Power Engineering International

Critical thinking: Ultra and supercritical technology focus

If coal has a future, then supercritical technology will be the key, writes Paul Breeze, who examines the latest developments

Eemshaven ultra-supercritical plant in The Netherlands.

Credit: Siemens

Supercritical steam plant technology is today the option of choice for most new coal-fired power stations.

These technically-advanced plants offer greater efficiency than older sub-critical designs and, most importantly, lower emissions.

The latter has become critical as the global warming spotlight falls squarely onto coal combustion. "It is a technical challenge for the fossil fleet," says Thomas Achter, steam turbine portfolio manager at Siemens.

Many governments and multilateral agencies concur - indeed the OECD has introduced financial regulations that encourage the use of both ultra-supercritical and supercritical steam generator technology for new coal plants.

The supercritical steam generator was first tested commercially during the late 1950s but at that time the materials available were barely adequate to support the technology successfully and operational problems were encountered.

Most developers continued to build sub-critical plants but the fleet of supercritical plants grew slowly as advances in metallurgical technology allowed the higher steam pressure and temperature conditions of the supercritical plant to be supported. By the last two decades of the 20th century, supercritical plants were the standard in countries like Germany and Japan and by the second decade of the 21st century this trend has spread to other countries.

New plants under construction reflect this shift. For example, GE has around 45 GW of steam capacity under construction according to Martin Boller, senior executive at GE Steam Power Systems. All the new capacity is supercritical and around 30 GW is ultra-supercritical.

Supercritical development

The supercritical power plant steam generator was developed in the US in the 1950s and the first plant, Philo unit 6 in Ohio built by Babcock and Wilcox and General Electric, was commissioned in 1957.

That 120 MW plant entered service with main steam conditions of 310 bar and 621°C, more extreme than many supercritical power plants operating today.

However, the materials available at that time were being pushed to the limits of their capabilities to support the temperature and pressure conditions inside the supercritical plant. In consequence, sub-critical designs continued to be the mainstay of the fossil fuel power generation industry.

While the first US plant was a pioneer, sub-critical plants remained the most common, the most reliable and the cheapest to build. However, average efficiencies were relatively low by today's standards at around 30 per cent. The advantage of the new technology was that it could increase efficiency substantially; the pioneering Philo unit 6 offered an efficiency approaching 40 per cent compared to a US national average of 30 per cent.

As the century advanced and new, stronger materials capable of operating at higher temperatures and pressures were developed, so more supercritical power plants were built.

However, the steam conditions were less severe than for the first plants. Typical is the Boxberg power plant that entered service in Germany the 1990s. This lignite-fired plant operated with a steam pressure of 260 bar and steam conditions of 540°C/580°C for reheat and main steam. Efficiency was around 42 per cent.

Thermodynamic efficiency in a steam turbine-based power plant depends on the temperature of the steam - the higher the temperature, the greater the potential efficiency.

As a consequence there has been a move to even higher temperatures and pressures leading to a new class designated the ultra-super critical power plant.

There is no definitive standard for ultra-supercritical steam conditions but they typically mean a steam temperature of greater than 600°C (593°C according to the OECD) and steam pressure in excess of 240 bar.

The first power plant of this type was commissioned in Japan in 1992 and numbers have increased substantially since then.

According to Platts, in August 2013 there were 165 operational ultra-supercritical power plants in operation around the world, together with 645 supercritical units and more than 7880 subcritical units.

Today these ultra-supercritical plants are becoming standard in many regions. "Modern supercritical units are being designed to produce main steam at 620°C and 300 bar and reheat steam at temperatures exceeding 620°C," Tariq Aziz, director of South Asia Power Generation Services for Black & Veatch told Power Engineering International.

Emissions has become "a technical challenge for the fossil fleet".

Credit: Siemens

"The cycle efficiency will be about five per cent higher than subcritical boiler designed to produce main steam and reheat at 565°C," he added.

However, these conditions make demands on the materials used to construct the steam generator and specialist alloys are required, especially for the superheater and for pipe-work carrying steam from the steam generator to the turbine. It is ironic, then that these steam conditions are virtually identical to those of the pioneering plant from 60 year ago.

