Excess Adsorption Capacity Research Articles

Adsorption Mechanisms of High-Pressure Methane and Carbon Dioxide on Coals

Carbon dioxide (CO2) geological sequestration and coal-bed methane (CBM) recovery in deep coal seams are usually operated with a pressure higher than 10 MPa. The adsorption mechanisms of methane (CH4) and CO2 on coals in such a situation, however, are not yet revealed. With the help of a high-pressure gas adsorption system, CH4 and CO2 adsorption isotherms were first conducted on two coal samples. Simplified local density (SLD) theory was then tailored and applied to describe the adsorption characteristics of specific CH4 and CO2 on coals. Next, the adsorption mechanisms of high-pressure CH4 and CO2 on coal samples were revealed on the basis of adsorbed and bulk density distributions within the matrix pores. The results show that the high-pressure gas adsorption on coals is different from that at low pressure. The excess adsorption capacity first increases and then decreases with pressure, and the maximum value occurs at a specific pressure referring to reverse pressure. The maximum excess adsorption capacity of CH4 is less than that of CO2, while its reverse pressure is greater than that of CO2. The reversal of excess adsorption with pressure depends on the relative increase in adsorption and bulk phases. However, the mechanisms of adsorption reversal vary with gas types. The reversal of CH4 excess adsorption is due to the saturation near the pore wall, while the CO2 excess adsorption reversal is due to the dramatic bulk density changes near the critical point. Advances in the mechanism of high-pressure gas adsorption on coals suggests that coal seams deeper than 1000 m have more recovery potential, and CO2-enhanced coal-bed methane recovery success depends on the optimal management of its injection pressure and the associated coal swelling.

Construction of Buertai Coal Macromolecular Model and GCMC Simulation of Methane Adsorption in Micropores.

With the increase in high gas mines in the low coal rank mining area in the northwestern part of China, high gas mines in the low-rank coal mining area have caused many gas emission accidents. Coal is a porous material, containing a large number of micropores (<2 nm), which can absorb large amounts of methane, so it is necessary to explore methane adsorption in micropores of low-rank coal. In this work, FTIR, HRTEM, and 13C-NMR were used to test the macromolecular structural parameters of Buertai coal, which was a kind of low-rank Jurassic coal in northwestern China. The results showed that the aromatic structural units in the Buertai coal structure mainly consist of naphthalene, anthracene, and phenanthrene. The fat structure mainly occurs in the form of aliphatic side chains, cycloalkanes, and other compounds. The oxygen atoms are present in the form of carbonyl groups, ether bonds, and phenol groups with a ratio of about 6:4:9. The nitrogen atoms are present in the form of pyrrole and pyridine compounds. Finally, the macromolecular structure model of Buertai coal was built, and the calculated NMR spectrum from the model was very consistent with the experimental NMR spectrum of Buertai coal. The relationship between the macromolecular density and energy of Buertai coal was explored using the Amorphous Cell module in the simulation software, Materials Studios 8.0 (MS 8.0), and the density value at the lowest energy was determined to be about 1.23 g/cm3. The pore structure parameters of Buertai coal were also calculated. It was found that both pore volume and void fraction decreased evenly as the diameter of the probe molecule increased, but the surface area decreased rapidly when the diameter of the probe molecule was 3.46 Å. All pore sizes were found to be smaller than 10 Å from the pore size distribution (PSD) curve of Buertai coal, which provided a lot of adsorption sites for methane (CH4). The results of the CH4 adsorption simulation from Grand Canonical Monte Carlo (GCMC) showed that CH4 is adsorbed inside the micropores of coal, and the adsorption capacity of CH4 depends on the diameters of micropores when the micropores are less than 8.5 Å. There are many micropores where CH4 did not appear because these micropores are closed and did not provide a channel for CH4 to enter. The results of experimental methane adsorption indicate that the excess adsorption capacity from the GCMC simulation was very close to the experimental results of Buertai coal. This work provides a new perspective to study the methane adsorption behavior in micropores of coal.

