Ultra-supercritical (USC) technology is one answer to prolonging the reign of King Coal, the most abundant and widely distributed fossil fuel on Earth. As we look beyond USC, PEi considers what challenges lie ahead for the power plant workhorse, the steam turbine.

Chris Webb, UK

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At a European Cooperation in Science and Technology (COST) meeting in 1994, a group of 40 power generators and manufacturers agreed to investigate the potential of nickel-based alloys as likely candidates for a new, advanced technology, named ‘AD700’. It would become a landmark in the ‘clean coal’ agenda and set the scene for further development of ultra-supercritical (USC) technology, where power plant designers were struggling to punch a way through steam parameters in the range of 300 bar and 600 °C – a barrier that had seemed impenetrable for some time.

Now, though technically feasible, the world has yet to see a commercial demonstration coal fired plant capable of operating at 700 °C and 350 bar. And there is little apparent market appetite for the higher capital costs – though manufacturers are reluctant to put a true figure on them – that such a move is expected to bring.

There is no doubt the European Union’s target of a reduction in greenhouse gas emissions of 20 per cent by 2020 will require an eclectic mix of technologies, including the replacement of older, inefficient power plants with new highly efficient units. While the selection of fuel for a new power plant is increasingly driven by CO2 emissions, this is spurring all the major turbine manufacturers to take USC to the next stage.

Until carbon capture and storage (CCS) becomes commercially available, increasing plant efficiency remains the only route to reducing CO2 emissions of coal fired power plants. Two of the main methods of improving steam power plant efficiency are steam cycle optimization and enhancing the behaviour of the steam turbine itself.

Today, coal fired power plants with USC steam cycle parameters and wet cooling tower can achieve net efficiencies of 45 per cent for hard coal and 43 per cent for brown coal at typical central European inland sites, with direct cooling providing an additional one per cent being possible.

But what of the move to that magic 50 per cent efficiency? Does it represent a quantum leap – a leap of faith, some would say – in USC technology and particularly in steam turbine design?

“Key USC advantages include higher efficiency, which lowers operating costs, reduces fuel consumption and cuts emissions,” says James M .Stagnitti, GE Energy’s manager of steam turbine design engineering. “Additionally, improved plant efficiency can help offset the costs of environmental programmes such as CCS, making such technologies more viable.” 

What is USC? 

Yet even the term USC, though it has been around for many years, remains vague. “GE typically considers USC to mean a combination of temperature over 565 °C and pressure over 241 bar,” Stagnitti continues. “Other definitions include 565 °C/275 bar. In any case, USC conditions push the boundaries of pressure and temperature for the steam turbine.”

If consensus exists in the industry, however, it is that transition to higher levels of USC – in terms of greater temperatures and pressures – represents not a quantum leap, but a natural progression. “It is a natural continuation of our, and possibly other companies’ development path. GE has a long history of advancement in steam turbine USC technology, including, in 1957 the first 1100 °F (593 °C) steam turbine in the USA and the most efficient for its time, 40 per cent. In 2000, came the most efficient USC turbine at 48.7 per cent, with steam parameters of 250 bar/600 °C/610 °C, in Japan.

GE continues to invest in fossil steam turbine platforms ranging from small 100 MW designs to large 1000 MW configurations. Ongoing work includes advancing state-of-the-art designs for turbine systems and components such as rotating and stationary airfoils, rotors, casings, valves and seals. Also, one of the key enablers for USC conditions is development of speciality materials that have enhanced capabilities to sustain elevated temperature and pressures without sacrificing life and reliability.

“Traditionally, cooling critical areas of the turbine has been an important part of high-temperature design. However, cooling is undesirable because of complexity and a negative impact on cycle efficiency,” says Stagnitti. This is where new materials come in. “Better materials will permit thinner sections, lower weight and improved ability to handle thermal transients. This in turn will lead to more efficient and flexible turbines for our customers.” 

Advances in materials 

Like all the other manufacturers, GE faces the challenges of coming up with materials that offer the right combination of performance, supply chain and cost-effectiveness. “A strong material supplier base is critical to provide the large forgings, castings and fabrications required for steam turbines. Advances in machining and joining technologies are required. It is important to create a balanced plant design that keeps the turbine, boiler and other components in tune and technologically in sync.” GE is part of a consortium working on a US Department of Energy initiative developing 1400 °F (760 °C) materials. Other programmes are targeting different temperature levels.”

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A steam turbine for the Neurath power plant in Germany Source: Alstom

Trevor Bailey is general manager of GE’s steam turbine product line. He sees the higher efficiencies offered by USC and the viability of CCS as two parts of a package that will underwrite coal’s future, both from an economic and environmental standpoint. “That’s really what drives this need for higher efficiency,” he says. “To be able to make the overall scheme competitive to produce electricity at a comparable rate in terms of US cents/kWh as other sources of generation.”

