Bio-valorization of CO2 for the sustainable production of biomethane as an alternative bioenergy storage system

Power to biomethane technology reported in the present work offers a solution to employ the surplus from renewable energy sources and to reduce CO2 emissions (from biogas upgrading or other CO2-rich gas streams), allowing to... [ view full abstract ]

Power to biomethane technology reported in the present work offers a solution to employ the surplus from renewable energy sources and to reduce CO₂ emissions (from biogas upgrading or other CO₂-rich gas streams), allowing to generate fully renewable and sustainable biomethane. The present work reports a CO₂ capture and bioconversion to biomethane system as an alternative energy storage strategy. Such technology produces biomethane using a bioelectrochemical system (BES) via electromethanogenesis (BES-EMG) promotion, from a CO₂-rich gas stream, wastewater and the intermittent electricity surplus from renewable energy sources.

In view of promising future applications of electromethanogenesis for biomethane production, this work evaluates several scenarios at lab-scale: (1) BES-EMG reactors inoculation; (2) start-up; (3) operation strategies in a scaling-up oriented vision. In terms of inoculation, start-up and operation of BES-EMG reactors, the best results were obtained operating the reactors in a 2-electrodes configuration applying 700 mV external voltage (between anode and cathode). An average biomethane production rate of 1-2 L_CH4/m²_cat·d was achieved, consuming a current density of 0.1 A/m². In a second stage, 4 BES-EMG identical reactors were operated in stack configuration and connected electrically in series, in order to increase the voltage, operating the stack at 2.8 V. A passive voltage balancing system was applied to control and limit the voltage drop at each individual cell of the stack. Developing a stack that requires a high voltage is mandatory to the future use of this technology as energy storage system with current electronic control systems.  

Finally, a system to solubilize CO₂ into real wastewater (collected from the primary settler of a local wastewater treatment plant) was developed. This system allows to solubilize up to 2-4 g/L of CO₂ coming from a CO₂-rich gas stream. A wastewater with a high load of inorganic carbon was tested in a stack of 4 BES-EMG cells already operating. Results showed that current density increased up to 0.15-0.2 A/m² and CH₄ was enhanced up to 2.5-3 L/d·m² (with 96% purity). At present, a 30 L volume prototype, composed by 45 cells connected in series and operating at a total voltage of 35 V, is under construction.