Patent Grant
8590463
The present invention relates to removing moisture from fuel for a solid fuel boiler.
Solid fuel boilers are commonly used by industry and utilities to generate steam for process requirements and to generate electricity. These boilers burn bark, coal, sludge, wood waste, refuse, tires, and other organic materials, often in combinations, and with fossil fuels. Generally, the organic materials have high moisture contents and are stored outdoors where they are often wet from rain water or, in the case of sludge, reclaimed from wastewater treatment facility.
In some cases, some of the moisture is removed before the fuel is delivered to the boiler by means of mechanical presses or drying chambers using hot gases from the discharge of the boiler. For example, U.S. Pat. No. 3,976,018 to Boulet teaches a fuel dryer for bagasse fuel. The dryer is separate from the boiler and uses alternating fixed and rotating conical trays over which the fuel passes downward while stack gases pass over the fuel to dry it. The fuel empties into a hopper for transporting to other equipment. U.S. Pat. No. 6,532,880 to Promuto teaches a system for drying sludge, including a shaftless spiral feed screw for moving sludge through a drying chamber as a high energy inductor draws hot gases through the chamber to dry the sludge as it advances through the chamber. U.S. Pat. No. 4,635,379 to Kroneld describes a dryer in which fuel travels on a conveyor bed while steam moves through the fuel from underneath. U.S. Pat. No. 4,254,715 to LaHaye et al. teaches a drying system in which heated air passes over the fuel in a combustion chamber, as burning occurs substantially at the bottom of the pile of fuel.
These drying systems are often troublesome, expensive, risky, and not particularly efficient. With many of these methods, the fuels still contain undesirably high moisture content. In any case, essentially all of the water in the fuel must be removed during the combustion process and when the fuels are wet the combustion process can be unstable and inefficient. The situation is further exacerbated by variable moisture contents of the fuel (e.g., from rain) that introduces variations into the combustion process and makes it more difficult to operate the boilers. Solid fuel boilers are typically constructed as large boxes (up to 100 square meters or more in floor area) with heavy steel tubing forming the walls of the box, typically referred to as the front, sides, and rear walls. The tubing typically has an outer diameter of 63.5 mm or 76.2 mm and is arrayed in parallel relationship forming flat panels, with the tubes running vertically. The tubes are typically spaced apart about 10-12 mm, with a steel membrane or fin bridging the gap. The whole assembly is seal welded together forming an air tight structure. The boiler walls, or tube panels, run vertically to the top of the boiler, up to 30 m or more tall. The walls are fed re-circulating water by headers at their lower extremity. Typically the front wall tubes are bent over more or less horizontally to form the roof of the boiler and the side walls end in relieving headers feeding back to a steam drum. The rear wall either ends in a header or feeds directly into the steam drum. In order to feed fuel and combustion air into the boiler, and to provide openings for other purposes, the boiler tubes are bent to spread them apart to form openings in the tube panel. The bottom of the boiler may be arranged to include a combustion support, such as a grate, a fluidized bed, or other arrangement. Grates include traveling grates, vibrating grates, tilting grates, and hydro-grates. Typically, the grates cover the bottom of the boiler and are made of heavy cast iron components with slots for combustion air to rise through the grate from a plenum below. The solid fuel lands on the grate and burns there. The ash is dumped off of the grate as the grate moves (rotates like a tank tread), vibrates, or tilts (in sections). Fluidized beds generally have a mass of sand or other media through which a stream of air or boiler flue gas is percolated to fluidize the bed. The fluidized bed acts as a heat sink, fuel drying system, turbulent fuel/air mixing system, fuel distribution system, and means for separating fuel and ash in the boiler. Additional combustion air ports, typically called “over fired air” (OFA) are arranged to blow air in above the grate or fluidized bed to help complete the combustion. In all of these arrangements, excessive moisture in the fuel causes poor combustion, which can result in poor operational efficiencies and high environmental emissions. The volume contained within the boiler walls is referred to herein as the “combustion chamber.” The region where most of the solid fuel burns, that is, on the grate or at the fluidized bed, is referred to herein as the “combusting zone.” It is understood that combustion of airborne combustible matter also takes place in the combustion chamber outside of the combusting zone.
