Note: Descriptions are shown in the official language in which they were submitted.
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PRETREATMENT OF FIBEROUS BIOMASS
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of the earlier filing date of U.S.
Provisional Patent
.. Application No. 62/408,523, filed on October 14, 2016, which is
incorporated herein by reference
in its entirety.
FIELD
The present invention concerns a system and process for converting fibrous
biomass raw
material into a product that facilitates subsequent torrefaction.
BACKGROUND
Torrefaction is a process whereby lignocellulosic materials are heated in an
oxygen free
environment to temperatures ranging from 240 C to over 300 C. In this
process residual moisture
is evaporated and the cellulosic fractions are decomposed to volatile, oxygen-
containing
compounds having a lower energy of combustion than the more concentrated,
carbon-containing
torrefied product. This results in a product having a higher energy based on
the weight of the
biomass.
However, the raw biomass, especially that from highly fibrous sources such as
shredded
cedar or Empty Fruit Bunches (EFB), which result from the harvest of palm oil,
has an extremely
low bulk density. As a result, the reactor systems used for torrefaction of
these low density fibrous
materials must be very large relative to their throughput and the fibrous
character makes the bulk
flow properties of these fibrous solids difficult to accommodate in the
reactor's design.
Conventional "pellet" mills, which produce dense pellets from these fibrous
raw materials, must
expend a great deal of energy to mill the material fine enough and make it
flowable to form good
pellets.
SUMMARY
Certain disclosed embodiments of the present application concern a process for
converting
fluffy, low density, fibrous biomass raw material into a form that facilitates
subsequent torrefaction.
Certain disclosed embodiments concern a process and system for pre-treating
fibrous biomass prior
to torrefaction comprising milling and densification to form the raw biomass
into a dense,
mechanically stable form.
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Certain disclosed embodiments also concern a new product form produced as an
intermediate in the torrefaction of fibrous biomass. The torrefaction process
involves heating
cellulosic material in the absence of oxygen. For certain embodiments,
torrefaction is preferably
conducted in a vertical bulk flow reactor by passing super heated steam
through a moving bed of
the fibrous biomass. But raw fibrous biomass, such as the EFB left over from
palm oil extraction
or simply the shredded stock from juniper, pine or other forest waste, is
nearly impossible to
process in a bulk flow torrefier due to its low bulk density and tendency of
the fibrous mass to
bridge and not flow through even a very large opening. Disclosed embodiments
of the new
intermediate product form may be produced as high density, short length
"pucks" by a process
comprising milling and compaction to form the raw biomass into a dense,
mechanically stable
form. This dense form allows for a more well-defined design for torrefaction
by a torrefier reactor
and associated facilities with the production of a more uniformly torrefied
biomass. The stronger,
denser treated feed stock has more predictable bulk flow properties, adequate
gas permeability to
allow for uniform torrefaction and reduced levels of fine particle fractions
that can be lost during
the torrefaction step. Post torrefication, the dense torrefied material is re-
milled easily because of
its more friable structure and is thus highly suitable for final compaction
into hard, water resistant
pellets or cubes. The final stable, water resistant and dense pellets or cubes
are a highly desirable
replacement fuel, such as fuel for powdered, coal-fired power plants. The
foregoing and other
objects, features, and advantages of the invention will become more apparent
from the following
detailed description, which proceeds with reference to the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a photographic image of certain disclosed embodiments of a wafer-
type puck
product having a substantially higher density than feed stock used to produce
the product, wherein
the product is now ready for torrefaction (the red size arrow is approximately
25 millimeters).
FIG. 2 is a photographic image of a cuber machine modified to make disclosed
product
embodiments.
FIG. 3 is a photograph of EFB biomass comprising a loose fibrous mass having a
bulk
density of about 1 ¨2 lbs/ft3.
FIG. 4 is a photograph of product made according to the present invention
subsequent to
torrefaction.
FIG. 5 is a schematic representation of a disclosed embodiment of a
torrefaction reactor.
FIG. 6 is a schematic representation of a disclosed embodiment of a
torrefaction reactor.
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FIG. 7 is a flow diagram illustrating process steps and associated components
of a
torrefaction system.
