Abstract:
A reagent drying system for use with a steam generation system [ 25 ] is described having a combustion chamber that produces exhaust flue gasses [FG 2 ]. A preheater [ 150 ] receives the exhaust flue gasses [FG 1 ] and transfers heat to create a heated input air stream [A 2 ] and a diverted air stream [A 2 ′]. The heated input air stream [A 2 ] is provided to the combustion chamber. The diverted air stream [A 2 ′] is provided to a dryer [ 196 ] as incremental air stream [IA]. Dryer [ 196 ] dries bulk reagents for dry milling into powder. The powder is then used to process the exhaust flue gasses to remove pollutants. The incremental air stream [IA] may also include leakage gasses [ 360 ] from preheater [ 150].

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. patent Application “Economical Use of Air Preheat” by Glenn D. Mattison and incorporates this patent application by reference as if set forth in its entirety herein. The Mattison patent application is being filed on the same day as the present patent application and both applications have the same owner. 
       FIELD OF THE INVENTION 
       [0002]    The present invention is directed to a system for capturing additional heat from flue gas output. More particularly, the present invention is directed to a system for capturing additional heat from flue gas for drying of reagents used in flue gas desulphurization operations. 
       BACKGROUND 
       [0003]    Many power generation systems are powered by steam generated via coal or oil fired boilers. These power generation systems will often incorporate an exhaust processing and heat recovery system (EPHRS) to reduce flue gas emissions and/or recover heat energy expelled via the flue gas stream from the boiler. 
         [0004]    A typical power generation system is generally depicted in the diagram shown as  FIG. 1 .  FIG. 1  shows a power generation system  10  that includes a steam generation system  25  and an exhaust processing and heat recovery system (EPHRS)  15  and an exhaust stack  90 . The steam generation system  25  includes a boiler  26 . The EPRS  15  includes an air preheater  50 , a particulate removal system  70  and a wet scrubber system  80 . A forced draft (FD) fan  60  is provided to introduce air into the cold side of the air preheater  50 . The particulate removal system  70  may be, for example, an electrostatic precipitator (ESP), a fabric filter system (Bag House) or the like. 
         [0005]    The air preheater  50  is a device designed to heat air before it is introduced to another process, such as combustion in the combustion chamber of a boiler  26 . The air preheater  50  receives air input A 1 , heats it, and provides it as air stream A 2  to the boiler. This is done by recovering heat expelled from the combustion chamber of the boiler  26  via the flue gas stream FG 1 . By recovering heat from the flue gas FG 1 , the thermal efficiency of the boiler  26  can be increased and the amount of heat lost is reduced. 
         [0006]    Rotary regenerative air preheaters generally exhibit air leakage that results in an increased flow of gasses to downstream gas treatment devices. If this leakage is recovered, its heat may be used for beneficial purposes. 
         [0007]    EPHR  15  is commonly configured to include a wet flue gas desulfurization system (WFGD), shown here as wet scrubber  80 , which reduces sulfur dioxide (SO 2 ) emissions that lead to acid rain. These require the use of milled limestone. Wet milling equipment, such as a wet mill  97 , is used to reduce the particle size of limestone and/or other reagents to a desired level of fineness. The milled reagents are mixed with additional water in a storage, mixing and injection tank  85  to produce a slurry. The mixed slurry is stored until it is injected into the wet scrubber  80  to neutralize and capture the SO 2 . 
         [0008]    Grinding dried solids, such as dried limestone, uses dry milling equipment that consumes significantly less energy than wet milling equipment. 
         [0009]    In order for dry grinding operations to take place, the moisture content of the solids must be below a specified level. Typically, this is accomplished by using a heated air dryer  96  fired by a fossil fuel burner  94  to evaporate excess moisture from the reagent as shown in  FIG. 2 . The dried reagents are then milled in a dry mill  98 . The drying process requires a significant amount of additional energy to operate. 
         [0010]    Elements of the system shown in  FIG. 2  function in the same manner as described for  FIG. 1  having the same reference numbers. 
         [0011]    Thus, an unaddressed need exists in the industry to provide a more efficient method of providing milled reagents for flue gas processing. 
