Thermodynamic comparison of cryogenic and membrane oxygen production for the supercritical NET power cycle

Oxy-combustion power plants, where high-purity oxygen instead of air is used for the combustion of fossil fuels, could become more common in the future because they are well-suited for carbon capture and storage (CCS). The supply method of high-purity oxygen is critical for the profitability of those power plants because it is highly energy-intensive and requires expensive equipment. Currently, the most mature method is cryogenic air separation, but an efficient alternative is membranes that separate oxygen by electronic and ionic conduction, The integration of these membranes in the Graz Cycle, a promising oxy-combustion power cycle, has already led to a remarkable improvement in efficiency. Therefore, it is studied whether membranes can also improve the NET Power cycle, another interesting oxy-combustion power cycle with supercritical CO₂ as working fluid, although its high efficiency is based on the utilization of heat from the cryogenic air separation. For this, thermodynamic simulations of the integration of two different membrane technologies were performed using the cloud-based process simulation platform IPSE GO. Case 1 has a 3-end membrane, on whose permeate side pure oxygen is under vacuum pressure while hot pressurized air is fed to the other side. In Case 2, cycle flue gas with low oxygen content is fed to the permeate side of a 4-end membrane. Both membrane cycles use a cascade of compressors, turbines and heat exchangers to achieve optimum efficiency. In this study the membrane area, the temperature differences of the heat exchangers in the oxygen production cycle as well as the membrane operating conditions were varied in search of an optimum. The net cycle efficiency of the optimized Case 1 is still only 50.73%, which is remarkably lower than the 52.72% of the base case with cryogenic air separation. But the efficiency of the optimized Case 2 is, with 52.66%, almost as high as that in the base case, which can make membranes an interesting alternative to cryogenic air separation.