U.S. patent application number 12/763355 was filed with the patent office on 2011-10-20 for method of drying biomass.
This patent application is currently assigned to River Basin Energy, Inc.. Invention is credited to Clinton B. Camper, Vijay Sethi.
Application Number | 20110252698 12/763355 |
Document ID | / |
Family ID | 44787030 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110252698 |
Kind Code |
A1 |
Camper; Clinton B. ; et
al. |
October 20, 2011 |
Method of Drying Biomass
Abstract
A process for torrefaction of biomass is provided in which
biomass are passed into a fluidized bed reactor and heated to a
predetermined temperature in an oxidizing environment. The dried
biomass is then fed to a cooler where the temperature of the
product is reduced to approximately 100 degrees Fahrenheit.
Inventors: |
Camper; Clinton B.;
(Billings, MT) ; Sethi; Vijay; (Laramie,
WY) |
Assignee: |
River Basin Energy, Inc.
Highlands Ranch
CO
|
Family ID: |
44787030 |
Appl. No.: |
12/763355 |
Filed: |
April 20, 2010 |
Current U.S.
Class: |
44/605 ;
44/626 |
Current CPC
Class: |
Y02E 50/15 20130101;
C10L 5/442 20130101; C10L 5/445 20130101; Y02E 50/10 20130101; Y02E
50/30 20130101; C10L 9/083 20130101 |
Class at
Publication: |
44/605 ;
44/626 |
International
Class: |
C10L 5/44 20060101
C10L005/44; C10L 5/40 20060101 C10L005/40 |
Claims
1. A single stage process for biomass torrefaction, comprising:
charging biomass to a fluidized bed reactor, charging air to said
fluidized bed reactor at a velocity of from about 4 to about 8 feet
per second, subjecting said biomass to a temperature of from about
230 to about 350 degrees Centigrade, and removing the water from
said biomass by torrefying said biomass, wherein: said biomass
charged to said fluidized bed reactor has an average moisture
content of from about 10 to about 50 percent, said fluidized bed
reactor is comprised of a fluidized bed with a fluidized bed
density of from about 20 to about 50 pounds per cubic foot, whereby
a dried, torrefied biomass is produced.
2. The process of claim 1, wherein said biomass charged to said
fluidized bed reactor has a minimum dimension of 3 mm to 10 mm.
3. The process of claim 1, wherein said biomass charged to said
fluidized bed reactor is wood, plant material or agricultural
waste.
4. The process of claim 1, wherein said biomass in said fluidized
bed reactor is maintained at said temperature of from about 230-350
degrees Centigrade.
5. The process of claim 1, wherein said fluidized bed is maintained
at a fluidized bed density of from about 20 to about 50 pounds per
cubic foot.
6. The process of claim 1, wherein said biomass has a residence
time in the reactor of 2 to 5 minutes.
7. The process of claim 1, further comprising the step of heating
said air prior to the time it is fed into said fluidized bed
reactor using heat recovered from heated gas taken from said
fluidized bed reactor.
8. The process of claim 1 wherein said cooler employs at least one
of a mixing screw conveyor, hollow flight screw conveyor, a rotary
drum and a rotary tube.
9. The process of claim 1, further comprising: feeding said dried
biomass into a cooler where it is cooled to a temperature near 100
degrees Centigrade and adding no more than 3% moisture to the dried
biomass.
10. The process of claim 1, wherein said cooler comprises at least
one water spray and at least one temperature sensor to allow water
to be applied to the biomass for at least one of progressively
lowering the temperature of the biomass to less about 100 degrees
Centigrade and adding up to 3 percent moisture to the biomass.
11. The process of claim 10 wherein the application of water is
continuous.
12. The process of claim 10 wherein the application of water is
intermittent.
13. The process of claim 1, wherein said reactor comprises at least
one water spray to aid in the control of reactor temperature.
14. The process of claim 13 wherein the application of water is
continuous.
15. The process of claim 13 wherein the application of water is
intermittent.
16. A process for drying a material, comprising: directing the
material to a fluidized bed reactor; predrying the material with
gasses exhausted from the fluidized bed reactor; and subjecting
said material within the fluid bed reactor to a temperature
sufficient to torrefy the material and remove water therefrom.
17. The process of claim 16, further comprising: adding fresh air
to said fluidized bed reactor to adjust the oxygen content of the
gasses within the fluidized bed reactor.
