Adsorption Characteristics Of CH4 Research Articles

Pore structure and gas adsorption characteristics in stress-loaded shale on molecular simulation

Since stress from overlying strata is one of the primary factors influencing pore structure and gas content in shale reservoirs, understanding its role is crucial for developing shale gas fields. In this study, Molecular Mechanics and Molecular Dynamics methods were employed to investigate the intrinsic relationship between pore structure, energy, and adsorption performance of the organic matter matrix under different stress conditions. The results show that the pore structure of the organic matrix can be categorized into three stages with the variation of external stress, with 0.2 and 0.8 GPa as the boundaries. The effect of external stress significantly impacts the methane adsorption capacity in the organic matrix. Changes in stress not only result in a reduction of adsorption space but also an increase in adsorption sites. These two factors have opposite effects on CH4 adsorption in the model. That is, the negative impact of the reduction in adsorption space outweighs the positive impact of the increase in adsorption sites, resulting in a decrease in the adsorption capacity. At 0.8 and 1 GPa, the maximum CH4 adsorption capacity was reduced by 70 % and 58 %, respectively. This study provides an important reference for understanding the pore structure of the organic matrix and the adsorption characteristics of CH4 in shale gas reservoirs.

Molecular simulation of CH4 adsorption characteristics in bituminous coal after different functional group fractures

To explore the effect of different functional group fractures in coal on the adsorption characteristics of CH4, molecular models of bituminous coal (Wiser) with fractured oxygen-containing functional groups (W–O5), sulfur-containing functional groups (W–S5), and nitrogen-containing functional groups (W–N5) were constructed. The microscopic mechanism by which the fracture of different functional groups in bituminous coal molecules affects CH4 adsorption was analyzed using Density Functional Theory (DFT), Molecular Dynamics (MD), and Grand Canonical Monte Carlo (GCMC) methods. The results show that functional groups facilitated the polarization of CH4 during the adsorption process, with the following influence relationship: oxygen-containing functional groups > sulfur-containing functional groups > nitrogen-containing functional groups; Fracturing functional groups in bituminous coal molecules weakened the adsorption capacity and reduced adsorption sites of CH4, the sequence of CH4 adsorption was Wiser > W–N5 > W–S5 > W–O5. High temperature reduced the effect of nitrogen-containing functional group fractures on CH4 adsorption, but had no obvious effect on sulfur-containing and oxygen-containing functional groups. The interaction between CH4 and the coal molecule decreased after functional group fractures, causing the enhanced diffusion of CH4 on bituminous coal molecules, and the order was W–O5 > W–S5 > W–N5 > Wiser. Additionally, the fracture of functional groups decreased the pore size of the bituminous coal molecules, which inhibited CH4 adsorption but facilitated its diffusion. The research provides theoretical support for enhancing CH4 desorption and improving the recovery rate of coalbed CH4.

Characterization of kerogen molecular structure and its effect on methane adsorption behavior: A comparative study on outcrop and core samples from Longmaxi shale

It has been known that adsorbed gas, mainly CH4, serves as the dominant source of shale gas yield in the middle and later stages of reservoir exploitation. Therefore, it is of great significance to study the microstructure of kerogen (organic matter rich in shale) and its effect on CH4 adsorption characteristics. In this paper, for the first time, a comparative study was conducted to experimentally characterize the microstructure of both outcrop and core kerogen samples from Longmaxi Shale, Sichuan Basin, SW China, using 13C NMR, FTIR, and XPS. Great differences in the chemical structure such as carbon skeleton (e.g., aromaticity, aliphatic branch) and heteroatoms (e.g., distribution and proportion of various types of functional groups) between the two kerogen samples have been identified. Molecular models of the outcrop and core kerogen samples were constructed based on the experimental data, that is, C237H219O21N5S4 for the former and C239H175O27N5S4 for the latter. CH4 adsorption isotherms obtained from gravimetric adsorption test and molecular simulation using proposed kerogen molecular models are in reasonable agreement with each other, especially at low-pressure range. Interestingly, the adsorption capacity of CH4 in the core sample is found higher than that in the outcrop sample under the same condition, indicating the effect of kerogen microstructure on CH4 adsorption behavior. As a result, to precisely evaluate the adsorbed gas content in shale gas reservoirs, it is strongly recommended to use a core sample rather than an outcrop sample.

