Patent Publication Number: US-10775039-B2

Title: Method for managing a shut down of a boiler

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European application 13191735.3 filed Nov. 6, 2013, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a method for managing a shut down of a boiler. 
     BACKGROUND 
       FIG. 1  shows an example of a boiler  1  having an evaporator  2  defined by walls  3  (tubed walls, preferably finned tubed wall); the walls  3  define a chamber  4  and the bottom of the walls  3  defines a hopper  5 . 
     One or also more than one walls  3  carry a firing system  6  comprising a fan for an oxidizer like air and a fuel supply  8  for coal, oil, gas, etc. 
     The tubed walls  3  are connected to inlet headers  9  and outlet headers  10 ; water is collected at the inlet headers  9  and is distributed through the tubes of the tubed walls  3  and, after passing through the tubed walls  3 , steam (or a mixture of steam and water or steam containing some water to a low extent) is collected at the outer headers  10 . The headers  9  and  10  are outside of the chamber  4 . Naturally also other types of evaporators are possible. 
     Above the evaporator  2 , the boiler  1  has a duct  12  that houses in series, from the bottom to the top, a superheater  13  for heating the steam directed to a high pressure user (like for example a high pressure turbine  13   a  of a power plant) and a reheater  14  for heating the steam discharged from the high pressure user and directed to a medium or low pressure user (like for example a medium or low pressure turbine  14   a  of a power plant). 
     The superheater  13  includes heat exchanging components having tubed heat exchanging surfaces  16  connected to inlet headers  17  and outlet headers  18 ; for example the tubed heat exchanging surfaces  16  can be tubed coils or tubed panels. 
     The attached FIGURE shows an example of a superheater  13  including three heat exchanging components each having tubed heat exchanging surfaces  16 , inlet header  17  and outlet header  18 . 
     The reheater  14  has a structure similar to the structure of the superheater  13 . 
     The reheater  14  includes heat exchanging components that comprise tubed heat exchanging surfaces  16 , such as tubed coils or tubed panels. The tubed heat exchanging surfaces  16  are connected to inlet headers  17  and outlet headers  18 . 
     The attached FIGURE shows an example of a reheater  14  including two heat exchanging components each having tubed heat exchanging surfaces  16 , inlet header  17  and outlet header  18 . 
     Above the reheater  14  there is provided an economizer  20 , to pre-heat water coming from a feedwater source  20   a  and directed to the evaporator  2 . The economizer  20  is also provided with inlet headers and outlet headers. 
     In the duct  12 , downstream the economizer  20 , there are typically installed a catalyzer  21  (if needed according to the emission requirements) for reducing the NO x  content of the flue gas, a preheater  22  for preheating air that is supplied into the chamber  4  for combustion of the fuel, a dust removal unit  23  such as a filter or electrostatic precipitator for solid particles removal from the flue gas; in some cases a damper  24  for regulating the opening of the flue gas duct  12  and a fan  7  for transportation of the flue gas to the stack  34  can also be provided. 
     In some cases, the economiser  20  can be separated in two parts, one upstream the catalyzer  21  and one downstream the catalyzer  21 . 
     During operation, water passes through the economizer  20  where it starts heating and then it is supplied through the headers  9  to the tubed walls  3 . While passing through the tubed walls  3  water evaporates, generating steam that is collected at the headers  10  and is directed (through a separating system  25  to remove possible liquid droplets) to the super heater  13  via the headers  18   a . The first stage of the super heater  13  can either be the upper (vertical) boiler enclosure wall or the internal hanger tubes ending in the first super heater bundle. 
     Downstream of the superheater  13 , superheated steam is directed to the high pressure turbine  13   a  for example of a power plant or for other high pressure user or to the reheater  14  inlet via the high pressure bypass valve  26 . 
     Steam from the high pressure turbine  13   a  or other high pressure user is collected at the inlet header  17  of the reheater  14  and, after passing through the reheater  14  it is collected in the outlet header  18  from which it is directed to the medium or low pressure turbine  14   a  or medium or low pressure user or via the low pressure bypass valve  27  to the condenser  35  provided downstream of the steam turbine. 
     Liquid droplets collected at the separating system  25  are directed back through the recirculation pump  29  to the economizer  20 . 
     During shut down the firing system  6  is stopped, the high pressure turbine  13   a  and the medium or low pressure turbine  14   a  are disconnected and the valves  26  and  27  are closed. 
     For this reason, the steam passing through the superheater  13  and reheater  14  is stopped, i.e. there is no further steam flow within the heating surfaces  16  of the superheater  13  and the reheater  14 . 
     Nevertheless, during shut down air keep circulating through the chamber  4 , this is due for example to purging or natural draft. For example, often the fan  7  operates for maintaining an underpressure inside the boiler enclosure also during shut down. This causes an air flow at temperature lower than the temperature of the steam within the superheater  13  and reheater  14 . 
     The flow increases the cooling of the steam contained within the tubed heat exchanging surfaces  16  of the superheater  13  and reheater  14 . This cooling can be large, because the thickness of the surfaces of the tubed heat exchanging surfaces  16  is usually small, such that the thermal storage capacity of the tube walls is low. 
     In contrast, the steam contained within the headers  17 ,  18  only undergoes a very limited cooling. 
     In fact, the headers  17 ,  18  have a large wall thickness and therefore they also have a large thermal storage capacity. 
     In addition, the headers  17 ,  18  are insulated such that substantial cooling from the outside of the headers  17 ,  18  is prevented; moreover, since there is no steam flow inside the headers  17 ,  18 , no substantial cooling from the inside of the headers  17 ,  18  occurs. 
     As a consequence, the temperature of the steam and of the header  17 ,  18  of the reheater  14  and superheater  13  (i.e. of the material of the header  17 ,  18 ) will decrease only with a very small gradient (i.e. the temperature of this steam slowly decreases), but the temperature of the steam contained in the tubed heat exchanging surfaces  16  of the reheater  14  and superheater  13  sensibly drops. 
     When the boiler  1  is start up again after shut down, the firing system  6  is started and the high pressure bypass valve  26  and the low pressure bypass valve  27  are opened. 
     Opening the high pressure bypass valve  26  and the low pressure bypass valve  27  causes steam circulation through the tubed heat exchanging surfaces  16  and the headers  17 ,  18  of the superheater  13  and the reheater  14 . This circulation causes steam at a low temperature (because it was contained within the tubed heat exchanging surfaces  16  during shut down) to pass through the headers  17 ,  18  that have a much higher temperature. 
     This circulation thus causes thermal stress of the material of the header  17 ,  18  and possibly a reduction of the lifetime. 
     SUMMARY 
     An aspect of the disclosure includes providing a method by which the thermal stress of the headers of the superheater and/or reheater can be limited. 
     These and further aspects are attained by providing a method in accordance with the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages will be more apparent from the description of a preferred but non-exclusive embodiment of the method, described with reference to the non-limiting accompanying drawings, in which: 
         FIG. 1  is a schematic view of a boiler. 
     
