Blade runner: improving gas turbine coatings
Tim Probert explores the changing nature of gas turbine operating regimes and the impact this has on materials selection, in particular the ceramic thermal barrier coatings applied to turbine blades.
The UK is set to close much of its coal-fired power generation capacity over the coming years due to the Large Combustion Plant Directive, which clamps down on sulphur dioxide, nitrous oxide and particulate emissions. Meanwhile, the UK’s much-vaunted new build nuclear programme has stalled, carbon capture and storage has yet to get off the ground, and the build-out of renewable capacity has begun to slow.
It is widely anticipated that gas-fired power plants will plug the gaps to keep the lights on. At around 36 months’ build time, combined-cycle gas turbine (CCGT) power plants are relatively quick to come online, cheaper than nuclear and, with lower carbon, nitrous oxide and zero sulphur dioxide emissions, more environmentally friendly than coal.
When the UK went on a dash for gas in the 1990s, plants were expected to provide baseload generation. The design of the gas turbines was for baseload, high-efficiency, low-cost electricity, but since then efficiency has been overtaken by other concerns.
In the near future, the grid will be dominated by gas, wind and nuclear generation. However, only gas plants can load-follow at scale. Wind power is intermittent, while nuclear power is relatively inflexible, running almost continuously at full capacity. This will result in gas plants running with more peaks and troughs. The subsequent reduction in capacity factors, ie the number of megawatt hours generated against the overall capacity of the plant, not only impacts on the capital cost of installing a gas-fired power plant but also on its reliability.
With efficiency no longer the only game in town, industrial gas turbine manufacturers are seeking ways to improve reliability and reduce the cost of parts. At the same time, utilities want to increase the service interval of their gas turbines to reduce downtime, but the trend towards flexibility makes this problematic.
Gas turbines have three main components: the compressor, combustion chamber and turbine. The drive towards higher efficiency requires higher combustion temperatures and subsequently the components are made from nickel-based superalloys able to withstand high temperatures.
Turbine blades go one step further, using state-of-the-art superalloys cast from single crystals, eliminating grain boundaries and therefore the main source of creep – critical for high-temperature applications. Superalloys have helped gas turbine thermal efficiency to reach 60% in combined-cycle operation, but even the most advanced cannot tolerate the conditions inside a turbine, where the gas temperature may reach 1,500°C and the hottest part of the blade may reach 950°C. To prevent blades from melting, their surfaces must be covered with a protective barrier called a thermal barrier coating (TBC).
Gas turbine TBCs are usually made from yttria-stabilised zirconia, a zirconium-oxide based ceramic chosen for its low thermal conductivity, high melting point, resistance to sintering and long life. TBCs are critical components in gas turbines because the gas temperatures are typically higher than the melting point of the underlying metal parts, so any TBC failure can endanger the turbine. Furthermore, TBCs are multifunctional, not only providing thermal insulation to protect the underlying superalloy engine parts, but also strain compliance to minimise thermal stress on the superalloy parts on heating and cooling, as well as reflecting much of the radiant heat from the hot gas, preventing it from reaching the metal alloy.
Thermal barrier coatings were developed primarily to allow power generation gas turbines to operate continuously at high temperatures. As their duty cycle becomes more irregular, towards a role of supporting renewable electricity generation, the changing mode of operation means there are more stops and starts, more ramping up and down, and more fatigue cycling of components, particularly the turbine blades.
With higher cycling, gas turbine blade fatigue and thermal stress issues become more dominant, particularly the high integrity, expensive single crystal alloys contained in the hot stages, which are designed to give good creep rupture performance and high strength at high temperatures to deliver continuous power demand.
The temperature variations have another impact. Parts of the turbines end up running for long periods in a temperature range for which they were not intended. They actually run cooler than expected, which diverts the hot gas path to areas of the turbine not designed to cope with it.
Moreover, increased cycling will exacerbate corrosion problems arising from fuel contaminants entering the turbine, which again may occur in unexpected locations in the turbine. Combined with the fluctuating stresses, it can lead to more complex damage such as corrosion fatigue or environmentally assisted cracking of turbine blades.
Research is now being conducted into how the chemical composition of TBCs can be tweaked to cope better with the changing workload. Altering the composition of TBCs, however, is not child’s play.
Rather than a single entity, TBCs are best thought of as a complex, interrelated material system, consisting not only of the yttria-stabilised zirconium oxide ceramic coating (topcoat), but also the underlying superalloy engine part, and two other layers in between. These include a metallic bondcoat layer that is more oxidation resistant than the superalloy and a thin, thermally grown oxide (TGO) layer that forms between the topcoat and the bondcoat as a result of in-service bondcoat oxidation. These intermediate barriers are necessary to prevent the coating from cracking as a result of thermal expansion.
The bondcoat also prevents oxidation and corrosion of the underlying alloy, as the yttria-stabilised zirconia topcoat will allow oxygen to pass through it, exposing it to oxidation and erosion.
Manufacturers are constantly fighting a battle between improving thermal resistance of TBCs while keeping them as thin as possible without degrading the life of the component, says Professor John Oakey, Professor of Energy Technology at Cranfield University, UK. He explains, ‘Thermal cycling is the killer for TBC coatings and potentially the bondcoats, because they don’t always have the ductility and the necessary fatigue-resistant properties.
‘What you want the TBC to do is to drop as much temperature over a very thin layer as possible, because you can’t make the coatings thicker on a rotating blade. This would result in an increase in weight of the components, which alters the stress distributions.’
