Adsorption Equilibrium Pressure Research Articles

Modeling and experimental study on methane diffusivity in coal mass under in-situ high stress conditions: A better understanding of gas extraction

Gas desorption and diffusion in pore is not only an important behavior to control the law of gas migration, but also an important theoretical basis for the proposal and optimization of extraction technology. At present, researches on gas diffusion in coal were mainly conducted on granular coal under the stress-free condition. However, on-site gas desorption and diffusion in coal seams occur in coal mass under the in-situ stress condition. In particular, high stress exerts a notable effect on gas diffusion in the deep mining stage. Therefore, experimental study and theoretical model establishment concerning the diffusion characteristics of coal mass under high stress require further investigation. In this work, first, with respect to the desorption and diffusion characteristics of coal mass under high stress, desorption and diffusion experiments were performed on coal mass under different high stress conditions with the aid of a self-developed experimental device, and the corresponding gas desorption laws were obtained. In addition, the law of coal mass permeability evolution under different stresses was obtained through the transient measurement method. Moreover, a coal mass diffusion model which conformed to the characteristics of high stress was established based on reasonable assumptions, a new physical structure model of coal under high stress and the relationship between diffusion length and stress. Moreover, this model, according to the laws of gas desorption under different conditions, it is found that the diffusion coefficient increases with the rise of adsorption equilibrium pressure, and stress is significantly proportional to the diffusion coefficient. Besides, with reference to the relationship between diffusion coefficient and stress value, the fundamental parameter that affects the desorption and diffusion amount, i.e., effective diffusion coefficient, was analyzed. Based on the variations of the effective diffusion coefficient with stress, the mechanism of how the classical pressure relief mining technology to improve gas extraction efficiency was further elaborated. The research results provide a more comprehensive theoretical support for the application of the pressure relief mining technology in deep high-stress coal seams.

Impact research of CH4 replacement with CO2 in hydrous coal under high pressure injection

Purpose. Based on high-pressure gas injection technology that enhances coal seam gas drainage, the effect of CH4 replacement with CO2 in aquiferous coal has been studied. Methods. Using the laboratory experimental method and the self-built high-pressure gas injection experimental device, high-pressure CO2 is injected into coal with different moisture contents to replace CH4 under different adsorption equilibrium pressures. Findings. With an increase in coal moisture content, the adsorption capacity of coal for CH4 and CO2 gradually weakens, the adsorption capacity for CO2 is always greater than that of CH4, and the CH4 replacement rate and the CO2 injection ratio gradually decrease. It is concluded that the CH4 replacement rate and the CO2 injection ratio are negatively correlated with the water content of coal. With an increase of the pre-adsorption equilibrium CH4 pressure (0.5, 0.75, 1.0, 1.3 and 2.0 MPa), the CH4 replacement rate and the CO2 injection ratio first sharply and then slowly increase. The transition point is 1.3 MPa (pre-adsorption equilibrium pressure of CH4). Originality. Based on the adsorption characteristics of coal seam gas injection, the influence of coal water content and gas injection pressure on CH4 replacement rate and CO2 injection ratio is analyzed, and the mechanism is studied. Practical implications. The experimental results have important guiding significance for selecting reasonable gas injection pressure and the source of gas to drive its injection into underground coal seam.

Gas desorption diffusion behavior in coal particles under constant volume conditions: Experimental research and model development

ABSTRACT Understanding gas diffusion characteristics of coal particles is a significant theoretical basis for the exploitation of coalbed methane. Generally, diffusion experiments are performed under constant pressure conditions, and scholars seldom discuss gas desorption experiments with constant volume, but the recognition of this characteristic is also equally important. Based on this, the paper conducted coal particles desorption experiments under conditions of constant volume to explore gas diffusion characteristics under different adsorption equilibrium pressures and initial emission environmental pressures. The experimental data showed that the variation law of desorption amount with time under constant volume conditions conformed to the Langmuir-like equation. Meanwhile, the rising of initial emission environmental pressure reinforced the linear relationship between the limitation desorption amount and the adsorption equilibrium pressure. When the initial emission environmental pressure was 0.10 MPa, the correlation coefficient of linear fitting between them was as high as 0.9826. To better describe the diffusion process of this condition, a dynamic diffusion model of coal particles gas diffusion under constant volume conditions was established. The calculated values of this model had a preferable consistency with the experimental data, and the correlation coefficients were all over 0.99, which verified the reliability of the model. This research makes up for the theory of gas diffusion and provides a theoretical basis for coalbed methane extraction.

