Method of Drying Biomass
A process for torrefaction of biomass is provided in which biomass are passed into a fluidized bed reactor and heated to a predetermined temperature in an oxidizing environment. The dried biomass is then fed to a cooler where the temperature of the product is reduced to approximately 100 degrees Fahrenheit.
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Embodiments of the present invention relate to a process for preparation of a solid fuel biomass by torrefaction. More specifically, in the process of one embodiment, the biomass material is heated directly in an oxidizing atmosphere while a portion of the processed biomass is used as fuel to produce heat required to support the torrefaction process.BACKGROUND OF THE INVENTION
“Biomass” refers to renewable organic materials such as wood, plant materials or agricultural waste. Biomass often contains about 10 to about 50 weight percent moisture, which does not add to the fuel value and increases the transportation cost of the material. “Torrefaction” refers to the treatment of biomass at a temperature between about 200° C. to about 350° 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.
Several U.S. patents and published patent applications are related to treating biomass using torrefaction. These include U.S. Pat. No. 4,553,978 (the “'978 patent”), entitled “Process for Converting Ligneous Matter of Vegetable Origin by Torrefaction, and Product obtained Thereby”; U.S. Pat. No. 4,787,917 (the “'917 patent”), entitled “Method for Producing Torrefied Wood, Product Obtained thereby, and Application to the Production of Energy”; U.S. Pat. No. 4,816,572 (the “'572 patent”), entitled “Thermocondensed Lignocellulose Material, and a Method and an Oven for Obtaining It”; U.S. Pat. No. 4,954,620 (the “'620 patent”), entitled “Thermocondensed Lignocellulose Material, and a Method and an Oven for Obtaining It”; U.S. Patent Application Publication No. 2003/0221363 (the “'363 application”), entitled “Process and Apparatus for making a Densified Torrefied Fuel”; U.S. Patent Application Publication No. 2008/0223269 (the “'269 application”), entitled “Method and Apparatus for Biomass Torrefaction Using Conduction Heating”; U.S. Patent Application Publication No. 2009/0007484 (the “'484 application”), entitled “Apparatus and Process for Converting Biomass Feed Materials into Reusable Carbonaceous and Hydrocarbon Products”; U.S. Patent Application Publication No. 2009/0084029 (the '“029 application), entitled “Process and Device for Treating Biomass”; U.S. Patent Application Publication No. 2009/0250331 (the “'331 application”), entitled “Autothermal and Mobile Torrefaction Devices”; and U.S. Patent Application Publication No. 2009/0272027, entitled “Method for the Preparation of Solid Fuels by Means of Torrefaction as well as the Solid Fuels thus Obtained and the Use of these Fuels”. The entire disclosure of each of the foregoing references is incorporated by reference herein.
All of the above references disclose torrefaction processes that employ a non-oxidizing or inert gas environment (that contain very low levels or no oxygen. Use of a non-oxidizing environment requires utilization of an external heating methodology to supply the heat necessary for torrefaction. In addition, the '978 patent discloses a torrefaction residence time of 0.5 to 5.0 hours and does not disclose a specific cooling methodology. The '917 patent discloses the use of a specific size feed material of 15 mm in length and 5 to 20 mm in diameter and cooling using an inert gas. The '572 and '620 patents disclose a torrefaction residence time of 30 minutes and do not disclose a specific cooling methodology. The '363 application discloses a process in which the air is removed from the processed biomass product at high pressure to make pellets, cubes or logs, i.e., densification, of the torrefied product immediately after torrefaction with subsequent cooling of the densified biomass pellets, but no specific cooling methodology is described. The '269 application discloses torrefaction of biomass using a specially designed oven that utilizes conduction to transfer heat from heated plates to the biomass material. The '484 application discloses the use of an externally heated ribbon channel reactor to progressively heat biomass material. The '331 application discloses combustion of vapors produced during torrefaction to supply the heat for torrefaction and the subsequent pelletizing of the torrefied product, but does not address product cooling.SUMMARY OF THE INVENTION
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.
