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Chapter 2. Wood carbonisation and the products it yields

2.1 Carbonisation
2.2 Efficiency in carbonisation
2.3 Measuring the yield
2.4 What happens during carbonisation
2.5 The stages in charcoal formation
2.6 Using heat efficiently in carbonisation
2.7 Continuous carbonisation
2.8 Classification of retort heating systems
2.9 Properties of carbonisation products

2.1 Carbonisation

Carbonisation is a particular form of that process in chemical technology called pyrolysis that is the breakdown of complex substances into simpler ones by heating. Carbonisation is the term used when complex carbonaceous substances such as wood or agricultural residues are broken down by heating into elemental carbon and chemical compounds which may also contain some carbon in their chemical structure. The term carbonisation is also applied to the pyrolysis of coal to produce coke.

2.2 Efficiency in carbonisation

The carbonisation stage in the charcoal making process is the most important step of all since it has such power to influence the whole process from the growing tree to the final distribution of the product to the user.

Yet carbonisation in itself is relatively not a costly step. Even though retorts may be of high capital cost they do not require very much labour per unit of production. Typically the carbonisation step may represent about 10% of total costs from growing and harvesting the tree to arrival of the finished charcoal into bulk store. But the conversion efficiency of the carbonisation step works its way back to the point where the wood is harvested. A high yield in conversion means that less wood has to be grown, harvested, dried, transported and loaded into the retort or other carbonising unit.

The specific way the wood is carbonised is also able to effect overall yield because of the effect it has on the amount of fines produced. Fines may have no market at all or may only be saleable after going through a fairly costly briquetting process.

The three major factors which influence the conversion yield are:

(a) The moisture content of the wood at time of carbonisation.
(b) The type of carbonising equipment used.
(c) The care with which the process is carried out.

2.3 Measuring the yield

Efficiency of carbonisation is expressed as the yield of charcoal in gross terms (at the side of the retort or kiln) expressed as a percentage of the wood charged or used-up to produce it. Normally only the wood actually used-up is reckoned. Thus unburnt wood which can be recycled is deducted from the wood used even though it represents a concealed form of inefficiency. On the other hand where indirect heating is used as in retorts or the Swartz type kiln, which employs an external fire grate, the amount of wood used-up in the heating must be included in the wood used to produce the charcoal. Account may be taken that in some cases this wood may be of lower quality.

Wood and charcoal must be measured using standardised methods. They need not be the same for both materials but they must be consistent so that results are comparable. In other words a consistent methodology of measurement must be adhered to. Properly measured conversion efficiencies allow different charcoal making methods to be compared. Also these measurements are essential in controlling large charcoal making enterprises.

The most accurate measuring system compares all quantities on a weight basis. To avoid complication due to differing moisture contents, the wood used is expressed on a bone dry basis and the charcoal is weighed bone dry and free of fines. where moisture is present it must be determined and allowed for. To apply such a system, equipment for weighing and determining moisture content of wood and charcoal must be available. Unfortunately this is rarely the case in most charcoal-making situations. It is the method most suitable for research on processing and for the large industrial enterprise. Being free of inbuilt errors it is the final reference system.

A practical method which has been widely standardized in South America, particularly in the steel industry of Brazil uses volume measurement. Both the wood used and the charcoal produced are measured in cubic meters corrected for stacking and compaction errors. The wood is measured in stores (stacked cubic meters) and each stere is taken an equivalent to 0.65 solid cubic meters. The system allows for the effect of shrinkage of the fuelwood on drying and the reduction in volume which occurs when charcoal is transported and handled due to settlement. This settlement is the result of abrading of sharp corners of the lump charcoal and the formation of fine charcoal which has practically no commercial value.

The shrinkage allowance for fuelwood is based on experiments on the effect of drying and destacking and restacking as happens when a pile of dry wood is transported from the forest to the charcoal plant. The results show that a pile of 100 stores of eucalypt wood shrinks to 84 stores after 3-4 months drying and when the same pile is restacked its new volume is only 79 steres. Thus a reduction of 15% is allowed for drying and 21% for drying and restacking. The true contents of a pile of fuelwood are also greatly influenced by the method of stacking. Experience is the only way to overcome this problem in order to tell if the volume of the wood has been inflated by dishonest stacking.

