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                        TECHNICAL PAPER #46
                     UNDERSTANDING WOOD WASTES
                              AS FUEL
                              Jon Vogler
                              Published by
                    1600 Wilson Boulevard, Suite 500
                      Arlington, Virginia 22209 USA
                 Tel:  703/276-1800 * Fax:   703-243-1865
                    Understanding Wood Wastes as Fuel
                          ISBN:  0-86619-260-3
              [C] 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
countries.  The 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 construction or
implementation details.  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 involved
in the production of the first 100 titles issued, contributing approximately 5,000
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 developing
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.
                    by VITA Volunteer Jon Vogler
We can define wood wastes as wastes arising from human operations
on wood; extracting it from forest, woodland, and plantation;
converting it into planks and other "stock"; fabricating these
into products--buildings, furniture, tools, and thousands of
other items; and finally, discarding these when broken or even
just "out of fashion."   To this definition may be added "nature's
wastes," such as leaves, twigs, and branches that fall from the
tree due to natural causes such as aging, wind, lightning, or
animal disturbance.
With that broad definition in mind, tree and wood wastes can be
categorized as follows:
Forest Wastes               Conversion Wastes            User Wastes
Thinnings(*)                Bark                         Sawdust
Reject Trees                Sawdust                      Shavings
Leaves                      Slabs(*)                     Sander Dust
Bark                        Edgings(*)                   End Trim(*)
Branches(*)                 Rejects(*)                   Off Cuts(*)
Topwood                                                  Veneer Clippings
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 chips
and shavings served as kindling.
This paper describes a number of uses of wood wastes as fuel,
which is how the greatest proportion of wood wastes are used.
Non-fuel uses of wood wastes, for example in building materials,
industry, and agriculture, are described in another paper, "Understanding
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 it
can be renewed.  Meanwhile, an insatiable demand for paper made
from wood pulp, wooden building components, furniture, and other
goods also contributes to deforestation.   Economical use of wood
wastes instead of new wood helps to preserve forests and woodland
in developed countries and is becoming essential to survival of
the poor in many parts of the Third World, as fuel becomes more
(*) widely used directly as domestic fuel, as kindling, and as the
raw material for charcoal.
This paper concentrates on three main uses for wood wastes as
o  Burning solid wood wastes or sawdust;
o  Using sawdust and tiny wood pieces to make small compact fuel
   pellets (briquettes) that can be burned in a manner similar
   to solid wood;
o  Making charcoal, a widespread (mainly cottage) industry for
   converting wood wastes into a lightweight, smokeless fuel.
Some experts describe certain wood waste processes as having
singular applications.  Many procedures for processing wood
wastes, however, can also accommodate a wide variety of agricultural
waste products such as husks and hulls.
All wood contains moisture; even kiln dried wood has an eight
percent moisture content.  When the kindling is first lit, white
smoke, containing a large percentage of water, rises from the
wood.  As the fire begins to burn, long tongues of yellow flame
indicate that the volatile substances, natural oils, and resins
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 [degrees] C.
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 reach
their flash point.  Thus much of the fuel value is lost.  Following
this, the wood burns with small white flames and hard, clear
outlines as the remaining fibrous matter (lignin) and carbon
Many years ago, stoves were made of cast iron panels bolted
together.  More 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 sheet
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 maintenance.
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 the
amount of air escaping, and the speed and amount of air that
passes through the mass of fuel.   Various features of stove design
affect this:
o  The fuel rests on a grate that allows air to pass through it
   from below.   Simple grates are usually parallel steel bars,
   close enough to prevent fuel from falling through the spaces
   between them.
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 control the
      quantity of air entering and fuels passing through the
      firebox.   This may vary from no air to a strong draft that
      causes the fire to blaze.
   -   Opening and closing the stoking doors varies the air
      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 not effective.
   -   Dampers regulate the draft by varying the size of the
      chimney opening.  The damper is a hinged flap in the flue,
      the pipe from the firebox to the chimney.