       SUMMARY OF THE INVENTION 
       [0012]    Embodiments of the present invention provide a reagent drying system for power generation systems that captures additional heat from a flue gas stream. Briefly described, in architecture, one embodiment of the system, among others, can be implemented as follows. A reagent drying system for use with a steam generation system [ 25 ] having a dryer [ 196 ] configured to receive said incremental air stream [A 2 ′] from an air preheater [ 150 ] to dry the bulk reagents. The air preheater [ 150 ] is preferably a rotary regenerative air preheater. 
         [0013]    The air preheater [ 150 ] is adapted to provide excess heated air [A 2 ′], being the additional air above the amount that said steam generation system [ 25 ] can use, the excess heated air [A 2 ′] and the leakage gasses [ 360 ] being at least part of the incremental air stream [IA] that is provided to the dryer [ 196 ]. 
         [0014]    The dried reagent is then allowed to be milled into powder by dry milling equipment, which requires significantly less energy as compared with wet milling equipment. 
         [0015]    Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which: 
           [0017]      FIG. 1  is a schematic block diagram depicting a prior art power generation system employing wet milling equipment. 
           [0018]      FIG. 2  is a schematic block diagram depicting a prior art power generation system employing dry milling equipment. 
           [0019]      FIG. 3  is a schematic block diagram depicting a power generation system employing a reagent drying system and dry milling equipment according to one embodiment of the present invention. 
           [0020]      FIG. 4  is another schematic block diagram depicting a power generation system employing a reagent drying system and dry milling equipment according to another embodiment of the present invention. 
           [0021]      FIG. 5  is a schematic diagram depicting the capture of heated leakage air from air preheater. 
           [0022]      FIG. 6  is another schematic block diagram depicting a power generation system employing a reagent drying system, dry milling equipment and a recuperative heat capture and transfer (RHCT) system according to another embodiment of the present invention. 
           [0023]      FIG. 7  is an enlarged schematic diagram depicting an embodiment of the RHCT system of  FIG. 6 . 
           [0024]      FIG. 8  is another schematic block diagram depicting a power generation system employing a reagent drying system, dry milling equipment and a dry scrubber according to another embodiment of the present invention. 
           [0025]      FIG. 9  is another schematic block diagram depicting a power generation system employing a reagent drying system, dry milling equipment and a dry scrubber according to another embodiment of the present invention. 
           [0026]      FIG. 10  is another schematic block diagram depicting a power generation system employing a reagent drying system, dry milling equipment, a dry scrubber and an RHCT according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0027]      FIG. 3  is a schematic block diagram depicting a power generation system  100  employing a reagent drying system and dry milling equipment according to one embodiment of the present invention. 
         [0028]    The present invention is directed to providing excess heat from the air preheater  150  to reagent drying operations. Excess heat may generally be defined as thermal energy that exceeds the thermal needs of the steam generation system  25 . By using excess heat from the air preheater to conduct reagent drying operations, it is possible to reduce, if not completely eliminate, the need (and thus, expense) for separate gas fired burners ( 94  of  FIG. 2 ) used to dry reagents prior to milling operations. 
         [0029]    In this embodiment a power generation system  100  is provided that includes a steam generation system  25 , an exhaust processing and heat recovery system (EPHRS)  115  and an exhaust stack  90 . In this embodiment, an incremental air stream IA is provided to reagent dryer  196 . Incremental air IA here is diverted air stream A 2 ′ that is a portion of the heated air stream A 2  expelled from the air preheater  150 . Diverted air stream A 2 ′ may be provided by diverting a portion of the airstream A 2  via use of a suitable damper type device (not shown) or appropriate ducting (not shown). In turn, the thermal energy from incremental air stream A 2 ′ is used in drying operation performed by the dryer  196 . 
         [0030]    The milled reagents are mixed with additional water in a storage, mixing and injection tank  85  to produce a slurry. The mixed slurry is stored until it is injected into the wet scrubber  80  to neutralize and capture the SO 2 . 
         [0031]    Dry mill  198  functions to mill the reagents into milled reagent of a desired particle size. The clean air from incremental air stream A 2 ′ is exhausted to the atmosphere. Air that needs to be processed is provided to particulate removal system  60  for cleaning. 
         [0032]      FIG. 4  is another schematic block diagram depicting a power generation system  100  employing a reagent drying system and dry milling equipment according to another embodiment of the present invention. As with all figures, the elements with the same reference numbers perform in the same manner. 