The charcoal volume is measured by placing it in a wire basket having the base one meter square and height somewhat more than a meter. A commercial cubic meter of charcoal is considered to have a true volume of one cubic meter only when measured at the side of the blast furnace, that is to say, in the bulk storage depot. At the side of the charcoal kiln a cubic meter of commercial charcoal is considered to have a true volume of 1.1 cubic meter. In this way the contraction of the charcoal in transport and the production of useless fines is allowed for. The standard yield of Brazilian charcoal kilns using this system is reckoned as 1 cubic meter of commercial charcoal from every 2.2 steres of fuelwood. Volume measurement for determining charcoal yield is subject to certain intrinsic errors but it is a simple method, easily understood and can be performed 'out in the open". It has a great advantage in the buying and selling of charcoal as it automatically discourages adulteration by wetting the charcoal and mixing it with sand and earth. The reason is that these actions have no effect on the volume. Further there is an incentive for the charcoal to be transported carefully so the reduction in saleable volume by settlement and production of fines is minimized. The temperature to which the charcoal is taken to in the kiln affects the measure of the yield by changing its content of volatile tarry material. Soft burned charcoal produced when the temperature does not rise above about 400°C can have a volatile matter content of about 30% and this is equivalent to a yield of about 42% on a bone dry weight basis. At 500°C the volatile matter is only about 13% and the yield about 33% on a bone dry basis. Hence, to compare equals with equals different kinds of charcoal must have about the same volatile matter content.

2.4 What happens during carbonisation

During pyrolysis or carbonisation the wood is heated in a closed vessel of some kind, away from the oxygen of the air which otherwise would allow it to ignite and burn away to ashes. Without oxygen we force the wood substance to decompose into a variety of substances the main one of which is charcoal, a black porous solid consisting mainly of elemental carbon. Other constituents are the ash from the original wood amounting to 0.5 to 6% depending on the type of wood, amount of bark, contamination with earth and sand, etc. and tarry substances which are distributed through the porous structure of the charcoal. As well as charcoal. Liquid and gaseous products are produced which may be collected from the vapours driven off if the charcoal is made in a retort. The liquids are condensed when the hot retort vapours pass through a water cooled condenser. The non-condensible gases pass on and are usually burned to recover the heat energy they contain. This wood gas, as it is called, is of low calorific value (around 10% of that of natural gas).

The products other than charcoal are usually referred to as by-products. Years ago recovery of the chemicals they contain was a flourishing industry in many developed countries. Since the advent of the petrochemical industry this by-product industry has become uneconomic since in most instances the chemicals can be produced from petroleum more cheaply. More information is given on this problem later.

2.5 The stages in charcoal formation

As the wood is heated in the retort it passes through definite stages on its way to conversion into charcoal. The formation of charcoal under laboratory conditions has been studied and the following stages in the conversion process have been recognised.

- at 20 to 110°C

The wood absorbs heat as it is dried giving off its moisture as water vapour (steam). The temperature remains at or slighly above 100°C until the wood is bone dry.

- at 110 to 270°C

Final traces of water are given off and the wood starts to decompose giving off some carbon monoxide, carbon dioxide, acetic acid and methanol. Heat is absorbed.

- at 270 to 290°C

This is the point at which exothermic decomposition of the wood starts. Heat is evolved and breakdown continues spontaneously providing the wood is not cooled below this decomposition temperature. Mixed gases and vapours continue to be given off together with some tar.

- at 290 to 400°C

As breakdown of the wood structure continues, the vapours given off comprise the combustible gases carbon monoxide, hydrogen and methane together with carbon dioxide gas and the condensible vapours: water, acetic acid, methanol, acetone, etc. and tars which begin to predominate as the temperature rises.

- at 400 to 500°C

At 400°C the transformation of the wood to charcoal is practically complete. The charcoal at this temperature still contains appreciable amounts of tar, perhaps 30% by weight trapped in the structure. This soft burned charcoal needs further heating to drive off more of the tar and thus raise the fixed carbon content of the charcoal to about 75% which is normal for good quality commercial charcoal.

To drive off this tar the charcoal is subject to further heat inputs to raise its temperature to about 500°C, thus completing the carbonisation stage.

