System and method for reheat steam temperature control in circulating fluidized bed boilers
This is a continuation of application Ser. No. 07/301,621 filed Jan. 24, 1989 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to circulating fluidized bed boilers and pertains particularly to a system and method for controlling reheat steam temperature. 
     Several methods are presently known for controlling reheat steam temperatures. 
     One method of reheater temperature control is the use of a system for gas bypass over the reheater. Two separate flue gas passes are provided in the convection pass of the boiler (one for superheater and one for reheater), with means such as dampers downstream of each to vary the amount of flue gas flow over each section. The outlet steam temperature of the reheater can be controlled by varying the amount of flue gas flow between the convection pass sections. The main disadvantage of this system is that the dampers are located in a higher temperature (500-700 degrees F.) dust laden flue gas path making them susceptible to erosion and mechanical failure. Also, the steam temperature control range is limited with this type of system. 
     Another method of reheater outlet steam temperature control is by the use of external heat exchangers. With this approach, a portion of the recirculated solids within the circulating fluidized bed system is diverted to an externally mounted fluidized bed heat exchanger, i.e. external heater exchanger (EHE), in which a section of or complete reheater is located. By varying the amount of solids flow to the EHE, the quantity of heat transfer to the reheater and the reheater outlet steam temperature is controlled. The main disadvantages of this system are that the solids flow control valve is a high maintenance item and the reheat tube surface within the EHE is subject to erosion. This effects the availability of the unit. 
     A further approach to the control of the reheater outlet steam temperature is by the use of spray desuperheater. This approach utilizes spraying water for desuperheating and thereby controlling reheater outlet steam temperature. This is a simple approach, but not generally accepted because it degrades the cycle efficiency. 
     Still another approach is by the use of excess air. Excess air supplied to the boiler can be used for reheat steam temperature control. This approach, however, is not favored because of its negative affect on boiler efficiency. 
     A still further approach is by the use of gas recirculation. By this approach, large quantities of flue gases are recirculated to achieve the rated reheater outlet steam temperature. This approach, however, requires the use of a gas recirculation fan for handling a hot dust laden gas and requires additional power consumption, which makes this approach disadvantageous. 
     Accordingly, the present invention is directed to an improved method and system for reheat steam temperature control. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is the primary object of the present invention to provide an improved system and method for controlling the reheater (outlet) steam temperature in circulating fluidized bed boilers. 
     In accordance with a primary aspect of the present invention, a steam generator having a fluidized bed combustion system that includes a fluidized bed combustor, at least one separator, and a reheater in a flue gas pass having at least first and second or final stages (possibly more), includes means for controlling the temperature of the second or final stage of the reheater comprising of a means for by-passing a selected portion of cold steam around said first stage reheater directly to said second or final stage reheater. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects and advantages of the present invention will become apparent from the following description when read in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic diagram illustrating a typical circulating fluidized bed boiler system embodying the present invention; and 
     FIG. 2 is a schematic diagram illustrating an arrangement of two typical boilers connected to a single turbine. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring to FIG. 1, a power plant embodying a typical circulating fluidized bed boiler with superheater and reheater is illustrated with the system incorporating a preferred embodiment of the present invention. The boiler system, designated generally by the numeral 10, comprises a fluid bed combustor 12 having a combustion chamber 14 into which combustible material, non-combustible material, possibly additives or recirculated material, primary air and secondary air are fed. In the combustion chamber, the bed is maintained in fluidized state by having the correct inventory of bed material and flow of air. The combustion chamber is provided with a bottom 16 having a grid-like construction through which fluidizing air is introduced. The combustion chamber walls are preferably constructed with membrane type tube walls, with or without a refractory covering. 
     First and second stages of superheaters 18 and 20 are located within the combustion chamber. The combustion chamber materials are carried from the combustion chamber by way of flues 22 to a hot separator 24 wherein the solids are separated from the flue gases for return by way of flues and flue system 26, 28 and 30 to the bottom of the combustion chamber for recirculation. These may be passed through fluidized bed coolers or the like prior to return to the combustion chamber. 
     The details of circulation circuit for the feed water and the primary superheaters are not illustrated as they do not form an essential part of the present invention. 
     Flue gases from the hot separator pass along by way of flue 32 to a convection pass 34. A single stage superheater 38 is placed or located in the convection pass with reheaters 40 and 42 located downstream of the superheater 38 and upstream of an economizer surface 44. The reheater is illustrated as two stages with 42 being a first stage and 40 being a second or final stage. The reheater may have more than two stages, with the final stage just down stream from superheater 38 such as 40 is located. These are arranged as counter flow heat exchangers with the gas flow direction down and the reheat steam flow direction up. The placement of the superheater 38 within this pass helps keep the temperature of the gas flow to reheater 40 below the critical temperature. This arrangement together with the bypass feature as will be explained enables a unique and effective control of the temperatures within the reheater sections. 