   -   Baffle plates:   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 iron
      obstruct the gas flow, and ensure the gases are heated to
      their flash point and radiate additional heat before
      escaping.   During "secondary combustion" the gases given
      off from the heated fuel-wood are drawn away from the
      main fire.   A secondary inlet admits air and the gases
      spontaneously ignite if they are at a sufficiently high
o  Heat exchangers, sometimes called smoke chambers or radiators,
   extract the maximum amount of heat from the hot fire
   gases.   They are additional chambers that can be bypassed
   during kindling.  By moving a valve, the hot gates may be
   directed through them when the fire has reached a certain
   temperature.   Heat exchangers and other parts of the fire
   boxes of stoves are often wrinkled or patterned in order to
   provide an increased surface area for transferring heat.  This
   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
   electric fan running in a steel tube.  This increases both the
   supply of air for burning and the rate at which heat is
   removed (transferred to the surrounding area).
Advanced Stove Designs
Five basic designs have evolved for wood-burning stoves, though
there are as many variations as there are stove manufacturers.
The main differences concern how air moves through the stove.
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 secondary air
    inlets above the wood for secondary combustion of the gases
    when the stove is burning well.
2.  Air enters the bottom of Diagonal stoves then moves diagonally
    through the fuel to the fire in the back of the stove.  A
    secondary air inlet above the wood assists secondary combustion.
    Heat exchangers are often fitted.
3.  Air enters near the bottom of Crossdraft stoves and leaves
    near the bottom at the back of the stove.  Secondary combustion
    of gases occurs in the main fuel bed.
4.  Downdraft stoves force air and combustion gases down through
    the burning fuel.  Air enters at or near the top of the stove
    and travels down through the grate to leave through a flue at
    the bottom.   These stoves are smokey when not burning properly,
    unless fitted 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 draft
    enters through the front and passes over the fuelwood, which
    then burns towards the back.  Baffle plates force the hot
    volatile gases to double back over the fire in order to reach
    the chimney flue.  They encounter secondary air and burn with
    high efficiency if the stove temperature is high.
Water Heating
Many wood-burning stoves are jacketed.   That is, the firebox is
surrounded by a jacket of water, which, when heated, convects
(moves upwards because hot water is less dense or lighter than
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.
Fuelwood accounts for at least half of all the wood used in the
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, as
fuel must be collected or purchased at a constantly increasing
expenditure of labor or money--a burden that falls mainly on
Part of the solution is, of course, to grow more trees.   Part is
to make better use of the fuelwood resources that remain.  The
introduction of cooking stoves that use less fuel than open fires
or traditional stoves can reduce the labor of fuel gathering and
conserve fuel, so as to extend the time available for long-term
measures (tree planting) to take effect.   However, the "advanced"
stoves described above are too costly for most Third World users.
Research programs have therefore been launched over the past few
years to develop better stoves than those in common use, yet
still simple, robust, of low cost, and suitable for local manufacture
and unskilled use.
Some Typical Improved Stove Designs
Early efforts concentrated on the development of massive stoves
made from mud.  Later more durable designs known as "pottery
insert" stoves were made from pottery by skilled artisans.  These
can be coated with an outside layer of mud to increase stability,
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 of
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 fire,
insulation to minimize heat loss, a grate to ensure good combustion,
and control of the air supply to regulate burning.   These
portable stoves also lend themselves to quality control and mass
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 demand,
especially in some urban areas of developing countries.   This type
of stove efficiently burns scraps that could not be effectively
used on an open fire.  Its fuel economy is excellent.  Because it
contains the heat well, the cook can remain seated close to the
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 upsets.
The charcoal-burning metal stove, known in Kenya as the "Jiko,"
is over 90 percent inefficient.   To replace it, the Umeme Stove
has been developed by UNICEF's Appropriate Technology Section in
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 liner
in the metal cladding.
VITA's work in Somalia and in West Africa has also yielded improved
stoves based on traditional designs.   In Somalia, soapstone
stoves carved to rigorous specifications to increase efficiency
are finding a ready market.  And in Burkina Faso, Mali, Guinea,
and elsewhere, traditional metal designs have been upgraded and
artisans trained in their production.   The use of templates in
these areas has permitted the manufacture of large numbers of
high quality stoves, thus bringing down the unit cost and making
them more attractive to purchasers.
