Abstract:
A system can quickly cool and de-pressurize a boiler arrangement under non-normal operating conditions such as loss of plant power. A discharge system injects into the furnace a combined stream of steam from a steam discharge system and ambient air, thereby both cooling components of the boiler arrangement and reducing pressure in the steam/water circuit. This reduces or eliminates the additional cost associated with providing extra capacity in a steam drum and/or an independently powered boiler water pump. The system is particularly useful for quickly cooling the U-beams of a circulating fluidized bed boiler during a black plant condition.

Description:
BACKGROUND INFORMATION 
       [0001]    Circulating fluidized bed (CFB) 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,  41st ed., Chapter 17 (2005; The Babcock &amp; Wilcox Company, Barberton, Ohio) and  Steam: Its Generation and Use,  42nd ed., Chapter 17 (2015; The Babcock &amp; Wilcox Company, Barberton, Ohio). 
     
    
       [0002]    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, typically by a staggered array of impact-type particle separators. The gaseous components of the stream navigate around the separator unit, while the entrained solids deflect and return to the furnace bottom. Impact-type particle separators, which generally are not cooled, protect the downstream heating surfaces, such as those of primary and secondary superheaters, from erosion by solid particles. 
         [0003]    While such separators can have a variety of configurations, a common version involves so-called U-beams, individual impingement members having U-shaped cross sections. 
         [0004]    Because of the extremely high temperatures experienced at a furnace outlet, the materials from which particle separator components, such as U-beams, are made must be sufficiently temperature resistant to provide adequate support and resist damage. Some impact-type particle separators are cooled or supported off a cooled structure; see, for example, U.S. Pat. Nos. 6,322,603, 6,454,824, and 6,500,221. 
         [0005]    A representation of a commercially available CFB boiler with impact-type separator is shown in  FIGS. 1, 2 and 2A . Furnace  10  has a gas-tight enclosure  11  suitable for operating with a positive pressure in furnace  10  and a flue gas flow path  15 . Primary air enters furnace  10  through windbox  80  and distribution grid  90  (also known as a distributor plate), and, downstream thereof, secondary air is injected through upper and lower overfire air headers. Fuel and sorbent are fed to the CFB bed through the lower walls of furnace  10 , with ash and spent sorbent being removed through drain pipes in the floor. The primary solids separation system, generally designated  30 , includes staggered rows of U-shaped channel members, i.e., primary particle separator U-beams  32  and in-furnace U-beams  34 , suspended from the roof or other pressure parts of the unit. Solids collected by U-beams  32 ,  34  and multi-cyclone dust collector are returned through the rear wall to the lower portion of furnace  10 . 
         [0006]    Situations where a plant loses power, sometimes referred to as a “black plant” condition, call for procedures and equipment which permit boiler pressure to decay and boiler temperature to cool to stable conditions as quickly as practical, all without allowing the water level to drop below the furnace roof. Typically, a main steam stop valve (MSV) closes to prevent rapid pressure reduction in the steam/water side and dropping of the water level in the boiler. Thermal inertia in the drum, tubes, headers and other boiler components continues to generate steam after the MSV closing, however. Opening of a steam relief valve prevents steam pressure buildup, which can trigger a safety valve opening with a corresponding rapid water level drop in the boiler and can provide cooling of superheater surface subjected to residual heat of the uncooled parts of the boiler components, such as a U-beam solids separator. The opened valve allows steam to bleed through the steam side of the superheater into the atmosphere or to the steam user (when the steam is used for heating), typically in a controlled manner. 
         [0007]    Such steam bleed lowers the water level in the boiler circulation system, however. If the water level recedes below the furnace roof, portions of the tubes become uncooled, and the residual heat of the uncooled parts of the solids separator can damage the uncooled tube portions. To prevent such damage by maintaining a safe water level in the boiler, the boiler can be provided with sufficient steam drum capacity and/or an independently powered boiler water pump, also known as a dribble pump. Both of these increase boiler cost, however. 
