[Stoves] ash layer beneath fire

Mon Jan 27 09:12:11 CST 2014

Dear folks,

Very interesting proposals, and much appreciated. The question was prompted by the overall need to stop the heat in the firebox from reaching the foundation (made of clay) and burning it up. As we install the liners in kitchens that have been using three stone open fires we find that we have to dig a bit deeper to get below the burnt up soil in before building the foundation.

The option of a grate, whatever else it does, will definitely increase either the cost of the process or of the stove, so that is a no-go zone, for this particular stove.
Also, since the installation process depends on the skills of diverse artisans scattered all over the country it means that any improvements must be 'transferable' and

Vincent Okello

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Dr.-Ing. Dieter Seifert
Sent: Thursday, January 23, 2014 3:21 PM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] ash layer beneath fire

Dear Crispin, Richard and Vincent,
I should like to make some remarks regarding the practice in the use of a stove with a grate fuelled with thin sticks or appropriate pellets:

The lighting is greatly simplified if one places igniting paper in the form of three or four rolls, crosswise to the steel bars. One can then ignite the paper rolls (with the kinlding on them) from the heating port, after having already inserted the pot. The smoke-phase is then short and heat loss is reduced.

With experience one can ensure that the insertion of the sticks is stopped in time, so that the sticks burn completely and the pot contents are parboiled - but the cooking is finished in the thermos-container (haybox, haybasket) as mentioned earlier. The wood consumption may be 330 g (corresponding to 5 MJ of heat content) for cooking 6 kg of food (requiring theoretically about 1.9 MJ heat supply). Thereby a little ash (about 2 g) is produced.

This method reduces the stress on the cook. A consumption of charcoal of more than 1.3 tons per year (more than 8000 kg chopped tree weight) by a household is reduced to about 600 kg of thin sticks, moreover with more comfort for all involved and less health burden.

I believe that this technology is in the sense of E. F. Schumacher that is promoted exemplary through Practical Action.

With kind regards
Dieter

Am 22.01.2014 23:51, schrieb Crispin Pemberton-Pigott:

Dear Richard and Vincent

Jambo.

I see just about everyone found that adding a grate to a flat-bottomed stove

improves the performance.

RS>.I would advise use of round bars as Crispin but I would suggest you

grind any round bars down to result in effectively a series of had cylinders

along their length with the flat side up.

I would advise the opposite and the metric to use in evaluating the

difference is the mass fraction of charcoal produced by the fire. We found

(for reasons that are not yet known) having a flat top definitely produces

more char thus losing more heat. It may have something to do with the

thermal contact the char piece makes with the metal, conducting the heat

away. This lowers the temperature of the 'coal' and causes it to burn poorly

or go out. There was a large mass difference for flat and round bars - more

than double.  The thermal contact between a coal and the round bar is much

smaller.

That is the only 'guess' I have so far. We will look into this further at

YDD this year. The two things tried were round bars from mild steel

(multiple welded pieces) and cast iron with a flat top (single piece). Cast

iron conducts less heat than mild steel but it still consistently produced

much more char. That is not useful as fuel in most homes and it is tossed

with the ash. Indar reported the round bar grate produced virtually no char

at all, finishing its burn at virtually the same time as the wood. That was

amazing.

You may need to restrict the air entry area to limit the air level below the

combustion chamber. If it is 'right' you will see large flames wandering

around in a low velocity environment below the pot - no flapping arms of

flame.  You should limit the pot gap (by shaping those tongues of clay) to

about 7mm for all pot sizes. This means trimming the slanted ones and

lowering the top ones (in most stoves).

The combination gives you the right balance of primary and secondary air.

Nzuri kuhusu

Crispin

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