How to transform wood to charcoal
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
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
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.
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 unburned 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
The shrinkage allowance for fuel wood 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 fuel wood 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
fuel wood. 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.
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-condensable 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
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
- at 20
The wood absorbs heat as it is dried giving off its moisture as water vapour (steam).
The temperature remains at or slightly above 100°C until the wood is bone dry.
- at 110
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
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
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 condensable vapours: water, acetic acid, methanol, acetone,
etc. and tars which begin to predominate as the temperature rises.
- at 400
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.
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
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
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 counter current 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
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.
Carbonisers can be classified by the type of heating system employed. There are three
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.
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).
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
fuel wood 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
Charcoal is the solid residue remaining when wood species, agro-industrial wastes and other forms of biomass are carbonised or burned under controlled conditions in a confined space such as a kiln. Charcoal-making is the transformation of biomass through the process of slow pyrolysis. The process takes place in four main stages governed by the temperature required in each stage (Seboka 2009): Stage 1: drying (110-200°C) Air-dry wood contains 12-15% of adsorbed water; after the first stage all the water is removed. This stage requires heat input, which is provided by burning a fraction of the biomass that would otherwise have been converted into charcoal. Stage 2: pre-carbonisation stage (170-300°C) During the pre-carbonisation stage endothermic reactions take place resulting in the production of some pyroligneous liquids such as methanol and acetic acid, and a small amount of non-condensable gases such as carbon monoxide and carbon dioxide. Stage 3: carbonisation (250-300°C) In this stage, exothermic reactions take place and the bulk of the light tars and pyroligneous acids produced in the pyrolysis process are released from the biomass. Stage 4 carbonisation (>300°C) During this stage, the biomass is transformed into charcoal, characterised by an increase in the fixed carbon content of the charcoal. The charcoal does, however, still contain appreciable amounts of tarry residue, together with the ash of the original biomass. It is important to notice that the processes in stage 1 and 2 demand heat, while in stage 3 and 4 surplus heat is produced. The following main types of charcoal kilns and more advanced retorts can be distinguished: earthen kilns, brick kilns, metal kilns, semi-industrial retorts and industrial retorts.