[Stoves] Chimneys

[dave kuchanny]

Hi Crispin / stovers

I've avidly followed your links, posts and advice for some time now, first time to write anything myself.

What about the zero cost stoves with chimneys? Surely these are amongst the most desirable in terms of economic effect to IAP and improved efficiency.

I have recently been running a study in Zambia on alleviation of IAP from cookingstoves using a participatory approach, not the 'top down' imposition of technologies. I did however showcase and example improved cookstove, promoted through GTZ in Uganda, the mud rocket stove called (I believe) the Lorena. This adobe (anthill soil and grass) based stove uses banana stem as formwork to build the fire box and chimney etc around, and has no financial costs to produce. The construction manual is straightforward, with simple diagrams, and with a 2 pot stove like this one, it is appropriate to the cooking dynamic of the region. It also comes with cleaning and maintenance guidelines to stop the sooting of the chimney becoming an issue.

The details are found here; http://www.energyandminerals.go.ug/pdf/gtz/HOUSEHOLD%20Stoves%20Construction%20Manual%20August%202008.pdf 

There are 2 issues I am aware of, but am looking for greater knowledge and further insights from the group.
1.  The stove cooks with small pieces of timber, but the local vernacular is split approximatley 50:50 wood/charcoal, so this will be inconvenient or completely inappropriate to some. Is it possible to use charcoal in stoves such as this, and what changes would be needed?
2.  I am aware of the improvement gained by adding insulation. This stove has large mass, insulative in part by the grass in the anthill soil 'mud' body. A fired clay insert that followed the heat pathway through the stove, in place of the banana stem formwork, would mean that initial heat would transfer more directly to the pot, not into the stove body, and increase fuel efficiency. Are there no cost methods to achieve this? What if there are no kilns nearby? Can we cerate this improved insulation by, say, adding vermiculite (not sure if locally available) or other material in larger ammount to the adobe mix surrounding the banana stems? Ash perhaps? Would we then need a stronger binder than the clay soils, like lime?

I am working towards briquette manufacture, more improved stoves and biochar potentials in the area but am looking to stagger introduction of these (probable) improvements. There are at present almost no improved stoves and extremely poor charcoaling and wood production methodologies, with little understanding of IAP andthe efferct of smoke. It makes sense for the transition to be gradual and community owned for there to be lasting adoption.

Any advice, contacts or cautions would be greatly appreciated.



--- On Mon, 1/11/10, Crispin Pemberton-Pigott <crispinpigott at gmail.com> wrote:

From: Crispin Pemberton-Pigott <crispinpigott at gmail.com>
Subject: Re: [Stoves] Chimneys
To: "'Discussion of biomass cooking stoves'" <stoves at lists.bioenergylists.org>
Date: Monday, 1 November, 2010, 3:04

Dear Joyce  There are two answers to your question. The first is that CO is not all that big a problem for most people. Yes it is a problem in certain places, Johannesburg and the col burning Highveld regions for example, but smoke exposure is a much larger concern in my experience.  The second is that chimneys are relative expensive. If you put a chimney on a stove that is not very clean burning, it quickly gets clogged and is a maintenance problem.  An example of this is the stoves made from clay and sand in Kenya. In the high regions (tea estates especially) there are ‘fuel efficient stoves’ promoted by the tea estate corporations as a beneficial idea. They have chimneys but are pretty dreadful is terms of combustion efficiency. In as little as 3 months a 3 inch diameter chimney gets clogged with condensed, boiled biomass vapours. The stove have chimneys but don’t really save much fuel and waste a great deal of it by simply not burning the
 gases.   So chimney are not as easy to work with as one would hope. Cleaning up the combustion is actually the most important if there is nearly zero money in the community.   Chimney stoves, in answer to your question about the effect of putting on a chimney, have to have pretty good air control or they are not very efficient.  Attached is a chart of a coal stove with a chimney attached, and no flue damper to control the draft. There is really no way for anyone to know how and when to close or partially close a damper for optimum efficiency. This is the result of an open chimney attached to a fairly large fire. The peak burning rate can be seen by looking for the steepest portion of the brown line. That is the mass burned during the operation.  As you can see the initial burn rate is low so the line is nearly horizontal, then it gets going like crazy to about 16 kW. Then the coal runs out and the burn rate slows. Then it is refuelled with a sharp
 jump up which tapers off in the end after about 200 minutes.  The thermal efficiency is the green lines, The darker one that moves up and down is the instantaneous efficiency calculated from the temperature of the gases in the chimney and the excess air at the time. The smoother green line is the cumulative efficiency, meaning how things have gone so far, all things considered. Two features are noticeable. The first is that it is pretty constant at about 65% efficient when the fire is large and burning at a high rate. The second is that as the fire dies down, the thermal efficiency drops to zero and in fact goes negative. Because it is negative (the fire is actually cooling the room by throwing more heat up the chimney than it is generating) the average for the whole burn drops from 60% at minute 100 to 33% at minute 200. That is amazing, eh?  So putting on a chimney does not guarantee overall success. The main reason for the poor performance is
 excessive draft – there is simply too much air getting into the stove, allowing it to operate at a high power level – too high to be useful actually. This is followed by a period when the stove cools the home drawing, as it does, about 50 cubic metres of -35 degree C air into the house to feed the fire.  So, chimneys make things a lot more complicated providing expected results and additional expense. The expense is not just for the chimney which might cost $5, but also for a stove that is air tight enough to control the combustion reasonably and now waste fuel.  Best regardsCrispin  ++++++++  Why is no one talking about chimneys that get rid of the CO safely? And doesn’t the addition of a chimney change the dynamics of any stove?  Joyce M Lockardrj.lockard at frontier.com503-533-4190 Home503-201-9548 Cell503-533-4209 Fax    
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