Analysis and design of electrochemically-mediated carbon dioxide separation

Abstract

Large-scale carbon dioxide (CO₂) separation is essential in the efforts to curb climate change, with applications in power stations, natural gas purification and enhanced oil recovery. Current CO₂ capture technology is energetically intensive, and challenging to deploy in existing power stations. Electrochemical CO₂ separation is a novel technology that has the potential to reduce CO₂ capture costs. By cycling of metal ions to modulate the CO₂ affinity of amine sorbents, energy and capital requirements can be significantly cut. The feasibility of this approach was previously demonstrated with a proof-of-concept device, but was limited by low energy efficiency and instability. This thesis describes a systematic effort to optimize this technology by exploring its design space, and identifying conditions for robust, energy efficient operation. The large effect of electrolytes on activation kinetics was explored via galvanostatic pulse voltammetry and bench-scale experiments. In the presence of halide electrolytes, energy efficiency was improved for short times, but bench-scale experiments showed an increase in resistance for longer operation, possibly due to electrolyte inclusion in the metal deposit. For the set of the electrolytes tested, nitrates were found to drive the most stable kinetics at moderate energy efficiencies. To explore the electrochemical cell performance for a range of designs and operating conditions, a modeling framework combining thermodynamics, electrode reactions and mass transfer was developed. Model predictions suggest the cell will operate in a mixed kinetic-mass transfer regime at the desired current densities. Model results further predict that introducing flow field disturbances to induce mixing between the bulk and boundary layer will improve energy efficiency significantly. A bench-scale system with modular internals was constructed and used to investigate performance effects of flow field designs. Model predictions were found to be in good qualitative agreement with experimental results. Under optimized conditions, an almost 70% lower voltage at 50 A/m2 was demonstrated. Electrochemical impedance spectroscopy experiments provide further evidence to the mixed kinetics-mass transfer regime of operation. A detailed energy and cost analysis was performed, and results suggest that this technology can cut capture costs significantly if the performance improvement can be sustained for longer operation.