Patent application title: SOLID FUEL GRAVITY FEED COMBUSTION DEVICE, SYSTEM AND METHOD
Steven J. Walker (Jaffrey, NH, US)
New England Wood Pellet LLC
IPC8 Class: AF23B5012FI
Class name: Furnaces including means to feed solid fuel supply hopper; e.g., magazine, etc.
Publication date: 2014-06-05
Patent application number: 20140150698
A solid fuel gravity feed combustion device, system and method are
presented. The device includes a chute having an inlet and outlet, such
that the chute inlet is configured to receive multiple pellets. A floor
is disposed substantially below the chute inlet, and adjacent to the
chute outlet. The chute outlet is configured to accommodate a pellet pile
including at least a portion of the multiple pellets disposed upon the
floor, where the pile surface is oriented substantially at a pellet angle
1. A gravity feed pellet feeder comprising: a chute further comprising a
chute inlet configured to receive multiple pellets and a chute outlet
comprising an outlet opening with an opening top and an opening bottom;
and a floor disposed substantially below said chute inlet, and further
disposed adjacent to said chute outlet, wherein said chute outlet is
configured to accommodate a pile comprising at least a portion of said
multiple pellets disposed upon said floor, said pile comprising a pile
surface oriented substantially at a pellet angle of repose from said
outlet chute opening top to the opening bottom.
2. The pellet feeder of claim 1, wherein said chute inlet receives said multiple pellets from a pellet hopper.
3. The pellet feeder of claim 1, wherein said floor comprises a burner grate.
4. The pellet feeder of claim 1, wherein said chute outlet is disposed adjacent to a burner grate.
7. The pellet feeder of claim 1, wherein said chute inlet comprises an airlock.
9. A method for conveying pellets to a pellet destination, the method comprising the steps of: providing a chute comprising a chute inlet, a chute outlet and a floor, said chute outlet disposed at said pellet destination; wherein said chute outlet is configured to accommodate a pile comprising multiple pellets disposed upon said floor, said pile comprising a pile surface oriented substantially at a pellet angle of repose; receiving said multiple pellets into said chute inlet; and at said chute outlet, removing a plurality of said multiple pellets from said pellet pile.
10. The method of claim 9, wherein receiving said multiple pellets into said chute inlet further comprises orienting said chute inlet adjacent to a hopper further comprising a hopper inlet and a hopper outlet.
11. The method of claim 10, further comprising the step of adding a plurality of pellets to said pellet hopper through said hopper inlet.
12. The method of claim 9, wherein removing a plurality of said pellets further comprises burning said plurality of pellets.
13. The method of claim 9, wherein removing said plurality of pellets further comprises the step of conveying said plurality of pellets away from said pellet pile.
14. A method for feeding pellets to a burner, comprising the steps of: configuring said chute to accommodate a pellet pile within said chute, wherein said pile comprises a surface oriented substantially at a pellet angle of repose; and adding pellets to said chute through said chute inlet.
15. The method of claim 14, wherein the step of adding pellets is performed by a hopper comprising a hopper inlet and a hopper outlet, wherein said chute inlet is connected to said hopper outlet and adding pellets to said chute comprises receiving pellets from said hopper outlet.
16. The method of claim 14, further comprising the step of removing pellets substantially at said chute outlet.
17. The method of claim 16, wherein the step of removing pellets substantially at the chute outlet comprises burning said pellets in said pellet burner.
18. The method of claim 17, wherein the step of removing pellets at said chute outlet further comprises clearing ash produced from spent pellets from the pellet burner.
19. A pellet transport system comprising: a pellet feeder comprising: a chute further comprising a chute inlet configured to receive multiple pellets and a chute outlet comprising an outlet opening with an opening top and an opening bottom; and a floor disposed substantially below said chute inlet, and further disposed adjacent to said chute outlet, wherein said chute outlet is configured to accommodate a pile comprising at least a portion of said multiple pellets disposed upon said floor, said pile comprising a pile surface oriented substantially at a pellet angle of repose from said outlet chute opening top to the opening bottom; and a conveyer configured to convey said plurality of pellets between a pellet repository and said pellet feeder.
