Advanced Gas Turbine Design
Abstract
Modern gas turbines operate at extremely high temperatures. In order to withstand these temperatures many of the critical components must be made of sophisticated materials. The most severe conditions are found inside the combustor and at the entry to the turbine stages. Here materials may need to be able to withstand 1600°C or more. This requires special superalloys, fabricated either from nickel or cobalt. These must then be cast into components as single crystals to give them the strength to withstand both the temperatures and the centrifugal forces that turbine blades have to endure. Where temperatures are lower, steels can often be used instead of these superalloys. However, both types of component may need special coatings that improve their resistance to corrosion, erosion and temperature.
Keywords
Nickel superalloy; cobalt superalloy; turbine blade cooling; thermal barrier coating; ceramics; corrosion resistance; blades; nozzles
The evolution of gas turbines since the middle of the 20th century has involved the development of a range of advanced materials and the optimization of the three major components, compressor, combustor and power turbine. Early development was all centered around aero engines and its primary aim was to produce high power levels from gas turbines to enable ever faster flight. This required a combination of high efficiency and low weight. Materials and component design soon became key to achieving these aims. Both of these features carried over into early aeroderivative gas turbines that exploited the key properties of flexible operation and easy deployment to apply the technology to both mechanical drive and to the power generation industry.
Towards the end of the 1970s it became clear that the two arms of the gas turbine industry were diverging. While weight was an issue for jet engines in aircraft it was much less important for the power generation sector. In consequence stationary gas turbines did not need to use titanium-based alloys that were important for aero engines. Instead efficiency became the most important aspect. High efficiency for power generation depends on optimizing the design of all the components of the gas turbine; compressor, combustion chamber and power turbine to deliver shaft power.
By the 1990s the advent of computer-aided design coupled with mathematical modeling had allowed both compressor and power turbine component design to be reach a high level of optimization. While there will always be room for improvement, it was now much more difficult to increase efficiency by modifying the design of these components.
The only remaining means of increasing efficiency was by changing the operating parameters of the machine. The two main parameters available to the designer are the compression ratio of the engine, determined by the compressor, and the power turbine inlet temperature. The former is a design choice which varies from manufacturer to manufacturer but remains relatively fixed within each design family. Typically the compression ratio is between 15:1 and 30:1 for modern gas turbines. That leaves power turbine inlet temperature. Raising this allows efficiency to be increased by increasing the overall temperature drop between inlet and outlet. It is in this area that most effort has been directed over the last 20 to 30 years.
Higher efficiency, obtained with a higher turbine inlet temperature, remains the key to improved performance of stand-alone gas turbines. However, for combined cycle power plants where power generation is shared between a gas turbine cycle and a steam turbine cycle, this must be combined with a balanced design. Extracting the maximum amount of energy and achieving the highest efficiency, which for the best systems is now in excess of 60%, demands careful integration of the two cycles and precise control of operating conditions. Moreover the best results are achieved when the system operates at a steady state and many of the best performing high efficiency combined cycle power plants operate under base load or intermediate load conditions.
In recent years, however, the duty cycle required of combined cycle plants has changed markedly, particularly in developed countries where the introduction of large volumes of renewable power into grid systems has become common. This has led to the need for combined cycle plants to be able to provide system back-up services involving much more frequent start-up and shutdown and the ability to ramp power both up and down rapidly. Achieving high efficiency under these conditions is much more difficult and major gas turbine companies now offer gas turbines and combined cycle plants that are capable of highly flexible operation. This comes at a price. Operating under frequency changing conditions will generally lead to a reduction in efficiency.
5.1 Gas Turbine Materials
Gas turbines operate at high temperatures and at extremely high speeds. This places great demands on all the components of the machine. In consequence designers and materials engineers have developed a complete range of specialist materials from which to build these components. Materials that are capable of providing great strength and durability at high temperatures are key among them.
As noted above, increasing the power turbine inlet temperature of a gas turbine represents the main means used by designers to achieve an increase in efficiency. An 8°C increase in the inlet temperature can lead to an increase in power output of 1.5% to 2.0% and an increase in efficiency of 0.3% to 0.6%.1
Inlet temperatures have risen steadily over the past century as materials have improved. The inlet temperature of the first gas turbine to operate successfully, built by Aegidius Elling in 1903, was 400°C, increasing to 500°C the following year. By 1967 the first stage inlet temperature of the most advanced gas turbines had reached 900°C and in 2000 temperatures of 1425°C were possible. Gains have continued into the 21st century and in 2011 Japanese company Mitsubishi Hitachi Power Systems developed an advanced gas turbine with inlet temperatures of 1600°C. Efforts are underway in Japan to push this to 1700°C in the near future. Beyond that, new targets will no doubt be set.
