Pushing the steam cycle boundaries

The efficiency of a steam turbine, more than any other component, will influence the overall efficiency of a steam power plant Source: Alstom

Breaking the 50 per cent efficiency barrier has become the four-minute mile for steam plants. But hitting this elusive target is becoming increasingly complex and will require a major step-change in technology.

Paul Breeze

Power plants based on the steam Rankine cycle have formed the backbone of the power generating industry for the best part of a century and are expected to continue to do so well into the present century. Most of these plants currently burn coal. In fact, coal fired plants using this cycle account for about 40 per cent of global power generation, more than any other single fuel or energy source. Against this background ” and with coal consumption set to rise until at least 2035, according to the US Energy Information Administration ” the efficiency of coal fired power plants is of enormous significance both from an economic and an environmental perspective.

Programmes across the globe now aim to push the net efficiency of existing steam plants ” which can reach 46 per cent, depending on the reference conditions ” beyond 50 per cent. But achieving this level of efficiency will require a major step-change in technology. Current materials cannot be used at temperatures and pressures beyond those already achieved so new, more expensive replacements will be required.

However, refinements in the overall steam cycle and in the design of the water steam cycle, along with the internal efficiency of the steam turbine and the boiler can all help to lift net efficiency further.

Increasing the steam cycle parameters is the most promising way to improve the efficiency of a modern coal fired power plant; the achievable improvement in efficiency is around 1 per cent (relative) for each 20 à‚°C rise in HP and RH temperature and 0.2 per cent (relative) for a 10 bar pressure increase.

Modern steam plants designed for high efficiency utilise supercritical steam condition (up to 250 bar, 565 à‚°C/565 à‚°C), which is so called because the temperature and pressure of water within the boiler are raised to a level where a distinction can no longer be made between the liquid and gaseous phase. Steam cycle parameters with a main steam pressure above 250 bar and temperature above 565 à‚°C and up to 600 à‚°C (main steam) and 620 à‚°C (reheat) are commonly called ultra supercritical (USC); these higher steam conditions can be achieved because of the development of new ferritic materials.

Efficiency in the water steam cycle can be obtained by: condenser pressure reduction; an increase in feedwater temperature to the boiler; an increase in the number of feedwater heating stages; and proper double reheat application.

The internal efficiency of the steam turbine, more than that of any other component, influences the overall power plant efficiency and the key features are: advanced 3D blade design to minimise profile and secondary losses; advanced scaling technology; and the application of large last stage blades to reduce exhaust losses.

Finally, boiler efficiency can be enhanced by reducing the flue gas temperature and combustion excess, as well as by minimising thermal and pressure losses, and the use of coal drying.

In addition, a further net efficiency increase can be obtained by reducing auxiliary consumption. This can be achieved primarily through the use of variable speed drives for pumps and fans.

Nonetheless, the target of 50 per cent efficiency is proving more elusive than was envisaged a decade ago. It appears unlikely that commercial plants that can achieve this target will be available much before the 2020s.

The state of the art

Estimates for the average efficiency of coal fired power plants in operation today vary but figures from power generation utilities group VGB suggest that average global efficiency is about 30 per cent. Other estimates put it a few per cent higher. Some regions perform better, with the EU average close to 38 per cent, according to VGB. Meanwhile, the US fleet probably averages only 33 per cent and in 2010 the average for the Chinese fleet was put at just under 37 per cent. All these figures are well short of the state-of-the-art in steam plant technology.

In modern steam plants, the supercritical phase simplifies boiler design, although it places greater demand on materials. More importantly, though, it allows efficiency gains because a Rankine cycle steam turbine is a thermodynamic engine and as its efficiency increases the greater the temperature gradient available for its operation.

However, a large base of subcritical plants remains in operation, particularly in developing nations, and plants of this type are still being built. The best and most modern of these can achieve efficiencies of 38″42 per cent. Supercritical power plants exceed this, with the first generation providing efficiencies of 42″44 per cent. Meanwhile, USC plants, which operate at higher temperatures and pressures than supercritical plants, can reach 44″46 per cent.

Some caution is required when talking about plant efficiency in this way, however, as the actual efficiency of any given plant depends upon the site and the reference conditions. A key condition is the cooling available and the resulting condensor vacuum at the steam turbine exhaust. At US inland sites, the best steam turbine exit pressure may be 7″9 kPa, whereas in Europe ” particularly where seawater cooling is available ” it can be as low as 3 kPa. This can make European plants 2 per cent more efficient than equivalent US plants.

Coal quality and a once-through boiler design common in Europe can help by reducing auxiliary power drain. It is also important to know if the higher heating value (HHV) or lower heating value (LHV) of the coal is being specified in the efficiency definition. Together, these factors can lead to as much as 4 per cent efficiency difference for virtually identical plants, according to the Electric Power Research Institute. Thus a typical subcritical plant in the US may be rated at 37 per cent efficiency while a state-of-the-art supercritical plant may have an efficiency of 42 per cent. In Europe the same plants might be rated at 41 per cent and 46 per cent efficient.