         [0033]    Incremental air stream IA is provided to the dryer  196  may be composed of “leakage” gasses  360  or diverted air A 2 ′ (shown in phantom) diverted from the air preheater  150 . By only using leakage gasses  360  from the air preheater  150 , the entire main heated airstream A 2  from the air preheater may be directed to the steam generation system  25 . 
         [0034]    In an alternative embodiment, it is possible that the incremental air stream IA includes at least part of the leakage gasses  360  and the diverted air stream A 2 ′ to both be sent to dryer  196  as incremental air IA. It is also understood that varying amounts of leakage gasses  360  and the diverted air stream A 2 ′ may be also used with the dryer  196  for all subsequent embodiments described in this application. 
         [0035]      FIG. 5  is a schematic diagram depicting the capture of heated leakage gasses  360  from air preheater  150 . Air preheater  150  configured to exhaust leakage air through exhaust conduits  361  from internal plenum  159  within the air preheater  150 . In this embodiment, a leakage outlet  325  is provided. This outlet may be implemented as an opening in the housing  154 , which allows access to the plenum  159 . An exhaust conduit  361  is provided for exhausting gas/air that may accumulate in the internal plenum  159 . A fan device  367  may be provided to allow the leakage gasses  360  to be exhausted from the internal plenum  159  more easily. 
         [0036]    A further leakage outlet may also be provided so that leakage gasses  360  accumulating within the internal plenum  365  may be readily exhausted through another exhaust conduit  363 . Fan  367  also draws the leakage flow from exhaust conduit  363 . However, a separate fan may be employed for each exhaust conduit if so desired or otherwise necessary. 
         [0037]    In an alternative embodiment, a pressure sensor  401  is positioned within the flue gas outlet to measure flue gas pressure (FG 2 ). Another pressure sensor  405  is positioned within exhaust conduit  361  to measure gas pressure there. A logic unit  409  is connected to sensors  401  and  405  and identifies pressure differences. 
         [0038]    A controller  413  is coupled to logic unit  409 , and takes action when the pressure difference exceeds a predetermined amount. Controller is connected to an actuator  417  that opens or closes a valve  421  allowing or restricting the leakage gasses in exhaust conduit  361  from flowing to fan  367  and drier  196 . 
         [0039]    Similarly, a pressure sensor  403  is positioned within the flue gas outlet to measure flue gas pressure (FG 2 ). Another pressure sensor  407  is positioned within exhaust conduit  363  to measure gas pressure there. A logic unit  411  is connected to sensors  403  and  407  and identifies pressure differences. 
         [0040]    A controller  415  is coupled to logic unit  411 , and takes action when the pressure difference exceeds a predetermined amount. Controller  415  is connected to an actuator  419  that opens and closes a valve  423  allowing or restricting the leakage gasses in exhaust conduit  363  from flowing to fan  367  and drier  196 . 
         [0041]      FIG. 6  is another schematic block diagram depicting a power generation system employing a reagent drying system, dry milling equipment, and a recuperative heat capture and transfer (RHCT) system, according to another embodiment of the present invention. 
         [0042]    Air preheater  150  will preferably be a high efficiency air preheater capable of outputting a greater volume of heated air than can be efficiently put to use by the steam generation system  25 . 
         [0043]    RHCT  300  is configured to receive an incremental air stream IA which may be diverted air A 2 ′ from air preheater  150 . RHCT  300  extracts thermal energy from incremental air stream IA. Diverted air stream A 2 ′ is a portion of the heated air stream A 2  expelled from the air preheater  150 . Diverted air stream A 2 ′ may be provided by diverting a portion of the air stream A 2  via use of a suitable damper or ducting system (not shown). In turn, the thermal energy extracted from incremental air stream IA is transferred to a heated air stream HA 1  and introduced to dryer  196 . 
         [0044]    Alternatively, leakage gasses  360  from exhaust conduits  361 ,  363  may also be used as the incremental airstream IA. 
         [0045]    RHCT  300  is configured so as to transfer thermal energy from the incremental air stream IA to heated air stream HA 1  without introducing any contaminates that may be contained in air stream A 2 /A 2 ′ or the leakage gasses  360 . 
         [0046]    Since no flue gas is used by the RHCT  300  to heat the heated airstream HA 1 , the RHCT  300  is not subjected to particulate matter that is often found in the flue gas streams. 