The grain size effect on pores structure characteristics of high-rank coal before and after the methane adsorption

Methane adsorption characteristics and pore structure are two essential parameters to evaluate the reservoir quality of coal. However, as a result of the frequent increased and decreased pressure on the coal during the methane adsorption, the pore structure of the coal can be altered. The high-rank coal was obtained from western Guizhou, China, and ground into three separate particle sizes, with methane adsorption measurements performed on these sub-samples. In addition, the dynamic change of the pore structure was calculated using low-temperature nitrogen adsorption and low-temperature carbon dioxide adsorption before and after the methane adsorption. The analysis indicate that the isothermal adsorption curves of the various grain sizes coal samples present no significant differences, while the adsorption rate and maximum excess adsorption capacity increase as particle sizes decrease. Until methane adsorption, the d(qde−qad)′ decreases as grain sizes decrease, while the d(qde−qad)′ decreases first and then increases for equal adsorption rate. Then, the d(qde−qad)′ for DY-5 coal sample decreases after methane adsorption, while it increases for the DY-6 and DY-7 coal samples, suggesting that the methane adsorption has a major impact on the meso- and macropore structure for the small grain sizes coal samples. Finally, at any time during the methane adsorption process, the pore structures in the coal might change. Pore connectivity is improved in coal samples with larger sizes, while after methane adsorption, the distribution of pores is more concentrated in coal samples with smaller grain sizes.

Functionalization of Silica SBA-15 with [3-(2-Aminoethylamino)Propyl] Trimethoxysilane in Supercritical CO2 Modified with Methanol or Ethanol for Carbon Capture

The CO2 adsorption process using amine-grafted silica is a promising technology for reducing the CO2 emissions from the power and industry sectors. In this work, silica SBA-15 was functionalized using [3-(2-aminoethylamino)propyl] trimethoxysilane (AEAPTS) in supercritical CO2 (scCO2) modified with 10% mol methanol or ethanol. The functionalization experiments were carried out at 323 K and 12.5 MPa, and with reaction times of 2 and 3 h. The molar fraction of AEAPTS in scCO2 plus 10% mol alcohol ranged from 0.5 × 10−3 to 1.8 × 10−3. It was found that as the molar fraction of AEAPTS increased, the amino-grafting density steadily rose, and the pore volume, surface area and pore size of the functionalized silica SBA-15 also decreased gradually. The scCO2 functionalization method was compared to the traditional toluene method. The diamine-SBA-15 prepared in the scCO2 process shows a slightly lower amine-grafting density but a higher surface area and pore volume than the ones obtained using the traditional method. Finally, the excess CO2 adsorption capacity of the materials at different temperatures and low pressure was measured. The diamine-silica SBA-15 displayed moderate excess CO2 adsorption capacities, 0.7–0.9 mmol∙g−1, but higher amine efficiency, ca. 0.4, at 298 K, due to the chemisorption of CO2. These findings show that diamine-grafted silica for post-combustion capture or direct air capture can be obtained using a media more sustainable than organic solvents.

Using Low-Pressure Methane Adsorption Isotherms for Higher-Throughput Screening of Methane Storage Materials.

A useful correlation between the low-pressure (up to 1.2 bar), low-temperature (195 K) and high-pressure (up to 65 bar), room temperature (298 K) methane storage properties of a range of porous materials is reported. Methane isotherms under these two sets of conditions show a remarkable agreement in both equilibrium adsorption and deliverable capacities for materials with pore volumes that are less than approximately 0.80 cm3/g. This trend holds well for the suite of metal-organic frameworks and porous coordination cages we studied, in addition to a zeolite and porous organic cage. Although it is well known that gravimetric gas storage capacity trends with gravimetric surface area, the 1.2 bar, 195 K excess adsorption capacity of a given framework is a better indicator of its room temperature, 65 bar capacity. Given the significantly smaller sample quantities needed for low-pressure measurements, greater accessibility to researchers around the world, accuracy of the measurement, and higher throughput, we envision this method as a rapid screening tool for the identification of methane storage materials. As excess/total adsorption and gravimetric/volumetric adsorption can be interconverted by simple utilization of the scalar quantities of pore volume or density, respectively, this method can be easily adapted to obtain both gravimetric and volumetric total adsorption capacities for a given adsorbent. In terms of volumetric methane adsorption, we further investigate the relationship between crystallographic and bulk density for the adsorbents studied here. With this analysis, it becomes apparent that in the absence of novel synthetic approaches, reported volumetric storage capacities should be viewed as an optimistic upper limit for a given material and not necessarily a true reflection of its actual adsorption properties as most MOFs have bulk densities that are less than half of their crystallographic values.

