Improvements in power, efficiency and environmental benefits equip the SGT-100 gas turbine for the increased demands of distributed generation

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1 Improvements in power, efficiency and environmental benefits equip the SGT-100 gas turbine for the increased demands of distributed generation Mr Brian Igoe, Product Manager, SGT-100, Siemens Industrial Turbomachinery Ltd, Lincoln, England. He is a professionally chartered Mechanical Engineer, having held the position of Product Manager for ten years the last five being for SGT-100 & SGT-200. Previously held roles include combustion and product development. His professional career commenced in the automotive industry before moving into rotating equipment in His current role includes responsibility for product capability, KPI's, improvements, etc and providing extensive support through Sales, Project and Service activities on products as well as activities related to fuels and the environment. Extra curricula activities include regular outings around Lincolnshire umpiring hockey matches, and last year Brian qualified in power boat safety operation. Ian Amos, Product Strategy Manager, Siemens Industrial Turbomachinery Ltd, Lincoln, England. Ian has 27 years of experience in the gas turbine business starting his career at Ruston Gas Turbines in the Advanced Engineering group. Since this time he has worked on the core engine design for all the current products produced in the Siemens Lincoln facility, in addition to aero engines and concept design for other gas turbine projects. In his current role Ian is responsible for ensuring that Siemens industrial gas turbine developments are in line with the changing requirements of the customer J Anbarasan Senior Manager, Sales and Marketing EO RE, Siemens Ltd, India. He has been in the rotating equipment business for the last 15 years selling spares, services, revamps, refurbishments and upgrades to both Gas and Steam turbines, initially from Sermatech for a period of six years, then for Siemens, Oil & Gas industry. During the last nine years he is in the business of selling small & medium range Gas Turbines, Steam Turbines in both Generating and Mechanical drive applications. In his current position he is serving the industry with all the rotating equipment solutions for the Oil & Gas Industry in the Indian market, which is one of the most critical equipment in the Oil & Gas supply chain. To address the market his key role is to identify customer needs and fulfill them with the most compliant and fitting solutions.

2 Abstract Over the next ten years it is expected that the use of distributed generation will increase between 15 and 30% above the current use of small power units located on or near customer sites. To ensure Siemens products are able to meet this potential growth, improvements have been made to the SGT-100 single shaft gas turbine. Targeting improvements in power, efficiency and environmental benefits, this gas turbine offers significant benefits to both simple-cycle and co-generation efficiency, with achievement levels of over 80% in the case of the latter. Described in this paper are the incremental improvements to the SGT-100 core engine, aimed at improving compressor- and overall engine efficiency; it also refers to other changes aimed at ensuring that long service intervals are achieved and maintained. In addition, and of some importance to many users, is the ability to use a wide range of fuels as an alternative to pipeline-quality gaseous fuels, and the commensurate improvements in emissions to atmosphere. All of these factors are considered essential to achieve the basic requirements of a local approach to provide an energy solution and the benefit such solutions offer.

3 Introduction In order to capitalize on the projected growth in distributed energy in the coming decade, incremental improvements have been made to the Siemens 5MW class gas turbine, the SGT-100. These improvements not only benefit the overall turbine efficiency but also maintain the reliability of the product over increased intervals of service and maintenance. Described in this paper are the improvements in the core engine in the compressor and turbine areas, showing how these have achieved both an improvement in overall engine efficiency as well an increase in power output. Further to the improvements to the core engine to achieve improved efficiency, several small improvements have been completed and introduced, aiming, at enhancing the product s good reliability. Some of these are mentioned. Improvement in the core-engine package is not the only aspect necessary to show a good approach to distributed energy. Other equally important aspects are considered, such as alternative forms of gaseous fuels and the impact on the environment. As a consequence, the end user is better prepared to make both a detailed and informed decision on how to approach his energy use in the future. The SGT-100 is the key component in efficient and cost effective distributed generation systems, providing reliable power and heat (and cooling) close to the point of use. The high thermal efficiencies of co-generation maximise the thermal efficiency and minimise greenhouse gas emissions SGT-100 Core Engine The 5MW frame size SGT-100 gas turbine (GT) is a highly efficient gas turbine configured with a combustion process capable of burning a wide variety of fuels, from pipeline-quality gas fuels through to waste gas produced, for example, by the decomposition of waste (includes landfill, digester and sewage gases). Electrical power is produced and can be used by the customer or exported to the local utility grid. The waste heat in the exhaust gases can be recovered and used in other processes to maximize the overall thermal efficiency. Energy recovered in the form of heat to water can provide medium pressure steam to be used through a steam turbine producing a further 2-3MW in electrical power, or generate between 13 and 26 tonnes per hour of (200 0 C) lower-grade steam for use in the customer s processes. Furthermore, the recovered heat can be used to drive absorption chillers, giving rise to tri-generation, or combined heat, power and cooling schemes.

