Connected-top-bottom-cycle to cascade utilize flue gas heat for supercritical carbon dioxide coal fired power plant

For coal fired power plant, the supercritical carbon dioxide Brayton cycle (S-CO₂) is difficult to absorb flue gas heat in a wide temperature range of 120–1500 °C. Here, novel methods are developed to cascade utilize flue gas heat, in which energies in high, moderate and low temperature levels are extracted by top cycle, bottom cycle or flue gas cooler (FGC), and air preheater, respectively. The cascade utilization shall satisfy the criterion that CO₂ temperature entering boiler for top cycle equals to CO₂ temperature leaving boiler for bottom cycle. The separate-top-bottom-cycle (STB) is proposed, in which no any component is shared by top and bottom cycles. Six possible bottom cycles are studied. The thermodynamics analysis is coupled with heat transfer and pressure drop analysis for whole power plant. It is found that the main vapor pressure of bottom cycle can be the “best” parameter to be adjusted over a wide range of 15–35 MPa to couple and optimize top and bottom cycles. Then, the parameter coordination principle is proposed to share specific components for top and bottom cycles. Thus, the separate cycles are converted into a connected cycle to simplify the whole system layout. The connected cycle has a power generation efficiency of 51.82% at main vapor parameters of 700 °C/35 MPa, significantly higher than available supercritical water-steam Rankine cycle power plant. The findings in this paper give a clue to further raise the power generation efficiency for large scale S-CO₂ coal fired power plant.