[Stoves] Damp fuel in TLUDs

Paul Anderson psanders at ilstu.edu
Tue Mar 22 22:16:28 CDT 2016


To those interested in TLUDs and char-making,

Yesterday I was with Dave Ely who operated a large double-walled TLUD 
for char making (30 gal barrel inside a 55 gal barrel, as TLUD, not as a 
retort).  We did not have much fuel-wood that was nicely dry.  Dave 
wrote (after I left before the end of the operations):
> after breaking down the 55 gal tlud ..., I observed a lot of wet tree 
> sap/oily residue. This  liquid had to contribute greatly in issues of 
> keeping the systems effeciently working. When mulberry brush limbs are 
> vertical in the column, and heated, the sap oozes out branch points 
> and bottom getting all surrounding wood wet, which does not allow 
> efficient heat transfer down the column. 
Commentary:  TLUDs operate with "glowing or flaming" pyrolysis at 
relatively low temperaturs of 550 to 650 deg C.  [The high and main heat 
is where the gases are being combusted].  As the MPF (migratory 
pyrolytic front) moves downward, some of the heat radiates downward, 
warming and drying the adjoining pieces of fuel.   In cases with fuel at 
20% Moisture Content (MC) or less, the moisture is vaporized and it 
moves upward.  I (and probably some others) commented about the "loss" 
of useful energy that was needed to vaporize the moisture.  I never 
thought much about other possible outcomes of the excessive moisture in 
damp fuel.  [Maybe others have already noted and reported on this issue, 
but it is new to me.]

If the fuel is sufficiently damp (but at what % MC is not known), the 
heat can do at least two things:

1.  Vaporize the water, but that water can condense back into liquid on 
cool fuel, making adjoining fuel increasingly more damp.

2.  Force sap (mainly water) out of the fuel in liquid form, making 
adjoining fuel increasely more damp, or drippings in the bottom of the 
fuel column.

In both cases, there is an increase in the amount of water in the next 
lower area that needs to be heated to pyrolysis temperatures. When the 
amount of water is too much to overcome (that is, to evaporate away from 
the zone of pyrolysis), the MPF is gradually weakened and eventually 
extinquished.   And ALSO the amount of water vapor in the rising hot 
gases could be too much to sustain the secondary combustion.

So, as Dave observed, as the TLUD proceeds to operate with damp fuel, it 
eventually "floods itself" with watery sap, etc.

Note:  In a "typical fire", damp wood does not burn well, but it does 
not have the physical vertical structure that results in the 
acccumulation of excessive moisture in the remaining fuel in lower levels.

Also, for char making with "damp-ish" fuel (perhaps above 25% MC?), the 
"flame cap" technology is better suited than is the TLUD technology 
(unless the TLUD heat of one batch is used to pre-dry the fuel for later 
batches).

Paul

-- 
Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype:   paultlud    Phone: +1-309-452-7072
Website:  www.drtlud.com





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