US20140075835A1

US20140075835A1 - Blanket for pyrolysis or drying of biomass

Info

Publication number: US20140075835A1
Application number: US14/115,060
Authority: US
Inventors: Kenneth B. Faires; Daniel T. Schwartz; Gregory M. Newbloom; Jeffrey J. Richards; Michael S. Noon; Jenny L. Knoth
Original assignee: University of Washington Center for Commercialization
Current assignee: University of Washington Center for Commercialization
Priority date: 2011-05-12
Filing date: 2012-05-14
Publication date: 2014-03-20
Also published as: EP2707463A4; EP2707463A2; WO2012155145A3; WO2012155145A2

Abstract

The disclosed embodiments are operable to produce biochar from a biomass or to dry a biomass through controlled heating. While prior art technologies are stationary enclosures, the embodiments provided herein are based on a portable, flexible laminated blanket that is draped over a biomass (e.g., a wood slash pile). In this way, the blanket functions as a portable and reusable kiln for pyrolyzing biomass into biochar or drying biomass.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/485,521, filed May 12, 2011, the disclosure of which is expressly incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with government support under DGE-0654252 awarded by National Science Foundation. The government has certain rights in the invention.

BACKGROUND

In the United States, forestry activities produce over 80 million green tons of slash (wood waste) annually. Current forest practices require that this material be piled and either left in the forest to decompose or burned. This wastes a potentially valuable renewable energy resource and produces greenhouse gases like methane and CO₂.
Biochar production is an age-old technique that utilizes partial combustion of a woody fuel source in an oxygen starved environment to convert the rest of the wood to charcoal. Biochar is useful as a renewable energy feedstock, among other applications. Established biochar production processes (e.g. the Missouri kiln, Brazilian beehive kiln, Slope type kiln) typically have the following features:

- 1. Wood is piled inside a large enclosure made of an air impermeable material.
- 2. The wood inside is ignited producing heat.
- 3. Small holes in the enclosure provide air flow to feed a small fire.
- 4. Heat produced from this fire is utilized to convert the majority of the wood to biochar.
- 5. When all the wood is converted to biochar, the enclosure is hermetically sealed and the charcoal allowed to cool until it is safe to remove and utilize.

Due to increasing interest in renewable energy, improved methods for producing biochar are of interest.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
In one aspect, a blanket is provided. In one embodiment, the blanket includes a material having the following properties:

- thermal insulation, such that the blanket is capable of withstanding temperatures of from 100 to 1000 degrees C.;
- gas impermeability; and
- flexibility, such that the blanket can be folded over on itself for storage;
- wherein the blanket is configured to conformally cover a combusting biomass to facilitate pyrolysis or drying of the biomass.

The combination of these properties is unique because it provides almost identical functionality as the brick and mortar used in stationary kilns, but because the material is flexible it provides a means to economically convert remote slash piles (e.g., at logging sites) into biochar.
In another aspect, a method is provided to produce biochar or dry biomass. In one embodiment, the method includes the steps of:
(a) combusting a biomass at a temperature sufficient to initiate pyrolysis or drying of the biomass;
(b) covering the biomass with a blanket as disclosed herein, wherein the temperature of the biomass covered by the blanket can be controlled and is sufficient to pyrolize or dry the biomass, and wherein air from outside the blanket only reaches the biomass in a controlled manner; and
(c) maintaining the temperature of the biomass covered by the blanket for a sufficient time to produce biochar or dry biomass.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a representative blanket covering a combusting biomass in accordance with aspects of the present disclosure;

FIGS. 2-4 are schematic illustrations of representative blankets in accordance with aspects of the present disclosure;

FIG. 5 is a schematic illustration of air flow and heat movement in relation to a representative blanket covering a combusting biomass in accordance with aspects of the present disclosure;

FIG. 6 is a theoretical temperature profile through a thickness of a representative blanket from a hot side near a combusting biomass to a cool side furthest from the combusting biomass in accordance with aspects of the present disclosure;

FIG. 7 is a photograph of an exemplary blanket having three layers in accordance with aspects of the present disclosure; and

FIGS. 8A and 8B are visual (8A) and thermal (8B) images of a representative blanket covering a combusting biomass in accordance with aspects of the present disclosure.

