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Power-to-Methane as one part of Power-to-Gas has been recognized globally as one of the key elements for the transition towards a sustainable energy system. While plants that produce methane catalytically have been in operation for a long time, biological methanation has just reached industrial pilot scale and near-term commercial application. The growing importance of the biological method is reflected by an increasing number of scientific articles describing novel approaches to improve this technology. However, these studies are difficult to compare because they lack a coherent nomenclature. In this article, we present a comprehensive set of parameters allowing the characterization and comparison of various biological methanation processes. To identify relevant parameters needed for a proper description of this technology, we summarized existing literature and defined system boundaries for Power-to-Methane process steps. On this basis, we derive system parameters providing information on the methanation system, its performance, the biology and cost aspects. As a result, three different standards are provided as a blueprint matrix for use in academia and industry applicable to both, biological and catalytic methanation. Hence, this review attempts to set the standards for a comprehensive description of biological and chemical methanation processes.

The present work reviews the role of biogas as advanced biofuel in the renewable energy system, summarizing the main raw materials used for biogas production and the most common technologies for biogas upgrading and delving into emerging biological methanation processes. In addition, it provides a description of current European legislative framework and the potential biomethane business models as well as the main biogas production issues to be addressed to fully deploy these upgrading technologies. Biomethane could be competitive due to negative or zero waste feedstock prices, and competitive to fossil fuels in the transport sector and power generation if upgrading technologies become cheaper and environmentally sustainable.

Microbial methanation, which produces CH 4 from H 2 and CO 2 (H 2 /CO 2 ) using hydrogenotrophic methanogens, converts CO 2 into a resource. The proposed system utilizes an isolated strain of hydrogenotrophic methanogen and an anaerobic microbial community from a deep aquifer where H 2 and CO 2 are produced via biodegradation of organic matter by fermentative bacteria, which CH 4 is then produced from H 2 and CO 2 by hydrogenotrophic methanogens. The strain and microbial community were added separately to liquid medium in serum bottles. An H 2 /CO 2 mixture was added to each bottle’s gas phase and incubated. CH 4 was produced continuously across both cultures. However, CH 4 production rates in the bottle with a single methanogen strain gradually decreased during cultivation with H 2 and CO 2 exchange. In contrast, the culture using the microbial community derived from anaerobic groundwater maintained consistent CH 4 production throughout the experiment. Analysis of organic acids in the media revealed that the accumulation of formate, acetate, and propionate in the microbial community culture was significantly lower than in the single-strain methanogen culture. These findings suggest that anaerobic bacteria efficiently decompose and consume organic acids that inhibit methanogenic activity, improving the culture environment. This study demonstrates the microbial community’s effectiveness in supporting and sustaining a microbial methanation system.

Background Biogas can be upgraded to methane biologically by adding H2 to biogas reactors. The process is called biological methanation (BM) and can be done in situ in a regular biogas reactor or the biogas can be transferred to a separate ex situ upgrading reactor. The hybrid BM concept, a combination of in situ and ex situ BM, has received little attention, and only a few studies have been reported. The hybrid BM has the advantage of resolving the issue of pH increment during in situ BM, while the size of the ex situ BM reactor could be reduced. Results In this study, the efficiency of in situ and hybrid biological methanation (BM) for upgrading raw biogas was investigated. The hybrid BM system achieved a CH4 yield of 257 mL gVS−1 when degrading a feedstock blend of manure and cheese waste. This represented an increase in methane yield of 76% when compared to the control reactor with no H2 addition. A 2:1 H2:CO2 ratio resulted in stable reactor performance, while a 4:1 ratio resulted in a high accumulation of volatile fatty acids. H2 consumption rate was improved when a low manure–cheese waste ratio (90%:10%) was applied. Furthermore, feeding less frequently (every 48 h) resulted in a higher CH4 production from CO2 and H2. Methanothermobacter was found to dominate the archaeal community in the in situ BM reactor, and its relative abundance increased over the experimental time. Methanosarcina abundance was negatively affected by H2 addition and was nearly non-existent at the end of the experiment. Conclusions Our results show that hybrid BM outperforms in situ BM in terms of total CH4 production and content of CH4 in the biogas. In comparison to in situ BM, the use of hybrid BM increased CH4 yield by up to 42%. Furthermore, addition of H2 at 2:1 H2:CO2 ratio in in situ BM resulted in stable reactor operation.

The fluctuating nature of renewable energies results in the need for sustainable storage technologies to defossilize the energy system without other negative consequences for humans and the environment. In this study, a pilot-scale trickle-bed reactor for biological methanation and various scale-up scenarios for 2024 and 2050 were investigated using life cycle assessment. A best- and worst-case scenario for technology development until 2050 was evolved using cross-consistency analysis and a morphological field, based on which the data for the ecological models were determined. The results show that the plant scale-up has a very positive effect on the ecological consequences of methanation. In the best-case scenario, the values are a factor of 23–780 lower than those of the actual plant today. A hot-spot analysis showed that electrolysis operation has an especially large impact on total emissions. The final Monte Carlo simulation shows that the technology is likely to achieve a low global warming potential with a median of 104.0 kg CO2-eq/MWh CH4 and thus can contribute to decarbonization.

