Abstract:
A combustion turbine having a compressed air cooling circuit that is connected to a nitrogen source. Compressed air is provided to the cooling circuit upon start-up and gradually switched to nitrogen cooling, as the nitrogen becomes available. Transition from compressed air to nitrogen cooling is supplied to the hottest components first in accordance with a pre-selected control scheme. Upon shutdown of the plant, the process is reversed.

Description:
FIELD OF THE INVENTION 
     This invention pertains generally to gas turbines and more particularly to high temperature gas turbines that employ a cooling medium to maintain the components within the working gas flow path within acceptable temperature limits. 
     BACKGROUND OF THE INVENTION 
     In most industrial combustion turbines, ambient air is drawn into the intake of a compressor, compressed and delivered to a combustion where it is combined with a fuel, ignited and transported through a transition member to a turbine wherein the working gas is expanded to produce mechanical energy. The compressor and turbine rotor are typically coupled to a common shaft so that rotation of the turbine rotor drives the compressor. Similarly, in power plant applications, the turbine rotor is also connected to a generator rotor to drive the generator to produce electricity. 
     A typical combustion turbine system  10  portion of such a power plant is illustrated in FIG.  1 . Generally, the combustion turbine is made up of a compressor section  12 , a combustion section  34  and a turbine section  41 . The compressor section  12  is made up of a plurality of stationary vanes  14  supported by the outer housing and rotating blades  16  which are mounted on a common shaft  18 . Each rotating blade row followed by a stationary vane row constitutes a compressor stage. FIG. 1 shows sixteen compressor stages. The compressor shown is also equipped with an inlet guide vane (IGV)  9 , and an outlet guide vane (OGV)  29 . The compressor is also arranged to form compressor bleed ports  22 ,  24  and  26  for bleeding compressed air for cooling high temperature turbine components. Ambient air  13  is introduced through an inlet  11  and successively compressed in each compressor stage, and it flows by all the bleed ports  22 ,  24  and  26 , and the rest of compressor stages  28 , and OGV  29  after which the compressed air travels through annular diffuser  30  to compressed air plenum  32  which surrounds the combustion  34  and transition member  36 . A portion of the compressed air  13  can be diverted from each of the bleed ports for cooling turbine components as discussed above. A portion of the compressed air, as shown by the arrow, reverses direction in the plenum  32  and travels between the combustion housing  38  and the combustion shell  40  where it is directed to a combustion inlet and combined with fuel introduced through the nozzle inlets  42 . The combined fuel and compressed air is burned in the combustion  34  to create a working gas which is directed through the transition member  36  to an inlet  44  to the first stage of the turbine  41 . The turbine section  41  is made up of a serial arrangement of stationary vanes  52  and rotating blades  54 . The rotating blades are supported by a common rotor system  56  and the vanes and blades are arranged in serial stages  44 ,  46 ,  48  and  50 , which form the first through fourth stages of the turbine section. The working gas exiting the transition member  36  then expands through the stages  44 ,  46 ,  48  and  50  causing rotation of the blades  54  which in turn impart mechanical, rotational power to the rotor system  56 . The turbine rotor system  56  is connected to the compressor shaft  18  so that rotation of the turbine rotor system  56  drives the compressor  12 . Normally, in power plant applications, the rotor system  56  is coupled to the rotor of a generator to drive the generator to create electricity. The working gas ultimately is exhausted at the exit to the turbine  58  and directed through an exhaust stack to the ambient atmosphere. 
     It is generally desirable to have the turbine work at the highest efficiency possible. It is also known that the higher the temperature of the working gas, the higher the efficiency of the turbine. However, the upper temperature that the working gas can practically function at is limited by the temperature characteristics of the materials that it interfaces with. In addition, the higher the temperature of the combustion process and working gas, the more pollutants such as NO x  that are created. Stringent environmental restrictions require such pollutants to be kept below a minimal level. These competing interests have been addressed by leaning out the combustion mixture to reduce flame temperature while maintaining an overall higher average thermal output and cooling the various components interfacing with the working gas flow path. 
     A system which has been employed for cooling the various turbine components is illustrated in FIG.  1 . The cooling system shown is an open loop cooling system wherein compressed air is introduced into the various components and after traversing a cooling path within a component, is exhausted into the working gas within the turbine, providing power augmentation. To assure that the working gas does not backflow through the cooling system, the pressure of the compressed air has to be greater than that of the working gas at the point at which the cooling air is introduced into the working gas flow path. In this regard, air  60  is bled from the first bleed port  22  of the compressor and introduced at the fourth stage  50  of the turbine to cool the turbine stationery components before being introduced into the working gas flow path around the rotating blades  54  at a point where the working gas is at its lowest pressure among the turbine stages. Similarly, air  62  is bled from a second bleed port  24  of the compressor  12  and introduced at the third stage  48  of the turbine  41 ; and air  64  is bled from the third bleed port  26  of the compressor  12  and introduced at the second stage  46  of the turbine. The compressed air exiting the compressor outlet  30  is used to cool the combustion shell and liner and the transition member before being introduced into the working gas path within those components. The air  66  exiting the compressor outlet  30  is also used to cool the first stage of the turbine and further diverted, as represented by reference character  68 , to cool the internal components of the rotor and the rotating blades  54 , before being introduced into the working gas flow path. In this manner, the internal components of the combustion, transition and turbine are able to accommodate higher temperatures for greater overall turbine efficiency. However, diverting compressed air from the compressor for cooling has a negative affect on the efficiency of the operation of the turbine in that there is less air available for combustion and to be introduced at the first stage of the turbine for power conversion from thermal power to mechanical power. Accordingly, it is desirable to find another means of cooling the turbine components interfacing with the working gas flow path that does not require or minimizes the diversion of air from the compressor. Accordingly, it is an object of this invention to provide a system that minimizes the use of compressed air for cooling a turbine&#39;s internal components along the working gas flow path. 
