[Gasification] Pyrolytic vs Carbon Gasification

[Daniel Chisholm]

On Wed, 2006-31-05 at 06:43 -0600, Thomas Reed wrote:


> In conventional downdraft gasifiers the reaction temperature is 
> typically >900 C and so the products of the pyrolysis (upper) of the 
> fuel react with the charcoal produced to have a clean HRG for the 
> operation of engines.

[..snip and shuffle...]

> In our WoodGas stove (TLUD or inverted downdraft), the flaming pyrolysis 
> zone passes down through the biomass and generates a rich pyrolysis gas 
> which has probably >0.1% combustible volatiles (tar).  The reaction zone 
> has a typical reaction temperature of 700 C and permits all kinds of 
> high energy but condensible products to exist.

Tom, why is it that the gas generator portion of your TLUD woodgas stove
ends up operating as a low(ish) temperature, tar-rich downdraft
gasifier?  Is it simply the (very small) size that prevents the
attainment of 1000C+ (?) combustion-zone and 800C+ reduction-zone
temperatures, or is there something else that I am missing?

Should I be thinking of the gas generator portion in a different manner
than an (inverted) downdraft stratified gasifier?



> It does not consume much 
> charcoal unless the fuel is wet (25% MC gives a char yield of 5%; 5% MC 
> yields 25% char).  The volatiles burn very well, but would require 
> higher temperature to convert to a clean, "hydrogen rice gas". 

Once the pyrolysis is complete, it seems that the fixed primary jets are
delivering enough air to give full combustion in the charcoal bed.  Is
this true?  Or is there enough CO that I ought to be getting combustion
with the secondary air, but it is so excessive (and at this late point
in the cycle, so far away from the char bed) that it is too cool to self
ignite or too maintain a flame?


> I have recently added an internal sleeve to our woodgas stove so that 

Aha, thanks for this tip!  I was thinking of coming up with something
similar, your idea is simpler and better.  I think I'll put some holes
in the sleeve that can line up with the secondary air ports, so that I
can rotate the sleeve and get 0-100% secondary air.  (It seems that in
many cases, the amount of secondary air is excessive, it is on the
borderline of blowing out the gas flame).




-- 
- Daniel
Fredericton, NB  Canada