Note: Descriptions are shown in the official language in which they were submitted.
Descri~tion
FUEL PELLETS
Technical _Field
Due to diminishing quantities of coal, petroleum,
and natural gas products, attention is being directed
~1
to other energy sources, including oil shale, solar
energy, and nuclear energy. One source which is
5 receiving considerable attention is biomass materials
such as wood ancl its byproducts. ~his i~ somewhat
ironic since the original source for energy in the
United States and the rest of the ~orld was wood. In
fact, U~S. Pa'cent 43,112 issued on June 14, 1864 was
10 directed to combining sawdust, tar, wood cuttings or
chippings, water, and coal -tar to form an artificial
uel.
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Background_Art
Recently, much attention has been directed to
preparing briquets from wood waste. For example, L. H.
Reineke wrote a U.S~ Forest Research ~ote entitled
"Briquets from Wood Residue'l, in November, 1964 des-
cribing various techniques available or briquetting
20 wood residue. In addition, U.S. Patent Numbers
3,227,530, 3,635,684, 3,843,336, 4,015~951 and
4r043~764 describes techniques or bricIuetting
cellulosic material.
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Use of available pelletized wood waste as a fuel
source has achie~ed only limited acceptance to date.
One reason for this is the relatively low heating value
o pelletized wood as compared to coal. Pelletized
wood can have a heating value of les~ than 7, aoo BTUIs
per pound, while coal generally has heating value in
excess of 9,000 BTU's per pound. Therefore, the
transportation and handling costs associated with
~ available pelletized wood are higher per BTU than for
; 10 coal.
Other problems with use of available pelletized
wood as a fuel source is that it has a slow burniny
rate and it exhibits incomplete burnout, resulting in
formation of carbonaceous residues and low combustion
efficiency. In addition, pellètized wood can be harder
to ignite than coal and pelletized wood can be more
fragile than coal, requiring special handlin~ to avoid
crumbling and to prevent weathering. To overcome the
crumbling and weathering problems, inorganic binders
such as cement and silicate of soda, and organic
binders such as tar, pitch, rosin, glues, and fibers
have been included in the pellets. However, no
binder has been found which solves the above problems,
and which also is inexpensive and does not reduce the
heati~g ~alue of the wood.
It has been attempted to use the self-binding
characteristics of various species of wood due to
lignin present to avoid the crumbling problem~ This
can be effected with some species of wood, but not all
species, by heating the wood above its minimum plastic
tempera~ure of 325 F as reported by Reineke in the
above-mentioned U.S. Forest Service Research Note, and
also as reported by Gunnerman in the above~mentiorled
Patent No. 4,015,951. However, such high ~emperatures
can severely limit the operating life of the pelletiz-
ing equipment and drive high BTU volatile components
from the wood.
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; Therefore, there is a need for a fuel pellet which
resists crumbling, is easily ignitable, burns fast and
completely, and has a heating value approaching that
of coal; and there is also a need for a method for
5 preparing the fuel pellet which does not require high
pelletizing temperature.
Disclosure of Invention
According to the invention, there is provided a
fuel pellet which comprises fxom about 50 to about 99~
by weight natural cellulosic material, and from about
1 to about 50% by weight synthetic polymeric thermo-
; plastic material, the synthetic thermoplastic being
substantially uniformly distributed throughout the fuel
pellet, wherein the thermoplastic material is chosen soit is solid at room temperature and has an injection
molding temperature of at least 200F.
Further according to the inventionr a fuel pellet
comprises an intimate mixture of at least 50~ by
weight material cellulosic material and suffi~ient
synthetic thermoplastic material that the pellet has a
substantially hydrophobic surface.
The thermoplastic material serves to bind the
pellet together, increases the heating value of the
pellet, lubricates the pelletizing die, and improves
the ignition and burning characteristics of the pellet.
Fuel pellets of the present invention exhibit complete
burnout, burn faster than pellets not containing
thermoplastic material, and can have a heating value
in excess of 9,000 BTU's per pound. Preferablyr
the thermoplastic material is uniformly distributed
throughout the fuel pellet.
Also according to the invention, a method for
preparing a fuel from particulate natural cellulosic
material and particulate synthetic polymeric thermo-
plas~ic material comprises the steps of:
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(a) providing particulate natural cellulosic
material having a free moisture content of from about
5~ to about 15% by weight;
(b) providing particulate synthetic polymeric
thermoplastic material which is solid at room tempera-
ture and has an injection molding temperature of at
least 200F, substantially all o~ the particulate
; thermoplastic material being minus 5 mesh;
(c) preparing a feed comprising from about 50~ to
about 99% by weight of the particulate cellulosic
material and from about 1% to about 50~ by weight of
the particulate thermoplastic material; and
(d) forming a fuel pellet by compressing the feed
; at a pressure whereby the temperature of the compressed
feed at the release of compression is from about 150
to about 250F.
The fuel pellet can be made by preparing a ~eed
comprising from about 50% to about 99% weight of
particulate natural cellu~osic material and from about
1% to about 50% by weight of particulate synthetic
thermoplastic material. The cellulosic material
has a free moisture content of from about 5 to about
15% by weight, and preferably substantially all of the
cellulosic material is -5 mesh. Substantially all of
the thermoplastic material is -5 mesh, and preferably
-10 mesh. The plastic and cellulosic material are
intimately combined by compressing the feed in a
die.
Drawings
: These and other features, aspects and advantages
of the present invention will become more apparent upon
consideration of the following description, appended
claims, and accompanying drawings where:
Fig. 1 illustrates in a prespective view a pellet
representative of pellets prepared according to the
present invention; and
5¢~
Figs. ~A and 2B illustrate a process embodying
feature of the process of the present invention. These
two figures are to be considered serially.
