French power engineering company Alstom is progressing well with a €1 billion joint repowering and new construction project in the southeast of the Netherlands. The 1280 MW plant combines an upgrade to the existing 640 MW Claus B unit with a new Claus C facility at Maasbracht for the Dutch utility, Essent. The concept is believed to be unique in Europe, and is also a milestone for Alstom, being its first installation to use a 3-on-1 configuration for increased efficiency with the GT26 gas turbine. PEi spoke to the parties involved in the project.

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Four years after negotiations began between Alstom and Dutch power utility Essent, construction is under way at the south-eastern town of Maasbracht on what will be a 1280 MW combined-cycle plant noted for bringing together client and contractor in an unusual partnership. The concept is believed to be unique in Europe, comprising as it does the repowering of an old steam turbine allied to three new gas turbines. It also represents a ‘first of a kind’ for Alstom, being its first installation to use a 3-on-1 configuration for increased efficiency with the GT26 gas turbine.

It was last year that the two companies signed a series of contracts worth up to €1 billion ($1.3 billion) for the repowering of Claus B and the construction of Claus C; together they will result in a highly efficient (58 per cent) combined-cycle plant based on Alstom’s acclaimed GT26 technology. Included in these contracts are long-term operations support and comprehensive maintenance agreements.

The Claus C contract fits neatly into the broader picture for the development of the power industry in the Netherlands. The government has set its sights on the incorporation of large-scale gas fired units, in addition to wind projects, as a means to control carbon emissions in line with its legal requirements.

For Essent, the Claus C investment arises from the company’s strategic objectives of providing reliable, affordable and sustainable energy. The Claus C power plant will provide an additional highly flexible plant in the Netherlands. With its environmental and economic advantages, this plant is another example of Alstom’s capability to deliver cleaner integrated power solutions.

An Out of the Ordinary Project

Awarded to Alstom under a design and build contract, the new Claus C plant is situated next to an existing power plant, Claus B, whose 640 MW steam turbine is to be retrofitted and integrated into the new combined-cycle plant. All of the main components, including three GT26 gas turbines and TOPAIR turbogenerators, heat recovery steam generators (HRSGs), distributed control system (DCS) and the relevant auxiliary equipment are being provided by Alstom.

Conceptually, the project is unusual. The Claus C project represents an opportunity to use the existing site facilities, i.e. parts of the existing Claus B, to develop an efficient and reliable unit with lower cost-to-build per unit of capacity than a comparable new build unit. Cost savings arise from replacing the old gas fired boiler of Claus B with three new gas turbines, while doubling the original plant’s output.

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As a result of this facility to reuse parts of the existing plant, it was decided that the best way to deliver the project would be to split the responsibilities between the client, Essent with its intimate knowledge of Claus B, and Alstom, who would be solely responsible for the new works. In effect, then, there would be a ‘brownfield’ site handled by Essent, and a ‘greenfield’ site delivered by Alstom. In between would be a host of key interfaces between the two, which would be engineered by Essent.

The complexity and essential nature of the works necessary to marry the brownfield and greenfield sites together makes success of the Essent/Alstom partnership a priority; without one, there would not be the other. Essent says the relationship is essentially symbiotic. A key facilitator of achieving an acceptable interface management solution was a complete in-depth review of both companies’ engineering practices at an early stage.

In respect of the operational considerations at the plant, moreover, the repowering configuration will increase the efficiency of the plant from its present 39 per cent to more than 58 per cent. Overall, Alstom claims, the project demonstrates the company’s capability to integrate state-of-the art technology into existing assets and to substantially increase plant performance.

Carbon emissions will be reduced by 40 per cent as a result of the unit’s much greater efficiency. The plant will have a design life of 25 years. There is the provision however, after a period of 15 years, to further modernize the plant, extending its life for a further 25 years of operation.

The Claus C project was the second combined-cycle power plant to be awarded to Alstom in the Netherlands last year. In January 2008, the company won an order from Electrabel Nederland, another Dutch utility, to build an 870 MW turnkey combined-cycle power plant – Flevocentrale – in Lelystad, located in the central region of the country.

