Enhanced coalbed methane recovery

Influences of long-duration NO exposure on CO2 adsorption on coals: insights into oxy-coal burning flue gas storage in coal reservoirs

Introducing oxy-coal burning flue gas into coal reservoirs has the advantages of mitigating emissions of CO2, NOx, and SO2, and producing in-situ coalbed methane (CBM). Given the characteristics of the geologic time scale for fluid sequestration, the long-duration NO exposure was conducted for various ranks of coal matrices to access the potential impacts of NO exposure on CO2 storage. Afterwards, the changes in critical physico-chemical properties and adsorbability of various ranks of coal matrices to CO2 because of long-duration NO exposure were revealed. Finally, the implications to implement oxy-coal burning flue gas sequestration in unmineable coal reservoirs with CBM production were indicated. The results demonstrate that the long-duration interaction with NO reduces the number of micro- and meso-pores of various ranks of coals, especially those with diametres below 4.00 nm. Moreover, the long-duration NO exposure reduces the oxygen-containing functional groups while significantly increasing the amine/amide-N for all the coals. The aforementioned alterations in the surface chemistry property imply that the coal matrix is capable of chemically adsorbing NO, thereby achieving its stable storage in target coal reservoirs. Given the electron donor-acceptor interactions between amine/amide-N and CO2, the NO exposure can elevate the CO2 storage capability of various ranks of coals under typical reservoir temperature and pressure. In summary, introducing oxy-coal burning flue gas into coal reservoirs is capable of stably storing critical gaseous pollutants and simultaneously enhancing the CO2 storage potential of coal reservoirs, thereby updating the existing oxy-coal burning technology and CO2 sequestration in unmineable coal reservoirs with the enhanced CBM recovery technology.

Experimental Study on the Effect of CO2 Injection Pressure on the Migration Characteristics and Extraction Effects of Replacement CH4

To study the effect of CO2 injection pressure on gas migration characteristics and coalbed methane (CBM) extraction, a platform for the experimental replacement of CH4 with CO2 was used to conduct experiments on the replacement of CH4 under different CO2 injection pressures and analyze the gas transport characteristics and CH4 extraction during the experiment. The results reveal that the rate of gas migration out of the coal seam accelerates with increasing gas injection pressure, as determined by comparisons of the migration rates between adjacent monitoring points. The change trend of the CH4 desorption rate under different gas injection pressures is divided into slow decline, sharp decline, and stability stages, and the maximum value of the effective diffusion coefficient increases from 2.3 × 10-5 to 3.4 × 10-5 and 4.6 × 10-5 cm2/s as the gas injection pressure increases from 0.6 to 0.8 and 1.0 MPa. Similarly, the change pattern of coal seam permeability can be divided into slow decline, sharp decline, and stability stages. After the gas injection pressure was increased from 0.6 to 0.8 and 1.0 MPa, the CH4 desorption volume increased from 90.2 to 94.1 and 97.8 L, whereas the coal seam CO2 sequestration volume increased from 269.2 to 274.2 and 322.8 L, respectively. In contrast, the CH4 extraction efficiency increased from 76.9 to 80.2 and 82.9%, respectively. The research results have important reference value and practical significance for optimizing the CO2 injection pressure and improving the CBM extraction.

Assessment of the Suitability of Coke Material for Proppants in the Hydraulic Fracturing of Coals

To enhance the extraction of methane gas from coal beds, hydraulic fracturing technology is used. However, stimulation operations in soft rocks, such as coal beds, are associated with technical problems related mainly to the embedment phenomenon. Therefore, the concept of a novel coke-based proppant was introduced. The purpose of the study was to identify the source coke material for further processing to obtain a proppant. Twenty coke materials differing in type, grain size, and production method from five coking plants were tested. The values of the following parameters were determined for the initial coke: micum index 40; micum index 10; coke reactivity index; coke strength after reaction; and ash content. The coke was modified by crushing and mechanical classification, and the 3-1 mm class was obtained. This was enriched in heavy liquid with a density of 1.35 g/cm³. The crush resistance index and Roga index, which were selected as key strength parameters, and the ash content were determined for the lighter fraction. The most promising modified coke materials with the best strength properties were obtained from the coarse-grained (fraction 25-80 mm and greater) blast furnace and foundry coke. They had crush resistance index and Roga index values of at least 44% and at least 96%, respectively, and contained less than 9% ash. After assessing the suitability of coke material for proppants in the hydraulic fracturing of coal, further research will be needed to develop a technology to produce proppants with parameters compliant with the PN-EN ISO 13503-2:2010 standard.

