Abstract:
In a process for the recovery of water, which arises from the combustion of a fuel ( 19 ), particularly natural gas, in a combined cycle power station ( 101 ) comprising a gas turbine plant ( 11 ), a waste heat boiler ( 33 ), and a steam turbine ( 25 ) arranged in a water/steam circuit with the waste heat boiler ( 33 ), and/or which is added in the form of water or steam ( 36 ), the water is condensed out from the flue gas ( 42 ) arising from the combustion of the fuel ( 19 ) and containing water in the form of water vapor after flowing through the waste heat boiler ( 33 ), and is separated in liquid form, in particular by expansion in a utilization turbine ( 20 ).

Description:
FIELD OF TECHNOLOGY  
         [0001]    The present invention relates to the field of combined cycle power stations (combined plant). It relates to a process according to the preamble of claim  1  and also to a combined cycle power station for carrying out the process according to the preamble of claim  10 .  
         STATE OF THE ART  
         [0002]    Combined cycle power stations normally burn natural gas, which produces water vapor during combustion. 1 kg of natural gas burns to about 2 kg of water and also CO 2 . The flue gas with the water contained therein is usually discharged through a chimney at elevated temperatures, without the water being used to advantage. The same also holds for water or steam, which in other cases is injected or sprayed in during combustion and thus becomes a component of the flue gas. On the other hand, water already forms an increasingly valuable resource at the present time.  
           [0003]    The simplified diagram of an exemplary combined cycle power station from the state of the art is reproduced in FIG. 1. The combined cycle power station  10  essentially comprises a gas turbine plant  11 , a waste heat boiler (heat recovery steam generator HRSG)  33 , and a steam turbine  25 , which are connected together. The gas turbine plant  11  consists of a compressor  14  and a turbine  17 , which are arranged on a rotor  16 , and also a combustion chamber  15 . In operation, the compressor  14  sucks in air through an air inlet  12 , compresses it, and delivers the compressed air to the combustion chamber  15 , where it enters combustion as combustion air, together with a liquid or gaseous fuel  19  (e.g., natural gas) which is fed in through a fuel supply duct  21 . In addition, water or steam  36  can be injected or sprayed into the combustion chamber  15  to reduce the combustion temperature. The hot combustion gases are conducted from the combustion chamber  15  into the turbine  17 , where they set the rotor  16  in rotation. The flue gas  42  exiting the turbine  17  is then conducted through the waste heat boiler  33  for the production of steam, where it flows in succession through a superheater  40 , an evaporator  39 , and a preheater (economizer)  34 , and gives up heat stepwise. The cooled flue gas  42  finally leaves the waste heat boiler  33  and is usually, possibly after a flue gas cleaning, discharged from a chimney.  
           [0004]    The superheater  40 , evaporator  39  and preheater  34  are connected in series as part of a water/steam circuit, in which the steam turbine is also connected. The exhaust steam from the steam turbine  25  passes into a condenser  26  and condenses there. The condensate is pumped by a condensate pump  28 , possibly with the addition of additional water  27 , through a feed water duct  29  to a feed water container  30  with degasser. The degassed condensate is then pumped as feed water  32  through the preheater  34  by a boiler feed pump  31 , and as pressurized feed water  35  to a steam drum  37  with the connected evaporator  39 . The steam then passes to the superheater  40 , where it is superheated, and finally drives the steam turbine  25  as superheated steam  41 . Both the steam turbine  25  and also the gas turbine plant  11  respectively drive a generator  13  or  24  which produces electrical current. Means for the recovery of water or water vapor contained in the flue gas are not provided here.  
         DESCRIPTION OF THE INVENTION  
         [0005]    The invention therefore has as its object to provide a process and also a combined cycle power station with which the water contained in the flue gas is recovered and can be advantageously reused.  
           [0006]    The object is attained by means of the entirety of the features of claims  1  and  10 . The core of the invention consists of configuring the combined cycle power station and conducting the process so that the water is condensed out of the flue gas after leaving the waste heat boiler and is separated in liquid form.  
