Efficient Cooking Stove fueled by Cow Dung: Difference between revisions

Culture of Cow Dung:

In South Asia, cows are immensely important. Most of the South Asian economies are based on agriculture and subsequent farming hence since time immemorial the benefits of cow dung have been explored and tested from being used as a fertilizer, medicine, fuel source and now most importantly to obtain biogas from. The potential of biogas from cow dung is immense and is currently being explored in Nepal extensively.
In India the cow is an object of worship. For centuries India and have used cow dung and urine for many purposes. Cow urine is used as bleach back in the days when India was under British occupation. It has various medicinal values- some use cow urine to treat acne! It is a seed protector, mosquitoes repellent and an ingredient in making bricks and can also treat skin sores.
Therefore it is not uncommon to see villages houses in rural South Asia splattered with round shaped patties on their houses, baking in the sun and one can almost get used to the smell!

Efficient Cooking Stove fueled by Cow Dung

Cow dung patties made by village women are used as a fuel while cooking when wood is scarce. Cow dung is dried and shaped into patties which are then burnt to produce heat[1]. Most cooking stoves are inefficient in producing heat from the cow dung- an energy efficient cooking stove have been designed to make the cow dung give off larger amounts of heat. In the past many modifications to the conventional kitchen stove have been made. Keeping those modifications in mind and cow dung as a fuel source, such stoves have been designed to maximize the heat output of burning cow dung. The devices operate on the basis of maximizing the surface area of the dung and blowing in more air. Efficient combustion takes place as more air is blown into the stove thus minimizing the formation of CO gases. The only by-product is water and carbon dioxide since complete combustion takes place. The goal is to have higher combustion efficiency since cow dung generally has a low one. The design is kept as simple as possible to ensure that local blacksmiths can make it themselves. Two major stoves that use cow dung as a fuel source have been developed. One uses the principle of fluidized bed combustion while the other one works on the basic principle of normal stoves. The first one was developed by Kumar and Shende of Madhav Institute of Technology and Science and the other developed by Rajendra Prasad of India Institute of Technology.

1. Kumar and Shende:

The cook stove developed by Kumar and Shende is based on the principle of fluidized bed combustion in which pulverized dry cow dung is burnt in preheated air. The design involves preheating air before it combusts and fluidizes simultaneously with the cow dung. Kumar and Shende project that this stove is up to 4 times more efficient than a normal stove. Air is blown in to the oven in the opposite direction of the flow of cow dung powder to produce fuel through ‘counter-current’. The air is initially heated up to the ignition temperature of fuel at the bottom of the chamber. Pulverized cow dung is introduced to the preheated air and combustion takes place in the fluidized state.

Cow dung mainly constitutes of carbon, hydrogen, oxygen and small amounts of nitrogen. In combustion carbon dioxide, water vapor and ash are produced. The ash is dealt with by installing a grate in the stove. The basic reactions involved in combustion are:

C + O2 -> CO2 + Heat and H2 + ½ O2 -> H2O + Heat

The main reason for having cow dung in powder form is that a higher surface area enables more efficient burning since less air needs to be blown to bring it into fluidized state. If some villages do not have access to electricity the air can be blown manually.

The design of the stove can be seen from the diagram. Air is blown in from the bottom of the chamber, which moves up as it is pre-heated. A funnel on the right side inserts powdered cow dung in to the chambers. A small chimney on the right side, above the ash storage tank, is used for the gases to escape from.

Calculations for this system are as follows (Kumar and Shende):

20gm dry powdered cow dung is burnt per minute.
The typical ultimate analysis of cow dung by weight is:
Carbon - 31.6%
Hydrogen - 05.18%
0xygen - 37.8%
Nitrogen - 06.12%
Ash - 19.3%
Net O2 required from fresh air is 17.368 gm.
Air supplied (with 20% excess) is 90.6 gm.
Total carbon dioxide leaving in flue gas is 22.18 gm.
Total water vapor formed is 9.324 gm.
Total nitrogen leaving in flue gas is 70.99 gm.
Total oxygen leaving in flue gas is 26.05 gm.

Air is preheated from 25oC to 200oC

Energy needed for preheating= Ma* Cp of air * (200 – 25)
91.725 * .241 *175
3868.50 calorie/minute

The material for the stove is iron sheets, which is readily available since. Since this stove is not yet in use, there is not estimated price for it, though ideally it should fall within the price range of about US$5, which is what most villagers pay for buying conventional cooking stoves.

2.Rajendra Prasad

Prasad’s cooking stove was designed after a study based on the existing 2 cow dung stoves in India. He outlined a few basic problems which he felt needed to be taken into consideration. He identified the problems as lack of maintaining a sufficient and proper supply of air to allow for complete combustion, the need for a large enough firebox that keep the fire burning for the entire cooking period and lastly the need to understand cow dung’s different combustion properties as compared to wood.

Properties of Cow Dung (Prasad):.
• It is bulky.
• It has large ash content
• It has large volatile content.
• Carbon content is low.
• Burning ratio is low.

The stove developed by Prasad has the following features:.
• Cylindrical double wall made from metal sheets with 2mm gap.
• 0.5cm thick iron rods that make the grate.
• Legs, handles and pot support.

The purpose of the cylindrical double wall is to create insulation so that less heat is lost to the surrounding air allowing the combustion to be efficient as higher temperatures can be maintained. The perforations in the inner cylindrical metal sheet at the bottom of the firebox draw hot air to enable efficient combustion.

Design:

It looks like a normal cylindrical stove, with two steel cylindrical walls separated by 2mm and open at the bottom and sealed from the top. The inner steel wall has perforations at the bottom. The grate is made from iron rods that are 0.5cm thick and are 0.5cm apart.
The grate functions to supply the necessary primary air needed for combustion and it lets the air flow freely without the ash clogging it up.

Performance:
• Reduces smoke.
• Reduces the need of constant refueling.
• Is easy to light.
• Long steady flames.
• 30 % efficiency (which is high and good for a cow dung cook stove).

References

• ‘Combustion of cow dung in fluidized state for household purposes’ Jha sharvan kumar and Shended Chemical eng. Dept., Madhav institute of Technology and Science, http://www.ese.iitb.ac.in/aer2006_files/papers/027.pdf
• ‘Fuel Efficient Cookstoves Using Cow Dung Cakes’ Dr Rajendra Prasad, Boiling Point, Issue 30 (1993) Sales and Subsidies, Centre for *Rural Development and Appropriate Technology, IIT, New Delhi http://www.hedon.info/goto.php/FuelEfficientCookstovesUsingCowDungCakes
  
• Herbshpere Website. ‘Holy Cow! There’s more to Dung than meets the Nose’ http://www.herbsphere.com/tester.htm

Interwiki links

• Wikipedia:Cow dung