DETAILED DESCRIPTION
I. Definitions
The following explanations of terms and methods are provided to better
describe the present
disclosure and to guide those of ordinary skill in the art in the practice of
the present disclosure.
The singular forms "a," "an," and "the" refer to one or more than one, unless
the context clearly
dictates otherwise. The term "or" refers to a single element of stated
alternative elements or a
combination of two or more elements, unless the context clearly indicates
otherwise. As used
herein, "comprises" means "includes." Thus, "comprising A or B," means
"including A, B, or A
and B," without excluding additional elements. All references, including
patents and patent
applications cited herein, are incorporated by reference.
Unless otherwise indicated, all numbers expressing quantities of components,
molecular
weights, percentages, temperatures, times, and so forth, as used in the
specification or claims are to
be understood as being modified by the term "about." Accordingly, unless
otherwise indicated,
implicitly or explicitly, the numerical parameters set forth are
approximations that may depend on
the desired properties sought and/or limits of detection under standard test
conditions/methods.
When directly and explicitly distinguishing embodiments from discussed prior
art, the embodiment
numbers are not approximates unless the word "about" is expressly recited.
Unless explained otherwise, all technical and scientific terms used herein
have the same
meaning as commonly understood to one of ordinary skill in the art to which
this disclosure
pertains. Although methods and materials similar or equivalent to those
described herein can be
used to practice or test technology according to the present disclosure,
suitable methods and
materials are described below. The materials, methods, and examples are
illustrative only and are
not intended to be limiting.
Biomass: Refers to various kinds of cellulose-containing materials including,
by way of
example and without limitation, forest waste, agricultural crops either grown
specifically for energy
production or as by-products of traditional agricultural activities, or
cellulosic biomass from urban
origin. Biomass may be obtained from forest thinning operations, as non-
commercial "slash" from
commercial logging operations or from purposeful agricultural operations that
encourage fast
growing cellulosic species, such as switch grass, corn stover, arundo donax.
These biomass
materials have a wide range of as-harvested physical sizes and shapes, as well
as a highly variable
amount of moisture, either as free water or bound water.
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Method for Making and Product Made
A conventional agricultural "cuber" machine was modified to transform fibrous,
low
density cellulosic biomass into a mechanically stable form suitable for use as
a feed stock to a
.. torrefier process without requiring the addition of a "binder" or other
such adjuvant. The
conventional "cuber" comprises a rotating press wheel that forces low density,
fibrous feed stock
into a tapered die. This device was modified to permit the product to be
produced in the form of
thin wafer type "pucks" of relatively uniform size and with a density
substantially higher than the
feed stock. Modifications of the standard cuber and method for its use
included altering the shape
of the dies to form cubes, adding a controller to provide a controlled
extrusion length for the thin
puck shapes, controlling the temperature of the dies to assure the extrusions
are mechanically
strong, such as by using a temperature of from about 70 C to about 150 C,
and controlling the
moisture content of the feed stock to a level of from about 3% to about 10% to
provide a suitable
mechanical strength of the extruded product. Lower moisture content typically
is better so long as
feedstock is properly compressed into a solid piece. If the moisture content
is higher than 10%,
cubes may crack and they also do not properly separate during the production
process.
Certain disclosed embodiments of the product concern a compact "cube" or "thin
puck" of
raw cellulosic biomass having a density of 20 to 32 lb/ft3, which is 4 to at
least 15 times the bulk
density of the shredded raw biomass. The moisture content is 10% or less and
more typically is
below 10%, such as from 3 to 8%. The product has more than adequate mechanical
strength for
commercial applications. Mecanical strength can be measured by a drop test.
For example, one
drop test that has been used to test product made according to the present
invention comprises
dropping product onto a hard surface from a height of 3 feet. Products
according to the present
invention typically produce less than 10% breakage. These drop tests establish
that the densified
.. biomass has sufficient mechanical strength to withstand normal handling
without break-down to its
original fine particle size. With no added moisture or other additives to
facilitate forming the
desired shape, disclosed product embodiments may be converted with low energy
utilization into a
high energy density product, such as is described in US 9,206,368, which is
incorporated herein by
reference.
