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Chapter 13 - Comparative performance of carbonization systems

13.1. Performance indices of carbonising equipment
13.2. Influence of wood characteristics on carbonization methods

Throughout the world wood is turned into charcoal by a surprising variety of systems. Choosing the optimum carbonization method is of interest to every potential charcoal producer. A close look reveals the reasons why so many wags of carbonising wood can coexist, different in detail, by relying on the same fundamental principle.

Most charcoal is made by small scale peasant type producers, either for their own local needs or for a restricted market. There is relatively little international trade in charcoal and hardly any competition between producers in one region with those in another. This fact tends to isolate charcoal producers in various countries and allows marked regional differences to continue.

The reason for carbonising the wood is a further factor. On the one hand, those who make charcoal for industry are interested in striving for maximum productivity and efficiency. At the other end of the scale are those who carbonise wood simply because they cannot subdivide the wood fuel in log form for use in domestic cooking by any other method. They have a totally different set of guidelines. The first group has access to capital and technology, the latter may not even possess an efficient axe or saw and must choose a method which requires the absolute minimum of capital investment. If this implies wasteful use of resources or human labour power, there seems no other alternative to them.

Tradition, the embodied wisdom of rural societies, plays an important part. To use the established method which is known to work successfully in a locality is the logical option for those who cannot afford to take risks because of their precarious economic situation. Where social factors are dominant, it is usually very difficult to introduce a new technology of charcoal-making unless the social factors are changed. Frequently one sees attempts to modify the technology of charcoal-making by providing aid: inputs such as chain saws, new kilns and so on. When these inputs are no longer available, economic necessity forces the producers to revert to the traditional, successful method with all its obvious technical faults. Therefore carbonising methods cannot be evaluated just on the basis of technical factors; social factors are of equal importance.

But good technology is important in the long run in improving social conditions. Therefore, if social factors permit, methods which give higher yields of better quality charcoal at lower cost should be used. These technical considerations are the concern of this chapter in comparing the different methods of converting wood to charcoal.

13.1. Performance indices of carbonising equipment

The various carbonising methods are classified (13). The method followed is based on reference 29 which gives a useful overall view of the range of methods available.

The first major difference is between systems which heat the wood by external means, using wood, oil, gas, etc., and systems which allow combustion on a limited scale to occur inside the carboniser by burning part of the wood charge and using this heat to dry and carbonise the remainder.

Fig. 13. Classification of carbonization systems

This method should be the most efficient since the heat is generated where it is needed, using low cost wood fuel. In practice, it is difficult to control the combustion and some extra wood is burned which lowers the yield.

Indirect (external) heating allows more precise control but to transmit the heat to the charge is difficult and inefficient and metal retorts are almost essential. By-products can be recovered free of contamination from the products of combustion. A hybrid method heats the charge of wood by passing hot gas through it. The hot gas is obtained by burning a fuel which can be wood, oil or gas. Precise control is needed to ensure that the hot gas is free of oxygen, otherwise some of the wood will be burned instead of being merely carbonised. Heat transfer from the hot gas to the wood is quite efficient and where the gases are recirculated under proper control, it is feasible to condense and collect by-products and the combustible wood gas.

Systems using internal generation of heat can be further divided by their method of construction. The three possibilities found are earth, which is lowest in cost, bricks or masonry of intermediate cost, and steel which is the most expensive. Steel kilns are further subdivided into portable and fixed types.

Steel kilns have two advantages: they can be moved easily, which may be very useful, and they cool quickly, allowing a shorter cycle time. However, portability is not always an efficient idea, since it makes it difficult to organise and supervise production efficiently and fixed brick kilns can be cooled quite rapidly using a slurry of clay and water (with care,) by injecting water spray into the kiln. Although cycle times are still around six to eight days, compared to two for steel kilns, the greater volume and much lower cost of brick kilns make them preferable except where portability is essential.

Earth kilns and pits even when operated efficiently, are slow burning and slow cooling and contaminate the charcoal with earth. However, where capital is limited or non-existent, they have real advantages.

Kilns heated by an external source of heat are subdivided into those heated by passing hot gases through the charge and those where heat is transferred through the walls of the retort. Most carbonisers in this subdivision are of metal but there is one exception, the Schwartz kiln, still commercially used, which is of brick and heats the charge by pressing hot flue gas from a bonfire burning wood built at the side of the kiln. Theoretically excellent since low quality wood and bark can be burned, in practice the kiln suffers in comparison to internally fired brick kilns by its high construction cost requiring steel and cast iron components, difficulty of precisely controlling the fire, and sealing the kiln for cooling, leading to air leaks and loss of charcoal.

Steel retorts heated through the walls are not used much today because of high cost and intrinsic low efficiency but some portable and semi-portable experimental retorts have appeared recently (14), e.g. the Constantine retort and the Jamaican oil drum retort. The steel retorts heated by circulating gases are efficient, produce charcoal of excellent quality and allow by-products to be recovered. However, their high capital cost makes them unattractive, except where the labour costs of traditional systems outweigh the high capital cost. These retorts are mainly applied today for making high grade charcoal for metallurgical and chemical use. Their use in the charcoal industry once seemed attractive but recent developments in making high purity iron without charcoal and changes in the world steel industry based on coal, make their use problematic until a lower capital cost version is developed. It seems unlikely that they can make any major contribution to the production of charcoal for domestic use in developing countries.

