| Application of biomass-energy technologies |
|II. Improved charcoal production|
Charcoal is an important household fuel and to a lesser extent, industrial fuel in many developing countries. It is mainly used in the urban areas where its ease of storage, high energy content (30 MJ/kg as compared with 15 MJ/kg few woodfuel), lower levels of smoke emissions, and, resistance to insect attacks make it more attractive than woodfuel. In the United Republic of Tanzania, charcoal accounts for an estimated 90 per cent (round wood equivalent basis) of biofuels consumed in urban centres (World Bank, 1988).
The production of charcoal spans a wide range of technologies from simple and rudimentary earth kilos to complex, large-capacity charcoal retorts. The various production techniques produce charcoal of varying quality. Improved charcoal production technologies are largely aimed at attaining increases in the net volume of charcoal produced as well as at enhancing the quality characteristics of charcoal. Typical characteristics of good-quality charcoal are shown in table 1.
Table 1. Typical characteristics of good-quality charcoal
5 per cent
Fixed carbon content
75 per cent
20 per cent
Efforts to improve charcoal production are largely aimed at optimizing the above characteristics at the lowest possible investment and labour cost while maintaining a high production volume and weight ratios with respect to the wood feedstock.
The production and distribution of charcoal consist of seven major stages:
1. Preparation of wood
2. Drying - reduction of moisture content
3. Pre-carbonization - reduction of volatiles content
4. Carbonization - further reduction of volatiles content
5. End of carbonization - increasing the carbon content
6. Cooling and stabilization of charcoal
7. Storing, packing, transport, distribution and sale
The first stage consists of collection and preparation of wood, the principal raw material. For small-scale and informal charcoal makers, charcoal production is an off-peak activity that is carried out intermittently to bring in extra cash. Consequently, for them, preparation of the wood for charcoal production consists of simply stacking odd branches and sticks either cleared from farms or collected from nearby woodlands. Little time is invested in the preparation of the wood. The stacking may, however, assist in drying the wood which reduces moisture content thus facilitating the carbonization process.
More sophisticated charcoal production systems entail additional wood preparation, such as debarking the wood to reduce the ash content of the charcoal produced. It is estimated that wood which is not debarked produces charcoal with an ash content of almost 30 per cent. Debarking reduces the ash content to between I and 5 per cent which improves the combustion characteristics of the charcoal.
The second stage of charcoal production is carried out at temperatures ranging from 110 to 220 degrees Celsius. This stage consists mainly of reducing the water content by first removing the water stored in the wood pores then the water found in the cell walls of wood and finally chemically-bound water.
The third stage takes place at higher temperatures of about 170 to 300 degrees and is often called the pre-carbonization stage. In this stage pyroligneous liquids in the form of methanol and acetic acids are expelled and a small amount of carbon monoxide and carbon dioxide is emitted (Fernandes, 1991).
The fourth stage occurs at 200 to 300 degrees where a substantial proportion of the light tars and pyroligneous acids are produced. The end of this stage produces charcoal which is in essence the carbonized residue of wood (Fernandes, 1991).
The fifth stage takes place at temperatures between 300 degrees and a maximum of about 500 degrees. This stage drives off the remaining volatiles and increases the carbon content of the charcoal.
The sixth stage involves cooling of charcoal for at least 24 hours to enhance its stability and reduce the possibility of spontaneous combustion.
The final seventh stage consists of removal of charcoal from the kiln, packing, transporting, bulk and retail sale to customers. The final stage is a vital component that affects the quality of the finally-delivered charcoal. Because of the fragility of charcoal, excessive handling and transporting over long distances can increase the amount of fines to about 40 per cent thus greatly reducing the value of the charcoal. Distribution in bags helps to limit the amount of fines produced in addition to providing a convenient measurable quantity for both retail and bulk sales.
Past efforts to improve charcoal production have largely focused on enhancing the efficiency of the combustion stages two to five through the design of new charcoal kilns. The improved charcoal kilns can be broadly classified into five categories, namely:
1. Earth kilns
2. Metal kilns
3. Brick kilns
4. Cement or masonry kilns
5. Retort kilns
The above categories are differentiated mainly by the technical sophistication and investment costs of the different kilns. The categories range from the rudimentary and low-cost earth kiln which is widely used in many developing countries to high-cost retort kilns, which in addition to charcoal, produces other valuable by-products. The main characteristics of the each of the five categories of kilns are given in table 2.
Suitable designs are available for the itinerant small-scale charcoal producers and large-scale industrialists who require the charcoal for steel production as in the case of Brazil where, in 1987, an estimated 7 million tons of charcoal were used to produce 35 per cent of local pig iron (Abracave, n.d., and Fernandes, 1991). Investment costs vary with the sophistication of the technology used but even simple designs such as the brick kilns and oil-drum kilns can be upgraded to large-scale complex charcoal production through the simultaneous installation and operation of a battery of kilns.
Retort kilns common in the developed world before and during the Second World War were used to produce a wide range of charcoal by-products that included acetic acid, methanol and tar. One of the largest retorts was operating in Pemery, France, in 1947 and had a capacity of 20,000 tons per year of charcoal. With the advent of the petro-chemical industry that was able to produce at much lower cost many of the chemicals previously produced by retorts, many of the large-scale retort operations have now been discontinued. The last known large-scale installation, closed in the late 1970s, was in Wundowie, Australia and had a capacity of 35 tons of charcoal per day (Fernandes, 1991).
In addition to cost, there are some general characteristics that differentiate the above five categories of charcoal production. The more complex designs are less labour intensive and include semi-automated operations. In addition, by-products in the high-cost designs are often as important, and sometimes more important than, the charcoal produced. The low-cost simpler designs are mostly found in developing countries where labour is abundant while the high-cost designs are mainly found in developed countries.
While most of the low-cost improved charcoal kilns have demonstrated high efficiencies under test conditions, none of the developed designs have attained substantive dissemination, largely because of the nature of charcoal production in many developing countries and the surprisingly high efficiency of traditional kilns under field conditions. Initially thought to be a grossly inefficient technology, a 1984/85 study in Sudan indicated that the efficiency of the traditional earth kiln is comparable with improved brick and metal portable kilns (Tebicke, 1991). A comparative study of five different kiln types showed that with the exception of the pit kiln, traditional kilns can attain similar levels of performance to improved metal kilns (World Bank, 1988) as shown in table 3.
This is also confirmed in the previous table on charcoal production technology which shows that there is no clear demarcation between the various designs in terms of yield. The critical factors appear to be operational and supervisional skill and moisture content of the utilized wood (Teplitz-Sempbitzky, 1990). The presence of a chimney that ensures optimum draught conditions also appears to be important.
The quality of charcoal, however, differs significantly since the more complex systems allow more elaborate control of combustion characteristics. The quality of charcoal is not as important in developing countries where it is largely used as a household fuel. Charcoal quality is, however, a crucial issue when it is used for industrial purposes such as the manufacture of steel or for tobacco drying and cement manufacture.
A large proportion of charcoal production in developing countries is carried out as a semi-illegal part-time activity since the wood used is often illegally procured. Consequently, few charcoal makers are willing to make the investment required by improved charcoal kilns are they willing to construct in-situ kilns since they would be vulnerable to punitive official measures such as imposition of tax and seizure. Consequently, dissemination of improved charcoal techniques to the informal sector has proved to be a difficult undertaking. Improved charcoal production technologies have proved more successful in areas where production is undertaken on a commercialized basis as in the case of Malawi, as shown in the case study below.