20. The system of claim 19, wherein said conveyer further comprises: a sensor configured to sense available capacity of said pellet feeder to receive additional pellets in said chute; and a sensor output configured to alert said conveyer to convey more pellets.
FIELD OF THE INVENTION
 The present invention relates to fuel transport, and more particularly, is related to conveying pellets.
 There are two major components in most typical solid fuel combustion systems: fuel and air (oxygen). The air contains the oxygen that is needed to react with the fuel to create combustion. Too little air, which, in effect, is too much fuel, will lead to incomplete combustion of the fuel, generally resulting in a buildup of partially combusted materials on the burning surface (grate). This buildup of partially combusted material generally starves the system of air, upsetting the functional balance of the system even more. As the worst case, this may lead to a shutdown of a burner. Even in the best of cases, this reduces efficiencies and increases emissions. In contrast, instances with too much air, or effectively too little fuel, may result in a flame that goes out as it becomes too cool, or possibly burning at low efficiently and higher emissions. A challenge in solid fuel combustion is metering precise amounts of fuel consistently into a combustion system.
 Four common ways of introducing solid fuels include suspension burning, stoker-feeding, a drop system, and a slinger system. In a stoker fed system, fuel is pushed into the combustion system, typically by either a ram or an auger. In a drop system, material is dropped from a conveyor or other means of metering fuel into a chute leading into the combustion chamber. In a slinger system, solid fuel is slung through an opening in the combustion chamber onto a burning grate. The stoker fed and the drop systems are common in smaller combustion systems. The slinging and suspension combustion are common in larger combustion systems, such as power plant-scale.
 In solid fuel combustion systems, "under air" refers to air that passes generally upward from beneath the fuel and up through the fuel, while "over air" refers to air that passes near or above the top of the fuel. Solid fuel is typically burned using a combination of under air and over air. Adjusting the amount of air, both over air and under air, can achieve higher or lower firing rates with the same size grate. There are both maximum and minimum limitations to the amount of air that can be introduced to a particular sized grate.
 Pelletized material is used in many applications. Materials may be pelletized to simplify handling, transport and use. Examples of pelletized materials include fuel pellets and animal feed pellets. Handling of pelletized material for pellet burners provides unique challenges. As discussed above, the pellets must be continuously fed to the burner to maintain constant heat output and high efficiency. While liquid fuel or gas and propane burners may be regulated by forcing the fuel by pressure or by gravity, pellet feeders typically require mechanical means to transport fuel from a pellet hopper to the burner. Examples include augers and conveyor belts. These mechanical pellet feeders add costs to the boiler, consume energy and require maintenance.
 Besides requiring power to operate and downtime to maintain, mechanical feeders may not feed the pellets to the burner at an optimum rate. Even if the feed rate is initially correctly tuned for an optimum burn rate, variations in the feed or other conditions may require adjustment of the feed rate. Such adjustment may be impractical, particularly if the feed pellet consistency varies often. Further, monitoring a burner to regulate feed for optimum burn rates adds expense and complexity to the system. Therefore it is desirable to develop a low cost and low energy pellet feeder that self-adjusts the feed rate to the optimal burn efficiency.
 Embodiments of the present invention provide a solid fuel gravity feed combustion device, system and method. Briefly described in architecture, a first aspect of the present invention is directed to an unpowered pellet feeder. The pellet feeder includes a chute with a chute inlet and a chute outlet, the chute inlet configured to receive multiple pellets. The pellet feeder includes a floor disposed substantially below the chute inlet, and further disposed adjacent to the chute outlet. The chute outlet is configured to accommodate a pile having at least a portion of the multiple pellets disposed upon the floor, the pile having a pile surface oriented substantially at a pellet angle of repose.