Thermodynamic analysis of the difference in adsorption characteristics of CH4 and CO2 on continental shale

In order to explain the difference in adsorption characteristics of CH4 and CO2 on continental shale from the perspective of thermodynamics, the isothermal adsorption experiments of CH4 and CO2 adsorbed by shale in Yanchang Formation in Ordos Basin were carried out, and the excess adsorption capacity was corrected to absolute adsorption capacity. Then the Clausius-Clapeyron equation was used to analyze the isosteric heat of adsorption of CH4 and CO2 on shale. The results show that, for calculating the absolute adsorption capacity, Ozawa empirical formula or Van der Waals approximation method should be used to calculate the adsorption phase density. The absolute adsorption capacity should be selected as the basic data for calculating the isosteric heat of adsorption. The reason is that the excess isosteric heat of adsorption has a negative value in the low adsorption capacity stage, which is contradictory to the fact that the adsorption process is exothermic, and is significantly higher than the absolute isosteric heat of adsorption. There is a good linear positive correlation between the isosteric heat of adsorption and the adsorption amount of CH4 and CO2 adsorbed by continental shale, and the isosteric heat of adsorption of CH4 is greater than that of CO2. The absolute initial isosteric heat of adsorption of CH4 and CO2 adsorbed by shale is 52.04 kJ/mol and 27.71 kJ/mol, indicating that the adsorption force of CH4 on Yanchang Formation shale is stronger than that of CO2.

Effect of dynamic fragmentation on microscopic pore structure in coal: New insights into CH4 adsorption characteristics

Tectonic coal is a necessary condition for coal and gas outburst, and its pore structure is dramatically altered. The pore structure of tectonic coal has unique characteristics in terms of mechanics and gas flow, which plays a critical role in eliminating outburst risk. Therefore, it is important to study the effects of dynamic fragmentation on microscopic pore structure and CH4 adsorption characteristics of coal body. In this study, coal samples from Xintian (XT) and Pingdingshan No.6 (P6) coal mines were crushed and sieved into three particle size fractions of 0.5–1 mm, 0.2–0.25 mm, 0.074–0.1 mm, respectively, simulating dynamic fragmentation by reducing the particle size. A novel index that evaluates the average single-particle equivalent pore length was proposed. This index, Lp, provides a more comprehensive quantitative characterization of the effects of dynamic fragmentation on microscopic pore structure by considering the dynamic particle size reduction process. The LP values of large-sized samples are 23.8 to 24.6 times that of medium-sized samples; the LP values of medium-sized samples are 12.8 to 17.1 times that of small-sized samples. The experimental results show that dynamic fragmentation significantly alters microscopic pore structure, as demonstrated by the positive correlation between the Lp and particle size. With the decrease in particle size, the amount of CH4 molecules adsorbed in XT samples increases from 105.870 × 1019 p/g to 106.934 × 1019 p/g, increasing by 1.0 %; that in P6 samples increases from 39.660 × 1019 p/g to 48.037 × 1019 p/g, rising by 21.1 %. Dynamic fragmentation has a relatively insignificant effect on CH4 adsorption capacity. The total adsorption equilibrium time for XT samples decreases from 242 h to 32 h, and that for P6 samples decreases from 1146 h to 100 h as the particle size decreases. Dynamic fragmentation significantly reduces CH4 adsorption equilibrium time. The results of this study contribute to the understanding of microscopic pore structure under the effects of dynamic fragmentation and new insights into CH4 adsorption characteristics, which are of great guiding significance to the occurrence mechanism of outburst prediction and risk.

