Microbial electromethanogenesis for co2 valorisation and electrical energy storage

Résumé

Carbon capture, utilisation, and storage (CCUS) is a group of technologies that are critical for the decarbonisation of the economy. CCUS is an umbrella concept that encompasses any technology that reduces or eliminates the emission of CO2 into the atmosphere, while redirecting the carbon into a sustainable carbon sink. Microbial electromethanogenesis (EM) can be included within this group of technologies as it is able to convert CO2 into organics and fuels by using microorganisms as biocatalysts. In addition, the EM process requires a certain amount of electricity to proceed, which makes it capable to convert any surplus of electrical energy into a more easily storable energy such as fuel gas. This feature allows EM to be grouped not only within CCUS but also within power-to-gas technologies. However, before achieving commercial development, EM needs to face a number of challenges, such as the efficient conversion of CO2 gas, the use of compatible electrode materials or the improvement of coulombic efficiencies. Thus, the aim of this thesis is to advance towards the practical application of EM as an energy storage and CCUS technology (operated either in stand-alone mode or integrated with other biotechnologies), by addressing some of the key issues mentioned above. In the process of integrating the EM system with other biotechnologies, it is necessary to understand the challenges that imply the use of complex wastes as substrates. This thesis compares the treatment efficiency of microbial electrolysis cell (MEC)- assisted anaerobic digestion (AD) and conventional AD for two real organic wastes: i) exhausted vine shoot fermentation broth (EVS) from the final stream of a gas stripping process and ii) cheese whey (CW) waste from the dairy industry. In the EVS treatment, the MEC-AD system produced 7 times more methane than the traditional AD, in addition to improving the electrical capacity and resistance of the system. For the CW valorisation tests, both the MEC-AD and the AD reactors were supplemented with two different carbonaceous materials (activated carbon (AC) and pyrolysed argan (PA)). The AC amendment allowed for a faster start-up, although it negatively affected the methane productivity. The PA had no visible benefit in terms of methane yield compared to nonsupplemented AD. Nevertheless, it slightly increased the methane production rate, something that could bring practical advantages in real-life AD facilities. Another aspect that needs to be addressed before scaling-up EM are the still relatively low current densities of the methanogenic biocathodes. The good results obtained with bioanodes modified with graphene oxide induced us to explore the use of this material in biocathodes. The current density generated by the graphene-modified electrodes was almost 30% higher than that of the control carbon felt electrode, with simultaneous increase in microbial abundance. It was also estimated that to produce high quality biogas (>95% methane concentration), a CO2 feed rate in the range of 15–30 g CO2 per m2 of electrode per day was required. Under certain circumstances biocathodes can behave as biological supercapacitors, which allows them to be used not only for long-term, but also for shortterm energy storage. This thesis investigates the electrical charge storage capabilities of graphene-modified carbon-felt-based bioelectrodes. Results seem to indicate that graphene-modifications have a positive effect in the electrochemical performance of biocathodes but deteriorates both the kinetics and the charge storage capabilities of the abiotic cathodes. Finally, this thesis also evaluates the technical feasibility of integrating an EM system into real biogas production plants (case study). It was found that the use of this technology for biogas upgrading or for CO2-rich streams valorisation can produce a biogas with a composition similar to that of natural gas. Results also showed that EM allows for a 38-54% improvement in methane production compared to the baseline scenario (conventional anaerobic digestion).