Design Research Of The Thermal System Of An1350MW Double Rehest Ultra Supercritical Cross-compound Unit

Double reheat generation technology is an important development direction in ultra supercritical parameter electricity generating technology, which has great utilization potentiality in the high parameter and large capacity generation units to be put into operation in the future. Designing the thermal system of the double reheat unit reasonably economically is of important significance to the implement of the double reheat generation technology and the embodiment of the efficiency advantage.This paper mainly focuses on the design of the thermal system and its parameters of an ultra supercritical parameter double reheat cross-compound unit. The steam’s main parameter is30MPa/600℃/620℃/620℃, and the steam turbine is placed in high and low platform. Several forms of thermal system such as extraction-backpressure boiler feedwater pump turbine(BFPT) system and condensing BFPT are designed basing on the fundamental principle of thermodynamic, with comprehensively considering the design problem of the boiler and turbine, and other parameters of the cycle are determined at the same time.The components of the modeling package Thermoflow, namely STEAM PRO, STEAM MASTER, THERMOFLEX, Thermoflow MACRO are used to modeling and calculating the thermal systems. The calculation of shows that:the1st reheat steam flow rate and2nd reheat steam flow rate of the extraction-backpressure BFPT system are less than those of the condensing BFPT system by266t/h and289t/h separately, which is to the benefit of the arrangement of the heat-absorbing surfaces in the double reheat boiler; as the steam bled from the main steam turbine is formerly expanded in the BFPT, its temperature(or superheat) is relatively low, which helps lower the heat exchange temperature difference in certain No.4and No.5heaters; but the heat rate of the extraction-backpressure BFPT system is6kJ/kWh more than that of the condensing BFPT system. Due to the greater1st and2nd reheat steam flow rate, the condensing system is to the disadvantage of the arrangement of the heat-absorbing surfaces. Exergy calculation of the main components of the above two system shows that:because of the bled steam temperature difference of No.4and No.5heaters, the exergy efficiency of the above two heater are different, the difference is7.47%and5.13%respectively. In the meantime, some other thermal systems are modeled and analyzed here such as back pressure BFPT system and Master Cycle.Calculation of the effect of1st reheat steam pressure and2nd reheat steam pressure on the plant gross efficiency and reheat steam flow rate of the back pressure system and conventional thermal system shows that:raising the1st reheat steam pressure(7.5～10MPa) and lowering the2nd reheat steam pressure(2.4～3.2MPa) can help improve the unit’s efficiency by about0.2%～0.3%. The calculation focused on the different combinations of steam source, bled steam discharge location and exhaust steam discharge location in the back pressure BFPT system shows that:the unit heat rate is lower by130kJ/kWh if the steam source is hot1st reheat steam or hot2nd reheat steam; but the inlet steam temperature will be very high(620℃) in the boiler feedwater pump turbine, causing the cost and operating price rising; the effect of steam source location on unit heat rate is greater than that of the bled steam or exhaust steam discharge location.