TECHNICAL PAPER #46
UNDERSTANDING WOOD WASTES
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Understanding Wood Wastes as Fuel
1986, Volunteers in Technical Assistance
This paper is one of a series published by volunteers in
Technical Assistance to provide an
introduction to specific state-of-the-art technologies of
interest to people in developing
papers are intended to be used as guidelines to help people choose technologies
that are suitable to their situations.
They are not intended to provide
People are urged to contact VITA or a similar organization for further
information and technical assistance if they find that a
particular technology seems to
meet their needs.
The papers in the series were written, reviewed, and
illustrated almost entirely by VITA
Volunteer technical experts on a purely voluntary
basis. Some 500 Volunteers were
in the production of the first 100 titles issued, contributing
hours of their time.
VITA staff included Marjorie Bowens-Wheatley as editor, Suzanne
Brooks handling typesetting and layout, and Margaret Crouch
as project manager.
VITA Volunteer Jon Vogler, the author of this paper, is
widely published in the field of
recycling. His book
Work From Waste, published by the Intermediate Technology development
Group, Ltd., London, England, describes how to recycle
paper, plastics, textiles,
and metals. Mr.
Vogler, an engineer, worked in Oxfam's "Wastesaver" program in
countries. He has
done much research in the field of recycling waste materials.
VITA is a private, nonprofit organization that supports
people working on technical problems
in developing countries.
VITA offers information and assistance aimed at helping individuals
and groups to select and implement technologies appropriate
to their situations.
VITA maintains an international inquiry Service, a
specialized documentation center, and a
computerized roster of volunteer technical consultants;
manages long-term field projects;
and publishes a variety of technical manuals and papers.
UNDERSTANDING WOOD WASTES AS FUEL
by VITA Volunteer Jon Vogler
We can define wood wastes as wastes arising from human
on wood; extracting it from forest, woodland, and
converting it into planks and other "stock";
into products--buildings, furniture, tools, and thousands of
other items; and finally, discarding these when broken or
just "out of fashion."
To this definition may be added "nature's
wastes," such as leaves, twigs, and branches that fall
tree due to natural causes such as aging, wind, lightning,
With that broad definition in mind, tree and wood wastes can
categorized as follows:
Conversion Wastes User
Stumps and Roots(*)
The use of waste wood is as old as humankind.
During early civilization,
stone-age people likely used wood waste to fuel fire
since greenwood is very difficult to burn.
Manufacture of items
from wood also began very early.
Wood was used for tools and
weapons and, no doubt, cut-offs from the production of long
implements were used for short axe-handles or pegs, while
and shavings served as kindling.
This paper describes a number of uses of wood wastes as
which is how the greatest proportion of wood wastes are
Non-fuel uses of wood wastes, for example in building
industry, and agriculture, are described in another paper,
the Non-fuel Uses of Wood Wastes."
The issue is
acute, because the poor throughout the world, both urban and
rural, continue to consume fuelwood and charcoal faster than
can be renewed.
Meanwhile, an insatiable demand for paper made
from wood pulp, wooden building components, furniture, and
goods also contributes to deforestation.
Economical use of wood
wastes instead of new wood helps to preserve forests and
in developed countries and is becoming essential to survival
the poor in many parts of the Third World, as fuel becomes
(*) widely used directly as domestic fuel, as kindling, and
raw material for charcoal.
This paper concentrates on three main uses for wood wastes
o Burning solid wood
wastes or sawdust;
o Using sawdust and
tiny wood pieces to make small compact fuel
(briquettes) that can be burned in a manner similar
to solid wood;
o Making charcoal, a
widespread (mainly cottage) industry for
wastes into a lightweight, smokeless fuel.
Some experts describe certain wood waste processes as having
Many procedures for processing wood
wastes, however, can also accommodate a wide variety of
waste products such as husks and hulls.
II. BURNING SOLID
COMBUSTION IN WOOD-BURNING STOVES
All wood contains moisture; even kiln dried wood has an
percent moisture content.
When the kindling is first lit, white
smoke, containing a large percentage of water, rises from
wood. As the fire
begins to burn, long tongues of yellow flame
indicate that the volatile substances, natural oils, and
within the wood have been released.
This chemical breakdown of
the wood into "char" and volatile gases occurs at
150-200 [degrees] C. The
gases do not all actually ignite until a temperature of 540
has been reached. In
an open fire, these volatile gases are given
off into the air in the rising smoke and hot air and do not
their flash point.
Thus much of the fuel value is lost.
this, the wood burns with small white flames and hard, clear
outlines as the remaining fibrous matter (lignin) and carbon
MODERN "AIR-TIGHT" STOVES
Many years ago, stoves were made of cast iron panels bolted
recently, welded steel sheet has replaced cast
iron. Cast iron
holds heat better, but is prone to cracking under
mechanical or thermal shock.
Although fire cement is sandwiched
in the joints, a cast iron stove is never as air tight as a
steel stove. Sheet
steel cannot crack, but may warp if overheated
unless made of thick (13 guage) plate.
Sheet steel stoves are
easier to move, being much lighter, and require little
The welded seams remain airtight for the life of a stove.
