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Oct 26, 2012

Technology in place to meet future steam turbine demands – Alstom

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Alstom’s steam turbine technology. Source: Alstom
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Steam turbines are the largest contributor to future power generation as the technology is expected to drive 40% of the global power market’s demand in the medium term, which will average between 230 GW/y and 270 GW/y for the next five years, says French energy equipment vendor Alstom.

Currently, 40% of the global installed base is ultra-supercritical (USC). Europe is close to 100% USC, with 50% of the global market expected to be USC in the next couple of years, foresees Alstom Thermal Power senior VP of steam Andreas Lusch.

“Alstom’s steam turbine technology for the future is ready,” adds Alstom Thermal Power steam turbines VP Martin Boller.

He explains that three critical areas exist: materials, casting and welding processes (with Alstom’s test casting and welding completed) and steam turbine and plant design, highlighting that Alstom has designed a 550 MW turbine train of 700 ˚C using double reheat technology, with design studies still ongoing.

“With all this in place, Alstom is ready to take steam turbine technology for 700 ˚C double reheat (720 ˚C /720 ˚C) into the future,” states Boller.

The rate of improvement of steam turbine technology is accelerating, says Alstom Thermal Power chief technology officer Charles Soothill, who adds that by combining the 700 ˚C double reheat and a 50% efficiency, a 40% reduction in specific carbon dioxide (CO2) emissions can be achieved. “This is a huge change in the environmental impact of power generation.”

However, the big question for power generators’ planning departments is whether the 50% efficiency barrier for steam turbine technology can be broken, notes Danish engineer specialising in the fields of steam turbines and steam cycles for power plants Sven Kjaer.

He says if power generators are able to get closer to the Carnot Cycle, a hypothetical heat engine cycle, efficiency can be improved. “The use of double reheat technology instead of single reheat technology will improve net efficiency by 2% to 2.5% and heat rate by 4% to 5%.”

“Cycle calculations show that a seawater-cooled 900 MW plant using double reheat technology can achieve a net efficiency of 49.3% and 50% with boiler flue gas coolers,” he adds.

Besides the numerous double reheat plants already in operation across the US, there are also two 400 MW USC plants in Denmark, namely in Skaerbaek and North Jutland, with a net efficiency on natural gas of 49% and a net efficiency on coal of 47%.

The 290 bar plants have a main steam temperature of 580 ˚C and reheat steam temperatures of 580 ˚C each, says Kjaer, who is a self-employed consultant targeting design and development of USC power plants and the modified double reheat cycle called Master Cycle. It is based on experiences at Eddystone, a coal-fired power station owned and operated by energy provider Exelon in Pennsylvania, US.

He says the main objections to the revival of this proven double reheat technology will not come from steam turbine manufacturers, but boiler manufacturers as their products are designed to suit single reheat plants. Kjaer hopes this is a problem that Alstom, a €20-billion company, can overcome.

Meanwhile, he also expects more compact designs in future to save building and steam line costs, and reduce pressure losses.

In terms of materials used in the manufacture of steam turbines, Kjaer says metallurgists worldwide are working on creating a new steel with 9% or 12% chrome that allows a maximum steam temperature of 650 ˚C over an operating life of 200 000 hours (about 30 years).

Those focusing on 9%-chrome steel have succeeded in achieving a higher creep strength but not in preventing oxidation. However, metallurgists focused on 12%-chrome steel have failed because the Z-phase formation eats up the steel’s other stabilising neutrons.

Therefore, Technical University of Denmark professor John Hald proposed a 12%-chrome steel in 2010 which might solve this problem and have a high corrosion resistance, even at 650 ˚C.

“The idea is to accelerate the formation of the Z-phase at the beginning of the steel’s life so that the formation cannot grow. The first two attempts have failed but tests are ongoing,” adds Kjaer.

Alstom is confident that it can supply the highest efficiency and most reliable product, says Alstom Thermal Power’s steam business VP of product/platform management Robert Gleitz.

“Alstom’s product strategy for power generation involves three levers, namely reducing the cost of electricity, lowering the environmental footprint of electricity generation and increasing the flexibility and reliability of operations,” he points out.

Fifty-five per cent of the global installed base is related to steam turbines, with 20% of the world’s installed steam turbines having been delivered by Alstom. More than 6 000 machines comprising various outputs have been installed globally, with outputs totalling 75 GW in North America, 328 GW in Europe, 66 GW in China, 13 GW in Latin America, 72 GW in the Middle East and Africa, and 21 GW in Asia.

“What sets Alstom’s steam turbines apart from other products is that the rotor is welded together and then machined to offer low stress level and a low susceptibility to stress corrosion cracking, making it safe and reliable,” says Soothill.

Further, shrink rings allow flexible operation owing to their light weight and the elimination of mass concentration, while also offering easy maintenance as no casing distortion takes place. A freestanding last-stage blade also ensures that the frequency field is safe and reliable. The longer the blade, the more efficient it is and the greater the power produced, he adds.

“Alstom has continued to develop these blades, with the latest technology developed being a 49 inch titanium blade used in USC steam turbines.”

He says Alstom is working towards creating balance between efficiency and flexibility of steam turbines.

Thin casings are needed to avoid thermal stress and low pressure is required to easily manage a steam turbine, but efficiency requires high pressure and thick casings. “Alstom is optimising these two factors to ensure flexibility in the operation of high efficiency turbines,” adds Soothill.

Further, steam turbines are constantly evolving to become more efficient and cost effective, as well as being designed to suit the use of new fuels, he says, adding that steam can be generated by various fuels, such as nuclear, biomass and coal, as long as what arrives in the turbine is steam. Therefore, one technology is able to serve a range of fuels.

Alstom has 55 years’ experience in supplying nuclear power stations with steam turbines, with 113 GW in operation and 26 GW under engineering, says Alstom Thermal Power nuclear senior VP Patrick Fragman.

“Around 30% of our technology is present in global nuclear power plants, meaning that out of the 475 nuclear power plants in operation worldwide, there are 136 Alstom steam turbines powering those units.”

Between 2003 and 2012, 94 units were ordered for nuclear power plants, of which 29 were Alstom units. There are currently 28 Arabelle steam turbines for nuclear applications in use or under construction worldwide, with Alstom planning to deliver 26 GW of new nuclear capacity by 2020.

Two types of steam turbines are used for nuclear applications: full-speed with two-pole generators and half-speed with four-pole generators. Half-speed dominates the market above 900 MW, with full-speed steam turbines dominating 700 MW and less.

Meanwhile, Gleitz says Alstom is ready for the future generation mix, with its customers experiencing a mindset change and integrating renewables into their portfolios. “In 2012, 50% of new installations (in value) will pertain to renewables.”

He notes that the demand for steam turbines suited to renewable power projects has increased over the past two decades, with impressive renewable growth expected in future.

Soothill adds that Alstom is also working to improve the flexibility of its steam turbines as it is becoming more and more important to integrate renewables into existing projects and the company is designing to meet this future demand.

“Substantial growth in energy production is expected regardless of the scenario. Increasing the renewable share is one of the key drivers for CO2 abatement,” says Gleitz, adding that, globally, there is a growing share of CO2-free technology.

• Hancock was Alstrom’s guest at its ‘A century of steam turbines’ media event in France.

Edited by: Martin Zhuwakinyu
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