Comparison and optimization of CO2 purification units for CCS applications

Several promising CO₂ capture technologies, like oxy-combustion, adsorption and membranes, feature a purity of the captured CO₂ stream which is insufficient for the storage site or the transport system. In these cases, a CO₂ Purification Unit (CPU) is required to lower the concentrations of O₂, N₂ and Ar at the limits allowed by the storage site/transport system. In this work, the available CO₂ Purification processes have been reviewed and the six main schemes have been simulated in Aspen Plus and optimized. Their performance have been ranked based on six selected key performance indicators: total annual cost, Specific cost per ton of captured CO₂, specific energy consumption, recovery, purity, and O₂ concentration in the purified CO₂. The techno-economic optimization is repeated for different carbon tax values and for three different feed streams compositions. The results of the optimization show that flash-based CPUs cannot meet the requirements for CO₂ storage due to a high concentration of O₂ (>1000 ppm) but they feature a low specific cost (5.8–25.9 €/tonCO₂ depending on the feedgas and plant size), low specific energy consumption (124.9–436.1 kJ/tonCO₂) and acceptable recovery (94.60–99.46 %). The distillation-based CPU can meet the requirements for CO₂ storage, but these CPUs have the highest cost (52–112 % higher than flash-based CPU) and the lowest recovery. The optimal CPUs are the ones which combine both distillation column and flash separation. These CPUs meet the oxygen requirements for CO₂ storage (<10 ppm) while providing the highest purity (99.997–99.999 %), high recovery (90.61–99.32 %) at a limited cost (6.1–36.0 euro/tonCO₂).