DETAILED DESCRIPTION

The disclosed embodiments are operable to produce biochar from a biomass or to dry a biomass through controlled heating. While prior art technologies are stationary enclosures, the embodiments provided herein are based on a portable, flexible laminated blanket that is draped over a biomass (e.g., a wood slash pile). In this way, the blanket functions as a portable and reusable kiln for pyrolyzing biomass into biochar or drying biomass. As used herein, the term “biomass” refers to any biomass known to those of skill in the art, and includes naturally occurring carbon sources.
In one aspect, a blanket is provided. In one embodiment, the blanket includes a material having the following properties:

The combination of these properties is unique because it provides almost identical functionality as the brick and mortar used in stationary kilns, but because the material is flexible it provides a means to economically convert remote slash piles (e.g., at logging sites) into biochar.
Referring now to FIG. 1, a representative blanket 105 is schematically illustrated covering a combusting biomass 110 resting on a surface 115 (e.g., dirt). The blanket 105 can be any blanket having the above-listed properties. Various embodiments of the blanket 105 are described further below.
So as to contain the thermal energy of the combusting biomass 110, the blanket 105 is a thermal insulator. The combusting biomass 110 may have a temperature of from 100 to 1000 degrees C., and in certain embodiments the blanket 105 withstands heat (i.e., maintains structural stability) across this range. In other embodiments, the blanket 105 is capable of withstanding temperatures of from 200° C. to 650° C. (e.g., for pyrolyzing biomass). In other embodiments, the blanket 105 is capable of withstanding temperatures of from 100° C. to 250° C. (e.g., for drying biomass).
The blanket 105 is also gas impermeable so as to prevent combustion fuel gasses to diffuse through the blanket 105. As used herein, the term “gas impermeable” defines a material that allows negligible oxygen diffusion through its thickness. By preventing gas diffusion, and particularly oxygen diffusion, the blanket 105 limits fuel to the combusting biomass 110 covered by the blanket 105. Because oxygen cannot pass through the blanket 105, the only oxygen provided to the combusting biomass must pass around the blanket 105. However, in one embodiment, a plurality of air vents 120 (see FIG. 5) are disposed peripherally on the blanket 105, wherein the plurality of air vents are configured to provide controllable air flow to pyrolyzing or drying biomass disposed beneath the blanket. The air vents 120 are positioned in the blanket 105 to provide controllable air flow to the combusting biomass 110. By controlling the flow of oxygen to the combusting biomass 110, the blanket 105 allows a user to pyrolize the biomass into biochar instead of combusting it into ash. In one embodiment, air from outside the blanket only reaches the biomass by passing around the periphery of the blanket.
The blanket 105 is also flexible, such that it can be folded over on itself for storage. By being flexible, it can be transported easily so as to facilitate pyrolysis of biomass in remote locations, such as logging sites and the like. Flexibility also allows the blanket 105 to conformally cover a combusting biomass 110. This allows the headspace between the blanket 105 and the combusting biomass 110 to be minimized, which facilitates pyrolysis and generally allows a user to control the temperature of the combusting biomass 110.
While the figures herein generally refer to use of the blanket 105 for use in pyrolysis of a biomass, it will be appreciated that the blanket 105 can similarly be used to dry the biomass, if the temperature is controlled properly, as will be discussed in more detail below.
Biomass drying is accelerated as the temperature of the biomass is elevated. Control of the oxygen flow under the blanket (e.g., via vent) can enable a small amount of combustion heat, to warm the biomass without pyrolysis, by restricting the oxygen input below that needed to sustain pyrolysis. Under these conditions, the blanket enables the warm combustions gases to circulate prior to exiting the blanket, thereby raising the temperature of the biomass. All but the most tightly bound water (generally less than 10% by weight) is liberated from the biomass as the temperature approaches 100° C., the boiling point of water.
