off and rises with the flow of
products from the reaction below. This hot mixture of products of the
reactions below and pyrolysis offgas heats and dries more fuel above
as it rises but in doing so is itself cooled and diluted by the water
from the drying. So in general, in an updraught device with a fire
underneath the pyrolysis gases cool as they rise.

Now the constituents of the reaction that supplies the heat for the
above varies depending on how the system is set up. In a gasifier the
intent is to react air ( a mixture of 20% oxygen and 80% inert
nitrogen) with the char that has worked itself to the bottom such that
the oxygen and char gasify to CO. For this to happen the char bed must
stay above ~800C and be deep enough to react out all the oxygen. In
practice this means the char bed is very hot at the bottom.



> So the configuration must have the hot char 
>between the secondary oxidation and the pyrolysis. Now I understand the 
>brilliant design of the TLUD stove. and the many challenges to get 
>everything fine tuned

Well, the TLUD  is an especial case, there is no need for the hot char
to be there once formed but there is no way it can be removed. It is
preserved by the fact that, though hot enough to burn, it is protected
by the hot pyrolysis offgas and the small amount of products of
combustion that were a result of the controlled combustion sustaining
the downward moving pyrolysis zone. So this rich offgas devoid of
oxygen percolates up through char formed earlier in the reaction  and
is then used to power a secondary flame with the addition of air
higher in the stove.


>
>If we have the primary air go through the char first and only CO 
>continuing to the fuel (and heat) we will have all CO AND pyrolysis 
>gases going to the secondary oxidation(?).

Yes

> Resulting in a dirty burn.

Not necessarily if the conditions can be maintained correctly but in
practice what happens is the fire loses too much heat and the
combustion in the char bed does not reduce CO2 formed when air first
combines with carbon back to CO. So the products of combustion low
down are largely CO2 and Nitrogen  these gases have no calorific value
and so they just dilute the offgas formed further up and make it
difficult to burn, hence lots of blue smoke.
>
>So the reason we do not want the white smoke is because that is 
>pyrolysis gases that still need to go through the char and turned to 
>CO(?).

No the white smoke is a mixture of combustion products, steam (or
water droplets) and pyrolysis offgas so dilute it will not support a
flame. In general the pyrolysis off gas and char do not react, the
driving reaction is the combustion of carbon to CO2 or CO.

AJH