     When the steam temperature leaving the particular section (in a counter flow heat exchanger arrangement) is close to the gas temperature entering that section, reducing the steam flow to that section will result in a considerable reduction in heat absorption. As the steam temperature approaches the gas temperature, the effective thermal head available for heat transfer is reduced. This provides the basis for the principle used for the reheat temperature control system in accordance with the present invention. 
     The generating system as illustrated in FIG. 1 is supplying steam to a two-stage turbine. In the illustrated arrangement, steam from superheater 38 flows via an outlet header 46 and supply line 48 by way of valve 50 to the inlet side of the high pressure turbine (HPT) 52. Cold steam leaving the turbine 52 returns by way of return line 53 to the reheaters 42 and 40. At the reheater, a bypass line 54 joins the return line 53 at 55 and bypasses a portion of the cold steam with the remaining portion of the steam going by way of differential control valve 56 to the inlet header 58 of the first stage reheater 42. 
     The steam passing through the reheater 42 exits by way of a header 60 and rejoins or recombines with the bypass portion of the cold steam at 62. A flow control valve 64 is provided in the bypass line 54 for control of the flow between the inlet manifold of the first stage reheater 42 and the bypass line. The recombined steam at 62 flows into inlet header 66 of the second or final stage reheater 40 where it is further heated and flows by way of outlet header 68, supply line 70 and valve 72 to the second stage or lower stage of the turbine (IPT) 74. The selective proportioning of the cold steam between the bypass line 54 and the second stage of the reheater 42 provides an effective and efficient means of controlling the temperature in the reheater stages. 
     The location of the first stage reheater 42 along the flue gas path is so chosen that bypassing the required portion of the cold reheat steam directly to the second stage reheater 40 cannot increase the steam temperature leaving the first stage reheater to more than the allowed metal temperature for the reheater tube material. A limit will be set to protect the first stage reheater materials from exceeding their allowable metal temperature. The value of 1050° F. is a typical limit and may vary depending upon the actual design conditions. The purpose of the system is such that the maximum tube outside surface temperature will not exceed the allowable metal temperature limit for the material selected. 
     The arrangement of the control valves 56 and 64 is so chosen that controlability is achieved throughout the steam temperature control range and permits all reheater surfaces to be placed in the convection pass of the boiler, eliminating the need for in furnace reheater surfaces. This also makes feasible a simplified start-up scheme when more than one boiler for example is connected to a common turbine system. In this arrangement, the set of valves provide a means for reheat steam flow balancing under various operating conditions. 
     In the circulating fluidized bed boiler, the combustion takes place in a fluidized bed of inert material. The fluidized bed material leaving the combustor is returned by means of a hot collector (such as a hot cyclone) through suitable sealing device. In operation, air and fuel are delivered to the combustion chamber 14 wherein the bed material is maintained in a fluidized state by having the correct flow of air and bed material. The fluidizing air is introduced through a grid-like grating or construction at 16 in the bottom of the chamber. The flue gas and combustion products, along with the carry over solids, first convey heat to the superheaters 18 and 20 and are conveyed by way of flue 22 into the hot separator 24 wherein the solids are separated and returned to the combustion chamber through the flue arrangement 26, 28 and 30. The hot flue gases are then conveyed from the hot separator(s) by way of flue 32 to the convection pass section 34 wherein the final stage superheater 38 and the reheater stages 40 and 42 are located. 
     Three superheater stages are disposed in the described system, these being 18 and 20 and 38, with 38 being in the flue gas convection pass. Desuperheaters may be positioned between the superheater stages for steam temperature control if necessary. The two stages 40 and 42 of the reheater are positioned in the convection pass 34 and in conjunction with the control valves and interconnecting piping so that precise control of the reheater outlet steam temperature is possible. The piping system is such that cold steam reentering this system at pipe 53 is selectively divided into two streams at the juncture 55 thereof with the bypass line 54. One stream passes to the first stage reheater and is distributed through inlet header 58. The other steam goes to the second stage reheater by way of valve 64 and inlet header 66. The selective division of the stream will be in proportion to the temperature control necessary, which is accomplished by the valves 56 and 64. 
