Abstract:
In a method for generating energy in an energy generating installation ( 10 ) having a gas turbine ( 12 ), in a first step, an oxygen-containing gas is compressed in a compressor ( 13, 14 ) of the gas turbine ( 12 ), in a second step the compressed gas is supplied, with the addition of fuel, for combustion in a combustion chamber ( 15 ), in a third step the hot flue gas from the combustion chamber ( 15 ) is expanded in a turbine ( 16 ) of the gas turbine ( 12 ) so as to perform work, and, in a fourth step, a branched-off part stream of the expanded flue gas is recirculated into a part of the gas turbine ( 12 ) lying upstream of the combustion chamber ( 15 ) and is compressed. A reduction in the CO 2  emission, along with minimal losses of efficiency, is achieved in that carbon dioxide (CO 2 ) is separated from the circulating gas in a CO 2  separator ( 19 ), and in that measures are taken to compensate for the efficiency losses in the gas turbine cyclic process which are associated with the CO 2  separation.

Description:
[0001]     This application is a Continuation of, and claims priority under 35 U.S.C. § 120 to, International application number PCT/EP2005/053838, filed 4 Aug. 2005, and claims priority therethrough under 35 U.S.C. § 119 to German application number 10 2004 039 164.5, filed on 11 Aug. 2004, the entireties of which are incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to the field of energy generating technology. It refers to a method for generating energy in an energy generating installation having a gas turbine, and to an energy generating installation useful for carrying out the method.  
         [0004]     2. Brief Description of the Related Art  
         [0005]     On account of their wide availability and their low price, fossil fuels are forecasted to remain the main energy source for power generation for the next 20 to 50 years. The demand for electrical energy will increase during this period at about 2-3% per year. At the same time, it is necessary to markedly reduce the CO 2  emitted by power stations, in order to stabilize the CO 2  concentration in the atmosphere.  
         [0006]     Increased CO 2  concentrations in the atmosphere have been associated with global warming. For this reason, international agencies and local governments are at the present time deliberating on the set-up of emission systems and will possibly introduce limitations on the future CO 2  emissions of power stations. Technological options are therefore required, which allow the continuing use of fossil fuels without the high CO 2  emissions associated with them. At the same time, high efficiency and low plant costs will remain critical factors in the construction and operation of a power station.  
         [0007]     Various projects have already been initiated, with the aim of developing low-emission processes based on gas turbines. There are three conventional ways of reducing the CO 2  emission from such power stations:  
         [0008]     1. Methods for capturing the CO 2  on the exit side: in these methods, the CO 2  generated from the exhaust gases during combustion is removed by means of an absorption process, membranes, refrigeration processes, or combinations of these.  
         [0009]     2. Methods for the carbon depletion of the fuel: in these methods, the fuel is converted before combustion into H 2  and CO 2 , and it thus becomes possible to capture the carbon content of the fuel before entry into the gas turbine.  
         [0010]     3. Oxygen/fuel processes (“oxy-fuel process”) with exhaust gas recirculation: in these, virtually pure oxygen is used, instead of air, as an oxidizing agent, with the result that a flue gas consisting of carbon dioxide and water is obtained.  
         [0011]     Each of these ways, however, has disadvantages which are reflected in a reduction in efficiency, in an increase in capital costs for the power station, or in necessary conversion measures for the turbomachines.  
         [0012]     There is, therefore, a high demand for a gas turbine cyclic process with maximum efficiency, low overall costs, and the option of the removal of CO 2 .  
         [0013]     In order to increase the efficiency of combined-cycle power stations equipped with gas turbines and to reduce costs, the following options may be envisaged: 
        Increasing the turbine inlet temperature.     Increasing the overall pressure ratio.     Using a gas turbine cyclic process with intermediate heating.        
 
         [0017]     The first two options are linked to certain physical limits. Thus, for example, NOx emissions increase with higher combustion temperatures, and the materials of the turbine blades have their strength limits at high temperatures. On the other hand, the pressure ratio for an uncooled single-shaft compressor is limited on account of the action of the high temperature of the compressed air on the rotor materials.  
