Patent Publication Number: US-2017356642-A1

Title: Circulating fluidized bed boiler with bottom-supported in-bed heat exchanger

Description:
RELATED APPLICATION DATA 
     This patent application claims priority to U.S. Provisional Patent Application No. 62/349,627 filed Jun. 13, 2016 and titled “CIRCULATING FLUIDIZED BED BOILER WITH BOTTOM-SUPPORTED IN-BED HEAT EXCHANGER.” The complete text of this patent application is hereby incorporated by reference as though fully set forth herein in its entirety. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure generally relates to the field of circulating fluidized bed (CFB) reactors or boilers such as those used in electric power generation facilities and, in particular, to a new and useful CFB reactor arrangement which permits temperature control within the CFB reaction chamber and/or of the effluent solids with an in-bed heat exchanger (IBHX). The CFB reactor arrangement provides a bottom-supported IBHX wherein the enclosure that defines the IBHX is supported from the dormant solids hoppers for the CFB and bubbling fluidized bed (BFB) of the IBHX. 
     2. Background Information 
     Circulating fluidized bed (CFB) reactors or boilers are used in the production of steam for industrial processes and electric power generation; see, for example, U.S. Pat. Nos. 5,799,593, 4,992,085, 4,891,052, 5,343,830, 5,378,253, 5,435,820, and 5,809,940. For an overview of the design and operation of CFB boilers, see  Steam/its generation and use,  42nd Edition, edited by G. L. Tomei, Copyright 2015, The Babcock &amp; Wilcox Company, ISBN 978-0-9634570-2-8, the text of which is hereby incorporated by reference as though fully set forth herein. 
     In a CFB boiler, upward gas flow carries reacting and non-reacting solids to an outlet at the upper portion of the furnace where the solids are separated from the gas, often by a staggered array of impact-type particle separators. The solids are used within the combustion process to transfer heat from the chemical process to the boiler water-cooled enclosure walls and other heating surfaces. The solids thus help control the overall furnace temperature that results in reducing NOx and SO 2 . The bulk of the solids reaching the top of the furnace are collected and returned to the furnace bottom. 
     U.S. Pat. No. 6,532,905 discloses a controllable solids heat exchanger called an in-bed heat exchanger (IBHX). The heat exchanger is immersed within a bubbling fluidized bed (BFB). Heat transfer in the heat exchanger is controlled by controlling the rate of solids discharge from the lower part of the BFB into the furnace. The discharge control is accomplished using at least one non-mechanical valve being operated by controlling flow rate of fluidizing gas in the vicinity of the non-mechanical valve. Reducing or completely shutting off fluidizing gas flow to the controlling fluidizing device (typically, a plurality of bubble caps are used to distribute the fluidizing gas) hampers local fluidization and, correspondingly, slows down solids movement through the non-mechanical valve thus allowing the control of the solids discharge from the BFB to the CFB. U.S. Pat. No. 8,434,430 discloses an example of a controllable non-mechanical valve for an IBHX in FIG. 3 of the patent. 
     An undesired drawback of reducing the flow rate of the fluidizing gas in the vicinity of the non-mechanical valve is bed material agglomeration. The decrease of the local fluidizing velocity and corresponding reduction of the bed mixing (while combustion takes place) can result in a local bed temperature rise sufficient for bed material agglomeration. Solids agglomeration may also happen elsewhere in the bed of the IBHX because generally lower fluidizing velocity in the BFB (compared to CFB) results in less vigorous mixing and thus higher potential for temperature and chemical non-uniformity leading to forming agglomerates. To be discharged from the IBHX through a dedicated drain opening, the agglomerates have to be moved towards this opening by the solids discharge flow. If the flow is not sufficient to move the agglomerates, they will eventually accumulate in the IBHX rendering its inoperable. 
     Using an open bottom design (see  Steam: Its Generation and Use,  41st ed., page 17-3 (2005; The Babcock &amp; Wilcox Company, Barberton, Ohio) allows draining agglomerates from any location of the IBHX thus greatly improving its operation reliability. Using an open bottom design with an IBHX, however, is associated with a substantial weight of bed material in the hopper(s) below the IBHX and corresponding load increase on the boiler support steel. 
