As well as gas repowering old oil fired units, Senoko Power has also upgraded two 425 MWe combined cycle plants as it prepares for privatisation.

With the transition to full “retail contestability” nearly complete and the three government owned power generating companies, PowerSeraya, Senoko and Tuas, being groomed for sale to the private sector in the near future (perhaps next year), players in the Singapore electricity market are facing ever tougher competitive pressures.

An added challenge is acute overcapacity, coupled with relatively modest annual load growth of a little over 4%/y. Singapore’s projected installed capacity in 2007 is 11 GWe – compared with an average demand in FY 2004/5 of around 4.2GWe, with a minimum of 2.9 GW and a maximum of 5.2 GWe.

Despite this, Keppel Energy continues to press ahead with its 470 MWe Merlimau combined cycle cogen plant, also on Jurong Island, due to enter service in 2007. Keppel, along with SembCorp (developer/owner/operator of Singapore’s first IPP and biggest cogen facility, the 815 MWe SembCogen CCGT plant on Jurong Island), is among potential buyers of the state owned generation assets when they are eventually put up for sale.

For some time Intergen (formerly a joint venture of Shell and Bechtel, but now owned by AIG Highstar Capital and an Ontario teachers’ pension fund) has been planning yet another CCGT on Jurong, the much delayed 720 MWe Island Power project (in which Malaysia’s Sime Darby Berhad once had a 50% stake and for which a gas sales agreement was signed with Conoco in 2003). But it remains to be seen whether the new owners of Intergen will want to proceed with this, and if so, on what schedule.

Meanwhile, the three main incumbent power generating companies, all now divested to the Singapore government’s investment arm, Temasek Holdings, in preparation for privatisation, have not been idle. Tuas Power is in the process of adding two more MHI 360 MWe CCGT blocks to its Tuas plant, doubling the installed capacity there, PowerSeraya is converting three of its 250 MWe oil fired units to orimulsion (under a contract signed with IHI), while Senoko Power has undertaken a major repowering project and transformed three of its 70s vintage 120 MWe oil fired plants (Senoko Stage I) into 1080 MWe of new natural gas fired CCGT capacity, employing three Alstom GT26 turbines (see MPS, August 2004). The full repowering was completed in December 2004.

Also in 2004, Senoko Power awarded a contract to Siemens for the upgrade and life extension of the existing 850 MWe V94.2-based combined cycle plant at the Senoko site. This initially consisted of four V94.2 gas turbines (now designated SGT5-2000E) running in open cycle mode, which started up in 1990/91. These were converted to two 425 MWe combined cycle blocks in the mid 90s with the addition of four HRSGs (one per gas turbine) serving two added steam turbine-generators (one steam turbine for each pair of GT/HRSGs).

The prospect of an increasingly aggressive Singapore power market has been a major driver behind this life extension and upgrade project says Roy Adair, CEO of Senoko Power, as well as the need to reduce emissions and to achieve a better return on investment than would be the case with a new build project.

He is, interestingly, a fan of the “blue ocean strategy” put forward by W Chan Kim and Renee Mauborgne, which advocates the creation of “uncontested market space” and making the “competition irrelevant.” While that may be a goal for longer term the present realities of the Singapore power market, require that one is “always looking for that extra competitive edge,” eg through increased efficiency and flexibility. With competition becoming increasingly intense, “we are having to run faster just to stand still,” he says.

But when the mid life point of the turbines was reached it was, says Roy Adair, a “straightforward investment decision to extend life at high performance levels for at least another ten years.” In particular it was considered a good opportunity to install new low NOx burners and to increase thermal efficiency by recoating the compressor section and tightening the clearance in the compressor and turbine sections.

And looking ahead, the life-extended units will have a “critical future role to play” says Roy Adair. But the over-capacity in the Singapore market will place considerable demands on the plant, with “exposure to a very different load pattern” from the baseload role that the two blocks have very ably performed since 1996, requiring much more flexibility in operation.

The life extension and upgrade contract for the two 425 MWe Senoko blocks was awarded on a turnkey basis to Siemens. The scope includes upgrade of the gas turbine instrumentation & control and electrical systems as well as the major overhauls of the turbines themselves, each of which have recorded 100 000 EOH.

Overhaul of the fourth and final gas turbine is scheduled for November 2005. The other three turbines were upgraded in November 2004, February 2005 and July 2005, with outage times of 31 days, 28 days and 25 days (planned) respectively, for mechanical work.This is impressive, particularly when the volume of work is taken into account (not so long ago outages of about 40 days would have been considered necessary for such upgrades). A key factor in shortening these outage times has been the existence of well motivated and well trained field maintenance group located in South East Asia.