Experimental investigation on the diffusion property of different form coal: Implication for the selection of CO2 storage reservoir

Gas diffusion is an important migration behavior for coal, and the diffusion ability serves as an important parameter to evaluate whether CH4 extraction in a coal reservoir is effective and whether the reservoir is suitable for CO2 storage. Granular coal, whose morphology differs greatly from that of coal mass in actual gas extraction, remains the research object of most current studies on the diffusion characteristics of coal. Consequently, problems occur in the application of the experimental results to engineering practice. In this study, first, different shape units and gas diffusion paths were proposed for coal matrix. Subsequently, an improved Uni-pore spherical diffusion model considering the gas loss at the beginning of the experiment was established based on the Uni-pore spherical diffusion model of granular coal. Then, a gas diffusion model suitable for coal mass was established based on Fick’s Second Law of Diffusion and the cylindrical matrix shape. In addition, a contrastive analysis was conducted on the desorption laws of coal with different forms under different adsorption equilibrium pressures. It is found that granular coal corresponds to a great gas desorption amount than coal mass. The gas diffusion coefficients of coal under different conditions were calculated based on the gas desorption curves and the corresponding diffusion models. To better grasp the diffusion coefficient law of coal with different forms, coal mass and 0.2–0.25 mm granular coal at the same matrix scale were contrasted. The results suggest that they are in the same order of magnitude and differ insignificantly. Finally, the primary reason why the desorption amounts of coal mass and 0.2–0.25 mm granular coal differ notably was revealed: The shape factor of spherical matrix of granular coal is larger than that of coal mass matrix. Furthermore, considering that different matrix shape factors caused by the structures of granular coal and coal mass lead to notably different gas adsorption capacities, this study held that tectonic coal seams may be more suitable for CO2 storage than intact coal seams under the same condition.

Effect of pore structure on competitive sorption and diffusion of mixed methane and carbon dioxide in anthracite, South Qinshui Basin, China

A deep understanding of the competitive adsorption and diffusion of methane (CH4) and carbon dioxide (CO2) in coal is significant for implementing the CO2-ECBM project. For three high-rank coal samples, industrial analysis, coal maceral, and vitrinite reflectance (RO) determination, mercury injection test, and low-temperature nitrogen/carbon dioxide specific surface test were carried out, and the composition and pore structure of coal samples were studied systematically. Meanwhile, through the three mixed gases (75%CH4 + 25%CO2, 50%CH4 + 50%CO2, 25%CH4 + 75%CO2) adsorption and desorption test, the absolute adsorption amount, free phase and adsorption phase of CH4 and CO2 under different equilibrium pressures, and the equivalent diffusion coefficients of gas migration in the process of adsorption/desorption equilibrium were calculated. Finally, the relationship among the coal composition, pore structure, and the competitive adsorption and diffusion characteristics of methane and carbon dioxide mixture in coal were studied. The results show that: (1) the composition of coal has an important influence on the development of coal pore structure and the adsorption and transport of gas in coal. Supermicropore (<2 nm in size) is the prominent adsorption place. Furthermore, its development, which essentially determines the adsorption capacity of coal samples, is positively correlated with fixed carbon content, vitrinite content, and vitrinite reflectance. (2) When the pores >100 nm in size relatively developed, the coal samples have higher porosity, lower micropore diffusion, and a smaller desorption hysteresis index. Because the pore's connectivity is limit, its content has no apparent correlation with the permeability of coal samples and most of the adsorption and diffusion parameters. (3) The desorption hysteresis is mainly caused by the opposite migration direction of gas in the process of adsorption and desorption and the resulting dislocation of coal matrix swelling and shrinkage. The competitive adsorption of CH4 or CO2 and their separation index (SCO2/CH4) in coal samples decreases with the increase of adsorption equilibrium pressure. Also, the desorption process often shows a higher SCO2/CH4 value than that of the adsorption process. (4) During the adsorption process, the equivalent macro-diffusivities of the coal sample decrease as a power function with increasing pressure and increase with the carbon dioxide content increasing in the mixed gas. However, the equivalent micro-diffusivities change little with the different CH4 and CO2 mixtures migration in different coal samples. The equivalent diffusion coefficient of coal is positively correlated with macropore, mesopore, transitional and micropore content in coal samples.