     SUMMARY OF THE INVENTION 
     The present disclosure improves reliability of the CFB boiler with IBHX while reducing its cost and widening the range of design options. 
     The disclosure provides a configuration wherein the enclosure of the IBHX is supported from the dormant solids hoppers for CFB and IBHX located under the distribution grids. 
     The disclosure provides a support configuration wherein the membranes between the tubes of the enclosure walls are removed to define loose tubes that extend through the hopper walls to accommodate thermal expansion. 
     The disclosure provides a support configuration wherein a skirt is disposed inside the IBHX hopper to prevent gas leakage from the IBHX hopper to the CFB hopper around the enclosure supports. 
     The disclosure provides a support configuration wherein a secondary gas conduit is supported by the CFB hopper with a secondary gas duct carried by the IBHX enclosure with nozzles to provide secondary gas to the CFB. 
     One embodiment of the invention discloses a circulating fluidized bed (CFB) boiler comprising: a CFB reaction chamber having side walls and an open-bottom grid defining a floor at a lower end of the CFB reaction chamber for providing fluidizing gas into the CFB reaction chamber; at least one bubbling fluidized bed (BFB) located within a lower portion of the CFB reaction chamber and being bound by enclosure walls and the floor of the CFB reaction chamber, with the fluidizing gas feed to the BFB portion of the grid controlled separately from the fluidizing gas feed to the CFB portion of the grid; at least one controllable in-bed heat exchanger (IBHX), the IBHX occupying part of the CFB reaction chamber floor and being surrounded by the enclosure walls of the BFB; bottom-supported hoppers containing dormant solids disposed under the CFB and the BFB; the enclosure walls of the BFB being supported off the bottom-supported hoppers; the enclosure walls of the BFB are of cooled membrane gas-tight design around the perimeter of the BFB, including: at least one top opening for CFB solids influx into the BFB; at least one overflow port for setting the BFB height; at least one underflow port for BFB solids controlled recycle back into the CFB; the gas-tight BFB enclosure extending below the grid to the elevation sufficient for not exceeding a preset percentage of leakage of the fluidizing gas from the BFB into the CFB through the bed of the dormant solids between the aforementioned elevation and the grid; and the tubes of the BFB enclosure below that elevation becoming of a loose design with sufficient flexibility for accommodating differences in thermal expansion of the tubes and the hoppers as the tubes penetrate the walls of the hoppers. 
     Another embodiment of the invention discloses a circulating fluidized bed (CFB) boiler comprising: a CFB reaction chamber having walls and an open-bottom grid defining a floor at a lower end of the CFB reaction chamber for providing fluidizing gas into the CFB reaction chamber; at least one bubbling fluidized bed (BFB) located within a lower portion of the CFB reaction chamber and being bound by enclosure walls and the floor of the CFB reaction chamber, with the fluidizing gas feed to the BFB portion of the grid controlled separately from the fluidizing gas feed to the CFB portion of the grid; at least one controllable in-bed heat exchanger (IBHX), the IBHX occupying part of the CFB reaction chamber floor and being surrounded by the enclosure walls of the BFB; hoppers containing dormant solids disposed under the CFB and the BFB; and the enclosure walls of the BFB being supported off the bottom-supported hoppers. 
     Yet another embodiment of the invention discloses a circulating fluidized bed (CFB) boiler comprising: a CFB reaction chamber having walls and an open-bottom grid defining a floor at a lower end of the CFB reaction chamber for providing fluidizing gas into the CFB reaction chamber; at least one bubbling fluidized bed (BFB) located within a lower portion of the CFB reaction chamber and being bound by enclosure walls and the floor of the CFB reaction chamber, with the fluidizing gas feed to the BFB portion of the grid controlled separately from the fluidizing gas feed to the CFB portion of the grid; the enclosure walls of the BFB are of cooled membrane gas-tight design; at least one controllable in-bed heat exchanger (IBHX), the IBHX occupying part of the CFB reaction chamber floor and being surrounded by the enclosure walls of the BFB; hoppers containing dormant solids disposed under the CFB and the BFB; and the enclosure walls of the BFB being connected to at least one of the bottom-supported hoppers with supports and becoming of a loose design with sufficient flexibility for accommodating differences in thermal expansion of the tubes and the hopper as the tubes penetrate the hopper wall. 