4 Figure 1: SGT 100 single shaft core engine Design The SGT-100 core engine shown in figures 1 and 2, comprises a 10-stage transonic compressor, with a pressure ratio of 14.7:1. A two-stage overhung compressor turbine provides the energy to drive the compressor with the excess torque taken via the input shaft through a generator thus providing electrical power. The turbine is configurable with either a conventional combustion system (diffusion flame) or a lean pre-mixed Dry Low Emissions (DLE) design. The former provides for many different types of fuels, whilst the latter lowers exhaust emissions emitted to the atmosphere. Variable guide vanes in the compressor section are used to optimize the compressor for starting and normal operation. An additional benefit of this feature is to reduce air massflow through the core at part loads and maintain low levels of carbon monoxide, CO. Twin ignition and cross lighting of remaining combustors is a normal feature where conventional combustion is used, whereas DLE requires an ignition source in each of the combustors. Control of the turbine is through a plc controller, incorporating a core engine controller, designed and manufactured by Siemens. This provides for control during all phases of operation, from pre-start through normal start and operation and finally during a manual intervention for a planned stop. The design of the turbine control module allows it to integrated into an overall plant control system is possible.

5 T P 4 variable guide vanes 6 can annular DLEcombustor 10 stage axial compressor 2 stage overhung turbine Tie-bolt Hirth serration Figure 2: SGT-100-1S cross section showing major features The SGT-100 single shaft engine is available in a number of ratings varying from 4.35MWe through to 5.4MWe. This last rating is the most recent and is presented in more detail below. Improvements Efficiency The newest rating in the SGT-100 family produces an ISO performance of the 5.4MWe. This new rating share many common features with its older sibling, (5.25MWe), but some key features have been redesigned thus enabling the higher rating to be achieved, figure 3. The improvements come principally through efficiency changes to both compressor and turbine sections. Compressor The first two stages of compressor blades (rotor 1 and 2, stator 1 and 2) have been changed to a new aerofoil shape. The controlled flow profile has been successfully introduced on other products, namely SGT-300 and SGT-400, with increasing field experience gained.

6 T P Compressor Blade Stator stages S1& S2 Rotor stages R1 & R2 HP Rotor blade SX4 material Triple fin shroud Step Tip seal Figure 3: Major area of change to SGT-100 As part of the engineering required to demonstrate this new blade aerofoil shape a technology demonstrator was made which provided all of the data necessary to prove the blade achieved the design intent. This demonstrator was based on the SGT-100 compressor, and the designs were then mechanically scaled to be incorporated into the other products. It was a natural extension to consider these blades for the SGT-100 production core engine. Figure4: Stage 1 Rotor, datum LHS, improved geometry RHS

7 Stages 1 and 2 of the compressor from the 5.2MWe rating were re-designed and optimized to achieve a 0.3% efficiency improvement. 3-D aerodynamic design tools used to customize MCA (multi-circular arc) and CD (constant discharge) airfoils which were subsequently optimized by 3-D CFD methods, figures 4 and 5. Figure 5 Current MCA stage 1 blade, left, compared to 3 D controlled flow blade for enhanced blade, right. 3D CFD simulation of the entire compressor was completed to check stage matching and determine potential performance improvements. Aero and mechanical design optimization was necessary to achieve High Cycle Fatigue (HCF) life requirements. Compressor variable guide vane (VGV) optimization was carried out using proprietary methods to achieve performance improvements over a range of loads and speeds while a number of mechanical design iterations were necessary to achieve a design free of any potential resonance frequency. Figure 4, above, shows the velocity profile change from the current Stage 1 blade to the optimized re-designed blade, figure 5. Turbine The HP rotor blade was also changed, both in geometric design and material. The newest generation of single-crystal material, CMSX4, was adopted from the onset, based on excellent operational experience gained on the current product. More significant improvements came with the changes aimed at improving the stage efficiency. Triple-fin shroud design was used to reduce the over-tip leakage associated with the current blade (single-fin shroud) along with more efficient ejection of blade cooling air from the aerofoil trailing-edge, rather than out through the blade shroud, figure 6. The change in airfoil shape also helped improve blade and stage efficiency. Adoption of blade coatings has further enhanced component life, and allows nonstandard fuels to be considered for use.

