Abstract:
A boiler system is provided comprising: a furnace adapted to receive a fuel to be burned to generate hot working gases; a fuel supply structure associated with the furnace for supplying fuel to the furnace; a superheater section associated with the furnace and positioned to receive energy in the form of heat from the hot working gases; and a controller. The superheater section may comprise a platen including a tube structure with an end portion and a temperature sensor for measuring the temperature of the tube structure end portion and generating a signal indicative of the temperature of the tube structure end portion. The controller may be coupled to the temperature sensor for receiving and monitoring the signal from the sensor.

Description:
PRIORITY 
       [0001]    This invention claims priority to U.S. Ser. No. 14/202,242, filed 1 Mar. 2014, for which Issue Notification was issued by the USPTO on 21 Dec. 2016, indicating invention shall be issued on 10 Jan. 2017 as U.S. Pat. No. 9,541,282, said Application/Patent is hereby incorporated herein in its entirety by the reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a boiler system comprising a controller for monitoring a temperature of a structure in a superheater section and controlling fuel provided to a furnace based on the monitored temperature. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a paper-making process, chemical pulping yields, as a by-product, black liquor, which contains almost all of the inorganic cooking chemicals along with lignin and other organic matter separated from the wood during pulping in a digester. The black liquor is burned in a recovery boiler. The two main functions of the recovery boiler are to recover the inorganic cooking chemicals used in the pulping process and to make use of the chemical energy in the organic portion of the black liquor to generate steam for a paper mill. 
         [0004]    In a kraft recovery boiler, a superheater structure is placed in the furnace in order to extract heat by radiation and convection from the furnace gases. Saturated steam enters the superheater section, and superheated steam exits from the section. The superheater structure comprises a plurality of platens. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with a first aspect of the present invention, a boiler system is provided comprising: a furnace adapted to receive a fuel to be burned to generate hot working gases; a fuel supply structure associated with the furnace for supplying fuel to the furnace; a superheater section associated with the furnace and positioned to receive energy in the form of heat from the hot working gases; and a controller. The superheater section may comprise a platen including a tube structure with an end portion and a temperature sensor for measuring the temperature of the tube structure end portion and generating a signal indicative of the temperature of the tube structure end portion. The controller may be coupled to the temperature sensor for receiving and monitoring the signal from the sensor. 
         [0006]    The temperature sensor may comprise a thermocouple. 
         [0007]    The controller may monitor the signal from the temperature sensor for rapid changes in temperature of the tube structure end portion. Rapid changes in temperature of the tube structure end portion may be defined by a rapid increase in temperature immediately followed by a rapid decrease in temperature. For example, rapid changes in temperature of the tube structure end portion may be defined by a monotonic increase in temperature of least about 25 degrees F. occurring over a time period of between about one to five minutes and immediately thereafter a monotonic decrease in temperature greater than zero in magnitude occurring over a time period of between about one to ten minutes. 
         [0008]    The controller may increase an amount of fuel supplied by the supply structure to the furnace after the temperature of the tube structure end portion has experienced rapid changes. 
         [0009]    The boiler system may further comprise a temperature measuring device for sensing the temperature of the working gases contacting the superheater section and generating a corresponding temperature signal to the controller. 
         [0010]    The controller may control the amount of fuel provided by the supply structure to the furnace such that the temperature of the working gases is below a threshold temperature until the temperature of the tube structure end portion has experienced rapid changes. 
         [0011]    In accordance with a second aspect of the present invention, a boiler system is provided comprising: a furnace adapted to receive a fuel to be burned to generate hot working gases; a fuel supply structure associated with the furnace for supplying fuel to the furnace; a superheater section associated with the furnace and positioned to receive energy in the form of heat from the hot working gases; and a controller. The superheater section may comprise a platen including a tube structure with an end portion and a sensor for measuring the temperature of the tube structure end portion and generating a signal indicative of the temperature of the tube structure end portion. The controller may be coupled to the sensor for receiving and monitoring the signal from the sensor and controlling an amount of fuel provided by the supply structure to the furnace based on the signal. 
         [0012]    In accordance with a third aspect of the present invention, a process is provided for monitoring a boiler system comprising a furnace for burning a fuel to generate hot working gases, a fuel supply structure for supplying fuel to the furnace, a superheater section comprising a platen including a tube structure with an end portion, and a sensor for measuring the temperature of the tube structure end portion and generating a signal indicative of the temperature of the tube structure end portion. The process may comprise monitoring the signal from the sensor, and controlling an amount of fuel provided to the furnace based on the signal. 
         [0013]    Monitoring may comprise monitoring the signal from the temperature sensor for rapid changes in temperature of the tube structure end portion. 
