The presence of mixed microplastics (MPs) in anaerobic wastewater treatment processes has been shown to impede fermentation performance by suppressing microbial activity. Microbial electrosynthesis ...(MES), with its extensive potential, offers a promising solution for refractory substances management and methane recovery, achieved through the enhancement of microbial metabolism and interspecies electron transfer. This study, therefore, delves into the functional impacts and the microbial response to MES in the remediation of wastewater contaminated with mixed-MPs. Results indicated that mixed-MPs could inhibit methane production (−52.38%) and substance removal (−26.59%), and MES could effectively mitigate this inhibitory effect (−22.86%, −19.01%). Concurrently, MES also boosts enzymatic activities pivotal for electron transfer, such as cytochrome c and nicotinamide adenine dinucleotide (NADH), as well as those linked to energy metabolism like adenosine triphosphate (ATP). Furthermore, MES bolsters microbial resistance to mixed-MPs, as evidenced by an increase in extracellular polymeric substances (EPS), albeit with a minor rise in reactive oxygen species (ROS) production and lactate dehydrogenase (LDH) release. Correspondingly, electric stimulation promoted the enrichment of functional microorganisms associated with fermentation, acetate production, electrogenesis, and methanogenesis, and stimulated elevated expression levels of genes related to methane metabolism. Notably, the Methanothrix-mediated acetoclastic pathway emerges as the predominant methanogenic route, succeeded by the Methanobacterium-driven hydrogenotrophic pathway. Lastly, the study underscores the supportive role of applied voltage and carriers in energy metabolism and substance transport, which are associated with methanogenesis. Overall, MES demonstrates efficacy in mitigating the biotoxicity induced by mixed-MPs exposure and in enhancing anaerobic wastewater treatment and methane recovery.
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•Applied voltage mitigated microplastic pollution.•Electric stimulation enhanced methane yield and reduced organic acid accumulation.•Functional enzyme activities and microbial resistance were enhanced.•Methane metabolism-related microbial abundance and gene expression were both elevated.•Methane metabolism was driven by acetoclastic pathways, via acetate-CoA ligase.
Thermal enhanced methane recovery inevitably aggravates the soil and water loss, causing severe harm to the sustainability of groundwater environment and the surrounding ecosystem. Therefore, ...quantitative analysis of the effect of thermal enhanced methane recovery on groundwater loss and ecological risk of coalbed methane development zone is necessary. In this study, a coupling model of gas drainage and groundwater loss is established. The model considers the dynamic gas diffusion of coal matrix, the two-phase flow of water and gas, and the influence of temperature on such flow. Based on this model, characteristics of groundwater loss of coal seam reservoir caused by enhanced methane recovery are analyzed, and the ecological risk assessment of methane recovery zone is realized. Results indicate that during heat injection, the permeability of the coal seam increases with distance from the borehole due to the competition between two-phase flow and temperature. High temperature develops the permeability, gas production, and water production of the reservoir. The change rules of water and gas productions are similar with initial increases and subsequent declines. The influence of coal gas diffusion on groundwater loss has a certain time lag. In the early stage, the dynamic attenuation of gas diffusion is not apparent. In the later stage, the supplement rate of gas from matrix to fracture decreases. The initial saturation has a significant influence on the water production rate in the early stage. A large Langmuir volume constant not only strengthens the peak value of gas drainage rate but also the gas drainage rate itself in the later declining period. Large scale coalbed methane development will face ecological risks such as water environment pollution, habitat destruction and soil degradation, which is the key aspect of ecological environment management and risk prevention.
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•Enhanced gas extraction can affect the sustainability of groundwater environment.•A thermal two-phase flow model is established to analyze groundwater loss.•The influence of the gas diffusion on groundwater loss has a certain time lag.•The influence of heat injection temperature on groundwater loss is analyzed.
Accurate acquisition of fluid flow patterns is crucial for economic exploitation and production prediction of hydrate-bearing sediments (HBSs). In this study, the clayey-silt marine sediments ...obtained from the South China Sea are used to remold core to investigate the effects of hydrate saturation, effective stress, pore pressure and osmotic pressure on the gas flow patterns. The results indicate that when the hydrate saturation is increased from 24.73% to 48.34%, the gas flow transitions from viscous flow to slip flow, and the gas slippage effect gets more and more pronounced with increasing hydrate saturation. The increase in effective stress leads to an increased sensitivity of gas permeability to changes in osmotic pressure, and it is more obvious at lower hydrate saturation. Elevated pore pressure leads to a small decrease in gas permeability but does not alter the flow pattern of fluid flow in the marine sediment. In addition, reducing the osmotic pressure can effectively weaken the slippage effect. Viscous flow and slip flow control the type of gas flow in HBSs, but slip flow dominates. Finally, a semi-empirical permeability model that considers the effect of hydrate saturation and effective stress on gas permeability is fitted with experimental data.
•Hydrate and effective affect the gas flow patterns in marine sediments.•Effective stress affects the sensitivity of gas permeability to osmotic pressure.•Gas slip flow pattern dominates in marine sediments.•A semi-empirical permeability model considering pressure effects is proposed.