Evolutions of CO2 Adsorption and Nanopore Development Characteristics during Coal Structure Deformation

The coal structure deformation attributed to actions of tectonic stresses can change characteristics of nanopore structure of coals, affecting their CO2 adsorption. Three tectonically deformed coals and one undeformed coal were chosen as the research objects. The isotherm adsorption experiments of four coal specimens were carried out at the temperature of 35 °C and the pressure of 0 to 7 MPa. Nanopore structures were characterized using the liquid nitrogen adsorption method. The results show that there exist maximum values of excess and absolute adsorption capacity, which increase with increasing coal deformation degree. As the degree of coal deformation increases, the pore volume and specific surface area present an obvious increasing trend in the case of micropores, exhibiting an increase at first (cataclastic coal and ganulitic coal) and then stabilization (crumple coal), in the case of mesopores, and showing a gradual decrease in the case of macropores. The mesopores are the key factor of CO2 adsorption of tectonically deformed coals, followed by the micropores and the limited effect of macropores at the strong coal deformation stage.

Pore structure characteristics and hydrocarbon generation potential of middle Jurassic lacustrine source rocks in the Yuka depression, Qaidam Basin, NW China: Implications from petrographic and organic geochemical analyses

The Yuka depression is one of the most important energy production bases in the Qaindam Basin. Although shales in the 7th member of the Dameigou Formation (J2d7) are key members of Middle Jurassic Petroleum System, organic geochemistry and petrography of different rock facies have not been fully analyzed. In this study we evaluate hydrocarbon generation potential of the J2d7 by applying integrated petrographic, geochemical and mineralogical analyses. In addition, pore structure is characterized using field emission scanning electron microscopy (SEM), CO2 and N2 physisorption. Results indicate that mudstone, carbonaceous-mudstone and siltstone have total organic carbon (TOC) values of 2.64%, 5.61% and 0.48%, respectively, as well as variations in the hydrogen index (HI). Kerogen types are mainly Type Ⅲ, with maturity varying from marginally mature to mature. Notably, excess methane adsorption capacities were higher in the carbonaceous-mudstone sample (4.66 cm3/g) than in the mudstone sample (1.78 cm3/g). Primary minerals in the mudstone were quartz and clay; kaolinite, illite and illite/smectite (I/S) were the dominant clay minerals. Clay minerals and TOC content, as well as positive correlations between illite, I/S and mesopore SSA, were factors controlling pore structure development; in contrast, kaolinite contents were negatively correlated with SSA. Liptinite is the dominant maceral composition and a high content of C29 regular sterane in the carbonaceous-mudstones suggests primary higher-plant input, associated with a low gammacerane index (0.28–0.32) and a high pristane/phytane (Pr/Ph) ratio (3.02–3.96). Petrographic and geochemical results indicate an alternating depositional environment across suboxic semi-saline to oxic fresh water changes. Higher-plants were predominant and a small quantity of algae were preserved through in-situ accumulation and migration during shale development; an oxic water column with carbonaceous-mudstone was also present. Although organic matter was marginally mature to mature, the abundance of liptinite (average of 63%) was derived from higher-plants, possibly being the primary materials producing gaseous hydrocarbons.

Molecular simulation of the adsorption characteristics of H2S in calcite slit-like pores

Hydrogen sulfide (H2S) in acid gas reservoirs is an important source of sulfur deposition in rock reservoirs. Studying the adsorption behavior of H2S in calcite slit-like pore is of great significance for predicting the amount of sulfur in natural gas. In this paper, the grand canonical Monte Carlo (GCMC) method is used to investigate the adsorption of H2S in calcite with slit-like pore. Moreover, the effects of different pressures, temperatures and pore sizes on the adsorption characteristics are discussed. The results showed that the adsorption of H2S goes up with the increasing of the pressure and gradually reaches a saturation level. And the adsorption of H2S decreases with the increasing of the temperature. The excess adsorption of H2S first increases and then decreases with the increasing of the pressure. The effect of the size on the excess adsorption capacity changes with the pressure. Meanwhile, the peak of the adsorbate near the surface of the adsorbent shows a nonmonotonic trend with the pressure.

