Thermo-economic analyses on a new conceptual system of waste heat recovery integrated with an S-CO_2 cycle for coal-fired power plants

摘要

Waste heat recovery from boiler exhaust flue-gas is an effective way to save energy in coal-fired power plants. The integration of a low-pressure economizer (LPE) is a conventional choice for waste heat recovery. In this study, thermodynamic analyses of a coal-fired power plant (CFPP) integrated with an LPE is conducted on a 600 MW CFPP as reference case. Standard coal consumption rate (SCCR) of the power plant could be decreased by 1.76 g.(kW h)(-1) by the LPE. Exergetic analysis reveals that significant irreversibility is exhibited by the air pre-heater (APH) of the waste heat recovery system with an LPE. Guided by the exergetic analysis of the conventional waste heat recovery system, a new conceptual system for waste heat recovery integrated with an S-CO2 cycle for CFPPs is designed in this study. In this novel system, the boiler flue-gas is split into two flows: one to heat air in APH, and another to drive an S-CO2 power cycle as the heat source. Parameters of the S-CO2 power cycle are thermodynamically optimized with the help of Genetic Algorithm. Optimal initial pressure and pressure ratio are 9.136 MPa and 5.84, respectively. Maximum cycle efficiency of the S-CO2 power cycle is 17.39%. With the optimal parameters, SCCR of the CFPP integrated with the S-CO2 power cycle decreases by 3.80 g.(kW h)(-1). If the LPE is further integrated, the reduction of SCCR can reach 5.19 g.(kW h)-1. The essential reason for the significant energy saving is revealed through exergetic analysis. Exergy losses and destructions also decrease significantly in the novel waste heat recovery system. Finally, the economic performance of the proposed system is evaluated. Results show that the novel waste heat recovery system is economically suitable, and that capital investment could be recycled in 3.067 years.