Commercial stoves, including an American product known as the Zip
Stove, are also currently being promoted in developing countries.
The Zip Stove, manufactured from light weight galvanized
steel, comprises a cylindrical combustion chamber with a removable
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 traditional
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 simple
stoves.  Burning any carbon fuel produces poisonous carbon monoxide.
In an enclosed room this can be very dangerous.   Simple
stoves are not very safe in this respect.   For example, the average
carbon monoxide content of gases emitted by traditional charcoal
stoves ranges from 0.9 percent to 0.3 percent.   By contrast,
European safety standards recommend carbon monoxide emissions
should be not more than 0.0005 percent in any enclosed area.  Even
so called improved stoves are no better in terms of gas emissions.
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 of
At present, there are no reliable measurements of carbon monoxide
and other emissions from open fires, but indications are that the
women who cook with these fires and improved stoves suffer respiratory
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 monoxide
in particular, and an air-tight firebox with baffles to achieve
more efficient burning of the combustion gases.
Huge quantities of sawdust are produced in sawmills and carpentry
workshops all over the world, but it is rarely recycled effectively.
It cannot be used for paper making because the fibers
are too short.  It will not burn on an open fire, except in the
smallest quantities.  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 briquettes
without expensive binders or very high pressures.   Only
large sawmills may find it economical to buy a briquetting press
and possibly to carbonize (make into charcoal) the finished
briquettes, let alone to recover the tar and combustible gases
that are the by-products of the carbonization process.   Nonetheless,
there are several ingenious ways people have found to burn
Tin Can Stove
The single chimney tin can stove is the simplest homemade stove
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 center of
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 down.  The
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 bottom.
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 can
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, but
still inexpensive to construct.   It consists of a 30-gallon steel
drum, supported on a false floor inside a 55-gallon steel drum. A
drawer, opening below the false floor, provides draft and catches
dropping ashes, which are then easily removed.   A hole in the
center of the false floor and the inner barrel bottom lets air
pass up to the fuel, and ashes fall into the drawer.   A tightly
fitting lid covers the outer barrel and two stovepipes exhaust
smoke.  It should stand at least two feet from any combustible
material and be set on a fireproof floor pad.   CAUTION:  DO NOT
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 can
heat a room 7 meteres square for six to eight hours with no tending. 
Wetter fuel heats less but lasts longer.   During the first
two hours of burning, there is enough heat at the center of the
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.   The sawdust
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.  Two
combustibles can heat enough water for a bath.   The special water
boilers have a grate at the bottom, on which the combustibles are
burned.  Above the grate is a chimney surrounded by a water jacket
with a water inlet and outlet that are plumbed into the household
hot water system.
The alternative to having a special stove for burning sawdust and
small wood wastes is to compress these into a briquette--a small,
compact fuel pellet.  The average calorific value of briquetted
wood waste or sawdust is 4,000 kilograms per cubic centimeter, so
every 100,000 tons of briquetted wood waste will be equivalent to
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 is
self-bonding at fairly high temperatures and requires no added
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 storage,
if protected from rain.
To achieve briquettes of the necessary strength and hardness, the
moisture content of the wood waste should be around 10 percent,
although in some cases, machines are capable of handling dry wood
chips.  Dryers may take the form of rotating drums through which
hot air is blown or steam-heated plates and pipes over which the
waste is cascaded.  A large proportion of the material may be
needed to provide sufficient heat to dry the feedstock from a
high moisture level.  It is usually necessary to grind the waste
to a suitable size and, before doing so, to strain it to remove
stones, soil, or metal, which would damage the grinder.
Briquetting machinery must be robust and powerful.   Attempts to
produce simple, low-cost, low-power machines for small-scale
operations have not been successful to date. Pressures of up to
1,000 kilograms per square centimeter can be involved.   To keep
die temperatures low and avoid burning the briquettes, dies need
water cooling.  Machines need motors giving between 25 and 100
kilowatts for every ton per hour of throughput, although not all
this is absorbed during operation.
