[Stoves] A heat-resistant insulation mix

[Fireside Hearth]

Hello Xavier, 

          Just a long shot here, but is your space between the walls sealed enough that you might be able to use plain old sand? Is sand available to you plentifully enough? While I can think of things like kao-wool insulating blanket which would offer a 1200 deg.c. insulation without burnout, this may be completely out of reach both cost and supply wise. Sand may not be the best at stopping heat transfer, but it might be good enough, and free. Another thought might be your inner wall surrounded by sand and a mid wall and more sand before the outer wall. One could maybe use some scrap metal to create a "midwall" with sand on both sides creating two thermal breaks (different material types coming into contact) as a way of focusing more heat back into the burn chamber and less radiant heat transfer through the outer wall.......food for thought.
Roger.

From: xvr.brandao at gmail.com
To: stoves at lists.bioenergylists.org
Date: Mon, 8 Aug 2011 23:07:14 +0100
Subject: [Stoves] A heat-resistant insulation mix
















Stovers,

 

I wanted to submit another question related
to the institutional rocket stoves (IRS).

We encountered a problem with the
insulation mix we used. In Benin, there are only a few oven to produce ceramics.
They are already used for bricks and NANSU (kind of Jiko) ceramics. Demand for
NANSU ceramics is huge from metal craftmen. So big that one of them grew tired
to wait, and made his own ceramics, with a simple wood fire. It is of poor
quality, and it breaks easily. Since this option and building our own oven was
not possible (too expensive and we are not ceramists), we needed to find an
insulation which didn’t need to be cooked. That is also why we want a
metal domestic charcoal stove.

So we followed a recipee described in the
IRS manual. Between the inox combustion chamber and the sheet metal outer body
of the stove, we used a mix of 6 parts wood saw dust and 1 part of clay. On the
stoves we made in the military camp (where they prepare from dawn till dusk),
the mix almost completely burnt after a few weeks of use, leaving only a thin
powder at the bottom of the space between the inox and sheet metal layer.

 

We don’t have anything to analyse it,
but we are pretty sure the powder pile at the bottom is clay powder, the same
clay powder we put in the mix. The sawdust burnt because of the heat. Small
smoke was coming out of the small holes in the outer layer of the stove when in
operation, certainly because the sawdust was burning. We thought that was the plan
: the sawdust would burn and leave holes in the mix, making a kind of
insulative ceramic. The strange thing is that it didn’t burn like that in
the two first stoves we made. At least not that we know.

Maybe is it because we then used water to mix
the sawdust to the clay? The mix clay-sawdust for the camp stoves was dry.

Or perhaps with such a thin layer of inox
(1mm), the sawdust is meant to burn completely, and 6 parts is too much for 1
part of clay. At one point or another, the only thing left would be a pile of
clay.

 

We are thinking about different things. We
need to do without ceramics.

For the next stoves we’d like to replace
inox layer + insulative mix by bricks. It will be less expensive. We want to
try to cut standard size clay bricks to the size of our combustion chamber (it
seems bold) and put them in the sheet metal box. Then to bind them with
something strong. And perhaps had an extra layer of cement + clay, I think this
is what you recommended Crispin, to prevent users from breaking the bricks when
pushing the wood. We need something heat and shock resistant, long-lasting,
efficient and cheap, and that seems like the best solution.

 

As for the stoves left empty, it seems that
the cooks don’t complain. Somehow, they are still efficient and clean.
Air is also in insulator. But we need to refill them. We think to avoid using
sawdust, and put a mix of clay + cement + water.

Do you think that would resist heat and
last? What are your experiences of making insulative layer, without the help of
ceramics? And with cheap and wide-spread material? Rock-wool for example is too
expensive and too rare. People here advise us to use laterite. It is cheap and
to be found everywhere, heat and shock resistant. Metal workers use laterite
bricks for furnaces. But I doubt about the energy efficiency, compared to clay.

 

Thanks in advance,

 

Xavier







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