Description
With reference to Fig. 1, there is shown a fuel
pellet 10 prepared from cellulosic material and thermo-
plastic material. Fuel pellet 10, which is cylindrical
in shape, has a minimum dimension of at least 3/16 inch
and comprises from about 50 to about 99% by weight
natural cellulosic material and from about 1 to about
50% by weight thermoplastic material. As is more
fully set forth below, these fuel pellets are easily
ignitable, burn evenly, quickly and completely, resist
weathering, and generally ha~7e a gross heating value in
excess of 9,000 BTU's per pound, and can have a gross
heating value in excess of 10,000 BTU's per pound.
The natural cellulosic material used to form the
pellets 10 can be particulate woody material such as
sawdust, wood shavings, sander's dust, hog fuel, peat,
` and bark. Agricultural waste such as banana and papaya
stalks, straw, bamboo, jute, bagasse, corn husks, corn
cobs, cotton "gin trash", sisal, seed hulls, and peanut
hulls can also be used. Also, paper and cardboard can
be included in the pellets. Combinations of the above
natural cellulosic materials can also be used.
Preferred natural cellulosic materials are these with
low moisture content to minimize drying costs and low
contamination levels to minimize pelletizer die wear.
As used herein, the term "cellu~osic material" includes
lignin.
Particulate wood material preferaby is used in the
pellets because it has a higher heating value and lower
moisture content than agricultural waste. Inclusion of
banana and/or papaya stalks in the pellets is desirable
because banana and papaya latex are good binding agents
and contribute to the cohesiveness of the pellets.
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The syn-thetic thermoplastic material can be
practically any available synthetic thermoplastic such
as, but not limited to, polys~yrene, polyethylene,
polypropylene, acrylonitrile-butadiene styrene, acetal
5 copolymer, acetal homopolymer, acrylic, polybutylene,
and combinations thereof. Although thermoplastics
containing a halogen such as polyvinylchloride can be
used, for most applications these are to be avoided,
because of corrosion and emission problems associated
10 wi th the combustion products of halogen-containing
thermopastics. It has been noted that for fast burning
and ease of ignition of the fuel pellets, polypropy-
lene and polyethylene are the preferred synthetic
thermoplastic materials.
The term "synthetic thermoplastic materials"
excludes naturally occurring thermoplastic materials
and naturally occurring cellulosic materials. For ease
of handling, the synthetic thermoplastic material must
be solid at room temperature. Pre~erably the synthetic
20 thermoplastic material has an injection molding tem-
perature of at least 200 F. The minimum injection
molding temperature of common thermoplastics as
reported in Modern Plas_lcs_Encyclopedia, Vol. 49,
I!~cGraw-Hill, 1972-3 Edition, is presented in Table
25 1.
TABLE 1
Minimum Injection
S~hetic ThermoplasticMolding Temperature (F.)
________
Polystyrene 325
Polye~hylene 250
Polypropylene 375
ABS 360
Cellulosics 335
Nylon 360
Polyesters 270
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It has been found difficult to pelletize a feed
containing more than about 1.25% by weight high impact
polystyrene. It was noted that pelletizer production
rate decreased with such a feed and it was dificult to
thoroughly disperse the high impact polystyrene in the
pellets. Thereforet when the pellets include high
impact polystyrene, it is preferred that feed to a
pelletizer contains only up to about 1.25~ by weight
high impact polystyrene. It is desirable to include
polystyrene in the pelletizer feed because it has been
found that polystyrene contributes greatly to the
cohesiveness of the fuel pellets. Such cohesiveness is
important because it is undesirable for the pellets to
break or disintegrate during handling and storage.
Such breakage and disintegraton can produce fines and
dust, which can be a serious fire and explosion
hazard.
It is critical to the present invention that at
least 1% by weight thermoplastic material be included
~0 in the fuel peIlets~ This is because fuel pellets
containing thermoplastic material have many significan~
advantages compared to fuel pellets containing only
cellulosic material. For example, inclusion of
thermoplastic material in fuel pellets allows the fuel
25 pellets to be formed easily in a pelletizer at temper-
atures lower than temperatures required for forming a
fuel pellet with only cellulosic material. Thus, the
thermoplastic material serves as a processing aid for
forming pellets from the cellulosic material. In
30 addition, the thermoplastic material has a higher
heating value than the cellulosic material, and the
resulting pellets have a correspondingly high heating
value.
Another advantage of the presence of synthetic
35 thermoplastic material in fuel pellets is that the
thermoplastic material provides a substantially water-
impervious coating, or sheath on the outside of the
pellets, thereby both preventing uptake of moisture by
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the pellets and resisting weathering in storage.
Becuase of the uniform distribution of the plastic in
the pellets, there is plastic even at the ends of a
cylindrical pellet. This also prevents uptake of water
by ~he pellets. Furthermore, the hydrophobic nature of
the plastic prevents water uptalce. Pellets of the
present invention have been left out overnight in the
rain and still maintained their cohesiveness, while
conventional wood pellets tend to disintegrate when
wet.
A portion of the thermoplastic material can be in
the fuel pellets in the form of discrete subparticles,
although it is preferred that the thermoplastic
material be substantially uniformly distributed
throughout the particles. The presence of discrete
thermoplastic subparticles in fuel pellets results in
easy ignition because the discrete subparticles
provide an ignition situs.
