Syngas Biomethanation Research Articles

Carbon Sequestration Through Syngas Biomethanation Coupled with H2 Supply for a Clean Production of Natural Gas Grade Biomethane

Biomass gasification generates a gas mixture (syngas) that constitutes a rich source of carbon and energy for the production of second-generation renewable fuels such as biomethane. However, the produced syngas composition (H2/CO/CO2) cannot be converted to natural gas grade biomethane due to stoichiometric limitations, and a waste stream of CO2 is released unexploited in the atmosphere. The present study introduces the concept of biomass gasification coupled to syngas biomethanation with in-situ exogenous H2 supply for the complete sequestration of the syngas carbon, the valorization of renewable excess electricity from wind and solar power and the production of biomethane satisfying the criteria for injection in the natural gas grid. Syngas biomethanation was executed by mixed microbial consortia in a trickle bed reactor at 37 °C and 60 °C. The assessment of the effects of the net inlet gas composition was performed according to a hereby proposed syngas quality index ( $$SQI$$ ), which is based on the syngas content in compounds able to act as carbon and electron donors and the stoichiometry of methane production. The SQI of the stoichiometrically ideal syngas composition is 4. Values below 4 correspond to a stoichiometric carbon-moles excess while values above 4 correspond to a stoichiometric electron-moles excess. It was demonstrated that switching the SQI of the supplied syngas from 1.44 to 3.67 increased the CH4 content in the outlet of the reactor from 30 to 72%, accompanied by an at least 1.2-fold increase of the CH4 productivity. A SQI of 4.78 (> 4) resulted in a significant deterioration of the quality of the produced biomethane due to a high content (52–54%) of unconverted H2 and because of thermodynamic limitations on carboxydotrophic hydrogenogenesis in thermophilic conditions. Maximal carbon sequestration and production of natural gas grade biomethane was shown to be feasible at a SQI = 3.98 in thermophilic conditions.

Scale up study of a thermophilic trickle bed reactor performing syngas biomethanation

Biomethane constitutes an important biofuel for the transition towards a biobased circular economy with a sustainable energy sector. An environmentally friendly pathway for its production is through the gasification of 2nd generation biomass that generates syngas (H2, CO, and CO2), followed by syngas biomethanation. A novel design thermophilic trickle bed bioreactor was successfully tested at semi-pilot scale. It was operated continuously performing biomethanation of an artificial syngas mixture (45% H2, 20% CO, 25% CO2 and 10% N2). Its effectiveness was compared to a primitive design lab scale trickle bed reactor with a 28 times lower bed volume under identical operating conditions. At an empty bed residence time of 0.6 h, the novel semi-pilot scale trickle bed reactor converted 100% and 98% of the influent H2 and CO, respectively and produced CH4 at a rate of 10.6 ± 0.2 mmol·lbed−1·h−1, whereas the lab scale reactor converted 89% of the H2 and 73% of the CO and achieved a CH4 productivity of 8.5 mmol·lbed−1·h−1. The maximum CH4 productivity achieved in the semi-pilot scale reactor was 17.6 ± 0.6 mmol·lbed−1·h−1 at an empty bed residence time of 0.33 h with a 99.2 ± 0.1% product selectivity. Furthermore, the semi-pilot scale reactor was connected in series with a fluidized bed gasifier fed with wood pellets to assess the biomethanation potential of the system when supplied with real syngas. The obtained results showed no process inhibition in the semi-pilot scale reactor, which accomplished 100% H2 conversion efficiency and 92.4 ± 0.6% CO conversion efficiency at an empty bed residence time of 0.6 h.

A novel two-stage process for biological conversion of syngas to biomethane

This study shows biomethane production in a novel two-stage syngas biomethanation consisting of the thermophilic anaerobic digestion of sewage sludge combined with an injection of syngas as the first stage. Since the syngas does not contain enough reducing equivalents, the produced biogas was connected to the second stage consisting of the trickle-bed reactor with the external H2 addition (ex-situ) to increase the CH4 content further. The aim was to evaluate the influence of different syngas compositions on the biomethane production in both stages. The results showed that H2 concentration in syngas is the main factor affecting the CH4 content in biogas. Moreover, the ex-situ reactor with H2 addition served to convert the residual CO and CO2, achieving the maximum CH4 content of 94.7% in the produced biomethane. In summary, the two-stage process enables biomethane production without any inhibitory effects on anaerobic sludge digestion.