 A second aspect of the present invention is directed to a method for conveying pellets to a pellet destination. The method includes the steps of providing a chute having a chute inlet, a chute outlet and a floor, the chute outlet disposed at the pellet destination. The chute outlet is configured to accommodate a pellet pile having multiple pellets disposed upon the floor, the pile having a pile surface oriented substantially at a pellet angle of repose, receiving the multiple pellets into the chute inlet, at the chute outlet, removing one of the multiple pellets from the pellet pile.
 Briefly described, a third aspect of the present invention is directed to a method for feeding pellets to a burner. The method includes the steps of providing a hopper having a hopper inlet and a hopper outlet, providing a chute having a chute inlet connected to the hopper outlet, and a chute outlet disposed at the burner, configuring the chute to accommodate a pellet pile within the chute, wherein the pile has a surface oriented substantially at a pellet angle of repose, and adding pellets to the pellet bin through the pellet bin inlet.
 Other systems, methods and features of the present invention will be or become apparent to one having ordinary skill in the art upon examining the following drawings and detailed description. It is intended that all such additional systems, methods, and features be included in this description, be within the scope of the present invention and protected by the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
 The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principals of the invention.
 FIG. 1 is a diagram of an embodiment of a simple gravity feed pellet feeder.
 FIG. 2A is a schematic diagram of a gravity feed pellet feeder attached to a hopper.
 FIG. 2B is a schematic diagram of a gravity feed pellet feeder with input level sensors.
 FIG. 3 is a detailed schematic diagram of a gravity feed pellet feeder.
 FIG. 4 is a diagram of a pellet feeding system embodiment.
 FIG. 5 is a flow chart of an embodiment of a method for feeding pellets.
 Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
 Exemplary embodiments of a gravity feed pellet feeder are presented. In one exemplary embodiment a gravity feed pellet feeder conveys pellets from a hopper to a destination, for example, a burner, by configuring the pellet feeder to accommodate pelletized material storage in accordance to the angle of repose of the pelletized material. As pelletized material is consumed or removed from the pellet feeder, it is replaced by fresh fuel that flows to the location vacated by the consumed/removed pellets.
Angle of Repose for Granular Materials
 When dropped onto a flat surface, granular material, such as fuel pellets, generally forms into a conical pile. When granular material is removed from the base of the conical pile of granular material, granular material uphill from the removed material generally flows downward under the force of gravity to fill the space created by the removed granular material. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose.
 For the purposes of this document, the angle of repose is defined as the steepest angle of descent of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. This angle may be between the angle of 0° and 90°. As noted above, when bulk granular material is poured onto an unconstrained horizontal surface a generally conical pile forms. Granular material piled at the angle of repose of that material may be thought of as being at an equilibrium point. If the angle of the material is increased past the angle of repose, material begins to slide or flow. If the angle is decreased below the angle of repose, additional material may accumulate before the material tends to flow.
 The angle of repose for granular material is related to physical properties of the granular material such as the density surface area and geometry of the particles, and the coefficient of friction of the material. Material with a low angle of repose forms flatter piles than material with a high angle of repose. Material with generally uniform physical properties tends to have a consistent and predictable angle of repose.
Gravity Feed Pellet Feeder
 FIG. 1 shows a first embodiment of a gravity feed pellet feeder 400. In the first embodiment, pellets are conveyed from a pellet hopper (not shown) containing a plurality of pellets to a pellet destination, for example, a pellet burner. The pellet hopper (not shown) is positioned above a chute inlet 410 of a pellet feeder housing 415. The size of the chute inlet 410 may be adjusted to regulate the flow rate of pellets. The pellets may flow through the chute leading to an outlet 420, where pellets may accumulate in a pile on a feeder bottom 430 at an opening 440 and exit the chute through an outlet 420 located, for example, at a burner (not shown).