Surprisingly, it has been found that the fuel
pellets exhibit burning and ignition characteristics
which are superior to the burning and ignition charac-
teristics of both the cellulosic material and thermo-
plastic material which make up the fuel pelletsO The
fuel pellets are a new composition of matter For
example, burning tests were conducted with (1) conven-
tional fuel pellets made only with sawdust, (~) poly-
propylene, and (3) fuel pellets according to the
present invention prepared with 91% by weight sawdust
(different from the sawdust used for the all sawdust
pellets~ and 9% by weight polypropylene. The all
sawdust fuel pellets burned at a rate equal to about
1/2 the rate of the fuel pellets of the present inven
tion. The two types of fuel pellets were about the
same size, but it should be noted that the all sawdust
fuel pellets were denser than the sawdust/polypropylene
fuel pellets, but this accounts for only part of the
difference in burning rate. Therefore, in a boiler of
a fixed size, the fuel pellets of the present invention
can be used to generate heat and steam at a faster rate
than conventional fuel pellets. In addition, the
sawdust/polypropylene fuel pellets left practically no
residue, while the conventional fuel pellets left a
carbonaceous residue. Furthermore, the uel pellets
consisting only of thermoplastic material did not burn
completely, but kept self-extinguishing. This was not
a problem with the fuel pellets of the present inven-
tion. Therefore, fuel pellets prepared from cellulosic
material and plastic material burn better than either
the cellulosic material alone or the plastic material
alone.
It is believed that thermoplastic material in fuel
pellets acts as a binder or the cellulosic materials,
and in fact is just about the perfec~ binder. This is
because the C05t of the thermoplastic materials is
minimal, since much scrap and waste plastic is rea~ily
available. Pellets containing at least 5~ by weight
thermoplastic material have been demonstrated to have
sufficient toughness to withstand exposure to the
shocks of transportationl storage, and stoking. When a
pellet includes thermoplastic material, crumbling and
excessive softening from weathering are avoided.
Furthermore, thermoplastic materials typically have a
higher heating value than cellulosic material.
The pellets should contain at least 1% by weight of the
thermoplastic material, and more preferably at least
; about 2.5~ by weight, to obtain these advantages.
The fuel pellets can contain as much as 50~ by
weight thermoplastic material to maximize their heating
value. However, preferably the pellets contain no more
than about 2S~ by weight thermoplastic,and more prefer-
ably no more than about 10% by weight, because
thermopla~tic material is more expensive than waste
cellulosic material. In addition, if the pellets
contain more than about ~5~ by weight thermoplastic
material, the burning properties of the pellets can be
adversely affected.
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Therefore, the pellets of the present invention
comprise from about 50 to about 99 ~ by weight cellu-
losic material, and from about 1 to about 50% by weight
thermoplastic material. Preferably, the fuel pellets
contain from about 1 to about 25~ by weight thermo-
plastic material, and more preferably from about 2.5
to about 10~ by weight thermoplastic material.
Materials other than natural cellulosic material
and synthetic termoplastic materials can be included in
the pellets. For example, materials such as comminuted
tires, thermosetting resins and/or petroleum distilla-
tion residue can be added to improve the heating value
of the pellets.
Oxidizing agents such as sodium perchlorate and
ammonium nitrate to facilitate combustion can also be
included in the pellets. Also, binding agents in
addition to thermoplastic materials can be used.
Exemplary of such binding agents are paraffin slack
wax, carnuba wax, and lignosulfonates, such as ammonium
lignosulfonate, sodium lignosulfonate, calcium ligno-
sulfonate, and magnesium lignosulfonate.
Certain cellulosic materials can be added to the
pellets as a pelletizing or processing aid. Preferred
materials in this category are oil seeds and their
25 products r which by their fatty acid content reduce wear
on the dies of the pelletizing equipment~ Exemplary of
such materials which can be included are coconut
husks, soy beans, peanuts, sunflower seeds, corn cake,
pressing residuals, and the like.
As used herein, the term "pellet" refers to a
discrete particle of any size or shape which contains
both natural cellulosic material and synthetic thermo-
plastic material. The pellet need not be symmetrical,
but it is preferred that the pel}et 10 be substantially
3s symmetrical in shape such as cylindrical, parallel-
piped or the like, having a diameter within the range
of from about 3/16 inch to about 1 inch. While it is
most practical to form the pellets in a cylindrical
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shape, the pellets can be in any suitable symmetrical
configuration such as the shape of a cube~ Pellets
have been produced which are cylindrical in shape,
such as the pellet shown in Fig. 1, having a length of
S about 1 inch and a diameter of about 3/8 inch. For
such a pellet, the "minimum dimension" of the pellet is
the diameter, i. e . 3/8 inch.
The larger the diameter of the particles, the
slower their burning rate. This is because of the fact
that as the diameter increases, the surface area to
volume ratio of the particles decreases. Depending
upon the flame temperature and burning rate required in
any given boiler, the optimum feed diameter for that
boiler can vary within tbe range of about 3/16 inch to
about 1 inch.
It is necessary that the particulate cellulosic
feed and particulate synthetic thermoplastic feed have
a maximum particle size less than about 60~ of the
; minimum dimension of the pellet to avoid crumbling of
the pellet in storage. For example, if the pellet is
cylindrical and has a diameter of 1/4 inch, then the
cellulosic feed and thermoplastic feed should have a
maximum particle size o about 0.15 inch (0.6 x 0.25),
i.e. about 5 mesh.
25The bulk density of ~he particles can vary in the
range of from about 30 to about 40 pounds per cubic
foot. It has been found that pellets 1 inch long and
1/4 inch in diameter made from aobut 90~ sawdust and
about 10% polyethylene thermoplastic can have a bulk
density of about 38 pounds per cubic foot.
