Patent Publication Number: US-9835056-B2

Title: Lignite drying integration with a water/steam power cycle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Patent Application No. 15290140.1 filed May 26, 2015, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to integrate lignite drying processes to improve both efficiency and cost-of-power generation of a dry-lignite coal power plant applicable with or without CO2 capture. The disclosure further relates to arrangements for using direct steam extraction from water/steam power cycle as an energy source for lignite coal drying. 
     BACKGROUND INFORMATION 
     General principle of Lignite Drying in a lignite fired plant is well known, using either hot flue gas extraction or steam extraction from Water &amp; Steam cycle or both to supply the lignite drying system that includes beater mills, rotary drum dryers and/or fluidized bed dryer. 
     Lignite drying techniques have been developed and tested in order to use medium or low enthalpy heat to achieve partial or high level of lignite pre-drying before pulverization, and gain typically up to 3% point efficiency gains without heat recovery of evaporation vapour of lignite moisture or 5% point efficiency gains with heat recovery of evaporation vapour of lignite moisture. The heat is either originating from low pressure steam extraction, or from exhaust flue gas. These techniques sometime additionally use mechanical or chemical dewatering processes. 
     U.S. Pat. No. 8,661,821 B2 in which superheated steam, which has done partial work in a steam turbine, is extracted from a water/stem power cycle and used as a drying medium to evaporate moisture from coal powder. Condensate from the drying is then fed into a deaerator of the steam turbine via a condensate pump for recirculation. As discussed, the drying steam can be extracted from any number of steam extraction points contained in the water/steam power cycle. 
     SUMMARY 
     A power plant is disclosed that is intended to provide an alternative means of thermally integrating a lignite dryer into a water/steam cycle of the power plant using steam extraction. 
     It attempts to address this problem by means of the subject matters of the independent claims. Advantageous embodiments are given in the dependent claim. 
     An aspect includes power plant with a water/steam power cycle, lignite dryer. The water/steam cycle comprises a pressure series of steam turbines including a high pressure steam turbine, an intermediate pressure steam turbine, and a low pressure steam turbine. The cycle further includes a re-heater that is fluidly located between the high pressure steam turbine and the intermediate pressure steam turbine. 
     The lignite dryer includes a heater connected to a steam portion of the steam/water power cycle so as to enable utilisation of steam energy in the lignite dryer ( 10 ). 
     The connection to the steam portion of the steam water power cycle comprises a first extraction line that is fluidly connected to the water/steam power cycle between the re-heater and the intermediate pressure steam turbine, or alternatively between the high pressure turbine and the re-heater and to the heater. The first extraction line further includes an ejector. The connection further includes a second extraction line that is fluidly connected to the water/steam power cycle between the intermediate pressure steam turbine ( 34 ) and the low pressure steam turbine ( 35 ). 
     The configuration and location of the ejector and the connection of the second extraction line to the ejector enables a lower pressure steam in the second extraction line to be fed into the heater together with a higher pressure steam in the first extraction line. 
     In an aspect the second extraction line includes a bypass that fluidly connects the first extraction line to the second extraction line so as to bypass the ejector. 
     In further aspect the power plant includes a de-superheater in the first extraction line upstream of the ejector. 
     In further aspect the power plant includes a throttle valve fluidly located between the connection of the second extraction line to the water/steam power cycle and the low pressure steam turbine. 
     Another aspect includes a method of controlling a power plant with lignite dryer. The method includes the steps of providing a water/steam power cycle having a pressure series of steam turbines including a high pressure steam turbine, an intermediate pressure steam turbine, and a low pressure steam turbine. The water/steam power cycle further includes a re-heater fluidly between the high pressure steam turbine and the intermediate pressure steam turbine and a throttle valve fluidly between the intermediate pressure steam turbine and the low pressure steam turbine. 
     The method further includes providing a lignite dryer having a heater fluidly connected to a steam portion of the steam/water power cycle so as to utilise steam energy in the lignite dryer, wherein the connection to the steam portion of the steam water power cycle comprises a first extraction line, connected to the water/steam power cycle between the re-heater and the intermediate pressure steam turbine or alternatively between the high pressure turbine and the re-heater, to the heater, including an ejector and further comprises a second extraction line that is fluidly connected to the water/steam power cycle between the intermediate pressure steam turbine and the throttle valve, the second extraction line including a bypass, with a bypass valve. 
     The method includes the further step of controlling a flow-rate to the heater by adjusting a pressure in the second extraction line in conjunction with the bypass valve. 
     In a further aspect the method includes providing a first control valve in the first extraction line upstream of the ejector and a second control valve in the second extraction line upstream of the ejector and then controlling the flow-rate to the heater in further conjunction with the first control valve and the second control valve. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example, an embodiment of the present disclosure is described more fully hereinafter with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic of a lignite fired power plant according to an exemplary embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  shows an exemplary embodiment of a power plant with a drying system to dry pulverised lignite. 
