Air supply systems for combustion of granular biomass fuels

Abstract

An air supply system for a pellet stove has an air supply system providing primary and secondary combustion air, the secondary combustion air being introduced into a burn pot above the primary air. The system and methods for the use thereof improve the emissions performance of stoves burning solid fuel in granular form, including solid biomass fuel. Stoves incorporating such air supply systems have reduced emissions, including reduced emissions of one or more of carbon monoxide, particulate matter (PM2.5), and black carbon.

Description

AIR SUPPLY SYSTEMS FOR COMBUSTION OF GRANULAR BIOMASS FUELSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims benefit of priority to U.S. Provisional Patent Application No. 63 / 570,764, filed March 27, 2024, titled “AIR SUPPLY SYSTEMS FOR COMBUSTION OF GRANULAR BIOMASS FUELS,” the entire contents of which is hereby incorporated by reference.BACKGROUND

[0002] Worldwide, stoves burning solid fuel are used extensively for cooking and heating. Solid fuel stoves, such as pellet stoves, have gained popularity in residential applications for heating and cooking due to their convenience, efficiency, and environmental friendliness compared to traditional wood-burning stoves. These devices use compressed biomass pellets, typically made from sawdust or agricultural residues, as fuel. Pellet stoves feature automatic feed mechanisms that deliver a controlled amount of pellets into the combustion chamber, ensuring consistent heat output. They also come equipped with exhaust systems and fans to enhance combustion efficiency and heat distribution throughout the home. The technology behind pellet stoves offers a promising alternative to fossil fuels and can significantly reduce greenhouse gas emissions when operated correctly.

[0003] However, pellet stoves are not without their shortcomings. One major concern is the emissions they produce. While they are generally considered cleaner than traditional wood-burning stoves, pellet stoves can release carbon monoxide (CO), particulate matter, and black carbon into the atmosphere, contributing to air pollution and potentially affecting indoor air quality. The emissions largely depend on the quality of the pellets used and the stove's combustion efficiency. Low-quality pellets or improper maintenance can lead to increased emissions. Therefore, while pellet stoves offer a more sustainable heating and cooking option compared to some alternatives, there is a need to improve the emissions performance which is essential to mitigate their environmental drawbacks.SUMMARY

[0004] According to some embodiments, a stove configured to burn granular solid fuel, such as biomass, pellets, or other suitable fuel source and includes a burn pot, the burn pot generally cylindrical in shape and having an outer wall, a top open end and a closed bottom end configured to support a fuel bed thereon, wherein the closed bottom end has a plurality of apertures formed therein, a fuel bed plenum disposed generally below the closed bottom end and configured to cause air within the fluid bed plenum to flow through the plurality of apertures formed in the closed bottom end; a primary air source in fluid communication with the fuel bed plenum, the primary air source configured to direct pressurized air to the fluid bed plenum; a secondary air plenum positioned within the burn pot above the closed bottom end, the secondary air plenum configured to direct secondary air radially outwardly from a center of the burn pot toward the outer wall; and a secondary air source in fluid communication with the secondary air plenum, the secondary air source configured to direct pressurized air to the secondary air plenum; wherein the primary air source and the secondary air source are individually controllable to vary the amount of primary air, the amount of secondary air, as well as the ratio of secondary air to primary air delivered to the burn pot.

[0005] According to some embodiments, a stove is configured to bum granular solid fuel, and includes a burn pot, the burn pot generally cylindrical in shape and having an outer wall, a top open end and a bottom closed end configured to support a fuel bed thereon, wherein a plurality of primary apertures are formed through the outer wall at and adjacent the fuel bed, the plurality of primary apertures being spaced apart from one another about a perimeter of the outer wall; a fuel bed plenum disposed generally about the perimeter of the outer wall and in communication with the plurality of primary apertures formed through the outer wall, the fuel bed plenum configured to cause air within the fuel bed plenum to flow through the plurality of primary apertures formed through the outer wall; a primary air source in fluid communication with the fuel bed plenum, the primary air source configured to direct pressurized air to the fuel bed plenum; a secondary air plenum positioned within the burn pot above the bottom closed end and above the plurality of primary apertures formed through the outer wall, the secondary air plenum configured to direct secondary air radially outwardlyfrom a center of the burn pot toward the outer wall; and a secondary air source in fluid communication with the secondary air plenum, the secondary air source configured to direct pressurized air to the secondary air plenum; and wherein the primary air source and the secondary air source are individually controllable to vary the amount of primary air, the amount of secondary air, as well as the ratio of secondary air to primary air delivered to the burn pot.

[0006] In some examples, the fuel bed plenum is coupled to the primary air source by a primary air conduit and further comprises an igniter disposed along the primary air conduit.

[0007] The secondary air plenum may be a conduit, pipe, or tube that extends generally orthogonally through the bottom closed end and into the bum pot. The secondary air plenum may include a plurality of secondary air delivery nozzles arranged circumferentially about the secondary air plenum. In some cases, the plurality of secondary air delivery nozzles are arranged in one or more rings about the secondary air plenum. In other words, a first plurality of air delivery nozzles are arranged in a first ring about the secondary air plenum at a first height, and a second plurality of air delivery nozzles are arranged in a second ring about the secondary air plenum at a second height.

[0008] The stove may additionally include a secondary air supply conduit extending through the outer wall and into the bum pot to the secondary air plenum, and the secondary air supply conduit may extend through the outer wall above the fuel bed. In some instances, the secondary air plenum comprises a plurality of secondary air delivery nozzles arranged circumferentially about the secondary air plenum. The plurality of secondary air delivery nozzles may be arranged in one or more rings about the secondary air plenum, such as a first ring of nozzles at a first height and a second ring of nozzles at a second height.

[0009] The stove may include a secondary air supply conduit configured to deliver secondary air from outside the bum pot into the burn pot and to the secondary air plenum, and the secondary air supply conduit may be spaced from the bottom closed end and above the fuel bed.

[0010] In some examples, a plurality of secondary apertures are formed through the outer wall at, and adjacent to, the secondary air plenum, the secondary apertures being spaced apartfrom one another about a perimeter of the outer wall and in communication with the secondary air plenum configured to cause air within the secondary air plenum to flow through the plurality of secondary apertures formed through the outer wall.

[0011] In some examples, the secondary air is injected in a secondary combustion zone within the burn pot, and wherein a diameter of the secondary combustion zone within the burn pot is at least 10% smaller or larger than the primary combustion zone. In other words, the secondary combustion zone has a diameter that is significantly different (e.g., >10%) from the diameter of the primary combustion zone.

[0012] In some cases, the primary air source is a first fan arranged to blow air into the fuel bed plenum. Similarly, the secondary air source may be a second fan arranged to blow air through the secondary air plenum.

[0013] In some instances, the secondary air plenum is configured to inject secondary air into the burn pot in a direction generally parallel to the direction of primary air injected into the bum pot. That is, where the primary air is injected in a horizontal plane into the burn pot, the secondary air may also be injected in a horizontal plane into the bum pot. It should be appreciated that the primary air and the secondary air may be injected in an outside-in direction, or an inside-out direction in relation to the bum pot, and may be injected in opposing directions.

[0014] According to some embodiments, a method of operating embodiments of the stove includes the steps of adding pelletized fuel to the burn pot; activating the primary air source; activating the secondary air source; activating an igniter to provide ignition heat to the pelletized fuel in the burn pot; and controlling the secondary air source and the primary air source such that a ratio of secondary air to primary air is greater than 3: 1.

[0015] The method may include activating the primary air source and the secondary air source such that the ratio of secondary air to primary air is 4: 1. In some cases, the amount of primary air is 20% to 35% of stoichiometric air

[0016] The method may include the step of adjusting, in response to a parameter associated with the stove, the ratio of secondary air to primary air. In some cases, theparameter is a power setting of the stove. In some cases, increasing or decreasing the power setting of the stove results in maintaining a constant ratio of secondary air to primary air.

[0017] In some examples, the method includes the step of adjusting the primary air source to inject primary air with a Reynolds number between 20 and 2,500. In some cases, the method includes adjusting the secondary air source to inject secondary air into the bum pot with a Reynolds number of between 200 and 3,000.

[0018] In some cases, the fuel bed plenum is coupled to the primary air source by a primary air conduit and further comprising an igniter disposed along the primary air conduit. In some cases, the igniter may be disposed in the burn pot.

[0019] In some examples, the secondary air plenum comprises a tube that extends generally orthogonally through the closed bottom end and into the burn pot. It may extend through a center of the closed bottom end upwardly into the bum pot.

[0020] According to some embodiments, the secondary air plenum comprises a plurality of secondary air delivery nozzles arranged circumferentially about the secondary air plenum. The nozzles may be arranged in one or more rings about the secondary air plenum, such as in one ring, two ring, three rings or more about the circumference of the secondary air plenum.

[0021] The plurality of apertures formed in the closed bottom end may be arranged in an array about the closed bottom end. This provides distributed primary combustion air into the burn pot.

[0022] In some cases, the primary air source may be a first fan arranged to blow air into the fuel bed plenum and the secondary air source may be a second fan arranged to blow air through the secondary air plenum.

[0023] The secondary air plenum may be configured to inject secondary air into the burn pot in a direction generally orthogonal to the direction of primary air injected into the burn pot. For instance, the primary air may be injected vertically upward into the burn pot while the secondary air may be injected horizontally. In some cases, the secondary air is injected near a center of the circular burn pot and in a direction that is radially outward from the perspective of the burn pot.

[0024] According to some embodiments, a method of operating the stove includes the steps of adding pelletized fuel to the bum pot; activating the primary air source; activating the secondary air source; activating an igniter to provide ignition heat to the pelletized fuel in the bum pot; and controlling the secondary air source and the primary air source such that a ratio of secondary air to primary air is greater than 3: 1.

[0025] In some cases, the stove is operated such that the ratio of secondary air to primary air is 4: 1. In some instances, the method includes adjusting, in response to a parameter associated with the stove, the ratio of secondary air to primary air. The parameter may be a power setting of the stove. For example, where the stove has power settings of low, medium, and high, the ratio of secondary air to primary air may be adjusted based, at least in part, on whether the stove power setting is set to low, medium, or high.

[0026] The method of operating the stove may further include adjusting the primary air source to inject primary air with a Reynolds number between 20 and 2,500.

[0027] The method of operating the stove may further include adjusting the secondary air source to inject secondary air into the burn pot with a Reynolds number of between 200 and 3,000.

[0028] One aspect described herein is a fuel bed air supply system comprising a plenum in fluid contact with an independently powered external air source, wherein: the plenum further comprises one or more nozzles, each configured to emit a jet of air; the plenum is deployed beneath a fuel bed in a stove configured to burn granular solid fuel; the jets of air from the one or more nozzles impinge on the fuel bed substantially perpendicular to the average plane of the fuel bed; and the jets of air from the one or more nozzles enter the fuel bed with sufficient momentum to deliver primary and secondary combustion air for combustion of the fuel in the fuel bed.

[0029] Another aspect described herein is an upper air supply system comprising a plenum in fluid contact with an independently powered external air source, wherein: the plenum further comprises one or more nozzles, each configured to emit a jet of air; the plenum is deployed above a fuel bed in a stove configured to burn granular solid fuel; the jets of air from the one or more nozzles exit the plenum substantially parallel to the average planeof the fuel bed; and the jets of air from the one or more nozzles deliver secondary combustion air for combustion of the fuel in the fuel bed.

[0030] A further aspect described herein is a combustion apparatus comprising a bum pot configured to burn granular solid fuel, a fuel bed air supply system further comprising a first plenum in fluid contact with a first independently powered air source, and an upper air supply system further comprising a second plenum in fluid contact with a second independently powered air source, wherein: the first plenum further comprises one or more nozzles, each configured to emit a jet of air; the first plenum is deployed beneath a fuel bed in the burn pot; the jets of air emitted from the first plenum impinge on the fuel bed substantially perpendicular to the average plane of the fuel bed; the jets of air emitted from the first plenum enter the fuel bed with sufficient momentum to deliver primary combustion air, or deliver primary and secondary combustion air, for combustion of the fuel in the fuel bed; the second plenum further comprises one or more nozzles, each configured to emit a jet of air; the second plenum is deployed above the fuel bed in the burn pot; and the jets of air emitted from the second plenum deliver secondary combustion air for combustion of the fuel in the fuel bed.

[0031] In one embodiment described herein is a combustion apparatus comprising a burn pot configured to burn granular solid fuel, a fuel bed air supply system further comprising a first plenum in fluid contact with a first independently powered air source, and an upper air supply system further comprising a second plenum in fluid contact with a second independently powered air source, wherein: the first plenum further comprises one or more nozzles, each configured to emit a jet of air; the first plenum is deployed beneath a fuel bed in the burn pot; the jets of air emitted from the first plenum impinge on the fuel bed substantially perpendicular to the average plane of the fuel bed; the jets of air emitted from the first plenum enter the fuel bed with sufficient momentum to deliver primary combustion air, or deliver primary and secondary combustion air, for combustion of the fuel in the fuel bed; the second plenum further comprises one or more nozzles, each configured to emit a jet of air; the second plenum is deployed above the fuel bed in the bum pot; the jets of air emitted from the second plenum exit the plenum substantially parallel to the average plane ofthe fuel bed; and the jets of air emitted from the second plenum deliver secondary combustion air for combustion of the fuel in the fuel bed.BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present application can be best understood by reference to the following descriptions taken in conjunction with the accompanying drawing figures, in which like parts may be referred to by like numerals.

[0033] Figs. 1 and 2 (l(a)-l(d) and 2(a)-2(f)) show exemplary air supply configurations that include a fuel bed air supply system and an upper air supply system, in accordance with some embodiments.

[0034] Fig. 1 A illustrates a top view of a burn pot including the sidewall of the generally cylindrical burn pot and top surface of the fuel bed air supply system plenum with air delivery nozzles, in accordance with some embodiments.

[0035] Fig. IB illustrates an array of air delivery nozzles, in accordance with some embodiments.

[0036] Fig. 1C illustrates an array of air delivery nozzles, in accordance with some embodiments.

[0037] Fig. ID illustrates a cross-sectional view of a bum pot showing the fuel bed air supply system plenum, air conduit, and top surface with an array of air delivery nozzles, in accordance with some embodiments.

[0038] Fig. 2A illustrates a top-down view of the burn pot showing a combination of the fuel bed air supply system and upper air supply system, in accordance with some embodiments.

[0039] Fig. 2B illustrates a circular array of holes of the upper air supply system, in accordance with some embodiments.

[0040] Fig. 2C illustrates a circular array of holes of the upper air supply system, in accordance with some embodiments.

[0041] Fig. 2D illustrates a circular array of holes of the upper air supply system, in accordance with some embodiments.

[0042] Fig. 2E illustrates an array of air delivery nozzles of the fuel bed air supply system, in accordance with some embodiments.

[0043] Fig. 2F illustrates a cross-sectional schematic view of the burn pot showing the fuel bed air supply system plenum with air conduit, the upper air supply system plenum with air conduit, and the air flow path through the fuel bed air supply system and upper air supply system, in accordance with some embodiments.

[0044] Fig. 3 A illustrates a top-down view of a bum pot, in accordance with some embodiments.

[0045] Fig. 3B illustrates a cross-sectional schematic view of a burn pot, in accordance with some embodiments.

[0046] Fig. 4A illustrates a perspective view of a burn pot, in accordance with some embodiments.

[0047] Fig. 4B illustrates a cross-sectional view of a burn pot, in accordance with some embodiments.

[0048] Fig. 5 illustrates shows a cross-section view of a bum pot, in accordance with some embodiments.

[0049] Fig. 6 illustrates an exemplary burn pot including a fuel bed air supply system and an upper air supply system installed in a heating stove, in accordance with some embodiments.

[0050] Fig. 7 illustrates a top-down view of the fuel bed supported on top of the fuel bed air supply system showing holes in the fuel bed where the combustion air released from the fuel bed air supply plenum nozzles bum a channel through the fuel bed, in accordance with some embodiments.

[0051] Figs. 8A-8C illustrate an exemplary burn pot including a fuel bed air supply system and an upper air supply system in operation burning wood pellets that further shows how the fuel bed air supply system air and the upper air supply system air influence combustion, in accordance with some embodiments.