The dense pucks of raw shredded biomass are produced, for example, using a
modified
Warren & Baerg Model 250W Cuber. Modifications to this exemplary known cuber
included
positioning a deflector bar 2 positioned radially outward from the end of the
cuber's dies as shown
in FIG. 2. The shredded biomass is initially milled in a hammer mill to a size
of less than 10 mm.
This shredded and milled biomass is then fed to the cuber. In the cuber
operation, as the biomass is
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extruded through the dies, the extrudate contacts the deflector bar, which
breaks the extrudate to
form puck-shaped solids (FIG. 1). The densified biomass pucks are then ready
for processing in a
bulk flow torrefier, as described by US 9,206,368 or similar process.
The products of the present invention can be produced having any desirable
dimensions.
The exemplary products illustrated by FIG. 1 have a substantially square,
rectangular or
parallelogram shape having dimensions corresponding to the dimensions of the
die 2 in FIG. 2. The
length of each extrudate is determined by the angle of the deflector plate 1
in FIG. 2. Certain
disclosed embodiments have a length that is between about 5 mm to about 30 mm.
As the cuber's
press wheel rotates past the inlet side of the die, the raw biomass is
compressed and forced into the
die. Each die may be tapered along its length to provide back pressure, which
increases the overall
compressive force. With each rotation of the press wheel, a fresh increment of
raw biomass is
pushed into the die. Some longitudinal compression occurs such that the final
extrudate exiting the
die has substantial longitudinal strength. When the extrudate contacts the
deflector plate 2, the
extrudate breaks and the result is the relatively small "pucks" shown by FIG.
1. The model 250
.. Cuber used for this demonstration example comprised 66 dies arranged around
the head of the unit.
III. Torrefaction
The densified biomass pucks made according to the present invention may be
processed by
a torrefier. One suitable torrefier and process for using the torrefier are
described below. A person
.. of ordinary skill in the art will appreciate, however, that other
torrefaction apparatuses and
processes also can be used to process densified biomass feed products made
according to the
present invention.
Certain embodiments of one suitable torrefaction reactor are represented by
FIGS. 5 and 6.
The illustrated reactor can be configured as a vertical disposed cylindrical
or rectangular vessel (5)
having an upper straight walled section (20), defining a reaction chamber, and
a lower tapered
bottom section (22) defining a cooling zone. One particular reactor is
configured as a cylindrical
vessel for ease of construction and management. The angle of the tapered
section has an angle
from the vertical of 25 or less, and advantageously is within the range of 10
to 25 , and more
advantageously from 12 to 18 .
One consideration for a suitable torrefaction system is the ability to control
the residency
time of the material within the reactor to achieve satisfactory and uniform
torrefaction of the
processed material. Presently disclosed embodiments typically operate in a
continuous mode, as
opposed to batch mode. The reactor may be equipped with particulate discharge
or discharge
means comprising an opening, typically an ovoid or spherical opening, located
at the base of the
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tapered bottom section. The particulate discharge or discharge means typically
has a dimension of
at least 200 mm or more in the shortest cross-section.
Torrefaction of biomass processed according to the present invention generally
involves
applying heat to induce conversion of the raw biomass to torrefied biomass.
Heat may be provided
by introducing hot gases into the torrefaction reactor. Accordingly, the
reactor is provided with a
heated gas input (19) positioned at the top of the tapered bottom section or
bottom of the upper
straight-walled section and comprises a device able to introduce the heated
gas around the
perimeter of the reactor. Typically such device includes one or more
injectors, or a plenum having
numerous orifice holes sized and spaced apart so as to assure even
distribution of the hot torrefying
gases while minimizing the system pressure drop. The region of the reactor
between the hot gas
inlet (19) and the gas outlet (27) is the torrefaction zone. The entry point
of the hot gases into the
torrefier reactor at a point (21) corresponds to where the stress on the
biomass is higher than in the
bulk of the reactor. That is, at a location where the cross sectional flow
area of the mass flow
reactor is becoming constrained, at the point where the straight sides of the
reactor meet a conical
shaped lower portion. The hot gases pass through the torrefaction zone
contacting enroute any
particulate biomass within the zone and exit from zone as torrefaction gases
from the reactor,
generally at the top of the reactor.
