U.S. patent application number 12/822863 was filed with the patent office on 2011-12-29 for method of simultaneously drying coal and torrefying biomass.
This patent application is currently assigned to River Basin Energy, Inc.. Invention is credited to Clinton B. Camper, Vijay Sethi.
Application Number | 20110314728 12/822863 |
Document ID | / |
Family ID | 45351174 |
Filed Date | 2011-12-29 |
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United States Patent
Application |
20110314728 |
Kind Code |
A1 |
Camper; Clinton B. ; et
al. |
December 29, 2011 |
Method of Simultaneously Drying Coal and Torrefying Biomass
Abstract
A process for simultaneously drying coal and/or torrefying
biomass is provided in which coal and/or biomass are passed into a
fluidized bed reactor and heated to a predetermined temperature.
The dried coal and/or biomass is then fed to a cooler where the
temperature of the product is reduced to approximately 200 degrees
Fahrenheit and water is added to further passivate the coal.
Inventors: |
Camper; Clinton B.;
(Billings, MT) ; Sethi; Vijay; (Laramie,
WY) |
Assignee: |
River Basin Energy, Inc.
Highlands Ranch
CO
|
Family ID: |
45351174 |
Appl. No.: |
12/822863 |
Filed: |
June 24, 2010 |
Current U.S.
Class: |
44/606 ; 44/605;
44/608; 44/626 |
Current CPC
Class: |
Y02E 50/30 20130101;
Y02E 50/15 20130101; C10L 5/445 20130101; Y02E 50/10 20130101; C10L
9/083 20130101; C10L 5/04 20130101; C10L 5/442 20130101 |
Class at
Publication: |
44/606 ; 44/605;
44/608; 44/626 |
International
Class: |
C10L 5/44 20060101
C10L005/44; C10L 5/00 20060101 C10L005/00 |
Claims
1. A method for drying coal, comprising: charging coal to a
fluidized bed reactor, said coal having an average moisture content
of from about 15 to about 50 percent, and 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;
charging air to said fluidized bed reactor at a velocity of from
about 4 to about 8 feet per second; subjecting said coal to a
temperature of from about 230 to about 350 degrees Centigrade; and
removing water from said coal wherein 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.
2. The process of claim 1, wherein said coal charged to said
fluidized bed reactor is under about 3 inches in size.
3. The process of claim 1, wherein said coal charged to said
fluidized bed reactor is classified as bituminous, sub-bituminous
or lignite.
4. The process of claim 1, wherein said coal in said fluidized bed
reactor is maintained at said temperature of from about 400-650
degrees Fahrenheit.
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 coal has a residence time
in the reactor of about 2 to about 5 minutes.
7. The process of claim 1, further comprising heating said air
prior to introduction into said fluidized bed reactor using heat
recovered from heated gas taken from said fluidized bed reactor or
from the downstream emissions control device.
8. The process of claim 1 further comprising a cooler that 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, wherein said reactor comprises at least
one water spray to aid in the control of reactor temperature.
10. The process of claim 1, further comprising feeding said dried
coal into a cooler where it is cooled to a temperature below 100
degrees Centigrade and mixed with water to restore a portion of the
product's equilibrium moisture.
11. The process of claim 10 wherein the application of water is
continuous.
12. A process for biomass torrefaction, comprising: charging
biomass to a fluidized bed reactor, said biomass charged to said
fluidized bed reactor has an average moisture content of from about
15 to about 50 percent, and 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; charging air to said fluidized bed
reactor at a velocity of from about 3 to about 8 feet per second;
subjecting said biomass to a temperature of from about 230 to about
350 degrees Centigrade wherein said air is heated using heat
recovered from heated gas taken from said fluidized bed reactor,
heated gas taken from the reactor associated with drying coal, or
heated gas taken from the process emissions control device; and
torrefying said biomass wherein: said biomass charged to said
fluidized bed reactor has an average moisture content of from about
15 to about 50 percent, and 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.
13. The process of claim 12, wherein said biomass charged to said
fluidized bed reactor has a minimum dimension of about 3 mm to
about 10 mm.
14. The process of claim 12, wherein said biomass charged to said
fluidized bed reactor is wood, plant material or agricultural
waste.
15. The process of claim 12, wherein said biomass has a residence
time in the reactor of 2 to 15 minutes.
16. The process of claim 12 further comprising a cooler that
employs at least one of a mixing screw conveyor, hollow flight
screw conveyor, a rotary drum and a rotary tube.
17. The process of claim 12, wherein said reactor comprises at
least one water spray to aid in the control of reactor
temperature.
18. The process of claim 12, further comprising feeding said dried
biomass into a cooler where it is cooled to a temperature below 100
degrees Centigrade and mixed with water to restore up to 3% of the
product's moisture.
19. The process of claim 18 wherein the application of water is
continuous.
20. A 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 15 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; and wherein heat used for said
single stage process is taken from exhaust associated with a coal
drying process.
Description
[0001] This application is related to pending U.S. patent
application Ser. No. 12/763,355, filed Apr. 20, 2010. This
application is also related to pending U.S. patent application Ser.
No. 12/566,174, filed Sep. 24, 2009 (the '174 application"), which
is a continuation-in-part of pending U.S. patent application Ser.
No. 11/316,508, filed Dec. 22, 2005, which is a
continuation-in-part of U.S. patent application Ser. No. 10/978,768
(the '768 application), now U.S. Pat. No. 7,537,622 filed Nov. 1,
2004, which is a continuation-in-part of abandoned U.S. patent
application Ser. No. 09/974,320 (the '320 application), filed on
Oct. 10, 2001, and a continuation-in-part of the '320 application.
The '174 application also claims the benefit of pending U.S.
Provisional Patent Application Ser. No. 61/212,668, filed Apr. 14,
2009. The entire disclosure of each of the identified patents and
patent applications is incorporated by reference herein.
FIELD OF THE INVENTION
[0002] A multi-stage process for simultaneous coal drying and
biomass torrefaction in fluidized bed reactors is provided. More
specifically, in the process of one embodiment, the biomass
material is treated by a torrefaction process in an oxidizing
atmosphere with heated exhaust generated by a coal drying
process.
BACKGROUND OF THE INVENTION
[0003] Co-firing is commonly employed to reduce emissions of
NO.sub.x, SO.sub.2 and CO.sub.2 associated with the combustion of
coal. Co-firing is generally defined as combusting a mixture of a
fossil fuel, i.e., coal, and a renewable fuel, i.e. a biomass.
"Biomass" is a renewable energy source derived from wood, lawn
waste and alcohol fuels, for example. Many coals contain about 15
to about 40 weight percent of moisture. Similarly, many biomass
materials contain about 10 to about 50 weight percent moisture.
This moisture not only does not add to the fuel value, but also
increases the transport cost of the material. Thus it is desirable
to dry both the coal and biomass.
[0004] "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.
[0005] 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 Torrefation 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.
[0006] All of the above references disclose torrefaction processes
that employ a non-oxidizing or inert gas environment that contains
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.
[0007] Several United States patents have issued to the applicant
for drying coal in a fluidized bed reactor. These include U.S. Pat.
No. 5,830,246, entitled "Process for Processing Coal", U.S. Pat.
No. 5,830,247 (the '247 patent), entitled "Process for Processing
Coal", U.S. Pat. No. 5,858,035, entitled "Process for Processing
Coal", U.S. Pat. No. 5,904,741, entitled "Process for Processing
Coal", U.S. Pat. No. 6,162,265, (the '265 patent) entitled "Process
for Processing Coal" and U.S. Pat. No. 7,537,622, entitled "Process
for Drying Coal". The entire disclosure of each of these United
States patents is hereby incorporated by reference into this
specification.
[0008] The '265 patent is illustrative of these patents that
teaches a process for preparing an irreversibly dried coal with a
first fluidized bed reactor that includes a first fluidized bed
with a fluidized bed density of from about 30 to about 50 pounds
per cubic foot is maintained at a temperature of from about 480 to
about 600 degrees Fahrenheit. Coal with a moisture content of about
15 to about 30 percent and a particle size such that all of the
coal particles in such coal are in the range of from 0 to 2 inches
is fed into the first fluidized bed. In addition, liquid phase
water, inert gas, and air is fed into the first fluidized bed that
is subjected, along with the coal, to a temperature of about 480 to
about 600 degrees Fahrenheit for about 1 to about 5 minutes while
simultaneously comminuting and dewatering said coal. This process
produces coal of coarse fraction and a fine fraction. The fine
fraction is sent to a cyclone wherein a portion of the fine
fraction is removed and fed to a cooler that reduces the
temperature of the fine fraction by at least about 300 degrees
Fahrenheit. The comminuted and dewatered coal is then passed to a
second fluidized bed reactor that includes a second fluidized bed
with a fluidized bed density of about 30 to about 50 pounds per
cubic foot and a temperature of about 215 to about 250 degrees
Fahrenheit. Water, inert gas and oil of about 0.5 to about 3.0
weight percent of mineral oil having an initial boiling point of at
least about 900 degrees Fahrenheit are also fed to the second
fluidized bed. The temperature change experienced by the coal as it
moves from the first fluidized bed reactor to the second fluidized
bed reactor, from about 480 to about 600 degrees Fahrenheit to
about 215 to about 250 degrees Fahrenheit, and associated processes
occur in less than about 120 seconds.