There are some similarities, in that both plants use Alstom’s GT26 gas turbines. Flevocentrale, however, involves the installation of two single-shaft arrangements (gas turbine, generator and steam turbine on the same shaft).

Alstom is a key player in the Dutch power generation market; its equipment generates approximately 60 per cent of the country’s electricity. Worldwide, there are now some 85 Alstom GT24 or GT26 gas turbines in commercial operation and the fleet has accumulated over three million firing hours.

Background to the Project

Designed in 1974 as a natural gas fired plant with heavy oil back-up and 41 per cent net efficiency, Claus B was commissioned four years later. The plant has been running principally on natural gas, supplemented at times by heavy oil and, in recent years, by an amount of biofuel.

A simplified flow diagram of the Claus C combined-cycle gas turbine power plant, which has a unique 3-on-1 configuration.
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Palm oil residues were used initially, but with concerns growing over the negative effects on land use in Southeast Asia, where the majority of this type of fuel originates, Essent decided not to renew supply contracts. Essent’s decision also coincided with the introduction of Dutch subsidies for renewable energy production, which also began to mitigate against palm oil use.

As long ago as 1995, EPZ, Essent’s predecessor, first raised the idea of utilizing the existing steam cycle at Claus B in a new facility to be built adjacent to it.

The original concept of two gas turbines was subsequently modified in 2003-2004 when the scheme was resurrected to include three gas turbines. This is the arrangement developed by Alstom.

The new ‘dash for gas’ may not be prevalent in Europe yet, but as Ward Gommeren, Alstom’s vice president for global power sales points out, the Claus C project is something of a departure from the norm, being part-new build, part retrofit in nature.

As a result, Gommeren says, the project has been a complex one from the very start. “As a plant integrator specialist, Alstom is providing Essent with a highly flexible plant in line with the stringent environmental regulations that are currently in place in the Netherlands.”

Alstom received a full notice to proceed (NTP) from Essent in December of last year.

“Market consultation started back in 2005. Essent carried out an in-depth market consultation for this project based on the technical complexity and contract format,” says Gommeren. “Essentially, it developed from an EPCM [engineering, procurement, construction management] form of contract to a full EPC [engineering, procurement, and construction] turnkey approach.”

Alstom was clearly focusing on the EPC approach from the very beginning, says Gommeren. “We started with lengthy technical discussions on the basic concept of the plant layout, coming from two gas turbines and HRSGs with supplementary firing and a bypass stack to the final solution of three gas turbines and a standard HRSG, along with repowering of the existing steam turbine.”

Effective Interface Management

As indicated earlier, the site is essentially split into two – a brownfield element containing the existing plant, and a greenfield area where the new Claus C facility is being built. The two merge, necessitating some 2000 plant interfaces. Essent is responsible for the majority of the works on the existing plant, while Alstom has responsibility for new build – a split of approximately 20/80 per cent of the total project.

“The main interfaces are within the brownfield,” says Gommeren. “That’s why we [Essent and Alstom] came jointly to the conclusion that the best workable solution to undertake the condenser retrofit, generator refurbishment, cooling water system and denim plant upgrades should be within the scope of Essent. We [Alstom] would do virtually everything else.”

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Management of these many interfaces was identified as being a key issue from the outset, says Gommeren. “Much effort was put in at the early stages in defining an acceptable interface management protocol. We had carried out an in-depth review of both companies’ engineering practices; Essent had its own engineering standard, based on experiences from conventional thermal plants, while and we [Alstom] had our Plant Integrator best practice approach for large combined-cycle power plants.”

A core management team was established to define a single common standard specification, which would enable the partners to reach an accord on the basic and detailed engineering of the plant. Aside from that, it was decided that Essent would be fully responsible for the de-coupling of the existing Claus A and Claus B units, Gommeren adds.

Essent’s project manager, Bart Beljaars continues: “The success of the Claus C depends as much on Essent as it does on Alstom,” he tells PEi. “Of course, Alstom has by far the most dominant role but there are some critical smaller items outside Alstom’s scope that are necessary to allow Claus C to function properly. These are contracted by Essent and include the dismantling of CCB, St-Geno, the 150 KV system, etc. Other items include the condenser retubing, main cooling water system works, the gas receiving station, 380 kV grid connection, the demin water supply, and the auxiliary boiler.” All of the interfaces between the Essent and Alstom works are managed by the former.

