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Modern gas turbines used for stationary power generation applications make enormous demands on the materials from which their key components are constructed.
The high combustor and turbine inlet temperatures, and the enormous centrifugal forces experienced by the turbine blades make the conditions within parts of the turbine some of the most extreme that any modern materials have to withstand.
The materials that are able to withstand them are both complex and expensive. Even so their lifetime is limited and they must be monitored regularly to ensure that they remain within the design parameters of the machine of which they form part.
The development of these materials involves a long process, part genius, part trial and error, as chemical compositions are tweaked in the search for the properties required; be that greater mechanical strength at high temperature or better corrosion resistance, then the results arduously tested. The designers have a range of fabrication techniques available too, and these are all deployed in the quest for better, more durable materials that allow the operating parameters of gas turbines to be extended to achieve both greater efficiency and greater flexibility.
As a consequence of such technological expertise, today’s turbines are fabricated from an array of advanced alloys and coatings that are tailored to resist the specific conditions within the gas turbine.
These have become so complex, says John Oakey, professor of Energy Technology at Cranfield University in the UK, that the industry now talks about ‘material systems’ rather than materials. Matching all the material systems’ components is crucial to enabling modern machines to achieve the performance they do.
Aero and stationary engines
Advanced materials have played a key role in the development of gas turbines ever since their inception. The earliest gas turbines were aero engines, and stationary engines were developed using these as models. The conditions experienced by materials in the early stationary engines were less extreme than those found in aero engines, and so initially the materials developed for aviation engines would be passed down to the stationary versions. Today there is still a strong relationship between the two but materials development has diverged to match differing demands of the two engine types.
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An aviation engine experiences two stressful periods during its normal duty cycle, at take-off and landing. While the aircraft is cruising, conditions are much less stressful. Additionally, an aero engine can be examined every day or each week to monitor the condition of its components.
In contrast, a stationary engine, while it might not experience quite the same temperatures as the aero engine at take-off, must be capable of operating for greatly extended periods of months in some cases at close to its maximum operating conditions. As a complement of this duty cycle, components can only be monitored during planned outages.
An additional difference, highlighted by Jon Schaeffer, senior manager, Materials and Processes Engineering at GE, is found in the size of the components. Where an aviation component is perhaps half a kilogram in weight, a similar stationary engine component could be 13 or 14 kg. This has a major effect on the way components can be fabricated.
Take for example the technology for casting turbine blades as single crystals. This was originally introduced for aero engines to provide improved mechanical performance. However, transferring that to stationary engines was no simple task when the mass of material that must be formed into a single crystal has increased by a factor of ten or more. In order to cope with such differences, development of industrial turbine components now follows a different path to that of aero engine components.
Every gas turbine has three main components: the compressor, combustion chamber and turbine. Each makes its individual demands on the materials from which it is constructed, but of the three the compressor is perhaps the least demanding. In an aero engine, all the components including the compressor need to be light and in the case of the latter much of it is made from titanium alloys.
Titanium is light but titanium alloys do not have the high-temperature performance of alternatives and so cannot be used in the high-temperature regions of the turbine and therefore other alloys are used. By contrast, weight is less of an issue in stationary applications and so here compressor components are often made of cheaper but heavier steels.
The turbine combustor experiences the highest temperatures of any part of the engine but it is not subject to high degrees of stress so relatively conventional materials can be used in its construction too. Not that any of the modern turbine materials are particularly conventional. In the case of the combustor, the material will usually be a nickel-based superalloy; the name ‘superalloy’ meaning that it is one of a class of modern alloys that have been developed specifically for high-temperature applications.
However, even the best superalloy is incapable of withstanding the gas temperatures in an advanced turbine combustor. Here, the metal must be cooled while at the same time its surface protected with a ceramic layer capable of resisting the high gas temperatures.
Professor Oakey has observed the trends within turbine materials development and the trade-offs that must be made. Excessive cooling air used in the combustor or turnine parts has the effect of reducing overall efficiency. One way of avoiding this, he notes, might be to use ceramic materials such as ceramic matrix composites (CMCs), which are capable of withstanding higher temperatures than alloys. However, in spite of various European development projects, combustors are still built with superalloys and there is no sign of CMCs being introduced in the immediate future.
While the combustor may be the hottest part of the complete gas turbine, it is within the turbine stage that the most extreme combination of conditions occurs and it is here that materials development has needed to be at its most creative.
The temperature of the gas entering the turbine is higher than the melting point of the superalloy components from which it is made so again thermal protection is needed.
Then half of the turbine blades are rotating at high speed, generating a massive centrifugal force that creates stress along the length of the blade and leads to creep, damage, which results in slow deformation of the blade under the extreme load.