[0009] The process described in the '265 patent works well with
reactors with a diameter of less than about 4 feet, which generally
have an output of about 200 tons per day. With larger reactors,
where the output(s) often exceed 1,000 tons per day, the process is
often not as efficient. Without being bound to any particular
theory, it is believed that, as the size of the reactor increases,
the gas velocity produced in the process increases geometrically,
often to the point where the desired density of the fluidized bed
used suffers. As the density of the fluidized bed declines, the
efficiency of the drying process decreases. It is thus a long felt
need to provide an improved process for drying coal and biomass or
a mixture thereof.
SUMMARY OF THE INVENTION
[0010] It is one aspect of the present invention to process coal
and/or biomass of high moisture content by employing a pre-drying
step. Thus, in accordance with various embodiments of the present
invention, a multi-stage process is provided for drying coal and/or
biomass. In the first stage of the process, a coal with a moisture
content from about 15 to about 40 percent and a biomass with a
moisture content from about 10 to 50 percent is heated in a first
fluidized bed reactor at a temperature of between about 400 to
about 650 degrees Fahrenheit until about 40 to about 60 percent of
the water is removed and until at least about 50 percent of the
particles less than about 400 microns are removed. During the first
stage process, air is fed into the first fluidized bed reactor at a
rate of from about 5 to about 8 feet per second.
[0011] In the second stage of the process, the dried coal and/or
biomass material treated by the first fluidized bed reactor is
heated in a second fluidized bed reactor at a temperature up to
about 250 degrees Fahrenheit until the coal and/or biomass contains
less than 1 weight percent of water. In one embodiment, the
temperature used in the second fluidized bed reactor is at least
about 50 degrees greater than the temperature used in the first
fluidized bed reactor. During the second stage process, air is fed
into the second fluidized bed reactor at a rate less than that
associated with the first fluidized bed reactor.
[0012] One of skill in the art will appreciate that although a
pre-drying step that utilizes a fluidized bed reactor is
contemplated, other methods may be employed to pre-dry the coal
and/or biomass without departing from the scope of the invention.
Furthermore, it will be appreciated that a pre-drying step may not
be required.
[0013] It is another aspect of the present invention to both cool
and add equilibrium moisture to the treated coal and/or biomass as
it exits the second stage or the initial drying stage if no
pre-drying is required. It is desirable to add moisture to the
processed coal and biomass to reduce the probability of post-drying
combustion of the coal portion. Thus, one embodiment employs a
fluid spray that cools the coal exiting the drying stage. The water
also replaces at least a portion of the coals equilibrium
moisture.
[0014] 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.
[0015] 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 fluidization 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 3 and 8 feet per second. At this velocity, the bed
temperature of the reactor would be maintained between about 230
and 350.degree. C.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] It is another aspect of the present invention to employ
pelletizing before or after cooling should the product market
justify product densification.
[0020] It is thus one aspect of embodiments of the present
invention to provide a method for drying coal, comprising: charging
coal to a fluidized bed reactor, the coal has an average moisture
content of from about 15 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;
charging air to the fluidized bed reactor at a velocity of from
about 4 to about 8 feet per second; subjecting the coal to a
temperature of from about 230 to about 350 degrees Centigrade; and
removing water from the coal wherein 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.
[0021] It is another aspect to provide a process for biomass
torrefaction, comprising: charging biomass to a fluidized bed
reactor, the biomass charged to the fluidized bed reactor has an
average moisture content of from about 15 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; charging air to the fluidized bed reactor at a velocity of
from about 3 to about 8 feet per second; subjecting the biomass to
a temperature of from about 230 to about 350 degrees Centigrade
wherein the air is heated using heat recovered from heated gas
taken from the fluidized bed reactor, heated gas taken from the
reactor associated with drying coal, or heated gas taken from the
process emissions control device; and torrefying the biomass
wherein: the biomass charged to the fluidized bed reactor has an
average moisture content of from about 15 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.
[0022] It is still yet another aspect of the present invention to
provide a 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, wherein: the biomass charged to
the fluidized bed reactor has an average moisture content of from
about 15 to about 50 percent, 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; and wherein heat used for the single
stage process is taken from exhaust associated with a coal drying
process.
[0023] 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
[0024] 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.
[0025] FIG. 1 is a schematic of one process for preparing a
coal-biomass-water slurry;
[0026] FIG. 2 is a schematic of one process for drying the coal
and/or biomass used in the process of FIG. 1;
[0027] FIG. 3 is a schematic of one apparatus that may be used in
the process of FIG. 2;
[0028] FIG. 4 is a schematic of another preferred apparatus that
may be used in the process of FIG. 2;
[0029] FIG. 5 is a schematic of another embodiment of the present
invention for cooling by immediately subjecting a product as it
exits the fluid bed to a water spray of sufficient quantity to cool
the blend and replace a portion of the blend equilibrium
moisture;
[0030] FIG. 6 is a schematic of one process for drying coal and/or
biomass;
[0031] FIG. 7 is a detailed portion of FIG. 6;
[0032] FIG. 8 is a detailed portion of FIG. 6 showing a cooling
portion;
[0033] FIG. 9 is a schematic of a biomass torrefaction process of
one embodiment of the present invention;
[0034] FIG. 10 is a detailed view of FIG. 9, showing a fluid bed
reactor;
[0035] FIG. 11 is a schematic of a process of drying biomass using
exhaust gases from a fluidized bed reactor for drying coal of one
embodiment of the present invention; and
[0036] FIG. 12 is a partial schematic view of stacked fluid bed
reactor used to dry coal and biomass.
[0037] 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
[0038] Referring now to FIGS. 1 and 2, a process for preparing a
coal-biomass-water slurry and drying the same is shown. In order to
remove water from coal and/or biomass, drying processes have been
employed that utilize fluidized bed reactors. A fluidized bed
reactor is a system wherein a fluid or gas is passed through a
granular solid material at high velocities to suspend the solid and
cause it to behave as if it were fluid (i.e. fluidization). In one
embodiment, the coal used in the process of this specification is
similar to the coal having about 5 to about 30 weight percent of
moisture and, more preferably about 10 to about 30 weight percent
of moisture. The biomass is similar to biomass having about 10 to
50 weight percent moisture. However, in the instant case, the coal
used may often contain up to about 40 weight percent of water. The
moisture content of coal and/or biomass may be determined by
conventional means in accordance with standard A.S.T.M. testing
procedures. Methods for determining the moisture content of coal
are well known in the art; see, e.g., U.S. Pat. No. 5,527,365
(irreversible drying of carbonaceous fuels), U.S. Pat. Nos.
5,503,646, 5,411,560 (production of binderless pellets from low
rank coal), U.S. Pat. Nos. 5,396,260, 5,361,513 (apparatus for
drying and briquetting coal), U.S. Pat. No. 5,327,717, and the
like. The entire disclosure of each of these United States patents
is hereby incorporated by reference herein.
[0039] In one embodiment of the present invention, the coal used in
the process of this invention contains from about 10 to about 25
percent of combined oxygen. For example, the coal used in the
process of FIG. 1 contains from about 10 to about 20 weight of
combined oxygen, in the form, e.g., of carboxyl groups, carbonyl
groups, and hydroxyl groups. As used herein, the term "combined
oxygen" means oxygen which is chemically bound to carbon atoms in
the coal. See, e.g., H. H. Lowry, editor, "Chemistry of Coal
Utilization" (John Wiley and Sons, Inc., New York, N.Y., 1963). . .
. The combined oxygen content of such coal may be determined, e.g.,
by standard analytical techniques; see, e.g., U.S. Pat. Nos.
5,444,733, 5,171,474, 5,050,310, 4,852,384 (combined oxygen
analyzer), U.S. Pat. No. 3,424,573, and the like. The disclosure of
each of these United States patents is hereby incorporated by
reference into this specification.