Many manufacturers of wood pulp and other wood products use
sawdust and other wastes as fuel for their manufacturing processes.
The wood wastes are briquetted in an ongoing process.   One
such process uses a machine that screws the waste wood (sawdust,
shavings, and other scrap, ground to the size of oatmeal particles)
first into a compression chamber at a pressure of 3,000
pounds per square inch (211 kilograms per square centimeter).  At
the outlet from this chamber, a secondary head cuts the compressed
material into a spiral ribbon and forces it into a mold
under a pressure of 25,000 (1757.7 kilograms per square centimeter)
to 30,000 pounds per square inch (2109.24 kilograms per
square centimeter).  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 chamber.
The molds are water cooled, and, by the time the wheel has moved
full circle, the briquette is cool enough to eject.   The machine
produces 4 by 12 inch briquettes that are fed manually into the
factory's furnaces.
For mechanical stoking an extruder is used that forces the waste
through one-inch round holes as continuous rods, which are cut
into one-inch lengths by rotating knives.   The machine is small
enough to be mounted on a truck and powered by a truck motor.
It is far less expensive to transport briquettes than loose
waste, so briquetting machinery should operate where the wood
waste arises.  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 for
fuel briquettes, transportation costs may not make the operation
cost-effective.  The finished briquette may need protection from
reabsorption of moisture and should be stored in dry areas or
packed in sacks.  Packing in plastic film or cellophane may be
necessary.  Careful handling and transportation are needed to
prevent crumbling.
Other processes include:
o   Briquetting between rollers with cavities that produce egg-shaped
    briquettes in 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);
o   Cubing--a modified form of pelleting (product size 2-5
o   Rolling/Compressing--where fibrous material is wrapped around
    a rotating shaft to produce a high density roll or log
    (product size 10-18 centimeters diameter).
Simple Sawdust Briquettes
Various attempts have been made to devise methods by which people
in rural areas can use sawdust to make briquettes.   The simplest
idea, for areas where dung is shaped by hand and sun dried for
use as fuel, is that the dung cakes will burn longer if wood ash
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 molding.
The length of the lever arm determines the briquetting pressure
and it is important that the mold be sturdy enough to withstand
this.  Approximately 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 briquettes.
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 cylinder
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.   There is
a simple device to eject the briquette, which is about 30 millimeters
in diameter and 10 millimeters thick.   Approximately 50
kilograms of briquettes can be produced in about eight hours.
A larger machine is powered by a single bullock.   It consists of
two sets of pistons and cylinders and turns at about four rpm to
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 the
School of Applied Research in India.   Further details can be
obtained from the National Research Development Corporation of
India, 20-22 Zamroodpur Community Center, Kailash Colony Extension,
New Delhi 110 048, India.
A Thai businessman, Sayan Panpinij, in collaboration with VITA,
has developed an extrusion machine that transforms rice husks
into burnable logs.  Approximately 75 kilograms of rice husk fuel
logs are produced per hour from each of twin extrusion heads,
with a density almost double that of firewood.   The machine is
powered by a 20-horsepower electric motor and works best with
husks that have been ground and dried to reduce moisture.  The
machine can be operated by one person who feeds the rice husks
into the hopper on top of the machine, removes the fuel logs from
below the extruder, and stacks these for cooling.   It is estimated
that three people will be necessary to operate four machines.  The
VITA extruder can also produce fuel logs from sawdust.   These have
a higher heat value than rice husk logs, produce less smoke and
ash when burned, and reduce wear and tear on the machine.  The
device is relatively new and has not yet been manufactured outside
of Thailand.
The life and maintenance of this extrusion machine is a primary
consideration for the user.  When the device is used for extruding
rice, the screw will need to be replaced every 120 hours.  The
extrusion cylinder has a life of about 450 hours and will probably
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.   Depending on
temperature, quality of the heater unit, and the length of operation,
the life of the heat unit varies between 240 and 350 hours.
In a four-unit plant, it is estimated that capital and operating
costs can be replaced within one year.
