Note: Descriptions are shown in the official language in which they were submitted.
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IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
ENERGY PELLET
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The contents of Provisional application Ser. No. 61/673,573 filed July
19, 2012,
on which the present application is based and benefit claimed under 35 U.S.C.
119(e), is
herein incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] (1) Field of the Invention
[0003] The present invention relates to energy pellets made from torrefied
wood and
whole-tree (white) wood and methods of making same. More particularly, this
invention
relates to energy pellets made from torrefied wood or white wood combined with
lignin
and a biomass-based plastizer to form a water-resistant and durable high-
energy-density
pellet.
[0004] (2) Description of Related Art
[0005] In recent years renewal energy sources have become more desirable and
thus
more important although the United States continues to be heavily reliant on
the
combustion of non-renewable fossil fuels to meet its energy needs.
Additionally, energy
from biomass mandates in the European Union has led to a rapidly growing
export
market for energy pellets. Exports of white-wood pellets to the EU from North
America
was 4.4 million metric tons in 2012 and is predicted to reach 25-70 million
metric tons
per year by 2020. To meet the growing energy needs biomass based materials
have been
pelletized to provide a dense, relatively high energy material.
[0006] One of the desirable biomass materials being developed is torrefied
wood.
Torrefied wood is wood that has been partially oxidized to drive off the water
and the
organic volatiles and a fraction of the cellulose, so that a "blackened" wood
chip is the
result. The torrefied wood chips are friable and can be easily formed into a
high-energy,
high-density pellet. When a pellet is made from southern yellow pine wood with
its high
resin content, it will remain as a pellet for days when immersed in water ¨
indicating
excellent water repellency. A pellet made from southern yellow pine needs no
binder,
since the pellets from that wood is exposed to the 40,000+ psi of the extruder
and
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temperatures exceeding 100 C, the natural components flow to form a water
repellant
binder naturally. However, when a pellet is made from torrefied wood, most of
the
natural binders are burned off so that the resulting pellet has poor water
repellency, and in
a worst case, the pellets turn into a black "mud" upon exposure to water.
[0007] Another problem with torrefied wood and even with white wood pellets,
especially those made from low-resin-containing biomass, is durability. The
pellets
themselves tend to be friable and create dust during mechanical processes
needed to
transport the pellets, such as loading and unloading rail cars and ships. As
much as 5-7%
loss of mass from dust has been reported for white-wood pellets. In addition
to product
loss, the dust creates hazards, from inhalation and the potential for dust
explosions.
[0008] Torrefied wood has the strong advantage of being closer to coal in its
burning
capabilities than white wood. And the energy (bulk) density of torrefied wood
is much
greater than white wood so its shipping costs are much lower.
[0009] The high-energy, high-density and coal-like mechanical properties of
torrefied
wood pellets are desirable for large biomass burning facilities ¨ like the
utilities but the
coal is normally transported in uncovered cars to and from ports and stored in
uncovered
piles at the combustion site, so water repellency is a key attribute. A single
utility would
have to spend $100 million or more to cover these pellet piles and rail cars,
and few if
any will make this capital expenditure, so water repellency is strongly
desirable.
[0010] Currently wood pellets are burned, but the lower energy density and
their
pulverizing and burning characteristics limit their use as a fuel since they
are so different
from coal. Energy pellets made entirely of lignin has approximately the same
energy
content as coal, about 12,000 Btu/lb, which is about 50% higher energy per
mass of low-
moisture wood pellets having about 8,000 Btu/lb.
[0011] Within the wood matrix, lignin is a macromolecule is chemically bound
to
cellulose and hemicellulose. In pulping processes or enzymatic biorefineries,
the lignin is
separated from the cellulose and hemicellulose and its molecular weight is
reduced (i.e.,
the lignin is "chopped up"). This isolated lignin at ambient conditions is a
solid, but it
can be melted at elevated temperature, with lower molecular weight lignin
generally
having a lower melt point. The problem is that isolated solidified lignin is
very friable,
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and its melt point may not be ideal for the pelletizing system so that the
lignin melts and
flows within the pelletizer but solidifies upon exiting and cooling.