The upgrade/life extension project encompassed a range of measures, including: the change from the Teleperm ME to the Teleperm XP I&C system, with ergonomic flat screens;

exchange of worn components in accordance with a lifetime assessment and condition based maintenance strategy; and retrofit of new static excitation equipment and start-up frequency converter units to improve the start-up reliability of the gas turbines. But perhaps the two most important modifications in terms of securing continued productive life for the assets are the burner and compressor efficiency upgrades.

Burner upgrade

Before the upgrade the NOx emissions from the V94.2 machines, with their original diffusion combustion system – reflecting the state-of-the-art at the time of commissioning – were around 2030 mg/Nm3 (ISO conditions, corrected to 12% CO2), which corresponds to about 400 ppm. There were also problems with temperature distribution caused by wear and tear.

Implementation of the dry low NOx upgrade package at Senoko has reduced NOx emissions to 194 mg/Nm3 or less (35-40 ppm), a 90% reduction. This takes it well below the existing requirements in Singapore, but also gives “room for further tightening of the limits in the future”, says Markus Spatz of Siemens operating plant services, with responsibility for the Far East and Pacific.

The upgrade has involved replacing the existing burners with dry low NOx remix burners of the HR3 type, combustion chamber dome refurb and upgrade, installation of more maintenance friendly flame tubes with tiles, tile holders, and a new flame tube bottom plate designed for premix operation, which also permits reduced cooling air consumption. New piping, valves and auxiliaries are also needed for the new pilot gas system and for premix operation.

The HR3 burner achieves better mixing of gas and air by injection of fuel through bores in diagonal swirler blades. This results in reduction or even elimination of temperature peaks in the combustion zone and reduces NOx. The diagonal swirlers also provide increased outlet velocity, reducing proneness to flashbacks.

Upgrade of the flame tube is always necessary as part of an HR3 retrofit. This is because the increased flow velocity due to the new diagonal swirlers results in a flame cone with a larger angle, which in turn shifts the hot recirculation zone within the silo combustion chamber from the centre towards the upper region of the chamber. Temperature distributions calculated using computational fluid dynamics also show that, with the new burner, higher temperatures are shifted to the upper region of the chamber. The new flame tube is designed for this altered temperature distribution.

The HR3 burner is by no means new technology, but the Senoko upgrade uses the latest version, resistant to the wet corrosion that has been encountered in ferritic gas premix “spiders.” These are the spider-shaped pipework assemblies that connect the diagonal swirler of each burner to the pear shaped central gas distributor.

Originally employing ferritic steel even though it is located in the combustion chamber, this component is prone to wet corrosion on its outer surface because of the presence of condensing water on the cold fuel gas pipes. The ferritic gas spider has therefore been subject to maintenance during hot-gas-path inspection outages, and, depending on remaining wall thickness, replacement of individual pipes has sometimes been necessary.

Markus Spatz recalls that “In one case this maintenance work was neglected during the hot-gas-path inspection and caused internal fire damage in the area above the flame tube bottom plates.”

The gas premix spider has now been redesigned using wet-corrosion-resistant material and is said to be maintenance free over an interval of 123 000 EOH.

This spider upgrade is part of the “41MAC” (41 000 EOH MAintenance Concept) upgrade package, which Siemens offers and, as the name suggests, extends the normal service interval to 41 000 hours, with the spider only requiring attention every three services.

Senoko is in fact currently on the 33MAC regime, with a 33 000 hour service interval.

Compressor recoating

Among the major successes of the Senoko life extension project to date have been the increased efficiencies recorded by the first two gas turbines to be upgraded, 0.22 percentage points and 0.38 percentage points, respectively. This corresponds to fuel consumption savings of 0.71% for the first gas turbine and a remarkable 1.2% for the second, which is of no small significance for a business in which fuel accounts for around 70% of total costs.

Compressor efficiency improvement has been a major contributor to these figures.

As part of the Senoko upgrade project, the compressors have been completely recoated with a duplex aluminium-ceramic coating, in two layers. This has been done in 16 days, within the scope of the turnkey outages.

The new coating reduces the surface roughness of vanes and blades, with the aluminium coating protecting the base material against wet corrosion and the ceramic layer providing protection against erosion due to dust and dirt particles. It is estimated that the investment in the advanced compressor coating will be paid back within less than one year.

More power to Senoko

Overall, as a result of the package of measures taken, the power output of the first unit to be upgraded has been increased by 2.8 MW and that of the second unit by 2.3 MW.

There is a range of further measures that can be undertaken to rejuvenate and uprate ageing SGT5-2000E gas turbines, including raising the turbine inlet temperature, increased compressor mass flow, wet compression, water injection and, only recently introduced, the option of retrofitting high efficiency (“Si3D”) blades (developed using 3D CFD). It remains to be seen whether Senoko opts for any of these in the future, but for now the SGT5-2000E machines appear well equipped to cope with the rigours of Singapore’s developing power generation market reliably and competitively.