1’300 MW Repowering Project Claus C A Way for Cleaner and Increasing Power
Production Using Existing Assets
Simone Turina, Roger Fehr, Terry Robinson, Richard P. Graham, Matthias Hiddemann and Mathieu Bloch
Alstom Power
Baden, Switzerland
1 Introduction
In June 2008, Alstom signed a contract with the Dutch utility Essent Energy to build a 1’300 MW combined cycle power plant in Maasbracht, located in the South East of the Netherlands.
Claus C, 1280 MWRepowering ESSENT-ALSTOM
Figure 1 - Location of Claus C The project consists of upgrading the existing 640 MW Claus B unit, a conventional steam power plant burning natural gas, to a re-powered combined cycle power plant, named Claus C. The output of the unit will increase to 1’280 MW, CO2 emissions will be reduced by 40%, while efficiency will increase from around 38% to 58.5%, bringing it in line with the most efficient combined-cycle power plants available today.
Alstom will design and build Claus C next to the existing power plant and provide the main power plant components, including three GT26 gas turbines along with the turbogenerators, HRSG (heat recovery steam generators) and the relevant auxiliary equipment including an ALSPA Distributed Control System. The old gas fired boiler of Claus B will be replaced by three new GT26 gas turbines. The existing 640 MW steam turbine will be retrofitted by Alstom and integrated into Claus C. Additionally, the steam turbine generator of Claus B will be re-used, as well as existing civil structures and BoP systems. Claus C will be the first 3-on-1 configuration for a KA26 combined cycle power plant.
2 Essent, Alstom and the Energy Market in The Netherlands
Essent is the largest energy company in the Netherlands: 2,6 million private and business customers purchase gas, electricity, heat and energy services. Essent (including its predecessors) has over 90 years experience of generating, trading, transmitting and supplying electricity and gas. Alstom is a well established player in the Dutch energy market since long time, with a largest installed capacity on gas-fired and coal-fired power plants corresponding to a market share of about 60%. These installations are being maintained by our local service organization in The Netherlands. The last turnkey projects awarded to Alstom are Flevocentrale, January 2008, a 870 MW, 2 x KA26-1 single shaft combined cycle power plant in Lelystad, and this Claus C repowering project. This represents adding 5 x GT26 based CCPP to the capacity of the Holland grid, or, in other terms, adding more than 2 GW to the actual 23 GW of installed capacity. Alstom is also supplying part of the scope of other 2 coal-fired steam power plants under construction. The Dutch electricity market has turned back to be buoyant since 2007 when its liberalization attracted many foreign countries players but mainly from Germany and France. In this new environment, and in a very similar way to the rest of West Europe, the need of flexible, efficient generation absorbing the peak load demand in compliancy to the new and more restrictive emission regulation is driving the design and construction of the new thermal power plants. In light of these drivers Essent developed the concept of Claus C repowering project.
3 The Project: Repowering Claus B (STPP) into Claus C (CCPP)
Claus B is a conventional steam power plant (STPP) commissioned on 1978 composed by a 640 MW steam turbine (ST) at around 40% efficiency, fed by a gas fired boiler providing 530 kg/s of 245 bar / 535°C supercritical steam. Essent started conceiving the idea of repowering the plant driven by the necessity to comply to much more restrictive emission regulations, to achieve a satisfactory level of efficiency and thus a profitable electricity production, to operate a plant with the necessary flexibility required by today market demands such as daily start/stop, part load operation with fast loading/deloading and frequency response, and, last but not least to transform a STPP into a CCPP at a reduced specific cost (EUR/kW) compared to an equivalent new build power plant.
