[Stoves] "Vom Pyrolysekocher zur terra preta" Workshop 11-12.06. --> Clay TLUDs

[Julien Winter]

Dear Rebecca;

Thanks for sharing your experience with rice hulls.  You have saved me some
trouble.

Rice hulls were worth testing, because they are plentiful, whereas sawdust
is harder to come by.

I have seen papers were rice hulls were used in bricks, but I have wondered
if they would work in refractory materials because some forms of silica
expand over a narrow temperate range and cause microcracks.  A diagram of
the phenomenon can be found here:
http://www.thaiceramicsociety.com/spaw/uploads/images/quartz%20inversion.jpg

Rice hulls can contain 20% silica, but I don't know what chemical structure
silica in raw hulls, or pyrolyzed hulls.  The surface of rice hulls may be
impervious to clay particles.  The exterior surface of plants is usually
coated with wax to prevent water loss.

By contrast, particles of sawdust will expand when added to wet clay, and
clay particles can form electrostatic bonds with glucose monomers.
Pyrolyzed, there is a little ash remaining.

Would rice hulls work better if they were pretreated, such as by exploding
them in boiling water?

Something else worth trying is "paper clay" where soaked and pulverized
waste paper is mixed in with the clay.  In some countries, waste paper is
abundant in the form of politicians' election posters ;-}

Of the organic matter added to the clay body, a herbivore manure, and straw
or fibrous material is a common component.  The manure provides fiber and
makes the green clay easier to work.  The fiber reinforces the outer walls
of the stove body against cracking.   Fiber can be added as straw or retted
plant stems.



----------------

Back to the subject of the TLUD in Werner's pdf attachment.

If you build a TLUD reactor in a single clay cylinder or pipe, it will
start out as hot at the top and cold at the bottom.  This creates
differential expansion in the clay and can cause cracking.

Notice how the reactor in Werner's pdf is built up as a stack of rings.
This is a good way to approach differential thermal expansion, because the
rings will expand and contract independently.  There is much less stress in
the stove walls, up and down the reactor, as the ignition front moves down
through the fuel.

The expansion stresses (thermal gradients) in the TLUD are mostly vertical,
so if a reaction chamber is built up of vertical staves (as we often see in
rocket stoves) the problem is not properly addressed.

If there is any air leakage at the junction of the rings, that leakage may
(or may not if the leakage is diffuse) serve a similar function as pilot
holes.

If a ring breaks, then the stove is not useless.  It is just shorter

An additional advantage of stacked rings, is that you can shorten the stove
for denser fuel (e.g. pellets), or for a shorter cooking time, or for short
cooks.

Cheers,
Julien.

-- 
Julien Winter
Cobourg, ON, CANADA
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