Referring now to FIG. 2, in one embodiment, the blanket 105 consists of a single layer 205 having all of the required properties. The blanket 105 will have a “hot” side nearest the combusting biomass 110, and a “cool” side furthest from the combusting biomass 110.
In one embodiment, the blanket comprises a thermal insulation layer capable of withstanding temperatures of from 100 to 1000 degrees C.; and a gas impermeable layer that is a different material than the thermal insulation layer, wherein the thermal insulation layer is disposed closer to the combusting biomass than the gas impermeable layer.
Referring now to FIG. 3, the blanket 105 comprises two layers: A thermal insulation layer 305, on the hot side, and a gas impermeable layer 310 on the cool side. The combination of the two layers (305 and 310) provides all of the required properties.
In one embodiment, the thermal insulation layer 305 comprises a ceramic material. In one embodiment, the ceramic material is selected from the group consisting of ceramic pressed particulate paper and woven ceramic fibers (e.g., basalt).
In one embodiment, the gas impermeable layer 310 is a metal foil. In one embodiment, the metal foil is selected from the group consisting of stainless steel foil, aluminum foil, or other refractory metals and alloys foils from them.
In one embodiment, the blanket has the additional property of durability, such that the blanket is not structurally damaged after repeated exposure to temperatures of from 100 to 1000 degrees C. Referring now to FIG. 4, a three (or more) layer blanket 105 is provided. In addition to the thermal insulation layer 305 and the gas impermeable layer 310 described with reference to FIG. 3, a first protective layer 410 is provided that confers the property of durability. The first protective layer 410 is disposed closest to the combusting biomass 110.
The protective layer 410 protects any of the other layers of the blanket 105 from being compromised (e.g., by ripping or puncturing) while in use. This is particularly desirable if the thermal insulation layer 305 is a ceramic material, which are typically fragile. Small holes in the blanket can serve as nucleation sites for larger tears and rips to form, which reduces reusability of the blanket. In one embodiment, the first protective layer is a metal mesh layer. In a further embodiment, the metal mesh layer is a stainless-steel mesh layer.
An optional second protective layer 415 can be added to provide durability to both the hot and cool sides of the blanket 105 for maximum durability. The second protective layer 415 can be the same or different in composition as the first protective layer 410.
In another aspect, a method is provided to produce biochar or dry biomass. In one embodiment, the method includes the steps of:
(a) combusting a biomass at a temperature sufficient to initiate pyrolysis or drying of the biomass;
(b) covering the biomass with a blanket as disclosed herein, wherein the temperature of the biomass covered by the blanket can be controlled and is sufficient to pyrolize or dry the biomass, and wherein air from outside the blanket only reaches the biomass in a controlled manner; and
(c) maintaining the temperature of the biomass covered by the blanket for a sufficient time to produce biochar or dry biomass.
In one embodiment, the biomass is pyrolized at a temperature of from about 200° C. to 650° C. In one embodiment, the biomass is dried at a temperature of from about 100° C. to 250° C.
In one embodiment, the step of maintaining the temperature of the biomass covered by the blanket for a sufficient time to produce biochar or dry biomass comprises adjusting the amount of air flowing to the biomass at least once.
In one embodiment, adjusting the amount of air flowing to the biomass comprises moving a peripheral edge of the blanket to increase or decrease airflow or operating peripheral vent ports.
The following example is provided for the purpose of illustrating, not limiting, the disclosed embodiments.