Techno-economic modeling of an integrated biomethane-biomethanol production process via biomass gasification, electrolysis, biomethanation, and catalytic methanol synthesis

Biological methanation (biomethanation) of syngas obtained from biomass gasification offers the opportunity to employ a low-pressure, low-temperature process to produce storable bio-derived substitute natural gas (bSNG), although its economic viability is limited by high energy and biomass costs. Research on syngas biomethanation techno-economic performance is limited and novel biomass-to-biomethane process configurations are required in order to assess opportunities for the enhancement of its efficiency and economic feasibility. In this study, we carried out the techno-economic modeling of two processes comprising integrated biomass gasification, electrolysis, and syngas biomethanation with combined heat and power recovery in order to assess and compare their fuel yields, energy efficiency, carbon efficiency, and bSNG minimum selling price (MSP). The first process operates standalone biomethanation (SAB) of syngas and can produce approximately 38,000 Nm3 of bSNG per day, with a total plant efficiency of 50.6%. The second process (integrated biomethane-biomethanol, IBB) exploits the unconverted carbon stream from the biomethanation process to recover energy and synthesize methanol via direct catalytic CO2 hydrogenation. In addition to the same bSNG output, the IBB process can produce 10 t/day of biomethanol, at a 99% purity. The IBB process shows little global energy efficiency gains in comparison with SAB (51.7%) due to the large increase in electrolytic hydrogen demand, but it shows a substantial improvement in biomass-to-fuel carbon efficiency (33 vs. 26%). The SAB and IBB processes generate a bSNG MSP of 2.38 €/Nm3 and 3.68 €/Nm3, respectively. Hydrogenation of unconverted carbon in biomass-to-biomethane processes comes with high additional capital and operating costs due to the large-scale electrolysis plants required. Consequently, in both processes, the market price gap of the bSNG produced is 0.13 €/kWhbSNG (SAB) and 0.25 €/kWhbSNG (IBB) even under the most optimistic cost scenarios considered, and it is primarily influenced by the cost of surplus electricity utilized in electrolysis, while the selling price of biomethanol exerts a very limited influence on process economics. Intensive subsidization would be required in order to sustain the decentralized production of bSNG through both processes. Despite their limited economic competitiveness, both processes have a size comparable with existing renewable gas production plants in terms of bSNG production capacity and the IBB process is of a size adequate for the supply of biomethanol to a decentralized biorenewable supply chain.

Modification and extension of anaerobic digestion model No.1 (ADM1) for syngas biomethanation simulation: From lab-scale to pilot-scale

The Anaerobic Digestion Model No.1 (ADM1) was extended for simulation of syngas biomethanation from lab-scale to pilot-scale, with the development of the gas–liquid mass transfer, the inhibition equations, and the biochemical reaction of CO and H2. In the lab-scale reactor, after increasing the syngas flow rate and syngas loading, the syngas conversion efficiency into methane increased from less than 68% to over 95%. The ADM1_modified was developed based on lab-scale data, and the simulation values fitted the experimental results well with R2 values higher than 0.86. The data from the pilot-scale reactor was then used for model validation, and methane production (MP) predictions matched well with the measured data (R2 > 0.83). Methanogenic pathways were further explored and compared by using high-throughput 16S rRNA gene sequencing and the ADM1_modified simulation, and the results from the two methods were highly similar. The established ADM1_modified could be used to describe or predict the syngas biomethanation process macroscopically and microscopically.