It is another aspect of the present invention to employ a fluid bed reactor to achieve the contemplated torrefaction. The fluid bed reactor uses air as a primary fluidizing gas with any additional fluidizing gas needed supplied by heated gas drawn from fluid bed exhaust, i.e., “offgas”. The rate of fluidizing gas introduction into the fluid bed reactor would be as required to produce an apparent gas velocity within the fluid bed reactor between about 4 and 8 feet per second. At this velocity, the bed temperature of the reactor would be maintained between about 230 and 350° C.
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.
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° 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.
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.
It is another aspect of the present invention to employ pelletizing before or after cooling should the product market justify product densification.
It is another aspect of the present invention to provide a single stage process for biomass torrefaction, comprising charging biomass to a fluidized bed reactor, charging air to the fluidized bed reactor at a velocity of from about 4 to about 8 feet per second, subjecting the biomass to a temperature of from about 230 to about 350 degrees Centigrade, and removing the water from the biomass by torrefying the biomass. The biomass charged to the fluidized bed reactor of this embodiment has an average moisture content of from about 10 to about 50 percent and the fluidized bed reactor is comprised of a fluidized bed with a fluidized bed density of from about 20 to about 50 pounds per cubic foot, whereby a dried, torrefied biomass is produced.
It is still yet another embodiment of the present invention to provided a process for drying a material by directing the material to a fluidized bed reactor, pre-drying the material with gasses exhausted from the fluidized bed reactor, and subjecting said material within the fluid bed reactor to a temperature sufficient to torrefy the material and remove water therefrom.
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.
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.
To assist in the understanding of one embodiment of the present invention, the following list of components and associated numbering found in the drawings is provided below:
- # Component
- 2 Biomass torrefaction system
- 6 Fluid bed reactor
- 10 Hopper
- 14 Conveyor
- 18 Surge bin
- 22 Feeder
- 26 Feed screw
- 30 Pre-dryer system
- # Component
- 34 Fluidized bed
- 38 Spreader
- 42 Feed screw
- 46 Off gas
- 50 Startup heater
- 54 Recycle fan
- 58 Air line
- 62 Air line
- 66 Air line
- 70 Air line
- 74 Air line
- 78 Air line
- 82 Air line
- 86 Air line
- 90 Air line
- 94 Fresh air fan
- 98 Vent valve
- 102 Emissions control device
- 106 Particulate removable device
- 110 Burner
- 114 Valve
- 118 Cooler
- 122 Dump valve
- 126 Conveyor
- 130 Storage system
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
Referring now to
The biomass torrefaction system 2 receiving feed biomass by railcar, by truck, or in other suitable containers, such as a Super Sack. The biomass feed material may contain, e.g., up to about 50 weight percent moisture. The biomass is initially fed into a hopper 10. In one embodiment, the hopper 10 is a custom feed hopper equipped with a screw conveyor or paddle screw feeder, which may be manufactured by Carolina Conveying of 162 Great Oak Drive, Canton, Canton N.C. 28716 that is adapted to controllably feed biomass to a feed conveyor 14. In another embodiment, the biomass is fed directly into a surge bin 18 from a railcar or truck unloading station (not shown) by a conveyor belt (not shown).
A feeder 22 positioned beneath the feed hopper 10 empties biomass onto the conveyor 14. In one embodiment, the feed conveyor 14 clears about 34 feet and provides about 6000 pounds of biomass per hour of biomass. The feed conveyor 14 may be manufactured by the New London Engineering Company of 1700 Division Street, New London, Wis. 54961. The feed conveyor 14 carries the biomass to the surge bin 18 that is equipped with a feed screw 26 at its bottom that is preferably speed controlled and adapted to supply the desired amount of feed at the desired rate to the reactor 6. In another embodiment, a rotary valve or lock hoppers may be used if the surge bin is alternatively located above the reactor, is used. In this alternative embodiment, the biomass to fill the surge bin 18 may come directly from a receiving facility (not shown). In one embodiment, the surge bin 18 employs low level and high level sensors that automatically control a rotary valve and/or associated feeder 22 located underneath the feed hopper 10 in order to maintain a minimum amount of feed biomass in the surge bin 18. In another embodiment, the level of Biomass in the surge bin 18 is controlled using a continuous level sensor such as, e.g., an ultrasonic level sensing unit. A feed screw 26 feeds biomass to the fluid bed reactor 6. The fluid bed reactor 6 may be a custom design or a commercially available design, e.g., fluid bed reactor model C-FBD-36/72 by Carrier Vibrating Equipment, Inc. PO Box 37070, Louisville, Ky.