Adaption of Theoretical Adsorption Model on Coal: Physical Structure

With the motivation to investigate the role of coal physical structure on the adsorption performance of coal reservoir, 18 different types of coal samples with different coal structures were collected from six coal profiles of four production mines located at China. The adsorption characteristics of CH4 on coal samples with different coal structures were examined, and then experimental results were fitted and analyzed by the Langmuir model and the adsorption potential model (D-R and D-A). The prominent factors in terms of adsorption capacity of coal with different coal structures and its adaptability to the model were discussed. Results indicate the following: a) under the condition of a similar coal rank, the adsorption performance of coal is governed by coal rock composition and adsorption heat, the effect of structural deformation on the adsorption performance of coal is not obvious; b) the Langmuir model has a certain adaptability to coal samples with different coal structures, while the D-R model is evidently not suitable to describe coal samples with scaly coal, part of broken coal with small vitrinite content; c) the D-A model has a high adaptability to coal samples with various coal structure types, and the stronger the coal deformation is, the higher the accuracy is.

Adsorption Characteristics of CH4 and CO2 in Shale at High Pressure and Temperature.

This work presents the adsorption behavior and appearance characteristics of CH4 and CO2 on the Longmaxi shale at high pressure and temperature. To investigate the variation of gas adsorption patterns under the constraint of pressure and temperature, the applicability of the theories of monolayer adsorption, multilayer adsorption, and micropore filling was discussed. The preferential selection coefficient of CO2 for CH4 under different conditions was characterized by the absolute adsorption capacity (Vabs) ratio of CO2 to CH4 (αCO2/CH4). Moreover, the implication of the CO2 injection to enhance gas recovery and the CO2 capture and storage (EGR–CCS) process was analyzed. The results exhibit that the excess adsorption curves of CH4 are smooth, and the experimental temperature has no noticeable effect on the shape of curves. At the same time, a “sharp peak” is recorded in the excess adsorption curves of CO2 at low temperatures (30 and 55 °C) near the critical pressure, which is quite distinct from the smooth curves at high temperatures (80 and 100 °C). Correspondingly, there are two “jump pressure” values in the density curves (30 and 55 °C) of the adsorption system and the density curves are divided into three stages. The Dubinin–Astakhov and Brunauer–Emmett–Teller (BET) models show an optimum degree of fit for CH4 and CO2 adsorption curves under all experimental temperature and pressure conditions. The Langmuir model fits the adsorption curves of 80 and 100 °C better, while the BET model is appropriate for 30 and 55 °C. The adsorption affinity of CO2 is higher than CH4, with the value of αCO2/CH4 in the range of 2.47–12.16. The value of αCO2/CH4 increases with a rise in pressure but is inhibited by high temperatures, while the inhibition is negligible when the experimental temperature exceeds 80 °C. The adsorption preferential of CO2 is stronger in the shallow reservoir (αCO2/CH4 > 10.5), and the application prospect of the EGR process is promising. In contrast, the adsorption preferential is slightly weakened in the deep reservoir (αCO2/CH4 < 4.5), which can be considered for CO2 capture, utilization, and storage. Results from this investigation provide novel insights on the adsorption characteristics of CH4 and CO2 on the shale matrix at high pressure and temperature. They are also expected to give certain enlightenment for the EGR–CCS process.

Mechanism and Characteristics of CH4/CO2/H2O Adsorption in Lignite Molecules

Adsorption characteristics of coalbed methane (CBM) are significant to investigate the absorption of coal, shale, and porous media. In particular, adsorption characteristics of CH4, CO2, and H2O play an important role in predicting CBM output and geologic sequestration potentials of CO2 in research fields of CO2-enhanced CBM recovery (CO2-ECBM) and sequestration of CO2. In this work, adsorption characteristics of CH4, CO2, and H2O in lignite molecules were simulated through the grand canonical Monte Carlo (GCMC) method and molecular dynamics (MD) method. Research results demonstrated that given the same temperature and pressure, the ultimate adsorption capacity of lignite per unit to H2O is the highest, followed by those of CO2 and CH4 successively. All isothermal adsorption curves conform to the “I-type” characteristics. In the saturated molecular configuration, gas molecules show different distribution patterns at two sides of the lignite molecule chain. Lignite has typical physical adsorption to CH4 and CO2, with adsorption energy provided by nonbonding energy. However, lignite has both physical adsorption and chemical adsorption to H2O, with adsorption energy provided by both nonbonding energy and hydrogen bond energy. High temperature is against adsorption of CH4, CO2, and H2O. Temperature might inhibit adsorption of gas molecules. Research conclusions lay foundations for the exploitation and development of CBM and relevant studies on sequestration of CO2.

