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WHAT IS WOOD DENSIFICATION?
West Virginia University
Extension Service
Publication No. 838
Wood Densification
WHAT IS WOOD DENSIFICATION?
Wood densification is the process of
taking wood by-products (manufacturing
residues) such as slabs, chips, or sawdust and
processing them into uniform sized particles
so they can be compressed into a fuelwood
product. When wood wastes are densified,
not only is a potential environmental hazard
being eliminated, but a high Btu, clean burning
fuel source is being created. Densified
wood in a log form will burn on the average,
three times longer than dry cordwood. A
densified fuel log will generally have a moisture
content of between 6 and 10 percent
with the average at about 8 percent. Some
other desirable characteristics of densified
wood fuels include: a low ash content of
around 2 percent, lack of creosote formation,
and a Btu rating of around 8000, making
it competitive with some types of coal. Handling,
transportation, and feeding of combustion
systems are also improved when
utilizing densified wood products.
Densified wood products come in a
variety of shapes and sizes for use as a fuel
source, including:
1. Logs - cylindrical shaped, with a
diameter of 2 to 4 inches and a length of 12
to 16 inches. A 12-inch log will weigh approximately
5 pounds.
2. Pellets - small cylinders about 1/2 to
1-1/2 inches in diameter and about 1 to 2 inches
long.
3. Briquettes - generally disk shaped and
produced from the same type of machines as
logs. Therefore, diameter is 2 to 4 inches and
length is about 1 to 2 inches.
Logs are generally produced for
residential markets as fireplace and wood
stove fuels. Popularity of logs, in part, is due
to the fact that they are clean burning, easy
to handle, and burn longer than traditional
cordwood. Logs are also used occasionally
in small industries as fuel for boilers. Pellets
are used more in commercial applications
for industrial boilers where ease of handling
and burning characteristics offer a competitive
alternative to coal. However, with the
newer types of boilers capable of using raw
sawdust, the need for pellets will decrease.
Furthermore, pellets require a high capital
investment, high output, and large markets
to be profitable. Briquettes have not been as
popular as pellets but can be used as a fuel
source for both residential and industrial applications.
Because of the market flexibility
of briquettes, this type of product could enhance
the economic feasibility for densified
fuelwood manufacturers.
HOW ARE DENSIFIED FUEL LOGS
PRODUCED?
Two different types of equipment are
available for manufacturing densified fuels:
a screw-type extruder and a compacting ram.
In the screw-type machine a large screw forces
the raw material through a die at a pressure
of around 20,000 psi, processing
approximately 2000 lbs of dry sawdust per
hour. The temperature created through this
process is above 400 degrees Fahrenheit. At
this temperature, lignin, the natural bonding
agent in wood, undergoes a softening
process, inducing a certain degree of bonding
among particles. However, bonding
within the log is probably due as much to the
strength derived from intertwined particles
as to the lignin bonding.
(1) Main Screw (8) Cylinder
(2) Front Screw (9) Roll Cutter
(3) Forming Head (10) Front Roll Seal
(4) Taper Sleeve (11) Shaft
(5) Cylinder Sleeve (12) Hopper
(6) Kikugata (13) V-Pulley
(7) Kikugata Case
Fig. 1 Screw-Type Extruder
The compacting ram operates like a
large piston which compacts the material at
around 8500 psi as it is forced through a die.
The log formed by this process consists of a
series of disks that are bonded together
primarily by the mechanical interaction of
particles and has very little to do with lignin
bonding. The resulting logs are not as dense
as the screw-type logs nor are they as
durable. The compacting ram is capable of
utilizing approximately 4000 lbs of dry sawdust
per hour.
Fig. 2 Compacting Ram
Briquettes can be produced by either
type of machine. However, the ram machine
is more readily capable of producing briquettes
since the disks produced at each stroke of
the piston can be easily separated into briquettes.
This can be accomplished by a punch
or a saw which separates the log at the weak
point between disks. A saw is generally the
best alternative for cutting logs or briquettes
produced on screw-type equipment.
PREPARATION OF THE RAW
MATERIAL
For production of densified fuelwood to
be successful, the raw material must be
properly prepared and of uniform size.
