CH4 Purity Research Articles

Using ligand regulation, metal replacement, and ligand doping strategies on Zr-FUM to improve methane separation from coalbed gas

Developing adsorbents suitable for industrial applications that can effectively enhance the separation of methane (CH4) from nitrogen (N2) in coalbed gas is crucial to improve energy recovery and mitigate greenhouse gas emissions. In this study, three modification strategies were implemented on Zr-FUM, including ligand regulation, metal replacement, and ligand doping, to synthesize Zr-FDCA, Al-FUM, and Zr-FUM-FA, with the aim of improving the performance of CH4/N2 separation under humid conditions. The results demonstrated that the promotion of robust orbital overlap and strengthened electrovalent bonding on adsorbents can selectively enhance CH4 adsorption. As a result, Zr-FUM-FA achieved a saturated CH4 adsorption capacity of 1.37 mmol/g, a CH4 working window of 307 s, and a CH4/N2 sorbent selection parameter (Ssp) of 47.31, exceeding the performance of most reported adsorbents. Analyses of the pore structure, surface morphology, and functional groups revealed that the presence of an ultramicropore proximity to CH4, reduced static resistance, and enhanced electrovalent bond were key factors for CH4 separation. Grand Canonical Monte Carlo and Density Functional Theory studies indicated that the introduction of -C-H- in FA played a crucial role in enhancing CH4 adsorption. Optimization of adsorption parameters using the Aspen adsorption package showed that in a dual-adsorbent bed system, the recovery and purity of CH4 in Zr-FUM-FA reach 99.5% and 97.3%, respectively, providing important theoretical support for the improvement of CH4 recovery in the pressure swing adsorption process from coalbed gas.

Coke oven gases processing by vacuum swing adsorption: Carbon capture and methane recovery

Coke oven gas (COG), a by-product gas from the steel industry with great potential for chemical utilization, contains multiple high-value gases including H2, CO2, and CH4. Our previous work has successfully achieved hydrogen production from COG with a purity greater than 99.99 %. For further separation of COG-to-hydrogen tail gas, a two-phase vacuum swing adsorption (VSA) strategy for CO2 capture and CH4 upgrading was carried out in this study. The accuracy of the 4-bed 8-step and 4-bed 7-step cycles established in the in-house mathematical model was verified through a series of multi-component breakthrough and VSA experiments for the two-stage CO2 capture and CH4 upgrading processes employing zeolite NaY and activated carbon RB3, respectively. To achieve more excellent separation performance, several novel VSA cycles were designed and developed in terms of each stage, and the Pareto fronts of purity and recovery of CO2 or CH4 for each cycle were obtained through a multi-objective optimization framework. Based on this, the separation performance and energy consumption of the optimal cycle for each stage were parametrically analyzed under various feed gas concentrations, vacuum pressures, and feed velocities. Moreover, the CO2 product with a purity of 95.58 % and a recovery of 90.11 % can be obtained ultimately by using a 4-bed 8-step cycle for the first stage and a 4-bed 7-step cycle for the second stage of the CO2 capture process under 0.1 bar vacuum pressure. The CH4 upgrading process using a 4-bed 7-step cycle at 0.1 bar vacuum pressure produced a product with 92.07 % purity and 98.81 % recovery. The specific energy consumption was 32.17 kJ/mol and 1.80 kJ/mol for CO2 capture and CH4 upgrading, respectively. Recovery of high-purity CO2 and CH4 at such medium vacuum pressure and low energy consumption through a two-phase VSA strategy from COG has great application potential.

Experimental demonstration of a sorption-enhanced methanation (SEM) cyclic process on a lab-scale TRL-3 fixed bed reactor