The individual blades are held in place by the turbine wheel and it is this that experiences the most extreme load, particularly in the later turbine rows using the longer and more massive blades. In each case materials have to be found that can resist the various conditions to which they are exposed.
Single crystal superalloys
While the future may yield ceramic components for gas turbines, today the materials technology revolves around two main areas, alloys and coatings. The alloys used to fabricate turbine components are complicated chemical mixes with each component in the mix included to achieve a particular purpose. However as the demands increase, so trade-offs become necessary.
Stainless steel offers an example. Normal stainless steel contains a small amount of chromium which forms a chromium oxide layer over the steel that protects it from further oxidation. However chromium oxide will not protect the steel above a certain temperature, and so for high-temperature alloys, aluminium is included. It is this that forms a protective layer, in this case of aluminium oxide.
Unfortunately, as the aluminium migrates to the surface of the alloy to form this layer it leaves a depleted layer in the underlying alloy material.The most prevalent superalloys for high-temperature turbine components are nickel-based. Since turbine components are often complex in form, they are usually cast and this is how they have generally been produced from superalloys.
|During operation, large volumes of air are pumped from inside via these openings, creating the cooling that flows evenly over the blades as a film of air
Traditional casting produces a solid metal mass with a structure of tiny microcrystals, or grains, as solidification takes place from seeds throughout the structure. These grains are held together by relatively weak forces, limiting mechanical strength, so elements such as carbon may be added to strengthen the bond between grains and improve mechanical performance.
It is clear that eliminating grain boundaries would help increase mechanical strength and the first method used to achieve this with gas turbine components was by utilising a technique called directional solidification. This involves cooling the casting at a controlled rate with the liquid/solid interface engineered to migrate slowly along the length of the component. Such controlled solidification leads to an oriented grain structure along the length of the piece so that grain boundaries across the width of the component are eliminated. This produces a component that has greater mechanical strength in the longitudinal direction; an advantage in a turbine blade that has to withstand massive centrifugal forces.
It is but a short step, conceptually, from directional solidification to single crystals. If a turbine blade can be made from a casting that is a single crystal, all grain boundaries are eliminated and the main source of creep, i.e. the creation of voids at grain boundaries, is eliminated. The melting point of the alloy is also increased allowing higher-temperature operation, while the elemental additions such as carbon, required to strengthen grain adhesion, can be eliminated.
Taking that conceptually simple step in reality involved an enormous degree of technical ingenuity. By careful management of crystal growth such that only a single crystal-forming dendrite from within the large crystallising mass of superalloy is allowed to enter the casting chamber, single crystal components can now be fabricated. In terms of superalloy components these are the state of the art today and are used when the highest resistance to creep at high temperature is needed.
Rotating turbine blades experience the greatest mechanical forces and these are fabricated from nickel-based superalloys because these offer the highest mechanical strength at high temperatures. However the nickel superalloys cannot withstand such high temperatures as some cobalt-based materials. While these offer less mechanical strength, they are ideally suited for the early stage fixed vanes in a gas turbine since these are subject to the highest temperatures of all within the turbine.
Meanwhile the large, last stage blades, exert the greatest force on the turbine wheel that holds them in place because of their greater mass and here GE is introducing titanium because of its lower mass.
Innovations in blade cooling
While superalloys have allowed great advances to be made, even the best superalloy cannot withstand the conditions inside a turbine where the gas temperature may be as high as 1500ºC and the hottest part of the blade reach 950ºC. In order to prevent blades from melting their surfaces must be protected by insulating coatings and then they must be cooled.
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Modern turbine blades from the hottest part of an engine are hollow so cooling air (or in some cases steam) can be pumped through them. This air is taken from the compressor stage of the turbine and so reduces overall efficiency. The air must be pumped through both the stationary vanes and rotating blades and in the latter case this leads to significant complexity as the air must be introduced into a component rotating at high speed.
The air, once it has cooled a blade, must then exit the blade. In most cases this takes place through holes in the back of the blades where it joins the hot combustion gases passing through the turbine. However, Professor Oakey says state-of-the-art turbines now use a different strategy called ‘film cooling’. This involves exhausting the cooling air through the front edge of the blade and elsewhere on the aerofoil surface so it forms a cooling film over the surface of the blade.
Blades for later stages often do not need to be cooled and can be solid. However all gas turbine blades are now extremely complex and expensive items, and they have to be capable of being repaired or refurbished and re-used.
Cooling air within a turbine blade can keep the body of the metal component below its melting point but if the surface was exposed to the highest gas temperature it would soon be softened and eroded.