Effect of supercritical CO2 saturation pressures and temperatures on the methane adsorption behaviours of Longmaxi shale

Abstract To investigate the effects of supercritical CO2 (SC–CO2) saturation pressures and temperatures on the methane adsorption capacity of shale, total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, scanning electron microscope and energy dispersive spectrometer (SEM/EDS) analysis, low-pressure N2 adsorption (LP-NA), and high-pressure methane adsorption (HP-MA) were conducted on raw and SC–CO2–saturated (8, 12, 16 MPa; 40, 60, 80 °C) shale collected from the Sichuan Basin. Results indicate that the excess adsorption isotherm of shale to methane clearly exhibited excess adsorption characteristics with increasing adsorption pressures. This excess methane adsorption capacity decreased after SC-CO2 saturation and is mainly attributable to decreases in the TOC content, clay minerals and specific surface area (SSA) in the SC–CO2–saturated shale. The density of adsorbed phase methane was considered at different adsorption models, and the results suggest that the Langmuir-Freundlich (LF) and Dubinin-Astakhov (DA) models better describe methane adsorption behaviours than the Langmuir and Ono-kondo models. The methane adsorption capacity of shale is closely related to the solubility of SC-CO2, which decreased significantly with increasing SC-CO2 saturation time and pressure, while the influence of SC-CO2 saturation temperatures was not evident at low-pressure (8 MPa). This study provides a theoretical reference for CO2 enhanced shale gas recovery.

Thermodynamic characteristics of CH4 adsorption by continental shale: A case study of the Upper Triassic Yanchang shale in the Yanchang Gasfield, Ordos Basin

In order to improve the calculation method of the isosteric adsorption heat, clarify thermodynamic characteristics of CH4 adsorption by continental shale and reveal the adsorption mechanism, this paper selected shale samples from the seventh Member of Yanchang Formation of Upper Triassic in the Yanchang Gasfield of the Ordos Basin as the research object. The isothermal adsorption experiment was carried out on the CH4 adsorption by continental shale and the excess adsorption curves were plotted. Then, the characteristics of the isosteric adsorption heat of the shale with different types of adsorption capacity were illustrated by analyzing and comparing the difference between excess and absolute adsorption capacity. And the following research results were obtained. First, under the same temperature and pressure, absolute adsorption capacity is higher than the excess values. The difference between them is higher under low temperature and high pressure and it is in the relationship of exponential function with the equilibrium pressure. If excess adsorption capacity is used to evaluate the adsorptive property of shale reservoirs, the evaluation result will be underestimated. Second, for CH4 adsorption by Yanchang Formation shale, the absolute and excess isosteric adsorption heat values have a linear positive correlation with absolute and excess adsorption capacity respectively, and the intermolecular force of adsorbate has a dominant effect on isosteric adsorption heat values. Third, absolute isosteric adsorption heat value is less than the excess heat. The relative error is in the range of 18.18–49.79%, and it is higher in the stage with low adsorption capacity. If excess adsorption capacity is taken as the basic data to calculate initial isosteric adsorption heat values, the calculation result will be overvalued, and consequently, the evaluation of the intermolecular force of adsorbent and adsorbate is overestimated.

Isothermal characteristics of methane adsorption and changes in the pore structure before and after methane adsorption with high-rank coal

The pore structure is an essential factor that influences the isothermal characteristics of methane adsorption of coal, and the pore structure is altered after methane adsorption. In this study, a high-rank coal sample was investigated via methane adsorption isothermal measurement, and changes in the pore structure were studied using low-pressure N2 adsorption and low-pressure CO2 adsorption before and after the methane adsorption. The excess adsorption capacity exhibits a rapid increase at low pressure, reaching a maximum when the test pressure is approximately 8 MPa. Following that, the excess adsorption capacity of the high-rank coal tends to decrease. After the methane adsorption, the pore volume and specific surface area of the micro-, meso-, and macropores increase as compared to those before the methane adsorption, especially for micropores with apertures greater than 0.8 nm and mesopores with apertures below 10 nm. This is mainly caused by high pressure in the methane adsorption, indicating a pressure effect on the pore structure after the methane adsorption. After the methane adsorption, the ratio of pores with various sizes in the high-rank coal is enhanced, but the connectivity for meso- and macropores presents a slight decrease.

Adsorption Characteristics of CO2/CH4/N2 Ternary Mixtures on Anthracite from 293.15 to 353.15 K and Pressures Up to 7 MPa.