In this work, the scaling-up of the sorption-enhanced methanation (SEM) process to a TRL-3 fixed bed reactor has been demonstrated. The SEM process for synthetic natural gas (SNG) production consists mainly of two stages: in the first stage, the zeolite facilitates the removal of a product from the methanation reaction, thereby enhancing CO/CO2 conversion; in the second stage, zeolite regeneration takes place. The functional materials chosen were a zeolite 4A and a Nickel-based catalyst, selected for their high H2O adsorption capacity and high catalytic activity, respectively. Various operational variables, including temperature and pressure, were optimized during both SEM and regeneration stages to enhance the performance of the process. Optimum regeneration conditions consisted of pressure swing operation—from 10 bar during the SEM reaction to atmospheric pressure—while maintaining a constant temperature at 230°C. These operating conditions were tested for more than 20 SEM/regeneration cycles using a feed gas containing H2/CO/CO2. Stable performance throughout the cycles was fulfilled, reaching a CH4 purity of 100 % during 25 min during SEM for the operating conditions chosen. Finally, the number of reactors needed to obtain a continuous stream of CH4 was calculated resulting in a process productivity of 8.00×10-4 molCH4/kgcat·s.

Biogas upgrading towards acetic acid production using Clostridium thailandense supplemented with granular activated carbon (GAC) and L-arginine: A genomic analysis approach

This study explores the impact of granular activated carbon (GAC) and L-arginine supplementation on biogas upgrading and acetic acid production employing Clostridium thailandense. GAC and L-arginine concentrations ranged from 0 to 20 g/L and 0 to 5 g/L, respectively, with H2 acting as the electron donor at an H2 to CO2 ratio of 2:1 (v/v). Experiments were conducted at 30 °C with an agitation speed of 150 rpm. Additionally, gene annotation of the C. thailandense genome using Rapid Annotations using Subsystems Technology (RAST) identified genes involved in CO2 to acetic acid conversion. Results indicate that adding 7.5 g/L GAC boosts CH4 purity in biogas, elevating CO2 and H2 consumption efficiencies to 88.3 % and 98.7 %, respectively. This enhancement leads to a CH4 content increase to 93.3 %, accompanied by 0.90 g/L acetic acid production. Conversely, L-arginine demonstrates no significant impact on CO2 conversion. Leveraging RAST, the study identifies hydrogenase genes and NADH-dependent ferredoxin-NADP+ oxidoreductase (Nfn), as crucial for heightened H2 consumption efficiencies and cell growth facilitated by GAC, thus enhancing biogas upgrading efficiency in C. thailandense. This research provides vital insights into optimizing sustainable biogas production through strategic GAC utilization and elucidates the roles of hydrogenase genes and Nfn.

Anaerobic digestion biogas upgrading using a two-stage membrane system under pilot-scale conditions.

In the present work, the construction, and operation of a pilot-scale biogas upgrading system is presented, employing 2 commercial polyimide (PI) membranes. The Upgrading system treats biogas produced via anaerobic digestion of the sludge, produced from the treatment of municipal wastewater in the facilities of Thessaloniki's Wastewater Treatment Plant. The goal of the separation unit is the production of high purity biomethane (>95%) for potential reuse in terms of energy. The fabrication of the pilot scale system includes the scale up of a laboratory setup separating CO2 from binary CH4–CO2 gas mixture. After the stability tests of the process, for the operation of 5 months (February to June 2023) the purity and recovery of CH4 in the final gas product. The experimental results showed an average recovery of CH4 of 95.7% for an average 55% feed composition, whereas the average purity in the final product was equal to 82.4%. The purity results were lower because of the N2 presence in the product stream (average 17.5%). After normalization with the help of the lab-scale binary results, the expected results assuming N2 absence would be 99.8% CH4 purity and 67% CH4 recovery. Finally, 3 different membrane configurations are compared in terms of their energy production, concluding to the efficiency of 2-stage configuration with recycling stream for the optimal combination of theoretical stage cut fractions.

Techno-economic analysis of CO2 cryogenic distillation from high CO2 content gas field: A case study in Indonesia

Indonesia has natural gas reservoirs with high CO2 content, such as Natuna and Bojonegoro gas fields. Various technologies have been developed, including cryogenic distillation separation technology suitable for application to gas wells with high CO2 content. Cryogenic distillation separates CO2 with high purity according to the operating conditions and the CO2 dew point. Cryogenic distillation was simulated in this study to determine the purity of the CO2 and to compare the economic potential of each approach. The highest CO2 purity using CFZ technology is 98.72% in the Natuna, while the highest purity of CH4 using Pellegrini Distillation technology is 99.95% in the Bojonegoro. Economically, the Natuna and Bojonegoro estimates using the CFZ technology have a lower total annual cost than the Pelegrini distillation. However, CFZ technology has a lower CO2 capture cost than Pellegrini Distillation technology, 7.9 USD/tonne CO2 and 10.26 USD/tonne CO2. In other words, separation using the CFZ technology is more effective for gas fields with higher CO2.

