Pilot-scale study of biomethanation in biological trickle bed reactors converting impure CO2 from a Full-scale biogas plant

Oxygen traces impact on biological methanation from hydrogen and CO2

Biomethane production from biological methanation of CO2 is promising both for biogas upgrading and surplus renewable energy storage. One of the questions for process upscaling is the impact of oxygen (in the biogas or in the purified CO2-rich off-gas) on the biological process. An adapted anaerobic thermophilic consortium was submitted to increasing amounts of oxygen in batch and continuous tests at partial pressures ranging from 0 to 50 mbar. Oxygen was quickly consumed and hydrogen uptake remained similar. In the same time, methane production dropped (-4 % in continuous tests). Part of the oxygen introduced was reduced biologically by hydrogen. The amount of hydrogen diverted to oxygen reduction (up to 15 % at 50 mbar O2) was proportional to the oxygen partial pressure. These results suggest that biological methanation systems tolerate the presence of oxygen. However, additional hydrogen should be added to maintain the conversion of CO2 into methane.

Ex-situ single-culture biomethanation operated in trickle-bed configuration: Microbial H2 kinetics and stoichiometry for biogas conversion into renewable natural gas

Biomethanation converts carbon dioxide (CO2) emissions into renewable natural gas (RNG) using mixed microbial cultures enriched with hydrogenotrophic archaea. This study examines the performance of a single methanogenic archaeon converting biogas with added hydrogen (H2) into methane (CH4) using a trickle-bed bioreactor with enhanced gas-liquid mass transport. The process in continuous operation followed the theoretical reaction of hydrogenotrophic methanogenesis (CO2 + 4 H2 → CH4 + 2 H2O), producing RNG with over 99 % CH4 and more than 0.9 H2 conversion efficiency. The Monod constants of H2 uptake were experimentally determined using kinetic modelling. Also, a dimensionless parameter was used to quantify the ratio between the H2 mass transfer rate and the maximum attainable H2 consumption rate. Single-culture biomethanation averts the formation of secondary metabolites and bicarbonate buffer interferences, resulting in lower demands for H2 than mixed-culture biomethanation.

Phenotypic and genomic characterization of Methanothermobacter wolfeii strain BSEL, a CO2-capturing archaeon with minimal nutrient requirements

A new variant of Methanothermobacter wolfeii was isolated from an anaerobic digester using enrichment cultivation in anaerobic conditions. The new isolate was taxonomically identified via 16S rRNA gene sequencing and tagged as M. wolfeii BSEL. The whole genome of the new variant was sequenced and de novo assembled. Genomic variations between the BSEL strain and the type strain were discovered, suggesting evolutionary adaptations of the BSEL strain that conferred advantages while growing under a low concentration of nutrients. M. wolfeii BSEL displayed the highest specific growth rate ever reported for the wolfeii species (0.27 ± 0.03 h-1) using carbon dioxide (CO2) as unique carbon source and hydrogen (H2) as electron donor. M. wolfeii BSEL grew at this rate in an environment with ammonium (NH4+) as sole nitrogen source. The minerals content required to cultivate the BSEL strain was relatively low and resembled the ionic background of tap water without mineral supplements. Optimum growth rate for the new isolate was observed at 64°C and pH 8.3. In this work, it was shown that wastewater from a wastewater treatment facility can be used as a low-cost alternative medium to cultivate M. wolfeii BSEL. Continuous gas fermentation fed with a synthetic biogas mimic along with H2 in a bubble column bioreactor using M. wolfeii BSEL as biocatalyst resulted in a CO2 conversion efficiency of 97% and a final methane (CH4) titer of 98.5%v, demonstrating the ability of the new strain for upgrading biogas to renewable natural gas.IMPORTANCEAs a methanogenic archaeon, Methanothermobacter wolfeii uses CO2 as electron acceptor, producing CH4 as final product. The metabolism of M. wolfeii can be harnessed to capture CO2 from industrial emissions, besides producing a drop-in renewable biofuel to substitute fossil natural gas. If used as biocatalyst in new-generation CO2 sequestration processes, M. wolfeii has the potential to accelerate the decarbonization of the energy generation sector, which is the biggest contributor of CO2 emissions worldwide. Nonetheless, the development of CO2 sequestration archaeal-based biotechnology is still limited by an uncertainty in the requirements to cultivate methanogenic archaea and the unknown longevity of archaeal cultures. In this study, we report the adaptation, isolation, and phenotypic characterization of a novel variant of M. wolfeii, which is capable of maximum growth with minimal nutrients input. Our findings demonstrate the potential of this variant for the production of renewable natural gas, paving the way for the development of more efficient and sustainable CO2 sequestration processes.

