Carbon dioxide (CO2) is an inorganic compound naturally occurring in the terrestrial atmosphere. From the industrial revolution onwards, burning of fossil fuels, production of electricity and other industrial processes have increased its concentration exponentially, leading to the current speed of global warming. Compared to other technologies for CO2 capture and conversion that use scarce and expensive materials to catalyse the reduction of carbon, microbial electro-synthesis (MES) utilises electro-active microorganisms. However, although successful outcomes have been obtained at laboratory scale, commercialisation is still not feasible.
Laura Rovira Alsina’s doctoral thesis addresses the challenges of scaling up microbial electrosynthesis of acetate (HA) from CO2. The researcher used real industrial off-gases for her experiments, investigated a mixed microbial culture able to work at high temperatures, and fed the process with electricity from renewable sources. The following results were obtained:
- Taking into account that the industrial sector emits CO2 at high temperatures, all experiments were carried out under thermophilic conditions (50 °C). This enhanced the kinetics of the reactions as well as the selectivity of the final product.
- To address one of the main challenges of the technology linked to the electricity utilisation as the main OPEX cost, renewable energy use was considered, and operation was simulated with only the surplus without battery storage, resulting in intermittent power supply. This reduced energy consumption by a factor of three and resulted in the combination of bioelectrochemical and microbial fermentation processes, achieving continuous acetate production (43 g m-2 d-1) and promising carbon conversion rates (2.2 kg CO2 kg HA-1). In terms of the intensification of the process, a developed thermodynamic model allowed to determine the most favourable operating conditions depending on the desired product. Analysis of the results showed that under thermophilic conditions, the chain elongation of HA to longer carboxylates was not spontaneous, rendering its conversion in successive anaerobic fermentation steps under mesophilic conditions as the most viable option.
- To bring the technology one step closer to field application, the systems were tested with real industrial off-gases containing impurities and a lower percentage of CO2 (from 100 to 14 %). The microbial community proved to be robust enough to maintain similar productivities compared to the operation with synthetic gas (2.5 % of difference) and to adapt to the new conditions, developing synergies to mitigate the impacts derived from the use of real gas with the presence of 12 % of oxygen (O2).
These findings were brought together to design, build, and operate the first pilot plant for microbial electrosynthesis from CO2 with digital monitoring and control of the key operational variables. This allowed defining control ranges with different levels of variability and immediate signal-response actions for the proper use and exploitation of the resources, achieving the best product/energy ratio obtained to date (483 g acetate kWh-1). This milestone approaches microbial electro-synthesis to market, although some relevant challenges remain, such as obtaining electrodes with biocompatible, cheap, and efficient materials, along with the constraints that limit production rates.