The ultra-supercritical future

14 August 2013

In 2009, coal-fired power generation accounted for 30% of CO2 emissions from the power sector and demand for this low-cost fossil fuel is set to increase. Elly Earls meets Dr John Topper of the IEA Clean Coal Centre to find out how deploying ultra-supercritical technology in coal-fired power plants could result in significant CO2 reductions, and why India and China are embracing this increasingly efficient technology.

Although demand for nuclear and renewable energy technologies is on the up, the growth of these non-fossil forms of power generation pales into
insignificance next to the increase in coal-fired power generation over recent years. According to the International Energy Agency (IEA) Clean Coal
Centre's report 'World Energy Outlook 2012', coal has dominated energy demand since the start of the 21st century, accounting for 45% of energy
demand growth from 2001-11.

This recent growth in coal use is directly related to increased emissions from the power sector; in 2009, coalfired power generation alone contributed 30% of total CO2 emissions. This is largely because the efficiency of existing coal-fired capacity is relatively low (around 33%),
which means that large amounts of coal must be combusted to produce each unit of electricity.

With demand for coal continuing to rise, the situation will deteriorate unless decisive action is taken by the power sector. Indeed, the IEA predicts that, unless something significant is done, energy-related emissions of CO2 will more than double by 2050, threatening the planet's low-carbon future. So, what can be done to ensure a sustainable future for the energy sector?

According to the World Coal Association (WCA), increasing the efficiency of electricity generation is essential in order to tackle climate change. A 1% improvement in the efficiency of a conventional pulverised coal combustion plant equates to a 2-3% reduction in CO2 emissions. Multiply that up, and the CO2 reductions that can be achieved by improving the efficiency of power plants are significant. Indeed, highly efficient modern coal plants emit almost 40% less CO2 than the average currently installed coal plant.

The multiple benefits of ultrasupercritical technology

According to the IEA's technology roadmap for high-efficiency low emissions (HELE) coal-fired power generation - which is based on the organisation's most ambitious scenario, that of limiting the average rise in global temperature by 2-3° by 2050 - ultra-supercritical (USC)
pulverised coal combustion is currently the most efficient HELE technology, with some units reaching 45% efficiency.

While the organisation's stated aim to halve energy-related CO2 emissions by 2050 (including reducing emissions from coal-fired power generation by around 90%) is extremely optimistic, there is no question that the use of USC technology can drive the power sector at least part of the way towards achieving it. In fact, combined with increasing deployment of carbon capture and storage through to 2050, virtually carbon-free generation from coal and gas is technically possible.

The conventional technology used for pulverised coal combustion is subcritical. This involves heating water to produce steam at pressures and
temperatures below the critical point of water, typically around 530-540°C, and the units are designed to achieve thermal efficiencies of up to 38%.

With supercritical (SC) technology, however, steam is generated at temperatures and pressures above the critical point of water, resulting in efficiencies of 42-43% when used with a typical internationally traded coal. USC technology simply takes this one step further, operating at even higher temperatures and pressures, meaning thermal efficiencies can reach up to 45%.

At present there is no agreed definition of USC technology, but it is generally accepted that the term refers to plants operating at steam temperatures over 600°C.

"Today, we have units operating around the world at temperatures of 605-615°C," says John Topper, managing director of the IEA Clean Coal Centre. "This adds 3-4% of efficiency, to a total of 6-8 points more than good subcritical technology, giving a significant operating advantage."

Of course, there are costs involved in building these more sophisticated units. The overnight cost of a subcritical unit is approximately 10-20% lower than for a supercritical unit, while the overnight cost of ultra-supercritical units may be up to 10% higher than that of supercritical units. This is due to the use of more advanced construction materials and techniques.

According to Topper, however, the higher costs are offset by the fuel savings, bearing in mind that power plants usually operate for more than
30 years.

"Although it costs a bit more to build the boiler, you save a lot of money on fuel," he confirms. "The higher you go on the temperature and
pressure of the steam, the greater the efficiency gains you can get." In turn, CO2 emissions are also reduced.

In the future, efficiencies could be improved further with advanced ultrasupercritical (A-USC) technology. Using the same basic principles as USC, A-USC will aim to achieve efficiencies of over 50% and will operate at temperatures of 700-760°C, delivering a 15% cut in CO2 emissions compared with SC technology.

There are currently no A-USC plants in operation, largely due to costs involved in constructing them and technical issues related to the far higher temperatures and pressures to which components in a A-USC system would be exposed; these require the use of super-alloys, which are much more expensive than the steel used in SC and USC units. Fabricating and welding the materials is also much more complicated.

USC technology: where are we now?

