Materials of construction for steam temperatures of over 700 °C
In order to realise steam power plants with higher efficiencies, designers require components that use appropriate materials of construction that can withstand fresh steam conditions of over 700 °C and pressures of up to 350 bar. New design concepts and joining and sealing techniques need to be investigated to deal with these conditions. This applies to boiler parts, steam turbines, valves and pipes, for example. Development cycles and costs can be significantly reduced using improved simulation tools. This also helps to reduce manufacturing costs.
There have been numerous attempts in the past to develop steels that withstand high steam temperatures of up to 650 °C. So far, none of these development projects have really been successful. Specialists do not believe that iron-based materials will be able to achieve any significant improvement on current temperature ratings. However, a new materials concept based on so-called nickel-base alloys is opening up new possibilities.
Nickel-base alloys are extremely resistant even at high temperatures. This would allow for a significant increase in steam temperatures to over 700 °C. In turn, this would make efficiencies of over 50% possible in steam power plants in the future. It should also be taken into account that the increasing fraction of imported coal from various sources in the future will mean that the fuel quality will fluctuate to a greater extent. Power plant technology will have to be designed to be particularly flexible in order to achieve the highest possible efficiencies despite these fluctuations.
- Development of nickel-base materials for increased steam temperatures (similar research is being carried out in the USA and Japan)
- Further development of iron-based materials for applications in the evaporator of a 700 °C power plant (up to approx. 610 °C) and in the gas turbine up to 650 °C
- Development of improved coating systems to reduce oxidation, corrosion and erosion in the materials of construction employed in the high-temperature area of a 700 °C power plant
- Improved seal technology for a 700 °C power plant
- Production of large cast and forged parts
- Welding of thick-walled components
- Development of a suitable manufacturing process for large-diameter pipes
New design concepts and joining and sealing techniques need to be investigated to deal with fresh steam conditions of over 700 °C with pressures of up to 350 bar. The problem of corrosion takes on a new dimension at these temperature levels and also with the co-combustion of renewable fuels that is planned or is to be expanded.
Corrosion-protection layers must also be developed in addition to suitable materials of construction. Applied materials research in this area is making use of institutional research being carried out by major research organisations with the support of COORETEC project funding (particularly with regard to the certification of materials). Development cycles and costs can be significantly reduced by improving simulation tools. This also helps to reduce manufacturing costs
The prospects for conventional steam power plants that use coal as a fuel can be summarised as follows:
Highest possible efficiency
Efficiencies of over 50% will be achievable by 2020. This will lead to possible CO2 reductions of around 30% compared to the plants currently in service in Germany and reductions of 15% compared to those plants currently being constructed. The basis for these improvements will be provided by optimised process design and improved individual components with high component efficiencies, such as steam generators and steam turbines. The degree to which processes and components can be optimised ultimately depends on the resistance of the materials of construction and the stage of development of material technologies. Fuels with high moisture contents, such as domestic lignite, have not been able to achieve these efficiencies so far. However, pre-drying of coal in external coal-drying plants may make this possible in the future.
The increase in the burning of imported coal from various sources and of various qualities, the use of domestic coal with poor combustion properties and the co-combustion of biomass and substitute fuels will require adapted combustion and steam generation technologies.
On the other hand, the co-combustion of biomass and substitute fuels with a sufficient fraction of biogenous material is advantageous as this further reduces CO2 emissions. Moreover, these fuels can be used here with a significantly higher efficiency than would be the case with mono-combustion. In order to achieve optimal combustion, new monitoring methods for combustion and steam generation are necessary.
The design and operational monitoring of pressure components is accomplished using innovative methods.
Higher process parameters and the flexibility expected in terms of load-cycle and start-up behaviour have to be taken into account here.
If new nickel-base materials are used for the high-temperature applications in place of the iron-based materials that have generally been used up to now, it will be possible to increase steam temperatures to over 700 °C in the future. Consideration was first given to power plants with a steam temperature of 700 °C back in the mid-1990s. These plans were grouped under the banner of the AD700 project, which was co-funded by the European Community's 4th Framework Programme for Research. One of the most important findings of this project was that it established that 700 °C technology is economically viable.
Further research projects were also conducted in parallel, which were generally coordinated with activities under AD700. These include the German MARCKO DE2 and MARCKO 700 projects that were co-funded by the German Federal Ministry of Economics and Technology. In the past, superheater test facilities have been financed by participating industrial companies.
A project named COMTES700 was initiated to test key components of a 700 °C power plant on a large scale. This component-testing facility was installed at the Scholven power plant and went into service in the summer of 2005. Fittings and the turbine inlet valve were tested here, alongside important steam generators and pipe components.
The project was funded by partners from industry and by the European Commission within the framework of the "Research Fund for Coal and Steel" (RFCS).
The COMTES700 project is being supported by the two MARCKO projects. The European energy suppliers E.ON, RWE, EnBW, Vattenfall, EDF, Electrabel, Elsam, Energi E2 and PPC and the four manufacturers ALSTOM Power, Hitachi Power Europe, Burmeister & Wain Energy and Siemens are participating in COMTES700. At the same time, experiments are being conducted in a number of power plants to verify corrosion behaviour at high temperatures in a realistic flue gas atmosphere and to investigate the oxidation behaviour of the inner surface of pipes.
The NRWPP700 project – a detailed engineering study – was initiated in 2006 under the leadership of VGB PowerTech e.V., supported by the Federal State of North Rhine-Westphalia. Increased prices for raw materials – e.g. nickel – and the experience gathered in purchasing semi-finished products and in manufacturing components for COMTES700 have provided the impetus for a detailed examination of engineering design as part of the PP700 project.