 The chute outlet 420 may be positioned adjacent to a burner grate (not shown), so that excess pellets spill onto the burner grate to be conveyed to the pellet burner. Alternatively, the chute bottom 430 may incorporate a burner grate 435 (FIG. 3), wherein a portion of the pellets on the burner grate 435 (FIG. 3) may be burned. While the chute bottom 430 is depicted as having a substantially horizontal surface, there is no objection to the chute bottom having an incline with respect to horizontal, or formed in other contours.
 A pellet pile 450 forms upon the chute bottom 430, with the surface of the pile 450 forming at approximately the angle of repose for the feed pellets. The angle of repose for wood pellet feed is typically in the range of 30 to 35 degrees, but may be beyond this range, depending upon the specific geometry and mass of the pellets. Therefore the pellets may continue to flow through the chute until the pellet pile 450 grows to reach the height of the opening top 445. Thereupon the flow of pellets ceases, until such pellets in the pellet pile 450 are removed or burned away, thereby lowering the pile 450 and allowing further pellets to flow from the hopper through the chute inlet 410. In this way the flow of pellets is regulated by the geometry of the feeder 400 and the burn rate or removal rate of the pellets.
 In general, the surface of the pellet pile 450 at the angle of repose may not be immediately adjacent to a ceiling surface, which may impede the flow through the chute 415. As shown in FIG. 1, the angle of repose surface of the pile of pellets 450 may be substantially uncovered, or the pile of pellets 450 may be covered such that a gap exists between the surface of the pile of pellets 450 and a cover (not shown), so as not to thereby obstruct the natural flow of pellets 450 down the pile as pellets 450 are either removed or consumed at the chute bottom 430.
 While the inlet 410 of the pellet feeder 400 is shown as rectangular in shape, there is no objection to the inlet 410 and/or housing 415 having other shapes, for example, circular. Similarly, while the pellet feeder opening 440 is shown to substantially expose the pellet pile 450, there is no objection to a smaller opening 440 that substantially covers the pellet pile 450, providing only enough space above the chute bottom 430 to facilitate removal of pellets from the pellet pile 450.
 FIG. 2A shows a second embodiment of a gravity feed pellet feeder 400 attached directly to a hopper 200. In this simplified system, wood pellets 450 are introduced into the hopper 200, such that the pellets 450 fall into the pellet feeder 400 form a pile in the pellet feeder 400. As pellets 450 are removed or consumed from the pellet feeder 400, replacement pellets 450 drop into the pellet feeder 400 from the hopper 200. In such a simple pellet feeding system continuous combustion may be supported without any electrical device at all. A proper chimney may maintain proper negative draft. A negative air system is preferably used to help prevent burn back into the hopper 200. As described below, several flow control mechanisms not indicated in FIG. 2A may control the flow of pellets 450 between the point pellets 450 are added to the system, the hopper 200 in this case, and the location in the pellet feeder 400 where the pellets 450 are removed or consumed.
 The chute inlet 410 (FIG. 1) may be closed, for example with an airlock, thereby blocking an air path from the chute opening 440 (FIG. 1) through the inlet chute to the hopper 200. The hopper 200 may also include an airlock opening for adding pellets to the hopper 200. The airlock eliminates a possible air path through the pellet feed path. During normal operation, the airlock may be sealed shut, to prevent a fire back burn up from a pellet burner (not shown) through the chute 410 (FIG. 1) and into the hopper 200.
 FIG. 2B shows a more detailed drawing of the second embodiment of the gravity pellet feeder 400, with the hopper 200 (FIG. 2A) not shown for clarity. A high level sensor 460 detects if pellets are present adjacent to the high level sensor 460. Similarly, a low level sensor 465 detects if pellets are present adjacent to the low level sensor 465. The outputs of the high level sensor 460 and the low level sensor 465 may be used to track both the current level of pellets 450 within the pellet feeder 400, and the consumption rate of the pellets 450, thereby alerting other components of the system to convey more pellets 450 to the pellet feeder 400. For example, the pellet consumption rate may be estimated based upon the difference between a first time when the pellet level falls below the high level sensor 465 and a second time when the pellet level falls below the pellet level low level sensor 460. The pellet feeder 400 under the second embodiment includes an angled back portion 416 that may be substantially parallel to the pellet angle of repose.