Characteristics of gas pressure change during the freezing process of gas-containing coal

To investigate the characteristics of gas pressure changes during the freezing of gas-containing composite coal, an experimental device for determining the freezing response characteristics of gas-containing coal was independently designed. Coal samples with different firmness coefficients from the No. 3 coal seam in Yuxi Coal Mine in Jincheng, Shanxi Province, were selected to determine the different freezing response characteristics. The gas pressure evolved under different temperatures (-10 °C-15 °C-20 °C-25 °C-30 °C) and different adsorption equilibrium pressures (1.0 MPa, 1.5 MPa, 2.0 MPa). The research results reveal that, during the freezing process of the gas-containing coal sample, the gas pressure in the coal sample tank changed as a monotonously decreasing function and underwent three stages: rapid decline, decline, and slow decline. The relationship between the gas pressure of the coal sample tank and the freezing time is described by a power function. Low temperatures promoted gas adsorption. As the freezing temperature decreased, the decrease of gas pressure in the coal sample tank became faster. During the freezing process, the adsorption capacity of soft coal was larger, and the gas pressure of soft coal was lower.

Mechanism of Gas Desorption in Coal Particles under Different Positive Pressures: Experiments and Numerical Model

The inaccurate calculation of the gas lost is the principal cause of inaccurate determinations of the coalbed methane content via air reverse circulation sampling (ARCS). The positive pressure environment has a significant impact on gas desorption during the sampling process. To obtain the gas desorption mechanism of coal particles under positive pressure, positive pressure desorption experiments were conducted on coal samples with different particle sizes under different adsorption equilibrium pressures using a self-designed positive pressure desorption experimental device. And a positive diffusion model for coal particles was established, in which the diffusion coefficient was calculated based on the results of the positive desorption experiments. The diffusion model was then used to simulate the diffusion capacity of coal particles and compared with the test data. The results show that the responses of the positive pressure desorption and atmospheric pressure desorption to the adsorption equilibrium pressure are similar. The gas desorption velocity increases as the adsorption equilibrium pressure increases. Positive pressure can effectively inhibit gas desorption. The initial gas desorption velocity decreases as the positive pressure and coal particle size increase. Concurrently, the entire sample desorption process during ARCS can be divided into three stages: a slow desorption stage, an accelerated desorption stage, and an atmospheric desorption stage. The relationship between the diffusion coefficient and the positive pressure exhibits an exponential distribution, and the positive pressure diffusion model can describe the gas diffusion from coal particles well. The results of this study could help to establish a gas loss compensation model for the ARCS process.

Selection of Characteristic Particle Size of Drilling Cuttings Based on Adsorption-Desorption Properties: Experiment and Simulation

The direct method for measuring gas content based on image processing, that is, comparing the desorption curves of drilling cuttings with the characteristic particle size with the desorption image in the database to determine the gas content of coal seams, can solve the problems of long measurement time and larger measurement errors of the traditional direct method. However, the precondition to realize this method is to define the characteristic particle size of drilling cuttings. Therefore, this paper performed adsorption and desorption experiments under different particle size ranges and dissimilar adsorption equilibrium pressures, solved the gas migration control equation in coal particles, and determined the characteristic particle size from experimental results and simulation effect. The results showed that gas desorption quantity was positively correlated with adsorption equilibrium pressure, while negatively correlated with the size of a particle size range. The greater the adsorption equilibrium pressure, the better the linear fitting relationship with the maximum desorption quantity, and the maximum correlation coefficient could reach 0.9658. Based on Darcy’s law, Langmuir adsorption equation, and mass conservation law, the theoretical control equation of gas migration in coal particles was derived and solved by Open source Field Operation and Manipulation (OpenFOAM). The accuracy of the calculation results of the solver was verified by the derived analytical solution of the gas pressure change of coal particles without considering the adsorption. From the experimental results and simulation results, the characteristic particle size of drilling cuttings was determined to be 0.5-1 mm. The research can provide help for the subsequent development of direct gas content measurement technology based on image processing.