[0040] In one embodiment of the present invention, the coal used in
the process of the instant invention contains about 10 to about 25
weight percent of ash. In one embodiment, the coal charged to
feeder 12 contains at least about 10 weight percent of ash. As used
herein, the term "ash" refers to the inorganic residue left after
the ignition of combustible substances; see, e.g., U.S. Pat. No.
5,534,137 (high ash coal), U.S. Pat. No. 5,521,132 (raw coal fly
ash), U.S. Pat. No. 4,795,037 (high ash coal), U.S. Pat. No.
4,575,418 (removal of ash from coal), U.S. Pat. No. 4,486,894
(method and apparatus for sensing the ash content of coal), and the
like. The disclosure of each of these United States patents is
hereby incorporated by reference into this specification. By way of
further illustration, one suitable ash containing coal which may be
used in this embodiment is Herrin number 6 coal, from Illinois.
[0041] The coal produced by the process of U.S. Pat. No. 5,830,246,
when subbituminous coal is used as the starting material, has a
particle distribution that renders it unsuitable for making a
stable slurry. When this coal is mixed with from about 25 to about
35 weight percent of water (by total weight of water and coal), the
slurry thus produced is unstable. It is thus an object of one
embodiment of this invention to provide a stable coal-water slurry
made from subbituminous coal, wherein said slurry has a solids
content of at least about 65 weight percent and a heating value
that is at least about 80 percent of the heating value of the
undried coal.
[0042] Referring to FIG. 1, in step 10 subbituminous coal and/or
biomass are dried to a moisture content of less than about 5
percent. In one embodiment, the process of U.S. Pat. No. 5,830,246
is utilized to dry such coal. A drying process for preparing an
irreversibly dried coal is used wherein a fluidized bed reactor is
provided that has a fluidized density of about 10 to about 40
pounds per cubic foot. The fluidized bed reactor is maintained at a
temperature of about 400-650 degrees Fahrenheit while coal with a
moisture content of about 5 to about 30 percent and a combined
oxygen content of from about 10 to about 20 percent and biomass
with a moisture content of about 10 to 50 percent is feed thereto.
In addition, mineral oil with an initial boiling point of at least
about 900 degrees Fahrenheit is fed into the reactor about 0.5 to
about 3.0 percent by weight of dried coal, thereby producing coated
coal and/or biomass. The coated coal and/or biomass is then held at
a temperature from about 400-650 degrees Fahrenheit in said reactor
for about 1 to about 5 minutes while simultaneously comminuting and
dewatering the coated coal and/or biomass. After the coated coal
and/or biomass is exposed to an ambient environment at about 25
degrees Centigrade (about 77 degrees Fahrenheit) and a relative
humidity of 50 percent, the coated coal and/or biomass contains
less than about 2.0 percent of moisture by weight, at least about
80 weight percent of the particles of said coated coal and/or
biomass are smaller than 74 microns, and the coal has a combined
oxygen content of from about 10 to about 20 weight percent.
[0043] In another embodiment, the process the '247 patent is used
in order to prepare the dried subbituminous coal and/or biomass.
More specifically, one embodiment of the present invention employs
a process for preparing an irreversibly dried coal and/or biomass
with a first fluidized bed reactor with a fluidized bed density of
about 20 to about 40 pounds per cubic foot, wherein the reactor is
maintained at a temperature of about 150 to about 200 degrees
Fahrenheit. Coal with a moisture content of about 15 to about 30
percent, an oxygen content of about 10 to about 20 percent, biomass
with a moisture content of 10 to 50 percent, and a particle size
such that all of the coal particles are in the range of from 0 to 2
inches are fed into the reactor. The coal in the reactor is then
subjected to a temperature of about 150 to about 200 degrees
Fahrenheit for about 1 to about 5 minutes while simultaneously
comminuting and dewatering the coal and/or biomass. The comminuted
and dewatered coal and/or biomass is then passed to a second
fluidized bed reactor with a fluidized bed density of about 20 to
about 40 pounds per cubic feet, wherein the reactor is at a
temperature of about 480 to about 600 degrees Fahrenheit. The
second fluidized bed reactor is also fed mineral oil with an
initial boiling point of at least about 900 degrees Fahrenheit of
about 0.5 to about 3.0 weight percent by weight of dried coal
and/or biomass, thereby producing a coated coal and/or biomass. The
coated coal and/or biomass is subjected to a temperature of about
480 to about 600 degrees Fahrenheit for about 1 to about 5 minutes
while simultaneously comminuting and dewatering the coated coal
and/or biomass, whereby a comminuted and dehydrated coal and/or
biomass is produced.
[0044] The use of a particular subbituminous coal with specified
properties, the drying step 10 is often important in order to
obtain a stable slurry. It should be noted that other coals often
do not require such a drying step in order to produce a stable
slurry. For example, by way of illustration and not limitation,
U.S. Pat. No. 4,282,006, (the '006 patent), the entire disclosure
of which is hereby incorporated by reference herein, discloses the
preparation of a 75 weight percent coal-water slurry using coal
from the Black Mesa mine is described (see Example 3). The
properties and chemical composition of such coal, however, is not
described in the '006 patent. Without wishing to be bound to any
particular theory, it is believed that the Black Mesa coal does not
have a combined oxygen content of from about 10 to about 25
percent. If the Black Mesa coal did have such an oxygen content,
one of skill in the art would not have been able to make a stable
slurry by drying. It has been discovered that, when coal with an
oxygen content of about 10 to about 25 percent is mixed with a
sufficient amount of water to produce a slurry with a solids
content of about 65 to about 75 weight percent, such slurry is
often not stable. When such coal is first dried and then modified
in accordance with the process of FIG. 1, a stable slurry may often
be made from such coal.
[0045] Referring again to FIG. 1, after the dried coal and/or
biomass has been produced in step 10, it is subject to a sieving
operation in step 12 to remove oversize particles. For example, in
such an operation all particles greater than about 700 microns are
removed. In another embodiment, all particles greater than about
500 microns are removed. The oversize particles are then fed via
line to a mill 16, wherein they are ground and then recycled via
line 18 to the dry subbituminous coal/biomass supply 10. The
undersize particles are fed via line 20 to a mixer 22 wherein a
sufficient amount of water is added via line 24 to produce a
coal/biomass/water mixture with a solids content by weight of dry
coal of about 65 to about 75 weight percent. Additionally, a
dispersing agent and/or an electrolyte may be added in accordance
with the process described in the '006 patent. The sieved, dried
coal is then fed via line 26 to a mill 28, for example, a ball
mill, in which the coal is ground to the particle size distribution
described in the '006 patent. In particular, the coal is ground
until at least about 5 weight percent of its particles are of
colloidal size, and until a coal compact is produced that is
described by the "CPFT" formula set forth in claim 1 the '006
patent. One of skill in the art will appreciate that one or more
other coal compacts may be added to the mill 28 via a line 30,
and/or water and/or surfactant and/or electrolyte may be added. In
one embodiment, the coal-biomass-water slurry produced in the mill
28 is also deashed 32. In one embodiment, the deashing process
described in U.S. Pat. No. 4,468,232 is used, the entire disclosure
of which is hereby incorporated by reference herein.
[0046] One embodiment of the present invention employs a process
for preparing a clean coal-biomass-water slurry wherein a
coal-water mixture comprised of about 60 to about 80 volume percent
of solids that are grinded until a coal-biomass-water slurry is
produced. The contemplated slurry has a yield stress of about 3 to
about 18 Pascals and a Brookfield viscosity at a solids content of
70 volume percent at ambient temperature and pressure, and a shear
rate of 100 revolutions per minute of less than 5,000 centipoise.
The slurry is comprised of a consist of finely divided particles of
coal dispersed in water wherein the specific surface area of about
0.8 to about 4.0 square meters per cubic centimeter and an
interstitial porosity of less than about 20 volume percent.
Furthermore, about 5 to about 70 weight percent of said finely
divided particles of coal in the water are of colloidal size, being
smaller than about 3.0 microns.
[0047] FIG. 2 is a flow diagram of a multi stage process 50 for
drying coal and/or biomass of one embodiment. The coal used in
process 50 is similar to the coal described in column 1 (see lines
16-61 of column 3) of the '265 patent, with the exception that it
preferably contains from about 15 to about 40 weight percent of
moisture, may contain from about 10 to about 25 weight percent of
combined oxygen, and may contain from about 10 to about 25 weight
percent of ash. The biomass used in process 50 would be wood or
agricultural products containing 10 to 50 weight percent
moisture.
[0048] Furthermore, coal used in process 50 may be lignitic or
sub-bituminous coal as disclosed at lines 62 et seq. of column 3 of
the '265 patent. These coals are also described in U.S. Pat. No.