Experimental Evaluation of Working Fluid Damage to Gas Transport in a High-Rank Coalbed Methane Reservoir in the Qinshui Basin, China

Formation damage induced by the injected working fluid runs through the whole life cycle of coalbed methane (CBM) extraction and ultimately reduces the production of CBM wells. The conventional method uses permeability as a parameter to evaluate the formation damage severity to coal by working fluids containing solids. However, less attention has been attracted to the formation damage of the pure liquid phase of the working fluid on the multiscale gas transport process of CBM. Therefore, we present a multiscale working fluid filtrate damage evaluation method considering the desorption, diffusion, and seepage and use it to evaluate high-rank coal in the Qinshui Basin of China. The results show that pure liquids with different pH values and salinities significantly damage the desorption-diffusion and seepage ability of CBM. The damage rates of alkaline fluid, hydrochloric acid fluid, and clear water on the methane desorption capacity of coal are 63.64, 17.63, and 24.34%, respectively, while those on the permeability of coal are 29.88, 42.38, and 46.66%, respectively. The formation damage severity in the seepage process is higher than that in the desorption-diffusion process, which proves the necessity of multiscale working fluid damage evaluation on CBM. Effective channel reduction and resistance increase in gas transport are the mechanisms of working fluid filtrate-induced formation damage, which are caused by water blocking, sensitive mineral swelling and clogging, and strengthened stress sensitivity. In addition to controlling the solid damage of the working fluid, reducing the invasion of the working fluid filtrate and maintaining its compatibility with the coal and formation fluids are even more important to protect the coal reservoir.

High pressure CO2 adsorption onto Malaysian Mukah-Balingian coals: Adsorption isotherms, thermodynamic and kinetic investigations

CO₂ sequestration into coalbed seams is one of the practical routes for mitigating CO₂ emissions. The adsorption mechanisms of CO₂ onto Malaysian coals, however, are not yet investigated. In this research CO₂ adsorption isotherms were first performed on dry and wet Mukah-Balingian coal samples at temperatures ranging from 300 to 348 K and pressures up to 6 MPa using volumetric technique. The dry S1 coal showed the highest CO₂ adsorption capacity of 1.3 mmol g^-1, at 300 K and 6 MPa among the other coal samples. The experimental results of CO₂ adsorption were investigated using adsorption isotherms, thermodynamics, and kinetic models. Nonlinear analysis has been employed to investigate the data of CO₂ adsorption onto coal samples via three parameter isotherm equilibrium models, namely Redlich Peterson, Koble Corrigan, Toth, Sips, and Hill, and four parameter equilibrium model, namely Jensen Seaton. The results of adsorption isotherm suggested that the Jensen Seaton model described the experimental data well. Gibb's free energy change values are negative, suggesting that CO₂ adsorption onto the coal occurred randomly. Enthalpy change values in the negative range established that CO₂ adsorption onto coal is an exothermic mechanism. Webber's pore-diffusion model, in particular, demonstrated that pore-diffusion was the main controlling stage in CO₂ adsorption onto coal matrix. The activation energy of the coals was calculated to be below -13 kJ mol^-1, indicating that adsorption of CO₂ onto coals occurred through physisorption. The results demonstrate that CO₂ adsorption onto coal matrix is favorable, spontaneous, and the adsorbed CO₂ molecules accumulate more onto coal matrix. The observations of this investigation have significant implications for a more accurate measurement of CO₂ injection into Malaysian coalbed seams.