In common practice the solid fuel is fed by gravity through large chutes, steeply mounted and having a cross section of about 500 mm square, from a hopper and/or conveyor system above, to the lower portion of the boiler just above the grate or fluidized bed. There are typically multiple chutes penetrating a wall or walls of the boiler. A solid fuel distributor is often integral with and at the bottom of the chute, right at the interface with the boiler wall. Mechanical distributors and pneumatic distributors are commonly used. Grate type boilers generally require some type of fuel distribution whereas fluidized bed boilers can be run without them as the fluidized bed can distribute the fuel, albeit inefficiently. Typically, the fuel slides down the chute and enters the boiler with high residual moisture content (up to 50% or more). The water in the fuel inhibits the combustion in the furnace, often requiring the continual use of supplemental fossil fuels to provide additional heat to compensate for the moisture. It is also very common for the load rate on these boilers to change frequently in reaction to changing steam demands. Inconsistent and high moisture content of the fuels makes it difficult for the boiler to respond effectively to the required load changes. This requires, again, the use of supplemental fossil fuels to improve the response of the boiler to load rate changes. Fossil fuels are typically used to start these boilers but continual use of fossil fuels is extremely expensive. Fluidized beds can help to compensate for varying moisture contents and load rates because they act as heat sinks, but they can have significant operational and mechanical problems such as sand sintering and sand erosion and they require a sand reclamation system. There is great demand for a simple means to dry solid fuels so that they are delivered to the boiler combustion chamber ready to burn. Such a system is preferably inexpensive to install and operate, reliable, effective, and safe. Various embodiments of the present invention, described below, address one or more of these challenges and provide one or more, and preferably all of these advantages.
It is an object of the invention to provide an efficient, robust means to reduce the moisture content of solid fuels so that they are delivered to the boiler combustion in a better condition to burn.
Embodiments of the invention dry fuel as it is being provided to the boiler through a delivery chute. In one embodiment, hot gases are drawn through the chute to dry the fuel as it is coming down the chute toward the combustion zone. In another embodiment, the fuel is dried by exposing it to the combustion zone environment as the fuel is being delivered.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
For a more through understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
To the right of the drawing is the interior of the boiler where the fuel is burned. To the left of the drawing is outside the boiler.
In different embodiments, the driving forces for the cooling media and the driving forces for the hot gas can be active or passive. Similarly, the control mechanisms for controlling the flow of the cooling media and the control mechanisms for controlling the flow of the hot gas can be active or passive. The systems for driving and controlling the cooling media flow can be interrelated or independent from the systems for driving and controlling the hot gas flow.
In some embodiments, the rate of flow of cooling media through the plenum can be passively controlled by the temperature of the chute, with the flow increasing as the temperature of the chute increases, without requiring sensors, electronic controllers, and controllable valves. In some embodiments, however, active controls, including temperature sensors, automatic or manual valves, and air moving devices, can be used to control the flow of the cooling medium in the plenum. A combination of active and passive controls may also be used to control the flow of cooling media in the plenum, with the cooling flow anticipating the heating flow, as described above.
In some embodiments, the flow of hot gas can be controlled by controlling, actively or passively, the flow of air through the plenum. In other embodiments, the flow of hot gas through the fuel is controlled independently of the cooling media flow, for example, by using sensors and controllable dampers, in additional to or instead of controlling the flow of air through the plenum.
A difference in pressure at different points of the chute containing fuel causes hot gas to be drawn through the fuel in the drying zone of the chute removing moisture. In the embodiment of
In other embodiments, a low pressure region for drawing the hot gas through the chute can be created by other means, such as by an inductor or other type of fan. Such air moving devices can be used to cause hot gas flow in addition to or instead of the hot gas flow caused by the low pressure region supplied by the air exiting the plenum. Alternatively, hot gases can be driven through the fuel by a high pressure region on one end of the chute. In one embodiment the hot gas is drawn from the combustion chamber through the same place where the fuel enters the combustion chamber. In other embodiments, the hot gas could enter the chute at a different location or a hot gas other than gas directly from the combustion chamber could be used. For example, a pneumatic fuel spreader at the bottom of the chute can create a low pressure zone at the bottom of the chute that draws hot gas through the drying zone from an opening further up the chute. In such a case, the hot gas flows in the same direction as the fuel in the chute. Hot gas can be drawn into the chute above the fuel, flow in the same direction as the fuel and exit with the fuel into the combustion chamber. In some embodiments, hot or cold gas can be injected at high pressure or volume into the chute to dry the fuel and to draw additional hot gas through the chute.
One embodiment can be easily controlled by monitoring the gas temperature in the drying chamber and is inherently safe because the air is only mixed with the boiler gas immediately before it is re-injected into the boiler.
The boiler tubes are bent out of the plane 40 of the tube wall to form a lower chute 32 that has only three sides, the fourth side is open to the boiler and exposed to the combustion in the boiler 33. Exposed chute 32 terminates at its bottom in fuel spreader, such as an angled portion 39 back to the plane of the tube wall. The sides and back of exposed chute 32 may be lined with a suitable high temperature and corrosion resistant material. A mechanical or pneumatic bark distributor can be used as a fuel spreader and can be arranged to fit at the bottom of chute 32 to distribute the fuel to the combusting zone 42. A fuel spreader need not be a complicated device, and can be any surface above the combustion region onto which fuel falls before falling onto the combusting zone.
