Qualitative and Quantitative Comparison of Two Promising Oxy-fuel Power Cycles for Co2 Capture

Since the Kyoto conference there is a broad consensus that the human emission of greenhouse gases, mainly CO2, has to be reduced. In the power generation sector there are three main alternatives which are currently studied world wide. Among them oxy-fuel cycles with internal combustion with pure oxygen are a very promising technology. Within the European project ENCAP – ENhanced CO2 CAPture the benchmarking of a number of novel power cycles with CO2 capture was carried out [1]. Within the category oxy-fuel cycles the Graz Cycle and the Semi-Closed Oxy-Fuel Combustion Combined Cycle (SCOC-CC) both achieved a net efficiency of nearly 50 %. In a second step a qualitative comparison of the critical components was performed according to their technical maturity. In contrast to the Graz Cycle the study authors claimed that no major technical barriers would exist for the SCOC-CC. In this work the ENCAP study is repeated for the SCOCCC and for a modified Graz Cycle variant as presented at the ASME IGTI conference 2006 [2]. Both oxy-fuel cycles are thermodynamically investigated based on common assumptions agreed with industry in previous work. The calculations showed that the high-temperature turbine of the SCOC-CC plant needs a much higher cooling flow supply due to the less favorable properties of the working fluid. A layout of the main components of both cycles is further presented which shows that both cycles rely on the new designs of the hightemperature turbine and the compressors. The SCOC-CC compressor needs more stages due to a lower rotational speed but has a more favorable operating temperature. In general, all turbomachines of both cycles show similar technical challenges and are regarded as feasible. NOMENCLATURE Aax = flow cross section cp,c = heat capacity of cooling flow cp,g = heat capacity of main flow dQ = differential convective heat transfer dT1 = differential temperature decrease due to polytropic expansion dT2 = differential temperature decrease due to convective cooling c m d & = differential cooling mass flow f = ratio of cooled surface to flow cross section