8 Current SGT-100 single shaft Turbine Arrangement HP Rotor Blade: Single Crystal CMSX4 Figure 6: HP turbine stage design change Design predictions, which were demonstrated during prototype testing of a development engine, resulted in an improvement of stage efficiency by more than 2%. Efficiency benefit Resultant benefit of these changes was a modest increase in output, commensurate with improvements in efficiency and fuel consumption as detailed in table 1 below. Although not specifically covered in this paper, the more significant change comes with the twinshaft variant, normally used for mechanical drive of a compressor or for pump duty. Configuring both single and twin-shaft variants with common compressor and turbine stage resulted in a 16% improvement in output for the twin-shaft variant. Rating 5.25MW(e) 5.4MW(e) Fuel: Natural Gas* Natural Gas* Frequency 50/60 Hz 50/60 Hz Electrical efficiency: 30.5% 31.0% Heat rate: 11,815 kj/kwh (11,119 Btu/kWh) 11,613 kj/kwh (11008 Btu/kWh) Turbine speed: 17,384 rpm 17,384 rpm Compressor pressure ratio: 14.6:1 15.6:1 Exhaust gas flow: 20.8 kg/s (45.8 lb/s) 20.6 kg/s (45.4 lb/s) Temperature: 530 C (986 F) 531 C (988 F) NOx emissions 25 ppmv 25 ppmv (with DLE, corrected to 15 % O2 dry):

9 Table 1: Comparison of ratings showing incremental benefit of efficiency improvements The exhaust conditions remain unchanged, thus giving very good heat recovery options. Performance: The improvements described above have been validated using both compressor and full gas turbine testing. Compressor testing including a series of engine maps resulted in a fully optimized set-up, maximizing the benefits of control of the VGV s, and achieved the design intent of +0.3% overall compressor efficiency improvement, with no significant changes to mass flow, table 1 above. Improvements - Reliability In common with the drive to continually improve all of the products, the SGT-100 has benefited from a number of improvements aimed at enhancing product life and extending periods of up-time i.e. the period of time before planned maintenance interventions. A number of improvements have been released and some of these are presented below. Calm Controls The driver for improved reliability has been the need to maximize operational up-time and to minimize unscheduled shut-downs for no real purpose. Control of the engine is still essential to prevent terminal events from causing extensive damage, but many lesser events can, and have been, re-classified from a shut-down trip to a warning or in some cases removed altogether. All new engine dispatches include this operational change as standard, and it is now being widely implemented into the existing fleet of turbines. CT1 Blade material The 5.4MWe rating has standardised on CMSX4 single crystal super-alloy for the use in the casting of the first stage turbine blade. This was introduced a number of years ago on the SGT-100 platform and experience is growing, with the lead engine having exceeded 32,000 operating hours and a number of other units exceeding 24,000 operating hours. VGV Improvements Variable guide vanes (VGV) applied to the first four rows of compressor (stator) blades allow the engine to operate throughout the transient speed range without potential surge issues. A recent change in the design minimizes ingress of compressor washing fluids and contaminants into the spindle area thus preventing corrosion as shown below, figure 7. This can result in blade seizure and subsequent failure, such as bent push rods. This change has been achieved without the need for casting or casing changes. The importance of VGV control is demonstrated where part load exhaust emissions control, particularly of carbon monoxide, CO, is required at reduced load. Lean pre-mix combustion design is inherently weak at low loads, due to increased air to fuel ratio, leading to a reduction in stability margin, resulting in higher CO emissions. Closure of the VGV s reduces the air passing to the combustor, thus achieving air fuel ratio similar to a higher load. Combustion stability improves with a result the CO emissions are much lower. It is possible to improve the CO turndown capability by this method by over 30%. A consequential benefit of this is to maintain the exhaust temperature at a much higher

