THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E St, New York, N.Y CT-11

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E St, New York, N.Y CT-11 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions on*... Sections, or printed in its publications. Discussion is printed only if the paper ie published in an ASME Journal. Authorization to photocopy material for internal or personal use under circumstance not falling within the fair use provisions of the Copyright Act is granted by ASME to ' libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of $0.30 per page is paid directly to the CCC, 27 Congress Street, Salem MA Requests for special permission or bulk reproduction should be addressed to the ASME Technical Pubrerring Department Copyright by ASME All Rights Reserved Printed in U.SA ABSTRACT A revolutionary step has been taken in the development of the Next Advance in Power Generation Systems - "FT' Technology Combined Cycle. This new gas turbine combined cycle system increases thermal performance to the 60% level by increasing gas turbine operating conditions to 2600 F (1430 C) at a pressure ratio of 23 to I. This represents a significant increase in operating temperature for the gas turbine. However, the potential for single digit NO levels (based upon 15% Oi. in the exhaust) has been retained. The combined effect of performance increase and environmental control is achieved by an innovative closed loop steam cooling system which tightly integrated the gas turbine and steam turbine cycles. Although a significant advance has been taken in performance, the new power generation system has been configured with a substantial number of proven concepts and technology programs are ongoing to validate the new features. The technical activities which support the introduction of the new turbine system have reached a point in the development cycle where the results are integrated into the design methods. This has permitted the :Tr Technology to achieve a design readiness status and the first unit will be under test in late INTRODUCTION A key advantage of gas turbine power generation systems is the ability to continue evolving to higher firing temperatures. Each increase in firing temperature yields a dual benefit: increased efficiency, that is, lower fuel use, and increased specific work, that is, output per unit of air flow, reducing capital cost. In a gas-turbine/steam-turbine combined cycle configuration, performance is optimized by selecting a pressure ratio to achieve high values of both efficiency and specific work. In order to produce an attractive system, pressure ratio and firing temperature must advance together (Carman, 1995). Environmental compatibility has become as important a consideration as performance. For a gas turbine, the main emission issue is nitric oxide (NO.). To achieve low NO, in a gas turbine, the maximum temperature in the combustion chamber must be reduced below the levels of conventional diffusion combustion. In order to achieve low H Gas Turbine Combined Cycle Technology & Development Status James C. Corman GM, Power Systems Engineering GE Power Generation General Electric Company Schenectady, NY mil a jig! combustion temperatures while maintaining high gas turbine firing temperatures, a new combustion concept called "premix lean" was developed. With premix lean, a large fraction of the compressor discharge air is mixed with the fuel prior to combustion, creating a lean mixture that produces low gaseous emissions when combusted in a fluidmechanics-stabilized combustion chamber. The dry low NO (DLN) premix combustion system has moved through the laboratory development stage and has been successfully introduced in commercial service. This combustion system is capable of achieving single-digit NO. levels. The long history of gas turbine technology development, successful transition of the resulting new products to the marketplace, and development and commercialization of a new combustion system, DLN, to meet increasingly stringent environmental constraints provide a solid foundation for the next major step to a new power generation system. RATIONALE FOR A NEXT-GENERATION POWER SYSTEM The goal of advanced gas turbine development has been to achieve increased performance through increased firing temperature. The increase in firing temperature is obtained by applying advanced airfoil cooling techniques and using new alloys. New cooling techniques and materials have been available for transfer directly from aircraft engine experience. The additional constraint of emission. control for environmental compatibility creates a conflicting set of parameters. Higher perfonnance requires higher firing temperatures, but lower NO,, emissions require lower combustion temperatures. Both performance and emission targets can be met by changing the gas turbine cooling system. The rationale for the change lies in the difference between the two controlling parameters for performance and emissions combustion temperature and firing temperature. The most critical element of an advanced gas turbine is its hot gas path, Fig. 1. The compressor discharge air and fuel are mixed and combusted in a chamber at a specific condition, combustion temperature. The flow stream of high-pressure, hightemperature combustion products is accelerated as they pass through the first stationary airfoil (first stage nozzle segment). The firing temperature the Presented at the International Gas Turbine and Amoengine Congress & Exhibition Birmingham, UK June 10-13, 1996