     The preceding non-limiting aspects, as well as others, are more particularly described below. A more complete understanding of the processes and equipment can be obtained by reference to the accompanying drawings, which are not intended to indicate relative size and dimensions of the assemblies or components thereof. In those drawings and the description below, like numeric designations refer to components of like function. Specific terms used in that description are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional side elevation view of a CFB boiler depicting a first exemplary configuration of the disclosure, illustrating a bubbling fluidized bed (BFB) enclosure within the CFB boiler. 
         FIG. 2  is an enlarged view of a portion of the BFB enclosure disposed below the distribution grid of the CFB. 
         FIG. 2A  is a view taken along line  2 A- 2 A of  FIG. 2 . 
         FIG. 3  is a plan view looking down along line  3 - 3  of  FIG. 1 . 
         FIG. 4  is a section view taken along line  4 - 4  of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     As shown in  FIGS. 1-4 , a circulating fluidized bed (CFB) furnace  1  includes walls  2  (including roof  2   a ) and an in-bed heat exchanger (IBHX)  3  immersed in bubbling fluidized bed (BFB)  4 . The circulating fluidized bed of furnace  1  predominantly includes solids made up of the ash of fuel  5 , sulfated sorbent  6  and, in some cases, external inert material  7  fed through at least one of walls  2  and fluidized by fluidizing gas (typically, primary air)  8  supplied through a distribution grid  9  fed from pipes  10 . Dormant solids below grid  9  effectively define a part of the furnace floor. Some solids are entrained by gases resulting from the fuel combustion and move upward (arrows  15 ) eventually reaching a particle separator  16  at the furnace exit. While some of the solids (arrow  17 ) pass separator  16 , the bulk of them (arrow  18 ) are captured and recycled back to the furnace. Those solids along with others (arrow  19 ), falling out of upflow solids stream  15 , feed BFB  4  that is being fluidized by fluidizing gas (typically, air)  22  supplied through a BFB distribution grid  24  fed from pipes  25 . Dormant solids below grid  24  effectively define another part of the furnace floor. The dormant solids under CFB and BFB are contained in hoppers ( 26  and  27 , correspondingly) equipped with outlets for draining solids from CFB and BFB ( 28  and  29 , correspondingly). Pipes  10  and  25  are supported off hoppers  26  and  27 , correspondingly (supports are not shown). 
     BFB  4  is separated from the CFB by an enclosure  30  made of gas-tight cooled membrane panels. Enclosure  30  surrounds the perimeter of BFB  4  but is essentially open from the top allowing solids influx from CFB into BFB (arrow  19 ). Enclosure  30  includes overflow ports (that can be formed as vertical slots connected to top opening  31 ; see  FIG. 3 )  32 , which lowest elevation essentially defines the height of BFB  4 . Enclosure  30  also includes underflow ports  34 . Controlling rate of solids recycle  35  through underflow ports  34  allows controlling the heat duty of IBHX  3 . The rate of solids recycle  35  is controlled by separately controlling (not shown) feed rate of fluidizing medium  22  to BFB plan areas adjacent to underflow ports  34 . 
     The pressure within enclosure  30  equals the pressure outside of it at the elevation of the top of BFB  4 . Due to higher bulk density of BFB compared to CFB, the pressure below that elevation is higher on the BFB side, i.e. within enclosure  30 . The highest pressure differential is at the elevation of the distribution grids ( 9  and  24 , located essentially at the same elevation). Cooled membrane panels  60  are used as stiffeners of enclosure walls  30  providing the rigidity necessary to withstand the pressure differential. The height of panels  60  depends on the amount of heat transfer surface required for the furnace heat duty. They can extend all the way through the furnace roof  2 A or be cut shorter and topped with headers  65 , from which pipes  70  continue up to roof  2 A. The lower ends of panels  60  penetrate through hoppers  27  and terminate with headers  61 . 