[0052] Fig. 9 illustrates a cross-section schematic view of a combustion chamber showing the primary air source, secondary air source, and igniter, in accordance with some embodiments.

[0053] Fig. 10 illustrates a perspective view of a primary and secondary air inlet, in accordance with some embodiments.

[0054] Fig. 11 illustrates an embodiment of a burn pot and secondary air supply conduit, in accordance with some embodiments.

[0055] Fig. 12 illustrates examples of secondary air plenums configured with secondary air injection holes, in accordance with some embodiments;

[0056] Fig. 13 illustrates a graph of experimental results showing total particulate matter emissions as a function of primary air flow rate as a percentage of stoichiometric air, in accordance with some embodiments;

[0057] Fig. 14 illustrates a graph of experimental results showing total particulate matter emissions as a function of primary air flow rate as a percentage of stoichiometric air, in accordance with some embodiments.

[0058] Fig. 15 illustrates a graph of experimental results showing the variation of black carbon emissions by varying a ratio of secondary air kinetic energy to fuel energy, in accordance with some embodiments.

[0059] Fig. 16 is a graph of various stoves, including a baseline stove and a concept stove according to embodiments herein, showing efficiency versus particulate emissions rate, in accordance with some embodiments.

[0060] Fig. 17 is a cross-section schematic view of another embodiment of a combustion system in which primary combustion air is injected radially inwardly into a fuel bed, in accordance with some embodiments.

[0061] Fig. 18 is a perspective view of a primary air supply plenum for use with a combustion system as in Fig. 17, in accordance with some embodiments.

[0062] Fig. 19 is a top view of a bum pot incorporating the air supply plenum of Fig. 18 installed in a heating stove, in accordance with some embodiments.

[0063] Fig. 20 is a cross-section schematic view of yet another embodiment of a combustion system in which secondary combustion air is delivered through a wall of the burn pot, in accordance with some embodiments.

[0064] Fig. 21A is a top-down schematic view of a component of an ash removal system, in accordance with some embodiments.

[0065] Figs. 21B-21F are cross-section schematic views depicting operation of one embodiment of an ash removal system, in accordance with some embodiments.

[0066] Figs. 22A-22B are cross-section schematic views depicting another embodiment of an ash removal system, in accordance with some embodiments.

[0067] Fig. 23 is a cross-section schematic view depicting a screw conveyor configured for use with another embodiment of an ash removal system, in accordance with some embodiments.

[0068] Fig. 24 is a cross-section schematic view of another embodiment of a combustion system with primary air apertures at varied heights in the fuel bed, in accordance with some embodiments.

[0069] Fig. 25 is a cross-section schematic view of still another embodiment of a combustion system in which different materials are used for different parts of the burn pot wall, in accordance with some embodiments.

[0070] Fig. 26 is a cross-section schematic view of yet another embodiment of a combustion system supplying secondary combustion air, in accordance with some embodiments.

[0071] Fig. 27 is a cross-section schematic view of a further embodiment of a combustion system having multiple stages of secondary combustion air injection, in accordance with some embodiments.

[0072] Fig. 28 is a cross-section schematic view of a yet further embodiment of a combustion system having a fuel bed, in accordance with some embodiments.

[0073] Fig. 29 is a cross-section schematic view of a still further embodiment of a combustion system delivering secondary combustion air within a burn pot and above a fuel bed, in accordance with some embodiments.

[0074] Fig. 30 is a cross-section schematic view of an embodiment of a combustion system wherein different portions of the burn pot have different diameters, in accordance with some embodiments.

[0075] Fig. 31 A includes cross-section schematic views of additional embodiments of combustion systems in which different portions of the burn pot have different diameters, in accordance with some embodiments.

[0076] Fig. 3 IB illustrates cross-section schematic views of additional embodiments of combustion systems in which different portions of the burn pot have different diameters, in accordance with some embodiments.

[0077] Fig. 32 is a cross-section schematic view of an embodiment of a combustion system having multiple igniters, in accordance with some embodiments.

[0078] Figs. 33A - Fig. 33B illustrates a graph of experimental results showing the variation of black carbon and carbon monoxide emissions with varying secondary air jet Reynolds number, in accordance with some embodiments.DETAILED DESCRIPTION

[0079] The following detailed description and provides a better understanding of the features and advantages of the inventions described in the present disclosure in accordance with the embodiments disclosed herein. Although the detailed description includes many specific embodiments, these are provided by way of example only and should not be construed as limiting the scope of the embodiments disclosed herein.

[0080] Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed embodiments belong. Any methods and materials similar to, or equivalent to, those described herein can be used in the practice or testing of the described embodiments.

[0081] The following description sets forth exemplary methods, parameters and the like. It should be recognized, however, that such description is not intended as a limitation on thescope of the present disclosure but is instead provided as a description of exemplary embodiments.

[0082] Numeric ranges provided herein are inclusive of the numbers defining the range.

[0083] The fuel bed air supply system (also referred to herein as the primary air supply) and upper air supply system (also referred to herein as the secondary air supply) and certain combustion characteristics apparent during use are described in further detail below.

[0084] Generally, a stove configured to bum granular fuels such as pellets or wood chips or other biomass may include an apparatus to contain and support the burning granular fuel herein referred to as a burn pot. The burn pot may contain conduits that supply air to different parts of the burn pot to facilitate combustion. The granular fuel that is in the bum pot is herein referred to as the fuel bed. The fuel bed may contain fuel particles that are not yet burning, burning fuel particles, burning charcoal particles and ash residue from previously burned particles. The burn pot may be contained in a stove that may further include an enclosed volume surrounding the burn pot where combustion takes place herein referred to as the firebox. The firebox may be in fluid connection with one or more fans that maintain the firebox at negative pressure and pulls combustion products from the firebox and expels them into the chimney exhaust herein referred to as an exhaust fan. The firebox may contain a window to view the flames emanating from the burn pot. To prevent accumulation of deposits from combustion onto the window and blocking the view of the flames, apertures adjacent to the window may allow air from outside the firebox to be drawn into the firebox by the negative pressure created in the firebox by the exhaust fan and pass over the window, which is referred to as airwash air herein. The air that enters the burn pot and impinges on the fuel bed is herein referred to as primary air, or primary combustion air. The air that enters the burn pot above the fuel bed where flames may be present is herein referred to as secondary air, or secondary combustion air. The airwash air and other outside air that enter the firebox from any locations that are not the primary air and secondary air conduits is herein referred to as tertiary air. The stove may contain an apparatus to store granular fuel and control the flow of fuel into the burn pot herein referred to as the fuel feed mechanism. The fuel feed mechanism may be controlled to adjust the fuel flow into the burn pot to control the heatoutput from the stove herein referred to as the power level. The fuel feed mechanism may be controlled manually or automatically. A conduit may guide the granular fuel from the fuel feed mechanism to the burn pot herein referred to as the fuel feed conduit. The fuel feed conduit may be above the burn pot and discharge fuel into the burn pot by the force of gravity herein referred to as top-feed. The fuel feed conduit may discharge fuel into the side of the burn pot adjacent the fuel bed with an apparatus such as a screw feeder herein referred to as horizontal-feed. The fuel feed conduit may be below the fuel bed and discharge fuel into the bottom of the fuel bed with an apparatus such as a screw feeder herein referred to as bottom-feed.

[0085] Generally, the fuel bed air supply system and upper air supply system include an air source that may output pressurized air, an air conduit to transport air from the air source to the burn pot, and plenums that distribute the air into the burn pot through one or more nozzles. The air conduit may be any suitable structure to transport air including a tube of round, square, or rectangular cross-section, a cylindrical pipe, or other shape. The fuel bed air supply system and the upper air supply system may be separately controlled, such that the amount of primary air, the amount of secondary air, as well as the ratio of primary air to secondary air may be adjusted. Of course, variations of individual control are possible such as using a single air source to supply air to both the fuel bed air supply system and the upper air supply system and using one or more valves or baffles to adjust the amount and ratio of air delivered as primary and secondary air. Another variation is to use a single air source to supply air to both the fuel bed air supply system and the upper air supply system while choosing the size and number of nozzles in the primary air plenum and secondary air plenum to fix the approximate ratio of primary and secondary air delivered to the bum pot at a preferred level. Such a simplified system may be preferred in some cases to reduce cost in a manufactured stove while still having reduced emissions due to the improvements described herein. In embodiment that use a single air source, the amount of air may be adjusted while maintaining an acceptable secondary to primary air ratio. Another variation is to use a single air source to supply air to both the fuel bed air supply system and the upper air supply system and choose the flow characteristics of the primary air conduit and the secondary air conduitto adjust the ratio of the primary and secondary air delivered to the burn put. One example of choosing the flow characteristics is to use a restrictor in one conduit to reduce that flow relative to the other. The terms “separate control” and “individual control” in relation to the primary and secondary air systems are used interchangeably herein.

[0086] With reference to Fig. 1A, a top-down view of the burn pot 110 is illustrated showing an array of air nozzles 101 having a suitable diameter. The burn pot 110 may be generally cylindrical in shape and have an opening at the top and a closed bottom end. The cross section of the generally cylindrical bum pot may be a circle, oval, ellipse, square, rectangle, triangle, hexagon, octagon or any other polygon. In some cases, the nozzle 101 diameter may be formed as a round nozzle, and in some cases, the nozzle is a hole formed in the fuel bed air supply system top surface 106. The nozzles may be a uniform size, or the hole size may vary. For example, in some cases, the nozzles closer to the edge of the bum pot 110 may have a hole diameter that is greater than a hole diameter of nozzles that are closer to the center of the fuel bed. In some cases, the nozzle diameter is on the order of 5 / 64 inch (2mm). In some examples, the top surface 106 of the fuel bed air supply system (which is also referred to herein as the closed bottom end of the bum pot) is a generally planar surface configured to support a number of fuel pellets within a generally cylindrical sidewall 112 of the bum pot 110. In some cases, the nozzles are formed as apertures formed in the top surface 106 of the fuel bed air supply system.

[0087] With reference to Fig. IB a sample array of air delivery nozzles 101 is depicted and correlates with air delivery nozzles utilized for the results presented in AirConfigl shown in Example 2 discussed below. In some cases, the diameter of the substantially circular top surface of the fuel bed air supply system plenum 105 is approximately 3.75 to 4.5 inches (95mm to 114mm), and in some cases was 4 inches (102 mm) in diameter. In some embodiments, the vertical spacing between rows 102 of air delivery nozzles is approximately 7 / 8 inch (22 mm). In some cases, the horizontal spacing between columns 103 of air delivery nozzles is approximately 13 / 16 inch (20 mm). In some embodiments, the diagonal spacing between adjacent air delivery nozzles 104 may be on the order of approximately 5 / 8 inch (15mm). Other embodiments of the fuel bed air supply system include nozzles that may be asubset of the array of nozzles shown in Fig. IB and may have the same dimensions. Of course, the disclosed dimensions for the arrangement and spacing of the nozzles 101 may be altered to adjust characteristics of the bum pot and may be selected to change combustion parameters, such as, without limitation, altering the mass flow rate of primary combustion air, adjusting the Reynolds number of the primary combustion air, and the velocity of the primary combustion air injected through the nozzles 101. In some experimental examples, the array of holes was arranged as 36x2mm holes, 88x2mm holes, and 88x4mm holes, although other configurations are contemplated.

[0088] With reference to Fig. 1C, an example array of air delivery nozzles 101 is shown and represents the configuration utilized for AirConfig2 defined in Example 2 below. As illustrated, the primary combustion air nozzles 101 may be formed in an annular ring about a center of the plenum top surface 106. In some cases, the annular ring may be located proximately to the outer wall of the burn pot 110. However, in some embodiments, the annular ring may be located about equidistant between the sidewall of the bum pot and the center of the upper surface 106 of the plenum. In some cases, additional primary combustion air may be introduced, such as by forming multiple annular rings of nozzles 101.

[0089] With reference to Fig. ID, a cross-section view of the bum pot 110 is depicted showing the sidewall 112 of the bum pot 110, the fuel bed air supply system plenum 105, primary combustion air conduit 109, top surface 106 of the fuel bed air supply system plenum with the array of air delivery nozzles that supports the fuel bed 107 and releases primary combustion air 108 to the granular solid fuel. In use, granular fuel is introduced into the bum pot and rests on the upper surface of the primary air plenum 106 to create a fuel bed107 and is ignited, such as by an igniter. Primary combustion air is forced into the plenum 105, thus pressurizing the plenum 105 and causing air to flow through the nozzles, through the fuel bed 107, and upwardly into the bum pot 110. The primary air 108 may be controlled, such as by causing a fan or pump to increase the mass flowrate of the primary combustion air108 into the plenum 105.

[0090] With reference to Fig. 2A, a top view of the bum pot 110 is depicted showing a combination of the fuel bed primary air supply system and secondary air supply system. Asshown in some embodiments, the fuel bed air supply system plenum 105 has a top surface 106 that the secondary air supply system plenum 201 passes through. In some cases, the secondary air supply system plenum 201 includes a flow pathway and may have one or more nozzles for secondary combustion air to enter the bum pot 110. In some cases, the secondary combustion air enters the burn pot 110 in a direction that is radially outward from the secondary air supply plenum 201 toward the burn pot sidewall 112 and parallel to the plane of the top surface of the fuel bed air supply system, as will be described in further detail below. The secondary air supply system may further be introduced into the burn pot 110 through alternative routing, such as for example, through the side of the burn pot side wall, or may be routed through the top of the burn pot and may be configured as a central tube that enters the burn pot from the top and may point downward into the burn pot. In many cases, however, the secondary air plenum is routed to provide secondary combustion air near the axial center of the bum pot and in a direction that flows radially outwardly from the center of the bum pot toward the side wall of the burn pot 112. In some embodiments, the performance of the stove may be agnostic as to the routing of the secondary air plenum, and may only require that the secondary air plenum is provided at a center of the bum pot and configured to direct secondary combustion air radially outward within the burn pot.

[0091] With reference to Figs. 2B, 2C, and 2D, examples of secondary air plenums 201 are illustrated. While the depicted plenums are constructed from cylindrical pipes, tubes of different cross-sectional shapes are possible, such as a circle, oval, ellipse, square, rectangle, hexagon, octagon or other polygon. Tubes with a circular cross section are preferred to facilitate an array of nozzles that point radially outward. With reference to Fig. 2B, in some cases, a circular array of holes 204 is formed in the secondary combustion air cap 210 of the upper air supply system. The secondary air plenum 201, in some examples, includes an air delivery conduit 212 to which the secondary combustion air cap 210 is attached. In some cases, the cap 210 is threaded onto the conduit 212, while in other cases it may be welded on, soldered, adhered, or affixed through any suitable method. The secondary air cap 210 may be a separate component that is affixed to the conduit 212, or it may be integrally formed withthe conduit 212. The performance of the arrangement of nozzles 204 is detailed as AirConfig3 defined in Example 2 below.

[0092] With reference to Fig. 2C, the circular array of holes 205 (e.g., nozzles) of the secondary combustion air cap 210 of the secondary air plenum 201 is illustrated. The secondary combustion air cap 210 may be formed to have any suitable number of nozzles, such as 4, 6, 8, 10, 12, 18, 24 or more. As illustrated, the cap 210 may be formed to have 8 nozzles equally spaced about the circumference of the cap 210. The performance of nozzles 205 depicted in Fig. 2C is detailed as AirConfig4 defined in Example 2 below.