2 flow stream temperature at the inlet to the first rotational stage (first stage bucket) establishes the power output (ultimately, the cycle efficiency). The difference between firing temperature and combustion temperature is the temperature drop (nozzle AT) required to cool the first stage nozzle, Fig. 2. In the current line of advanced gas turbines, the first stage nozzle is cooled with compressor discharge air flowing through the airfoil and discharging out into the combustion gas stream as the airfoil is film cooled. This cooling process causes a temperature drop of up to 280 F (155 C) across the first stage nozzle. If the nozzle can be cooled with a closed-loop coolant without film cooling, the temperature drop across the first stage nozzle would be less than 80 F (44 C), which would permit a 200 F (110 C) rise in firing temperature with no increase in combustion temperature. The key to single-digit NO capability at increased performance (ircrea Red firing temperatures) is closed-loop cooling of the first stage nozzle. The cooling concept has a dual effect: allowing higher firing temperatures to be achieved without combustion temperature increases and permitting more compressor discharge air to flow to the headend of the combustor for fuel premixing. A change in cooling strategy for the first stage stationary nozzle will therefore resolve the conflicting requirements for higher performance and lower emissions. However, once a closed-loop cooling system is established, it can do much more to benefit performance. In conventional gas turbines, compressor air is also used to cool the remaining rotational and stationary airfoils. This air also enters the combustion gas flow path and is classified as "chargeable air," reducing cycle performance. If this chargeable air is also replaced with a closed-loop coolant system, a cycle benefit of up to two points can be achieved, a dramatic improvement. The move to a higher level of gas turbine power generation performance while meeting emission control targets can be achieved by a change from open-loop air cooling to closed-loop cooling of the gas turbine hot gas path parts. To achieve effective cooling while maintaining hot gas path part integrity, however, an improved coolant medium is required. In a power generation application, the high levels of performance for gas turbine systems are achieved by means of a combined cycle. The high temperature exhaust from the gas turbine is used to generate steam in a heat recovery steam generator (HRSG). This steam is then expanded through a steam turbine. For the gas turbine combined cycle power generation application, there is another coolant alternative steam. The airfoil cooling requirements of the gas turbine are met by using steam a cooling fluid that is much more effective than air to cool the hot gas path parts without relying on film cooling. This coolant is provided in a closed-loop system integrated with the steam bottoming cycle. The use of closedloop steam cooling for advanced gas turbines was studied under a GE/EPRI Program (GE/EPRI RP ). ADVANCED H COMBINED CYCLE SYSTEM The integration features of the H Combined Cycle System include the flow of coolant steam from the steam cycle to the gas turbine, as shown in Fig. 3. The high-pressure steam from the HRSG is expanded through the high-pressure section of the steam turbine. The exhaust steam from this turbine section is then split. One part is returned to the HRSG for reheating; the other is used for cooling in the gas turbine. Steam is used to cool both the stationary and rotational parts of the gas turbine. After cooling the hot gas path parts, the steam temperature is increasi-d to approximately reheat temperature. The gas turbine cooling steam is collected and returned to the steam cycle, where it is mixed with the reheated steam and introduced to the intermediate-pressure steam turbine section for energy recovery. The H Combined Cycle is composed of the H gas turbine, at increased operating conditions, the threepressure-level HRSG, and a reheat steam turbine. These three components can be configured in a single-shaft or multi-shaft arrangement for an efficient, low emission power generation system. H GAS TURBINE The attractive operational characteristics of the H Technology Combined Cycle system are a direct result of the gas turbine performance level. This new gas turbine, Fig. 4, features closed-loop steam cooling for the first and second stages of its four-stage turbine. In order to optimize the efficiency and specific work with the 2600 F (1426 C) class of firing temperatures, a higher pressure ratio compressor derived from the GE Aircraft Engine CF6 80C2 has been used The DIN combustion system now in service across the product line has been adapted to the H gas turbine. These are the three key components for the H Technology Gas Turbine. The characteristics for these key components have previously been described (Corman, 1995). NEW GAS TURBINE DESIGN AT LOW RISK The next-generation power system features both a new gas turbine configuration and new integration concepts linking the gas and steam turbine cycles. A key consideration in the step to this next system was reduction in technical risk. This was achieved by using as many proven components as possible and in parallel initiating technology development programs to establish the design basis for any new areas. 2

