A thermodynamic approach to energy requirements for CO2 capture and a comparison between the minimum energy for liquid and solid sorbent processes

There has been an increased interest in the use of solid sorbents for CO₂ capture from flue gases to reduce emissions from fossil energy. This work uses a simple Carnot engine-like model to compare the energy requirements for a CO₂ capture process using a solid adsorbent in a circulating fluidized bed with its minimal thermodynamic needs and with the performance of a conventional liquid solvent process. The energy requirements for CO₂ capture using thermal swing separation sorption are dominated by the standard Gibbs free energy of separation from the sorbent ($\Delta g_{0,sep}$), the sensible heat needed to reach the desorption temperature, and loading optimization to avoid thermodynamic pinching effects. The $\Delta g_{0,sep}$ is an invariant of the system, so only its value at reference conditions is required and it is independent of the desorption temperature or the heat of evaporation of a liquid solvent. A baseline is established using the $\Delta g_{0,sep}$ as well as the equivalent work for a well-established amine process. In all cases the energy requirements are found to be well above the minimum thermodynamic values and those of conventional liquid absorption. Higher-capacity solid sorbents and challenging improvements on heat recovery will be needed to close the gap.