[0093] With reference to Fig. 2D, the circular array of holes (e g., nozzles) 206 of the performance of secondary combustion air cap 210 is detailed as AirConfig5 defined in Example 2 below. In some examples, the center of holes 206 of the secondary combustion air supply system is located above the top surface of the primary combustion air plenum by a distance h 207. In some cases, the secondary air nozzles may be located approximately 2.25 inches (57 mm) above the plane of the top surface of the primary air supply system. In other cases, the secondary air nozzles may be located between about 0.5 inches (13mm) and 4 inches (102 mm) above the top surface. According to some embodiments, such as the experiments detailed below, and with reference to Figs. 2B, 2C, and 2D, hole arrays 204, 205, and 206 formed within the upper cap 210 may be positioned at the same height relative to the top surface of the fuel bed air supply system 207. In some cases, the secondary air nozzles may be located relative to the bum pot diameter and / or the burn pot height. For example, in some instances, the secondary air nozzles may be positioned at a height equal to about 25%, or 40%, or 50%, or 60%, or 70%, or 80%, or 90, or 100%, or 110%, or 120%, or 130%, or 140%, or 150% or more of the diameter of the bum pot. In some examples, the secondary air nozzles may be positioned at a height that is equal to 20%, 25%, 30%, 40%, 50%, 60%, or 70%, or 75% of the height of the bum pot.

[0094] In some cases, the secondary combustion air supply system provides a stream of secondary combustion air that enters the burn pot in a direction generally orthogonal to the primary combustion air. For example, as shown in in Fig. 2A, the upper air supply system plenum 201 may extend upwardly through the top plate 106 near the center of the top plate106, and have nozzles that inject air radially from the plenum 201. This provides a volume of secondary combustion air delivered from the center of the bum pot 110 which initially flows radially outward from the plenum 201 within the bum pot 110. This radial air injection is generally orthogonal to the primary combustion air, which is injected to flow upwardly within the burn pot 110. According to some embodiments, this secondary air is injected near the center (inside) of the burn pot 110 and flows from the center of the bum pot toward the outside of the bum pot, which may be described as an inside-out air flow.

[0095] As used herein, the terms fuel bed air supply system refers to the air supply that passes through the fuel bed, and may also be referred to as the primary combustion air supply system. Similarly, the upper air supply system may also be referred to as the secondary combustion air supply system.

[0096] With reference to Fig. 2E, an example array of air delivery nozzles 101 of the fuel bed air supply system is shown that was used in the reported results for AirConfig3, AirConfig4, and AirConfig5 described in Example 2 below. In some examples, the upper air supply system plenum passes through the center of the fuel bed air supply system plenum 214.

[0097] With reference to Fig. 2F a schematic cross-sectional view of the burn pot 110 is depicted showing the sidewall 112 of the bum pot 110, the fuel bed air supply system plenum 105, air conduit 109, top surface 106 of the fuel bed air supply system plenum 105 with the array of air delivery nozzles that supports the fuel bed 107 and releases primary combustion air 108 to the granular solid fuel, the upper air supply system plenum 201, upper air supply system air conduit 202, secondary combustion air 203, at a height h 207 above the top surface 106. As described, the height h 207 may be any suitable height, and in some cases is between about 0.5 inches and 6 inches (12.7mm and 152.4mm), or between 1 inch and 5 inches (25.4mm and 127mm), or between 1.5 inch and 5 inches (38mm and 127mm), or between 2 inches and 4 inches (51mm and 102mm), or between 2.25 inches and 3 inches (57mm and 76mm). In one particular example, the height h 207 is 2.25 inches (57mm).

[0098] In some examples, the fuel bed air conduit 109 is independent from the secondary air conduit 202. In some cases, a first air source is coupled to the fuel bed air conduit 109 anda second air source is coupled to the secondary air conduit 202. The air source may be a pump, a fan, a compressor, or other device capable of introduction air through the fuel bed air conduit 109 and the secondary air conduit 202. According to some embodiments, a controller may be configured to control the mass air flow through one or more of the primary air conduit 109 and the secondary air conduit 202. In some cases, the controller is configured to establish a ratio between the mass air flow between the primary air conduit 109 and the secondary air conduit 202. In some cases, the ratio is 1 :4, with the secondary air conduit delivery 4 times as much air by mass as the primary air conduit 109. Of course, other ratios may be used based upon the fuel, temperature setting of the stove, and the desired heat output of the stove. For example, the controller may adjust the ratio to be 1 : 1, 1 :2, 1 :3, 1 :5, 1 :6 or some other desired ratio. However, based on experimentation, in some examples, a ratio of 1 :4 provides a surprising decrease in emissions of the stove as will be detailed herein below.

[0099] With reference to Fig. 3A, a top-down view of burn pot 110 is illustrated and is the burn pot 110 used as OutletConfigl defined in Example 2 below. The bum pot 110 includes a sidewall 112 that forms a generally cylindrical burn pot 110 and a top surface 106 of the primary air plenum with formed nozzles 101 that allow primary combustion air to be injected through the top surface 106 of the primary air plenum and into the burn pot 110.

[0100] Fig. 3B illustrates a cross-sectional view of the burn pot 110 according to some embodiments, the performance of which is represented as OutletConfigl in Example 2 detailed below. In the illustrated example, the diameter of the generally cylindrical bum pot sidewall 112 is approximately 4 inches (about 101.6mm). The primary air plenum 105 receives air from the primary air conduit and allows air to flow from the primary air plenum 105 through the nozzles in the top surface 106 of the plenum to provide primary combustion air to the burn pot 110 to facilitate burning of the fuel arranged into a fuel bed onto the top surface 106 of the plenum 105.

[0101] Fig. 4A illustrates a perspective view of a burn pot 110, the experimental performance of which is designated as OutletConfig2 in Example 2 below. In some examples, the burn pot 110 has an outlet 402 that has a smaller diameter than the bum pot sidewall 112. In some cases, the bum pot 110 may have a diameter about 4 inches(101.6mm) and the outlet 402 may have a diameter of about 2 inches (50.8mm). The outlet 402 may be formed by a restrictor plate 401 that may cause the outlet 402 to have a diameter that is less than the diameter of the burn pot. The restrictor plate may be formed to be orthogonal to a longitudinal axis of the generally cylindrical burn pot 110, or may be frustoconical in shape such that it gradually narrows along the longitudinal axis of the generally cylindrical burn pot 110.

[0102] Fig. 4B illustrates a cross-sectional view the burn pot 110 of Fig. 4A, the experimental performance of which is designated as OutletConfig2 in Example 2 below. In some cases, the diameter of the generally cylindrical burn pot sidewall 112 is approximately 4 (101 6mm) inches and the restrictor 401 reduces the outlet diameter to 2 inches (50.8mm). Of course, the disclosed dimensions are exemplary and should not be limited on the embodiments described herein.

[0103] With reference to Fig. 5 a cross-sectional view of an embodiment of a burn pot 110 is illustrated, the performance of which is detailed as OutletConfig3 in Example 2 below. In some cases, a collar 501 may be formed, attached, and / or sealed to restrictor 401 to further influence the combustion gases within the burn pot 110. In some cases, the collar 501 and has a diameter that is the same as the outlet, and may have a height that is between 0.75inch (19mm) and 1.5 inch (38mm), and in some embodiments, is 1.25 inch (31.75mm). It is believed that the collar may cause edge recirculation of the smoke and particulates contained in the exhaust and force the exhaust to circulate back down through the burn pot 110 and improve total combustion.

[0104] Fig. 6 illustrates an exemplary burn pot 110 including a fuel bed air supply system and an upper air supply system installed in a heating stove that bums wood pellets. In some heating stoves, the fuel feed conduit 601 drops pellets onto fuel ramp 602 such that the pellet fuel falls within the bum pot sidewall 112 and rests on the top surface 106 of the fuel bed air supply system plenum and receives air from the fuel bed air supply system plenum and the upper air supply system plenum 201. Primary combustion air passing through the fuel bed air conduit 109 and secondary combustion air passing through upper air conduit 202 into the air delivery plenums may be monitored by mass air flow sensors 603a, 603b. The mass air flowsensors 603a, 603b may be any suitable mass or volume flow sensor, but in some cases are selected from one or more of a moving vane meter, hot wire sensor, cold wire sensor, velocimeter, membrane sensor, orifice meter with differential pressure sensors, Coriolis mass flow meter, impeller turbine mass flow meter, laminar flow elements, or other suitable flow sensor. In some cases, the mass air flow sensors 603a, 603b may be in communication with a controller that is configured to determine an appropriate mass flow rate and / or a ratio of primary combustion air to secondary combustion air and selectively adjust the air source to meet the ratio of primary combustion air to secondary combustion air. In some cases, this ratio is about 1:4 The mass flow rate of primary and / or secondary air can be related to the fuel feed rate in terms of stoichiometric air, the amount of air necessary to completely bum the fuel. In some cases, the primary air mass flow rate is about 20% of the stoichiometric airflow rate. In some cases, the primary air mass flow rate is in the range of 15% to 50% of the stoichiometric airflow rate. In some cases, the controller can adjust the primary mass airflow rate and / or secondary mass airflow rate and / or the ratio of primary to secondary mass air flow according to the fuel feed rate.

[0105] Fig. 7 illustrates a top-down view of the fuel bed 107 supported on top of the fuel bed air supply system showing holes 701 where the combustion air released from the fuel bed air supply plenum nozzles 101 burns a channel through the fuel bed 107. In some cases, the fuel bed 107 is distributed across the top surface 106 of the primary air plenum. As primary air is fed into the burn pot 110, it cooperates with the fuel bed 107 to cause combustion of the fuel pellets in the fuel bed 107. The primary combustion air, as a result of one or more of combustion, fuel fed rate, primary air mass flow, primary air hole number and size, will burn a channel through the fuel bed and create hot spots 701 within the bum pot 110. The hot spots will generally be distributed throughout the fuel bed 107 above fuel bed air supply plenum nozzles 101.

[0106] With reference to Figs. 8A-8C, exemplary burn pots including a fuel bed air supply system and an upper air supply system in operation burning wood pellets is depicted.

[0107] Fig. 8A illustrates that, in practice, flames 801 form where air from the fuel bed air supply system has burned through the fuel bed and reaches the top of the fuel bed. Theseflames form as primary air forms a channel through the fuel bed and is allowed to escape the fuel bed with velocity that cause the flames 801 to rise above the fuel bed. Where the fuel bed may not be evenly distributed, the flames may form at only a few discrete locations where the thickness or density of the fuel bed is less than at other locations.

[0108] Fig. 8B illustrates that, in practice, flames 801 form where air from the fuel bed air supply system has burned through the fuel bed 107 and formed channels in the fuel bed 701. When compared to FIG. 8A, Fig. 8B illustrates many smaller flames distributed about the fuel bed, indicating a more uniform distribution of fuel pellets within the fuel bed. In some cases, secondary combustion air from the secondary combustion air supply system plenum 201 creates bright spots 802 on the bum pot sidewall 1 12.

[0109] With reference to Fig. 8C, flames 801 form where air from the fuel bed air supply system has burned through the fuel bed and reaches the top of the fuel bed. Fig. 8C illustrates a reduced primary air mass flow rate, which results in a more even bum and less char being lofted out of the burn pot. In some cases, this is accomplished by increasing the number and / or size of the nozzles in the top surface of the primary air plenum.

[0110] In some embodiments the air supply systems disclosed herein are used in stoves configured for the combustion of solid fuel in granular form. In some embodiments the solid fuel comprises biomass, coal, charcoal, or combinations thereof. In some embodiments the solid fuel comprises biomass. In some embodiments the biomass comprises agricultural waste or residues, forestry waste or residues, municipal waste or residues, or combinations thereof. In some embodiments the biomass comprises agricultural waste or residues. In some embodiments the biomass comprises forestry waste or residues. In some embodiments the biomass is derived from trees. In some embodiments the biomass comprises wood.

[0111] In some embodiments the solid fuel is used in granular form. In some embodiments the granular form includes one or more of pellets, chips, or briquets. In some embodiments the granular form includes pellets. In some embodiments the granular form includes chips. In some embodiments production of the granular form of the solid fuel comprises chipping, pelletizing, or combinations thereof.

[0112] In some embodiments the solid fuel in granular form comprises wood pellets or wood chips. In some embodiments the solid fuel in granular form comprises wood pellets. In some embodiments the solid fuel in granular form comprises wood chips. In some embodiments the solid fuel in granular form is wood pellets.

[0113] Fig. 9 schematically illustrates an example combustion system 900 including a burn pot 110 of a stove along with a primary combustion air pathway 109 and a secondary combustion air pathway 202. In some cases, a primary air fan 902 is energized and provides primary combustion air 108 along the primary air conduit 109 into the primary air plenum 105 and through nozzles in the top surface 106 and into the bum pot. Similarly, a secondary air fan 904 may be energized to provide secondary combustion air 203 through the secondary air conduit 202, into the secondary air plenum 201 and into the burn pot. In some cases, the primary air fan 902 and the secondary air fan 904 may be separately controllable, such as to change the mass flow rate of air being sent through the respective air conduits 109, 202. The control may include adjusting a voltage of the primary air fan 902 and / or the secondary air fan 904 to control the speed of the motor in each of the fans. The primary combustion air 108 and / or the secondary combustion air 203 may be pressurized such that fans 902, 904 push air into the burn pot 110. In some cases, the fans 902, 904 create a positive pressure relative to ambient within the primary air conduit 109 and the secondary air conduit 202. The positive pressure within the air conduits 109, 202 causes the air to flow into the bum pot 110, which may have a lower pressure, especially when the stove is in operation and the exhaust extractor fan 908 maintains the firebox (not shown) at negative pressure relative to ambient to prevent leaks of exhaust gasses into the indoor environment near the stove.

[0114] In some examples, an igniter 906 is provided to generate initial heat to cause the fuel pellets within the fuel bed to ignite along with the primary combustion air 108. In some cases, the igniter 906 is positioned to heat the primary air 108 to a temperature sufficient to ignite the fuel pellets in the fuel bed on the top surface of the primary air plenum 106. While the igniter is shown as being disposed within the primary air conduit 109, it should be appreciated the igniter 906 may be located within the burn pot 110 in order to heat the fuel pellets directly in order to establish and / or sustain combustion. In some cases, an additionaligniter or heater may be provided to preheat the secondary combustion air 203 in the secondary air conduit 202. However, in any case, as the secondary air conduit 202 passes through the primary air plenum 105 and fuel bed and into the secondary air plenum 201 in the bum pot 110, the secondary combustion air will be preheated as it passes through the fuel bed, or above the flames in configurations in which the secondary air plenum enters through the sidewall of the burn pot or from overhead.

[0115] In some cases, the stove may have an exhaust fan 908 that draws air and exhaust from the burn pot out of the burn pot and through a chimney or other suitable exhaust. The exhaust fan 908 negative pressure relative to ambient may draw additional air into the firebox (not shown) enclosing the burn pot through a passage under the viewing door window (not shown) of the stove, through the fuel feed system or leaks in the stove body. This additional air introduced into the firebox is called tertiary air. In some cases, the tertiary air supplies additional combustion air to the flames emanating from the burn pot allowing complete combustion and influencing emissions and stove thermal efficiency. In some cases, the exhaust fan can be controlled to maintain a desired negative pressure relative to ambient to draw in a specified amount of tertiary air to optimize combustion and transfer thermal energy from the fuel into the room for heating. In some cases, a pressure sensor can be used to allow a controller to adjust the exhaust fan rotation speed to maintain a desired negative pressure in the firebox. In some cases, the desired negative firebox pressure can vary with fuel feed rate.

[0116] The combustion system 900 may include a primary air plenum 105 into which the primary combustion air 108 is delivered. The top surface 106 of the primary air plenum 105 is formed with nozzles, as described in embodiments throughout this disclosure, and the primary combustion air 108 passes through the nozzles and into the burn pot.

[0117] According to some embodiments, it has been found that injecting too much primary combustion air 108 into the bum pot 110 causes char to be lofted out of the burn pot due to the high momentum of the primary combustion air. This is less desirable as lofting char leads to an increase in particulate emissions. In some cases, reducing the momentum of the primary combustion air, such as by reducing the mass flowrate of the primary combustion air, or increasing the number and / or the size of the primary air nozzles in the top surface ofthe primary air plenum 106 results in less char being lofted, but if the mass flow rate is too low, this can result in an inefficient burn and an increase in soot and ash.