       SUMMARY OF THE INVENTION  
       [0018]     One of numerous aspects of the present invention includes providing a method for generating energy, based on a gas turbine cyclic process, and an energy generating installation useful for carrying out the method, which allow the efficient removal of carbon dioxide without appreciable losses of efficiency.  
         [0019]     Another aspect of the present invention includes providing CO 2  separation with a partial recirculation of the flue gas and, at the same time, to take measures for compensating for the efficiency losses in the gas turbine cyclic process which are associated with the CO 2  separation.  
         [0020]     A preferred, exemplary embodiment of the invention is distinguished in that the carbon dioxide (CO 2 ) is separated only partially from the circulating gas. Owing to the partial separation of the CO 2  from the recirculated and compressed flue gas, higher CO 2  concentrations, and therefore improved separation effectiveness, can be achieved.  
         [0021]     In another preferred, exemplary embodiment, to generate the oxygen-containing gas supplied to the compressor of the gas turbine, air is enriched with oxygen. The oxygen enrichment improves the CO 2  separation. It would increase the combustion temperature if at the same time more flue gas were not recirculated or water or steam were not added.  
         [0022]     A further preferred, exemplary embodiment of the invention is distinguished in that, before the part stream is branched off, the expanded flue gas is used for generating steam in a waste heat recovery steam generator.  
         [0023]     In a first alternative development of the invention, the oxygen-containing gas is compressed in the compressor in at least two compressor stages connected in series, the oxygen-containing gas is intermediately cooled between the two compressor stages, the recirculated flue gas is added to the oxygen-containing gas upstream of the first compressor stage, and the carbon dioxide (CO 2 ) is separated from the intermediately cooled oxygen-containing gas before entry into the second compressor stage. The CO 2  separation downstream of the intermediate cooling in a multistage compressor integrates the partial CO 2  separation into a gas turbine cyclic process with high efficiency. Components derived from the aeronautics sector, which have pressure ratios of above 30 bar, typically 45 bar, may be employed. The temperatures (15° C. to 100° C., at best between 50° C. and 60° C.) which are reached after intermediate cooling are well suited to standard CO 2  separation methods, such as, for example, CO 2  membrane units.  
         [0024]     In particular, to separate the carbon dioxide (CO 2 ), the oxygen-containing gas is put through a CO 2  separator, and the quantity of gas flowing through the CO 2  separator is set by means of an adjustable valve which is arranged in a bypass to the CO 2  separator. Preferably, the valve, also serving for regulation, is opened completely during the starting phase, during part-load operation, or during an emergency shutdown, in order to short-circuit the CO 2  separator.  
         [0025]     A further improvement arises when the branched-off part stream of the flue gas is cooled in a cooler before recirculation, water optionally being extracted from the part stream. This gives rise to lower compression work in the first compressor stage and to increased water extraction. In addition, the cooler may be used in order to regulate the temperature at entry into the compressor.  
         [0026]     A flexible type of operation is obtained in that the branched-off part stream is interrupted when the gas turbine cyclic process is to be run in a standard mode without the separation of carbon dioxide (CO 2 ).  
         [0027]     It is particularly beneficial if the carbon dioxide (CO 2 ) is separated in the CO 2  separator in a wet method by means of membranes. In this case, the membranes are saturated with water. As a result, the cooled gas stream is saturated with water. It thereby becomes possible to integrate the CO 2  separator into plant concepts with spray cooling or with what is known as inlet fogging in the case of medium pressure upstream of the high-pressure compressor stage (for inlet fogging see, for example, the article by C. B. Meher-Homji and T. R. Mee III, Gas Turbine Power Augmentation by Fogging of Inlet Air, Proc. of 28th Turbomachinery Symposium, 1999, pages 93-113).  
         [0028]     It is accordingly conceivable that, for intermediate cooling, water is sprayed into the stream of oxygen-containing gas, or that water is sprayed into the stream of oxygen-containing gas in the manner of inlet fogging at the inlet of the second compressor stage.  