Fire Control in Air-Tight Stoves
Control of the rate of burning is achieved by controlling
amount of air escaping, and the speed and amount of air that
passes through the mass of fuel.
Various features of stove design
o The fuel rests on
a grate that allows air to pass through it
Simple grates are usually parallel steel
close enough to
prevent fuel from falling through the spaces
o An important
requirement is an air-tight firebox constructed
so that all air
admitted is controllable, by one of the
Air intakes positioned below the grate
quantity of air
entering and fuels passing through the
This may vary from no air to a strong draft
causes the fire
Opening and closing the stoking doors varies
supply to the
fire, but the doors are usually above the
grate level, so
air passes over, not through the fuel and
the draft is
Dampers regulate the draft by varying the
size of the
opening. The damper is a hinged flap in
the pipe from
the firebox to the chimney.
the volatile gases are given off at 150-200 [degrees] C.
If these hot
gases escape up the chimney their
fuel value is
lost. Baffle plates of steel or cast
gas flow, and ensure the gases are heated to
point and radiate additional heat before
During "secondary combustion" the
off from the
heated fuel-wood are drawn away from the
A secondary inlet admits air and the gases
ignite if they are at a sufficiently high
o Heat exchangers,
sometimes called smoke chambers or radiators,
maximum amount of heat from the hot fire
They are additional chambers that can be
kindling. By moving a valve, the hot
gates may be
them when the fire has reached a certain
Heat exchangers and other parts of the fire
boxes of stoves
are often wrinkled or patterned in order to
increased surface area for transferring heat.
is one of the
functions of the patterns and traditional
scenes that are
cast into the surfaces of many Scandinavian
o Heat output can be
increased by forced draft provided by an
running in a steel tube. This increases
supply of air for
burning and the rate at which heat is
(transferred to the surrounding area).
Advanced Stove Designs
Five basic designs have evolved for wood-burning stoves,
there are as many variations as there are stove
The main differences concern how air moves through the
1. Updraft Stoves
allow air to enter through inlets at the
bottom, move up
through the grate into the burning wood, and
flow out of the
flue. Many updraft stoves have
inlets above the
wood for secondary combustion of the gases
when the stove is
2. Air enters the
bottom of Diagonal stoves then moves diagonally
through the fuel
to the fire in the back of the stove. A
inlet above the wood assists secondary combustion.
are often fitted.
3. Air enters near
the bottom of Crossdraft stoves and leaves
near the bottom
at the back of the stove. Secondary
of gases occurs
in the main fuel bed.
4. Downdraft stoves
force air and combustion gases down through
fuel. Air enters at or near the top of
and travels down
through the grate to leave through a flue at
These stoves are smokey when not burning
with a flap valve to allow the smoke to
leave at the top
of the stove until the fire is burning
5. Front end
combustion or "S" draft stoves.
In this model, logs
burn from front
to back much like a cigar. The primary
the front and passes over the fuelwood, which
towards the back. Baffle plates force
volatile gases to
double back over the fire in order to reach
flue. They encounter secondary air and
if the stove temperature is high.
Many wood-burning stoves are jacketed.
That is, the firebox is
surrounded by a jacket of water, which, when heated,
(moves upwards because hot water is less dense or lighter
cold water) or is pumped away to be used.
Back boilers are also
common. Water to be
heated flows through a chamber (usually made
of copper) behind the flue.
Water heating reduces the heat radiated
from the stove itself; however, the heated water may be
passed through radiators to heat areas away from the stove.
WOODBURNING IN THE THIRD WORLD
Fuelwood accounts for at least half of all the wood used in
world each year and for more than 85 percent of wood used in
Third World countries.
No other source of energy is available (or
seems to be) on a scale large enough to satisfy the billion
people who depend upon fuelwood.
Demand is now outstripping supply
and the situation worsens with constant population growth,
fuel must be collected or purchased at a constantly
expenditure of labor or money--a burden that falls mainly on
Part of the solution is, of course, to grow more trees.
to make better use of the fuelwood resources that
introduction of cooking stoves that use less fuel than open
or traditional stoves can reduce the labor of fuel gathering
conserve fuel, so as to extend the time available for
measures (tree planting) to take effect.
However, the "advanced"
stoves described above are too costly for most Third World
Research programs have therefore been launched over the past
years to develop better stoves than those in common use, yet
still simple, robust, of low cost, and suitable for local
and unskilled use.
Some Typical Improved Stove Designs
Early efforts concentrated on the development of massive
made from mud. Later
more durable designs known as "pottery
insert" stoves were made from pottery by skilled
can be coated with an outside layer of mud to increase
durability, and insulation.
These high mass stoves were, however,
found to suffer from a number of design flaws.
The stoves themselves
absorbed tremendous amounts of heat, which while useful
for space heating in some areas, used up excessive amounts
fuel. The mud or
clay walls disintegrate in rain or high humidity,
and the individual construction of the stoves precludes
effective quality control unless the builder is very well
trained. As a result,
many high mass stoves use more fuel, not
less, than traditional stoves.
Because of these and other problems,
subsequent research focused on smaller, portable metal and
ceramic stoves based on traditional designs.
The result of scientific
research by VITA and others has been a series of guidelines
for the design of such stoves.
