Performance of a Semi-Closed Oxy-Fuel Combustion Combined Cycle (scoc-cc) with an Air Separation Unit (ASU)

The objective of this paper is to evaluate the performance of a semi-closed oxy-fuel combustion combined cycle (SCOC-CC) and its power penalties. The power penalties are associated with CO₂ compression and high-pressure oxygen production in the air separation unit (ASU). The paper discusses three different methods for high pressure oxygen (O₂) production. Method 1 is producing O₂ directly at high pressure by compressing the air before the air separation takes place. Method 2 is producing O₂ at low pressure and then compressing the separated O₂ to the desired pressure with a compressor. Method 3 is alike the second method, except that the separated liquid O₂ is pressurized with a liquid oxygen pump to the desired pressure. The studied SCOC-CC is a dual-pressure level steam cycle due to its comparable efficiency with three pressure level steam cycle and less complexity. The SCOC-CC, ASU and CO₂ compression train are modeled with the commercial heat and mass balance software IPSEpro. The paper analyzed the SCOC-CC performance at different combustion outlet temperatures and pressure ratios. The combustion outlet temperature (COT) varied from 1200 °C to 1550 °C and the pressure ratio varied from 25 to 45. The study is concerned with mid-sized SCOC-CC with a net power output 100 MW. The calculations were performed at the selected design point which was at 1400°C and pressure ratio at 37. The calculated power consumption of the O₂ separation at a purity of 95 % was 719 kJ/kgO₂. The power consumption for pressurizing the separated O₂ (method 2) was 345 kJ/kgO₂ whereas it was 4.4 kJ/kgO₂ for pumping liquid O₂ to the required pressure (method 3). The calculated power consumption for pressurizing and pumping the CO₂-enriched stream was 323 kJ/kgCO₂. The SCOC-CC gross efficiency was 57.6 %. The SCOCCC net efficiency at method 2 for air separation was 46.7 %. The gross efficiency was reduced by 9 % due to ASU and other 2 % due to CO₂ compression. The SCOC-CC net efficiency at method 3 of the air separation was 49.6 %. The ASU reduced the gross efficiency by 6 % and additional 2 % by CO₂ compression. Using method 3 for air separation gave a 3 % gain in cycle efficiency.