Figure 2 - Claus C The first idea of Essent was to convert this plant into a combined cycle power plant (CCPP) consisting out of two high efficient gas turbines (GT) with heat recovery steam generator (HRSG) and supplementary firing. Essent started to broadly query the market for technical solutions and feasibility studies back in 2005 and finally Alstom succeeded to provide effective technical consultancy and worked out the most effective solution with the goal to provide the best possible efficiency together with highest flexibility and at the same time maximizing the re-use of existing equipment. After a long concept development phase, Essent and Alstom agreed on a configuration with three gas turbines without supplementary firing, which allows to reach 58.5% plant efficiency at ISO conditions, therefore in the highest class of today CCPP, matching almost perfectly the maximum capacity of the condenser (only 5% difference in the condensing steam flow) and the cooling system. Due to a complete different profile of all steam parameters like pressure/temperature/flow, with exclusion of the condensing steam flow that was fixed around the same value of the existing plant as technical target, a substantial modification of the ST appeared evident. In fact, a conventional STPP presents a steam flow pattern with several extractions (7 in this case!), already starting right after the HP module and feeding different stages of water pre-heaters. This configuration lead to a flow de-loading of the IP and LP module, exactly opposite to a ST fed by an HRSG where the low pressure steam comes both from the IP steam turbine module and directly from the boiler, thus presenting a much higher steam flow downstream of the ST instead of upstream. The ST modification was proposed and agreed with Essent having the target to reach the maximum efficiency. This was achieved via a detailed site survey aiming to check the details of the steam turbine, which was originally manufactured from another manufacturer, including all the interfaces with the water steam cycle (WSC) in view to fit the re-designed ST but saving the most possible existing structure and piping arrangement. The result of this intensive re-engineering effort was to configure the retrofit ST with a new HP and IP module and a new internal body of the LP but saving the outer casing and all the ST bearings.
With the progressing involvement of Alstom into the concept study, a tight collaboration of the two parties appeared clear and of high the importance. This was achieved via a jointly written order specification in which functionality, scope, design codes & standards, performance guarantees, etc. are defined. The second step of this challenging buyer-contractor partnership was to clearly define the division of work/scope, the long list of interfaces and the division of responsibility for the optimal managing of those interfaces. It has been agreed that Essent takes care of all to be re-used systems from Claus B due to the specific owner’s knowledge required. A difficult point was to define how to integrate the completely new Distributed Control System (DCS) with the reused equipment but due to the good understanding and alignment between Essent and Alstom this critical interface has been smoothly handled.
4 Claus C Repowering: Project Scope and Features
Division of Work/Scope Step 1: Division of Work (DOW) The total scope is split into re-used systems and new systems: Alstom is mainly responsible for all the new systems while Essent takes over the work to the remaining systems. Step 2: Define clear interfaces First has been created a list of all identified interfaces such as control, electrical or mechanical connections (more than 4000 Interfaces!), secondly a division of responsibility has been defined reflecting the scope and the easiness to control them. Step 3: Functional responsibility Since the whole plant is operated from a complete new DCS delivered by Alstom, Essent delivers the process function plans for the DCS for the equipment, which remains under Essent responsibility. Alstom will write and implement the software for integrating that scope into the overall plant operating concept. Step 4: Commissioning Alstom and Essent together will do the cold and hot commissioning of such re-used systems. Alstom delivers the control support, where Essent ensures that the functionality and settings are in accordance with the requirements of the system. Figure 3 shows the project activity share:
5 The Steam Cycle
The Alstom GT26 is the core part of the repowering project. Its hot exhaust gas above 600°C is used to produce steam in the coupled HRSG, which is designed as horizontal triple pressure reheat HRSG without duct firing. Figure 4 shows a simplified flow diagram.
The feed water is taken from a large single feed water tank, which is combined with the deaerator. It serves as the buffer volume for the whole water steam cycle and directly feeds the water to the below located combined HP/IP feed water pumps from where it is pumped into the HP and IP drums. The LP feed water is throttled down from the IP water line. In a dedicated economizer loop at the end of the HRSG heat is taken to pre-warm the feed water, that helps to increase the efficiency and lowering the flue gas temperature. The steam produced in the HP drum is further superheated and feeds then the HP steam turbine from where it flows back as cold reheat steam to the HRSG’s. The cold reheat steam is once again heated up to hot reheat steam and is then feeding the IP steam turbine. The IP steam produced in the HRSG is mixed with the hot reheat steam and flows into the IP steam turbine. From the exhaust of the IP steam turbine the steam flows into the three LP steam turbines where it expands to condenser pressure. Four can type vertical condensate pumps return the condensate as feedwater back to the combined deaerator-feed water tank. One pump is in stand-by at base load. Three horizontal HRSG will be delivered. Each of them can be fully double isolated from the rest of the steam circuit to achieve maximum reliability of the full plant. Provisions are made to distribute cold reheat steam equal to all the three HRSG’s while operating with three or two HRSG’s. Taking into account that 4 x 100% HP/IP feed water pumps are delivered the pumps can be staggered in their operating regime which will lead to better utilization of the energy used for pumping. In addition all the 4 feed water pumps are equipped with variable speed hydraulic couplings for smooth operation. Peculiar of GT26 technology, the high pressure cooling air for the hot gas path components of the GT, extracted from the compressor, is re-cooled by two once-trough coolers ), which are part of the water steam cycle. The steam produced in these once trough coolers is HP steam and is routed, together with the HP steam from the drums, to the super heater. During start up and shut down HP steam is bypassed to hot reheat steam which is then directly routed to the condenser.