Experiments were carried out to investigate the influence of liquid flow modulation of trickle bed reactors (TBR) on biological hydrogen methanation (BHM). The modulation promises to improve the gas‐liquid mass transfer and has already been demonstrated in trickle bed reactors of other fields of application. Therefore, the influence of four different circulation intervals with pauses from two to 1,440 min was investigated in TBR for BHM. The results showed that as pause intervals without sprinkling became longer, the methane content increased from 88.61 ± 1.58 vol‐% at a circulation interval of 2 min to up to 97.19 ± 0.46 vol‐% at a circulation interval of 1,440 min. The analysis of the process liquid indicated a stable biological process at any trial phase. This study demonstrated that the performance of TBR on BHM can be significantly improved by liquid flow modulation, thus significantly reducing operating costs. To increase the performance of biological hydrogen methanation with trickle bed reactors, the influence of four different circulation intervals with pauses from two to 1,440 min was investigated. The results showed that as pause intervals without sprinkling became longer, the methane content increased from 88.61 ± 1.58 vol‐% at a circulation interval of 2 min to up to 97.19 ± 0.46 vol‐% at a circulation interval of 1,440 min.

Biological methanation is the production of CH4 from CO2 and H2. While this approach to carbon capture utilization have been widely researched in the recent years, there is a gap in the technology. The gap is towards the flexibility in biomethanation, utilizing biological trickling filters (BTF). With the current intermittent energy system, electricity is not a given surplus energy which will interfere with a continuous operation of biomethanation and will result in periods of operational downtime. This study investigated the effect of temperature and H2 supply during downtimes, to optimize the time needed to regain initial performance. Short (6 h), medium (24 h) and long (72 h) downtimes were investigated with combinations of three different temperatures and three different flow rates. The results from these 27 experiments showed that with the optimized parameters, it would take 60 min to reach 98.4% CH4 in the product gas for a short downtime, whereas longer downtimes needed 180 min to reach 91.0% CH4. With these results, the flexibility of biomethanation in BTFs have been proven feasible. This study shows that biomethanation in BTFs can be integrated into any intermittent energy system and thereby is a feasible Power-2-X technology.

The increased demand for resources and energy that is developing with rising global consumption represents a key challenge for our generation. Biogas production can contribute to sustainable energy production and closing nutrient cycles using organic residues or as part of a utilization cascade in the case of energy crops. Compared to hydrogen (H2), biogas with a high methane (CH4) content can be fed into the gas grid without restrictions. For this purpose, the CH4 content of the biogas must be increased from 52 to 60% after anaerobic digestion to more than 96%. In this study, biological hydrogen methanation (BHM) in trickling-bed reactors (TBR) is used to upgrade biogas. Design of experiments (DoE) is used to determine the optimal process parameters. The performance of the reactors is stable under all given conditions, reaching a “low” gas grid quality of over 90%. The highest CH4 content of 95.626 ± 0.563% is achieved at 55 °C and 4 bar, with a methane formation rate (MFR) of 5.111 ± 0.167 m3/(m3·d). The process performance is highly dependent on the H2:CO2 ratio in the educts, which should be as close as possible to the stochiometric ratio of 4. In conclusion, BHM is a viable approach to upgrade biogas to biomethane quality and can contribute to a sustainable energy grid.

The selection of a local and sustainable use of biogas, and biogas feedstocks, towards the upgrading process to biomethane, is a key aspect towards more consistent energy planning within the frame of the EU Green Deal and Sustainable Development Goals. In this paper, four biomethane production and utilization pathways were assessed in the view of economic, environmental, technological, and social dimensions compared to a reference scenario in which direct biogas use in a cogeneration unit is assumed. The technologies analyzed included membrane systems, amine scrubbing, water scrubbing, and biological methanation, regarding the Sardinian context. The impact assessment was carried out using the TOPSIS method. As an output, thirteen consistent indicators reflecting the holistic aspect of sustainability were designed and proposed based on an in-depth literature review and the authors’ technological knowledge. The results show that the reference scenario was the preferable one. In terms of environmental and social considerations, biological methanation emerged as the most environmentally and socially responsible alternative. From the economic perspective, all the upgrading options depicted similar results. The technological-oriented weighting showed that the two most widespread upgrading options highlighted the optimal results.

Biological methanation is driven by anaerobic methanogenic archaea, cultivated in different media, which consist of multiple macro and micro nutrients. In addition, a reducing agent is needed to lower the oxidation–reduction potential (ORP) and enable the growth of oxygen-sensitive organisms. Until now, sodium sulfide (Na2S) has been used mainly for this purpose based on earlier published articles at the beginning of anaerobic microbiology research. In a continuation of earlier investigations, in this study, the usage of alternative reducing agents like sodium dithionite (Na2S2O4) and L-Cysteine-HCl shows that similar results can be obtained with fewer environmental and hazardous impacts. Therefore, a newly developed comparison method was used for the cultivation of Methanothermobacter marburgensis. The median methane evolution rate (MER) for the alternatives was similar compared to Na2S at different concentrations (0.5, 0.25 and 0.1 g/L). However, the use of 0.25 g/L Na2S2O4 or 0.1 g/L L-Cys-HCl led to stable MER values over consecutive batches compared to Na2S. It was also shown that a lower concentration of reducing agent leads to a higher MER. In conclusion, Na2S2O4 or L-Cys-HCl can be used as a non-corrosive and non-toxic reducing agent for ex situ biological methanation. Economically, Na2S2O4 is cheaper, which is particularly interesting for scale-up purposes.