Study on the effect of multiple non-breathable gangue occurrence on gas migration in coal seams

Abstract As the mining environment gradually moves toward deep, the form and occurrence mode of gangue are more complex. The occurrence of non-breathable gangue seriously worsens gas migration conditions, which greatly reduces the gas extraction efficiency and increases the possibility of coal and gas outburst. Therefore, in order to explore the effect of multiple non-breathable gangue occurrence on gas migration in coal seams, the self-developed the constant pressure coal and rock mass gas seepage test system is used to systematically study the characteristics of coal seam gas migration under different layers of gangue and the differentiated occurrence of multiple gangue. The equivalent characterization of coal seam gas migration under the condition of multiple inclusions is proposed through theoretical analysis and numerical simulation. Research shows: the permeability factor of the whole coal sample, sample containing a single layer gangue and sample containing double layers gangue exhibits a “V-shaped variation law with the increase of the gas adsorption equilibrium pressure. There is a critical gas pressure point when the permeability factor is the lowest, and with the increase in the number of gangue layers, the greater the pressure value of the critical gas pressure point. According to the characteristics of gas pressure distribution, the sample containing gangue can be roughly divided into four influence areas: gas expansion affected area, gas stable seepage area and upper and lower effective seepage area. The more the number of gangue layers and the larger the area of gangue layers, the upper and lower effective seepage area is smaller, the closer the positions of gangue are to the gas source, gas expansion affected area is smaller, both of them will cause more difficult in gas migration and the lower of the sample permeability factor. When different sizes of gangue are used at two positions of the gangue in the samples, with the increase of gas adsorption equilibrium pressure, the attenuation rate of permeability factor in the early stage of the sample is significantly higher than that of the sample with the same size of gangues in the front and at the back. The results of the paper have certain actual guiding significance for improving gas extraction efficiency and preventing mine gas disasters.

Experimental study on methane diffusion kinetics in three typical metamorphic coals

During coalbed methane (CBM) depletion, the diffusion property of methane is one of the key factors affecting the production of CBM recovery. In this work, the pore structure of three typical coals with different metamorphic degrees are tested by scanning electron microscopy and low field nuclear magnetic resonance. The high precision diffusion experiment system is developed to study the diffusion property of methane under different pore structure, gas adsorption pressure and coal sample size. The effects of tortuosity and pore length on methane diffusion is quantitatively analyzed, and the multistage clustering effect of coal particle microscopic composition is discussed. The results show that the pore structure leads to different diffusion resistance, which affects the initial diffusion rate of methane. Coals with high metamorphic degree correspond to complex pore structure and low diffusion rate of methane, while coals with low metamorphic degree correspond to simple pore structure and high diffusion rate of methane. With the increase of methane adsorption equilibrium pressure, the maximum diffusion ratio increases gradually, but the growth rate decreases gradually. The diffusion behavior of methane is closely related to the size of coal particles, and this change has an obvious size effect. As the particle size gradually increases, the effective diffusion coefficient of coal samples changes inversely, but when the coal particle reaches the critical size range, the change of diffusion coefficient begins to stop. The structure model of multistage clustering of coal particles is established and the causes of coal particle size effect are revealed. The research results can effectively guide the development of CBM, gas migration during CO2 enhanced coalbed methane and CO2 geological storage.

Study on the mechanism of the influence of HNO3 and HF acid treatment on the CO2 adsorption and desorption characteristics of coal

In order to cooperate with the application of acid fracturing enhanced permeability technology, this paper proposes using CO2 adsorption and desorption system and low-temperature nitrogen adsorption method to conduct experimental research on the changes in the CO2 adsorption and desorption characteristics of coal after treatment with HF solution and HNO3 solution. In addition, Combined with X-ray diffraction experiments, the indirect relationship between mineral dissolution and adsorption and desorption characteristics is analyzed. The research results show that: The acid solution transforms the pore structure of coal through the dissolution of mineral impurities. In the microporous stage, hydrofluoric acid is mainly used for pore formation and nitric acid for pore expansion. Acid treatment changed the adsorption characteristics of the coal sample, reduced the saturated adsorption capacity a, and increased the adsorption constant b. And a and b were linearly related to the specific surface area and pore volume, respectively. This paper has determined that adsorption equilibrium pressure 1.91 MPa and 2.57 MPa are the enhanced adsorption thresholds of hydrofluoric acid and nitric acid, respectively. After exceeding the threshold, acid treatment will no longer play a role in increasing CO2 adsorption. In addition, the acid treatment also changed the desorption characteristics of the coal sample, increased the initial desorption amount and the final desorption amount, shortened the desorption time. The effect of hydrofluoric acid was more evident than that of nitric acid.

