Adsorption Capacity Of Coal Research Articles

Effects of indigenous microorganisms on the CO2 adsorption capacity of coal

The adsorption capacity of coal is a crucial criterion for CO2 sequestration in deep unminable coal seams. Previous studies have documented how indigenous microorganisms interact with supercritical carbon dioxide (ScCO2) in coal. But how these interactions affect the adsorption capacity were rarely investigated. In this study, we treated bituminous coal samples with ScCO2-H2O and ScCO2-H2O-microorganism interactions in high-pressure reactors for 120 days at 35 °C and 10 MPa, respectively. After the treatments, we characterized both coal pore types and structures. The comparisons between two different post-treatments indicate that no significant change in coal pore types but notable alterations in pore structures are observed. Specifically, the treated coal with microorganisms exhibits a significant increase in micropore specific surface area (SSA) and surface roughness than the one without them. However, changes in meso/macropore structures and overall pore complexity of the coal sample treated with microorganisms were less pronounced than in coal sample treated with ScCO2-H2O alone. FTIR spectra analysis indicated that the absorption peak intensity of oxygen-containing groups and aliphatic functional groups in the sample treated with microorganisms decreased much more significantly than the sample treated without microorganisms. The CO2 adsorption capacity slightly increased in all treated samples at equilibrium pressures below 2.5 MPa. Above this pressure, the coal sample treated with microorganisms maintained a higher CO2 adsorption capacity, while the sample treated without microorganisms exhibited lower CO2 adsorption capacity compared to untreated coal sample. The increased SSA and the reduced oxygen-containing functional groups of coal after the treatment exerted antagonistic effects on CO2 adsorption. These findings suggest that indigenous microorganisms in coal seams affect CO2 adsorption through antagonistic effects, necessitating site-specific microbial assessments.

Analysis of the influence of temperature on the adsorption characteristics of coal: Insights into low-temperature outburst prevention

To investigate the impact of temperature reduction on the adsorption characteristics of coal gas, isothermal adsorption experiments were conducted with environmental temperature and moisture content as variables. The changing patterns of coal gas adsorption characteristics, adsorption theories, and the temperature response of gas expansion were analyzed. Furthermore, engineering measures for preventing gas outbursts in low-temperature frozen coal seams were discussed. The research findings indicate that the gas adsorption capacity of coal samples increases with decreasing temperature. When the temperature decreases from 20 to −20 °C, the gas adsorption capacity decreases from 34.48 to 19.90 cm3/g. At the same pressure, the adsorption capacity of coal is negatively correlated with temperature. The Langmuir theoretical model better reflects the adsorption law of coal gas under different environmental temperature conditions, wherein the adsorption constant “a” gradually decreases with the increase in environmental temperature. In a closed space, the gas pressure decreases with the decrease in coal sample temperature. Under the new adsorption equilibrium, the gas pressure in the coal sample tank decreases linearly with temperature. Coal samples with different moisture contents exhibit varying degrees of decrease in gas pressure at the same temperature, with higher moisture content resulting in a smaller decrease in gas pressure. Gas expansion energy decreases linearly with the decrease in coal temperature, indicating a proportional relationship between gas expansion energy and coal gas release pressure. As the temperature decreases, the gas pressure decreases. Based on the experimental results, a synergistic approach for preventing gas outbursts through low-temperature freezing is proposed.

Research on high-pressure adsorption of supercritical CO2 and the characterization of coal structure change in anthracite

CO2-Enhanced Coal Bed Methane ( CO2-ECBM) Technology has been widely used in deep coalbed methane (DCBM) extraction and CO2 sequestration. CO2 is in the supercritical state ( ScC - O2 ) in deep coal seam reservoir, and has mechanical, physical and chemical effects on the coal body, making understanding ScCO2 adsorption crucial. Based on the self-developed supercritical isothermal adsorption device, ScCO2 high-pressure adsorption experiments were carried out in anthracite. Besides, using multiple characterisation methods to analyse the changes in microstructures of coal before and after ScCO2 adsorption and reveal the mechanism of processing. The results indicate that an obvious mutation zone in adsorption during high-pressure conditions is carried out, with the minimal adsorption capacity in the mutation zone; 2) The effect of ScCO2 on the microstructure of the coal mainly focuses on the micropores and mesopores (below 50 nm ), resulting in an improved coal adsorption capacity; 3)Providing three assessment indexes of potential of DCBM resource exploitation and CO2 sequestration includes the buried depth, porosity and fracture permeability. The above research outcomes provide important theoretical foundation for the development of DCBM resources, assessment of CO2 sequestration potential and engineering application.

