Ex-situ biogas upgrading in thermophilic up-flow reactors: The effect of different gas diffusers and gas retention times

Spiral-Pipe Gas Anaerobic Digester

The reduction of carbon dioxide to methane using hydrogen is an important process in biogas production. However, designing gas anaerobic digesters (GADs) based on this reaction presents several challenges. In this study, we developed an innovative spiral-pipe gas anaerobic digester (SGAD) to increase the displacement distance between the bubbles, thus prolonging the gas retention time and facilitating the reduction of CO2 to CH4 via H2. The process was successfully demonstrated by using a CO2/H2 ratio of 1:3 and a gas-feeding rate of 3.9 L Lr -1 d-1. During the experiment, more than 98% of the CO2 and 96% of the H2 were consumed, resulting in biogas containing ca. 86-96% CH4. Additionally, we applied our proposed evaluation methodology for assessing GAD performance to evaluate the performance of the SGAD. This methodology serves as a reference for evaluating and designing GAD systems. The innovative design of the SGAD and the corresponding evaluation methodology offer new insights into the design of reactors.

A dynamic model of growth phase of bio-conversion of methane to polyhydroxybutyrate using dynamic flux balance analysis

Biological conversion of waste methane to biodegradable plastics is a way of reducing their production cost. This study addresses the computational modeling of the growth phase reactor of the process of polyhydroxybutyrate production. The model was used for investigating the effect of gas recycling and inlet gas retention time on the reactor performance. The model was run by the use of a genome-scale metabolic network of Methylocystis hirsuta in a dynamic flux balance analysis framework. The reactor has been modeled for two separate feeding scenarios: a pure methane feed and a biogas feed. The mass transfer coefficient parameter was predicted as a function of superficial gas velocities by the regression of data from published experiments. The results show an increase of removal efficiency by 38% and biomass concentration by 2.8 g/L with the increase of gas recycle ratio from 0 to 30 at the empty bed residence time of 60  min .

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.

Insights into the recent advances of agro-industrial waste valorization for sustainable biogas production

Recent years have seen a transition to a sustainable circular economy model that uses agro-industrial waste biomass waste to produce energy while reducing trash and greenhouse gas emissions. Biogas production from lignocellulosic biomass (LCB) is an alternative option in the hunt for clean and renewable fuels. Different approaches are employed to transform the LCB to biogas, including pretreatment, anaerobic digestion (AD), and biogas upgradation to biomethane. To maintain process stability and improve AD performance, machine learning (ML) tools are being applied in real-time monitoring, predicting, and optimizing the biogas production process. An environmental life cycle assessment approach for biogas production systems is essential to calculate greenhouse gas emissions. The current review presents a detailed overview of the utilization of agro-waste for sustainable biogas production. Different methods of waste biomass processing and valorization are discussed that contribute towards developing an efficient agro-waste to biogas-based circular economy.

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.

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.

Overview of recent progress in exogenous hydrogen supply biogas upgrading and future perspective

With the rapid development of renewable and sustainable energy, biogas upgrading for producing high-quality biomethane as an alternative to natural gas has attracted worldwide attention. This paper comprehensively reviews the current state of biogas upgrading technologies. The advances in physicochemical, photosynthetic autotrophic, and chemical autotrophic biogas upgrading technologies are briefly described with particular attention to the key challenges. New chemical autotrophic biogas upgrading strategies, such as direct and indirect exogenous hydrogen supply, for overcoming barriers to biogas upgrading and realizing highly efficient bioconversion of carbon dioxide are summarized. For each approach to exogenous hydrogen supply for biogas upgrading, the key findings and technical limitations are summarized and critically analyzed. Finally, future developments are also discussed to provide a reference for the development of biogas upgrading technology that can address the global energy crisis and climate change issues related to the application of biogas.

Effect of pressure on biomethanation process and spatial stratification of microbial communities in trickle bed reactors under decreasing gas retention time

The current study investigated the effect of elevating gas pressure on biomethanation in trickle-bed reactors (TBRs). The increased pressure led to successful biomethanation (CH₄ > 90 %) at a gas retention time (GRT) of 21 min, due to the improved transfer rates of H₂ and CO₂. On the contrary, the non-pressurized TBR performance was reduced at GRTs shorter than 40 min. Metagenomic analysis revealed that the microbial populations collected from the lower and middle parts of the reactor under the same GRT were more homogeneous compared with those developed in the upper layer. Comparison with previous experiments suggest that microbial stratification is mainly driven by the nutrient provision strategy. Methanobacterium species was the most dominant methanogen and it was mainly associated with the bottom and middle parts of TBRs. Overall, the increased pressure did not affect markedly the microbial composition, while the GRT was the most important parameter shaping the microbiomes.

Ex-situ biogas upgrading in thermophilic trickle bed reactors packed with micro-porous packing materials

Two thermophilic trickle bed reactors (TBRs) were packed with different packing densities with polyurethane foam (PUF) and their performance under different retention times were evaluated during ex-situ biogas upgrading process. The results showed that the TBR more tightly packed i.e. containing more layers of PUF achieved higher H₂ utilization efficiency (>99%) and thus, higher methane content (>95%) in the output gas. The tightly packed micro-porous PUF enhanced biofilm immobilization, gas-liquid mass transfer and biomethanation efficiency. Moreover, applying a continuous high-rate nutrient trickling could lead to liquid overflow resulting in formation of non-homogenous biofilm and severe deduction of biomethanation efficiency. High-throughput 16S rRNA gene sequencing revealed that the liquid media were predominated by hydrogenotrophic methanogens. Moreover, members of Peptococcaceae family and uncultured members of Clostridia class were identified as the most abundant species in the biofilm. The proliferation of hydrogenotrophic methanogens together with syntrophic bacteria showed that H₂ addition resulted in altering the microbial community in biogas upgrading process.

Hydrogenotrophs-Based Biological Biogas Upgrading Technologies

Biogas produced from anaerobic digestion consists of 55–65% methane and 35–45% carbon dioxide, with an additional 1–2% of other impurities. To utilize biogas as renewable energy, a process called biogas upgrading is required. Biogas upgrading is the separation of methane from carbon dioxide and other impurities, and is performed to increase CH₄ content to more than 95%, allowing heat to be secured at the natural gas level. The profitability of existing biogas technologies strongly depends on operation and maintenance costs. Conventional biogas upgrading technologies have many issues, such as unstable high-purity methane generation and high energy consumption. However, hydrogenotrophs-based biological biogas upgrading offers an advantage of converting CO₂ in biogas directly into CH₄ without additional processes. Thus, biological upgrading through applying hydrogenotrophic methanogens for the biological conversion of CO₂ and H₂ to CH₄ receives growing attention due to its simplicity and high technological potential. This review analyzes the recent advance of hydrogenotrophs-based biomethanation processes, addressing their potential impact on public acceptance of biogas plants for the promotion of biogas production.