     SUMMARY OF THE INVENTION 
     The instant invention takes advantage of a nitrogen source in a power plant to supply nitrogen in lieu of compressed air to at least a portion of the cooling circuit in a combustion turbine. In a preferred arrangement, the nitrogen is supplied from an air separation unit that separates oxygen from the nitrogen in the air for use in an integrated gasification combined cycle (IGCC) plant. Upon startup of the plant, air is initially supplied to the cooling circuit of the combustion turbine until the nitrogen becomes available. In one preferred arrangement, a cooling control system monitors the availability of nitrogen and controls valves within separate legs of the combustion turbine cooling circuit to supply the nitrogen in lieu of the compressed air sequentially, one leg at a time as the nitrogen becomes available. In a preferred scheme, the cooling leg corresponding to the hottest turbine components is supplied nitrogen first along with compressed air until sufficient nitrogen is available to replace the compressed air in that cooling leg completely. The control system next adds nitrogen to the next successive leg corresponding to the next hottest component, one leg at a time until preferably the nitrogen completely supplants the cooling air in the second, third and fourth stage of the turbine stator and the turbine rotor. 
     In the preferred arrangement, the cooling system controller also monitors the temperatures of the components being cooled and regulates the volume of nitrogen supplied to those components to maintain the temperature in an acceptable range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description when read in conjunction with the drawings in which: 
     FIG. 1 is a schematic diagram of a combustion turbine illustrating a prior art cooling system scheme; and 
     FIG. 2 is a schematic illustration of the combustion turbine of FIG. 1 modified to include an embodiment the cooling system of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 illustrates the combustion turbine of FIG. 1 with the cooling system modified in accordance with this invention. Like reference characters refer to like components on the several figures. Aside from the cooling system, the operation of the combustion turbine system  10  is the same as that previously described with respect to FIG.  1 . 
     The cooling system illustrated in FIG. 2 includes valves V 5 , V 6 , V 7  and V 8  respectively in the cooling legs  68 ,  64 ,  62  and  60 . An optional, additional cooling leg  80  is also shown together with nitrogen supply line  78  and will be discussed hereafter. 
     In accordance with this invention, nitrogen supply lines  76 ,  74 ,  72  and  70  are respectively connected to cooling legs  60 ,  62 ,  64  and  68  downstream of the corresponding air supply valves V 8 , V 7 , V 6  and V 5 . Each of the nitrogen supply lines  70 ,  72 ,  74  and  76  is provided with its own flow control valve respectively, V 1 , V 2 , V 3  and V 4 . In the illustrated embodiment, the nitrogen is supplied from an air separation unit normally used to provide a supply of oxygen, for example, to a coal gasification process as part of an integrated gasification combined cycle plant. Nitrogen is a by-product of that process that might otherwise be vented to the atmosphere. 
     In the case of the embodiment illustrated in FIG. 2, upon startup of the plant, valves V 1 , V 2 , V 3  and V 4  are closed and valves V 8 , V 7 , V 6  and V 5  are opened to respectively provide compressed air from the compressor bleed ports  22 ,  24  and  26  and the compressor outlet  30  to the cooling circuits  60 ,  62 ,  64  and  68 . As the nitrogen pressure starts building up, valve V 1  is gradually opened and valve V 5  is gradually closed so that the nitrogen in supply line  70  supplants the air in cooling leg  68  to supply the rotor with nitrogen. When sufficient nitrogen is available to more than completely supplant the rotor cooling air in cooling leg  68 , valve V 2  in nitrogen supply line  72  is gradually opened and valve V 6  in cooling leg  64  is gradually closed. The control of valves V 2  and V 6  is coordinated to continue to convey a sufficient amount of cooling gas to the second stage of the turbine stator through cooling leg  64  to maintain the pressure of the coolant above that of the working gas in the second stage and to satisfy cooling requirements. When sufficient nitrogen is available to completely supplant the compressed air and maintain adequate cooling in the second stage, then the same process is repeated for the third stage and then the fourth stage. 
