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close this book Boiling Point No. 17 - December 1988
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Appropriate energy stoves - for residues plus charcoal

by P C Bhardwaj - Kenya

Summarised by Ian Grant of ITDG

"The human survival and eating habits, over centuries, depend upon the cooked food and warm habitat during the cold weather. Hungry stomach has no eyes, no understanding, for it, environment does not exist, prime concern is to procure food. Irony of fate is, as the population is growing, people are either migrating towards cities, or towards forests, where water and land are available. There is no way to stop this migratory trend, only solution is to develop sound and energy efficient devices".

P C Bhardwaj of Appropriate Energy Enterprises Ltd. P O Box 11927, Nairobi, has been designing, making and selling steel cooking stoves since 1984. He has continually improved his designs for higher fuel efficiency and greater convenience and now markets a range of domestic and institutional stoves some of which include hot water systems.

He has throughout pursued two basic principles:

1. to burn agricultural residues, loose or briquetted in conjunction with charcoal.

2. to improve the efficiency of the pyrolysis process by maintaining a high temperature in the combustion chamber and by supplying oxygen in the form of steam.

He quotes the following definition of pyrolysis:

"According to Dictionary of Scientific and Technical Terms, "Pyrolysis (chemical process); the breaking apart of complex molecules into simpler units by the use of heat, as in the Pyrolysis of heavy oil to make gasoline". The above definition portrays the process as very simple, but in reality it is very complex. On pyrolysis, the biomass releases volatile matter which is non-flammable and to ignite it requires thermal cracking at about 400-500C. This posed the problem of insufficient oxygen supply, because the process generated more and more heat which in turn produced more and more gas, a point of saturation was reached and gas remained unburnt".

Note: some volatiles are very flammable - Ed

Domestic Stoves

The first stoves he designed burnt rice husk and used pre-heated stoves to provide the extra heat needed for more complete combustion. Although this was fuel efficient, it was too heavy and inconvenient to use.

The "Majiko" stove and "Modified Majiko" replaced the hot stones with steam produced from a water jacket surrounding the stove and an annular ring around the top of the stove to generate the steam. The steam was piped into the combustion chamber. Figure 2 shows various stages of development of these stoves which would burn "rice husks, sawdust, coffee husks, dried cowdung, pine needles, peat or any biomass in granule or powder form".

The inventor described the stove as follows:

"It has inbuilt water heating and is divided into independently replaceable five components. These are Base, Gasifier, Steamer, Grate and the Water Storage Tank. This stove proved to be extremely efficient and for next two years no changes were made. It has the following advantages:

The surrounding temperature did not go beyond 50-60C.

The base heat was utilised for heating about 120 litres of water through thermo-syphon system, without any extra usage of fuel.

The side wall chamber heat, which is normally cladded with clay liner to stop the heat from being wasted, was utilised for gasification of biomass.

The heat which escaped from the sides of the cooking pan, at the top of the stove, was partially utilised for producing steam. The steam was introduced into the burner to provide extra oxygen needed to burn the gas.

The gasifier could be quickly removed from the fire, without spilling the charcoal outside. This charcoal could be utilised, by putting the steamer on top of it, for slow fire cooking, short meal cooking or roasting of meat, potatoes or maize.

The gasifies that weighed about 12kg, had to be physically lifted for filling and emptying, which was quite cumbersome. The thickness of the burner and liner needed to be increased, for enhancing the life span, but it was not possible, because it would have become too heavy to be used by average cook. It also created packing problems for export purposes.

The latest model in this series has been modified to overcome these short comings. The water storage tank has been redesigned, so that all the components of the stove can be packed into it for transportation. The steamer has been incorporated in the gasifier thus avoiding the use of rubber hose and other connections. The pot-rest is removable, and is designed to go over the base, when gasifier is removed, for slow and short span cooking. The gasifier need not be lifted for filling and emptying.

It is mounted on a trolley, that has two locking devices, which makes it stable for cooking. Also eliminates the need of physical lifting, it can be pushed in and pulled out from the base. The gasifier has two openings placed at 180C. Once the locking devices are removed, the gasifier can be rotated for removal of the biomass. Filling is also equally easy.

For short cooking, the pot rests can go over the base, thus lengthening the life of steamer. When gasifier is removed, the trolley can take the meat roasting grill and sticks. The water storage tank has folding legs and much larger water holding capacity. Tank can be insulated to store hot water." Figure 3 shows the companies latest type of domestic stove - "Common Man Stove" which is described as follows:

"Due to popular demand, four types of stove for low and middle income groups have been designed. The main fuel for these stoves remains to be waste biomass, which is subjected to partial pyrolysis and partial direct burning. Due to direct burning, though there is no smoke, but the pans become stained. The stoves are so economical that a bag of good quality charcoal can last for 6-8 weeks to cook meals for 6-8 persons.