[0012] The lignin content in wood has recently been shown (Thomas Wilson, PhD
Thesis, Penn State University, 2010) to have no effect on the durability of
white wood
pellets. In the Penn State study, a wide variety of wood with varying lignin
contents was
used; however, all that lignin was in the macromolecular form and chemically
bound to
cellulose. Contrary to the Penn State study, lignin with lower molecular
weight that has
been isolated from wood has better binding capability. And the macromolecular
lignin
used in the Penn State study may not have the required melt-point profile.
[0013] For a binder to work well in forming energy pellets, the binder must
melt and
flow under the extruder conditions of elevated temperature and pressure. As
the pellets
emerge from the extruder and cool, the binder must reform as a solid, coating
and
protecting the smaller elements - loose fibers of white wood or particles of
torrefied
wood.
[0014] Lignosulfonate is a byproduct of the sulfite pulping process.
Lignosulfonates
have demonstrated commercial utility as binders for animal foods.
Lignosulfonates are a
poor choice as energy pellet binders because: (1) lignosulfonates have high
levels of
organically-bound sulfur ¨ as high as 10% by mass ¨ that converts to sulfur
dioxide upon
combustion; and (2) lignosulfonates contain residual sugar from the pulping
process.
These sugars are hydrophilic and deleteriously affect the water resistance of
energy
pellets. The sulfonate groups on the lignin are also hydrophilic, exacerbating
the water
resistance problem.
SUMMARY OF THE INVENTION
[0015] There is provided a high-energy water-resistant pellet having at least
75%
torrefied wood and the remainder being a two-component binder comprising from
about
2% to about 25% by total weight of the pellet. The starting material is
torrefied wood
having a moisture content of below 15% water, with the equilibria water
content being
about 10-15% water. The binder is a two-component system ¨ with a plastizer to
adjust
the melt point, such as tall oil pitch, rosin, fatty acids, animal oils,
vegetable oils, or corn-
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oil proteins, preferably from 3% to about 20% of the binder, and lignin from
about 80-
97% of the binder.
[0016] There is also provided at method for making a high-energy water-
resistant pellet
from torrefied wood and a two component binder.
[0017] It is therefore the general object of the present invention to provide
a high-
energy, high density fuel pellet made from torrefied wood or white wood and a
binder
that is water repellent.
[0018] Another object of the present invention is to provide a high-energy,
high-density
water-repellant fuel pellet from torrefied wood or white wood that includes a
two-
component binder.
[0019] Yet another object of the present invention is to provide a process for
producing
a high-energy pellet.
[0020] Other objects, features and advantages of the invention will be
apparent to those
skilled in the art from the following detailed description of the preferred
embodiments of
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0021] The present invention now will be described more fully hereinafter in
which
preferred embodiments of the invention are shown. This invention may, however,
be
embodied in many different forms and should not be construed as limited to the
embodiments set forth herein; rather these embodiments are provided so that
this
disclosure will be through and complete and will fully convey the scope of the
invention
to those skilled in the art.
[0022] A preferred fuel pellet of the present invention is produced from
torrefied wood
having a moisture content of below 15% water, with the equilibria water
content being
about 10-15% water, that has been mixed with a binder and compressed by
passing
through an extruder, typically at a temperature at which the binder is in a
molten state
while the torrefied wood is compressed. Forming the pellets commonly involves
hydraulic extrusion through a fuel pellet die. Of course, other suitable
compression
equipment known to those skilled in the art may be used. The temperature of
the
torrefied wood and binder in the extruder is above 100 C but is not allowed to
exceed
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280 C to avoid adverse exothermal tonefication conditions. Preferably the
temperature is
maintained between 100 C and 165 C. Another embodiment of the present
invention
uses whole-tree (white) wood in place of torrefied wood. For use in this
invention the
white-wood is ground to a size that allows free-flow into the throat of the
extruder.