18. The process of claim 16 wherein said material is at least one
of biomass and coal.
19. The process of claim 16 wherein said fluidized bed reactor has
an aspect ratio is no greater than about 2.
20. The process of claim 16 further comprising directing heated air
to the fluidized bed reactor with a startup heater.
Description
FIELD OF THE INVENTION
[0001] Embodiments of the present invention relate to a process for
preparation of a solid fuel biomass by torrefaction. More
specifically, in the process of one embodiment, the biomass
material is heated directly in an oxidizing atmosphere while a
portion of the processed biomass is used as fuel to produce heat
required to support the torrefaction process.
BACKGROUND OF THE INVENTION
[0002] "Biomass" refers to renewable organic materials such as
wood, plant materials or agricultural waste. Biomass often contains
about 10 to about 50 weight percent moisture, which does not add to
the fuel value and increases the transportation cost of the
material. "Torrefaction" refers to the treatment of biomass at a
temperature between about 200.degree. C. to about 350.degree. C.
wherein water and volatile carbon molecules are vaporized. In
addition, during the torrefaction process, molecules of
hemicelluloses contained in the biomass decompose into smaller,
less complex carbon molecules, some of which are also vaporized.
Molecules of cellulose and lignin also found in the biomass can
also be decomposed in the process but to a much lesser extent than
the hemicelluloses molecules. After torrefaction, the biomass is
easier to grind, has a significantly reduced moisture content (less
than about 3%), and possesses an increased heating value.
[0003] Several U.S. patents and published patent applications are
related to treating biomass using torrefaction. These include U.S.
Pat. No. 4,553,978 (the "'978 patent"), entitled "Process for
Converting Ligneous Matter of Vegetable Origin by Torrefaction, and
Product obtained Thereby"; U.S. Pat. No. 4,787,917 (the "'917
patent"), entitled "Method for Producing Torrefied Wood, Product
Obtained thereby, and Application to the Production of Energy";
U.S. Pat. No. 4,816,572 (the "'572 patent"), entitled
"Thermocondensed Lignocellulose Material, and a Method and an Oven
for Obtaining It"; U.S. Pat. No. 4,954,620 (the "'620 patent"),
entitled "Thermocondensed Lignocellulose Material, and a Method and
an Oven for Obtaining It"; U.S. Patent Application Publication No.
2003/0221363 (the "'363 application"), entitled "Process and
Apparatus for making a Densified Torrefied Fuel"; U.S. Patent
Application Publication No. 2008/0223269 (the "'269 application"),
entitled "Method and Apparatus for Biomass Torrefaction Using
Conduction Heating"; U.S. Patent Application Publication No.
2009/0007484 (the "'484 application"), entitled "Apparatus and
Process for Converting Biomass Feed Materials into Reusable
Carbonaceous and Hydrocarbon Products"; U.S. Patent Application
Publication No. 2009/0084029 (the '"029 application), entitled
"Process and Device for Treating Biomass"; U.S. Patent Application
Publication No. 2009/0250331 (the "'331 application"), entitled
"Autothermal and Mobile Torrefaction Devices"; and U.S. Patent
Application Publication No. 2009/0272027, entitled "Method for the
Preparation of Solid Fuels by Means of Torrefaction as well as the
Solid Fuels thus Obtained and the Use of these Fuels". The entire
disclosure of each of the foregoing references is incorporated by
reference herein.
[0004] All of the above references disclose torrefaction processes
that employ a non-oxidizing or inert gas environment (that contain
very low levels or no oxygen. Use of a non-oxidizing environment
requires utilization of an external heating methodology to supply
the heat necessary for torrefaction. In addition, the '978 patent
discloses a torrefaction residence time of 0.5 to 5.0 hours and
does not disclose a specific cooling methodology. The '917 patent
discloses the use of a specific size feed material of 15 mm in
length and 5 to 20 mm in diameter and cooling using an inert gas.
The '572 and '620 patents disclose a torrefaction residence time of
30 minutes and do not disclose a specific cooling methodology. The
'363 application discloses a process in which the air is removed
from the processed biomass product at high pressure to make
pellets, cubes or logs, i.e., densification, of the torrefied
product immediately after torrefaction with subsequent cooling of
the densified biomass pellets, but no specific cooling methodology
is described. The '269 application discloses torrefaction of
biomass using a specially designed oven that utilizes conduction to
transfer heat from heated plates to the biomass material. The '484
application discloses the use of an externally heated ribbon
channel reactor to progressively heat biomass material. The '331
application discloses combustion of vapors produced during
torrefaction to supply the heat for torrefaction and the subsequent
pelletizing of the torrefied product, but does not address product
cooling.