Critical points include close
matching of pot to stove to ensure maximum contact with the
insulation to minimize heat loss, a grate to ensure good
and control of the air supply to regulate burning.
portable stoves also lend themselves to quality control and
production, as exact templates can be put into the hands of
artisans who are trained in their use.
Portable metal cooking stoves are proving to be much in
especially in some urban areas of developing countries.
of stove efficiently burns scraps that could not be
used on an open fire.
Its fuel economy is excellent.
contains the heat well, the cook can remain seated close to
stove while cooking.
This is not possible with the open fire or
traditional coal-pot stove.
With improved stoves, smoke is reduced.
They are also more stable than the traditional coal-pot,
and the pot can be stirred vigorously without the risk of
The charcoal-burning metal stove, known in Kenya as the
is over 90 percent inefficient.
To replace it, the Umeme Stove
has been developed by UNICEF's Appropriate Technology
Nairobi. It is
designed so that the cooking pot sits inside the
stove. There is a
sloping inner chamber made of metal, which is
insulated from an outer metal cladding by a layer of
ash. A newer
model, known as the Kenyan Ceramic Jiko, uses a fired clay
in the metal cladding.
VITA's work in Somalia and in West Africa has also yielded
stoves based on traditional designs.
In Somalia, soapstone
stoves carved to rigorous specifications to increase
are finding a ready market.
And in Burkina Faso, Mali, Guinea,
and elsewhere, traditional metal designs have been upgraded
artisans trained in their production.
The use of templates in
these areas has permitted the manufacture of large numbers
high quality stoves, thus bringing down the unit cost and
them more attractive to purchasers.
Commercial stoves, including an American product known as
Stove, are also currently being promoted in developing
The Zip Stove, manufactured from light weight galvanized
steel, comprises a cylindrical combustion chamber with a
grate and an outer casing, with a layer of refractory (heat
resistant material) insulation between them.
This stove, and
others like it, is much more costly than the improved
stoves that are produced locally, and may not be any more
Dangers of Simple Stoves
Fuel efficiency is not the only concern in the design of
stoves. Burning any
carbon fuel produces poisonous carbon monoxide.
In an enclosed room this can be very dangerous.
stoves are not very safe in this respect.
For example, the average
carbon monoxide content of gases emitted by traditional
stoves ranges from 0.9 percent to 0.3 percent.
European safety standards recommend carbon monoxide
should be not more than 0.0005 percent in any enclosed
so called improved stoves are no better in terms of gas
The average carbon monoxide flue gas composition of the
Zip Stove for burning wood is 1.3 percent and for charcoal 2
percent. When damp
wood is burned, the production of carbon
monoxide increases 1.8 percent with considerable quantities
At present, there are no reliable measurements of carbon
and other emissions from open fires, but indications are
women who cook with these fires and improved stoves suffer
damage that is equivalent to smoking several packs of
cigarettes a day.
The solution to this problem lies in creating designs with a
chimney to remove gases from the room, the lethal carbon
in particular, and an air-tight firebox with baffles to
more efficient burning of the combustion gases.
Huge quantities of sawdust are produced in sawmills and
workshops all over the world, but it is rarely recycled
It cannot be used for paper making because the fibers
are too short. It
will not burn on an open fire, except in the
Its lignin structure makes it unsuitable for
fertilizer, animal feed, or biogas production.
Unless it contains
very high proportions of resin, it is difficult to use in
without expensive binders or very high pressures.
large sawmills may find it economical to buy a briquetting
and possibly to carbonize (make into charcoal) the finished
briquettes, let alone to recover the tar and combustible
that are the by-products of the carbonization process.
there are several ingenious ways people have found to burn
Tin Can Stove
The single chimney tin can stove is the simplest homemade
to use sawdust for cooking.
A hole is cut in the bottom of one
side of a five-gallon can.
A short length of broomstick is placed
horizontaly in the hole so that it reaches just to the
the can. Another
stick is held upright in the center of the
stove, with the ends of the two sticks touching.
The can is
filled with sawdust, tamped down with a wooden block during
filling and sprinkled with water to keep the dust level
sticks are removed, some diesel oil or kerosene is dripped
through the hole where the center stick was.
The oiled area is
lighted with a burning rag through the air hole at the
The mass will burn for six to seven hours.
The burning rate can
be controlled by obstructing the air flow through the bottom
passage. A simple
"trivet" (three-legged stand for cooking pots)
can be placed on top of the can and a cooking pot or kettle
be heated on it.
Food cooked on this stove will tend to smell and
taste of wood-smoke.
Other Sawdust Stoves
The double drum stove is even larger and more complicated,
still inexpensive to construct.
It consists of a 30-gallon steel
drum, supported on a false floor inside a 55-gallon steel
drawer, opening below the false floor, provides draft and
dropping ashes, which are then easily removed.
A hole in the
center of the false floor and the inner barrel bottom lets
pass up to the fuel, and ashes fall into the drawer.
fitting lid covers the outer barrel and two stovepipes
smoke. It should
stand at least two feet from any combustible
material and be set on a fireproof floor pad.
open the lid while the fuel is burning.
