[Gasification] Pyrolytic vs Carbon Gasification

[Thomas Reed]

Dear Dan and All:

In conventional downdraft gasifiers air and gas are pulled DOWN through 
the fuel, even though the hot gas would like to rise, so gas velocity 
must exceed the natural convection velocity.  Also the charcoal bed is 
supported and can't escape.  The pyrolysis superficial velocity (PSV) is 
typically 0.3 m/s (m3 gas/sec-m2), very high intensity (1.8 MW/m2 for 6 
MJ/m3 gas) and  very high temperatures, 900-2200 C. With a PSV of 0.3, a 
3 kW stove would have an area of 3/1800 X 10^4 = 16.6 cm^2 or a diameter 
of 4.6 cm, < 2", a very inconvenient size for fueling (and the gas would 
come out the bottom). 

In the inverted downdraft gasifier too high a gas velocity will blow the 
charcoal bed out the top, so the upper limit to PSV is ~0.06 m/sec, and 
temperatures are much lower, typically 700-750.

So downdraft and inverted downdraft are complimentary in their 
functions, temperatures and intensities. 

TOM REED        BEF



Daniel Chisholm wrote:
> 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).
>
>
>
>
>