Study of recirculating water-based and carbon-based working fluids on the combustion flow field and the cycle performances of semi-closed CO2-capturing Brayton-derived cycles

Abstract

The transition into new power technologies that utilize fossil fuels with near zero emissions to the ambient is an urgent need to mitigate the worldwide environmental problems. In this study, detailed analyses of carbon-based and water-based oxy-combustion power cycles are introduced. The applied methodologies to perform these analyses comprise CFD analysis of reacting flows for dual recuperative cycle (DRC) and reheated cycle (RHC). The employed CFD methodologies used the non-simplified Navier-Stokes formulation of reacting flows to avoid the use of extra models or assumptions. It was found that recycling carbon-based fluids would allow better overall performances of the resulting cycles. Recycling water-based fluids, however, would provide some interesting features regarding the temperature field in the combustion chamber. Hence, it would avoid in a better way the possibility of facing overheating caused by oxy-combustion, as well as provide some valuable aerodynamic features for the turbines. When analyzing the obtained combustion flow field, CO2 and H2O working fluids create a more important temperature abatement in the flame surrounding areas and trail than Air. When comparing CO2 and H2O working fluids between them, the H2O working fluid initially showed a slightly bigger high-temperature zone than the CO2 working fluid. Despite this fact, and after those initial zones, the H2O working fluid showed a more important temperature abatement than the CO2 working fluid, except for the transversal tip of the flame trail, where temperatures are in a safe range for superalloys. When considering the temperature on the fluid domain symmetry axis and at the outlet, for the 30 atm case (30.4 bar), it was found that the CO2 fluid presented a temperature equal to 86.9% of the Air one in the same location and pressure, whereas the H2O fluid presented an 82% of the Air one. In addition, thermoeconomic analyses were conducted for the DRC and RHC that are working at high pressure of less than 42 bar and operating temperatures of 1100 K to 1450 K to ensure feasible design for the cycle components. Furthermore, it is found that a maximum efficiency of 47.50% is obtained by the carbon-based RHC under wet-cooling conditions and a minimum cycle efficiency of 33.54% is obtained by the water-based DRC under dry-cooling conditions. From an economic point of view, the average LCOE of the present carbon-based and water-based cycles is 4.17 ¢/kWh, which is 28% lower than the average LCOE of the supercritical carbon-based and integrated gas-turbine-based cycles. Moreover, the LCOE of the carbon-based RHC is minimum (3.92 ¢/kWh) at minimum cycle temperature (Tmin) of 305 K (wet cooling) and identical to the water-based RHC (4.00 ¢/kWh) at Tmin of 323 K (dry-cooling).