METHOD OF DRYING BIOMASS

Abstract

A process for torrefaction of biomass is provided in which biomass are passed into a fluidized bed reactor and heated to a predetermined temperature in an oxidizing environment. The dried biomass is then fed to a cooler where the temperature of the product is reduced to approximately 100 degrees Fahrenheit.

Claims

1. A process for producing torrefied biomass, comprising: directing a feed stream of biomass to a fluidized bed reactor, wherein the fluidized bed reactor has an aspect ratio no greater than about 2; directing a gas to the fluidized bed reactor, wherein the gas comprises a reactive oxygen; heating the biomass in the fluidized bed reactor with a first heat source, which provides heat energy into the fluidized bed reactor, to a first temperature sufficient to evaporate water in the biomass and to convert a portion of the biomass to vaporized organic compounds, wherein a dry biomass is produced that contains less than about 10 wt. percent moisture; and heating the dry biomass with the first heat source and a second heat source associated with combustion of the vaporized organic compounds, wherein heat from the first heat source and the second heat source intermingle to provide heat energy to produce a torrefied biomass from the dry biomass.

2. The process of claim 1, further comprising adding air to the fluidized bed reactor to adjust an oxygen content of the first heat source and the second heat source within the fluidized bed reactor.

3. The process of claim 1, further comprising adding coal to the feed stream.

4. The process of claim 1, further comprising directing heated air to the fluidized bed reactor with a startup heater.

5. The process of claim 1, wherein the first temperature does not initiate a torrefaction reaction.

6. The process of claim 1, wherein torrefied biomass is cooled in a mixer or a hollow flight screw cooler.

7. The process of claim 1, wherein the torrefied biomass is consolidated into pellets or briquettes before cooling.

8. The process of claim 1, wherein energy recovery of the torrefied biomass is between about 80% and 85% of the content of the biomass of the feed stream, and wherein substantially all sulphur is removed from the biomass of the feed stream.

9. The process of claim 1, wherein mass of the torrefied biomass has a mass of between 50% and 65% of that of the biomass of the feed stream.

10. The process of claim 1, wherein a pressure in the fluidized bed is near ambient.

11. The process of claim 1, wherein the moisture content of the biomass of the feed stream is between about 10 and 50 wt. percent.

12. The process of claim 1, wherein the moisture content of the torrefied biomass is less than about 1 wt. percent.

13. The process of claim 1, further comprising adding water to the torrefied biomass to increase a moisture content to about 3 wt. percent.

14. The process of claim 1, wherein the gas has an oxygen content of greater than about 10 volume percent.

15. The process of claim 1, wherein the gas comprises air.

16. The process of claim 1, wherein a temperature of the produced by the first heat source and the second heat source is between about 230 C. and about 350 C.

17. The process of claim 16, wherein the biomass is exposed to the first temperature between 15 minutes and 20 minutes.

18. The process of claim 1, wherein the gas comprises an offgas.

19. The process of claim 18, wherein a percent of water vapor in the offgas is between about 10 and 17 vol. percent, and wherein a percent of carbon dioxide in the offgas is between about 4 and 5 vol. percent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] 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.

[0024] FIG. 1 is a schematic representation showing the relationship between biomass, coal, and charcoal torrefaction;

[0025] FIG. 2 is a schematic of a biomass torrefaction process of one embodiment of the present invention;

[0026] FIG. 3 is a detailed view of FIG. 2 showing a fluid bed reactor used in the process of one embodiment of the present invention;

[0027] FIG. 4 is a table showing wood biomass data; and

[0028] FIG. 5 is a table showing bio-coal data.

[0029] 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:

TABLE-US-00002 # Component 2 Biomass torrefaction system 6 Fluid bed reactor 10 Hopper 14 Conveyor 18 Surge bin 22 Feeder 26 Feed screw 34 Plate 46 Off gas 50 Startup heater combustion air fan 54 Recycle fan 58 Recycle Gas line 62 Recycle Gas line 66 Recycle Gas line 70 Heated Fluidizing Gas line 74 Heated Fluidizing Gas line 78 Heated Fluidizing Gas line 82 Offgas line 86 Recycled Gas line 90 Recycled Gas line 94 Fresh air fan 98 Valve 102 Emissions control device 106 Particulate removable device 110 Startup heating system 114 Valve 118 Cooler 122 Dump valve 126 Conveyor 130 Storage system

[0030] 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

[0031] FIG. 1 is a schematic representation showing the relationship between biomass, coal, and charcoal. It is one goal of embodiments of the present invention to provide a system and process suited for altering biomass, regardless of its source, such that it behaves like coal. One advantage of providing biomass that behaves like coal is that existing coal burning electrical power plants can use the processed biomass without substantial modifications. To make biomass a viable alternative, moisture content must be reduced, heating value must be increased, grindability and handling must be improved, hydrophobicity must be imparted, shelf life must be increased, energy density must be increased, and homogeneity must be improved. To achieve these objectives, embodiments of the present invention treat biomass by torrefaction wherein water, carbon dioxide, carbon monoxide, light volatile organic chemicals, sulphur dioxides, and hydrochlorides are driven out of the raw biomass. The end result is a coal like product that can be used in coal burning electricity generation plants of current design.

[0032] More specifically, the torrefaction contemplated by embodiments of the present invention include thermally processing biomass at temperatures of about 250-325 C. (480-620 F.) under near atmospheric pressure and in the presence of oxygen. This process will remove water and light volatiles from biomass and will reduce the oxygen content of the biomass. Importantly, the amount of fixed carbon in the biomass is increased and the biopolymers, cellulose, hemicelluloses, and lignin, are decomposed.