PLANT: Cycle concept/engineering/arrangement EQUIPMENT: 3 GT26 and Generators 3 HRSG and Auxiliaries 1 Feedwater Tank 4 Feedwater Pumps 1 HP ST 1 IP ST 3 LP ST inner bundles 3 Step up Transformer Plant Air System Complete Electrical System Control Systems and DCS Civil Structures Erection and Commissioning
Demolition works Water Treatment Plant Primary Roads Gas Receiving Station Refurbishment of: MCW System Condenser Vacuum System ST-Generator
Each HRSG is connected independently from the other HRSG to one of the three single condenser elements. The existing exhaust steam header of the removed auxiliary steam turbine (driver of the feed water pumps) is redesigned to accept LP steam from the common LP bypass and distribute the steam uniformly into the three existing condensers. To keep the HRSG’s as hot as possible the stack is equipped with an automatic damper. The whole plant arrangement has been conceived to allow the operation of one or two or three GT without any penalization on the equipment performance.
Plant operating concept: maximum flexibility and part load efficiency The whole water/steam cycle arrangement has been customized in order to achieve the highest possible flexibility of operation, taking account that the plant can be operated from 30% to 100% load staggering the activation of 1 or 2 or 3 GT which are supposed to operate between 60% and 100% of their load. The specific configuration and design has been chosen to guarantee at the same time the highest efficiency through all the plant load span. For example, the excellent part load behaviour of GT26 has been matched with the custom designed characteristic of the retrofitted ST, such that the ST operated at certain part load level (with 2 GT in operation) assures a minimal loss of efficiency that is, in addition, offset by the increased vacuum of the condenser. The result is an outstandingly good efficiency of the plant both at full and part load operation (see graph below) in full compliancy to the emission limits.
Figure 5 - Efficiency vs. GT Load for 1, 2 and 3 Gas Turbines Furthermore: from the beginning it was a clear requirement for Essent that the Claus C should be able to operate in a daily start/stop mode. The plant shall have maximum flexibility such as fast loading/deloading, low emissions over a wide range of operation, high reliability and availability combined with the possibility to maintain one or two GT train while the rest of the plant remains in service. Therefore Alstom and Essent together optimized the plant. Several provisions have been made to maximize the flexibility:
o Stack dampers to seal the hot gas path thus stretching the time window of the fast hot start capability
o Variable speed drive for the feed water pumps for smooth operation during start, stop and continuous very low load operation (LLOC mode)
o HRSG design with small headers to minimize the thermal stress during gradients
o Condenser evacuation system using water ring vacuum pumps to keep the condenser under pressure over night
o Small auxiliary boiler to supply gland seals The implementation of a fast loading capability of the ST via a manually activated steam management logic, gives to the customer the advantage to choose between life time consumption of the steam turbine and the possibility to make more profit when responding faster to grid requirements. Frequency response operating mode of GT26 was also implemented. In addition to the mentioned features, the full plant has been designed according the “Low Load Operating Concept” (LLOC). This is a feature that only Alstom can offer exploiting the dual stage combustion system of the GT26. Running on LLOC means that the GT can be even further deloaded to a point where the SEV combustor is switched off and the EV combustor reaching its minimum thermal load at which it still runs on premix mode thus keeping the NOx and CO limits (approx. 12% GT load). At that level the plant runs at a load <25% within the emission consent. This operating regime will have the advantage of very fast response to grid operator signals and reduced OH consumption compared with start-stop while the GT keeps running on a minimum of fuel consumption and overall emissions production. See below the arrangement of the three GT power island blocks beside the existing ST (in green the LP module)
Figure 7 - GT26 Rotor In the mid 1990s, Alstom introduced two similar sequential combustion gas turbines: the GT24 for the 60-Hz market and the GT26 for the 50-Hz market. Since its first launching in 1995, the advanced class GT24/GT26 gas turbines have demonstrated that this technology platform does offer significant advantages (see Figure 1) that are going to be explained.