Partially decayed and processed cellulosic materials give a much
higher heating value than if the materials are simply dried.  For
example, dried rice straw (10 percent moisture content) has a
heat value of only 3,000 BTU/pounds (7 million joules/kilogram
[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 GJ/kg) and
12,000 (28 million J/kg or 0.0279 GJ/kg) when the material has
partially rotted before it is dried.   In the Philippines, the
MAPECON research group has set up a pilot plant producing such
fuel, with 25 percent moisture content and an average of 10,000
BTU/pounds (23 million J/kg or 0.0232 GJ/kg) which they call
`green charcoal,' at the rate of one ton per hour.   The group
reports that it is very competitive with other types of fuel.
Retting--soaking in water for several days or longer at normal
air temperatures--allows chopped, moistened woody residues to be
biodegraded (partially decayed).   This process is used to produce
mats that can be pressed into fiberboard, but a simple hand press
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
during pressing.
Tying brushwood into compact bundles for ease of transportation
and use is the simplest means of densifying wood wastes.  Twigs,
straw, hay, dry leaves, and other woody wastes are bundled all
over the world, using cord, vines, wire, or any locally available
tying material.  Where large-scale bundling is carried out, stands
or racks have been developed to assist in the bundling process,
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.
In contrast to the heavy weight and high smoke level of the wood
from which it is made, charcoal is a light, smokeless fuel of
high calorific value.
When wood is heated in the absence of air, changes take place in
several stages.  At 100 to 120 [degrees] C, water is emitted into the air.
Green wood contains between 50 to 70 percent water, which must be
evaporated before the wood temperature can rise higher. Carbonization
(conversion into carbon or charcoal) begins at 270 to 400 [degrees]
C.  The reaction, technically named pyrolysis, gives out heat.  The
wood chars and gives off gases and vapors--carbon dioxide, carbon
monoxide, hydrogen, methane, water vapor, methanol, acetone, tar,
and pitch.
The yield of charcoal and its composition depend on the species
of wood, the carbonizing temperature, and other factors.  Yield is
generally about 25-40 percent by weight of dry wood.   Although low
carbonization temperatures produce a higher yield (because the
charcoal still contains matter that has not been given off as
gas) the charcoal quality is poor.   It smokes and flames.  Temperatures
that are too high, on the other hand, shorten the life of
equipment, so care should be taken to keep carbonizing temperatures
between 400 and 700 [degrees] C.
The energy value of the gases represents some 40 percent of all
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 recover
these compounds.  In small-scale processing, however, they help
maintain burning in the kiln.
Charcoal is made by placing wood in a kiln, igniting it in the
air, and then, when it is burning thoroughly, reducing the supply
of air almost completely.  Many types of kilns are in use.  Some
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 deep.
Stakes are set in the ground around the pile to support a wall
made with interleaved branches or scrap corrugated iron.  The kiln
is then lit and allowed to burn fiercely until smoke comes out at
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 charcoal is
put into sacks for sale.  It is reported that two experienced
charcoal makers can produce about six tons of charcoal a month by
this process, which needs no capital money just a sack, a spade,
and an ax.
The CUSAB or Oil Barrel Kiln
Kilns for carbonizing small wood pieces are made from oil drums,
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 holes.  The
screw connectors can then be fitted with plugs to cut off the
emerging air.  Holes should face the wind and a stick can be used
to keep the openings clear of debris during the early hours of
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 oil
drums makes this a cost-effective technology.
The Steel Kiln
The steel kiln can produce an average of 500 kilograms of charcoal
every two days from two and a half tons of wood, depending
on moisture content and density of the timber used as feedstock.
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 loading,
unloading, and moving to new sites.   It has been designed to
withstand rough usage and extreme temperature conditions.  There
are no underground fittings.
To operate, logs are placed, with kindling between them, in the
lower cylinder, which rests on eight smoke boxes.   The lower
cylinder is then densely packed with logs. When full, its rim is
filled with mud to form an air seal and the upper cylinder is
mounted on top.  The upper cylinder is also packed to a height
such that the top cover does not quite meet the cylinder. Flaps
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 for
fifteen minutes to reduce burning.   After 16 to 24 hours, smoke
will cease.  Each chimney can then be removed and the smoke box
closed.  Cooling takes 8 to 12 hours.