Carbon monoxide conversion and syngas biomethanation mediated by different microbial consortia

Syngas biomethanation is an attractive process for extending application of gasification products. In the present study, anaerobic sludges from three methanogenic reactors feeding cattle manure (CS), sewage sludge (SS) and gaseous H2/CO2 (GS) were used to investigate the effect of microbial consortia composition on syngas biomethanation. The results showed that CS presented the highest CO consumption rate due to its highest relative abundance of CO consuming bacteria. The CO was mainly converted to acetate, and syntrophic acetate oxidization (SAO) bacteria converted acetate to H2/CO2 for hydrogenotrophic methanogenesis in CS and SS. However, acetate was accumulated in GS for lacking acetoclastic methanogens and SAO bacteria, leading to lower biomethanation efficiency. Additionally, adding stoichiometric H2 could convert CO and CO2 to nearly pure methane, while, the CO consumption rate declined in H2 added systems. The results present novel insights into microbial consortia on CO conversion and syngas biomethanation.

Temperature effects on syngas biomethanation performed in a trickle bed reactor

Abstract Syngas, mainly consisting of CO, H2 and CO2, can be generated from the gasification of biomass and organic waste and constitutes an important energy and carbon source. However, its biological conversion is still challenging due to the low solubility and the toxic nature of its components. In this study, enriched mixed microbial consortia were inoculated in trickle bed reactors operated in continuous mode with the supply of artificial syngas (45% H2, 25% CO2, 20% CO and 10% N2) under mesophilic (37 °C) and thermophilic (60 °C) conditions. The results revealed a clear superiority of the thermophilic conditions exhibiting higher methane productivities, higher conversion efficiencies and lower yields of byproducts at all steady states tested compared to mesophilic temperature. The highest methane productivity achieved was 8.49 mmol∙lbed−1∙h−1. The microorganisms related to syngas biomethanation were investigated through metagenomic analysis of samples obtained from the inoculum, the liquid phase and the biofilm of the reactors. The continuous operation altered completely the dominant species in mesophilic conditions compared to the inoculum. Accumulation of volatile fatty acids (VFAs) under mesophilic conditions was attributed to the high relative abundance of the genus Sporomusa. A large quantity of acetogenic cell debris scavengers and potential acetogenic metabolism of the genus Thermincola could justify accumulation of VFAs under thermophilic conditions. Absence of aceticlastic methanogens in both reactors was also noticeable. The archaeal communities were enhanced in the biofilm compared to the liquid phase presenting a 6.2 fold and a 1.8 fold higher relative abundance at 37 °C and 60 °C, respectively.

Techno-economic assessment of an integrated biomass gasification, electrolysis, and syngas biomethanation process

Biological methanation (biomethanation) of biomass-derived syngas can be a promising alternative to catalytic methanation, due to its milder operating conditions, and could improve the feasibility of power-to-gas and syngas upgrading systems. However, the feasibility of integrating syngas biomethanation with other processes, i.e., electrolysis and gasification, has not been thoroughly assessed so far by the existing literature. In this study, we carried out the techno-economic analysis of such integrated system and we compared it with the production of pure hydrogen. The results indicate that the two processes could produce 0.39 Nm3 of bio-derived substitute natural gas (bSNG) or 0.07 kg of bio-hydrogen (bH2) per kg of dry biomass, respectively. The process cold gas efficiency associated with the produced bSNG is estimated at 50.6%, with a 97.4% input hydrogen utilization efficiency. For bH2, the cold gas efficiency is 36.6%, with 85% hydrogen utilization. Gasification and gas compression were identified as the unit operations with the highest energy demand in both processes. The minimum selling prices (MSP) of the two products were estimated at 2.68 €/Nm3 for bSNG and 15.35 €/kg for bH2. While delivery costs and a limited production capacity pose additional challenges to the development of bH2 production on decentralized gasification plants, bSNG production for grid injection could become a more feasible alternative under appropriate incentive schemes. Key optimization opportunities for such process rely on better heat integration, lower pressure operation, and the use of waste biomass as feedstock.

