Materials for turbines and zero-CO2 power plants
Ceramic thermal barrier coatings boost the efficiency and lifetime of turbines in advanced power plants. Gas separation membranes have a high potential for efficient CO2 separation.
- Thermal barrier coatings
- Gas separation membranes
Thermal barrier coatings
Thermal barrier coatings are an integral part of both aircraft and stationary gas turbines, since only these coatings can withstand the high fuel gas temperature needed for efficient operations. The standard that has been established worldwide for thermal barrier coatings is yttria-stablized zirconia (YSZ) that has been partially stabilized with 7-8 weight % Y2O3.
This material has a number of outstanding properties for this application, including low thermal conductivity, high expansion coefficients for reducing thermal tensions in the composite with the metallic substrate and good fracture toughness.
At present, two manufacturing processes are used in industrial series production of thermal barrier coatings: electron beam physical vapour deposition (EB-PVD) and atmospheric plasma spraying (APS).
Gas separation membranes
In this research area, IEK-1 is concerned with the development of inorganic membranes for gas separation. These membranes will allow the separation of the technically relevant gases O2/N2 CO2/H2 and CO2/N2 for various fossil power plant concepts, in order to achieve a CO2 flow that is as pure as possible and free from residual gases and which is to be subsequently stored underground. In contrast to conventional separation methods, membrane processes have the advantage that they cause significantly lower losses in efficiency than methods such as chemical gas scrubbing. This makes them particularly interesting for applications in power plants.
For each separation task, different ceramic membranes are developed at the institute. Among the different inorganic membrane types, a distinction is made between dense ion-conducting or mixed-conducting membranes and microporous crystalline or amorphous membranes.
All membrane types have development needs with regard to permeability, selectivity and stability. IEK-1 is concerned with the development and production of new materials and membrane coatings using different process technologies.
Other coating developments
Barrier coatings for composite fibre materials
Environmental barrier coatings (EBCs) are employed when high thermal stress and corrosive atmospheres makes the use of components difficult or impossible. They are used in thermal barrier systems of space re-entry vehicles, as seeker head covers for highly agile missiles, or as coatings for combustion chamber shingles in aircraft gas turbines. When used as barrier coatings in gas turbines, they not only provide protection for the particular components, but frequently also boost efficiency and reduce emissions.
Abradables, or abradable coatings, have been used since the 1960s, primarily in compressors and high-pressure turbines of aircraft engines. Recently, however, they also have been increasingly used in stationary gas turbines.
An abradable is a ceramic coating that is applied to the stationary parts of a turbine (guide vanes) and in which the rotating parts (in this case, the blade tips of the turbine rotor blades) can "abrade" or "run in" during operation. In this way, the gap width between stationary and movable turbine parts can be minimized, which allows performance and efficiency of the turbine to be significantly increased. This in turn results in reduced fuel consumption. Figure 1 shows a stationary guide vane manufactured by Rolls Royce, IEK-1's project partner. Atmospheric plasma spraying was used to apply a ceramic abradable coating to the guide vane.
At IEK-1, suitable materials and process parameters for manufacturing these types of abradable coatings are developed in collaboration with Rolls Royce. Abradable coatings have to meet particularly tough requirements. On the one hand, they must demonstrate good abradable behaviour (which requires certain porosity, meaning that the coatings must not be too hard). On the other hand, they must of course also be mechanically stable and withstand the high operating temperatures within the turbine.
Yttrium-stablized zirconia is frequently used for abradable coatings. Since the application limit for coatings made of this material is reached at approx. 1200°C, the focus of our institutes's research activities is on the development of coatings made of aluminium magnesium spinel, which can still be used at operating temperatures of over 1300°C.
Optimizing such coatings until they are ready for industrial application is a main focus of IEK-1's research activities.