         [0029]     A second alternative development of the invention includes that the branched-off part stream of flue gases is compressed in a separate compressor before recirculation into the gas turbine, in particular the carbon dioxide (CO 2 ) being separated from the compressed part stream of flue gas, and the compressed part stream subsequently being added to the oxygen-containing gas upstream of the combustion chamber, and, to separate the carbon dioxide (CO 2 ), the compressed part stream is put through a CO 2  separator and the quantity of gas flowing through the CO 2  separator is set by means of an adjustable valve which is arranged in a bypass to the CO 2  separator. Furthermore, before entry into the CO 2  separator, the compressed part stream is cooled in a cooler.  
         [0030]     It is also advantageous if the branched-off part stream of flue gas is cooled in a cooler before recirculation and water is in this case optionally extracted from the part stream, and if the flue gas expanded in the turbine of the gas turbine is intermediately heated and is expanded anew in a further turbine, and the further turbine is used for driving the separate compressor. The use of a separate compressor for the recirculated flue gas makes it possible to have a higher CO 2  concentration during CO 2  separation. Separation takes place at the full compressor pressure (at best at about 30 bar) by means of a single compressor stage. Intermediate heating affords a higher energy density in the cyclic process and reduces the NOx emissions in the process. Furthermore, the intermediate heating (by means of a second combustion chamber) allows more stable combustion in the first combustion chamber on account of the higher oxygen excess ratio in the case of a predetermined overall recirculation rate. This also results in higher flexibility in process management, such as, for example, in varying the release of heat in the first and the second combustion chamber.  
         [0031]     A third alternative development of the invention includes that the carbon dioxide (CO 2 ) is separated from the flue gas expanded in the turbine of the gas turbine, and, after the separation of the carbon dioxide (CO 2 ), a part stream is branched off and is recirculated to the inlet of the compressor of the gas turbine, in particular the flue gas expanded in the turbine of the gas turbine being cooled in a cooler before the separation of the carbon dioxide (CO 2 ), and water in this case being extracted from the flue gas, and the flue gas is expanded to a few bar in the turbine of the gas turbine and the flue gas is expanded further in an exhaust gas turbine after the separation of the carbon dioxide (CO 2 ). The CO 2  is separated here at a low pressure, but, due to the extraction of water, a high CO 2  partial pressure is nevertheless achieved.  
         [0032]     In a preferred embodiment of the energy generating installation according to the invention, an oxygen enrichment device preferably having air separation membranes and intended for enriching with oxygen the air sucked in by the compressor is arranged upstream of the inlet of the compressor of the gas turbine, and a waste heat recovery steam generator is arranged in the exhaust gas line.  
         [0033]     A particularly high efficiency of the installation can be achieved when the compressor of the gas turbine includes two compressor stages, when the CO 2  separator is arranged between the two compressor stages, when an intermediate cooler is provided between the outlet of the first compressor stage and the inlet of the CO 2  separator, and when the recirculation line is returned to the inlet of the first compressor stage. The CO 2  separator is preferably bridged by means of a bypass in which an adjustable valve is arranged.  