In operation, biomass processed according to the present invention enters the
top of the
mass flow reactor (25) and after descending the reactor in a mass flow mode,
moving against the
.. upward flow of hot gases, the torrefied biomass leaves the lower discharge
point of the reactor (24).
By suitable design of the sloped walls of the mass flow reactor, the entire
mass remains in the
torrefaction zone of the reactor for a uniform and controlled period of time.
As noted above
discharge of the torrefied biomass occurs via the particulate discharge means.
To further facilitate
control of the rate of discharge the system may be equipped with a discharge
regulating device
.. located externally to the particulate discharge means and wherein the
device comprises a conveyor
or airlock.
In a preferred embodiment, the system disclosed herein is further equipped
with a
temperature sensing means, or temperature sensor, able to determine the
temperature of the
particulate biomass within the reaction temperature. The temperature sensing
means, or
temperature sensor, is further in communication with the discharge regulating
device and together
function to control the rate of discharge of torrefied particulate biomass
from the system. In this
manner, the residency time of the particulate biomass within the torrefaction
reactor is controlled
by function of its temperature, thereby ensuring a correct and desired degree
of torrefaction. And
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by using a reactor having mass flow characteristics the uniformity of the
degree of torrefaction is
consistent across the bulk mass of the material.
For successful torrefaction of biomass processed according to the present
invention for use
as a solid fuel, uniform and controlled torrefaction of biomass is desired.
Incomplete torrefaction
results in a product which will be problematic in grinder operations due to a
higher modulus
(flexibility and toughness). Over-torrefied material loses more of its energy
as high fuel value
compounds are driven off at long residence time or higher temperatures.
A functional block flow diagram of the overall process is shown in FIG. 7.
Biomass is
processed according to the present invention for use as feed material to the
reactor. The feed
.. material (1) is first converted to a specified size in a conventional
grinder (3). The grinder reduces
this size to a maximum of 13 mm x 75 mm. Any conventional grinder may be used,
such as a
horizontal tub grinder commonly used in the forest products industry.
The processed biomass sized by the grinder is dried in a continuous direct air
heated dryer
(4). This dryer may be of the bulk flow type or any dryer suitably configured
for this service. The
.. dryer (4) delivers a product having a controlled residual moisture content
of 25 wt% or less based
on total weight of the biomass, and advantageously the residual moisture
content is from about 12
wt% to about 25 wt%. The heated air for the dryer is a combination of hot
combustion gases from
an auxiliary heater (11) combined with cooled combustion gases (18) from the
thermal oxidizer (9)
associated with the torrefaction reactor (5). Fuel (13) combined with
combustion air (12) in the
auxiliary heater (11) provides the balance of thermal energy for operating the
dryer (4). The dryer
(4) delivers a product having a controlled residual moisture content of 25 wt%
or less to the
torrefier (5).
From the dryer (4) the biomass having 25% or less moisture content and with a
size of from
about 13 mm to about ¨ 75 mm in the longest dimension is fed to the bulk flow
torrefier (5). Based
on the particle size of the pre-sized biomass feed the vertical angle of all
non-vertical surfaces (A,
B) have been previously determined by a series of tests conducted on
representative samples of the
biomass. In the case of Eastern Oregon Juniper, for example, the maximum angle
(A) in FIG. 5
would be 16 . The biomass enters the torrefier (5) at the top via a rotary air
lock or similar
atmosphere control device (25). Such rotary air lock or atmospheric control
device is required to
prevent ingress of oxygen into the reactor as torrefaction of biomass occurs
at elevated
temperatures in the substantial absence of oxygen. The torrefier is of the
mass or bulk flow
configuration (FIGS. 5, 6). That is, it has a cylindrical (FIG. 5) or
rectangular (FIG. 6) body with a
diameter or diagonal (D) and height (reaction zone) sized to provide the
required residence time for
the reaction. The volume of the reaction zone of the vessel allows the biomass
to be heated to the
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torrefaction temperature of from 240 C to 280 C. Controlling charging and
discharging rates
provides a residence time at the maximum temperature of from about 5 minutes
to about 15
minutes. Exposure to temperatures greater than this promotes pyrolysis of the
biomass and detracts
from its calorific value as torrefied biomass. If the residency time is too
short the raw biomass does
not undergo full conversion to a torrefied biomass.