SUMMARY OF THE INVENTION
[0005] It is one aspect of the present invention to provide a
process for biomass torrefaction using combustion vapors and a
portion of the solids within a reactor to supply heat necessary for
torrefaction. The reactor may be directly associated with the
torrefaction process or be associated with another process, for
example, a coal drying process. One of skill in the art will
appreciate that "solids" as referred to herein on occasion shall
mean biomass, coal in a combination of coal and biomass
(hereinafter "biomass" for simplicity). In one embodiment of the
invention, combustion rate within the reactor is controlled by
adjustment of the feed rate, control of the amount of air added to
the reactor and/or the addition of water directly into the reactor.
The torrefied product is cooled using the direct application of
water wherein the water addition rate is controlled so that the
cooled, torrefied biomass product has a moisture content below 3%.
Prior to or subsequent to cooling, the product may be
pelletized.
[0006] It is another aspect of the present invention to employ a
fluid bed reactor to achieve the contemplated torrefaction. The
fluid bed reactor uses air as a primary fluidizing gas with any
additional fluidizing gas needed supplied by heated gas drawn from
fluid bed exhaust, i.e., "offgas". The rate of fluidizing gas
introduction into the fluid bed reactor would be as required to
produce an apparent gas velocity within the fluid bed reactor
between about 4 and 8 feet per second. At this velocity, the bed
temperature of the reactor would be maintained between about 230
and 350.degree. C.
[0007] It is another aspect of the present invention to provide a
torrefaction process that employs a rotary drum reactor. In one
embodiment, the biomass flows countercurrent to the flow of
reaction gas, e.g. air. The amount of air would be that required to
supply sufficient oxygen concentration to maintain the necessary
combustion rate of volatiles compounds to supply sufficient heat to
torrefy the biomass.
[0008] It is yet another aspect of the present invention to employ
water sprays and a mixing device, such as a mixing screw or rotary
drum, to cool the processed biomass. Hot torrefied product would be
discharged directly from the reactor into the cooler and water
would be sprayed onto the hot product through the use of a
multiplicity of sprays to provide cooling through evaporation of
water. The total amount of water added would be that to provide
cooling to approximately the boiling point of water (100.degree. C.
at sea level) without raising the moisture content of the cooled
product above approximately 3 weight percent. The mixing/tumbling
action of the cooler would provide particle to particle contact to
enhance distribution of the water added for cooling. The direct
application of water may be achieved by methods disclosed in U.S.
patent application Ser. No. 12/566,174, which is incorporated by
reference in its entirety herein.
[0009] In an alternative embodiment of the present invention, an
indirect cooler to reduce the temperature of the torrified biomass
is employed in the event that a minimum moisture content is
required. For example, an indirect cooler with cooling surfaces
such as a hollow flight screw cooler or a rotary tube cooler may be
employed to achieve this goal. The contemplated indirect cooler
would not necessarily employ water sprays.
[0010] It is another aspect of the present invention to employ
pelletizing before or after cooling should the product market
justify product densification.
[0011] It is another aspect of the present invention to provide a
single stage process for biomass torrefaction, comprising charging
biomass to a fluidized bed reactor, charging air to the fluidized
bed reactor at a velocity of from about 4 to about 8 feet per
second, subjecting the biomass to a temperature of from about 230
to about 350 degrees Centigrade, and removing the water from the
biomass by torrefying the biomass. The biomass charged to the
fluidized bed reactor of this embodiment has an average moisture
content of from about 10 to about 50 percent and the fluidized bed
reactor is comprised of a fluidized bed with a fluidized bed
density of from about 20 to about 50 pounds per cubic foot, whereby
a dried, torrefied biomass is produced.
[0012] It is still yet another embodiment of the present invention
to provided a process for drying a material by directing the
material to a fluidized bed reactor, pre-drying the material with
gasses exhausted from the fluidized bed reactor, and subjecting
said material within the fluid bed reactor to a temperature
sufficient to torrefy the material and remove water therefrom.
