Refurbishment & Repowering: Senoko moves on

The second phase of the Senoko repowering project is now underway in Singapore. The conversion of two more units to combined cycle will result in efficiency, environmental and economic gains for owners Senoko Power.

Siàƒ¢n Green

In January 2002, Alstom announced that it had been awarded the contract for the second phase of the Senoko Power Station Stage 1 repowering project. By 2005, the repowered plant will have started operation, supplying power into Singapore’s newly deregulated electricity market.

Civil work is coming to an end on site as delivery of the first main equipment approaches
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The Phase 2 contract follows on from the contract for Phase 1 of the repowering project, which was awarded to Alstom (then ABB) in 1999. Phase 1 involved the repowering of one unit of Senoko Stage 1 with a gas turbine and heat recovery steam generator (HRSG), increasing the output of the unit from 120 MW to 360 MW. This unit entered operation in February 2002, becoming one of Singapore’s most advanced power generation plant.

The repowering project ” the first ever undertaken in Singapore and in the Asia region ” involves the conversion of the conventional oil-fired thermal units of Senoko Stage 1 into combined cycle units. As in Phase 1, each of the two remaining units will be repowered with a gas turbine and HRSG, while the existing steam turbine will be refurbished. When the last unit enters commercial operation in March 2005, the total output of Senoko Stage 1 will have increased from 360 MWe to just under 1100 MWe.

The Senoko power station is owned and operated by Senoko Power Ltd., a subsidiary of Singapore’s Temasek Holdings (Pte) Ltd. Senoko Power is one of the largest generating companies in Singapore, meeting about 30 per cent of the country’s electricity demand. The company was created in 2001 when the government divested it out of its former parent company ” Singapore Power Ltd ” in preparation for the sale of generating assets in Singapore to further liberalise the electricity market

Old and new

Prior to the repowering, the Senoko power plant consisted of a total of 1610 MWe of steam plant and 850 MWe of natural gas fired combined cycle plant ” a combination of some of the oldest and the newest generating plant in the country. The three oil fired generating units which comprised the Senoko Stage 1 plant were commissioned in 1975. In the late 1990s, Senoko Power realised that they were reaching the end of their physical and economic life, and decided to transform them into three advanced combined cycle units.

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The decision to undertake a repowering of the existing units, rather than decommission them and build a new combined cycle plant on a greenfield site, was based on project economics and projections of future electricity demand in Singapore. In the late 1990s, Singapore’s economy had been affected by the Asian financial crisis. Senoko Power was confident, however, that the economy would recover. “This investment is a clear demonstration of our commitment to ensuring the continuous supply of efficient, reliable and clean power in Singapore both now and in the future,” the company said in a statement.

One of the main challenges during the repowering project has been the available space on site
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It continued: “The bottom line is cost savings for our customers. Using the existing site and retaining the civil infrastructure, construction costs will be lowered by ten per cent compared to a new construction on a greenfield site. Furthermore, higher plant efficiency and operational productivity will eventually lead to a lower cost of electricity generated.”

The environmental benefits of a natural gas fired combined cycle plant also played a part in the company’s decision. The completed repowered plant will have an efficiency, based on higher heating value, of some 50 per cent, which is 14 percentage points of 36 per cent higher than the existing plant.

Alstom embarked on the first phase of the repowering project in late 1999. The conversion of this first unit increased the output of the plant by 240 MWe with a total project cost of $200 million. A phased construction programme was implemented to minimize interruption of the operation of units 2 & 3 of Senoko Stage 1. This meant that the gas turbine and HRSG had to be erected alongside the existing operational boiler, and that there were only 15 months in which to refurbish the existing steam turbine and modify existing buildings.

Before the first repowered unit entered operation, and before Alstom started work on Phase 2 of the repowering, the company had a short window in which to refurbish the entire cooling water system for all three Stage 1 units. Alstom then started work on Phase 2 of the repowering in early 2002.

Phase 2 underway

As the output of the newly repowered unit was equal to that of the three Stage 1 oil fired units, the two units being repowered under Phase 2 were immediately taken out of service. Under its Phase 2 contract, Alstom is to supply two gas turbines, two HRSGs and a water treatment plant. It will also refurbish the existing steam turbine, and provide maintenance services for a six-year period. As in Phase 1, Alstom’s advanced GT26 gas turbines will be used.