5,145,489, the entire disclosure of which is hereby incorporated by
reference into this specification. In one embodiment, the coal
and/or biomass used in step 52 is 2''.times.0'', and more
preferably, 2'' by 1/4''. As is known to those skilled in the art,
2'' by 1/4'' material has all of its particles within the range of
about 0.25 to about 2.0 inches. Crushed coal conventionally has an
about 2''.times.0'' to 3''.times.0'' particle size distribution.
This crushed coal can advantageously be used in at least some of
the process disclosed herein.
[0049] Initially, raw coal and/or biomass is fed into a first
reactor and is shown as step 52 wherein the raw coal and/or biomass
is preferably fed from a feeder 102 (see FIG. 3; also see FIG. 4).
The feeder 102 may be similar to, or identical to the feeder 12
described in the '265 patent, i.e., any coal feeder commonly used
in the art. Thus, e.g., one may use one or more of the coal feeders
described in U.S. Pat. Nos. 5,265,774, 5,030,054
(mechanical/pneumatic coal feeder), U.S. Pat. No. 4,497,122 (rotary
coal feeder), U.S. Pat. Nos. 4,430,963, 4,353,427 (gravimetric coal
feeder), U.S. Pat. Nos. 4,341,530, 4,142,868 (rotary piston coal
feeder), U.S. Pat. No. 4,140,228 (dry piston coal feeder), U.S.
Pat. No. 4,071,151 (vibratory high pressure coal feeder with
helical ramp), U.S. Pat. No. 4,149,228, and the like. The
disclosure of each of these United States patents is hereby
incorporated by reference into this specification. The feeder of
one embodiment is comprised of a hopper and a star feeder. It is
also contemplated that feeder be capable of continually delivering
coal and/or biomass to fluidized bed. The star feeder is a metering
device that may be operated by a controller that controls the rate
of coal removal from a hopper; see, e.g., U.S. Pat. No. 5,568,896,
the entire disclosure of which is hereby incorporated by reference
into this specification. In another embodiment, the coal and/or
biomass have separate feeders to allow proportions to be adjusted
more quickly during operation.
[0050] Referring now to FIGS. 2 and 3, and in step 54 thereof, air
is introduced into the first fluidized bed reactor 110 via line
106. The air may be introduced by conventional means such as, e.g.,
a blower (not shown). In one embodiment, the air so introduced is
hot air at a temperature of about 250 to about 400 degrees
Fahrenheit, and preferably of about 300 to about 350 degrees
Fahrenheit. The air is introduced via line 106 into a fluidized bed
112 in order to maintain the temperature of such fluidized bed 112
at a temperature of about 300 to about 550 degrees Fahrenheit and,
preferably of about 450 to about 500 degrees Fahrenheit. Without
wishing to be bound to any particular theory, it is believed that
this hot air helps oxidize a portion of the coal and/or biomass in
the first reactor 110, thereby supplying energy required to
vaporize the water in such coal and/or biomass. It is also believed
that this oxidation occurs on the most reactive sites on the coal
and/or biomass. In the case of the coal fraction, this oxidation
reduces the natural equilibrium moisture thereof. Equilibrium
moisture is defined as the amount of water that will chemically
combine with the coal and is measured by ASTM procedure D 1412.
Reducing the equilibrium moisture of the coal reduces the amount of
water the coal can absorb from the environment which reduces the
tendency to self-heat that can result in spontaneous combustion of
the coal.
[0051] In one embodiment of the present invention, the air is
introduced via line 106 into fluidized bed 112 at a fluidizing
velocity in the reactor vessel of greater than about 4 feet per
second; the air is injected and preferably greater than about 5
feet per second. In one aspect of this embodiment, the air is
introduced via line 106 at a fluidizing velocity of from about 5 to
about 8 feet per second. In another aspect of this embodiment, the
air is introduced via line 6 at a fluidizing velocity of from about
6 to about 8 feet per second. Without wishing to be bound to any
particular theory, it is believed that maintaining the air flow
within the desired ranges is essential for maintaining the desired
conditions within the fluidized bed 112 and for conducting an
efficient drying operation.
[0052] In step 56 of the process, the reactor 110 is fluidized,
i.e., a fluidized bed is established therein. One may establish
such a fluidized bed by conventional method such as, e.g., the
method disclosed in the '265 patent. In one embodiment, the
fluidized bed 114 is provided in a reactor vessel 110 and is
comprised of a bed of fluidized coal and/or biomass particles, and
it preferably has a density of from about 20 to about 40 pounds per
cubic foot. In one embodiment, the density of the fluidized bed 20
is from about 20 to about 30 pounds per cubic foot. The fluidized
bed density is the density of the bed while its materials are in
the fluid state and does not refer to the particulate density of
the materials in the bed. The fluidized bed may be provided by any
of the means well known to those skilled in the art. Reference may
be had, e.g., to U.S. Pat. Nos. 5,145,489, 5,547,549, 5,546,875
(heat treatment of coal in a fluidized bed reactor), U.S. Pat. No.
5,197,398 (separation of pyrite from coal in a fluidized bed), U.S.
Pat. No. 5,087,269 (drying fine coal in a fluidized bed), U.S. Pat.
No. 4,571,174 (drying particulate low rank coal in a fluidized
bed), U.S. Pat. No. 4,495,710 (stabilizing particulate low rank
coal in a fluidized bed), U.S. Pat. No. 4,324,544 (drying coal by
partial combustion in a fluidized bed), and the like. In the
process of this instant invention, air is fed into the fluidized
bed to heat the fluidized bed and to maintain the bed at the
desired density. Without wishing to be bound to any particular
theory, it is believed that, in order to efficiently maintain the
fluidized bed 114 at the desired density, the air flow into the
fluidized bed should preferably be from about 5 to about 8 feet per
second. Air flow outside of these ranges may not yield the desired
results.
[0053] The reactors 110 and 138 are often cylindrical and may be
large when used with one-stage processes and often require gas
velocities of about 18 feet per second or more. Without wishing to
be bound to any particular theory, it is believed that velocities
of this magnitude often result in excessive entrainment of the
fluidized bed and/or may distort the fluidization in the fluidized
bed. In any event, velocities of this magnitude do not produce the
drying results obtained by embodiments of the present
invention.
[0054] In step 58, the fluidized bed 114 is heated by conventional
means such as, e.g., using hot air provided in another reactor (not
shown) and/or another device. Thus, e.g., one may provide the hot
air to line 106 from a separate fluidized bed reactor. The
fluidized bed is preferably maintained at a temperature of about
150 to about 200 degrees Fahrenheit. In a more preferred
embodiment, the fluidized bed 14 is maintained at a temperature of
about 165 to about 185 degrees Fahrenheit. Various means may be
used to maintain the temperature of fluidized bed 114 at a
temperature of from about 150 to about 200 degrees Fahrenheit.
Thus, e.g., one may use an internal or external heat exchanger (not
shown). See, e.g., U.S. Pat. Nos. 5,537,941, 5,471,955, 5,442,919,
5,477,850, 5,462,932, and the like. In one embodiment, hot gas
from, e.g., a separate fluidized bed reactor is fed into fluidized
bed 114. This hot gas preferably is at temperature of about 480 to
about 600 degrees Fahrenheit and, more preferably, at a temperature
of about 525 to about 575 degrees Fahrenheit. In another embodiment
the air fed via line 106 is hot air provided by a heat exchanger,
not shown. Thus, e.g., one may use an internal or external heat
exchanger (not shown). See, e.g., U.S. Pat. Nos. 5,537,941,
5,471,955, 5,442,919, 5,477,850, 5,462,932, and the like; the
entire disclosure of each of these United States patents is
incorporated by reference herein.
[0055] It will be seen that a portion of the air fed via line 106
is diverted via line 108 into reactor 138, thereby effecting step
74 (the heating of the fluidized bed 113 in reactor 138). The air
fed into reactor 113 is preferably fed at a velocity of from about
8 to about 12.2 feet per second. Without wishing to be bound to any
particular theory, it is believed that this rate of air flow in
reactor 138 is essential to maintain the fluidized bed under the
desired conditions and to obtain the desired efficiency of drying;
the use of lower or higher air flow velocities is undesirable and
ineffective.