[0013] The Summary of the Invention is neither intended nor should
it be construed as being representative of the full extent and
scope of the present invention. Moreover, references made herein to
"the present invention" or aspects thereof should be understood to
mean certain embodiments of the present invention and should not
necessarily be construed as limiting all embodiments to a
particular description. The present invention is set forth in
various levels of detail in the Summary of the Invention as well as
in the attached drawings and the Detailed Description of the
Invention and no limitation as to the scope of the present
invention is intended by either the inclusion or non-inclusion of
elements, components, etc. in this Summary of the Invention.
Additional aspects of the present invention will become more
readily apparent from the Detail Description, particularly when
taken together with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate embodiments of
the invention and together with the general description of the
invention given above and the detailed description of the drawings
given below, serve to explain the principles of these
inventions.
[0015] FIG. 1 is a schematic of a biomass torrefaction process of
one embodiment of the present invention;
[0016] FIG. 2 is a detailed view of FIG. 1, showing a fluid bed
reactor;
[0017] To assist in the understanding of one embodiment of the
present invention, the following list of components and associated
numbering found in the drawings is provided below: [0018] #
Component [0019] 2 Biomass torrefaction system [0020] 6 Fluid bed
reactor [0021] 10 Hopper [0022] 14 Conveyor [0023] 18 Surge bin
[0024] 22 Feeder [0025] 26 Feed screw [0026] 30 Pre-dryer system
[0027] # Component [0028] 34 Fluidized bed [0029] 38 Spreader
[0030] 42 Feed screw [0031] 46 Off gas [0032] 50 Startup heater
[0033] 54 Recycle fan [0034] 58 Air line [0035] 62 Air line [0036]
66 Air line [0037] 70 Air line [0038] 74 Air line [0039] 78 Air
line [0040] 82 Air line [0041] 86 Air line [0042] 90 Air line
[0043] 94 Fresh air fan [0044] 98 Vent valve [0045] 102 Emissions
control device [0046] 106 Particulate removable device [0047] 110
Burner [0048] 114 Valve [0049] 118 Cooler [0050] 122 Dump valve
[0051] 126 Conveyor [0052] 130 Storage system
[0053] It should be understood that the drawings are not
necessarily to scale. In certain instances, details that are not
necessary for an understanding of the invention or that render
other details difficult to perceive may have been omitted. It
should be understood, of course, that the invention is not
necessarily limited to the particular embodiments illustrated
herein.
DETAILED DESCRIPTION
[0054] Referring now to FIG. 1, a biomass torrefaction system 2
implements a process for preparing torrefied biomass. More
specifically, one embodiment of the present invention employs a
fluidized bed reactor 6 wherein a fluid or gas is passed through a
granular solid biomass material at high velocities to suspend the
solid and cause it to behave as if it were fluid (i.e.
fluidization). The biomass may be wood that has been reduced in
size by a commercially available wood chipper. The size of the
biomass will vary, but the smallest dimension is typically about 3
mm to 10 mm thick. One of skill in the art will appreciate that
straw or other agricultural waste may be used without departing
from the scope of the invention. In one embodiment, biomass having
about 10 to 50 weight percent moisture is processed The weight
percentage is, preferably, determined by conventional methods in
accordance with standard A.S.T.M. testing procedures.
[0055] The biomass torrefaction system 2 receiving feed biomass by
railcar, by truck, or in other suitable containers, such as a Super
Sack. The biomass feed material may contain, e.g., up to about 50
weight percent moisture. The biomass is initially fed into a hopper
10. In one embodiment, the hopper 10 is a custom feed hopper
equipped with a screw conveyor or paddle screw feeder, which may be
manufactured by Carolina Conveying of 162 Great Oak Drive, Canton,
Canton N.C. 28716 that is adapted to controllably feed biomass to a
feed conveyor 14. In another embodiment, the biomass is fed
directly into a surge bin 18 from a railcar or truck unloading
station (not shown) by a conveyor belt (not shown).
[0056] A feeder 22 positioned beneath the feed hopper 10 empties
biomass onto the conveyor 14. In one embodiment, the feed conveyor
14 clears about 34 feet and provides about 6000 pounds of biomass
per hour of biomass. The feed conveyor 14 may be manufactured by
the New London Engineering Company of 1700 Division Street, New
London, Wis. 54961. The feed conveyor 14 carries the biomass to the
surge bin 18 that is equipped with a feed screw 26 at its bottom
that is preferably speed controlled and adapted to supply the
desired amount of feed at the desired rate to the reactor 6. In
another embodiment, a rotary valve or lock hoppers may be used if
the surge bin is alternatively located above the reactor, is used.
