Advanced Steam Turbine Technology for Unique Double Reheat Steam Power Plant Layout

Abstract

Environmental aspects have increased the pressure on the fossil power generation industry to reduce carbon dioxide (CO2) emissions. One way to achieve this is by increasing the overall plant efficiency, which also fosters an economical plant operation. How can the efficiency of a next generation coal fired ultra super critical (USC) steam power plant (SPP) be increased significantly in the nearest future while maintaining its familiar reliability and availability at the same time? In China’s national USC SPP demonstration project, Pingshan Phase II, this challenge is met by a double reheat cross compound turboset with one elevated and one conventional turbine layout, together with increased steam parameters of up to 325 bar and steam temperatures of up to 630°C. The nominal electrical capacity of the plant will be 1350 megawatts (MW). With this set up, a ‘half-net’ efficiency of more than 52.2 percent is expected [‘half-net’ = gross efficiency with generator power reduced by boiler feed water pump power consumption]. The first, elevated turbine train consists of two high-pressure modules having different pressure stages and one generator and it is located close to the main headers of the boiler at a height of appr. 83 meters. This unique turbine arrangement allows the expensive high-temperature pipes to be shortened, leading to substantially reduced pipe pressure losses and costs. The second turbine train will be installed on a conventional turbine deck at a height of appr. 17 meters and consists of two intermediate pressure and three low pressure turbine modules as well as a second generator. In this paper, the advanced steam turbine technology for this power plant concept is presented and discussed in detail. To achieve the next level of efficiency with an SPP today, the application of the 700°C material class is not possible to due to the slow progress of the associated technology development. It is more expedient to exploit the limits of the 600°C material class to the highest possible extent in USC conditions i.e. to the pressures and temperatures mentioned above. Design concept studies have shown that 52.2% ‘half-net’ efficiency cannot be achieved with a single reheat layout, so a double reheat (DRH) layout has been chosen. In addition, 1350 MW cannot be achieved with one turbine train (tandem compound), but only with two turbine trains (cross compound). In order to achieve the highest reliability possible, proven turbine design topologies and features have been used. The major change to the Siemens barrel type VHP turbine was a material change from 10% Chromium steels to FB2 and CB2. The HP turbine received increased wall thicknesses as well as a similar material change compared to a standard USC design. In order to control the oxidation at these elevated temperatures, oxidation protection measures have been applied where required. The startup procedure has been tailored specifically to the needs of a double reheat cross compound configuration.