Thermodynamic analysis of oxy-fuel combustion integrated with the sCO2 Brayton cycle for combined heat and power production

Abstract

The supercritical CO2 (sCO2) Brayton cycle is highly efficient in terms of its power production efficiency and is also compact in structure. It is therefore expected to substitute the steam Rankine cycle in various applications. In addition, the high temperature heat that is disposed to the heat sink in the sCO2 cycle can be further utilized to increase the economy and profitability of the plant. In this paper, we report the integration of the sCO2 cycle with single-stage reheating and recompression with the pulverized coal oxy-fuel combustion process for combined heat and power production. The integrated system was analyzed in terms of its thermodynamic performance, including its electrical efficiency and heating efficiency. For oxy-fuel combustion, different flue gas recycling modes, i.e., wet mode and dry mode, were compared. Under the basic sCO2 cycle conditions of 30 MPa/600 °C/600 °C, the electrical efficiencies for the wet and dry modes were 36.6 and 35.3%, respectively, while the corresponding heating efficiencies were 15.4 and 14.9%, respectively, with a total efficiency of 52.0% for the wet mode and 50.2% for the dry mode. Furthermore, the CO2 capture rate was determined to be ~99.94%. Sensitivity analysis was conducted, and the influences of the turbine inlet temperature, turbine inlet pressure, oxygen concentration, turbine outlet pressure, and split ratio in the high-temperature recuperator were investigated. Finally, the residual heat from the air separation unit and the flue gas was recovered and integrated with the sCO2 cycle, and it was found that the heat recovery decreased the split ratio to recompression and could add a 0.3–2.7 percentage point increment to the electrical efficiency, in addition to a 0.3–2.1 percentage point increment to the heating efficiency.