United States
Environmental Protection
Agency
Municipal Environmental Research'
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-180 Oct. 1981
Project Summary
Fundamental Considerations
for Preparing Densified
Refuse-Derived Fuel
Edward Winter
A series of pilot-scale tests were
conducted to determine the effects of
various parameters on the densifi-
cation of refuse-derived fuel (RDF).
The experiments included a series of
bench-scale experiments involving a
single die arrangement, as well as
larger-scale studies in which a com-
mercial pallet mill was used.
The bench-scale tests in which the
pellets were individually formed were
conducted both to provide data
needed for an analysis of the basic
dynamics of pellet formation and as an
aid in the interpretation of results
obtained with the pellet mill. The
energy required to overcome die
friction was studied independently of
the energy consumed in material
deformation and compression. By so
doing, it became possible to determine
the specific effects of die length,
diameter, and taper. The results also
suggested explanations for the ex-
cessive die wear and why less energy
is needed to increase the mass
throughput observed in commercial
pellet mills.
With the use of data obtained in the
tests with the pellet mill, the relation-
ship between specific energy of
densification and mass flow rate
through a mill was found to be as
follows:
where a and b depend upon the die
dimensions and characteristics of the
feedstock. Other data were developed
that relate feed moisture content to
pellet density and feed size to maxi-
mum achievable palletization rate.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental Research Laboratory. Cincin-
nati, OH, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The U.S. Environmental Protection
Agency (EPA) assigned the Municipal
Environmental Research Laboratory
(MERL) in Cincinnati, Ohio, major
responsibility for research and devel-
opment in the field of recovery and use
of municipal solid waste. One concept
investigated involves the recovery of
energy from solid waste. Refuse is
combusted either directly for steam
recovery or in combination with fossil
fuels for power generation. The latter
involves processing the refuse to
remove the combustibles for use in a
modified power generation boiler,
usually in combination with coal. The
processed refuse is usually referred to
as refuse-derived fuel (RDF).
The RDF concept in the United States
has generally been limited to power
generating facilities that burn pulverized
coal. The use of RDF need not be limited
to large users, however; it may, in fact,
be more valuable to small power
generating facilities. Small industrial
and institutional boiler owners may find
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RDF an attractive and cheaper alter-
native to fossil fuels, for which they
receive no quantity discounts, as do the
large users. In addition, small users may
have increased flexibility in negotiating
contracts for RDF (especially with
regard to length of commitment). Many
small power generators are economically
marginal because their boiler facilities
are older, coal-burning models that
require costly air pollution equipment.
The use of RDF may help such facilities
absorb the cost for such controls.
RDF prepared for large utility boilers
is typically composed of the light
fraction of shredded refuse that has
been air-classified, screened, or other-
wise processed to remove the noncom-
bustibles. In this fluffy form, it can be
pneumatically fed into the suspension
utility boiler. For the smaller, stoker-fed
boilers, however, a densified form of
RDF is used.
Densification imparts to RDF many
attractive features in terms of combus-
tion characteristics, not the least of
which are compatibility with other
(conventional) fuels in co-firing and
efficiency of transport. A growing
awareness of these characteristics is
expanding the potential for using
densified RDF (d-RDF) as an energy
source. Before this potential can be fully
realized, however, insight into the
relatively complex densification process
must be acquired. This acquisition is
difficult because the machines currently
used to produce d-RDF were originally
designed to densify much different
materials, e.g., animal feedstocks and a
number of agricultural products.
The limited field experience with the
densification of RDF has not been
entirely satisfactory, e.g., the actual
output from a pellet mill typically falls
short of its rated output; dies, rollers,
and other moving parts show signs of
excessive wear; and the machines are
difficult to feed and easily jammed.
Because little information was available
on the production and burning of d-RDF,
EPA implemented parallel programs to
(1) determine the engineering and
economic aspects of preparing d-RDF
and (2) assess the technical and
environmental implications of using d-
RDF as a coal substitute. In addition to
the report summarized here, the follow-
ing reports have been prepared as part
of these programs: "Densification of
Refuse-Derived Fuels: Preparation,
Properties, and Systems for Small
Communities", "Coal: d-RDF Demon-
stration Tests in an Industrial Spreader
Stoker Boiler"; and "A Field Test Using
Coal: d-RDF Blends in Spreader Stoker-
Fired Boilers."