         [0034]     A development of this embodiment is that the recirculation line is returned to the inlet of the combustion chamber, in that a separate compressor and the CO 2  separator are arranged in series in the recirculation line, in that a cooler is provided between the separate compressor and the CO 2  separator, and in that the CO 2  separator is bridged by means of a bypass in which an adjustable valve is arranged. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     The invention will be explained in more detail below with reference to exemplary embodiments, in conjunction with the drawing in which:  
         [0036]      FIG. 1  shows a simplified installation diagram of an energy generating installation according to a first exemplary embodiment of the invention, with a two-stage compressor having intermediate cooling in the gas turbine;  
         [0037]      FIG. 2  shows a simplified installation diagram of an energy generating installation according to a second exemplary embodiment of the invention, with a second gas turbine for compressing the recirculated flue gas; and  
         [0038]      FIG. 3  shows a simplified installation diagram of an energy generating installation according to a third exemplary embodiment of the invention, in which the recirculation of the flue gas takes place after the separation of the CO 2 . 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0039]      FIG. 1  reproduces a simplified installation diagram of an energy generating installation  10  according to a first exemplary embodiment of the invention. The energy generating installation  10  includes a gas turbine  12  with two compressor stages  13  and  14  connected in series, with a combustion chamber  15  and with a turbine  16  which drives a generator  28 . The compressor stages  13 ,  14  and turbine  16  are seated on a common shaft in the usual way. Of course, the compressor stages and the turbine may also be arranged on a plurality of shafts, in which case the turbine may additionally be subdivided likewise into two or more stages. The first compressor stage  13  sucks in air  23  which, before compression, is enriched with oxygen by the extraction of nitrogen N 2  in an oxygen enrichment device  11 . Flue gas recirculated from the outlet of the installation is admixed to the air, optionally enriched with oxygen. The resulting gas enriched with oxygen is precompressed in the first compressor stage  13 , subsequently intermediately cooled in an intermediate cooler  18 , and then supplied for postcompression to the second compressor stage  14 . Before the intermediately cooled gas enters the second compressor stage  14 , carbon dioxide (CO 2 ) is extracted from it in a CO 2  separator  19 . A bypass  33  led past the CO 2  separator  19  and provided with a first adjustable valve  21  makes it possible to set the throughput through the CO 2  separator  19  and consequently the quantity of the CO 2  separated overall. A second valve  21 ′ arranged upstream of the CO 2  separator  19  serves both for shutting off in the event of short-circuiting by the bypass  33  and for regulation.  
         [0040]     The gas postcompressed in the compressor stage  14  is conducted for the combustion of a fuel into the combustion chamber  15 . The hot flue gas occurring during combustion is expanded in the turbine  16  so as to perform work and subsequently flows through a waste heat recovery steam generator  17  where it generates steam for a steam turbine or other purposes. After leaving the waste heat recovery steam generator  17 , the flue gas is discharged via an exhaust gas line  24 . Branching off from the exhaust gas line  24 , part of the flue gas is recirculated to the inlet of the first compressor stage  13  via a recirculation line  34  and, as already described above, is admixed to the air (optionally) enriched with oxygen. A valve  22  and a cooler  20  are arranged in the recirculation line  34 . With the aid of the valve  22 , the recirculation rate can be set or recirculation can be interrupted completely. The cooler  20  reduces the compression work by cooling the flue gas. It may, furthermore, extract water from the recirculated flue gas.  
         [0041]     An advantageous aspect of the gas turbine cyclic process illustrated in  FIG. 1  is the combination of flue gas recirculation with partial separation of CO 2  and of a highly efficient turbine cyclic process with multistage compression and intermediate cooling. The air quantity required for stoichiometric combustion (with λ=1) determines the maximum recirculation ratio for the flue gas. A higher recirculation ratio is advantageous because it maximizes the CO 2  concentration in the gas flowing through the intermediate cooler  18  and the CO 2  separator  19 . The enrichment of the intake air with oxygen, which can be achieved within the oxygen enrichment device  11 , for example, using air separation membranes operating at low temperatures, makes it possible, with a predetermined combustion temperature of the gas turbine  12 , to have a higher recirculation of the flue gas.  