In an alternative embodiment, the biomass is dried to a moisture content of less than about 40 weight percent and, more preferably, less than about 30 weight percent prior to introduction to the reactor 6. The biomass may be pre-dried by conventional means including, e.g., rotary kilns (see, e.g., U.S. Pat. No. 5,103,743 of Berg), cascaded whirling bed dryers (see, e.g., U.S. Pat. No. 4,470,878 of Petrovic et al.), elongated slot dryers (see, e.g., U.S. Pat. No. 4,617,744 of Siddoway et al.), hopper dryers (see, e.g., U.S. Pat. No. 5,033,208 of Ohno et al.), traveling bed dryers (see, e.g., U.S. Pat. No. 4,606,793 of Petrovic et al.), vibrating fluidized bed dryers (see, e.g., U.S. Pat. No. 4,444,129 of Ladt) and fluidized-bed dryers or reactors (see, e.g., U.S. Pat. No. 5,537,941 of Goldich, U.S. Pat. No. 5,546,875 of Selle et al., U.S. Pat. No. 5,832,848 of Reynoldson et al. U.S. Pat. No. 5,830,246, U.S. Pat. No. 5,830,247, U.S. Pat. No. 5,858,035 of Dunlop, U.S. Pat. No. 5,637,336 of Kannenberg et al., U.S. Pat. No. 5,471,955 of Dietz, U.S. Pat. No. 4,300,291 of Heard et al. and U.S. Pat. No. 3,687,431 of Parks), which are incorporated by reference herein. The heat source for pre-drying the biomass may be of the form of waste heat, other available heat sources, or auxiliary fuels. The waste heat may be drawn from the reactor 6. In one embodiment, the biomass is pre-dried to a moisture content of about 12 to about 20 weight percent. In another embodiment, two or more biomass materials, each with different moisture contents, are blended together to provide a raw feed with a moisture content of less than about 40 weight percent.
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,
Referring again to
One embodiment employs a startup heater air fan system 50 that provides the air for an in-duct, natural gas-fired burner used for preheating the fluidizing gas during startup. The startup heater air fan system 50 may, e.g., be a blower provided with a burner system that is manufactured by, e.g., Stelter & Brinck, Ltd., 201 Sales Avenue, Harrison, Ohio 45030. In addition, a recycle fan 54 is used to move the fluidized gas in a loop comprised of lines 58, 62, 66, 70, 74, 78, 82, 86 and 90 during startup and shutdown of the system. The recycle fan 54 may be, e.g., a New York Blower Type HP Pressure Blower manufactured by The New York Blower Company.
A fresh air fan 94 is used to add fresh air to the fluidizing gas in order to adjust the oxygen content thereof. The fresh air fan 94 may, e.g., be a New York Blower Type 2606 Pressure Blower manufactured by The New York Blower Company. In another embodiment, the fan 94 may be replaced with a control valve and a suitable control valve added to line 86. During startup and shutdown, as fresh air is added to the fluidizing gas, a vent valve 98 is used to release an equal amount of gas to the emissions control device 102 to maintain a consistent flow of fluidizing gas through the reactor 6.
During normal operations, vent valve 98 remains open to vent all of the fluidizing gas to the emissions control device 102. Gases exiting the reactor 6 enter a particulate removal device 106 where fines are separated. The particulate removal device 106 may be, e.g., a Flex-Kleen Pulse Jet Baghouse manufactured by the Flex Kleen Division of the Met Pro Corporation of Glendale Heights, Ill. or a cyclone e.g., manufactured by Fisher-Kloterman, Inc., (a CECO Environmental Company) of 822 South 15th Street, Louisville, Ky. 40210. Multiple fines removal devices may be employed to allow coarser particulate to be recovered as additional product or as a separate product.
