Patent Application
20090151902
The present invention relates generally to fluidized bed type fossil fuel fired heat generating systems, and more particularly to the re-circulating of heated solids in a fluidized bed type fossil fuel fired heat generating system.
Heat generating systems with furnaces for combusting fossil fuels have long been employed to generate controlled heat, with the objective of doing useful work. The work might be in the form of direct work, as with kilns, or might be in the form of indirect work, as with steam generators for industrial or marine applications or for driving turbines that produce electric power. Modern water-tube furnaces for steam generation can be of various types including fluidized-bed boilers. While there are various types of fluidized-bed boilers, all operate on the principle that a gas is injected to fluidize solids prior to combustion in the reaction chamber. In circulating fluidized-bed (CFB) type boilers a gas, e.g., air, is passed through a bed of solid particles to produce forces that tend to separate the particles from one another. As the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed then becomes fluidized, with the gas cushion between the solids allowing the particles to move freely and giving the bed a liquid-like characteristic. The bulk density of the bed is relatively high at the bottom and decreases, as it flows upward through the reaction chamber where fuel is combusted to generate heat.
The solid particles forming the bed of the circulating fluidized bed boiler typically include fuel particles, such as crushed coal or other solid fuel, and sorbent particles, such as crushed limestone, dolomite or other alkaline earth material. Combustion of the fuel in the reaction chamber of the boiler produces flue gas and ash. During the combustion process, the sulfur in the fuel is oxidized to form sulfur dioxide (SO2), which is mixed with the other gasses in the furnace to form the flue gas. The ash consists primarily of unburned fuel, inert material in the fuel, and sorbent particles, and is sometimes referred to as bed materials or re-circulated solids.
The ash is carried entrained in the flue gas in an upwardly flow and is exhausted from the furnace with the hot flue gas. While entrained therein and being transported by the flue gas, the sorbent particles that are present within the reaction chamber, i.e., furnace or combustor, capture, i.e., absorb, sulfur from the SO2 in the flue gas. This reduces the amount of SO2 in the flue gas that ultimately reaches the stack and as such the amount of SO2 that is exhausted into the environment.
In order to replenish the solid particle materials that are consumed in or exhausted by the furnace, fresh fuel and sorbent particles as well as recycled ash are continuously introduced to the bed of the circulating fluidized bed boiler. Continuing, after being exhausted from the furnace, the flue gas and ash are directed to a separator, such as a cyclone, to remove the ash from the flue gas. Two parallel paths are then typically provided for re-circulating the separated ash back to the bed of the circulating fluidized bed boiler. At any given time, the separated ash may be directed along either or both of said parallel paths by a solids flow control valve located between the separator and said two parallel paths. Such solid flow control valves are well known in the art and may be controlled pneumatically, hydraulically or in some other functionally equivalent manner.
Circulating fluidized bed boilers are designed so as to operate within a narrow temperature range in order to thereby promote the combustion of fuel, the calcination of limestone and the absorption of sulfur. This narrow range of furnace temperatures must be maintained over a range of furnace loads, from full load down to some level of partial loading. The furnace temperature is controlled through absorption of heat from the flue gas and bed ash that is produced as a result of combustion in the reactor chamber of the furnace. While most of the heat absorption is through the furnace walls and the in-furnace panels, on larger circulating fluidized bed boilers, heat absorption by the furnace enclosure walls and in-furnace panels is insufficient to achieve the desired operating temperatures. For these larger circulating fluidized bed boilers, therefore, external heat exchangers are employed to absorb heat from the ash that is removed from the flue gas in the cyclone or other separator, before the ash is re-circulated to the to the circulating fluidized bed boiler. Such external heat exchangers are commonly referred to as External Heat Exchangers (EXE) or Fluid Bed Heat Exchangers (FBHEs).
Accordingly, if directed along one of the two parallel re-circulating paths, the sorbent and other ash particles are fluidized and these fluidized ash particles are then transported to and are made to flow through a FBHE by means of injected high pressure gas, e.g., air, which is normally at a pressure of about 200 inches water gage (WG). Heat is transferred from the fluidized particles to a working fluid such as water, steam, a mixture of both or some other coolant flowing through a tube bundle within the FBHE. The flow of cooled fluidized particles is then reintroduced into the furnace. The amount of cooling of the fluidized particles that is performed in the FBHE is typically controlled based on the gas temperature within the furnace that is desired.
