Designing, building and operating an up-scaled methane producing bioelectrochemical system for power-to-methane

A 17 L scale up methane producing bioelectrochemical system (BES) for power-to-gas application was built taking into account: (1) use of granular activated carbon as cathode material to provide a large surface area for the reaction to take place; (2) a commercially available tubular membrane separating the anaerobic cathodic reaction from the anodic oxygen production; (3) an hexagonal prism design that can be further up-scaled in modules, specially avoiding the hurdle of producing electrodes in non-commercially available sizes. The BES was operated for 470 days with step wise increase in applied current density between −6 and −125 A m⁻³_BES, with resulting methane production rates between 10 NL m⁻³_BES d⁻¹ and 280 NL m⁻³_BES d⁻¹ and faradaic efficiency ranging between 80% and 100%. A stable performance was observed after a start-up period of around 70 days. Furthermore, changes in the electrolyte composition and anode dimensions were made in order to decrease losses associated to the electrolyte and anode reaction, respectively. The distribution of the voltage losses is assessed in this paper and for the majority of the experimental time the energy efficiency ranged around 40%, specifically when operating with an electrolyte with [Figure presented] / [Figure presented] carbonate/bicarbonate concentrations of 0.2 M, 0.4 M and 0.6 M and with an anode surface area equal to or larger than 0.02 m². Besides the BES itself, the reactor line-up included a bubble column, an oxygen stripping unit and an interchange vessel, all of which essential for its operation.