[0118] Fig. 10 illustrates the interior components of the burn pot showing the top surface 106 of the primary air plenum and the secondary air plenum 201. As can be seen, the top surface 106 is configured with apertures, nozzles, or holes 101 that allow primary combustion air to pass through the top surface from the primary air plenum 105 and through the top surface 106 into the bum pot. The secondary air is delivered to the burn pot through the secondary air plenum 201 which may be configured with a series of radial holes 1002. The radial holes 1002 may be provided in a single ring around the secondary air conduit 212 of the secondary air plenum 201, however, the radial holes 1002 may be disposed in two rings (as illustrated), three rings, or more to provide a sufficient mass flow of secondary combustion air. The secondary air plenum 201 is disposed generally in the center of the top surface 106 and delivers secondary combustion air in a radially outward direction from the inside of the bum pot toward the outside of the burn pot.

[0119] Fig. 11 illustrates a burn pot 110 and shows a secondary combustion air inlet 1104 which is in fluid communication with the secondary air plenum. The secondary air conduit can be coupled to the secondary air inlet 1004 so that secondary air can be delivered to the secondary air plenum. In some cases, the bum pot 110 is configured as a replacement burn pot for a commercially available stove. In some cases, a burn pot may be sold separately as a retrofit item to improve the efficiency and emissions of a commercially available pellet stove. The burn pot can be swapped into a stove, and the primary and secondary combustion air systems can be connected to the burn pot to realize the advantages described herein. In some cases, the burn pot 110, combustion air systems and other components described herein can be integrated into new stove designs and included at the time of manufacture.

[0120] Fig. 12 illustrates several examples of secondary air plenums 201. While the depicted plenums are constructed from cylindrical pipes, tubes of different cross-sectional shapes are possible, such as a circle, oval, ellipse, square, rectangle, hexagon, octagon or other polygon. Tubes with a circular cross section are preferred to facilitate an array of nozzles that point radially outward. In some cases, the secondary air plenum 201 includes afirst threaded end 1202 that is configured to be threaded into a coupling carried by the secondary air delivery system or connected in other ways such as a single continuous piece or welded joint. In some cases, the secondary air plenum 201 includes a positive stop 1204 that may be used to set the height of the ring of holes 1002 above the top plate of the primary air plenum. The ring of holes 1002 may be any suitable size, shape, location, and number of holes; however, several preferred sizes and number of holes are described herein and provide exceptional results when compared to a baseline stove. In some cases, the size and number of holes results in a flow area through which the secondary air is introduced into the burn pot. The size and number of the holes 1002 can be selected to allow a predetermined mass flow rate given a certain fan speed and to also allow a predetermined velocity of secondary combustion air at a predetermined Reynolds number to flow through the holes to provide mixing of combustion air, which has been shown to improve combustion.

[0121] In some cases, the holes are configured to be large enough or have a large enough quantity of smaller holes to deliver the mass air flow required at a reasonable pressure for blower fans without making too much noise. In some cases, the holes are smaller to provide sufficient momentum and kinetic energy to effectuate turbulent mixing of the secondary combustion air with the flames above the fuel bed. In some cases, the turbulent air jets expand at a 24-degree angle from the nozzle, therefore 15 holes around the periphery of the secondary air plenum will provide complete coverage around the bum pot. Of course, other numbers of holes can be provided, and the holes may be provided in multiple positions along the length of the secondary air plenum, such as by providing multiple annular rings of holes.

[0122] Fig. 13 illustrates a graph of experimental results showing total particulate matter emissions as a function of primary air flow rate as a percentage of stoichiometric air, in accordance with some embodiments, and is discussed in detail below.

[0123] Fig. 14 illustrates a graph of experimental results showing total particulate matter emissions as a function of primary air flow rate as a percentage of stoichiometric air, in accordance with some embodiments, and is discussed in detail below.

[0124] Fig. 15 illustrates a graph of experimental results showing the variation of black carbon emissions by varying a ratio of secondary air kinetic energy to fuel energy, in accordance with some embodiments, and is discussed in detail below.

[0125] Fig. 16 is a graph of various stoves, including a baseline stove and a concept stove according to embodiments herein, showing efficiency versus particulate emissions rate, in accordance with some embodiments, and is discussed in detail below.

[0126] With reference next to Fig. 17, another embodiment of a combustion system 1700 is illustrated schematically in cross section. In this embodiment, the closed end or bottom surface 1702 of the burn pot is configured to support the fuel bed 107, and can comprise a solid wall with no holes or air-delivery openings. A primary air distribution plenum 1704 can be arranged circumferentially about the fuel bed, and can make up the outer wall of the bum pot adjacent the fuel bed. A plurality of spaced-apart primary air holes are formed through the outer wall of the burn pot at and adjacent the fuel bed. The primary air distribution plenum 1704 is configured to receive primary combustion air 108 from a primary air conduit 109 and to deliver such primary air circumferentially around the burn pot so that primary air, pressurized by a primary fan, is delivered to the fuel bed through the primary air holes. The primary air holes can be distributed evenly or randomly about the primary air distribution plenum so as to deliver the primary air directly to the fuel bed 107 from the side. In the illustrated embodiment, the primary air holes are arranged to be in a lower half of the fuel bed. As shown, the primary air 108 is injected under pressure so that the primary combustion air flows in a direction radially inwardly, and generally horizontally, through the fuel bed.

[0127] Continuing with reference to Fig. 17, a secondary air conduit 202 extends upwardly through the bottom wall 1702 of the burn pot to deliver pressurized secondary combustion air 203 via a secondary air plenum 201 vertically above the fuel bed. In the illustrated embodiment, as in some embodiments discussed above, the secondary air conduit 202 terminates in a secondary air plenum 201, and the secondary combustion air 203 is injected via several secondary air holes disposed about the circumference of the secondary air plenum so have a radially-outwardly directed flow direction. As in other embodiments, the relative flow volumes of primary and secondary air can be adjusted as desired. Also, as inother embodiments, an exhaust fan (not shown) draws combustion products 1706, as well as tertiary air 1708 within the firebox, to an exhaust. In some embodiments a fuel feed conduit 601 is above the bum pot 110 in a top feed arrangement to deliver granular fuel into the burn pot 110 and onto the fuel bed 107.

[0128] Fig. 18 is a perspective view showing an embodiment of a primary air plenum 1800 constructed in accordance with testing of an embodiment employing principles as in the schematic diagram of Fig. 17. As shown, the primary air plenum 1800 includes a primary air intake 1802. The primary air plenum includes an air space 1704 that extends about its entire circumference and, as shown, includes primary air holes 1804 formed through its inner surface 1806. In the illustrated embodiment the primary air plenum has an inner diameter of 4.5 inches - matching the inner diameter of the test burn pot - and the primary air holes comprise 18 evenly-spaced 9 / 64 inch holes.

[0129] Fig. 19 is a perspective view showing an embodiment of a bum pot in which the primary air plenum of Fig. 18 has been installed so that the inner wall of the primary air plenum 1806 makes up the outer wall of the bum pot adjacent the fuel bed. This illustration concerns the testing apparatus, which is performed using the same Breckwell SP1000 stove used in tests described below. The primary air intake 1802 of the primary air plenum is attached to a primary air conduit 109 configured to supply primary combustion air motivated by a primary air fan as in embodiments discussed herein. As depicted in Fig. 19, the illustrated embodiment is a burn pot 110 having a diameter of 4.5 inches. The closed end 1702 of the burn pot is a solid wall, and the plenum inner wall 1806 extends about 2 inches upwardly from the bottom wall 1702. The primary air holes 1804 are centered one inch from both the bottom wall and one inch from the top of the primary air plenum. The upper wall of the bum pot 112 extends 3.5 inches upwardly from the top of the plenum, and an upper shield 1902 extends another 1.5 inches above the upper wall, overlapping the fuel feed conduit 601 and maintains approximately the same diameter of 4.5 inches as the inner wall of the bum pot 110. In this example test stove configuration, the secondary air plenum 201 extends upwardly from the bottom wall 1702 along an axis of the bum pot 110. The secondary combustion air plenum 201 in the depicted embodiment is formed by a pipe that is34 NPT trade size, having an outer diameter of 1.05 inches. The particular secondary combustion air plenum 201 comprises 18 evenly-spaced 9 / 64 inch diameter holes 1904 distributed about its circumference. The secondary air holes 1904 are 4 inches above the bottom wall 1702. For purposes of testing, a thermocouple 1906 was arranged so as to extend above the burn pot at the top edge - even with the top edge of the upper shield 1902.

[0130] With reference next to Fig. 20, another embodiment of a combustion system 2000 is shown comprising a primary air plenum 1704 disposed about the outer wall of the fuel bed and configured to inject primary combustion air 108 into the fuel bed 107 in a radially- inward flow direction. In the illustrated embodiment, the secondary air supply conduit 202 - comprising a tube - extends in a direction transverse to the burn pot axis through the upper wall of the burn pot 112 and above the fuel bed 107. The secondary air supply conduit communicates with a secondary air plenum 201 disposed along the axis of the bum pot above the fuel bed 107. The secondary air plenum having a plurality of spaced-apart holes about its circumference and configured to inject secondary combustion air 203 in a radially- outwardly-directed flow direction spaced above the fuel bed 107. In this manner the secondary air supply does not interfere with the fuel bed.

[0131] Continuing with reference to Fig. 20, an ash removal system 2002 can be arranged directly below the fuel bed 107. It is to be understood that several embodiments of specific ash removal systems can be employed, including ash removal systems that enable removal of ash during continued operation of the stove, ash removal systems for use only when the stove is inoperable, and ash removal systems that simultaneously tend to terminate bum operations of the stove. The ash removal system 2002 supports the fuel bed and encloses the bottom end of the bum pot 110.

[0132] With reference next to Figs. 21 A-F, one embodiment of an ash removal system 2002 is depicted schematically. In this embodiment, a hollow rectangular housing 2102 (see Fig. 21B) supports a slider 2104 that comprises a hole 2105 (see Fig. 21A) that is typically arranged under the primary air plenum 1704 and fuel bed 107 during regular operation (see Fig. 2 IB). The housing can include a similar hole 2106. In some variations, the bottom surface 2108 of the housing 2102 can support the fuel bed. During operation, ash 21 10 willcollect within the hole, as shown in Fig. 21B. To dispose of the ash 2110, the slider 2104 can be slid outwardly as shown in Fig. 21C. During this operation, the upper surface of the slider adjacent the hole 2106 blocks the remaining fuel bed 107 from falling downwardly (see Fig. 21 C). As shown in Fig. 2 ID, ash 2110 that has been collected in the hole 2105 will simply fall out due to gravity as the slider 2104 is slid out of its housing 2102. The slider 2104 can then be slid back into place (see Fig. 2 IE) so that the hole 2105 in the slider 2104 is again aligned with the hole 2106 in the housing 2102 directly below the fuel bed 107. In some variations, and as shown in Fig. 2 IF, when the hole is again aligned with the bum pot, ash and portions of the fuel bed 107 can be expected to fall into the hole in the slider.

[0133] Figs. 22A and 22B present another embodiment of an ash removal system 2002 that can be fit below the fuel bed 107 when the primary combustion air 108 is delivered from the side of the fuel bed 107 rather than from below the fuel bed. In the embodiments illustrated in Figures 22A and 22B, one or two (or more) pairs of rotating rollers 2202 are configured to roll with each other. As the pairs roll, protrusions on the rollers crush cinders, clinkers or other tramp material that may be within the ash 2110 so that ash falls with gravity through the rollers and to an ash collection volume 2204 below the rollers 2202. Such rotating rolls can borrow structure and operation ability as discussed in US Patent No. 5027719, the entirety of which is hereby incorporated by reference.

[0134] In yet another embodiment, and with reference next to Fig. 23, an ash removal system 2002 can comprise one, two or more screw conveyors 2302 disposed beneath the fuel bed 107 - or can be disposed beneath an ash treatment system such as the rollers in the embodiment just discussed. The illustrated screw conveyor 2302 periodically, or continuously, removes ash from below the fuel bed.

[0135] Although a few specific ash removal structures have been disclosed, it is to be understood that a broad range of structures and methods of ash removal are contemplated. For example, in another embodiment the ash removal system can comprise a simple sliding drawer that can be removed, emptied, and returned by the user during periods of nonuse.

[0136] With reference next to Fig. 24, another embodiment of a combustion system 2400 includes a primary air distribution plenum 1704 disposed circumferentially about the fuel bed107 of the burn pot. In this variation multiple rows of primary holes are provided so that the primary plenum 1704 supplies primary combustion air 108 to the fuel bed at multiple elevations 108A and 108B. In some variations a row of spaced-apart primary holes is provided in a bottom half of the fuel bed 107 and a second row of spaced-apart primary holes is provided in a top half of the fuel bed. In another variation more than two rows of primary holes are provided. In still another variation a randomized distribution of primary holes is provided in the fuel bed. Combinations of such variations are also contemplated. Most preferably, each of the primary air holes is configured to direct a flow of primary air 108 directed radially inwardly and generally horizontally. In further variations such flows of primary air 108 can be generally radially-inwardly directed, but can also be disposed at various non-horizontal dispositions and / or combinations of horizontal and non-horizontal flow directions.

[0137] With reference to Fig. 25, in another embodiment of a combustion system 2500, the upper section of the burn pot outer wall 112 can be constructed of a temperature-resistant material such as a ceramic material rather than typical burn pot materials such as steel or stainless steel. In such an embodiment, when operated at a low primary air 108 to secondary air 203 ratio such as 1 :4 with the primary airflow 108 approximately equal to 20% of the stoichiometric air required by the fuel flow from the fuel feed conduit 601 into the bum pot 110 and onto the fuel bed 107 as in some of the experimental examples described below, the primary air plenum 1704 remains at a lower temperature and amenable for construction of materials such as steel and stainless steel, which have lower temperature resistance than that which is used for the upper section above the fuel bed.

[0138] With reference next to Fig. 26, yet another embodiment of a combustion system 2600 provides a secondary combustion air delivery system 2602 with an outer secondary plenum and an inner secondary plenum 201. As shown, the outer secondary plenum 2604 is positioned and configured to extend about the sidewall of the burn pot 112 above the fuel bed 107 and, similar to the circumferential primary air plenum 1704 discussed in embodiments above, comprises a plurality of spaced-apart outer secondary air holes configured to direct a flow of secondary air 203 radially inwardly 203 A from the bum pot wall 1 12. However, theillustrated secondary combustion air delivery system additionally comprises a secondary air conduit 2606 extending from a side wall of the burn pot above the fuel bed, preferably in communication with the outer secondary air plenum 2604 at a point vertically spaced from the outer secondary air holes so as to not interfere with such outer secondary air injections 203A. The secondary air conduit 2606 communicates with and delivers secondary air 203 to the inner secondary air plenum 201 positioned axially within the burn pot and having inner secondary air holes about its circumference so as to inject flows of secondary air radially outwardly 203B.

[0139] In the illustrated embodiment the outer secondary air holes and inner secondary air holes are disposed at the same vertical height above the fuel bed and direct air flows 203 A and 203B generally horizontally. In variations, the inner secondary air holes can be vertically spaced from the outer secondary air holes, and one or more or several of such air holes can be configured to inject air in a non-horizontal or non-radial flow path. In still further variations, the inner secondary air holes can be spaced to direct air flows radially outwardly but directed generally between the radially-inwardly-directed air flows of the outward secondary air holes. In yet other variations the outer and inner secondary air holes can be positioned and configured to direct flows of secondary air directly at one another so as to encourage turbulent mixing.

[0140] With reference next to Fig. 27, another embodiment of a combustion system 2700 resembles the embodiment of Fig. 26, except that the outer 2604 and inner 201 secondary air plenums are both extended to each include two spaced-apart rows - or stages - of secondary air holes. It is to be understood that further embodiments can contemplate multiple (2, 3, or more) such stages of secondary air holes, and that such stages can be configured in various ways, such as multiple stages of aligned inner and outer secondary air holes resulting in both radially-outward 203B and radially-inward-directed 203 A air injections. Other stages can be staggered by direction, such as a stage of radially-inward-directed air injections followed by a stage of radially -outward-directed air injection, and the like. A vertical distance 7 / 2702 between stages can vary from embodiment to embodiment. In some variations, the vertical distance between stages is a function of the radius of the bum pot sidewall 112, and thevertical distance between adjacent stages is at least equal to the radius of the burn pot sidewall 112.