Wood particles must be 1/4 inch or smaller
and cannot have a moisture content in excess
of 10 percent. Above 10 percent, moisture
adversely affects the system, slowing down
production and causing wear on the internal
parts. Material larger than 1/4 inch should
first be processed through a hammermill in
order to achieve the necessary size and
uniformity. Larger material like slabs,
edgings or end-trim may have to be
processed through a chipper or hog prior to
hammermilling. All systems should have a
metal detection device to insure that
processing equipment is not damaged by
foreign objects.
Green (wet) sawdust must be dried
prior to densification. Most driers are
powered by natural gas, although some
recent dryers are equipped to burn sawdust.
The dryer has a large heating unit that blows
hot air through the drying unit. Hot air mixes
with the sawdust as it falls from the side of
the rotating dryer. Once dried, the sawdust
is generally placed into a dry storage bin and
fed into a metering bin as needed. The
metering bin feeds the extruder(s) or compacting
ram(s) premeasured volumes of sawdust
as needed.
POST - DENSIFICATION PROCESSING
Once the densified log product has
been formed in either the screw-type or compacting
ram-type machine it enters the cooling
stage. The cooling process is a very
important step, because the logs are still
fragile after leaving the densifying process
and must have time to cool in order to maximize
lignin bonding efficiency. The lignin in
the wood will bond more securely as it cools
in its new shape.
The ram-type log exits the densification
equipment as one continuous log. After
traveling about 100 to 150 feet in the cooling
line, logs are then cut. This distance is required
not only to let the log cool, but to
provide added back pressure to the ram to
help increase the density of the log being
manufactured.
The screw-type machine does not need
a long cooling line to generate back pressure
since the process is designed to produce a
very dense log. The logs are cut to length immediately
after leaving the extruder. The
logs are about 400 degrees Fahrenheit at this
time and need to be cooled either on a
straight line conveyor (about 100 feet is sufficient)
or cycled through a spiral conveyor
or cooling tower which has fans that circulate
air for cooling.
Packaging fuel logs or briquettes is
often a very labor and/or capital intensive
operation. The most cost effective means of
packaging logs is in groups of 3 or 6. The most
common types of packaging include:
1) Cardboard boxes that hold 3 to 6 logs.
More than 6 logs can pose a problem in lifting.
2) Trays covered with a heavy layer of
plastic. Each tray is then passed through a
shrink tunnel. Shrink wrapping yields a very
durable package, plus the added benefit is
that the consumer can see the product they
are buying. In addition, this type of packaging
is generally more cost effective than
boxes.
The most effective way of packaging briquettes
is bagging. Typical bag weights are
25 and 50 pounds. Briquettes are simply conveyed
to a bagging machine (a hopper with
manually controlled feeder) where the bags
are filled and then passed through a machine
that seals the bag.
Transportation of the packaging
material to the processing plant is also an important
consideration. Empty trays can be
shipped in a ratio of about two to every one
box shipped.
After filling the boxes or trays they are
generally stacked on pallets and each pallet
is shrink wrapped in plastic to insure that the
logs are kept free of moisture while in
storage or shipment. The shrink wrapping of
pallets also helps keep the load stable, permitting
the higher stacking of pallets without
danger of damaging the product.
When the raw material being utilized is
already dry, the space requirements of the
plant can be reduced substantially. In this
case, the wet storage bin, the metering bin for
the dryer, the dryer, and associated blower
systems could be eliminated.
PLANT LAYOUT
CAPITAL COSTS
A key factor in determining plant size is
the condition of the raw material being utilized.
With green (wet) sawdust a drying system
is necessary, which will require a plant
size of at least 100 feet by 100 feet. The
machinery that will need to be under roof in
this type of system includes: a portion of the
belting for transporting the raw material into
the plant, the metal detector, a hammermill
(a chipper or hog may also be required to
break down large pieces for the hammermill),
wet storage bin, metering bin, dryer,
dry storage bin, screw conveyor from the dry
storage bin to the extruders, cooling line,
packaging equipment, and inventory storage
for the palletized product. Moving the sawdust
through the different stages of the
process will require blowers, which must also
be under roof.
Fig. 3 Flow Diagram for Wet and Dry
Materials
Capital costs of a densified woodfuels
plant will depend on the production
capabilities desired and whether the raw
material is green (wet) or dry. Capital costs
can be divided into two areas: the initial cost
of equipment and the initial cost for the
structure to house the equipment and the
final product.
The densifying equipment will cost approximately
$70,000 for the capability to
produce 2000 pounds an hour of finished
product. The cost differential between the
screw-type machine versus the ram-type is
negligible when examining the capital cost in
units of productivity per hour.
