System modification and thermal efficiency study on the semi-closed cycle of supercritical carbon dioxide

Supercritical carbon dioxide (sCO₂) semi-closed cycle is an advanced, efficient, and economical power generation system that can meet the needs of low cost and efficient utilization of coal and low carbon emissions. The Advanced System for Process Engineering software (Aspen Plus) was used to simulate different sCO₂ systems. In the simplified sCO₂ closed cycle, multi-stage compression and inter-cooling of CO₂ compressors together can increase the system efficiency by about 5%. The system thermal efficiency increases by 0.5% with every 10 bar increase of the combustion chamber pressure between 200 bar and 500 bar, and increases by 0.35% with every 10 °C increase of the combustion temperature between 800 °C and 1500 °C. In the typical sCO₂ semi-closed cycle–Allam cycle, the gas temperature in the combustion chamber decreases with the increase of CO₂ backflow, and it would decrease with increase of the excess O₂ ratio. The system thermal efficiency could be improved by 3% by adding reheat and graded expansion, and by 2% with ASU coupling and CO₂/O₂ mixing. Adding heat exchangers and proper stream distribution can improve the system efficiency by up to 10%. When the combustion chamber temperature is 1300 °C, turbine inlet pressure is 300 bar, turbine outlet pressure is 30 bar, and the split ratio of CO₂ is 5:95, the sCO₂ semi-closed cycle with precompression, recompression or partial cooling reaches the highest thermal efficiency (~64%). Depending on the results of the system modification and sensitivity analysis, ideas can be provided for the improvement and demonstration of the system.