Cleaned gas passes a vent valve 98 where an appropriate amount of gas is vented to an emissions control device 102. The purpose of the emissions control device 106 is to destroy any carbonaceous components in the offgas after removal of particulate. The emissions control device could be, e.g., a thermal oxidizer manufactured by John Zink Company, LLC of 11920 East Apache, Tulsa, Okla. 74116. Alternatively, the emissions control device could be, e.g., a catalytic oxidizer manufactured by McGill AirClean, LLC 1779 Refugee Road, Columbus, Ohio 43207. Extra fuel may be added to the venting gas to raise the temperature thereof. The heated gas is then fed to a turbine to generate electricity to be used by the plant or for sale.
In one embodiment, a typical startup procedure involves, e.g., starting the heater air fan 50 and the recycle fan 54. Recycle fan speed is selected to ensure sufficient gas flow to achieve bed fluidization, preferably the apparent gas velocity in the reactor is in the range of about 4 to 8 feet per second. Biomass feed is started to partially fill the reactor 6 to a predetermined startup bed height. An in-duct natural gas-fired burner 110 is started, and the temperature of the fluidizing gas is slowly increased. When the biomass in the reactor 6 reaches a temperature within the range of about 175 to about 250 degrees Centigrade, it begins to release heat as it consumes oxygen present in the fluidizing gas. Small amounts of biomass are added to the reactor 6 to maintain a steady rise in the temperature of the fluidized bed. It is preferred that the temperature of the fluidized bed be maintained at about 230 to about 350 degrees Centigrade and, more preferably, about 310 to about 330 degrees Centigrade. As biomass is processed it exits reactor 6 through valve 114 into a cooler 118. A dump valve 122 can be used to remove material buildup in the bed, or in case of emergency, be actuated to quickly empty the reactor 6 contents into the cooler 118. Once the fluidized bed 6 reaches the operating temperature, the startup burner is turned down. In one embodiment, hot gasses taken from the emissions control device 106 are used to preheat the fluidizing gas (for example, by the process of
At steady state, reactor 6 operation is a balance between biomass particle size, the reactor temperature, the residence time required for decomposition of organics material such as hemicelluloses, the residence time required for moisture and volatile organics to diffuse from the interior of the biomass particles, the reaction rate of oxygen with the volatile organics, and the gas velocity required for maintaining proper levels of fluidization. In one embodiment, the smallest biomass particle dimension is from about 3 mm to about 10 mm, the apparent velocity of the fluidizing gas is from about 4 to about 8 feet per second, the temperature of the fluidized bed is maintained at about 230 to about 350 degrees Centigrade and, more preferably, at about 310 to about 330 degrees Centigrade, and the average biomass particle residence time is from about 2 minutes to about 5 minutes.
The gases leaving the reactor 6 via line 82 have an oxygen content of less than about 15 volume percent, whereas the oxygen content of the fluidizing gas is maintained at greater than about 18 volume percent (and, more preferably, closer to that of fresh air) to maximize the rate of biomass processing. At the preferred steady state conditions, the amount of heat released via the combustion of the biomass is balanced by the amount of heat required to accomplish torrefaction and dry the biomass added to the reactor 6.
The off gas from reactor 6 is run through a particle separation step to remove particles entrained in the reactor offgas. In one embodiment, this step consists of a single unit such as bag house (not shown) or a cyclone 106. In another embodiment, the particle separation step includes multiple devices to facilitate recovery of entrained particles on the basis of particle size or density. Larger particles may be directed to the cooler for recovery as product.
The biomass produced in reactor 6 is typically at a temperature of about 310 to about 330 degrees Centigrade, and it typically contains about 0 to about 1 weight percent of moisture. This product is discharged through valve 114 which may be, e.g., a rotary valve, lock hoppers, etc. to a cooling apparatus 118.