EXAMPLE

Development and Testing of Pyrolysis Blanket

The experimental development of a representative pyrolysis blanket will now be described. The primary goal during development was to create a portable, reusable mechanism for pyrolyzing biomass.
The first prototype was a semipermeable ceramic material. The blanket provided multiple functions: (1) Capture and redistribute heat generated in the local regions of combustions, and (2) permit hot volatile off-gas to vent slowly out of the pile.
The blanket was made from a high-temperature semi-permeable ceramic fiber blanket material. This material is by its very nature a very good insulator because it is made of woven basalt fibers, which have a low thermal conductivity. The gas permeability of the blanket was also within a range that was expected to be functionally appropriate for the scale of the biomass pile that was targeted for pyrolysis (200 lbs of green wood).
During the progress of the burn the temperature under the blanket reached as high as 450° C., a temperature indicating that a significant amount of combustion was occurring. Another critical finding was that because of the semipermeable air barrier, combustion could not be reduced over time. Therefore, as more and more charcoal was produced, it was being combusted. Removing the blanket only caused the charcoal to ignite, effectively degrading our desired product, biochar.
One issue was the difference in max temperature between the covered pile, 750° F. versus the uncovered pile 1050° C. This is strong evidence that the blanket is serving to limit some of the combustion, but not enough toward the end of the process to affect efficient charcoal recovery.
There were a number of significant insights gleaned from the first prototype that are summarized below:

- 1. The combustion of wood locally in a pile is sufficient to carbonize even large diameter wood as long as the heat is captured and redistributed.
- 2. A semi-permeable barrier is effective at suppressing combustion but not easily sealed when the wood is done carbonizing and the process needs to be “shut-off” such that the biochar can be collected.
- 3. Free convection of volatile/heated gases is a significant factor in design of inlets and outlets and providing heat throughout the pile to affect wood conversion.

While the design recommendations above suggested that the semipermeable blanket system was not sufficient for large pile conversion, we were able to use it effectively to produce high quality charcoal on a small scale. Some modifications to the blanket to improve durability and control over the temperature within the blanket were made for a second prototype.
The other major change that was made involved the insertion of an outlet into the top of the blanket. This was to improve the rate of volatile release created during the biochar production process. This simple design was hypothesized to provide a means to control the temperature by changing the outlet diameter.
Testing of the second prototype required manually tuning the diameter of the outlet in order to control the temperature within the blanket. While this is not realistic on a large scale, this demonstrated that the outlet diameter had a strong influence over the temperature and that with appropriate outlet size, the temperature could be varied from 250° C. up to 650° C. reproducibly with 20° C. accuracy. This temperature spans the relevant range to achieve biochar with different properties for various applications.
There were a number of significant insights gleaned from the second prototype that are summarized below:

- 1. Orifice diameter should be something that is controllable because as more biochar is produced, it needs to be adjusted to maintain a constant temperature within the pile.
- 2. A semi-permeable barrier is not suitable to produce biochar on a large scale because its combustion cannot be effectively stopped.
- 3. Metal mesh is effective at providing durability and added structure to the blanket to make more complicated geometries over the wood possible.

After critical evaluation of the requirements of scale, several changes were made to the blanket and operational design to achieve the final prototype. The first change was to incorporate a metal foil into the laminate. This foil serves to reflect radiative heat, but most importantly, provides an air impermeable layer behind the ceramic fiber insulation to stop permeation of air through the blanket. This laminate design is shown in FIGS. 4 and 7. In the third prototype, the foil is placed such that it is on the cool side of the blanket as its melting point is rather low compared to the expected operating temperatures.
Implementing the impermeable layer is critical to the ability to mediate temperature under the blanket, but also to recovering the biochar after the pyrolysis is completed. Without this layer, air will continually permeate into the biochar, potentially maintaining sufficient combustion to lose the entire product if not actively quenched. The use of an impermeable material also required reconsideration of how to control airflow into the pile. In this regard, controllable inlets and outlets were been placed radially around the blankets bottom in order to drive the buoyant convection that results in good mixing within the pile. A simple schematic of this design is shown in FIG. 5.
The blanket has good thermal performance and gas impermeability. FIG. 6 is a theoretical temperature profile through a thickness of a representative blanket from a hot side near a combusting biomass to a cool side furthest from the combusting biomass in accordance with aspects of the present disclosure.
FIG. 7 is a photograph of an exemplary blanket having three layers: a ceramic fiber thermal insulation layer, an aluminum foil gas impermeable layer, and a stainless steel mesh protective layer.
Tests of the blanket of FIG. 7 successfully demonstrated a blanket system that possesses the necessary features to produce biochar in a scalable and inexpensive manner. The blanket is flexible and inexpensive, can be simply lain over an existing wood pile once a fire is ignited in order to convert the rest of the wood to biochar. Inlets and outlets have been integrated into the blanket which can control air flow and thus temperature inside the pile dynamically. The blanket survived testing with minimal wear. The blanket can be formed to any size, including larger sizes to accommodate larger piles of biomass. Pictures of the blanket field test are shown in FIGS. 8A and 8B, which are visual (8A) and thermal (8B) images of a representative blanket covering a combusting biomass. Note that the thermal image in FIG. 8B registers a temperature of 166° C. on the cool side of the blanket, indicating a relatively low temperature (estimated to be about 300° C.) on the hot side of the blanket, which is conducive to pyrolysis of the biomass into biochar.
While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A blanket comprising a material having the following properties:

