Coupling of the S-CO<sub>2</sub> cycle and ORC to realize an efficient utilization scheme of flue gas waste heat for the coal-fired power generation system

Abstract

<p indent="0mm">The multistage compression regeneration process can significantly improve the thermal efficiency of the supercritical carbon dioxide (S-CO₂) cycle. Because of the improvement of regeneration, when the multistage compression cycle is used for coal-fired power generation systems, the residual heat in the tail flue cannot be fully absorbed. To solve this problem, a combination of the tri-compression cycle with reheating and organic Rankine cycle (ORC) is constructed. By optimizing the energy flow process between the system and the environment, the system performance is gradually improved. First, the heat in the cooler of the S-CO₂ cycle and tail flue is partially absorbed by ORC (R123). Then, the outlet flue gas temperature (<italic>T</italic>_fg,ex) decreases, and the boiler efficiency increases. When the main vapor parameters of the S-CO₂ cycle are 620°C/<sc>30 MPa,</sc> <italic>T</italic>_fg,ex decreases from 132°C to 123°C, and the net power efficiency (<italic>η</italic>_e) increases from 47.13% to 47.24%. This combined cycle can effectively absorb the heat of flue gas at the tail flue; therefore, high boiler efficiency and net power efficiency can be obtained. Second, based on the coupling of the S-CO₂ cycle and ORC, the characteristics of the heat release process in the cooler of the S-CO₂ cycle under different temperature conditions are further analyzed. The air and ORC jointly absorb the heat in the high-temperature range of the cooler, and the net power efficiency is further improved from 47.24% to 48.90%. This study provides an effective method for residual heat absorption in the tail flue and cooler.