Improvement of biofuel recovery from food waste by integration of anaerobic digestion, digestate pyrolysis and syngas biomethanation under mesophilic and thermophilic conditions

An integrated treatment system with co-fermentation of food waste (FW) and syngas generated from digestate pyrolysis was verified as a promising approach to improve biofuel recovery. Syngas was injected and co-fermented with the FW at different injection volumes starting from 560 mL (Stage II). More stable methane production (MP) was exhibited in the thermophilic reactor than the mesophilic reactor. As syngas injection volume gradually increased to 5300 mL in stage V, the MP in the thermophilic reactor was about 22% higher than that in the mesophilic reactor. It was calculated that the syngas was not only almost stoichiometrically converted into CH4 but also accelerated the digestion process of FW, resulting in the improvement of biofuel recovery. The different pathways of syngas biomethanation under the different temperatures were revealed by 16S rRNA genes analysis. Methanosaeta spp. was dominant in the mesophilic reactor and CO and H2 could be converted into acetate and then CH4. On the other hand, hydrogenotrophic methanogens were dominant in the thermophilic reactor indicating that CO may be converted with H2 as the intermediary. This integrated treatment system has the potential for industrial applications and provides a useful approach for the development of a circular bio-economy.

Modeling of syngas biomethanation and catabolic route control in mesophilic and thermophilic mixed microbial consortia

The syngas biomethanation process is a promising bioconversion route due to its high versatility, as it could be applied as a stand-alone technology, coupled to gasification plants, and integrated in anaerobic digestion or bioelectrochemical conversion systems. The biomethanation of syngas typically takes place through a rather complex network of interspecies metabolic interactions, which may vary significantly depending on the operating conditions applied and the diversity of microbial groups present. Despite there are several benefits derived from using microbial consortia, these also present challenges associated with limited process control and low product selectivity. To address the latter, the syngas biomethanation process carried out by mesophilic and thermophilic microbial consortia was modelled with the ultimate goal of studying possible catabolic route control strategies through the modulation of key operating parameters. The results showed that the thermophilic microbial consortium presented much higher apparent specific methane productivity (18.8 mmol/g VSS/d) than the mesophilic (4.6 mmol/g VSS/d) at an initial PCO of 0.2 atm, and that the difference increased with increasing initial PCO. This difference in productivity was found to derive from the catabolic routes used rather than the kinetic parameters of each microbial consortium. Additionally, the thermodynamic considerations included in the models revealed the possibility of controlling the catabolic routes used by each consortium through the modulation of the mass transfer and PCO2. Our results strongly indicate that modulating the PCO2 is a promising operational strategy for boosting the product selectivity towards CH4, the productivity of the system and the biomethane quality simultaneously.

Effects of temperature, hydrogen/carbon monoxide ratio and trace element addition on methane production performance from syngas biomethanation

Synthesis gas (Syngas) biomethanation is an environmentally friendly technology for fuel calorific value improvement. However, the slow mass transfer and poor product quality limit its development. In this study, the effects of temperature, hydrogen/carbon monoxide (H2/CO) ratio and trace element addition on simulated syngas biomethanation were investigated in three batches of experiments. Results showed that (1) the temperature influenced little on the quality of produced biogas; (2) the methane content in the biogas production were 66.37 ± 4.04%, 70.61 ± 6.06% and 73.35 ± 2.39% respectively with the H2/CO ratio of 3:1, 4:1 and 5:1; (3) after the addition of Fe, Co and Ni elements, the biogas quality was significantly improved (methane content was 79.76 ± 7.35%), but the microbial community structure did not change. This experiment provided a guidance for improving the biogas production performance of syngas biomethanation.

Pyrolysis of difficult biodegradable fractions and the real syngas bio-methanation performance

Sustainable approaches for the anaerobic digestion treatment of food waste to minimize environmental challenges and recover energy are some economic goals receiving global attention. Meeting global bio-energy demand from food waste and associated waste residues requires exploring alternative optimization routes. Pyrolysis process for the treatment and recycling of difficult biodegradable fractions in food waste processing facility, and the subsequent utilization of real syngas in anaerobic digestion as a food waste treatment were investigated to evaluate their effects on biofuel production via bio-methanation. Different proportions of syngas that are transformed into methane show that the carbon monoxide content in the reactors (G21, G23, G61, and G63) was completely degraded on 195, 195, 95, and 145 h. The higher hydrogen concentration in the simulated syngas expedites the carbon monoxide degradation rate. Application of two-stage pyrolysis to difficult biodegradable fractions treatment generated real syngas (Hydrogen 60: Carbon monoxide 20 vol%) for improved bio-methanation (100% methane) compared to the blank. The simulated syngas from the anaerobic digestion treatment of food waste generated methane (75–80%) than the control (45%). Results from both the simulated and real syngas show that high volume percent of hydrogen in the syngas positively influenced the degradation and transformation of carbon monoxide and promoted a higher rate of bio-methanation. Further study showed that pH values in the range 7.21–7.78, and low concentrations of volatile fatty acids and ammonia nitrogen indicated no toxicity from syngas bio-methanation with anaerobic digestion. This route is beneficial for enhancing the value of resources recovered from difficult biodegradable fractions pyrolysis, achieving a higher bio-methanation rate, and improving sludge stabilization with anaerobic digestion treatment of food waste.

