[Gasification] [Stoves] Limiting factor for secondary burn?

[Thomas Reed]

Dear Frank, Andrew and All interested in gasifiers and stoves:

Andrew is right on the money.  A *bottom lit updraft gasifier*, burns a 
small amount of charcoal or coke to make hot CO and N2 which then 
pyrolyse the descending fuel to make a VERY tarry  (high volatile) gas.  
These are both pyrolytic gasifiers, and produce lots of charcoal.  A 
true gasifier also converts the charcoal to more CO in a second stage 
and minimizes the volatiles. 

For coal the updraft gasifier is a Lurgi type.  The volatiles are 
benzene, toluene, xylene and coal oil, enough to be commercial products 
and power the Wehrmacht during WWII. The resulting coke is useful for 
making steel. 

For biomass the volatiles are a few hundred monomers, oligomers and 
fragments of cellulose, hemicellulose and lignin, most of which are 
laboratory curiosities, but typically 2 % methanol from hardwoods only, 
some acetone and acetic acid which were commercially produced by wood 
pyrolysis until about 1950 when it was easier to make them from oil or 
natural gas.
------------------------------------------------------------------------
A *toplit* *updraft gasifier*,  burns most of the volatiles to make 
combustible gas and charcoal.  It is the basis of our WoodGas stoves. 

It isthe first stage of a true biomass gasifier which then converts the 
resulting charcoal to more CO.
So choose your poison and products before designing the gasifier...

Yours truly,

TOM REED              BEF
------------------------------------------------------------------------

frank wrote:
> Dear AJH, and stovers,
>
> see below:
>
> AJH wrote:
>
>   
>> 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.
>>  
>>
>>     
> Thanks for this!
> 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. 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
>
> 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(?). Resulting in a dirty burn.
>
> 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(?). After reading how the theater fog maker works I think the white 
> smoke is like what is produced in the theater fog maker .
>
> Thanks for the time and all this info all have offered to help me 
> understand how this works.
>
> Frank
>
>
>
>
>   
>>  
>>
>>     
>>> 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
>>
>>
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>>  
>>
>>     
>
>