While the increase from 400°C to 1600°C represents an enormous advance in the space of almost 110 years, gas turbine combustors could deliver gas at a higher temperature still. The limiting factor remains the material from which the first stage of the power turbine is made. There are no modern materials that can withstand even the existing temperatures inside a combustor with ease and great technical sophistication has been required to be able to exploit the current highest inlet temperature, using a mixture of advanced cooling techniques and sophisticated materials technology.
Moreover, it is not just in the power turbine that high performance materials are required. The combustor liner must withstand extremely high temperatures too while the compressor stages, though not exposed to such high temperatures, have to be able to withstand corrosion from air laden with moisture containing salts and acidic materials that are drawn into the machine.
5.2 Compressor
The primary metal used for compressor blades in aero engines is titanium, in the form of a range of titanium alloys. The metal is preferred for its low weight and relatively high temperature resistance. Since the 1950s the amount of titanium in an aero engine has increased from around 3% to 33%.2 The best titanium alloys are resistant to up to around 540°C. However, this is too low for the last stages of the compressor of modern aircraft engines and these must be made from higher temperature nickel alloys which weigh almost twice the equivalent titanium component.
For stationary applications weight is not usually a consideration and the compressor blades and nozzles for heavy duty gas turbines and aeroderivative gas turbines can be made from steels. These steels usually contain chromium and carbon and some more recent ones contain nickel and molybdenum too. These steels have high tensile strength and high cycle fatigue strength. They are resistant to acidic salts too, but compressor blades are often provided with a special coating to improve their corrosion and erosion resistance.
5.3 Combustor
The combustion liner is exposed to much higher temperatures that the compressor components and neither titanium nor iron-based alloys exhibit suitable resistance to the various types of heat induced failure that can occur. Instead, the most common type of material for combustor liners has been nickel superalloys. Hastelloy X was used between the 1960s and the 1980s when it was replaced by an alloy called Nimonic 263. However, even nickel alloys could not cope with the subsequent increase in combustion temperatures and cobalt based superalloys are now replacing them in the most demanding situations. Where temperatures exceed even the ability of these alloys, special ceramic coatings with low heat conductivity are added to help with thermal resistance. This is combined with air cooling of the metal components to keep the temperatures within the operating range of the superalloys. The same materials and coatings may be used for transition pieces too.
5.4 Turbine Components
The most demanding conditions within a gas turbine are normally found in the first stages of the power turbines and it is here that the most advanced materials technologies are applied. Iron alloys have been used in the past to fabricate both stationary and rotating components but as temperatures increased, these were replaced by a range of nickel-iron and nickel-based superalloys. Typical of these advanced materials is an alloy denoted by the title Alloy 718 which is used for large rotating power turbine components in large industrial gas turbines. It is made from nickel, chromium, iron, molybdenum, titanium, aluminum, cobalt, and carbon. The composition is shown in Table 5.1.3 Even these nickel-iron and nickel superalloys have not proved adequate for the latest power turbine inlet stages and they are now being replaced by cobalt superalloys for the most demanding situations.
Table 5.1
| Element | Proportion |
| Nickel | 53.0 |
| Chromium | 19.0 |
| Iron | 18.5 |
| Molybdenum | 3.0 |
| Titanium | 0.9 |
| Aluminum | 0.5 |
| Cobalt | 5.1 |
| Carbon | (0.03) |
Source: Intech.
These most demanding conditions are met in the first stage vanes and blades of the power turbine. The vanes are stationary but they are the first components that the gases from the combustor reach and they must be able to withstand the highest temperatures, as well as being resistant to corrosion from any impurities carried through with the hot, high pressure gases. The first stage turbine blades are also exposed to extremely high temperatures. However, in their case the difficulty is compounded by extremely high rotational speeds that produces massive centrifugal forces within the blades themselves, forces that can tear the component apart. The forces manifest themselves through what is known as creep rupture of the metal component. Creep rupture is specific to high temperature conditions and can lead to component fracture. It can be found in turbine blades and also in the discs that hold the blades in position on the turbine shaft since these have to support all the centrifugal force generated by the mass of the blades.