2.6 Using heat efficiently in carbonisation

In carbonisation there are substantial flows of heat into and out of the wood being carbonised. Correct control of them affects the efficiency and quality of charcoal production. The heat flows can be calculated and shown on a heat balance diagram of the process. This needs a knowledge of heat engineering but the basic principles are not hard to understand. A heat input must come from the burning of a fuel of some kind which will usually mean wood in the case of charcoal making. Even if we use the exothermic heat from carbonisation or the heat liberated by burning the off-gas from the retort any additional heat will come from burning some wood and hence represents a loss. Wood which is burned cannot be turned into charcoal.

The three main stages requiring heat inputs in charcoal making are:

- The drying of the wood.

- Raising the temperature of the oven dry wood to 270°C to start spontaneous pyrolysis which itself liberates heat.

- Final heating to around 500-550°C to drive off tar and increase the fixed carbon to an acceptable figure for good commercial charcoal.

An ideal carbonising process would be one which required no external heat to carry out the carbonisation. The exothermic heat of the process would be captured together with the heat produced by burning off-gas and liquid by-products and this in total would be sufficient to dry out the residual moisture in the wood, raise it to spontaneous pyrolysis temperature and then heat it to a temperature sufficient to drive-off residual tars. In practice due to losses of heat through the walls of the carboniser and poor drying of the feedstock it is almost impossible to achieve this aim. However some systems particularly the large hot rinsing gas retorts come close to the ideal where the climate of the locality permits proper drying of the wood raw material.

No wood will carbonise until it is practically bone dry. The water in green wood however is typically about 50% of the green weight of the wood and this must all be evaporated before the wood will start to pyrolyse to form charcoal.

It is most economic to dry out as much of this moisture as possible using the sun's heat before the wood is carbonised. In dry savannah regions this is fairly simple as the wood can be left 12 months or more to dry without serious loss due to insect attack or decay. In the humid tropics two or three months may be the practical limit before insect and decay losses become intolerable. The loss in charcoal yield due to excessive moisture content has to be balanced against the loss of wood substance due to biological deterioration.

The important factors in drying and storing the wood raw material are described in Chapter 4.

2.7 Continuous carbonisation

One of the most important steps forward in the production of charcoal was the application of the concept of continuous carbonisers. By causing the raw material wood to pass in sequence through a series of zones where the various stages of carbonisation are carried out it is possible to introduce economies in use of labour and heat thus reducing production costs and increasing the yield from a given amount of wood.

The concept of a continuous carboniser where the wood travels vertically downwards as it is heated and carbonised follows fairly obviously from the idea of the iron smelting blast furnace. But it proved necessary in order to get charcoal in lump form to abandon the idea of obtaining the heat for drying the charge and heating it to carbonisation point by burning part of the wood charged. This proved too difficult to control. The heating process had to be changed to use of hot oxygen-free gas produced externally and blown through the descending charge of wood. In this way the operation was under complete control and it proved possible to produce properly burned charcoal and yet ensure that it still emerged in lump form. Furthermore, the charcoal was never contaminated with ash since the carboniser always operates at a temperature below glowing combustion point.

Recovery of the heat emerging from the top of the carboniser was achieved by burning the gas and vapours under controlled conditions in hot blast stoves similar to those used in iron smelting and then blowing this hot gas into the retort at appropriate points so that carbonisation was completed by the hot gas first impinging on the charcoal emerging from the spontaneous pyrolysis zone. The gas then passed up the tower giving up its heat in countercurrent form to the descending charge of wood. The finished charcoal in the lower part of the retort was cooled before it reached the base by blowing in cold oxygen-free fuel gas and extracting it just below the point of entry of the hot gas coming from the hot blast stove. The fuel gas, warmed through cooling the charcoal then entered the hot blast stoves to be burned with air to produce the hot rinsing gas to be blown back into the unit to strip the residual tar from the charcoal and then proceed up the tower giving up its heat to the descending charge of wood. The position of the different zones in the tower could be controlled by regulating the gas injection rate and its temperature and the rate at which wood was admitted at the top and the charcoal was removed at the base.

