Simultaneous Biogas Upgrading and Bicarbonate Product Delivery via Electrochemical pH Swing

Biogas upgrading through the separation of carbon dioxide (CO2) to produce high-quality biomethane can significantly enhance the usability of biogas product. This study demonstrates an energy-efficient approach to biogas upgrading by leveraging electrochemical pH swing principles and integrating ionically conductive materials into bipolar membrane electrodialysis (BMED). The developed BMED process effectively captures CO2 from biogas, producing high-quality biomethane while generating a bicarbonate solution product. The bicarbonate solution product can be further utilized in various applications such as algae cultivation, fertilizers, and a direct carbon supplementation to the crop-soil ecosystem. In this study, both theoretical (thermodynamic) and practical (experimental) perspectives are employed to evaluate the effect of different voltages and gas flow rates on CO2 capture efficiency, current density, operational capacity, and specific energy consumption. Experimental results indicate that gas flow rate is a critical factor influencing upgrading performance. The specific energy consumption of the BMED ranges from 1.11 to 3.58 kWh per kg of CO2, with current densities between 60 and 130 A/m2. To validate the experimental data, thermodynamic models are developed based on gas–liquid equilibrium (NaOH-H2O–CO2) and activity coefficients using the electrolyte nonrandom two-liquid model. This study also explores the interrelationships among current density, energy consumption, and CO2 capture scale. Furthermore, response surface models are constructed to identify optimal trade-offs between specific energy consumption and bicarbonate product purity, with thermodynamic modeling supporting future scale-up design. One of the optimal operating conditions could achieve a product purity of 1.95 g HCO3–/L. Overall, this work offers engineering insights into the operation of BMED enhanced with ionically conductive materials, presenting a promising energy-efficient solution for biogas upgrading.