Abstract:
A combined cycle power plant includes a gas turbine, a condensing stage, a steam turbine, and a heat recovery steam generator (HRSG). The HRSG is configured to generate steam for driving the steam turbine in response to heat transferred from exhaust gas received from the gas turbine at a first temperature and to transmit the exhaust gas to the condensing turbine at a second temperature that is lower than the first temperature.

Description:
BACKGROUND 
       [0001]    This invention relates generally to combined cycle plants, and more particularly to a system and method for increasing efficiency and water recovery of a combined cycle power plant using a gas turbine operated at a pressure ratio greater than about 30. 
         [0002]    A combined cycle power plant utilizes a gas turbine and a steam turbine in combination to produce power. The power plant is arranged such that the gas turbine is thermally connected to the steam turbine through a heat recovery steam generator (“HRSG”). The HRSG is a non-contact heat exchanger that allows feedwater for the steam generation process to be heated by otherwise wasted gas turbine exhaust gases. The HRSG is always located downstream of a gas turbine in a conventional design. 
         [0003]    A variety of techniques have been employed to reduce the size of HRSGs for combined cycle power plants. One known technique includes reducing the heat transfer surface area of the HRSG, which reduces the electrical efficiency of power plants. A variety of techniques have also been employed to recover water from the turbine exhaust gas. Some known techniques include the use of a liquid desiccant system or air-cooled condenser. 
         [0004]    There is a need for a combined cycle power plant that provides increased electrical efficiency in a manner that is more cost effective than presently achievable with combined cycle power plants using known techniques. The HRSG for the combined cycle power plant should be smaller, resulting in reduced cost. The combined cycle power plant should also be capable of recovering water from the gas turbine exhaust gas in a more cost effective manner than combined cycle power plants presently using known techniques. 
       BRIEF DESCRIPTION 
       [0005]    According to one embodiment, a combined cycle power plant comprises: 
         [0006]    a gas turbine; 
         [0007]    a condensing stage; 
         [0008]    a steam turbine; and 
         [0009]    a heat recovery steam generator (HRSG), wherein the condensing stage is configured to recover water from exhaust gas generated via the gas turbine, and further wherein the gas turbine, condensing stage, steam turbine and HRSG are together configured to convert latent heat of water vapor generated from the recovered water into useful electricity. 
         [0010]    According to another embodiment, a combined cycle power plant comprises: 
         [0011]    a gas turbine; 
         [0012]    a condensing turbine; 
         [0013]    a steam turbine; and 
         [0014]    a heat recovery steam generator (HRSG) connected downstream from the gas turbine and upstream from the condensing turbine in the combined cycle, wherein the HRSG is configured to generate steam for driving the steam turbine. 
         [0015]    According to yet another embodiment, a combined cycle power plant comprises: 
         [0016]    a gas turbine; 
         [0017]    a condensing turbine; 
         [0018]    a steam turbine; and 
         [0019]    a heat recovery steam generator (HRSG) configured to generate steam for driving the steam turbine in response to heat transferred from exhaust gas received from the gas turbine at a first temperature and to transmit the exhaust gas to the condensing turbine at a second temperature that is lower than the first temperature. 
     
    
     
       DRAWINGS 
         [0020]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawing, wherein: 
           [0021]      FIG. 1  illustrates a combined cycle power plant according to one embodiment; and 
           [0022]      FIG. 2  illustrates an aero derivative gas turbine combined cycle power plant according to one embodiment. 
       
    
    