In order to create materials that can operate under such conditions, not only have new materials been developed but new methods of preparing them have had to be introduced. The traditional method of manufacturing metallic components is to forge them by machining from ingots of the material. Alloys designed to be forged have to exhibit specific properties compatible with forging. As inlet temperatures have risen it has proved difficult to combine the properties that lead to ease of forging with the properties required to retain high temperature strength and corrosion resistance. To get around this, there was a shift to the use of investment casting4 to form the components precisely without the need for forging.
Casting using a mold and molten metal produces a cast component which has a microcrystalline structure with randomly sized and oriented grains. The rate at which the cast is cooled can be used to control grain size to some extent. These cast components enabled higher operating temperatures to be reached but eventually even they reached their limit.
The main failure mechanism in turbine blades made from such castings involves cavities forming along grain boundaries that occur perpendicular to the length of the blade and to the direction in which the centrifugal force is acting. To overcome this, a technique called directionally solidified casting was developed which preferentially reduced the number of these transverse grain boundaries by extending the grains along the length of the blade. This helped increase the temperature capability of such castings by around 14°C compared to a conventionally cast component.
Directionally solidified cast components remain adequate in some extreme situations but in others even these were not capable of providing adequate lifetimes under the first stage power turbine conditions. The solution was single crystal casting. In this type of casting a component such as a turbine blade is cast as a single crystal of the superalloy. Further, the orientation of the crystal can be controlled so that the direction of greatest strength is exactly that which will experience the greatest tension and stress.
There are additional advantages of single crystal component growth. As these contain no grain boundaries, elements that are added to the superalloy mix to control strength at grain boundaries can be eliminated and this has led to higher melting point alloys, improving further their high temperature strength. Even so these single crystal castings have their limitations; even they begin to soften at the power turbine inlet temperatures. To overcome this, special coatings have been designed that can protect the alloy surface from the most extreme temperatures.
5.5 Component Coatings
One of the main ways of improving the performance of superalloys in gas turbines is with coatings. These coatings can be used to increase the resistance to corrosion or oxidation of the metal surface. Other types of coating, when used in conjunction with a cooling regime, can help protect the component from the most extreme temperatures.
Superalloys for high temperature components have to combine high physical strength such as resistance to creep fatigue and erosion with resistance to chemical erosion of the metal surface via either corrosion or oxidation. However, as the conditions that the metals must resist become more and more extreme, so it becomes more difficult to combine all these properties in a single superalloy. Coatings offer a way of enhancing the superalloy performance.
There are two types of environmental degradation to which hot turbine components can be subjected, hot corrosion and high temperature oxidation. High temperature corrosion occurs when there are alkali metal contaminants such as sodium and potassium present as well as sulfur. Sodium can be found, for example, if the plant is operating near the sea and there is salt water spray in the air. Sulfur usually comes from the fuel. Natural gas contains little sulfur but many natural gas plants are also designed to operate on liquid fuel too and this may contain some sulfur. Meanwhile oxidation is a natural process, the reaction between a metal and oxygen from air, that takes place more rapidly the higher the temperature.
In order to combat high temperature corrosion and oxidation, the normal strategy is to add elements to the alloy that help resist the corrosive reaction. However, these can have adverse effects on the high temperature performance of the alloy.
Coating the surface of the superalloy can help prevent or slow these phenomena. The most common type of coating is a diffusion coating, usually of an aluminum compound called an aluminide. This is formed by depositing a thin layer of aluminum onto the surface of a component made of a nickel or cobalt superalloy and heating it to around 1000°C. At this temperature some of the aluminum diffuses into the surface of the alloy to create cobalt-aluminum or nickel-aluminum compounds called aluminides which have higher corrosion and oxidation resistance than the superalloy alone. At the same time the aluminum will react with oxygen at high temperature to preferentially form aluminum oxide which also helps form a barrier coating. If this oxide layer becomes damaged and eventually flakes off, a new layer forms provided there is still aluminum present to form it.
These aluminide coatings rely on the substrate superalloy providing a part of the coating. An alternative is to apply an overlay coating that does not require reaction with the substrate. This type of coating can contain a variety of components that are tailored to provide the necessary resistance. In addition the thickness of the coating does not depend on the ability of the diffusion coating to react with the substrate. Overlay coatings can be laid down using a vacuum plasma spray method. A typical overlay coating will contain nickel (or cobalt), chromium, aluminum and yttrium.
5.6 Thermal Barrier Coatings
While the coatings discussed above are primarily designed to increase resistance to corrosion or oxidation there is another type of coating that is designed to improve the ability of the component to withstand higher temperatures. This type of coating is called a thermal barrier coating. Thermal barrier coatings are usually applied on top of an overlay or diffusion coating.
