Combined Cycle Power Plants
Abstract
The most effective way of increasing the efficiency of a gas turbine based power plant is to turn it into a combined cycle plant. The most common configuration of this type involves adding a steam turbine bottoming cycle to the gas turbine. Hot gases from the exhaust of the gas turbine are used to raise steam in a heat recovery steam generator and this steam is used to drive a steam turbine. The best plants of this type can achieve an energy conversion efficiency of over 60%. More versatile combined cycle plants can be created by adding additional heating by burning more natural gas in the steam generator. A further configuration uses solar heat add to the heat in the gas turbine exhaust.
Keywords
Combined cycle; topping cycle; bottoming cycle; heat recovery steam generator; steam turbine; integrated solar combined cycle
Adapting a simple cycle gas turbine in the ways outlined in Chapter 6 offers a means of increasing the efficiency of the cycle by reducing parasitic loads such as the excess air compressor load and of capturing energy that might otherwise be wasted. However, this is not always the most effective way of increasing overall efficiency of energy conversion. For large gas turbine based plants in particular, the best way of improving efficiency is to add a steam turbine bottoming cycle, creating a combined cycle power plant.
A combined cycle plant is simply what its name suggests. Instead of relying on a single thermodynamic cycle to convert energy into electricity the plant uses more than one. These piggy-back one another with the first cycle using the highest temperature thermodynamic working fluid, followed by a second using the intermediate temperature fluid and—in principle at least—a third using a lower temperature working fluid. In fact combined cycle plants with more than two cycles are not used commercially although they are theoretically possible.
One way in which a gas turbine can be used in this way is as a topping cycle. There are a number of power plant concepts that involve the generation of a high pressure, high temperature gas stream. A pressurized fluidized bed combustion plant, a high pressure molten carbonate fuel cell and some high temperature solar power plants can all produce a stream of gas that could be used to drive a gas turbine before the hot gas is used either to generate steam to drive a steam turbine, or in the case of the fuel cell, to provide fuel for the fuel cell. Topping cycles are not the subject of this book but more can be found about these individual topping cycle plants in other books in this series.
Much more significant for the modern power generation industry is the addition of a bottoming cycle to a gas turbine power plant. In this case the bottoming cycle is usually a steam turbine cycle, with heat from the gas turbine exhaust exploited to raise steam. This is the most common combined cycle power plant.
It would be possible to add a third cycle to exploit the low grade heat remaining after steam generation. This could be achieved with a closed cycle turbine such as an organic Rankine cycle. Such turbines can exploit low grade heat to produce electricity and are used in some geothermal plants where the temperature of the geothermal reservoir is relatively low. However, it is unlikely to be economically viable to add this third cycle to a modern combined cycle plant.
7.1 Evolution of the Combined Cycle Concept
The first gas turbine that was ever sold commercially for use in a power station to generate electricity operated in a sort of combined cycle mode. The 3.5 MW unit was installed at the Belle Isle Station in Oklahoma in 1949. Belle Isle was an established fossil fuel-fired steam power station and the heat in the exhaust gases of the gas turbine was used to heat the feed-water for the boiler of the steam plant, boosting steam plant output. In later applications of gas turbines, during the 1950s and 1960s, the exhaust gas from a gas turbine was used as the combustion air for a conventional fossil fuel-fired boiler. This added a few percentage points to overall efficiency. Other early combined cycle power plants were created by repowering fossil fuel plants, replacing the coal furnace with a gas turbine but using the steam generator and steam turbine of the original plant to exploit the heat in the gas turbine exhaust.
The development in the 1950s of more advanced boiler tubes with spiral fins welded to the central tube gradually made it possible to build a more effective heat recovery steam generator (HRSG) for energy capture from a gas turbine exhaust. Many of the first plants to employ this new technology were cogeneration plants that produced electricity and steam but a small number of these steam generators were used in utility power plants. This established the modern gas turbine power station.
The basic combined cycle configuration consists of a gas turbine, a HRSG and a steam turbine. In the early incarnations of this configuration the gas turbine operated as if it were a stand alone gas turbine generating plant with its own generator producing electrical power. The steam bottoming cycle is then added. As we have seen in earlier chapters the temperature of the exhaust gases exiting a gas turbine is relatively high, usually 400°C to 500°C, and sometimes higher. This is hot enough for the heat to be captured in a HRSG where it is used to raise steam and this steam is then exploited in a steam turbine generator which produces more electricity. The basic combined cycle configuration is shown in Fig. 7.1.