Other Simple Kilns
There are many other simple kiln designs available.   One version
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 consisting
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 charcoal.  In
Papua New Guinea, two cylinders made from 44-gallon drums, lying
on their sides over a stone or concrete fire trench, produce high
quality charcoal.  A group of drum kilns wired together will allow
the heat to be distributed more efficiently and produce charcoal
Retorts are designed to use the gases (including condensed gases
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 necessary
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 in
road construction, preserving timber, or sealing flat roofs.  Some
retorts can recover gases that are directed to the firebox where
they are burned to fuel the process during its later stages,
saving solid fuels.
Industrial Processes
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 on
site, lifting and lowering the cover during operation.   No concrete
foundation is required.  It takes only 48 hours to produce
four tons of charcoal, which can be discharged directly from a
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 year.
Permanent Vertical Kiln:  This is available in sizes to produce
between 5,000 and 10,000 tons of charcoal per year.   The material
is handled mechanically and can be passed through a continuous
dryer.  Little labor is necessary.
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 conversion,
packaged coal-fired boilers, and gas turbine power generation.
Within the timber industry, wood-fired fluidized bed furnaces
have become commercially available for steam raising.   There is
increasing interest in processing wood waste into upgraded fuels
such as gas, charcoal, or oil fluidized beds.
Further details may be obtained from manufacturers.
If charcoal can be sold near the site where it is made, transportation
and storage costs will not be high.   If it is to be
transported a long distance or sold later when the market price
is better, it is desirable to compress it into small, dense
briquettes.  This 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 such
machines are expensive (about US$100,000), but easily obtainable
and not difficult to operate or maintain.   So far, no company has
produced an inexpensive small briquetting press that produces
sufficient pressure to make briquettes that do not crumble without
a binder, so large presses have to be used.   Some models need
to be fed by at least eight steel kilns, which result in additional
transportation costs.
Charcoal Briquetting Processes
The production of charcoal briquettes may be accomplished either
by preparing the charcoal first and then pressing it, or by
preparing wood briquettes to be carbonized after forming.  One
method produces semicharcoal briquettes by preheating sawdust
until the lighter gases have been given off and tar begins to
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 pressure,
until it has partially carbonized, then applies a pressure of 350
pounds per square inch until carbonization is complete.   The
resulting briquettes are further heated to drive off gases that
would create smoke.
Another process distills finely ground wood to produce granulated
charcoal, which is mixed with the wood tar produced in the process
and briquetted.  The briquettes are reheated in a retort to
drive off and recover the lighter fractions of the tar.   The
remaining particles may then be bound firmly together to form a
dense briquette.  This process is sometimes referred to as"coking."
It is reported that these processes are commercially unsuccessful
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 being
pressed into uniformly-sized, strong, dense briquettes, free from
charcoal dust.
Practical briquetting operations entail four steps:
1.  Preparation of charcoal fines.  Lump charcoal is crushed, then
    milled using a screen with 1/10 inch or 1/8 inch holes to
    produce material with enough fines to fill the voids between
    the larger pieces and to prevent them from being crushed
    during briquetting.
2.  Mixing to coat the charcoal particles with a film of binder.
    A kneader-type, double shaft mixer is often used.  Another
    method, however, involves simultaneously feeding pre-crushed
    charcoal and cassava flour into a hammermill.  The mixture is
    stirred continuously, then steamed until the flour forms a
    binding paste.
3.  Briquetting the mixture between two cylindrical rolls that
    rotate in opposite directions.  Each roll is designed with
    rows of hollowed half molds, aligned so the halves match.
    Hundreds of briquettes can be produced at every turn of the
4.  Drying the briquettes continuously or in batches.  Dryers are
    similar to agricultural dryers in operation.  Briquettes
    produced with asphalt or pitch binders do not need artificial
    drying, only cooling.