Optimization of CO2/N2 injection ratios in goaf by saturation adsorption capacity

The inerting technology is a common means to prevent fires in goaf of coal mine, in which adsorption capacity is the key index to measure the inerting effect. Therefore, this paper obtained the coal spontaneous combustion (CSC) stage of Qianyingzi coal mine through programmed heating experiments, and determined the physical adsorption stage temperature (below 323.15 K) matching the actual environment. The oxidation characteristics of coal samples with different adsorption degrees were further studied by adsorption experiment and TG-DSC test. The results indicate that the combustion activation energy of coal samples follows the sequence of second-stage adsorption coal > dried coal > raw coal > first-stage adsorption coal. This implies that the CO2 saturated adsorption can exert a specific inhibitory impact on coal combustion. Therefore, the index of the saturation adsorption capacity (SAC) was proposed to evaluate the inerting effect. We further studied the adsorption capacity of coal at different ratios of CO2/N2 by molecular simulation under different pressures (0–5 MPa). The results show that CO2 dominates in both single-component adsorption and competitive saturated adsorption. The average of ratios (AOR) and average of differences (AOD) threshold are determined. The priority of SAC is obtained in the order of single-component (CS), CO2/N2 mixture ratio at 4:6 (C4:6), CO2/N2 mixture ratio at 2:8 (C2:8) from largest to smallest. Furthermore, a fuzzy optimization method for injection ratio of CO2/N2 under different pressures was established. During dynamic changes in environmental pressure, it is appropriate to inject CO2 separately at 0.2–2.5 MPa. At 3 MPa, a mixture of CO2/N2 at a ratio of 2:8 is the best for prevent fires in goaf. Similarly, an inert gas with a CO2/N2 ratio of 4:6 is injected at 3–5 MPa.

Differential Analysis of Various Moisture Phases on N2 Displacement of CH4 in Coal.

The aim of the study was to elucidate the impact of different moisture phases during gas injection on coalbed methane displacement. The coal samples were treated with two methods: water vapor adsorption and liquid water stirring. The differences in the coal samples treated with various moisture phases during gas injection for coalbed methane displacement were investigated by using the isothermal adsorption curves of CH4, N2, and H2O in coal and N2 displacement of CH4 in coal. The results indicate that variations in the gas adsorption capacity of coal are treated with different moisture phases. The gas adsorption capacities and displacement capacities of the coal samples treated with the water vapor adsorption methods are better than those treated with the stirring methods. In the isothermal adsorption experiment, for the coal samples treated with different moisture phases, at a moisture content of 2.75%, the saturated adsorption capacities of CH4/N2 are 0.204/0.189 (cm3/g), and at a moisture content of 5.63%, the saturated adsorption capacities of CH4/N2 are 0.151/0.139 (cm3/g). In addition, in the displacement experiment, for the coal samples treated with different moisture phases, at a moisture content of 2.75%, the difference in the total gas adsorption capacities is 0.62 cm3/g and the difference in the CH4 adsorption capacities is 0.473 cm3/g, and at a moisture content of 5.63%, the difference in the total gas adsorption capacities is 0.3 cm3/g and the difference in CH4 adsorption capacities is 0.22 cm3/g. For the coal samples treated with various moisture phases, the differences in the CH4/N2 adsorption and displacement capacities are greater at a moisture content of 2.75% than at 5.63%. Notably, the moisture phase has only a marginal influence on the CH4 desorption capacity and desorption rate. The study is important to understand the interactions between coal and moisture.