Improved Permeability Model of the Binary Gas Interaction within a Two-Phase Flow and its Application in CO2-Enhanced Coalbed Methane Recovery

In this paper, we develop a dual-porosity dual-permeability model for binary gas migration to explore the permeability evolution in the matrix and fracture in the process of a gas-water two-phase flow during CO₂-enhanced coalbed methane (CO₂-ECBM) recovery in coal reservoirs. This mechanistic model accommodates the effects of elastic deformation caused by the effective stress change in the matrix and fracture, the swelling/shrinkage deformation of the matrix caused by adsorption/desorption, the convection and diffusion of gas, and the discharge of water. Specifically, the time-dependent matrix swelling, from initially completely reducing the fracture aperture to finally affecting the coal bulk volume, is considered by the invaded volume fraction involving binary gas intrusion. The model is validated through laboratory data and applied to examine the permeability evolution of CO₂-ECBM recovery for 10 000 days. Furthermore, we analyze the sensitivity of some selected initial parameters to capture the key factors affecting CO₂-ECBM recovery. Our modeling results show that the permeability evolution can be divided into two stages during the process, where stage I is dominated by effective stress and stage II is dominated by adsorption/desorption. Increasing the injection pressure or initial permeability advances the start of stage II. The decrease in initial water saturation causes the permeability to change more drastically and the time of stage II to appear earlier until a time long enough, after which little effect is seen on the permeability results.

Integrated assessment of CO2-ECBM potential in Jharia Coalfield, India

Coalbed methane (CBM) production is effectively achieved by utilizing two processes, viz. primary and secondary recovery. In this paper, the primary recovery of CBM was studied using the adsorption isotherm while CO₂-ECBM process for the secondary recovery was simulated with realistic parameters. The adsorption isotherm for CH₄ was drawn up to the pressure of 1200 psi for four coal samples and Langmuir isotherm curves for both CH₄ and CO₂ was measured for one sample up to 2000 psi. The adsorption isotherm of four samples was further utilized for finding the primary recovery factor of methane, showing that the average primary recovery is ~ 54% with the highest recovery factor of ~ 76% for one sample. Hence, CO₂-ECBM process could be further implemented to enhance gas recovery. Then, a 3D heterogeneous coalbed model at a depth of 3219 ft was constructed using the COMET3 simulator to demonstrate the potential of CO₂-ECBM recovery technique. A concept of break-even time was introduced in this study for the comprehension of CO₂-ECBM process. It is found that coalbed reservoirs may opt to implement this technology with economically sound recovery.

Experimental Study of CO2-ECBM by Injection Liquid CO2

Coal mine gas disasters have severely restricted production safety. Improving gas extraction efficiency can effectively reduce disasters. Scholars have confirmed that CO2 successfully displaces coal seam CH4. This study conducted displacement and in situ experiments and compared gas drainage under different injection pressures. The displacement experiments indicated that CH4 production rates increased under increased pressures while the displacement ratios decreased. The pressure had a positive effect on sweep efficiency. The in situ experiment showed that CH4 and CO2 concentration trends in the inspection hole remained consistent. Through observing the data of the original and inspection holes, the average gas drainage concentration during low- and medium-pressure injections increased by 0.61 times and 1.17 times, respectively. The low-pressure average gas drainage scalar was increased by 1.08 times. During the medium-pressure injection, the average gas drainage purity increased by 1.94 times. The diffusion ranges of CO2 under low- and medium-pressure injections were 20–25 m and 25–30 m, respectively. The sweep efficiency of medium-pressure injection was 26% better than that of the low-pressure injection, with average pressures of 2.8 MPa and 1.4 MPa, respectively, for sweep efficiency. This study proposes an effective method for improving coal mine gas drainage efficiency.

Surface tensions at elevated pressure depend strongly on bulk phase saturation

HYPOTHESIS

Understanding interfacial phenomena at elevated pressure is crucial to the design of a variety of processes, modeling important systems, and interpreting interfacial thermodynamics. While many previous studies have offered insight into these areas, current techniques have inherent drawbacks that limit equilibrium measurements.

EXPERIMENTS

In this work, we adapt the ambient microtensiometer of Alvarez and co-workers into a high pressure microtensiometer (HPMT) capable of experimentally quantifying a wide range of interfacial phenomena at elevated pressures. Particularly, the HPMT uses a microscale spherical interface pinned to the tip of a capillary to directly measure surface tension via the Laplace equation. The stream of microscale bubbles used to pressurize the system ensures quick saturation of the bulk phases prior to conducting measurements. The HPMT is validated by measuring the surface tension of air-water as a function of pressure. We then measure the surface tension of CO₂ vapor and water as a function of pressure, finding lower equilibrium surface tension values than originally reported in the literature.

