Developing Cobalt-Boron Alloyed Steel

Sande Stahlguss GmbH (SSG) manufactures steel castings mainly for energy generating purposes. Since 2011 SSG has been casting parts in a newly developed high alloyed 9% chrome cobalt-boron steel (called CB2, GX13CrMoCoVNbNB9-2-1). Due to the use of this material, the efficiency of power plants can be raised and C02-emissions can be reduced significantly.

In the face of the global growth of energy consumption and reduction of fossil fuel reserves, power plant operators and designers have been trying to improve plant efficiency throughout the entire industrialized world. Several industrialized countries have committed to reduce CO2 emissions. Just recently the U.S. administration confirmed its goal to reduce carbon-dioxide emissions by 30% by 2030. As nuclear energy in Germany will be phased out through 2022, a significant need for the construction of new fossil power plants and upgrades to existing fossil power plants is evident as renewable resources will not completely meet the rising energy demand in the medium run. As developing and newly industrialized countries are focused on the robustness of power plants and a favorable cost performance ratio, the industrialized countries are looking for measures to improve the power plant efficiency and reduce the ecological footprint. An example for a highly efficient power plant technology is the combined cycle power plant “Irsching 4” constructed by SIEMENS.

Globally, the average coal fired power plants have an efficiency of approximately 44%. The major part of energy input remains unused. One option to enhance the efficiency is to make use of the combined cycle cogeneration plants (CCCP). Doing this can raise the efficiency level by up to 50% (Fig. 1).

Notwithstanding, the design optimization of combined cycles dictates the efficiency of power plants can be enhanced by raising the operating temperature. The goal is to minimize the loss of heat and to operate turbines at higher temperatures and pressures. A proven side effect of enhancing efficiency by raising operating temperatures is the reduction of carbon dioxide emissions.

Conventional steels for energy purposes have to withstand a temperature of 560C (1,040F) and a pressure of 250 bar (3600 psi) in long-term operation. High alloyed 9% chrome steel, such as P91, are applied wherever the material has to withstand up to 600°C (1112°F) and 270 bar (3900 psi). The net-efficiency of power plants using P91-steel is roughly 45.3% (Fig. 2). By using the newly developed CB2-steel, the operating temperature can be raised up to 620C (1,150F), and the efficiency output meanwhile rises by nearly 1%, up to 46%. Enhancing the efficiency by one percent point implies a reduction of carbon dioxide emissions of 2.4 million metric tons CO2 based on a power plant operation time of 30 years and the use of a 700 MW turbine. Moreover, 2,000 tons NOx, 200 tons SO2, and 500 tons particulates could be saved.

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