A process for preparing fuel pellets is shown
schematically in Figs. 2A and 2B. Cellulosic feed
material, plastic feed particles, and plastic feed
sheet are delivered by trucks (not shown) and stored in
storage bins 20a, 20b, and 20c, respectively. Addi-
tional feed storage bins can be provic3ed for segragat~
ing different types of feed. The feed, either before
or after introduction into the feed bins, can be
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treated to separate foreign materials such as metallic
impurities and soil. This can be done by means of such
equipment as pneumatic conveyers, screen, magnets, and
combinations therof. Magnets conventionally are built
into the e~uipment, described below, used for comminu-
ting the feed materials. The feed from the cellulosic
feed storage bin 20a is transferred via a belt conveyer
24a to a classifying device such as a vibxating screen
26 to separate oversize particles 28 from particles 30
which are suitable for direct feed to a pelletizing
operation. The size of the holes in the screen depend
upon the size of the pellets ~o be made, but in any
case~ the size of the holes is necessarily smaller
than the mininum dimension of the pellets. For exam-
ple, if cylindrical pellets having a diameter of 3/16inch are to be made, then the size of the holes in the
screen is necessarily less than 3/~6 of an inch. In
the version of the process shown in Fig. 1, the screen
segregates particles greater than 1/8 inch in diameter,
and passes these particles to a comminution device
such as a hammermill 32.
In the communition device, the feed cellulosic
material is comminuted to a desired particle size. As
used herein, the term "comminution" refers to any
physical act of size reduction, including, but not
limited to chopping, crushing, and grinding by suitable
machinery. There are at least three types of machines
useful for reducing the size of wood. Veneer and
comparable fine scrap can be reduced to chips in a
hammmermill, in which rotating bars of various designs
break up the material by impact. A disk chipper can be
used for solid scrap and round wood of various sizes.
This chipper has knives set in radial slots. A knife
hog is similar in action to the chipper, but the
3s knives are set in the sloping surfaces of a V shaped
drum. The kni~e is suitable for solid wood and for
scraps that may be somewhat smaller than the disk
chipper can handle. Preferred comminution equipment
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for the cellulosic waste is a hammermill sold by
American Pulverizer Company of St. Louis, Missouri
under the tradename American Swing Hammer.
In general, preferably the comminution device is
operated so that substantially all of the cellulosic
feed is comminuted to -5 mesh and at least 50% by
weight is -10 mesh.
Exemplary of the opera~ion of the hammermill 32 is
comminuting cellulosic feed for making cylindrical
pellets having a diame~er of 3/8 inch and cylindrical
pellets having a diameter of 1/4 inch. For pellets
having a diameter of 3/8 inch, preferably all of the
particles are -5 mesh, and at least 50% of the parti-
cles are -10 mesh~ If the pellets have a diameter of
1/4 inch, then prefereably all of the cellulosic
material is comminuted to -10 mesh. PreEerably, the
comminuting equipment is operated so that substantially
all of the particulate cellulosic material has a
particle size greater than about 30 mesh. This is to
avoid the presence of fines and dust in the feed to the
pelletizer, and the explosion hazard associated with
such small particles of cellulosic material.
The par~icles 30 not requiring comminution and the
comminuted particles 34 from the hammermill 32 are
collected on a belt conveyor 36 and passed via ducts 37
to two rotary driers 38 in parallel to reduce the
moisture content of the cellulosic material. To
develop the necessary strength and hardness in the
pellets, it is essential that the free moisture content
30 o~ the cellulosic material be reduced ~o less than
about 15% by weight. By "free moisture" there is meant
moisture which can be removed by evaporation at normal
temperatures and does not include any bound water
such as chemically bound water that might be present in
35 the feed material. Various types of dryers such as
steam-heater plates, and dry steam pipes over which the
feed is cascaded can be used to bring the Eeed to the
desired moisture content. Flash dryers using a short
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exposure to hot gases can be used. The heat from
drying can be provided by burning the fuel pellets
and/or fines produced by this process in a heater 40
which supplies hot gas via ducts 41 to the driers.
5When the free moisture content of the cellulosic
material is reduced to less than about 5% by weight,
the pellets upon discharge from the pelletizer burst
and demonstrate a "Christmas tree" effect. These
pellets are unsatisfactory because they tend to form
fines in storage and handling. This problem can be
overcome by introducing steam, as necessry, at the
pelletizer. However, it is undesirable to remove
moisture from the cellulosic feed in a drier, thereby
expending energy for thîs purpose, only to put the
moisture back into the feed at the pelletizer.
Therefore, it is preferred that the free moisture
content of the cellulosic material be reduced to no
less than about 5% by weight. In summary then, prefer-
ably the driers reduce the moisture content of the feed
; 20 to about 5% to a~out 15% by weight, the same as re-
quired for feed to the pelletizer.
For high production rates from a pelleti~er, and
for production of pellets which e~hibit excellent
cohesiveness and high strength, preferably the free
moi~ture content of the feed to the pelletizer is from
about 8% to about 12~ by weight, and most preferably
about 10% by weight.
To aid in drying the cellulosic feed material, dry
slaked lime, i.e. cacium carbonate, can be combined
with the dryer feed. The calcium carbonate combines
with water of the feed material and then releases
moisture more easily in the dryer, thereby aiding more
rapid drying of the feed material. Use of calcium
carbonate in an amount of from about 2 to about 10%
by weight of the feed, and preferably in an amount of
about 5% by weight, significantly aids in the drying
process. The preferred grade of calcium carbonate is a
; fine grade having a particle size of less than 100
5~5~
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mesh. When this drying technique is used, the product
fuel pellets contain at least 1% by weight calcium.