Having classified the various types of carboniser, they can then be compared, using various calculated indexes (29) such as production per unit of internal volume, unit area of space occupied, unit of capital invested, etc. These calculations are best carried out to compare types within a subdivision when the basic type of carboniser needed has been chosen on broad social and technological grounds. In practice, as far as the developing world is concerned, the choices are limited to deciding between pits, earth kilns, brick kilns and steel kilns, all internally heated. Where capital is the limiting resource, and wood is available, earth kilns are preferable. Where some capital is available and a serious effort is to be made to produce quality charcoal efficiently, brick kilns will probably be preferred. Steel kilns may find use where mobility is of such overriding importance that it overcomes high capital and repair costs.

13.2. Influence of wood characteristics on carbonization methods

13.2.1. Species
13.2.2. Moisture content
13.2.3. Wood size

The characteristics of the wood raw material have a significant effect on the choice and performance of carbonization equipment. The three important-factors are species, moisture content, and dimensions of the wood itself.

13.2.1. Species

Generally all species of wood can be carbonised to produce useable charcoal. There is a variation in the ash content of different woods but this is generally not significant. Bark, however, has an unacceptably high ash content and the structure of bark charcoal is too friable to be useful for most purposes. Therefore, where possible, bark should not be used or the amount of bark charged with the wood should be minimised.

Softwoods generally produce a softer, more friable charcoal than hardwoods but where available in quantity at a suitable price, they are a good raw material and can produce all types of charcoal.

Where a choice of wood supply is possible, such as where plantations are being established to provide wood, it is worthwhile to choose the species and manage its growth rate to optimise charcoal properties. Eucalypt species produce good dense charcoal and are the favoured plantation species for the purpose. Careful tests should be made before unproven, little known species are planted.

What counts in the long run is the mass of saleable charcoal produced per unit mass of wood substance. The volume of wood grown per hectare is only a rough indicator of the mass of wood substance produced. A high volume increment may correspond to low density and hence low yield of charcoal per unit volume of wood. Also denser wood usually produces a denser, less friable charcoal. Therefore research to determine what species and what management regime produces the maximum yield of wood substance by weight from plantations is worthwhile. This is an area of active research and definite answers are not yet available. But eucalypts are still the favoured genus.

13.2.2. Moisture content

Moisture in wood charged to the kiln has to be evaporated by burning extra wood and this lowers overall yield. Also the time to complete a carbonization cycle is extended, thus increasing costs. The volume of unseasoned wood is also higher than dry wood and the packing fraction of the kiln is thus marginally reduced when green wood is used. Wood will dry in the air without any heating cost. Air drying costs are mainly financial plus the wood loss due to fungal decay and insect attack. It is necessary to strike the optimum time balance in drying so that the maximum amount of moisture is lost in the early period when water loss is rapid. Financial costs are less and wood loss due to insect and fungi is still low. About three months of drying is roughly optimum but this varies with climate and kind of wood. Effective drying is difficult in the humid tropics.

13.2.3. Wood size

Carbonization rate is closely related to wood size. Large wood pieces carbonise slowly since the transfer of heat into the interior of the wood is a relatively slow process. Sawdust, for example, can be flash carbonised very rapidly but the powdered charcoal produced is of low market value. On the other hand, large diameter trunks of dense species may shatter when carbonised making the charcoal more friable than otherwise. Studies have shown that charcoal with optimum properties for the iron industry is produced with wood pieces measuring about 25-80 mm across the grain. Length along the grain has little influence. (26).

With plantation grown wood uniformity in wood size is possible but natural forests yield a wide range of sizes. Cutting and splitting of wood is costly in labour, fuel and capital and should be avoided wherever possible. For carbonising large diameter trunks and mixed size charges of wood the slow cycles are best. The pit system is optimum. Of the masonry kilns the slower cycle, larger kilns are best. They are a well proven method for carbonising large diameter (around 0.5 m) dense wood from natural forests. Trouble in carbonising can be reduced by placing the large diameter blocks in the centre of the charge. Metal kilns which lose much heat through the walls and cool quickly are ineffective in carbonising large section wood.

The cost of cutting up wood is a serious and growing one as fuel, labour and capital costs increase and this favours the use of earth pits, mounds and brick kilns. It is also usually easier and faster to charge kilns with large size wood, especially if its length conforms with the size of the kiln, pit or mound. It is worthwhile carefully studying the relation between growing, harvesting, drying and kiln charging to decide the optimum dimensions of the wood both in length and diameter, so that overall handling and carbonising costs are minimised and charcoal of optimum properties for the final end use is obtained.

Poor rural dwellers who cannot afford saws and axes often convert large diameter wood to charcoal so that it can be broken up for use in cooking fires. When the relative efficiency of wood to charcoal conversion and burning efficiency of wood and charcoal cooking fires are compared, the practice has much to commend it. Further charcoal is dry and can be stored indefinitely without deterioration. Calculation shows that carbonising large diameter wood and burning the charcoal is about twice as efficient thermally as burning the wood direct in an open cooking fire. Furthermore, without axes, saws and wedges, large diameter wood is unused and may rot before it can be burned.

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