Abstract:
A power plant with a fossil fuel fired boiler, an air feed system, flue gas system and condensate system. A unitary flue gas heat exchanger spans a bypass line in the flue gas system and the condensate system so as to improve the thermal efficiency of the power plant while minimising complexity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Application No. PCT/EP2014/068073 filed Aug. 26, 2014, which claims priority to EP Application No. 13185151.1 filed Sep. 19, 2013, both of which are hereby incorporated in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to power plants comprising fossil fuelled boilers and in particular to energy efficient configurations thereof. 
       BACKGROUND 
       [0003]    A typical fossil fuel power plant, described for example in EP 2351914 A, has a boiler comprising an economizer, an evaporator, a super heater, and a reheater, as well as several steam turbine modules comprising high-pressure, intermediated-pressure and low-pressure stages mounted on a shaft that drives a generator. Steam generated by the boiler is first expanded in the high-pressure turbine stage after which it is reheated in the reheater before being subsequently fed into the intermediate-pressure turbine stage. Steam exhausted from the intermediate stage, after being further expanded in the low-pressure turbine stage, is fed into a condenser. Condensate collected in the well of the, condenser is pumped by one or more extraction pumps to condensate preheaters that preheat the condensate using steam extracted from the low- and intermediate-pressure steam turbines. The plant may further comprise one or more high pressure preheaters located downstream of a feed water pump system used to boost the pressure of the condensate downstream of the low-pressure preheaters. Steam extracted from the high pressure turbine stage may be used as a heat source for these preheaters. Preheated condensate from the high pressure preheaters is then feed into the boiler/ economiser thus completing a closed loop steam condensate cycle. 
         [0004]    A typical fossil fuel power plant includes a fossil fuel system, which may comprise a coal milling facility that feeds the coal into the boiler, an air system for supplying combustion air, and a flue gas exhaust system for directing flue gas from the boiler. The flue gas system typically includes an air preheater for preheating air before it enters the boiler and a particulate collection system, such as an electrostatic precipitator or fabric filter and a desulphurization unit that polishes the flue gas before it is emitted through a stack or cooling tower. Preheating of air using flue gas is one means of improving the thermal efficiency of the plant. There is, however, a constant need to provide plants that balance increasing thermal efficiency demands at reduced cost which may be achieve through reduced complexity. 
       SUMMARY 
       [0005]    A fossil fuel power plant arrangement is provided that addresses the dual problem of plant complexity and thermal efficiency. 
         [0006]    This problem is addressed by means of the subject matter of the independent claim. Advantageous embodiments are given in the dependent claims. 
         [0007]    The disclosure is based on the general idea of providing a unitary flue gas heat exchanger that spans the bypass line and the second condensate line and is configured and arranged to transfer thermal energy from flue gas to condensate. 
         [0008]    An aspect provides a power plant that has a fossil fuel fired boiler, an air feed line for feeding air into the boiler, a flue gas line connected to the boiler for exhausting flue gas from the boiler, an air preheater configured and arranged in both the air feed line and the flue gas line to exchange thermal energy from flue gas exiting the boiler to air feed into the boiler. The aspect further provides a bypass line that forms part of the flue gas line and is arranged to bypass the air preheater, a first condensate line, connected to the boiler for feeding condensate into the boiler, with a plurality of preheaters, and a second condensate line with first and second distal ends connected to the first condensate line. The aspect further includes a unitary flue gas heat exchanger, spanning the bypass line and the second condensate line. The unitary flue gas heat exchanger provides an efficient yet simple means of transferring thermal energy from the flue gas to the condensate in the condensate lines. 
         [0009]    In an aspect, the condensate line comprises one or more low pressure preheaters, one or more high pressure preheater and a feed water pump system that is fluidly located between the one or low pressure preheaters and the one or more high pressure preheaters. The feed water pump system is configured and arranged to boost condensate pressure in the first condensate line. A first end of the second condensate line is located downstream of the low pressure heater and upstream of the feed water pump system and an auxiliary feed water pump system is located in the second condensate line. 
         [0010]    In different aspects the auxiliary feed water pump system is located either upstream or downstream of the flue gas heat exchanger. 
         [0011]    In an aspect, the second end of the second condensate line is located downstream of the one or more high pressure preheaters, while in another aspect the second end of the second condensate line is located upstream of the feed water pump system. 
         [0012]    In an aspect, the second condensate line between the flue gas heat exchanger and the second end passes through at least one of the one or more high pressure preheaters on the steam side so as to enable heating of condensate passing through the at least one or more high pressure preheaters. 