If directed along the other one of the two parallel re-circulating paths, the sorbent and other ash particles are also fluidized and are entrained therewithin and are transported by an injected high pressure gas, such as air, again normally at a pressure of around 200 inches WG. In this case, in accordance with this path, the fluidized particles are directed through an ash re-circulation pipe having a seal, commonly referred to as a seal pot or siphon seal, that is suitably installed so as to be operative to ensure proper flow of gas and ash in the primary loop, which is defined as the furnace, the separator, i.e., cyclone, seal pot and FBHE. The seal pot functions to prevent a backflow of gas and solid particles from the furnace into the re-circulation pipe. From the seal pot, the sorbent and other solid ash particles are then reintroduced into the furnace without being cooled.
U.S. Pat. Nos. 6,779,492 and 6,938,780, which are also assigned to the same assignee as that of all of the rights in the present application, provide detailed descriptions of conventional circulating fluidized bed boilers having seal pots and FBHEs.
There remains a need for a more efficient and less expensive means for recycling ash in circulating fluidized bed boiler heat generating systems. For example, it would be beneficial if the relatively high pressure fluidizing air required by conventional FBHEs and seal pots could be eliminated, since this would reduce not only the expense of providing the required high pressure blowers and fluidizing nozzles of conventional construction, but also would reduce the dynamic loading to which the structural steel, which is required to support the FBHEs and seal pots of conventional construction is subjected, and in addition the consumption as well of power that is required to operate such high pressure blowers in order to thereby provide the necessary supply of high pressure air. Additionally, it would be beneficial to have higher heat transfer rates in the FBHE than those that are now possible when FBHEs of conventional construction are employed. Heat transfer is typically defined y the equation Q=R×S×LMTD where Heat transferred (Q=Btu/hr), Heat Transfer Rate (R=Btu/hr−Ft2−F), Surface (S=Square Feet (Ft2)) and Log Mean Temperature Difference (LTMD=Deg. F). For a constant transfer rate (R), increasing the LMTD results in a reduction of required heat exchanger surface (S) for a given heat loading. The moving bed heat exchanger (MBHE) constructed in accordance with the present invention improves on the LMTD over that in typical FBHEs by permitting full counter-flow of solids and working fluid.
Accordingly, it is an objective of the present invention to provide an improved technique for recycling the ash that is produced from the combustion of fossil fuels, such as, for example, the recycling of the ash that is produced from the combustion of fossil fuels in a circulating fluidized bed boiler.
It is another object of the present invention to provide an improved technique for removing heat during the recycling of the ash that is produced from the combustion of fossil fuels.
Additional objects, advantages, and novel features of the present invention will become apparent to those skilled in the art from the disclosure of this patent application, including the following detailed description thereof, as well as by practice of the present invention. While the present invention is described below with reference to a preferred embodiment(s), it should be understood that said invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present invention as said invention is disclosed and claimed herein and with respect to which said invention could be of significant utility.
In accordance with the present invention, a moving bed heat exchanger (MBHE) is provided. The MBHE could, for example, be installed in the primary recirculation loop of a circulating fluidized bed boiler with said MBHE having a vessel, a plurality of tubes, and a plurality of air inlets.
The vessel of the MBHE includes an upper portion with a feed opening, a lower portion with a floor having a discharge opening, and an intermediate portion disposed between said upper portion and said lower portion. The vessel of the MBHE receives hot ash particles, such as hot limestone particles with absorbed sulfur, via the feed opening thereof. These hot ash particles are typically received from a cyclone or other type separator after these hot ash particles have been removed from the flue gas that is exhausted from a furnace, such as the furnace of a circulating fluidized bed boiler. The vessel of the MBHE is suitably configured, i.e., is sized, shaped and/or has structural components, so as to be operative to direct a gravity flow of the hot ash particles, which are received thereby, from the upper portion of the vessel through the intermediate portion of the vessel to the floor of the lower portion of the vessel, and so as to be operative as well to collect the ash particles on the floor of the lower portion of the vessel. This directed gravity flow of the ash particles may be referred to as a “moving bed”.