Simultaneous biogas upgrading and medium-chain fatty acids production using a dual membrane biofilm reactor

Utilizing H2-assisted ex-situ biogas upgrading and acetate recovery holds great promise for achieving high value utilization of biogas. However, it faces a significant challenge due to acetate's high solubility and limited economic value. To address this challenge, we propose an innovative strategy for simultaneous upgrading of biogas and the production of medium-chain fatty acids (MCFAs). A series of batch tests evaluated the strategy's efficiency under varying initial gas ratios (v/v) of H2, CH4, CO2, along with varying ethanol concentrations. The results identified the optimal conditions as initial gas ratios of 3H2:3CH4:2CO2 and an ethanol concentration of 241.2 mmol L−1, leading to maximum CH4 purity (97.2 %), MCFAs yield (54.2 ± 2.1 mmol L−1), and MCFAs carbon-flow distribution (62.3 %). Additionally, an analysis of the microbial community's response to varying conditions highlighted the crucial roles played by microorganisms such as Clostridium, Proteiniphilum, Sporanaerobacter, and Bacteroides in synergistically assimilating H2 and CO2 for MCFAs production. Furthermore, a 160-day continuous operation using a dual-membrane aerated biofilm reactor (dMBfR) was conducted. Remarkable achievements were made at a hydraulic retention time of 2 days, including an upgraded CH4 content of 96.4 ± 0.3 %, ethanol utilization ratio (URethanol) of 95.7 %, MCFAs production rate of 28.8 ± 0.3 mmol L−1 d−1, and MCFAs carbon-flow distribution of 70 ± 0.8 %. This enhancement is proved to be an efficient in biogas upgrading and MCFAs production. These results lay the foundation for maximizing the value of biogas, reducing CO2 emissions, and providing valuable insights into resource recovery.

CO2 capture and conversion to methane with Ni-substituted hydrotalcite dual function extrudates

Nickel-substituted hydrotalcites promoted with cesium were synthesized, and their dual-function ability for acting as a high-temperature CO2 adsorbent and methanation catalyst was assessed. The Ni-substituted hydrotalcites were synthesized by the co-precipitation method with (Ni + Mg)/Al and Ni/(Ni + Mg) molar ratios of 3 and 0.4, respectively. Extrudates of these materials were prepared and tested for CO2 capture from a synthetic flue gas mixture (with 15 vol% CO2) and subsequent methanation. Two wetness impregnation routes with cesium were considered: i) powder impregnation before the extrudates shaping (PI) or ii) the direct impregnation of the extrudates (EI). EI materials were able to sorb more CO2 due to their higher number of basic sites, especially those with strong strength. However, while the amount of CH4 produced by both types of samples is similar, EI materials are less selective to methane (i.e., they present a slightly higher formation of CO). For this reason, Cs-doped PI extrudates were selected and used in the parametric study. Under cyclic steady-state and operation at 350 °C and 1 bar, the reactor packed with this material allowed capturing ca. 0.24 molCO2 kgDFM-1 and the complete conversion of the captured CO2 into CH4. However, a substantial amount of H2 is released from the reactor without being converted, hindering the CH4 purity level. This indicator can be increased at the expense of some product loss by performing a short-duration purge with CH4 between the capture and methanation stages. Remarkedly, this material showed outstanding stability during the continuous cyclic operation, without CO formation.

