(202c) An Alternative CO2 Capturing Process by Using An Electrodialysis Method

Carbon capture and sequestration (CCS) has been recognized as an effective measure to mitigate global warming. The key issue for the implementation of the CCS scenario is process cost reduction. Especially, power consumption for the capture and recovery process of CO₂ accounts for a major portion of the total cost for the CCS, and therefore, it is required to develop an effective and low-cost capturing and recovery method of CO₂ for the deployment of the CCS. In this study, a new type of CO₂ capturing and recovery process from flue gas by electrodialysis is proposed. The CO₂ in the flue gas is captured by an alkaline hydroxide absorbing solution to form an alkaline carbonate solution with a normal gas-liquid absorption tower. CO₂ + MOH → MHCO₃ (1) CO₂ + 2MOH → M₂CO₃ + H₂O (2) The recovery of CO₂ from the alkaline carbonate solution could be conducted by reversing reactions of (1) and (2) with an electrodialysis method, and the alkaline solution will be regenerated simultaneously. The basic unit of the CO₂ recovery system is composed of two cells separated by a cation exchange membrane, and the cells are sandwiched by two sheets of a bipolar membrane. The carbonate solution is introduced into one cell (desalination cell, hereafter) and an electrolyte solution is introduced into the other cell (electrolyte cell, hereafter). When an electrical potential difference larger than the electrolysis voltage of water is applied to the system, water molecules in the bipolar membrane will be dissociated into protons and hydroxyl ions. When the electric potential is applied so that the electric field is directed from the desalination cell to the electrolyte cell, the protons will be transported to the desalination cell while the hydroxyl ions will be transported to the electrolyte cell. At the same time, the alkaline ions in the desalination cell will be transported toward the electrolyte cell through the cation exchange membrane. As a result, the pH in the desalination cell will be reduced, and CO₂ gas will be recovered from the carbonate solution as, CO₃^2- + H⁺ → HCO₃^- (3) HCO₃^- + H⁺ → CO₂ + H₂O (4) In the electrolyte cell, the alkaline ions transported from the desalination cell and hydroxyl ions produced in the bipolar membrane will be regenerate the alkaline hydroxide solution, which can be reused for the absorbing solution for the CO₂ capture from the flue gas. Thus, regeneration of the alkaline metal hydroxide solution and recovery of CO₂ gas can be achieved by the electrodialysis method with the two-cell system.

An experimental study was conducted in a laboratory-scale electrodialysis apparatus to examine the feasibility of the proposed method. Sodium bicarbonate solution (0.6~1.0 M) was used as a feed carbonate solution, and sodium carbonate (0.1~0.5 M) was used as an electrolyte solution in the recovery cell. The electrodialysis was operated with a current density in the range of 2.4 ~ 9.5 mA/cm². A number of two-cell units (5~10) are placed between two electrode cells at the both ends. The membrane effective area was 210 cm² and the distance between two membranes was 0.75 mm. When an electric potential difference was applied along the stack, the pH of the desalination cell was decreased with time, while that of the electrolyte cell was increased with time. At the same time, generation of CO₂ gas from the desalination solution was observed. The CO₂ generation rate was increased with an increase in the current density, however, the power consumption per unit amount CO₂ recovery was also increased with increasing the current density. When the number of cell units was increased, the CO₂ generation rate was increased and the power consumption per unit amount of CO₂ recovery was decreased. The power consumption was slightly decreased by increasing the initial concentration of the carbonate solution in the desalination cell and that of the electrolyte solution in the electrolyte cell. The generation rate of CO₂ was in the range of 0.5 ~ 1.4 x 10^-4 mol/s, and that of the power consumption was in the range of 0.57 ~1.47 kWh/kg-CO₂ or 0.57 ~1.47 MJ/t-CO₂. It is expected that the efficiency could be improved by optimizing the operation conditions such as the number of unit cells, membrane properties.