We present the adsorption data of CO2, CH4, and N2 ternary mixtures on anthracite from 293.15 to 353.15 K and pressures up to 7 MPa obtained via a volumetric method. The experimental data are given at different conditions, and simple analysis is made. The results show that the excess adsorption capacity of the CO2/CH4/N2 mixture and its component gases decreases linearly with the increase of temperature for constant composition. The total adsorption capacity is between pure CO2 and pure N2 in the entire pressure range. The effect of total pressure on total adsorption capacity is mainly influenced by the CO2 mole fraction in the feed gas. However, the adsorption capacities of CH4 and N2 decrease, and CO2 presents a maximum with the decrease in total pressure. For the feed gases of 10% CH4 and 16% CO2, the total adsorption capacity increases when the ratio of CO2 and CH4 increases, respectively, and the increasing magnitudes for 10% CH4 are larger than that of 16% CO2. The obtained experimental data are in agreement with the extended three-parameter Ono–Kondo model.

Supercritical Methane Adsorption on Shale over Wide Pressure and Temperature Ranges: Implications for Gas-in-Place Estimation

Methane adsorption experiments over wide ranges of pressure (up to 30 MPa) and temperature (30-120 °C) were performed using a gravimetric method on the Longmaxi shale collected from the northeast boundary of Sichuan Basin, China. Organic geochemical analyses, shale composition determination, and porosity tests were also conducted. The experimental supercritical methane excess adsorption isotherms at different temperatures initially increase and then decrease with increasing pressure, giving a maximum excess adsorption capacity (Gexm = 1.86-2.87 cm/g) at a certain pressure P (6.71-12.90 MPa). The excess adsorption capacity decreases with increasing temperature below 28 MPa, while this effect reversed above 28 MPa. However, the absolute adsorption capacity decreases as the temperature increases over the full pressure range. Supercritical methane adsorption on shale is of temperature dependence because it is a physical exothermic process supported by calculated thermodynamic parameters. P is positively correlated with the temperature, while the decline rates (0.021-0.058 cm g MPa) in excess adsorption negatively correlate with the temperature. Meanwhile, Langmuir volume G (3.07-4.04 cm/g) decreases while Langmuir pressure P (1.44-4.31 MPa) increases with temperature elevation. In comparison to the actual adsorbed gas (absolute adsorption), an underestimation exists in the excess adsorption calculation, which increases with increasing depth. The conventional method, without subtracting the volume occupied by adsorbed gas, overestimates the actual free gas content, especially for the deep shale reservoirs. In situ adsorbed gas is simultaneously controlled by the positive effect of the reservoir pressure and the adverse effect of the reservoir temperature. Nevertheless, in situ free gas is dominated by the positive effect of the reservoir pressure. Lowerature overpressure reservoirs are favorable for shale gas enrichment. Geological application of gas-in-place estimation shows that, with increasing depth, the adsorbed gas content increases rapidly and then declines slowly, whereas the free gas content increases continuously. There was an equivalence point at which the contents of adsorbed and free gas are equal, and the equivalence point moved to the deep areas with increasing water saturation. Moreover, the adsorbed gas and free gas distribution are characterized by the dominant depth zones, providing the reference for shale gas exploration and development.

Molecular Simulation of CH4 and CO2 Competitive Adsorption in Moisture Coals

Molecular simulations of CH4/CO2 competitive adsorption in moisture coals are carried out. The synergistic effect of micropore walls has a significant impact on the spatial distribution of adsorbates. Water molecules in micropores can prevent CO2 and CH4 from smoothly diffusing and adsorbing in the pore. The micropore is more favorable to the advantage of CO2 in the competitive adsorption. The variation of excess adsorption capacity is related to the density difference between adsorbed phase and free phase. Relative to CH4 density, CO2 density is easier to affect the variation trend of adsorption capacity in the competitive adsorption.

Supercritical-CO2 Adsorption Quantification and Modeling for a Deep Coalbed Methane Reservoir in the Southern Qinshui Basin, China.