“Essent developed the integral approach in order to successfully manage all these interfaces between the contractors,” Beljaars continues. “For this role, Essent has 50 full-time staff on the site. It’s Essent that oversees the integration of all the contracted scopes. Since July 2008, Essent has expended some 100,000 man-hours in the execution phase on site preparation, dismantling CCB, separating the CCB from CCA and project management.”

Overall Design Concept

Flexibility and fuel efficiency are the main drivers behind the overall design of the power plant. Initially, Claus B will continue to operate as a peak load power plant. The new plant, Claus C, designed around a multi-shaft arrangement utilizing three power trains (gas turbine and generator) in a single row is designed to be competitive in a market place where volatile power rates are the order of the day.

Each of the largely standard GT26 gas turbines, rated at 288 MW, is directly coupled to a 50 Hz TOPAIR air-cooled turbogenerator. And the exhaust lines from each are fitted with Alstom HRSGs, featuring three-stage pressure reheat, designed specifically for cycling operations such as those envisaged at Claus C. There is no provision for supplementary firing. The steam produced by these HRSGs will be transferred by a common header to a 640 MW steam turbine.

The plant is designed for daily start/stop operation. Built into the specification is the requirement that the loss of a single gas turbine HRSG train during normal operation should not result in an instant loss of power of more than 435 MW. Similarly, the loss of the common steam turbine system under comparable conditions should not result in the instant loss of more than 500 MW. The condenser is fit for adequate steam bypass operation.

Claus C has been designed for a net efficiency of 58.5 per cent at full load under normal operational conditions with three gas turbines running, falling to only 58.3 per cent with two gas turbines. Any reduced request for power will be met by reducing the load of the two turbines in operation to 548 MW, corresponding to 42 per cent of the rated plant output.

Under such circumstances, the efficiency remains high, says Alstom, and still in the range of 56 per cent, guaranteeing a proportionately high operating flexibility. Night stops and hot restarts to full load can be accommodated in just one hour, so that power production can be minimized during off-peak periods.

Steam Turbine Retrofit

Although wishing to take advantage of the economic benefits of using equipment from the old steam cycle, Alstom has been keen to ensure that the re-use of the equipment from the old steam cycle does not compromise efficiency. To this end, substantial changes to the steam turbine unit have been necessary to accommodate disparate steam conditions provided by the HRSGs and compared to the old supercritical boiler.

While the low pressure (LP) section remains largely unmodified, save for reconditioning work, new units were required for the high pressure (HP) and intermediate pressure (IP) sections. The steam flow to the condenser of Claus C (320 kg/s) is very similar to that for Claus B (310 kg/s), and is therefore acceptable to the new plant. On the high pressure side, however, the three HRSGs produce only 260 kg/s, compared to that from the gas fired boiler of precisely twice that amount, resulting in a sub-optimal HP steam pressure of 120 bar. This, coupled with the fact that the HRSGs produce steam at a much higher temperature, render the old and relatively inefficient boiler redundant.

The Claus C CCGT plant project is located at Massbracht in the southeast of the Netherlands.
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Claus B’s steam turbine will remain connected to the 150 kV grid, while the three gas turbines will be independently connected to a 380 kV facility.

The proposal is to repower the standard set-up of Claus B (steam boiler and steam turbine) into a natural gas fired combined-cycle power plant via the:

  • Installation of three Alstom-GT26 gas turbines (F-class, > 250 MW) which is considered to be proven technology
  • Installation of connections, by underground cabling, for the gas turbines to the 380 kV grid
  • Installation of three HRSGs, with no supplementary firing. The ability to directly implement a DeNOx module and a CO catalyst in the future is also part of the design of the HRSG
  • Retrofit of the Claus B steam turbine, which includes a new steam turbine controller, new HP and IP modules, a new LP steam path (inner casing and rotor) and the re-use of the LP outer casing
  • Condenser re-tubing to prevent copper particles depositing on the steam turbine (design procedure Alstom) – re-tubing in titanium to prevent fouling (reduced heat transfer rate) and pitting
  • Renewal of Claus B generator rotor and exciter
  • Complete revision of the remaining Claus B re-use scope (generator stator, step-up transformer, cooling water systems, civil structures)
  • Installation of a new overall DCS; all instruments of the re-use (i.e. the brownfield) to be renewed.