Decoding wettability in coal-water-CO2 system for enhanced sequestration security

The injection of CO2 into deep unminable coal seam is a most promising carbon storage method that can simultaneously achieve the dual objectives of mitigating CO2 emission and enhancing coalbed methane recovery. In this context, the wettability of the coal-water-CO2 system plays an important role in the CO2 injection rate, storage capacity, and sequestration security. However, the factors affecting wettability alternation in the coal-water-CO2 system are still unclear until now. In this study, the sessile drop method was used to measure the contact angle (CA) in the coal-water-CO2 system under different pressures (3 – 8 MPa) and temperatures of (40–70 °C) with four different rank coals. Results indicate that with increasing pressure, the CA shows a steady increase below 5 MPa followed by a rapid increase at 5 – 8 MPa. The CA decreases with increasing temperature and coal rank. The CA is negatively correlated with the mes- and macro-pores, but positively correlated with micropores. It was found that these relationships are attributed to the CO2 density and CO2 adsorption capacity of coal. As the CO2 density increases, the coal-CO2 interaction force intensifies, leading to a tendency towards CO2 wetting for the coal-water-CO2 system. The adsorption capacity illustrates the strength of the coal-CO2 interaction, where the affinity of polar adsorption sites for CO2 molecules exceeds that of nonpolar adsorption sites. Finally, we conducted a comprehensive assessment of the wettability of the coal-water-CO2 system as it varies with burial depth by developing an integrated model that considers four influential factors, including pressure, temperature, coal rank, and pore structure. The findings of this study can be utilized to determine the optimal CO2 injection pressure in CO2-ECBM projects and enhance sequestration safety.

Prediction Model of Lean Coal Adsorption of Power Plant Flue Gas.

To minimize errors in calculating coal flue gas adsorption capacity due to gas compressibility and to preclude prediction inaccuracies in abandoned mine flue gas storage capacity for power plants, it is imperative to account for the influence of compression factor calculation accuracy while selecting the optimal theoretical adsorption model. In this paper, the flue gas adsorption experiment of a power plant with coal samples gradually pressurized to close to 5 MPa at two different temperatures is carried out, and the temperature and pressure data obtained from the experiment are substituted into five different compression factor calculation methods to calculate different absolute adsorption amounts. The calculated adsorption capacities were fitted into six theoretical adsorption models to establish a predictive model suitable for estimating the coal adsorption capacity in power plant flue gas. Results reveal significant disparities in the absolute adsorption capacity determined by different compression factors, with an error range of 0.001278-7.8262 (cm3/kg). The Redlich-Kwong equation of state emerged as the most suitable for the flue gas of the selected experimental coal sample and the chosen composition ratio among the five compression factors. Among the six theoretical adsorption models, the Brunauer-Emmett-Teller model with three parameters demonstrated the highest suitability for predicting the adsorption capacity of coal samples in power plant smoke, achieving a fitting accuracy as high as 0.9922 at 49.7 °C.

Ethylene and propylene sorption kinetics on hard coals

Ethylene and propylene are gases released to the mine air together with the increase in the temperature of coal as part of self-heating. The concentrations of these hydrocarbons in the mine air are used as indices to assess the development of the coal self-heating process.This paper presents the results of tests involving coal adsorption capacity as well as ethylene and propylene sorption kinetics, conducted on five samples of Polish hard coals collected from coal beds under extraction in Polish mines.The adsorbed hydrocarbon quantities depend on the degree of metamorphism, the pore volume and the internal structure accessibility of the coals. Based on the performed kinetic measurements, it was concluded that propylene sorption is a process more rapid than ethylene sorption – the time required to reach the propylene sorption equilibrium is shorter. A decrease in the time required to reach the sorption equilibrium of both the hydrocarbons was also observed together with the increase in pressure, primarily for coals with low adsorption capacity. Using kinetic data made it possible to determine diffusion coefficients based on a unipore model and kinetic parameters based on modified unipore model. The values of above-mentioned parameters depend on the pressure and type of gas. As the pressure increases, their increase is observed. The values of both diffusion coefficients and kinetic parameters are higher for propylene than for ethylene.

Production and application of activated carbon obtained from the thermochemical degradation of corn cob

