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Coal briquetting and clays for Zambian stoves

Boiling Point
Issue 36 (1995) Solar Energy in the Home
Article	Coal briquetting and clays for Zambian stoves
Author	N R Hill

Two projects of the Zambian National Council for Scientific Research (NCSR), Director W C Mushipi.

The twin projects of ceramic stoves for domestic cooking (A) and briquetting of coal (B) resulted from official concern over the rate of deforestation caused by the demand for fuelwood as well as for the production of charcoal. Both projects, which began over five years ago, are housed in a purpose-built pilot plant about 0.5 km from the main NCSR site in Lusaka.

[top] [end]Project A - Coal briquetting

The main raw material for the briquettes is a washed, sub-bituminous coal having a carbon content of 58 per cent and ash content of 24 per cent.

The first stage is carbonization of the coal which reduces the volatiles to 6 per cent - but also reduces the calorific value by 25 per cent. To ensure the briquettes will ignite readily, some bagasse is separately carbonized and mixed in with the coal. The mixture is formed into solid, part-rounded, smooth surfaced briquettes, less than 100mm across (see Figure 1), which are then passed through a de-smoking machine (Figure 2).

The briquettes can burn for 6 to 8 hours, and when 10 are loaded in one of the NCSR ceramic stoves the fire keeps burning for 9 to 10 hours. A disadvantage of the process is that as much as a quarter of the heat value of the coal is lost, and further energy is consumed in running the de-smoking machine. It is understood that the initial carbonizing of the coal enables it to be briquetted without the need for a binder.

No details of costs of the new items of plant or for making the finished products, were obtained.

(Editorial note. Unfortunately NR Hill's summary does not give any information about the quality and quantity of the fumes emitted when the briquettes are made or when burned in the NCSR stove. For domestic situations this is now probably more important than their calorific value or fuel economy.)

[top] [end]Project B - Clays for Zambian stoves

The Geological Survey of Zambia has published a comprehensive document on the clay deposits of Zambia entitled 'Chikankata clay deposit: a candidate material for heat-resistant, energy-saving, cook stoves' by N C H Lubaba, J S Banda, S Mambwe & Y Minamikawa

So far, following six or seven ears of evaluation work, the clay-most suitable for stoves, of the three samples studied in detail, is that from Chikankata, which contains smectite clay mineral, talc and quartz. It is considered that the content of MgO, 14-15 per cent, from the talc as well as the 10 per cent of quartz are important in its performance. Sufficient plasticity is given by the 30 to 40 per cent of the particles which are less than 2 microns.

Cross section of the NCSR, Lusaka, domestic clay stove

The Chikankata clay is the most economic as the firing can be done at 850°C, and it then attains a higher density than the other clays. It also has low thermal conductivity and the lowest thermal expansion. Its performance is claimed to be better than that of clay used for the University of Zambia's improved 'mbaula' stove.

Cook stoves made from Chikankata clay lasted more than 12 months and, using charcoal as a fuel, can maintain for one hour a fire temperature 100°C higher than a traditional brazier. The durability of the stoves was influenced by design or geometrical factors, heat-treatment, and the size, shape and toughness of the grog. More durable stove compositions were obtained at lower firing temperatures and by incorporating tough and large grog particles with a high aspect ratio.

In 1993 2,000 NCSR stoves were sold and it is expected about 4,000 will be sold in 1994.

[top] [end]Contents: Boiling Point 36: Solar Energy in the Home

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Parameters for a Solar Cooker Programme - The Sunstove Solar Box Cooker - Sunstoves in the Republic of South Africa - Gaining Ground in Solar Box Cooking in Kenya - Solar Cookers - A Cause Worth Promoting - Free Energy from the Sun - A Solar Box Cooker with a Reflecting Lining - The Solar Puddle - A New Water Pasteurization Technique - Renewable Energy - A World Bank View - ESMAP study points toward village-level management of woodfuel resources in Chad - Burning Charcoal Issues - A Dangerous Trade - Saving Wood by Burning Coal - Haitis Domestic Fuel Project - Coal briquetting and clays for Zambian stoves - Improving the three-stone fire - Comparative tests of solar box cookers - Parabolic Solar Reflector and Heat Storage Cooker - An Affordable Parabolic Solar Cooker

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