Abstract:
A method of and an apparatus for operating a circulating fluidized bed reactor having a furnace with a discharge port for exhaust gas, and a particle separator having an inlet connected to the exhaust gas discharge port and an outlet duct for the exhaust gas and a return duct for separated solids. The method includes the steps of arranging a bypass duct bypassing the particle separator, and conducting a partial flow of exhaust gas along the duct for increasing the fly ash content in the exhaust gas after the separator. The bypass duct is advantageously provided with means for controlling the flow of exhaust gas in the bypass duct.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method of and an apparatus for operating a circulating fluidized bed reactor having a separator for separating entrained solid particles from the exhaust gas and recycling the separated particles to the combustion chamber. The invention particularly relates to the composition of the bed material, and seeks to solve problems relating to the control of bed inventory and bottom ash quantity.  
         [0003]     2. Description of the Related Art  
         [0004]     Circulating fluidized bed reactors have been used for decades and are known to those skilled in the art of power generation, for instance. The circulating fluidized bed reactors may be gasifiers, combustors, steam generators, or any other similar type of apparatus as those skilled in the art will recognize. The fluidized bed reactors normally have an upright furnace, or reaction chamber, to the lower part of which the fuel is introduced. Primary and secondary gases, usually air, are supplied through the bottom and the sidewalls of the furnace. The combustion of the fuel takes place in a fast fluidized bed, which , in addition to fuel particles, usually also contains limestone.  
         [0005]     A particle separator is in communication with the upper end of the furnace through a discharge port. A discharge duct connects the discharge port with the particle separator. Hot exhaust gas is discharged from the furnace, and flows through the discharge port and the discharge duct into the particle separator. The particle separator of a circulating fluidized bed boiler is usually a cyclone. When using a cyclone, the discharge duct transmits the exhaust gas with entrained solid particles tangentially into the upper portion of the cyclone separator. The cyclone separator, or other particle separator, for example, an impact separator, separates solid particles from the hot exhaust gas, which solid particles are gravity-fed to the lower end of the separator.  
         [0006]     The lower end of the particle separator, wherein the solid particles are collected, is connected to the upper end of a vertical return duct. The opposite or lower end of the return duct has an outlet connected to the furnace for returning the separated solid particles from the particle separator to the furnace. Solid particles removed from the bottom of the furnace are referred to as bottom ash, whereas the portion of the solid material leaving the particle separator with the exhaust gases is called fly ash.  
         [0007]     Circulating fluidized bed boilers having particle separators, for example, of the cyclone type, for separating entrained solid particles from exhaust gas and recycling the separated particles back to the combustion chamber of the boiler are well known. Examples of such systems are set forth in U.S. Pat. Nos. 4,733,621 and 5,281,398. In the former patent, the particles separated in the cyclone separator are recycled to the boiler through a split loop seal. U.S. Pat. No. 5,281,398 discloses a centrifugal separator made of flat watertube panels. This type of cyclone can be integrated with the furnace so that there is no discharge duct between the furnace and the cyclone.  
         [0008]     The cyclone separators for circulating fluidized bed reactors have been improved over the past decades so that they have become very efficient. In normal running conditions, they may separate about 99.9 percent of the solid material leaving the combustion chamber with the exhaust gas. An efficient separation of solid particles from exhaust gases is always a property worth striving for. For instance, the better efficiency the separation has, the higher is the combustion efficiency. However, very high separation efficiency may also bring about some problems or drawbacks in the process. For instance, it may lead to a high bottom ash content compared to fly ash. When the proportion of bottom ash is high, an efficient bottom ash removal is required to maintain the required bed inventory (i.e., the composition of the bed material) in proper condition.  
         [0009]     Since the temperature of the bottom ash is on the order of about 600 to about 900° C., ash coolers are needed to bring the ash temperature down to about 300° C., so that the ash may be safely discharged from the reactor. The more bottom ash that has to be removed from the reactor, the more expensive (i.e., of higher capacity) the equipment for both the discharge and the cooling of the bottom ash that is required.  
         [0010]     Very high separation efficiency may also become a problem, e.g., when the quality of fuel varies in a way leading to the formation of an excessive amount of fine particles in the bed. If the particle separator recirculates a very high proportion of the fine particles, the resulting high fine particle bed inventory may lead, e.g., to a very high heat transfer efficiency in the furnace. If the heat transfer rate exceeds its designed value, the bed has a tendency to cool to a lower temperature, leading, e.g., to increased emissions to the environment.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention provides an improved method of and an apparatus for operating a circulating fluidized bed reactor.  
