Gas turbines and green bananas: 60 per cent efficiency

Siemens’ latest gas turbine is set to offer efficiency in combined-cycle operation of more than 60 per cent. PEi visits the test site of the SGT5-8000H, E.ON Kraftwerke’s Irsching power plant near Vohburg, Bavaria, to check on the progress of the world’s largest and most powerful gas turbine.

Tim Probert, Associate Editor

Not far from River Danube-bisected Vohburg, 70 km north of Munich, a quiet revolution is happening. The small Bavarian city à‚— the smallest in Germany to be considered as such à‚— is the home to E.ON Kraftwerke’s Irsching power plant, which serves the Munich-Nuremburg grid connection.

Irsching is an unremarkable power station, the original two of its three gas and fuel oil fired units have been put into cold reserve, and the 440à‚ MW Unit III is a peaking capacity unit commissioned in the 1980s that is fired up just two or three times a week. Unit IV, however, is radically different and ‘uber’-modern.

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Unit IV is the site of Siemens’ prototype SGT5-8000H gas turbine, which is the largest and most powerful gas turbine yet produced. Ordinarily, Siemens would conduct the testing of a new gas turbine at its test bed in Berlin, but the SGT5-8000H is no ordinary gas turbine.

Notwithstanding the dimensions of the SGT5-8000H à‚— 13.2 m from flange to flange by 5 m wide, as well as weighing 440 tonnes à‚— its 340 MW in open cycle operation (more than 20 per cent bigger than the SGT5-4000F) makes it too powerful to be accommodated by the Berlin site, which was designed to test turbines up to a capacity of 220à‚ MW.

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Moreover, the SGT5-8000H is not generating wastewater but electricity for the grid, albeit classified as ‘unreliable’, and had produced approximately 200 000 kWh at the time of writing. The revenue this brings in, however, is but a drop in the ocean compared to the €50 million ($79 million) being spent by Siemens on natural gas during the testing of the new turbine.

Finding the right location

An expansion of the test bed was briefly considered but, having watched other gas turbine manufacturers learn the lessons of introducing a large engine too quickly to their enormous financial cost, other options were sought. Moreover, the limited gas supply to the test centre, which would have posed additional problems for the testing of the gargantuan SGT5-8000H, hastened Siemens’ decision to build a unique test centre elsewhere.

The Irsching site, 50 m from the Danube, has a ready supply of cooling water and is close to an E.ON Ruhrgas pipeline, while a 400 kV transmission line from Munich connects to Irsching via an existing 380 kV switchyard. Infrastructure, which, according to Willibald Fischer, project manager of the 8000H, is “perfect”.

Eventually, Siemens approached E.ON Kraftwerke, a subsidiary of the giant German utility E.ON, which it knew was wanting to erect a combined-cycle gas turbine power plant in southern Germany to provide peak load back-up capacity to its wind farm projects located in or near the North Sea. So, a combined-cycle unit is what E.ON will get. From 2011, when Siemens hands over the commercial operation of Irsching IV, E.ON Kraftwerke will be the proud owner of a combined-cycle power plant with a total output of about 530 MW and a peak load efficiency of over 60à‚ per cent. This means converting the 340 MW open cycle SGT-8000H turbine, or ‘green banana’ as Fischer puts it, to a fully ripened, 530 MW combined-cycle unit.

Executing such a plan is easier said than done. It also means working backwards, namely that the operating parameters for combined-cycle operation have had to be tested in open cycle operation.

Opting for air-cooled technology

The SGT8000H is the result of an intensive R&D programme to produce a competitive, efficient and flexible engine. It is also the first new frame to be developed since the merger of Siemens and the former fossil fuel business of Westinghouse.

After a comprehensive feasibility analysis during the conceptual design phase, a fully air-cooled concept was chosen from four thermodynamic cycle designs and 20 variants of air-cooled engines. Fischer said that increased flexibility was a major driver behind the choice of air-cooled technology, and, as there are no ties to the steam cycle required, it makes an excellent choice for repowering.

The engine includes a single tie-bolt rotor with compressor and turbine disks; hydraulic clearance optimization (whereby the gaps between turbine blades and the casing are expanded and reduced by shifting the rotor towards the front end); an advanced sealing system for low leakage of cooling air; and advanced materials to increase the firing and exhaust temperature.

The engine is also optimized for a high pressure à‚— 17 000 kPa à‚— and a high-temperature combined-cycle process with a Benson boiler.

Testing testing

The SGT5-8000H (50 Hz) prototype was built in Berlin in 2007 and was shipped to Irsching on 30 April last year. The first firing of the new unit was successfully completed on 20 December.

Field validation commenced on 28 January 2008, and the first synchronization with the grid took place on 7 March. The next milestone was reached on 20 March, when 60 per cent load firing was established, and on 10 April, the unit reached ‘baseload’, or 90à‚—95 per cent capacity.

The H-class turbine is located in a compact, structural steel building that also houses the generator and steam turbine. The main gas turbine auxiliaries are arranged on a steel platform, while the auxiliary components for the water/steam cycle and the closed cooling water system are located in an annex to the turbine hall.

