USC Steam Turbine technology for maximum efficiency and operational flexibility

State of the art ultra supercritical (USC) steam parameters provide highest efficiencies (i.e. lowest fuel costs and emissions like CO2) for coal fired steam power plants. At the same time these plants also face the requirements of deregulated electricity markets with an increasing portion of renewable energy. Thus the units have to be capable of flexible load operation and frequency support. The overload admission has proven to be very effective for flexible turbine operation at an excellent heat rate. Both main turbine components and controls have been optimized to minimize thermal stresses, leading to an improved start-up performance. Despite increased steam parameters and number of load cycles and starts maintenance intervals have not been shortened. As a result overall life cycle costs are minimized. Siemens has more than 10 years of experience with USC steam turbines and continues to optimize associated designs and technologies. This paper presents the current Siemens steam turbine technology and products for USC applications and references for operating units. Copyright © Siemens AG 2008. All rights reserved. 2 Introduction According to common forecasts, the worldwide demand for power will increase significantly over the next two decades, and the current power plant capacity will need to double by the year 2030. A considerable share of this huge increase in demand has to be covered by coalfired units. To save primary energy resources i.e. to reduce fuel consumption, and to reduce emissions, maximum power plant efficiency is a crucial parameter for power plants. Therefore steam parameters will have to be maximized to an economically reasonable extent. State of the art temperatures and pressures for ultra supercritical (USC) applications are 600°C at 270 bar for the main steam and 610°C at 60 bar for the reheat steam. Furthermore, the share of renewable capacities will increase further to fulfill global emission reduction targets. As a result electric grids will face a growing share of volatile capacities. This again requires additional flexibility of the conventional units and, in addition to combined cycle units, coal-fired units will also operate in the mid merit market. Thus these units will experience an increased number of starts and load changes, including rapid load changes, especially within small electric grids. Siemens USC steam turbines 50 Hz full speed tandem compound turbo-sets for USC steam power plants (SPP) are available for gross power outputs from 600 – 1200 MW per unit. The steam turbine set-up for ultra-supercritical applications depends on the unit rating, the number of reheats selected, and the site backpressure characteristics. A typical turbo-set comprises three separate turbine modules operating at different pressure and temperature levels. These modules are the high pressure turbine (HP), intermediate pressure turbine (IP) and, depending on the cooling water conditions, one, two or three low pressure turbines (LP). The generator is directly coupled to the last LP turbine. Figure 1 shows an SST-6000 series Siemens steam turbine with two LP turbines. A comprehensive description of the specific turbine features of the whole turbo-set is given in [1]. Copyright © Siemens AG 2008. All rights reserved. 3