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Fuel Efficient Cookstoves Using Cow Dung Cakes


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Boiling Point
Front cover of Boiling Point issue 30
Issue 30 (1993) Sales and Subsidies
ArticleFuel Efficient Cookstoves Using Cow Dung Cakes
AuthorRajendra Prasad V


A study sponsored by CORT and made by Dr Rajendra Prasad, Centre for Rural Development and Appropriate Technology, IIT, New Delhi

Reproduced from Changing Villages: Rural News and Views Vol. 11 No. 2 April-June 1992

In India, there are basically two major designs of metal stoves for burning cow dung cakes. Both were studied and the lessons learnt were as follows:
  1. There has to be a proper and sufficient supply of primary and secondary air for the efficient combustion of the fuel and the volatiles generated. Care has to be taken that the ash created by the combustion of the cowdung cakes does not restrict the flow of primary air. It should be possible to remove the ash as and when it is formed.
  2. The firebox size, both in terms of its height and diameter (width) has to be large enough to accommodate enough fuel to last the total cooking period desired. This does not mean that all the fuel required is fed at the beginning. It can be fed in batches to ensure that there is a continuous building up of the fuel inside the stove. The firebox has to be designed to accommodate this accumulation of the burning fuel.
  3. Cowdung as a fuel is very different from wood. Its combustion is different; it is much more bulky, the ash content is large, and the volatile content is large. The carbon content is lower, the burning ratio is lower and the ratio of primary to secondary air needed is different. The temperatures generated and the I lame size and patterns arc different. All these things must be kept in mind when designing a stove for efficient combustion of cowdung is, therefore, obvious that a stove designed to be efficient for the combustion of wood may not perform so well with cowdung cakes. Certainly one can try lo achieve some sort of optimal conditions by designing a stove for mutifuel combustion.

Figure 1: Trends in solid fuel use
Figure 1: Trends in solid fuel use
Based on the above understanding, an attempt was made to design a stove for efficient combustion of cowdung cakes. This does not mean that the stove cannot take other fuels, say wood, but that has not been the main objective here. That will be dealt with in a separate study. The objective here has been very clearly defined.

[top] [end]Description

This is a metal stove, easily transportable, strong and sturdy, easy to handle, safe to operate, much less smoky, fast and efficient burning, not too costly (about 200 Indian rupees (USA) for the large size and about 15() Indian rupees (US$6) for the medium one).

This is in a way a double walled cylindrical stove, the outer wall is a thin plain sheet metal one while the inner one carries several rows of small (a few mm dia) holes. The 2mm gap between the walls is open towards the bottom of the stove while the top is sealed. The grate is made of 0.5 cm thick iron rods, at 0.5 cm spacing having about the same amount of spacing between them. The stove is provided with legs, handles and pot supports. It is simple to manufacture in any village sheet metal workshop. All the materials of construction are available in the market without difficulty. No special skill is required to manufacture or to use it.

[top] [end]Technical Design

The gap between the two walls has a double function. One, is to act as an insulator reducing the heat losses to the atmosphere. This results in higher temperatures inside the firebox, easy ignition and more complete fuel combustion.

Second, the hot air in the gap is drawn through three rows of small holes in the firebox wall to supply hot secondary air which is extremely useful for the efficient combustion of the fuel and the volatile generated. Clear blue flames appear at those holes where the hot air meets the volatile gases.

The grate supplies the required primary air for the combustion of the fuel and allows the ash to fall down without choking the air flow. This type of grate is not needed for wood fuel as the ash content is much lower and such large spacings create too much free space causing heat losses, cooling off the fuel and reducing temperatures inside the fuel thus causing difficulties in the ignition of the fuel.

[top] [end]Performance

The performance of the stove is very good. It reduces the smoke output, which is normally bad with cowdung fuel, makes lighting up easier, needs less attention and reduces the risk of the fire going out when re-fueling. Over feeding or under feeding or delays in feeding do not cause the fire to go out immediately. The control and range of operation of the stove at low power outputs and high power outputs is improved. The long steady flames generally visible with wood combustion are a rare sight with cowdung cakes. In this stove, cowdung cakes almost compete with the wood in this regard. The high volatile content generally found a nuisance during combustion by causing intense smoke has been converted to an advantage by burning after mixing with preheated secondary air.

[top] [end]Results

There is a gain both in terms of better combustion efficiency of the fuel and better heat transfer to the vessel. Assuming a net calorific value of 2500 kcal/kg for the cowdung cakes, efficiency over 30% has been achieved for the large sized stove (suitable for cooking around 10 kg of material at a time) and over 20% for the medium sized stove of about half that capacity. Cooking is quicker and pans are cleaner.

Ed Note: Many developing countries do not have the quantities of cowdung available in India and so would not have use for these Chulhas. as. However, the procedure described and the basic principles of combustion involved are instructive. There may be other biomass residues which can be burnt in stoves designed in a similar way.

[top] [end]Contents: Boiling Point 30: Sales and Subsidies

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Why Commercialization for Stoves - Report of the International Seminar on Stove Commercialization - Commercial Marketing for the Indian NPIC - A Commercial Drop in an Ocean of Subsidy - Commercialization of Kenyas Rural Stove Programme - Ahibenso - The Improved Ghana Coalpot - Cooking Stoves for Commercial, Sustainable Production and Dissemination in Africa - Poor Project Planning and Unsuitable Stoves - Chulhas for Tibetan - Communities in India - Stove Dissemination in China - Stove Designing For Successful Marketing - Practical Tips for a Marketing Strategy - ESMAP in the 1990s - Fiji Woodburning Stoves - The Health Impacts of Biomass and Coal Smoke in Africa - Smoke Gets in your Eyes - and Forms Cataracts - Fuel Efficient Cookstoves Using Cow Dung Cakes - Solar Pond Method to Heat Water

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Pagename: FuelEfficientCookstovesUsingCowDungCakes @HEDON: AFHA