A thermal barrier coating is a thin layer of a material that has a very low thermal conductivity. When it is applied to a turbine blade it creates a thermal barrier layer between the hot gases from the gas turbine combustor and the superalloy blade itself. The thin layer will conduct heat only slowly. On its own, this confers no great advantage because eventually the heat will be transmitted to the component underneath and the latter will reach the external temperature. However, if the blade is also cooled internally, then the metal material can be kept within the temperature range at which it can operate while the barrier coating withstands the external temperature. These thin barrier coatings can support a temperature difference of hundreds of degrees centigrade across them.
The superalloys that are used to make turbine blades start to soften in the 1200°C to 1400°C temperature range. If a blade is coated with thin layer of a material such as zirconium oxide or yttrium oxide then the temperature at the metal surface can be kept below the softening temperature. In order to do this the thermal barrier coating must be able to support a temperature drop of 200°C or more across a very thin film.
Maintaining the integrity of the thermal barrier coating can be extremely difficult. It requires that the coefficient of expansion of the superalloy is closely matched to the barrier coating. Otherwise one will expand at a different rate to the other and the two will separate. The intermediate overlay coating for corrosion resistance may be used to help match the expansion coefficients across the surface layers. Thermal barrier coatings are used both on power turbine components and also on the inner surface of the combustor liner. The latter is cooled externally with air from the compressor.
5.7 Turbine Blade Cooling
In order for a thermal barrier coating to be effective, a turbine blade must be cooled internally so that the temperature of the underlying superalloy can be maintained below the point at which the metal will start to soften. Internal cooling involves creating cooling ducting and channels within the shaft or discs that support the blades to feed the cooling fluid to the hollowed blades. Depending upon the design there will also be holes and slots in the blades themselves to allow the cooling fluid to pass through them and out. This is shown schematically in Fig. 5.1.
The simplest form of cooling involves stealing some hot air from later stages of the power turbine or from the compressor and using this to cool the early stages of the power turbine. The cooling air enters the blade through its root and is vented into the gas turbine through holes in the blade. This air then forms a film across the surface of the blade which protects the blade material, hence its name, film cooling. Eventually this cooling air becomes mixed with the flow of air through the machine. Borrowing air in this way will reduce the efficiency slightly but offers the simplest way of providing cooling.
The alternative is steam cooling. This can be applied when the gas turbine forms part of an integrated combined cycle system with a steam generator. Steam is an effective cooling fluid and it carries a smaller efficiency penalty than using borrowed air. However, it is much more difficult to implement because the steam cooling circuit must be kept apart from the air passing through the turbine. Its use can also have an impact on the flexibility of the overall plant. As a result of the complexity, some companies use steam cooling for stationary components only and air cooling for rotating components. Steam cooling can also be applied effectively in the combustor.
5.8 Advanced Materials
Of an increase of roughly 500°C in power turbine inlet temperature over the past six decades it is estimated that around 150°C of the advance is due to the introduction of new superalloys and use of directionally solidified and single crystal fabrication techniques while a further 100°C can be attributed to the use of thermal barrier coatings. However, existing materials are now reaching the limit of their applicability and new ones are needed to enable temperatures to rise further. Candidates include using alternative metals as the basis for alloys and the use of ceramics.
Among the alternative metals that might be used are chromium, molybdenum and platinum. Chromium-based alloys are known to have higher melting points, good oxidation resistance and lower density that nickel-based alloys as well as thermal conductivity that is up to four times greater than many superalloys. However, chromium is extremely brittle at higher temperatures and this is exacerbated if it is exposed to nitrogen which is present in large quantities in air. This has so far prevented the development of chromium superalloys that might be suitable for gas turbines.
Molybdenum alloys also have a very high melting point and can be used in very high temperature applications. Unfortunately the metal oxidizes very rapidly above 500°C so the alloys can only be used in an oxygen-free atmosphere. Platinum-based alloys are attractive because of their stability at high temperatures, resistance to oxidation, ductility and thermal conductivity. Price is the main handicap that prevents more widespread use but it is envisaged that platinum superalloys may find application in some extremely demanding stationary components in gas turbines.
The other alternative is to use ceramic materials. Silicon carbide and silicon nitride have been considered as potential candidates since the 1960s. They can operate at much higher temperatures than metal alloys, they are resistant to corrosion and oxidation, they are much lighter than high temperature superalloys and they are much cheaper. However, all ceramics so far tested have been far too brittle to be useful. Until this problem can be overcome, it is unlikely they will find widespread application in gas turbines.