Biogas is experiencing a period of rapid development and biogas upgrading is attracting increasing attention. Consequently, the market for biogas upgrading is facing significant challenges in terms ...of energy consumption and operating costs. Selection of upgrading technology is site-specific, case-sensitive and dependent on the biogas utilisation requirements and local circumstances. Therefore, matching the technology selected for use to specific requirements is significantly important. This paper systematically reviews the state-of-the-art of biogas cleaning and upgrading technologies, including product purity and impurities, methane recovery and loss, upgrading efficiency and the investment and operating costs. In addition, the potential utilisation of biogas and the corresponding requirements on gas quality are investigated in depth. Based on the results of comparisons between the technical features of upgrading technologies, the specific requirements for different gas utilizations and the relevant investment and operating costs, recommendations are made regarding appropriate technology.
Enhanced coalbed methane recovery and CO2 geostorage in coal seams are severely limited by permeability decrease caused by CO2 injection and associated coal matrix swelling. Typically, it is assumed ...that matrix swelling leads to coal cleat closure, and as a consequence, permeability is reduced. However, this assumption has not yet been directly observed. Using a novel in situ reservoir condition X‐ray microcomputed tomography flooding apparatus, for the first time we observed such microcleat closure induced by supercritical CO2 flooding in situ. Furthermore, fracturing of the mineral phase (embedded in the coal) was observed; this fracturing was induced by the internal swelling stress. We conclude that coal permeability is drastically reduced by cleat closure, which again is caused by coal matrix swelling, which again is caused by flooding with supercritical CO2.
Key Points
In situ reservoir condition micro‐CT imaging of coal flooded by supercritical CO2
Microcleat closure due to supercritical CO2 injection
Internal swelling stress induced fracturing of the mineral phase (inside the coal)
•Changing characteristics of the permeability in both CO2-ECBM test and N2-ECBM test were studied.•A two-stage flushing process on flowing capacity was proposed based on the flow state in coal.•The ...hysteresis of pressure transfer plays a decisive role in the changing characteristics.•Both the adsorption stress and volumetric strain are different in N2-ECBM and CO2-ECBM test.
Compared with the pre-modification technique, such as hydraulic fracturing, the gas injected for enhanced coalbed methane recovery (ECBM) technique offers not only a good economic advantage by enhancing the CH4 recovery, but also provides an environmental benefit with CO2 storage, and the changing characteristics of the flow capacity in coal reservoir critical affect both the CH4 production and CO2 geo-sequestration. In this pursuit, the present study envisaged the determination of the change in characteristics of the gas flow capacity in ECBM using N2 and CO2 core flood experiments on CH4 saturated bituminous coal. The critical controlling factors of the changing characteristics were discussed. The whole experimental process was divided into two stages, in which the first stage was an unsteady flow in coal that directly led to a sharp drop on both the volumetric strain and the outflow rate, while the second stage was a steady flow in coal. It was found that the changing trend of permeability in both tests followed the same relationship: rise (Ⅰ), stable (Ⅱ), slowly drop (III), steady-state (IV). The differences on both the flowing ability and the displacing effect between CO2 and N2 mainly played the role in the outflow rate, volumetric strain, and the duration of each stage in permeability. Total recovery of CH4 was 66% at the stages Ⅰ + Ⅱ + III in N2-ECBM test, while it was obtained 80% after the unsteady flow stage in CO2-ECBM test, thereby showing the different characteristic between two tests. Based on the effective stress principle, the changing characteristics in stress caused by adsorption were discussed to analyse the differential flowing characteristics of two tests. Our results provide new insight into the changing characteristics of flow capacity on coal in ECBM process.
The microscopic mechanism of the competitive adsorption of CH4, CO2 and N2 in coal is the theoretical basis for enhancing coal seam gas recovery by injecting CO2 (CO2-ECBM). Based on this principle, ...this work used Grand Canonical Monte Carlo and Molecular Dynamics to investigate the microscopic adsorption mechanism of single, binary, and ternary component gases in Wiser bituminous coal molecules. The adsorption mechanism was explored by changing gas composition and concentration. The comparison of adsorption separation coefficients suggested that CO2 had the highest adsorption capacity, whereas N2 had the lowest capacity. When the CO2 concentration in the gas mixture was high, the adsorption amount was large and the adsorption separation coefficient was small. This finding indicated that high concentrations of CO2 had negative effects on competitive adsorption. Energy changes were also evaluated. The potential energy between CO2 and the framework was the strongest among the two other gases. The inhibition of CH4 adsorption intensified with decreasing molar fraction of CO2. This phenomenon was explained from the perspective of heat of adsorption. As the molar fraction of CO2 in the adsorption system decreased, the heat of the isotherm adsorption increased. Meanwhile, the adsorption system became unstable and the capacity of CH4 adsorption on the framework weakened. Results provide a theoretical basis for the use of CO2-ECBM.