         [0047]    The present invention is applicable to embodiments having an air preheater  150  that have leakage gasses  360 . The leakage gasses  360  may be collected and fed via exhaust conduits  361 ,  363  to fan  367 . Even though this is not specifically shown on some of the embodiments, it is assumed that this general feature may be used on other embodiments. 
         [0048]      FIG. 7  is an enlarged schematic diagram depicting an embodiment of the RHCT system of  FIG. 6 . In this embodiment, the RHCT  300  includes heat exchanger  310 . Heat exchanger  310  is preferably configured to receive the diverted air A 2 ′ from the air preheater  150 . It may also be configured to receive leakage gasses  360  from the air preheater  150 . 
         [0049]    Since the RHCT  300  is not subjected to the particulate matter typically found in the flue gas streams, it is possible for the heat exchange elements (not shown) used in the heat exchanger  310  to be placed in much closer proximity to each other and thereby provide for more surface area available to contact the incremental air stream IA. In this way, the efficiency of the heat exchanger  310  can be significantly enhanced since the greater the surface area of the heat exchange elements that is provided, the more heat that can be captured for a given volume. Further, since the heat exchange elements are not subjected to much particulate matter, the threat of blockage due to accumulations of particulate matter in the heat exchanger  310  is greatly reduced, if not completely avoided. This reduces the amount of normal maintenance required. 
         [0050]      FIG. 8  is another schematic block diagram depicting a power generation system  100  employing a reagent drying system and dry milling equipment according to another embodiment of the present invention. 
         [0051]    This embodiment shares many of the elements of the embodiment shown in  FIG. 3 . Elements with the same numbers perform the same functions. However, in this embodiment, a dry scrubber  180  is used in place of the wet scrubber  80  of  FIGS. 1-4 . This eliminates the need for the storage, mixing tank  85  since aqueous solutions are not used as they are in the wet scrubbers  80 . 
         [0052]    Dry powder reagents are sprayed into flue gasses FG 2  in dry scrubber  180 . The powder is distributed as evenly as possible within the flue gas to react with the pollutant gasses in flue gas FG 1 . 
         [0053]    Since dry scrubber  180  employs powders that are sprayed into the flue gasses, it is important to collect the powder prior to exhausting the flue gasses. Therefore, dry scrubber  180  is positioned before the particulate removal system  70  that collects the particulate matter and separates out the gasses that are released through stack  90 . 
         [0054]    In an alternative embodiment, the dry scrubber may be injection lances feeding powder into a conduit. 
         [0055]    These injection lances and/or dry scrubber  180  may also be located between the steam generator system  25  and the air preheater  150  to process the flue gasses FG 1 . 
         [0056]      FIG. 9  is another schematic block diagram depicting a power generation system  100  employing a reagent drying system and dry milling equipment according to another embodiment of the present invention. 
         [0057]    This embodiment, shares many of the elements of the embodiment shown in  FIG. 4 , which perform the same functions here. However, a dry scrubber  180  is used in place of the wet scrubber  80  of  FIGS. 1-4 . As described above, this embodiment employs dry powder reagents to process the flue gasses FG 2  in dry scrubber  180 . 
         [0058]    The dry scrubber  180  is positioned before the particulate removal system  70  that collects the particulate matter, and separates out the gasses that are released through stack  90 . Again, in an alternative embodiment, the dry scrubber  180  may be positioned to process flue gasses FG 1 . 
         [0059]    Please note that in the embodiments of  FIGS. 3 ,  4 ,  6 ,  8 ,  9  and  10  the function of drying the reagents performed by dryer  196  prior to milling, may alternatively be performed in the dry mill  198 . This effectively would equate to merging the functionality of the dryer  196  and dry mill  198  into a single element. 
         [0060]    Please note that this invention is also applicable to other types of air preheaters. For example the scope of this invention covers its use with tri-sector and quad-sector air preheaters commonly known in the industry. A bi-sector air preheater has one duct for receiving hot flue gasses and transfers the heat to one air intake duct. 
         [0061]    A tri-sector air preheater has one duct for receiving hot flue gasses and transfers heat to one primary air intake duct and one secondary air intake duct. 
         [0062]    A quad-sector air preheater has one duct for receiving hot flue gasses and transfers heat to one primary air intake duct and two secondary air intake ducts. The primary intake duct typically sandwiched between the secondary ducts. 
         [0063]    It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.