This type of retort known under the generic name of 'continuous vertical hot rinsing gas retort' is commonly called the Lambiotte retort after its inventor, (Lambiotte, 1942, 1952). It is probably the most sophisticated charcoal making process because of the quality and yield of the charcoal it produces but there are other continuous charcoal making systems which are in successful commercial use. The best known of these uses the continuous multiple hearth roasting furnace also known as the Herreshoff roaster after its inventor. Just as the rinsing gas retort borrows much of its technology from the blast furnace so the multiple hearth furnace is a simple transfer of technology from the chemical and metallurgical industries where it is a familiar unit used for roasting sulphide ores prior to further processing.

The Herreshoff roaster is at a disadvantage compared with the rinsing gas retort in that it can only process finely divided wood or bark, etc, and hence can only produce powdered charcoal which must be briquettes for sale. Such briquettes are of no use for ordinary metallurgical use. The only economic market is for barbecues which requires a fairly sophisticated consumer market.

The Herreshoff roaster produces powdered charcoal and a mixture of hot gases and vapours. This gas mixture is an environmental pollutant. Since it is uneconomic to recover by-products from it nowadays the only use is to burn it to produce process heat such as for driving briquettes or making steam which might be passed through turbines to generate power. If no economic use can be found for the heat then the gas is merely burned to waste in a tall chimney.

The Herreshoff roaster is of interest because of its simplicity. It operates continuously obtaining the heat needed for final drying and carbonisation of the feedstock by burning part of it by the controlled admission of air to the hearths as the material progresses from top to bottom. If it could handle wood in lump form it would be an ideal continuous system.

All other continuous systems proposed, and there are many, based on moving belts, screw conveyers, fluidised beds and the like, while they can produce charcoal, generally fail on economic grounds.

Recently, particularly after the rise in oil price of the seventies a number of systems emerged which aimed to produce hot gas for process heating to replace oil or gas. They are based on burning finely divided wood or bark, etc, in combustion chambers with controlled admission of air and using in some cases the combustion principle of the fluidised bed. with this system a bed of saw dust or other fuel is kept in suspension by blowing air through it and the wood is allowed to burn in suspension using the oxygen in the air blast. Such systems can produce charcoal in powdered form by arranging the rate of feed so that the carbonised wood particles are removed from the fluidised bed at a sufficient rate to prevent them from being entirely burned. Keeping the system operating continually without the furnace getting too hot or too cold with feedstock of varying moisture content and fineness calls for good control. Such systems may appeal because they can be built much smaller than the well proven Herreshoff roaster which needs about 100 tons of feedstock per 24 hours as a minimum input. Extravagant claims have been made for the benefits especially from by-product recovery to be obtained from such systems but it seems they have still to be proved industrially. By-products can be collected if desired from the gas stream issuing from the converter or the hot gas can be burned in a boiler or furnace. Since they can only produce powdered charcoal a material of rather limited commercial usefulness they are hardly a solution to the problems of making charcoal by improved methods in the developing world.

2.8 Classification of retort heating systems

Carbonisers can be classified by the type of heating system employed. There are three different types.

Type 1. Heat for carbonisation is generated by allowing part of the wood charged to burn to provide the heat to carbonise the remainder. The rate of burning is controlled by the amount of air admitted to the kiln, pit, mound or retort. This is the traditional system used to produce most of the world's charcoal. It is the method used in the well-proven Herreschoff roaster. It is an efficient system if properly controlled as the heat is produced exactly where it is needed and there are no problems of heat transfer. Fluidised and other types of agitated bed carboniser also rely on this system. It's main disadvantage in simple equipment is that excessive amounts of wood are burned away because the air admitted is not closely controlled.

Type 2. Heat for carbonisation by this method is obtained by burning fuel, usually wood or perhaps wood gas, outside the retort and allowing it to pass through the walls to the wood contained in the sealed retort. Most of the early retort systems built to supply wood chemicals before the rise of the petrochemical industry were heated by this system. The system is rather inefficient in its use of heat energy since it is difficult to get a good flow of heat through the metal walls of the retort into the wood packed inside because the contact of the wood with the walls is so irregular. Overheating of the retort walls often occurs causing damage. The method is still used today for some simple type retorts such as the 'oil drum retort' which has been promoted in the Caribbean and the Constantine retort developed in Australia (19).