Adsorption Characteristics and Controlling Factors of CH4 on Coal-Measure Shale, Hedong Coalfield

Adsorbed gas is one of the crucial occurrences in shale gas reservoirs; thus, it is of great significance to ascertain the adsorption capacity of shale and the adsorption characteristics of CH4. In this investigation, the Taiyuan–Shanxi Formations’ coal-measure shale gas reservoir of the Carboniferous–Permian era in the Hedong Coalfield was treated as the research target. Our results exhibit that the shale samples were characterized by a high total organic carbon (TOC) and over to high-over maturity, with an average TOC of 2.45% and average Ro of 2.59%. The mineral composition was dominated by clay (62% on average) and quartz (22.45% on average), and clay was mainly composed of kaolinite and illite. The Langmuir model showed a perfect fitting degree to the experimental data: VL was in the range of 0.01 cm3/g to 0.77 cm3/g and PL was in the range of 0.23–8.58 MPa. In addition, the fitting degree depicted a linear negative correlation versus TOC, while mineral composition did not exhibit a significant effect on the fitting degree, which was caused by the complex pore structure of organic matter, and the applicability of the monolayer adsorption theory was lower than that of CH4 adsorption on the mineral’s pore surface. An apparent linear positive correlation of VL versus the TOC value was recorded; furthermore, the normalized VL increased with the growth of the total content of clay mineral (TCCM), decreased with the growth of the total content of brittle mineral (TCBM), while there was no obvious correlation of normalized VL versus kaolinite, illite and quartz content. The huge amount of micropores and complex internal structure led to organic matter possessing a strong adsorption capacity for CH4, and clay minerals also promoted adsorption due to the development of interlayer pores and intergranular pores.

Methane Adsorption on Hybrid and Non Hybrid Activated Carbon Synthesized from Coconut Shells and Poly Ether Ether Ketone

The shorter driving range is the challenge of compressed natural gas (CNG) as a vehicular fuel. In this study, adsorbents were prepared from coconut shells and Poly Ether Ether Ketone (PEEK) using KOH microwave activation to overcome the challenge of CNG storage system. The CNG storage system has some disadvantages which include high-pressure operation with less safety guard, and heavy storage cylinders. The adsorbents were used as a potential Sorbents for methane (CH4) storage at different pressures. The coconut shell and PEEK were carbonized from ambient temperature to 700 ± 20 oC at 10 oC min-1 heating rate with 1 L min-1 N2 flow rate. The carbonization temperature of the precursor was determined using thermo-gravimetric and derivative thermogravimetric (TG/DTG) analysis. The activation was achieved with well modified microwave equipment operated at 500 W and 5 minutes. The adsorbents were characterized by Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption and scanning electron microscopy (SEM). The CH4 adsorption characteristics were conducted using volumetric adsorption equipment at an ambient temperature and pressures of 5-17 bar. The highest CH4 uptake achieved from hybrid adsorbent at 5, 7.5, 11 and 17 bar are 2.35, 3.04, 4.80 and 7.15 mmol/g respectively. The experimental data simulated using three common adsorption models: Langmuir, Freundlich and Sips. The Freundlich, had high correlation coefficient up to 0.9989 and lower root mean square deviation (RMSD) which fitted our data better than others. The findings revealed the potential of coconut shell-PEEK as sorbents for CH4 adsorption applications and isotherm models equations used in adsorption