Accurate depiction of the adsorption capacity of supercritical CO2 (ScCO2) by existing adsorption models is an important focus for deep coal seams in CO2-enhanced coalbed methane (CO2-ECBM) recovery. To investigate the applicability of different adsorption models for the adsorption isotherms of ScCO2, the validities of 10 different adsorption models were analyzed, based on analyses of the adsorption characteristics of ScCO2 from deep coal seams of the Southern Qinshui Basin, China. These models include the Langmuir (L) model, two-parameter Langmuir (TL) model, Toth (T) model, Langmuir–Freundlich (LF) model, extended Langmuir (EL) model, double parameter Brunauer–Emmett Teller model, three-parameter BET (TBET) model, Dubinin–Radushkevich (D–R) model, Dubinin–Astakhov (D–A) model, and Ono–Kondo lattice (OK) model. These models were tested for both the excess and absolute adsorption capacities of ScCO2 under various temperatures and pressures. The simulation accuracy of the different adsorption models was analyzed. The optimal models for the adsorption of ScCO2 in deep coal seams were selected based on a comprehensive analysis of the simulation parameters, standard error, and residual sum of squares. There were obvious differences in the validity of the different adsorption models in terms of the excess adsorption capacity and absolute adsorption capacity of ScCO2. The D–A and D–R models are the optimal adsorption models for the adsorption isotherms of the excess adsorption of ScCO2 for the whole tested pressure range. The T, TL, and D–R models are the optimal adsorption models in simulation of the excess adsorption capacity of ScCO2 for the selected adsorption models when the equilibrium pressure is divided into two sections at the point of 8.13 MPa. In simulation of the absolute adsorption capacity of ScCO2, the TBET and LF models are the optimal adsorption models among the selected models when the equilibrium pressure is less than or equal to 8.13 MPa. The linear, exponential, logarithmic, power function, and polynomial adsorption simulation all have good precision in the simulation of the absolute adsorption capacity of ScCO2 when the pressure is beyond 8.13 MPa.

Hydrogen storage in a two-liter adsorbent prototype tank for fuel cell driven vehicles

A two-liter prototype hydrogen adsorbent tank, filled with MOF-5 material, was built, tested and modeled as part of the work carried out within the U.S. Department of Energy Hydrogen Storage Engineering Center of Excellence. The hydrogen was stored adopting the flow-through cooling concept. This approach exploits the characteristics of low temperature recirculating hydrogen to provide the cooling power required to adsorb the gas. The heating power, required to discharge hydrogen, was provided adopting a honeycomb finned heat transfer system, powered with a resistive heater. The system demonstrated the ability to achieve excess adsorption capacities on the order of 6.5 wt% at 77 K and at pressures between 40 bar and 80 bar. Adopting the flow-through cooling charging approach, gravimetric and volumetric capacities of 12 wt% and 31 g/L (on a material basis) were achieved, respectively, in approximately 10 min, at temperatures of about 90 K and pressures of 80 bar. The hydrogen discharging tests were carried out at pressures between 80 bar and 2.5 bar with a single resistive rod operating at an electric power on the order of 40 W. The prototype system demonstrated the ability to drive continuously a fuel cell of approximately 1 kW (corresponding to the prototype scale), at its nominal power for about 1 h 10 min.

Molecular simulation of methane adsorption within illite minerals in the Longmaxi Formation shale based on a grand canonical Monte Carlo method and the pore size distribution in southeastern Chongqing, China

In order to investigate the pore structure and the adsorption capacity of illite with respect to the methane in the Longmaxi Formation, isothermal adsorption experiments utilizing mercury intrusion, liquid nitrogen, and low-temperature carbon dioxide techniques were applied to the shale samples from southeastern Chongqing. The adsorption characteristics of illite slit pores varying in diameters were simulated using a Monte Carlo method. The results reveal that the pore volume and the specific surface area of the shale are primarily supplied by pore diameters measuring less than 2 nm. Illite is one of the primary components of the clay mineralogy within the shale that forms parallel or nearly-parallel plate pores. For pore sizes ranging from 0.5 nm to 0.9 nm (at conditions of 303.15 K and 8 MPa), the methane molecules are affected by van der Waals and electrostatic forces that leads to a large excess in the adsorption capacity of methane. Once the pore size becomes less than 0.9 nm, the methane adsorption becomes primarily affected by van der Waals forces. At the said size, the excess adsorption capacity of methane initially decreases, after which it remains constant with an increase in the pore size. The free gas content surges with the growing pore diameters. The average equivalent adsorption heat reflects that the adsorption of methane onto illite is characterized by physical adsorption. During the adsorption process, when the pore size is between 0.5 nm and 1.2 nm, the average equivalent adsorption heat decreases rapidly with an increase in the pore diameter. In pore size that exceeds 1.2 nm, the adsorption intensity between the methane molecules and the illite slit becomes stable. In this case, the average adsorption heat is measured to be 6.72 kJ/mol. Meanwhile, the monolayer of methane is adsorbed onto the pore wall and the local density of methane exhibits the characteristics evident of that of a single peak when the pore size is between 0.5 nm and 0.8 nm. The adsorption mode changes from single-layer adsorption to double-layer adsorption when the pore size is between 0.8 nm and 1.2 nm. In addition, the local density curve changes from unimodal to bimodal. In pores sized larger than 1.2 nm, the free volume of methane adsorption can be larger, wherein the local density curve is bimodal.