         [0012]     According to one aspect, the present invention provides a method of operating a circulating fluidized bed reactor having a firnace with a discharge port for exhaust gas, a particle separator connected to the discharge port and having an outlet for the exhaust gas and a return duct for the separated solids. The method comprises the steps of arranging a bypass duct bypassing the particle separator, and conducting a partial flow of exhaust gas along the duct for increasing the fly ash content in the exhaust gas after the separator.  
         [0013]     In accordance with this method, the exhaust gas stream bypassing the particle separator decreases the amount of solid particles separated by the separator, whereby the solids inventory and the accumulation of bottom ash in the furnace are decreased.  
         [0014]     According to another aspect, the present invention, provides an apparatus enabling the adjustment of the bed inventory such that the composition of the bed material may be kept in an optimal condition. Preferably, such an apparatus includes a furnace, a discharge port for removing exhaust gas with entrained solid particles from the furnace, a particle separator (preferably, a cyclone separator) connected to the discharge port for separating solid particles from the exhaust gas, the particle separator having an outlet for the exhaust gas connected to an exhaust gas duct and a solids outlet connected to a return duct for recycling the separated solid material back to the bottom of said furnace, and conducting a portion of the exhaust gas past the particle separator for decreasing the quantity of solid material entering the separator.  
         [0015]     The conducting means advantageously comprises a bypass duct, having its first end connected upstream of the particle separator, and its second end connected to the exhaust gas duct downstream of the particle separator.  
         [0016]     In a preferred embodiment of the present invention, the first end of the bypass duct is connected to the top of the furnace.  
         [0017]     In another preferred embodiment of the present invention, the first end of the bypass duct is connected to the discharge duct between the top of the furnace and the particle separator.  
         [0018]     Accordingly, the present invention advantageously provides a novel and an improved method and apparatus for adjusting the composition of the bed material so that the amount of bottom ash is maintained within acceptable limits.  
         [0019]     The present invention brings about numerous advantages in addition to the already mentioned smaller and less expensive bottom ash treatment apparatus. For instance, it provides operational flexibility to the furnace and, thus, allows changing the bottom ash and fly ash proportions, it is easier to make changes in the fuel, the heat loss is less, and it may be used for controlling the temperature and/or heat transfer in the furnace.  
         [0020]     The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the currently preferred, but nonetheless illustrative, embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a schematic, side elevational view of a circulating fluidized bed reactor illustrating the manner in which the exhaust gas is treated in prior art combustion processes.  
         [0022]      FIG. 2  is a schematic, side elevational view of the upper portion of a circulating fluidized bed reactor illustrating a preferred embodiment of the present invention.  
         [0023]      FIG. 3  is a schematic, side elevational view of the upper portion of a circulating fluidized bed reactor illustrating another preferred embodiment of the present invention.  
         [0024]      FIG. 4  is a schematic, side elevational view of the upper portion of a circulating fluidized bed reactor illustrating yet another preferred embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Referring now in greater detail to the drawings, in  FIG. 1 , the overall schematic of a typical circulating fluidized bed reactor system  10  is shown. Particulate fuel, inert bed material, and possible auxiliary material, such as limestone, are introduced to the furnace  12  of the reactor system  10  by solid material feeders  14 , such as screw feeders or pneumatic feeders. The solid materials form a bed, which is fluidized by primary gas  16  introduced through a bottom grid  18 . In a circulating fluidized bed, the velocity of the fluidizing gas in the furnace is typically from about 4 m/s to about 9 m/s. The reactions, such as combustion, of the fuel are completed by secondary gas  20  introduced through the sidewalls  22  of the furnace  12 .  
         [0026]     The reactions in the furnace  12  produce gases, such as flue gases, which are discharged together with particles entrained with the gases from the furnace  12  through a discharge port  24  to a discharge duct  26 , and further to a particle separator  28 . In the particle separator  28 , which is usually a cyclone separator, most (for example 99.9%) of the particles entrained with the exhaust gases are separated from the exhaust gases. The separated particles are conducted along a return duct  30  connected to the bottom of the separator  28  via a loop seal  32  back to the lower portion of the furnace  12 .  
         [0027]     Cleaned exhaust gases are discharged from the particle separator  28  through a central gas outlet  34 , usually arranged at the top of the separator, to an exhaust gas duct  36 . In the exhaust gas duct  36 , the gases are usually conducted through a heat recovery area  38  and a dust separator  40  to a stack  42 . The exhaust gas duct  36  may comprise further components, such as gas cleaning components, or the like, which are known to those skilled in the art, but such are not shown in  FIG. 1 .  
