Patent Application
20140150698
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.
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.
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.
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.
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 (
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
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.
The chute inlet 410 (
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 (
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
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.
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.
