Enhancement of the Roundtrip Efficiency of Air Separation Units through Cryogenic Energy Storage and Waste Heat Recovery and Utilization

The production of oxygen (O2) through the cryogenic separation of air is a vital initial step for oxy-firing and gasification processes. The process is energy intensive due to the multistage compression of feed and product gases. The latter negatively impacts the process roundtrip efficiency (RTE) and economic viability. Integrating the cryogenic air separation unit (ASU) with a liquefied natural gas (LNG) regasification and combustion unit for cold energy utilization, is one of the recent cost-effective and environmentally beneficial designs developed in the literature. Here, we propose to enhance the RTE of the LNG pre-cooled ASU, through combining it with three different technologies. First, we propose to utilize an organic Rankine cycle (ORC) to recover waste heat produced by the nitrogen (N2) product multistage compressors in order to generate electricity. A systematic study will be done to optimize the cycle conditions, and to choose an appropriate low global warming potential (GWP) working fluid for this application. Second, we propose to recover the enthalpy of combustion from oxy-firing the natural gas for power generation using an absorption heat pump (AHP). Ammonia-water will be used as the primary working fluid for this application. The AHP role will be to amplify this recovered heat and transfer it to the liquid nitrogen energy storage (LNES) unit. Other absorbent-refrigerant pairs will also be explored and tested in this project. Third, we propose to store the liquid N2 produced by the ASU at cryogenic temperatures and near ambient pressures. At peak electricity hours, the liquid N2 is pumped, heated using the amplified heat recovered from the AHP unit, then expanded to generate electricity. We expect that converting the conventional ASU into a trigeneration system with combined cooling, heating, and power (CCHP) would enhance the process roundtrip significantly. Thermophysical properties and phase behavior of fluids considered in this research project will be predicted using molecular theory. Molecular dynamic simulations will also be carried out to validate the theory predictions.