Plants based on this configuration became common during the 1970s and 1980s. However, these were still based on a standard open cycle industrial gas turbine. Change came towards the end of the 1980s when the major manufacturers began to explore the idea of designing both gas and steam turbines specifically for combined cycle power stations. This eventually led to the modern generation of extremely high efficiency combined cycle plants. In 1990 the best efficiency for a combined cycle plant was 50%. In 2011 a combined cycle power plant in Germany achieved 60.75% efficiency.
7.2 The High Efficiency Combined Cycle Power Plant
The great advances achieved in combined cycle efficiency can be attributed to two principle sources. The first is the tight integration of all the components of the power plant. This allows the two cycles to operate at optimum efficiency and reduces energy loss. The second has been the advance in efficiency of the stand alone gas turbines as a result of the increase in first stage temperature that has been achieved with new materials and designs. The current designs in which the highest inlet temperature can reach 1600°C have broken the 60% efficiency barrier. Efforts are underway to push this temperature to 1700°C, potentially leading to an efficiency of 65%. To put this in perspective, the best coal-fired power stations using the most advanced boiler technology can only achieve around 47% efficiency.
Integration of the two cycles in a combined cycle plant means balancing the energy capture in each cycle. A stand alone gas turbine will offer its highest efficiency when the exhaust gas temperature is as low as possible. However, for a combined cycle plant it is advantageous to allow the exhaust gases from the gas turbine to leave at a significantly higher temperature since this makes the steam turbine cycle more efficient. So while the gases may leave a stand-alone gas turbine at a temperature as low as 400°C it is not uncommon to see the exhaust gas temperature for an integrated combined cycle unit to be over 600°C.
Much of the design effort that led to these high efficiency systems has been aimed at achieving high turbine inlet temperatures. Heavy duty industrial gas turbines have traditionally been given a letter of the alphabet that denotes the series to which they belong. Later letters in the alphabet denote a turbine with a higher inlet temperature. During the 1990s the biggest industrial turbines were F class turbines. H class gas turbines began to appear at the end of the 1990s and by 2011 there was at least one J class turbine.
The H class turbines generally have a turbine inlet temperature of between 1400°C and 1500°C. To manage this, companies have had to resort to extremely advanced turbine materials and blade cooling configurations. The first company to announce an H class turbine was GE which developed the machine under the US Department of Energy’s advanced turbine system program. Inlet temperature is 1430°C and the compression ratio is 23:1, higher than the 15:1 ratio in the company’s earlier F class machines. In the H turbine the first stage vanes and turbine blades both use single crystal superalloy with a thermal barrrier coating. In addition, the close integration with the steam turbine cycle in the H machine means that steam from the steam generator can be used to cool the vanes and blades of the first two turbine stages. The third stage uses more conventional air cooling and the fourth stage is uncooled.
There is a complex steam cycle too. The HRSG provides steam at three pressure levels. The highest pressure steam is used in the first, high pressure stage of a two-stage steam turbine. Steam is then reheated before entering the second, intermediate stage steam turbine. The third pressure level is used for steam cooling. A typical HRSG layout is shown in Fig. 7.2.
Steam cooling offers an efficiency advantage over the more traditional air cooling because the latter usually borrows air from the compressor and so represents a further parasitic load. Steam from the combined cycle steam generator does not carry the same penalty. However, steam cooling is more complex to implement and it also affects overall flexibility. So while GE and the Japanese company Mitsubishi Hitachi Power Systems have both adopted steam cooling, Siemens has exclusively used air cooling for its high efficiency combined cycle plants. Alstom too, retained air cooling in its large gas turbines which are novel for their use of gas turbine reheat to avoid the high inlet temperatures used by other manufacturers. (Alstom sold its gas turbine division to GE in 2014, reducing the number of major manufacturers of large industrial gas turbines to four including Ansaldo.)
7.3 Heat Recovery Steam Generators
The HRSG is a key component of a combined cycle power plant. Its role is to convert as much of the heat as possible from the exhaust gas of the gas turbine into steam for a steam turbine. The temperature of the exhaust gases from the steam turbine will be between 400°C and perhaps 650°C, low compared to the gases exiting the boiler of a coal-fired power plant. The heat in this gas is captured by water and steam that flows through tubes placed in the path of the hot gases. These tubes will have fins welded to them to increase their surface area so that they can absorb more heat from the hot gases. These tubes are arranged in modules, sometimes called racks, with each module serving a slightly different function.