The present work aims to perform the thermal degradation of corn cob for the production of activated carbon, which is used in a column adsorption system for the dechlorination of supply water. The biomass under study and the product generated by pyrolysis in a fixed bed reactor were analyzed using classical methods (moisture, volatile materials, ash and fixed carbon, lignin, cellulose, hemicellulose and apparent density) and instrumental methods (thermogravimetry and CNH). In the pyrolysis process, the following parameters were studied: temperature (360 to 640 oC) and heating rate (13 to 27 oC/min). The reaction time was 30 minutes. The dechlorination test using the activated carbon obtained in this research was carried out in an adsorption column in continuous flow at an average flow rate of 6 mL/min. The iodine number of the adsorbent produced in this research was calculated. The results obtained showed that the carbon content (44%) and the lignin content (28%) point to the use of residual biomass studied in this research for the production of activated carbon via the thermal route. The highest activated carbon yield was 71.12% at a temperature of 360 °C with a heating rate of 20 °C/min. While the minimum yield was 21.6% with a temperature of 600 °C and a heating rate of 25 °C/min. The iodine number of the activated carbon produced at 640 oC was 820 mgI2/g. In the adsorption test, the following coefficients from the Thomas model were obtained: kTH = 0.0093 mL/mg.min and qo = 98 mg/g, when a liquid flow equal to 9.9 mL/min was used. The adsorption capacity of coal is related to the flow rate of the dye solution, being noticed that the higher the liquid flow rate in the column, the higher the adsorbate mass transfer coefficient.

Analysis of the influence of different pore ratios on methane adsorption characteristics of coal

The diverse pore structures in coal are pivotal in coalbed methane storage and transportation, making it highly significant to explore methane adsorption characteristics across varying pore sizes for effective prevention and control of coal mine gas disasters. Using the proposed graphene nanotube filling method, eight coal molecule models with varying pore size ratios were constructed, including four different pore radii: 0.35 nm, 0.55 nm, 0.75 nm, and 2 nm. Methane adsorption amount, methane density, and methane concentration distribution were obtained through Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods. The research results show that coal consisting of multiple pore sizes had a better methane adsorption capacity than a single pore size. Combining pore sizes of 0.35 nm, 0.55 nm, and 0.75 nm in a 1:2:1 ratio resulted in the highest excess methane adsorption capacity in coal, reaching 5.623 mmol/g. The single pore size of 0.55 nm in coal had the lowest excess adsorption amount of methane, which was 2.236 mmol/g. The presence of mesopores weakened the methane adsorption capacity of coal, and this weakening effect increased with the increase of mesopore proportion. In the model composed of single pore size micropores, the methane concentration inside the pores was uniformly distributed. The coexistence of multiple micropores may lead to methane filling in the micropores in a monolayer adsorption form. The coexistence of micropores and mesopores does not alter methane filling in the micropores and monolayer adsorption in the mesopores. The research results provide new insights into our understanding of the impact of pores on methane adsorption in coal.

Effect of Inhibitors on Hydrogen Bond Reduction and Kaolinite Dissolution for Enhanced CO2 Adsorption in Coal.

The application of an inhibitor to the remaining coal in the goaf not only prevents spontaneous combustion of the coal seam in the mining area but also greatly enhances the capacity of coal to adsorb CO2. To investigate the mechanism by which inhibitors improve the CO2 adsorption capacity of the coal seam in the goaf, we conducted swelling experiments, infrared spectroscopy, scanning electron microscopy, and X-ray diffraction analyses to examine the microstructural changes in the adsorption of CO2 before and after inhibition. The results indicate that after inhibition, the number of hydrogen bonds between coal macromolecules decreased, and the samples exhibited approximately 5% swelling. This swelling of the coal macromolecular structure and the increased distance between coal particles create additional space for CO2 sequestration, which is a critical factor contributing to the enhanced CO2 adsorption capacity of coal. The mineral composition of coal consists of 75.6% kaolinite, and inhibition leads to a reduction in kaolinite content by 0.8-7.9%. After inhibition, the swelling and disintegration of kaolinite cause uneven stress, resulting in changes to the pore structure. Closed pores filled with kaolinite transform into open pores, and the original pores crack, forming new pores and pore channels. The dissolution of kaolinite particles increases the porosity of the coal, further facilitating gas adsorption. Among the three inhibitors tested, the most effective in enhancing CO2 sequestration by bituminous coal in the mining area was the urea solution. This study holds significant importance in improving the CO2 sequestration capacity of residual coal in goaves.