The IEA's technology roadmap states that there are nowhere near enough USC plants operating at present, despite the recent growth in the use of this technology. In 2011, roughly 50% of new coal-fired power plants used HELE technologies, predominantly SC and USC pulverised coal combustion units, but, according to the agency, there are still far too many subcritical units being constructed.

Indeed, the IEA found that around three quarters of operating units use non-HELE technology, and that more than half of current capacity is over 25
years old and comprises units of less than 300MW. But that's not to say progress isn't being made. A handful of countries, including China, India, Japan and Korea have prioritised improving the efficiency of their coal-powered plants.

In Japan and Korea, average efficiencies are now above 40% as the two countries started adopting SC technology before 2000, and, since the
mid-2000s, China has experienced huge growth in coal-fired generation, with the use of SC and USC increasing rapidly.

"China has so many units now installed and operating that they are really the market leaders," says Topper. "I predict that they will soon overtake
Western suppliers in the export market for USC technology."

While still a little behind, India too has since 2010 made significant strides in improving the efficiencies of its coal fleets and is set to follow a similar path to that of China over the coming years.

For Topper, it is these countries, as well as South Africa (which is about to adopt supercritical pulverised coal firing for power) that will really drive
the growth of USC technology in years to come.

"The imperative for emerging nations, particularly India and China, is to support their economic growth with a relatively cheap source of energy, but they also want to take full advantage of their knowledge and ability to use coal as effectively as possible," he notes.

In addition to building USC plants, companies in India and China are experimenting with A-USC technology. In India, for example, a consortium comprising Bharat Heavy Electricals, National Thermal Power Corporation and the Indira Gandhi Centre for Atomic Research has submitted a 'project design memorandum' for an 800MW pilot project based on A-USC technology. Completion is expected in 2018 and plant efficiency is estimated at 46%.

"This is a very ambitious schedule, because the associated materials testing and proving is very timeconsuming," Topper remarks.

Of course, allied to all this is the clean air agenda in highly polluted cities in Asia.

"At the IEA Clean Coal Centre, we're trying to encourage organisations in these countries to go for very best practice in terms of efficiency," Topper says. "That doesn't just mean efficiency in terms of carbon mitigation, but also best practice in terms of sulphur and nitrogen reductions. If they're going to use coal, we want them to use it as well as possible."

Collaboration: the key to a lowcarbon future

There are still many challenges to be overcome before HELE technologies become commonplace. For example, many subcritical plants are already paid for and provide a continuous source of revenue for plant owners; replacing these units would be expensive and would also increase the cost of the electricity generated.

Moreover, the only way to substantially reduce CO2 emissions caused by energy generation is by using carbon capture and storage (CCS) in
combination with advanced technologies such as USC and A-USC. But, at present, the capital and operating costs of CCS are high and a significant amount of energy is needed for CCS, reducing plant efficiencies by 7-10%.

Clearly, the solution is far from simple and will involve collaboration between governments, power companies and research institutes. At the IEA Clean Coal Centre, work on this front has already started.

"We have some convening power and we're seen as independent and credible, so we have started a workshop series on A-USC technology," says Topper. "We're conscious that there has been too much working independently, so we decided to run a workshop in September 2012 in Vienna to invite everyone that was a player in the advanced game to get together and talk about it."

The workshop was attended by 77 delegates from industrial companies worldwide, including a large Chinese delegation, and there were 25 presentations covering national programmes, development of new nickelbased alloys and subsequent proposals for alternative plant configurations in A-USC plants.

One of the most interesting developments was China's instigation of a comprehensive national programme, which involves steel production
companies, boiler and turbine manufacturers, and design institutes. As part of this initiative, the Huaneng group is building a component test
facility based on the EU COMTES700.

While the Chinese have made significant progress since they started the programme in 2010, and are confident the boiler side will progress quickly, they are less certain when it comes to turbine developments. This is a gap that could potentially be filled through international collaboration, perhaps by importing key steam turbine components. For Topper, building bridges like this is the key aim of the IEA Clean Coal Centre's workshop series.

"We want to try and get some of the Western organisations to reveal as much as they're prepared to reveal to the emerging nations - even if it's just telling them what's possible," he says. "Clearly, there's a lot of proprietary interest in this, so there's a bit of a dilemma there. But people are talking to one another and exchanging; cooperation is beginning to grow."

USC and A-USC technology have a vital role to play in ensuring the sustainable development of the global energy industry. Demand for coal is
not slowing, so the only way to meet global energy demands and reduce CO2 emissions associated with coalfired power generation is to improve the efficiency of the process, with the aim of linking that to widespread introduction of CCS.

While the International Energy Agency's ambitious goal of halving energy-related CO2 emissions by 2050 is unlikely to be met, the increasing deployment of USC and A-USC technology will go some way towards making one of our most important industries as efficient and environmentally friendly as possible.

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