    
    
     DETAILED DESCRIPTION 
     In the following reference to the boiler of  FIG. 1  is made. 
     The method can be applied to any boiler also different from the one shown. For example the walls  3  can extend up to the top of the boiler (i.e. they can define the duct  12  and house the tubed coils or tubed panels  16 ). The walls can either be completely used as evaporator or can be divided in evaporator (lower part) and superheater (upper part). In addition the evaporator can have a different structure than the tubed walls  3 . 
     The method is preferably implemented to limit the stress of the headers  17 ,  18  of the superheater  13 , but it can also be conveniently used to limit the stress to the headers  17 ,  18  of the reheaters  14  or of other parts of the boiler  1 . 
     The method comprises regulating the temperature of the headers  17 ,  18  during shut down to a target temperature that is a function of the expected temperature for the steam moving from the tubed heat exchanging surfaces  16  into the headers  17 ,  18  at a starting up following the shut down. The target temperature is for example the expected temperature for the steam moving from the tubed heat exchanging surfaces  16  into the headers  17 ,  18  or a temperature preferably close to this expected temperature and in this last case the temperature is lower than the expected temperature. 
     In particular this temperature regulation is a cooling of the headers  17 ,  18 . 
     This cooling is mainly done after shut down, that means without additional use of expensive fuel, only by using the boiler pressure storage capacity and the boiler heat content in an appropriate way. 
     Thanks to this controlled cooling of the headers  17 ,  18 , when the boiler  1  is started up after shut down, the steam moves from the tubed heat exchanging surfaces  16  through the the headers  17 ,  18  and since the temperature of the steam does not differ from the temperature of the headers  17 ,  18  or the difference is a limited controlled and calculated difference, the thermal stress undergone by the headers  17 ,  18  is limited. 
     Preferably regulating the temperature of the heaters  17 ,  18  comprises maintaining a flow through the headers  17 ,  18  during the shut down or at least part of the shut down. 
     In fact, if steam keeps circulating through the tubed heat exchanging surfaces  16  and headers  17 ,  18 , the headers  17 ,  18  are cooled by the steam that circulates through them and that is in turn cooled by the flow through the duct  12 . 
     Maintaining the flow through the headers  17 ,  18  can be implemented by maintaining a steam flow through the control valve  26  and valve  27 . In fact, the flow through the valve  26  allows cooling of the headers  17 ,  18  of the superheater  13  and the flow through the valve  27  allows to cool the headers  17 ,  18  of the reheater  14 . Preferably the mass flow through the valve  26  and  27  is less than 10% of the nominal mass flow. 
     In a preferred embodiment, the method is implemented in connection with the tubed heat exchanging surfaces  16  of the superheater  13  and the control valve  26  is downstream of the superheater  13 . 
     In addition, a gas flow is preferably maintained through the duct  12  during shut down. Maintaining a gas flow through the duct  12  includes operating the fan  7 . For example the fan  7  is operated at minimum load or at a load less than 10% of its nominal mass flow. Operating the fan  7  is anyhow not mandatory and natural draft can suffice for air circulation. 
     The method can also comprise regulating the pressure within the boiler, i.e. within the heat exchanging components; pressure regulation can be done before shut down or during shut down. Preferably such a regulation aims at increasing the pressure within the boiler  1 . 
     In a first example, regulating the pressure includes regulating the high pressure by-pass control valve  26  or the turbine inlet valve. 
     In a different example, regulating the pressure includes circulating water through the economizer  20  and evaporating at least partly water passing through the economizer  20 . Circulation through the economizer  20  can be achieved by stopping the recirculation pump  29  and opening the line  30  (eco steaming line) provided between the top level of the economiser and the separating system  25 . 
     Continuously operating the fan  7  for a certain time after shut down or using the natural boiler draft causes a permanent heat input on the economiser surfaces with steam production. This steam production is used to improve the pressure maintenance during the header cooling process. Maintaining a small feedwater flow (continuous or discontinuous) avoids a complete steaming of the economiser. 
     Naturally the features described may be independently provided from one another. 
     In practice the materials used and the dimensions can be chosen according to requirements and to the state of the art.