all data at ISO conditions, 45 mbar condenser pressure
Plant Net Output MW 424.0 850.3 857.7
Plant Net Efficiency % 58.3 58.5 59.0
Plant Net Heat Rate kJ/kWh 6’172 6’156 6’103
KA26-1Single Shaft
1 on 1
KA26-2Multi Shaft
2 on 1
Gross Gas Turbine ISO Rating
GT26 288.3 MW 38.1 %
KA26-2 ICSTM*
Multi Shaft2 on 1Net Combined Cycle Plant Output
*Integrated Cycle Solution
Figure 8 - GT26 Performance Data
6.1.1 Sequential Combustion The main technology differentiator of Alstom’s GT24/GT26 gas turbines is the sequential combustion principle, which was already introduced in 1948 into the market as a way of increasing efficiency at low turbine inlet temperature levels. The GT24/GT26 combustion system is based on a well-proven Alstom combustion concept using the EV (EV = EnVironmental) burner in an annular combustor followed by the SEV (Sequential EnVironmental) burner in the second, annular combustion stage. Integrating the concept of dry low NOx EV-burner and sequential combustion into a single shaft gas turbine resulted in the
GT24/GT26 – a machine with high power density, thus small footprint, and high exhaust temperature in order to maximize the efficiency in combined cycle. The EV burner uses the vortex breakdown of a strongly swirling core flow to stabilize a premix flame without the need for a swirler body or centre body in the region of the ignitable mixture, ensuring intrinsic safety against auto ignition or flashback events. The EV burner special geometry and premix technique provides a high mixing effectiveness leading to a low emission signature. In the annular SEV combustor, the lean premixed combustion principles are repeated in a similar fashion as in the EV: vortex generation, fuel injection, premixing. The SEV combustor consists of annular distributed burners followed by a single annular combustion zone. Exhaust gases from the high-pressure turbine enter the SEV combustor through the diffuser. The loading / de-loading of the gas turbine is carried out by varying the set points for the HP turbine inlet and LP turbine inlet temperature, as well as the variable inlet guide vanes (VIGV). The loading consists of several phases from the initial ignition of the EV burners, the subsequent ignition of the SEV burners, and then the loading of the GT by opening the VIGV to allow a greater air mass flow through the GT. When the VIGV is fully open, the TIT setting of the SEV combustor is increased further to achieve the base load value.
6.1.2 Operational Flexibility Today, the operational flexibility of a power plant is a key factor, crucial for its long-term commercial success. This ability to adapt to changing operating requirements has already been well demonstrated in numerous GT26 power plants and the features that define this flexibility can be summarised as:
o Superior part load efficiencies o High operational flexibility from “daily start and stop” to “base load” operation o Low emissions over a wide operating range, 30% to 100% load o High fuel flexibility (natural gas composition; oil No. 2) o Very low combined cycle start-up times o Overnight parking of combined cycle with Low Load Operation Concept
More and more plants need to be able to cope with different operating regimes. Variations can be seasonal, such as with base loads during peak seasons and intermediate or higher cycling duty together with part load operation during times of lower demand, or occur over longer time spans as market changes. The GT24/GT26 platform in combined cycle is already providing such flexibility. Moreover the GT26 with its sequential combustion technology offers further enhanced operational flexibility providing a solution for plant operators’ typical dilemma of either having to operate the plant at relatively high partial loads in order to fulfil emission requirements or to shut down the plant. The so called “Low Load Operation Concept” (LLOC) utilises the possibility of shutting down the sequential SEV combustor at low part loads while keeping the EV combustor on nominal conditions and is therefore a unique feature of the GT26 technology. This concept allows the plant to be operated in combined cycle mode at a very low combined cycle load (less than 25%)
with the combustor operating in lean premix mode. This ensures low emission levels as well as a homogenous turbine inlet temperature distribution, whilst at the same time keeping the water-steam cycle up-and-running. The following are the major advantages of the Low Load Operation Concept:
o allows fast re-loading when power is demanded, since there are no start-up related delays as plant preparation, GT run-up, HRSG purging, water/steam cycle warming up etc.
o does not affect the GT maintenance cost and intervals negatively o provides a reasonable plant efficiency and at the same time a low fuel consumption o provides an on-line operation reserve, readily & quickly available on demand. o avoids start-stop cycles and the related cyclic thermal stress to the topping and bottoming
cycle equipment o avoids the potential risk of start-up failures as the plant remains in operation o avoids increased noise and water plume emission, possible during start-ups o emission levels are similar or lower to base load and well within typical permit
requirements o reduces cumulative emissions compared to parking a plant at a higher partial load
assures a homogenous turbine inlet temperature distribution, which is not the case for non-reheat engines due to combustor piloting or staging at part load.
