US20100116182A1

US20100116182A1 - Resistance heater based air heating device

Info

Publication number: US20100116182A1
Application number: US12/562,718
Authority: US
Inventors: Thomas A. Chodacki; Louis Castriotta, III; Michael W. Tanguay
Original assignee: Saint Gobain Ceramics and Plastics Inc
Current assignee: Coorstek Inc
Priority date: 2008-09-18
Filing date: 2009-09-18
Publication date: 2010-05-13
Also published as: EP2331876A1; EP2331876A4; WO2010033797A1

Abstract

Featured is a resistance heater air heating device that includes a low thermal conductivity enclosure and an electric resistance heater that is disposed within such an enclosure. In more particular embodiments, such an enclosure is configured so as to provide a high surface area which in combination with the heater heats the air passing through the enclosure. In more particular embodiments, the ceramic enclosure is configured and arranged so as to include a through aperture in which is disposed the electric resistance heater and through which the air flows along the length of the through aperture.

Description

This application claims the benefit of U.S. provisional application No. 61/192,641 filed Sep. 18, 2008, which is incorporated herein by reference in its entirety.

FIELD OF INVENTION

The present invention relates to devices and structures for igniting the biomass within a furnace and methods related thereto.

BACKGROUND OF THE INVENTION

Biomass is one of the oldest fuels known to man and is vegetation or fuel from plants, agricultural waste products or the like. During photosynthesis, plants combine carbon dioxide from the air and water from the ground to form carbohydrates that are the building blocks of biomass. Burning biomass efficiently extracts the energy stored in the chemical bonds and produces carbon dioxide and water. Generating energy and heat by burning biomass displaces more polluting forms of energy generation and also provides other environmental benefits, such as reducing acid rain, soil erosion, water pollution and pressure on landfills. Additional environmental benefits include mitigating climate changes, providing wildlife habitat, and helping to maintain forest health through better management.
Biomass fuel is both abundant and renewable. There is biomass in virtually every part of the world that can be tapped to create power. At present, the world population uses only about 7% of the available annual production of biomass. As a result, biomass is not only the logical alternative fuel of the future but is also currently a logical source of energy.
Stoves or furnaces for burning biomass fuel to produce energy are not new. There are many stoves and furnaces for burning biomass fuel, however, there currently is not widespread acceptance of these furnaces or stoves by consumers. Cost is one of the main motivators leading consumers to use a stove or furnace that burns biomass fuels. Consumers of current biomass fuel stoves or furnaces many times have to compromise in terms of cleanliness and convenience when switching to a furnace that burns biomass fuels. One main area of inconvenience is starting and running the furnace. For example, many conventional igniters locate a wound resistive element within a tubular member that extends along the length of the tubular member. These wound resistive element in combination with the tubular member under certain conditions takes an extremely long time to heat the biomass fuel to the point of ignition.
It thus would be desirable to provide a new device that can hest biomass and cause it ti ignite and methods related thereto. It would be particularly desirable to provide such a device and method that would result in the biomass being ignited in mush shorter time periods than that seen for conventional devices. It also would be desirable to provide such a device that is scalable so as to be useable for different size heating devices. It also would be desireable to provide such heating devices that can be used for other applications involving the rapid heating of air to high temperatures.