Experimental research on the law of the deformation and damage characteristics of raw coal/ briquette adsorption-instantaneous pressure relief

In order to clarify the characteristics of gas-bearing coal adsorption-instantaneous pressure relief deformation, and reveal the mechanism of the effect of gas desorption on the outburst. Through the gas-bearing coal adsorption-instantaneous pressure relief test system, the adsorption-instantaneous pressure relief test using briquette/raw coal samples was used to conduct a comprehensive comparison and analysis of the deformation characteristics of the two coal samples. Studying the damage of coal with different adsorption characteristics gases under the condition of adsorption-instantaneous pressure relief, and use strain gauges to monitor the microscopic deformation of the coal, and analysis of the adsorption of briquette/raw coal at constant pressure-instantaneous similarities and differences of deformation characteristics during pressure relief. “The results showed that under the same test conditions of adsorption equilibrium pressure, the degree of crushing of coal samples after instantaneous pressure relief is CO2 > CH4 > N2. At the moment of instantaneous pressure relief, the strain of the raw coal/briquette suddenly decreases, characterized by swelling deformation. Under the same temperature and pressure conditions, the expansion energy of CO2 is the highest, followed by CH4 and N2. Due to the raw coal has not completely destroyed the joints, only elastic deformation occurs, and the strain rises rapidly within a short period of time, resulting in shrinkage deformation. However, during the instantaneous pressure relief of the briquette, swelling and deformation occurred, and the coal was destroyed. The research results have certain significance for the exploration of coal and gas outburst mechanism.

Study on gas desorption and diffusion kinetic behavior in coal matrix using a modified shrinking core model

An accurate model of gas desorption and diffusion is the necessary prerequisite to accurately predict gas transfer and evaluation of gas content in the coal. A new modified shrinking core model was proposed and modified based on the shrinking core model and taking into account the time-depend diffusivity. It was to predict the kinetic process of gas desorption and diffusion including three processes: (1) desorption on the undesorbed core surface, (2) diffusion through the desorbed layer, and (3) diffusion through the gas film. The kinetic data of gas desorption and diffusion was obtained from a series of experimental tests conducted under isothermal conditions. The model showed the best agreement with the experimental data, comparison with the shrinking core model, the simplified unipore diffusion model, and the simplified bidisperse diffusion model. Comparison and validation of simulated curves and experimental data indicated the diffusion through the desorbed layer had a dominant role in the whole process of gas transfer in coal particle. The effect of adsorption equilibrium pressure, temperature, and particle size on the initial diffusivity was discussed by using the modified shrinking core model to model the experimental data. The initial diffusivity was affected slightly by the adsorption pressure and increased with the increase in temperature and the particle size. This research provides a new modified shrinking core model to describe the gas transfer process in coal particles better.

Effect of ambient pressure on gas adsorption characteristics of residual coal in abandoned underground coal mines

To study the influence of ambient pressure on gas adsorption of residual coal and provide a basis for establishing a reasonable calculation model of gas resource adsorbed in residual coal in abandoned underground coal mines, the residual coal was divided into loose-type residual coal and column-type residual coal after an analysis on the ambient gas pressure variation process. Then the gas adsorption experiments were conducted under adsorption equilibrium pressure range of 0.5–2.5 MPa using a self-made experimental device. The results show that (1) In the range of 0.5–2.5 MPa, the amount of methane adsorption in the loose-type residual coal under the intense replenishment condition can be 19.5% less than that under the gradual replenishment condition. (2) With a continuous decrease in ambient pressure, the methane adsorption amount in the column-type coal is reduced. The adsorption capacity at a unit pressure drop gradually increases from a minimum of 0.24 mL▪g-1 × 0.1 MPa−1 to a maximum of 2.375 mL▪g-1× 0.1 MPa−1. The initial adsorption equilibrium pressure shows little effect on the methane adsorption in column-type residual coal. (3) The bigger difference of the methane adsorption amount, with a maximum of 2.347 mL▪g-1, between the loose-type residual coal and column-type residual coal under the intense replenishment condition can be observed in the pressure range of 0.8–1.2 MPa. Therefore, the coal type should be considered to establish a gas storage model of residual coal in abandoned coal mines.

