Abstract:
A gas turbine system and a combined plant including the gas turbine system provides a higher effect in realizing a high plant efficiency and reduction of NOx generation. The gas turbine system includes a compressor ( 1 ) for compressing combustion air, a combustor ( 2 ) for burning fuel with the combustion air, and a gas turbine ( 3 ) driven by high temperature gas generated at the combustor ( 2 ). A portion of the exhaust gas discharged from the gas turbine ( 3 ) is recirculated back into the combustor ( 2 ). A combined plant can also include the gas turbine system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high efficiency gas turbine system and a combined plant comprising this gas turbine system. 
     2. Description of the Prior Art 
     One example of a gas turbine system and a combined plant comprising this gas turbine system in the prior art will be described with reference to FIG.  3 . FIG. 3 shows a main construction of a prior art example of a combined plant comprising gas turbine system and a steam turbine. 
     In FIG. 3, combustion air a is compressed at a compressor  01  and is then mixed with fuel b at a combuster  02  to effect a combustion for generating a high temperature combustion gas. This high temperature combustion gas drives a gas turbine  03  to expand and is then supplied into an exhaust gas boiler  05  as exhaust gas c for generating steam there. 
     A high temperature high pressure steam generated at the exhaust gas boiler  05  drives a steam turbine  06  to expand and is then condensed at a condenser  08 . The pressure of the condensed water is elevated by a feed water pump  09  so that the condensed water is circulated into the exhaust gas boiler  05 . 
     A generator  04  is fitted to the gas turbine  03  and another generator  07  is fitted to the steam turbine  06  and so the construction is made such that work each of the turbines  03 ,  06  may be taken out. 
     In the combined plant as so constructed, there is currently a high demand for realizing a higher efficiency thereof. Especially, in order for a gas turbine system which constitutes a first stage portion of the combined plant to effect a high temperature combustion of the fuel, further enhancement of the efficiency and reduction of NOx generation are both required. Also, if the gas turbine system comprises a gas turbine unit only, enhancement of the efficiency and reduction of NOx generation are likewise required. 
     SUMMARY OF THE INVENTION 
     In view of the mentioned high demand for realizing a higher efficiency and a further reduction of NOx generation in the gas turbine system and the combined plant comprising the same in the prior art, it is an object of the present invention to provide a more effective gas turbine system and combined plant comprising the same. 
     In order to achieve the mentioned object, the present invention provides the following embodiment. 
     In a first embodiment, a gas turbine system comprises a compressor for compressing combustion air, a combustor for burning fuel with the combustion air and a gas turbine driven by a high temperature gas generated at the combustor. The gas turbine system is constructed such that a portion of exhaust gas discharged from the gas turbine is recirculated into the combustor. 
     In the conventional gas turbine system, the combustion gas has a high excess air ratio due to a limitation in the turbine inlet temperature and because the exhaust gas has a high oxygen concentration. However, in the first embodiment of the present invention, a portion of the exhaust gas is recirculated into the combustor and thereby the oxygen concentration in the exhaust gas can be reduced, exhaust loss of the gas turbine system is largely reduced, and generation of NOx is also reduced. Further, as the exhaust gas to be circulated into the combustor has a high steam concentration, work output of the gas turbine is increased because of the physical properties of the high steam concentration. 
     In a modified embodiment, a gas turbine system as mentioned above further comprises an exhaust gas compressor and is constructed such that the portion of exhaust gas is pressurized by the exhaust gas compressor to be partly recirculated into the combustor and to be partly mixed into the fuel before the fuel is supplied into the combustor. 
     In the conventional gas turbine system, the combustion gas has a high excess air ratio due to a limitation in the turbine inlet temperature and the exhaust gas has a high oxygen concentration. However, in the present invention, the exhaust gas is circulated into the combustor and thereby the excess air ratio of the combustion gas can be made lower. Hence, oxygen concentration in the exhaust gas can be reduced and exhaust loss from the gas turbine system can be reduced largely. 
     Also, the exhaust gas of the low oxygen concentration and the fuel are mixed together before the fuel is supplied into the combustor so that a low BUT gas may be formed, and this low BUT gas is mixed with air to burn. Hence, flame temperature can be maintained lower and NOx generation quantity can be reduced. Moreover, the exhaust gas recirculated into the combustor or the fuel has a high steam concentration, work output of the gas turbine is increased because of physical property of the high steam concentration. 
     In another modified embodiment a gas turbine system as mentioned above is designed so that the exhaust gas pressurized by the exhaust gas compressor is used as cooling gas of the gas turbine. 
     According to the embodiment above, in addition to the features of the first modified embodiment, the exhaust gas pressurized by the exhaust gas compressor is used as a cooling medium for the gas turbine. Because the exhaust gas has a high steam and CO 2  gas concentration, a high cooling effect can be obtained because of physical properties of the high steam and CO 2  gas concentration. 