Changes in Physicochemical Properties of Coal and Their Mechanism Due to Supercritical CO2–H2O Treatment

The dissolution of supercritical carbon dioxide (ScCO2) in water forms a ScCO2–H2O system, which exerts a transformative influence on the physicochemical characteristics of coal and significantly impacts the CO2-driven enhanced coalbed methane (CO2-ECBM) recovery process. Herein, the effect of ScCO2–H2O treatment on the physicochemical properties of coal was simulated in a high-pressure reactor. The migration of major elements, change in the pore structure, and change in the CH4 adsorption capacity of coal after the ScCO2–H2O treatment were detected using plasma emission spectroscopy, the low-temperature liquid nitrogen adsorption method, and the CH4 adsorption method, respectively. The results show that (1) the ScCO2–H2O treatment led to mineral reactions causing a significant migration of constant elements in the coal. The migration of Ca ions was the most significant, with an increase in their concentration in treated water from 0 to 16–970 mg·L−1, followed by Na, Mg, and K. Al migrated the least, from 0 to 0.004–2.555 mg·L−1. (2) The ScCO2–H2O treatment increased the pore volume and pore-specific surface area (SSA) of the coal via the dissolution and precipitation of minerals in the coal pores. The total pore volume increased from 0.000795–0.011543 to 0.001274–0.014644 cm3·g−1, and the total pore SSA increased from 0.084–3.332 to 0.400–6.061 m2·g−1. (3) Changes in the CH4 adsorption capacity were affected by the combined effects of a mineral reaction and pore structure change. The dissolved precipitates of the minerals in the coal pores after the ScCO2–H2O treatment caused elemental migration, which not only decreased the mineral content in the coal pores but also increased the total pore volume and total pore SSA, thus improving the CH4 adsorption capacity of the coal. This study provides theoretical support for CO2 sequestration and ECBM recovery.

Natural gas desorption versus laboratory gas adsorption with pore shrinkage

To perform hydro-mechanical experiments on sorptive geomaterials, the gas is injected into bulk of the specimen to re-saturate it representing its in-situ state, and adsorption is monitored until gas equilibrium is reached. We show in this study that this experimental procedure does not reflect the in-situ state of the sample leading to potentially significant errors. We measured the carbon dioxide (CO2) adsorption capacity of the coal specimen (free of gas) and its powder with different particle sizes and compared the results with field desorption data. Experimental results indicate an increase in coal adsorption capacity with decrease in coal particle size. Interestingly, the field data exhibits much higher desorbed gas amount compared to laboratory-measured adsorption capacity of coal powder with smallest measurable particle size under the same gas pressure. To shed light on the observation of field desorption – laboratory adsorption hysteresis, the micro-scale gas adsorption experiments combined with X-ray micro-Computed Tomography (XRCT) measurements were conducted where an intact specimen was imaged before (dry state) and after Krypton injection. The analysis of the XRCT images indicates that the Krypton adsorption closes/reduces the aperture of pre-existing fractures in the specimen. In addition, the Krypton diffuses only in a small region near pre-existing fractures despite its adsorption measurement indicating gas equilibrium and specimen saturation. This observation proposes that significant portion of pores exposed to Krypton experienced a size reduction by swelling resulting in partial saturation of the specimen. This patrial saturation at laboratory conditions evidently cannot resemble the in-situ state of gas sorption capacity.

Energy Variation Features during the Isothermal Adsorption of Coal under High-Temperature and High-Pressure Conditions

This paper aims to describe methane adsorption in coal under the conditions of high temperature and high pressure, as well as quantitatively decipher the change rule of energy in the isothermal adsorption process. The isothermal adsorption test was carried out with four groups of middle-rank coals from the Linxing area with different degrees of metamorphism. The impacts of the degree of deterioration of coal, temperature, and pressure on adsorption were analyzed with regard to the adsorption amount, adsorption potential, and adsorption space. Additionally, the energy change during the adsorption of methane by the coal was considered. The results show that the coal adsorption capacity hinges on the degree of deterioration of the coal, as well as the pressure and temperature. Additionally, the impact of temperature upon coal methane adsorption under depth conditions is highlighted. Like the adsorption space, the adsorption potential is an important parameter used to quantitatively characterize the adsorption ease and adsorption capacity; furthermore, the adsorption potential of millipores exceeds that of mesopores, as they are capable of offering a larger specific surface area for adsorption. The total decrease in the surface free energy during adsorption increases as the pressure increases; simultaneously, the increase rate is fast and then slow. The total decrease in the above-described free energy diminishes as the temperature escalates. Under the same pressure, the total decrease in the aforementioned free energy increases as the reflectance of the specular body of the coal increases. The decrease in the aforementioned free energy at each point of pressure lessens as the pressure grows; notably, when the pressure is comparatively low, the reduction is very fast. As the pressure escalates continuously, the decrease speed is slow. Regarding the effect of pressure and temperature upon adsorption, the adsorption gas volume of coal exists in a conversion depth from 1200 m to 1500 m; at the same time, the impact of pressure upon adsorption is dominant up to this depth. Additionally, beyond this depth, temperature gradually comes to have the greatest impact on adsorption.