CO2 storage capacity of anthracite coal in deep burial depth conditions and its potential uncertainty analysis: a case study of the No. 3 coal seam in the Zhengzhuang Block in Qinshui Basin, China

The storage of CO2 in deep unminable coal seams can mitigate greenhouse gas emissions. However, CO2 storage in deep anthracite coal is complex with some uncertainties in the estimation of CO2 storage capacity. Based on isothermal adsorption experiments and gas solubility experiments under high temperature and pressure conditions, the total storage capacity of CO2 in anthracite coal is discussed. The results show that the absolute adsorption amount is over 44 cm3/g at temperatures of 318.15, 335.65, and 353.15 K as well as adsorption equilibrium pressures of 10 MPa. The storage capacity of adsorbed and free gas is 35–70 cm3/g and 5–8 cm3/g, respectively, within a depth range of 1000–2000 m. The soluble gas can be ignored for its low content between 0.22 cm3/g and 0.28 cm3/g with a proportion of less than 1%. The storage capacity of CO2 may be estimated inaccurately because of the heterogeneity and uncertainty of the macroscopic geological conditions and coal reservoir parameters. Taking the No. 3 coal seam in Zhengzhuang block as an example, the storage priority area was divided into supercritical area and subcritical area with five sub-areas according to storage conditions, and the storage capacity was calculated, showing a relatively good storage potential.

Influence of water injection on the desorption characteristics of coalbed methane

AbstractHydraulic methods have been extensively employed to improve the production rate of commercially viable coalbed methane and prevent potential mining disasters resulting from the production of this methane by increasing the permeability of coal seams. In this paper, an experimental system with a stirrer was utilized to study the effects of water injection on the gas desorption characteristics of coal seams. Pressure‐released desorption values and total gas desorption values under conditions of different adsorption equilibrium pressures and water contents were determined. The results show that both the free gas and the adsorbed gas could be replaced by the injected water, causing the system pressure to increase significantly. When releasing the system pressure, the gas desorption value of the water‐injected coal sample was reduced. After water injection, the total desorption value of the coal was increased. Thus, water injection could help to promote coal gas desorption.

Individual amounts of Lewis and Brønsted acid sites on metal oxides from NH3 adsorption equilibrium: Case of TiO2 based solids

The present study is dedicated to the development of an original method for the measurement of the individual amounts QASi of the acid sites ASi (acidity of Lewis and Brønsted) present on three TiO2 based solids of increasing composition complexity: TiO2-P25, 6% WO3/TiO2-P25 and a sulfated 0.7 % V2O5/9% WO3/TiO2 NH3-SCR catalyst. The method is based on quantitative characterizations of the NH3 adsorption equilibrium (adsorption temperature Ta and pressure Pa) by the association of (a) the AEIR method providing the individual coverage θASi(Ta, Pa) of the adsorbed NH3 species on the ASi sites and (b) the total amount (in μmol/g) of the adsorbed NH3 species: QTNH3(Ta, Pa) by using a mass spectrometer. For a solid having n types of ASi sites, the QASi values are obtained from the numerical solution of linear equation systems (with at least n equations) obtained considering that QTNH3(Ta, Pa) is equal to the sum of the contribution of each adsorbed NH3 species: QASi θASi(Ta, Pa).This imposes the measurement of at least n QTNH3(Ta, Pa) in a Ta range preventing the contribution of parallel surface processes (i.e NH3 oxidation). On TiO2-P25, n = 3 (two Lewis and one weak Brønsted acid sites) and the QASi amounts are obtained from three QTNH3(Ta, Pa) values. The others solids having two Lewis (L1 and L2) and two Brønsted (B1 and B2) acid sites impose a series of m measurements of QTNH3(Ta, Pa) with m>> n: the QASi are obtained by optimization between theoretical and experimental QTNH3(Ta, Pa) curves such as 109, 202, 70 and 130 μmol/g for QL2, QL1, QB2 and QB1 respectively of 0.7 % V2O5/9% WO3/TiO2. It is shown that these four amounts of sites permit to conclude that the L2 Lewis acid site is the one forming the pivotal NH3ads-L2 species of the reaction by using nitrogen mass balances between the amounts of adsorbed NH3 species and the N2 production in the presence of NO.