         [0014]    Controlling may comprise increasing an amount of fuel supplied by the supply structure to the furnace after the temperature of the tube structure end portion has experienced rapid changes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein: 
           [0016]      FIG. 1  is a schematic view of a kraft black liquor recovery boiler system constructed in accordance with the present invention; 
           [0017]      FIG. 2  illustrates a portion of a superheater section of the boiler system of  FIG. 1 ; wherein tube structures defining platens are illustrated schematically as rectangular structures; 
           [0018]      FIG. 3  illustrates first, second and third tube structures of a platen; and 
           [0019]      FIG. 4  is an example plot of a tube structure clearing event. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration, and not by way of limitation, specific preferred embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
         [0021]      FIG. 1  illustrates a kraft black liquor recovery boiler system  10  constructed in accordance with the present invention. Black liquor is a by-product of chemical pulping in a paper-making process. The initial concentration of “weak black liquor” is about 15%. It is concentrated to firing conditions (65% to 85% dry solids content) in an evaporator  20 , and then burned in the recovery boiler system  10 . The evaporator  20  receives the weak black liquor from washers (not shown) downstream from a cooking digester (not shown). 
         [0022]    The boiler system  10  comprises a recovery boiler  12  comprising a sealed housing  12 A defining a furnace  30  where a fuel, e.g., black liquor, is burned to generate hot working gases, a heat transfer section  32  and a bullnose  34  in between the furnace  30  and the heat transfer section  32 , see  FIG. 1 . The boiler system  10  further comprises an economizer  40 , a boiler bank  50  and a superheater section  60 , all of which are located in the heat transfer section  32 , see  FIG. 1 . The hot working gases resulting from the burning of the fuel in the furnace  30  pass around the bullnose  34 , travel into and through the heat transfer section  32 , are then filtered through an electrostatic precipitator  70  and exit through a stack  72 , see  FIG. 1 . It is noted that when the furnace  30  is initially fired, another fuel other than black liquor, such as natural gas or fuel oil, may be provided to the furnace  30  via injectors  137 . Once the furnace  30  has reached a desired temperature, black liquor instead of natural gas or fuel oil may be used as the fuel in the furnace  30 . 
         [0023]    Vertically aligned wall tubes  130  are incorporated into vertical walls  31  of the furnace  30 . As will be discussed further below, a fluid, primarily water, passes through the wall tubes  130  such that energy in the form of heat from the hot working gases generated in the furnace  30  is transferred to the fluid flowing through the wall tubes  130 . The furnace  30  has primary level air ports  132 , secondary level air ports  134 , and tertiary level air ports  136  for introducing air for combustion at three different height levels. Black liquor BL is sprayed into the furnace  30  out of spray guns  138 . The black liquor BL is supplied to the guns  138  from the evaporator  20 . The injectors  137  and the spray guns  138  define fuel supply structure. 
         [0024]    The economizer  40  receives feedwater from a supply FS. In the illustrated embodiment, the feedwater may be supplied to the economizer  40  at a temperature of about 250° F. The economizer  40  may heat the water to a temperature of about 450° F. The hot working gases moving through the heat transfer section  32  supply energy in the form of heat to the economizer  40  for heating the feedwater. The heated water is then supplied from the economizer  40  to a top drum (steam drum)  52  of the boiler bank  50 , see  FIG. 1 . The top drum  52  functions generally as a steam-water separator. In the embodiment illustrated in  FIG. 1 , the water flows down a first set of tubes  54  extending from the top drum  52  to a lower drum (mud drum)  56 . As the water flows down the tubes  54 , it may be heated to a temperature of about 400-600° F. From the lower drum  56 , a portion of the heated water flows through a second set of tubes  58  in the boiler bank  50  to the upper drum  52 . A remaining portion of the heated water in the lower drum  56  is supplied to the wall tubes  130  in the furnace  30 . The water flowing through the second set of tubes  58  in the boiler bank  50  and the wall tubes  130  in the furnace  30  may be heated to a saturated state. In the saturated state, the fluid is mainly a liquid, but some steam may be provided. The fluid in the wall tubes  130  is returned to the boiler bank  50  at the top drum  52 . The steam is separated from the liquid in the top drum  52 . The steam in the top drum  52  is supplied to the superheater section  60 , while the water returns to the lower drum  56  via the first set of tubes  54 . 
         [0025]    In an alternative embodiment (not shown), the upper and lower drums  52 ,  56  may be replaced by a single drum, as is known to those skilled in the art, whereby steam is supplied by the single drum to a superheater section. 