In this embodiment, the fuel when moving through exposed chute 32 is preferably inside the combustion chamber, that is, it is within the tube walls of the combustion chamber. The fuel traveling in exposed chute 32 is preferably not directly over combusting zone 42 and is preferably not falling directly into the grate or bed. That is, the fuel preferably falls in a chute above and to the side of the solid fuel combustion region. The fuel exiting exposed chute 32 is then distributed by fuel spreader 41 onto the solid fuel combustion region, typically a bed or grate. The fuel is exposed to the interior of the combustion chamber before reaching the fuel spreader. The distance between the points where the fuel enters the combustion chamber and the fuel spreader is sufficiently great so that a significant amount of moisture is removed from the fuel as it falls toward the fuel spreader. For example, more than 10%, of the moisture can be removed. The distance through which the fuel falls while being exposed to the combustion chamber interior is preferably at least two meters. While falling within the chute inside the combustion chamber, the fuel is exposed to both radiant heating and convective heating by the hot gas in the combustion chamber. In a preferred embodiment, incorporated into vertical chute 30 are multiple adjustable barriers, such as stop plates 34 and 35, that control the rate of flow of fuel through the chute, for example, by controlling the height from which the fuel falls by gravity.
Wet fuel enters chute 29 at 36 and flows into chute 30 at point 37. If the fuel is relatively dry, stop plates 34 and 35 will be in the open position (35 is shown open, 34 is shown closed) and the fuel will accelerate by gravity to a higher velocity as it passes through exposed chute 32 where it dries from exposure to the hot gas and radiation from the combustion in the boiler. If the fuel is relatively wet, stop plates 34 or 35 will be closed to momentarily interrupt the flow of the fuel. When the stop plate reopens, the fuel is dropped from a lesser height; therefore it passes through exposed chute 32 more slowly with more time to dry. The vertical heights of exposed chute 32 and vertical chute 30 are determined by the drying requirements of the fuel. Likewise the number of stop plates is determined by the variability in the moisture content of the fuel. The stop plates can be arranged to operate manually by a counterweight arrangement so that the selected stop plate will automatically open when a certain amount of fuel accumulates on it. Alternatively, the stop plates can be controlled automatically by a variety of means. The section 43 of the tube wall through which fuel passes on its way into the combustion chamber is preferably not vertical, that is, the tube wall is tilted and has a horizontal component where the chute enters the combustion chamber. The second embodiment has the advantage of being extremely simple in design, and although the boiler gas is not drawn through the fuel, the fuel is exposed to radiation from the boiler. Embodiments preferably use gravity feed in the chute without mechanical feed devices, such as the moving trays of Boulet or the screw of Promuto. Unlike LeHaye et al., embodiments of the present invention preferably remove moisture from the fuel before it reaches the grate or bed in or on which it is burned.
The third embodiment has the advantage of much higher heat tolerance than the first embodiment while still requiring little modification to existing boilers. Furthermore, the downward flow of gas requires less energy to create as it is flowing in the direction of the falling fuel and will not be counter to the direction of the injection of the fuel and gas by the fuel distributor. While three embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The term “solid fuel” includes wet fuels, such as sludge, wet coal, and other such fuels that are useful in boilers of the type described above. The term “chute” includes any passage by which fuel is delivered to the boiler. The cooling plenum shown outside could also be formed as conduits such as pipes inside.
In each embodiment, sufficient moisture is removed from the fuel to improve combustion. The amount of moisture removed will depend on the moisture of the incoming fuel, the size of the fuel particles, the differential temperature and velocities between the combustion gas and the fuel, and the contact time between the fuel and the gas. The size of the fuel particles vary and typically range from sawdust to several inches long pieces of wood or bark. The contact time is limited therefore only the smallest particles will dry completely in the chutes and will enter the boiler ready to burn. Mid size particles will lose their surface moisture and some internal moisture. Larger particles will lose only surface moisture. In the described embodiments of the present invention, the transfer of heat to the fuel is thought to be dominated by radiation. In the prior art, the internal temperature of fuel chutes typically does not exceed 350 degrees F. Fahrenheit or so. In embodiments of the present invention, the interior of the chute may be heated to more than 350 degrees F., more than 500 degrees F., more than 700 degrees F., or more than 1000 degrees F., measured away from the entrance to the combustion chamber. Boiler combustion gas, however, is typically over 2000 degrees Fahrenheit, even as hot as 2500 degrees. Radiant heat transfer, among other parameters, is a function of differential temperature to the 4th power. Therefore embodiments of the present invention may increase the heat transfer to the fuel by from 500 to 1000 times over existing fuel chutes with no auxiliary heating. Physical constraints around the boiler limit the height of the drying chute and therefore the contact time between the fuel and the hot gas. There are also practical limits on how much gas can be drawn through the chute. These constraints limit the drying in the chutes in most embodiments for large particles to removing more than 10%, more than 15% or removing about 25% of the moisture carried by the fuel for some fuels. For fuels such as sawdust, having small particles, the embodiments of the invention can remove, for example, more than 25%, more than 50% or more than 75% of the moisture from the fuel before it reaches the combusting zone.
Although embodiments of the present invention and various advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.
This application claims priority from U.S. Prov. Pat. App. No. 61/055,802, filed May 23, 2008, which is hereby incorporated by reference.
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| Number | Date | Country | |
|---|---|---|---|
| 61055802 | May 2008 | US |