     Enclosure  30  is topped with a header  72  that is connected with the outside of the furnace through pipes  74 . If temperature of the cooling medium in enclosure  30  and/or panels  60  differs from that of walls  2 , corresponding penetrations through roof  2 A are equipped with expansion joints  76  and  78 . The lower part of enclosure  30  extends below grid  24 . The weight of enclosure  30  is supported off hoppers  26  and  27 . An exemplary configuration of a supports  79  and  80  for supporting enclosure  30  is depicted in  FIGS. 2 and 2A . Support  79  is welded to the walls of the hoppers  26  and  27  while support  80  is welded to membranes  81 . Horizontal pads  82  and  83  are welded to supports  79  and  80 , respectively. The pads  82  and  83  can slide against each other that allows for independent thermal expansion of enclosure  30  and hoppers  26  and  27 .  FIGS. 2 and 2A  depict one exemplary configuration but other support arrangements can be used to support enclosure  30  from one or both of hoppers  26  and  27 . Below the support elevation, the membranes  81  in the panels forming enclosure  30  terminate, and the resulting configuration of loose tubes  84  provides flexibility to accommodate differences in thermal expansion of tubes  84  and hoppers  26  and  27  as tubes  84  penetrate the walls of hopper  26 . Skirt  86  is attached to the inside of enclosure  30  above support  80  and extends into hopper  27 . Positive pressure in hopper  27  (compared to hopper  26 ) pushes skirt  86  against the wall of hopper  27  creating a seal (along with the resistance of the layer of dormant solids below grid  24 ) that essentially eliminates fluidizing gas leakage between hoppers  26  and  27 . Loose tubes  84  are connected to headers  88  outside hoppers  26  and  27 . 
     IBHX  3  can be supported off platework between hoppers  27  or off enclosure  30  or some combination thereof. IBHX  3  terminates at headers  89 . 
     Enclosure  30  also includes a duct  92  for supplying part of secondary gas (typically, secondary air)  95  through nozzles  98  into the CFB. Nozzles  98  can be formed of enclosure  30  tubes. Another part of secondary gas  95  is supplied through nozzles  99  on walls  2 . The combination of nozzles  98  and  99  allows effective coverage of furnace  1  plan area by secondary gas  95 . One type of nozzle that can be used is disclosed in U.S. Pat. No. 8,622,029, the text of which is hereby incorporated by reference as though fully set forth herein. At certain conditions, e.g. for smaller furnace sizes, it is possible to provide an acceptable secondary gas coverage by using only nozzles  99  on walls  2 . In such a configuration, duct  92  is not required and can be removed. 
     Duct  92  is supplied with secondary gas  95  through a conduit  102  made of membrane panels  104 . As shown in  FIG. 4 , part of the panel  104  between duct  92  and conduit  102  turns into screen  105  to allow a passage for the secondary gas from conduit  102  into duct  92 . Panels  104  at the upper end can terminate at header  72  and/or dedicated headers (not shown). Their lower ends extend downward to essentially the same elevation as where gas-tight BFB enclosure  30  turns into a loose-tube type design. At that elevation conduit  102  made of panels  104  is connected gas-tightly to plate-type conduit  106  that continues to the wall of hopper  26  and penetrates the wall. Conduit  106  is equipped with expansion joints  107  on its both ends for accommodating its thermal expansion versus conduit  102  and hopper  26 . Upon the connection with conduit  106 , membrane panels  104  turn into loose tubes  108 , which configuration allows accommodation of the difference in thermal expansion between tubes  108  and hopper  26  as the tubes penetrate the hopper wall and terminate at header  109 . 
     Furnace walls  2  are supported off top steel  110  and expand downwards. Hoppers  26  and  27  have bottom supports  115  and expand upwards. A pressure seal allowing both expansions is provided by expansion joint  120  around the perimeter of furnace  1 . At certain conditions, e.g. lower furnace height due to high fuel reactivity and/or relaxed combustion efficiency requirements and/or relaxed emissions requirements, etc., the entire boiler can be bottom-supported. This would eliminate the need in expansion joint  120 . 
     The foregoing description has been made with reference to exemplary embodiments. Modifications and alterations of those embodiments will be apparent to one who reads and understands this general description. The present disclosure should be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or equivalents thereof. 
     The relevant portion(s) of any specifically referenced patent and/or published patent application is/are incorporated herein by reference.