WO2024073625A1 - Air supply systems for combustion of granular biomass fuels - Google Patents

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, and orthogonal to, 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 (PM_2.5), and black carbon.

Description

AIR SUPPLY SYSTEMS FOR COMBUSTION OF GRANULAR BIOMASS FUELS

CROSS-REFERENCE

[0001] This application claims the benefit of United States Provisional Application No. 63/410,948, filed September 28, 2022, entitled “AIR SUPPLY SYSTEMS FOR COMBUSTION OF GRANULAR BIOMASS FUELS” the entire disclosure 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 ratio of secondary air to primary air delivered to the burn pot.

[0005] 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.

[0006] In some examples, the secondary air plenum comprises a cylindrical pipe that extends generally orthogonally through the closed bottom end and into the burn pot. It may extend through a center of circular center of the closed bottom end upwardly into the bum pot.

[0007] 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. [0008] 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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. [0015] 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 emitting 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.

[0016] 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 emitting 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 air from the one or more nozzles deliver secondary combustion air for combustion of the fuel in the fuel bed.

[0017] 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 emitting a jet of air; the first plenum is deployed beneath a fuel bed in the bum 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 emitting 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. [0018] In one embodiment 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 emitting a jet of air; the first plenum is deployed beneath a fuel bed in the bum 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 emitting a jet of air; the second plenum is deployed above the fuel bed in the burn 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.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] 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.

[0020] 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.

[0021] Fig. 1 A illustrates a top view of a bum pot including the sidewall of the substantially cylindrical burn pot and top surface of the fuel bed air supply system plenum with air delivery nozzles, in accordance with some embodiments.

[0022] Fig. IB illustrates an array of air delivery nozzles, in accordance with some embodiments. [0023] Fig. 1C illustrates an array of air delivery nozzles, in accordance with some embodiments.

[0024] 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.

[0025] 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.

[0026] Fig. 2B illustrates a circular array of holes of the upper air supply system, in accordance with some embodiments.

[0027] Fig. 2C illustrates a circular array of holes of the upper air supply system, in accordance with some embodiments.

[0028] Fig. 2D illustrates a circular array of holes of the upper air supply system, in accordance with some embodiments.

[0029] Fig. 2E illustrates an array of air delivery nozzles of the fuel bed air supply system, in accordance with some embodiments.

[0030] 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.

[0031] Fig. 3A illustrates a top-down view of a bum pot, in accordance with some embodiments.

[0032] Fig. 3B illustrates a cross-sectional schematic view of a bum pot, in accordance with some embodiments.

[0033] Fig. 4A illustrates a perspective view of a burn pot, in accordance with some embodiments.

[0034] Fig. 4B illustrates a cross-sectional view of a burn pot, in accordance with some embodiments. [0035] Fig. 5 illustrates shows a cross-section view of a bum pot, in accordance with some embodiments.

[0036] 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.

[0037] 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 burn a channel through the fuel bed, in accordance with some embodiments.

[0038] 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.

[0039] Fig. 9 illustrates a schematic view of a combustion chamber showing the primary air source, secondary air source, and igniter, in accordance with some embodiments.

[0040] Fig. 10A illustrates a perspective view of a primary and secondary air inlet, in accordance with some embodiments.

[0041] Fig 10B illustrates a burn pot, in accordance with some embodiments.

[0042] Fig. 11 illustrates various embodiments of a secondary air supply conduit, in accordance with some embodiments.

[0043] Fig. 12 illustrates examples of secondary air plenums configured with secondary air injection holes, in accordance with some embodiments;

[0044] Fig. 13 illustrates a graph of experimental results showing total particulate matter emissions as a function of primary air and secondary air ratios, in accordance with some embodiments;

[0045] Fig. 14 illustrates a graph of experimental results showing total particulate matter emissions as a function of primary and secondary air ratio, in accordance with some embodiments. [0046] Fig. 15 illustrates a graph of experimental results showing the effects of black carbon emissions by varying a ratio of secondary air kinetic energy to fuel energy, in accordance with some embodiments.