Bart Beljaars, Essent’s project manager for the scheme, explains a key consideration in the retrofitting of the steam turbine is that the old Claus B design was based on a steam temperature of 535 °C.

“With Claus C we are upgrading the system to 565 °C; what this means is that a completely new steam path is required. The extra investment in the steam turbine is easily outweighed by accomplishing a significant increase in efficiency. Only the three outer casings of the low pressure modules are re-used, as these are directly coupled to the re-used condenser. The through-flow, in terms of kg/s of steam, is the same as in the original Claus B design.”

Project Philosophy

Alstom’s project director, Thomas Gross, says having a strong project team is one of the key parameters for being successful in such a complex project. The nature of the brownfield interface requires specific Claus B knowledge from Essent to be integrated in the team.

For example, specific owner’s knowledge is also crucial in preventing a ‘Claus A-trip’; Claus A is in operation during the implementation phase and in direct connection with the Claus B brownfield area. It is anticipated that up to 60 per cent of the time will be required to properly manage this part of the project scope.

In order to properly manage all interfaces between the different lots, the project team is designed according to what Gross calls an ‘integral approach’, in which one team deals directly with all suppliers, being responsible for the complete Claus C scope; there are no ‘double functions’ or ‘double checking’. Management of the project is aimed at avoiding sub-groups within the team. Instead, a uniform approach considers specifications and procedures for the complete Claus C scope, preventing unnecessary increased complexity on the interfaces.

Alstom also brings to the table its ‘Plant Integrator’ concept, which, says the company, maximizes performance, while its range of environmental products and solutions enable it to supply cleaner power than ever before. Plant Integrator approach is designed to harness the company’s expertize at the component and plant level, as Alstom is not only an equipment designer but also a manufacturer and an EPC contractor. The result, says Alstom, is shorter lead-times to produce electricity faster, and higher plant efficiency for increased power output.

The GT26 Gas Turbine

Alstom says it developed the GT26 (and the GT24) in response to the changes arising from electricity deregulation and tougher competition, shifting consumption trends and more stringent emissions legislation.

Customers also need to ensure reliability of supply yet reduce the cost per kWh of producing electricity, meaning that raising efficiency remains a constant challenge. Then there’s the issue of rising to the challenge of addressing flexibility to fuel gas composition needs in times of global transport of fuel gas.

The Claus C CCGT plant will feed steam to the steam turbine in the exisitng Claus B unit.
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The GT26 is Alstom’s latest solution to lowering kWh costs without compromising availability by raising the efficiency close to 60 per cent in combined-cycle operation.

The GT26 has low environmental emissions achieved using sequential combustion technology burning the fuel in two dry low NOx combustors. It achieves this with low firing temperatures and with burners that are robust enough to cope with the wide fuel gas compositions that are seen in the market today. In addition, while efficiency levels and power density increase, operational flexibility of the GT26 is significantly improved.

Designed to operate in new or existing power plants, or as part of a turnkey project, the GT26 is capable of operation in baseload, intermediate or peak duty. Alstom claims to be the only manufacturer in the world to have chosen the sequential combustion path, having installed its first unit in Switzerland in 1948.

With the advent of metallurgically superior hot gas materials and advanced blade cooling technology in the 1970s, the technology has advanced to where it is today and, specifically, the GT26.

The development of the compressor in both the GT26 is the result of an evolutionary process with a gradual increase in the pressure ratio to over 30 bar. Both units employ controlled diffusion airfoil (CDA) blading, where each compressor stage is individually optimized according to specific requirements and boundary layer conditions.

This, says Alstom, leads to higher overall compressor efficiency, while retaining a high surge margin. In addition, three rows of variable guide vanes are used to optimize the operation concept at every load.