To produce satisfactory briquettes economically, the binding
substance must meet certain requirements.   It must produce a
briquette strong enough to withstand damage during transport,
storage, and stoking.  Exposure to weather must not cause crumbling
or softening and, during use, the heat must not cause disintegration
and loss of fine pieces through the grates.   It must
burn without smoke and unpleasant smell and not be too dusty.
Ideally the binder should have as high a heat value as the charcoal.
Binders fall into three categories:   inorganic materials, organic
materials, and fibers.
o    Inorganic materials, such as cement and silicate of soda
     are appropriate for wood fuel.  These substances are poor
     because they give more ash, reduce the heat value, and fall
     apart while burning.
o    Organic materials such as tar, pitch, resin, and glue
     usually increase the heat value and create no extra ash.
o    Various types of fibrous material may serve as binding
     agents.   The cheapest is hydrated wood fiber-wood waste--ground,
     pulped wood waste, which, when dry, binds together
     in the same way as paper.
Some binders permeate the material to be briquetted; others coat
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).   Tar, pitch,
asphalt, and sugar cane molasses are used in less than 30 percent
of the cases.  They are moisture resistant but not smokeless.  This
is no drawback in industrial uses, such as smelting and heating,
but would be inappropriate for home fuel or cooking.
Secondary distillation (heating a second time) can drive off the
smokey gases, but increases cost and does not completely remove
objectionable smells during burning.   A good smokeless charcoal is
one that contains at least 75 percent fixed carbon and not more
than 24 percent "volatile" (able to be emitted as gases) matter.
Uses of Briquetted Charcoal
Briquetted charcoal has many industrial uses and can be used as
domestic fuel as well.  The product is a high quality industrial
fuel for production of steel, cement, copper, rubber, gun powder,
and other products.
In the chemical industry, very pure briquettes are used as
activated carbon for air and water purification, for filtration,
decolorization, purification of sugar, and as a chemical
catalyst.  Activated carbon commands prices five to six times
higher than those of briquetted charcoal.
Appropriate Technology International has several reports on the
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 dozen
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 number of
"Briquettes From Wood Waste," Madison, Wisconsin, Forest Products
  Laboratory, U.S. Department of Agriculture, 1947.
Bryant, B.S. et al, Fuel Briquettes from Fibrous Residues Using a
  Hand-Operated Lever Press, Volunteers in Technical Assistance,
  Arlington, Virginia, USA.
Cosgrove-Davies, Mack.  "Understanding Briquetting," a Technical
  Paper by Volunteers in Technical Assistance, Arlington,
  Virginia, USA.
Currier, R. A., Manufacturing Densified Wood and Bark Fuels,
  Oregon State University Extension Service Special Report 490,
Klages, A., 1953, Economic Aspects of Wood Briquetting, Australian
  Timber Journal 19, pages 414-441.
Smith, A. E., Flynn G., & Breag G. R., A Profile of the Briquetting of
  Agricultural and Forestry Residues, Tropical Development and
  Research Institute, 127, Clerkenwell Road, London, EC1R 5DB,
  United Kingdom.
"Briquettes From Wood Waste," Madison, Wisconsin, Forest Products
  Laboratory, U.S. Department of Agriculture, 1947.
Bryant, B. S. et al, Fuel Briquettes from Fibrous Residues Using
  a Hand-Operated Lever Press, Volunteers in Technical Assistance,
  Arlington, Virginia, USA.
Currier, R. A., 1977, "Manufacturing Densified Wood and Bark
  Fuels."   Oregon State University Extension Service Special
  Report, 490.
Foley, G., Moss, P., and Timberlake, L., Stoves and Trees, Earthscan,
Joseph, S., and Hassrick P., Burning Issues:   Implementing pilot
  stoves Programmes--A guide for Eastern Africa.  London:  Intermediate
  Technology Publications, 1984.
Klages, A., 1953, Economic Aspects of Wood Briquetting, Australian
  Timber Journal 19, pages 414-441.
Reineke, L. A., 1955, Briquettes from Wood Waste, Forest Products
  Laboratory Report No. 1666-13.