A serious flare-up may
With dry sawdust and a good draft, one charge of this stove
heat a room 7 meteres square for six to eight hours with no
Wetter fuel heats less but lasts longer.
During the first
two hours of burning, there is enough heat at the center of
lid to boil water or cook.
As burning progresses, the heat on the
lid is distributed more toward the rim.
Stoves can also provide
hot water. A coil of
metal (preferably copper) pipe placed inside
the stovepipe will heat water that is circulated through it.
Mexican Water Heater
A sawdust-fire water heater is widely used in Mexico.
is lightly sprinkled with petroleum or fuel oil and loosely
packed in polythene bags that are sealed.
The full bag is known
as `combustible' and is sold by grocers and hardware stores.
combustibles can heat enough water for a bath.
The special water
boilers have a grate at the bottom, on which the
burned. Above the
grate is a chimney surrounded by a water jacket
with a water inlet and outlet that are plumbed into the
hot water system.
III. COMPACTING WOOD
The alternative to having a special stove for burning
small wood wastes is to compress these into a briquette--a
compact fuel pellet.
The average calorific value of briquetted
wood waste or sawdust is 4,000 kilograms per cubic
every 100,000 tons of briquetted wood waste will be
42,850 tons of fuel oil, making it a valuable fuel that will
repay substantial costs of manufacture and transport.
High-Tech Briquetting Processes
The process is based on the recognition that most wood waste
self-bonding at fairly high temperatures and requires no
binder. Sawdust is
preheated to above 163 [degrees] C to destroy its
"elasticity" and to eliminate moisture.
This decreases the weight
by about one-third and almost doubles the heating value per
pound. It is then
moistened and briquetted hot without a binder.
Pressure is retained during cooling.
The resulting briquettes
are firm and strong enough to withstand rough handling and
resist weathering to an extent that permits shipment and
if protected from rain.
To achieve briquettes of the necessary strength and
moisture content of the wood waste should be around 10
although in some cases, machines are capable of handling dry
chips. Dryers may
take the form of rotating drums through which
hot air is blown or steam-heated plates and pipes over which
waste is cascaded. A
large proportion of the material may be
needed to provide sufficient heat to dry the feedstock from
high moisture level.
It is usually necessary to grind the waste
to a suitable size and, before doing so, to strain it to
stones, soil, or metal, which would damage the grinder.
Briquetting machinery must be robust and powerful.
produce simple, low-cost, low-power machines for small-scale
operations have not been successful to date. Pressures of up
1,000 kilograms per square centimeter can be involved.
die temperatures low and avoid burning the briquettes, dies
Machines need motors giving between 25 and 100
kilowatts for every ton per hour of throughput, although not
this is absorbed during operation.
Many manufacturers of wood pulp and other wood products use
sawdust and other wastes as fuel for their manufacturing
The wood wastes are briquetted in an ongoing process.
such process uses a machine that screws the waste wood
shavings, and other scrap, ground to the size of oatmeal
first into a compression chamber at a pressure of 3,000
pounds per square inch (211 kilograms per square
the outlet from this chamber, a secondary head cuts the
material into a spiral ribbon and forces it into a mold
under a pressure of 25,000 (1757.7 kilograms per square
to 30,000 pounds per square inch (2109.24 kilograms per
Friction at this extreme pressure generates
enough heat to achieve self-bonding.
The molds are parallel to
the axis of the wheel.
The mold is closed by a hydraulic piston
that retracts as the mold fills.
When one mold has been filled,
the wheel rotates to align the next with the compression
The molds are water cooled, and, by the time the wheel has
full circle, the briquette is cool enough to eject.
produces 4 by 12 inch briquettes that are fed manually into
For mechanical stoking an extruder is used that forces the
through one-inch round holes as continuous rods, which are
into one-inch lengths by rotating knives.
The machine is small
enough to be mounted on a truck and powered by a truck
It is far less expensive to transport briquettes than loose
waste, so briquetting machinery should operate where the
Briquetting presses are best located at sawmills,
furniture factories, or oil mills.
However, if these are far from
populations or industrial centers where there are markets
fuel briquettes, transportation costs may not make the
finished briquette may need protection from
reabsorption of moisture and should be stored in dry areas
packed in sacks.
Packing in plastic film or cellophane may be
handling and transportation are needed to
Other processes include:
between rollers with cavities that produce egg-shaped
sizes between one and four centimeters.
o Pelleting where
waste is forced by pressure rolls through the
holes in a
die-plate (product size 0.5 centimeter);
modified form of pelleting (product size 2-5
Rolling/Compressing--where fibrous material is wrapped around
a rotating shaft
to produce a high density roll or log
10-18 centimeters diameter).
Simple Sawdust Briquettes
Various attempts have been made to devise methods by which
in rural areas can use sawdust to make briquettes.
idea, for areas where dung is shaped by hand and sun dried
use as fuel, is that the dung cakes will burn longer if wood
is added. Most
efforts have been devoted to making simple machines.
Most hand-operated machines use a mechanical lever to apply
greater compacting pressure than is possible with hand
The length of the lever arm determines the briquetting
and it is important that the mold be sturdy enough to
four to five hours work of by a competent
blacksmith or welder are all that is needed for the simplest
devices. A steel
pipe provides a good briquetting mold.