[0141] With reference next to Fig. 28, another embodiment of a combustion system 2800 is shown employing a primary air plenum 1704 that injects primary combustion air 108 radially inwardly 108B from primary air holes disposed about the circumference of the burn pot and having a secondary air conduit 202 disposed vertically above the fuel bed 107. In the illustrated embodiment, the fuel bed 107 is comparatively thick, for example having a thickness greater than the radius of the burn pot in the fuel bed. In one variation the thickness of the fuel bed is greater than the radius but less than the diameter. In another variation the thickness of the fuel bed is between about 1 .5-4 times the burn pot radius at the fuel bed. The fuel bed thickness may be maintained by adjusting the flow of granular fuel onto the fuel bed 107 and the flow of primary air 108. The primary air plenum 1704 is also comparatively tall and approximately matches the thickness of the fuel bed. The primary air conduit 109 is positioned in the upper portion of the primary air plenum 1704 and the holes that inject primary air radially inward 108B into the fuel bed 107 are located in the bottom portion of the primary air plenum such that the flow of air in the annulus of the primary air plenum is downward 108C. This downward flow of air from the top of the fuel bed 107 towards the bottom acts to cool the top of the fuel bed. The comparatively thick fuel bed surrounded and cooled by the primary air plenum may reduce the temperature at the top of the fuel bed, leading to slower devolatilization and combustion of fresh fuel particles that arrive on the fuel bed 107 from the fuel feed conduit 601. This slower devolatilization may contribute to a more constant fuel burning rate, especially at low fuel feed rates when a screw feeder may release granular fuel intermittently. This more constant burning rate can maintain the instantaneous air-fuel ratio closer to the desired average air-fuel ratio and improve combustion. Lower fuel bed outlet temperatures may also reduce particulate emissions. In this embodiment, the fuel feed conduit 601 is connected to the bum pot sidewall 112 to deliver granular fuel to the top of the fuel bed through the burn pot sidewall 112 below the secondary air conduit 202 that delivers air to the secondary air plenum. In the illustrated embodiment the fuel feed conduit 601 is angled downward where granular fuel falls onto thefuel bed with gravity. Other possible embodiments are contemplated such as where the fuel feed conduit 601 is horizontal or angled upward and granular fuel is pushed by a feed apparatus such as a screw feeder. An ash removal system 2002 of any suitable form that supports the fuel bed 107 and allows for ash removal during use or during periods of non-use is contemplated.

[0142] Fig. 29 shows yet another embodiment of a combustion system 2900 in which a secondary air conduit 202 enters the bum pot 110 from above the burn pot outlet and descends axially within the bum pot to the secondary air plenum 201 that is spaced above the fuel bed 107. As above, the secondary air plenum 201 comprises a plurality of spaced-apart secondary air holes configured to direct flows of secondary air radially outwardly 203. In some embodiments a portion of the secondary conduit 202 can be placed into contact with the bum pot wall 112 so as to impart combustion heat to the secondary air 203.

[0143] Fig. 30 schematically presents yet another embodiment of a combustion system 3000 in which the burn pot at and adjacent the fuel bed 107 has a first diameter 3002 and the secondary bum zone above the fuel bed 107 at and adjacent the secondary air plenum 201 has a second diameter 3004 that is less than the first diameter. In this configuration, the larger first diameter 3002 at the fuel bed 107 can be configured to spread out combustion and lower temperatures in the fuel bed to reduce particulate matter emissions. The lesser second diameter in the secondary burn zone 3004 reduces the cross-sectional area, concentrating gasses so as to create a more intense flame zone at higher temperature in the secondary combustion zone, reducing soot, particularly at low fuel feed rates when there typically are less combustible vapors available in the secondary combustion zone. The secondary air conduit 202 may pass through the sidewall of reduced cross section 3004 as illustrated, or may also pass through the sidewall of larger cross section 3002 above the fuel bed. The transition from the fuel bed diameter 3002 to the secondary combustion diameter 3004 may be abrupt as depicted or a tapered transition.

[0144] Through testing, as described in more detail below in the examples, Applicant has learned that black carbon (soot) emissions tend to increase substantially at low fuel feed rates. However, if the Watts firepower per cross sectional area of the bum pot (i.e., W / cm2)is greater than about 50 W / cm2, soot emissions are observed to be reduced substantially. Thus, the embodiment illustrated in Fig. 28 provides a relatively low-diameter combustion area with a higher firepower per cross-sectional area so as to maintain a higher W / cm2, while the thick fuel bed simultaneously lowers fuel bed temperatures, thus reducing particulate matter emissions. The wider fuel bed of Fig. 30 also can lower such fuel bed temperatures for lower particulate matter emissions, while the reduced-diameter secondary combustion chamber can increase W / cm2 to reduce black carbon emissions. Another reason for having a fuel bed that is relatively wide as compared to a secondary combustion area is to accommodate low bulk-density fuels such as corn cobs or non-densified biomass. The large fuel bed accommodates such low bulk-density fuels, while the reduced-diameter secondary combustion zone concentrates combustible gasses for more effective secondary combustion than where such gasses are thinly distributed over a wider cross-sectional area.

[0145] Figs. 31 A and 3 IB present embodiments 3100 in which the burn pot diameter grows significantly at and adjacent the secondary combustion zone 3102. As shown, the burn pot diameter becomes larger at the secondary combustion zone 3102 at a location where secondary combustible vapors reach the injected secondary combustion air 203. Notably, up to this point the diameter of the bum pot may remain the same so as to keep combustible vapors together and accessible to the injected secondary combustion air 203. The wider outlet portion of the secondary combustion zone 3102 spreads secondary flames to a wider cross section thus lowering the velocity of the combustion products 1706 to increase the residence time at high temperature within the secondary combustion zone of the burn pot to assist complete combustion and reduce emissions. The transition from the fuel bed diameter 3002 to the diameter of the secondary combustion zone 3102 may be abrupt as depicted or a tapered transition.

[0146] Fig. 32 presents a still further embodiment 3200 in which a primary igniter 3202 is provided in the primary air plenum 1704 adjacent the side wall at the fuel bed 107. A second igniter 3204 can be placed at or adjacent the burn pot wall 112 in the secondary combustion zone above the secondary air plenum 201. Upon sensing a decrease in temperature within the burn pot, typically due to a loss of combustion (herein referred to as a flameout), thesecondary igniter can be signaled to activate so that the stove can recover from such a flameout and restore combustion in the secondary zone.

[0147] Fig. 33A and Fig. 33B illustrate graphs of experimental results showing the variation of black carbon and carbon monoxide emissions with varying secondary air jet Reynolds number, in accordance with some embodiments and is discussed in detail below.

[0148] The embodiments described herein contain different features and design elements that have been described and depicted in certain combinations. These features and design elements may be used in any combination thereof and is not limited to the specifically described combinations.

[0149] Generally, the efficiency of a pellet stove is measured using a parameter known as the "thermal efficiency." This metric quantifies how effectively the stove converts the energy content of the pellets into usable heat for heating a space or cooking. The efficiency is expressed as a percentage and is calculated by comparing the heat output to the energy input using the formula: Efficiency (%) = (Useful Heat Output / Energy Input) x 100.

[0150] The Useful Heat Output represents the actual heat generated by the stove and released into the room for heating or cooking purposes. It can be measured in energy units, such as British Thermal Units (BTUs), kilowatt-hours (kWh), or kilojoules (kJ).

[0151] The Energy Input refers to the total energy content of the fuel consumed by the stove during its operation. It's usually measured in BTUs, kilowatt-hours (kWh), or kilojoules.

[0152] The efficiency is based on both the Useful Heat Output and Energy Input. While the efficiency of a stove can vary based on factors like the stove's design, combustion technology, maintenance, and the quality of the pellets used, it is a general metric used to evaluate stoves, along with the emissions of the stove.

[0153] Based on example embodiments as described herein, a pellet stove can be modified, created, and / or operated in a way to significantly reduce the emissions and increase the efficiency of the stove. These improvements are made available by certain features disclosed herein, which include one or more of the following: (i) primary combustion air distributed throughout the fuel bed having a desirable momentum necessary to providesufficient primary combustion air, but not so high as to loft char out of the burn pot; (ii) secondary combustion air provided by a plenum spaced above the fuel bed in an inside-out direction from the central axis of the burn pot toward the burn pot sidewall, the secondary combustion air being introduced to mix the combustion vapors emanating from the fuel bed; (iii) a ratio of secondary combustion air to primary combustion air within the range of 2: 1 to 5: 1, and in some cases, is 4:1; (iv) providing separately controllable primary air source and secondary air source, the air sources providing positive pressure of the primary and secondary combustion air.

[0154] EXAMPLES

[0155] The following examples are intended to illustrate, but not limit, embodiments described herein. For all the runs summarized below (Example I and Example 2), the pellet stove was fueled with either 100% Douglas Fir softwood pellets (moisture, 2.0-3.0%; ash, 0.2-0.4%; Pellet Fuels Institute certified "Premium Grade Pellet Fuel"; Golden Fire® brand) or 100% hardwood pellets compliant with Pellet Fuels Institute certified “Premium Grade Pellet Fuel” sold under the product labeling “Lignetics® Premium Quality Wood Pellet Fuel Made from the Finest Hardwood Sawdust” by Lignetics of West Virginia, Inc. Glenville, WV. Hardwood is known to have higher ash content and different combustion properties from Douglas Fir softwood. Both fuels were assumed to have 3% wet basis moisture content. The hardwood pellet fuel used in run 27 was from a different lot meeting the Pellet Fuels Institute standard for premium grade pellet fuel with a measured wet basis moisture content of 3.3%. The Golden Fire brand Douglas Fir pellet fuel used in run 28 was from a different lot with a measured wet basis moisture content of 5.9%.

[0156] Example 1 : Baseline Emissions Performance

[0157] Baseline emissions performance indicates an example of emissions performance of a representative commercial pellet stove, was determined for a Breckwell SP1000 pellet stove, available from Breckwell Hearth of South Pittsburg, TN, and operated without modification according to the owner's manual. Air supply was controlled solely by setting the damper on the air inlet to the bum pot: A) damper= full open, for power level setting 5, high"; B) damper= 2 cm open, for power level setting 3, "medium"; and C) damper= closedposition, for power level setting 1, "low". When the damper is in the closed position the air supply is sufficient to support "low" power output because the "closed position" engages a restrictor plate that does not completely close the air inlet to the burn pot. The commercial Breckwell SP1000 is equipped with a combustion blower that draws air into the firebox. Under standard operating conditions, the combustion blower, which is configured as an exhaust extractor fan, withdraws exhaust from the firebox, and operates at constant speed at all power level settings. The firebox operates at a net negative pressure relative to the ambient environment. Because the firebox operates at a net negative pressure relative to the ambient environment, at all power settings, air flow into and through the firebox is induced through: a) the air inlet to the bum pot (which is controlled by the damper), b) a separate conduit to the igniter and through the igniter into the burn pot, c) the window air wash, d) the fuel feed apparatus (including the hopper, the pellet feed auger, and the feed chute), and e) any incidental leaks that may be present.

[0158] For operation at each of three power level settings, 5, 3, and 1 (corresponding to "high", "medium", and "low" power), values of the following emission rates (mass pollutant / time) of different pollutants in the stove exhaust stream were determined: carbon monoxide (CO, reported in gram / minute [g / min]); particles with a diameter less than 2.5 microns known as fine particulate matter orPNU s (or "PM2.5", reported in gram / hour [g / hr]); dark carbon particles from incomplete combustion known as black carbon (BC, reported in g / hr); carbon dioxide (CO2 or "CO2", reported in g / min); and the fuel feed rate (reported in kilograms / hr, [kg / hr], dry basis). CO, PM2.5, BC, and CO2 were measured as is known in the art. CO and CO2 were measured continuously (sampling rate = 1Hz, at one-second intervals) and PM2.5 and BC were measured using a gravimetric filter for each run at a given configuration. CO, PM2.5, BC, and CO2 were measured with the aid of a 4000 Series Laboratory Emissions Monitoring System (LEMS), commercially available from Aprovecho Research Center of Cottage Grove, OR, and operated according to the manual "Instructions for Use of the Laboratory Emissions Monitoring System (LEMS)", Aprovecho Research Center, updated November 2018, for ISO 19867-1. In addition, the BC measurement use the Nexleaf Analytics photographic analysis method of filter color as described in the journalarticle Ramanathan, N., et al. "A cellphone based system for large-scale monitoring of black carbon." Atmospheric environment 45.26 (2011): 4481-4487. This method has been found to agree with reference analytical techniques of BC to within 20%. Therefore, the BC measurements are semi-quantitative, sufficient to indicate trends. Test runs 1-26 used the Nexleaf website to analyze the BC content of the filters. Test runs 27 and 28 used the same technique implemented in the Python programming language. The Python implementation was found to report BC that was on average 24% lower than the website. The estimate of BC for runs 27 and 28 was adjusted to be comparable to the other test runs.

[0159] The fuel feed rate, for a given stove operating condition for which CO, PM2.5, BC, and CO2 values are reported, was calculated based on the amount of CO and CO2 emitted during the period in which the given stove operating condition was in effect (integration of the CO and CO2 values recorded over the relevant period), and converted to the corresponding fuel feed rate, assuming that the Douglas Fir and hardwood fuel was 50% carbon, by weight. The total fuel fed during an experimental session (consisting of start-up, one or more combustion runs at one or more power settings, and shut down), calculated as described immediately above, agreed +2.5% / -l 5% (the observed range over all runs reported here, Table 1 and Table 3) with the measured total mass of fuel pellets consumed during an experimental session (and assuming the fuel pellets contained 3% moisture).

[0160] The values of the emission rates for CO, PM2.5, BC, and CO2 described above, measured for a particular combustion device (e.g., a Breckwell SP1000 pellet stove) operating under specified conditions (such as selected for the baseline emissions performance determination described here) can be used to characterize the emissions performance of the particular device operating under the specified conditions and to assess the emissions performance against certain regulatory requirements in certain jurisdictions. In certain other jurisdictions and / or against certain other regulatory requirements, a corresponding emission factor is more preferably used to characterize emissions performance. The emission factor is the mass of CO, PM2.5, BC, or CO2 emitted per unit mass of fuel consumed. The emission factors for CO, PM2.5, BC and CO2 are inherent in the emission rate data, when the fuel feed rate is also known. For convenience the emissions factors are tabulated and reportedseparately below. Since the fuel feed rate can vary even at the same nominal fuel feed setting the emissions factors are a better comparison of relative performance between configurations and used in subsequent discussion below. The units of the emission factors are grams CO per kilogram fuel [g / kg] for CO, grams PM2.5 per kilogram of fuel [g / kg] for PM2.5, milligrams of BC per kilogram of fuel [mg / kg] for BC, and grams CO2 per kilogram fuel [g / kg] for CO2.

[0161] Baseline performance was determined in one experimental session consisting of starting the stove, completing the pre-programed start-up routine, warming up the stove by operating at power level setting 5 for one hour, running at power level setting 5 while sampling for PM2.5 and BC on a gravimetric filter (Run Al, Table 1, below), turning down to and running at power level setting 3 while sampling for PM2.5 and BC on a separate gravimetric filter (Run A2, Table 1, below), turning down to and running at power level setting 1 while sampling for PM2.5 and BC on a separate gravimetric filter (Run A3, Table 1, below), and shutting down the stove. CO and CO2 were monitored continuously throughout the experimental session. The 100% Douglas Fir softwood fuel was used for the baseline testing.

[0162] Transition from steady state operation at one power level setting to another was fast relative to the duration of a run at any given power level setting (about 0.5 min v.s. 60 min or more), and accomplished by setting the damper on the air inlet, as described above, and changing the power level setting on the stove's control, which has the sole effect of changing the fuel feed rate by changing the duty cycle of the pellet feed auger. The baseline performance results, in units described above, are reported in Table 1 and Table 2, immediately below.

