CO₂ Capture from Flue Gas Using an Electrochemically Reversible Hydroquinone/Quinone Solution

Abstract

Electrochemical methods are potentially an energy-saving way to capture CO2 from flue gas. Unlike the regeneration of amine through high-temperature thermal distillation in commercial CO2 absorption processes, the CO2 absorption solution is regenerated by electrolysis at a low temperature. In this work, tiron (disodium 4,5-dihydroxy-1,3-benzenedisulfonate, QH2) was employed as a pH mediator because its redox reactions can change the pH of the solution. Na2Q, which was prepared by QH2 and NaOH in a molar ratio of 1:2, was used to capture CO2 because of its alkalinity. Then, CO2 was desorbed by the oxidation of QH– and QH2 formed in the CO2-rich aqueous solution to quinone (Q) to release protons, and the alkalinity was recovered by the reduction of Q to the quinone dianion (Q2–). The redox performance of Na2Q in aqueous solution was investigated using cyclic voltammetry, and the CO2 capacity of Na2Q solutions at different concentrations (0.1–0.7 M) was measured. The results show that the redox behavior of Na2Q was reversible in the neutral or weakly alkaline solutions. However, the reduction of Q to Q2– by electrolysis was difficult in a high pH solution. During adsorption, the Na2Q solution absorbed CO2 at a molar ratio of about 1:1 (CO2/Q2– ≈ 1). The CO2-saturated Na2Q solution was electrolyzed in the anode zone under a constant current. The CO2 desorption rate reached 100%, and Q2–, QH–, and QH2 were oxidized to give Q. In the cathode zone, Q was reduced to Q2–, which could be used to adsorb CO2 from flue gas. On the basis of the potential difference between the cathode and anode, the regeneration energy consumption was estimated to be about 2.4 GJ/tonne of CO2.