The preferred method for cooling, rehydration, and stabilization occurs in one process piece of process equipment. This could be a screw conveyor, a mixing screw conveyor, a rotary drum, rotary tube cooler or any other device that would provide cooling through the application of water as well as mixing. The cooler 118 would be equipped with a multiplicity of water sprays and temperature sensors to allow water to be applied to the product for either progressively lowering the temperature of the product to less than the ambient boiling point of water (100 degrees Centigrade at sea level) and/or adding up to about 3 percent moisture to the product. The application of water may be continuous or intermittent. The control of water application could be on the basis of temperature, the mass flow rate of product and/or a combination thereof.
In one embodiment, the cooling device would be a mixing screw such as those manufactured by Austin Mac, Inc. 2739 6th 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.
The cooled biomass from cooler 118 is discharged 70 to a conveyor 126. The conveyor 126 conveys the cooled biomass product to a storage system 130, a load out system for trucks or railcars (not shown), or directly to the end user. Any gases emitted in the cooler are directed to the emissions control device 106.
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.
1. A single stage process for biomass torrefaction, comprising:
- charging biomass to a fluidized bed reactor, charging air to said fluidized bed reactor at a velocity of from about 4 to about 8 feet per second, subjecting said biomass to a temperature of from about 230 to about 350 degrees Centigrade, and removing the water from said biomass by torrefying said biomass, wherein: said biomass charged to said fluidized bed reactor has an average moisture content of from about 10 to about 50 percent, said fluidized bed reactor is comprised of a fluidized bed with a fluidized bed density of from about 20 to about 50 pounds per cubic foot, whereby a dried, torrefied biomass is produced.
2. The process of claim 1, wherein said biomass charged to said fluidized bed reactor has a minimum dimension of 3 mm to 10 mm.
3. The process of claim 1, wherein said biomass charged to said fluidized bed reactor is wood, plant material or agricultural waste.
4. The process of claim 1, wherein said biomass in said fluidized bed reactor is maintained at said temperature of from about 230-350 degrees Centigrade.
5. The process of claim 1, wherein said fluidized bed is maintained at a fluidized bed density of from about 20 to about 50 pounds per cubic foot.
6. The process of claim 1, wherein said biomass has a residence time in the reactor of 2 to 5 minutes.
7. The process of claim 1, further comprising the step of heating said air prior to the time it is fed into said fluidized bed reactor using heat recovered from heated gas taken from said fluidized bed reactor.
8. The process of claim 1 wherein said cooler employs at least one of a mixing screw conveyor, hollow flight screw conveyor, a rotary drum and a rotary tube.
9. The process of claim 1, further comprising:
- feeding said dried biomass into a cooler where it is cooled to a temperature near 100 degrees Centigrade and adding no more than 3% moisture to the dried biomass.
10. The process of claim 1, wherein said cooler comprises at least one water spray and at least one temperature sensor to allow water to be applied to the biomass for at least one of progressively lowering the temperature of the biomass to less about 100 degrees Centigrade and adding up to 3 percent moisture to the biomass.
11. The process of claim 10 wherein the application of water is continuous.
12. The process of claim 10 wherein the application of water is intermittent.
13. The process of claim 1, wherein said reactor comprises at least one water spray to aid in the control of reactor temperature.
14. The process of claim 13 wherein the application of water is continuous.
15. The process of claim 13 wherein the application of water is intermittent.
16. A process for drying a material, comprising:
- directing the material to a fluidized bed reactor;
- predrying the material with gasses exhausted from the fluidized bed reactor; and
- subjecting said material within the fluid bed reactor to a temperature sufficient to torrefy the material and remove water therefrom.
17. The process of claim 16, further comprising:
- adding fresh air to said fluidized bed reactor to adjust the oxygen content of the gasses within the fluidized bed reactor.
18. The process of claim 16 wherein said material is at least one of biomass and coal.
19. The process of claim 16 wherein said fluidized bed reactor has an aspect ratio is no greater than about 2.
20. The process of claim 16 further comprising directing heated air to the fluidized bed reactor with a startup heater.