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close this book Boiling Point No. 03 - October 1982
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View the document Biobriquets and Hybrid Stoves
View the document Sawdust Burning Cooker
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Sawdust Burning Cooker

Introduction

There is presently a need to utilize all existing fuel resources to minimize the demand on traditional fuels. The situation in developing countries is rather grim where wood is used by the majority of the population for cooking and reserves are dwindling rapidly. In addition, other factors which emphasize the need to develop cookers using other fuels are:

- the increasing costs of traditional fuels;

- the large amounts of other potential fuels available,

- the high demand for low grade heat for cooking;

- the relatively tedious method of cooking with traditional fuels;

- the high level of smoke generated by traditional fuels.

Considering the above, the author has studied methods of using waste material as fuel in many rural applications. This paper discusses a cooker using sawdust as a fuel.

Design Features of the Cooker

The cooker is made of galvanized iron 0.16cm thick. The dimensions of the cooker are shown in Fig 1. The two galvanised iron handles are riveted to the body of the cooker, as are the three base stands. The removable top cover is shaped so as to direct the hot gases on to the base of the cooking pot. The base plate has seven holes l.9cm in diameter and spaced 6.4cm apart between centres. This configuration, deduced from earlier work, provides for the use of a minimum mass of fuel, high heat production, and minimizes the material used for constructing the cooker. The base plate rests on one of three racks: thus various heights of packed fuel can be achieved. The cooker can be made with simple tools, such as a chisel, hammer and rivets.


FIGURE

Use of the Cooker

The fuel should be in solid particles that are suitable for close packing. Sawdust has been used in initial tests. Before loading the cooker with fuel, rods (of a slightly smaller diameter than the base-plate holes) are inserted through the holes, and the sawdust packed around the vertical rods by hand; it is not necessary to use much force or any binder to keep the particles together. The rods are then extracted, leaving holes through the fuel.

The fuel can be lit by sprinkling kerosene around the top of the holes and then applying a flame, or simply applying a flame under the base plate to the holes, so the fuel burns upwards. Alternatively bits of paper inserted into the tops of the rod holes, and scattered over the surface, can be lit to ignite the fuel. Start-up time is usually 3-10 minutes which is faster than many traditional cookers.

After lighting, the pot is placed on the supports which keep it 1.25cm and 3.8cm above the top of the cooker and the burning surface of the fuel respectively. Heat lost from the hot gases is therefore minimized and adequate combustion is produced by the air entering the bottom of the cooker.

When loaded to maximum height the cooker holds about 1.7kg of sawdust, and depending on wind conditions can burn for up to about 4 hours. Using a packing height of about 5cm and the 7 burner holes, it consumed in tests about 0.65kg of fuel in about 3 hours. The heat produced depends on the number of burner holes and the height of fuel used so the cooker can be adjusted to the needs of the user. For general use for up to 4 hours it is recommended that the cooker is loaded with the base plate in the lowest position, using all 7 burner holes. For cooking periods of 2 hours or less the base plate should be in the centre or top position. In all cases the sawdust should be packed to the top of the cooker.

Under use the cooker has been shown to have an average efficiency of about 15% over the period of burning. Loading is simple, and once lit, the cooker does not have to be tended as the fuel burns well throughout the cooking period. In practical use it is found that, in about three days, the user can learn to assess the depth of fuel and number of burner holes required to give particular cooking times, and therefore avoid waste of fuel.

An advantage of this cooker using sawdust is the negligible amount of smoke produced, which is only visible during the first few minutes until the burner holes burn with a red glow.

Future Developments

Work on this cooker is being extended to utilise all possible forms of waste material. It is also necessary for other traditional fuels to be used in order to minimise the rejection of this cooker by the would-be users. Various materials can be used for construction and further work is in progress to consider those which will reduce the dependence on imported materials.

Several demonstrations have been made of this cooker in Sierra Leone. There is keen interest shown by many people, and some have fabricated their own models. However, certain design guidelines must be adhered to in order to reduce fuel wastage, improve efficiency, reduce smoke production and minimize accidents. A popularizing programme will therefore have to be carefully initiated so that effective use is made of the present cooker design.

Michael Bassey

University of Sierra Leone