A cyclic sorption-reaction process for continuous synthetic methane production from flue gas and green hydrogen

In the scope of Power-to-Methane processes, green H2 and CO2 (captured from flue gas, for instance) can be catalytically converted to CH4 (or synthetic natural gas), an easily stored and distributed energy vector. From the perspective of process intensification, the CO2 capture (by sorption) and conversion can be performed in the same multifunctional reactor, thus requiring fewer operational units and potentiating the synergy between the exothermic methanation and the endothermic CO2 desorption. In this regard, two reactors were individually packed with alternating layers of two commercial materials, a CO2 sorbent (K-promoted hydrotalcite) and a methanation catalyst (Ru-based), and tested to carry out a cyclic sorption/reaction process. The feed during the sorption stages consists of a synthetic flue gas stream containing 15 %vol. of CO2 balanced in N2. In the reactive regeneration stage, the inlet is pure H2, which reacts with the previously sorbed CO2, producing CH4. The two sorptive reactors were operated in parallel and out of phase until a cyclic steady-state was reached. The employed materials showed no signs of deactivation during over 150 h of time on stream. The performance of the cyclic unit was tested under different operating conditions, in a parametric study that assessed the influence of each variable on several performance indicators like CH4 productivity and CH4 purity. The study was carried out at atmospheric pressure and centered on a reference experiment in which a CH4 productivity of 1.12 molCH4·kgcat–1·h−1 and a purity of 60.8 % were obtained, using 200 mLN·min−1 of synthetic flue gas (15 %vol. CO2) and 100 mLN·min−1 of H2, at 350 °C and a switching time of 30 min. Nevertheless, all performance indicators could be improved in other operating conditions (CH4 purity up to 71.6 % and CH4 productivity up to 1.32 molCH4·kgcat–1·h−1).

Investigating two synthetic routes for gas hydrate formation to control the trapping of methane from natural gas

Gas hydrate is considered as an emerging technique for natural gas storage and CH4 separation from natural gas, because assorted nano-sized cages of gas hydrate allow selective trapping of specific gaseous guests such as CH4, C2H6, and C3H8. When a mixture of CH4, C2H6, and C3H8 forms gas hydrates with a liquid promoter at 5.6 mol%, sII large cages (sII-L) can be fully occupied by a promoter, while only CH4 remains in the hydrate phase by occupying sII small cages (sII-S). Based on this, can CH4 be selectively stored or separated from natural gas? Here, we aimed to elucidate the cage-filling behavior of gaseous and liquid guests in CH4 + C2H6 + C3H8 hydrates formed with the addition of tetrahydrofuran (THF), trimethylene oxide (TMO), or 1,3-dioxolane (DIOX). We examined feasible strategies for synthesizing gas hydrate focusing on ice-borne or water-borne route. Our results suggested that ice-borne gas hydrates were a promising option for CH4 separation, because only CH4 was trapped in ice-borne gas hydrates. In contrast, C2H6 and C3H8 shared sII-L with promoters in water-borne gas hydrates. Although partial occupation of sII-L by C2H6 and C3H8 reduced CH4 purity in the hydrate phase, it resulted in enhancing the CH4 uptake in THF hydrate formed via water-borne route, unlike TMO and DIOX. The present findings will broaden the understanding of the cage occupation behavior in ternary gas hydrates formed with liquid promoters, and provide feasible strategies for synthesizing gas hydrates for storing natural gas or CH4 separation from natural gas.

Multi-factorial central composite and complete mixing model for optimizing industrial biogas valorization

There is an increasing interest to use CO2 selective membrane to purify methane from natural or biogas because of the ease of access to this membrane configuration. The use of this membrane, however imposed an additional strain on the system because there was H2S contamination in the enriched product stream and requirement of high CO2/CH4 selectivity. To address this issue, an empirically validated complete mixing model integrated with a multi-factorial central composite design was applied to optimize the membrane process in methane enrichment from biogas containing H2S. The optimization resulted in 100% methane purity and 91.51% recovery using a methane selective membrane at 837.43 CH4/CO2 selectivity, 35.00 bar feed pressure, and 0.643 stage cut, without H2S contamination in the product stream. For comparison with CO2 selective membrane, methane purity of 91.74% and 90.26% recovery were achieved using CO2/CH4 selectivity of 1086.87, 31.82 bar feed pressure, and 0.310 stage cut. Methane selective membrane was performed better than CO2 selective membrane on four fronts, namely lower selectivity, higher CH4 recovery, higher CO2 recovery, higher CH4 purity, and the product stream was H2S free. CO2 selective membrane was superior in other aspects, namely higher CO2 purity, lower stage cut and feed pressure.

