Gas turbine system and combined plant comprising the same

Abstract: Gas turbine system and combined plant comprising same provide higher effect in realizing high efficiency of plant and reduction of NOx generation. The gas turbine system comprising compressor (1) for compressing combustion air, combustor (2) for burning fuel with the combustion air and gas turbine (3) driven by high temperature gas generated at the combustor (2) is constructed such that portion of exhaust gas discharged from the gas turbine (3) is circulated into the combustor (2). The combined plant comprising the gas turbine system is also provided.

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 and to a method of operating a 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. 2. Fig. 2 shows a main construction of a prior art example of a combined plant comprising a gas turbine system and a steam turbine. In Fig. 2, combustion air a is compressed at a compressor 01 and is then mixed with fuel b at a combustor 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. 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 of 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, 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 demanded. Also, in case the gas turbine system comprises a gas turbine unit only, enhancement of the efficiency and reduction of NOx generation are likewise demanded. US-A-5 595 059 discloses a gas turbine system in which flue gas and fuel (natural gas) are mixed together and heated by exhaust gas of the gas turbine to be reformed at a reformer before the mixture is supplied into a combustor. At the reformer, the endothermic reaction of water vapour and carbon dioxide in the natural gas and the flue gas takes place to thereby produce hydrogen and carbon monoxide. DE 198 10 820 discloses a gas turbine system with the features of the preamble portion of claim 1. In this system, the portion of the entire exhaust gas portion discharged from the gas turbine and pressurized by the exhaust gas compressor is mixed with the compressed air from the compressor before this mixture is supplied into the combustor. The fuel is separately introduced into the combustor. 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, it is an object of the present invention to provide a more effective gas turbine system and a combined plant comprising the same. In order to achieve this object, the present invention provides a gas turbine system as defined in claim 1, a combined plant comprising the gas turbine system of the invention and a method of operating a gas turbine system as defined in claim 4. Preferred embodiments are defined in the dependent claims. According to the gas turbine system of the invention, while in the conventional gas turbine system, the combustion gas has a high excess air ratio due to limitation in the turbine inlet temperature and the exhaust gas has a high oxygen concentration, in the present invention, a portion of the exhaust gas is circulated into the combustor and thereby the oxygen concentration in the exhaust gas can be reduced, exhaust loss of the gas turbine system is reduced largely 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 physical property of the high steam concentration. Further, as the system is constructed such that the portion of exhaust gas is pressurized by the exhaust gas compressor to be partly circulated into the combustor and to be partly mixed into the fuel before the fuel is supplied into the combustor while, in the conventional gas turbine system, the combustion gas has a high excess air ratio due to limitation in the turbine inlet temperature and the exhaust gas has a high oxygen concentration, 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 of 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 BTU gas may be formed and this low BTU 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 circulated 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 a preferred embodiment the exhaust gas so pressurized by the exhaust gas compressor is used as cooling gas of the gas turbine. Accordingly, in addition to the feature above, the exhaust gas pressurized by the exhaust gas compressor is made use of as cooling medium of the gas turbine and as the exhaust gas has a high steam and CO 2 gas concentration, a high cooling effect can be obtained because of physical property of the high steam and CO 2 gas concentration. According to the invention, a combined plant having the feature of the gas turbine system mentioned above above 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 as an embodiment according to the present invention. Fig. 2 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 comprising the same as an 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 as the first 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. 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 so compressed exhaust gas c is led into the combustor 2 as dilution gas, a portion of the same 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 cooling medium of the gas turbine 3 and the remaining portion of the same flows through a supply pipe 15 of the fuel b to join in 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 and so the construction is made such that work of each of the turbines 3, 6 may be taken out. In the conventional gas turbine system, the combustion gas has an excess air ratio of 2.6 or more due to limitation in the turbine inlet temperature and hence oxygen concentration in the exhaust gas is as high as 13% or more, but in the present embodiment as mentioned above, the exhaust gas c circulates into the combustor 2 and thereby the combustion gas can be made to have the excess air ratio of about 1.1 and the oxygen concentration in the exhaust gas can be made to about 2%. 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. By the matter mentioned above, while exhaust loss in the conventional gas turbine system has been about 13%, the exhaust loss in the gas turbine system of the present embodiment can be reduced to about a half thereof and 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 combustor 2 so that a low BTU gas of about 1000 Kcal/Nm 3 may be formed and this low BTU gas is mixed with air to burn and thereby 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 circulating into the combustor 2 through the supply pipe 15 of the fuel b has a high steam concentration and work output of the turbine can be increased because of physical property of the high steam concentration. Also, a portion of the exhaust gas c, as cooled (or as it is), is made use of as cooling medium of the gas turbine and the exhaust gas c has a high steam and CO 2 concentration. Hence, because of physical property thereof, a high cooling effect can be obtained In the above, while the invention has been described based on the embodiment as illustrated, the invention is not limited thereto but, needless to mention, may be added with various modifications in the concrete construction thereof 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.

Claims: A gas turbine system comprising a compressor (1) for compressing combustion air (a), a combustor (2) for burning fuel (b) with said combustion air (a), a gas turbine (3) driven by a high temperature gas generated at said combustor (2), and an exhaust gas compressor (10), wherein said gas turbine system is constructed such that a portion of exhaust gas (c) discharged from said gas turbine (3) is pressurized by said exhaust gas compressor (10) and is circulated into said combustor (2), characterized in that said gas turbine system is constructed such that said portion of exhaust gas pressurized by said exhaust gas compressor (10) is partly circulated into said combustor (2) and is partly mixed into said fuel (b) before said fuel (b) is.supplied into said combustor (2). A gas turbine system as claimed in claim 1, characterized in that said gas turbine system is constructed such that the exhaust gas (c) so pressurized by said exhaust gas compressor (10) is also used as cooling gas of said gas turbine (3). A combined plant comprising the gas turbine system as claimed in claims 1 or 2. A method of operating a gas turbine system comprising a compressor (1) for compressing combustion air (a), a combustor (2) for burning fuel (b) with said combustion air (a), a gas turbine (3) driven by a high temperature gas generated at said combustor (2), and an exhaust gas compressor (10), wherein a portion of exhaust gas (c) discharged from said gas turbine (3) is pressurized by said exhaust gas compressor (10) and is circulated into said combustor (2), characterized in that said portion of exhaust gas pressurized by said exhaust gas compressor (10) is partly circulated into said combustor (2) and is partly mixed into said fuel (b) before said fuel (b) is supplied into said combustor (2). A method of operating a gas turbine system as claimed in claim 4, characterized in that said exhaust gas (c) so pressurized by said exhaust gas compressor (10) is also used as cooling gas of said gas turbine (3).

Inventors: AKITA EIJI, MANDAI SHIGEMI, MORI HIDETAKA, SUGISHITA HIDEAKI, AKITA, EIJI, MANDAI,SHIGEMI, MORI, HIDETAKA, SUGISHITA,HIDEAKI, Akita, Eiji, c/o Takasago Machinery Works, Mandai,Shigemi,c/o Takasago Research &amp; Development, Mori, Hidetaka,c/o Takasago Research &amp; Development, Sugishita,Hideaki, Takasago Research &amp; Development
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 2000
Main classification: F01K23
IPC codes: F01K23, F02C3, F01K21, F02C7