In this alternative embodiment, the biomass to fill the surge bin
18 may come directly from a receiving facility (not shown). In one
embodiment, the surge bin 18 employs low level and high level
sensors that automatically control a rotary valve and/or associated
feeder 22 located underneath the feed hopper 10 in order to
maintain a minimum amount of feed biomass in the surge bin 18. In
another embodiment, the level of Biomass in the surge bin 18 is
controlled using a continuous level sensor such as, e.g., an
ultrasonic level sensing unit. A feed screw 26 feeds biomass to the
fluid bed reactor 6. The fluid bed reactor 6 may be a custom design
or a commercially available design, e.g., fluid bed reactor model
C-FBD-36/72 by Carrier Vibrating Equipment, Inc. PO Box 37070,
Louisville, Ky.
[0057] In an alternative embodiment, the biomass is dried to a
moisture content of less than about 40 weight percent and, more
preferably, less than about 30 weight percent prior to introduction
to the reactor 6. The biomass may be pre-dried by conventional
means including, e.g., rotary kilns (see, e.g., U.S. Pat. No.
5,103,743 of Berg), cascaded whirling bed dryers (see, e.g., U.S.
Pat. No. 4,470,878 of Petrovic et al.), elongated slot dryers (see,
e.g., U.S. Pat. No. 4,617,744 of Siddoway et al.), hopper dryers
(see, e.g., U.S. Pat. No. 5,033,208 of Ohno et al.), traveling bed
dryers (see, e.g., U.S. Pat. No. 4,606,793 of Petrovic et al.),
vibrating fluidized bed dryers (see, e.g., U.S. Pat. No. 4,444,129
of Ladt) and fluidized-bed dryers or reactors (see, e.g., U.S. Pat.
No. 5,537,941 of Goldich, U.S. Pat. No. 5,546,875 of Selle et al.,
U.S. Pat. No. 5,832,848 of Reynoldson et al. U.S. Pat. No.
5,830,246, U.S. Pat. No. 5,830,247, U.S. Pat. No. 5,858,035 of
Dunlop, U.S. Pat. No. 5,637,336 of Kannenberg et al., U.S. Pat. No.
5,471,955 of Dietz, U.S. Pat. No. 4,300,291 of Heard et al. and
U.S. Pat. No. 3,687,431 of Parks), which are incorporated by
reference herein. The heat source for pre-drying the biomass may be
of the form of waste heat, other available heat sources, or
auxiliary fuels. The waste heat may be drawn from the reactor 6. In
one embodiment, the biomass is pre-dried to a moisture content of
about 12 to about 20 weight percent. In another embodiment, two or
more biomass materials, each with different moisture contents, are
blended together to provide a raw feed with a moisture content of
less than about 40 weight percent.
[0058] In one embodiment, the raw biomass feed is spread with an
integrated spreader and contacted with, e.g., off-gas from the
fluidized bed reactor in order to pre-dry the biomass before it
enters the fluidized bed 6. More specifically, FIG. 2 is a
schematic of an integrated fluid bed and pre-dryer system 30
comprised of a fluidized bed 34 with an integrated spreader 30.
Here, a feed screw 42 feeds the raw biomass onto the spreader 38
that distributes the incoming feed material so that off gases from
the fluidized bed 34 that flow, e.g., in the directions of arrows
46 contact and pre-dry the feed material before it reaches the
fluidized bed 34.
[0059] Referring again to FIG. 1, the reactor 6 is fluidized, i.e.,
a fluidized bed is established therein which may establish such a
fluidized bed by conventional means as described above. In one
embodiment, the fluidized bed reactor 6 is cylindrical and has an
aspect ratio (bed height divided by diameter) of about 2 or less;
in one embodiment, the aspect ratio ranges from about 6/3 to about
1/3. The bed within the cylindrical fluidized bed reactor
preferably has a depth of from about 1 to about 8 feet and, more
preferably, from about 2 to about 5 feet. Non-cylindrical fluidized
beds also may be used, but in one embodiment, the aspect ratio
thereof (the ratio of the bed height to the maximum cross sectional
dimension) ranges from about 2/1 to about 1/3. Non-cylindrical
fluidized beds may also include enlarged upper sections to
facilitate particle disengagement, e.g., fluid bed reactor model
C-FBD-36/72 by Carrier Vibrating Equipment, Inc. PO Box 37070,
Louisville, Ky. Bed fluidization may be achieved by fluidizing gas
that enters the reactor 6 through a perforated plate (not shown).