Floating Membrane Bioreactors with High Gas Hold-Up for Syngas-to-Biomethane Conversion

The low gas-to-liquid mass transfer rate is one of the main challenges in syngas biomethanation. In this work, a new concept of the floating membrane system with high gas hold-up was introduced in order to enhance the mass transfer rate of the process. In addition, the effect of the inoculum-to-syngas ratio was investigated. The experiments were conducted at 55 °C with an anaerobic mixed culture in both batch and continuous modes. According to the results from the continuous experiments, the H2 and CO conversion rates in the floating membrane bioreactor were approximately 38% and 28% higher in comparison to the free (suspended) cell bioreactors. The doubling of the thickness of the membrane bed resulted in an increase of the conversion rates of H2 and CO by approximately 6% and 12%, respectively. The highest H2 and CO consumption rates and CH4 production rate recorded were approximately 22 mmol/(L·d), 50 mmol/(L·d), and 34.41 mmol/(L·d), respectively, obtained at the highest inoculum-to-syngas ratio of 0.2 g/mL. To conclude, the use of the floating membrane system enhanced the syngas biomethanation rates, while a thicker membrane bed resulted in even higher syngas conversion rates. Moreover, the increase of the inoculum-to-syngas ratio of up to 0.2 g/mL favored the syngas conversion.

Enrichment of Mesophilic and Thermophilic Mixed Microbial Consortia for Syngas Biomethanation: The Role of Kinetic and Thermodynamic Competition

Mixed culture-based syngas biomethanation is a robust bioconversion process with high versatility in terms of exploitable feedstocks and potential applications, as it could be operated independently, or coupled to anaerobic digestion systems and in-situ biogas upgrading processes. Typically, the syngas biomethanation consists in the stepwise conversion of syngas into methane through a number of catabolic routes, which may vary considerably depending on the operating conditions. In this study, two enrichments were performed at 37 °C and 60 °C to investigate the effect of the incubation temperature on the microbial selection process and the dominant catabolic routes followed. This was carried out through the characterization of the catabolic routes and the microbial composition of the enriched cultures, and a thermodynamic feasibility study on their metabolic networks. The enrichments resulted in two stable microbial consortia with different patterns of activity. The mesophilic enriched consortium presented a more intricate metabolic network composed by four microbial trophic groups, where aceticlastic methanogenesis contributed to 64.9 ± 8.3% of the CH4 production. The metabolic network of the thermophilic enriched consortium was much simpler, consisting in the syntrophic association of carboxydotrophic hydrogenogens and hydrogenotrophic methanogens. This led to significant differences in methane productivity, corresponding to 1.83 ± 0.27 and 33.48 ± 0.90 mmol CH4/g VSS/h for the mesophilic and the thermophilic enriched consortium, respectively, which would potentially make the thermophilic consortium more suited for industrial applications. 16S rRNA gene amplicon analysis indicated the presence of strains with similarity to Acetobacterium sp., Methanospirillum hungateii, Methanospirillum stamsii and Methanothrix sp. at mesophilic conditions, and Thermincola carboxydiphila and Methanothermobacter sp. at thermophilic conditions, implying a role in the conversion of syngas. The thermodynamic feasibility study demonstrated that the microbial selection was not driven solely by kinetic competition, since thermodynamic limitations also played a significant role defining the dominant catabolic routes.