Type 3. In this system the wood is heated by direct contact with hot inert gas circulated under fan pressure through the retort. Heat transfer by this system is good since the hot gas directly contacts the wood to be heated. Since the gas is free of oxygen there is no combustion inside the retort and the heat transfer cools the gas which must be withdrawn and reheated to enable it to be used again for heating purposes.

The best known examples of this system are the lambiotte and the Reichert retort systems. The lambiotte or continuous hot rinsing gas retort has been described in 2.7 above. The Reichert retort is a batch type retort which heats the wood charge to convert it into charcoal by circulating hot oxygen-free gas through the charge by means of a fan and a system of heating stoves. In many ways this system resembles a batch type rinsing gas retort without the advantage of continuous feed. Another example is the Schwartz kiln developed many years ago in Europe. This kiln has an external firebox or grate and the hot flue gas from fuelwood burned in this grate is passed through the charge to heat it. The combined effluent gases pass up the chimney of the kiln into the air.

This system of heating, while technologically excellent, is more complicated than System 1 (burning part of the charged wood) and unless there is a compelling reason for its use as is the case with the hot rinsing gas retort, the cost of using it cannot be justified compared with the simple process of System 1. More information is given on these aspects in Chapter 3 and Reference (33).

2.9 Properties of carbonisation products

2.9.1 Charcoal
2.9.2 Pyroligneous acid

Carbonisation of wood gives rise to a complex range of products; solid, liquid and gaseous. Dozens of chemicals could be extracted from the liquid condensate if it were economically practical.

Today, with the eclipse of the wood based distillation industry, the primary reason for carbonising wood is to obtain charcoal. Any benefits which can be obtained from the working up of by-products nowadays are marginal and in the case of new installations probably uneconomic. Below are given the properties of the main products which can be obtained from wood carbonisation. Charcoal because of its importance is treated in greater detail.

2.9.1 Charcoal Moisture content Volatile matter other than water Fixed carbon content Ash content Typical charcoal analyses Physical properties Adsorption capacity The chemical composition of charcoal

Most of the specifications used to control charcoal quality have originated in the steel or chemical industry. when charcoal is exported, buyers tend to make use of these industrial quality specifications even though the main outlet of the imported charcoal may well be the domestic cooking or barbecue market. This factor should be borne in mind since industrial and domestic requirements are not always the same and an intelligent appraisal of actual market quality requirements may allow supply of suitable charcoal at a lower price or in greater quantities beneficial to both buyer and seller.

The quality of charcoal is defined by various properties and though all are inter-related to a certain extent, they are measured and appraised separately. These various quality factors are discussed below. Moisture content

Charcoal fresh from an opened kiln contains very little moisture, usually less than 1%. Absorption of moisture from the humidity of the air itself is rapid and there is, with time, a gain of moisture which even without any rain wetting can bring the moisture content to about 5-10%, even in well-burned charcoal. When the charcoal is not properly burned or where pyroligneous acids and soluble tars have been washed back onto the charcoal by rain, as can happen in pit and mound burning, the hygroscopitity of the charcoal is increased and the natural or equilibrium moisture content of the charcoal can rise to 15% or even more.

Moisture is an adulterant which lowers the calorific or heating value of the charcoal, where charcoal is sold by weight, keeping the moisture content high by wetting with water is often practised by dishonest dealers. The volume and appearance of charcoal is hardly changed by addition of water. For this reason bulk buyers of charcoal prefer to buy either by gross volume, e.g. in cubic metres, or to buy by weight and determine by laboratory test the moisture content and adjust the price to compensate. In small markets sale is often by the piece.

It is virtually impossible to prevent some accidental rain wetting of charcoal during transport to the market but good practice is to store charcoal under cover even if it has been bought on a volume basis, since the water it contains must be evaporated on burning and represents a direct loss of heating power. This occurs because the evaporated water passes off into the flue and is rarely condensed to give up the heat it contains on the object being heated in the stove.

Quality specifications for charcoal usually limit the moisture content to around 5-15% of the gross weight of the charcoal. Moisture content is determined by oven drying a weighted sample of the charcoal. It is expressed as a percentage of the initial wet weight.