           [0007]    A first preferred embodiment of the process according to the invention is characterized in that the flue gas is expanded for condensing the water out, with output of work. The expansion is preferably carried out by means of a utilization turbine. In particular, the waste heat boiler is operated for this purpose at a pressure exceeding the ambient air pressure by several bar, preferably 2-5 bar, and the flue gas is brought to the ambient air pressure by the subsequent expansion. Alternatively to this, the waste heat boiler can be operated at about the ambient pressure with respect to the flue gas, the flue gas subsequently being expanded into a vacuum, and the flue gas being compressed again to ambient air pressure after the separation of water. In both cases, the water is separated from the flue gas particularly during the expansion or in the utilization turbine itself, and/or in a droplet separator following the expansion or the utilization turbine.  
           [0008]    A second preferred embodiment of the process according to the invention is characterized in that the waste heat boiler is operated with respect to the flue gas at a pressure exceeding the ambient air pressure by several bar, preferably 2-5 bar, in that the water is condensed out of the flue gas on cold surfaces after leaving the waste heat boiler, in particular on the cold tubes of a heat exchanger, and in that the dewatered flue gas is brought to the ambient air pressure by a subsequent expansion; here also, the expansion is preferably carried out by means of a utilization turbine.  
           [0009]    A preferred embodiment of the combined cycle power station according to the invention is distinguished in that the means for condensation and separation comprise means for the expansion of the flue gas, preferably in the form of a utilization turbine.  
           [0010]    A first development of this embodiment is characterized in that the expansion means or the utilization turbine is followed by a droplet separator.  
           [0011]    A second development of this embodiment is characterized in that the expansion means comprises a utilization turbine working in vacuum, and that a compressor follows the utilization turbine.  
           [0012]    A third development of this embodiment is characterized in that condensation means, particularly in the form of a heat exchanger, is arranged between the waste heat boiler and the expansion means. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention will be explained in detail hereinafter using embodiment examples in combination with the accompanying drawings.  
         [0014]    [0014]FIG. 1 is a simplified diagram of an exemplary combined cycle power station from the state of the art, to which the invention can be applied;  
         [0015]    [0015]FIG. 2 is a T-S [temperature-entropy] diagram illustrating the conduct of the process in a preferred embodiment example of the process according to the invention;  
         [0016]    [0016]FIG. 3 is a diagram of a combined cycle power station comparable to FIG. 1, according to a first embodiment example of the invention, with an expansion of the flue gas to ambient air pressure;  
         [0017]    [0017]FIG. 4 is a diagram of a combined cycle power station comparable to FIG. 1, according to a second embodiment example of the invention, with an expansion of the flue gas into vacuum and subsequent compression to ambient air pressure;  
         [0018]    [0018]FIG. 5 is a diagram of a combined cycle power station comparable to FIG. 1, according to a third embodiment example of the invention, with an expansion of the flue gas to ambient air pressure after a preceding condensation of the water vapor. 
     
    
     PREFERRED EMBODIMENTS OF THE INVENTION  
       [0019]    The basic idea of the invention can be characterized as “supercharged waste heat boiler of a combined cycle power station with water separation by way of temperature drop or partial pressure”. The idea can be explained using the T-S diagram shown in FIG. 2 of an example of the performance of the process. From the point P 1  on the isobar of the ambient air pressure p A , the air sucked in by the compressor ( 14  in FIG. 1) is compressed along the curve a to the combustion chamber pressure (isobar P BK ) and is heated in the combustion chamber at constant pressure (curve b). The gas turbine ( 17  in FIG. 1) in a combined plant now expands the flue gas along the curve c to a pressure p K  in the waste heat boiler ( 33  in FIG. 1), which is at several bar, e.g., 2-5 bar, above the ambient air pressure p A . The “supercharged” waste heat boiler removes heat from the flue gas for the production of steam, and thus cools the flue gas to about 80-90° C. (curve d).  
