A Study on Superheat Utilization of Extraction Steam in a 1000MW Double Reheat Ultra-Supercritical Unit

Abstract

The world’s first 1000MW double reheat ultrasupercritical unit has been in operation since September 25th, 2015 in Taizhou, China. The thermal efficiency at turbine heat-rate acceptance (THA) condition is around 51%, which is the highest among all condensing units in coal-fired power plants around the world. However, the resultant superheat degree of the extraction steam is relatively high, leading to a large temperature difference in heat transfer process in the regenerative system, thereby a great exergy loss. In order to utilize the superheat of turbine bleeds more effectively, we present a scheme by employing an outer steam cooler (OSC) after the last high pressure heater in series to use the superheat to heat the feed water. Based on the newly installed unit in Taizhou, we examine the energy saving effect of the superheat utilization of different bleeds and their possible combinations respectively. The influencing factors of the mass flow rate, superheat, and effective superheat of the extraction steam are studied. Thermodynamic analyses revealed that the second extraction steam has not only high effective superheat, but also large mass flow rate, so in the overall efficiency improvement it ranks first and the third extraction steam ranks second. Although the fourth extraction steam has the largest superheat, it ranks third as the result of relatively lower mass flow rate. It was found that at nominal load, by adopting OSC’s to utilize the superheat of the second to sixth extraction steam, temperature of the feed water can increase by 8.1 °C, 3.5 °C, 2.6 °C, 1.1 °C, and 1 °C respectively, and the net coal consumption reduces by 0.73g/kWh, 0.47g/kWh, 0.40g/kWh, 0.21g/kWh and 0.22g/kWh accordingly. Consequently, three possible schemes are recommended for future design: one is to adopt one OSC to utilize the superheat of the second extraction steam, in return of 8.1°C increment in feed water temperature and 0.73g/kWh reduction of the net coal consumption; the second is to adopt two OSC’s to utilize the superheat of the second and third extraction steam at the same time, in return of 11.4 °C increment in feed water temperature and 1.21g/kWh reduction of the net coal consumption; and the last is to apply three OSC’s to utilize the superheat of the second to the fourth extraction steam simultaneously, to achieve 13.9°C increment in feed water temperature, and 1.62g/kWh reduction of the net coal consumption.