safety & health
Wood, like coal, is a versatile fuel in that it can be converted to more convenient forms - e.g., charcoal, liquid, and gaseous fuels. We will only consider charcoal since it is a principal fuel used by the urban low-income population in many developing countries, particularly in East Africa, the Caribbean Islands, and Thailand.
Charcoal is produced from wood by a carbonization process. When wood is heated in the absence of air, the volatiles are driven out and what remains is charcoal. In carbonization processes part of the wood is burnt to provide the necessary heat. The traditional method of producing charcoal is to use an earthen or a pit kiln. From such a system, only 20-30of the original heat content of the wood is left in the charcoal. The fact that a charcoal stove has a higher efficiency has to be tempered with the rather poor conversion efficiency of wood to charcoal.
A more efficient system permits the collection of the volatiles in the form of a variety of organic liquids and a combustible gas. Two major difficulties arise in the application of this system in developing.countries. The first is the capital cost, which is quite high (for example, the Lambiotte system developed in France is estimated to cost $2 million/unit of 55 tons of charcoal/day). The second is the utilization of the by- products, which will inevitably entail additional investments. The technical factors determining the design and performance of pyrolytic conversion systems have been reviewed by Zaror and Pyle (1983).
The principal advantage of charcoal as a domestic fuel is its smoke-free combustion. This property combined with the absence of volatile matter makes the task of stove designing relatively simple. Chimneys, which appear to be essential for burning wood, can be dispensed with. Second, power control can be easily accomplished by controlling the combustion air supply to the fuelbed. The third point in favor of charcoal is that there exists a well-developed marketing system for the fuel as well as the appliance. This makes it much simpler to introduce improved designs. Finally, the preference for charcoal in urban areas appears primarily due to the transportation costs of wood. If the distance exceeds 50 km in many countries, even an inefficient charcoal production system fares better costwise than does wood. A combination of situations that favour the use of charcoal has been considered by Karch and Boutette (1983).
Charcoal has a specific combustion value on the average of 28,900 kJ/kg. Its density is strongly dependent on the wood used in its production. An average value is about 400 kg/m3 according to Earl (1975).