         [0026]    In the embodiment illustrated in  FIG. 2 , the superheater section  60  comprises first, second and third superheaters  62 ,  64  and  66 , each of which may comprise between about 20-50 platens  62 A,  64 A and  66 A. Steam enters the platens  62 A,  64 A and  66 A through a corresponding manifold tube called an inlet header  62 B,  64 B and  66 B, is superheated within the platens  62 A,  64 A and  66 A, and exits the platens  62 A,  64 A and  66 A as superheated steam through another manifold tube called an outlet header  62 C,  64 C and  66 C. The platens  62 A,  64 A and  66 A are suspended from the headers  62 B,  64 B,  66 B,  62 C,  64 C and  66 C, which are themselves suspended from overhead beams (not shown) by hanger rods  200 . The hot working gases moving through the heat transfer section  32  supply the energy in the form of heat to the superheater section  60  for superheating the steam. It is contemplated that the superheater section  60  may comprise less than three superheaters or more than three superheaters. 
         [0027]    A platen  62 A from the first superheater  62  is illustrated in  FIG. 3 . The remaining platens  62 A in the first superheater  62  as well as the platens  64 A and  66 A in the second and third superheaters  64 ,  66  are constructed in generally the same manner. The platen  62 A may comprise first, second and third separate metal tube structures  160 - 162 , see  FIG. 3 . In  FIG. 2 , the platens are schematically illustrated as rectangular structures, but are defined by tube structures. The tube structures  160 - 162  comprise inlet portions  160 A- 162 A, which communicate with the inlet header  62 B and end portions  160 B- 162 B, which communicate with the outlet header  62 C. The tube structure inlet portions  160 A- 162 A and end portions  160 B- 162 B are located above a roof  12 B of the boiler housing  12 A, see  FIGS. 1 and 3 , while intermediate portions  160 C- 162 C of the tube structures  160 - 162  extend within the boiler housing  12 A and are located within the heat transfer section  32 . The tube structures  160 - 162  define pathways through which fluid, e.g., steam, passes from the inlet header  62 B, though the tube structures  160 - 162  and out the outlet header  62 C. It is contemplated that the platen  62 A may have less than or more than three tube structures, e.g., one, two, four or five tube structures. 
         [0028]    The steam is heated to a superheated state in the superheater section  60 . Prior to boiler/furnace start-up, cooled liquid water may settle in lower bends of the tube structures  160 - 162  in the platens  62 A,  64 A and  66 A. Until the liquid water is boiled away during boiler/furnace start-up, the liquid water prevents steam from passing through the tube structures  160 - 162 . The steam moving through the tube structures  160 - 162  functions as a cooling fluid for the metal tube structures  160 - 162 . When no steam moves through a tube structure  160 - 162 , the tube structure may become overheated, especially at an end portion  160 B- 162 B, which may cause damage to the tube structure  160 - 162 . 
         [0029]    In the present invention, start-up of the furnace  30  is monitored by a controller  210  to ensure that the furnace  30  is heated slowly until any liquid water in the tube structures  160 - 162  of the superheater section platens  62 A,  64 A and  66 A has safely evaporated before the furnace  30  is heated to an elevated state. 
         [0030]    A temperature measurement device  170 , which, in the illustrated embodiment, comprises an optical pyrometer, may be provided in or near the heat transfer section  32  to measure the temperature of the hot working gases in the heat transfer section  32  and entering the superheater section  60 . The temperature measuring device  170  generates a corresponding temperature signal to the controller  210 . The temperature sensed by the temperature measurement device  170  provides an indication of the amount of energy in the form of heat being generated by the furnace  30 . Until the controller  210  has verified that liquid water in the tube structures  160 - 162  has been cleared, the amount of fuel provided by the injectors  137  or the spray guns  138  to the furnace  30  is controlled by the controller  210  at a low level. That is, in the illustrated embodiment, the amount of fuel provided by the injectors  137  or the spray guns  138  to the furnace  30  is controlled by the controller  210  such that the temperature of the hot working gases in the heat transfer section  32  and entering the superheater section  60 , as measured by the temperature measuring device  170 , is less than a predefined initial working gas threshold temperature, such as a threshold temperature falling within the range of 800-1000 degrees F. If the temperature of the hot working gases exceeds the threshold temperature, the amount of fuel provided to the furnace  30  is reduced. Once the controller  210  has verified that liquid water in the tube structures  160  has been cleared, then the controller  210  will allow the rate at which fuel is provided to the furnace  30  to increase such that the temperature of the hot working gases entering the superheater section  60  exceeds the threshold temperature. 
         [0031]    The controller  210  comprises any device which receives input data, processes that data through computer instructions, and generates output data. Such a controller can be a hand-held device, laptop or notebook computer, desktop computer, microcomputer, digital signal processor (DSP), mainframe, server, other programmable computer devices, or any combination thereof. The controller  210  may also be implemented using programmable logic devices such as field programmable gate arrays (FPGAs) or, alternatively, realized as application specific integrated circuits (ASICs) or similar devices. 