Earth rams, simple hand-powered presses currently in use for
making building blocks, can be easily modified to make
The Combustaram, similar to the CINVA-Ram and Tersaram,
is commercially available or can be locally manufactured.(*)
Another device consists of a piston that reciprocates in a
on which there is a hopper to feed the sawdust (or other
agricultural waste) to be compacted.
The piston is driven by a
hand-turned crankshaft, on which a flywheel is mounted.
a simple device to eject the briquette, which is about 30
in diameter and 10 millimeters thick.
kilograms of briquettes can be produced in about eight
A larger machine is powered by a single bullock.
It consists of
two sets of pistons and cylinders and turns at about four
produce two briquettes per revolution.
The capacity is around 150
to 200 kilograms of briquetted fuel per eight hours.
(*) Both machines were designed, fabricated, and tested by
School of Applied Research in India.
Further details can be
obtained from the National Research Development Corporation
India, 20-22 Zamroodpur Community Center, Kailash Colony
New Delhi 110 048, India.
A Thai businessman, Sayan Panpinij, in collaboration with
has developed an extrusion machine that transforms rice
into burnable logs.
Approximately 75 kilograms of rice husk fuel
logs are produced per hour from each of twin extrusion
with a density almost double that of firewood.
The machine is
powered by a 20-horsepower electric motor and works best
husks that have been ground and dried to reduce
machine can be operated by one person who feeds the rice
into the hopper on top of the machine, removes the fuel logs
below the extruder, and stacks these for cooling.
It is estimated
that three people will be necessary to operate four
VITA extruder can also produce fuel logs from sawdust.
a higher heat value than rice husk logs, produce less smoke
ash when burned, and reduce wear and tear on the
device is relatively new and has not yet been manufactured
The life and maintenance of this extrusion machine is a
consideration for the user.
When the device is used for extruding
rice, the screw will need to be replaced every 120
extrusion cylinder has a life of about 450 hours and will
need to be rebored every 150 hours for its most efficient
operation. When the
device is used for extruding sawdust, however,
the life of the machine is nearly double.
temperature, quality of the heater unit, and the length of
the life of the heat unit varies between 240 and 350 hours.
In a four-unit plant, it is estimated that capital and
costs can be replaced within one year.
RETTING AND PRESSING
Partially decayed and processed cellulosic materials give a
higher heating value than if the materials are simply
example, dried rice straw (10 percent moisture content) has
heat value of only 3,000 BTU/pounds (7 million
[J/kg] or 0.0698 gigajoules/kilogram [GJ/kg]), but this will
increase to between 7,500 (17.4 million J/kg or 0.0174
12,000 (28 million J/kg or 0.0279 GJ/kg) when the material
partially rotted before it is dried.
In the Philippines, the
MAPECON research group has set up a pilot plant producing
fuel, with 25 percent moisture content and an average of
BTU/pounds (23 million J/kg or 0.0232 GJ/kg) which they call
`green charcoal,' at the rate of one ton per hour.
reports that it is very competitive with other types of
Retting--soaking in water for several days or longer at
air temperatures--allows chopped, moistened woody residues
biodegraded (partially decayed).
This process is used to produce
mats that can be pressed into fiberboard, but a simple hand
can also be used to make briquettes from retted agricultural
residue or wood wastes.
The lever is made from steel pipe and the
timber mold has holes on each side to allow water to escape
Tying brushwood into compact bundles for ease of
and use is the simplest means of densifying wood
straw, hay, dry leaves, and other woody wastes are bundled
over the world, using cord, vines, wire, or any locally
tying material. Where
large-scale bundling is carried out, stands
or racks have been developed to assist in the bundling
and to allow for drying before use.
Brush bundling machinery is
also available, but indescriminate use can seriously damage
ground cover, leading to soil erosion and loss of fertility.
IV. MAKING CHARCOAL
FROM WOOD WASTES
In contrast to the heavy weight and high smoke level of the
from which it is made, charcoal is a light, smokeless fuel
high calorific value.
THE CARBONIZATION PROCESS
When wood is heated in the absence of air, changes take
several stages. At
100 to 120 [degrees] C, water is emitted into the air.
Green wood contains between 50 to 70 percent water, which
evaporated before the wood temperature can rise higher.
(conversion into carbon or charcoal) begins at 270 to 400
C. The reaction,
technically named pyrolysis, gives out heat.
wood chars and gives off gases and vapors--carbon dioxide,
monoxide, hydrogen, methane, water vapor, methanol, acetone,
The yield of charcoal and its composition depend on the
of wood, the carbonizing temperature, and other
factors. Yield is
generally about 25-40 percent by weight of dry wood.
carbonization temperatures produce a higher yield (because
charcoal still contains matter that has not been given off
gas) the charcoal quality is poor.
It smokes and flames.
that are too high, on the other hand, shorten the life of
equipment, so care should be taken to keep carbonizing
between 400 and 700 [degrees] C.
The energy value of the gases represents some 40 percent of
the heat value of the original dry wood.
Some of the gases contain
valuable chemical compounds.
Unfortunately, production on an
industrial scale is necessary before it is economical to
these compounds. In
small-scale processing, however, they help
maintain burning in the kiln.