         [0008]    An attempt to ameliorate these additional costs is described in U.S. Pat. No. 8,047,162, where steam bleed is controllably discharged into the boiler furnace. The steam bleed temperature typically is on the order of 400° to 600° C. (750° to 1100° F.) lower than that of the uncooled parts of the solids separator, so introduction of steam into the solids/gas flow path accelerates the cooling of (potentially) uncooled tubes in the vicinity to a safe temperature (˜500° C.). This solution can reduce the amount of the extra steam drum capacity, but it does not usually altogether eliminate the need for additional capacity nor for the independently powered boiler water pump. 
       SUMMARY 
       [0009]    The processes, systems and equipment described herein can protect power generation devices and their components against thermal damage due to abnormal operating conditions, while reducing or altogether avoiding the additional costs associated with providing steam drums with additional capacity and/or an independently powered boiler water pump. 
         [0010]    Advantageously, the cooling of boiler components, such as U-beams and associated support structures, is accomplished with a simultaneous reduction in boiler pressure. 
         [0011]    In one aspect is provided a CFB boiler arrangement that includes a furnace, a solids separator system, a steam transporting circuit, and at least one secondary air inlet mechanism adapted to introduce into the CFB boiler furnace steam and air when needed (e.g., during abnormal operating conditions such as a black plant trip). The introduced stream of steam and air can accelerate cooling of solids separator system, which in turn can reduce or eliminate the cost of means necessary to prevent damage to uncooled boiler tubes, such as additional drum capacity and/or an independently powered boiler water pump. 
         [0012]    In the CFB boiler arrangement, steam for injection can be obtained from an attemperator inlet header, the steam drum, or any other point in the steam circuit. The steam circuit optionally also can include a pressure reducing station connected to a steam supply line. 
         [0013]    The CFB boiler arrangement optionally can include a dribble pump connected to a steam drum in the steam circuit to maintain water flow to the steam drum, thereby offsetting steam lost from the steam circuit due to injection into the furnace. 
         [0014]    In yet another aspect of the present invention is a circulating fluidized bed boiler arrangement that comprises a furnace with at least one primary air inlet and one or more secondary air inlets, a solids separator system, a steam/water circuit for circulating steam and water, and a steam discharge system, a system for cooling components of said boiler arrangement during abnormal operating conditions comprising: a) at least one of said one or more secondary air inlets comprising a valve which, in an opened conditions, permits ingress of air external to said boiler arrangement; b) conduit for conveying steam out of and away from the steam discharge circuit, said conduit providing ingress to said at least one secondary air inlet; and c) in or associated with said secondary air inlet, an eductor in communication with said steam conveyance conduit, said eductor being capable of outputting a combined stream of said external air and said conveyed steam to said furnace during abnormal operating conditions. 
         [0015]    Also provided is a method for cooling hot components of a boiler arrangement that includes a boiler enclosure defining a gas flow path for transporting flue gas during normal operation. The method finds particular utility in connection with boiler parts such as impact-type particle separator components, particularly during abnormal operation conditions such as a black plant trip. The method includes providing a source of steam and discharging a combined stream of the steam and ambient air into the gas flow path, thereby cooling the hot boiler components. 
         [0016]    The method optionally can involve monitoring the temperature of one or more of the components and continuing the steam/air discharge step until the temperature of the component(s) in the vicinity is safe, typically ˜450° to ˜480° C. (850° to 900° F.). 
         [0017]    In yet another aspect of the present invention is a method for facilitating the cooling of components of a circulating fluidized bed boiler arrangement that comprises (1) a furnace with at least one primary air inlet and one or more secondary air inlets, wherein at least one of said one or more secondary air inlets is adapted to communicate with an air valve and encloses an eductor, (2) a solids separator system and (3) a steam/water circuit for circulating steam and water, said method comprising: a) conveying steam out of and away from said steam/water circuit to said eductor; and b) allowing said eductor to combine said steam with air introduced through said air valve, said air originating from a source external to said boiler arrangement; and c) introducing said combined stream into said furnace, thereby reducing the internal temperature of said furnace and helping to cool said components. 
