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

[Crispin Pemberton-Pigott]

Dear Julien and Rebecca

 

The matter of the impact of silica on the cracking tendency of ceramic components is central to making stoves that survive.

 

I have worked on this with CermaLab of Pretoria for ProBEC and the result are still available at the ProBEC website.

 

The suppression of the silica transition from state A to B, which happens at 573°C, crack glass and ceramic. This transition can be smoothed to take place over a larger range of temperature (with glass it is almost instantaneous) by adding various ingredients and optimising the firing temperature.

 

The addition of lithium is a common approach, so lithium petalite is one solution. We used a mix that included 55% lithium petalite in one project. Lithium carbonate is much better by very expensive.

 

The transition is also accomplished by adjusting the chemistry of the clay (which could include all sorts of biomass) so as to bind the silica. This is what happens with a baldosa tile made the traditional Aztec way. I found that the free silica content of a baldosa tiles was essentially zero even though the silica content was more than 50%.

 

There is a book by Hugh Allen available on line (1991, ATI) about how to make improved Kenyan jiko stoves. It is the ‘poor man’s way’ to develop a clay body – what to do when you have no access to a lab (XRF, XRD) and you want to use what is available.  I think we can do better with more modern understandings + Hugh’s advice which is iterative and takes about a year.

 

Thermal shock resistance is not only from suppressing the silica transition, or having a low thermal expansion coefficient. It also should be strong enough to have a good service life. Not a lot of information on making domestic stove ceramics is available because people who work out how to do it keep the knowledge secret. 

 

I someone wants to embark on a research project into this for a particular location they can contact for some pointers.

 

Regards

Crispin

 

 

 

 

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.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20160505/265f62ac/attachment.html>