[0056] In step 62 of the process, coal and/or biomass "fines" are
removed from the reaction mass disposed within the fluidized bed
112. These fines portions (i.e., those with a particle size less
than about 400 microns) are entrained from the top 116 of the
fluidized bed to the cyclone 120 via line 118. The coarser
component of the entrained stream will preferably be cooled in
cooler 128, as are the coarser components from cyclone 124. The
finer fraction from cyclone 120 is preferably passed via line 122
to cyclone 124. The coarser component from cyclone 124 is then fed
to cooler 128 and the fraction so cooled is then passed to storage
132. The exhaust gas fed via line 134 is blended with the air in
line 108 and the blended hot gases are then fed into the reactor
138. One may use any of the cyclones conventionally used in fluid
bed reactors useful for separating solids from gas. Thus, e.g., one
may use as cyclone, the cyclones described in U.S. Pat. No.
5,612,003 (fluidized bed with cyclone), U.S. Pat. No. 5,174,799
(cyclone separator for a fluidized bed reactor), U.S. Pat. Nos.
5,625,119, 5,562,884, and the like. The entire disclosure of each
of these United States patents is hereby incorporated by reference
herein.
[0057] Water in reactor 110 is removed from the coal and/or biomass
fed via line 104. This step is also indicated, as step 68, in FIG.
2. The raw coal fed via line 104 of one embodiment contains from
about 15 to about 40 weight percent of water. The biomass fed via
line 104 of one embodiment contains about 0 to 50 weight percent of
water. By comparison, the coal and/or biomass withdrawn via line
136 contains about 40 to about 60 percent less water. Put another
way, the ratio of the water concentration in the raw feed divided
by the water concentration in the dried product is from about 1.6
to about 2.5. The water removed from the coal and/or biomass within
the reactor 110 is passed together with flue gas and fines via line
118 to cyclone 120 and thence, via line 122 to cyclone 124.
Thereafter, it passes via line 134 to condenser 135, wherein it is
removed. The gas passing from condenser is preferably substantially
dry, containing less than about 5 weight percent of water.
Thereafter, this dry gas is mixed with the air in line 108 and
thence fed into the fluidized bed 113 as its fluidizing medium. The
raw coal and/or biomass from feeder 102 is maintained in reactor
110 for a time sufficient to remove from about 40 to about 60
weight percent of the water in the feed stock. Generally, such
"residence time" is preferably less than about 15 minutes and
frequently is from about 5 to about 12 minutes.
[0058] In step 60, the dried coal and/or biomass from reactor 110
is removed from such reactor and fed into reactor 138 via line 136.
Simultaneously, or sequentially, in step 72 exhaust gas is fed via
line 108 from line 106, and is preferably mixed with dry gas from
condenser 135, and it is then fed into fluidized bed 113.
[0059] In step 74 of the process, the fluidized bed 113 is heated
to a temperature that preferably is at least about 50 degrees
Fahrenheit, higher than the temperature at which fluidized bed 112
is maintained. The temperature in fluidized bed 113 preferably is
about 450 to about 650 degrees Fahrenheit and, more preferably,
about 550 to about 600 degrees Fahrenheit. The fluidized bed 113 is
preferably heated by both the hot coal and/or biomass fed via line
136, and/or the heat in the gas fed via line 108, and/or the
combustion processes involved in said fluidized bed (often referred
to as "off gas"). In a manner similar to that depicted for reactor
110, water is removed from the coal in fluidized bed 113, and such
coal and/or biomass is then discharged via line 154; in general,
the water content of such coal and/or biomass is preferably less
than about 1 weight percent.
[0060] The water removed from the coal and/or biomass in reactor
138 is fed via line 140 (together with "fines" and as) to cyclone
142 and thence via line 144 to a condenser 146; the waste water
from condenser 146 is then removed via line 150. This step is
depicted as step 84 in FIG. 2.
[0061] In step 76, the fines are removed from the reactor 138 via
line 140. The solid product from cyclone 142 is then fed via line
152 and preferably blended with the dry coal and/or biomass from
line 154. The blend is then fed to cooler 156, wherein it is
preferably cooled to a temperature less than 200 degrees
Fahrenheit. In one embodiment, FIG. 5, cooling is accomplished by
immediately subjecting the product as it exits the fluid bed to a
water spray of sufficient quantity to cool the blend and replace a
portion of the blend's equilibrium moisture. In another embodiment,
only a portion of the needed water is added at the exit of the
reactor and the remaining amount is added in the cooler. This
cooler could be any number devices that facilitate adding water and
mixing the product e.g. a mixing screw conveyor as manufactured by
Austin Mac, Inc. 2739 6.sup.th Avenue South, Seattle, Wash. In
another embodiment, all of the cooling and equilibrium moisture
addition occurs in the cooler. In another embodiment, a cooler that
incorporates indirect cooling surfaces such as a hollow flight
screw conveyor as manufactured by the Therma-Flite Company of 849
Jackson Street, Benica, Calif. 94510 or a Roto-Tube cooler as
manufactured by FMC Corporation of 400 Highpoint Drive Chalfont,
Pa. 18914 is used to cool the product before water is added to
replace a portion of the equilibrium moisture. After cooling the
blend is fed via line 158 to storage.
[0062] FIG. 4 is a schematic of a preferred apparatus which is
similar to the apparatus depicted in FIG. 2 but utilizes a single,
compartmentalized vessel instead of the two reactor vessels 110 and
138.
[0063] FIG. 5 is an alternate embodiment of the present invention
wherein the coal and/or biomass product is cooled after it exits
the last fluidized bed. The temperature of the coal product is
sensed by a temperature sensor 160 as the coal is directed to a
water spray 102 that cools the coal product to a temperature less
than the ambient boiling point of water (212 degrees Fahrenheit at
sea level). The water spray may be continuous and/or be selectively
controlled by a rotary valve 164 motor that is indexed to open a
solenoid valve to inject water. The temperature of the coal and/or
biomass product is also sensed by sensor 160 after being cooled.
The coal and/or biomass product may be further cooled in a screw
168, which will be described in further detail below with reference
to FIGS. 6-8. The contemplated temperature sensors can be any
temperature sensing device, but, preferably an optical sensor. One
of skill in the art will appreciate that although one embodiment
employs water to cool the coal and/or biomass, other cooling
techniques may be employed such as direct contact cooling that may
use an inert gas or a heat exchanger, without departing from the
scope of the invention.
[0064] FIG. 6 is a schematic of a preferred coal drying and/or
biomass torrification system 210 wherein feed coal and/or biomass
are received at the site of the system 210 by railcar, by truck, or
in Super Sacks. The feed material may contain, e.g., up to about 40
weight percent of moisture. In one embodiment, the coal used in the
process and system has an equilibrium moisture of about 18 to about
26 percent. The term equilibrium moisture is the amount of moisture
in the coal that is bonded to the atoms in the coal and is measured
by ASTM procedure D1412. The coal and/or biomass are fed into feed
hopper 214. In one embodiment, the feed hopper 214 is a custom feed
hopper equipped with a 6 inch Plattco rotary valve 212
(manufactured by the Plattco Corporation of 7 White Street,
Plattsburgh, N.Y. 12901) at its bottom that is adapted to
controllably feed coal to the feed conveyor 218. In one aspect of
this embodiment, valve 212 is connected via a link (not shown) to a
controller (not shown). In another embodiment, the coal and/or
biomass are fed directly into surge bin 220 from a railcar or truck
unloading station (not shown) by a conveyor belt (not shown).
[0065] The rotary valve 212 empties coal and/or biomass onto a
conveyor 218 that, in part runs underneath the feed hopper 214. In
one embodiment, the feed conveyor 218 is New London Feed Conveyor
clearing about 34 feet and capable of providing about 6000 pounds
per hour of coal and/or biomass; this feed conveyor is manufactured
by the New London Engineering Company of 1700 Division Street, New
London, Wis. 54961. The feed conveyor 218 carries the coal and/or
biomass to a surge bin 220 equipped with a feed screw 222 at its
bottom that is preferably speed controlled and adapted to supply
the desired amount of feed at the desired rate to the reactor. In
another embodiment, not shown, instead of feed screw 222 one could
use a rotary valve or lock hoppers if the surge bin was above the
reactor. In this embodiment, the coal and/or biomass to fill the
surge bin may come directly from a receiving facility (not shown).
In one embodiment, the surge bin 220 is comprised of low level and
high level sensors (not shown) that automatically control the
rotary valve 212 located underneath the feed hopper 214 in order to
maintain a minimum amount of feed coal in the surge hopper. In
another embodiment, the surge bin 220 level is controlled using a
continuous level sensor such as, e.g., an ultrasonic level sensing
unit. In another embodiment, the coal and/or biomass have separate
feed systems similar to those described above. The feed screw 222
feeds coal and/or biomass via line 224 to fluid bed reactor 226.
The fluid bed reactor 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.