There is evidence that charcoal with a high moisture content (10% or more) tends to shatter and produce fines when heated in the blast furnace, making it undesirable in the production of pig iron. Volatile matter other than water

The volatile matter other than water in charcoal comprises all those liquid and tarry residues not fully driven-off in the process of carbonisation. If the carbonisation is prolonged and at a high temperature, then the content of volatiles is low. When the carbonisation temperature is low and time in the retort is short, then the volatile matter content increases. (33)

These effects are reflected in the yield of charcoal produced from a given weight of wood. at low temperatures (300 C) a charcoal yield of nearly 50% is possible. At carbonisation temperatures of 500-600 C volatiles are lower and retort yields of 30% are typical. At very high temperatures (around 1,000 C) the volatile content is almost zero and yields fall to near 25%. As stated earlier, charcoal can reabsorb tars and pyroligneous acids from rain wash in pit burning and similar processes. Thus the charcoal might be well burned but have a high volatile matter content due to this factor. This causes an additional variation in pit burned charcoal in! wet climates. The resorbed acids make the charcoal corrosive and lead to rotting of jute bags - a problem during transport. Also it does not burn cleanly.

The volatile matter in charcoal can vary from a high of 40% or more down to 5% or less. It is measured by heating away from air, a weighed sample of dry charcoal at 900°C to constant weight. The weight loss is the volatile matter. Volatile matter is usually specified free of the moisture content, i.e. volatile matter - moisture or (V.M. - moisture)

High volatile charcoal is easy to ignite but may burn with a smoky flame. Low volatile charcoal is difficult to light and burns very cleanly. A good commercial charcoal can have a net volatile matter content - (moisture free) of about 30%. High volatile matter charcoal is less friable than ordinary hard burned low volatile charcoal and so produces less fines during transport and handling. It is also more hygroscopic and thus has a higher natural moisture content. Fixed carbon content

The fixed carbon content of charcoal ranges from a low of about 50% to a high of around 95%. Thus charcoal consists mainly of carbon. The carbon content is usually estimated as a "difference", that is to say, all the other constituents are deducted from 100 as percentages and the remainder is assumed to be the per cent of "pure" or "fixed" carbon. The fixed carbon content is the most important constituent in metallurgy since it is the fixed carbon which is responsible for reducing the iron oxides of the iron ore to produce metal. But the industrial user must strike a balance between the friable nature of high fixed carbon charcoal and the greater strength of charcoal with a lower fixed carbon and higher volatile matter content to obtain optimum blast furnace operation. (33) Ash content

Ash is determined by heating a weighed sample to red heat with access of air to burn away all combustible matter. This residue is the ash. It is mineral matter, such as clay, silica and calcium and magnesium oxides, etc., both present in the original wood and picked up as contamination from the earth during processing.

The ash content of charcoal varies from about 0.5% to more than 5% depending on the species of wood, the amount of bark included with the wood in the kiln and the amount of earth and sand contamination. Good quality lump charcoal typically has an ash content of about 3%. Fine charcoal may have a very high ash content but if material less than 4 mm is screened out the plus 4 mm residue may have an ash content of about 5-10%. Typical charcoal analyses

To illustrate the range of composition found in commercial charcoal Table 1 lists the composition of random samples of charcoal from various kinds of woods and various kinds of carbonisation systems. In general, all woods and all systems of carbonisation can produce charcoal falling within the commercial limits.

Table 2 records the variations in charcoal composition as found in the blast furnace charge at a large charcoal iron works in Minas Gerais, Brazil. All of this charcoal was made using beehive type brick kilns. The wood used was either mixed species from the natural forest of the region or eucalypt wood from plantations.

Table 1. Some Typical Charcoal Analyses


Wood species Production Method

Moisture content %

Ash %

Volatile matter - %

Fixed carbon %

Bulk density raw -kg/m3

Bulk density pulverised kg/m3

Gross calorific value kJ/kg Oven dry basis



Earth pit








Pulverised fuel for rotary kilns 1/





















Mixed Tropical Hardwood







Low grade charcoal fines 1/








Domestic charcoal 1/


Earth mound





      Well burned sample 1/







      Soft burned sample 1/


Portable steel kiln





    32500 2/

Coconut shells






    30140 4/

Eucalyptus Saligna







1/= Guyana. 2/= U.K. 3/= Brazil. 4/= Fiji.