Experimental study of the effects of gas adsorption on the mechanical properties of coal

A coupled gas-solid testing system was developed to quantitatively investigate the effects of gas adsorption on the strength, volumetric strain, and fracture development of coal. Uniaxial loading tests were conducted on coal with samples that had adsorbed helium (He), nitrogen (N2), methane (CH4), and carbon dioxide (CO2) gases. Gas adsorption during the testing process was graphed using MATLAB, and the fractal dimensions of the fractures in coal were obtained. Under the same adsorption pressure, the adsorption capacity and degradation rate of coal were found to decrease in the following order for the four gases: CO2 > CH4 > N2 > He. The adsorption of CO2 had a salient negative affect on coal strength, while the effect of non-adsorptive He was negligible. The peak strength decreased by 48.27% after CO2 adsorption at a pressure of 2.0 MPa, and the deterioration rate of coal strength increased logarithmically under adsorption equilibrium pressure. The fractal dimension and density of coal fractures increased with adsorption capacity. The degree of fracture development in coal with adsorbed CO2 at a gas pressure of 2.0 MPa was 2.28-fold that of coal without adsorbed gas, and fish scale-shaped swelling and shedding were observed. The novel findings of this study provided a scientific basis for the quantitative evaluation of the mechanical properties of coal with adsorbed gases.

Methane displacement characteristic of coal and its pore change in water injection

Coalbed methane as one type of clean energy has become an important gas resource recently. High-pressure water injection in coal seams is an effective approach for improving gas extraction efficiency, which is determined by the gas displacement characteristic and pore structure of coal. To investigate the gas displacement characteristics in coal and its pore response and influential factors, gas adsorption and water injection experiments were conducted under different conditions. The results show that the gas displacement caused by the water injection undergoes three stages: rapid increase, slow increase, and almost constant. The wetting process in water injection includes three processes: wetting, soaking, and spreading, and the wettability of coking coal is best, followed by lean coal and anthracite. The amount of gas driven by the water increases with increasing water injection pressure, and it is more favorable to increase the injection pressure to improve the gas displacement effect under the relatively low injection pressure. The lower the coal rank, the better the gas displacement effect due to the higher porosity of the coal, and the longer the early gas displacement stage. The high adsorption equilibrium pressure can improve the gas displacement effect; for the relatively high adsorption equilibrium pressure, the gas displacement effect is better. After water injection in coal, the large fractures and pores dramatically increase in size, especially for the low metamorphic coals coking coal, contributing to the majority of the increase in porosity. The results of this study can provide a theoretical foundation for the wide application of water injection technology for efficient gas drainage in coal mines.

Adsorption Mechanism and Kinetic Characterization of Bituminous Coal under High Temperatures and Pressures in the Linxing‐Shenfu Area

AbstractThe majority of coalbed methane (CBM) in coal reservoirs is in adsorption states in coal matrix pores. To reveal the adsorption behavior of bituminous coal under high‐temperature and high‐pressure conditions and to discuss the microscopic control mechanism affecting the adsorption characteristics, isothermal adsorption experiments under high‐temperature and high‐pressure conditions, low‐temperature liquid nitrogen adsorption‐desorption experiments and CO2 adsorption experiments were performed on coal samples. Results show that the adsorption capacity of coal is comprehensively controlled by the maximum vitrinite reflectance (Ro, max), as well as temperature and pressure conditions. As the vitrinite reflectance increases, the adsorption capacity of coal increases. At low pressures, the pressure has a significant effect on the positive effect of adsorption, but the effect of temperature is relatively weak. As the pressure increases, the effect of temperature on the negative effect of adsorption gradually becomes apparent, and the influence of pressure gradually decreases. Considering pore volumes of pores with diameters of 1.7–100 nm, the peak volume of pores with diameters 10–100 nm is higher than that from pores with diameters 1.7–10 nm, especially for pores with diameters of 40–60 nm, indicating that pores with diameters of 10–100 nm are the main contributors to the pore volume. The pore specific surface area shows multiple peaks, and the peak value appears for pore diameters of 2–3 nm, indicating that this pore diameter is the main contributor to the specific surface area. For pore diameters of 0.489–1.083 nm, the pore size distribution is bimodal, with peak values at 0.56–0.62 nm and 0.82–0.88 nm. The adsorption capability of the coal reservoir depends on the development degree of the supermicroporous specific surface area, because the supermicroporous pores are the main contributors to the specific pore area. Additionally, the adsorption space increases as the adsorption equilibrium pressure increases. Under the same pressure, as the maximum vitrinite reflectance increases, the adsorption space increases. In addition, the cumulative reduction in the surface free energy increases as the maximum vitrinite reflectance increases. Furthermore, as the pressure increases, the surface free energy of each pressure point gradually decreases, indicating that as the pressure increases, it is increasingly difficult to adsorb methane molecules.