To define 2nd law efficiency targets for novel separation technologies, a simplified model of a power plant with two forms of CO2 capture was developed. In this investigation, oxy-fuel combustion and post-combustion CO2 separation were compared on an exergetic basis. Using exergy balances and black-box models of power plant components, multiple scenarios were run to determine the impact of plant configuration and separation unit efficiency on overall plant performance. Second law efficiency values from the literature were used to set the baseline performance of various CO2 separation configurations. Assumed advances in 2nd law efficiency were used to determine the potential for overall system performance improvement. It was found that the 2nd law efficiency of air separation must reach a critical value before the thermodynamics of oxy-fuel combustion become favorable. Changes in operating equivalence ratio significantly move the tipping-point between post-combustion and oxy-fuel strategies. (c) 2007 Elsevier Ltd. All rights reserved.