Table 2. Characteristics of Charcoal for Blast Furnaces

Chemical and Physical Composition of Charcoal Dry Basis - by weight

Range Max. Min.

Yearly Average

Charcoal considered good to excel lent





75 - 80%





3 - 4%

Volatile matter




20 - 25%

Bulk density - as received (kg/m³)




250 - 300

Bulk density - dry




230 - 270

Average Size (mm) as received




20 - 50

Fines content - as received (<6.35 mm)




10% max.

Moisture content -as received




10% max.

The ranges and yearly averages refer to charcoal used by the steelworks. This is a mixture of 40% eucalyptus charcoal produced in company operated kilns and 60% heterogenous natural wood charcoal manufactured by privately operated kilns. "Good to excellent" charcoal refers to that produced from eucalyptus wood in company kilns. Physical properties

The properties described so far are referred to as chemical properties but physical properties, especially for industrial charcoal, are no less important. It is in the charcoal iron industry that physical properties have great importance. The charcoal is the most expensive raw material in the blast furnace charge. Charcoal's physical properties influence the output of the blast furnace whereas chemical properties are more related to the amount of charcoal needed per ton of iron and the composition of the finished iron or steel. (29)

Blast furnace charcoal must be strong in compression to withstand the crushing load of the blast furnace charge of "burden". This compression strength, always less than charcoal's rival, metallurgical coke made from coal, determines the practical height and hence efficiency and output of the blast furnace. The ability to resist fracturing when handled is important to maintain constant permeability of the furnace charge to the air blast which is vital in maintaining furnace productivity and uniformity of operations.

Various tests have been developed to measure fracture resistance; a rather difficult property to define in objective terms. These tests rely on measuring the resistance of the charcoal to shattering or breakdown by allowing a sample to fall from a height onto a solid steel floor or by rumbling a sample in a drum to determine size breakdown after a specified time. The result is expressed as the percentage passing and retained on various sized screens. Charcoal with poor shatter resistance will produce a larger percentage of fines when a sample is tested. Fine charcoal is undesirable in the blast furnace since it blocks the flow of air blast up the furnace. Fragile charcoal may also be crushed by the weight of the charge and cause blockages. Adsorption capacity

Wood charcoal is an important raw material for activated charcoal. (See Chapter 6). Some data could be useful where charcoal producers are selling charcoal to be turned into activated charcoal by specialist factories. (27)

As produced, normal wood charcoal is not a very active adsorption material for either liquids or vapours because its fines structure is blocked by tarry residues. To convert the charcoal to "activated" this structure must be opened up by removing the tarry residues. The most widely used method today consists in heating the pulverised raw charcoal in a furnace to low red heat in an atmosphere of superheated steam. The steam prevents the charcoal from burning away by excluding oxygen. Meanwhile the volatile tars can be distilled away and are carried off with the steam, leaving the pore structure open. The treated charcoal is run off into closed containers and allowed to cool. Activation furnaces are usually continuous, i.e. the powdered charcoal passes continuously cascade fashion through the hot furnace in the steam atmosphere.

After activation the charcoal is tested to quality specifications to determine its power to decolorize, by adsorption, watery solutions such as raw sugar juice, rum wine, and so on; oils such as vegetable oil and to adsorb solvents such as ethyl acetate in air. Adsorbtive power tends to be specific. Grades are made for aqueous solutions, others for oils and others for vapours. The tests measure the adsorptive power. There are small differences in the finished product made from raw charcoals of different origin but generally all are useable if properly burned. A good basic charcoal for making activated charcoal can be made from the wood of Eucalyptus grandis in brick type kilns.

Charcoal for adsorption of gases and vapours is usually made from coconut shell charcoal. This charcoal has high adsorptive power and resists powdering in the adsoption equipment -a very important factor. The chemical composition of charcoal

The constituents of charcoal are carbon, tar and ash. The relative proportions of each reflect the ash content of the wood from which the charcoal was made and the temperature at which carbonisation was terminated. To give an idea of the way these values can vary the following data derived from work on Australian eucalyptus is given. See Tables 3 and 4. (11, 24). Although many species were studied only the results for two species of international interest, Eucalyptus saligna and camaldulensis are quoted here. A more complete tabulation of these results is quoted in (20).