thermal insulation, such that the blanket is capable of withstanding temperatures of from 100 to 1000 degrees C.;

gas impermeability; and

flexibility, such that the blanket can be folded over on itself for storage;

wherein the blanket is configured to conformally cover a combusting biomass to facilitate pyrolysis or drying of the biomass.

2. The blanket of claim 1, wherein the blanket consists of a single layer.

3. The blanket of claim 1, wherein the blanket comprises two or more layers.

4. The blanket of claim 3, wherein the blanket comprises a thermal insulation layer capable of withstanding temperatures of from 100 to 1000 degrees C.; and

a gas impermeable layer that is a different material than the thermal insulation layer, wherein the thermal insulation layer is disposed closer to the combusting biomass than the gas impermeable layer.

5. The blanket of claim 3, wherein the thermal insulation layer comprises a ceramic material.

6. The blanket of claim 5, wherein the ceramic material is selected from the group consisting of ceramic pressed particulate paper and woven ceramic fibers.

7. The blanket of claim 3, wherein the gas impermeable layer is a metal foil.

8. The blanket of claim 7, wherein the metal foil is selected from the group consisting of stainless steel foil or other refractory metals and alloys foils from them.

9. The blanket of claim 1, wherein the blanket has the additional property of durability, such that the blanket is not structurally damaged after repeated exposure to temperatures of from 100 to 1000 degrees C.

10. The blanket of claim 9, wherein the blanket further comprises a first protective layer that confers the property of durability, wherein the first protective layer is disposed closest to the combusting biomass.

11. The blanket of claim 9, wherein the first protective layer is a metal mesh layer.

12. The blanket of claim 9, wherein the metal mesh layer is a stainless-steel mesh layer.

13. The blanket of claim 9, wherein the blanket further comprises a second protective layer, disposed furthest from the combusting biomass.

14. The blanket of claim 1, further comprising a plurality of air vents disposed peripherally on the blanket, wherein the plurality of air vents are configured to provide controllable air flow to pyrolyzing or drying biomass disposed beneath the blanket.

15. A method, comprising the steps of:

(a) combusting a biomass at a temperature sufficient to initiate pyrolysis or drying of the biomass;

(b) covering the biomass with the blanket of any one of claims 1-14, wherein the temperature of the biomass covered by the blanket can be controlled and is sufficient to pyrolize or dry the biomass, and wherein air from outside the blanket only reaches the biomass in a controlled manner; and

(c) maintaining the temperature of the biomass covered by the blanket for a sufficient time to produce biochar or dry biomass.

16. The method of claim 15, wherein the biomass is pyrolized at a temperature of from about 200° C. to 650° C.

17. The method of claim 15, wherein the biomass is dried at a temperature of from about 100° C. to 250° C.

18. The method of claim 15, wherein the step of maintaining the temperature of the biomass covered by the blanket for a sufficient time to produce biochar or dry biomass comprises adjusting the amount of air flowing to the biomass at least once.

19. The method of claim 18, wherein adjusting the amount of air flowing to the biomass comprises moving a peripheral edge of the blanket to increase or decrease airflow or operating peripheral vent ports.

20. The method of claim 15, wherein air from outside the blanket only reaches the biomass by passing around the periphery of the blanket.