Evaluation of Property Methods for Modeling Direct-Supercritical CO2 Power Cycles

Direct supercritical carbon dioxide (sCO(2)) power cycles are an efficient and potentially cost-effective method of capturing CO2 from fossil-fueled power plants. These cycles combust natural gas or syngas with oxygen in a high pressure (200-300 bar), heavily diluted sCO(2) environment. The cycle thermal efficiency is significantly impacted by the proximity of the operating conditions to the CO2 critical point (31 degrees C, 73.7 bar) as well as to the level of working fluid dilution by minor components, thus it is crucial to correctly model the appropriate thermophysical properties of these sCO(2) mixtures. These properties are also important for determining how water is removed from the cycle and for accurate modeling of the heat exchange within the recuperator. This paper presents a quantitative evaluation of ten different property methods that can be used for modeling direct sCO(2) cycles in Aspen Plus VR. Reference fluid thermodynamic and transport properties (REFPROP) is used as the de facto standard for analyzing high-purity indirect sCO(2) systems, however, the addition of impurities due to the open nature of the direct sCO(2) cycle introduces uncertainty to the REFPROP predictions as well as species that REFPROP cannot model. Consequently, a series of comparative analyses were performed to identify the best physical property method for use in Aspen Plus VR for direct-fired sCO(2) cycles. These property methods are assessed against several mixture property measurements and offer a relative comparison to the accuracy obtained with REFPROP. The Lee-Kessler-Plocker equation of state (EOS) is recommended if REFPROP cannot be used.