System evaluation for high temperature ultra supercritical steam power plants

High efficiency pulverized coal power plant technology, especially the system design for high temperature turbines, is discussed. It is believed that a 700-deg C class steam turbine system will be realized with Ni-based super alloys. The super alloys are more expensive than current materials like 12%Cr steels though its high temperature strength is excellent. In addition, the size of the alloys is limited for the current design because the quality of large Ni alloys is not high enough. Therefore, it is necessary to study new design concepts to utilize the alloys. The key concept is the optimized combination of steam turbine cooling and the super alloys. By adopting cooling to all turbines, it becomes possible not to use the super alloys for the turbines except valves, and relative thermal efficiency reduction from the system without cooling is 0.4%. If reheat turbines are cooled, the relative thermal efficiency reduction from the system without cooling is only 0.2% though it is necessary to use the super alloys for VHPT (Very High Pressure Turbine).