SUMMARY OF THE INVENTION

The present invention features a heating device comprising an electric resistance heater that is disposed within a low thermal conductivity enclosure or an enclosure having low thermal diffusivity. In more particular embodiments, such an enclosure is configured so as to provide a high surface area which in combination with the heater heats the air passing through the enclosure. In more particular embodiments, such an enclosure is configured and arranged so as to include a through aperture in which is disposed the electric resistance heater and through which the air flows along the length of the through aperture. An enclosure having low thermal conductivity or low thermal diffusivity is particularly advantageous as the heat energy being developed by the electric resistance heater is maintained in a location where it can be picked up by the air flowing through the through aperture/heating device.
In more specific embodiments, the inner surface of the through aperture of the low thermal conductivity enclosure is configured with a plurality of convolutions or flutes that create hills and valleys that extend along the length of the enclosure. The inner surface also is adaptable so as to present any of a number of configurations so as to adjust the performance of the heating device to suit a given application. In yet more specific embodiments, the low thermal conductivity enclosure is heated by convection and radiation by the electrical resistance heater, so that the inner surface of the enclosure is at a high temperature. In yet further embodiments, such a device further includes an outer member that surrounds the low thermal conductivity enclosure.
In further embodiments, the low thermal conductivity enclosure is composed of a ceramic or other material that is appropriate for use under the temperature conditions for a heating device according to the present invention and which exhibit low thermal conductivity or low thermal diffusivity. In further embodiments, such materials can be porous or non-porous. In yet further embodiments, such materials also exhibit electrical insulating characteristics.
In more particular embodiments the electric resistance heater is characterized as being capable of reaching a high temperature and having a high watt density. In more specific embodiments, such electric resistance heaters include, but are not limited to hot surface igniters, silicon carbon igniters, silicon carbide hot surface igniters, ceramic/intermetallic hot surface igniters, silicon nitride igniters and other high wattage heating devices/elements. In illustrative exemplary embodiments, such hot surface igniters include Norton (St. Gobain Industrial Ceramics Norton Igniter Products) Mini Igniters®, Norton CRYSTAR Igniters®, Surface Igniters, hot surface igniters, and I²R hot surface igniters.
In the case of hot surface igniters, the heating tip or element thereof is heated by electricity to a desired temperature that will lead to the ignition of the biomass fuel. Such an igniter typically includes a heating element that extends outwardly from an end of the base which it is secured to. There are several manufacturers of igniters used and an igniter from any one manufacturer, because of its particular material composition, mass, and physical configuration, will generally heat up at a different rate to a different final temperature than an igniter from another manufacturer. For example, igniters from one manufacturer may heat up to a temperature of approximately 1600° F., in approximately 5 seconds, and to a relatively stable final temperature of approximately 2500° F. when energized for 20-30 seconds or longer. The rate of temperature change and the final temperature attained also depends on the value of the applied voltage.
In further embodiments, a heating element of the electric resistance heater is heated to a first temperature at or less than a first period of time and to a final temperature that is greater than the first temperature at or less than a second period of time. In yet further embodiments, the first temperature is at or above 1000° F. and the second temperature is at or about 2000° F.
In more particular embodiments, the heating device is configured so as to include a plurality of electric resistance heaters or that are disposed within the ceramic enclosure. In yet more particular embodiments, such a heating device is configured so as to include a plurality of low thermal conductivity enclosures and a plurality of electric resistance heating devices, at least one electric heating device for each enclosure.
The heating device of the present invention also is adaptable by increasing the size of the electric resistance heater, increasing the number of such resistance heaters within an enclosure, and increasing the number of enclosures and/or electric resistance heaters per enclosure so as to meet the heating requirements for a given application. For example, for larger size wood pellet (biomass) furnaces, one could configure a heating device to include two enclosures, each including an electric resistance heater, so as to provide sufficient heat energy to heat up the incoming air to the desired temperature within a desired period of time.
As compared to prior art devices using a metal enclosure, the air is heated up about twice as fast when using a heating device of the present invention as well as showing an increase in efficiency of about 15% in delivering the desired temperature. For example, for a given application using a wound wire resistance heater, it has taken 20 minutes for the heating device to heat air to the temperature necessary for causing the wood pellet (biomass) fuel to ignite. In addition, the low thermal conductivity enclosure of the present invention exhibit resistance to thermal shock, such as that which occurs after energizing the electric resistance heaters contemplated for use with the present invention. Such electric resistance heaters are designed to attain a high temperature very rapidly after being energized which can in turn shock the inner surface of the low thermal conductivity enclosure.
Also featured are furnaces embodying such one or more of such heating devices and methods related thereto.
Other aspects and embodiments of the invention are discussed below.

BRIEF DESCRIPTION OF THE DRAWING

For a fuller understanding of the nature and desired objects of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawing figures wherein like reference character denote corresponding parts throughout the several views and wherein:

FIG. 1 is an illustrative view of an exemplary furnace having a combustion chamber, adapted to use a heating device of the present invention.

FIG. 2 is a partially cut away of the exemplary furnace of FIG. 1.

FIG. 3 is an exploded illustrative view of a portion of the combustion chamber of the exemplary furnace of FIG. 1.

FIG. 4 is one illustrative view of a resistance heater air heating device according to the present invention more particularly showing one end of such a device.

FIG. 5 is another illustrative view of the resistance heater air heating device of FIG. 4 more particularly showing another end of such a device.