6.1.3 Fuel Flexibility Increasing demands are being placed on today‘s global gas turbine fleet to burn natural gas with higher C2+ (higher-order hydrocarbons with more than one carbon atoms) contents and also with greater, more rapid fluctuations in the C2+ content. As gas supply disruptions are becoming more common and Liquefied Natural Gas (LNG) shipments are ramping up globally, this trend is set to continue. Furthermore operators may encounter sudden changes in fuel composition because a gas supplier is changing from one well to another, a different LNG gas source is being used or tankers are being refuelled, etc. Gas turbines must be able to handle such changes without interruption of power generation. The GT24/GT26 combustion system is able to handle high hydrocarbon gases and high inert contents and in addition can handle high speeds of change of the gas composition. The current field experience covers over 20 engines with high C2+ content in the range 9% to 16% (see Figure 4). The inert contents in the field have been as high as 20%. The operating concept has been implemented into the GT24 and GT26 fleet with the help of a fast on-line gas sensor. This type of gas sensor is especially suited for the fast changes of the gas composition and Alstom’s control concept uses fast response infrared sensors to detect changing C2+ contents in the fuel. Whereas traditional gas chromatographs have response times in the order of 15 minutes, the 20-second response time of infrared sensors allows near real-time re-optimizations of the operating concept for the current fuel composition. In total, the GT24/GT26 fleet experience with high C2+ gas supplies is more than 800,000 fired hours burning high hydrocarbon fuels.
6.1.4 Summary Since day one the GT26 has been designed to meet combined cycle applications. By using the unique sequential combustion efficiencies of up to 59% and beyond are achievable while maintaining superior operational flexibility. As a result Clean Power down to 30% of relative GT load can be delivered and the GT is able to offer attractive start up times to provide a significant spinning reserve that can be rapidly dispatched in response to scheduled and unscheduled variations in demand. The EV burners, which are implemented in the complete Alstom gas turbines range, provide superior fuel flexibility for operating reliable also with changing fuel qualities from different source – the GT26 is today ready for the introduction of Liquefied Natural Gas (LNG).
6.2 Steam Turbine: The Retrofit Design In December 2008 Claus B unit stopped generating and decommissioning commenced. During 2009 the existing steam turbine and it’s auxiliary equipment will be removed to be replaced in 2010 by a retrofit steam turbine supplied by Alstom Power. As part of the repowering project at the Claus Centrale Power Station Alstom Steam Turbine Retrofit group will update the existing non-Alstom 640 MWe rated steam turbine. The retrofit concept is to define a steam turbine and turbine island solutions to match at best the new steam conditions and the layout of the existing Power Plants, therefore Alstom team has re-designed the steam path/blading in order to achieve the maximum possible full and part load output within the operating range. All the optimizations put in place considering the specific steam conditions and operating flexibility requested by Claus C have been achieved without penalizing the maintainability and the lifetime of the components. The retrofit steam turbine will generate 487MWe from the steam supplied by the new 3x Heat Recovery Steam Generators (HRSG).
