Charcoal Kilns From Small to Biggest
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
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
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
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
Earth mound kiln
This type of kiln dominates charcoal production in Africa. The biomass is gathered,
cut to size, and placed on the ground. The mound or pile of biomass is covered
with earth. The earth forms the necessary gas-tight insulating barrier behind
which carbonisation can take place without leakage of air, which would allow the
charcoal to burn to ash. The kiln is fired and the biomass heats up and begins to
pyrolyse. The kiln is mostly sealed, although a few air pockets are initially left
open for steam and smoke to escape. As the kiln emissions change colour, the
charcoal producer may seal some air pockets. When the production process has
ended, the kilns are opened or dug up and the charcoal is removed. The
conversion efficiency of this type of kiln is typically about 10-15%.
Improved earth mound kiln (Casamance kiln)
The Casamance kiln was developed in Senegal and is an earth mound kiln
equipped with a chimney. This chimney, which can be made of oil drums, allows a
better control of air flow. In addition, the hot fumes do not escape completely but
are partly redirected into the kiln, which enhances the yield.
Figure 4 Cacamance kiln. Source: Energypedia https://energypedia.info/
Due to this reverse draft carbonisation is faster and more uniform giving a higher
quality of charcoal and efficiency. Disadvantages of this kiln type are that it
requires some capital investment for the chimney and it is more difficult to
Earth pit kilns
Earth pit kilns form another way of making charcoal used in many parts of the
world. Pit kilns are preferred where the soil is well drained, deep and easy to
excavate. The earth is excavated to the required depth, width and length. Wood is
heaped into the trench, making provision for air passages. The wood is loaded
horizontally into the pit and covered with grass, leaves and then earth to ensure
that it is airtight and that it has sufficient thermal insulation. The pit is then left for
about 4 days to allow cooling to take place; the complete process takes about 7
days. Charcoal yields from such pits are low (10-15%). Ventilation may be difficult
to control and frequently carbonisation is incomplete, producing only low quality
charcoal. Furthermore, pit kilns are labour intensive since a pit must be dug into
Unlike woody biomass, agricultural residues such as cotton stalks, or process
residues such as sawdust and coffee husk, cannot be carbonised using earth
mounds or pit kilns. Due to their physical characteristics (shape, size and bulk
density), such biomass materials tend to flare up and hence other carbonisation
technologies with better air control need to be employed.
Brick kilns have generally a substantial capacity, in the case of the Missouri kiln
sometimes as much as hundred m3
. The carbonisation time is up to 28 days,
which is relatively long. The stationary nature of brick kilns has the disadvantage
that the biomass has to be collected and transported to the kiln. An advantage is
that tar recovery is possible: (Foley 1986) describes a tar removal method from
beehive kilns with a recovery rate of 120 kg tar/tonne charcoal; (Pari, Hendra, and
Setiawan 2004) show the collection of wood vinegar in a Yoshimura kiln by leading
the pyrolysis smoke through bamboo poles. Brick kilns have a lifetime of up to 10
years; the investment costs are estimated at 500 Euro or more, depending on the
type and size of kiln and prices of local materials (bricks, iron, transport costs).
These investment costs are modest compared to (semi-)industrial retorts, but are
still high for most small business in developing countries.
The development of demountable metal kilns has been described in detail in (Foley
1986). Early reports on metal kilns date back to the 1890s, and have resulted in
robust metal kilns like the Uganda Mark V kiln (1960s) and the TPI kiln (1970s).
Furthermore, drum charring units have been developed, made from 200 litre oil
drums. Because of the controlled air supply and gas flows during the carbonisation
process, metal kilns are able to carbonise all kinds of non-wood materials into
charcoal at a reasonably high efficiency of 20-30%. Most metal kilns are
transportable, which is very practical in case of biomass that needs to be collected
from large areas. The total production cycle of the carbonisation process takes
only 2-3 days. The biomass must be cut and/or split to size to fit into the kiln.
Figure 6 the Mark V (left) and TPI metal kiln (right)
If the metal kiln has a bottom plate, the charcoal produced in the process can be
recovered without mixture with earth or sand. Metal kilns, if designed to shed
water from the cover, can be operated in areas of high rainfall, providing the site
has adequate drainage. The capital costs of metal kilns are as low as zero or so for
a very simple metal kiln made out of three oil drums without chimney and around
1000 Euro for more advanced kilns, while the lifespan of 2-3 years is relatively
short. Nevertheless, since efficiency is high and carbonisation is very quick, metal
kilns can be a suitable technology. Recovery of tars in transportable metal kilns is
not common, but possible.
55 GALLON DRUM CHARCOAL RETORT
Semi-industrial charcoal retorts
The Improved Charcoal Production System (ICPS), also called Adam-retort after
its German inventor, may be presented as an example of semi-industrial retort
technology. The kiln returns the wood gases back to the carbonisation chamber,
burns a higher proportion of the tar components and uses the heat for the
carbonisation process. Efficiency can be as high as 40 % and noxious emissions
are reduced by 70 %. In addition the production cycle is completed within 24 to
30 hours. The retort is suitable for semi-industrial production. The kiln is
stationary, indicated investment costs are about 300 to 400 Euro (comparable to
brick kilns). The Adam-retort has been introduced in Kenya on a pilot basis.
Currently, the kiln is further refined in order to make it portable.
Figure 7 the Improved Charcoal Production System (ICPS) or Adam retort
Continuous Carbonisation System for Biomass (CCS)
The CCS system consists of a large tower (height 7 m, length 3 m) in which the
biomass falls in through the chimney and is dried and pre-carbonised while falling
through the hot volatiles towards a carbonisation zone in the middle of the tower
(an area with limited air supply). The CCS is a viable carbonisation system for light
biomass such as coffee hulls, rice husks, shredded biomass and wood chips.
Investment costs are approximately 20,000 Euros.
Advanced industrial retorts, applying separate combustion chambers, may achieve
efficiencies of up to 40%. Due their high investment costs (at least USD 100,000
for a batch-type twin retort; and several million USD for a large continuous retort)
these are not discussed further in this report.