[0163] Table 1: Baseline Emission Results

[0164] Table 2: Baseline Emissions Factor Results

[0165] Example 2: Emissions Performance of Examples of Certain Embodiments

[0166] Stove Modification and Setups

[0167] The Breckwell pellet stove used to determine baseline emissions performance, as described above, was modified in order to test and characterize emissions performance of specific examples of certain embodiments described throughout the specification. The stock stove's air inlets to the bum pot (damped air inlet and air conduit to the igniter) were closed completely. The stock stove's burn pot was replaced by different examples of a combustion apparatus comprising a burn pot and fuel bed air supply system as described herein and shown in the various figures. In some examples, the combustion apparatus further comprised an upper air supply system (e.g., secondary combustion air). The fuel bed air supply system, and upper air supply system (when present), comprised independently controllable fan(s) which delivered independently controllable air flows to the bum pot via a plenum deployed so that air delivered to the plenum by the fan(s) was injected into the burn pot via nozzles in the plenum, as described herein and shown in the figures. In the examples described here, the nozzles were straight-through holes of circular cross section.

[0168] In the fuel bed air supply system, the plenum was deployed below the fuel bed, substantially parallel to the average plane of the fuel bed, and the nozzles injected air from below the fuel bed and substantially perpendicular to the fuel bed. For clarification, the fuel bed includes fuel spread in a generally horizontal plane and the nozzles injected air from below the fuel bed in a vertically upward direction through the fuel bed. In the upper air supply system, the plenum was deployed above the fuel bed and the nozzles injected air substantially parallel to the average plane of the fuel bed (e g., horizontally, which is orthogonal to the primary combustion air flow). The five specific examples of a fuel bed airsupply system and an upper air supply system (when present) tested in Example 2 are further described in Figs. 1 A - 2F and referred to as air supply configurations 1 - 5 in Table 2 (AirConfigl - AirConfig5). In AirConfigl, the combustion system comprised a fuel bed air supply system and the plenum had 37 holes, arranged in a regular array and 5 / 64 inch in diameter. In AirConfig2, the combustion system comprised a fuel bed air supply system and the plenum had 16 holes, approximately evenly spaced, approximately concentric with the perimeter of the substantially circular bum pot, and 5 / 64 inch (2mm) in diameter. In AirConfig3, the combustion system comprised a fuel bed air supply system and the plenum had 36 holes, arranged in a regular array and 5 / 64 inch (2mm) in diameter, and an upper air supply system and the plenum had eight holes approximately evenly spaced in a circular array and 5 / 64 inch (2mm) in diameter. In AirConfig4, the combustion system comprised a fuel bed air supply system and the plenum had 36 holes, arranged in a regular array and 5 / 64 inch (2mm) in diameter, and an upper air supply system and the plenum had eight holes approximately evenly spaced in a circular array and 3 / 16 inch (4.75mm) in diameter. In AirConfig5, the combustion system comprised a fuel bed air supply system and the plenum had 36 holes, arranged in a regular array and 5 / 64 inch (2mm) in diameter, and an upper air supply system and the plenum had 12 holes approximately evenly spaced in a circular array and 5 / 64 inch (2mm) in diameter.

[0169] Additional specific examples are described by Figs. 9-12 and referred to as air supply configurations 6 - 9 and 11 - 13. In AirConfig6, the combustion system comprised a fuel bed air supply system and the plenum had 36 holes, arranged in a regular array and 5 / 64 inch (2mm) in diameter, and an upper air supply system and the plenum had 24 holes approximately evenly spaced in a circular array and 5 / 64 inch (2mm) in diameter. In AirConfig7, the combustion system comprised a fuel bed air supply system and the plenum had 36 holes, arranged in a regular array and 5 / 64 inch (2mm) in diameter, and an upper air supply system and the plenum had 24 holes approximately evenly spaced in a circular array and 5 / 64 (2mm) inch in diameter. In AirConfig8, the combustion system comprised a fuel bed air supply system and the plenum had 88 holes, arranged in a regular array and 5 / 64 inch (2mm) in diameter, and an upper air supply system and the plenum had 18 holesapproximately evenly spaced in a circular array and 9 / 64 inch (3.6mm) in diameter. In AirConfig9, the combustion system comprised a fuel bed air supply system and the plenum had 88 holes, arranged in a regular array and 5 / 32 inch (4mm) in diameter, and an upper air supply system and the plenum had 18 holes approximately evenly spaced in a circular array and 9 / 64 inch (4mm) in diameter. In AirConfigl 1, the combustion system comprised a fuel bed air supply system and the plenum had 88 holes, arranged in a regular array and 5 / 64 (2mm) inch in diameter, and an upper air supply system and the plenum had 12 holes approximately evenly spaced in a circular array and 7 / 32 inch (5.5mm) in diameter. In AirConfigl2, the combustion system comprised a fuel bed air supply system and the plenum had 88 holes, arranged in a regular array and 5 / 64 (2mm) inch in diameter, and an upper air supply system and the plenum had 6 holes approximately evenly spaced in a circular array and 27 / 64 inch (10.7mm) in diameter. In AirConfigl3, the combustion system comprised a fuel bed air supply system and the plenum had 88 holes, arranged in a regular array and 5 / 64 inch (2mm) in diameter, and an upper air supply system and the plenum had 24 holes approximately evenly spaced in a circular array and 5 / 64 inch (4mm) in diameter.

[0170] An additional specific example is a modified form of the burn pot described in Figs. 9-12 (not pictured) and referred to as air configuration 10. In AirConfiglO the secondary air plenum 201 was removed and the hole where it passes through 106 was blocked to make an approximately flat plate. The side wall of the burn pot 110 was modified to create a double-walled plenum where the secondary combustion air enters from the outside of the bum pot wall toward the center (outside-in airflow) through 18 holes approximately evenly spaced in a circular array and 9 / 64 inch (3.6mm) in diameter (Fig. 18). The fuel bed air supply system plenum had 88 holes, arranged in a regular array and 5 / 64 inch (2mm) in diameter. AirConfiglO is a modified form of AirConfig8 where the secondary combustion plenum of AirConfiglO provides secondary air from the outside in where AirConfig8 provides air from the inside-out, the hole number and diameters of the primary and secondary air plenums are otherwise the same.

[0171] An additional specific example is a modified form of the burn pot described in Figs. 9-12 and is pictured in Fig. 19 and denoted AirConfig 14. The fuel bed air supplysystem was replaced with the double walled plenum pictured in Fig. 18 where the primary combustion air enters from the outside of the bum pot wall toward the center (outside-in airflow) through 18 holes approximately evenly spaced in a circular array and 9 / 64 inch (3.6mm) in diameter. The upper air supply system and the plenum had 18 holes approximately evenly spaced in a circular array and 9 / 64 inch (3.6mm) in diameter.

[0172] For AirConfigl to AirConfig5 test runs 1-9, the internal diameter of the burn pot sidewall 110 was 3.875 inches (98mm) and the secondary air plenum used 14 NPT trade size pipe. For AirConfig5 test runs 10-13 the internal diameter of the bum pot sidewall 110 was 3. 75 inches (95mm) and the secondary air plenum used 14 NPT trade size pipe. For AirConfig6, 8, 9, 11 , 12, 13, and 14 the internal diameter of the bum pot sidewall 110 was 4.5 inches (114mm) and the secondary air plenum used % NPT trade size pipe. For AirConfig7 the internal diameter of the burn pot sidewall 110 was 3.75 inches (95mm) and the secondary air plenum used 14 NPT trade size pipe. For AirConfiglO the internal diameter of the burn pot sidewall 110 was 4.5 inches (114mm) and the secondary air plenum was not present.

[0173] The burn pots of the example combustion apparatuses further comprised one of three examples of bum pot outlets. The three specific embodiments of the burn pot outlets are as shown in Figs. 3 - 5 and described in the accompanying text and referred to as outlet configurations 1 - 3 in Table 2 (OutletConfigl-OutletConfig3), respectively. Briefly: OutletConfigl was an open bum pot (open top on a generally cylindrical burn pot, Fig. 3A and Fig 3B); OutletConfig2 included an approximately 2-inch (51mm) diameter restrictor plate, concentric with an approximately 4 inch (102mm) diameter generally cylindrical burn pot (Figs. 4A and 4B. OutletConfig3 has a 2 inch (51mm) diameter restrictor plate and collar descending into the burn pot, as further shown in Fig. 5 and described in the accompanying text.

[0174] Experimental Protocols and Operation

[0175] Generally, for any given experimental session described here (which included from one to five runs at from one to three power level settings): 1) the stove was started by charging the bum pot with 100 g of pellets, soaking the pellets in 12 g of denatured ethanol,and lighting the combination with a match or by turning on the igniter 906; 2) the stock stove's pre-programmed start-up routine was allowed to run; 3) the experimental protocol, including warm-up and / or equilibration and sampled runs at specified power level settings, was executed; and 4) the stove was shut down. CO and CO2 were monitored continuously, as described for the runs in Example 1.

[0176] The following experimental protocols (Step 3 in the paragraph immediately above) were used:

[0177] Runs 1-4: the stove was set to the chosen power level setting, warmed up at the setting for about 20 min to about 60 min, and a run of from about 2 hours to about 3 hours was conducted at the chosen power level setting with filter(s) installed to determine PM2.5 and BC, according to the methods described, and referenced, in Example 1, above.

[0178] Runs 5-7 and 9-11 : the stove was set to power level setting = 1, warmed up at 1 for about 20 min to about 60 min, a run of one hour at 1 was conducted with filter(s) installed to determine PM2.5 and BC at power level setting 1, the power level setting was increased to 3, the stove was equilibrated at 3 for 20 min, a run of one hour at 3 was conducted with new fdter(s) installed to determine PM2.5 and BC at power level setting 3, the power level setting was increased to 5, the stove was equilibrated at 5 for 20 min, and a run of one hour at 5 was conducted with new filter(s) installed to determine PM2.5 and BC at power level setting 5.

[0179] Run 8: the same experimental protocol as used for runs 5-7 and 9-11 was followed, except that when the power level setting was not changed (between runs 8b and 8c, and between runs 8d and 8e), the equilibration period of 20 min was omitted; the filter(s) were changed, and the new run at the same power level setting commenced.

[0180] Runs 12 and 13: the stove was set to power level setting = 5, warmed up at 5 for about one hour, a run of one hour at 5 was conducted with filter(s) installed to determine PM2.5 and BC at power level setting 5, the power level setting was decreased to 3, a run of two hours at 3 was conducted with new filter(s) installed to determine PM2.5 and BC at power level setting 3, the power level setting was decreased to 1, and a run of three hours at 1 was conducted with new filter(s) installed to determine PM2.5 and BC at power level setting 1.

[0181] Runs 14-17 and 20-22 were run at the corresponding settings using the same protocol as runs 12 and 13 above.

[0182] Runs 18, 19, and 23 were operated steady-state at the indicated settings for a period of 60 to 90 minutes with equilibration periods before data was recorded, similar to runs 5-7 as described above.

[0183] Runs 24, 25, and 26 were operated steady-state at the indicated settings for a period of 20 minutes on power level 5, 40 minutes on power level 3, and 60 minutes on power level 1 with equilibration periods before data was recorded, similar to runs 5-7 as described above.

[0184] Runs 27 and 27 were operated as described above for runs 12 and 13.

[0185] Where indicated power level 1&4 is a slightly increased fuel feed rate from power level 1 as described in the SP1000 owner’s manual.

[0186] The stock stove's combustion blower (exhaust extractor fan) control was modified to allow variable speed control.

[0187] For runs 1-11 (Tables 3, 4, and 5), the combustion blower was set to the speed which delivered a total firebox pressure of -0.35 ± 0.05 inches water column with AirConfig5 installed and not operating, and the stove shut down and at ambient temperature. This speed setting on the combustion blower was used during experimental sessions and the fuel bed air supply system and upper air supply system were set to deliver the respective air flows shown in Table 2 (Fuel Bed Flow and Upper Flow, respectively; reported in units of standard liters per minute [SLPM] which is a mass flow reported as a volume flow at standard conditions of 20 degrees Celsius and 101,325 Pascals). (Therefore, for purposes of reporting in Table 2, and since firebox pressure was not measured during stove operation, it is not shown for runs 1-11 [N / A].)

[0188] For runs 12 and 13 (Tables 3, 4, and 5), the combustion blower was set to the speed that gave the indicated firebox pressure (Table 3, inches water column) during stove operation and with the reported fuel bed air supply system air flow (Table 3, Fuel Bed Flow) and upper air supply system air flow (Table 3, Upper Flow).

[0189] Specifics of the pellet stove setups and operating parameters tested, as described above, (runs 1-28) are summarized in Table 3, immediately below. Power level setting has the same meaning in Table 3 as in Table 1, as described above in Example 1. The fuel type is indicated as “DF” for the 100% Douglas Fir softwood pellets or as “Hard” for the hardwood pellets.

[0190] Table 3: Setups and Operating Parameters

[0191] Measured Emission Results

[0192] The values of CO, PM2.5, BC, CO2, and fuel feed rate for runs 1-28 (as described in Table 3, above) were determined as described above for Example 1, reported using the same units as described above for Example 1, and are reported in Table 4 and Table 5, immediately below.

[0193] Table 4: Emission Rate Results of Examples of Certain Embodiments

[0194] Table 5: Emission Factor Results of Examples of Certain Embodiments

[0195] In Table 4 and Table 5 above during runs 24, 25, and 26 the PM2.5 emissions rates and factors are marked N / A and certain values of BC emissions rates and factors are marked N / A. In these cases, the test runs were shorter than other test runs at those power levels and the filter loading for the PM2.5 and BC measurements was too light to be compared with the other test runs so they are not included.

[0196] During runs 24, 25, and 26 an automotive exhaust oxygen sensor was in contact with the exhaust stream as it exited the stove to measure the volume or molar concentration of oxygen (02) in the exhaust. A Bosch LSU 4.9 sensor run with an Innovate Motorsports LC-2 controller was used to measure lambda, the ratio of the measured air-fuel ratio to the stoichiometric air-fuel ratio. The LSU 4.9 data sheet gives a formula relating lambda toexhaust oxygen concentration allowing the oxygen concentration to be calculated. Table 6 below gives the average, minimum, and maximum lambda and oxygen concentration values during the corresponding test runs.

[0197] Table 6: Exhaust Oxygen Results of Examples of Certain Embodiments

[0198] Derived Parameters

[0199] Other parameters can be estimated from the data presented in Table 3, 4, and 5 if certain reasonable assumptions are made. If the chemical composition of a fuel is known the stoichiometric amount of air can be calculated. The chemical composition of both the hardwood and Douglas Fir softwood fuel was assumed to have the following mass fractions of chemical constituents: 50.7% carbon, 6.1% hydrogen, 39.3% oxygen, 0.1% nitrogen, 0.8% ash, and 3% water. If air is simplified to be 20.95% oxygen and 79.05% nitrogen air has a molecular weight of 28.838 grams per mol [g / mol]. These assumed values give a stoichiometric air requirement of 6.23 grams air per gram of wood fuel for complete combustion. The primary, secondary, and total (sum of primary and secondary air) amount of air injected into the burn pot can be compared to the amount of stoichiometric air as apercentage through appropriate unit conversions. The ratio of secondary to primary air can also be calculated by simple division of the mass flow rates. Below in Table 7 for test runs 1- 28 as described in Table 3, the primary, secondary and total airflow rates are tabulated in SLPM mass flow and as a percentage of stoichiometric air and the ratio of secondary to primary air is tabulated.

[0200] Table 7: Burn Pot Airflow of Certain Embodiments

[0201] If a temperature of the injected primary and secondary air is assumed the ideal gas law can be used to calculate the density of the air as it enters the burn pot from the primary and secondary air plenums. The density can then be used with the total hole area of a plenum and the mass flow rate of air to calculate the velocity of the air jets as they leave the plenum. The viscosity of air at the assumed temperature can be found in reference texts. The jet Reynolds number can then be calculated as the air density multiplied by the jet velocity multiplied by the jet hole diameter divided by the air viscosity.