By 8 April, the unit had completed 44 operating hours over 14 days and 14 starts. Fischer estimates that by the time the project reaches ‘Gate IV’ (scheduled for August 2008), when the SGT5-8000H series is ready for commercial release and can be marketed, the prototype unit will have completed 300-400 hours. By this time, all commercial data will be finalized, which will confirm emissions data, start-up data, stable operation performance at normal conditions and so forth.

Between August 2008 and June 2009, the SGT5-8000H will undergo endurance testing and ‘semi-commercial’ operation in grid conditions under the auspices of E.ON Kraftwerke. The aim is to demonstrate that the turbine’s hot gas parts achieve their designed lifetime, as well as to demonstrate that the designated maintenance schedules are correct.

By June 2009, when the 18-month testing and validation process is completed, the turbine will have been in operation for approximately 2000 hours.

The testing process itself is being executed in different phases. Phases 1 and 2 include hot commissioning of the gas turbine and auxiliaries; start-up testing and optimization; full-speed no-load compressor tests; and synchronization, loading to baseload and mechanical verification.

Test phase 3a comprises baseload testing, inlet guide vane/variable guide vane optimization; first combustor testing, and performance testing/growth testing. Phase 3b includes performance testing and more extensive combustor testing.

All in all there are more than 2830 test sensors on the SGT5-8000H, including 1688 temperature sensors, 616 for pressures, 357 strain gauges, 48 for clearance and 56 for blade vibration. The sensors, which are wired up to the front and back end and rotate with the engine when in operation, send telemetric signals to the rotating receiver.

Manufacturing of the SGT5-8000H at Siemens’ plant in Berlin, Germany
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The data, much like a Formula One racing car sending information to the pits, are sent back to the 20 test engineers and 15 design engineers on-site at Irsching. The data are also sent worldwide to three other Siemens’ test centres: Mulheim, Germany; Orlando, Florida, USA and Jupiter, Florida, USA.

So far, Fischer has been very pleased with the testing progress overall. “All in all, the first phases of the validation programme were longer compared to a commissioning schedule for a ‘normal’ unit,” he said.

“The duration from first firing to first synchronization has taken a bit longer. However, this was expected and accounted for in the schedule. Overall, results are currently very satisfying. Ignition behaviour is extremely good,” said Fischer.

Converting to combined cycle operation

Once the 18-month testing process is completed in June 2009, completion of development of the SGT6-8000H gas engine will follow after the 50 Hz version. At Irsching, Unit IV’s SGT5-8000H will be extended to a combined-cycle power plant. Phase 2 will see the SGT-8000H modified from a fully instrumented to a commercial engine in combined-cycle operation: the SGCC-8000H.

In order to revert to what Fischer calls ‘proven plant concepts’, the new turbine is based on the SCC5-4000F single shaft design. This also means that the SCC5-8000H combined-cycle unit will be equipped with existing technologies and components such as the SGEen5-3000W water-cooled generator, an SST5-5000 steam turbine and a Benson boiler.

The SGT5-8000H is being tested day and night
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One of the targets of the development of the SGT-8000H/SCC-8000H was a net efficiency of more than 60 per cent. This target will be achieved through an aforementioned improvement of the gas turbine itself compared with Siemens’ current F-class series.

The SGT5-8000H has a 600 à‚°C bottoming cycle, driven by exhaust gas at 625 à‚°C. By comparison, conventional combined cycle plants have bottoming cycles at around 565 à‚°C; the 35 à‚°C increase improves efficiency by 0.2à‚—0.3 per cent.

The turbine exhaust at full load is 625 à‚°C, which is taken through the diffuser into two heat recovery steam generators, which are expected to be ordered in this month. The Benson drumless, once-through boiler has fast start-up capabilities (45 minutes tested, 40 minutes untested, believed optimal).

The SGT5-8000H turbine with generator at the Irsching site
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The pressure ratio of 19 is achieved by a 13-stage compressor, which has been tested at the Berlin works as a scaled 60 Hz version on an F-class machine. The compressor had a correspondingly scaled mass flow and was operated on different regimes.

As well as a movable inlet guide vane, three additional movable vanes are provided for the first three stages. This provides better control over airflow at part load and start up, and eventually better turndown and control over the combustion process.

Siemens says that the H-class turbine could easily be an intermediate load machine, despite its size and design as a combined-cycle unit. Indeed, one of the chief aims of the testing process is to see whether the SGT5-8000H can demonstrate high turndown efficiency and could therefore be operated profitably at low loads without the need for shutdowns.

The German company is also highlighting the environmental benefits of its flagship gas turbine. The two percentage points increase in operational efficiency in the SCC5-8000H will result in a reduction of carbon dioxide emissions of 40 000 tonnes per year compared with current combined-cycle standards, equivalent to 9500 medium-sized cars driving 20 000à‚ km a year.

Siemens is pouring a great deal of time, money, blood, toil, tears and sweat into the new H-Class. By the time E.ON takes over the running of the combined-cycle SCC5-8000H turbine in mid-2011, they will know whether it is has proved a success, both on a technical and commerical basis.

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