         [0042]     The installation illustrated in  FIG. 1  has the following properties and advantages: 
        Due to the partial separation of the CO 2  from the recirculated and precompressed flue gas, higher CO 2  concentrations, and therefore higher efficiencies in CO 2  separation, can be achieved by the CO 2  separator  19 .     By the valve  21 , it is possible to set optimally the fraction of the gas passing through the CO 2  separator  19 . During the starting phase, in part-load operation or during a rapid shutdown, the valve  21  can be opened fully in order to short-circuit the CO 2  separator  19 .     The valve  22  in the recirculation line  34  can be used, during faults, in part-load operation or in the starting phase, for running the process in the standard mode without CO 2  separation.     The arrangement of the CO 2  separator  19  downstream of the intermediate cooler  18  of a multistage compressor  13 ,  14  integrates CO 2  separation into a gas turbine cyclic process with high efficiency. Components originating from aeronautics and having pressure ratios above  30  bar, typically at  45  bar, may be used. The temperatures (20° C. to 100° C., in particular between 50° C. and 60° C.) reached at the outlet of the intermediate cooler  18  are adapted to those of the standard CO 2  separation process, such as, for example, in a CO 2  membrane unit.     Specific CO 2  membrane units are usually operated in a wet mode (saturated with water). Consequently, the membranes saturate the cooled gas stream with water. The CO 2  separator  19  can thus be integrated into concepts with intermediate spray cooling or with inlet fogging in the case of medium pressures upstream of the postcompressor stage.     Optional enrichment with oxygen allows an increased recirculation of the flue gas (note: the enriched O 2  increases the combustion temperature if the diluting constituent is not at the same time increased, which may take place either by increased flue gas recirculation or by the addition of water or steam).     The cooler or condenser  20  in the recirculation line  34  allows an increased recovery of water at the expense of greater cooling.        
 
         [0050]     The installation diagram of the exemplary embodiment shown in  FIG. 2  includes two gas turbines  12  and  12 ′ in an energy generating installation  30 . The first gas turbine  12  includes a compressor  25 , a combustion chamber  15 , and a turbine  16  which drives a first generator  28 . Here, too, air  23  sucked in the gas turbine  12  is (optionally) enriched with oxygen in an oxygen enrichment device  11 , compressed in the compressor  25 , and used for the combustion of fuel in the combustion chamber  15 . The hot flue gases are expanded first in the turbine  16  of the first gas turbine  12  and subsequently in the turbine  16 ′ of the second gas turbine  12 ′. Additional heating in an intermediate heater  27  (sequential combustion) may optionally be carried out between the two turbines  16  and  16 ′. The expanded flue gas is subsequently conducted through a waste heat recovery steam generator  17  and discharged in an exhaust gas line  24 . Part of the flue gas is recirculated again and admixed, directly upstream of the combustion chamber  15 , to the oxygen-enriched and compressed air. The necessary compression takes place in the compressor  25 ′ of the second gas turbine  12 ′, which may at the same time drive a second generator  28 ′. In a similar way to  FIG. 1 , after compression, the recirculated flue gas is cooled in a cooler  26 ′ and is subsequently partially freed of the carbon dioxide in a CO 2  separator  19 . To set the separation rate, hereto, a bypass  33  with a valve  21  may be provided. To regulate and shut off the stream through the CO 2  separator  19 , once again a second valve  21 ′ can be used upstream of the CO 2  separator  19 . Upstream of the cooler  26 ′, a regenerative heat exchanger  26  may additionally be arranged, in which the CO 2 -depleted gas leaving the CO 2  separator  19  is preheated, before combustion, in a thermo dynamically efficient way and a large part of the cooling power of the heat exchanger  26  is thus recovered. The valve  22  and the cooler  20  in the recirculation line  34  fulfill the same functions as in  FIG. 1 . The bypass  33  should necessarily bridge the CO 2  separator  19  and the two coolers  26  and  26 ′, since otherwise cooling takes place upstream of the combustion chamber  15 , this being unfavorable in thermo dynamic terms.  
         [0051]     The separate compressor  25 ′ makes it possible to have a higher CO 2  concentration and therefore an increase in the effectiveness of CO 2  separation. At the same time, the efficiency of the process rises due to the intermediate heating. The installation illustrated in  FIG. 2  has, correspondingly, the following properties and advantages: 
        CO 2  separation takes place at full compressor pressure (optimally about 30 bar) by a single compressor stage on account of the separate compressor.     the use of intermediate heating gives higher energy density in the process.     the use of intermediate heating reduces the NOx emission in the process.     the use of intermediate heating makes it possible, because of the higher oxygen excess ratio, in the case of a predetermined overall recirculation rate, to have more stable combustion in the first burner (combustion chamber  15 ). This affords higher flexibility in the control of the process, that is to say, a greater range of variation in the heat release in the first and the second burner (intermediate heater  27 ).        