The effect of pressure on the concentration of methane and carbon dioxide absorption in biogas

Biogas is playing a vital role in the emerging market for renewable energy. Biogas is an alternative energy that can be used as a substitute for fossil energy. The main composition of biogas is methane gas (CH4) and carbon dioxide gas (CO2) with a small amount of hydrogen sulfide (H2S). Therefore, it is necessary to do treatment to enrich gas methane and reduce CO2 and H2S content in optimizing the use of biogas. Absorption is a CO2 capture technology often used in the chemical industry. A chemical absorption technique using a solvent solution is one of the most widely used techniques to capture CO2. This research uses MDEA (Nmethyldiethanolamine) solvent as an alkanolamine solution dissolved in water. The unit operation usually used to carry out the absorption process is an absorber, and the type of absorber used is a spray column absorber. Simulating variations in pressure on the absorber was conducted in this research. The result of this research is that the increase in pressure will increase the composition percent of CH4 in sweet gas and increase the amount of CO2 absorption in biogas. The best result of absorber pressure is obtained at a pressure of 48 bar. The purity of CH4 and %removal of CO2 in biogas can be solved simply by using the evaluation parameters of the various absorber pressures presented, which we expect to contribute to the biogas industry in the future.

Modelling of methane sorption enhanced reforming for blue hydrogen production in an adiabatic fixed bed reactor: unravelling the role of the reactor's thermal behavior

Methane sorption enhanced reforming (SER) is investigated in this work as a promising route for blue H2 production. A 1-D dynamic heterogeneous model is developed to evaluate the thermal behavior of a fixed bed reactor under adiabatic conditions. The heterogeneous model allows to decouple the feed gas temperature from the initial solid one in order to investigate the behavior of the reforming step in a temperature swing reforming/regeneration process. The effects of the feed gas temperature, the initial bed temperature, and the bed thermal capacity are studied by evaluating the global impact of each parameter through a set of key performance indices (CH4 conversion, H2 yield and purity, carbon capture ratio) calculated as integrals over the duration of the reforming step. The results highlight the minor effect of the initial bed temperature on the process performances showing the potential of minimizing the extent of a cooling step between regeneration and reforming stages. Besides, due to the endothermic nature of the methane sorption enhanced reforming process at high temperatures, thermal energy must be provided to the SER process to achieve high CH4 conversion and high carbon capture ratio. This can be made either in the form of high feed temperature or by utilizing the energy stored in the bed benefiting from the bed thermal capacity.

Novel monoethanolamine absorption using ionic liquids as phase splitter for CO2 capture in biogas upgrading: High CH4 purity and low energy consumption

Ionic liquid, as a green absorber, could be effectively combined with amine solution to obtain a mixed system that effectively improved the upgrading effect and reduced regeneration energy consumption in the biogas upgrading process. In this work, an aqueous biphasic solvent consisting of 30 wt% monoethanolamine (MEA) and 40 wt% 1-butyl-3-methylimidazolium tetrafluoroborate (BF) was prepared, which had advantages such as excellent biogas upgrading effect, high CO2 recycling loading and low regeneration energy consumption. The effect law of ionic liquid addition to amine solution on biogas upgrading and mass transfer characteristics was comprehensively evaluated through the analysis and prediction of physical parameters, and the excellent biogas upgrading characteristics of the mixed system and the favorable effect of ionic liquid on mass transfer were verified. The solvent effect of the ionic liquid could appropriately lower the energy barrier for the reaction to produce carbamate and improve the CO2 capture effect. The higher polarity of the ionic liquid solvent also provided the driving force for the phase separation and accelerated the phase separation process. The distribution pattern of CO2 loading with time and space was comprehensively studied by using amplified phase separator, covering the whole process variation of phase separation. The important role of intermolecular forces in the phase separation process was discussed, and the important reasons for the occurrence of phase separation in phase separators were explained.