It is believed that to make good pellets with
bark, it is necessary to first comminute the bark, then
5 dry the comminuted bark and then comminute the dried
bark one more time before feed to the pelletizer. This
is because raw bark is usually available only as large
particles which are difficult to dry efficiently.
Water can be removed from the eed material
~lO upstream of the driers when the feed material contains
-gross quantities of water For example, water can be
removed from peats, bark, or sawdust with presses that
operate on the roller or clothes-wringer principle.
Screw presses, using tapered screws, are also useful
15 for dewatering of bark. The drying operation can be
run as a batch operation to avoid the expense of
duplicating drying, cooling and cvnveying equipment for
different cellulosic feed materials.
The gases and water evolved in the driers 38 are
20 withdrawn from the driers via lines 42 into two cy-
clones 44 in parallel, one for each drier, by an
exhaust fan 46. The discharge from the fan 46 can be
passed to a dust collector (not shown) or passed
directly to the atmosphere. Particulate matter with-
25 drawn via line 42 is separated in the cyclones 44 anddropped into a fines bin 47. The particulate matter in
bin 47 is fed by a rotary valve 48 to a fines bin 77
(Fig.2A). The dried feed material is transferred by a
storage bin tank feed conveyor 49 to one or more
30 storage bins 52a or 52b (Fig. 2B). The different
storage bins are used for storing different types of
feed material. More storage bins than the two storage
bins shown in Fig. 2A can be used. The storage bins
52a and S2B preferably are tumbled bins to avoid
35 compaction of the feed material and to maintain dehy-
;dration of the feed. A rotary cooler (not shown) using
ambient air to cool the material discharged by the
drier can be used if required to avoid caking of the
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feed material in storage.
The plastic feed is passed from the plastic feed
bins 20b and 20c via belt conveyor 24b and 24c, respec-
tively, to comminution devices such as granulators 56a
S and 56b respectively. The smaller the particle size of
the thermoplastic feed, the stronger the fuel pellets
and the more even and uniform their burning character-
istics, and the less plastic required in the fuel
pellets. In addition, when the pellets are to be
pulverized before burning, it is important that the
plastic be comminuted to a small size so that each
particle resulting from the pulverization contains
both plastic and cellulosic materialO Therefore, the
granulators are operated so that substantially all of
the particulate thermoplastic material is minus 5 mesh.
Preferably, the bulk7 i.e.,at least 50~ by weight of
` the particulate thermoplastic material is minus 10
mesh, and more preferably substantially all is minus 10
mes`h. It is believed that optimumly substantially all
of the plastic is -20 mesh. The comminuted plastic
feed discharged by the granulators 56a and 56b passes
to belts 57a and 57b, respectively, for transport to
plastic feed storage bins 52c and 52d, respectively.
More than two plastic storage bins can be used if
required.
Each of the storage bins has associated with it a
weigh belt conveyor 62a, 62b, 62c, or 62d. The four
conveyors 62a, 62b, 62c, and 62d are u~ed to provide
the proper weight ratios of the feed materials to a
pellet mill 70. The four con~eyors drop their feed
onto a belt conveyor 64 which carries it to a chamber
65 for preheating of the feed with dry steam, if
desired. From the chamber 65 the feed passes into a
mixer 66 such as a comblnation mill to obtain uniform
mixing of the different types of feed material. The
mixer discharges mixed ~eed onto a belt conveyor
67 which lifts the feed to a pellet mill feed bin 68.
The feed is gravity fed from the bin 68 to a conveyor
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69 which drops the feed into the pellet mill 70 in
which the pellets of the present invention, such as a
pellet shown in Fig. 1, are formed. Any suitable
pelletizing machine can be used such as, for example,
the one produced by the California Pellet Mill Company
of San Francisco, California or the mill produced by
Koppers Sprout-Walden Company. In this apparatus, the
material is fed into a hopper and pressed into dies
having the desired configuration and shape.
10The pellet mill must be capable of producing a
pressure in the die during compression which causes the
temperature of the feed material to increase so that
the pellets have a temperature of from about 150 to
~; about 250F where they are discharged from the pelle~
mill, i.e. where the pressure is released. When
the discharged pellets are at a temperature in excess
of about 250F, degradation and carbonization of the
thermoplastic material can occur, and when the dis-
charged pellets are at a temperature of less than aobut
150F, the pellets can ha~e insufficient cohesiveness.
Preferably, the discharge temperature of the pellets is
from about 190 to about 210F to produce pellets with
excellent burning properties and good cohesion. As the
discharge temperature of the pellets increases, their
density increases. For example, pellets conta:ining 5%
by weight polyethylene and 95~ by weight sawdust had a
density of 31 pounds per cubic foot when discharged
from a pelletizer at 190F, and a density of 34 pounds
per cubic foot wben discharged from tbe pelletizer at a
temperature of 199F.
Supplemental heat and moisture for the pellet mill
70 can be provided by steam 71 which can be generated
in a boiler 72 fueled by pellets produced by this
process or reject fines. The steam can be used for
drying the feed in the dryers 38.
California pellet mills produce a high pressure
at the impact point of the rollers to produce the
desired temperature during pelletizing. A portion of
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-18-
the thermoplastic material forms a surface skin on the
pellet at these temperatures. This skin protects the
pellets from shattering and from significant changes in
moisture content.
The temperatures and pressures required for making
these pellets are substantially less than those des-
cribed in U.S. Patent No. 4,015,951 lssued ~o Gunner
man. Therefore, it is expected that the pelletizing
equipment used for forming pellets in this process will
have a substantially longer life and require sub~tan-
tially less maintenance and fewer replacement dies than
pelletizing equipment used according to the method
described by Gunnerman.