Fresh air is used for fluidizing, but a mixture of fresh air and
recycled gas, i.e., gas taken from the fluidized bed reactor, may
be used. It is preferred to use a blower to control the amount and
the composition of the fluidizing gas. In other embodiments,
multiple blowers may be used.
[0060] One embodiment employs a startup heater air fan system 50
that provides the air for an in-duct, natural gas-fired burner used
for preheating the fluidizing gas during startup. The startup
heater air fan system 50 may, e.g., be a blower provided with a
burner system that is manufactured by, e.g., Stelter & Brinck,
Ltd., 201 Sales Avenue, Harrison, Ohio 45030. In addition, a
recycle fan 54 is used to move the fluidized gas in a loop
comprised of lines 58, 62, 66, 70, 74, 78, 82, 86 and 90 during
startup and shutdown of the system. The recycle fan 54 may be,
e.g., a New York Blower Type HP Pressure Blower manufactured by The
New York Blower Company.
[0061] A fresh air fan 94 is used to add fresh air to the
fluidizing gas in order to adjust the oxygen content thereof. The
fresh air fan 94 may, e.g., be a New York Blower Type 2606 Pressure
Blower manufactured by The New York Blower Company. In another
embodiment, the fan 94 may be replaced with a control valve and a
suitable control valve added to line 86. During startup and
shutdown, as fresh air is added to the fluidizing gas, a vent valve
98 is used to release an equal amount of gas to the emissions
control device 102 to maintain a consistent flow of fluidizing gas
through the reactor 6.
[0062] During normal operations, vent valve 98 remains open to vent
all of the fluidizing gas to the emissions control device 102.
Gases exiting the reactor 6 enter a particulate removal device 106
where fines are separated. The particulate removal device 106 may
be, e.g., a Flex-Kleen Pulse Jet Baghouse manufactured by the Flex
Kleen Division of the Met Pro Corporation of Glendale Heights, Ill.
or a cyclone e.g., manufactured by Fisher-Kloterman, Inc., (a CECO
Environmental Company) of 822 South 15th Street, Louisville, Ky.
40210. Multiple fines removal devices may be employed to allow
coarser particulate to be recovered as additional product or as a
separate product.
[0063] Cleaned gas passes a vent valve 98 where an appropriate
amount of gas is vented to an emissions control device 102. The
purpose of the emissions control device 106 is to destroy any
carbonaceous components in the offgas after removal of particulate.
The emissions control device could be, e.g., a thermal oxidizer
manufactured by John Zink Company, LLC of 11920 East Apache, Tulsa,
Okla. 74116. Alternatively, the emissions control device could be,
e.g., a catalytic oxidizer manufactured by McGill AirClean, LLC
1779 Refugee Road, Columbus, Ohio 43207. Extra fuel may be added to
the venting gas to raise the temperature thereof. The heated gas is
then fed to a turbine to generate electricity to be used by the
plant or for sale.
[0064] In one embodiment, a typical startup procedure involves,
e.g., starting the heater air fan 50 and the recycle fan 54.
Recycle fan speed is selected to ensure sufficient gas flow to
achieve bed fluidization, preferably the apparent gas velocity in
the reactor is in the range of about 4 to 8 feet per second.
Biomass feed is started to partially fill the reactor 6 to a
predetermined startup bed height. An in-duct natural gas-fired
burner 110 is started, and the temperature of the fluidizing gas is
slowly increased. When the biomass in the reactor 6 reaches a
temperature within the range of about 175 to about 250 degrees
Centigrade, it begins to release heat as it consumes oxygen present
in the fluidizing gas. Small amounts of biomass are added to the
reactor 6 to maintain a steady rise in the temperature of the
fluidized bed. It is preferred that the temperature of the
fluidized bed be maintained at about 230 to about 350 degrees
Centigrade and, more preferably, about 310 to about 330 degrees
Centigrade. As biomass is processed it exits reactor 6 through
valve 114 into a cooler 118. A dump valve 122 can be used to remove
material buildup in the bed, or in case of emergency, be actuated
to quickly empty the reactor 6 contents into the cooler 118. Once
the fluidized bed 6 reaches the operating temperature, the startup
burner is turned down. In one embodiment, hot gasses taken from the
emissions control device 106 are used to preheat the fluidizing gas
(for example, by the process of FIG. 2) to reduce the amount of
combustion of biomass required to maintain the temperature of the
fluidized bed. The reactor 6 is preferably equipped with several
water spray nozzles (not shown) to assist in the control the
temperature of the fluidized bed. The reactor 6 is also preferably
equipped with several temperature sensors to monitor the
temperature of the fluidized bed.
