On-line current control for continuous conversion of CO2 to CH4 in a microbial electrosynthesis cell

Abstract

This study demonstrates the continuous conversion of carbon dioxide (CO2) to methane (CH4) in a microbial electrosynthesis (MES) cell with on-line CO2 concentration measurements in the cathode off-gas and feedback control of the current. First, a dynamic non-linear model describing CO2 conversion to acetate and CH4 was used to evaluate the impact of MES cell current on electricity consumption and cathode off-gas composition, and to select proportional–integral–derivative (PID) controller parameters. Next, on-line current control based on CO2 measurements was demonstrated in laboratory-scale 0.5 L and 1 L (cathode volume) MES cells fed with pure CO2 or synthetic biogas (40% CO2 and 60% CH4). In all tests, feedback control resulted in stable long-term CH4 production at a volumetric rate of 0.8–1.4 L (LR d)−1, near-constant CO2 content in the cathode off-gas, and Coulombic efficiency of 65–80%. Acetate (500 – 1000 mg L−1) was observed in cathodic liquid in all tests, suggesting relatively low activity of acetoclastic methanogens in the cathodic biofilm. The model and the proposed approach for current control can be also applied for MES of products other than CH4, such as carboxylic acids.