Before introducing the feed to the pelletizer~ it
can be combined with a binding agent such as an a~ueous
solution of sodium silicate. For example, the material
can be sprayed with about 5% by weight based on the
total feed of 40 Baume' alkali stabilized sodium
silicate solution added to the mixer 66. During the
drying step, the moisture content needs to be adjusted
to compensate for the water added by spraying with
the silicate solution. It is believed that destabil-
ized alkali sodium silicate solubilizes lignin of the
cellulosic feed and the lignin then polymerizes,
resulting in a stronger pellet.
From the pellet mill, the formed pellets are
cooled in a cooler 72 by ambient air supplied ~y a
blower 73, and transferred to a screen 74 for separa-
tion of any fines 75 which are carried by a conveyor 76
to a ines storage bin 77. The fines are transferred
from ~he storage bin 77 by a rotary valve 78 and a
blower 79 for feed to the boiler 72 used to generate
steam for the pellet mill. The product pellets 80 can
be sent to storage, bagged, or transf0rred to trucks or
railroad cars or shipment.
The pellets prepared according to this process
exhibit high heating value, which can be in excess of
10,000 B~U's per pound, are easily ignitable, burn
, - .
r
5~
--19--
rapidly, resist weathering, and are easy to s~ore and
handle. They are moisture resistant, and produce very
little ash and essentially no oxides of sulfur on
burning. Therefore, they are a premium fuel which in
many respects is superior to coal. In general, because
of the plastic, fuel pellets of the present invention
have a heating value of over l~000 BTU~s per pound
yreater than the heating value of the cellulosic
material in the pelle~ by itself Also, by varying
the amount of thermoplastic in the fuel pellet, the
heating value of the pellet can be tailor made for a
customer's re~uirements. Pellets with a heating value
of 15,000 BTU per pound have been made.
The fuel of the present invention can be com-
minuted before it is burned. Such comminution raisesthe flame temperature when the fuel is burned. Flame
; temperatures in excess of about 2500F can be achieved,
- particularly when less than about 150~ stoichiometric
; air is used. Such high temperatures are valuable
~ 20 because they allow the uel pellets of the present
; invention to be burned in applications requiring high
temperature such as the manufacture of cement. It has
been noted that it is difficult to burn conventional
all wood fuel pellets with less than 150~ stoichio-
metric air and maintain clean stack gas.
These and other features of the present invention
will become better understood with reference to the
following examples.
Example l
Seven types of fuel pellets were prepared using
seasoned Douglas fir sawdust and plastic feed. The
sawdust was dried to a moisture content of less than
15% by weight in a drier made by the Heil Company of
Milwaukee, Wisconsin, Model Number 75-22. The sawdust
was then comminuted to about minus 5 mesh in an Elms
Hammermill equipped with a 50 horsepower motor, and
operated at 3600 RPM. The plastic feed was also ground
in the same hammermill~ The ground plastic feed and
. . , ''-`-
.
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~5~
-20-
ground sawdust were combined and fed to a California
Pellet Mill, Model 125C. For each of the seven differ-
ent types of pellets made, the plastic type, the
plastic content of the pellets~ bulk density of the
pellets, and the pellet temperature as the pellets
emerge from the die are reported in Table 2. The
pellet temperature was measured by collecting a cubic
foot of the pellets as they emerged from the die in a
preheated container and inserting a preheated temper-
ature probe into the container. The container was held16 inches below the die discharge. The pellet temper-
ature was recorded after the measured temperzture
stabilized. The pellets were cylindrical in shape,
having a diameter of about 3/8 inch. Pellet number
seven7 which contained 10% by w~ight high impact
polystyrene, was a very strong pellet, but was diffi-
cult to produce, and jammed the die. Therefore, only a
small quantity of this type o~ pellet was produced.
The sulfur content of the pellets ranged from 0.02% by
weight up to 0.15% by weight. All the pellets had a
heat content in excess of 8,000 BTU's per pound.
TAB~E 2
Pellet Bul~ Plastic Quantity
Pellet Temp. Density Content Plastic Burned
~umber (~) (lb/~t) (~ wt ) Type~l) (pounds)
1 190 40 5 Low Density 2700
Polyethylene
2 195 41 5 High Density 600
Polyethylene
w/Yellow Dye
3 198 38 10 High Density 2100
Polyethylene
4 ~ lS Migh Density 2160
Polyethylene
195 35 10 Virgin High Density 540
Polyethylene
6 208 40 10 Polypropylene(2)1740
7 -~ 10 High Impact 0
Polystyrene
(1~ 50% by weight ~5 mesh.
(2) Reground battery casings.
~ ' :
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-21-
The quantity of each type of pellet, as identi~ied
in Table 2 was burned in a boiler. The boiler used in
the test had a 3-water wall furnace equipped with 3
spreader stokers and a vibrating grate. A forced draft
fan forced air through the grate, and overfire air was
used as an option. A dust collector was provided
between the boiler exit and the stack. The unit had
been used with coal firing and with all-wood pellets.
The excess air meter was constant throughout the
operation at about 75~.
Prior to operation on the pellets identiEied in
Table 2, the boiler was operated with commercially
available all-wood pellets. The all-wood pellets were
burned without any overfire air because the overfire
air caused turbulence which carried fines from the
pellets out the stack. With the all-wood pellets,
to avoid smoke, 75~ excess air was required, and the
grate was shaken every 45 minutes.