Effect of microstructure and chemical composition of coal on methane adsorption

Coalbed methane (CBM) is mainly adsorbed in coal reservoir. To explore the methane (CH4) adsorption mechanism in coal, this paper investigated the relationships among CH4 adsorption characteristics and microstructure and chemical composition of the coal samples through different solvent treatments. The results indicate that after THF and CH3CH2OH treatments, aromatic and aliphatic functional groups decline significantly as hydrocarbon low molecular weight compounds are dissolved, while oxygen-containing functional groups show a remarkably increase after acid solvent treatment. However, it can be found that the inter-layer spacing (d002) and stacking height (Lc) values of the basic structure unit of coal show enlarged characteristics, except for the residue treated by CH3COOH. The macropore content of coal samples under THF treatment increases greatly and the micropores all but disappear, while other samples show an increase. Actually, chemical structure also contributes to nanopore structure. Both the specific pore volume and specific surface area show an opposite trend slightly with microcrystalline structure parameters, while the d002 value gives a ‘U’ shape distribution for pore structure parameters. Further, combined with adsorption curves, adsorption potential theory and Clausius-Clapeyron equation, the adsorption capacity and adsorption heat indicate a physical adsorption process. However, there is no perfect correlation among microcrystalline structure parameters, pore structure distribution and CH4 adsorption. On the other hand, low molecular weight compounds such as aromatic hydrocarbons and aliphatic hydrocarbons combined to macromolecular structures by non-covalent bonds, are easy to form competitive adsorption with CH4 molecule by occupying CH4 adsorption sites. Meanwhile, these aromatic and aliphatic structures as the non-polar functional groups on the surface of coals can also promote CH4 adsorption, but the polar oxygen-containing functional groups (especially –OH and C–O) weaken CH4 adsorption capacity greatly. Totally, the more complex the chemical composition of coal surface is, the more important it is on CH4 adsorption.

Effect of Surface Area and Micropore Volume of Activated Carbons from Coal by KOH, NaOH and ZnCl2 Treatments on Methane Adsorption

Abstract This paper investigates the methane adsorption characteristics of activated carbons produced from coal by activation with KOH, NaOH and ZnCl2 treatments at different agent to coal ratios (1:1–6:1) and temperatures (600–800 °C) under N2 flow. CH4 adsorption capacities and desorption behaviors of the activated carbons were examined at 0 °C and 25 °C. The relationship between CH4 adsorption characteristics and surface properties of activated carbons, such as BET surface area determined by N2 adsorption at −196 °C, and micropore volume determined by CO2 adsorption at 0 °C were investigated. Optimal results for CH4 adsorption at 0 °C and 25 °C were obtained for the activated carbon samples obtained with KOH treatment at 800 °C at 4:1 ratio, as 2.67 and 1.12 mmol/g, respectively. The results have shown that CH4 adsorption increased proportionally with micropore volume of activated carbons, whereas BET surface area does not exhibit an exact consistency. CH4 adsorption-desorption isotherms at 25 °C have shown that an increase in mesopore formation caused a decrease in adsorption; but allowed desorption to be reversible. Higher methane adsorption capacities were obtained from activated carbons produced from coal by various treatments in this study than most of the reported results in literature at the similar conditions, indicating the suitability of the evaluated materials for industrial applications of methane storage.

Methane adsorption characteristics of shale from different basins and its geological significance

The CH4 adsorption characteristics of shale from different basins was studied based on determination of mineral compositions, Rock-eval pyrolysis parameters and CH4 gas adsorption isotherms. The results show that TOC contents, contents of quartz and maturity stage of shale can influence the CH4 adsorption capacity. Whereas, the results show that there exists unobvious relationships between CH4 adsorption capacity and the contents of clay and carbonate materials can be obtained in this study.

Methane adsorption on koh microwave treated porous carbon from sustainable coconut solid waste material

The shorter driving range is the challenge of compressed natural gas (CNG) as a vehicular fuel. In this study, adsorbent is prepared from coconut shells using KOH microwave activation to overcome the challenge of CNG storage system. The CNG storage system has some disadvantages which include high-pressure operation with less safety guard, and heavy storage cylinders. The adsorbent is used as a potential Sorbent for methane (CH4) storage at different pressures. The coconut shell was carbonized from ambient temperature to 700 ± 20 oC at 10oC min-1 heating rate with 1 L min-1 N2 flow rate. The carbonization temperature of the precursor was determined using thermo-gravimetric and derivative thermogravimetric (TG/DTG) analysis. The activation was achieved with well modified microwave equipment operated at 500W and 5 minutes. The CH4 adsorption characteristics were conducted using volumetric adsorption equipment at an ambient temperature and pressures of 4, 5 and 7 bar. The CH4 uptake achieved at 4, 5 and 7 bar are 2.9707, 3.0559 and 3.6685 mmol g-1. The experimental data simulated using two common adsorption models: Langmuir and Freundlich. The experimental data was also evaluated using the common adsorption kinetic namely pseudo-first order, pseudo- second, order kinetics and Elovich. For the three initial pressures of 4, 5 and 7 bar, CH4 adsorption show more fits to pseudo-second order with R2> 0.967, R2 > 0.967 and R2> 0.960. The results reveal that coconut shell is a viable and sustainable material for synthesizing of the adsorbents for methane adsorption.