Incremental improvements

The state of the art today is the USC plant with a maximum efficiency of 46 per cent using European reference conditions. A plant of this type will operate at a maximum steam pressure of 250″290 bar, a steam temperature of 600 à‚ºC at the exit of the boiler superheater and 620 à‚ºC for reheat steam. These conditions have been achieved by incremental improvements in boiler and steam turbine material design over the last 10 to 20 years but that progress has now come to a halt ” although Alberto Torre, steam turbine expert at Ansaldo Energia, points out that the Danish Avedore 2 plant (300 bar/580 à‚ºC/600 à‚ºC) has been in operation since 2001 with a net efficiency of approximately 48 per cent.

According to Michael Wechsung, Siemens’ principal expert for steam turbines, it might be possible to push the temperature to 620 à‚ºC or only a little higher using existing materials. But that is probably the limit. Rich Kehl, product manager for GE’s large fossil fuel steam turbines, agrees that the limit has been reached. As he points out, while it has been possible to introduce a small number of stages using higher-performance materials to try and push this envelope, the gains that can be made in that way have come to an end.

What is required now is a wholesale move from traditional steels alloys to new nickel-based materials such as Inconel 740, which has just been approved by ASME for use in new steam plant boilers. High-temperature materials are not new. John Marion, R&D director for boilers at Alstom in the US, notes that gas turbines operate at much higher temperatures than steam turbines so there is already a body of experience to draw on. Even so, a shift to the new materials is not without difficulty.

The first problem is that nickel-based materials are much more expensive than the chromium steel alloys used for existing plants. The second is that steam plants place much greater demands than gas turbines. For example, steam turbines are much more massive than gas turbines and operate under much greater pressures. On top of that, steam turbine components might be expected to operate for the entire life of the steam plant, which could be over 200 000 hours, whereas gas turbine hot components are replaced after 25 000 hours.

So while the experience with gas turbines will be valuable for steam plant development, the technology transfer cannot be direct. It is not just turbine components that are needed, either. Superheaters, reheaters, valves, pipework and other components will all need to be built from high-performance materials and all need developing and testing before plants can be built.

New steam conditions

The cost of developing and building a more efficient steam cycle power plant operating at more extreme steam conditions has led to unprecedented co-operative development programmes in the US, Europe and elsewhere. The major steam plant companies have acknowledged that it will be too expensive to try to develop the new technology alone and so they are working together. In Europe, the US, Japan, China and India, government funding is helping to bring the new technology to the demonstration project stage.

In addition, the extra cost of the nickel-based alloys needed to push steam plant technology to its next stage of development means that another incremental change in conditions is not economically feasible. In order to make the new investment pay for itself, efficiency must increase by a significant amount. With this in mind, the target in Europe is for a plant operating at a steam temperature of 700 à‚ºC. Pressures will be increased too, perhaps to 350 bar, or even higher. This steam temperature increase of 80 à‚ºC and pressure increase of 60 bar or more should be sufficient to push the overall efficiency to 50 per cent, at least based on European reference standards.

In the US, the 700 à‚ºC target is also recognised but alongside it there is a much more ambitious target. A US Department of Energy sponsored programme is aiming to develop an advanced ultra supercritical (A-USC) plant operating at a steam temperature of 760 à‚ºC and 345 bar. The temperature increase of 140 à‚ºC over current USC conditions should also achieve 50 per cent using US reference conditions and based on the HHV of coal.

Carbon emission reduction

Efficiency is the primary benchmark for these advanced plants but carbon emissions are also vital. Based on average global fleet efficiency of about 30 per cent, VGB estimates global carbon dioxide (CO2) emissions from coal fired power plants average 1115 g CO2/kWh. With a state-of-the-art USC plant, this falls to 727 g CO2/kWh, while a plant with 50 per cent efficiency would emit just 669 g CO2/kWh. This still falls short of a recent proposal by the US Environmental Protection Agency to limit coal plant emissions to 1000 lb/MWh, or 454 g/kWh, but a plant operating at 760 à‚ºC might come close without the need for carbon capture.

Emission performance is important in the developed world where the most stringent regulations are being applied, but it is arguably more important in the developing world. Countries such as China and India are expanding their coal generating capacity rapidly to meet demand and while they are unlikely to start adding carbon capture to the bulk of these plants in the near future, they are committed to high efficiency steam cycles as a means of limiting emissions. The sooner 50 per cent efficiency plants are available, the sooner these countries will deploy them. This may prove crucial for the future control of global atmospheric temperatures.

Demonstration projects

With efficiency and emission performance as the driving force, the development of a 50 per cent efficient or 700 à‚ºC steam cycle has been underway for 15 years in Europe under the auspices of the AD700 programme. As part of this programme, a large-scale demonstration programme (COMTES 700) was carried out at the E.ON coal fired Scholven power plant in Gelsenkirchen, Germany.