Drying costs can differ depending on the
style and the size of the dryer. A rotating gas
fueled dryer will cost approximately
$160,000 with the heating unit. This is the
most commonly used dryer in the wood densifying
business. An impact drying system has
the capabilities of reducing the wood, like a
hammermill, as well as drying it. The impact
dryer can handle large quantities of sawdust,
is of heavy design, and costs approximately
$550,000.
Hammermills and the conveying systems
for a densified fuel wood plant would
cost approximately $20,000 each, while the
metering bin has a price of approximately
$30,000. Storage bins vary in cost depending
on the size and shape required and for that
reason many storage bins are fabricated. A
fabricating operation charges $3.25 to $5.00
per pound to manufacture an item such as a
bin. The price varies due to the complexity of
the design or the type of metal that has to be
used.
Packaging equipment has an approximate
cost of $50,000 for an automated
system. This will take the logs and box them
or put them into trays and shrink wrap them.
The cost of automated packaging equipment
will reduce labor costs and may make the
machine well worth the investment.
OPERATING COSTS
Operating costs include utilities, fuel
costs for machinery, labor, repair and maintenance
of equipment, packaging supplies,
transportation, and raw material expense.
While these costs will vary with type of raw
material and level of production, some rough
estimates can be made.
A plant utilizing three extruders is
capable of 6000 pounds of logs per hour. If
the plant operates 16 hours a day, four days
per week, 1,140,500 pounds of logs will be
produced in one month. In one month the
plant described would use approximately
59,000 kwh of electricity. If the raw material
has to be dried the dryer would use approximately
20,500 therms per month of
natural gas in the same plant. Maintenance
cost of approximately $5000 a month can be
expected from wear and tear on the moving
parts of the system including bearings,
augers, dies, and other parts. The major portion
of this cost will be born by the extruder
or ram, since this equipment has the
most mechanical stress of any others in the
system.
Labor requirements to operate the
above plant would be 6 to 9 employees. This
would include one person to feed the system
from raw material storage, one to supervise
the drying operation, one or two persons to
operate the densification equipment and the
remainder to handle packaging and inventory
storage. A plant manager would be
necessary under this type of operation.
However, for a plant using dry sawdust, ramtype
densification equipment and bagging
briquettes, the labor requirements would be
3 to 4 employees, including a plant manager.
The cost of packaging supplies varies
depending on the type of packaging required.
Boxes in general will cost 10 to 15
cents more per container than trays with a
heavy shrink wrap. New types of shrink wraps
will allow a tray to easily hold the weight of
3 to 6 logs. Also, transportation of empty
trays and shrink wrapping requires much less
space when shipping, reducing costs even
further.
Raw material costs will vary with the accessibility
and availability. Ideally, a densification
facility would be located in an area
of abundant raw material supplies. The ideal
densified fuelwood plant for wet sawdust
would be a part of or adjacent to an already
existing sawmill, where the handling costs of
the raw material would be greatly reduced.
For dry sawdust the ideal location would be
near a secondary manufacturer of wood
products such as a dimension mill, flooring
mill, millwork manufacturer, etc.
OUTLOOK FOR DENSIFIED
FUELWOOD PRODUCTS IN
WEST VIRGINIA
Fortunately, West Virginia is located
within 500 miles of 50 percent of the U.S.
population. The key to success of densified
wood products is the effort and finances that
will be allocated to the marketing of the
product within this heavily populated area.
At this time, the authors are aware of only
one densified log producer on the East coast,
which indicates that a significant opportunity
exists for the production and marketing of
densified fuelwood products.
September 1988
Herman C. Sims Dr. Curt C. Hassler Dr. Thomas L. Bean
Graduate Assistant Extension Specialist Extension Specialist
Wood Products Safety
The preparation of this report was financially aided through a grant from the Southeastern Regional
Biomass Energy Program.
This development of a Biomass Energy Program was prepared as a cooperative effort with the Fuel
and Energy Office, Governor’s Office of Community and Industrial Development.
Programs and activities offered by the West Virginia University Cooperative
Extension Service are available to all persons without regard to race, color,
sex, disability, religion, age, veteran status, sexual orientation or national origin.
Cooperative Extension Work in Agriculture and Home Economics, West
Virginia University and the United States Department of Agriculture,
Cooperating. Director, Morgantown, West Virginia.
Published in Furtherance of Acts of Congress of May 8 and June 30,1914.
