Experimental and mathematical analyses of bio-electrochemical conversion of carbon dioxide to methane

Electromethanogenesis (EM) is a reaction in a bio-electrochemical system (BES) in which microorganisms catalyse the reduction of CO2 to methane by electric current at the cathode. We have proposed a novel application of EM combined with CO2 Capture and Storage (CCS), in which geologically-sequestrated CO2 is reduced to methane using electricity from variable renewables. To develop this technology, further investigation and establishment of the mathematical model are indispensable. In this study, experimental and mathematical analyses were conducted to develop mathematical model. A mathematical model for EM is developed based on BES models constructed in previous studies. As a result of mathematical analysis, it is suggested that EM reaction is dominated by the direct pathway at high cathode potential, and the methane production rate is proportional to the electrical current generated in the system. To calculate methane production rate via the indirect pathway requires measurement of microorganism concentration, and it will enable completion of mathematical model. Moreover, to examine the practical application of EM, a scale-up experiment was conducted in fed-batch mode using a 3 L volume reactor. As electrode material, carbonized coconut shells were chosen due to their large surface area, good biocompatibility, and good chemical stability with relatively low cost. Thermophilic microorganisms originated from a subsurface formation (a depleted petroleum reservoir) were inoculated in the reactor and incubated at 55 degrees C with applied voltages of 1.0 V or 0.7 V. The maximum CH4 production rate was 340 mmol m(-3) day(-1) with current-capture efficiency of >30 %. A rises of current generation were observed every time after the medium was exchanged, which indicates the microbial activity is responsible for current generation and CH4 production. The maximum CH4 production rates in 1st, 2nd and 3rd cycle were 108, 179 and 340 mmol m(-3) day(-1) respectively, which shows a growth of performance in microbial catalytic ability. These results suggest that it is feasible to scale up the EM system and, however, further studies are required to commercialize the system as a CCS technology. (c) 2017 The Authors. Published by Elsevier Ltd.