Exploring challenges in microbial electromethanogenesiseffect of power interruptions, temperature, pH and gas composition

Abstract

Renewable energy and carbon capture and utilisation technologies have experienced a rise in recent years as a result of increased awareness of fossil fuel consumption and associated pollution. In this context, biogas from thermal process or anaerobic digestion has become a critical technology to simultaneously achieve waste management and bioenergy production. To meet natural gas specifications, the CH4 content of biogas must be upgraded. Conventional biogas upgrading technologies use separation and sorption techniques, and although they are mature and applicable technologies, they are generally energy intensive. Bioelectrochemical systems (BES) have recently emerged as an alternative to these traditional biogas upgrading systems. By means of electromethanogenesis (EM) in the biocathode of a BES, the CO2 fraction of the biogas can be directly reduced to CH4 in a biocathode, using surplus energy produced with renewable energies. However, this technology is still in an early stage of development and suffers from several challenges such as the intermittency of the power source, the influence of temperature, the influence of pH and the pollutants present in the biogas to be upgraded. In this context, the main objective of this thesis will be to investigate the influence that these factors have on EM, to perform a preliminary study on EM variability, understand their impact on the electrotrophic hydrogenogenic and methanogenic stages and to explore the technical feasibility of using a real biogas as feedstock. The need to accommodate fluctuations intrinsic to renewable energy (mainly solar and wind) requires an understanding of the impact this power inconstancy would have on EM. This thesis explores the impact of 24 to 96 h power outages on EM reactors to determine their effect on methane production rates, current density consumption, current conversion efficiency, and on the microbial communities that compose the cathode biofilm. During the power outages, the cathodes were operated with and without external H2 supplementation to determine how the power outages affect the hydrogenogenic and methanogenic pathways. EM was resilient to power fluctuations, although process efficiency decreased in the absence of H2 supplementation. Another important aspect of EM is the effect that medium-low temperatures have on the electrotrophic and methanogenic stages. To address this issue, EM reactors were subjected to different temperatures (between 30 and 15 °C). Decreasing the temperature affected the methane richness of the product. Methanogenesis, rather than hydrogenesis, was affected and proved to be the main source of variability in EM. Selectivity is another challenge faced by EM systems. It mainly dependns on the microbial communities that finally grow on the cathode and our hypothesis is that pH could play a key role. This thesis studies the impact of pH on the EM process both during start-up and during normal operating conditions. The acidic environment allowed a faster onset of methane production, and dropping pH improved performance up to pH of 4.5. Results also seemed to indicate that high local pH on the surface of the cathode prevented severe physiological disruptions on the microbial communities caused by low bulk pH. The last challenge to be addressed in this thesis is the use of real biogas. The CO2-rich off-gas phase from hydrothermal carbonisation (HTC) was used as a real substrate for an EM system. The work demonstrated that off-gas HTC can be used as raw material in an EM system although there is a decrease in methane production of up to 50% probably caused by the presence of CO.