[0066] In one embodiment, not shown in FIG. 6, prior to the time
the coal and or biomass is fed to reactor 226 it is dried to a
moisture content of less than about 40 weight percent and, more
preferably, less than about 30 weight percent. The coal and/or
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. Nos. 5,830,246,
5,830,247, and 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) all of which are incorporated by reference herein. The
heat source for pre-drying the coal may be of the form of waste
heat, other available heat sources, or auxiliary fuels. In one
embodiment, the coal and/or biomass is pre-dried to a moisture
content of about 12 to about 20 weight percent. In one embodiment,
two or more coals, as well as 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.
[0067] In one embodiment, the raw feed is spread with a spreader
and contacted with, e.g., off-gas from the fluidized bed reactor in
order to dry it to the desired moisture content. FIG. 7 is a
schematic of an integrated fluid bed and pre-dryer system 300
comprised of a fluidized bed 302 and a spreader 304. Here, a feed
screw 308 feeds the raw coal and/or biomass via line 310 onto the
spreader 304. The spreader 304 distributes the incoming feed
material so that off gases from the fluidized bed 302 that flow,
e.g., in the directions of arrows 306 contact and pre-dry the feed
material prior to the time it reaches the fluidized bed 302. One
skilled in the art would recognize that spreader 304 enhances the
operability of the system but its exclusion would not prevent the
system from being operated. One skilled in the art would also
recognize that the single reactor feed point could be replaced with
a multiplicity of feeders spaced around the perimeter of the
reactor either in the vapor space or beneath the surface of the
bed.
[0068] Referring again to FIG. 6, the reactor 226 is fluidized,
i.e., a fluidized bed is established therein. One may establish
such a fluidized bed by conventional means as described above. In
one embodiment, the fluidized bed reactor is cylindrical and has an
aspect ratio (bed height divided by diameter) of 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 4 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 226 through a perforated plate (not shown). Fresh air is
used for fluidizing, but a mixture of fresh air and recycle gas may
be used. It is preferred to use one blower to control the amount
and the composition of the fluidizing gas. In other embodiments,
multiple blowers may be used.
[0069] One embodiment employs a startup heater air fan system 228
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 228 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 230 is used to move the fluidized gas in a loop
comprised of lines 232, 234, 236, 238, 240, 242, 244, 252, and 254.
The recycle fan 230 may be, e.g., a New York Blower Type HP
Pressure Blower manufactured by The New York Blower Company.
[0070] A fresh air fan 256 is used to add fresh air to the
fluidizing gas in order to adjust the oxygen content of the
fluidizing gas. The fresh air fan 256 may, e.g., be a New York
Blower Type 2606 Pressure Blower manufactured by The New York
Blower Company. In another embodiment, the fan 256 may be replaced
with a control valve and a suitable control valve added to line
252. As fresh air is added to the fluidizing gas, a vent valve 264
is used to release an equal amount of gas to the emissions control
device 270 to maintain a consistent flow of fluidizing gas through
the reactor. In one embodiment, the velocity of the fluidizing gas
is less than about 12 feet per second and, preferably, from about 4
to about 9 feet per second. Gases exiting the reactor 226 enter a
dust removal device 262 where coal fines are separated; the dust
removal device 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. In another embodiment, there
may be multiple fines removal devices to allow coarser dust to be
recovered as additional coal product or as a separate product.
[0071] Clean gas passes a vent valve 264 where an appropriate
amount of gas is vented to an emissions control device 270 and the
balance becomes a part of the "closed loop recirculation." The
purpose of the emissions control device 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 Catalytic Product,
International of 980 Ensell Road, Lake Zurich, Ill. 60047-1557. The
emissions control device could also be a gas turbine where extra
fuel is added to the gas to raise the temperature and the gases are
then expanded to generate electricity for the plant use or for sale
to the utility.
[0072] In one embodiment, a typical startup procedure involves,
e.g., starting the heater air fan 228 and the recycle fan 230.
Recycle fan speed is selected to ensure sufficient gas flow to
achieve bed fluidization. Coal and/or biomass feed is started to
fill the reactor 226 to the desired bed height. An indication of a
full bed is overflow of coal from the reactor 226 to the screw
cooler. An in-duct natural gas-fired burner 266 is started, and the
temperature of the fluidizing gas is slowly increased. The burner
266 may be, e.g., a Stelter & Brink 2,000,000 btu per hour
process air heater. When the coal and/or biomass in the reactor 226
reaches a temperature within the range of about 350 to about 400
degrees Fahrenheit, it begins to release heat as it consumes oxygen
present in the fluidizing gas. Small amounts of coal are added to
the reactor 226 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 450 to about 650 degrees
Fahrenheit and, more preferably, about 580 to about 620 degrees
Fahrenheit. As coal and/or biomass are processed it exits reactor
226 through valve 268 into the cooler 260. Once the fluidized bed
reaches the operating temperature, the startup burner is shut down.
In one embodiment, hot gasses produced by the emissions control
device are used to preheat the fluidizing gas to reduce the amount
of combustion of coal and/or biomass required to maintain the
temperature of the fluidized bed. The reactor 226 is preferably
equipped with several water spray nozzles (not shown) to assist in
the control the temperature of the fluidized bed. The reactor is
also preferably equipped with several temperature sensors to
monitor the temperature of the fluidized bed. Dump valve 66 can be
used to remove buildup in the bed or in case of emergency, be
actuated to quickly empty the reactor contents into the cooler
260.
[0073] The gases leaving the reactor 226 via line 244 have an
oxygen content of less than 15 volume percent, whereas the oxygen
content of the fluidizing gas is maintained at greater than about
15 volume percent (and, more preferably, closer to that of fresh
air) to maximize the rate of coal processing. In one embodiment,
the temperature of the fluidized bed is maintained at about 450 to
about 650 degrees Fahrenheit and, more preferably, at about 580 to
about 620 degrees Fahrenheit. At the preferred steady state
conditions, the amount of heat released via the combustion of the
coal and/or biomass is balanced by the amount of heat required to
heat and dry the coal and/or biomass added to the reactor. As will
be apparent, the temperature of the fluidized bed can be controlled
by varying the rate of coal and/or biomass feed into the system, by
the amount of oxygen supplied to the system through the fluidizing
gas and/or by injection of water in the fluidized bed.
[0074] The off gas from reactor 226 is run through a particle
separation step to remove particles entrained in the reactor
offgas. In one embodiment, this step consists of a single baghouse
such as baghouse 262 or a cyclone (not shown). 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.
Cooling and Stabilizing of the Dried Coal
[0075] The coal and/or biomass produced in reactor 226 is typically
at a temperature of about 580 to about 620 degrees Fahrenheit, and
it typically contains about 0 to about 2 weight percent of
moisture. This product is discharged through feeder 268 which may
be, e.g., a rotary valve, lock hoppers, to a cooling/rehydration
step. This hot, substantially dry product is then cooled and
partially rehydrated in the next step of the process.
[0076] The cooling, rehydration and stabilizing step can be applied
to coal and/or biomass produced via the process described herein
above. Alternatively, or additionally, it can be applied to heated,
and partially or fully dried coal and/or biomass produced by other
processes. Thus, e.g., one may produce such a heated, partially
dried coal and/or biomass using one or more of the processes
described in the above-identified patents related to the drying of
coal.
[0077] The preferred method for cooling, rehydration, and
stabilization occurs in one process piece of process equipment 260.
This could be a screw conveyor, a rotary drum, rotary tube cooler
or any other device that would provide cooling through heat
transfer surfaces and or the application of water as well as
mixing. This cooler 260 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 (212
degrees Fahrenheit at sea level) and/or adding about 5 to 12
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. One of skill in the art will
appreciate that the apparatus shown in FIG. 5 may generally be
employed.
[0078] In one embodiment, the cooling, rehydration and
stabilization device would be a hollow flight screw cooler as
manufactured by the Therma-Flite Company of 849 Jackson Street,
Benica, Calif. 94510. FIG. 8 is a schematic of a hollow-flight
screw cooler 400 that is comprised of a chamber 402, a hollow screw
404 disposed in the center of said chamber, a device for rotating
the hollow screw (not shown), a hollow jacket 406 disposed around
said chamber, a first cool water inlet port 408, a second cool
water inlet 410, a first hot water outlet 412, and a second hot
water outlet 414. The rotating screw assembly 404 is comprised of a
hollow shaft 418 and a multiplicity of hollow flights 420. Cold
water is fed into inlet port 408 and exchanges heat with the
material that contacts the flights 420 and the shaft 418. Cold
water is also fed into jacket inlet port 410 and exchanges heat
with the material that contacts the chamber 402.
