A Thermodynamic Analysis of Two Competing Mid-Sized Oxyfuel Combustion Combined Cycles

Abstract

A comparative analysis of two mid-sized oxy-fuel combustion combined cycles is performed. The two cycles are the Semi Closed Oxy-fuel Combustion Combined Cycle (SCOC-CC) and the Graz cycle. In addition, a reference cycle was established as the basis for the analysis of the oxy-fuel combustion cycles. The fuel for all three cycles is natural gas. A parametric study was conducted where the pressure ratio and the turbine entry temperature (also called combustor outlet temperature) were varied. The optimum net efficiency for the reference cycle is 56% at a pressure ratio of 26.2 and turbine entry temperature of 1400 C. The optimum net efficiency for the SCOC-CC was 46% at a pressure ratio of 57.3 and a turbine entry temperature of 1450 C . The optimum net efficiency for the Graz cycle was also 46% at a lower pressure ratio than the SCOC-CC, at 36.5 at the same turbine entry temperature of 1450 C. The main reduction in effifciency for the oxy-fuel combustion cycles comes from the O2 production and compression. The layout and the design of the SCOC-CC is considerably simpler than the Graz cycle while it achieves the same net efficiency. The fact that the efficiencies for the two cycles are close to identical differs from previously reported work. Earlier studies have reported around a 3% points advantage in efficiency for the Graz cycle, which is attributed to the use of a second bottoming cycle. This additional feature, possible to include in both cycles, is omitted to make the two cycles more comparable in terms of complexity. Even in its simplified form the Graz cycle requires the use of intercooling and steam cooling, in contrast to the SCOC-CC. The Graz cycle, however, has substantially lower pressure ratio at the optimum efficiency and has much higher power density for the gas turbine than both the reference cycle and the SCOC-CC.