Off-Design modelling and part-load control strategies of the semi-closed oxy-combustion combined cycle

The semi-closed oxy-combustion combined cycle (SCOC-CC) is a promising oxy-fuel combustion power cycle designed to achieve near-zero emissions. The power plant operates on a Joule-Brayton cycle, in which a portion of the exhaust gases is recirculated to moderate the temperature of the combustion of natural gas with oxygen produced by an air separation unit. This study presents a detailed evaluation of the part-load performance of the SCOC-CC, built upon the design proposed by the authors in a previous study. A rigorous part-load model of the twin-shaft gas turbine is developed in Modelica, integrating operating maps for the compressors and accounting for turbine cooling flows at partial loads. Two different control strategies for the gas turbine are compared. The gas turbine part-load model is validated using measured data from a real twin-shaft gas turbine. Additionally, a part-load model of the heat recovery steam cycle is developed using Thermoflex®, while the CO2 capture and compression unit is modelled in Aspen Plus®. The SCOC-CC achieves a net electric efficiency of 51.11% at full load, which decreases to 45.51% at the minimum load of 34.3%. The part-load strategy with a higher turbine outlet temperature improves electric efficiency by approximately 0.5 percentage point.