Conceptual Design of Semi-Closed Oxy-Fuel Combustion Combind-Cycle

Sammanfattning: Accelerating actions to mitigate CO2 emissions was one of the important messages at the UN Climate Summit 2014. About 85 % of the world’s energy is provided through the burning of fossil fuels, which is the main source of CO2 emissions. Increasing the share of renewable energy, and making energy efficiency improvements and savings were actions that were agreed upon to meet CO2 emission reductions. At the same time, global energy demand is rising along with a significant reliance on fossil fuels as an energy source. Carbon Capture and Storage (CCS) is considered as the technology which can reduce CO2 emissions from fossil fuel-fired power plants. The aim of this doctoral thesis was to investigate one of the CCS technologies, the Semi-Closed Oxy-Fuel Combustion Combined Cycle (SCOC-CC). The doctoral thesis covered the preliminary thermodynamic and turbine design. Different methods and tools were used to perform the studies. Thermodynamic studies were achieved using the commercial heat and mass balance program IPSEpro. Turbine mean-line design studies were performed using the in-house tool developed in Matlab, LUAX-T. The commercial software AxCent from Concepts NREC was used to complete the turbine through-flow design. The results of this doctoral thesis were published in seven peer-reviewed papers. The results can be summarized after the doctoral objectives. The study recommended a two pressure level SCOC-CC. The thesis studied a mid-sized SCOC-CC with net power around 100 MW. The studied SCOC-CC had a pressure ratio of 37 and COT 1400 °C. The net cycle efficiency was 46.7 % and energy penalties due to ASU and CO2 compression were 10 and 2 percentage points, respectively. The calculated power consumption of O2 separation at 95 % purity was 719 kJ/kgO2. The power consumption for pressurizing the separated O2 by compression was 345 kJ/kgO2, while it was 4.4 kJ/kgO2 for pumping liquid O2. Pumping O2 could give a 3 % gain in efficiency and thus increase SCOC-CC net efficiency to 49.6 %. The thesis studied both single- and twin-shaft oxy-fuel gas turbines. Single-shaft gas turbine rotational speed was set to 5200 rpm. The designed turbine had four stages, while the compressor had 18 stages. Twin-shaft turbine gas generator rotational speed was set to 7200 rpm and power turbine rotational speed was set to 4800 rpm. A twin-shaft turbine was designed with five turbine stages while the compressor with 14 stages. The studies showed that the required cooling for the oxy-fuel gas turbines was higher than for the conventional gas turbines at a specific power or specific COT. The cooled area in the oxy-fuel gas turbines was greater. The required cooling per square meter of cooled area was used as a parameter to compare the required cooling for oxy-fuel and conventional gas turbines. The study showed that the required cooling per cooled area was close in both studied turbines. The through-flow design was performed on the twin-shaft power turbine last stage. The last stage was designed with damped forced conditions. The study showed the parameter distribution of relative Mach number, total pressure and temperature, relative flow angle and tangential velocity. Through-flow results in a 50 % span and mean-line results showed reasonable agreement between static pressure, total pressure, degree of reaction and total efficiency. Total temperature and relative Mach number showed some difference, which can be attributed to the working fluid in the study being set to pure CO2. The relative tip Mach number at rotor exit was 1.03, which is lower than the maximum typically allowed value of 1.2.