[0202] Reynolds number is a dimensionless parameter to characterize the flow of a fluid, such as air through a conduit, tube, pipe, hole, or nozzle as described in the disclosed embodiments. It plays a role in determining the type of flow regime and can be used to predict whether the flow is laminar or turbulent. In the case of turbulent flow, the Reynoldsnumber also characterizes the length scales and structure of the turbulent eddies. According to embodiments described herein, increasing the flow velocity within an air supply nozzle will increase the Reynolds number, and higher velocities result in greater turbulent mixing, momentum, and kinetic energy. The air supply nozzle diameter is directly related to the Reynolds number, with a larger nozzle diameter resulting in a higher Reynolds number for the same velocity. However, the velocity through an array of nozzles depends on the total open area of the nozzles, depending on both the number of nozzles and their diameter. By adjusting nozzle number and diameter the velocity and Reynolds number of the air jets can be set for a given air flow rate. Finally, changes in air density and dynamic viscosity will also affect the Reynolds number. In some cases, the primary and / or secondary air supply is modified so that the combustion air provided to the burn pot is within a determined range of Reynolds number. For example, in some cases, the primary air may be caused to flow in order to have a Reynolds number of between 20 and 2,500, or between 100 and 2000, or between 400 and 1500, or between 800 and 1200. In some cases, the secondary air may be caused to flow such that it has a Reynolds number between 200 and 3000, or between 400 and 2500, or between 800 and 2000, or between 1000 and 1600. In some cases, the ratio of Reynolds numbers between the secondary air and primary air is controlled such that the ratio is between 1 :1 : and 6:1. The temperature of the inj ected air is assumed to be 500 Celsius for all test runs due to heating from the burn pot as the air travels through the primary and secondary air plenums which gives an air density of 0.455 kilograms per cubic meter [kg / m3]. The total kinetic energy flow of the primary and secondary air is given by one half the mass flow of air multiplied by the velocity squared after appropriate unit conversion with units of joules per second [J / s] or watts [W], The thermal energy flow of the fuel can be calculated as the fuel mass flow rate multiplied by the higher heating value of the fuel (assumed to be 20,634 kilojoules per kilogram [kJ / kg] for Douglass Fir softwood and 19,734 [kJ / kg] for hardwood) to give firepower in kilowatts [kW] or [kJ / s] after appropriate unit conversion. The kinetic energy flow of the primary and secondary air can be divided by the thermal energy flow of the fuel to give a ratio of kinetic energy (KE) of the air injected from a plenum to the thermal energy of the fuel in units of joule kinetic energy to megajoule offuel energy [J / MJ], The kinetic energy flow of the primary and secondary air can be divided by the mass flow rate of air to give the kinetic energy of the airflow per mass of fuel in [J / kg] after unit conversion. As described above the velocity of primary and secondary air, Reynolds number of primary and secondary air, air kinetic energy ratio to the fuel energy, and air kinetic energy per mass fuel are below in Table 8 with corresponding units in the table heading according to run number operating as described in Table 3.

[0203] Table 8: Calculated Parameters of Certain Embodiments

[0204] Based upon the experimental results, it becomes apparent that at high fuel feed rates, there is an accumulation of ash and particulate emissions that are sensitive to primary combustion air amounts. At a low fuel feed rate, the levels of soot have a tendency to increase. There is a surprising result at certain parameters. For example, during a high fuel feed rate (e.g., a high heat setting of the stove), having a ratio of secondary combustion air to primary combustion air of 4: 1 shows drastically reduced particulate emissions. This result is displayed in Fig. 13 which graphs test results 1300 of primary air percentage of stoichiometric air versus total particulate matter emissions for test runs 16a, 17a, 18a, 19a, 19b, and 20a operating at high fuel feed rate with hardwood fuel with the air supply configurations 6, 7, 8, and 9 as indicated in Table 3 above. The results show a near linearcorrelation between primary air percentage of stoichiometric air and particulate matter emissions. For example, where the primary air percentage made up 85% of the total stoichiometric air supply, the particulate matter emitted was near 1 g / kg, shown as result 1302. However, as the primary air percentage was reduced and the secondary air percentage was increased to maintain stoichiometry, the particulate emissions reduced significantly. For example, where the primary air percentage was only 20% of the total air, the particulate matter drops to 0.3 and 0.35 in two experimental tests. These results are shown as 1304 and 1306. Thus, there is a direct correlation between primary air percentage and particulate matter emissions, with a surprising reduction in particulate matter emissions where the secondary air to primary air ratio is about 4: 1 and about 20% of stoichiometric air is provided as primary air and the remaining 80% of stoichiometric air is supplied from a secondary air plenum at the center axis providing secondary air jects radially outward toward the bum pot sidewall.

[0205] Fig 14 illustrates a graph 1400 depicting test results showing the percentage of primary air to secondary air at a medium power setting (e.g., medium fuel feed rate) with hardwood fuel. While the particulate matter trend may not be as strong as the high-power setting, there is still a significant decrease in particulate matter emissions when comparing the primary air at above 35% as compared to closer to 22%. The reduction in the primary air to secondary air ratio from 35% to 22% shows a particulate matter decrease of about 40% in some cases.

[0206] Fig 15 illustrates a graph 1500 showing the black carbon emissions as a result of secondary air kinetic energy (KE) / fuel energy as described above but in units of joule kinetic energy per kilojoule fuel energy [J / kJ] when burning softwood pellets at a low power setting for certain test runs. As can be seen, as the KE Ratio increases, that is, as the secondary air kinetic energy increases, the amount of black carbon reduces exponentially. Initially, where the kinetic energy ratio is near zero, the black carbon is near 30 mg / kg. However, as the kinetic energy ratio increases to 0.2, the black carbon emission drops by a factor of 3 to below 10 mg / kg. Therefore, the kinetic energy of the secondary air plays an important role in the reduction of black carbon, which is believed to be due to the effect ofmixing within the burn pot. Incidentally, the test results show that the release of carbon monoxide follows a very similar trend to the Fig. 15 graph at both medium power and low power operation of the stove. This highlights the benefits of the inside-out secondary air delivery system described herein.

[0207] Fig. 33A and Fig.33B illustrate two graphs showing the black carbon emissions 3302 and carbon monoxide emissions 3304 as a result of secondary air jet Reynolds number. For these selected test runs as indicated 3306 the air and fuel mass flow was relatively constant with the secondary air jet number and diameter changing. The different number and diameter holes at relatively constant secondary air mass flow caused the air jets to have different Reynolds number. As can be seen as the jet Reynolds number decreases the black carbon emissions and carbon monoxide emissions tend to increase. Therefore, secondary air jet Reynolds number plays an important role in the reduction of black carbon and carbon monoxide, especially at low fuel feed rates.

[0208] Fig. 16 illustrates a graph of the improved stove 1600 according to embodiments described herein in comparison with the baseline stove described above, as well as other stoves based on a database of stove efficiencies and emissions provided by the Environmental Protection Agency (EP A). As can be seen, the SP1000 Base results 1602 indicate that the SP1000 Base stove produces PM emissions at a rate of about 1.5 g / hr, and a High Heating Value (HHV) efficiency % of about 66%. While this is better than many of the stoves included in the EPA database, it should be appreciated that the modified stove, shown as the SP1000 Concept, showed dramatic improvements in both PM emissions and HHV efficiency. Specifically, the SP100 Concept result 1604 shows a PM emission of 0.2g / hr, a 91% reduction in PM emission and an HHV efficiency of 83%, which is a 16% improvement in efficiency. In addition, the SP1000 Concept shows a 36% reduction in PM emission from the next cleanest stove in the EPA database. These test results were taken by an independent testing lab in accordance with the requirements of ASTM E2779 and ASTM E2515 while operating the bum pot equivalently to run 15 as described in Table 3.

[0209] These improvements are a direct result of the improvements described herein, namely, a bum pot that introduces primary air vertically from underneath the fuel bed,secondary air introduced above the fuel bed in a radially outward direction, a secondary air to primary air ratio of 4: 1, and independently controlled positive pressure for the primary air and secondary air. This is a departure from traditional stoves that rely on an exhaust fan to draw air into the burn pot, rather that positive pressure that pushes air into the bum pot. Moreover, the ratio of secondary air to primary air may be adjusted depending on the fuel type and stove power setting. As was discussed in more detail in embodiments presented above, however, it is to be understood that such stoves can also benefit from alternative configurations of delivering primary and secondary air. For example, rather from vertically underneath the fuel bed, some embodiments can provide primary air injected into the fuel bed in a radially-inward direction from primary holes along the perimeter of the burn pot at and adjacent the fuel bed. Variations in delivery of secondary air can also be beneficial. And principles such as positive pressure and secondary air to primary air ratios can be applicable to such variations and embodiments.

[0210] With reference next to Fig. 17, another embodiment of a combustion system is illustrated schematically. In this embodiment, the bottom surface, or floor, of the bum pot is configured to support the fuel bed, and can comprise a solid wall with no holes or airdelivery openings. A primary air distribution plenum can be arranged circumferentially about the fuel bed, and can make up the outer wall of the bum pot adjacent the fuel bed. A plurality of spaced-apart primary air holes are formed through the outer wall of the bum pot at and adjacent the fuel bed. The primary air distribution plenum is configured to receive primary combustion air from a primary air conduit and to deliver such primary air circumferentially around the bum pot so that primary air, pressurized by a primary fan, is delivered to the fuel bed through the primary air holes. The primary air holes can be distributed evenly or randomly about the primary air distribution plenum so as to deliver the primary air directly to the fuel bed from the side. In the illustrated embodiment, the primary air holes are arranged to be in a lower half of the fuel bed. As shown, the primary air is injected under pressure so that the primary combustion air flows in a direction radially inwardly, and generally horizontally, through the fuel bed.

[0211] Continuing with reference to Fig. 17, a secondary air conduit extends upwardly through the bottom wall of the burn pot to deliver pressurized secondary combustion air via a secondary air delivery tube vertically above the fuel bed. In the illustrated embodiment, as in some embodiments discussed above, the secondary air delivery tube terminates in a secondary air plenum, and the secondary combustion air is injected via several secondary air holes disposed about the circumference of the secondary air plenum so have a radially- outwardly directed flow direction. As in other embodiments, the relative flow volumes of primary and secondary air can be adjusted as desired. Also, as in other embodiments, an exhaust fan draws combustion products, as well as tertiary air within the firebox, upwardly to an exhaust.

[0212] Fig. 18 is a photograph showing an embodiment of a primary air plenum constructed in accordance with testing of an embodiment employing principles as in the schematic diagram of Fig. 17. As shown, the primary air plenum includes a primary air intake. The primary air plenum includes an air space that extends about its entire circumference and, as shown, includes primary air holes formed through its inner surface. In the illustrated embodiment the primary air plenum has an inner diameter of 4.5 inches - matching the inner diameter of the test burn pot - and the primary air holes comprise 18 evenly-spaced 9 / 64 inch holes.

[0213] Fig. 19 is a photograph showing an embodiment of a burn pot in which the primary air plenum of Fig. 18 has been installed so that the inner wall of the primary air plenum makes up the outer wall of the burn pot in the fuel bed. This photograph concerns the testing apparatus, which is performed using the same Breckwell SP1000 stove used in tests described above. The primary air intake of the primary air plenum is attached to a primary air conduit configured to supply primary combustion air motivated by a primary air fan as in embodiments discussed herein. As depicted in Fig. 19, the illustrated embodiment is a bum pot having a diameter of 4.5 inches. The floor of the fuel bed is a solid wall, and the plenum inner wall extends about 2 inches upwardly from the bottom wall. The primary air holes are centered one inch from both the bottom wall and one inch from the top of the primary air plenum. The upper wall of the burn pot extends 3.5 inches upwardly from thetop of the plenum, and an upper shield extends another 1.5 inches above the upper wall, overlapping the pellet fuel feed chute. The upper shield is angled radially outwardly relative to the upper wall. In this example test stove configuration, the secondary conduit extends upwardly from the bottom wall along an axis of the burn pot. The secondary combustion air plenum is formed by a pipe that is % NPT trade size, having an outer diameter of 1.05 inches. The particular secondary combustion air plenum comprises 18 evenly-spaced 9 / 64 inch diameter holes distributed about its circumference. The secondary air holes are 4 inches above the floor. For purposes of testing, a thermocouple was arranged so as to extend above the bum pot at the top edge - even with the top edge of the upper shield.

[0214] Testing was performed on a bum pot configuration example using the Breckwell SP1000 stove used for previous testing, but with the bum pot configured as shown in Figures 18 and 19 with primary air injected radially inward, and following testing protocols similar to those described above. Test results indicated particulate matter (PM) emissions for hardwood pellets was 0.39g / hr (time weighted average of test runs 27a-c in Table 4). This is a substantial improvement in light of the baseline measurement of 1.5g / hr for hardwood pellets from the US Environmental Protection Agency database of certified heaters. Test results with Douglas fir pellets indicated PM emissions rate of 0.077g / hr (time weighted average of test runs 28a-c in Table 4), which is a dramatic improvement over the baseline measurement of 0.98g / hr for Douglas fir pellets (time weighted average of test runs Al -A3 Table 1), and even an improvement over the 0.082g / hr test measurement for an embodiment in which the primary air plenum directed air upwardly through the fuel bed from below the fuel bed (time weighted average of test runs 21a-c in Table 4). These improvements are a direct result of the improvements described herein, namely, a burn pot that introduces primary air radially inward adjacent to the fuel bed, secondary air introduced above the fuel bed in a radially outward direction, a secondary air to primary air ratio of 4:1, and independently controlled positive pressure for the primary air and secondary air.

[0215] With reference next to Fig. 20, another embodiment of a combustion system is shown comprising a primary air plenum disposed about the outer wall of the fuel bed and configured to inject primary combustion air into the fuel bed in a radially-inward flowdirection. In the illustrated embodiment, the secondary air supply conduit - comprising a pipe - extends in a direction transverse to the burn pot axis through the upper wall of the burn pot and above the fuel bed. The secondary air supply conduit communicates with a secondary air delivery tube disposed along the axis of the bum pot above the fuel bed. The secondary air delivery tube includes a secondary air plenum having a plurality of spacedapart holes about its circumference and configured to inject secondary combustion air in a radially-outwardly-directed flow direction spaced above the fuel bed. In this manner the secondary air supply does not interfere with the floor of the fuel bed.

[0216] Continuing with reference to Fig. 20, an ash removal system can be arranged directly below the fuel bed. It is to be understood that several embodiments of specific ash removal systems can be employed, including ash removal systems that enable removal of ash during continued operation of the stove, ash removal systems for use only when the stove is inoperable, and ash removal systems that simultaneously tend to terminate burn operations of the stove.

[0217] With reference next to Figs. 21A-F, one embodiment of an ash removal system is depicted schematically. In this embodiment, a hollow rectangular housing (see Fig. 2 IB) supports a sliding section that comprises a hole (see Fig. 21A) that is typically arranged under the bum pot during regular operation (see Fig. 21B). The housing can include a similar hole. In some variations, the housing can include a grate that functions as the floor of the fuel bed. During operation, ash will collect within the hole, as shown in Fig. 21B. To dispose of the ash, the slider can be slid outwardly as shown in Fig. 21C. During this operation, the upper surface of the slider adjacent the hole blocks the remaining fuel bed from falling downwardly (see Fig. 21C). As shown in Fig. 2 ID, ash that has been collected in the hole will simply fall out due to gravity as the slider is slid out of its housing. The slider can then be slid back into place (see Fig. 2 IE) so that the hole is again directly below the fuel bed. Grating, if present, can continue to support the fuel bed. In some variations, and as shown in Fig. 21F, when the hole is again aligned with the burn pot, ash and portions of the fuel bed can be expected to fall into the hole in the slider.

[0218] Figs. 22A and 22B present another embodiment of an ash removal system that can be fit below the fuel bed when the secondary combustion air is delivered from a position above the fuel bed rather than through the floor of the fuel bed. In the embodiments illustrated in Figures 22A and 22B, one or two (or more) pairs of rotating rollers are configured to roll with each other. As the pairs roll, protrusions on the rollers crush cinders, clinkers or other tramp material that may be within the ash so that ash falls with gravity through the rollers and to an ash collection volume below the rollers. Such rotating rolls can borrow structure and operation ability as discussed in US Patent No. 5027719, the entirety of which is hereby incorporated by reference.

[0219] In yet another embodiment, and with reference next to Fig. 23, an ash removal system can comprise one, two or more screw conveyors disposed beneath a grate that makes up the fuel bed - or can be disposed beneath an ash treatment system such as the rollers in the embodiment just discussed. The illustrated screw conveyor periodically, or continuously, removes ash from below the fuel bed.