[0047] 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.

DETAILED DESCRIPTION

[0048] 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.

[0049] 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.

[0050] 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 the scope of the present disclosure but is instead provided as a description of exemplary embodiments.

[0051] Numeric ranges provided herein are inclusive of the numbers defining the range. [0052] 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.

[0053] 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 fuel bed air supply system and the upper air supply system may be separately controlled, such that the ratio of primary air to secondary air may be adjusted.

[0054] With reference to Fig. 1 A, 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. 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 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 burn 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 substantially cylindrical sidewall 112 of the burn pot 110. In some cases, the nozzles are formed as apertures formed in the top surface 106 of the fuel bed air supply system.

[0055] 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 a subset 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.

[0056] 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 1 12 of the bum pot 110 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. [0057] 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 101 that supports the fuel bed 107 and releases primary combustion air 108 to the granular solid fuel. In use, a fuel bed 107 is introduced into the burn pot and 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 101, 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 air 108 into the plenum 105.

[0058] With reference to Fig. 2A, a top view of the bum pot is depicted showing a combination of the fuel bed primary air supply system and secondary air supply system. As shown 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 a nozzle 205. The nozzle 205 may have one or more exit pathways for secondary combustion air to enter the burn pot 110. In some cases, the secondary combustion air enters the burn pot 1 10 in a direction that is orthogonal to the primary combustion air 108, as will be described in further detail below. The secondary air supply system may further be introduced into the bum 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. 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 bum pot.

[0059] With reference to Figs. 2B, 2C, and 2D, examples of secondary air plenums 208 are illustrated. 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 208, 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 212 may be a separate component that is affixed to the conduit 212, or it may be integrally formed with the conduit 212. The performance of the arrangement of nozzles 204 is detailed as AirConfig3 defined in Example 2 below.

[0060] With reference to Fig. 2C, the circular array of holes 205 (e g., nozzles) of the secondary combustion air cap 210 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.

[0061] 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 plate 106 of the primary combustion air 108 system. In some cases, the secondary air nozzles may be located approximately 2.25 inches (57 mm) above the plane of the top surface 106 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 106. According to some embodiments, such as the experiments detailed below, hole arrays 204 and 205 formed within the upper cap 210 may be positioned at the same height relative to the top surface 106 of the fuel bed air supply system. 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 burn 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.

[0062] In some cases, the secondary combustion air supply system provides a stream of secondary combustion air 203 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 plate 106, and have nozzles that inject air radially from the cap 210. This provides a volume of secondary combustion air delivered from the center of the bum pot 110 which initially flows radially outward from the cap 210 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 bum 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.

[0063] 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. [0064] 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 201 passes through the center of the fuel bed air supply system plenum 105.

[0065] 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 101 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 A 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). [0066] 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 and a 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. [0067] 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 burn pot 110 includes a sidewall 112 that forms a 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.

[0068] Fig. 3B illustrates a cross-sectional view of the burn pot 110 according to some embodiments, the performance of which is represented as OutletConfig 1 in Example 2 detailed below. In the illustrated example, the diameter of the substantially cylindrical burn pot sidewall 110 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 101 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.

[0069] 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 110. In some cases, the burn 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 cylindrical burn pot 110, or may be frustoconical in shape such that it gradually narrows along the longitudinal axis of the cylindrical bum pot 110.

[0070] 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 substantially 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.

[0071] 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 402, 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 bum pot 110 and improve total combustion.

[0072] 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 heating stove pellet fuel feed chute 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 105 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 flow sensors 603a, 603b may be any suitable mass 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, 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.

[0073] 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 bums a channel through the fuel bed. In some cases, the fuel bed 701 is distributed across the top surface 106 of the primary air plenum. As primary air is fed into the burn pot 1 10, 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 within the bum pot 110. Where the fuel bed 107 is distributed across the burn pot 110, the hot spots will generally be distributed throughout the bum pot 110.