FINDINGS

This work both introduces further development of a useful experimental technique for probing interfacial phenomena at elevated pressures and demonstrates the importance of establishing bulk equilibrium to measure surface tension. The true equilibrium state of the CO₂-water surface has a lower tension than previously reported. We hypothesize that this discrepancy is likely due to the long diffusion timescales required to ensure saturation of the bulk fluids using traditional tensiometry. Thus we argue that previously reported elevated pressure measurements were performed at non-equilibrium conditions, putting to rest a long standing discrepancy in the literature. Our measurements establish an equilibrium pressure isotherm for the pure CO₂-water surface that will be essential in analyzing surfactant transport at elevated pressures.

Promotion effects of microwave heating on coalbed methane desorption compared with conductive heating

As a clean energy resource, coalbed methane (CBM) has drawn worldwide attention. However, the CBM reservoir has strong adsorption capacity and low permeability and thus requires stimulation. As a means to stimulate coalbed methane recovery, thermal injection faces geological and economic challenges because it uses conventional conductive heating (CH) to transfer heat. Realized by the conversion of the electromagnetic energy into the thermal energy, microwave heating (MH) may be a sound stimulation method. Although previous research suggested that MH had potential as a stimulation method for coalbed methane recovery, it is not clear if MH is superior to CH for enhancing coalbed methane recovery. This paper compares the effect of MH and CH on methane desorption from coal using purpose-built experimental equipment. To compare the MH and CH experimental results, the desorption temperature for each CH desorption test was set to the maximum temperature reached in the correlative MH desorption test. The results show that although the cumulative desorbed volume (CDV) of methane under MH was less than that desorbed by CH in the initial desorption stage, the final total CDV under MH for the three different power settings was ~ 12% to ~ 21% more than that desorbed by CH at the same temperatures. CH and MH both change the sample’s microstructure but MH enlarges the pores, decreases methane adsorption, promotes methane diffusion, and improves permeability more effectively than CH. Rapid temperature rise and the changes in the coal’s microstructure caused by MH were the main reasons for its superior performance. These findings may provide reference for selecting the most appropriate type of heating for thermal injection assisted coalbed methane recovery.

Measurement and interpretation of unary supercritical gas adsorption isotherms in micro-mesoporous solids

Gas adsorption at high pressures in porous solids is commonly quantified in terms of the excess amount adsorbed. Despite the wide spectrum of adsorbent morphologies available, the analysis of excess adsorption isotherms has mostly focused on microporous materials and the role of mesoporosity remains largely unexplored. Here, we present supercritical CO2 adsorption isotherms measured at $$T=308$$ T = 308  K in the pressure range $$p=0.02{-}21$$ p = 0.02 - 21  MPa on three adsorbents with distinct fractions of microporosity, $$\phi_2$$ ϕ 2 , namely a microporous metal-organic framework ($$\phi_2=70$$ ϕ 2 = 70 %), a micro-mesoporous zeolite ($$\phi_2=38$$ ϕ 2 = 38 %) and a mesoporous carbon ($$\phi_2<0.1$$ ϕ 2 < 0.1 %). The results are compared systematically in terms of excess and net adsorption relative to two distinct reference states–the space filled with gas in the presence/absence of adsorbent–that are defined from two separate experiments using helium as the probing gas. We discuss the inherent difficulties in extracting from the supercritical adsorption isotherms quantitative information on the properties of the adsorbed phase (its density or volume), because of the nonuniform distribution of the latter within and across the different classes of pore sizes. Yet, the data clearly reveal pore-size dependent adsorption behaviour, which can be used to identify characteristic types of isotherm and to complement the information obtained using the more traditional textural analysis by physisorption.