[0220] Although a few specific ash removal structures have been disclosed, it is to be understood that a broad range of structures and methods of ash removal are contemplated. For example, in another embodiment the ash removal system can comprise a simple sliding drawer that can be removed, emptied, and returned by the user during periods of nonuse.

[0221] With reference next to Fig. 24, another embodiment of a combustion system includes a primary air distribution plenum disposed circumferentially about the fuel bed of the bum pot. In this variation multiple rows of primary holes are provided so that the primary plenum supplies primary combustion air to the fuel bed at multiple elevations. In some variations a row of spaced-apart primary holes is provided in a bottom half of the fuel bed and a second row of spaced-apart primary holes is provided in a top half of the fuel bed. In another variation more than two rows of primary holes are provided. In still another variation a randomized distribution of primary holes is provided in the fuel bed.Combinations of such variations are also contemplated. Most preferably, each of the primary air holes is configured to direct a flow of primary air directed radially inwardly and generally horizontally. In further variations such flows of primary air can be generally radially-inwardly directed, but can also be disposed at various non-horizontal dispositions and / or combinations of horizontal and non-horizontal flow directions.

[0222] Increased efficiency of a stove can lead to a hotter burn, and particularly a hotter secondary burn. In another embodiment, and as depicted in Fig. 25, the upper section of the burn pot outer wall can be constructed of a temperature-resistant material such as a ceramic material rather than typical burn pot materials such as steel or stainless steel. In such an embodiment, the primary air plenum intentionally has a lower temperature, as a lower temperature in the fuel bed can lead to reduced volatile ash generation. As such, the primary air plenum remains amenable for construction of materials such as steel and stainless steam, which have lower temperature resistance than that which is used for the upper section above the fuel bed.

[0223] With reference next to Fig. 26, yet another embodiment of a combustion system provides a secondary combustion air delivery system with an outer secondary plenum and an inner secondary plenum. As shown, the outer secondary plenum is positioned and configured to extend about the perimeter of the burn pot above the fuel bed and, similar to the circumferential primary air plenum discussed in embodiments above, comprises a plurality of spaced-apart outer secondary air holes configured to direct a flow of secondary air radially inwardly from the burn pot wall. However, the illustrated secondary combustion air delivery system additionally comprises a secondary air conduit extending from a side wall of the bum pot above the fuel bed, preferably in communication with the outer secondary air plenum at a point vertically spaced from the outer secondary air holes so as to not interfere with such outer secondary air injections. The secondary air conduit communicates with a secondary air tube that delivers secondary air to the inner secondary air plenum positioned axially within the burn pot and having inner secondary air holes about its circumference so as to inject flows of secondary air radially outwardly.

[0224] In the illustrated embodiment the outer secondary air holes and inner secondary air holes are disposed at the same vertical height above the fuel bed and direct air flows generally horizontally. In variations, the inner secondary air holes can be vertically spaced from the outer secondary air holes, and one or more or several of such air holes can beconfigured to inject air in a non-horizontal flow path. In still further variations, the inner secondary air holes can be spaced to direct air flows radially outwardly but directed generally between the radially-inwardly-directed air flows of the outward secondary air holes. In yet other variations the outer and inner secondary air holes can be positioned and configured to direct flows of secondary air directly at one another so as to encourage turbulent mixing.

[0225] With reference next to Fig. 27, another embodiment resembles the embodiment of Fig. 26, except that the outer and inner secondary air plenums are both extended to each include two spaced-apart rows - or stages - of secondary air holes. It is to be understood that further embodiments can contemplate multiple (2, 3, or more) such stages of secondary air holes, and that such stages can be configured in various ways, such as multiple stages of aligned inner and outer secondary air holes resulting in both radially-outward and radially- inward-directed air injections. Other stages can be staggered by direction, such as a stage of radially -inward-directed air injections followed by a stage of radially-outward-directed air injection, and the like. A vertical distance between stages can vary from embodiment to embodiment. In some variations, the vertical distance between stages is a function of the combustion chamber radius, and the vertical distance between adjacent stages is at least equal to the combustion chamber radius.

[0226] With reference next to Fig. 28, another embodiment is shown employing a primary air plenum that injects primary combustion air radially inwardly from primary air holes disposed about the circumference of the burn pot and having a secondary air conduit disposed vertically above the fuel bed. In the illustrated embodiment, the fuel bed is comparatively thick, for example having a thickness greater than the radius of the bum pot in the fuel bed. In one variation the thickness of the fuel bed is greater than the radius but less than the diameter. In another variation the thickness of the fuel bed is between about 1.5-4 times the bum pot radius at the fuel bed. The comparatively thick fuel bed reduces the temperature at the fuel bed, leading to less ash volatilization and cooler outlet, tending to recondense volatile components of ash. In this embodiment, the pellet fuel feed chute opens below the secondary air conduit line that delivers air to the secondary air plenum.

[0227] Fig. 29 shows yet another embodiment of a combustion system in which a secondary air conduit enters the burn pot from above the burn pot outlet and descends axially within the burn pot to the secondary air plenum that is spaced above the fuel bed. As above, the secondary air plenum comprises a plurality of spaced-apart secondary air holes configured to direct flows of secondary air radially outwardly. In some embodiments a portion of the secondary conduit can be placed into contact with the burn pot wall so as to impart combustion heat to the secondary air.

[0228] Fig. 30 schematically presents yet another embodiment of a combustion system in which the burn pot at and adjacent the fuel bed has a first diameter and the secondary bum zone above the fuel bed at and adjacent the secondary air plenum has a second diameter that is less than the first diameter. In this configuration, the larger first diameter at the fuel bed can be configured to spread out combustion and lower temperatures in the fuel bed to minimize ash volatilizations. The lesser second diameter in the secondary bum zone reduces the cross-sectional area, concentrating gasses so as to create a more intense flame zone at higher temperature in the secondary combustion zone, reducing soot, particularly at low fuel feed rates when there typically are less combustible vapors available in the second combustion zone.

[0229] Through testing Applicant has learned that black carbon (soot) emissions tend to increase substantially at low fuel feed rates. However, if the Watts firepower per area (i.e., W / cm2) is greater than about 50 W / cm2, soot emissions are reduced substantially. Thus, the embodiment illustrated in Fig. 28 provides a relatively low-diameter combustion area with more fuel per cross-sectional area so as to boost W / cm2, while the thick fuel bed simultaneously lowers fuel bed temperatures, thus reducing ash volatilization. The wider fuel bed of Fig. 30 also can lower such fuel bed temperatures, while the reduced-diameter secondary combustion chamber can increase W / cm2. Another reason for having a fuel bed that is relatively wide as compared to a secondary combustion area is to accommodate low bulk-density fuels such as corn cobs or non-densified biomass. The large fuel bed accommodates such low bulk-density fuels, while the reduced-diameter secondarycombustion zone concentrates combustible gasses for more effective secondary combustion than were such gasses thinly distributed over a wider cross-sectional area.

[0230] Figs. 31 A and 3 IB present other embodiments in which the burn pot diameter grows significantly at and adjacent the secondary combustion zone. As shown, the bum pot diameter becomes larger just as secondary combustible vapors reach the injected secondary combustion air. Notably, up to this point the diameter of the burn pot may remain the same so as to keep combustible vapors together and accessible to the injected secondary combustion air. The wider outlet portion spreads secondary flames more evenly, leading to more even heating of the heat exchanger for improved heat transfer and thermal efficiency. This can also shorten the flame height.

[0231] Fig. 32 presents a still further embodiment in which a primary igniter is provided in the primary air plenum adjacent the side wall at the fuel bed. A second igniter can be placed at or adjacent the burn pot wall in the secondary combustion zone above the secondary air plenum. Upon sensing a decrease in temperature within the burn pot, typically due to flameout, the secondary igniter can be signaled to ignite so that the stove can recover from such a flameout.

[0232] A person of ordinary skill in the art will recognize that any process or method disclosed herein can be modified in many ways. The process parameters and sequence of the steps described and / or illustrated herein are given by way of example only and can be varied as desired. For example, while the steps illustrated and / or described herein may be shown or discussed in a particular order, these steps do not necessarily need to be performed in the order illustrated or discussed.

[0233] The various exemplary methods described and / or illustrated herein may also omit one or more of the steps described or illustrated herein or comprise additional steps in addition to those disclosed. Further, a step of any method as disclosed herein can be combined with any one or more steps of any other method as disclosed herein.

[0234] Unless otherwise noted, the terms “connected to” and “coupled to” (and their derivatives), as used in the specification and claims, are to be construed as permitting both direct and indirect (i.e., via other elements or components) connection. In addition, the terms“a” or “an,” as used in the specification and claims, are to be construed as meaning “at least one of” Finally, for ease of use, the terms “including” and “having” (and their derivatives), as used in the specification and claims, are interchangeable with and shall have the same meaning as the word “comprising”.

[0235] As used herein, the term “or” is used inclusively to refer items in the alternative and in combination.

[0236] As used herein, characters such as numerals refer to like elements.

[0237] Embodiments of the present disclosure have been shown and described as set forth herein and are provided by way of example only. One of ordinary skill in the art will recognize numerous adaptations, changes, variations and substitutions without departing from the scope of the present disclosure. Several alternatives and combinations of the embodiments disclosed herein may be utilized without departing from the scope of the present disclosure and the inventions disclosed herein. Therefore, the scope of the presently disclosed inventions shall be defined solely by the scope of the appended claims and the equivalents thereof.

[0238] The disclosure also includes the following numbered clauses:

[0239] Clause 1 : A fuel bed air supply system comprising a plenum in fluid contact with an independently powered external air source, wherein: the plenum further comprises one or more nozzles, each configured to emit a jet of air; the plenum is deployed beneath a fuel bed in a stove configured to bum granular solid fuel; the jets of air from the one or more nozzles impinge on the fuel bed substantially perpendicular to the average plane of the fuel bed; and the jets of air from the one or more nozzles enter the fuel bed with sufficient momentum to deliver primary and secondary combustion air for combustion of the fuel in the fuel bed.

[0240] Clause 2: An upper air supply system comprising a plenum in fluid contact with an independently powered external air source, wherein: the plenum further comprises one or more nozzles, each configured to emit a jet of air; the plenum is deployed above a fuel bed in a stove configured to burn granular solid fuel; the jets of air from the one or more nozzles exit the plenum substantially parallel to the average plane of the fuel bed; and the jets of airfrom the one or more nozzles deliver secondary combustion air for combustion of the fuel in the fuel bed.

[0241] Clause 3: A combustion apparatus comprising a burn pot configured to burn granular solid fuel, a fuel bed air supply system further comprising a first plenum in fluid contact with a first independently powered air source, and an upper air supply system further comprising a second plenum in fluid contact with a second independently powered air source, wherein: the first plenum further comprises one or more nozzles, each configured to emit a jet of air; the first plenum is deployed beneath a fuel bed in the burn pot; the jets of air emitted from the first plenum impinge on the fuel bed substantially perpendicular to the average plane of the fuel bed; the jets of air emitted from the first plenum enter the fuel bed with sufficient momentum to deliver primary combustion air, or deliver primary and secondary combustion air, for combustion of the fuel in the fuel bed; the second plenum further comprises one or more nozzles, each configured to emit a jet of air; the second plenum is deployed above the fuel bed in the bum pot; the jets of air emitted from the second plenum exit the plenum substantially parallel to the average plane of the fuel bed; and the jets of air emitted from the second plenum deliver secondary combustion air for combustion of the fuel in the fuel bed.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. A stove configured to burn granular solid fuel, comprising: a bum pot, the burn pot generally cylindrical in shape and having an outer wall, a top open end and a bottom closed end configured to support a fuel bed thereon, wherein a plurality of primary apertures are formed through the outer wall at and adjacent the fuel bed, the plurality of primary apertures being spaced apart from one another about a perimeter of the outer wall; a fuel bed plenum disposed generally about the perimeter of the outer wall and in communication with the plurality of primary apertures formed through the outer wall, the fuel bed plenum configured to cause air within the fuel bed plenum to flow through the plurality of primary apertures formed through the outer wall; a primary air source in fluid communication with the fuel bed plenum, the primary air source configured to direct pressurized air to the fuel bed plenum; a secondary air plenum positioned within the burn pot above the bottom closed end and above the plurality of primary apertures formed through the outer wall, the secondary air plenum configured to direct secondary air radially outwardly from a center of the burn pot toward the outer wall; and a secondary air source in fluid communication with the secondary air plenum, the secondary air source configured to direct pressurized air to the secondary air plenum; and wherein the primary air source and the secondary air source are controllable to vary the amount of primary air and the amount of secondary air delivered to the burn pot.

2. The stove as in claim 1, wherein the primary air source and the secondary air source are individually controllable to vary the ratio of secondary air to primary air delivered to the burn pot.

3. The stove as in claim 1, wherein the fuel bed plenum is coupled to the primary air source by a primary air conduit and further comprising an igniter disposed along the primary air conduit.

4. The stove as in claim 1, wherein the secondary air plenum comprises a tube that extends generally orthogonally through the bottom closed end and into the bum pot.

5. The stove as in claim 4, wherein the secondary air plenum comprises a plurality of secondary air delivery nozzles arranged circumferentially about the secondary air plenum.

6. The stove as in claim 5, wherein the plurality of secondary air delivery nozzles are arranged in one or more rings about the secondary air plenum.

7. The stove as in claim 1, additionally comprising a secondary air supply conduit extending through the outer wall and into the burn pot to the secondary air plenum, the secondary air supply conduit extending through the outer wall above the fuel bed.

8. The stove as in claim 7, wherein the secondary air plenum comprises a plurality of secondary air delivery nozzles arranged circumferentially about the secondary air plenum.

9. The stove as in claim 8, wherein the plurality of secondary air delivery nozzles are arranged in one or more rings about the secondary air plenum.

10. The stove as in claim 1, additionally comprising a secondary air supply conduit configured to deliver secondary air from outside the bum pot into the bum pot and to the secondary air plenum, the secondary air supply conduit being spaced from the bottom closed end and above the fuel bed.

11. The stove as in any one of claims 1-10, additionally comprising a plurality of secondary apertures formed through the outer wall at and adjacent the secondary air plenum,the secondary apertures being spaced apart from one another about a perimeter of the outer wall and in communication with the secondary air plenum configured to cause air within the secondary air plenum to flow through the plurality of secondary apertures formed through the outer wall.

12. The stove as in any one of claims 1-11, wherein the secondary air is injected in a secondary combustion zone within the bum pot, and wherein a diameter of the secondary combustion zone within the bum pot is at least 10% smaller or larger than the primary combustion zone.

13. The stove as in any one of the preceding claims, wherein the primary air source is a first fan arranged to blow air into the fuel bed plenum.

14. The stove as in any one of preceding claims 1-13, wherein the secondary air source is a second fan arranged to blow air through the secondary air plenum.

15. The stove as in any one of claims 1-142, wherein the secondary air plenum is configured to inject secondary air into the bum pot in a direction generally parallel to the direction of primary air injected into the burn pot.

16. A method of operating the stove of claim 1, comprising: adding pelletized fuel to the bum pot; activating the primary air source; activating the secondary air source; activating an igniter to provide ignition heat to the pelletized fuel in the burn pot; and controlling the secondary air source and the primary air source such that a ratio of secondary air to primary air is greater than 3: 1.

17. The method of operating the stove as in claim 16, wherein the ratio of secondary air to primary air is 4: 1.

18. The method of operating the stove as in claim 16, wherein the amount of primary air is 20% to 35% of stoichiometric air19. The method of operating the stove as in claim 16, further comprising, adjusting, in response to a parameter associated with the stove, the ratio of secondary air to primary air.

20. The method of operating the stove as in claim 19, wherein the parameter is a power setting of the stove.

21. The method of operating the stove as in claim 16, further comprising adjusting the primary air source to inject primary air with a Reynolds number between 20 and 2,500.

22. The method of operating the stove as in claim 16, further comprising adjusting the secondary air source to inject secondary air into the bum pot with a Reynolds number of between 200 and 3,000.

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