Analysis of Thermodynamic Performance and Contribution of a Double-reheat BEST Unit under Variable Working Conditions
In order to solve the problem of increased superheat degree and heat loss in the regenerative extraction steam after the main steam parameters were increased,taking a double-reheat supercritical unit as a prototype,a thermal system model for a back pressure extraction steam turbine(BEST)unit and a thermos-economics model were established,and the changes of thermal efficiency and exergy efficiency with loads of each heater for a traditional unit and BEST unit were compared.The contribution of boiler feedwater,main steam turbine and auxiliary steam turbine to the energy-saving effect of BEST unit was analyzed,and the impact of auxiliary steam turbine extraction and heating feedwater on the energy-saving potential was discussed.The results indicate that the BEST design scheme significantly reduces heater ex-ergy losses and optimizes boiler feedwater,which is the main factor in improving the thermal efficiency.However,a reduction in primary/secondary reheated steam flowrate decreases the output of the main tur-bine and weakens the energy-saving effect of the unit.The weakening effect can be offset by the auxiliary steam turbine.The contribution of auxiliary steam turbine to the thermal efficiency of BEST unit increases with decreasing unit load,thereby ensuring the energy-saving effect of the unit at low loads.
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