[Stoves] Ceramic Philosophy: Request for Pyrolysis Biochar Stove Designs

Mon Feb 13 12:16:44 CST 2023

Dear Friends of Ceramics and Such

The question as to the materials one might use for a TLUD reactor is an interesting one.  There are some basic rules which will help a selection process.

  1.  The firing temperature of a clay (ceramic) material has to be higher than the temperature that the TLUD will reach Or there are very likely to be disastrous results. It is very difficult to get the temperature of a clay component above 900 C when it is running, so a practical firing temperature for that application will be 1100 C (which is not difficult to attain).
  2.  The tendency of a clay material to fracture is caused by differential thermal expansion.  If the material is "insulative" it is more likely to break because there is the maximum temperature difference between the hot and cold sides.
  3.  The thermal expansion can be characterized using a metric expressing the % increase in size per degree C.  An expansive material like ordinary clay or red clay is 5x10-6 to 8x10-6.  A low thermal expansion material such as was used making the POCA charcoal stove in Maputo was 5 x10-8 which is only 1% of the usual clays used for making bricks and stoves.
  4.  The strength of a fired material is independent of the thermal expansion coefficient. You can have an exceedingly strong material with a low thermal expansion, or a porous material with the appearance of pumice.
  5.  Low expansion conventional materials tend to have a high lithium or lithium carbonate content. By conventional, I mean that is a standard way to lower the expansion coefficient - add lithium.  Iron tends to be the worst additive for expansion, so the material from Bikita Mine in Zimbabwe is used a lot because it has no iron in it. It has about 4% lithium and some good results were achieved using this material 55% by mass in the total mix.
  6.  When the inside surface of the TLUC gets hot, it expands stretching the outer which cracks because the tensile strength is much less than the compressive strength.  If you can create a mix that has a high tensile strength, and a low compressive strength, you will become famous.
  7.  The materials used to make clay pots that are placed on a fire, and the mitad cooking plate used in Ethiopia are usually made from a blue clay that has a naturally low thermal expansion coefficient.  There is one site in Zambia where the natural clay is 2.3 x10-6.  That is the material used for making clay cooking pots sold in Lusaka.
  8.  In Thailand there is a well-developed technique for using ordinary clays to make stoves that do not crack.  The idea is to make them with a vertical slice so it is not actually a complete tube.  The two-pot clay stove made in Thailand with one pot higher than the other has such a vertical slit in both of the chambers.  There was a problem firing them because the clay twisted and the pot resting surface (which seals against the bottom) would be slightly spiraled.  This was solved by placing a strip of clay across the top of the slit before firing, and pressing it ever-so-gently against the outside surface.  This strip was fired along with the stove and when it was done, the strip was knocked off.   Then the fire is lit, the whole chamber expands and contracts (within limits) and stops the cracking. When the inside is hot, the crack expands and contracts as it cools.
  9.  This approach works up to a temperature in the range of 500 or 600 C, but it will develop mini-cracks rapidly if pushed past that.
  10. Making a TLUD from clay present a special set of problems because it requires as tightly controlled primary air flow.  Combining a metal strip to cover the vertical slit might turn a hopeless clay cylinder into a viable reactor.  This would be cheaper than making the whole stove out of metal.
  11. The future, I believe, is not to use metal or clay, but to make the whole stove out of a geopolymer.  Primitive geopolymers can be made from a clay and a mix of 1:3 sodium silicate and sodium hydroxide (the latter being what is used to make soap). I am making something very complicated overly simple, but in short, mixing this blend into certain clay produces an extremely strong polymerized rock which can take high temperature without being fired first, something that cannot be said for ceramics.
  12. There are fired geopolymers as well, however my inclination is to try to make something that is superior to ceramic and which can be produced without firing to a high temperature.  I have seen a foamed geopolymer that was slightly flexible, extremely heat resistant (over 1100) and very insulating. It fact it can be nailed to a wall without breaking.  That would be perfect for a TLUD.
  13. The book on the subject is "Geopolymers - 5th Edition" by Joseph Davidovits, in France.  Some of you may have heard of the artifact "Venus from Dolni Vestonice".  It is 25,000 years old and is made from a geopolymer, not fired clay.  Wikipedia<mailto:https://en.wikipedia.org/wiki/Venus_of_Doln%C3%AD_V%C4%9Bstonice> says it is low temperature fired clay. In fact it is a clay mixed with alkaline salts and wood ash.  There are a number of ways to make low temperature (300 C) geopolymers.
  14. At Pumapunku<mailto:https://en.wikipedia.org/wiki/Pumapunku> there are all manner of "impossible" stone slabs. They are said to be "carved" but this group had no metal tools capable of carving stone to sharp inside corners.  In fact all the stones shown with smooth holes and sharp corners are cast from a ferro-sialate geopolymer.  It particular the "red sandstone"  blocks (up to180 tons) are much larger than any host material deposit in the area.
  15. Have a look at this photo: https://en.wikipedia.org/wiki/File:Puma_Punku10.JPG  This is a cast object, not carved from a solid stone. Note the cramp sockets cast into the top.  These are identical to the metal ones used in ancient Athens and identical in function to the stone ties used at the top of Borobadur in Java.  What does that tell us?
  16. In fact, at Pumapunku there are alkali and acidic geopolymers (the result of which are very different colour).  These materials have far greater strength and heat resistance than fired clay. The activator was probably Na2CO3 (natron).  The Si/(Al Fe) ratio is 3.22.  This is important for strength in order to create the -Fe-O-Si-O-Al-O-Fe... geopolymer.  There are various ratios that work, but read the book to get a grip on that.  Importantly, there is a high iron content in these, which goes against standard low expansion ceramic thinking.

If we want to make low cost TLUD chambers it is likely that the best materials will be "modern" in their chemistry and ancient in their technology.

It is likely they will be made of polymers chains of -Si-O-Al-O-Si- cross-linked with -O-Si-O- which has a Si:O ratio of exactly 3.  It will take 1000 C.

Regards
Crispin

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