[0079] Hot and dry coal and/or biomass enters the hollow-flight
screw cooler through port 416, and contacts rotating screw assembly
404. As the hot coal and/or biomass contacts the rotating screw
assembly 404, it is not only moved from inlet port 416 toward
outlet port 422, and during such passage it exchanges heat with the
flights 420, shaft 418 and chamber 402 and is also cooled. The
hollow-flight screw cooler assembly 400 is comprised of a
multiplicity of temperature sensors 420 that are adapted to sense
the temperature at various points in the screw assembly. The screw
cooler assembly is also comprised of a multiplicity of water sprays
424.
[0080] In the cooling, rehydration and stabilization step, the goal
is to reduce the temperature of the hot coal received in from the
reactor from a temperature of about 580 to about 620 degrees
Fahrenheit to a temperature of less than about 250 degrees
Fahrenheit using heat exchange surfaces 420, 418 and 402 described
above. Using a heat exchange device for the initial cooling slows
the rate of heat removal from the product which reduces thermal
shock and may improve the mechanical integrity of the product coal.
Further cooling of the product to a temperature between about 170
degrees Fahrenheit and about 200 degrees Fahrenheit is obtained
using direct application of water using the water sprays 424. The
direct application of water also enhances the stability of the
product by restoring the moisture content of the coal to
equilibrium moisture content, a value typically in the about 6 to
12 weight percent range.
[0081] The multiplicity of temperature sensors and/or the mass flow
rate of the product are used to control the water sprays 424. The
temperature sensors may also be used as a part of the overall
process control. For example, if the rate of temperature decrease
in the hollow-flight screw cooler is too low, and/or too high, the
rate may be modified by modifying the coal and/or biomass feed rate
into the system, and/or by modifying the rate at which the screw
turns and/or the rate at which cooling water is circulated and/or
the rate at which water is applied using the sprays. The water is
preferably sprayed onto the coal and/or biomass after it has
reached a temperature of less than about 250 degrees Fahrenheit.
The water spray may be continuous, and/or it may be intermittent.
In one preferred embodiment, water is introduced only in the last
half of the hollow-flight screw cooler assembly 400.
[0082] The water sprayed into the system has at least two
functions. The first is to cool the coal and/or biomass by
transferring heat from the coal and/or biomass to the water and
possibly vaporizing some of the water. The second function of the
sprayed water is to rehydrate the coal until it preferably contains
about 6 to about 12 weight percent of moisture. By rehydrating the
coal in the hollow-flight screw cooler, the applicants claim to
restore the coal to a state such that when stored in an open-air
coal pile, further moisture pick-up is either slowed or eliminated.
The rehydration step in that regard pacifies the processed coal
against further hydration. The water so introduced preferably
reduces the temperature of the coal and/or biomass to less than 212
degrees Fahrenheit. In one preferred embodiment, water is
introduced only in the last half of the hollow-flight screw cooler
assembly such that the coal and/or biomass at the outlet port 422
is at less than 180 degrees Fahrenheit.
[0083] Referring again to FIG. 6, the cooled rehydrated and
stabilized coal and/or biomass from cooler 260 is discharged via
line 270 to conveyor 272. The conveyor 272 conveys the dried
rehydrated stable coal and/or biomass product to a storage system
274 (not shown), a load out system for trucks or railcars, or
directly to the end user. Any gases emitted in the hollow-flight
screw cooler are directed to the emissions control device 270.
Properties of the Coal and/or Biomass Produced in One Embodiment of
the System
[0084] In one embodiment, the coal and/or biomass produced in the
process of the invention shown in FIGS. 6-8 contains about 36 to
about 39 weight percent of volatile matter and about 50 to about 65
weight percent of fixed carbon, and such coal and/or biomass has a
heating value of about 9,300 to about 11,000 btu (British thermal
units) per pound. The coal and/or biomass produced in the process
of the invention preferably contains about 6 to about 12 weight
percent of moisture. It is "irreversibly dried," i.e., its
equilibrium moisture is reduced from the native coal which reduces
its tendency to absorb moisture. The coal is stabilized wherein 50
to 100 percent of the equilibrium moisture is added back to the dry
coal product to minimize the reaction rate and heat generation
associated with moisture absorption.
The Use of a Vibrating Fluidizing Bed
[0085] In one embodiment, a vibrating fluidizing bed is utilized in
assembly 10. This type of fluidizing bed is well known to those
skilled in the art and is described, e.g., in U.S. Pat. No.
4,444,129 (the '129 patent), the entire disclosure of which is
hereby incorporated by reference into this specification. The '129
patent discloses a process for drying wet coal fines smaller than
28 mesh in size that employs both a vibrating fluidized bed type
dryer and a coal fired burner for supplying hot drying gases to the
dryer. In the apparatus used in this process, a regenerative
separator is interposed between the coal fired burner and the
fluidized bed type dryer to satisfactorily remove particle matter
from the gases without unacceptable pressure losses. Hot gases
exhausted from the fluidized bed type dryer are also cleansed to
remove particulate coal particles which are used as fuel for the
coal fired burner.
[0086] For example, in one embodiment a method of drying fine coal
particles smaller than approximately 28 mesh in size is employed
wherein gases are heated by a coal fueled burner. The heated gases
are passed through a regenerative separator to remove substantially
all particulate ash particles therefrom to prevent ignition of the
coal particles. The passing coal particles are passed from an inlet
to an outlet of a fluidized bed type dryer wherein the gases from
the regenerative separator are directed through the fluidized bed
dryer to dry the coal particles as they are passed therethrough.
The dried coal particles are collected for future use as fuel and
the bulk of the gases are recirculated to the burner prior to
passing the gases through the regenerative separator, gases not
recirculated being limited to that amount required to discharge
water equivalent to that removed from the coal stream to be dried.
In one embodiment, passing the heated gases through the
regenerative separator includes providing rotational movement to
the gases by intersecting curved blades positioned in the path of
the heated gases, and removing the ash particles during the
rotation of the gases through the side of the separator. Further,
at least a portion of the gases to collect particulate coal
exhausted from the dryer and utilizing the collected particulate
coal to fuel the coal fueled burner. In addition, at least some of
the coal particles from gas exhausted from the dryer for mixing
with dry coal paths delivered through the outlet of the dryer. The
rotational movement of the gases may also be stopped in the
regenerative separator before entry into the fluidized bed dryer,
by intersecting blades curved in the opposite direction from the
rotation.
[0087] Referring again to FIG. 6, in the system 210, fluidizing gas
may be introduced into the fluidized bed reactor 226 (and, in
particular, into the fluidized bed thereof) at a fluidizing
velocity in the reactor vessel of greater than about 4 feet per
second and, more preferably, greater than about 5 feet per second.
In one aspect of this embodiment, the fluidizing gas is introduced
at a fluidizing velocity of about 5 to about 8 feet per second. In
another aspect of this embodiment, the fluidizing gas is introduced
at a fluidizing velocity of about 6 to about 8 feet per second. In
one embodiment, the fluidized bed reactor 226 is a cylindrical
reactor that may be heated by conventional means such as, e.g.,
using hot air provided in another reactor (not shown) and/or
another device. The fluidized bed 214 is preferably maintained at a
temperature of about 150 to about 200 degrees Fahrenheit. In a more
preferred embodiment, the fluidized bed 214 is maintained at a
temperature of about 165 to about 185 degrees Fahrenheit. Various
means may be used to maintain the temperature of fluidized bed at
its desired temperature. Thus, e.g., one may use an internal or
external heat exchanger (not shown). See, e.g., U.S. Pat. Nos.
5,537,941, 5,471,955, 5,442,919, 5,477,850, 5,462,932, and the like
which are incorporated by reference herein. In another embodiment,
not shown, hot fluidizing gas from a heat exchanger may be fed to
fluidized bed reactor 226. One may use an internal or external heat
exchanger (not shown). See, e.g., U.S. Pat. Nos. 5,537,941,
5,471,955, 5,442,919, 5,477,850, 5,462,932, and the like which are
hereby incorporated by reference herein.
[0088] Referring now to FIG. 9, a biomass torrefaction system 500
implements a process for preparing torrefied biomass. More
specifically, one embodiment of the present invention employs a
fluidized bed reactor 506 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
percent moisture is, preferably, determined by conventional methods
in accordance with standard A.S.T.M. testing procedures.
[0089] The biomass torrefaction system 500 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 510. In one embodiment, the hopper 510 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 514. In another embodiment, the biomass
is fed directly into a surge bin 518 from a railcar or truck
unloading station (not shown) by a conveyor belt (not shown).