Progress in carbon capture technologies

Human factors are one of the key contributors to carbon dioxide emissions into the environment. Since the industrial revolution, the atmospheric carbon dioxide levels have increased appreciably. This has been attributed to the utilization of fossil fuels for energy generation coupled with the clearing of forests and extensive manufacturing of some industrial products such as cement. The increase in atmospheric concentrations of carbon dioxide has been widely linked to climate change and the Earth's temperature. A drastic approach is therefore needed in terms of policy formulation to address this global challenge. Carbon capture and storage are reliable tools that can be introduced to the industrial sector to address this issue. Therefore, this review presents a thorough investigation of the various technologies that can be harnessed to capture carbon dioxide. The cost associated with the capture, transport, and storage of the carbon dioxide is discussed. Socio-economic aspects of carbon capture and storage technologies are also presented in this review. Factors influencing public awareness of the technology and perceptions associated with carbon capture and storage should be a point for consideration in future research activities relating to this novel technology. This, in effect, this will ensure effective expert knowledge communication to the general public and foster social acceptance of this technology.

Significance of Enhanced Oil Recovery in Carbon Dioxide Emission Reduction

Limiting the increase in CO2 concentrations in the atmosphere, and at the same time, meeting the increased energy demand can be achieved by applying carbon capture, utilization and storage (CCUS) technologies, which hold potential as the bridge for energy and emission-intensive industries to decarbonization goals. At the moment, the only profitable industrial large-scale carbon sequestration projects are large-scale carbon dioxide enhanced oil recovery (CO2-EOR) projects. This paper gives a general overview of the indirect and direct use of captured CO2 in CCUS with a special focus on worldwide large-scale CO2-EOR projects and their lifecycle emissions. On the basis of scientific papers and technical reports, data from 23 contemporary large-scale CO2-EOR projects in different project stages were aggregated, pointing out all the specificities of the projects. The specificities of individual projects, along with the lack of standardized methodologies specific for estimating the full lifecycle emissions resulting from CO2-EOR projects, pose a challenge and contribute to uncertainties and wide flexibilities when estimating emissions from CO2-EOR projects, making the cross-referencing of CO2-EOR projects and its comparison to other climate-mitigation strategies rather difficult. Pointing out the mentioned project’s differentiations and aggregating data on the basis of an overview of large-scale CO2-EOR projects gives useful information for future work on the topic of a CO2-EOR project’s lifecycle emissions.

Study on the Impact Pressure of Swirling-Round Supercritical CO2 Jet Flow and Its Influencing Factors

Supercritical carbon dioxide (SC-CO2) jet is capable of decreasing the threshold pressure of rock breakage and mitigating formation damage, owing to its low viscosity, high diffusivity, and extremely-low surface tension. The swirling-round jet holds the advantages of both a swirling jet and a round jet. Therefore, the comprehensive technique, swirling-round SC-CO2 (SR-SC-CO2) jet, is expected to substantially enhance rock-breaking efficiency. However, theoretical analysis of the flow field characteristics of SR-SC-CO2 has not been reported yet. This work aims to lay a theoretical foundation for employing SR-SC-CO2 in drilling and fracturing. The flow field is simulated using Naiver-Stokes equations and the RNG k-ε turbulence model. Sensitivity analysis, regarding pressure drop of the nozzle, confining pressure, fluid temperature, jetting distance, the diameter of the nozzle’s central hole, and grooving area, are performed. We show that the combined swirling-round SC-CO2 jet flow could maintain a relatively larger axial as well as tangential velocity compared to a single approach of swirling jet or round jet, enabling one to acquire a deeper oillet and expand the perforation area effectively. The simulation results substantiate the enormous potential of SR-SC-CO2 in improving rock-breaking efficiency and clarify the influence of relevant parameters on the impact pressure of the jet flow.

Laboratory Measurements of the Relative Permeability of Coal: A Review

The relative permeability of coal to gas and water is an essential parameter for characterizing coalbed methane (CBM) reservoirs and predicting coal seam gas production, particularly in numerical simulations. Although a variety of studies related to the relative permeability of coals have been conducted, the results hardly meet the needs of practical engineering applications. To track the dynamic development of relative permeability measurements in the laboratory, discover the deficiencies, and discuss further work in this field, this paper investigates the relative permeability measurement preparation work and laboratory methods and summarizes the development of techniques used to determine the water saturation during experimentation. The previously determined relative permeability curves are also assembled and classified according to coal rank and the absolute permeability. It is found that the general operations in the relative permeability measurement process are still not standardized. The techniques applied to determine the water saturation of coal in experiments have been refined to some extent, but no optimal technique has been recognized yet. New techniques, such as the incorporation of high-precision differential pressure gauges, can be used to determine the water production during relative permeability measurement. In addition, the existing relative permeability data are limited, and no study has focused on supercritical carbon dioxide-water and mixed gas (methane and carbon dioxide)-water relative permeability measurements. To meet the requirements of actual projects, further research on this topic must be conducted.