[0033] Referring now to FIG. 2, the biomass torrefaction system 2 of one embodiment of the present invention employs a fluidized bed reactor 6. 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. In one embodiment, biomass having about 10 to 50 weight percent moisture is processed. The biomass is initially fed into a hopper 10 that in one embodiment is a feed hopper equipped with a screw conveyor or paddle screw feeder 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.

[0034] A feeder 22 positioned beneath the feed hopper 10 empties biomass onto the conveyor 14. In one embodiment, the feed conveyor 14 provides up to 6000 pounds (2721.6 kg) of biomass per hour to the surge bin 18. The surge bin 18 is equipped with a controllable feed screw 26 that supplies the desired amount of feed at the desired rate to the fluid bed reactor 6. In another embodiment, a rotary valve or lock hoppers may be used if the surge bin is located above the reactor 6. 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 predetermined 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 directs biomass to the fluid bed reactor 6. The fluid bed reactor 6 may be a custom design or a commercially available design.

[0035] The biomass is dried to a moisture content of less than about 40 weight percent before introduction to the reactor 6. The biomass may be pre-dried by conventional means including, e.g., air drying, rotary kilns, cascaded whirling bed dryers, elongated slot dryers, hopper dryers, traveling bed dryers, vibrating fluidized bed dryers, and other methods that do not employ a fluidized bed reactor. Those of skill in the art will appreciate that fluidized-bed dryers or reactors may also be used. 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 or an emissions control device 102. In one embodiment, the biomass is pre-dried to a moisture content of about 5 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 an average moisture content of less than about 40 weight percent.

[0036] FIG. 3 is a schematic of an integrated fluid bed reactor 6 and pre-dryer system of one embodiment of the invention. Off-gases 46 from the fluidized bed 6 contact and pre-dry the feed material before it reaches a plate 34. The fluidized bed reactor 6 is cylindrical and has an aspect ratio (bed height divided by diameter) of about 2 or less, in one embodiment, the aspect ratio ranges from about 2 to about 1/3. The bed is positioned within the cylindrical fluidized bed reactor at 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 to about 1/3. Bed fluidization is achieved by directing fluidizing gas through the perforated plate 34. A mixture of fresh air and recycled gas, i.e., gas taken from the fluidized bed reactor 6, is used as the fluidizing gas. It is preferred to use a blower to control the amount and composition of the fluidizing gas. In other embodiments, multiple blowers may be used.

[0037] A startup heater system 110 is used to provide the heat needed for preheating the fluidizing gas during startup for flame stabilization during normal operation. 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.

[0038] A fresh air fan 94 is used to add fresh air to the fluidizing gas in order to adjust the oxygen content thereof. 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.

[0039] Gases exiting the reactor 6 enter a particulate removal device 106 where fines are separated. 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 102 is to destroy any carbonaceous components in the offgas after removal of particulate. The emissions control device could be, e.g., a thermal oxidizer.

[0040] In one embodiment, a typical startup procedure involves, e.g., starting the heater system 110 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. The temperature of the fluidizing gas is slowly increased using the heater system. When the biomass in the reactor 6 reaches a temperature within the range of about 446 to 482 F. (230 to 250 C.), biomass is fed to the reactor to fill the reactor bed. When the biomass reaches a temperature of approximately 250 C. (480 F.), it begins to release heat as it consumes oxygen present in the fluidizing gas. Small amounts of biomass are then 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 and 350 C. (450 to 670 F.) and, more preferably, about 270 to about 300 C. (520 to about 570 F.).

[0041] 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. As the process reaches steady state, the temperature of the recycle gas in line 66 increases and the burner system 110 controls automatically reduces the firing rate. In one embodiment, hot gasses taken from the emissions control device 106 are used to preheat the fluidizing gas (for example, by the process of FIG. 3) to reduce the amount of combustion of biomass required to maintain the temperature of the fluidized bed as well as the amount of fuel required by the burner system 110. The reactor 6 is preferably equipped with several water spray nozzles (not shown) to assist in the control the temperature of the fluidized bed. The reactor 6 is also preferably equipped with several temperature sensors to monitor the temperature of the fluidized bed.

[0042] At steady state, reactor 6 operation is a balance between biomass particle size, the reactor temperature, the residence time required for decomposition of biomass polymers, 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 fluidizing gas velocity is from about 4 to about 8 feet per second, the temperature of the fluidized bed is maintained at about 230 and 350 C. (450 to 670 F.) and, more preferably, at about 270 to about 300 degrees C. (520 to about 570 F.), and the average biomass particle residence time is from about 5 minutes to about 30 minutes.

[0043] The gases leaving the reactor 6 via line 82 have an oxygen content of less than about 8 volume percent, whereas the oxygen content of the fluidizing gas is maintained at greater than about 10 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.

[0044] 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.

[0045] The biomass produced in reactor 6 is typically at a temperature of about 275 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.

[0046] 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.

[0047] In one embodiment, the cooling device would be a mixing screw. In another embodiment, the cooling device could be a hollow flight screw cooler. 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 flow rate or temperature of the water in the screw jackets and/or the rate at which water is applied using the sprays. The water spray may be continuous, and/or it may be intermittent.

[0048] In yet another embodiment, torrefied biomass is consolidated into briquettes or pellets and then cooled.

[0049] 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.

[0050] Referring now to FIG. 4 shows a Proximate and Ultimate analysis for an example woody biomass feed. FIG. 5 shows a Proximate and Ultimate analysis for the torrefied product produced from the woody biomass feed of FIG. 4.

[0051] 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.