New, resistant materials of construction will be necessary due to the rise in component temperatures of around 100 K in the superheater and in the steam reheater as part of the changeover to 700 °C steam turbines. Innovative austenites and nickel-base materials are the main candidates for use in superheaters at these increased temperatures. However, these materials still have to be certified. Ferritic/martensitic steels are particularly suitable for use in evaporators. The effect of ash deposits from the flue gas also has to be investigated.
Of particular interest is the issue of how deposits interact with the pipe materials under typical combustion and boiler-operation conditions in a 700 °C power plant. As in the case of 600 °C plants, programmes to investigate corrosion and fouling deposits as well as materials research and certification programmes will have to focus on 700 °C power plants.
The amount of knowledge available about 700 °C power plants is now sufficient enough to allow for the construction of a demonstration plant. However, the development potential has not yet been fully harnessed. Increasing steam conditions to over 700 °C is now considered to be technically feasible. Intensive work on this approach will continue. One of the goals of the ongoing research is to improve the composition of nickel-base materials so that steam temperatures of up to 800 °C will be possible.
An important contribution here would be the development of material models to optimise the composition of the basic materials accompanied by a corresponding improvement in welding filler materials.
Further verification work is to be carried out on 700 °C technology. For this purpose, a demonstration plant with an output of at least 500 MW is to be planned and constructed. Assuming that current research projects and those planned for the near future are successful, this plant could be realised by 2014. The groundwork for the detailed design of the demonstration power plant has been established by the PP700 pre-engineering study that was to be completed by the middle of 2008.
Numerous research projects will have to be carried out to put in place the prerequisites for the construction and operation of the demonstration plant. The goal here is to develop optimal materials and to certify the processes required for the production of high-temperature components.
14 current research projects
Testing of welded joints and of thick-walled components made of nickel alloys using non-destructive testing procedures
Organisations carrying out research:
Materials Testing Institute (MPA) Otto-Graf-Institut University of Stuttgart
Fraunhofer Institute for Non-Destructive Testing (IZFP)
Federal Institute for Materials Research and Testing (BAM) - Department 8 - Division 8.4 Non-destructive Testing; Acoustical and Electromagnetic Methods
Project number: 0327705R
Investigations of the long-term strength and deformation behaviour of pipes and forged parts made of nickel-base alloys
Organisation carrying out research: FDBR Forschungsstiftung, Fachverband Dampfkessel, Behälter- und Rohrleitungsbau e.V, Düsseldorf
Project number: 0327705Y
Long-term cyclic deformation behaviour of large steam turbine components in the high-temperature range
Organisation carrying out research: Siemens AG - Energy Sector PG P11M11, Hanover
Project number: 0327717D
Fundamental experimental investigations of corrosion and fouling deposits
Organisation carrying out research: University of Stuttgart - Faculty 4 Energy Technology, Process Engineering and Biological Engineering - Institute of Combustion and Power Plant Technology (IFK)
Project number: 0327744A
Investigation and modelling of steam-side oxidation and formation of protective coatings for materials of construction for 700 °C power plants
Organisation carrying out research: Materials Testing Institute (MPA) Otto-Graf-Institut University of Stuttgart
Project number: 0327744B
Mineral release, mineral conversion and transport of minerals to material, innovative wall-thickness measurements using ultrasound (can be used during operation)
Organisation carrying out research: Technische Universität Carolo-Wilhelmina zu Braunschweig - Fakultät 4 - Maschinenbau - Institut für Wärme- und Brennstofftechnik
Project number: 0327744C
Avoidance of corrosion on the combustion chamber walls
Organisation carrying out research: Technische Universität Darmstadt - Department of Mechanical Engineering - Department of Energy Systems and Technology
Project number: 0327744D
Investigation of the fundamentals of fouling deposits and corrosion on new materials of construction in 700 °C steam power plants
Organisation carrying out research: Forschungszentrum Jülich GmbH - Institut für Werkstoffe und Verfahren der Energietechnik (IEK-1) - Microstructure and Properties (IEK-2)
Project number: 0327744E
Strength and deformation behaviour of 625/700 °C materials of construction and welded joints of these materials under long-term realistic loading conditions
Organisation carrying out research: Technische Universität Darmstadt - Fachbereich Maschinenbau - Staatliche Materialprüfungsanstalt Darmstadt - FG und Institut für Werkstoffkunde
Project number: 0327754A
Analytical and numerical evaluation of welded joints between dissimilar materials
Organisation carrying out research: Materials Testing Institute (MPA) Otto-Graf-Institut University of Stuttgart
Project number: 0327754B
Development of models to describe thermo-cyclical behaviour
Organisation carrying out research: Fraunhofer Institute for Mechanics of Materials IWM
Project number: 0327754C
Investigations of the long-term operating behaviour of pipes and forged parts made of nickel-base alloys for the highly efficient power plants of the future
Organisation carrying out research: Grosskraftwerk Mannheim Aktiengesellschaft
Project number: 0327799
GKM test system - Subproject: 725 °C bypass valve – Testing of materials and technology at service conditions for steam control fittings for use in low-CO2 power plants with steam parameters at 725 °C
Organisation carrying out research: Welland & Tuxhorn AG, Bielefeld
Project number: 0327814
Service-life concepts and evaluation of failure mechanisms for high-temperature steam turbines
Organisation carrying out research: Siemens Aktiengesellschaft - Power Generation - Dep. P11M2, Mühlheim/Ruhr
Project number: 0327715P