3 The extensive existing technology base has permitted the new gas turbine to be configured using proven components and design methods. The fact that the compressor itself was derived from an aircraft design with millions of operational hours reduces the inherent risk of using a new compressor design. An extensive DLN combustor experience base is being developed for the current FA line of gas turbine products, which operate at combustion conditions very close to the actual H combustion temperature (not firing temperature). The construction materials for the critical hot gas path parts have been derived from proven aircraft and power generation designs. The design methodology for the closed-loop airfoil cooling system was validated against aircraft engine, advanced condition, practices. The steam cooling system and TBC application are the two unique elements of the H gas turbine. Both have undergone many years of development with H.S. Department of Energy (DOE) Advanced Turbine System Program (GE/DOE Contract), supporting parallel GE internal programs in which the focus has been on establishing the enabling technologies. The technical information generated by these programs has been integrated into the design process. TECHNOLOGY DEVELOPMENT The technical studies to expand the design data base deal mainly with characterizing the new coolant concept. The construction materials are proven alloys when air is used as a coolant. The compatibility of the materials with steam as a coolant was validated at H operational conditions. The steam purity characteristics for use in the airfoil coolant paccages were confirmed through the acquisition of data at an operating combined cycle power plant. The new coolant medium required validation of heat transfer correlations under H operating conditions. The TBCs for airfoil parts are being evaluated in extensive laboratory and field tests. Subscale compressor tests and full condition DLN combustion tests are being used to validate the operational characteristics of these key components. Steam Materials Compatibility The materials of construction for the hot gas parts will be operated in a steam environment at different conditions. These life limiting parameters are: Fatigue crack propagation, low cycle fatigue and creep. Test facilities have been designed and tests are conducted in steam environments on all materials at H operating conditions. An example of the facilities being used to perform these tests is shown in Fig. 5. This specific test setup has been used to evaluate material creep. The effects of steam on creep behavior of candidate materials for structural and gas path components was investigated. Initial efforts were concentrated on measuring the creep resistance of candidate materials in the steam environment as compared to air. Standard creep rupture tests of smooth bar specimens under static load were run in both steam and air environments under a range of stresses and temperatures. The results were evaluated using the Larson- Miller parametric (LMP) approach of quantifying material creep strength versus time and temperature. The creep strength of the materials has been shown to vary with LMP in a predictable manner. Steam Purity Characteristics In order to evaluate the purity characteristics for steam as a coolant medium, an experimental facility was set up at an operational gas turbine/steam turbine combined cycle plant. This permitted a realistic assessment to be made of the particulates in the coolant stream and to ass the needs for a filter system. The results from this study were previously reported (Woodmansee, et al., 1995). The results of these studies indicated that particles were mainly in the form of Fe304 and were less than 1 PPb., (parts per billion by weight). The particle size was less than 20p with the majority of the particles < A "commercial" full flow filter system has been specified and tests conducted with a sample element. The particulate capture confirmed a design specification for 24,000 hours to filter replacement. The particles remaining after filtration are projected to result in less than 1 mil deposit in coolant channels during the 24,000 hour time period. Turbine Cooling Technology The use of steam as a gas turbine coolant medium introduces changes in the design correlations. The closed loop coolant flow circuits rely on forced convection as the primary heat transfer mode. This transport medium can be evaluated in stationary components with existing correlations. However, in the rotational components, e.g., bucket coolant channels, the dimensionless parameters which exist due to steam properties, are outside the region of existing data. An' experimental facility was constructed to obtain data on rotational passages (Staub, et al., 1995). The experimental thermal transport data has been taken for both smooth and turbulated paccages at the Reynolds and Buoyancy numbers of design interest for the various bucket coolant passages. Thermal Barrier Coatings Thermal Barrier Coatings (TBC) are currently in use in gas turbine hot gas path parts e.g., combustion liners and transition pieces. In the H Technology Gas Turbine, it is projected that WC will be employed in these locations as well as the high temperature airfoils. 3