10 level suited for heat recovery systems, although the mass flow is reduced. The VGV schedule is controlled by a turbine control parameter called T-fire, derived from actual engine parameter measurements. Figure 7: VGV corrosion concerns and revised design Siemens recommend High Efficiency Particle Arrestor (HEPA) filtration is installed wherever practicable. When this has been retrofitted operators have often found it removes the need for compressor washing (subject to local ambient environmental conditions). Eliminating or reducing the need for off-line washing increases engine availability time in excess of 0.5%, in addition to removing a potential initiator of corrosion. Fuel Flexibility One of the key benefits of the SGT-100 is its ability to operate on a wide range of fuels, varying from very rich gaseous fuels such as LPG through to weak gas fuels, containing high levels of inert species (predominately carbon dioxide and nitrogen). The ability to operate on such a wide variety of fuels enables the use of the SGT-100 to be used in areas where power and heat are required, but not standard fuels, such as good-quality natural gas may not. The chart below, figure 8, shows the types of fuels encountered and used over the years. In most cases a diffusion flame combustor is required, but DLE operation has been, and is continually being, expanded to exploit these fuel types. Although not specific to the SGT-100, the fuels range has been expanded for DLE operation on both the SGT-300 and SGT-400 product models. Deriving gases from the decomposition of waste, either in a landfill site, or under a more controlled process such as anaerobic digestion results in a gas which is somewhat weak compared to natural gas, but which still allows combustion to take place, producing the electrical power and simultaneously providing heat from the exhaust. Just the same as when other normal fuels are used. These weak gases contain higher quantities of inert species such as carbon dioxide and nitrogen, which suppress the fuel calorific value to typically half or less than half that of a pipeline-quality gas fuel.

11 Also covered in this range of fuels are well-head and associated gas fuels, which, in addition to the inert species mentioned above, may also contain higher hydrocarbon species. Air blown Biomass Gasification 3.5 Landfill & Sewage Gas Coke oven gas high Hydrogen High Hydrogen Refinery Gases Diffusion flame operating units DLE operating units Ethane LPG Low Calorific Value (LCV) Off-shore lean Well head gas Tri fuel SGT-300 PLG/NG/Liquid Off-shore SE Asia lean well head gas Medium Calorific Value (MCV) IPG Ceramics Pipeline Quality NG Normal Siemens Diffusion Operating Experience Off-shore rich gas Siemens DLE Units operating Standard DLE fleet Capability Expanded DLE Capability High Calorific Value (HCV) SITL. Definition Wobbe Index (MJ/Nm³) Figure 8: Fuel flexibility SITL Products Active Pilot Control A feature that has been shown to offer significant benefits on other products when operating on weaker fuels in a lean pre-mix (DLE) combustor is a means of controlling the fuel flow to the pilot burner in a more active manner. The intelligent control of engine parameters has resulted in improved operation where, for example, wide fuel variation has been witnessed and control of a low-emissions signature throughout these fuel variations has been necessary. Active control reduces the need to include any online monitoring of the incoming gas in terms of composition, calorific value or Wobbe Index. Low Emissions Combustion Configuration Lean pre-mix combustion uses an excess of oxygen along with good fuel/air mixing to achieve the homogeneous mixture necessary for low combustion temperatures and hence production of low levels of oxides of nitrogen, NOx, carbon monoxide, CO. This is generally referred to as Dry Low Emissions (DLE). In the SITL range of products fixed air swirlers are used, rather than a variable orifice. In a single shaft configuration the speed, hence air mass flow is constant. As load is reduced the amount of fuel required reduces, thus resulting in a leaning out of the Fuel Air Ration (FAR). There is a limit at which point combustion instability occurs, resulting in a rapid increase in CO emissions. Closure of the VGV near this limit improves lower load operation as described earlier. Combustion hardware for DLE is shown in figure 9 below and comprises: Pilot burner housing the ignitor (one per combustor) and is provides fuel during start and transient operation Main burner provides fuel during load operation