The plurality of tubes of the MBHE, which preferably are in the form of finned tubes, are disposed in the intermediate portion of the vessel of the MBHE and are configured so as to be operative to direct a flow of working fluid, such as water, steam, a mixture of water and steam, or some other fluid, in a direction substantially orthogonal to the direction of the directed gravity flow of the aforereferenced hot ash particles through the intermediate portion of the vessel. If the direction of the gravity flow of the aforereferenced hot ash particles is vertically downward, the flow in a direction substantially orthogonal to the direction of such gravity flow of the aforereferenced hot ash particles would be a substantially horizontal flow. The flow of the working fluid is such that heat from the hot ash particles is transferred to the working fluid to thereby cool said hot ash particles as the latter are directed to the lower portion of the vessel of the MBHE.
The plurality of air inlets of the MBHE, which will typically be in the form of air nozzles, are suitably configured so as to be operative to inject air into the lower portion of the vessel of the MBHE in order to thereby control the amount of the previously hot ash particles, which have now been cooled, that are collected and discharged through the discharge opening of the vessel of the MBHE. The amount of heat that is transferred from the hot ash particles to the working fluid will normally correspond to the amount of the previously hot ash particles, which have now been cooled, that are collected and discharged through the discharge opening of the vessel of the MBHE. Preferably, the amount of such cooled ash particles, which are collected and discharged, is controlled based on either the temperature of the gas in the furnace or the temperature of the working fluid leaving the MBHE.
Typically, in circulating fluidized bed boilers of conventional construction, air is injected at multiple locations and at various pressures. Fluidizing air injected into the furnace thereof through nozzles installed at the bottom of the furnace requires a pressure in the range of 65 inches WEG at the inlet of the nozzles. On the other hand, fluidizing air that is injected through nozzles into seal pots and FBHEs of conventional construction requires higher pressures in the range of 200 inches WG at the inlet of such nozzles. Such higher pressure is required as a direct result of the greater amount of ash that is present in terms of the height required in the seal pot and in the FBHS as compared to the height in the furnace.
In accordance with other preferred aspects of the present invention, the air that is injected fluidizes the now cooled ash particles, which have been collected, and transports these now fluidized cooled ash particles through the discharge opening of the MBHE. A discharge pipe suitably configured so as to be operative to direct the transported now fluidized cooled ash particles through the discharge opening of the MBHE may be provided. Beneficially, such a discharge pipe will have an inlet disposed within the lower portion of the vessel of the MBHE at a distance that is located above the floor of said lower portion of the vessel of the MBHE. Said inlet could, for example, be located 12 inches above the floor of the lower portion of the vessel of the MBHE, although this may vary depending on the implementation without departing from the essence of the present invention. If such a discharge pipe is provided, the now fluidized cooled ash particles are accordingly transported into the inlet of said discharge pipe and from there through the discharge opening of the vessel of the MBHE.
According to yet another aspect of the present invention, a hood is preferably disposed within the lower portion of the vessel of the MBHE at a distance that is located above the inlet of the aforereferenced discharge pipe. This hood is suitably configured so as to be operative to support the weight of the ash that is above the hood and so as to as well direct the transported ash particles that are below the hood into the inlet of the aforereferenced discharge pipe.
In accordance with still other aspects of the present invention, the above described upper, intermediate and lower portions of the vessel of the MBHE form a first compartment of the vessel of the MBHE, and said vessel also includes a second compartment that includes another separate feed opening and another floor having another separate discharge opening. Said vessel receives other ash particles, which are also hot, via the other feed opening thereof. Said vessel is also further configured so as to be operative to direct a gravity flow of the hot other ash particles received thereby to the floor of the second compartment thereof and so as to be operative as well to collect said hot other ash particles on this other floor thereof. A plurality of other air inlets preferably is also provided. Said plurality of other air inlets, which will typically also be in the form of air nozzles, are suitably configured so as to be operative to inject air into the second compartment of the vessel of the MBHE in order to thereby control the amount of the hot other ash particles, which are collected and discharged through the other discharge opening of the vessel of the MBHE. Thus, both cooled particles from one compartment and hot particles from the other compartment can be discharged, e.g., for recycling to the furnace of a circulating fluidized bed boiler.