       [0023]    While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0024]      FIG. 1  illustrates a combined cycle power plant  10  according to one embodiment. The power plant  10  comprises a high pressure gas turbine system  12  with a combustion system  14  and a turbine  16 . The gas exiting turbine  16  may be at a pressure, for example, of about 45 psi for one particular application. The power plant  10  further comprises a steam turbine system  18 . The steam turbine system  18  comprises a high pressure section  20 , an intermediate pressure section  22 , and one or more low pressure sections  24 . The low pressure section  24  exhausts into a condenser  26 . 
         [0025]    The steam turbine system  18  is associated with a heat recovery steam generator (HRSG)  32 . The HRSG  32  is a counter flow heat exchanger such that as feedwater passes there through, the water is heated as the exhaust gas from turbine  16  gives up heat and becomes cooler. The HRSG  32  has three (3) different operating pressures (high, intermediate, and low) with means for generating steam at the various pressures and temperatures as vapor feed to the corresponding stages of the steam turbine system  18 . The present invention is not so limited however; and it can be appreciated that other embodiments, such as those embodiments comprising a two-pressure HRSG will also work using the principles described herein. The steam turbine system  18  may comprise, for example, a low pressure steam turbine  34 , an intermediate steam turbine  36  and a high pressure turbine  38 . Each section  20 ,  22 ,  24  generally comprises one or more economizer, evaporators, and superheaters. 
         [0026]    The combined cycle power plant further comprises a low pressure condensing turbine  40 . The temperature of the gas  42  exiting the high pressure turbine  16  is an optimized value based upon the particular application requirements. This temperature may be, for example, about 1100° F. for one particular application. The optimized gas  42  enters the HSRG  32  to drive the bottoming cycle, where the gas is cooled down to a lower temperature that may be for example, about 180° F. for one particular application. The cooled gas enters the low pressure condensing turbine  40  to produce more power. 
         [0027]    The lower pressure condensing turbine  40  expands the cooled high pressure turbine exhaust gas to about atmospheric pressure. Generally, the temperature of the gas leaving the low pressure turbine  40  is below the dew point, resulting in formation of water droplets in the outlet of the low pressure turbine  40 . The wet gas exiting the low pressure turbine  40  enters a moisture separator  44  where the water is collected and where the water-depleted exhaust gas is vented to atmosphere through an exhaust gas stack  46 . 
         [0028]    The HRSG  32  uses the heat of the turbine exhaust gas  42  to produce three (s) steam streams, a high pressure steam stream  48 , an intermediate pressure steam stream  50 , and a low pressure steam stream  52 . These three steam streams  48 ,  50 ,  52  enter the high, intermediate and low pressure steam turbines  38 ,  36 ,  34  to produce power. A high pressure steam  54  stream extracted from the high pressure steam turbine  38  is injected to the gas turbine combustor  14 . 
         [0029]    Subsequent to exiting the low pressure steam turbine, the steam stream enters the condenser  26  where the steam is condensed into liquid water. The liquid water exiting the condenser  26  along with make-up water  56  and water streams  58 ,  60  from the moisture separator  44  and HRSG  32  enters a water collector  62 . 
         [0030]    An appropriate amount of water is pumped  64  from the water collector  62  to the HRSG  32  where the water absorbs the heat from the high pressure gas turbine exhaust to generate the steam streams  48 ,  50 ,  52 . The three steam streams  48 ,  50 ,  52  enter the steam turbines  38 ,  36 ,  34  to complete the bottoming cycle. 
         [0031]    In summary explanation, a combined cycle power plant scheme has been described that significantly increases the efficiency of a combined cycle with a gas turbine operated at a high pressure ratio and that recovers water from the turbine exhaust gas. At least one known combined cycle is increased by at least three (3) percentage points and recovers water using a condensing turbine stage according to one aspect using the principles described herein. The HRSG is always located downstream of a gas turbine in a conventional combined cycle power plant scheme, while a HRSG is connected between a high pressure gas turbine stage and a low pressure gas turbine (condensing) stage according to embodiments described herein. Since the HRSG in the embodiments described herein recovers heat from a high pressure turbine exhaust gas to generate steam for the bottoming cycle, the size of the HRSG can be significantly reduced. According to one embodiment, the high pressure turbine exhaust gas may be about 50 psi, depending upon the particular turbine design and application. Since the bottoming cycle is driven by a turbine exhaust gas at a temperature of about 1100° F. according to one embodiment, the bottoming cycle efficiency is increased above that achievable using a conventional combined cycle power plant scheme in which the bottoming cycle is driven by a turbine exhaust gas at a temperature between 700° F. and 800° F. Further, the use of a low pressure (condensing) gas turbine in combination with a high pressure gas turbine using the principles described herein allows for expansion of gas leaving the HRSG and condensing of water from the exhaust gas. The embodiments described herein thus employ a condensing stage of a gas turbine to recover water from the turbine exhaust gas and to transform the latent heat of water vapor to useful electricity, leading to a higher combined cycle efficiency. 
         [0032]      FIG. 2  illustrates an aero derivative gas turbine combined cycle power plant  100  according to one embodiment. The power plant  100  is a combined cycle power plant that employs an LMS  100  gas turbine produced by General Electric Company having a place of business in Schenectady, N.Y. The efficiency of the LMS  100  combined cycle has been demonstrated to increase by at least three (3) percentage points and to recover water using a condensing turbine stage when employed according to the principles described herein with reference to  FIG. 1 . 
         [0033]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.