The new gas-fired Mellach plant is Austria’s most powerful thermal power plant

Siemens has high hopes that the flagship CCPP it has built for Austria’s Verbund will be the first of many state-of-the-art cogeneration plants that are economically efficient and a strong match for renewables.

In December 2008, the European Union agreed ambitious objectives for 2020. The new combined-cycle power plant (CCPP) at Mellach in Austria’s Styria, to the south of regional capital Graz, is a vital building block in the country’s plans to meet these targets. The plant was built by Siemens as a turnkey project, and was officially handed over to Austrian power provider VERBUND Thermal Power (VTP) on 22 June 2012, having started operating in May 2012. With an electrical generating capacity of 838 MW and an efficiency of 59.2% in straight power generation, the new plant surpasses the contractually agreed performance values and represents the state-of-the-art at international level. A district heating output of 400 MWth, supplied to Graz, means about 81% of the energy in the fuel is put to effective use. This makes the new plant in Mellach not only the most powerful, but also the most efficient thermal power plant in Austria. Compared with the latest coal-fired power plants, which offer efficiency levels of about 46%, the Mellach CCPP brings significant advantages in CO2 emissions.

The Mellach combined-cycle plant runs on natural gas supplied via a pipeline. The two new units (numbered 10 and 20) are designed as single-shaft configurations in which the major components – gas turbine, generator and steam turbine – are on the same shaft. In addition to generating 5 TWh of electric power per year, the plant can produce about 800 GWh of district heating. Siemens supplied two model SGT5-4000F gas turbines, two SST5-5000 steam turbines, two SGen5-2000H series generators, all the electrical equipment, and the SPPA-T3000 instrumentation and control (I&C) system. The company has also been awarded a maintenance contract. The two transformers were supplied by Siemens Transformers Austria’s manufacturing plant at Weiz, also in Styria, which was responsible for planning, designing, fabricating and testing the transformers, as well as their final assembly, erection and commissioning.

The Weiz manufacturing plant is the Siemens Transformer Group’s largest facility anywhere in the world. To comply with regional water pollution control regulations, the new units feature two different cooling systems: unit 10 is cooled by fresh water from the nearby River Mur, while unit 20 has a forced-draft wet-cell cooling tower. The plant’s power transmission system is also distinctive, with one line connected to the local 110 kV network, and one to the regional high-voltage (400 kV) grid. Otherwise, the two lines are identical.

When it was founded in 2001, VTP had an installed generating capacity of about 1900 MW, most from coal-fired plants. But the power plant fleet was largely ageing, leading to the shutdown of the St Andrä, Zeltweg, Pernegg, Werndorf 1 and Voitsberg plants in southern Austria – as well as Korneuburg in the north – cutting power capacity by about 850 MW in 2000–06 in the south of the country. At the same time, demand for electricity in Austria was rising by up to 3% per year. As a result, Vienna Technical University forecast a national power shortfall of 20–40 Twh by 2020. In response, VTP began planning work as early as 2003 on a new thermal power plant in the 800 MW output class with an eye to addressing this shortfall and ensuring compliance with future climate protection rules.

The key objectives in designing the new plant were:

  • Use of the best available technology;
  • High energy conversion through high operating efficiency;
  • High CO2 efficiency to meet climate protection targets;
  • Use of available potential for heat and power cogeneration;
  • Low specific emissions in accordance with the state-of-the-art technology;
  • High plant operating flexibility.

A comparison of future developments of plants over 2000–20 reveals that combined-cycle technology offers clear benefits over coal in parameters such as efficiency, CO2 emissions and fuel consumption. In 2000, efficiency of combined-cycle power plants around the world averaged about 54%, compared with just 25% for coal-fired plants. Current CCPP units offer efficiency levels of over 59%, such as Mellach (59.2%), or even higher, such as Unit 4 at Irsching in Bavaria, which hit 60.75% in May 2011.

New designs have also enabled CO2 emissions to be reduced from 365 to 330 g/kWh. Experts predict that energy conversion efficiency levels will rise to around 63% by 2020, resulting in even further reduction in CO2 emissions to 313 g/kWh. While the development of coal-fired power plants is also set to continue, with today’s efficiency levels of 45% forecast to hit 50% by 2020, the much higher carbon content of coal means emissions from coal-fired plants will remain significantly higher (1116 g/kWh in 2000, falling to 669 g/kWh in 2020).

The bottom line is that combined-cycle power plants firing natural gas offer the best energy and CO2 efficiency levels. Carbon dioxide emissions from the latest CCPPs are about 55% lower than from state-of-the-art coal-fired plants. Since CO2 separation is not yet available on an industrial scale, but is only being trialled in small-scale demonstration plants, VTP opted for combined-cycle technology. The company’s existing sites in southern Austria were investigated to assess their suitability for erecting an 800 MW class CCPP, based on the following criteria:

  • Fuel supply (natural gas),
  • Power transmission,
  • Cooling water supply (river water),
  • Potential for district heating,
  • Synergies among sites in terms of power plant infrastructure and personnel.