[0074] 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. [0075] Fig. 8 A illustrates that, in practice, flames 801 form where air from the fuel bed air supply system has burned through the fuel bed 107 and reaches the top of the fuel bed 107. These flames 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 107. Where the fuel bed 107 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. [0076] 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. [0077] 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 107. Fig. 8C illustrates a reduced primary air mass flow rate, which results in a more even burn 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 106 of the primary air plenum 105.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] Fig. 9 schematically illustrates an example combustion system 900 including a burn pot 110 of a stove along with a primary combustion air pathway 108 and a secondary combustion air pathway 212. In some cases, a primary air fan 902 is energized and provides primary combustion air 108 along the primary air conduit 109. Similarly, a secondary air fan 904 may be energized to provide secondary combustion air 203 through the secondary air conduit 212. 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, 212. 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 bum 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 212. The positive pressure within the air conduits 109, 212 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.

[0082] 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 107. 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 additional igniter or heater may be provided to preheat the secondary combustion air 203 in the secondary air conduit 212. However, in any case, as the secondary air conduit 212 passes through the fuel bed 107 and into the burn 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.

[0083] 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.

[0084] 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 fuel bed 107.

[0085] 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, results in less char being lofted, but if the mass flow rate is too low, this can result in an inefficient bum and an increase in soot and ash.

[0086] 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 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 bum 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, 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 212 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 burn pot toward the outside of the burn pot.

[0087] Fig. 11 illustrates a burn pot 110 and shows a primary combustion air inlet 1104 which is in fluid communication with the primary air plenum. The primary air conduit can be coupled to the primary air inlet 1004 so that primary air can be delivered to the primary air plenum. In some cases, the burn pot 110 is configured as a replacement burn pot for a commercially available stove. In some cases, a bum 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 bum pot 110, combustion air systems and other components described herein can be integrated into new stove designs and included at the time of manufacture.

[0088] Fig. 12 illustrates several examples of secondary air delivery conduits 212. In some cases, the secondary air delivery conduit 212 includes a first 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 delivery conduit 212 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 bum 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 to flow through the holes to provide mixing of combustion air, which has been shown to improve combustion.

[0089] 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 primary combustion air. 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 burn 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. [0090] Generally, the efficiency of a pellet stove is measured using a parameter known as the "combustion efficiency" or "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.

[0091] 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) or kilowatts (kW).

[0092] 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 joules. [0093] 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.

[0094] Based on example embodiments as described herein, a pellet stove can 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 provide sufficient primary combustion air, but not so high as to loft char out of the burn pot; (ii) secondary combustion air provided by an inside-out injector spaced above the fuel bed, the secondary combustion air being introduced to mix the primary combustion air; (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.

[0095] EXAMPLES

[0096] 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. [0097] Example 1 : Baseline Emissions Performance

[0098] 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= closed position, 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 bum 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.

[0099] 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 orPM25 (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 journal article 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.

[0100] 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).

[0101] 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 reported separately 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.

[0102] 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.

[0103] 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.

[0104] Table 1: Baseline Emission Results

[0105] Table 2: Baseline Emissions Factor Results

[0106] Example 2: Emissions Performance of Examples of Certain Embodiments

[0107] Stove Modification and Setups

[0108] 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 instant 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 burn pot via a plenum deployed so that air delivered to the plenum by the fan(s) was injected into the bum 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.

[0109] 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 air supply 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.

[0110] 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 holes approximately 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 AirConfigl 2, 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. [OHl] 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 212 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 burn 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 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.

[0112] For AirConfig l 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 % 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 NPT trade size pipe. For AirConfig6, 8, 9, 11, 12, and 13 the internal diameter of the burn pot sidewall 110 was 4.5 inches (114mm) and the secondary air plenum used ³A 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 'A 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.

[0113] 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 substantially 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 substantially 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.

[0114] Experimental Protocols and Operation

[0115] 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 106; 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.

[0116] The following experimental protocols (Step 3 in the paragraph immediately above) were used:

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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 fdter(s) installed to determine PM2.5 and BC at power level setting 1.

[0121] Runs 14-17 and 20-22 were run at the corresponding settings using the same protocol as runs 12 and 13 above.

[0122] 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.