Number 1 pellets were burned for about Z7 minutes,
and the stack showed a light bluish smoke. Number 2
pellets were then burned for about 18 minutes, and the
flame continued to be smokey. Number 3 pellets were
then burned!for!about 40 minutes. The amount of smoke
in the stack began to decrease. This was partially
attributed to the use of overfire air, which was
introduced about 8 minutes before starting up on the
number 3 pellets. It is possible to use overfire air
with fuel pellets of the present invention because they
have few, if any, fines. The use of overfire air
cleaned up the presence of smoke from the flame on the
~ bed of fuel particles, and substantially decreased the
; amount of smoke in the stack. For unknown reasons, it
was impossible to produce steam at a rate of 10,000
pounds per hour at 50 PSIG with the number 3 pellets~
The number 5 pellets were then burned for about 40
minutes, and an attempt was made to increase the steam
rate to 15,000 pounds per hour. This resulted in an
increase in smoke in the stack.
` ~
- ~ :
The number 6 pellets were then burned for about 20
minutes. The amount of smoke began to decrease, and
very little smoke was present in the stack and above
the fire in the boiler. Earlier problems involving a
smoky flame, and pellets sticking on the pounds per
hour resulted in stack temperatures of 600 F. At the
end of the run, number 4 pellets were burned for 45
minutes. There was very good combustion in the fur-
nace. The flames were very short, about 4" tall, and
very intense. The flames with all-wood pellets were on
the order of 12" to 15". The steam rate was lS,OOO
pounds per hour with a 590 F stack temperature.
Based on the results of these tests, the following
conclusions were reached:
(1) O~erfire air is useful for cleaning up a
smokey bed flame.
(2) It is tentatively believed that higher firing
rates improve ~omplete combustion without smoke,
because higher firing rates were used with pellets
numbers 4, 5 and 6, which produced less smoke.
(3) Smaller and/or shorter pellets burn faster.
This conclusion is based on the results with the number
6 pellets which contained some fines.
Based upon the results of this test, it is postu-
lated that reducing the input plastic size of thepellets is desirable because it would improve the
lubricity in the pellet machine and reduce the tendency
for a smoky flame. It is believed that distinct
particles of plastic in the fuel pellets contribute to
a smokey flame. Therefore, the paxticle size of the
plastic should be reduced to a small a size as poss-
ible.
Example 2
i Using the same eguipment as used for Example 1,
fuel pellets were prepared from 90% by weight seasoned
Douglas fir sawdust feed and 10~ by weight high-density
polyethylene. In a first test, the sawdust feed was
dried to free moisture content of 14.7% by weight, and
.,
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-23-
was then comminuted in the hammermill by using a
1/8" screen. The pellets produced were not as cohesive
as desired, and quite a few ~ines were present.
In a second test, the same sawdust feed was dried
to 180 5~ moisture content, and comminuted in the
hammermill using a 3/16" screen. Because of the high
moisture content of the sawdust, the pellets from the
pelletizer had insufficient cohesiveness and tended to
crumble.
In a third test, the sawdust was dried to 4.4%
moisture content and comminuted in the hammermill with
a 3/16" screen. It was necessary to add steam at the
pellet mill or else the pellets exhibited a "Christmas
tree" effect.
In a fourth test, about 25 tons of pellets were
satisfactorily produced where the sawdust feed was
dried to a moisture content of an average of about
10.~% by weight. The screen si~e of the hammermill was
about 3/16". The plastic used was high-dansity poly-
ethylene ground to minus 5 mesh, 50% by weight minus lO
mesh. ~hese pellets produced little, if any, fines,
and were satisfactorily transferred by truck over lO0
miles to the site of the boiler described in Example l.
The sawdust before drying had a moisture content of
about 54% by weight.
A series of four tests were run under different
operating conditions using the boiler of Example l to
burn the 25 tons of pellets. During these four tests,
measurements were performed for stack emissions.
Approved Environmental Protection Agency procedures
were used (isokinetic sampling, etc.) and the effluent
was filtered for particulate analysis and cooled
in ice water for condensate collection. The stack gas
was analy~ed for oxygen and carbon dioxide content.
Stack gas temperature was also recorded.
Prior to the tests the furnace was operated on all
wood waste pellets, which made the transition to the
pellets of the present invention fairly easy. No
,,
-24-
changes in operating conditions were required with the
new fuel. There were some changes in the structure of
the 1ames within the furnaceO The height of the
flames was considerably increased when compared to coal
or all-wood pellets. During the test the tips of the
flames were estimated to be in the 6 to 10 foot level
versus 2 to 2 1/2 feet for all-wood pellets. This is
probably due to the evolution of combustible gases from
the pellets. These gases rise through the combustion
air and burn very much like a slow gas diffusion
flame, resulting in the tall flames. All-wood pellets
burn more like charcoal or coal with a low flame
structure above the glowing pellets. Another charac-
teristic of the flame structure is the higher flame
temperature up to 2500F, making the pellets satisfac-
tory as a fuel for applications requiring high flame
temperature such as cement manufacturing operations.
These temperatures compare with 2100F to 2200F for a
pellet made of all wood waste. Temperatures o~ about
2~00F have been attained by burning the pellets of
this invention.
On the second of the four tests over-fire air was
used to see its effect on flame structure. Not much
difference was observed e~cept to lower the flame
height somewhat. It is postulated that had the over-
fire jets been located at a higher level than the
current 2-1/2 feet, it may have reduced the flame
height more. This is based on previous observations of
~he effect of this over-fire air on a flame structure
of 2 to 3 feet flames.
It has been standard operating procudure with
all-wood pellets to vibrate the grates for 2 ~o 3
seconds on 45 minute intervals. On test No. 3, this
intarval was shortened to 15 minutes. The stack gas,
while completely acceptable, was not as clean as was
observed on the regular schedule of grate vibration.
