A promising candidate for CO2 neutral power production is Semi-Closed Oxyfuel Combustion Combined Cycles (SCOC CC). Two alternative SCOC-CCs have been investigated both with recirculation of the Working Fluid (CO2 and H2O) but with different H2O content due to different conditions for condensation of water from the Working Fluid. The alternative with low moisture content in the re-circulated Working Fluid has shown highest thermodynamic potential and has been selected for further study.
The necessity to use recirculated exhaust gas as the Working Fluid will make the design of the gas turbine quite different from a conventional gas turbine. For a combined cycle using a steam Rankine cycle as a bottoming cycle it is vital that the temperature of the exhaust gas from the Brayton cycle is well suited for steam generation that fits steam turbine live steam conditions. For oxyfuel gas turbines with a combustor outlet temperature of the same magnitude as conventional gas turbines a much higher pressure ratio is required (close to twice the ratio as for a conventional gas turbine) in order to achieve a turbine outlet temperature suitable for combined cycle.
Based on input from the optimized cycle calculations a conceptual combustion system has been developed, where three different combustor feed streams can be controlled independently: the natural gas fuel, the oxidizer consisting mainly of oxygen plus some impurities, and the re-circulated Working Fluid. This gives more flexibility compared to air-based gas turbines, but introduces also some design challenges. A key issue is how to maintain high combustion efficiency over the entire load range using as little oxidizer as possible and with emissions (NOx, CO, UHC) within given constraints. Other important challenges are related to combustion stability, heat transfer and cooling, and material integrity, all of which are much affected when going from air-based to oxygen-based gas turbine combustion.
Matching with existing air-based burner and combustor designs has been done in order to use as much as possible of what is proven technology today. The selected stabilization concept, heat transfer evaluation, burner and combustion chamber layout will be described. As a next step the pilot burner will be tested both at atmospheric and high pressure conditions.