         [0028]     A portion of the solid particles discharged from the furnace  12  through the discharge port  24 —so-called fly ash—is not separated from the exhaust gases in the particle separator  28 , but escapes through the gas outlet  34 . A portion of the fly ash may be collected in a hopper  44  arranged in the exhaust gas duct  36 , but most of it is collected by the dust separator  40 . The portion of the solid material in the furnace  12  that does not escape through the gas outlet  34 , is eventually discharged from the furnace as bottom ash  46 . While the bottom ash is usually at a temperature of about 650 to about 850° C., it is cooled by a bottom ash cooler  48  to a lower temperature (e.g., about 300° C.), before it is discharged from the reactor  10 .  
         [0029]     In the first preferred embodiment of the present invention, schematically shown in  FIG. 2 , the furnace  12 , the particle separator  28 , the discharge duct  26  therebetween, the return duct  30 , the gas outlet  34 , and the upstream part of the exhaust gas duct  36  have been shown just like in  FIG. 1 .  FIG. 2  also shows a bypass duct  50  coupled between the top of the furnace  12  and the exhaust gas duct  36 . Due to the pressure difference between the furnace  12  and the exhaust gas duct  36 , a stream of gas and entrained fine solids tends to flow through the duct  50 , thus bypassing the particle separator  28 .  
         [0030]     In the embodiment shown in  FIG. 2 , the top of the furnace  12  is provided with another outlet opening  52  connected to the first end of the bypass duct  50 . As another alternative, the first end of the bypass duct  50  may be connected to the same outlet  24  with the discharge duct  26  by, for instance, a branch pipe. As yet another alternative, the first end of the bypass duct  50  may be connected to the discharge duct  26  somewhere between the outlet opening  24  and the inlet into the particle separator  28 .  
         [0031]     Regardless of the exact position and structure of the bypass duct  50 , the purpose of the bypass duct  50  is to receive a portion of the exhaust gases, and some solid particles entrained with the exhaust gases, from the furnace  12  and to take the received portion of the exhaust gas to the exhaust gas duct  36  downstream of the particle separator  28 . By doing this, a portion of the solid particles are positively taken out of the fluidized bed circulation, and not returned back to the furnace  12 . Thereby, the amount of fly ash, collected in the hopper  44  and the dust separator  40 , is increased. Correspondingly, the amount of bed material circulating in the furnace  12  and the particle separator  28  is decreased. Eventually, also the amount of bottom ash  46 , to be discharged from the bottom of the furnace  12 , is decreased. In this embodiment, the sizing and geometry of the bypass duct  50  determine the quantity of solids taken to the cyclone separator outlet stream.  
         [0032]     Another preferred embodiment of the present invention is shown in  FIG. 3 . In this embodiment, the bypass duct  50  is provided with an additional means for controlling flow of the exhaust gas in the bypass duct  50 . In this embodiment, the controlling means comprises gas piping  54  equipped with a control damper  56 , such as a butterfly valve. The additional gas piping  54  is used for introducing gas, such as air, to the bypass duct  50  so as to decrease the amount of exhaust gases and solid particles flowing through the bypass duct  50  from the furnace  12  to the exhaust gas duct  36 . By using the damper  56 , the amount of introduced gas, and the amount of bypassing gas and particles, can be adjusted. When more gas is introduced via the gas piping  54 , less exhaust gases and solid particles bypass the particle separator  28 . The medium through piping  54  can be, for example, air or recirculated flue gas.  
         [0033]     Yet another preferred embodiment of the present invention is shown in  FIG. 4 . In this embodiment, the controlling means comprises a control valve  58  installed directly in the bypass duct  50 . This embodiment gives the broadest possible flexibility, because, as the exhaust gas flows in, the bypass duct can be adjusted between totally blocked and fully open positions. Other suitable controlling means may include a passage and a port allowing exhaust gas and entrained particles to enter the flue gas channel upstream of the particle separator.  
         [0034]     As will be readily apparent to those skilled in the art, the first end of the bypass duct could also be connected in various different ways upstream of the particle separator  28  in the embodiments of  FIGS. 3 and 4 , as was described above with reference to the embodiment shown in  FIG. 2 .  
         [0035]     The result in the second and third embodiments, compared to the first one, is that the composition of the bed material can be better controlled. That is, the amount and particle size distribution of the bed material can be adjusted to better meet the demands of the fluidized bed process.  
         [0036]     The bypass duct  50  may be manufactured of refractory lined pipes or conduits, or they may be pipes or components lined with an appropriate metal and/or ceramic material. It is self-evident that the lining has to endure both high temperature and high velocity of the solids. Suitable lining materials will be readily apparent to those skilled in the art.  
         [0037]     While the invention has been described by way of examples of what are at present considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of the features and applications included within the scope of the invention, as defined in the appended claims.