Pellet Mill Studies
Procedure
To gain insight into the physics of the
densification of RDF, i.e., of the so-
called "light" fraction of municipal solid
waste (MSW), two interrelated courses
of action were persued: one in which
pellets were produced by means of a
single die under laboratory conditions
and the other in which a commercial
densification machine was used.
The processing began with the
delivery of a packer-truck load of MSW
to the processing facility. The MSW was
then processed through shredding, air
classification, and trommel screening
unit operations. A model 48-4 horizontal
Gruendler swing hammermill was used
for size reduction; a vertical-type,
straight, rectangular column device was
used for air classification. A California
Pellet Mill* was installed in the light
fraction processing line. By suitably
arranging the conveyor system, the
densifier could be fed either with the
air-classified light fraction or with the
screened light fraction. Three sizes of
pellets were produced by interchanging
the dies, which had different length-to-
diameter ratios (1/2 by 4 in., 3/4 by 3
in., and 1 by 5 in.). The effect of die
configuration and feed material on the
net power and specific energy con-
sumption were evaluated. Measurements
were made of power consumption,
throughput, pellet density, and moisture
content. A portable, power-measuring
apparatus determined mill power con-
sumption. The free-wheeling power of
the mill was subtracted from its gross
power consumption to determine the
net power consumption.
Results
The general nature of the results from
this series of exploratory tests indicated
that net power and net specific energy
consumption are a function of the mill
throughput. For certain situations, it
was found that the basic relationship
between energy and throughput was
also parametic with pellet density, and
•Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
in some cases, with both pellet density
and moisture content.
The specific energy data for the
screened light fraction processed
through a 0.75- by 3.0-in. die indicate a
discernable dependence upon density.
In attempts made to describe the
relation of net power to mill throughput
analytically in terms of either exponen-
tial or power law relations, a power law
relation of the type
En = aMD"
provided the best analytical description.
The coefficient, n, and exponent, b, vary
with density. In this case, the mill
throughput was evaluated on a net
basis. The moisture content of the feed
material was on the order of 15 percent.
Laboratory Densification
Studies
Procedure
A special die arrangement was
constructed for formating pellets in a
single die. The basic assembly consisted
of a rod, container, die, and support
plate. The dies used had inside diameters,
of 1 /2, 5/8, and 3/4 in. In addition to
straight dies, taper angles of 2.4, 7.1
and 14 degrees on the radius were
evaluated for the 3/4-in. die. Tapers of 0
(straight die) and 14 degrees were
evaluated for the 1.2- and 5/8-in.
diameter dies. The assembly was
mounted in a standard laboratory Tinius
Olsen Universal Testing Machine. Its
motion, i.e., the distance the die is
pressed into the container, is measured
with a deflectometer. A graph of force
versus deflection is generated on a high
magnification recorder.
In experiments concerned with the
effects of temperature and moisture
content on the densification process,
the moisture content of the material
was varied by adding a measured
amount of water to air-dried material.
To evaluate the effect of temperature, it
was necessary to heat the material
while it was in the die and container. In
commercial applications, the material
most likely would be heated in a feeding
chamber arrangement before being
introduced into the die.
To determine die friction, a rod,
slightly smaller in diameter than the
internal diameter of the die, was used to
push the material through the preloaded
die. Essentially, the rod replaced the
container in the test apparatus.
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Results
The force-compression curve (Figure
1) is an example of the actual plots made
by a recorder controlled by the Tinius-
Olsen compression tester and deflecto-
meter. Each of the four curves in the
figure represent results obtained with
one of four pellets. Essentially, each
curve is a plot of the cortipressive force
(in pounds) versus the distance to which
the die is pressed into the container.
Each division on the horizontal axis
corresponds to 0.4 in. of compression
(or crosshead motion).
The curves form four distinct regions
of interest:
• In region 1, the loose material is
compressed to form a compact
cylinder in the container. The force
in this region rises exponentially as
the material is compcessed.
• .In region 2, the material begins to
move through the die. The onset of
motion results in a precipitous drop
in force that correspondes to the
transition from static to kinetic
friction.
• In region 3, the pellet is extruded
from the die. The force fluctuations
are attributable to nonhomogeneity
of the material.
• I n region 4, the force surges rapidly
as the amount of material remain-
ing in the container approaches
zero. The surge occurred consis-
tently whenever the length of the
material remaining in the container
was less than about 1 /4 in.
Within the range of conditions pre-
vailing or applied in the experiments,
flow rate and temperature did not
significantly influence the deformation
pressure. The influence of moisture
content is somewhat analogous to that
observed in soil mechanics. Essentially,
as the moisture content increases, the
material becomes less viscous. As a
result it becomes easier to deform and,
in effect, easier to push through the die.