     In another modified embodiment, a gas turbine system as mentioned above further comprises a condenser through which the exhaust gas passes so that water content in the exhaust gas may be condensed, and the portion of exhaust gas is the water content in the exhaust gas so condensed at the condenser. 
     According to the embodiment above, if the combustion gas in the combustor has an excessively high temperature even if the excess air ratio is set to a lower level, water injection or steam injection is effected using the water content in the exhaust gas to be circulated into the combustor so that a predetermined turbine inlet temperature may be obtained and the excess air ratio of the combustion gas may be reduced. Therefore, oxygen concentration in the exhaust gas can be reduced and exhaust loss can be largely reduced as compared with the conventional gas turbine system. 
     Also, flame temperature and oxygen concentration in the combustor can be maintained lower so that NOx generation quantity can be reduced. Moreover, water content concentration in the combustion gas becomes higher so that work output of the gas turbine is increased because of the physical properties of the higher water content concentration. 
     It is to be noted that if the oxygen concentration in the exhaust gas is maintained lower, then necessarily steam concentration in the exhaust gas becomes higher. Hence, the water content in the exhaust gas can be recovered easily at the condenser so that the water injection or steam injection becomes possible without a make-up water. 
     Another embodiment is directed to a combined plant comprising the gas turbine system as mentioned in any one of the above embodiments. 
     According to the embodiment above, a combined plant having a feature of the gas turbine system mentioned in any one of the previous embodiments can be obtained, and a combined plant having a high efficiency and a low NOx generation as a whole can be realized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory view of a gas turbine system and a combined plant comprising the same elements as a first embodiment according to the present invention. 
     FIG. 2 is an exemplary view of a gas turbine system and a combined plant comprising the same elements as a second embodiment according to the present invention. 
     FIG. 3 is an explanatory view of a gas turbine system and a combined plant comprising the same in the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A gas turbine system and a combined plant of a first embodiment according to the present invention will be described with reference to FIG.  1 . 
     FIG. 1 shows a main construction of the combined plant comprising the gas turbine system of the fist embodiment. In FIG. 1, numeral  1  designates a compressor for compressing combustion air a. A gas turbine  3 , an exhaust gas compressor  10  and a generator  4  are fitted coaxially to the compressor  1 . The combustion air a compressed at the compressor  1  is mixed with fuel b at a combustor  2  to effect a combustion for generating a high temperature combustion gas. This high temperature combustion gas drives the gas turbine  3  to expand and is then discharged as exhaust gas c. 
     Numeral  5  designates an exhaust gas boiler for recovering waste heat of the exhaust gas c. A high temperature high pressure steam generated at the exhaust gas boiler  5  drives a steam turbine  6  to expand and is then condensed at a condenser  8 . The pressure of the condensed water is elevated by a feed water pump  9  so that the condensed water is circulated into the exhaust gas boiler  5 . 
     About a half quantity of the exhaust gas c coming out of the exhaust gas boiler  5  diverges into the exhaust gas compressor  10  to be compressed. A portion of the compressed exhaust gas c is led into the combustor  2  as dilution gas, a portion of the compressed exhaust gas c is led into a cooler  14  to be cooled (or is led as it is) to be further led into the gas turbine  3  as a cooling medium for the gas turbine  3 , and the remaining portion of the compressed exhaust gas flows through a supply pipe  15  of the fuel b to mix with the fuel b of gas or liquid before the fuel b is supplied into the combustor  2 , so that a low BTU gas may be formed. 
     The generator  4  is fitted to the gas turbine  3  and a generator  7  is fitted to the steam turbine  6 . Thus, the construction is such that work done by each of the turbines  3 ,  6  may be transformed into power. 
     In the conventional gas turbine system, the combustion gas has an excess air ratio of 2.6 or more due to limitations in the turbine inlet temperature and, hence, oxygen concentration in the exhaust gas is as high as 13% or more. However, in the present embodiment as mentioned above, the exhaust gas c recirculates into the combustor  2  and thereby the combustion gas can have an excess air ratio of about 1.1, and an oxygen concentration in the exhaust gas can be about 2%. It is noted that the oxygen concentration in the exhaust gas is preferably 10% or less, or more preferably 5% or less. Further, if the oxygen concentration in the exhaust gas is reduced, then it will also suppress a generation of NOx. 
     Thus, while exhaust loss in a conventional gas turbine system has been about 13%, exhaust loss in the gas turbine system of the present embodiment can be reduced to about a half thereof. This results in enhancement by about 7% of a thermal efficiency of the combined plant as a whole. 