Wettability Alteration by Carbonated Brine Injection and Its Impact on Pore-Scale Multiphase Flow for Carbon Capture and Storage and Enhanced Oil Recovery in a Carbonate Reservoir

Carbon capture and storage is key for sustainable economic growth. CO2-enhanced oil recovery (EOR) methods are efficient practices to reduce emissions while increasing oil production. Although it has been successfully implemented in carbonate reservoirs, its effect on wettability and multiphase flow is still a matter of research. This work investigates the wettability alteration by carbonated water injection (CWI) on a coquina carbonate rock analogue of a Pre-salt reservoir, and its consequences in the flow of oil. The rock was characterized by routine petrophysical analysis and nuclear magnetic resonance. Moreover, micro-computed tomography was used to reconstruct the pore volume, capturing the dominant flow structure. Furthermore, wettability was assessed by contact angle measurement (before and after CWI) at reservoir conditions. Finally, pore-scale simulations were performed using the pore network modelling technique. The results showed that CWI altered the wettability of the carbonate rock from neutral to water-wet. In addition, the simulated relative permeability curves presented a shift in the crossover and imbibition endpoint values, indicating an increased flow capacity of oil after CWI. These results suggest that the wettability alteration mechanism contributes to enhancing the production of oil by CWI in this system.

Variation in Permeability during CO2-CH4 Displacement in Coal Seams. Part 2: Modeling and Simulation

Having a clear understanding of the permeability variation mechanism is important for controlling the process of displacement of CH₄ with CO₂ in deep coal seams. Based on the stress-strain equation of porous elastic media and horizontal strain variations of coal, a mathematical model predicting permeability variation after CO₂ injection into gas saturated coal seams was established. The model shows that, during the displacement of CH₄ with CO₂, the shrinkage strain of the coal matrix increases logarithmically with the decrease of pore pressure. With a decrease in the reservoir pressure, permeability rebound occurs with the influence of matrix shrinkage and gas slippage. Under low confining pressures, the rebounded permeability is high, and its associated rebound pore pressure is also high. For coals with a high cleat compression coefficient, the permeability decreases range is obvious. And permeability rebound only happens under low reservoir pressures. Coal properties, e.g., Poisson's ratio and Langmuir volume, show obvious influences in permeability variation during gas production. The model was also extended to predict permeability variation for a well-control area. During gas drainage process, the permeability in the well-controlled area first increases, then decreases, and then slowly returns to the original state with the lengthening of well-controlled radius. Under high confining pressures, the permeability decline range is more obvious. Also, correspondingly, the attenuation range of permeability increases and the rebound range decreases. The proposed model is beneficial in predicting permeability variations during the displacement of CH₄ with CO₂, as well as guiding CO₂ injection into coal seams.

Research on the Processes of Injecting CO2 into Coal Seams with CH4 Recovery Using Horizontal Wells

The paper presents a research study on modeling and computer simulation of injecting CO2 into the coal seams of the Upper Silesian Coal Basin, Poland connected with enhanced coal bed methane (ECBM) recovery. In the initial stage of the research activities, a structural parameter model was developed specifically with reference to the coal-bearing formations of the Upper Carboniferous for which basic parameters of coal quality and the distribution of methane content were estimated. In addition, a lithological model of the overall reservoir structure was developed and the reservoir parameters of the storage site were analyzed. In the next stage of the research, the static model was supplemented with detailed reservoir parameters as well as the thermodynamic properties of fluids and complex gases. The paper discusses a series of simulations of an enhanced coalbed methane recovery process with a simultaneous injection of carbon dioxide. The analyses were performed using the ECLIPSE software designed for simulating coal seam processes. The results of the simulations demonstrated that the total volume of CO2 injected to a designated seam in a coal mine during the period of one year equaled 1,954,213 sm3. The total amount of water obtained from the production wells during the whole period of the simulations (6.5 years) was 9867 sm3. At the same time, 15,558,906 sm3 of gas was recovered, out of which 14,445,424 sm3 was methane. The remaining 7% of the extracted gas was carbon dioxide as a result of reverse production of the previously injected CO2. However, taking into consideration the phenomena of coal matrix shrinking and swelling, the total amount of injected CO2 decreased to approximately 625,000 sm3.