         [0018]    Steam itself, with a typical temperature range of ˜150° to ˜400° C. (300° to 750° F.), is substantially cooler than the temperature of uncooled boiler parts such as components of a solids separating unit, typically ˜750° to ˜925° C. (1400° to 1700° F.). Nevertheless, ambient air has a temperature that is substantially lower than that of steam. Thus, a combined discharge of steam and air into the furnace of a boiler significantly reduces the amount of time needed to cool boiler parts, e.g., solids separating unit components, to a temperature level considered safe in view of the materials of components (such as tubes) in the vicinity. 
         [0019]    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 
         [0020]    The embodiments disclosed herein may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting. The foregoing and other aspects will become apparent to those skilled in the art to which the present examples relate upon reading the following description with reference to the accompanying drawings, in which: 
           [0021]      FIG. 1  is a schematic representation of a CFB boiler arrangement of the prior art. 
           [0022]      FIGS. 2 and 2A  are schematic illustrations of the upper portion of the CFB boiler of  FIG. 1 . 
           [0023]      FIG. 3  is a schematic representation of a CFB boiler arrangement adapted to inject, when needed in view of plant operating conditions, a combined stream of steam and ambient air into a boiler. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    A more complete understanding of the components, processes, systems, methods and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. The figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and is, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments. 
         [0025]    Although specific terms are used in the following description for the sake of clarity, these terms 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. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function. 
         [0026]    The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
         [0027]    As used in the specification, various devices and parts may be described as “comprising” other components. The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that do not preclude the possibility of additional components. 
         [0028]    As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially,” may not be limited to the precise value specified, in some cases. The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4 also discloses the range “from 2 to 4.” 
         [0029]    To the extent that explanations of certain terminology or principles of the fluidized bed arts, systems, processes, and related arts may be necessary to understand the present disclosure, the reader is referred to Steam/its generation and use, 40th Edition, Stultz and Kitto, Eds., Copyright 1992, The Babcock &amp; Wilcox Company, and to Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright 2005, The Babcock &amp; Wilcox Company, and Steam/its generation and use, 42nd Edition, G. L. Tomei, Ed., Copyright 2015, The Babcock &amp; Wilcox Company, the texts of which are hereby incorporated by reference as though fully set forth herein. 
         [0030]    While not intended to be limiting, the following description is based in large part on possible operation of equipment during abnormal operating conditions, such as those which occur during a black plant trip. 
         [0031]    Unless the surrounding text explicitly indicates a contrary intention, any value given in the form of a percentage in connection with a gaseous stream, input or product is a volume percentage (v/v), while all other values given in the form of percentages are weight percentages (w/w). 
         [0032]    The word about and “˜” symbol, when used in connection with a number, has the meaning dictated by the surrounding context and includes the number itself as well as at least the degree of error commonly associated with measurements of the particular quantity in question. 
         [0033]    The terms “downstream” and “upstream” refer to spatial relationships based on where combustion gases are generated, with the area of primary generation being considered as the most upstream point. 
         [0034]    While the components and operation of a CFB boiler have been known for some time and are more fully described in one or more of the patents mentioned previously, a brief overview is provided below to assist in the understanding of the articles and processes of the invention. 
         [0035]    Referring to  FIG. 3 , CFB boiler arrangement  1  includes furnace  10  which is supplied with primary air through windbox  80  and distribution grid  90 , flow thereof being controlled by valve  92 , and with secondary air through headers  52  and nozzles  50 , flow thereof being controlled by valves  54 . The two points of secondary air introduction shown in  FIG. 3  is exemplary and not limiting. The shape and mechanism of operation of headers  52  and nozzles  50 , as well as the source of the air introduced by valves  54 , likewise can vary widely. 
         [0036]    The header associated with any nozzle  50  in which is situated an eductor  57  (discussed in more detail below) is equipped with at least one valve  56  which can be remotely controlled so as to connect that header to a source of ambient air located external to CFB boiler arrangement  1 . 
         [0037]    Gases and solid products of combustion occurring in furnace  10  move upwardly (downstream), away from locations where primary and secondary air are introduced. Gases pass through to primary and secondary superheaters  41  and  42 , where they act to superheat steam flowing therethrough, and beyond. 