[0065] At steady state, reactor 6 operation is a balance between
biomass particle size, the reactor temperature, the residence time
required for decomposition of organics material such as
hemicelluloses, the residence time required for moisture and
volatile organics to diffuse from the interior of the biomass
particles, the reaction rate of oxygen with the volatile organics,
and the gas velocity required for maintaining proper levels of
fluidization. In one embodiment, the smallest biomass particle
dimension is from about 3 mm to about 10 mm, the apparent velocity
of the fluidizing gas is from about 4 to about 8 feet per second,
the temperature of the fluidized bed is maintained at about 230 to
about 350 degrees Centigrade and, more preferably, at about 310 to
about 330 degrees Centigrade, and the average biomass particle
residence time is from about 2 minutes to about 5 minutes.
[0066] The gases leaving the reactor 6 via line 82 have an oxygen
content of less than about 15 volume percent, whereas the oxygen
content of the fluidizing gas is maintained at greater than about
18 volume percent (and, more preferably, closer to that of fresh
air) to maximize the rate of biomass processing. At the preferred
steady state conditions, the amount of heat released via the
combustion of the biomass is balanced by the amount of heat
required to accomplish torrefaction and dry the biomass added to
the reactor 6.
[0067] The off gas from reactor 6 is run through a particle
separation step to remove particles entrained in the reactor
offgas. In one embodiment, this step consists of a single unit such
as bag house (not shown) or a cyclone 106. In another embodiment,
the particle separation step includes multiple devices to
facilitate recovery of entrained particles on the basis of particle
size or density. Larger particles may be directed to the cooler for
recovery as product.
[0068] The biomass produced in reactor 6 is typically at a
temperature of about 310 to about 330 degrees Centigrade, and it
typically contains about 0 to about 1 weight percent of moisture.
This product is discharged through valve 114 which may be, e.g., a
rotary valve, lock hoppers, etc. to a cooling apparatus 118.
[0069] The preferred method for cooling, rehydration, and
stabilization occurs in one process piece of process equipment.
This could be a screw conveyor, a mixing screw conveyor, a rotary
drum, rotary tube cooler or any other device that would provide
cooling through the application of water as well as mixing. The
cooler 118 would be equipped with a multiplicity of water sprays
and temperature sensors to allow water to be applied to the product
for either progressively lowering the temperature of the product to
less than the ambient boiling point of water (100 degrees
Centigrade at sea level) and/or adding up to about 3 percent
moisture to the product. The application of water may be continuous
or intermittent. The control of water application could be on the
basis of temperature, the mass flow rate of product and/or a
combination thereof.
[0070] In one embodiment, the cooling device would be a mixing
screw such as those manufactured by Austin Mac, Inc. 2739 6.sup.th
Avenue South, Seattle, Wash. In another embodiment, the cooling
device could be a hollow flight screw cooler as manufactured by the
Therma-Flite Company of 849 Jackson Street, Benica, Calif. 94510.
The screw cooler assembly is also comprised of a multiplicity of
water sprays and temperature sensors to control the application of
water on the basis of product temperature. For example, if the rate
of temperature decrease in the cooler is too low, and/or too high,
the rate may be modified by modifying the biomass feed rate into
the system, and/or by modifying the rate at which the screw turns
and/or the rate at which water is applied using the sprays. The
water spray may be continuous, and/or it may be intermittent.
[0071] The cooled biomass from cooler 118 is discharged 70 to a
conveyor 126. The conveyor 126 conveys the cooled biomass product
to a storage system 130, a load out system for trucks or railcars
(not shown), or directly to the end user. Any gases emitted in the
cooler are directed to the emissions control device 106.
[0072] While various embodiments of the present invention have been
described in detail, it is apparent that modifications and
alterations of those embodiments will occur to those skilled in the
art. However, it is to be expressly understood that such
modifications and alterations are within the scope and spirit of
the present invention, as set forth in the following claims.
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