FIG. 6 is a side view of the resistance heater air heating device of FIG. 4.

FIG. 7 is a cross-sectional view along line 7-7 of FIG. 6.

FIGS. 8A, B are end views respectively of the resistance heater air heating device of

FIG. 4 (FIG. 8A) and the resistance heater air heating device of FIG. 5 (FIG. 8B).

FIG. 9 is an view of a cross-section of a resistance heating device according to another embodiment of the present invention with the electric resistance heating element removed for clarity.

FIG. 10 is an illustrative view of the resistance heating device of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the various figures of the drawing wherein like reference characters refer to like parts, there is shown in FIG. 1 an illustrative view an exemplary furnace 100 having a combustion chamber 110 in which is located a resistance heater air heating device 500 of the present invention. The furnace 100 includes a housing 120 and the combustion chamber 110 is within the housing 120. In the illustrated embodiment, at least a portion of the combustion chamber 110 a viewable through a window 122, which is sealed with respect to the housing 120. The housing 120 also includes an access panel 124 that allows access to a portion of the interior of the furnace 100 located below the combustion chamber. The access panel, in some embodiments, allows users to remove combustion products from the furnace 100. As shown more clearly in FIG. 4, the housing 120 also includes a hopper and a feed mechanism for controllably placing biomass combustibles into the combustion chamber 110. The housing 120 of the furnace includes a door 126 that allows access to the hopper (not shown in FIG. 4). Biomass fuels are placed into the hopper after opening door 126. Any type of biomass can be used as a fuel. For example, corn, wood chips, or pellets of biomass material are among the fuel sources. Although the exemplary furnace 100 shown in FIG. 1 is a space heater application, other applications include, but are not limited to a forced air furnace, a hot water heater, an electrical generator, a swimming pool heater, or for heating water for circulation within a hot water heating system.
Referring now to FIG. 4, there is shown a partially cutaway view of the exemplary furnace 100 of FIG. 1. The furnace includes a housing 120 and includes a hopper 420 which holds fuel 426. Positioned within the hopper is a feed mechanism 430 which is attached to a motor 432. The feed mechanism 430 can be termed as a feeder wheel which has slots therein. A portion of the fuel 426 falls within the slots while the motor 432 turns the feeder wheel to move the fuel in the slots from the lower portion of the hopper to a feeder tube 440. The feeder tube is a tube that connects the hopper 420 to the combustion chamber 110. Specifically the feeder tube 440 is positioned so that the fuel within a slot drops through the feed tube 440 and into a burn pot 300 within the combustion chamber 110. The rate at which the fuel is input to the combustion chamber and specifically to the burn pot 300 can then be controlled by controlling the motor 432 which rotates or turns the feeder mechanism or feeder wheel 430. The furnace or stove 100 also includes a controller 500 which controls many aspects of the stove or furnace. The controller 500, for example, controls the rate at which fuel is input into the burn pot 300.
Referring now to FIG. 2 there is shown an exploded illustrative view of a portion of the combustion chamber 110 and the burn pot 300 of the furnace 100. The combustion chamber 110 is bounded by a top burner plate assembly 210 and a bottom plate 220. The combustion chamber also includes a back wall 212. Attached to the bottom plate 220 is a first pin 222 and a second pin 224. The burn pot 300 includes a first burn pot portion 310 and a second burn pot portion 320. The first burn pot portion includes a side wall 312, which has openings, such as opening 314 therein, for directing combustion air around the burn pot 300. The second portion of the burn pot 320 also has a side wall 322. The sidewall 322 also includes openings, such as opening 324, for directing air entering from outside the burn pot 300 to within the burn pot assembly. Also attached to the side wall 322 of the second burn pot portion 320 is a mounting wing 326. The mounting wing 326 includes openings that allow the mounting wing 326 to fit over the first pin 222 and the second pin 224 attached to the bottom plate 220 of the combustion chamber 110. Attached to the side wall 312 of the first burn pot portion is another mounting wing 316, which has opening therein so that the mounting wing 316 also fits over the first pin 222 and the second pin 224 of the bottom plate 220 of the combustion chamber 110.
Also located within the combustion chamber is a movable floor 240 and a translating plate 250. The movable floor includes a grill 242 and an opening 244. The movable floor 240 is attached to a pivot pin 245 so that the moving floor 240 can pivot around the pivot pin 245. The translating plate 250 also has an opening 254 therein. The translating plate 250 also includes a solid surface area 252. The translating plate 250 also is pivotally attached to the pivot pin 245. An actuator rod 400 is attached to the movable floor 240 as well as the translating plate 250. The actuator rod 400 is used to move the movable floor 240 and the translating plate 250 between a first position and a second position. In some embodiments, separate actuator rods are used to move the movable floor 240 and the translating plate 250.