Another feature of the GT26 is Alstom’s EV (EnVironmental) burner technology, which has operated for millions of hours throughout the company’s gas turbine fleet, giving long burner life, no maintenance between hot gas path inspections and low emissions. The EV burner gives the benefit of dry low NOx combustion for operation with different natural gases, with the option to run with liquid fuel as an alternative.

The burner is shaped like two half cones slightly offset laterally to form two inlet slots of constant width running the component’s full length. Combustion air enters the cone through these slots and fuel is injected through a series of fine holes in their edges. With this arrangement, fuel and air spiral into a vortex form and are intensively mixed.

Two fully annular combustion chambers distribute the circumferential temperature evenly, while avoiding problem zones such as cross-firing tubes or transition pieces. In addition the annular combustion systems of Alstom do not need a so-called ‘Combustor Inspection’ as with can-annular systems, which reduces the amount of maintenance leading to higher availability.

The sequential combustion concept results in a gas turbine exhibiting high power density, and therefore affording smaller blade dimensions. The five rows of turbine blades are anchored in fir tree slots. Air from the compressor cools the high pressure turbine stage and the first three low pressure turbine stages utilizing a combination of film and convection cooling techniques.

Cooling air for the hot gas path components is taken from four extraction points along the compressor. Air from two of these secondary air flows is used directly, while the two other streams are cooled by heat exchangers (once through coolers) before entering the hot gas path components. The heat rejected is recovered in the water-steam cycle, which maximizes the performance of the GT26 in combined-cycle applications. In simple cycle applications, the cooling is achieved by quenching water, which is introduced directly into the secondary air stream.

Principle of Sequential Combustion

Sequential combustion breaks the link between higher efficiency and higher inlet temperature. In sequential combustion, the process is characterized by splitting the combustion process into two stages, separated by an expansion to an intermediate pressure level. In this so-called ‘reheat’ process, energy is added part way through the expansion process, resulting in high gas turbine efficiency and high power density.

The sequential combustion principle, applied to the large, heavy duty GT26 gas turbine model distinguishes it from conventional machines. In effect, sequential combustion can be visualized as a gas turbine comprising two combustor-turbines in series, where the exhaust gases from the first turbine feed the combustor of the second.

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An efficient 22-stage subsonic compressor feeds combustion air into the first combustor. Once there fuel is mixed with the high-pressure air and burnt in the first combustor – the annular EV combustor. The hot gases drive the first turbine, the single-stage high-pressure turbine.

Unlike conventional turbines, fuel is now injected into a second burner set and ignites spontaneously in the following annular combustion zone, the SEV (Sequential EV) combustor, thereby reheating the air and expanding it further through four low pressure turbine stages.

The GT26 is fitted with 24 retractable EV burners, each operating over the entire load range. Compared to other combustor arrangements, the annular combustor distributes the hot gas, circumferentially, at a much more uniform temperature.

Radial temperature uniformity is accomplished by pre-mixing virtually all incoming compressor air with the fuel in the EV burner, and by the absence of film cooling in the convection-cooled combustor walls. This produces a single, uniform flame ring in the free space of the EV combustion zone. Alstom points out that a key benefit of this is that the flame has no contact with the walls of the burner.

In the annular SEV combustor, the combustion process is repeated as in the EV: vortex generation, fuel injection, pre-mixing and flame stabilization in a vortex. The SEV combustor consists of 24 burners distributed in a ring, followed by an annular combustion zone surrounded by convection-cooled walls. Exhaust gas from the high pressure turbine enters the SEV combustor through the diffuser area.

Combustion temperature uniformity in the SEV, as in the EV, is achieved by the uniform premixing of fuel with air in a vertical flow with high mixing intensity. Each SEV burner contains four delta-shaped vortex generators on each burner wall in order to form four pairs of vortices in the combustion air.

The GT26 gas turbine lowers kWh costs without compromising availability by raising the efficiency close to 60 per cent in combined-cycle operation.
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Fuel is then injected through an air-cooled fuel lance, distributing with four jets into the vortex pairs in such a way that it forms a perfect fuel/air mixture prior to combustion. The fuel jet is surrounded by cool carrier air that delays spontaneous ignition until the fuel/air mixture has reached the combustion zone.