         [0013]    A further aspect includes a third condensate line with a first end located fluidly between the feed water pump system and the one or more high pressure preheaters and a second end located in the first condensate line downstream of the one or more high pressure preheaters . The aspect also includes a condensate heat exchanger in the third condensate line. In this aspect, the second end of the second condensate line is located fluidly between the one more low pressure preheaters and the feed water pump system while a portion of the second condensate line, located downstream of the flue gas heat exchanger, passes through the condensate heat exchanger thereby enabling condensate passing through the third condensate line to be heated by condensate heated within the flue gas heat exchanger. 
         [0014]    In an aspect, the condensate line comprises one or more low pressure preheaters, one or more high pressure preheaters and a feed water pump system that is fluidly located between the one or more low pressure preheaters and the one or more high pressure preheaters and further configured and arranged to boost condensate pressure in the first condensate line. The first end of the second condensate line is located fluidly between the feed water pump system and the one or more high pressure preheaters and the second end of the second condensate line is located downstream of the one or more high pressure preheaters. 
         [0015]    It is a further object of the invention to overcome or at least ameliorate the disadvantages and shortcomings of the prior art or provide a useful alternative. Other aspects and advantages of the present disclosure will become apparent from the following description, taken in connection with the accompanying drawings which by way of example illustrate exemplary embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    By way of example, an embodiment of the present disclosure is described more fully hereinafter with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is a schematic of a prior art steam plant arrangement; 
           [0018]      FIG. 2  is a schematic of an exemplary embodiment of a steam plant having an intermediate hot/warm condensate heat exchanger; 
           [0019]      FIG. 3  is a schematic of a further exemplary embodiment having a unitary flue gas/condensate heat exchanger combined with a cascade of condensate preheaters; 
           [0020]      FIG. 4  is a schematic of a further exemplary embodiment having a unitary flue gas/condensate heat exchanger combined from a low pressure feed water tank and an auxiliary feed water pump system; and 
           [0021]      FIG. 5  is a schematic of a further exemplary embodiment having a unitary flue gas/condensate heat exchanger combined from a low pressure feed water tank to boiler inlet with an auxiliary feed water pump system. 
           [0022]      FIG. 6  is a schematic of a further exemplary embodiment having a unitary flue gas/condensate heat exchanger combined from a low pressure feed water tank to boiler inlet with a dedicated feed water pump system. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Exemplary embodiments of the present disclosure are now described with references to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiment disclosed herein. 
         [0024]    Exemplary embodiments herein described may be applied to a fossil fuel fired power plant as shown in  FIG. 1 . Such a fossil fuel power plant comprises, a boiler  10 , an air line  30  for feeding mainly combustion air into the boiler  10 , a flue gas line  20  for exhausting flue gas from the boiler  10 , and a condensate line  40  that forms parts of a closed loop water/steam circuit for feeding condensate into the boiler  10 . 
         [0025]    Typically, an air preheater  32  spans the flue gas line  20  and the air line  30 . The flue gas line  20  may further include a particulates collection unit  24 , for example an electrostatic precipitator and a desulphurisation unit  26  connection prior to an exhaust stack  28  or cooling tower  28 . 
         [0026]    The condensate line  40  includes a plurality of preheaters  50 ,  52  that may include one or more low pressure preheaters  50  and one or more high pressure preheaters  52 . In such an arrangement, a feed water pump system  44 , located between the low pressure preheaters  50  and the high pressure preheaters  52  boosts the pressure of condensate passing through the condensate line  40 . An optional low pressure feed water tank  48 , forming part of the condensate line  40 , may be located between the feed water pump system  44  and the one or more low pressure preheaters  50 . 
         [0027]    In an exemplary embodiment shown in  FIG. 2 , the flue gas system of the power plant has a bypass line  22  around the air preheater  32  that acts as a means to enable as least some flue gas from the boiler to bypass the air preheater  32  partially located in the flue gas line  20 . In addition, the condensate system includes a second condensate line  42  that extends from the condensate line  40  between the one or more low pressure preheaters  50  and the feed water pump system  44 , through an auxiliary feed water pump system  46  and a flue gas heat exchanger  54  located on the bypass line  22  and then through a condensate heat exchanger  56  before returning back to the same region of the condensate line  40 , either directly to the line portion or else to a low pressure feed water tank  48  that forms part of the condensate line  40 . 
         [0028]    The flue gas heat exchanger  54 , which is a unitary heat exchanger, that is the only heat exchanger in the bypass line  22 , enables the transfer of thermal energy from the flue gas system into the condensate system. 
