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close this book Boiling Point No. 28- August 1992
View the document Biomass Combustion, Chimneys & Hoods
View the document WOOD FUEL :
View the document Chimneys & Hoods for Smoke Removal
View the document Biomass Combustion & the Environment
View the document Charcoal & the Environment - Pros & Cons
View the document Smoke Measurement
View the document Stove Emission Monitoring
View the document Successful Mud Brick Chimneys
View the document Alternative Approach to Wood Combustion
View the document Triple Cone Stove Burning Ricehulls & Woodsmoke
View the document "Energy Assistance Revisited - A Discussion Paper"
View the document Clays for Stoves
View the document ITDG & The Maendeleo Review
View the document NEWS
View the document HEDON
View the document PUBLICATIONS
View the document Research & Development News
View the document Letters To The Editor

Letters To The Editor

Dear Editor

Woodburning stove for rural areas of Southern Africa

I am a final year Mechanical Engineering student at the University of the Witwatersrand, Johannesburg. I am working on the design of a low-cost fuel-efficient woodburning stove and am writing for information.

A survey of the area in which the stove is to be used has revealed that a two-pot ceramic stove is desired. I have studied the manual written by Bill Stewart et Al, Improved Wood, Waste and Charcoal Burning Stoves.

* Could you elaborate on the reasons for choosing a spherical, integral firebox in the two pot stoves.

* It is well known that energy can be released through two reactions:

1) C + O2 -> CO2

2) 2C + O2 -> CO followed by

2CO + O2 -> 2CO2

The first reaction releases 97.6S Btu/Ib and the second 58.8 + 136.4 Btu/lb.

Which process should be encouraged and by what means to give maximum energy release? Is there such a thing as an optimum combination of the two reactions? CO is poisonous - how can it be ensured that none is released from the stove? What sort of effect does airflow and the quantity of air have on combustion? There is obviously an optimum - how can I find this optimum airflow?

* You suggest that the firebox height should be half the diameter of the outer pot. Why?

* Do you have any information on the best tunnel inclination and size and how it affects the airflow. Can I get the tunnel to draw like a chimney?

I would be grateful if you could assist me in any way with these questions.

Yours faithfully Grant Tremeer, South Africa


Dear Mr Tremeer

I thank you for your letter and I will try to answer your questions line by line.

Spherical fire boxes are important for a number of reasons. Since most pots are round, it's usually more convenient to make a round firebox, less materials are required and certainly for ceramic or mud stoves, sharp corners are weak points. From the combustion point of view, corners can become cold spots and disturb the flow of gases by creating resistance to the draught.


Your combustion equations refer to pure carbon. Well made charcoal would consist of:




Oxygen Ash




11.7% 3.4%

Wood of course is even more different: (for Douglas fir)











I enclose a section from "Biomass Stoves" by Samuel Baldwin's giving a fairly good description of the combustion process which is not as simple as your equations suggest, (pages 175-186).

Table 6, page 178 gives the stoichiometric quantities required for biomass fuels. For complete combustion, every fuel molecule must meet an oxygen molecule in conditions particularly temperature that allow them to react. This requires perfect mixing. Since perfect mixing is virtually impossible to achieve, the chances of fuel and oxygen molecules meeting can be enhanced by increasing the number of oxygen molecules, i.e. by having excess air.

One of the problems with air is that it only contains 21% oxygen, the remaining gases are mostly nitrogen which provides no useful function at all but acts as a heat sink i.e. it absorbs heat in the combustion chamber and takes it away as it leaves via the exhaust.

In coal-fired boilers, the excess air is often around 1.2-1.4 times the stoichiometric quantity. For enclosed stoves with doors and dampers, excess air is, in practice, 1.6-2 times.

The general rule of thumb for combustion is the 3 'T's: Time for the reactions to occur, Temperature to allow the reactions to occur and Turbulence to ensure there is perfect mixing. In simple stoves, the 3 'T's are difficult to achieve because the cost constraint inhibits the use of high insulation materials and forced draught systems to induce turbulence.

One of the best methods to use when designing for complete combustion is to observe the fire. In an academic environment you may want to have gas analysis also, but I prefer to watch the fire through clear pyrex dishes placed over the pot hole. Keep water in them otherwise they overheat and explode. If you change the stove, it gives different flame envelope patterns. The best is a bright yellow name with lots of turbulence. In charcoal stoves, this is less visible, therefore it is best to monitor CO emissions.

Sam Baldwin in Burkina Faso did a survey of open fire places and the optimum relationship between the pot diameter and the height it was from the fire was H = D/ 2. This is a useful starting point but experimentation is important for different fuel and stove types. There are other ways of predicting height and I enclose some useful references (pages 251-364 - 'A Woodstove Compendium').