         [0020]    If now this flue gas is expanded (along the curve e) by means of a utilization turbine or comparable means, low temperatures arise at the point P 2  after the turbine, corresponding to the pressure before the turbine. The water content of the flue gas condenses, particles which may be present in the flue gas serve as condensation nuclei. The water can then be separated in the turbine itself or in a subsequent droplet separator, and subsequently drawn off.  
         [0021]    A combined cycle power station  101  designed for the performance of this process is reproduced in FIG. 3, which is comparable to the illustration of FIG. 1 (the same parts are given the same reference numerals). The combined cycle power station  101  of FIG. 3 differs from the combined cycle power station  10  of FIG. 1 in the flue gas sequence following the waste heat boiler  33 . In the combined cycle power station  101 , a utilization turbine  20  (which for example drives a generator  22 ) for the exiting flue gas  42  containing water vapor, and also a droplet separator  23 , follow the waste heat boiler  33 . The flue gas  42  under pressure is cooled in the utilization turbine  20  by expansion. The water vapor then condenses and can be removed either already at the utilization turbine  20  or in the following droplet separator  23 . The “dewatered” flue gas  43  then leaves the droplet separator  23 . This kind of water recovery has the following advantages:  
         [0022]    The plant is very compact; the gas turbine can, e.g., be embodied as a variant of a standard machine without end stage;  
         [0023]    good heat transfer coefficients are obtained in the waste heat boiler;  
         [0024]    a chimney can be omitted, since the “dewatered” flue gas leaves the plant at low temperatures;  
         [0025]    the “cold” of the flue gases can be further utilized, e.g., for cooling purposes or at the intake side of the gas turbine (booster).  
         [0026]    On the other hand, a slight performance loss results, since the expansion line of the gas turbine is made smaller, and this is only partially compensated by the recuperation in the steam turbine and in the utilization turbine after the waste heat boiler. In order to remedy it, the water recovery can also be carried out in a modified form:  
         [0027]    In an alternative manner of conducting the process, for which the combined cycle power station  102  according to FIG. 4 is designed, the waste heat boiler  33  is not “supercharged”, but operates at about atmospheric pressure. The following utilization turbine  18  (with generator  22 ) expands the flue gas 42 into a vacuum. After the separation of the water (H2O) in the utilization turbine  18  or in a following droplet separator  23 , the “dewatered” flue gas  43  is again compressed to ambient air pressure in a compressor  44  (with a reduced flue gas mass flow).  
         [0028]    In another alternative manner of conducting the process, for which the combined cycle power station  103  according to FIG. 5 is designed, the waste heat boiler  33 —as in the combined cycle power station  101  of FIG. 3—is run “supercharged”. After leaving the waste heat boiler, the water vapor of the flue gas  42  is condensed on cold surfaces or tubes of a heat exchanger  45 , making use of the high partial pressure. An expansion of the “dewatered” flue gas  43  takes place thereafter in a utilization turbine  20 .  
                                             List of Reference Numbers                                    10, 101-103   combined cycle power station           11   gas turbine plant           12   air inlet           13, 22, 24   generator           14   compressor           15   combustion chamber           16   rotor           17   turbine           18, 20   utilization turbine           19   fuel           21   fuel supply duct           23   droplet separator           25   steam turbine           26   condenser           27   additional water           28   condensate pump           29   feed water duct           30   feed water container           31   boiler feed pump           32   feed water           33   waste heat boiler (HRSG)           34   preheater           35   pressurized feed water           36   water or steam           37   steam drum           38   saturated steam           39   evaporator           40   superheater           41   superheated steam           42   flue gas (water vapor containing)           43   flue gas (dewatered)           44   compressor           45   heat exchanger           P A     ambient air pressure           P BK     combustion chamber pressure           P K     boiler pressure           P1, P2   point           a-e   curves