Anthropogenic emissions of carbon dioxide (CO2) from fossil-fuel electric power plants potentially can lead to undesirable global climate change. Consequently, significant research effort has focused on reducing the carbon intensity of fossil fuel use through energy conservation, energy efficiency improvements and Carbon Capture and Storage (CCS).
CCS involves CO2 separation from flue gas, transportation to a storage location and permanent storage in an appropriate geological reservoir. The US Department of Energy (DOE) has challenged researchers to develop separation processes capable of 90% CO2 capture at 95% purity, and subsequent compression to 140 bar for transport and storage, without increasing the cost of electricity (COE) by more than 35%. Membrane processes are one of many options under development.
Because of the relatively low CO2 concentration in post-combustion flue gas, most of the reported process configurations for membrane systems have been sought to generate affordable CO2 partial pressure driving forces for permeation. This can be achieved by feed compression, vacuum permeation, internal gas recycle, and feed air sweep configurations. Additionally, most process designs involve CO2 concentration in a two-stage configuration followed by cryogenic separation and liquefaction. These schemes utilize highly selective membranes. While such membranes may reduce energy consumption, the required membrane area will be higher than if lower selectivity, higher intrinsic permeability materials are used.
Process designs that utilize membrane materials with higher permeabilities and lower selectivites are considered. Such materials offer the potential to reduce membrane capital cost at the expense of greater energy consumption. Moreover, the low pressure ratios being considered for membrane capture processes may not be able to exploit economically the potential of higher selectivity materials to concentrate CO2.
A super-structure for staged membrane capture processes with final cryogenic concentration is proposed which accommodates the previously proposed configurations and the new configurations for lower selectivity materials. The superstructure allows for two types of CO2 recycle loops: one through the boiler and the other after the boiler; the post-boiler recycle loop avoids undesirable reductions in oxygen concentration. The tradeoff between capital and operating costs is evaluated by determining the levelized cost of electricity for various embodiments of the superstructure to identify the optimal design.