Also attached to the burn pot 300, and specifically to the second portion of the burn pot 320, are tow igniters 260 and an igniter 262. The igniters 260, 262 place heated air into the burn pot 300 and thus, the igniters 260, 262 are in fluid communication with the interior portion of the burn pot assembly. The igniters 260, 262 are used to initially fire the furnace or to initially ignite biomass fuel added to the burn pot 300. In further embodiments, once the biomass fuel within the burn pot has been started, the igniters 260, 262 no longer place heated air into the burn pot 300.
Positioned below the bottom plate 220 is a combustible product tray 270. The combustible product tray 270 includes a floor 272 as well as at least one side wall. Attached to the floor 272 of the combustible product tray 270 is a distributor 274. The distributor 274 is positioned so that when a portion of an ash column is removed from the burn pot 300, the distributor 274 prevents the product from merely stacking up on the floor 274 of the combustible product tray 270. In other words, the distributor 274 distributes the byproduct of combustion from the burn pot over the floor 272 of the combustible product tray 270.
When in a first position, the grill 242 having openings therein of the movable floor 240, the second portion of the burn pot 320, the opening 254 in the translating plate 250, and the first portion of the burn pot 310 are substantially aligned to form the burn pot 300. When the translating plate 250 and the movable floor 240 are in the first position, the biomass material can be inserted into the burn pot 300 and specifically can drop to the grill portion 242 of the movable floor 240. The igniters 260, 262 are turned on to initially ignite the biomass material. Once the biomass material is burning, additional biomass material is placed through an opening 211 in the top burner plate assembly 210 and into the burn pot 300.
Combustion air can be forced through the openings 314 within the first burn pot portion 310 and through the openings 242 in the second burn pot portion 320, respectively, to provide sufficient oxygen for the biomass fuel to burn completely. As burning continues, an ash column builds within the burner pot 300. The ash column eventually builds up to a point where the ash column is above the second portion of the burn pot 320, and above the translating plate 250.
Each of the igniters 260, 262 can be arranged so as to embody the resistance heater air heating device 500 of the present invention which heating device is configured and arranged so to heat incoming air to a temperature that eventually causes the biomass material to ignite and eventually reach a sustained combustion. After the biomass reaches a sustained combustion, the furnace control circuitry cause the resistance heater air heating device 500 of the present invention to be turned off.
Referring now to FIGS. 4-8 there are shown one illustrative view of a resistance heater air heating device 500 according to the present invention showing one end of such a device (FIG. 4); another illustrative view of the resistance heater air heating device showing another end of such a device (FIG. 5); a side view of the resistance heater air heating device of FIG. 4 (FIG. 6); a cross-sectional view along line 7-7 of FIG. 6 (FIG. 7) and end views respectively of the resistance heater air heating device of FIG. 4 (FIG. 8A) and the resistance heater air heating device of FIG. 5 (FIG. 8B). Such a resistance heater air heating device 500 includes an outer member 510, an electric resistance heater 530 and a low thermal conductivity enclosure 520 in which is disposed at least the heating element 534 of the electric resistance heater.
Such a resistance heater air heating device 500 also is configured so as to include one or more air vents or passages in addition to the open ends 512 a,b so that a source of air enters into the device and exits the device after being heated by the electric resistance heater 530 in combination with the low thermal conductivity enclosure 520.
In more particular embodiments the electric resistance heater 530 is characterized as being capable of reaching a high temperature and having a high watt density. In more specific embodiments, such electric resistance heaters 530 include, but are not limited to, hot surface igniters, silicon carbon igniters, silicon carbide hot surface igniters, ceramic/intermetallic hot surface igniters silicon nitride igniters and other high wattage heating devices/elements. In illustrative exemplary embodiments, such hot surface igniters include Norton (St. Gobain Industrial Ceramics Norton Igniter Products) Mini Igniters®, Norton CRYSTAR Igniters®, Surface Igniters, hot surface igniters, and I²R hot surface igniters.
In the case of hot surface igniters, the heating tip or heating element 534 is heated by electricity to a desired temperature such as for example, a temperature necessary to cause a fuel/air mixture to ignite. Such an igniter typically includes a heating element 534 that extends outwardly from an end of the base 532 which it is secured to. There are several manufacturers of igniters used and an igniter from any one manufacturer, because of its particular material composition, mass, and physical configuration, will generally heat up at a different rate to a different final temperature than an igniter from another manufacturer. For example, igniters from one manufacturer may heat up to a temperature of approximately 1600° F., in approximately 5 seconds, and to a relatively stable final temperature of approximately 2500° F. when energized for 20-30 seconds or longer. The rate of temperature change and the final temperature attained also depends on the value of the applied voltage.