From the perspective of water heating, the first module is called an economizer. This takes relatively low grade heat and uses it to heat the feed-water that is returning from the steam turbine to the boiler. This hot water is then fed into a module called the evaporator which heats the water to its boiling point. The temperature at which the water boils may be much higher than 100°C because the system is operated under pressure. At the top of the evaporator is a steam drum where water and steam are separated. The water cycles back through the evaporator while steam is collected a taken to the third module, called the superheater. This dries the steam and raises its temperature above the boiling point, before piping it to the steam turbine.
The three modules are arranged in the hot gas path so that the superheater is exposed to the hottest gas. This is followed by the evaporator while the economizer takes heat from the coolest exhaust gas, before it is released through the plant stack. Complexity can be added because many modern combined cycle plants use HRSGs with two or three of these economizer–evaporator and drum–superheater arrangements to provide steam at two or three pressures. In this case the multiple modules are arranged in order of decreasing steam and water temperature. In some designs there may also be one further module, called a reheater, which reheats steam from a high pressure steam turbine before it is fed to an intermediate pressure steam turbine as in the GE H system described above.
There are a number of design for HRSGs although they all share many common features. The variants include steam generators in which the exhaust gas path is vertical and the boiler tubes are arranged horizontally in the gas path and steam generators with a horizontal gas path and vertical steam tube modules. The way in which water and steam circulates within the boiler tubes varies too, with some using forced or pumped circulation and others using natural circulation. Natural circulation HRSGs usually have a horizontal exhaust gas path while forced circulation boilers are often vertical.
The temperature of the turbine exhaust gases fall as they pass through the various stages of the HRSG. Emission control to remove nitrogen oxides requires the gases to be at a specific temperature for the process to be carried out efficiently. The point at which the exhaust gases reach this temperature often lies within the HRSG. This means that these emission controls systems must be installed inside the boiler.
It is possible to add heat to the exhaust gases by installing gas burners within the steam generator, a technique known as supplementary firing. Supplementary firing can make the system more flexible by allowing more steam generation where needed or to supply additional heat if the gas turbines are operating at less than full load. Supplementary firing is less thermally efficient and is rarely used in very large combined cycle plants. However, it is common in stations that supply steam for process heat as well as electricity.
An advanced HRSG design that is being introduced to add greater operational flexibility to combined cycle power plants is the once-through steam generator. Fig. 7.3 compares the drum-type and once-through steam generator. This design eliminates the steam drum from the conventional HRSG design by completing the conversion from water to steam within the evaporator sections of the boiler. The steam drum of the more conventional design is a handicap for fast start-up because it is massive and therefore takes a long time to reach its operating temperature. Eliminating it allows the steam cycle to start more quickly. Once-through steam generators can be both horizontal and vertical.
7.4 Flexible Combined Cycle Power Plants
The development of combined cycle gas turbine power plants has led to impressive gains in performance and, at over 60% energy conversion efficiency, the best modern stations offer the highest energy conversion efficiency of any large scale power plant. This equates to lower overall emissions of carbon dioxide for each unit of electricity generated.
These impressive performance figures can only be achieved under relatively steady state operating conditions. However, some of the strongest markets for large combined cycle plants are in developed countries that have sophisticated grid systems where large quantities of renewable electricity are being generated and delivered into the grid. The main types of renewable generation in use, wind power and solar power, are both intermittent and can display high levels of unpredictability. To manage this type of power delivery while maintaining a stable electricity network that can balance demand and supply requires a fleet of power stations that can take up the load when renewable input falls and back out when it rises. In many cases new combined cycle power plants are required to perform this function. This has led to the need to develop flexible combined cycle power plants.
The demands on a flexible plant are different to those experienced by one that operates under relatively stable conditions. A plant that is supporting renewable generation must be able to start-up quickly and be able to change its output level quickly too. This involves rapid changes in temperature which can create large temperature gradients in components. Such gradients are a major source of stress and fatigue leading to high maintenance and repair costs. Simplification of designs, such as the reduction of thermal mass and inertia in the once-through steam generator, can help reduce these stresses by allowing components to heat up more quickly. Other changes to the operation of combined cycle plants are intended to reduce the temperature swings.
