(10c) Low CO2 Concentration Post-Combustion Capture: Process Insights

For post-combustion CO₂ technologies applicable to power-plants, amine-based solvent CO₂ capture has seen considerable development for commercial deployment among other available options. Although amine-based solvent CO₂ capture has been in use for several decades in gas sweetening applications for example, a key technological challenge for post-combustion CO₂ capture in fossil-fuel power generating plants is the low CO₂ concentration in the flue gas. Typical concentrations range from 12-14 vol% for coal-fired plants and 3-5 vol% for natural-gas fired plants. The low concentration reduces the driving force for CO₂ transfer from the gas phase into the solvent with its consequent impact on the energy of regeneration of the solvent. With technology suited to both natural and coal-fired gas plants, the even lower drying force for CO₂ transfer from natural gas generated-flue gas would require stringent process conditions to overcome the effect of this limitation such as the potentially higher energy implications for solvent regeneration, with the associated high cost of electricity.

In this work, results from experiments conducted in a 0.1 MWth pilot post-combustion CO₂ capture unit at low CO₂ concentration (~5 vol%) mimicking natural gas-fired plant conditions is presented. Impacts of different liquid circulation, gas flow rate and stripper pressure were examined. The results of the study was compared with findings from studies done with coal-derived flue gas. It is shown that appropriate hydrodynamic conditions in the column are required for the low CO₂ concentration based on reduced liquid circulation rates to enhance solvent performance as a result of the low driving force for capture. The corresponding higher oxygen content in the low-content CO₂ gas did not contribute to increased oxidative solvent degradation compared to the impact of the stripping temperature. Leveraging extensive work done with high CO₂ flue gas conditions (applicable to coal-fired plants), comparisons will highlight applicability of knowledge to natural-gas fired plants together with recommended areas for process enhancement, particularly as these plants increase in prominence for power generation.