     The drying includes an inlet line  11  for directing lignite in the lignite dryer  10 , a vapour outlet line  16  for exhausting moisture laden gas from the lignite dryer and a solids outlet line  12  for discharging dried lignite for use in a combustor. The lignite dryer  10  can be a Steam Fluidized Bed Dryer or a Steam Heated Rotary Tube Dryer. 
     In an exemplary embodiment shown in  FIG. 1 , the power plant includes a water/steam cycle a water/steam power cycle having a pressure series of steam turbines  32 , 34 , 35 , a condenser  38  at a low pressure end of pressure series of steam turbines  32 ,  34 , 35  configured and arrange to condense steam exhausted from the low pressure end of the pressure series of steam turbines, a low pressure condensate system  40  arrangement downstream of the condenser  38 , adapted to preheat condensate from the condenser  38 , a high pressure condensate system  44  separated from the low pressure condensate system  40  by a feed water tank  66 , and boiler  50  for boiling and superheating condensate from the high pressure condensate system  44  and to further and optionally performs the function of a re-heater  52  for reheating steam between the pressure series steam turbines  32 , 34 , 35 . 
     In an exemplary embodiment shown in  FIG. 1 , a first extraction line  104  extends from a point in the water/steam power cycle between the re-heater  52  and the intermediate pressure steam turbine to the heater  13  of the lignite dryer  10 . This enables extraction steam to be used as an energy source for the lignite dryer  10 . In an exemplary embodiment the first extraction line  104  includes a de-superheater. 
     In an exemplary embodiment, shown in  FIG. 1 , includes an additional extraction line  101  with an ejector  106 . This additional extraction line  101  extends from a point of the water/steam cycle located between the intermediate pressure steam turbine  34  and the low pressure steam turbine  35  to the first extraction line  104  at the ejector  106 . 
     The ejector  106  is a device that operates using the venturi principle. The device utilises higher pressure steam from the first extraction line  104  to generate a high-velocity jet at the throat of a convergent-divergent nozzle thus creating a low pressure at that point. The low pressure point, which is the point at which additional extraction line  101  connects to the first extraction line  104 , draws extraction steam from the lower pressure additional extraction line. In this way lower pressure steam in the second extraction line can be fed into the heater together with higher pressure steam of the first extraction line  104 . 
     In an exemplary embodiment shown in  FIG. 1  the first extraction line  104  includes a de-superheater  104   b.  In an exemplary embodiment where the first extraction line  104  includes an ejector  106 , the de-superheater  104   b  is located upstream of the ejector  106 . 
     In an exemplary embodiment shown in  FIG. 1  in which the first extraction line  104  includes an ejector  106 , the additional extraction line  101  includes a bypass  103  with a bypass valve  101   a,  connecting the first extraction line  104  to the second extraction line  101  so as to bypass the ejector  106 . This arrangement can be used when the steam plant is operating a high or maximum load such that the steam pressure in the additional extraction line  101  has sufficient pressure and energy to supply the lignite dryer  10  while maximising energy recovery in the intermediate pressure steam turbine  34  by minimising extractions from this turbine. 
     In further exemplary embodiments shown in  FIG. 1 , a throttle valve  102  is located in the water/steam power cycle between the connection of the second extraction line  101  and the low pressure steam turbine so as to enable control extraction pressure in the additional extraction line  101 . This can be achieved by coordinated operation of the throttle valve  102  with the bypass valve  101   a.  For example at full and very high loads throttle valve  102  is fully opened while the bypass valve  101   a  is used to control supply pressure at adequate level. As load decreases, resulting in a lower low pressure steam turbine  35  pressure, the bypass valve  101   a  is opened further until in the fully opened position. At this point, or else at a pre-set opening point, the throttle valve  102  begins to close thus maintain the required intermediate pressure steam turbine  34  exit pressure at level required to supply steam to the lignite dryer  10 . The operation limit of the additional extraction with this arrangement, without use of the first extraction may be limited by the maximum low pressure steam turbine  35  steam temperature limit. As a result, this solution is most applicable for high loads, for example above 70%, as low pressure steam turbine  35  temperature limitations typically limit throttling at lower load. 
     In an exemplary embodiment where temperature limitations of the low pressure steam turbine  35  are reached, the bypass valve  101   a  is closed while steam in the additional extraction line  101  is mixed with hot reheat extraction steam from the first extraction  104  using the ejector  106 . This arrangement may be used for middle and low water/steam power cycle loads down, for example, 35% or even lower, depending on the design limits of the water/steam power cycle components. The control of the exemplary embodiment may be further enhance by providing a first control valve  104   a  in the first extraction line  104  upstream of the ejector  106  and a second control valve  101   b  in the second extraction line  101  upstream of the ejector  106 . In this arrangement the flow-rate to the heater is further controlled in further conjunction with the first control valve  104   a  and the second control valve  101   b.    
     Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment, the present disclosure can be embodied in other specific forms. 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.