Preliminary investigations identified Mellach as the most suitable site for constructing a combined-cycle power plant. And the plant’s performance over the first few months of operation justifies this decision. Comparative analyses demonstrate that the environment has clearly benefitted from replacing VTP’s previous generating capacity in southern Austria with the new units at Mellach: through the use of natural gas and the high efficiency level of the CCPP, CO2 emissions have been reduced by about 60%, NOx emissions are down by about 90%, and dust emissions have been cut by 90%, saving up to 2 million tonnes of CO2 emissions every year. The plant is also involved in the emissions trading scheme for CO2 certificates, so climate protection goals for Austria and for the Styria region are unaffected. The use of the Mellach plant to supply district heating has also improved the overall environmental and climate protection balance of the Graz metropolitan area.

The plants previously in operation at the Mellach/Werndorf site played a key role in supplying district heating to Graz, providing about 80% of the 1100 GWh needed by the city every winter. The new plant also had to have a connection to the district heating system, whereby steam is extracted from the exhaust end of the intermediate-pressure section of the steam turbine, and from a tap on the low-pressure section, and conveyed to the heating condenser. Each line can deliver a maximum capacity of 200 MW of heat to the heating network.

At the heart of the new units at Mellach stand two model SGT5-4000F gas turbines, with a gross capacity of 292 MW and low heat consumption. Optimized flow and cooling conditions enable gross efficiency of 39.8%, ensuring economical power generation. The key design features of this gas turbine are:

  • Accessible annular combustion chamber with 24 hybrid burners,
  • Ceramic heat shields in combustion chamber,
  • 15-stage axial compressor with optimized flow distribution (CDA blading),
  • Single-crystal turbine blades with thermal barrier coating,
  • State-of-the-art film cooling technology,
  • Hydraulic clearance optimization, and
  • Low-NOx combustion system.

According to the Siemens assembly and erection concept, gas turbines of this size are completely pre-assembled at Siemens’ Berlin Manufacturing Plant before shipment to the power plant site. But the clearance of bridges on Austria’s autobahns prevented this approach with Mellach. The gas turbines were therefore split into different sections (upper casing, lower casing and rotor) for shipment, and assembled on-site. Delivery of the generators and main transformers (weighing about 380 tonnes each) also presented a logistics challenge.

The plant’s generators and steam turbines are also designed to meet the most stringent requirements. Generator capacity (540 MVA) has been improved through a hydrogen cooling system. The steam turbines used at Mellach are model SST5-5000 units with a capacity of 140 MW, designed with a combined high-pressure and intermediate-pressure section and a double-flow low-pressure section. The discharge cross-section of the fresh water-cooled steam turbine unit is 12.5 metres, while that of the unit with cooling tower is 8 metres.

The cogeneration concept applied at Mellach is designed to satisfy growing energy demand while meeting climate protection goals. This is achieved by utilizing waste heat from the power generation process. Thanks to this approach, heat and power cogeneration facilities can achieve significantly higher levels of fuel utilization (over 80% for the Mellach power plant).

The countries of Eastern Europe and Scandinavia, in particular, have recognized the economic and ecological benefits of cogeneration. In Denmark, for example, cogeneration plants already account for 52% of the country’s total installed generating capacity. Finland and Latvia, at almost 40%, and the Netherlands, with around 30%, also make extensive use of cogeneration. Austria, at about 14%, and Germany (12%) are some way behind. The same variable picture is reflected across the European Union, with different regulatory frameworks, and no common approach to this key pillar of the energy supply of the future.

The European Parliament declared heat and power cogeneration a top priority in Directive 2004/8/EC, and the Heat and Power Cogeneration Act passed by the German government in 2009 called for cogeneration plants to supply 25% of Germany’s electricity by 2020 – double their current contribution. The overdue revision of the cogeneration law was approved by the German Federal Government on 24 May 2012. The new legislation strengthens support for CHP and seeks to give a major boost to its use in large power plants by providing significant incentives for investment.

Siemens has many years of experience constructing CHP plants in Europe, having built over 50 for process and district heating applications since 1990. Key references include:

  • The Mainz-Wiesbaden power plant, which achieved an efficiency level of 58% – a world record for a cogeneration plant – in 2002, and which has cut CO2 emissions to just 345 g/kWh. A maximum heat output of 250 MWth means more than 80% of the energy in the fuel is put to effective use.
  • Repowering of Simmering power station in Vienna, which involved upgrading an existing facility. Simmering, Austria’s largest power plant, can operate on an extremely flexible basis. A maximum heat output of 450 MWth ensures over 81% of energy in the fuel is put to effective use.
  • Mellach, which is optimally adapted to local needs and can supply 400 MWth of district heating, so that more than 80% of the energy in the fuel is put to effective use.

In late May 2012, Siemens Energy and the utility company Stadtwerke Düsseldorf paved the way for a further major step into the future with a contract to construct Lausward Power Station. The plant, to be built in Düsseldorf’s Rhine port area, will be the world’s most modern heat-and-power cogeneration facility. A flagship project for future power generation in Germany and beyond, the plant will employ Siemens H-class combined-cycle gas and steam turbine technology. An electrical unit output of about 595 MW and a net efficiency of over 61% will set a new world record for both parameters, with 300 MWth of district heat boosting overall utilization of the natural gas fuel to about 85%.

The project represents – for the time being – the crowning moment in Siemens’ long history of success, which has seen more than 200 cogeneration plants based on Siemens components enter service around the world. The Mellach plant and the future facility in Düsseldorf demonstrate that modern, combined-cycle heat-and-power cogeneration plants can make a major contribution to the world’s energy transformation, are economically efficient, and ideal for supplementing volatile power generation from renewable energy sources such as sun and wind.

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