Once there, the mixture ignites spontaneously and, as in the EV, combustion occurs in a single, stable flame ring, operating across its entire load range. Neither the EV nor SEV combustor contains any moving parts, so no so-called ‘combustor inspection’ is needed for either the EV or SEV combustor.

This mechanical simplicity determines the high reliability and availability of the GT26 design.

DCS Enables High Level Automation

The Distributed Control System (DCS) is a state-of-the-art control system for power plant automation. The DCS performs all of the functions for the operation of the combined-cycle power plant necessary to optimize efficiency, while ensuring safety, as Beljaars explains.

“We agreed with Alstom to upgrade the DCS following the ‘VGB’ and ‘Blockleit concept’. We minimize the amount of local control boxes in the field and at the same time we are able to optimize the routing of signals into the DCS. Moreover, Essent was keen to have a major input into the set-up of the DCS overall; this is typical of Essent’s involvement in projects of this type, and fits in with out general O&M philosophy. We are focused in ensuring that the system conforms in principle – and operationally – to those installed at our other plants. That’s where the Blockleit Betrieb and VGB comes from; it’s an Essent initiative. I believe that the DCS is technically one of the project’s biggest challenges.”

“There is a high level of automation enabling start-up and shutdown with just two employees in the control room and one in the field. During stable operation only one employee is needed in the control room and one outside. This high-level automation philosophy is typical for Essent.”

The main features of the DCS are:

  • An integrated system for analogue and binary control, and protection of the combined-cycle process
  • Interfacing of the gas turbine and steam turbine control system
  • Interfacing of the package control systems
  • The facility to maintain operation within safe limits
  • State-of-the-art, ergonomic human machine interface (HMI).

The layout of the DCS is based on an open architecture and is defined by the requirement for safe and reliable operation of the power plant at all times.

The power plant process, in addition to the operational concept of the plant, determines the control system functions as well as the degree of automation performed by the control system.

The control system configuration is based on Alstom’s standard concepts for combined-cycle plants, developed after analysis of the process requirements and an optimum outlay of the DCS. The high degree of modularity increases the flexibility of the design, testing, erection and commissioning, and facilitates the expansion of the control and monitoring system as and when necessary.

The hardware configuration of the DCS consists of the following main components:

  • The processing units with CPU and I/O modules, providing control of the power plant process, are connected to the field instruments and motor control centres (MCC) through control cables
  • The redundant control network, which represents the plant-wide data highway, interconnected to the processing units and the operator stations, historical data storage and retrieval system, and the engineering station
  • An operator station, which performs system-wide monitoring and control of the combined-cycle process
  • An emergency panel, which is hard-wired directly to the gas turbine and steam turbine control system as back-up for trip releases of the rotating machines in emergency operation situations
  • The HDSR system on which historical data storage occurs and retrieval functions are performed
  • Numerous printer devices for the reporting of system events, plant status information and plant statistical reports
  • The engineering station for programming the processing unit controllers
  • Workstations and peripherals, which are interfaced via the plant network LAN.

Construction Timeline

While negotiations between the two companies began in 2005, the formal timeline for realization of the Claus C project is as follows:

  • March 2007: Alstom and Essent began their co-operation together and jointly developed the order specification as the basis for an agreement
  • June 2008: The agreement was signed between Alstom and Essent
  • August 2008: Site preparation executed by Essent
  • September 2008: Start of building permit engineering and general permit obtained
  • September 2008: Essent began the dismantling of Claus B, including the mechanical, electrical and I&C technical separation from the Claus A plant
  • February 2009: Alstom mobilizes on site and piling works begin
  • April 2009: The 500th pile driven
  • Fourth quarter 2010: Completion of construction, start hot commissioning and backfeed and gas become available
  • First quarter 2011: First firing of gas turbines
  • October 2011: PAC (Provisional Acceptance Certificate).

To date, the project is progressing according to the programme, and the plant is be fully operation by the third quarter of 2011. Unfortunately, all too often the delivery of large scale projects can be characterized by wary, if not adversarial, relations between client and contractor, but Claus C proves the exception.

Through meticulous planning from the very start, the project demonstrates that a very complex project can engender harmony and co-operation. And that is a rare thing in today’s world of power contracting.