Higher plant efficiencies are a goal the world over, not just in Europe or the US. Collaborative programmes elsewhere, including in the Far East, are looking into developing materials that can operate at the much higher temperatures and pressures that USC progression represents.

“The thing about materials development is that it is really about understanding the operating conditions and environment in which the material has to live. Materials have to undergo strength tests, creep characteristic tests, and satisfy a whole array of issues if we are to be able to control a piece of machinery like a steam turbine and satisfy ourselves that they will remain within the limits we require for a 30–35 year operating life.

“So it’s about playing with different material combinations. And you are really using base materials and small percentages – sometimes fractions of a percentage – of other metals that will change the characteristics of the [base] material to achieve the properties we are looking for.”

The purpose of these programmes is to bring the materials progressively to commercial availability. “Once that’s done, the materials issue goes away and it becomes a question of being able to pull that technology together. That’s what the manufacturers are doing to make their products commercially attractive and competitive.”

“In the case of USC, we need to have the boiler and steam turbine technology that can work at those higher temperatures. We need the steam turbine and all its associated pipework and systems that make up the overall boiler/turbine cycle or system to function together at those elevated temperatures. But we’re really some years away from achieving commercial level operations.

“A key impetus will be emissions legislation. I think there is a greater sense of urgency and need in Europe because of the geopolitical structure, in the sense that you have a number of countries, all of which are looking for fuel diversity in their electricity production regime. Coal and lignite are plentiful in a number of countries in Europe and there are issues around gas supply. So I think there is more of a sense of urgency in Europe to find solutions for cleaner coal than maybe in the US.” 

Improvements in power density 

To this end, improvements in the power density of steam turbines have been driven largely by the development of improved rotor and bucket alloys capable of sustaining higher stresses and enabling the construction of longer final stage buckets for increased exhaust area per exhaust flow. Improvements in efficiency have been brought about largely in two ways.

Firstly, improvements in mechanical efficiency by reducing aerodynamic and leakage losses as the steam expands through the turbine have been achieve. Secondly, there have been improvements in thermodynamic efficiency by increasing the temperature and pressure at which heat is added to the power cycle. This is where USC comes in.

Siemens is focusing a great deal of effort on higher plant efficiencies in preparing for the coal fired power station of the future, which should use as little fuel as possible in order to keep atmospheric emissions to a minimum. Crucially, the steam temperature needs to be increased from the level currently found in the best power plants, of around 600 °C up to 700 °C — the temperature to which the metals are being subjected in Siemens’ Mülheim laboratory, for instance. Only then will it become possible to achieve 50 per cent efficiency.

“Temperature is the key factor,” says Ernst-Wilhelm Pfitzinger, the project manager in charge of developing the 700 °C turbine in Mülheim. There are good practical reasons why designers are determined to leap from 600 °C to 700 °C and 285 to 350 bar pressure. “Above 600 °C, we have to use new materials anyway; traditional metals just wouldn’t be able to withstand the temperatures,” says Pfitzinger. “And we want to make as much use as possible of these materials, so we’re going to go straight to 700 °C.”

The higher pressure is necessary to optimize efficiency. The objective is to raise efficiency by four percentage points over that at 600 °C, and to cut coal consumption by 6–7 per cent, thus also reducing CO2 emissions. 

Nickel alloys required – but at a price 

By new materials, Pfitzinger means nickel alloys, which are a sophisticated mix of high-strength metals like nickel and chromium, with a pinch of iron. Such alloys are expensive. After a painstaking process they cost five to ten times as much as the chromium steel used today. That is not exactly peanuts in a turbine requiring some 200 tonnes of such alloys.

To reduce material costs, the turbine need not be entirely of nickel alloy, but can be composed of different alloys, depending on the temperatures different areas are subjected to. For example, the inner and outer housings are to be thermally separated by a layer of cooler steam, so that normal steel will be adequate for the outside, which will have to withstand a temperature of 550 °C. In addition, the 1 metre-thick shaft can be forged in several pieces, with the nickel alloy only employed in the hottest area.

The need for action is urgent. On average, coal fired power plants consume 480 g of coal to produce 1 kWh. In doing so, they release between 1000 g and 1200 g of CO2 into the air, or some 8 billion tonnes a year. The industry is responding. One of the most efficient coal fired power plants in the world, the Block Waigaoqiao III in China, for which Siemens delivered two 1000 MW turbines, burns only 320 g of coal per kWh, and thus emits only 761 g of CO2.