     Further, the exhaust gas c of the low oxygen concentration and the fuel b of gas or liquid are mixed together before the fuel b is supplied into the combustion  2 . Therefore, a low BTU gas of about 1000 Kcal/Nm 3  may be formed, and this low BTU gas is mixed with air to burn so that flame temperature can be maintained lower even in a diffusion type combustor and NOx generation quantity can be reduced to 10 ppm or less. 
     Moreover, the exhaust gas c recirculating into the combustor  2  through the supply pipe  15  of the fuel b has a high steam concentration. Thus, work output of the turbine can be increased because of the physical properties of the high steam concentration. 
     Also, a portion of the exhaust gas c, as cooled (or as it is), used as a cooling medium for the gas turbine, and the exhaust gas c has a high steam and CO 2  concentration. Hence, because of the physical properties thereof, a high cooling effect can be obtained. 
     A gas turbine system and a combined plant comprising the same as a second embodiment according to the present invention will be described with reference to FIG.  2 . 
     FIG. 2 shows a main construction of the combined plant comprising the gas turbine system of the second embodiment. In FIG. 2, numeral  101  designates a compressor for compressing combustion air a. A gas turbine  103  and a generator  104  are fitted coaxially to the compressor  101 . The combustion air a compressed at the compressor  101  is mixed with fuel b at a combustor  102  to effect a combustion for generating a high temperature combustion gas. This high temperature combustion gas drives the gas turbine  103  to expand and is then discharged as exhaust gas c. 
     Numeral  105  designates an exhaust gas boiler for recovering waste heat of the exhaust gas c. A high temperature high pressure steam generated at the exhaust gas boiler  105  drives a steam turbine  106  to expand and is then condensed at a condenser  108 . The pressure of the condensed water is elevated by a feed water pump  109  so that the condensed water is circulated into the exhaust gas boiler  105 . 
     The exhaust gas c coming out of the exhaust gas boiler  105  heats injection water d, to be described below, at a feed water heater  111 , and then passes through a condenser  112  so that water content in the exhaust gas c is condensed and separated. The pressure of the condensed water is elevated by a pump  113 , and then the condensed water is heated at the feed water heater  111 . The condensed water so pressure-elevated and heated is sent to the combustor  102  of the gas turbine  103  to be used as injection water d, which is injected into the combustor  102 . 
     As the combustion gas generated at the combustor  102  has an excessively high temperature even if excess air ratio thereof is set to about 1.1, water injection or steam injection is effected into the combustor  102  using the injection water d. Therefore, the temperature of the combustion gas is set to a predetermined gas turbine inlet temperature and, at the same time, the excess air ratio of the combustion gas can be reduced. 
     It is to be noted that the generator  104  is fitted to the gas turbine  103  and a generator  107  is fitted to the steam turbine  106 . Therefore, the arrangement is such that work done by each of the turbines  103 ,  106  may be transformed into power. 
     In the present embodiment as mentioned above, the water content in the exhaust gas c is recirculated to the combustor  102  to be water-injected or steam-injected thereinto. Thus, the combustion gas can have an excess air ratio of about 1.1, and the oxygen concentration in the exhaust gas can be as low as about 2%. Hence, as compared with the conventional gas turbine system, exhaust loss becomes smaller and thermal efficiency of the entire combined plant can be enhanced. It is to be noted that the oxygen concentration in the exhaust gas is preferably 10% or less, or more preferably 5% or less. Further, if the oxygen concentration in the exhaust gas is reduced, then it will also suppress a generation of Nox. 
     If the oxygen concentration in the exhaust gas c is maintained lower, then steam concentration in the exhaust gas c necessarily becomes higher. Hence, the water content in the exhaust gas c can be recovered easily at the condenser  112  and thereby the water injection or steam injection becomes possible without make-up water. 
     Also, flame temperature and oxygen concentration in the combustor  102  can be maintained lower by the water injection or steam injection using the injection water d. Hence, NOx generation quantity can be reduced. 
     Furthermore, because of the physical properties of the high water content concentration in the exhaust gas, work output of the gas turbine  103  can be increased and the exhaust gas temperature also can be maintained higher. 
     That is, the gas turbine  103  of the present embodiment uses not only the conventional combustion gas but also a high temperature steam as the working medium. Thus, it has advantages both of the conventional gas turbine and steam turbine. 
     In the above, while the invention has been described based on the embodiments as illustrated, the invention is not limited thereto. Needless to mention, various modifications in the concrete construction of the invention may be made within the scope of the claims as appended herebelow. 
     For example, while the gas turbine system employed in the combined plant has been described in either of the embodiments, the gas turbine system of the present invention may be employed with the same function and effect in a plant having a gas turbine only without an exhaust gas boiler or a steam turbine.