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Fuel from Three Stages of Pyrolysis

Boiling Point
Issue 34 (1994) Smoke Removal
Article	Fuel from Three Stages of Pyrolysis
Author	International Network for Sustainable Energy

A seminar was held between 27th September and 15th October 1993, in the Ivory Coast, to discuss the pyrolysis wood. Many forestry organizations from francophone African countries took part. The seminar considered the relative merits of three alternative ways of obtaining fuels from wood. The three fuel forms: charred wood (bois torréfié;, wood charcoal (charbon de bois) and wood gas are all produced by heating wood in a closed container to different temperatures and in atmospheres with different levels of oxygen/air reduction. This process of thermal decomposition is known as pyrolization and takes place in two phases. In the first, or endothermic phase, heat is absorbed from an external source. The second, exothermic stage occurs, when heat is produced from the combustion of some of the volatile gases released as the first stage develops. If the pyrolization process is halted at the end of the first phase (a temperature of about 270° to 280°C) the product is charred wood. After the second phase is completed, the product is charcoal. If more air is then admitted and the temperature is allowed to rise to 1500°C (by the combustion of more of the gases produced), then wood gas is produced which can be burned in an internal combustion engine. An external source of heat is needed (for example gas), during the initial, endothermic phase of producing charred wood. (see figure 1.) In tropical Africa, the process of using wood to produce charred woodfuel or gas is not common. Charcoal, however, is a traditional and popular fuel produced in different types of kiln: the traditional kiln, known as a casamancaise made from earth, mud and grass; more pennanent types of brick kiln; metal kilns, which may be quite simple and portable, or more complex, incorporating means of collecting the liquid or gaseous byproducts. To produce gas suitable for internal combustion engines, generator of the type shown in Figure I must be used.

[top] [end]The properties of charred wood

Charred wood has all the good properties of woodfuel without the disadvantage of producing smoke and, unlike charcoal, it does not absorb moisture and it is not dirty. It is a stable material and can be crushed and compacted for storage and use. In addition to these desirable properties, charred wood contains 90 per cent of the initial energy of the wood. Its increased calorific value is shown in Table 1.

Table 1: Comparison of calorific values of wood fuels

Fuel	Calorific value in Mj/kg
Fuel wood	17
Charcoal	29
Charred wood	22

Figure 1: Wood gas generator

For most fuel purposes, it can be used to replace charcoal or wood as it can be crushed and compacted to give specific densities from 800 to 1000kg/m3.

In the humid tropics, the hydrophobic properties of charted wood allow it to provide more energy than charcoal - which may have absorbed moisture before reaching the consumer. So far. it is a little-known commodity in tropical Africa. Its market acceptance will depend more on economics than on its technical merits, but to be marketable it must be shown to be superior to its competitors. Unlike wood, charcoal and petroleum products whose price structures are well known, charred wood would need investment for marketing and for production in acceptable forms.

In conclusion, it can be expected that charred wood could compete in availability and cost with wood and charcoal. In Senegal it has been competing with bottled gas' which required a larger investment in equipment and was, therefore, too expensive for the, poorer sections of the population.

[top] [end]Contents: Boiling Point 34: Smoke Removal

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Smoke in the Kitchen - Any Stove Will Smoke if You Dont Use it Correctly - Acute Respiratory Infection, Conjunctivitis and Accidental Burns - Exposure to Air Pollution From Transitional Household Fuels In A South African Population - Smoke Removal in Kenya - Chinese Chimneys - Indoor Air Pollution in Rural Tigray - Removing Smoke from Nepali Kitchens - A Breath of Fresh Air for Smoky Houses - Vietnames Kitchens - Reducing Greenhouse Gas Emissions and Improving Environmental Degradation - Indian Governments Stove Programme in Question - Cooking energy Efficiency in Indonesia - Phillipines Ricehull Stove - Stoves for Cafes and Food Stalls

• Fuel from Three Stages of Pyrolysis - An Electric Metad - Crack Reduction in Clay Stoves

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