[Stoves] stoves by the millions

[Crispin Pemberton-Pigott]

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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