12 Radial swirler, provides high velocity air to mix with the fuel in the pre-chamber prior to burning in the combustor Double-skin impingement cooled combustor For liquid operation a liquid core is required and locates inside the main burner/radial swirler DLE combustion comprises multiple fuel streams, pilot fuel for ignition and transient operation and main fuel for load operation. The fuel schedule ramps both across the speed range during the start and the load range such that the pilot reduces from approximately 80% at ignition to 5% at full load (actual starting and full load values vary from product to product and duty requirements). The pilot provides stable operation for rapid load change such as load rejection and also controls the level of NOx emissions. Pilot Burner Igniter Liquid Core Main Burner and Radial Swirler Pre-Chamber Cyclone Fuel Schedules Fuel Control Schedule Pilot Split (%) Tfire (K ) G a s L iq u id Double-skin Impingement cooled combustor Figure 9: Dry Low Emissions Combustion system and fuel schedule SGT-100 Gas Turbine Application in Power Generation The SGT-100 is ideally suited to distributed power generation schemes and supplied as an underbased mounted generator package with minimum site connects to fuel, services and electrical distribution system. Co-generation in particular provides the end-user with the benefits of: reduced energy costs compared to the import of electricity from the grid and local heat production.

13 Reduced emissions of C02 due the extremely high overall thermal efficiency when there is a use for the exhaust heat. Security of supply, reducing reliance on grid connections. Co-Generation Arrangement and Efficiency Combined heat and power (CHP) or Co-generation offers significant advantages over other, more traditional, methods to provide power and heat. In a co-generation facility, figure 10, power and heat are produced simultaneously from a single fuel source, with the gas turbine providing the power and the hot exhaust gas passed through a waste heat recovery unit (or heat recovery steam generator) to generate steam or hot water for factory or facility processes. Utilizing the exhaust heat in this way can in most instances lift the overall efficiency of conversion of the fuel provided from 30% to more than 80%. This latter figure is usually deemed the benchmark for good-quality CHP schemes. Cool Exhaust Gas Factory Water Returned to CHP Plant Steam Supply to factory process WHRB Fuel Gas Turbine Generating Set Figure 10: Typical CO Generation arrangement ~ * Waste Heat Recovery Boilers are also known as Heat Recovery Steam Generators (HRSG) Tri-generation

14 An extension to co-generation is the additional circuit taking water through an absorption chiller, hence tri-generation, figure 11. This results in the production of cold water used in air-conditioning systems. This provides an ideal solution for applications involving large public spaces, such as shopping centers and office accommodation. The type of solution provides good flexibility, allowing adjustments in both steam-raising and cold water production, depending on season and climate conditions. Cool Exhaust Gas Building/ Facility Water Returned to CHP Plant Steam Supply to factory Water return in Chiller Circuit Chilled Water for Air Conditioning Absorption Chiller Air Fuel Steam supply ~ Gas Turbine Generating Set Figure 11: Tri generation, incorporating adsorption chiller The turbine improvements described earlier in this paper offer incremental improvements to the plant efficiency, resulting in reductions in the carbon footprint. Table 2 identifies the potential benefits associated with a good co-generation application realizing in excess of 80% efficiency and providing a reduction in annual energy costs of up to 30%.

15 Site Requirements Assumptions 6000 kwe electrical Gas Turbine Power 5252 kwe kw thermal Gas Turbine Heat input kw Gas Turbine Exhaust Heat 8667 kw Gas Price /kwh Electricity price /kwh GT running hrs 8400 hrs/yr Boiler efficiency 90% External supply Power kw Description hours run Cost Power kw Description Hours run Cost 6000 electric import GT not running boiler gas fuel electric import Toal Cost boiler gas fuel GT running Annual savings Payback Table 2: Benefit for GT Co generation application Gas Turbine Cogeneration Solution 748 electric import gas turbine fuel boiler fuel Total cost Summary Improvements to the single-shaft SGT-100 have been presented resulting in efficiency and output gains without impacting the good-quality exhaust conditions necessary for heat recovery systems such as co-generation. These gains, when considered with fuel and environmental considerations, make the SGT-100 an ideal solution for the distributed generation market.