But before any equipment delivery and installation could take place for Phase 2, Alstom had to undertake eight months of site preparation work. This involved demolition of the 180 m-high stack at the site as well as the three original oil fired boilers. The challenge here was that space for demolition was extremely limited and the unit repowered under Phase 1 was in the ‘danger area’. According to the project management team at Alstom, stack demolition therefore had to be done very carefully using concrete cutting equipment.

The stack was demolished by using a concrete circular saw installed on a steel platform on top of the stack. This saw cut off slices, which were then cut up and passed down the inside of the stack. The accumulated material at the base of the stack was removed during night shift when cutting was stopped. Stack demolition took a total of five months.

Stack demolition started in January 2002; when this was complete, and the other existing structures had been removed, civil works were able to start in August 2002.

Mechanical & electrical erection started in January 2003, and the lack of space on site continues to be a challenge. For example, the erection of the HRSGs can only take place after the erection of the GT generator and the main components of the gas turbines. However, a lack of space is a common challenge in a retrofit project, because you start with an existing plant and have to fit the new kit in where the old kit was.

Advanced equipment

For Phase 2 of the repowering, Alstom is replacing each of the two existing oil fired boilers with a GT26B gas turbine and HRSG. The HRSGs are of natural circulation design and are supplied by CMI. Live steam parameters for the HRSGs are: mass flow: 91.6 kg/s; pressure: 130.2 bar; temperature: 563à‚°C.

The GT26 is a 50 Hz machine optimized for combined cycle applications and designed for a high degree of operational flexibility. The first GT26 unit to enter operation was at Alstom’s Birr test facility in 1996. The machine uses a sequential combustion system with two annular combustors: the EV combustor and the sequential EV (SEV) combustor. There is a single-stage high pressure (HP) turbine between the two combustors, while downstream of the SEV combustor are four low pressure (LP) turbine stages.

Air discharged from the 22-stage compressor, which achieves a pressure ratio of 30:1, before entering the EV burners. Here fuel is mixed with the high pressure air and ignited, producing the hot gas to drive the HP turbine. The gas is reheated in the SEV combustor, into which fuel is injected and ignites spontaneously, and undergoes its final expansion through the four stages of the LP turbine. The result is high power density and low NOx, with high exhaust temperatures, up to 640à‚°C, over a wide part load range.

The Senoko repowering project is scheduled to be completed by early 2005
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Improvements implemented to address the GT24/GT26 introductory technical issues (identified during commercial introduction of the second generation, B type, in 1999/2000) have now enabled highly flexible and reliable operation of the fleet at performance levels commensurate with industry norms. Operation has seen positive feedback from a recent GT24/GT26 user’s group meeting.

Many are now reporting that availability, reliability, startup, shutdown and system load-following characteristics of the GT24/GT26 units are greatly improved. This is confirmed by third-party statistics showing GT26 reliability better than 97 per cent last calendar year. Future modifications aimed at delivering further enhancements to output, efficiency and operational flexibility are already designed and tested. These include:

  • High efficiency compressor ” successful demonstration of 5 per cent electrical output improvement at Alstom’s Birr GT26 full scale test facility. Components are now being delivered to the first GT24 customer field validation site.
  • High fogging inlet system ” successful demonstration of better than 6 per cent electrical output in both the test facility and field validation units. The system can be applied to both existing and new gas turbine installations
  • Dual fuel capability ” successful demonstration in both the test facility and field validation units. The system is now available for commercial application on both existing and new gas turbine installations.

As of February 19, 2003, the GT24/GT26 fleet achieved another milestone surpassing the 500 000 operating hours (fired hours).

Current status of Senoko

Senoko Phase 1 has now accumulated over 6000 operating hours. The flexibility of the plant allows the client to operate at the most favourable load levels and to date this has been mostly base load operation with occasional part load periods and weekend shutdowns. During the last months, the plant has also been achieving 100 per cent availability.

For Phase 2, the main focus on-site at present is the civil works. The demolition of the redundant equipment is complete and the new civil works are well progressed. The first gas turbine unit will arrive on-site in May 2003, and the second one month after that. HRSG erection will start in the middle of May 2003. The first unit will start commercial operation in September 2004 and the second in March 2005, marking the completion of Singapore’s first repowering project.

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