[Stoves] Space between biomass fuel and cookpot in an open-fire cookstove

[K McLean]

Crispin,

Thanks for your always helpful advice.

For widespread adoption, we sometimes must sacrifice efficiency for
simplicity.  With the cooking hole, that probably means using bricks
without curves.

The cooking hole is new and I hope others test variations.  Here is a video
<https://drive.google.com/file/d/1Wn-wudQ_5InNPaKrSi6MtJS2rUvjeaKi/view?usp=sharing>
of one cooking hole heating two cookpots.  We found it was smoky without
the hole between the bottom bricks,

As a reminder, the cooking hole is a 22x22x24 hole in the ground.  The fuel
is piled up so the top is 10 cm above ground.  Sticks and maize stalks are
stacked in a criss cross manner to allow air to flow within the fuel pile.
The pile is lit on top.  It burns down without tending for 30-90 minutes
without adding fuel.  Water in both pots boils in about 15 minutes and
simmers for the rest of the 30-90 minutes.  I am told that the visible
smoke is much less than from firewood in unmodified three stone cookstoves.

[image: Youtube Photo.jpg]

I think custom bricks that are not fired in a kiln ("unburnt bricks") will
harden while cooking meals.  We are testing this and I'm optimistic.
Ripple Africa has found that their unburnt bricks harden while cooking. And
our custom bricks that line the cooking hole are starting to harden.  Using
custom, unburnt bricks to support cookpots will allow us to determine the
best dimensions and just distribute molds.  Our networks make the
distribution of molds in Africa and local production of the custom bricks
easy.  It sounds like they should be about 10 cm high so that two bricks
will elevate the cookpot 10 cm above the top of the fuel pile.

Kevin

On Fri, Jul 22, 2022 at 6:16 PM Crispin Pemberton-Pigott <
crispinpigott at outlook.com> wrote:

> Dear Kevin
>
> I looked into this when developing the Vesto and there are two answers.
> One is the correct height (minimum?) Above the fire to get good draft to
> pull air into the fire at a high enough velocity to get good mixing, and
> the flame space needed to complete combustion.
>
> For wood, I found the vertical space needed is at least 100 mm, with a
> preference for 110 mm, and for charcoal 50 mm with  preference for 60 mm.
>
> You drawing shows two parallel rows of bricks. This should be two
> semi-circles in order to limit drawing in useless air. Cooling the flame
> with unnecessary excess air is a major cause of PM with wood stoves and CO
> in charcoal stoves.
>
> Having a fuel hole and an air hole is important, closing everything else
> if possible. Putting a stone/pebble grate as you have been, is a great
> help. That allows incoming air to burn the accumulating char, raising the
> combustion temperature. That of course reduces all emissions from
> incomplete combustion.
>
> Sheltering a fire on three sides is so-o much better than two parallel
> walls. So if they have bricks, even mud bricks as are common in the Eastern
> Cape, the result can be pretty good.
>
> As a general rule, put the air entrance at the back bottom, opposite the
> fuel entrance, not on the same side as per rocket stove. It will keep the
> flame centered and away from the walls. That plus >100 mm flame space will
> give a reasonable result.
>
> Keep up your good work!
>
> Regards
> Crispin
> *From:* kmclean56 at gmail.com
> *Sent:* July 22, 2022 9:41 AM
> *To:* stoves at lists.bioenergylists.org
> *Reply to:* stoves at lists.bioenergylists.org
> *Subject:* [Stoves] Space between biomass fuel and cookpot in an
> open-fire cookstove
>
> What is the minimum distance from the top of the fuel to the bottom of the
> cookpot?  I've heard that 8 cm is the minimum to get the best combustion.
>
> We are training cooks to use common bricks to elevate two cookpots over
> one cooking hole.  I want to make sure the cookpots are high enough.
>
> [image: 2 burner drawing.jpg]
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