         [0032]    Preferably, for each of the tube structures  160 - 162  in the platens  62 A,  64 A and  66 A, a temperature sensor  220 , such as a thermocouple in the illustrated embodiment, is provided at the end portion  160 B- 162 B of the tube structure  160  to measure the temperature of the tube structure  160 - 162  at that location, see  FIG. 3 . The temperature sensors  220  generate corresponding temperature signals to the controller  210 . Each tube structure end portion  160 B- 162 B is located near its corresponding outlet header. It is contemplated that a temperature sensor  220  may not be provided for all of the tube structures  160 - 162  in each of the platens  62 A,  64 A and  66 A. However, it is preferred that a temperature sensor  220  is provided for at least one tube structure  160 - 162  in each platen  62 A,  64 A and  66 A. 
         [0033]    Liquid water evaporating in a tube structure  160 - 162  after furnace startup is referred to herein as a “tube structure clearing event.” Such a tube structure clearing event is characterized by rapid changes in temperature at the end portion of the tube structure. In the illustrated embodiment, “rapid changes in temperature” of the end portion  160 B- 162 B of a tube structure  160 - 162 , as measured by a corresponding temperature sensor  220 , are characterized by the temperature increasing monotonically, rapidly, e.g., over a 1-5 minute period, and significantly, e.g., by a temperature increase of at least 25 degrees F., and immediately thereafter, decreasing monotonically, rapidly, e.g., over a 1-10 minute period, by a temperature magnitude decrease equal to or less than the magnitude of the temperature increase but, in any event, the magnitude of the decrease in temperature is greater than zero. 
         [0034]    In  FIG. 4 , a plot is illustrated corresponding to a measured tube structure clearing event. As shown in  FIG. 4 , the temperature of a tube structure end portion, as measured by a corresponding temperature sensor  220 , began to monotonically increase in temperature at about 8075 seconds from about 550 degrees F. to a maximum temperature of about 700 degrees F. at about 8225 seconds. Hence, over a time period of about 150 seconds, the tube structure end portion increased in temperature by about 150 degrees F. After reaching the maximum temperature at about 8225 seconds, the temperature of the tube structure end portion immediately began to decrease monotonically to a temperature of about 610 degrees F. at about 8725 seconds. Hence, over a time period of about 500 seconds, the tube structure end portion monotonically decreased in temperature by about 90 degrees. 
         [0035]    Hence, the temperature sensors  220  are monitored by the controller  210  for rapid temperature changes, i.e., a rapid increased in temperature immediately followed by a rapid decrease in temperature, indicating that fluid is moving through the entire length of their corresponding tube structures  160 - 162 . In the illustrated embodiment, once all of the temperature sensors  220  have provided signals indicating that rapid temperature changes have occurred at their corresponding tube structure end portions, the controller  210  may cause the injectors  137  or spray guns  138  to increase the amount of fuel provided to the furnace  30  since the temperature of the hot working gases in the heat transfer section  32  and entering the superheater section  60  can safely exceed the predefined initial working gas threshold temperature (800-1000 degrees F. in the illustrated embodiment). 
         [0036]    An “increase in the amount of fuel provided to the furnace” is intended to encompass increasing the rate at which fuel is input into the furnace  30  by either the injectors  137  or the spray guns  138 . Hence, an increase in the amount of fuel provided to the furnace  30  may result when the injectors  137  increase the rate at which natural gas or fuel oil is input into the furnace  30 ; when the injectors  137  stop inputting natural gas or fuel oil while, at that same time, the spray guns  138  begin inputting black liquor into the furnace  30  at a rate which exceeds the rate at which natural gas or fuel oil was injected into the furnace  30 ; or when the spray guns  138  increase the rate at which black liquor is input into the furnace. 
         [0037]    In accordance with a further aspect of the present invention, once all of the temperature sensors  220  have provided signals to the controller  210  indicating that rapid temperature changes have occurred at their corresponding tube structure end portions, the controller  210  may generate a message or otherwise indicate to an operator that a tube structure clearing event has occurred and/or request that the operator input a tube structure clearing verification signal. In this embodiment, the controller  210  will not automatically cause the injectors  137  or spray guns  138  to increase the amount of fuel provided to the furnace  30  once all of the temperature sensors  220  have provided signals to the controller  210  indicating that rapid temperature changes have occurred at their corresponding tube structure end portions, as is done by the embodiment discussed above. Instead, the controller  210  will wait until it receives a verification signal input from the operator, via a keypad, keyboard or other input device, indicating that the operator has verified that a tube structure clearing event has occurred. Only after receiving the verification signal input by the operator will the controller  210  cause the injectors  137  or spray guns  138  to increase the amount of fuel provided to the furnace  30 . 
         [0038]    While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.