New material for power plants

Working with E.ON and Hitachi Power, engineers at ThyssenKrupp VDM have developed an alloy that could improve the efficiency of coal-fired power plants while reducing carbon-dioxide emissions.

The efficiency of power plants can be increased by raising the steam temperature in the boiler and turbine from 600oC (at a pressure of 250 bar) to around 700oC (350 bar) – but that is a temperature that the materials currently used in power plants cannot withstand permanently.

To resolve that issue, ThyssenKrupp VDM researchers have taken an existing nickel-based steel alloy and significantly enhanced its properties.

The result of their work is Alloy 617B, with an optimised chemical composition (OCC) that offers high strength and ductility at the same time as good weldability. In the power plant of the future it could be used in tubes, valves and connectors, as well as in the form of sheet and plate.

Alloy 617B OCC owes its special properties to three factors. First, it is melted and remelted in a vacuum, preventing the pickup of undesirable substances from the air. Second, it contains a minimal amount of boron, whose content has to be precisely defined and controlled. And finally, its molybdenum and carbon contents have been optimised to enhance weldability while retaining the same strength.

A precursor of the now optimised Alloy 617 B OCC has been tested successfully for around five years in various laboratories and pilot plants. The results and experience gained played a major role in the development of the new material. From 2012, the high-performance material is to be tested in pilot plants in parallel with further laboratory investigations.

’To validate the lab tests it is necessary to manufacture components from the nickel alloys and test them under actual power-plant conditions,’ said E.ON Energie’s Helmut Tschaffon. ’This includes demonstrating that the materials can withstand the normal operating stresses in a power plant, such as startups, shutdowns and load changes.’

’We are confident that the positive properties of Alloy 617 B OCC will be confirmed in the tests and that we can start marketing the material soon,’ said Dr Jutta Klöwer, head of research and development at ThyssenKrupp VDM. ’This takes us a big step closer to our goal of building more efficient, low-CO2 power plants.’