[Stoves] stoves by the millions

[mtrevor]

Dear Chrispin

Thank you for the edifying comparison and explanation on refractory issues. Dr Goreau advise he believes biorock 
would not work in an internal stove application, unless we want to make quick lime or use with betel nut. However a mesh
or wire frame for the external housing might work. 



----- Original Message ----- 
  From: Crispin Pemberton-Pigott 
  To: 'Discussion of biomass cooking stoves' 
  Sent: Saturday, June 01, 2013 5:18 AM
  Subject: Re: [Stoves] stoves by the millions


  Dear Paul O

   

  You have given another good example of the difference between a refractory material (which means it can withstand very high temperatures) and a material designed to resist thermal shock. High temperature shock resistance is not synonymous with 'refractory'. The word 'refractory' is often used incorrectly in stove discussion. Stoves do not necessarily benefit from having a refractory rating on the combustion chamber.

   

  Stove combustion chambers must resist rapid temperature changes in a short period of time at a moderate temperature (600-1000° C). Industrial ceramics designed for refractory applications often have a very slow heating rate, particularly when fired for the first time.

   

  When a refractory material is capable of doing both jobs (unusual) and it is new, it usually has to be fired at a very low rate - taking from 1 to 2 days to reach max temperature when initially commissioned. This to remove moisture and in some cases, to do a final firing. Castable ceramics are all in this category (the ones I have seen, anyway). They have to breathe out the crystal water otherwise steam pockets break through the material.

   

  There is a paper on the ProBEC website explaining why ceramic parts crack and what to do about it.

   

  Stress and Strain:  

  A Rocket type-stove has all the air coming from one side and this drives the flame against the back all of the combustion tube. This is about the most demanding possible arrangement as the temperature of the flame varies rapidly and frequently applying large internal stresses on the material particularly on the top 1mm which flakes off, even if the body does not crack (though usually it does).

   

  In order to minimise this effect, you can bring the air from both sides (as one list member has done recently with a distillation device) to counter each other and keep the flame in the centre. Xavier was also experimenting with this layout in Benin and was successful in bring the ceramic failure problem under control (right Xavier?).

   

  Of critical importance when creating a ceramic for stove combustion chambers is a low thermal expansion rate and a strong suppression of the silica transition from α to β phase at 573 degrees.

   

  The wall temperature can easily exceed 573° C but does not often reach 1200 so the challenge is not a refractory one but an expansion issue. If a simple clay body is hot on one side and cool on the other (insulating) is it very likely to fail catastrophically after a few cycles.

   

  Regards

  Crispin

   

  Art,

  Not long ago I made reactor using refractory ceramic pipe. This pipe is designed to handle molten metal and is supposed to withstand temperatures up to 1,750 C. In less than two minutes into a TLUD burn with rice hulls, it cracked. It is hard to imagine that a reef-formed reactor will last very long.

  Paul Olivier



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