4 For steam cooled gas turbines, TBC for airfoils are subjected to high thermal gradients. The TBC integrity and degradation mechanics have been assessed using a unique experimental procedure, E- Beam Thermal Gradient Test Facility, which simulates this unique operating environment. The parameters which are being adjusted in order to provide a TBC system r.vhich meets the design criteria are application techniques (process control) and bond coat chemistries. The evaluation tests are run in both a steady state and transient manner to assess TBC durability. COMPONENT VALIDATION TESTS In addition to the fundamental studies which have acquired data for the gas turbine and system design, a series of experiments have been set up to validate key components designs. Compressor Tests Although the compressor for the H gas turbine is based upon a proven CF6-80C2 design, a component validation test has been conducted with a 1/3 scale compressor model, Fig. 6. This petmitted the performance "map" to be validated as well as to set the operational strategy for the variable geometry airfoils. Combustor/First Stage Nozzle A turbine simulator (combustor, transition piece and first stage nozzle) has been constructed at full scale (for a 50 Hz machine). This facility is operated at H conditions and validates the combustor performance parameters and the airfoil thermal and mechanical design characteristics. H PRODUCT STATUS The technology development tasks have been completed to a stage that the information is integrated into the design methods. This has permitted a design to be frozen for the 9H, 50 Hz, product. The market assessment has indicated that this should be the first machine to test. The factory test of this 911 is scheduled for the 4th quarter of It is expected that the 7H, 60 Hz, product will be installed as a first commercial unit with a field validation test under the US DOE Advanced Turbine System Program (GE/DOE Contract). This Government/Industry Cooperative Agreement now in the component validation/detailed design phase is underway. The component validation experiments are continuing to provide data on the viability of the H Gas Turbine Combined Cycle System. A select number of technology programs are also continuing in order to provide the basis for an evolution path far into the next decade. H TECHNOLOGY PRODUCT PERFORMANCE The H combined cycle power generation systems are designed to achieve 60% net plant efficiency. The operational and performance characteristics for the H technology gas turbine/combined cycle products are summarized in Table 1. The significant efficiency increases over the F technology product line are achieved by advancing the operating conditions pressure ratio and firing temperature. These advantages are achieved with the STAG 109H (50 Hz) and STAG 1071i (60 Hz) systems, while maintaining single-digit NO and CO capability. CONCLUSION The H family of unique integrated gas turbine/steam turbine systems provides an opportunity to move up to the next plateau for power generation systems. The innovative cooling system for the H gas turbine permits a step change in firing temperature, reaching record levels of efficiency and specific work, while retaining low emission compatibility. The design for this next generation system has proceeded through the development stage and has achieved a design freeze status. This will permit both 50 Hz and 60 Hz gas turbine combined cycle systems to enter commercial service before the end of the &rade, The H family of advanced power generation systems will set the performance, emission control, and operational standards well into the 21st century. REFERENCES Corman, IC., 1995, "New Technology Trends for Improved IGCC System Performance Proceedings AWE IGTI Turbo Expo '95, ASME 95-GT-227, Corrnan, IC., 1995, "Challenges and Opportunities for Advanced Gas Turbine Systems", Proceedings 1995 Yokohama International Gas Turbine Congress, Yokohama, Japan. GE/DOE Contract, DE-AC21-93MC30244, Cooperative Agreement DE-FC21-95MC 31176, "Advanced Turbine Systems. GEffiPRI Contract, "Advanced Gas Turbine Cycle Studies", GEffiPRI Resnrch Project RP , TR , Final Report to be published. Staub, F., Maddaus, A., Telaiwal, P., 1995, "Rotational Effects on Heat Transfer at Advanced Engine Conditions," F. Staub, A. Maddaus, P. Tekriwal, IGTI Houston, June 1995, 95-GT-417. Woodmansee, D., Tolpadi, A., Hwang, A., Maddaus, A., 1995, "Ensuring Adequate Cooling Purity for Advanced Gas Turbines", IGTI Conference, Houston, TX, June 1995, 95-GT

5 _ " s.r; cs." ir! mtlifib ro Figure 1. Gas Turbine - Hot Gas Flow Path Air Higher Firing Temperature Maximizes Output Fuel law Nozzle AT Minimizes NOT Combustion Temperature = Thing Temperature + Nozzle A Combustor First-Stage Nozzle Firing Temperature Produces Work Nozzle AT First-Stage Bucket Figure 2. Reducing Combustion Temperature/ Increasing Firing Temperature 5

6 Air Gas Turbine Steam Turbine Figure 3. STAG 109H and 107H Combined Cycle System Description n As%.lima I _ issik it Mill -iiilltilfeav=th Fours 4. Cross-Section H Gas Turbine

7 Test Vessel Thermocouple Creep Specimen Extensometer Steam Generator Flowmeter Head To Conditioned Water Lever Arm Metering Valve ' Data Acquisition Feed thru Pull Rod Weights Figure 5. Schematic of the CREEP Testing Apparatus Compressor Rotor Test Vehicle Figure 6. Subsonic Compressor Test 7

8 Characteristics Table 1 H vs. FA CHARACTERISTICS AND PERFORMANCE Firing Temperature Class, F(C) 2350(1300) Air Flow, Lb/Sec (kg/sec) 974(442) Pressure Ratio 15 Specific Work, MW/Lb/Sec (MW/kg/sec).26 (.57) Performance Advanced Machines 7F / / (.72) jea 2350(1300) 1327(602) (.58) ill 2600(1430) 1510 (685) (.70) Simple Cycle Output, MW Simple Cycle Efficiency, % Combined Cycle Net Output, MW Combined Cycle Net Efficiency, % NO: (ppmvd at 15% 02)