During 17 000 hours of operation, components such as the superheater and reheat panels and valves were tested at 700 à‚ºC. Testing was completed in 2009 and led to updated boiler codes for higher temperature operation. A full-scale demonstration was planned for E.ON’s plant at Wilhelmshaven, which was due to have begun operating in 2014. However, economic conditions in Europe mean that this project is currently suspended.

The US programme has been in place for about ten years and has been pursuing a range of material developments similar to those in Europe. This programme is now approaching the same state as that in Europe. The next stage is a demonstration project but so far no such project is planned.

Meanwhile the focus has shifted to other parts of the world. Three new development programmes are underway in China, India and Japan. The Japanese programme has as yet no stated target for a demonstration scheme while China proposes to demonstrate the technology by 2020. India is more bullish, with a target date of 2017, which is achievable but ambitious and could be the basis for deploying the technology commercially by the beginning of the next decade. As Alstom’s John Marion observes, the bet now is that the first demonstration plant will be in China or India, not the US or Europe.

Parallel strategies

While the most important method of improving steam plant efficiency is to increase the steam cycle operating temperature to raise thermodynamic performance, there are other means to boost performance levels. One is coal drying, a technique that is particularly useful when burning lignite or brown coal.

In Germany, where lignites can contain up to 60 per cent water, two advanced drying techniques are being tested. One involves mechanical drying, in which the coal is heated to about 140″200 à‚ºC. The other is a fluidised bed drying technique, which has been tested at RWE’s Niederaussem lignite fired power plant, where it has demonstrated an efficiency improvement of 4″5 percentage points over conventional drying at the same plant.

In the US, where lignite is also burned extensively but with a lower moisture content of typically around 30″35 per cent, the US Department of Energy has sponsored a clean coal project that uses power plant waste heat to dry the incoming fuel. This project, at Great River Energy’s Coal Creek power station, has already shown efficiency and emission improvements. With lignite representing about 17″20 per cent of global coal reserves, such advanced coal drying techniques could prove an important strategy in many regions in the future.

Elsewhere, improvements in the use of heat within a power plant to keep heat losses to a minimum could help nudge up efficiency. Meanwhile, over the longer term it might prove effective to introduce bottoming cycles to steam plants based on organic Rankine cycle turbines.

A final strategy that is attracting wide interest is the re-introduction of the double reheat cycle first tested over 30 years ago but found uneconomical at that time. The technique is known to lead to an increase in efficiency. A Chinese implementation being developed by engineers from Shanghai Waigaoqiao No.3 Power Generation Company foresees a double reheat unit with a cross-compound arrangement: the first turbine-generator train including the high-pressure (HP) turbine and the first reheat (RH) turbine is located very close to the boiler, while a second train with the second RH turbine and the low-pressure turbines is located in the turbine house. This arrangement minimises the cost of high-pressure piping and pressure losses.

An important advantage of the double reheat design is that it can be implemented using existing material and USC designs and could provide another incremental increase in efficiency, perhaps to 48 per cent. Both Siemens and Alstom are taking an interest in this development.

Another area that could prove important is the design of coal plants that can operate more flexibly at part load rather than always at full load. In the past, many coal fired plants have been specified to be capable of operating at 100 per cent output for 7000″8000 hours each year. But modern grids require plants that can be operated at 40 per cent load or lower and still maintain good efficiency. Siemens has published some ideas for more flexible operation and is now looking for a customer to work out these ideas in more detail.

Significant steam turbine material design improvements have occurred over the last 10″20 years, but progress now appears to have halted Source: GE

Markets and plant sizes

Many of these technical advances will be implemented over the coming decade. However, the technical changes are taking place alongside a shift in the centre of gravity of the market for coal fired power plants that will also have a profound effect.

Trevor Bailey, general manager for GE’s steam turbine product line, believes that the market for new coal fired plants in Western Europe and the US will be negligible in the coming years. Instead, the focus will be on Eastern Europe and Asia, with countries such as Poland, India, China and South Korea leading the way. Given this shift, it will be the markets that will drive the technology. For example, markets may be looking for smaller, high-efficiency coal plants with capacities of 400 MW to 600 MW instead of 1000 MW. Technology developments should make it possible to build such smaller USC plants while maintaining good efficiency, though perhaps not as high as the largest plants. As an illustration of this trend, there are already 350 MW supercritical plants operating in China, according to Alstom’s Marion.

At the other end of the size range, the economies of scale that can be gained by increasing size may make it more cost-effective to build 50 per cent efficiency plants based on advanced materials at larger sizes than are typical today, perhaps up to 1200 MW or 1300 MW. The cost per kW is likely to fall as size increases and steam turbine efficiency generally improves with size too.

From a business perspective, there is a lot to play for. As markets shift, so major Western manufacturers will find themselves having to work harder as they compete for business in these new markets. With the potential advent of carbon capture too, the steam cycle power plant industry is facing a decade of excitement and change.

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