Exergy-based sustainability analysis of biogas upgrading using a hybrid solvent (imidazolium-based ionic liquid and aqueous monodiethanolamine)

Energy may be generated in large quantities from fossil fuels, but this comes with environmental concerns. Thus, renewable resources like biogas, comprising carbon dioxide and methane, should be used alone or in combination with fossil fuels to mitigate the environmental footprints of energy generation systems. In this study, a new concept of hybrid solvent was presented, which combines 1-octyl-3-methylimidazolium tetrafluoroborate with aqueous mono diethanolamine for biogas upgrading process to provide high purity (≥ 99 wt%) and recovery (≥ 99 wt%) of biomethane. The process was simulated in ASPEN Plus® V.11. The thermodynamic framework was validated against experimental data, and rigorous regression was conducted to obtain binary parameters. To establish the efficacy of the suggested hybrid solvent, three scenarios were studied by altering the concentration of ionic liquid (5–20 wt%) linked with amine and compared to aqueous mono diethanolamine as the base case (50 wt%). The results showed that a hybrid solvent with 5 wt% 1-octyl-3-methylimidazolium tetrafluoroborate could increase CH4 purity to 99% (mol%). The hybrid solvent led to an energy saving of 64.94% compared to the amine-based system. Thermodynamic irreversibilities showed that 5 wt% 1-octyl-3-methylimidazolium tetrafluoroborate improved exergy efficiency by 54% over the amine-based procedure. Environmentally, the hybrid solvent system also achieved a higher capture rate (99%) and lower emissions (0.017 kW/kmol). Comparing the economic prospects, 5 wt% 1-octyl-3-methylimidazolium tetrafluoroborate saved 56% on total capital cost, making it competitive from an investment perspective.

CO2 methanation enhanced with a cyclic SERP process using a commercial Ni-based catalyst mixed with 3A zeolite as adsorbent

Valorization of greenhouse gas CO2 is needed to mitigate and reduce climate change impact. The sustainable production of methane to obtain synthetic natural gas (SNG) is one of the most attractive solutions, given current needs. The methanation reaction, known as the Sabatier reaction, is limited by the formation of CO as an intermediate at high temperatures and the formation of H2O as a by-product. For this reason, recent research has focused on the use of catalysts/adsorbents mixtures capable of retaining the water produced as well as being methane-selective at low temperatures. In this work, the kinetic model of a commercial Ni/SiAl catalyst in an experimental reaction bed was obtained. Moreover, the water adsorption capacity of the catalyst has been studied in the typical reaction temperature range (473–623 K), and the adsorption equilibrium and diffusional parameters were determined. With the data obtained from these experiments, the methane production in a cyclic Sorption Enhanced Reaction Process (SERP) has been simulated, adding zeolite 3A as a water selective adsorbent. The process consists of three consecutive stages: an adsorption/reaction stage and two other stages, rinse and purge, to desorb the adsorbed water and remove it by condensation. A sensitivity analysis has been carried out to determine the effect of the operational variables on the process performance to obtain methane suitable for domestic natural gas consumption. A CO2 conversion of 99.7 %, methane selectivity of 99.9 %, molar purity of CH4 of 98.2 % as product, and compression energy consumption (6.97 kJ/mol CH4) 99 times lower than methane combustion energy has been obtained, proposing, therefore, a new process of methanation.

Production of H2 via sorption enhanced auto-thermal reforming for small scale Applications-A process modeling and machine learning study

Small scale production of H2 via sorption enhanced auto-thermal reforming (SEATR) of methane is simulated using Ni based catalyst and CaO sorbent for the capturing of CO2. One dimensional heterogeneous reactor model was developed using gPROMS model builder to study the performance of SEATR reactor. At low pressure mode, the process was evaluated for varying temperature, pressure, gas flux and steam to carbon ratio. Chemical equilibrium with application (CEA), an equilibrium based software was employed so as to compare both equilibrium and simulation results. Under a range of temperature (500–1000 K), pressure (1–10 bar), S/C ratio (1–6), and O/C ratio (0.2–0.6) close to equilibrium conditions, model outputs satisfactory results with regard to CH4 conversion, CO2 capturing, H2 yield and purity. At 750 K, 2.9 bar, Gs of 0.4 kg/m2 s, S/C of 3 and O/C of 0.45, H2 purity and CH4 conversion achieved was 97% and 94% respectively in comparison with 66% and 77% from conventional auto-thermal reforming. In Bayesian Regularization (BR), Mean square error(MSE) and R value is minimum for neural network algorithm comparison. It accounts for 1.2e−10 and 0.999 respectively. BR produces minimum error with increase in Epochs and gradients values highlighting maximum performance with optimize computation time for process modeling data-integration studies and generalization.