TYPES OF KILNS
Charcoal is made by placing wood in a kiln, igniting it in
air, and then, when it is burning thoroughly, reducing the
of air almost completely.
Many types of kilns are in use.
are industrial size, some are much smaller.
They will be described
here in order of complexity, starting with the simplest.
The Earth Kiln
An earth kiln usually occupies about eight square meters of
ground. Logs of wood
are placed on the ground with space between
them to allow air passage in the early stages.
The pile is built
to a meter high and covered with leafy vegetation 30 cm
Stakes are set in the ground around the pile to support a
made with interleaved branches or scrap corrugated
iron. The kiln
is then lit and allowed to burn fiercely until smoke comes
various places. The
pile is then covered with earth and left to
burn for about two days.
Burning is complete when the kiln slumps
down to half its original height.
More soil is added to exclude
air totally for three or four days until the kiln is
cold. It is
uncovered, allowed to cool for a few hours, then the
put into sacks for sale.
It is reported that two experienced
charcoal makers can produce about six tons of charcoal a
this process, which needs no capital money just a sack, a
and an ax.
The CUSAB or Oil Barrel Kiln
Kilns for carbonizing small wood pieces are made from oil
45 gallons or 250 liters in volume.
Each oil drum is fitted with
holes of approximately five centimeters.
Threaded pipe fitting
the same approximate diameter are then welded to the
screw connectors can then be fitted with plugs to cut off
emerging air. Holes
should face the wind and a stick can be used
to keep the openings clear of debris during the early hours
burning. It is
reported that five to six kilns can produce four
to five tons of charcoal per month.
Although kilns have a short
life, the pipe fittings can be reused, and the low cost of
drums makes this a cost-effective technology.
The Steel Kiln
The steel kiln can produce an average of 500 kilograms of
every two days from two and a half tons of wood, depending
on moisture content and density of the timber used as
This represents up to 12 tons of charcoal per month.
The kiln is
simple to operate and does not normally require attention at
night nor water for cooling purposes.
It is, however, an expensive
object and very hard work to transport across rough roads.
Two strong men can barely handle two kilns, including
unloading, and moving to new sites.
It has been designed to
withstand rough usage and extreme temperature
are no underground fittings.
To operate, logs are placed, with kindling between them, in
lower cylinder, which rests on eight smoke boxes.
cylinder is then densely packed with logs. When full, its
filled with mud to form an air seal and the upper cylinder
mounted on top. The
upper cylinder is also packed to a height
such that the top cover does not quite meet the cylinder.
of the smoke boxes are open for lighting.
Then, when plenty of
smoke is emitted, some flaps are closed.
In approximately an
hour, the cover will settle down onto its rim.
Chimneys are then
fitted to the smoke boxes.
If blue smoke comes from a chimney,
the chimney is removed and the smoke box below it is capped
fifteen minutes to reduce burning.
After 16 to 24 hours, smoke
will cease. Each
chimney can then be removed and the smoke box
takes 8 to 12 hours.
Other Simple Kilns
There are many other simple kiln designs available.
uses a drum lying on its side.
It has been found very satisfactory
by the Fiji Department of Forestry.
In the Philippines tests
have been made on various improved simple designs, mostly
of two drums welded together to increase capacity to 160
kilograms of wood.
Improved air vents and chimneys can cut heating
time to four hours, and yield up to 40 percent
Papua New Guinea, two cylinders made from 44-gallon drums,
on their sides over a stone or concrete fire trench, produce
quality charcoal. A
group of drum kilns wired together will allow
the heat to be distributed more efficiently and produce
Retorts are designed to use the gases (including condensed
or liquors) more effectively.
They give a higher yield because
they carbonize all of the raw materials.
Kilns on the other hand,
burn away some of the raw material in order to provide the
heat. Heat for
carbonization is provided by otherwise useless
materials such as coconut shells, pigeon pea bushes, palm
leaves, and woodworking scraps.
A tar condenser may be fitted, in
which the gases are condensed and the tars collected for use
road construction, preserving timber, or sealing flat
retorts can recover gases that are directed to the firebox
they are burned to fuel the process during its later stages,
saving solid fuels.
Increased sizes and complexities of kiln are available as follows:
Mobile Vertical Bath Kiln:
This 19-foot-high kiln weighs nearly
three tons and has a built-in crane to assist in erecting it
site, lifting and lowering the cover during operation.
foundation is required.
It takes only 48 hours to produce
four tons of charcoal, which can be discharged directly from
chute into bags.
Liquors (condensed gases) are recovered.
Demountable Vertical Kiln:
This semi-permanent design can be set
up in an area of forest.
When cleared, it can be re-erected on a
new concrete foundation in another area.
It can be moved using
large road vehicles.
Skilled erection is, however, a requirement.
This kiln can produce around 3,000 tons of charcoal per
Permanent Vertical Kiln:
This is available in sizes to produce
between 5,000 and 10,000 tons of charcoal per year.
is handled mechanically and can be passed through a
dryer. Little labor
Larger Kilns: These
are usually horizontal and include continuous
drying, briquetting, and bagging plants.