 FIG. 3 shows the second embodiment of the gravity pellet feeder 400 from a viewpoint showing the underside of the bottom 430, in this embodiment, formed as a grate 435 having a plurality of under air holes 433. The under air holes 433 are preferably small enough so unburned fuel does not pass through them, but large enough to allow ash to pass through. The bottom grate 435 may be a static grate or a moving grate for more difficult ash characteristics. Over air holes 470 are positioned below a plenum inlet hole 475. Air is forced into the plenum inlet hole, for example, via a ventilator hose (not shown). The air is then forced through the over air holes 470 onto burning pellets 450 (FIG. 2B) at a desired rate of flow to optimize combustion. In the second embodiment, the over air holes 470 are positioned just above the chute opening top 445, however, in alternative embodiments the over air holes 470 are positioned below a plenum inlet hole 475 may be positioned elsewhere around the fire for desired results.
 An advantage of the gravity feed pellet feeder 400 over prior art pellet feeders is that prior art gravity feeders have difficulty ensuring that oxygen flow and pellet feed rate are correctly coordinated to ensure a consistent charcoal burn rate. Too fast a burn rate, for example, in the presence of too much oxygen, may starve the fire as fuel is consumed faster than it is supplied, unless a faster fuel feed is provided. Too slow a burn rate, for example in the absence of sufficient oxygen, may result in a buildup of excess fuel on the burner unless the fuel feed rate is reduced accordingly. In contrast a gravity feed pellet feeder 400 may be self-regulating, such that consumed charcoal is replaced only when the spent fuel is substantially exhausted, and the charcoal burn rate may increase and decrease according to the available oxygen, resulting in a more consistent fuel burn rate. The burn rate may be further controlled by the amount of area of the fuel pile 450 (FIG. 2B) that is exposed to air/oxygen.
 The gravity feeder of this invention is advantageous in its simplicity, cost-savings and ability to provide consistent fuel burn. In contrast, a traditional stoker-fed system is generally more complex and difficult to maintain a consistent burn. In the case of a ram feed system, the ram travels forward and backward creating periods where excess fuel is provided, and times where insufficient fuel is provided, resulting in variations in combustion and therefore variations in efficiencies and emissions. This is also true of the drop system, where there may be inconsistencies in the amount of solid fuel that is dropped in any given amount of time. And as the fuel is dropped, it is unevenly introduced to the combustion zone. In addition, external upset conditions such as changes in voltage, slight differences in the solid fuel consistency, and uneven air supply, caused for example by plugged air hole may lead to various inefficiencies and irregularities in fuel consumption.
 This invention introduces fuel at a consistent rate, and is self-regulating. Fuel is replaced at substantially the same rate as it is burned. If more or less air is introduced to the system, it will naturally feed more or less fuel. This may reduce or eliminate the need for burn rate control or fuel level monitoring technology. The system becomes self-regulating by using the angle of repose to expose a consistent amount of fuel to the combustion zone.
 In an alternative embodiment where the bottom 430 incorporates a burner grate, as pellets are burned away and the ash is removed, the pile 450 drops and additional pellets flow through the chute 410 to replace them, until the pile again rises to the level of the opening top 445. The burner may be incorporate an updraft burner, where flames are drawn upward from the burner grate, or a downdraft burner, where flames are drawn downward through the bottom of the burner grate, for example, by an impelled air flow. For a larger pellet pile, the chute opening top 445 may be positioned relatively higher from the bottom 430. For a smaller pellet pile, the chute opening top 445 may be positioned closer to the bottom 430. In this way, the rate and volume of the pellet flow may be adjusted.