[0023] The water-resistant high-energy pellet comprises at least 75% torrefied
wood
and preferably at least about 85% torrefied wood, and the remainder a binder
comprising
from about 2% to about 25%, preferably about 3% to about 15%, by total weight
of the
pellet. The binder comprises of a plasticizer, such as tall oil pitch,
preferably from 5% to
about 20% by weight of the binder, and lignin from about 80% to about 97% by
weight
of the binder. The optimal ratio of the lignin to plasticizer will depend upon
the
characteristics of the torrefied wood, the lignin, and the plasticizer that
are contained
within the pellet. Those choices typically, are made with economics being the
primary
factor.
[0024] Lignin, a by-product of the pulp and paper industry, of sufficiently
low Tg will
flow and provide protective coverage within the pellet that is needed for
water repellency
but may not flow under the conditions of the pelletizer which operates at 105
C and
>40,000 psi. The individual characteristics of the lignin chosen may not be
suitable
without a plasticizer since its Tg may be too high which inhibits its flow at
the
temperature of the pelletizer. Adding a plasticizer that itself is water
resistant can
provide the requisite Tg allowing the binder to flow within the pelletizer yet
solidify at
ambient temperature. Again this ratio will be driven by performance and
economics
since lignin generally will be much less expensive than tall oil pitch once
the multiple
large-volume lignin production facilities come on-line from papermaking
operations and
enzymatic conversion of biomass that will have lignin as a byproduct stream.
[0025] The binders contemplated for use in this invention are totally natural
organics,
preferably tall oil pitch, fatty acids, rosin and the like. Tall oil pitch is
the bottoms
product from the first distillation column of a refinery that has tall oil as
a feedstock and
makes distilled rosins and fatty acids as products. The tall oil pitch may be
used, or the
"spent" tall oil pitch from which the valuable sterol fraction has been
removed could be
used.
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[0026] The binders of this invention may also include vegetable or animal oils
to
plasticize the lignin while providing water repellency. Corn oil and corn
protein isolated
from ethanol processing provides a ready commercial source of natural
plasticizers. Corn
protein has demonstrated binding capability, and provides additional binding
capability to
the lignin.
[0027] Tall oil pitch is an excellent binder for fire logs, and TOP is
currently
commercially used for this application. The cost of TOP can be relatively high
($400-
$600/ton). The "spent" tall oil pitch, which has the valuable sterol fraction
removed is
less expensive.
[0028] Torrefied wood pellets have a bulk density (40-45 lbs/ft3), about twice
that of
non-pelletized torrefied wood (16-20 lbs/ft3), so making pellets dramatically
reduces
volume and subsequent transportation costs. Torrefied wood pellets have an
energy
density about the same as coal, 12,000 Btu/lb. Size-reduction characteristics
of wood
pellets ¨ how they break down under mechanical forces ¨ are much different
than that of
coal, which is a problem when large fractions of wood pellets are fed into
existing
pulverizing equipment used by all coal-burning power-generation equipment.
However,
size-reduction characteristics of torrefied wood pellets are very similar to
those of coal.
Thus the mechanical handling characteristics and energy density of torrefied
wood pellets
are very similar to coal, making those pellets much more acceptable as a
direct substitute
for coal.
[0029] As noted, one of the critical characteristics of the high-energy-
content torrefied
wood pellets of this invention is that they are water repellant. Another
important
property is pellet strength and attrition resistance, since dust creates not
only worker
health and safety issues but also dust severely reduces the bulk density of
the pellets so
that a ship loaded with pellets crossing an ocean carries a lower mass loading
causing the
shipping rates per ton to be higher. This densification effect is counter-
intuitive because
the dust particles separate the individual pellets thereby decreasing the bulk
density
instead of partitioning entirely in the interstitial areas of the bulk
pellets.
[0030] Many modifications and other embodiments of the invention set forth
herein
will come to mind to one skilled in the art to which this invention pertains
having the
benefit of the teachings presented in the foregoing descriptions. Therefore,
it is to be
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understood that the invention is not to be limited to the specific embodiments
disclosed
and that modifications and other embodiments are intended to be included
within the
scope of the appended claims. Although specific terms are employed herein,
they are
used in a generic and descriptive sense only and not for purposes of
limitation.
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