The hot gases enter the torrefier through air inlet(s) (19) located at the
junction of the
straight sides of the reactor (20) and the elongated cone shaped lower section
(22). The location of
the hot gas inlet is at a point where the stress on the mass charge is
greatest. An internal flow
splitter (23) having sloped sides with angles equal to the slope of the walls
further increases the
stress in the particulate solids mass. At that point there is the least
tendency for the charge to
become fluidized and this location promotes the greatest and most even
distribution of the hot gases
throughout the downward moving mass. The hot gases entering at a temperature
of about 300 C
move upward through the downward moving mass. The decomposition of the biomass
and
removal of the last amount of moisture and torrefaction reaction gases occurs
as the hot gases move
upward. This method permits the maximum temperature of the torrefied biomass
to be limited by
modulating the temperature of the torrefying gases circulated through the bulk
flow reactor.
The reaction gases exit the top of the torrefier (27) and flow through an
external heat
exchanger (10) where the gases are reheated by combustion gases from a thermal
oxidizer (9). A
portion of the reaction gases, which represents the residual moisture and the
released
decomposition gases (16), flow to the thermal oxidizer (9) where, combined
with metered air (14)
and if necessary auxiliary fuel (15), are combusted. After re-heating the
circulating torrefier gases
(17) in heat exchanger (10) the cooled combustion gases (18) flow to the dryer
(4) to augment the
heat required there.
Below the gas inlet(s) (19) of the torrefier (5), the torrefied biomass is
cooled by contact
with a jacketed section (26) of the torrefier (5). The coolant in this area
may be any suitable
coolant, such as water or other heat transfer fluid. The temperature of the
coolant is maintained
above the dew point of the hot gases in the torrefier, generally above about
80 C. The downward
moving mass is cooled to below 150 C, which is its auto-ignition temperature
in air.
The cooled torrefied biomass is discharged from the lower conical section of
the torrefier
via an opening (26) to a rotary air lock, or preferably a graduated pitch
screw conveyor. The
discharge opening (24) is an elongated slot whose smallest dimension was
determined by a series
of tests using the typical process biomass. The minimum dimension of the
elongated discharge
opening to provide for a bulk flow condition generally is about 200 mm. The
torrefied biomass as
discharged may still have a temperature significantly greater than the ambient
air temperature;
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accordingly, it is desirable to manage this temperature by advantageously
subjecting it to a cooling
step to mitigate any risk of spontaneous combustion on exposure to ambient
air.
This method permits the properties of the torrefied biomass to be controlled
by modulating
the rate of withdrawal of the torrefied biomass from the bulk flow torrefier.
A useful discharge
means is a screw conveyor in which the flights of the conveyor increase in the
direction of the
discharge flow in order to facilitate bulk flow from the reactor.
The cooled torrefied product from the reactor (5) is optionally milled (7) to
a smaller size
suitable for densification (8) to yield a final, torrefied, high density fuel
(2).
IV. Example
The following example is provided to illustrate certain features of a
particular working
embodiment. A person of ordinary skill in the art will appreciate that the
scope of the present
invention is not limited to these particular features.
Raw EFB biomass shown in FIG. 3 is a loose fibrous mass having a bulk density
of about 1
¨ 2 lb/ft3. Its low density and highly interlocking fibrous nature makes it
very difficult if not
impossible to process in a desirable bulk flow reactor.
Dried whole EFBs were hammer milled. Chopped EFB was cubed. A thin puck insert
design was then developed, as well as a method of cooling the dies.
After processing through the modified cuber, the bulk density of the biomass
material was
increased to about 25 lb/ft3. (FIG. 1). After torrefaction, the material shown
in FIG. 1 became a
brownish-black mass (see FIG. 4) having a high energy density.
In view of the many possible embodiments to which the principles of the
disclosed
invention may be applied, it should be recognized that the illustrated
embodiments are only
preferred examples of the invention and should not be taken as limiting the
scope of the invention.
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