         [0038]    Solid components are removed by impact-type separators  32  (U-beams) which serve to protect downstream heating surfaces from erosion. Such solids are collected and recycled back to furnace  10 . U-beams  32  can be equipped with a temperature sensor, designated  139  in  FIG. 3 , which assists in monitoring the temperature of U-beams, as discussed in more detail below. Temperature sensor  139  is adapted to output, or used in combination with devices capable of outputting, data that can be read or monitored remotely. 
         [0039]    CFB boiler arrangement  1  also includes steam delivery path  43  and steam discharge system  100 , which includes steam bleed line  160  for transporting steam  115  from a steam source located at any point in the boiler steam path of steam/water circuit  60 , starting with steam drum  20  or preferably, and as shown in  FIG. 3 , attemperator inlet header  140  associated with attemperator  46 , a device which reduces and controls the temperature of a superheated fluid passing therethrough by, for example, spraying high purity water  44  into an interconnecting steam pipe, usually between superheaters  41  and  42 . Steam discharge system  100  also includes line  61  connected to windbox  80  as well as valve  165 , which preferably can be controlled remotely so as to permit introduction of steam  115  into windbox  80  via line  61  when needed or desired, as more fully described below. 
         [0040]    After an abnormal operating event such as a black plant trip, several events occur, either by default or by design. 
         [0041]    For example, the aforementioned solids generally collapse to the floor of furnace  10 . These solids are initially at the bed operating temperature just prior to the interrupting event and continue to transfer heat to the lower walls of furnace  10  and generate steam for some time. With MSVs closed, the additional steam generation, if not controlled, leads to lifting one or more of the safety valves on main steam outlet  65  and drum  20 . Corresponding massive loss of steam results in a rapid drop of the boiler water level, which presents the risk of the water level going below the furnace roof 
         [0042]    In an abnormal operating situation such as a black plant trip, U-beams  32  represent a significant thermal storage mass which continues to radiate heat to surrounding areas for an extended period of time. Specifically, water-cooled U-beam/rear wall support tubes  37  (see  FIG. 2A ) continue to receive heat from U-beams  32  at an elevated temperature similar to that from normal boiler operation. As long as these tubes contain water, they maintain acceptable temperatures and stress values. However, if the water level falls below the furnace roof, some portion of these tubes may have only steam cooling, and the temperature of the tube metal quickly rises. Even though low alloy steel tubes typically are used for the U-beam and rear wall support tubes  37 , shown as SW membrane panel in  FIG. 2A  (with ability to maintain normal operation stress levels to temperatures over normal working temperature), loss of water in the tubes while U-beams  32  are still near their normal operating temperature can result in a tube temperature where the normal operation stress in the tube exceeds its allowable stress at that temperature. 
         [0043]    A number of actions or steps, which can occur in series, in parallel or some combination thereof, are envisioned to counter the conditions that lead to rapid water loss to below the furnace roof. 
         [0044]    Controlled venting of steam  115 , into furnace  10  alone or to furnace  10  and the atmosphere, can be undertaken as required to suppress pressure rise and reduce the chance of safety valves being lifted. 
         [0045]    For example, 5-10% boiler maximum continuous rating (BMCR) steam flow can be vented through injection headers and nozzles (not shown) of steam discharge/injection system  100 , described above, which helps cool U-beams  32 . 
         [0046]    Further, the pressure rise at main steam outlet  65  can be monitored, as is known in the art, with opening of power operated vent  70  occurring if pressure continues to rise and approaches the lift pressure of the outlet safety valve of secondary superheater  42  by about 25-30 psig. This can result in venting of an additional 5-10% BMCR steam through power operated vent valve  70 . 
         [0047]    If present, optional dribble pump  170  can maintain water flow to drum  20  to offset water lost through continued production, as well as venting, of steam. Commencement of flow can be manual or automatic, usually in less than 10 minutes and preferably within no more than 5 to 7 minutes, and preferably is capable of lasting for ˜45 minutes from point of initiation. Dribble pump  170  preferably is capable of supplying drum  20  with at least 10% of maximum continuous rating (MCR) feed water flow at normal operation pressure and can keep the level of water in drum  20  stable at or within 7.5-10 cm (3 to 4 inches) below normal water level. 