Selective Adsorption and Selective Transport Diffusion of CO2-CH4 Binary Mixture in Coal Ultramicropores

The adsorption and diffusion of the CO2-CH4 mixture in coal and the underlying mechanisms significantly affect the design and operation of any CO2-enhanced coal-bed methane recovery (CO2-ECBM) project. In this study, bituminous coal was fabricated based on the Wiser molecular model and its ultramicroporous parameters were evaluated; molecular simulations were established through Grand Canonical Monte Carlo (GCMC) and Molecular Dynamic (MD) methods to study the effects of temperature, pressure, and species bulk mole fraction on the adsorption isotherms, adsorption selectivity, three distinct diffusion coefficients, and diffusivity selectivity of the binary mixture in the coal ultramicropores. It turns out that the absolute adsorption amount of each species in the mixture decreases as temperature increases, but increases as its own bulk mole fraction increases. The self-, corrected, and transport diffusion coefficients of pure CO2 and pure CH4 all increase as temperature or/and their own bulk mole fractions increase. Compared to CH4, the adsorption and diffusion of CO2 are preferential in the coal ultramicropores. Adsorption selectivity and diffusivity selectivity were simultaneously employed to reveal that the optimal injection depth for CO2-ECBM is 800-1000 m at 308-323 K temperature and 8.0-10.0 MPa.

Revisiting Bangham's law of adsorption-induced deformation: changes of surface energy and surface stress

Adsorption-induced deformation has to be described in terms of the change of the surface stress Δfand not the surface energy Δγ. The former explains both expansion and contraction.

Competitive adsorption of a binary CO2-CH4 mixture in nanoporous carbons: effects of edge-functionalization

The effect of edge-functionalization on the competitive adsorption of a binary CO2-CH4 mixture in nanoporous carbons (NPCs) has been investigated for the first time by combining density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulation. Our results show that edge-functionalization has a more positive effect on the single-component adsorption of CO2 than CH4, therefore significantly enhancing the selectivity of CO2 over CH4, in the order of NH2-NPC > COOH-NPC > OH-NPC > H-NPC > NPC at low pressure. The enhanced adsorption originates essentially from the effects of (1) the conducive environment with a large pore size and an effective accessible surface area, (2) the high electronegativity/electropositivity, (3) the strong adsorption energy, and (4) the large electrostatic contribution, due to the inductive effect/direct interaction of the embedded edge-functionalized groups. The larger difference from these effects results in the higher competitive adsorption advantage of CO2 in the binary CO2-CH4 mixture. Temperature has a negative effect on the gas adsorption, but no obvious influence on the electrostatic contribution on selectivity. With the increase of pressure, the selectivity of CO2 over CH4 first decreases sharply and subsequently flattens out to a constant value. This work highlights the potential of edge-functionalized NPCs in competitive adsorption, capture, and separation for the binary CO2-CH4 mixture, and provides an effective and superior alternative strategy in the design and screening of adsorbent materials for carbon capture and storage.

Multidimensional quantitative imaging of gas adsorption in nanoporous solids

X-ray computed tomography is applied to image gas adsorption in nanoporous solids. The equations are developed to calculate rigorous measures of adsorption, such as the excess adsorbed amount, by applying a dual-scanning technique. This approach is validated by considering the CO2/13X zeolite system in a fixed-bed adsorber, and multidimensional patterns are obtained of key characteristic properties, such as bed porosity, excess adsorption, and density of the adsorbed phase. The quantification of the spatial variability of the adsorbed amount within the system represents a major novelty with regards to conventional techniques. The ability to quantify adsorption with such a level of observational detail discloses unparalleled opportunities to interrogate and revisit adsorption processes in porous media.

