Modelling of a supercharged semi-closed oxyfuel combined cycle with CO2 capture and analysis of the part-load behavior

Based on different sources a gas turbine model including a cooling model has been set up using the simulation software EBSILON®Â Professional. A gas turbine (GT) in simple cycle and a natural gas combined cycle (NGCC) including a triple pressure HRSG have been investigated and used as reference cycles. The NGCC has been modified to a semi-closed oxyfuel combined cycle (SCOC) with CO2 capture. Results from the ENCAP project [1] have been used to evaluate the simulation results of this work. As a novel configuration a supercharged SCOC process (S-SCOC) has been presented. Both design-point and part-load simulations have been performed. In design-point the NGCC achieves a net efficiency of 56.6 %. For the SCOC process the net efficiency is lowered by 8.3 %-points due to the capture of CO2. At part-load a sliding pressure (variable supercharge) has been introduced for the S-SCOC process. The sliding pressure for part-load leads to a constant volumetric flow rate through the GT over the whole load range. Therefore, the supercharged process yields a constant pressure ratio and a constant exhaust temperature at part load. Sliding pressure operation allows for a superior performance of the GT for the whole load range. The influence of a higher pressure of the exhaust gas stream on the heat transfer in the HRSG has been investigated. Fuel mass flow and flow conditions have been kept constant for the different processes. It has been found that the flow conditions for both processes (SCOC: constant density, volumetric flow rate changes; S-SCOC: vice versa) lead approximately to the same results for the Reynolds number on the flue gas side at part load. Although the conditions differ for the atmospheric SCOC process and the S-SCOC process with sliding pressure, the impact of part-load operation on the heat transfer is very similar. Since the efficiency of the supercharged GT hardly changes at part load, the net efficiency of the S-SCOC at part load remains very high. At 50% load the S-SCOC achieves an efficiency of 96 % of the design point efficiency compared to 91 % for the NGCC.