H2 mass transfer – A key factor for efficient biological methanation: Comparison between pilot-scale experimental data, 1D and CFD models

Biological methanation is an emerging Power-to-Gas technology that provides a flexible, efficient, and long-term storage system to address the renewable energy intermittency issue. However, scaling-up a biological methanation plant requires overcoming the bottleneck of H2 gas–liquid mass transfer. The current work demonstrates analytically, experimentally and numerically the importance of H2 mass transfer on CH4 purity and productivity. The influence of sparger design was investigated in a pilot-scale bubble column bioreactor. An increase of 265 % in kLa and subsequently 81 % in CH4 purity is obtained when a uniform porous plate is used over a 4-points porous sparger. Numerical simulation of the bioreactor was performed using a multiscale modeling approach coupling 1D and CFD models. The numerical simulations were validated satisfactorily by the experimental data. Parametric studies were also performed using the 1D model to reveal the impact of operating conditions on the bioprocess. An analytical solution for the productivity of biomethanation reactors at steady-state is proposed based on mass balances and provides new insights into biological methanation. It is found that any modification of operating parameters that improve H2 mass transfer, such as higher gas recirculation rate and smaller bubble size, leads to a better bioreactor performance.

Performance study of a methanation process for a syngas obtained from a sorption enhanced gasification process

Synthetic Natural Gas (SNG) produced from renewable sources constitutes a solution for the transition to net-zero emissions for natural gas relying sectors. From the different biomass-to-SNG routes, the gasification of biomass to produce a syngas that is further converted into SNG has a large potential in a near future. In this work, the methanation process under quasi commercial conditions was demonstrated for a syngas generated from biomass gasification. The influence of process variables like pressure, steam/CO ratio and syngas composition (i.e. content of CO2, CH4 and light hydrocarbons) were analyzed to evaluate the maximum CH4 purity achieved in a commonly used two-step methanation process. Moreover, the need of adding green H2 produced from renewable sources was studied to adjust the H2/CO/CO2 ratio for methanation. A product gas with 78 %vol. CH4 was produced using a commercial Ni-based catalyst, being the conversion of CO2 the limiting factor towards the fulfilment of a higher CH4 content (only 37 % of CO2 conversion was reached in the second methanation reactor at 270 °C, 4 bar and 2140 h−1 of gas space velocity). Alternative operating conditions and/or intensified process schemes stand as the most promising alternatives for the conversion of syngas into high purity SNG.

Nitrogen replacement strategy to efficiently enhance methane recovery of the methane purification process using nitrogen equilibrium selective sorbent

MIL-100(Cr) is a very promising sorbent based on an equilibrium effect for rejecting nitrogen from substandard quality natural gas. In this work, methane purification from a 25% N2/75% CH4 mixture has been studied using MIL-100(Cr) as a sorbent, on the basis of which an N2 displacement-assisted vacuum-pressure swing adsorption (N2DIS-VPSA) process is proposed. Adsorption equilibrium isotherms of N2 and CH4 have been measured over MIL-100(Cr) powder and pellets, and N2/CH4 selectivity has also been calculated based on ideal adsorbed solution theory (IAST). A rigorous mathematical model has been used to investigate gas- and solid-phase concentration evolution histories to deeply understand the internal mechanism of the N2 replacement process. Breakthrough experiments have revealed that a returned N2 flow could replace almost all of the co-adsorbed CH4, greatly improving the CH4 recovery. At the same time, both experimental and simulation results showed that the N2 replacement process had no effect on the purity of CH4. However, it should be noted that an excessively long N2 replacement time led to incomplete subsequent regeneration of the sorbent, thereby reducing the purity of CH4 in the N2DIS-VPSA process. By optimizing the N2 replacement duration, a high purity CH4 (97%) product gas could be obtained with 92% recovery, much higher than that of MIL-100(Cr) applied into common PSA process (58%).