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•Molecular simulation was carried out to study the microscopic adsorption mechanism of single- and multi-gases in bituminous coal.•The greater the content of strongly adsorbed gas, the weaker the gas adsorption selectivity.•Van der Waals and electrostatic energy are the main potential energy affecting gas adsorption in coal molecules.•The energy distribution of different types and contents of gas in the adsorption system is analyzed.•Clarified the linear relationship between the heat of adsorption and the amount of adsorption.
Liquid CO2 (LCO2) enhanced coalbed methane recovery had been studied in laboratory experiments and field applications, supporting many improvements and achievements. Previous studies primarily ...investigated the gas bursting, flooding effect and adsorption effect; however, the freezing-thawing phenomenon (drikold formation and gasification) that commonly occurs during the LCO2 injection process was insufficiently studied. The freezing-thawing phenomenon might enhance the pore volume and change the permeability evolution of the coalbed; thus, cyclical LCO2 injection was proposed to exploit the phenomenon, and the influence of multiple freezing-thawing cycles on the coal pores was investigated in this paper. Nuclear magnetic resonance (NMR) and infrared thermal imagery (ITI) were used to monitor the pore variation and surface temperature distribution, respectively. Low temperatures could make the saturated water in the pores freeze and undergo a 9% volume increase. The three coals used in this experiment displayed different crack intensities and forms with ITI. After cyclical LCO2 injection, the NMR amplitude increased, and the T2 range was widened under a saturation condition, while the cores under a centrifuge state had lower amplitudes and a narrower T2 range; this difference indicated that the pore structure could be altered by multiple freezing-thawing cycles of LCO2. The more freezing-thawing cycles the cores experienced, the greater the change in pore structure was. The total porosity φt and effective porosity φe increased while the residual porosity φr and T2cutoff values decreased with more freezing-thawing cycles. However, the variations with coal rank were observed; with higher coal ranking, φt and φe increased less, and the φr and T2cutoff values decreased less, which suggests that lower ranking coals could be most easily affected by the LCO2 enhanced recovery method and have the most improved pore connectivity. Moreover, the enhancement ratio of φt and φe increased for all three coals tested, which could be fit with quadratic functions with fit coefficients greater than 0.99. The increasing relative ratio De/t of anthracite was fit with a linear function, while the lignite and bitumite were fit with quadratic functions. These functions all indicate that the multiple freezing-thawing cycles of LCO2 injection had a positive impact on the enhancement efficiency of pore porosity. Finally, a potential field application of cyclical LCO2 injection was also discussed to improve the fracturing effect.
This study aimed to evaluate the feasibility of degassing membrane (DM) technology for recovering dissolved methane from AnMBR effluents. For that purpose, a PDMS membrane module was operated for ...treating the effluent from an AnMBR prototype-plant, which treated urban wastewater (UWW) at ambient temperature. Different transmembrane pressures and liquid flow rates were applied for evaluating methane recovery efficiency. Maximum methane recoveries were achieved when increasing the vacuum pressure and reducing the liquid flow rate, reaching a maximum methane recovery efficiency of around 80% at a transmembrane pressure (TMP) of 0.8 bars and a treatment flow rate (QL) of 50 L h−1. The results revealed that the combination of PDMS DMs and AnMBR technology would allow to reduce the energy demand of UWW treatment, achieving net energy productions while reducing greenhouse gas emissions. Optimum operation was determined at a TMP of 0.8 bars and a QL of 150 L h−1 when combining energy, environmental and economic targets. Under these operating conditions, the combination AnMBR + DM resulted in energy requirements and greenhouse gases emissions of −0.040 kWh and 0.113 kg of CO2-eq per m3 of treated water, respectively, resulting in a DM payback period of around 10.5 years.
•PDMS membranes allow a suitable methane capture from AnMBR effluents.•AnMBR energy efficiency can be improved by employing PDMS membranes.•Greener wastewater treatments can be reached by coupling PDMS membranes with AnMBR.•PDMS membranes showed a feasible potential for full-scale applications.
•Micro mechanism of hydrate CH4-CO2 replacement are studied using DSC, Raman and QM.•Replacement depends on the difference between potential energy of destruction and binding energy.•Replacement is a ...process in which hydrate cage is broken followed by preparing.•Replacement only happens when the concentration CO2 around CH4 hydrate is higher than 20%.
New energy development and carbon emission reduction are two important issues of sustainable development for human beings. Hydrate-based methane - carbon dioxide (CH4-CO2) replacement is the promising technology for it can realize CH4 production from natural gas hydrate (NGH) and CO2 capture and sequestration (CCS) in the strata simultaneously and stably. This work reported the micro mechanism and the influence of the hydrate-based CH4-CO2 replacement by differential scanning calorimetry (DSC) measurements and in situ Raman spectroscopy measurements. It was found the replacement is affected by the potential energy of destruction (Eped) and the binding energy (Eb). The replacement happens only when the Eped is larger than the Eb. The concentration of CO2 around the hydrate is the controlling influence on the replacement. The higher the concentration of CO2, the greater the Eped, and the higher CH4 production efficiency.
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