Optimization of the Ex Situ Biomethanation of Hydrogen and Carbon Dioxide in a Novel Meandering Plug Flow Reactor: Start-Up Phase and Flexible Operation

With the increasing use of renewable energy resources for the power grid, the need for long-term storage technologies, such as power-to-gas systems, is growing. Biomethanation provides the opportunity to store energy in the form of the natural gas-equivalent biomethane. This study investigates a novel plug flow reactor that employs a helical static mixer for the biological methanation of hydrogen and carbon dioxide. In tests, the reactor achieved an average methane production rate of 2.5 LCH4LR∗d (methane production [LCH4] per liter of reactor volume [LR] per day [d]) with a maximum methane content of 94%. It demonstrated good flexibilization properties, as repeated 12 h downtimes did not negatively impact the process. The genera Methanothermobacter and Methanobacterium were predominant during the initial phase, along with volatile organic acid-producing, hydrogenotrophic, and proteolytic bacteria. The average ratio of volatile organic acid to total inorganic carbon increased to 0.52 ± 0.04, while the pH remained stable at an average of pH 8.1 ± 0.25 from day 32 to 98, spanning stable and flexible operation modes. This study contributes to the development of efficient flexible biological methanation systems for sustainable energy storage and management.

Ex-situ biomethanation for CO2 valorization: State of the art, recent advances, challenges, and future prospective

Ex-situ biomethanation is an emerging technology that facilitates the use of surplus renewable electricity and valorizes carbon dioxide (CO2) for biomethane production by hydrogenotrophic methanogens. This review offers an up-to-date overview of the current state of ex-situ biomethanation and thoroughly analyzes key operational parameters affecting hydrogen (H2) gas-liquid mass transfer and biomethanation performance, along with an in-depth discussion of the technical challenges. To the best of our knowledge, this is the first review article to discuss microbial community structure in liquid and biofilm phases and their responses after exposure to H2 starvation during ex-situ biomethanation. In addition, future research in areas such as reactor configuration and optimization of operational parameters for improving the H2 mass transfer rate, inhibiting opportunistic homoacetogens, integration of membrane technology, and use of conductive packing material is recommended to overcome challenges and improve the efficiency of ex-situ biomethanation. Furthermore, this review presents a techno-economic analysis for the future development and facilitation of industrial implementation. The insights presented in this review will offer useful information to identify state-of-the-art research trends and realize the full potential of this emerging technology for CO2 utilization and biomethane production.

Comparative study on packing materials for improved biological methanation in trickle Bed reactors

Packing materials improve biological methanation efficiency in Trickle Bed Reactors. The present study, which lies in the field of energy production and biotechnology, entailed the evaluation of commercial pelletized activated carbon and Raschig rings as packing materials. The evaluation focused on monitoring process indicators and examining the composition of the microbial community. Activated carbon resulted in enhanced methane purity, achieving a two-fold higher methane percentage than Raschig rings, maintaining a stable pH level within a range of 7-8 and reducing gas retention time from 6 h to 90 min. Additionally, the digestate derived from biogas plant was found to be a sufficient nutrient source for the process. Fermentative species with genes for β-oxidation, such as Amaricoccus sp. and Caloramator australicus could explain the production of hexanoic and valerate acids during reactor operation. Based on the physical properties of packing materials, the efficiency of biological methanation could be maximized.

Biological methanation in trickle bed reactors - a critical review

Biological methanation of H₂ and CO₂ in trickle bed reactors is a promising energy conversion and storage approach that can support the energy transition towards a renewable-based system. Research in trickle bed reactor design and operation has significantly increased in recent years, but most studies were performed at laboratory scale and conditions. This review provides a comprehensive overview of the trickle bed reactor concept and current developments to support the decision-making process for future projects. In particular, the key design and operational parameters, such as trickling or nutrient provision, are presented, introducing the most recent advances. Furthermore, reactor operation, including the inoculation, long-term and dynamic operation, is described. To better assess the reactor upscaling, several parameters that enable reactor comparison are discussed. On the basis of this review, suitable operational strategies and further research needs were identified that will improve the overall trickle bed reactor performance.

