Comparison of cryogenic and membrane oxygen production implemented in the Graz cycle

One of the most promising technologies to decrease greenhouse gas emissions is carbon capture and storage (CCS). Oxy-fuel combustion, in which high-purity oxygen mixed with flue gases is used to burn fuels, reduces the complexity of CCS systems. Several methods have been studied for oxygen production to feed the process, where cryogenic air separation is the most mature technology. However, it is a highly energy-intensive method, which motivates the research of other alternatives such as ceramic membranes. In order to compare the performance of both oxygen separation methods, the coupling of an oxygen production cycle based on ceramic membranes with a high-efficient power cycle, the Graz cycle, is studied. This cycle initially operates using cryogenic air separation. The calculations of both cycles are carried out using the simulation software IPSEpro. For the membrane cycle, two cases are identified, whose main difference is the method to reduce the oxygen partial pressure on the permeate side of the membrane: vacuum generation (Case 1) and membrane sweeping (Case 2). Both cases are optimized, considering the thermodynamic conditions of the membrane operation and its effects on the energy consumption of oxygen production. Membrane cases achieve 54.08% and 55.76% of net efficiency for Case 1 and 2, respectively, being 0.61% and 2.30% points higher than the base case. Furthermore, the differences in turbomachine performances and streams variations are discussed, considering the effects of energy integration of membrane cases.