[0090] A feeder 522 positioned beneath the feed hopper 510 empties
biomass onto the conveyor 514. In one embodiment, the feed conveyor
514 clears about 34 feet and provides about 6000 pounds of biomass
per hour of biomass. The feed conveyor 514 may be manufactured by
the New London Engineering Company of 1700 Division Street, New
London, Wis. 54961. The feed conveyor 514 carries the biomass to
the surge bin 518 that is equipped with a feed screw 526 at its
bottom that is preferably speed controlled and adapted to supply
the desired amount of feed at the desired rate to the reactor 506.
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 518 may come directly from a receiving facility (not shown). In
one embodiment, the surge bin 518 employs low level and high level
sensors that automatically control a rotary valve and/or associated
feeder 522 located underneath the feed hopper 510 in order to
maintain a minimum amount of feed biomass in the surge bin 518. In
another embodiment, the level of Biomass in the surge bin 518 is
controlled using a continuous level sensor such as, e.g., an
ultrasonic level sensing unit. A feed screw 526 feeds biomass to
the fluid bed reactor 506. The fluid bed reactor 506 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.
[0091] 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 506. 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 506.
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.
[0092] 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. 10 is a
schematic of an integrated fluid bed and pre-dryer system 530
comprised of a fluidized bed 534 with an integrated spreader 530.
Here, a feed screw 542 feeds the raw biomass onto the spreader 538
that distributes the incoming feed material so that off gases from
the fluidized bed 534 that flow, e.g., in the directions of arrows
546 contact and pre-dry the feed material before it reaches the
fluidized bed 534.
[0093] Referring again to FIG. 9, the reactor 506 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 506 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 2/1 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.
[0094] One embodiment employs a startup heater air fan system 550
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 550 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 558, 562, 566, 570, 574, 578, 582, 586 and 590
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.
[0095] A fresh air fan 594 is used to add fresh air to the
fluidizing gas in order to adjust the oxygen content thereof. The
fresh air fan 594 may, e.g., be a New York Blower Type 2606
Pressure Blower manufactured by The New York Blower Company. In
another embodiment, the fan 594 may be replaced with a control
valve and a suitable control valve added to line 586. During
startup and shutdown, as fresh air is added to the fluidizing gas,
a vent valve 598 is used to release an equal amount of gas to the
emissions control device 602 to maintain a consistent flow of
fluidizing gas through the reactor 506.
[0096] During normal operations, vent valve 598 remains open to
vent all of the fluidizing gas to the emissions control device 602.
Gases exiting the reactor 506 enter a particulate removal device
606 where fines are separated. The particulate removal device 506
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.
[0097] Cleaned gas passes a vent valve 598 where an appropriate
amount of gas is vented to an emissions control device 602. The
purpose of the emissions control device 606 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. In one embodiment, extra
fuel is added to the vented gas in order to further raise the
temperature so that the gas exiting the emissions control device
may be fed to a turbine to generate electricity to be used by the
plant or for sale.
[0098] In one embodiment, a typical startup procedure involves,
e.g., starting the heater air fan 550 and the recycle fan 554.
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 610 is started, and the temperature of the fluidizing gas is
slowly increased. When the biomass in the reactor 506 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 506 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 506 through
valve 614 into a cooler 618. A dump valve 622 can be used to remove
material buildup in the bed, or in case of emergency, be actuated
to quickly empty the reactor 506 contents into the cooler 618. 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 606 are used to preheat the fluidizing gas
(for example, by the process of FIG. 10) to reduce the amount of
combustion of biomass required to maintain the temperature of the
fluidized bed. The reactor 506 is preferably equipped with several
water spray nozzles (not shown) to assist in the control the
temperature of the fluidized bed. The reactor 506 is also
preferably equipped with several temperature sensors to monitor the
temperature of the fluidized bed.
[0099] At steady state, reactor 506 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 3 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.
[0100] The gases leaving the reactor 506 via line 582 have an
oxygen content of less than about 12 volume percent, whereas the
oxygen content of the fluidizing gas is maintained at greater than
about 15 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.
[0101] The off gas from reactor 506 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 606. 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.
[0102] The biomass produced in reactor 506 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 614 which may be, e.g., a
rotary valve, lock hoppers, etc. to a cooling apparatus 618.
[0103] 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 618 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.
[0104] 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.
[0105] The cooled biomass from cooler 618 is discharged 570 to a
conveyor 626. The conveyor 626 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 606.
[0106] Referring now to FIG. 11, a schematic of one embodiment of
the present invention used to dry both coal and biomass is provided
that uses the heated off gases from the coal fluid bed reactor 702
to dry biomass in a biomass fluid bed reactor 716 associated with a
biomass torrefication process. In operation, a coal fluid bed
reactor 702 receives un-processed coal via a feed screw 706. During
startup, A burner 710 heats ambient air that is transferred to the
coal fluid bed reactor 702 by a startup fan (not shown) via line
718. Once the reactor begins generating its own heat, the startup
burner is no longer needed. Once the coal is dried it exits the
coal fluid bed reactor 702 and is deposited on a cooler 720 that
cools the dried coal and optionally adds fluid thereto to prevent
unwanted combustion. The heated gases exiting the coal fluid bed
reactor 702 via a line 722 are directed to a particulate removal
device 726. Heated gas exiting the particulate removal device 726
via line 730 can be directed to line 734 to be directed to line 738
and used to temper the oxygen content of the fluidizing gas for
Reactor 702. Normally, line 734 has no flow.
[0107] The exhaust gases travel in line 730 to line 774 to supply
hot gas for fluidizing the biomass fluid bed reactor 746. Excess
gas produced by the coal fluid bed reactor 702 is sent directly to
an emissions control device 794 via line 742. Air is supplied via
fan 778 and mixed with offgas from coal fluid bed reactor 702 in
exhaust line 774 to provide fluidizing gases with controlled oxygen
content to line 782. The fan 784 of line 750 boosts the gas
pressure therein. The gas in line 750 is heated in heat exchanger
786 and transferred to line 754, the line that supplies heated gas
used to fluidize the biomass that is fed to the biomass fluid bed
reactor 746 by a feed screw 758. The gas in line 750 may be
alternatively heated by a start up burner similar to that employed
in the coal drying process. After the biomass is dried it is
directed to a cooler 760 where fluid may be added thereto, a
process similar to that employed in the coal drying process. Hot
exhaust gases from the biomass fluid bed reactor 746 travel via
line 762 to a particulate removal device 766. Hot gases exiting the
particulate removal device 766 are directed via line 770 to the
emissions control device 794. The scrubbed hot gas/air mixture
exiting the emissions control device 794 is directed to heat
exchanger 786 via line 750 that is used to transfer heat to the
biomass fluidizing gases to supplement the heat provided by the
coal fluidized bed reactor 702 exhaust.
[0108] The emissions control device 794 also receives any gas
produced during cooling i.e., steam, of the dried coal and dried
biomass, via lines 798 and 802, respectively. After the clean hot
gas leaves the emission control device 794 via line 806, is fed to
heat exchanger 786 via line 810 and to heat exchanger 814, via line
812. The gas leaving heat exchanger 786 is vented to atmosphere via
line 818. Optional heat exchanger 814 uses heat from the gas taken
from the emissions control device 794 to heat ambient air moved by
fan 822 that is fed via line 826 to line 738. The relatively cool
gas leaving heat exchanger 814 via line 830 is also vented to
atmosphere.
[0109] FIG. 12 shows a stacked fluid bed reactor system. In this
embodiment, the two reactors depicted in FIG. 11 are replaced by
the single stacked reactor system. The ratio of the biomass bed
area and the coal bed area is between 2.0/1 and 3.5/1 so that
apparent gas velocities in both beds are maintained in the range
required to achieve fluidization. Coal is treated in a lower fluid
bed reactor 702 wherein the exhaust thereof directly fluidizes a
bed of biomass positioned in an upper portion 746 of the combined
fluid bed reactor. The operating conditions for both reactors are
the same as described for the separate reactor system described
earlier. If additional oxygen is needed to maintain oxygen
concentration in the fluidizing gas for the upper reactor, it would
be injected in the section between the beds. The processed coal is
sent to a coal cooler 726 and the processed biomass is sent to a
cooler 760. Exhaust associated with both the coal processing and
the biomass processing is sent to a particulate removal device and
then to an emissions control device. Heated gases exiting the
emissions control device is used to preheat the fluidizing air
and/or the air injected into the reactor section between the two
reactor beds. One skilled in the art will appreciate that the
offgas from the lower bed will contain dust that would be removed
by particulate removal device 726 in FIG. 11. Equipment (not shown)
to alleviate grid plate fouling would include enlargement of the
openings in the upper bed grid plate and installation of high
pressure nozzles that would remove particulate from grid holes
during operation.
[0110] 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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