Table 3 Volatile Matter and Yield of Charcoal at Various Temperatures



Carbonization Temperature C









Euc camaldulensis

% volatiles









% yield









Euc saligna

% volatiles









% yield









Mean of 15 spp

% volatiles









% yield









Table 4 Inorganic Content of Bark Sapwood and Heartwood



parts per million

% Ash

% Silica












Euc camaldulensis











































Euc saligna











































2.9.2 Pyroligneous acid Acetic acid Methanol and acetone The tars

The watery condensate of the vapours leaving the retort is known as pyroligneous acid. water insoluble tars condense at the same time and separate from the watery phase on standing. The composition of pyroligneous acid is extremely complex and only the major constituents can be mentioned. The yield is important for determining the economics of recovery and varies with the type of wood carbonised. European beech, the hardwood which formed the based of the European industry, has a high content of pentosan sugars and this gives a high yield of the valuable acetic acid. Eucalypt wood on the other hand gives a much lower yield of acetic acid and other products. The type of carbonisation plant also influences yields. It is not possible to give definite predictions of yield; accurate large scale tests must be made before investing money in by-product recovery.

For guidance the following are typical yields obtained from pyroligneous acid produced by carbonising northern hemisphere deciduous hardwoods.

Yield per 1,000 kg of air dry wood

Acetic acid

50 kg


16 kg

Acetone and Methyl Acetone

8 kg

Soluble tars

190 kg

Insoluble tars

50 kg Acetic acid

Acetic acid is the most valuable product in terms of total cash return that can be recovered from the pyroligneous acid. Although the amount of acetic acid marketed as a by-product of wood distillation nowadays is rather insignificant the acid from wood distillation is sought after for certain uses because it is fairly pure. The method used to recover the acid from the condensate is usually by solvent extraction of the crude acid liquor using ethyl acetate after the soluble tars and the methanol/acetone have been separated. The acetic acid passes into the ethyl acetate phase. The ethyl acetate is recovered in a still and returned to the extraction column. The acetic acid is purified by distillation. Several grades may be produced which vary in their purity and acid content. Methanol and acetone

Because of the low prices ruling for these products made by the petro-chemical route and the high cost of separating them as pure grades from the pyroligneous acid it is usual to recover them as a mixture which also contains methyl acetone. The mixture is sold as a solvent for use in the paint industry.

The mixed solvent is recovered by distilling the water phase after the insoluble tar has been decanted. The liquid is distilled in a primary still and the acetic acid, methanol, acetone, etc. is varpourised. The soluble tars remain in the still. The vapours are fractionated in a column and the crude mixed methanol solvent fraction (about 85% methanol) is separated from the mixture of acetic acid and water. This latter mixture is purified as described above by solvent extraction of the acetic acid. The crude methanol cut could be further purified but price does not allow it as a rule and it is sold as mixed solvent. The tars

The insoluble tar is a useful product in veterinary medicine as an antiseptic and as a wood preservation agent and caulking compound. When produced from softwood distillation it is usually called Stockholm tar. It's recovery by decantation from the condensate is simple. Aromatic substances valuable in medicine and perfumery can be separated from this tar by complex chemical processes. If this tar were produced in the developing world it would probably find local markets at a reasonable price.

The soluble tar is more difficult to market. This material is a complex mixture of highly condensed yet water miscible susbstances for which very few uses seem to exist. It has been used as an admixture with clay in brickmaking to produce porous bricks and of course can be burned as a fuel.

The tars from wood distillation must be recognised as pollutants of the environment and hence cannot be allowed to escape into streams. Waste liquors of all kinds from by-product recovery must be run into closed shallow ponds and the water allowed to evaporate leaving behind the tarry residues. These, after they have accumulated can be burned to remove the risk they pose to stream life, fish, water supplies and so on. This method works well in areas where the net evaporation exceeds the net precipitation, i.e. where charcoal is being made in a semi-arid climate but is an obvious failure in the humid tropics.

Alternatively the tars and all the volatile material except the water component can be burned as fuel. In many ways this is the best way of using the material rather than investing in by-product recovery schemes. Because of the large amount of energy needed to evaporate the water it is best to burn the mixture of gas and condensibles as hot uncondensed gas as close as possible to the carbonising equipment.

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