The addition of moisture does not,
however, serve to lubricate the die
walls.
No difference could be discerned
between the strength of pellets formed
in the 0° and those formed in 7° taper
dies. On the other hand, pellets formed
with the 14° die were consistently
better in quality than were those formed
with the 0° and the 7° dies. Even though
the pellets produced with the 14° die
had a lower density, they were in effect
better formed because the material was
4000
2000-
Figure J.
1.0 2.0
Extrusion (in.)
Typical force versus extrusion curves for single die studies.
3.0
interwoven such that the pellets could
not easily be broken.
At the lower moisture content, the
outside surface of the pellet tended to be
flaky. Increasing the moisture content
brought about the formation of a
smoother surface. As the moisture
content began to exceed 20 percent, the
surface again tended to flake and the
pellet became weak and spongy.
Conclusions
Pellet Mill Tests
1) The specific net energy of pelletiza-
tion can be related to mass through-
put by the equation
En = aMDb
where a and b depend on die and
feed characteristics.
2) Over a throughput range of 0.05 to
1.3 metric tons per hour (MTPH) the
specific energy requirements de-
crease from 180 to 8 kWh/metric
ton.
3) The maximum throughput capacity
of the pellet mill (rated at 2 MTPH) for
a characteristic feed size of 1.0 cm is
0.8 MTPH. The capacity decreases
almost linearly to a value of 0.35
MTPH for 2.0 cm feed.
4) With pellets formed of screened light
fraction material that has been
passed through a shredder equipped
with grates having 1 /2-in. openings,
and then desified in a 1 /2-in. x 4-in.
die, the density of the pellets falls
from 1.31 to 1.12 g/cc as the
moisture content is increased from
13.6 percent to 26.4 percent,
Laboratory Densification
Studies
Trends exhibited in a single die
studies must be considered in light of
the basic differences between the
procedure used and a full scale densi-
fication process. With this precaution in
mind, the following conclusions can be
drawn:
1) Energy is required in the pallati-
zation process to accomplish
three distinct functions: precom-
press the loose feed, deform the
feed as it enters the die, and
balance the die frictional force as
the pellet passes through the die.
2) The frictional force can be related
to die length and diameter by the
equation
F=F0exp4_£_L
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3) Static friction necessitates apply-
ing a greater force to begin the
flow of a pellet than is needed to
maintain the flow. Thus, stopping
and starting the flow results in an
increase in energy requirements.
4) The force required for deformation
rises dramatically when the
thickness of feed above the die
inlet is less than one-half of the
die diameter. This phenomenon
may be attributed to changes in
the orientation of material flow
lines, and it might significantly
contribute to the wear, encoun-
tered in a commercial pellet mill
as well as to the energy require-
ments.
5) Although the pressure required
for pellet deformation increases
with extent of deformation, the
classical model for homogenous
crystaline materials, PD = Y In
(A/a) does not consistently and
accurately predict the magnitude
of this pressure.
6) A 1 percent increase in the
moisture content of the feed
decreases the deformation pres-
sure by approximately 300 psi.
However, if the moisture content
exceeds 20 percent, a very weak
pellet is formed.
7) Increasing the proportion of
newsprint in the feed is accom-
panied by a dramatic rise in
pressure.
8) Variations in feed and die tem-
peratures apparently exert no
effect on extrusion pressures.
9) Pellets formed by a die having an
inlet taper of 14° consistently
have a higher quality and a lower
deformation pressure than do
those formed by dies having a 0°,
2°, or 7° taper.
10) Typically, the specific energy
consumed in deformation and
friction at a moisture content of
15 percent is 14.4 kWh/MT.
The full report was submitted in
fulfillment of Grant No. R-805414-010
by the University of California, Berkeley,
CA, under the sponsorship of the U.S.
Environmental Protection Agency.
The EPA author Edward Winter is with the Municipal Environmental Research
Laboratory. Cincinnati. OH 45268.
Car/ton C. Wiles is the EPA Project Officer (see below).
The complete report, entitled "Fundamental Considerations for Preparing
Densified Refuse-Derived Fuel," authored by G. J. Trezek, G. M. Savage, and
D. B. Jones of the University of California, Department of Mechanical Engi-
neering, Berkeley. CA 94607 (Order No. PB 82-101 668; Cost: $8.00, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
U.S. GOVERNMENT PRINTING OFFICE, 1981 — 559-017/7379
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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