     The hot steam leaving the first stage reheater from the outlet header 60 is mixed with the cold steam via the bypass line 54 after or down stream of the flow control valve 64 and the blended stream enters the second stage reheater by way of the inlet header 66. The flow through the first stage reheater is controlled by proper manipulation of the two control valves 56 and 64, which in turn control the steam temperature leaving the second stage reheater 40. Hot steam from the second or final stage reheater is directed back to the turbine by way of the hot reheat steam line 70. 
     A differential pressure responsive control unit 80 controls the setting of valve 56 for controlling the pressure differential available for the control valve 64. The control unit 80 is responsive to the pressure differential between the cold steam return line 53 and the outlet pressure at juncture 62 of the outlet of reheater 42 and the bypass line 54. This is indicated by phantom line 84 in FIG. 1. The control unit 80 is set to control the valve 56 as a function of load on the boiler. 
     The valve 64 in the bypass line 54 is controlled by temperature responsive control unit 82 which responds to the temperature of the outlet steam from the second or final stage reheater 40. This is indicated by phantom line 86 in FIG. 1. In the illustrated embodiment, as an example the temperature of reheater 40 is maintained within the limit of about 1000 degrees F., plus or minus 10 degrees F. As the temperature of the steam leaving reheater 40 begins to increase above 1010° F., the valve 64 is opened to bypass additional cold steam directly to reheater 40. As the temperature begins to fall below 990° F. The valve 64 is closed to reduce the flow of bypass cold steam to the second stage 40. 
     Referring to FIG. 2, a system identical to FIG. 1, but with a duplicate boiler, is disclosed. In this system, the components of the first boiler arrangement are identified by the same reference numerals as in FIG. 1, with the second boiler arrangement being identified with the same numbers primed. Therefore, in this arrangement, a boiler turbine system is disclosed wherein two boilers are supplying steam to a single turbine. One essential feature required for this type of system is that means be provided for controlling the amount of reheat steam flow to each boiler, so that the steam temperature at reheat outlet is within limits at all possible operating conditions. In the illustrated system, duplicate controls and piping are provided for the two boilers. 
     The control valves 56 and 64 for the reheat steam temperature control can be used for flow balancing and maintaining the reheater outlet temperature within limits under both normal and abnormal operating conditions. In this arrangement, pressure reducing valves 80 and 82, along with desuperheaters 76 and 78, provide for flexibility during cold start-up, hot start-up and also when starting the second unit while the first one is on line. This simple system eliminates the need for a sophisticated steam blending system. It provides a simple and effective system and method for reheat outlet steam temperature control under varying load conditions. 
     In operation, from a cold start, combustion is initiated in the combustion chamber 14 with the introduction of fuel and combustion air. As heat is generated as a result of the combustion, the hot gases of combustion move upward in the combustion chamber transferring heat to the water in the combustion chamber walls and to superheaters 18 and 20. The hot gas, combustion products, and solids pass from the combustion chamber along flue 22 into the hot separator 24 where the solids are separated for return to the combustion chamber. The hot flue gas passes along flue 32 into the the convection pass 34 where the heat is transferred in sequence to the superheater 38, the second or final stage reheater 40, and the first stage reheater 42. The flow of hot gas through the system begins before the flow of cold steam. The boiler is fired and fuel burns for a period of time providing hot gas before steam is generated and starts the turbine. Cold reheat steam doesn&#39;t start flowing until after the turbine starts. 
     As the hot gas give up their heat to the water and steam in the water walls, in superheaters and reheaters, the temperature drops so that it is less at each successive stage. It should be noted that the gas temperature leaving the combustion chamber outlet at full load will be in the range of 1550 degrees F. to 1700 degrees F. The greater the temperature differential between the gas and the water, the greater the heat transfer will be, and the cooler the gas will be as it passes from the respective heater. Therefore, as the gas passes superheater 38, it will be below the critical temperature for reheater 40 up to some load of the boiler. Thus, with superheater 38 in the gas pass ahead of the reheater 40, the gas temperature will be below the critical temperature for reheater 40 until after about 40% to 50% load is reached. At this time cold steam is available for control of the temperature in accordance with this invention. This point of not requiring flow through the reheater until the unit is at 50% load is another advantage of this invention. Most standard systems require flow through the reheater during the earlier stages of start-up (hot or cold), to protect some from burn out. Thus, an expensive by-pass system must be utilized. However, with this system&#39;s physical layout, a bypass is not required and system start-up periods can be shortened. 
     Other modifications and changes are possible in the foregoing disclosure and in some instances, some features may be employed without the corresponding use of other features. Accordingly, while the present invention has been illustrated and described with respect to a specific embodiment, it is to be understood that numerous changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.