One way of improving the ability of a plant to react quickly is to decouple the gas and steam turbine cycles. Gas turbines can start rapidly but the HRSG and steam turbine sections may require more time to reach operating temperature. Decoupling leads to complications for steam cooled gas turbines because the gas turbine cannot operate until the HRSG is operating at its correct temperature. Manufacturers have had to adapt. For example, GE has now developed a version of its H class gas turbine that is fully air-cooled,1 so avoiding this problem. The new version offers faster start-up and ramp rates than the earlier steam cooled versions of the same turbine.
The period of greatest thermal stress in a combined cycle power plant is during a cold start after the temperature of all the components has been allowed to fall close to ambient. One way of avoiding this is to keep the plant warm by idling at very low power. “Parking” a power plant in this way not only avoids the stresses of a cold start but it means that the plant can be brought up to full power much more quickly. There is an economic price since the plant will burn fuel while it is parked but this may be more than offset by the savings in maintenance costs due to the wear and stress introduced by frequent cold starts.
Another issue that needs to be addressed in flexible combined cycle plants is the efficiency at part load. While a combined cycle plant may be able to achieve high efficiency at full load this will often fall off at part load as the operating conditions change. It may be necessary to design slightly lower full load efficiency in order to get high efficiency at less than full load. Typical part load efficiency for a flexible combined cycle plant will be around 50% to 55% under low load compared to 58–59% at full load.
The other area that is affected by flexible operation is power plant emissions. Emissions can be controlled relatively easily under steady state conditions but when conditions are varying, then emissions can rise significantly. (Think of a diesel-engined car accelerating and producing a cloud of black smoke.) If efficiency drops then the amount of carbon dioxide produced for each unit of electricity rises. Nonsteady state operation will usually increase the quantity of NOx generated in the turbine combustor too. The latter can usually be removed using advanced emission control technology but it can still lead to an overall rise in emissions compared to the steady state and varying conditions make control of emissions more difficult to maintain. These are problems that gas turbine and combined cycle plant manufacturers are addressing in the middle of the second decade of the 21st century.
7.5 Integrated Solar Combined Cycle Power Plant
The combined cycle power plant is a flexible concept and it can be adapted in various ways to accommodate different sources of energy. One of the most interesting of these is the integrated solar combined cycle (ISCC) power plant. This type of plant collects solar heat energy and adds it to the energy from fuel burnt in a conventional combined cycle plant in order to reduce the cost of power.
Solar thermal energy can be harvested in various ways. In the case of the ISCC plant the usual arrangement is for solar energy to be collected using an array of parabolic trough solar collectors. These are aligned so that they can track the sun across the sky from morning to night, collecting as much solar energy as possible and focusing it onto a heat collecting tube that runs along the length of each trough at the focus of the parabola. A heat transfer fluid is pumped through these heat collection tubes. By passing the fluid through several parabolic trough collectors in succession, the temperature of the fluid can be raised as high as 550°C.
In the simplest ISCC configuration the high temperature heat transfer fluid is passed through a heat exchanger that forms part of the HRSG of a combined cycle plant, as shown in Fig. 7.4. The heat is extracted and used to raise steam for the steam cycle of the plant. Provided the solar input is kept low compared to the total energy input into the steam cycle, the efficiency is much higher than for a stand alone solar plant and improves the efficiency of the combined cycle plant. However, the solar power input is only available during the day so that at night the steam temperature and pressure will fall because less energy is available. Depending upon the amount of solar energy available, this can have a significant impact on overall efficiency. Ideally the solar input should be kept to 10% or less of the total energy input.
A way of using more solar energy and maintaining the steam operating conditions is to equip the plant with thermal energy storage. Various storage systems have been tested including water and molten salts. If thermal energy can be stored then it will be available over 24 hours instead of only being available during daylight hours.
An ISCC plant offers a very efficient way of using solar heat energy. It also offers an advantage in a flexible combined cycle plant because the solar energy can be used to maintain the steam turbine cycle at operating temperature, allowing much faster start-up when power from the plant is needed.
The first ISCC plant of this type was inaugurated in 2010 in Italy when a solar collection field was added to an existing combined cycle power plant near Syracuse in Sicily. Since then plants have been built in Florida, Egypt, Iran, Algeria and Morocco. This type of power plant is considered particularly attractive in regions such as North Africa where there is abundant sunlight and a ready supply of natural gas.