 Note that while the first and second embodiments of the gravity feed pellet feeder feeds fuel pellets to a pellet burner, other embodiments are possible. For example, a third embodiment of a gravity feed pellet feeder may be an animal pellet feeder, where pellets are fed to, for example, a feed trough instead of the burner. In the third embodiment, the pellets are animal feed pellets, and the pellet feeder advances feed pellets to the pellet trough as pellets are consumed from the pellet trough. Under the third embodiment, an airlock may not be useful. Advantages of such a feed system include the ability to supply feed at the rate of consumption until the pellets in the bin are exhausted without, for example, mechanical or electrical feed level sensing devices in the trough.
 For another example, a fourth embodiment of a gravity feed pellet feeder may be a pellet bagging system, where pellets, such as fuel pellets, fertilizer pellets, or animal feed pellets, are stored in a bin and fed to a pellet bagging device. Pellets may flow through the chute into a bag provided by the pellet bagging device until the bag fills to a level corresponding to the level of the chute egress end, whereupon the full bag is removed and replaced by an empty bag. Advantages of such a bagging system over a system with a constant mechanized feed include the reduction of spillage and overflow, as the pellets cease to feed until the full bag is removed. This automatic cessation of feed may be accomplished without, for example, sensors or timers.
 A first exemplary embodiment of a pellet feeding system 600 is shown in FIG. 4. Under the first embodiment system, fuel pellets 450 are stored in a remote hopper 610, or other pellet repository. The pellets 450 may be conveyed from the hopper 610 by a mechanized conveyor 620, for example, an auger driven tube, driven, for example, by a conveyer motor 622, such that pellets 450 may drop down from the conveyor 620 into an angle of repose pellet feeder 400. As shown by FIG. 4, the flow of pellets between the conveyer 620 and the pellet feeder 400 may be further regulated, for example, with a secondary feeder motor 624 controlling an airlock.
 The pellets 450 accumulate in a pile in the feeder 400, where the surface of the pile of pellets 450 is substantially at the angle of repose according to the geometry of the pellets 450. The fuel pellets 450 are consumed by a burner (not shown). As pellets 450 are consumed they are replaced by additional pellets 450 as space becomes available in the feeder 400. The mechanized conveyor 620 may typically only operate intermittently, for example, conveying pellets until the feeder 400 is filled above high level sensor 460, whereupon the mechanized conveyor is dormant until enough pellets 450 are consumed so the top of the pellet pile 450 falls below the low level sensor 465.
 Additional embodiments are also possible, for example, where the burner is a downdraft burner, so that flames pass downward through the pellet feeder 400. In other embodiments, an agitating grate may serve as the mechanized conveyer 620. As described above, an air lock may typically be used between the auger 620 and the feeder 400 for burn-back protection. An air lock allows fuel to pass through while allowing only minimal air to pass through.
 FIG. 5 is a flowchart of an exemplary method for feeding pellets. It should be noted that any process descriptions or blocks in flow charts should be understood as representing modules, segments, portions of code, or steps that include one or more instructions for implementing specific logical functions in the process, and alternative implementations are included within the scope of the present invention in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present invention.
 The first exemplary method includes the step of configuring a chute to accommodate a pellet pile within the chute, wherein the pile has a surface oriented substantially at a pellet angle of repose, as shown by block 510. As shown in block 520, pellets are added to the chute through the chute inlet. A hopper having a hopper inlet and a hopper outlet is provided, as shown by block 530, wherein the chute inlet is connected to the hopper outlet. Optionally, pellets may be removed from the chute substantially at the chute outlet, as shown by block 540.
 As used within this document, the term "pellet" refers to a plurality of particles formed of substantially similar material into a substantially uniform shape, for example, a cylinder, sphere, or block. While a pellet may be typically created by compressing an original material into a pellet form, pellets may be created by paring down a larger object into a pellet size, for example, a lump of coal, or pellets formed substantially naturally, for example kernels of grain. The term "pelletize" refers to a process of forming a pellet.
 In summary, a solid fuel gravity feeding system, device and method have been presented. It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
Patent applications by Steven J. Walker, Jaffrey, NH US