A novel concept for syngas biomethanation by two-stage process: Focusing on the selective conversion of syngas to acetate

Thermal gasification of nonrenewable and renewable sources produces syngas, containing CO, H2, CO2 and N2. Anaerobic conversion of syngas to CH4 is a promising way to replace natural gas. However, the high N2 content (>50%) in syngas would result in the low CH4 content in the biogas and CO in syngas also had serious inhibition on methanogens. The present study proposed a two-stage anaerobic process for syngas biomethanation, and syngas was first anaerobically converted to acetate by mixed culture, which could be further converted to methane easily without the negative effects of N2 and CO. The results showed that mesophilic condition was more suitable for the conversion of syngas to acetate compared to thermophilic and ambient conditions at pH 5.5 considering the higher acetate yield and syngas conversion rate. Although CO was efficiently converted at thermophilic condition, it was mostly converted to H2, which was then converted to acetate. CO was much easier to be converted compared to H2. Further study showed that pH 6.5 and 7.5 were optimal for selective conversion of syngas to acetate. The other products including butyrate and ethanol were also detected in relatively higher amounts at pH 4.5 and 9.5. Although pH 5.5 and 8.5 had relatively lower syngas conversion rates compared to pH 6.5 and 7.5, they might inhibit methanogens naturally without adding methane inhibitors. Finally, batch experiments showed that the acetate concentration had obvious inhibition on syngas conversion when the acetate concentration was higher than 2 g/L.

Syngas biomethanation: state‐of‐the‐art review and perspectives

AbstractSignificant research efforts are currently being made worldwide to develop more efficient biomethane production processes from a variety of waste streams. The biomethanation of biomass‐derived syngas can contribute to increasing the potential of methane production as it opens the way for the conversion of recalcitrant biomasses, generally not fully exploitable by anaerobic digestion systems. Additionally, this biological process presents several advantages over its analogous process of catalytic methanation such as the use of inexpensive biocatalysts, milder operational conditions, higher tolerance to the impurities of syngas, and higher product selectivity. However, there are still several challenges to be addressed for this technology to reach commercial stage. This work reviews the progress made over the last few years in syngas biomethanation processes in order to provide an overview of the current state of the art of this technology. The most relevant aspects determining the performance of syngas biomethanation processes are extensively discussed here, including microbial diversity and metabolic interactions in mixed microbial consortia, the influence of operating parameters and bioreactor designs, and the potential of modelling as a tool for the design and control of this bioprocess. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd

Syngas Biomethanation in a Semi-Continuous Reverse Membrane Bioreactor (RMBR)

Syngas biomethanation is a potent bio-conversion route, utilizing microorganisms to assimilate intermediate gases to produce methane. However, since methanogens have a long doubling time, the reactor works best at a low dilution rate; otherwise, the cells can be washed out during the continuous fermentation process. In this study, the performance of a practical reverse membrane bioreactor (RMBR) with high cell density for rapid syngas biomethanation as well as a co-substrate of syngas and organic substances was examined in a long-term fermentation process of 154 days and compared with the reactors of the free cells (FCBR). The RMBR reached maximum capacities of H2, CO, and CO2 conversion of 7.0, 15.2, and 4.0 mmol/Lreactor.day, respectively, at the organic loading rate of 3.40 gCOD/L.day. The highest methane production rate from the RMBR was 186.0 mL/Lreactor.day on the 147th day, compared to the highest rate in the FCBR, 106.3 mL/Lreactor.day, on the 58th day. The RMBR had the ability to maintain a high methanation capacity by retaining the microbial cells, which were at a high risk for cell wash out. Consequently, the system was able to convert more syngas simultaneously with the organic compounds into methane compared to the FCBR.

Rapid bio-methanation of syngas in a reverse membrane bioreactor: Membrane encased microorganisms

The performance of a novel reverse membrane bioreactor (RMBR) with encased microorganisms for syngas bio-methanation as well as a co-digestion process of syngas and organic substances was examined. The sachets were placed in the reactors and examined in repeated batch mode. Different temperatures and short retention time were studied. The digesting sludge encased in the PVDF membranes was able to convert syngas into methane at a retention time of 1day and displayed a similar performance as the free cells in batch fermentation. The co-digestion of syngas and organic substances by the RMBR (the encased cells) showed a good performance without any observed negative effects. At thermophilic conditions, there was a higher conversion of pure syngas and co-digestion using the encased cells compared to at mesophilic conditions.