The excess air drifted down somewhat during this
test and the free oxygen at one time was down to 5% at
-25-
14.7% carbon dioxide. This would suggest that future
tests should be run over longer periods at low excess
air levels to see if more satisfactory combustion can
be obtained. If so, this would result in higher
overall efficiency. Test results and fuel analysis are
presented in Tables 3 and 4I respectively.
The test results with the Frajon fuel pellet~ were
very good. No adjustments to operating conditions had
to be made. All tests passed the EPA particulate
emission standards. The average particulate emission
level was 0.28 pounds per million BTU of heat imput. A
very clean stack appearance was obtained with Opacity
values of 0, 0, 15 and 0. There were no Yisible
emissions from the stack at any time except for the
brief interval when the grates were being vib~ated.
Very good efficiencies were obtained compared to the
experience with coal and plain wood pellets. At an
average steam production rate of 27,900 l~/hr. of steam
over a period of 10.25 hours, twenty-three tons of
pellet~ were consumed. This amounts to an average
steam yield of 6.22 pounds of steam per pound of
fuel. This compares to steam yields of 5.7 and 5.
for conventional wood pellets and coal, respectively,
in the same boiler under comparable operating condi-
tions. The heating value of the fuel was 9,160 BTU'sper pound. The heat requirement was therefore 1,470
BTU's of heat input per pound of steam. It is believed
that these results can be further improved by lowering
the excess air that was used.
~ ;~6 ~
.
Table 3: SUNMARY 0~ TESTS
Run No. Steam Rate Overfire Shake Flame %C %CO P3rticulate Stack ~g Opa-
lbs.lhr, Grate Temp. gr./~u/ft. Lbs. F city
Interval F Avg. Avg. at 12% CO parti-
Minutes cula~e
per 10
. in stack BTU
1 28,100 ~o 45 2184 800 ~.8 0.12 0.23 585 0
: 2 31,600 Yes 45 2294 6.2 11.6 0.12 0.26 620 0
3 34,400 Yes 15 2302 602 12.2 0.15 0.32 643 15
4 30,000 ~o 45 2415 ~.3 9.9 0.13 0.30 643 0
***********************************************************************~*******************
, .
Table 40 FUEL ANALYSIS
Run No. Moisture Sulfur Ash Bulk Density
Percent Percent Percent BTUllb. lbs./cu. ft.
: 1 7.7 less than 0~01 0.73 9106
' 2 7.5 " " " 0.73 ~g68
: 3 7.9 " " " 0.66 9382
4 7.7 " " " 0.78 9180
. AVG. 7.7 " " " 0.72 9159 32.8
********************************************
~.,
~ Table 5: COMPARISO~ OF FUELS
: Fuel Fuel Analysis Stack * Ringel- Particulate * Ashpit * lbs steam *
Temp. man * gr/scf residue lbs. fuel
. % Ash % HO BTU/lb. F meter at 12% CO % of fuel
Coal 10.2 2.210,760 610 3 High 24 4.95
; Conven-
tional 1.9 11.07,050 620 0 0.13 3 4.70
Wood
Pellets
~i Pellets
of Exam-0.7 7.79,160 620 0 0.13 1 .6.22
.,
~ * Same boiler used
,.
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-27-
Very good burn-out of the fuel was experienced.
Le S5 than a 20 gallon drum of ash was collected during
the whole burn from 25 tons of fuel pellets. This is
of the same order as the ash content of the fuel
(0.7~)~ A comparison of the fuel pellets burned during
this test and coal and conventional wood pellets
is presented in Table 5.
Example 3
Pellets were successfully produced from Canadian
sorghum peat, high density polyethylene, and high
impact polystyrene using the same equipment used for
Example 1.
The moisture content, sulfux conten~, and gross
heating value of the raw peatl the peat after drying,
and the fuel pellets are presented in Table 6. The
peat was dried and ~hen comminuted with a 3/16 screen
in the hammermill. The polyethylene and polystyrene
were comminuted to minus 5 mesh, with 50% by weight of
the particles ~10 mesh. The peat and plastic were
mixed in proportions of 95 parts by weight dried
peat, 4.5 parts by weight polyethylene and 0.5 parts by
weight polystyrene. The mixture was formed into
cylindrical pellets of 1/4 inch diameter. Steam was
used at tie pelletizers.
TABLE 6
(~ by weight)Heat Content
aterlalMoisture Ash_ SulfurBTU/lb
Raw Peat35.5 8.3 .08 5490
Dried Peat 9.1 7.1 .09 - --
Pellet 9.3 6.5 .09 9~70
Example 4
Using the same equipment as used for Example 1,
six ~ons of fuel pellets were prepared from 97.5% by by
weight seasoned Douglas fir sawdust feed and 2.5~ by
weight high-density polyethylene. All of the poly-
ethylene was ground to -10 mesh. The pellets were
burned in the same boiler used for the pellets of
r ~ ~
--28--
Example 1. The pellets had handling and burning
properties comparable to pellets containing 5~ by
weight thermoplastic. T t is believed that the amount
of thermoplastic required to produce a pellet of good
handling and burning properties can be reduced as the
particle size of the plastic is reduced.
Although the process and the` pellets of the
present invention have been described in considerable
detail with reference to certain versions thereof,
other versions are possible. For example, the pellets
can be comminuted for use in pulveri~ed coal burners or
can be gasified. In addition, equipment other than a
conventional pelleti2er can be used for combining the
plastic and cellulosic material to produce an intimate
mixture of the cellulosic material and plastic.
Therefore, the spirit and scope of the appended claims
should not necessarily be limited to the description of
the preferred versions contained herein.