         [0048]    A more detailed description of these ameliorative measures can be found in the aforementioned U.S. Pat. No. 8,047,162. 
         [0049]    In addition to the foregoing common post-trip events, the present equipment and processes transport some of steam  115  in steam discharge system  100  to eductor  57 , with the flow of that steam being controlled by valve  58 . 
         [0050]    Steam discharge into furnace  10  through eductor  57  starts and valve  56  opens. Eductor  57  is a device that uses the kinetic energy of a moving fluid (in this case steam  115 ) to entrain another fluid (in this case ambient air). Suction created by eductor  57  induces ambient air flow into furnace  10  through opened valve  56 . The mixed steam and air are discharged into furnace  10  through nozzle  50 . Steam velocity may be of 500 ft/sec or 800 ft/sec or 1100 ft/sec. 
         [0051]    Each nozzle of a furnace, or any lesser number, can be fit with an eductor and accompanying piping and valves. 
         [0052]    The induced air flow rate not only adds to the discharge steam flow rate but, because ambient air temperature (˜15° to ˜35° C.) is substantially lower than that of steam (˜150° to ˜300° C.), the cooling capacity of the combined flow of steam and air into furnace  10  is substantially higher than that of the steam alone. This results in faster cooling of U-beam  32 . Further, use of less steam in the cooling process means that the amount of extra capacity designed into drum  20  can be reduced and, depending on the efficacy of a given cooling arrangement, dribble pump  170  can be likewise downsized or even eliminated altogether. 
         [0053]    Using ambient air for such cooling during abnormal operating conditions partially decouples steam discharge flow rate from the cooling needs. If more steam needs to be discharged for maintaining drum pressure than required for cooling needs (i.e., by inducing ambient air flow), additional steam discharge can be accomplished through remotely-controlled relief valve  70 . Cooling steam discharge is expected to be on the order of ˜3% to 10% of BMCR. 
         [0054]    With the ID fan (not shown) idled due to abnormal operating (e.g., black plant) conditions, furnace  10  has positive pressure due to the pressure drop across the boiler convection pass generated by the combined cooling steam/air flow from eductor  57 . To prevent combustible gases generated within the hot bed material on the floor of furnace  10  being forced through distribution grid  90  into windbox  80 , the pressure in windbox  80  preferably is maintained higher than that in furnace  10 , as evidenced by the outputs of pressure sensor  94  (in windbox  80 ) and of pressure sensor  96  (in furnace  10 ). Ensuring higher pressure in windbox  80  can be achieved by injecting steam  115  into windbox  80  through line  61 , while maintaining valve  92  in a closed position. Steam flow rate through line  61  is controlled by valve  165  so as to maintain an acceptable preset pressure differential, again as evidenced by the relative outputs of pressure sensors  94  and  96 . Flow rate of steam  115  through line  61  typically does not exceed 1.5%, 1%, or even 0.5% of BMCR. 
         [0055]    A portion of steam injected into windbox  80  moves through distribution grid  90  while another portion condenses. The latter can be removed through windbox drain valve  82 . 
         [0056]    All venting to furnace  10 , including that through eductor  57 , can be ceased when temperature sensor  139  indicates that the local temperature has fallen to a preset temperature of, for example, ˜540° C. (1000° F.), ˜510° C. (950° F.), ˜480° C. (900° F.) or even ˜450° C. (850° F.). 
         [0057]    The CFB boiler unit can be returned to normal operation configuration after power supply thereto is re-established. 
         [0058]    The foregoing description has been made with reference to exemplary embodiments. While various aspects and embodiments have been disclosed herein, other aspects, embodiments, modifications and alterations will be apparent to those skilled in the art upon reading and understanding the preceding detailed description. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting. It is intended that the present disclosure be construed as including all such aspects, embodiments, modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 
         [0059]    The relevant portion(s) of any specifically referenced patent and/or published patent application is/are incorporated herein by reference.