[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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