Conceptual Mean Line Design of Single and Twin Shaft Oxy Fuel Gas Turbine in a Semiclosed Oxy Fuel Combustion Combined Cycle

Abstract

The aim of this study was to compare singleand twinshaft oxyfuel gas turbines in a semiclosed oxyfuel combustion combined cycle (SCOC–CC). This paper discussed the turbomachinery preliminary meanline design of oxyfuel compressor and turbine. The conceptual turbine design was performed using the axial throughflow code luaxt, developed at Lund University. A tool for conceptual design of axial compressors developed at Chalmers University was used for the design of the compressor. The modeled SCOC–CC gave a net electrical efficiency of 46% and a net power of 106 MW. The production of 95% pure oxygen and the compression of CO2 reduced the gross efficiency of the SCOC–CC by 10 and 2 percentage points, respectively. The designed oxyfuel gas turbine had a power of 86 MW. The rotational speed of the singleshaft gas turbine was set to 5200 rpm. The designed turbine had four stages, while the compressor had 18 stages. The turbine exit Mach number was calculated to be 0.6 and the calculated value of AN2 was 40 آ·â€‰106 rpm2m2. The total calculated cooling mass flow was 25% of the compressor mass flow, or 47 kg/s. The relative tip Mach number of the compressor at the first rotor stage was 1.15. The rotational speed of the twinshaft gas generator was set to 7200 rpm, while that of the power turbine was set to 4800 rpm. A twinshaft turbine was designed with five turbine stages to maintain the exit Mach number around 0.5. The twinshaft turbine required a lower exit Mach number to maintain reasonable diffuser performance. The compressor turbine was designed with two stages while the power turbine had three stages. The study showed that a fourstage twinshaft turbine produced a high exit Mach number. The calculated value of AN2 was 38 آ·â€‰106 rpm2m2. The total calculated cooling mass flow was 23% of the compressor mass flow, or 44 kg/s. The compressor was designed with 14 stages. The preliminary design parameters of the turbine and compressor were within established industrial ranges. From the results of this study, it was concluded that both singleand twinshaft oxyfuel gas turbines have advantages. The choice of a twinshaft gas turbine can be motivated by the smaller compressor size and the advantage of greater flexibility in operation, mainly in the offdesign mode. However, the advantages of a twinshaft design must be weighed against the inherent simplicity and low cost of the simple singleshaft design.