Efficiency, cost, environment

Power plant technology has always had to balance performance with cost. In the case of supercritical power plants, that balance has been determined to a large extent by the cost of the materials needed for critical components. As Steven Borsani, director of business development Asia for Babcock and Wilcox points out, "designing the boiler for any temperature is not the issue."

The first supercritical boilers of the late 1950s were designed for a main steam temperature of 650°C but the temperature had to be reduced because suitable alloys were not then available.

Having backed off in the middle of the last century, temperatures and pressures have been edging up over the past two decades as the demand for higher efficiency and lower emissions have increased. This has been reinforced more recently by both government and multilateral agency intervention. For example, the OECD introduced stringent financing rules in November 2015 that now make sub-critical coal-fired power plants ineligible for funding except for plants under 300 MW in International Development Agency-eligible countries.

For plants between 300 MW and 500 MW, ultra-supercritical technology is preferred but supercritical technology is permitted, again limited to IDA-eligible countries. For plants larger than 500 MW, the financing rules allow funding only for ultra-supercritical power plants with a steam temperature of greater than 593°C and a pressure of greater than 240 bar, or with emission of less than 750g CO2/kWh.

One of the most important implications of this ruling is that power plants in the 300 MW to 500 MW range must now be supercritical or ultra-supercritical.

Previously plants of this capacity would often have been subcritical. This means that fluidized bed technology manufacturers are introducing supercritical technology to their plants too.

These have traditionally been smaller than pulverized coal-fired plants. Today both types can achieve efficiency in the range of 45-46 per cent, depending upon the operating conditions. "We don't see any major difference today," says Gerd Heiermann, head of USC boiler technology at Doosan.

What is clear, however, is that development of both pulverised coal and circulating fluidized bed ultra-supercritical boilers has advanced as far as it can with the existing materials.

The ambition exists to push to even higher steam temperatures and pressures but that means shifting to new types of alloys and these new alloys are expensive.

The material question

The components used in high performance power plants today rely almost exclusively on steels. Typical are materials such as P92 which is designed to withstand temperatures of 600°C-650°C in plant boilers. Primary components of the alloy, other than iron, are chromium and tungsten but there are small amounts of many other elements including nickel, molybdenum, vanadium and manganese. Another commonly used material is 347H, an austenitic steel with good high temperature corrosion resistance while 310HCbN is "the battle-hardened material," according to Borsani.

These materials are mostly well established and generally available from multiple suppliers so availability is good and costs are competitive. However, they are not suitable for the next stage in ultra-supercritical development.

"The next-generation design will focus on pushing main steam pressure to 400 bar and steam temperatures above 700°C," says Aziz. This will require nickel and nickel/chromium super-alloys. Programmes around the world are working to develop materials for what is known as the advanced ultra-supercritical power plant. These include initiatives supported by the US Department of Energy in the US, a similar programme in Japan supported by the Ministry of International Trade and Industry and parallel work in China, Europe and India.

GE's best performing ultra-supercritical plant to date, Rheinhafen-Dampfkraftwerk in Germany

GE, meanwhile, has developed a supercritical plant with steam conditions that are intermediate between those of the current generation ultra supercritical plants and the next generation as defined by Aziz. The GE plant is designed to achieve a steam temperature of 650°C-670°C and a steam pressure of 330 bar. The company's best performing ultra-supercritical plant to date, the Rheinhafen-Dampfkraftwerk facility in Karlsruhe, Germany, has achieved 47.5 per cent efficiency. A plant using the new design should be able to increase this by 1.5 per cent, pushing close to 50 per cent net efficiency.

To achieve this performance, GE will use nickel super alloys in critical parts of the plant such as the boiler headers, main steam piping and in the inlet stages of the steam turbine. The new alloys are more expensive but GE believes the plant will offer economic advantages that make the additional cost worthwhile. "The aim is to use as little nickel as possible to reduce cost," Boller explains. "We believe now is the time to offer this into the market."

The need for flexibility is today greater than ever.

Credit: Siemens

The advantages of a new generation of power plants with more extreme steam conditions are clear. In principle it might be possible to raise efficiency to around 50 per cent with a steam temperature of 700°C. A further temperature increase to 750°C could push this as high as 52 per cent. However, cost will be critical. There is another consideration too: with market conditions changing, absolute efficiency might not be the only goal.

Flexible power plants

One change that can be seen today is the need for greater operational flexibility for fossil fuel power plants, particularly, but not exclusively, in support of renewable power generation. "In Germany we have seen this for more than ten years now," says Achter.