Although a single electric resistance heater 530 is shown disposed in the low thermal conductivity enclosure, this shall not be limiting. It is contemplated and thus within the scope of the present invention for one or more, e.g., a plurality, of electric resistance heaters 530 to be located within the through aperture 526 of the low thermal conductivity enclosure 530.
It is contemplated that the resistance heater air heating device 500 of the present invention can be adapted for use with any of a number of surface heating igniters or heaters as is known to those skilled in the arts. Also it is contemplated such a resistance heater air heating device 500 also can be adapted for use with any of number of electric resisting heating elements as is known to those skilled in the art.
The low thermal conductivity enclosure 520 or an enclosure that exhibits low thermal diffusivity, is configured and arranged so the heat energy being generated by the electric resistance heater 530 is contained in the area between the enclosure and the electric resistance heater and so as to be exposed to the flow of air. As shown more clearly in FIGS. 7-8, the low thermal conductivity enclosure is configured and arranged so as to include a through aperture 526 in which is disposed the electric resistance heater 530 and through which the air flows along the length of the through aperture. An enclosure having low thermal conductivity or low thermal diffusivity is particularly advantageous as the heat energy being developed by the electric resistance heater is maintained in a location where it can be picked up by the air flowing through the through aperture/heating device.
In more particular embodiments, such an enclosure 520 is configured so as to provide a high surface area which in combination with the heater heats the air passing through the enclosure. In further embodiments, the inner surface of the through aperture 526 of the low thermal conductivity enclosure is configured with a plurality of convolutions or flutes that create hills 522 and valleys 524 that extend along the length of the enclosure. The inner surface of the through aperture 526 is adaptable so as to present any of a number of configurations to adjust the performance of the heating device to suit a given application.
In yet more particular embodiments, the low thermal conductivity enclosure 520 is composed of a material such that the inner surface of the through aperture thereof, is heated by one of convection or radiation by the electrical resistance heater, so that the inner surface is at a high temperature. In this way, the air as it passes along the through aperture is not significantly cooled by the low thermal conductivity enclosure 520. In further embodiments, the inner surface is maintained at a temperature such that the air is heated by the inner surface. In more particular embodiments, the low thermal conductivity enclosure 520 is composed of a low thermal conductivity ceramic such as for example, low density cordierite (e.g., a density cordierite tube) or other material that is appropriate for use under the temperature conditions for a heating device according to the present invention and which other material exhibits low thermal conductivity or low thermal diffusivity. In further embodiments, such materials can be porous or non-porous. In yet further embodiments, such materials also exhibit electrical insulating characteristics so as to provide protection from electrical shock.
The outer member 510 is configured and arranged so as that it surrounds the low thermal conductivity enclosure 530. In particular embodiments, the outer member 510 is a metallic member (e.g., stainless steel) that is appropriate for the intended use and provides some protection to the low thermal conductivity enclosure 520 disposed therein. In more particular embodiments, the outer member 510 and the low thermal conductivity enclosure 520 are arranged so that an outer surface of the low thermal conductivity enclosure is in contact with an inner surface of the outer member. In further embodiments, the outer member 510 and the low thermal conductivity enclosure 520 are configurable so that the outer surface of the low thermal conductivity enclosure 520 is spaced from the inner surface of the outer member 510.
Referring now to FIGS. 9 and 10 there are shown various views of a resistance heater air heating device 600 according to another embodiment of the present invention. In the illustrated embodiment, such a resistance heater air heating device 600 includes a plurality of low thermal conductivity enclosures 620 a,b that are located within an outer member 610. In this embodiment, one or more electric resistance heaters 630 are disposed in the through aperture 626 a,b of the corresponding low thermal conductivity enclosure.
The configuration and arrangement of the low thermal conductivity enclosure 520, 620 (e.g., size and shape) alone or in combination with the electric resistance heater 530, 630 (e.g., power output and size) are selectable and adaptable so as to allow the resistance heater air heating device 500,600 of the present invention to be easily adapted for use in any of a number of applications, including those not specifically for a biomass furnace.
In other aspects of the present invention there is featured a furnace such as that described herein which embodies any of the above-described resistance heater air heating devices 500, 600 of the present invention for purposes of igniting the biomass fuel (e.g., wood pellets) being burnt within the furnace. In yet other aspects of the present invention there is featured methods for igniting biomass fuel using the above-described resistance heater air heating devices 500, 600 and the steps described above for igniting the fuel.
Although a preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.