In a project led by Trianel Power-Projektgesellschaft, Siemens is building a comparable power plant for a consortium of 27 municipal utilities on a site at Lünen in northern Germany. The plant is scheduled to go into operation by next year. However, with an efficiency of around 46 per cent, even these power plants fall short of the Holy Grail of 50 per cent. 

Siemens: 50 per cent in sight 

Siemens’ aim is to achieve 50 per cent efficiency by 2015. Such a power plant would consume only 288 g of coal per kWh, and thus produce only 669 g of CO2. Such a step would have significant consequences because each percentage point in improved efficiency, if applied to all coal burning power plants, would translate into an estimated 260 million tonnes of CO2 emissions mitigated each year.

Peter Walker is another who struggles with the ambiguities of the term ‘USC’. The director of Steam Turbine R&D Execution at Alstom, he says the term has no real physical meaning. “It is just somehow beyond supercritical. Does the industry have a specific meaning for it? I think the answer today is no. People use it, but it doesn’t have an agreed definition. In Alstom for marketing purposes we define USC as having a live steam temperature above 600 °C at boiler outlet, but this is not very meaningful in technical terms. So when we develop steam turbines or other components we refer to a steam class by its pressure and temperature because that gives us an exact meaning. I’ve also heard USC used by other organizations for [conceptual] plants up to 700 °C, for example, and that’s purely arbitrary.”

While the progression to higher temperature and pressure will bring efficiency gains with less fuel and CO2, Walker says these come with both a higher capital cost and some technical consequences. “It is a technical challenge to give you the same level of flexibility in terms of start-up and shutdown times, particularly, since the higher steam conditions tend to lead to worse load cycle fatigue.”

Nevertheless for Alstom, at least says Walker, and just as for GE, moving up to higher steam values represents merely a natural progression. “Alstom has a design that we have used in the supercritical range that is very robust and that is relatively easily scalable to develop to these higher conditions.

“That’s not to say that developing it has not been hard work, but it was not a quantum leap. These are the machines that we’ve supplied to many of our projects in Germany like the RDK8 plant in Karlsruhe and Neurath near Cologne. The key issues for the steam turbine at these very high pressures and temperatures include how to keep your cylinder round. Being round is very important for clearance control and ensuring the machine doesn’t distort through high levels of stresses during transients and so-forth.”

“With our HP construction, because we have the shrink ring design, that wasn’t a big issue. Again, for a long time we’ve had welded rotor construction which meant that we could have a different material on the very hot part of the HP and then to have a material more suitable for the lower conditions lower down. We’ve continued to use that concept. We use more creep resistant and more oxidation resistant materials [where necessary].” 

The flexibility factor 

Walker says the biggest challenges concerning building the first commercial demonstration 700 °C plant are not to do with the steam turbines themselves but are, in fact, to be found elsewhere. To understand the real challenges, he says, you need to step slightly away from the cycle question and focus on flexibility. “The key issues facing the market right now, with all the renewables coming onto the grid, are to do with start-up and shutdown times and low load idling.”

The main problem, living with enormous pressures and thick walled components is that plant components do not like fast transients, says Walker. “This is where we and other companies are focused on in terms of technology development. So far as the 600–620 ºC turbines are concerned – which are just coming online at the moment – these machines do represent a moderate step in technology, so we can’t be sure that there will be absolutely no issues when they first operate. However, we are very confident that the designs are robust so we don’t expect any significant problems. The step to 700 ºC in order to get that 2.5 per cent or so increase in efficiency will call for more innovation in the design of machines and materials. But even then, for Alstom it would be an evolution of the turbine design and not a revolution. Alstom already has its design ready for a 700 ºC at 500 MW size.

“The steam turbine won’t be the limiting part of the cycle. What seems to control the economics of a 700 °C machine is neither the turbine nor the boiler – it’s the cost of the main steam pipe from the top of the boiler down to the turbine. It has to be flexible while carrying full steam temperature and pressure. If the main steam pipe has to be made of a nickel alloy then you pay more for your main steam pipe than you do for your turbine.”

“Whether or not the high cost of nickel alloy main steam pipe negates the benefits of higher efficiency will depend on the market price of fuel and CO2, Walker says. “One of Alstom’s strategic areas is CO2 capture and we’ve done a lot of work on this. A more economical solution might be a 620 °C plant with carbon capture with the same amount of CO2 as a 700 °C plant without CCS. It depends on how capital cost balances out against the cost of CO2 and fuel. The market conditions are changing rapidly today, with the cost of international fuels currently being very high and extremely volatile. Similarly there is a lot of uncertainty on the future market for CO2 or the possibility of CO2 penalties and taxes. I personally believe 700 ºC power plants will only be built once there is a very clear regulatory framework that would justify the capital expenditure.” 

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