Factors Influencing the Methane Adsorption Capacity of Coal and Adsorption Heat Variations

Factors Influencing the Methane Adsorption Capacity of Coal and Adsorption Heat Variations

Experimental Study on Influencing Factors and Thermal Effects of CO2 Adsorption by Coal.

The utilization of CO2 is extremely important to solve the environmental problems and coal spontaneous combustion in goaf. There are three kinds of CO2 utilization in goaf: adsorption, diffusion, and seepage. Since adsorption will consume CO2 in goaf, the optimization of CO2 injection amount is very critical. A self-developed adsorption experimental device was used to determine the CO2 adsorption capacity of three different particle sizes of lignite coal samples at 30-60 °C and 0.1-0.7 MPa. The factors affecting CO2 adsorption by coal and its thermal effect were studied. In the coal and CO2 system, the CO2 adsorption characteristic curve is not affected by temperature, but there are differences in that with different particle sizes. The adsorption capacity increases with the increase of pressure, while it decreases with the increase of temperature and particle size. Under atmospheric pressure, the adsorption capacity of coal is a logistic function relationship with temperature. Furthermore, the average adsorption heat of CO2 on lignite shows that the interaction force between CO2 molecules has a stronger effect on CO2 adsorption than the effect of heterogeneity and anisotropy on the coal surface. Finally, the existing gas injection equation is improved theoretically with CO2 dissipation, which provides a new idea for the work of CO2 prevention and fire suppression in goaf.

Effect of Soaking Time on the Spontaneous Combustion Characteristics of Lignite

ABSTRACT When mining deep coal seams, the overlying mined-out area must be pumped and drained, and the remnant coal in the mining void area is vulnerable to spontaneous combustion via water immersion and air drying. To investigate the effect of soaking time on the spontaneous combustion characteristics of lignite, low-temperature N2 adsorption experiments and Fourier transform infrared spectroscopy (FTIR) experiments were used in this study to investigate the changes in pore structure and functional group content of coal samples by increasing the soaking time. Based on programmed warming experiments, macroscopic characteristic indicators of coal spontaneous combustion (CSC) oxidizability, such as the oxygen consumption rate and index gas, were derived. Results showed that the effect of soaking caused the total pore volume and specific surface area of lignite to increase to different degrees, which enhanced the adsorption capacity of coal to oxygen and improved the effective contact of oxygen with the active sites on the coal surface. However, there was no consistency in the effect of different soaking times on the content of reactive functional groups of the coal, with higher methyl and methine contents after 60 days of soaking (S60) and 120 days of soaking (S120) than in the raw coal (RC), and less in 90 days of soaking (S90). The propensity of coal to spontaneously combust is determined by both the pore structure and reactive functional groups, which shows the propensity of spontaneous combustion S60>S120>RC>S90. Results from this study are important in understanding the mechanism of CSC of water-soaked lignite in mined-out areas and can provide theoretical guidance to prevent CSC in mining areas.

Model for Predicting CO2 Adsorption in Coal Left in Goaf Based on Backpropagation Neural Network