[0123] 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.

[0124] 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.

[0125]

[0126] The stock stove's combustion blower (exhaust extractor fan) control was modified to allow variable speed control.

[0127] 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].)

[0128] 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).

[0129] Specifics of the pellet stove setups and operating parameters tested, as described above, (runs 1 -26) 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.

[0130] Table 3: Setups and Operating Parameters

[0131] Measured Emission Results

[0132] The values of CO, PM2.5, BC, CO2, and fuel feed rate for runs 1-26 (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.

[0133] Table 4: Emission Rate Results of Examples of Certain Embodiments

[0134] Table 5: Emission Factor Results of Examples of Certain Embodiments

[0135] 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.

[0136] 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 to exhaust 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.

[0137] Table 6: Exhaust Oxygen Results of Examples of Certain Embodiments

[0138] Derived Parameters

[0139] 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 a percentage 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- 26 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.

[0140] Table 7: Burn Pot Airflow of Certain Embodiments

[0141] 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.

[0142] Reynolds number is a dimensionless parameter to characterize the flow of a fluid, such as air through a 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. According to embodiment described herein, increasing the flow velocity within an air supply pipe will increase the Reynolds number, and higher velocities result in greater inertia. The air supply pipe diameter is directly related to the Reynolds number, with a larger pipe diameter resulting in a higher Reynolds number for the same 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 injected 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 of fuel 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. [0143] Table 8: Calculated Parameters of Certain Embodiments

[0144] 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 versus total particulate matter emissions for certain test runs operating at high fuel feed rate with hardwood fuel. The results show a near linear correlation between primary air percentage 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.

[0145] 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.

[0146] 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. However, as the kinetic energy ratio increases to 0.2, the black carbon emission drops by a factor of 3 to below 10. 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 of mixing 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.

[0147] 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.

[0148] 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 independent 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.

[0149] 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.

[0150] 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.

[0151] Unless otherwise noted, the terms “connected to” and “coupled to” (and their derivatives), as used in the specification and claims, are to be constmed 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”.

[0152] As used herein, the term “or” is used inclusively to refer items in the alternative and in combination.

[0153] As used herein, characters such as numerals refer to like elements.

[0154] 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.

[0155] The disclosure also includes the following numbered clauses:

[0156] 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 emitting 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.

[0157] 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 emitting 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 air from the one or more nozzles deliver secondary combustion air for combustion of the fuel in the fuel bed.

[0158] 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 emitting 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 emitting a jet of air; the second plenum is deployed above the fuel bed in the burn 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

CLAIMS WHAT 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 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 fuel 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 fuel 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; and wherein the primary air source and the secondary air source are individually controllable to vary a ratio of secondary air to primary air delivered to the burn pot.

2. 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.

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

4. The stove as in claim 3, wherein the secondary air plenum comprises a plurality of secondary air delivery nozzles arranged circumferentially about the secondary air plenum.

5. The stove as in claim 4, wherein the plurality of secondary air delivery nozzles are arranged in one or more rings about the secondary air plenum.

6. The stove as in claim 4, wherein the plurality of apertures formed in the closed bottom end are arranged in an array about the closed bottom end.

7. 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.

8. The stove as in any one of preceding claims 1-6, wherein the secondary air source is a second fan arranged to blow air through the secondary air plenum.

9. The stove as in any one of claims 1-6, wherein the secondary air plenum is configured to inject secondary air into the bum pot in a direction generally orthogonal to the direction of primary air injected into the burn pot.

10. A method of operating the stove of in 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.

11. The method of operating the stove as in claim 10, wherein the ratio of secondary air to primary air is 4: 1.

12. The method of operating the stove as in claim 10, further comprising, adjusting, in response to a parameter associated with the stove, the ratio of secondary air to primary air.

13. The method of operating the stove as in claim 12, wherein the parameter is a power setting of the stove.

14. The method of operating the stove as in claim 10, further comprising adjusting the primary air source to inject primary air with a Reynolds number between 20 and 2,500.

15. The method of operating the stove as in claim 10, 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.