Beneficially, the amount of hot other ash particles, which are collected and discharged through the other discharge opening of the vessel of the MBHE, is controlled such that the amount of the hot other ash particles collected on the floor of the second compartment of the vessel of the MBHE is sufficient to seal the second compartment of the vessel of the MBHE against a flow of an external gas through the discharge opening of the vessel of the MHE into the second compartment of the vessel of the MBHE. Accordingly, the present invention can be implemented to provide a MBHE and a seal pot unit, which are integrated.
In
In addition, with further reference to
Continuing a plenum 105, as illustrated in
The flue gas and ash generated in the furnace of the circulating fluidized bed boiler 100 are exhausted from the furnace of the circulating fluidized bed boiler 100 via a conveying line 125. As is well understood, the flue gas serves as a carrier and transports the ash entrained therewith from the furnace of the circulating fluidized bed boiler 100.
A cyclone 130 is employed to separate from the flue gas the ash that is entrained therewith. From the cyclone 130, the flue gas, which is now substantially free of the ash previously entrained therewith, is transported via a conveying line 135 preferably to any downstream processing equipment, e.g., heat exchangers, air pollution control (APC) equipment, and thereafter ultimately to an exhaust stack.
The ash after being separated from the flue gas in the cyclone 130 is directed from the cyclone 130 to a moving bed heat exchanger (MBHE) 155 via a first path 140 and then to a seal pot 165 via a second path 145. As best understood with reference to
In
With further reference to
The MBHE 155 is suitably sized, shaped and/or has structural components (not shown in the interest of maintaining clarity of illustration in the drawings) so as to be operative to direct the moving bed of hot ash particles 140 from the upper portion 200 thereof to the immediate portion 205 thereof of the MBHE 155. The intermediate portion 205 includes a heat exchanger 215 typically consisting of boiler pressure parts. These pressure parts preferably include a bundle of finned tubes (not shown in the interest of maintaining clarity of illustration in the drawings) through which a working fluid, generally in the form of steam and/or of water, flows. This working fluid serves as a coolant, and is used to recover heat from the moving bed of hot ash particles 140 as the hot ash particles 140 are made to flow through the heat exchanger 215.
The bundle of finned tubes of the heat exchanger 215 are preferably oriented such that the flow of the working fluid therethrough is substantially orthogonal to the gravity flow of the moving bed of hot ash particles through the heat exchanger 215. The fins beneficially extend from the tubes in a direction that is substantially parallel to the direction of flow of the moving bed of hot ash particles. After passing through the heat exchanger 215, the cooled ash particles denoted in
As best understood with reference to
As best seen with reference to
By controlling the injection of air 240 into the MBHE 155, the amount of collected cooled ash particles 252 that are discharged through the discharge openings 220 in the MBHE 155 can be controlled. Similarly, by controlling the injection of air 240′ into the seal pot 165, the amount of the collected hot ash particles 260 that are discharged through the discharge openings 220′ can also be controlled. By controlling the injection of low pressure air 240 to the MBHE 155, the amount of heat transferred from the hot ash particles 140 to the working fluid flowing in the heat exchanger 215 can also be controlled. That is, the amount of heat transferred from the hot ash particles 140 to the working fluid will correspond to the amount of collected cooled ash particles 250 that are discharged through the discharge openings 220. This control is preferably effected based on the temperature of the gas in the furnace of the circulating fluidized bed boiler 100 or the steam/water temperature in the MBHE 155, but could equally well be based on other furnace related parameters without departing from the essence of the present invention.
