4.1. How wood is transformed into charcoal
4.2. Industrial safety in carbonization
4.3. Incentives and labour management
The carbonization stage may be decisive in charcoal production even though it is not the most expensive one. Unless it is carried out as efficiently as possible, it puts the whole operation of charcoal production at risk since low yields in carbonisation reflect back through the whole chain of production as increased costs and waste of resources.
Wood consists of three main components: cellulose, lignin and water. The cellulose and lignin and some other materials are tightly bound together and make up the material we call wood. The water is adsorbed or held as molecules of water on the cellulose/lignin structure. Air dry or "seasoned" wood still contains 12-18% of adsorbed water. Growing, freshly cut or "unseasoned" wood contains, in addition, liquid water to give a total water content of about 40 to 100% expressed as a percentage of the oven dry weight of the wood.
The water in the wood has all to be driven off as vapour before carbonization can take place. To evaporate water requires a lot of energy so that using the sun to pre-dry the wood as much as possible before carbonization greatly improves efficiency. The water remaining in the wood to be carbonised, must be evaporated in the kiln or pit and this energy must be provided by burning some of the wood itself which otherwise would be converted into useful charcoal.
The first step in carbonization in the kiln is drying out of the wood at 100°C or below to zero moisture content. The temperature of the oven dry wood is then raised to about 280°C. The energy for these steps comes from partial combustion of some of the wood charged to the kiln or pit and it is an energy absorbing or endothermic reaction.
When the wood is dry and heated to around 280°C, it begins to spontaneously break down to produce charcoal plus water vapour, methanol, acetic acid and more complex chemicals, chiefly in the form of tars and non-condensible gas consisting mainly of hydrogen, carbon monoxide and carbon dioxide. Air is admitted to the carbonising kiln or pit to allow some wood to be burned and the nitrogen from this air will also be present in the gas. The oxygen of the air is used up in burning part of the wood charged.
The spontaneous breakdown or carbonization of the wood above a temperature of 280°C liberates energy and hence this reaction is said to be exothermic. This process of spontaneous breakdown or carbonization continues until only the carbonised residue called charcoal remains. Unless further external heat is provided, the process stops and the temperature reaches a maximum of about 400°C. This charcoal, however, will still contain appreciable amounts of tarry residue, together with the ash of the original wood. The ash content of the charcoal is about 3-5%; the tarry residue may amount to about 30% by weight and the balance is fixed carbon about 65-70%. Further heating increases the fixed carbon content by driving off and decomposing more of the tars. A temperature of 500°C gives a typical fixed carbon content of about 85% and a volatile content of about 10%. The yield of charcoal at this temperature is about 33% of the weight of the oven dry wood carbonised - not counting the wood which was burned to carbonise the remainder. Thus the theoretical yield of charcoal varies with temperature of carbonization due to the change in its content of volatile tarry material. (24, 26, 31). Table 4 shows the effect of final carbonisation temperature on the yield and composition of the charcoal.
Table 4. Effect of carbonisation temperature on yield and composition of charcoal
|
Carbonisation Temperature |
Chemical analysis of charcoal |
Charcoal yield based on oven dry wood | |
|
°C |
% of fixed charcoal |
% volatile material |
(0% moisture) |
|
300 |
68 |
31 |
42 |
|
500 |
86 |
13 |
33 |
|
700 |
92 |
7 |
30 |
Low carbonization temperatures give a higher yield of charcoal but this charcoal is low grade, is corrosive due to its content of acidic tars, and does not burn with a clean smoke-free flame. Good commercial charcoal should have a fixed carbon content of about 75% and this calls for a final carbonising temperature of around 500°C.
The yield of charcoal also shows some variation with the kind of wood. There is evidence that the lignin content of the wood has a positive effect on charcoal yield. A high lignin content gives a high yield of charcoal. Therefore, mature wood in sound condition is preferred for charcoal production. Dense wood also tends to give a dense, strong charcoal, which is also desirable. However, very dense woods sometimes produce a friable charcoal because the wood tends to shatter during carbonization. The friability of charcoal increases as carbonization temperature increases and the fixed carbon content increases as the volatile matter content falls. A temperature of 450 to 500°C gives an optimum balance between friability and the desire for a high fixed carbon content.
The many variables possible in carbonization make it difficult to specify an optimum procedure - generally the best results will be obtained by using sound hardwood of medium to high density. The wood should be as dry as possible and usually be split to eliminate pieces more than 20 cm thick. Firewood which will be burned up inside the kiln or pit to dry out and start carbonization of the remainder can be of inferior quality and smaller in cross section. Its sole function is to produce heat to dry out and heat up the remainder to carbonising temperature. One should try and reach a final temperature of around 500°C through the whole of the charge. With pits this is difficult since the air circulation and cooling effects are irregular and cold spots occur. These produce "brands" of uncarbonised wood. Trying to reach a final overall temperature of 500°C with a pit or kiln having poor and irregular air circulation usually results in burning part of the charcoal to ashes, while leaving other parts of the charge only partly carbonised. Hence the importance of using well designed kilns properly operated for an efficient charcoal operation. More information on the technical aspects of carbonization can be found in references 6, and 7.
Carbonisation produces substances which can prove harmful and simple precautions should be taken to reduce risks.
The gas produced by carbonization has a high content of carbon monoxide which is poisonous when breathed. Therefore, when working around the kiln or pit during operation and when the kiln is opened for unloading, care must be taken that proper ventilation is provided to allow the carbon monoxide, which is also produced during unloading through spontaneous ignition of the hot charcoal, to be dispersed.
The tars and smoke produced from carbonization, although not directly poisonous, may have long-term damaging effects on the respiratory system. Housing areas should, where possible, be located so that prevailing winds carry smoke from charcoal operations away from them and batteries of kilns should not be located in close proximity to housing areas.
Wood tars and pyroligneous acid can be irritant to skin and care should be taken to avoid prolonged skin contact by providing protective clothing and adopting working procedures which minimize exposure.
The tars and pyroligneous liquors can also seriously contaminate streams and affect drinking water supplies for humans and animals. Fish may also be adversely affected. Liquid effluents and waste water from medium and large scale charcoal operations should be trapped in large settling ponds and allowed to evaporate so that this water does not pass into the local drainage system and contaminate streams.
Fortunately kilns and pits, as distinct from retorts and other sophisticated systems, do not normally produce liquid effluent - the by-products are mostly dispersed into the air as vapours. Precautions against airborne contamination of the environment are of greater importance in this case.
Burning of charcoal is a responsible operation which requires skill, patience, experience and readiness to observe correct working methods at all times and in all weathers. The yield achieved in the burning stage effect the economics of the whole operation. Therefore, it is worthwhile to pay the charcoal burning crew an incentive over their normal salary based on the quality and quantity of charcoal they produce. Such a scheme requires proper measurement of the wood entering and the charcoal leaving the system. Measurement can be by volume or by weight but, in any case, must be carried out seriously if it is to function properly and satisfy all parties.
Charcoal production is often a seasonal activity. The rainy season may close down operations or the labour force may traditionally be employed at certain times in harvesting or planting operations in agriculture. This makes it difficult to attract and hold a labour force who are well trained and motivated. But good results depend on a stable and contented work force. Therefore, it is important to endeavour to develop a stable labour regime in charcoal production. This feature will be noted as a characteristic of all successful large scale charcoal operations.
