Integration of Membrane Contactors and Bioelectrochemical Systems for CO2 Conversion to CH4

Abstract

Anaerobic digestion of sewage sludge produces large amounts of CO2 which contribute to global CO2 emissions. Capture and conversion of CO2 into valuable products is a novel way to reduce CO2 emissions and valorize it. Membrane contactors can be used for CO2 capture in liquid media, while bioelectrochemical systems (BES) can valorize dissolved CO2 converting it to CH4, through electromethanogenesis (EMG). At the same time, EMG process, which requires electricity to drive the conversion, can be utilized to store electrical energy (eventually coming from renewables surplus) as methane. The study aims integrating the two technologies at a laboratory scale, using for the first time real wastewater as CO2 capture medium. Five replicate EMG-BES cells were built and operated individually at 0.7 V. They were fed with both synthetic and real wastewater, saturated with CO2 by membrane contactors. In a subsequent experimental step, four EMG-BES cells were electrical stacked in series while one was kept as reference. CH4 production reached 4.6 L CH4 m−2 d−1, in line with available literature data, at a specific energy consumption of 16–18 kWh m−3 CH4 (65% energy efficiency). Organic matter was removed from wastewater at approximately 80% efficiency. CO2 conversion efficiency was limited (0.3–3.7%), depending on the amount of CO2 injected in wastewater. Even though achieved performances are not yet competitive with other mature methanation technologies, key knowledge was gained on the integrated operation of membrane contactors and EMG-BES cells, setting the base for upscaling and future implementation of the technology.

How Operational Parameters Affect Electromethanogenesis in a Bioelectrochemical Power-to-Gas Prototype

“…Moreover, the carbon conversion efficiency (η CONV ) from dissolved CO 2 to gaseous CH 4 was estimated (Eq. 3), as explained by Rodríguez-Alegre et al (2019).…”

“…Sulfates presence was mainly due to wastewater pre-treatment, where H 2 SO 4 was adopted for pH neutralization (see Figure 2). This neutralization was needed to balance NaOH addition, which in turn was required to increase the pH and efficiently dissolve the CO 2 in the wastewater (Rodríguez-Alegre et al, 2019). The same amount of NaOH was added for all the tested conditions, in order to replicate the same pre-treatment process, with and without CO 2 capture.…”

“…When the CO 2 capture process was active, CO 2 was constantly detected in the biogas, sometimes at non-negligible concentration (1.8 ± 1.5%). Indeed, the CO 2 was intentionally dissolved in excess in the wastewater, compared with cathodic reduction capacity, in order to achieve non-limiting EMG conditions (Rodríguez-Alegre et al, 2019). Interestingly, also H 2 was detected at the relatively high volumetric concentration of 0.02 .…”

“…As organic matter oxidation generates CO 2 , the process can be run efficiently with a regular feeding of wastewater, or sludge (Moreno et al, 2016;Xu et al, 2020). On the other hand, CO 2 injection can contribute to EMG (and generally to bioelectrochemical processes) by increasing the electrical conductivity of the medium, reducing cathode overpotentials and voltage requirements (Rodríguez-Alegre et al, 2019). Water scrubbing is the most used downstream technique for CO 2 capture, adopted by 33.8% of the European biogas upgrading plants (Batlle-Vilanova et al, 2019).…”

“…Moreover, the contactors have a compact and scalable architecture, allowing to reach high gasliquid contact areas (Nogalska et al, 2017). The integration of membrane contactors with a stack of laboratory-scale EMG-BES reactors was previously tested by the authors, achieving good results in terms of CO 2 capture and CH 4 production (Rodríguez-Alegre et al, 2019).…”