In summary, the integrated MBHE and seal pot unit 150 can be used to control the combustion temperature in the furnace of the circulating fluidized bed boiler 100. Since the ash moves through the MBHE 155 and across the heat exchanger 215 in a gravity flow, the injection of high pressure air in order to thereby transport the ash and induce the heat transfer is not required. Thus, there is no requirement in accordance with the present invention for employing any high pressure fluidizing blowers. As a result, this significantly reduces not only material cost but also power consumption. The counter current flow of the moving bed of ash vertically downward in the MBHE 155 results in higher log mean temperature difference (LMTD), which contributes to higher heat transfer rates in the MBHE 155 and thus reduced heat exchanger surface requirements. Furthermore, because the MBHE 155 is capable of utilizing a plurality of finned tubes that embody a high fin density without hindering the flow of ash therethrough, the heat transfer surface can be arranged in a very compact design. The extended surface resulting from the use of a plurality of tubes that embody high density fins coupled with the high LMTD, renders it possible to realize as a consequence thereof significant reductions in pressure part surfaces and refractory compared with that which is necessary when fluidized bed heat exchangers (FBHEs) of conventional construction are being employed. Furthermore, because the rate of ash flow is controlled in the MBHE 155 by means of the controlling of the discharge of ash downstream of the heat exchanger, there is no need for an ash control valve to be employed upstream of the seal pot 165 and the MBHE 155. This is in contrast to the need for employing an upstream ash control valve to control the solid flow in FBHEs that embody a conventional construction.
The air, which is injected into the MBHE 155 and the seal pot 165, is controlled in order to thereby cause a specific level, i.e., quantity, of ash to be maintained in the MBHE 155 and the seal pot 165 so as to thus provide the required furnace to cyclone seal. In addition, the injection of air into the MBHE 155 is also controlled in order to thereby control the flow of ash across the heat exchanger 215 so as to thus achieve a specific steam generator parameter, such as, for example, a specific gas or steam temperature within the furnace of the circulating fluidized bed boiler 100. Finally, the injection of air into the MBHE 155 and the seal pot 165 is also controlled in order to thereby maintain an even distribution of the cooled and hot ash particles in the ash return lines 160 and 170 to the furnace of the circulating fluidized bed boiler 100. By virtue of the arranging of the discharge openings in rows and the regulating of the air, which is injected for purposes of effecting the transport of the ash through each row of the discharge openings 220 or 220′, an even ash flow can be thereby ensured across the width of the MBHE 155 and of the seal pot 165 and in each of the return lines 160 and 170 as well. Furthermore, the regulation of the ash discharge from the rows A-F is further operative to promote even coolant temperatures within the tubes of the heat exchanger 215. Moreover, because the MBHE 155 and seal pot 165 are capable of being controlled independently of each other without departing from the essence of the present invention, if such is desired, the MBHE 155 is capable of being operated with the seal pot 165 shut down or visa versa. With the seal pot 165 and MBHE 155 being arranged in parallel relation to each other, large particles, which are discharged from the cyclone 130 can without departing from the essence of the present invention, if such is desired, be channeled away from the MBHE 155 for purposes of being discharged out through the seal pot 165.
In
Air denoted in the drawings by the reference numeral 475 from a suitable source thereof (not shown in the interest of maintaining clarity of illustration in the drawings) is fed via a duct 405 to the plenum 235 or 235′ which is operative to distribute such air, which in turn effects the feed thereof to a manifold 412, in the case of the MBHE 155, or to the manifold 412′, in the case of the seal pot 165. From the manifold 412 or 412′ such air is distributed to the individual low pressure air nozzles 415, in the case of the MBHE 155, and to the lower pressure air nozzles 415′, in the case of the seal pot 165, for injection thereafter into the MBHE 155 or the seal pot 165, as applicable. The flow of air via the duct 405 to the plenum 235 or 235′ is controlled, in accordance with the preferred embodiment of the present invention, by a variable air flow valve 410 in response to instructions received thereby from the controller 450. The controller 450 is operative to effect the control of a separate variable air flow control valve 410 that is associated with each controller 450. All the valves 410 may, if such is desired, be controlled without departing from the essence of the present invention by a single controller 450.
In
In operation, a small amount of low pressure air 240 or 240′ is injected to control the solids within the MBHE 155 and the seal pot 165. To this end, the pressure of the injected air is much lower than the surrounding pressure of the solids.