The existing single flow HP and double flow IP turbines will be removed completely with the exception of the bearings and pedestals which will be reused. The inner casings and rotors of the three double flow LP turbines will be removed and replaced a new design fitting into the existing outer casing. The existing combined control and lube oil system will be replaced by a separate control oil system and a refurbished lube oil system. The remaining systems, instrumentation and controller will be new. Turbine Retrofit scope includes, as components:
o One single flow HP turbine complete module (RTB) o One single flow IP turbine complete module (RTB) o Three double flow LP turbine inner blocks (rotor + inner casing) o Stop and Control Valves with Actuators o Control Oil system o Lube Oil system o Gland Steam Systems o Turbine Drain System o Steam Turbine controller o Set of Steam Turbine Instrumentation o Steam turbine data recorder – life time calculation module
The five cylinder steam turbine retrofit concept features the following:
o All components designed for 565 °C live and hot reheat temperature o Double shell design for all cylinders o Shrink ring design for the HP turbine o Reaction type blading o Welded HP/IP/LP rotors o Integral expansion sleeve couplings
Alstom will be also responsible of commissioning and performance test of the steam turbine and the whole plant. Alstom has gained unrivalled experience in retrofitting several hundreds of steam turbines from a wide variety of EOMs in locations across the world during the last 40 years. Alstom engineering team have a wide scope of expertise and experience in worldwide operations. Eight of these engineers spent a week on site measuring and photographing the existing turbine and auxiliaries. The key factors to be considered were the pedestal locations and the generator, which is being refurbished by the Client. The loading on existing pedestals and bearings has been recalculated for the Alstom equipment. The steam conditions are a result of the gas turbines and heat recovery steam generators and were used to calculate the performance across the wide range of operating conditions. The new HP and IP modules are members of the Alstom range of steam turbine components that will be adapted to the existing bearing pedestal positions of the existing unit. The existing double flow IP turbine will be replaced by a single flow unit.
Figure 10 - Claus C, Overall Thermodynamic Principle The design and engineering responsibility for the retrofit and coordination of engineering interfaces of all the involved Alstom teams is being undertaken by the Turbine Retrofit team. The design is well advanced with concept reviews having been held with the participation of Essent. Eighty percent of all the retrofitted ST necessary equipment will be manufactured by Alstom factories in Poland, France, Germany and Switzerland. The steam turbine controller is a proprietary Alstom system and will be fully integrated into the station control system. It features a dedicated logic to handle both full and very low part load steam flows and it is interfaced with lifetime data recorder tracking the operational data that will be used in the future to analyze the performance and condition of the plant. This is a challenging project with a comprehensive scope, accelerated timescales, and a Client with an overwhelming focus on Quality.
6.3 HRSG: A Customized Steam Generator for Claus C Alstom is providing the engineering, fabrication and construction of the three Heat Recovery Steam Generators (HRSG’s) for the Claus C project. The Claus C HRSG’s have been specifically designed for application behind the GT26 gas turbines incorporating Alstom’s unique OCCTM (Optimized for Cycling and Constructability) features and with a thermal design optimized for the Claus C three-on-one steam cycle.
Figure 11 - Alstom OCCTM HRSG’s for KA-26 Plant Alstom has the benefit of years of expertise in high pressure/temperature steam generation as evidenced by hundreds of successful HRSG installations worldwide. Alstom HRSG design begins with a standard design platform, which is then customized as needed to meet specific customer, site and operational requirements. This approach allows Alstom and its customers to capitalize on standard proven design elements while minimizing engineering time in order to meet plant completion schedules. The Alstom HRSG incorporates that expertise in the form of unique design features that ensure highly reliable, efficient operation under the most demanding cycling duties. The Alstom HRSG incorporates the most rigorous design choices to ensure life-long reliability and low maintenance requirements. Some of these standard features include:
o Single straight row of tubes per header to provide for smaller diameter header to reduce thermal stresses
o Multiple harp-header nozzle connections to promote uniform metal temperatures o No use of division walls inside headers to ensure that headers and parallel tubes do not
experience differential expansion stresses o Enhanced drain arrangements to prevent condensate flooding
In addition to the features described above, the Claus C HRSG design incorporates other standard modularity features that will result in reduced fieldwork at the project site. These features include:
o Two module wide by five module deep pressure part configuration (ten total modules) reducing the number of pressure part modules typical for large HRSG’s for the 50Hz market
o Module casings with full-length panels shipped in shop-integrated “piggyback” configuration with overlapping insulation & liners that utilize external butt-welded field joints
Figure 12 - HRSG Casing Steel with Integrated “Piggyback” Panels
o Columns with shop-drilled holes for platforms to reduce the number of field welded connections
o Shop-installed collection manifolds utilized to connect the harp headers and connecting pipes
o Feedwater, evaporator and steam piping that penetrates the roof casing is shop-installed o Lower harp header connecting manifolds are shop-installed to the extent that permits the
module to be lowered into the structure o Top-supporting steel, roof casings, module-support components and casing-penetration
bellows are shop installed o Large bore piping is shipped in large, shop-fabricated spools to reduce the field weld
count
Figure 13 - Alstom OCCTM Module Delivery and Erection
6.4 Generator for GT26 of Claus C GT26 gas turbine, in a multi-shaft configuration, like Claus C project, is coupled with an air-cooled generator called TOPAIR (see figure below).