Equilibrium Theory–Based Analysis of Nonlinear Waves in Separation Processes

Different areas of engineering, particularly separation process technology, deal with one-dimensional, nonstationary processes that under reasonable assumptions, namely negligible dispersion effects and transport resistances, are described by mathematical models consisting of systems of first-order partial differential equations. Their behavior is characterized by continuous or discontinuous composition (or thermal) fronts that propagate along the separation unit. The equilibrium theory (i.e., the approach discussed here to determine the solution to these model equations) predicts this with remarkable accuracy, despite the simplifications and assumptions. Interesting applications are in adsorption, chromatography and ion-exchange, distillation, gas injection, heat storage, sedimentation, precipitation, and dissolution waves. We show how mathematics can enlighten the engineering aspects, and we guide the researcher not only to reach a synthetic understanding of properties of fundamental and applicative interest but also to discover new, unexpected, and fascinating phenomena. The tools presented here are useful to teachers, researchers, and practitioners alike.

Adsorption of carbon dioxide, methane, and their mixtures in porous carbons: effect of surface chemistry, water content, and pore disorder

The adsorption of carbon dioxide, methane, and their mixtures in nanoporous carbons in the presence of water is studied using experiments and molecular simulations. Both the experimental and numerical samples contain polar groups that account for their partially hydrophilicity. For small amounts of adsorbed water, although the shape of the adsorption isotherms remain similar, both the molecular simulations and experiments show a slight decrease in the CO2 and CH4 adsorption amounts. For large amounts of adsorbed water, the experimental data suggest the formation of methane or carbon dioxide clathrates in agreement with previous work. In contrast, the molecular simulations do not account for the formation of such clathrates. Another important difference between the simulated and experimental data concerns the number of water molecules that desorb upon increasing the pressure of carbon dioxide and methane. Although the experimental data indicate that water remains adsorbed upon carbon dioxide and methane adsorption, the molecular simulations suggest that 40 to 75% of the initial amount of adsorbed water desorbs with carbon dioxide or methane pressure. Such discrepancies show that differences between the simulated and experimental samples are crucial to account for the rich phase behavior of confined water-gas systems. Our simulations for carbon dioxide-methane coadsorption in the presence of water suggest that the pore filling is not affected by the presence of water and that adsorbed solution theory can be applied for pressures as high as 15 MPa.

Synergetic effect of carbon nanopore size and surface oxidation on CO2 capture from CO2/CH4 mixtures

We have studied the synergetic effect of confinement (carbon nanopore size) and surface chemistry (the number of carbonyl groups) on CO2 capture from its mixtures with CH4 at typical operating conditions for industrial adsorptive separation (298 K and compressed CO2-CH4 mixtures). Although both confinement and surface oxidation have an impact on the efficiency of CO2/CH4 adsorptive separation at thermodynamics equilibrium, we show that surface functionalization is the most important factor in designing an efficient adsorbent for CO2 capture. Systematic Monte Carlo simulations revealed that adsorption of CH4 either pure or mixed with CO2 on oxidized nanoporous carbons is only slightly increased by the presence of functional groups (surface dipoles). In contrast, adsorption of CO2 is very sensitive to the number of carbonyl groups, which can be examined by a strong electric quadrupolar moment of CO2. Interestingly, the adsorbed amount of CH4 is strongly affected by the presence of the co-adsorbed CO2. In contrast, the CO2 uptake does not depend on the molar ratio of CH4 in the bulk mixture. The optimal carbonaceous porous adsorbent used for CO2 capture near ambient conditions should consist of narrow carbon nanopores with oxidized pore walls. Furthermore, the equilibrium separation factor was the greatest for CO2/CH4 mixtures with a low CO2 concentration. The maximum equilibrium separation factor of CO2 over CH4 of ~18-20 is theoretically predicted for strongly oxidized nanoporous carbons. Our findings call for a review of the standard uncharged model of carbonaceous materials used for the modeling of the adsorption separation processes of gas mixtures containing CO2 (and other molecules with strong electric quadrupolar moment or dipole moment).