Strategies to overcome mass transfer limitations of hydrogen during anaerobic gaseous fermentations: A comprehensive review

Fermentation of gaseous substrates such as carbon dioxide (CO₂) has emerged as a sustainable approach for transforming greenhouse gas emissions into renewable fuels and biochemicals. CO₂ fermentations are catalyzed by hydrogenotrophic methanogens and homoacetogens, these anaerobic microorganisms selectively reduce CO₂ using hydrogen (H₂) as electron donor. However, H₂ possesses low solubility in liquid media leading to slow mass transport, limiting the reaction rates of CO₂ reduction. Solving the problems of mass transport of H₂ could boost the advance of technologies for valorizing industrial CO₂-rich streams, like biogas or syngas. The application could further be extended to combustion flue gases or even atmospheric CO₂. In this work, an overview of strategies for overcoming H₂ mass transport limitations during methanogenic and acetogenic fermentation of H₂ and CO₂ is presented. The potential for using these strategies in future full-scale facilities and the knowledge gaps for these applications are discussed in detail.

Enhanced process control of trickle-bed reactors for biomethanation by vertical profiling directed by hydrogen microsensor monitoring

Biomethanation is an emerging Power-to-X technology enabling CO₂ valorisation to produce biomethane using renewable H₂. A promising reactor for facilitating biomethanation is the trickle bed reactor (TBR), however, these bioreactors are conventionally operated with a black-box approach, where the system is solely described by the input and output characteristics. This study employed a novel approach for process surveillance of internal dynamics in TBRs by installing multiple H₂ microsensors along its vertical axis. The H₂ microsensor monitoring was demonstrated for 135 days in a TBR integrated into a full-scale biogas plant. Despite achieving an overall CH₄ productivity of 12.6 L L^-1 d^-1, the vertical positioning of microsensors revealed a clear zonation with CH₄ productivity zones reaching 54.8 L L^-1 d^-1 and enabled early warning detection of deteriorating process performance days before detecting it in the product gas. Thus, vertically positioned microsensors present a promising solution for securing process stability.

Removal of gas phase methanol and acetonitrile mixture in an air membrane bioreactor (aMBR) under steady and transient-state operations

A laboratory scale air membrane bioreactor (aMBR) was used to treat a gas-phase mixture of methanol (MeOH) and acetonitrile (ACN), with an inoculum comprising of a mixed culture of microorganisms. The aMBR was tested under both steady-state and transient modes, with inlet concentrations ranging from 1 to 50 g/m³ for both compounds. Under steady-state conditions, the aMBR was operated at various empty bed residence times (EBRT) and MeOH:ACN ratios, while intermittent shutdown was tested during transient-state operations. The results showed that, the aMBR demonstrated > 80% removal efficiencies for both MeOH and ACN. An EBRT of 30 s was found to be the best treatment time for the mixture, providing>98% removal, with<20 mg/L of the pollutant accumulation in the liquid-phase. The microorganisms also showed preferential utilization of ACN compared to MeOH from the gas-phase and good resilience capacity after three days of shutdown/re-start operation.

Biogas upgrading in a pilot-scale trickle bed reactor - Long-term biological methanation under real application conditions

The biological methanation of H₂ and CO₂ in trickle bed reactors is one promising energy conversion technology for energy storage, but experiences at pilot-scale under real application conditions are still rare. Therefore, a trickle bed reactor with a reaction volume of 0.8 m³ was constructed and installed in a wastewater treatment plant to upgrade raw biogas from the local digester. The biogas H₂S concentration of about200 ppm was reduced by half, but an artificial sulfur source was required to completely satisfy the sulfur demand of the methanogens. Increasing the ammonium concentration to > 400 mg/L was the most successful pH control strategy, enabling stable long-term biogas upgrading at a CH₄ production of 6.1 m³/(m³_RV·d) with synthetic natural gas quality (CH₄ > 98%). The results of this study with a reactor operation period of nearly 450 days, including two shutdowns, represents an important step towards the necessary full-scale integration.

Preliminary gas flow experiments identify improved gas flow conditions in a pilot-scale trickle bed reactor for H2 and CO2 biological methanation

Biological methanation of H₂ and CO₂ is a potential energy conversion technology that can support the energy transition based on renewable sources. The methanation performance in trickle bed reactors can be improved by approaching the gas flow through the reactor towards plug flow. Through preliminary gas flow experiments without biological conversion, this study investigated operational and constructional conditions that enhance plug flow in a pilot-scale trickle bed reactor with 1 m³ gas volume. An improved gas flow was observed when the feed gas was applied in a top-to-bottom direction and when the process liquid was not trickled through the packing bed. Furthermore, the gas flow experiments identified reactor-specific properties, such as unused or dead volumes. Applying gas flow experiments prior to reactor start-up is recommended as a simple and convenient method to identify individual reactor properties and optimization potentials for higher methanation performance.