To prevent this, the surface of the blade is coated with a ceramic insulating material (e.g. 0.3mm thick) capable of withstanding the gas temperature and also of creating a temperature drop of more than 200ºC to the metal surface, depending on the coating thickness. These thermal barrier coatings (TBCs) were first developed for aero engines but are now widely deployed in stationary engines too, another example of technology transfer between the two industries.
Although recipes vary from company to company, a typical TBC would be made from yttria-stablised zirconia, says Professor Oakey. According to Siemens, its coatings can withstand a temperature of 1200ºC, or even as high as 1350ºC for their most advanced coatings.It is not possible to put a TBC of this type directly onto the superalloy metal surface, however because they have different thermal coefficients of expansion; one will expand at a different rate to the other. That means the coating will soon crack and flake away. To avoid this an intermediate barrier coating is necessary. This serves two purposes. The first is to minimise the effects of the different expansion coefficients – which is why such coatings are sometimes called ‘bondcoats’. The second purpose is to avoid oxidation and corrosion of the underlying alloy. The TBC layer at high temperature will allow oxygen to pass through it so without protection, the metal beneath would soon be oxidized and eroded.
This barrier layer is normally composed of what is referred to in the industry as ‘MCrAlY’ (metal-chromium-aluminium-yttrium).The ‘metal’ can be nickel, cobalt, iron and a number of other exotic ingredients, such as rhenium, which Siemens has used. The aluminium in this coating selectively oxidises, creating a protective oxide layer. Meanwhile, rhenium improves the mechanical strength of the coating and also helps reduce the diffusion of aluminium into the superalloy substrate.
Changes in inspection frequency
The mechanical strength of the coating is an increasingly important issue in modern combined-cycle power plants (CCPPs) because whereas in the past they would often have operated continuously for long periods, their duty cycle is becoming much more irregular as they move towards a role of supporting renewable electricity generation.
The changing mode of operation means there are more stops and starts, more ramps up and down, and more fatigue cycling of components. This can lead to coating failure. Failure can also be caused by the aluminium oxide protective coating growing too thick. Neither process can be eliminated and therefore even the best blade coatings have a limited life.
Thus, not only are turbine components threatened by extended high-temperature operation leading to creep deformation of components, thermal cycling and other operational issues can cause coating failure. Furthermore, modern gas turbine fuels are not as clean and reliable as they once were. Turbines can find themselves burning fuels with unexpected impurities that can cause other types of corrosion and this too will threaten coating integrity.
With so many processes threatening the integrity of a gas turbine component, it is accepted today that they will age and deteriorate. To combat this, turbine blades are regularly removed and inspected. A typical blade lifecycle will involve it being removed every few years (manufacturers are aiming at three years), all the coatings on the blade stripped, the metal inspected and refurbished if necessary, and then recoated and returned to service.
This type of recycling allows blade life to be extended but eventually the distortion of a blade because of stress will become so great that it is no longer useful. In order to keep track of this, each blade must be numbered and a history maintained so that it can be withdrawn after a fixed number of hours, after which the blade will be destroyed and the material it is made from reused.
Are ceramic blades the future?
The demands placed on gas turbine components is constantly changing as operating conditions change, fuels change and as designers try to wring more performance from them by increasing operating temperatures and pressures. Whereas five years ago the primary goal was higher-temperature operation, today issues such as being able to survive load cycling and contaminated (but cheap) fuels are becoming the primary issues.
These problems are being solved with complex metal components and multi-layered coatings but it has long been a dream to manufacture high temperature gas turbine components from ceramics. This is still an active goal for development in institutions and within the major gas turbine manufacturers but success has been extremely elusive because while ceramics are extremely temperature resistant they are brittle and difficult to work with.Two types of ceramic are being actively explored. Siemens, for example, is building on experience gained in Westinghouse, which the company now owns, to pursue oxide-based ceramics. GE, meanwhile, has been working on ceramics based on silicon carbide.
Ceramics on their own do not have the structural properties needed for reliable turbine components so much development is directed towards producing CMCs, as mentioned by Professor Oakey, in which the ceramic is reinforced with a second fibrous ceramic that endows it with increased strength. This composite does have its drawbacks. The ceramic alone can withstand a very high temperature of up to 1700ºC. However the reinforcing materials are not so durable at high temperature so they must be insulated to protect them.
Work on ceramics has been continuing for many years now. While there are promising signs, it is probable that it will be many years yet before major gas turbine components are made from CMCs. Yet however distant their practical realisation, they will eventually be needed if progress is to continue. Even with the best will and the greatest ingenuity, superalloys cannot be expected to operate at much higher temperatures and yet it is higher temperatures that will be necessary to improve efficiency further.
Paul Breeze is a UK-based freelance journalist, who writes on energy-related matters.
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