         [0029]    A third condensate line  56  has ends joining the condensate line  40  at a point between the one or more high pressure preheaters  52  and the feed water pump system  44  and at a point between the boiler  10  and the one or more high pressure preheaters  52 . The third condensate line  56  further passes through the condensate heat exchanger  56 . The second condensate line  42  and the third condensate line  43  to the condensate are thermally connected via a heat exchanger  56 . 
         [0030]    In an exemplary embodiment shown in  FIG. 3 , the flue gas system of the power plant has a bypass line  22  around the air preheater  32  that acts as a means to enable as least some flue gas from the boiler to bypass an air preheater  32  that is partially located in the flue gas line  20  and the air line  30 . The condensate system additionally includes a second condensate line  42  that extends from the condensate line  40  between the one or more low pressure preheaters  50  and the feed water pump system  44 , through and auxiliary feed water pump system, a flue gas heat exchanger  54  located on the bypass line  22  and then through the one or more high pressure preheaters  52  before returning back to the same region of the condensate line  40 , either directly to the line or via a low pressure feed water tank  48 . 
         [0031]    The flue gas heat exchanger  54 , which is a unitary heat exchanger, that is the only heat exchanger on the bypass line, enables the transfer of thermal energy from the flue gas into the condensate system. The further connection of the second condensate line  42  to the one or more high pressure preheaters  52  enables the transfer of thermal energy gained by condensate in the flue gas heat exchanger  54  into the main condensate flow passing through the one or more high pressure preheaters  52 . 
         [0032]    In an exemplary embodiment shown in  FIG. 4 , the flue gas system of the power plant has a bypass line  22  around the air preheater  32  that acts as a means to enable at least some flue gas from the boiler  10  to bypass an air preheater  32  that is partially located in the flue gas line  20  and the air line  30 . In addition, the condensate system includes a second condensate line  42  that extends from the feed water pump system  44 , through a flue gas heat exchanger  54  located on the bypass line  22 , and then through an auxiliary feed water pump system  46 , before returning back to a region of the condensate line  40  between the boiler  10  and the one or more high pressure preheaters  52 . 
         [0033]    The flue gas heat exchanger  54 , which is a unitary heat exchanger, that is the only heat exchanger in the bypass line  22 , enables the transfer of thermal energy from the flue gas into the condensate system. 
         [0034]    In an exemplary embodiment shown in  FIG. 5 , the flue gas system of the power plant has a bypass line  22  around the air preheater  32  that acts as a means to enable at least some flue gas from the boiler to bypass an air preheater  32  that is partially located in the flue gas line  20  and the air line  30 . In addition, the condensate system includes a second condensate line  42  that extends from the condensate line  40  between the one or more high pressure preheaters  52  and the feed water pump system  44 , through a flue gas heat exchanger  54  located on the bypass line  22 , and then through an auxiliary feed water pump system  46 , before returning back to a region of the condensate line  40  between the auxiliary feed water pump system  46  located in the condensate line  40  downstream of the one or more high pressure preheaters  52  and the one or more high pressure preheaters  52 . 
         [0035]    The flue gas heat exchanger  54 , which is a unitary heat exchanger, that is the only heat exchanger in the bypass line  22 , enables the transfer of thermal energy from the flue gas into the condensate system. 
         [0036]    In an exemplary embodiment shown in  FIG. 6 , the flue gas system of the power plant has a bypass line  22  around the air preheater  32  that acts as a means to enable at least some flue gas from the boiler  10  to bypass an air preheater  32  that is partially located in the flue gas line  20  and the air line  30 . In addition, the condensate system includes a second condensate line  42  that extends from the condensate line  40  between the one or more high pressure preheaters  52  and the feed water pump system  44 , through an auxiliary feed water pump system  46  and then through a flue gas heat exchanger  54  located on the bypass line  22 , before returning back to a region of the condensate line  40  between the boiler  10  and the one or more high pressure preheaters  52 . 
         [0037]    The flue gas heat exchanger  54 , which is a unitary heat exchanger, that is the only heat exchanger in the bypass line  22 , enables the transfer of thermal energy from the flue gas into the condensate system. 
         [0038]    In an alternate not shown exemplary embodiment, the auxiliary feed water pump system  46  is located downstream of the flue gas heat exchanger  54  instead of upstream of the flue gas heater exchanger  54  as shown in  FIG. 6 . 
         [0039]    Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment, it will be appreciated that the present disclosure can be embodied in other specific. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather that the foregoing description and all changes that come within the meaning and range and equivalences thereof are intended to be embraced therein.