Fractal characteristics of coal samples utilizing image analysis and gas adsorption

Coal is a porous medium with fractal characteristics. In order to investigate the effect of fractal dimensions on methane adsorption capacity, fractal characteristics of 11 coal samples were analyzed, using scanning electron microscopy (SEM) and low-pressure nitrogen gas adsorption (LP-N2GA). Data from SEM image analysis and LP-N2GA experiments were applied to assess the heterogeneities of pore distribution (D1) and the irregularities of coal surface (D2) on the basis of box-counting method and Frenkel–Halsey–Hill (FHH) theory, respectively. The relationship between fractal dimensions and coalification was investigated. Based on the physical description of fractal surfaces and pore distributions, the influence of fractal dimensions (both D1 and D2) on CH4 adsorption characteristics was also discussed. The results indicate that these coal samples have different CH4 adsorption characteristics and fractal geometries, with D1 ranging between 1.5380 and 1.8267, and D2 varying from 2.2656 to 2.6541. The U-shaped curve relationship between D values (including D1 and D2) and volatile matter (Vdaf) is observed, demonstrating that coalification makes coal surfaces and pore networks comparatively smoother and more regular for lower rank coals (Vdaf>15%), but rougher and more complex for higher rank coals (Vdaf<15%). The Langmuir volume (VL) shows a positive linear correlation with the fractal dimension D2 values, but little correlation with D1 values. While, the Langmuir pressure (PL) is affected by both D1 and D2. Fractal dimensions comprehensively reflect the difference in physical properties of coal, which can be used to evaluate CH4 adsorption capacity. Fractal analysis is of great significance for better understanding of the surface irregularity and methane storage capacity of a coal reservoir.

Calcium decorated and doped phosphorene for gas adsorption

In this paper, we present the results from first-principles study based on the electronic structure and adsorption characteristics of CH4, CO2, H2 and NH3 adsorbed on Ca decorated/doped phosphorene. Our study finds that phosphorene exhibits n-type behaviour on decorating calcium, and p-type on doping calcium. Gas molecules are physisorbed on both pristine and calcium-mediated phosphorene, visible through their lower binding energy and charge transfer values. Ca decorated phosphorene is suitable for hydrogen storage due to its higher binding energy for H2. Ca doped structures shows increased binding affinity towards CH4 and NH3 in zigzag1 direction and armchair directions respectively. The extracts of our study implies that Ca doped phosphorene possess increased binding affinity towards gas molecules, and the results are highly helpful for gas adsorption and to design gas sensors based on calcium doped or decorated phosphorene.

Adsorption characteristics of methane on Maxsorb III by gravimetric method

Adsorption characteristics of CH4 on the carbonaceous porous material is evaluated for possible application in adsorbed natural gas (ANG) system. Adsorption uptakes at assorted temperatures (25–80 °C) and pressures ranging from ambient to relatively high pressure i.e., 8.0 MPa are experimentally investigated. Surface characteristics such as pore surface area, micropore volume and pore size distribution of the adsorbent (Maxsorb III) are first evaluated using Classical Volumetric Method i.e., the manometric method with N2 gas adsorption at 77 K. The sorption measurements for methane, CH4 gas are carried out by thermogravimetric (TGA) method using magnetic suspension balance coupled with the automatic dosing system. The buoyancy measurements were first conducted using Helium gas as adsorbate. Buoyancy correction is applied to all sets of measured data and the specific uptake capacities (g/g of adsorbent) at various temperatures and pressures were calculated. The isotherm data were then fitted using Langmuir and Tòth isotherm models. It is observed that the data can be satisfactorily fitted using Tòth model with excellent fitting accuracy around 2.2% within the experimental range. The outcome of the present study especially the adsorption data at high pressures is applicable to accurate design and modeling of Adsorbed Natural Gas (ANG) systems.