Fluidized Bed Kilns:
Fluidization is a well known technique, a
developing technology in such applications as coal
packaged coal-fired boilers, and gas turbine power
Within the timber industry, wood-fired fluidized bed
have become commercially available for steam raising.
increasing interest in processing wood waste into upgraded
such as gas, charcoal, or oil fluidized beds.
Further details may be obtained from manufacturers.
BRIQUETTING OF CHARCOAL
If charcoal can be sold near the site where it is made,
and storage costs will not be high.
If it is to be
transported a long distance or sold later when the market
is better, it is desirable to compress it into small, dense
also uses the fine dust, which cannot otherwise
be sold or used. The
disadvantage is the cost of a binding substance,
such as starch from cassava.
If no binder is used, a
briquetting press with high working pressure is needed and
machines are expensive (about US$100,000), but easily
and not difficult to operate or maintain.
So far, no company has
produced an inexpensive small briquetting press that
sufficient pressure to make briquettes that do not crumble
a binder, so large presses have to be used.
Some models need
to be fed by at least eight steel kilns, which result in
Charcoal Briquetting Processes
The production of charcoal briquettes may be accomplished
by preparing the charcoal first and then pressing it, or by
preparing wood briquettes to be carbonized after
method produces semicharcoal briquettes by preheating
until the lighter gases have been given off and tar begins
distill. The partly
charred sawdust, brownish in color, is then
cooled to 100 [degrees] C, moistened with water, and pressed
into a mold.
Another method heats dry sawdust in molds, under low
until it has partially carbonized, then applies a pressure
pounds per square inch until carbonization is complete.
resulting briquettes are further heated to drive off gases
would create smoke.
Another process distills finely ground wood to produce
charcoal, which is mixed with the wood tar produced in the
and briquetted. The
briquettes are reheated in a retort to
drive off and recover the lighter fractions of the tar.
remaining particles may then be bound firmly together to
dense briquette. This
process is sometimes referred to as"coking."
It is reported that these processes are commercially
because the charcoal briquettes produced are too brittle
to be used. An
alternative is for finely ground charcoal or
charcoal dust to be mixed with a suitable binder before
pressed into uniformly-sized, strong, dense briquettes, free
Practical briquetting operations entail four steps:
1. Preparation of
charcoal fines. Lump charcoal is
milled using a
screen with 1/10 inch or 1/8 inch holes to
with enough fines to fill the voids between
the larger pieces
and to prevent them from being crushed
2. Mixing to coat
the charcoal particles with a film of binder.
double shaft mixer is often used.
involves simultaneously feeding pre-crushed
cassava flour into a hammermill. The
continuously, then steamed until the flour forms a
3. Briquetting the
mixture between two cylindrical rolls that
opposite directions. Each roll is
rows of hollowed
half molds, aligned so the halves match.
briquettes can be produced at every turn of the
4. Drying the
briquettes continuously or in batches.
agricultural dryers in operation.
asphalt or pitch binders do not need artificial
To produce satisfactory briquettes economically, the binding
substance must meet certain requirements.
It must produce a
briquette strong enough to withstand damage during
storage, and stoking.
Exposure to weather must not cause crumbling
or softening and, during use, the heat must not cause
and loss of fine pieces through the grates.
burn without smoke and unpleasant smell and not be too
Ideally the binder should have as high a heat value as the
Binders fall into three categories:
inorganic materials, organic
materials, and fibers.
materials, such as cement and silicate of soda
for wood fuel. These substances are
give more ash, reduce the heat value, and fall
materials such as tar, pitch, resin, and glue
the heat value and create no extra ash.
o Various types of
fibrous material may serve as binding
The cheapest is hydrated wood fiber-wood
waste, which, when dry, binds together
in the same way
Some binders permeate the material to be briquetted; others
the surface. Starch
binders, such as cassava, corn, and others
are smokeless, but not moisture resistant.
They are normally used
in the proportions of four percent (dry basis).
asphalt, and sugar cane molasses are used in less than 30
of the cases. They
are moisture resistant but not smokeless.
is no drawback in industrial uses, such as smelting and
but would be inappropriate for home fuel or cooking.
Secondary distillation (heating a second time) can drive off
smokey gases, but increases cost and does not completely
objectionable smells during burning.
A good smokeless charcoal is
one that contains at least 75 percent fixed carbon and not
than 24 percent "volatile" (able to be emitted as
Uses of Briquetted Charcoal
Briquetted charcoal has many industrial uses and can be used
domestic fuel as well.
The product is a high quality industrial
fuel for production of steel, cement, copper, rubber, gun
and other products.
In the chemical industry, very pure briquettes are used as
activated carbon for air and water purification, for
decolorization, purification of sugar, and as a chemical
carbon commands prices five to six times
higher than those of briquetted charcoal.
REFERENCES AND RESOURCES
Appropriate Technology International has several reports on
use of both charcoal and wood stoves.
For information contact
ATI, 1331 H Street, N.W., Washington, D.C. 20005, USA.
Intermediate Technology Publications (ITP) includes over a
titles on this subject in their catalogue.
The catalogue can be
ordered from I.T. Publications, Ltd., 9 King Street, Covent
Garden, London, WC2E 8HW, United Kingdom.
Institute of Natural Resources (1978).