     Preferably, nitrogen cooling is first supplied to the hottest components because they are most likely otherwise to experience the most oxidation. The inert properties of nitrogen reduce the possibility of high temperature chemical reactions such as oxidation and thus enhance the life of the components. Upon shutdown of the air separation unit, the process is reversed and valves V 5 , V 6 , V 7  and V 8  are opened and valves V 1 , V 2 , V 3  and V 4  are closed and the cooling system is switched to compressed air cooling. 
     In another preferred embodiment, the cooling system of this invention includes a cooling system controller  82  that receives pressure and temperature signals P 1 , T 1 , P 2 , T 2 , P 3 , T 3 , P 4  and T 4  corresponding to the pressure and temperature of the nitrogen in supply lines  70 ,  72 ,  74  and  76  respectively. The cooling system controller  82  also receives temperature and pressure signals T 5 , P 5 , T 6 , P 6 , T 7 , P 7 , T 8  and P 8  respectively from cooling legs  68 ,  64 ,  62  and  60 . From these signals, the cooling system controller  82  computes the availability of nitrogen to supply lines  70 ,  72 ,  74  and  76  and the cooling capacity of the nitrogen in those lines as well as the cooling capacity of the compressed air in cooling legs  68 ,  64 ,  62  and  60 . Desirably, the cooling system controller  82  also receives temperature signals T 13 , T, 14 , T, 15  and T, 16  corresponding to the rotor and respective turbine stages that are connected to the nitrogen cooling system. From these latter temperature signals, the cooling system controller  82  can determine the cooling requirements of the turbine components and control the valves V 1 , V 2 , V 3 , V 4 , V 5 , V 6 , V 7  and V 8  accordingly to assure there is adequate cooling for those components. The algorithms for making those calculations are well within the state of the art. 
     Adding nitrogen cooling to the combustion shell, transition and first stage of the turbine requires additional modification to the cooling circuit which is schematically illustrated by the additional cooling leg  80 . While substantial benefit can be received form this invention without the cost of this additional cooling leg, an additional reduction in compressor bleed for cooling purposes can be achieved with such a modification. It is not practical to introduce the nitrogen directly into the compressed air plenum  32  surrounding the combustion and transition because that would substantially dilute the compressed air entering the combustion, which is necessary for the combustion process. However, the combustion shell can include a closed cooling path and the additional cooling leg  80  can be connected directly to that cooling path as well as to the transition and first stage so that the nitrogen would be introduced into the working gas downstream of the combustion zone in the combustion  34 . Temperature signals T 11 , and T 12  can then be routed to the cooling system controller along with temperature and pressure signals T 9 , P 9  from the additional cooling leg  80  and temperature signals T 10 , P 10  from the additional nitrogen supply line  78 , to control valves V 9  and V 10 , to provide the required nitrogen cooling to the combustion, transition and first stage of the turbine, as described for the other components heretofore. In this manner, a maximum amount of compressor air is available for power augmentation at the first stage of the turbine. Additional efficiency is achieved through power augmentation supplied by the nitrogen cooling gas exhausted into the working gas stream. 
     The use of nitrogen from an air separation unit in place of compressed air from the turbine compressor for turbine component cooling has a number of benefits. Firstly, the invention provides additional turbine power augmentation as a result of the additional air that is available at the first turbine stage, while still making the air available for cooling upon startup, shutdown or the unavailability of the nitrogen supply. The blade cooling air only produces power from its point of insertion into the blade path to the turbine exit. Substituting nitrogen as a coolant allows more air to produce power along the entire length of the blade path. A second benefit is reduced loading on the cooling air cooler normally used to reduce the temperature of the compressed cooling air to an acceptable level. Lowering air cooling temperatures discards some heat produced by the gas turbine compressor before it can be used to produce power in the expander sections, resulting in reduced turbine efficiency. A third benefit is the relatively low temperature of the nitrogen, coming from the air separation unit, which is typically about 200° F. (111° C.) cooler than the cooling air would have been, reducing the requirement for coolant flow through the rotor and blades. A fourth benefit of replacing cooling air with nitrogen is the inert property of the nitrogen, which can be beneficial to the metal of the turbine components by reducing the possibility of high temperature chemical reactions such as oxidation, which would be a potential problem with cooling air. 
     Either full or partial nitrogen cooling can be used in the gas turbine. Full cooling means that the air is replaced by nitrogen in all cooling networks, while partial cooling means that air is replaced by nitrogen in only some of the cooling networks. The substitution of nitrogen for air in the rotor cooling network is the most beneficial to the turbine performance because the cooling air would have been taken from the compressor at its highest pressure condition, then cooled to meet the cooling requirements of the rotor. The next most beneficial location for nitrogen substitution is stage  2 , then stage  3 , and finally stage  4 , which shows the least performance improvement. The additional cooling leg  80  is not significant to increasing the efficiency of the turbine since the compressed air would be introduced into the working gas upstream of the first stage of the turbine. However, it does provide additional mass flow through the turbine, which produces additional power, and it provides the advantage of minimizing the chemical reaction of the cooling gas on the turbine components. Accordingly, the additional leg  80 , while not necessary to this invention, can provide benefit. 
     While a specific embodiment of the invention has been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.