Charcoal Production, Kiln Selection

Charcoal is widely used as a domestic fuel for cooking in many towns and cities in developing countries as it is cleaner and easier to use than wood.

Small-scale charcoal production is labor-intensive. It can be divided into different stages of operation.

  • Growing the fuel
  • Harvesting the wood
  • Drying and preparing the wood for carbonization
  • Carbonizing the wood to charcoal
  • Screening, storage and transport to warehouse or distribution point

However, it can have a detrimental effect on the surrounding environment when demand for fuel increases beyond what can be supplied on a sustainable basis.

The amount of charcoal produced varies, with the methods employed to produce it, and the skill of the operator.

This brief looks at some of the approaches in the production of charcoal and on a small scale in developing counters where efficiencies can be greatly improved through the adoption of better techniques and equipment.

Although wood is the most common fuel source many other sources have been tried including agricultural waste such as millet stems and corn cobs as well as coconut shell. These biomass materials are made up of cellulose, lignine and volatile substances and water. During the production process the volatile components are driven off and the cellulose and lignine are decomposed. The process is divided into the following stages.

  • Combustion: oxygen supply is high and temperature rises from ambient to over 500°C and when the fire is established, the oxygen supply is reduced after the firing point is closed and temperature drops to about 120°C.
  • Dehydration: free water is driven out at a reduced temperature of about 100°C and the kiln gives out thick, white and moist steam.
  • Exothermic reaction: when the wood has dried, temperatures rise to about 280°C and the wood begins to break down into charcoal, water vapour and other chemicals; the smoke at this stage is yellow, hot and oily and the temperature is maintained by controlling the air flow through holes and vents to help burn more wood.
  • Cooling: when carbonization is complete, the kiln cools to below 100°C and charcoal can be removed for further cooling.

The process of carbonization is greatly dependent on the carbonisation temperature, the moisture content of the wood used (the drier the better), the skill of the producer and the condition of the wood (lignin content).

Kilns

Traditional charcoal production is an acquired skill. The most critical factor in the efficient conversion of wood to charcoal is the careful operation of the kiln. Wood must be dried and carefully stacked to allow an even flow of air through the kiln and sufficient time for reactions to take place. If kilns are not operated correctly, yields can be half the optimum level.

Traditional Kilns

Much charcoal for domestic consumption in developing countries is produced in pit kilns (holes dug in the ground), or in mound kilns (piles of wood stacked on the ground and covered with soil), by farmers and landless laborers. Yields (weight of charcoal/weight of wood) from pits vary from less than 10 per cent to over 25 per cent.

Brick and Concrete Kilns

Kilns made of bricks can be more efficient than earth mounds, can be operated all year round and have longer lifetimes than metal or mud kilns, and are less susceptible to poor operator practices. However, the high-grade charcoal that they produce may not be acceptable to domestic users, since it is difficult to ignite. Switching to large, efficient kilns, has many economic and social implications, as most charcoal is still produced by farmers and landless peasants who, under normal circumstances, might not be able to benefit from the switch and, indeed, might suffer from it.

Brick kilns are ideal for replacing traditional kilns when consistent high-quality charcoal is required in large quantities. The throughput of a battery of seven “beehive” kilns, for example, is around 15,000 cubic meters per year. However, the construction of such kilns requires a relatively high level of brick-building skills, as well as a supply of bricks. This restricts the scope of such kilns in many countries, but in areas where they can be cheaply built and maintained, they have proved to be a very effective method of charcoal-making.

One of the major advantages of the brick kiln over earth kilns of similar size earth kilns is that their carbonization cycle is much quicker. Typically, a 50 cubic meter brick kiln has a carbonization cycle of 8-10 days, whereas that of the comparable earth kiln is, at least, twice as long. Moreover, the labor involved in operating the brick kiln is very much less than that required to construct and manage the earth kiln. Furthermore, the operation of the brick kiln is generally much simpler than the earth kiln: workers can be trained in its use relatively easily and shortages of skilled labor are not likely to be a constraint on production.

Brick kilns, however, are usually permanent structures: they are, thus, only suitable in locations where there is a supply of wood within easy transport distance and sufficiently large to last the working life of five or more years of the kilns.

Portable Steel Kilns

Portable steel kilns are in the form of a cylinder with a conical top. A kiln breaks down into three components which are designed to be easily rolled along the forest floor to new burn areas or to be transported by truck. Portable steel kilns have a small output: the annual production from a typical demountable kiln with a volume of 7 cubic meters is in the range of 100-150 tons. They are not, therefore, particularly suitable for areas where there is a need for highvolume production. Their ideal application is where the source of wood is dispersed and charcoal-making is carried out on a relatively small scale.

The advantages of the portable steel kiln are that it requires less labor than the small earth kiln and has a generally greater yield of more consistent and higher quality charcoal. It is also much quicker: the total carbonization cycle with a 7 cubic meter demountable steel kiln is 3-4 days; with a similar size earth mound, the cycle is likely to be 10-14 days.

The mobile steel kiln, like the brick kiln, has the substantial advantage over the earth kiln in that training in its use is very easy. The steel kiln can, therefore, be used even in areas where there is no tradition of charcoal-making.

Issues of Production and Use

Charcoal is important in terms of energy and economies within most third-world countries. The production of charcoal employs a considerable number of people in rural areas. However, charcoal users as the group are most strongly exposed to carbon monoxide (CO), followed by wood users. Charcoal use also results in high volumes of carbon dioxide (CO2) emissions contributing to global warming.

Increasing end-use efficiency requires the promotion of improved stoves. Traditional stoves are normally made by the informal sector; models with higher heat transfer efficiencies should be developed in collaboration with end-users and stove producers, and manufactured by the private sector.

Inefficiencies inherent to the production and use of charcoal, rapid urbanization and the preference of urban dwellers for charcoal place a heavy strain on local wood resources. Financial loans helped people cover the costs of converting as cost was seen as the dominant constraint. Introducing LPG or Kerosene reduces the particle pollutants, which result in improved long-term health benefits when compared with traditional fuel wood or charcoal use for cooking.

Conclusion

Improved charcoal kilns require some capital outlay but also require better understanding and control of the carbonization process. Drying of wood, better stacking methods, and better process control, in combination with a chimney to force inverted draught, can greatly increase carbonization efficiency. However, they take more time and effort to prepare the kiln and control the carbonization process.

In areas where wood is freely available traditional charcoal makers may not have an incentive to improve their production and may use several traditional kilns. Increasing the efficiency of charcoal production requires regulatory measures, systematic training, and demonstration programs.

source: www.practicalaction.org

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