The pressure of the solids on the floor of the compartment that defines the MBHE 155 and on the floor of the compartment that defines the seal pot 165, corresponds to the height of the solids in the respective one of the aforementioned compartments. In most cases, the pressure of the solids in such compartments will be well in excess of 200 inches WG. However, the pressure of the air 240 or 240′ injected into the respective compartment need only be a low pressure. Such low pressure air can be provided for this purpose from a primary or secondary air source that is commonly available at circulating fluidized bed boiler plants. For example, such primary air, which is generally so available at a pressure of 65 inches WG, can be utilized as the source of the air 475.
The short height of the discharge pipe 225 or 225′ above the floor surface 275 or 280 effectively enables the height of the bed of collected ash 252 or 260 to be reduced concomitantly, and hence the amount of pressure that is required in order to effect the transport of the solids to the discharge pipe inlet 227 or 227′. The injected air 240 or 240′ is designed to effectively bubble up through the collected ash 252 and 260 and is then deflected by the hood 230 or 230′ into the discharge pipe inlet 227 or 227′, and through the discharge pipe 225 or 225′ into the conveying line 160 or 170. During this process, the low pressure air effects the transport of the ash from the MBHE 155 and/or seal pot 165 to the furnace of the circulating fluidized bed boiler 100. As the ash is so transported from the respective compartment, the bed of ash moves in a downwardly direction thereby promoting a heat transfer therefrom to the working fluid flowing through the tubes of the heat exchanger 215.
In
The controller 450 is capable of controlling the variable air flow valve 410 in order to thereby effect a pulsation of air through the nozzles 415 or 415′ in an on-off sequence. Alternatively, the controller 450 also is capable of controlling the variable air flow valve 410 such that the injectors 415 or 415′ inject a continuous stream of low pressure air at varying flow rates into the respective compartment.
In summary, a non-mechanical control of ash flow across the MBHE 155 and the seal pot 165 is provided utilizing air at a pressure far lower than the surrounding pressure of the ash collected on the respective compartment floor. Because only low pressure air is required, the power usage of the circulating fluidized bed boiler plant can thereby be reduced, and hence the circulating fluidized bed boiler plant can operate at a higher energy efficiency, e.g., a higher plant heat rate. Furthermore, the amount of ash being discharged from the MBHE 155 and the seal pot 165 can be effectively controlled to the desired extent over the full load range of the circulating fluidized bed boiler 100.
As described above, in accordance with the present invention a more efficient and less expensive technique for recycling ash in circulating fluidized bed heat generating systems is provided. This technique to which the present invention is directed beneficially eliminates the need for the relatively high pressure fluidizing air that is required by FBHEs and seal pots, which are of conventional construction, and can reduce not only the expense of the high pressure blowers and fluidizing nozzles that are commonly required therefor, but also the dynamic loading to which the structural steel, which is required for purposes of supporting FBHEs and seal pots that embody a conventional construction, is subjected. The consumption of power conventionally required to operate such blowers in order for them to thereby provide the supply of high pressure air is also eliminated. Additionally, this technique to which the present invention is directed beneficially facilitates higher heat transfer rates in the heat exchanger than those now possible using conventionally constructed FBHEs because of the relatively low log mean temperature difference LMTD of the fluidized ash flow within such conventionally constructed FBHEs.
While a preferred embodiment of our invention has been described and illustrated herein, it will be appreciated that modifications thereof, some of which have been alluded to hereinabove, may still be readily made thereto by those skilled in the art. We, therefore, intend by the appended claims to cover the modifications alluded to herein as well as all the other modifications that fall within the true spirit and scope of our invention.
The present application is related to U.S. application Ser. No. 09/740,356, filed Dec. 18, 2000 and entitled “Recuperative and Conductive Heat Transfer System” (now U.S. Pat. No. 6,554,061, issued on Apr. 29, 2003), U.S. application Ser. No. 10/451,830, filed Oct. 29, 2002 and entitled “Circulating Fluidized Bed Reactor Device” (now U.S. Pat. No. 6,779,492, issued on Aug. 24, 2004), and U.S. application Ser. No. 10/451,769, filed Oct. 29, 2002 and entitled “Centrifugal Separator in Particular for Fluidized Bed Reactor Device” (now U.S. Pat. No. 6,938,780, issued on Sep. 6, 2005), the disclosures of which are incorporated herein in their entirety by reference.