Proceedings of the Seminar
on Wood as an Alternative Energy Resource.
Suva, Fiji, University
of the South Pacific.
Volunteers in Technical Assistance (VITA) also offers a
"Briquettes From Wood Waste," Madison, Wisconsin,
Department of Agriculture, 1947.
Bryant, B.S. et al, Fuel Briquettes from Fibrous Residues
Press, Volunteers in Technical Assistance,
"Understanding Briquetting," a Technical
Paper by Volunteers
in Technical Assistance, Arlington,
Currier, R. A., Manufacturing Densified Wood and Bark Fuels,
University Extension Service Special Report 490,
Klages, A., 1953, Economic Aspects of Wood Briquetting,
Timber Journal 19,
Smith, A. E., Flynn G., & Breag G. R., A Profile of the
Forestry Residues, Tropical Development and
127, Clerkenwell Road, London, EC1R 5DB,
"Briquettes From Wood Waste," Madison, Wisconsin,
Department of Agriculture, 1947.
Bryant, B. S. et al, Fuel Briquettes from Fibrous Residues
a Hand-Operated Lever
Press, Volunteers in Technical Assistance,
Currier, R. A., 1977, "Manufacturing Densified Wood and
Oregon State University Extension Service
Foley, G., Moss, P., and Timberlake, L., Stoves and Trees,
Joseph, S., and Hassrick P., Burning Issues:
Programmes--A guide for Eastern Africa.
Klages, A., 1953, Economic Aspects of Wood Briquetting,
Timber Journal 19,
Reineke, L. A., 1955, Briquettes from Wood Waste, Forest
Smith, A. E., Flynn G., & Breag. G. R., A Profile of the
of Agricultural and
Forestry Residues, Tropical Development
Institute, 127 Clerkenwell Road, London,
EC1R 5DB, United
Wartluft, J., Double Drum Sawdust Stove, a technical
published by VITA,
Arlington, Virginia, USA. ISBN
Charcoal Making for Small-Scale Enterprises:
An illustrated manual.
International Labour Office, 1975.
Grato, N., "Charcoal Manufacture," Liklik Buk
pp: 132-133, Lae,
Guinea: Liklik Buk Information Centre,
New Guinea, 1977.
Little, E. C. S., 1978, The Mini CUSAB Kiln for Rapid
Charcoal from Scrub, Coconut Wood, and Coconut
Technology 5 (1): 12-14.
Medrano, E. M., "Design, Fabrication and Operation of Drum
Coconut Shells." Technology
Journal 1 (2): 26-35,
Papua New Guinea Building Research Station,
"Manufacture of Charcoal
Boroko, PNG Building Research Station Technical
Bulletin No. 10.
Richolson, J. M., and Alston, A., Coconut Palm Wood Charcol:
A Potential Source
of Heat Energy Suva, Fiji Department of
Testing the Efficiency of Wood-Burning Cookstoves,
Assistance, Arlington, Virginia USA, 1985.
The Comparative Performances of Kenyan Charcoal Stoves, ITDG
Technical Paper No. 1.
Vil lanueva, E. P., and Banaag, N. F., "Sawmill Waste
Domestic Use and
Its Quality as Compared to Ipil-Ipil (Leucaena
glauca benth) and
Coconut (Cocos nucifer L) Shell Charcoals,
Project No. 33-11,
Second Progress Report, The Lumber, August-September
SOURCES OF HELP AND INFORMATION
Asian and Pacific Coconut Community (APCC)
Department of Agriculture
Fibre Building Board Development Organization, Ltd.
1 Hanworth Road
Feltham, Middlesex TW13 5AF
Ministry of Agriculture
Fisheries and Forests
P.O. Box 358
Forest Products Research and Industries Development
Wood Stoves Project
9 King Street
Covent Garden, London WC2E 8HW
New Zealand Forest Service (NZFS)
Wellington, New Zealand
Kristian Institute of Technology of Weasisi (KITOW)
P.O. Box 16
South Pacific Bureau for Economic Cooperation (SPEC)
Timber Research and Development Association
Bucks, United Kingdom
Tropical Products Institute (TPI)
56 Grays Inn Road
London WC1X 8LU
United Nations Industrial Development Organization (UNIDO)
P.O. Box 707
Volunteers in Technical Assistance (VITA)
1815 North Lynn Street, Suite 200
Arlington, Virginia 22209 USA
Wood Stove Group
Post Bus. 513
5600MB, Eindhoven, Netherlands
Aldred Process Plant
Oakwood Chemical Works
Worksop, Notts S80 3EY
SUPPLIERS OF BRIQUETTING EQUIPMENT
Air Plant (Sales). Ltd., (Spanex)
295 Aylestone Road
Leicester, LE1 7PB
Aldred Process Plant
Oakwood Chemical Works
Worksop, Notts S80 3EY
Chuo Boeki Goshi Kaisha
P.O. Box 8
Eco Briquette APS
P.O. Box 720
Fred Hausmann AGH
Universal Wood Limited
11120 Roselle Street
San Diego, California 99121
VS Machine Factory
90/20 Ladprao Soi 1 Road
Woodex International, Ltd.
P.O. Box 400
Canada M5W 1E1