Smith, A. E., Flynn G., & Breag. G. R., A Profile of the Briquetting
  of Agricultural and Forestry Residues, Tropical Development
  and Research Institute, 127 Clerkenwell Road, London,
  EC1R 5DB, United Kingdom.
Wartluft, J., Double Drum Sawdust Stove, a technical bulletin
  published by VITA, Arlington, Virginia, USA.  ISBN 0-86619-109-7.
Charcoal Making for Small-Scale Enterprises:   An illustrated manual.
  Geneva:   International Labour Office, 1975.
Grato, N., "Charcoal Manufacture," Liklik Buk pp:  132-133, Lae,
  Papua New Guinea:  Liklik Buk Information Centre, Lae, Papua
   New Guinea, 1977.
Little, E. C. S., 1978, The Mini CUSAB Kiln for Rapid Small-Scale
  Manufacture of Charcoal from Scrub, Coconut Wood, and Coconut
  Shells, Appropriate Technology 5 (1):  12-14.
Medrano, E. M., "Design, Fabrication and Operation of Drum Kilns
  for Charcoaling Coconut Shells."  Technology Journal 1 (2):  26-35,
Papua New Guinea Building Research Station, "Manufacture of Charcoal
  by Retorts," 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
  Forestry (mimeo), 1977.
Testing the Efficiency of Wood-Burning Cookstoves, Volunteers in
  Technical Assistance, Arlington, Virginia USA, 1985.
The Comparative Performances of Kenyan Charcoal Stoves, ITDG
  Stoves Project, Technical Paper No. 1.
Vil lanueva, E. P., and Banaag, N. F., "Sawmill Waste Charcoal for
  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
Asian and Pacific Coconut Community (APCC)
Box 343
Jakarta, Indonesia
Department of Agriculture
Box 14
Nuku'alofa, Tonga
Fibre Building Board Development Organization, Ltd.
1 Hanworth Road
Feltham, Middlesex TW13 5AF
United Kingdom
Forestry Division
Ministry of Agriculture
Fisheries and Forests
P.O. Box 358
Suva, Fiji
Forest Products Research and Industries Development Commission
NSDB college
Laguna 3720
Wood Stoves Project
9 King Street
Covent Garden, London WC2E 8HW
United Kingdom
New Zealand Forest Service (NZFS)
Private Bag
Wellington, New Zealand
The Principal
Kristian Institute of Technology of Weasisi (KITOW)
P.O. Box 16
Isangel, Tanna
New Hebrides
South Pacific Bureau for Economic Cooperation (SPEC)
Box 856
Suva, Fiji
Timber Research and Development Association
Hughenden Valley
High Wycombe
Bucks, United Kingdom
Tropical Products Institute (TPI)
56 Grays Inn Road
London WC1X 8LU
United Kingdom
United Nations Industrial Development Organization (UNIDO)
P.O. Box 707
Vienna, Austria
Volunteers in Technical Assistance (VITA)
1815 North Lynn Street, Suite 200
Arlington, Virginia 22209 USA
Wood Stove Group
Eindhoven University
Post Bus. 513
5600MB, Eindhoven, Netherlands
Aldred Process Plant
Oakwood Chemical Works
Sandy Lane
Worksop, Notts S80 3EY
United Kingdom
Air Plant (Sales). Ltd., (Spanex)
295 Aylestone Road
Leicester, LE1 7PB
United Kingdom
Aldred Process Plant
Oakwood Chemical Works
Sandy Lane
Worksop, Notts S80 3EY
United Kingdom
Chuo Boeki Goshi Kaisha
P.O. Box 8
Ibaraki City
Osaka 567
Eco Briquette APS
P.O. Box 720
Frederikshavn DK-9900
Fred Hausmann AGH
Hammerstrasse 46
4055 Basel
Sukkulakatu 3
Universal Wood Limited
11120 Roselle Street
Suite J
San Diego, California 99121
VS Machine Factory
90/20 Ladprao Soi 1 Road
Bangkok, Thailand
Woodex International, Ltd.
P.O. Box 400
Terminal A
Toronto, Ontario
Canada M5W 1E1

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