There is evidence of a similar move in India too where the plant load factor for coal-fired power plants has fallen from 78.9 per cent in 2007-2008 to 62 per cent in 2015-2016 and is expected to fall to 48 per cent by the end of 2022, according to Sachin Deole, business development director, Power Generation, at Black & Veatch India.

Flexible operation means more startups and shutdowns and more frequent output ramping. This presents a different challenge to coal-fired power plant designers. Rapid change in output leads to large temperature gradients in components and this creates stress. The result is usually a shorter component lifetime and greater operation and maintenance costs.

Designing a plant that can operate flexibly means making changes to the design strategy. Already many supercritical plants operate in sliding pressure mode to allow for variable output with minimum variation in efficiency. Another approach noted by Achter was to adopt a higher mass flow but with a slow temperature rise to facilitate fast load increase. Meanwhile, Borsani says that Babcock and Wilcox has adapted its header design to achieve a more even temperature distribution during startup.

However, the issue is not just new power plants. Existing coal-fired power plants that may originally have been designed for baseload or steady-state operation are now being asked to operate more flexibly. As Borsani points out, in the independent power producer market today, plants are required to cycle daily. These plants are often parked at low load at night in order to facilitate a rapid startup next day. This means that low load costs have to be reduced as far as possible.

One key element that enables flexible operation is digital - the control system. According to GE, a modern coal-fired power plant has about 10,000 sensors, which monitor everything from steam temperature and pressure to equipment's vibrations. This permits a much larger degree of control of plant components but also enables output changes to be managed across the whole plant. "I think it is absolutely critical for the power plant to control not only the individual components but the integration of the plant," Boller emphasises. Digital control means that plant operation can be optimised to minimise component stress, or to reduce fuel consumption, or to respond to critical grid demand.

The growth of coal generation

As coal plant designs adapt to a changing market, coal-fired power generation continues to expand in spite of global warming worries and growth is likely to continue well into the 21st century.

The IEA Current Policies scenario in the 2016 World Energy Outlook would see coal capacity increase by 60 per cent between 2014 and 2040, though growth is more modest under more aggressive emissions control scenarios. However, this growth is not evenly spread across the globe. In Europe there is very little new coal capacity being built as a switch to renewables accelerates and a similar pattern can be seen in the US, accompanied by a move to natural gas.

A modern coal-fired power plant has about 10,000 sensors

However, in both these regions, while there is little new capacity being built, there is a strong retrofit market with older plants being updated to provide higher efficiency, lower emissions and greater flexibility. This offers attractive opportunities for many companies. Only in Eastern Europe is there any new capacity under construction.

For most companies looking for new projects, the biggest markets are in Asia. China and India dominate when it comes to new coal plant construction but in both, local companies are responsible for many of the projects. In India, "supercritical plants will be set up predominantly by Central and state governments," Deole confirms.

For Western suppliers, other countries, particularly in Southeast Asia, appear to offer better opportunities. Borsani identifies Indonesia and Vietnam, and to some extent the Philippines, as offering the biggest opportunities from Babcock and Wilcox' perspective.

In Vietnam, typical plant size is 600 MW or 1000 MW, whereas in the Philippine 300 MW plants are more common with some 600 MW. Doosan's Heirermann also identifies the Philippines, Indonesia and Vietnam as strong markets. However, he points out that Chinese companies were likely to start competing with Western suppliers in these markets. That could be an important feature in years to come.

Other countries and regions offer opportunities too, though often on a smaller scale. For example Siemens has seen a demand for plants of around 300 MW in Africa, the Middle East and South America. These tend to be individual projects, often supplying power to small grids. There may be a market for even smaller plants too. Babcock and Wilcox has seen demand for 100 MW plants in Africa. Meanwhile Doosan has seen some potential in Turkey too, though the demand here is for low cost plants which may lead to sub-critical technology.

Looking to the future, the power generation landscape is changing more rapidly that was expected even at the beginning of this decade. IEA analysis shows renewables advancing much more swiftly that was expected and this will impact on future coal plant demand. Companies everywhere will be hoping to be able to chart a path through this new landscape.

If coal has a future, then supercritical technology will be the key.

Paul Breeze is a journalist specializing in energy topics.

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