Figure 14 - Alstom TOPAIR Generator
TOPAIR is Alstom’s air-cooled Turbogenerator product line that has accumulated more than 40 million successful operating hours and a significant fleet has proved a reliability of up to 99.985% based on data collected by Strategic Power Systems, an independent assessor. Furthermore, the high reliability of the TOPAIR Turbogenerators has been demonstrated in various types of environmental conditions such as high altitude in Columbia, extreme temperatures of Alaska and the deserts or high humidity in tropical areas. TOPAIR can be operated up to 400 MVA (50Hz) providing a simple alternative solution to hydrogen-cooled generators. Therefore, the TOPAIR Turbogenerator product line allows significant cost savings, since there is less civil works, a simpler foundation, a simpler engineering, no hydrogen treatment system or seal oil system to operate or maintain and less piping. To optimize the maintenance and operating costs, it is equipped with the following low maintenance features. Its stator concave/convex wedging system provides a sustained pressure on the stator bars and can compensate for the initial settling and thermal expansion of the bars during operation. This wedging system has demonstrated excellent condition after more than 50,000 operating hours and inspection can be performed with a robot, without rotor removal. In the case that a re-wedging would be necessary, this could be done simply by adjusting the position of the lower wedges. The TOPAIR Turbogenerator is equipped with a stator end winding support system that ensures flexibility in the axial direction, to compensate thermal expansions in cycling mode, while providing a high stiffness in the radial and tangential directions, to withstand high electromagnetic forces in fault cases. The system is designed such that the wedges are pressed together by means of Belleville washers, enabling the system to compensate for settling effects,
which naturally occur in stator end winding. This system retightening can be done during normal inspection in just a few hours. The maintenance free stator core is held under a constant axial pressure by Bellville shaped press plates. These press plates are made from Aluminium that has excellent flux screening capabilities and heat conductivity, providing an extended reactive power range. A special stator core suspension system is used to reduce the transmission of the stator core vibrations to the housing. TOPAIR has demonstrated excellent RAM figures in various types of stringent operating conditions, electrical grid conditions and over millions of operating hours. Its unique low maintenance features and its robustness make it a real contributor to the plant operators’ profitability. Alstom continuously improves its Turbogenerator technology and manufacturing processes by using the experience from the field and increases the unit ratings. As a result, Alstom has delivered the world’s largest air-cooled Turbogenerator in successful operation in Bahrain in 2004.
7 Conclusions
Claus C repowering is an engineering intensive retrofit project that has been made possible by the experience and engineering skills of Essent and Alstom joined together in a very collaborative and effective way. This fruitful team work has allowed Essent to reach their project targets and Alstom to provide the highest possible value as EPC and full chain OEM contractor. Claus C demonstrates how a conventional gas fired steam plant built 3 decades ago can be converted into a modern, efficient, flexible, profitable combined cycle plant, environmentally compliant with the increasingly stringent actual emission regulations.
Figure 15 - 3D Drawing of the Power Plant after Repowering
This huge quantum improvement will be achieved (and guaranteed!!!) without conceding anything to the overall performances compared to a new combined cycle power plant and at the same time reducing the investment compared to a green field project due to the maximized re-use of the existing equipment, and such achieving a very competitive specific cost (EUR/kW). The teamwork between the two parties as allowed to understand the residual life of the saved components and to find the best technical solution to integrate the new with the existing equipment. In Claus C project Alstom has found the best way to act as plant integrator: all the equipment components (excluding the GT and its generator) have been customized both in their design and in their configuration; further, the overall plant itself comes into a configuration (3xGT26 + 1xST) never installed before by Alstom. This deep tailor made approach together with the excellent performance of the single components and the optimized water/steam cycle around them allows to reach a level of performance in full and part load that can be considered top-class for a CCPP and even unrivalled being the result of a retrofitted plant. Essent from its side will benefit from a state of the art combined cycle power plant with a reduced investment and minimized permitting efforts and time.