Injecting power plant flue gas into a goaf stores CO2 in the flue gas and effectively prevents the spontaneous combustion of the coal remaining in the goaf. Here, we investigated the adsorption behavior of three types of coal at normal temperature and pressure using a self-developed adsorption experimental device. We used a specific surface area and porosity analyzer to study the effects of pore structure, mineral content, and moisture content on CO2 adsorption in coal. Based on the experimental data, we designed a multifactor CO2 adsorption prediction model based on a backpropagation (BP) neural network. The results indicated that the pore size of most micropores in coal was in the range of 0.5–0.7 and 0.8–0.9 nm. The specific surface area and pore volume were positively correlated with the CO2-saturated adsorption capacity, whereas the mean pore diameter, mineral content, and moisture content were inversely associated with the CO2-saturated adsorption amount. The accuracy of the multifactor BP neural network prediction model was satisfactory: the determination coefficients (R2) of the training and test sets were both above 0.98, the root mean square error (RMSE) and mean absolute error (MAE) of the test set were both less than 0.1, and the prediction results satisfied the requirements. To optimize the prediction performance of the model, we used the random forest algorithm to calculate the importance of each factor. The sum of the importance weights of the specific surface area, moisture content, and pore volume was 91.6%, which was much higher than that of the other two factors. Therefore, we constructed an optimization model with specific surface area, moisture content, and pore volume as input variables. The R2 values of the training and test sets in the simplified model were improved compared with those of the multifactor model, the RMSE and MAE were reduced, and the fitting effect was ideal. The prediction model of CO2 adsorption in coal based on the BP neural network can predict the CO2 adsorption capacity of coal under different physical and chemical conditions, thereby providing theoretical support for the application of CO2 storage technology in goafs.

Fractal Dimension and Nuclear Magnetic Resonance Characteristics of Surfactants for Coal Gas Desorption

In this paper, the fractal dimension of coal surfaces was calculated using the fractal theory, and the influence of different surfactants on the fractal dimension of coal surface was studied. Six kinds of sustainable and traditional surfactants used in coal gas desorption processes were compared and analyzed. We used mineral gas fertilizer coal from Huainan Liuzhuang, China, as the research object and studied sodium dodecyl benzene sulfonate (SDBS), cohol diethanolamide (CDEA), nonylphenol polyoxyethylene ether (NP-10), tea saponin, sucrose ester and rhamnolipid surfactants used to modify coal. The methane adsorption capacity of coal before and after surfactant modification was measured by low field nuclear magnetic resonance methane adsorption, and Langmuir volume and Langmuir pressure were obtained according to the Langmuir fitting equation. The results showed that from the perspective of fractal dimensions, the best surfactant in the context of sustainability (in order) is rhamnolipid, CDEA, tea saponin, sucrose ester. According to the two important parameters VL and PL in the Langmuir equation, the coal sample modified by sucrose ester had the strongest gas adsorption capacity. According to the numerical analysis of the surface fractal dimension DS of the coal modified by surfactants, the soluble organic matter in the raw coal samples dissolved, and the micropore morphology evolved to become mesoporous or macroporous, which is more favorable for desorption. The coal samples modified by rhamnolipid and SDBS had the strongest gas desorption ability.

Competitive adsorption of N2 and CO2 on coal by a self-balancing triaxial equipment: A case study

ABSTRACT A self-balancing coal triaxial loading adsorption-desorption-replacement tester was independently developed to investigate the influence of in-situ stress on the competitive adsorption of a N2 and CO2 mixture while providing guidance for coal seam gas injection and extraction. A binary gas mixture of N2 (20%) and CO2 (80%) was prepared using a high-pressure gas distribution system. The isothermal competitive adsorption and desorption experiments of the binary mixed gas under different stress states (6 MPa, 9 MPa, 12 MPa, 15 MPa) were performed on the coal samples from Xinzhuangzi coal mine, Huainan. The results showed that when the confining pressure remained unchanged and the adsorption equilibrium pressure was 0.6 MPa, the axial pressure increased from 6 MPa to 12 MPa (the stage before coal yielded), the coal adsorption capacity for N2 and CO2 decreased, and the competitive adsorption capacity of N2 decreased by 14.4%. When the axial pressure increased from 12 MPa to 15 MPa (after the coal yielded), the coal adsorption capacity for N2 and CO2 increased, and the competitive adsorption capacity of N2 increased by 22.3%. When the stress remained unchanged, the adsorption equilibrium pressure increased, adsorption growth rate of N2 was greater than that of CO2, and competitive adsorption capacity of N2 was enhanced. Therefore, in the coal pre-yield stage, the CH4-displacement ability of the mixed gas decreased with an increase in stress and increased with an increase in air pressure. This phenomenon indicates that low stress and high gas pressure promoted the replacement of CH4 by the mixed gas and increased the permeability of the low-permeability coal seam. Beyond the yield point of the coal, the addition of N2 promoted the replacement of CH4 by gas injection; the addition also promoted the permeability enhancement of the coal seam. Thus, the amount of N2 in the mixed gas should be increased as in-situ stress increases.This research can contribute to CO2 recovery and energy exploitation and utilization.