Study of High Pressure Isothermal Methane Adsorption on Shales

In order to research the problem that the Langmuir model cannot well described the experiment results of high pressure isotherm adsorption about shale gas, the Longmaxi formation shale samples which came from the CD-2 well in the Chengkou area, northeast of Chongqing were selected to the high pressure isothermal methane adsorption experiments under the reservoir temperature and pressure conditions, and used the liquid methane density under the atmospheric pressure boiling point as adsorption phase density to correct the excess adsorption capacity. The experiment results show that the excess adsorption capacity increase at the beginning and then decrease with the pressure rise and that’s the characteristic of supercritical methane adsorption. Under the high pressure isothermal conditions, the excess adsorption capacity is different from the absolute adsorption capacity, the liquid methane density under the atmospheric pressure boiling point could be selected as adsorption phase density to calculate the absolute adsorption capacity. Langmuir model can be used to explain the absolute adsorption capacity of methane under the high pressure isotherm adsorption.

Supercritical CO2 adsorption in a simulated deep coal reservoir environment, implications for geological storage of CO2 in deep coals in the southern Qinshui Basin, China

AbstractCO2 sequestration in deep, unmineable coal seams has been proposed as a method for offsetting increasing anthropogenic CO2 emissions, while increasing natural gas production. Through laboratory experiments on gaseous/supercritical CO2 adsorption using a manometric procedure, we investigated anomalies in supercritical CO2 adsorption and determined CO2 density‐temperature effects on adsorption capacity, which are closely related to the CO2 storage potential in deep coal reservoirs. Results indicate that the revised Dubinin‐Radushkevich (D‐R) model can fit supercritical CO2 adsorption data well, and there is a significant difference between excess and absolute adsorption capacity as CO2 density increases. In the supercritical isochore, a rapid change in CO2 density results in a transition from Langmuir‐like monolayer adsorption to multilayer adsorption, and there is an unsaturated multilayer system during this transition. Above the temperature at the cross point, the increasing of density decreases, and a negative dependence on temperature causes a gradual decrease in adsorption capacity. Coal reservoirs near the boundary between the supercritical isochore stage and fully supercritical CO2 stage should be the prime target for geological storage of CO2. In order to better understand supercritical CO2 adsorption behavior and identify target coal seams, further supercritical CO2 adsorption experiments are needed in the southern Qinshui Basin.

Sorption capacity of Indian coal and its variation with rank parameters

The study of gas sorption characteristics is important for practical assessment of coal bed methane (CBM) production and CO2 sequestration in coal seam. Adsorption isotherm is one of the critical parameters for the establishment of production as well as injection well. Adsorption isotherm provides information about the reservoir conditions and critical desorption pressure as well as volume of gas that can be sequestered in deep coal seam. Alteration in sorption isotherm reflects the increase or decrease of the gas production as well as CO2 sequestration. Therefore, in this paper, experimental investigation was carried out to determine the CO2/CH4 sorption capacity of five different coal samples taken from different locations of Jharia coalfield (Moonidih area) of Gondwana basin. Gas sorption capacity was determined at 27 °C temperature and up to 7.5 MPa pressure. CO2 adsorption was observed to be higher than that of the CH4. The sorption ratio of CO2/CH4 varied from 1.6:1 to 1.2:1 for all coal samples. Furthermore, the experimental results were correlated using established Langmuir, Freundlich, Temkin and D–R isotherm models. Experimentally obtained values satisfactorily fitted to the Langmuir and Freundlich model with comparable accuracy. The excess adsorption capacity of coal was also compared with different rank parameters to understand the variation of sorption capacity with rank of coal.