A new recipe
Siemens, one of the world’s leading manufacturers and holder of the record for the most efficient gas turbine efficiency rating (60.75%), has begun to add rhenium to its bondcoat recipe to reduce oxidation, which can cause the thermally grown oxide layer to grow too thick and cause failure.
Rhenium is a rare metal characterised by a high melting point and high density. Adding 1–2% rhenium to a mixture of cobalt, nickel, chromium, aluminium and yttrium (MCrAlY coatings) forms a 300-micron-thick barrier of aluminium oxide on the MCrAlY surface that improves oxidation resistance.
Without it, the nickel base alloy in the blade would survive 4,000 hours of operation at maximum operating temperatures. With the coating, however, the alloy can hold out against the oxygen for more than 25,000 hours – longer than power plant operators demand as a minimum.
Siemens’ coating also serves as an adhesive agent for the ceramic thermal insulation layers, in conjunction with a special blade-cooling setup that blows air from narrow jets onto the blades, reducing the surface temperature on the metal in the first row of blades from 1,200°C to around 950°C. This allows turbines to be operated at a higher temperature and increased efficiency.
Fuel flexibility: an additional challenge
Professor Oakey notes that another flexibility issue is rapidly rising up the agenda – fuel flexibility. In the UK, which is accustomed to natural gas from the North Sea, the type of fuel used in gas turbines is changing. The increase in imported LNG and the possibility of shale gas in future are not all perfectly interchangeable. The different fuels burn differently, have different compositions and potentially differing contaminants.
Gas turbine power plants located near to the coast are more likely to ingest sodium through the air, increasing the chances of corrosion damage to blades. Similarly, sulphurous environments can also cause damage.
With increased fuel flexibility, however, Oakey expects more British gas turbines to suffer from calcium-magnesium-aluminium-sulphate (CMAS) attacks that cause discolouration in large gas turbines running on natural gas, due to contaminants fed into the turbine degrading the durability of the TBC.
Oakey continues, ‘Utilities will want to use the cheapest available fuel. The ability to handle different fuels, including biogas and biomethane, is another issue for manufacturers to contend with. From a materials point of view, the varying gases will have an impact on the life of coatings.
‘The impact is quite modest compared to the cycling frequency, but in terms of the plant itself, it is a further significant issue. The OEM guarantees performance of gas turbines based on utilising specified fuels, usually sweet natural gas.’
Are ceramic blades really the future?
While heat-resistant and heat-insulating protective coatings such as Siemens’ rhenium-enhanced bondcoat reduce oxidation and improve thermal efficiency, the enhancement of the thermal expansion coefficients of TBCs could also improve the coatings’ thermal cycling lifetime.
If gas turbine manufacturers were able to produce the high-temperature gas turbine blades themselves from ceramics, rather than merely the coatings, operating life could be significantly increased still further. Siemens’ research is focused on using oxide ceramics while other companies in the sector, such as GE, are opting for a base material of silicon carbide, whose structure and properties resemble those of diamond.
Silicon carbide is a high-strength material that has one key disadvantage – it oxidises when in contact with oxygen at high temperatures and oxygen is something gas turbines have plenty of. Siemens researchers are therefore focusing on the development of oxide ceramics that have already reacted with oxygen. The German firm says the lower rigidity of its material is not a drawback because the most important thing is its actual useful expansion, which is greater than that of silicon carbide.
However, success has so far proven elusive, due to the brittle nature of the ceramics and the complexities of working with them. Ceramic blades need to be reinforced if they are going to survive the minimum 25,000 hours of operation that customers demand of them. Siemens is therefore developing and testing fibre-reinforced ceramic matrix composites (CMCs) that are capable of withstanding higher temperatures than superalloys. The fibres provide a reserve for handling stress and keep the ceramic intact, even if it already has cracks in some places.
The combination of two brittle materials – a ceramic matrix and fibre – results in high tolerance to strain and damage. The oxidised fibres of aluminium oxide and silicon dioxide nevertheless remain the weakest link in the chain. Although they too no longer react with oxygen, they can only withstand temperatures up to 1,200°C. Ceramic alone can handle up to 1,700°C, therefore when used in certain gas turbine components it requires no cooling. The fibre compound thus has to be protected from the extreme temperature of the heated gas by a thick ceramic insulation.
In May, Siemens opened a facility in Charlottesville, Virginia, USA, for the commercial production of airfoil ceramic cores for gas turbine blades and vanes using tomo-lithographic moulding (TOMO) technology. TOMO is a manufacturing platform designed to enable rapid development and production of high-performance products made from metals, ceramics, polymers and composite material systems.
The technology uses lithography and lithographic machining techniques, and uses a photochemical process to create a highly defined 3D model of the finished part. It works by taking a computer-aided design file, slicing it into layers and putting those layers into a lithographer. The layers are put together to make airfoils, produce a mould and reproduce the parts in a cost-effective way. It allows for different airfoil designs with improved cooling characteristics, which leads to improved efficiency and higher operating temperatures.
The advancements are expected to improve the cooling capability of gas turbine blades, therefore enabling higher levels of engine performance and efficiency for future Siemens gas turbines. However, much like nuclear fusion, gas turbine materials made from CMCs are said to be almost 15–20 years away from reality.
The development of CMCs is ultimately essential if there is to be a stepchange in gas turbine performance. In the meantime, the quest continues for small, incremental improvements in materials systems more durable in complex environments and operational conditions in a changing energy world.