INCORPORATION BY REFERENCE

All patents, published patent applications and other references disclosed herein are hereby expressly incorporated by reference in their entireties by reference.

EQUIVALENTS

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents of the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims

1. A resistance heater air heating device, comprising

a low thermal conductivity enclosure having a through aperture;

an electric resistance heater that is disposed within the low thermal conductivity enclosure through aperture;

wherein the electric resistance heater is configured and arranged so as to heat air flowing through the through aperture to at least a predetermined temperature within a predetermined time; and

wherein an inner surface of the low thermal conductivity enclosure is configured and arranged so as to provide a high surface area which in combination with the electric resistance heater heats the air passing through the through aperture.

2. The resistance heater air heating device of claim 1, wherein the inner surface of the low thermal conductivity enclosure is configured with a plurality of convolutions that extend along the length of the enclosure.

3. The resistance heater air heating device of claim 2, wherein the convolutions create a plurality of hills and valleys that extend along the length of the enclosure.

4. The resistance heater air heating device of claim 1, wherein the inner surface of the low thermal conductivity enclosure is configured with a plurality of flutes that extend along the length of the enclosure.

5. The resistance heater air heating device of claim 4, wherein the flutes create a plurality of hills and valleys that extend along the length of the enclosure.

6. The resistance heater air heating device of claim 1, wherein the low thermal conductivity enclosure is composed of a material so the enclosure inner surface is heated by convection and radiation by the electrical resistance heater.

7. The resistance heater air heating device of claim 1, further comprising an outer member that surrounds the ceramic enclosure.

8. The resistance heater air heating device of claim 1, wherein the electric resistance heater is configured and arranged so as to reach a high temperature and has a high watt density.

9. The resistance heater air heating device of claim 1, wherein the electric resistance heater is selected from a group consisting of hot surface igniters, silicon carbon igniters, silicon carbide hot surface igniters, ceramic/intermetallic hot surface igniters and silicon nitride igniters.

10. The resistance heater air heating device of claim 1, wherein the electric resistance heater is selected from the group consisting of Norton Mini Igniters®, Norton CRYSTAR Igniters®, Surface Igniters, hot surface igniters, and I²R hot surface igniters.

11. The resistance heater air heating device of any of claims 8-10 wherein a heating element of the electric resistance heater is heated to a first temperature at or less than a first period of time and to a final temperature that is greater than the first temperature at or less than a second period of time.

12. The resistance heater air heating device of claim 11, wherein the first temperature is at or above 1000° F. and the second temperature is at or about 2000° F.

13. The resistance heater air heating device of claim 1, further comprising a plurality of low thermal conductivity enclosures and a plurality of electric resistance heaters, at least one electric heater for each low thermal conductivity enclosure.

14. The resistance heater air heating device of claim 1, further comprising at least one electric heating device for each low thermal conductivity enclosure.

15. A furnace for burning biomass fuel, comprising

a combustion chamber in which is received the biomass fuel and combustion air; and

a resistance heater air heating device that is fluidly coupled to the combustion chamber such that at least some of the combustion air passes through the resistance heater air heating devices and onto the combustion chamber, the resistance heater air heating device including

a low thermal conductivity enclosure having a through aperture;

wherein the electric resistance heater is configured and arranged so as to heat combustion air flowing through the through aperture an onto the combustion chamber, to at least a predetermined temperature within a predetermined time; and