 
         [0056]     Moreover, the compressors and turbines may also be connected to one another in a way different from  FIG. 2 , in order to make it possible to use a power turbine running freely (on a separate shaft). Furthermore, it is also conceivable to provide multistage compression with intermediate cooling of the recirculated flue gas. In this case, CO 2  separation would take place at a lower pressure, but a higher system pressure overall could be achieved. The bypass would then include only the CO 2  absorber unit, but not the coolers which, moreover, would not be designed regeneratively.  
         [0057]     The installation diagram of the exemplary embodiment shown in  FIG. 3  includes an energy generating installation  32  with a gas turbine  12  having a compressor  25 ′, combustion chamber  15 , and turbine  16  and following waste heat recovery steam generator  17 . After running through the waste heat recovery steam generator  17 , the flue gas is dewatered in a cooler  20  and subsequently freed partially from carbon dioxide in the CO 2  separator  19 . Only after CO 2  separation is part of the flue gas recirculated to the inlet of the compressor  25 ′ via the recirculation line  34  and mixed with the oxygen-enriched intake air  23 . The rest of the flue gas can be expanded further in an optional following exhaust gas turbine  29 . In addition, the air  23  present at the inlet and enriched with oxygen in the oxygen enrichment device  11  may be precompressed in a compressor  25  and optionally cooled intermediately in an intermediate cooler  35 . Thus, for example, a pressure ratio of  10  in the precompression (compressor  25 ) of the oxygen-containing gas and a pressure ratio of 10-20 in the main compression ( 25 ′) could be selected. If highly enriched air is then used, an efficient process can thus be achieved.  
         [0058]     In this version, the carbon dioxide is separated before recirculation. Although the CO 2  is separated at a lower pressure, the dewatering results in a high CO 2  partial pressure. The installation illustrated in  FIG. 3  has, correspondingly, the following properties and advantages: 
        in contrast to  FIG. 1  and  2 , the flue gas is subjected overall to CO 2  separation. Part of the flue gas is then recirculated. However, this procedure may also be employed in concepts with intermediate cooling (similar to  FIG. 1 ) and intermediate heating (similar to  FIG. 2 ).     water may be injected (not illustrated in  FIG. 3 ), in order to reduce the NOx emissions of the combustion and to reduce the degree of flue gas recirculation required for a predetermined CO 2  exhaust gas concentration.        
 
         [0061]     Other possibilities arise when a cyclic process with a high degree of water injection (intermediate spray cooling, water or steam injection into the combustion chamber) is combined with the model of partial flue gas recirculation: 
        when the high fraction of water in the flue gas is removed, the CO 2  concentration rises. As a result, the efficiency of CO 2  separation is improved, specifically both in the “tail-end” configuration according to  FIG. 3 , that is say in a solution with following CO 2  separation at the end of the process, and in separation in the medium-pressure range according to  FIGS. 1 and 2 .     the addition of water makes it possible to have the same combustion temperature with less flue gas recirculation. This may have effects on efficiency in cases where the water supply is uncritical.     water injection may also be employed in processes without flue gas recirculation, in order to allow efficient “tail-end” CO 2  separation after water condensation. In a limit situation, sufficient water could be added to the process to allow combustion with X near to 1 at reasonable temperatures without flue gas recirculation.        
 
       LIST OF REFERENCE SYMBOLS  
       [0000]    
       
           10 ,  30 ,  32  energy generating installation  
           11  oxygen enrichment device  
           12 ,  12 ′ gas turbine  
           13 ,  14  compressor stage  
           15  combustion chamber  
           16 ,  16 ′ turbine  
           17  waste heat recovery steam generator (HRSG)  
           18 ,  35  intermediate cooler  
           19  CO 2  separator  
           20 ,  26 ′ cooler  
           21 ,  21 ′,  22 ,  31  valve  
           23  air  
           24  exhaust gas line  
           25 ,  25 ′ compressor  
           26  regenerative heat exchanger  
           27  intermediate heater  
           28 ,  28 ′ generator  
           29  exhaust gas turbine  
           33  bypass  
           34  recirculation line  
       
     
         [0085]     While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.