Three types of stoves do not have inbuilt water heating system and others have it. There is a model, where two stoves are connected to one water tank, to accommodate the need of cooking more than one dish at a time at the same time keeping the cost low".

Institutional Stoves

Based upon the technology already explained, a large stove has been developed, which can cook meals for 100-600 people. These stoves are in four parts, ie, the Base, Insulated Water Storage Tanks, Gasifier, Cooking Trolley. Gasifier and Trolley are made out of stainless steel with cast iron replaceable liners and burner tubes. It has inbuilt steam production system, to supply extra oxygen to the burner tubes. The cooking pan is not directly placed on the stove. For this purpose, a trolley has been designed, which is wheeled over the stove, carrying the cooking pan. After the food has been cooked, it is wheeled away from the stove to the place of serving, thus avoiding the need of picking up hot pans from the stove. The four wheels of the trolley can be locked on the ground so that it does not shake during cooking.

In about six hours of cooking, one pair of stove gives over 1000 litres of hot water at about 70C, which can meet all the requirements of kitchen, eg, hot water for cooking, washing etc. Stoves can cook meals for 500-600 people only utilising maximum of 25-30 kg of charcoal per day. The cost of main fuel, ie biomass is only collection and transportation, which in comparison is negligible. Optimum utilization of the existing kitchen space, with least modification, is encouraged, by supplying a custom built complete kitchen unit. The unit contains a chimney, two stoves, one or two storage tanks, coupled with plumbing, and containers for storing enough charcoal and waste biomass for a week. The chimney is designed to eliminate smoke while lighting the charcoal and later to take the cooking vapours away from kitchen. Various components have different life spans as follows:

 

Base, Storage Tank, Gasifier, Trolley five to seven years.

Grate, (cost is very nominal) one year.

The Cast Iron Liner three to four years.

Burner Tubes one year.

The Burner Tubes, the Liner and the Grate can be replaced by the user. This stove, if biomass (which in any case is waste) is not taken into consideration, results in saving of 70-80% of charcoal. The cost of unit, can be met, within 18 months from the saving in fuel-bills.

Ovens

There are all sizes of portable ovens to go with all the models of stoves. The smallest which can bake six 500gm breads to large one with 100 bread capacity. All ovens are fitted with precision temperature gauges and are on wheels for convenient handling. These prove most useful in rural areas, where there are no existing baking facilities.

P C Bhardwaj concludes his description of his stoves as follows:

"Conclusion

AEE stoves compare very favourable with the other improved stoves eg Bellerive and Alpha Laval and have a very strong point to their credit. These are the only ones, which convert waste into useful energy. The side wall temperature remains low at 50C. The most of the heat which normally escapes from the sides is utilised. It has the following advantages over other conventional stoves:

a. Extremely energy efficient, utilises waste as main fuel, hence saves 80-85% charcoal when compared to any stove using charcoal or wood fuel.

b. It has an efficiency of over 27% when even the specific consumption of the biomass is taken into account.

c. Has a well spread broad flame with a very low emmision content, which is well below the human tolerance level. Because of large even flame, cooking is faster.

d. There is no element of environment pollution.*

e. Surrounding temperatures are low thus comfortable and conducive for cook.

f. Easy to handle.

g. It produces the bye-product of hot water. Water storage tank can be installed at a higher place outside the kitchen. Water can be connected to sinks, etc. for regular supply.

h. Gasifier has thick cast iron liner which has a long life span and cast iron burner tubes are easily replaceable.

i. It can do all types of cooking, including boiling, frying and roasting.

j. It is convenient to use because of the trolley which eliminates the need of physical lifting of pans or transferring of cooked food.

k. It is priced in such a way that after a year it would pay its installation cost by saving fuel.

It has following disadvantages:

a. It needs some care in handling and demands regular maintenance, because of sophistication of design and use of two types of fuel.

b. It attains a constant high temperature within 35 minutes of lighting. This can only be reduced by introduction of baffle plates, which results in drop in efficiency. However, after about 1 1/2 hour, if fresh charcoal is not added, flame can be controlled for slow and short cooking.

c. Installation cost is higher than all other stoves."

Although we have no reason to doubt P C Bhardwaj's claims for high fuel efficiencies, ITDG has not yet had an opportunity to carry out tests on the stoves. We have not been given prices of the stoves, but we expect that even the cheapest will not be competitive with the KCJ and the initial cost will be beyond the means of the rural housewives who are most likely to have free access to suitable residue fuels. The company deserves support for its innovative work in developing improved stoves in Kenya (Ed).