With regards to the angle of inclination of tunnels, there is no known process for calculating it. I normally set this during experimentation, based around how much heat I need in the 2nd pot. This is normally split - 2 parts heat in the 1st pot and 1 part heat to the 2nd pot under high power operation.

I enclose a picture of a 2 pot stove that achieves this. (small picture of Anagi 11).

Yours sincerely

Peter Young

Senior Technical Manager

Stoves & Household Energy Programme


Dear Editor

In BP 27 April, 1992 edition, you wrote about "Diambar Stoves in Senegal". The "SAKKANAL" is very similar to this.

We often claim that improved stoves allow a reduction by half of fuel needed for cooking but more realistically it is only a saving of 30% in the consumption of energy.

A strong interest in this question in Senegal has again arisen from the Lorena stove in Guatemala which, for the first time, has brought a solution to the problem of price. In developing countries stoves that are really used are those which arc sufficiently cheap (10 dollars and even less).

A similar stove called the Ban Ak Suuf, which also means mud and sand in the local language, is being introduced little by lime into Senegal. However, the Lorena and Ban Ak Suuf only last on average one to two years. It is necessary to increase research activity to find a cheap and durable stove.

I propose that the SAKKANAL stove should be produced and provided. Here are some of the advantages of the SAKKANAL stove:

* the Sakkanal uses very little charcoal

* the Sakkanal uses 30-50% less fuel

* it produces less smoke than open fires and protects children from the flames

* women prefer it to the Ban ak Suuf because they can repair the walls and the stove lasts a long time.


Yours sincerely


Project Manager, CPRS, Ex Centre Social


Dear Editor

Lately we are hearing the complaint that research on stove testing and design has declined in favour of marketing, dissemination and impact studies. To us this does not seem such a surprising development since funds for research on stove testing and design have been decreasing for quite a number of years.

Our main objectives at Eindhoven for the last 12 years have been research on stove testing and design and dissemination of the resulting body of knowledge.

Stove research is not very popular with donors perhaps because results cannot be guaranteed beforehand (we cannot guarantee to improve the efficiency of a stove by 10 or 20% points within a few months). They do not always see the necessity of research and the complexity of the problem is not understood. Non technical people tend to underestimate the complexity of the interaction of all the different processes taking place in a stove. Donors tend to expect a construction drawing of a stove that can be used all over the world under all circumstances with an efficiency of over 60% and no smoke emission. They do not see that this is about the same as asking for a car that is able to transport 100 tons of goods, that can be easily parked in small places and is able to accelerate from 0 to 100 km/h in 4 seconds. If we try to explain what our problems are in research we often get a reaction along the following lines: So you build a stove, put fuel in it, light it and it works; so what is the problem?

More cooperation and a better exchange of information is needed. We have been sending around our reports with test results to all the institutes that we know of but unfortunately we hardly every get a report in return. In fact we sometimes have the impression that we are the only institute where research on stoves is done and the only subject we hear of is stove testing in laboratory and field.

Donors also want research to take place in the developing countries themselves regardless of the situation or possibilities. Although this can have a very positive effect it is a waste of money if the necessary equipment and expertise are not available locally. Assistance should be offered by an experienced institute for some time while later on cooperation on equal terms can evolve. This is the approach we stand for.

We have had a number of researchers based at other institutes working with us for periods of 6 to 12 months. This is a way to disseminate knowledge and can also be beneficial to the institute. We would like to take this opportunity to invite institutes or researchers to contact us if they want to cooperate with us on research. Our research work calls for a good background knowledge of physics, mechanical engineering or chemistry and a good knowledge of English.

Yours sincerely

E Moerman, K Krishna Prasad, Eindhoven University, The Netherlands


Ed - The Eindhoven Institute has contributed many articles to BP and we hope to publish a further article on the downdraught stove in our next edition of Boiling Point. As can be seen from page 38 of this edition, BP has now established a Research & Development News column to report on research in all aspects of stove work and to encourage networking.


Workshop on Commercialisation of Woodstoves

Organised by IDEA & Sponsered by RWEDP/FAO, ARECOP and ITDG, December 1992, Sri Lanka

IDEA (Integrated Development Association) in co-operation with ITDG/GTZ/FAO/RWEDP and ARECOP has organised a Workshop on Stove Commercialisation to be held in Sri Lanka from 2nd December - 7th December 1992. The Seminar will attempt to answer two questions:

1. When is commercialization a suitable approach for woodstove dissemination?

2. If commercialisation is appropriate what are the implications, changes and challenges for existing stove projects?

15 participants from 12 countries will be selected to participate at this workshop. The participants will have adequate experience and knowledge in stove dissemination activities in their countries and are required to present a country paper at the workshop.

Boiling Point will review the results of this seminar in April 1993.