Coal Porosity Research Articles

Relationship between wave speed variation and microstructure of coal under wet conditions

To explore the influence of coal microstructure on the wave speed in different wetting conditions, we first reconstructed a 3-D coal body model with computed tomography technology. We then performed ultrasonic measurements on wet coal samples and analyzed the effects of water on the wave speeds of coal samples with different structures. At last, we examined the relationships of the longitudinal wave speed to water-bearing saturation, porosity and fractal dimension of coal. The results showed that 1) longitudinal wave speed is relatively high in type I sample at dry condition, and slowly increases after wetting, presenting a poor wetting effect; 2) longitudinal wave speed rapidly increases in type I samples with water-saturation >80%, and in type II samples with water-saturation > 75%. These results indicate that the wave speed in type II samples is more sensitive to water than that in type I samples, which can be explained by that type II samples have more micropores and greater micropore wall tortuosity; 3) wave speed increment is positively correlated with porosity and fractal dimension, obeying an exponential function relationship; 4) wave speed is smaller at nature water saturation than at vacuum water saturation due to the distribution of pores and cracks and their connectivity; and 5) using wave speeds in different water saturation states can predict coal's cementation, connectivity as well as the number of isolated pores.

The Porosity Dynamic Model of the Compacted Broken Coal

In order to study the dynamic change law of the porosity of the compacted broken coal under different axial stress loading, based on the environment of the broken and compacted coal in the gob, aiming at the influence of the porosity on the spontaneous combustion of the coal, combined with the fractal theory, the fractal model of the porosity of the broken coal is established. A self‐designed “testing device for permeability evolution and spontaneous combustion characteristics of crushed coal under pressure” is used to carry out axial loading test on selected coal samples in the gob. By comparing and analyzing the calculated results of void dynamic evolution model and experimental data, it is found that the relative error of void dynamic evolution model is between 2.8% and 6.2%, which meets the engineering needs. According to the stress‐strain curve, initial accumulation state parameters, fractal dimension of initial crushing, and particle size distribution, the change of porosity under different compacted conditions can be predicted by the model, which has certain significance for identifying the change of compacted broken coal porosity and analyzing the process of coal spontaneous combustion and oxidation.

Effects of pulse wave on the variation of coal pore structure in pulsating hydraulic fracturing process of coal seam

Coal pore structure is not only closely related to gas adsorption, but also affect the dynamic characteristics of gas desorption and diffusion. In order to understand the influence of pulsating hydraulic fracturing on the variation of coal pore structure, in this paper, the mercury intrusion method and liquid nitrogen adsorption method were used to quantitatively analyze the changes of coal porosity and pore size distribution under the action of pulse wave. And combined with scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) test to study the variation of minerals on the surface of coal samples. The results show that the influence of pulsating load on coal porosity is greater than that of static pressure load. The cumulative volume of micropores and their proportion decrease with the increase of pulsating frequency and pressure; the cumulative volume and proportion of macropores show an increasing trend; the cumulative volume of mesopores increases with increasing frequency, but the proportion increases in the low frequency phase, and decreases in the high frequency phase. There are two types of pores in the coal sample affected by the pulse waves: one is the expansion of the original pores, and the other is the new pores because the mineral crystals embedded in the coal body are washed out. The results have an important theoretical basis for the analysis of gas microscopic dynamics during pulsating hydraulic fracturing.

Analysis of pulverized tectonic coal gas expansion energy in underground mines and its influence on the environment.

Production of a large amount of gas during outbursts will cause greenhouse effects, which will impact the atmospheric environment. In this study, some inherent properties of pulverized tectonic coal were investigated. The results indicate that tectonic coal was more broken and exhibited a higher gas adsorption volume. No obvious changes were found in the micropore and mesopore volumes, whereas the macropore volume and pulverized tectonic coal porosity were significantly increased compared with those of intact coal. Additionally, the initial gas desorption capacities of pulverized tectonic coal were enhanced by tectonism, which might be related to the development of macropore structures and porosity. Analysis of gas expansion energy at the same particle size showed that the values increased with the increasing pressure. Pulverized tectonic coal had a higher gas expansion energy, which could result in a larger outburst of potential energy. Almost all outbursts occurred in tectonic development zones and released a large amount of gas, which greatly damaged the ecological environment. From the perspective of environmental protection, attention should be paid to gas control in the tectonic development zone.

In-situ SAXS study of pore structure during carbonization of non-caking coal briquettes

Carbonization can enhance the utilization of coal. Pore structure is the main feature of the physical structure of coal and its carbonization products as char and coke. In this work, we performed an in-situ small angle X-ray scattering (SAXS) study on high temperature carbonization (1200 °C) of a kind of lignite and a kind of sub-bituminous coal. The variation of pore structure including pore size distribution, porosity and specific surface area of coal illustrates the different stages of the carbonization process.

Improved floatability of low-rank coal through surface modification by hydrothermal pretreatment

Abstract Low-rank coal (LRC) contains a developed pore structure and rich oxygen-containing functional groups (OCFG). Therefore, it cannot easily be upgraded using flotation technology. In this study, simple and effective hydrothermal pretreatment (HP) was performed to reduce the porosity and increase the hydrophobicity of LRC surface. LRC characteristics, such as coal composition, surface morphology, pore structure, and surface functional groups were analyzed before and after HP. The results showed that, after HP, the coals exhibited lower amounts of moisture and volatile matter, but substantially greater amounts of fixed carbon than raw coal. Pore structure analysis indicated the collapse and closure of many pores on the coal surface, indicating in low porosity in the pretreated coal. In addition, X-ray photoelectron spectroscopy revealed more hydrophobic functional groups and less OCFG on the coal surface. It is, therefore, HP resulted in excellent flotation recovery, with increased recovery of 70.7% combustible material.

Specific Surface and Porosity of Coal

The specific surface and porosity of Belovo coal are determined. Coal samples undergo adsorption–structural analysis at the liquid-nitrogen temperature and at different relative partial pressures, on Sorbi N.4.1 and Termosorb TPD 400 instruments and also on the Micromeritics ASAP-2020 static vacuum system.

Experimental Study on Pore-fracture Evolution Law in the Thermal Damage Process of Coal

ABSTRACT High temperature causes thermal damage to coal, alters its pore-fracture structure, and then influences its permeability and spontaneous combustion characteristics. In this paper, to study the evolution law of coal pore-fracture in the thermal damage process and the influence mechanism of thermal damage on pore-fracture development, the microfracture evolution characteristics and pore morphology of coal samples treated under different temperatures were qualitatively studied by susing a scanning electron microscope (SEM). Besides, the evolution of coal pore structure as a function of temperatures in the heating process was quantitatively studied based on Nuclear magnetic resonance (NMR) technique. In addition, the pore structure of coal samples was quantitatively characterized in combination with the fractal theory, and the variation of fractal dimension of coal sample pore structure with heat treatment temperature was obtained. The results show that after heat treatment, the development of fractures of coal was promoted, and the connectivity between the fractures was effectively enhanced. Moreover, the higher the temperature is, the more favorable it is for the development, expansion, and connectivity of fractures and for the formation of microfracture networks in coal. The high-temperature thermal damage changes the pore structure of coal, increases the pore volume and porosity of coal and enhances the pore connectivity. Moreover, after high-temperature heat treatment, the pore structure of DL coal with low metamorphism changed more significantly than that of XG coal. The porosity of DL coal and XG coal increased with the rising temperature, while the pore fractal dimension DmNMR, DMNMR, and DNMR of the two kinds coal samples show the opposite trend. Furthermore, the variation amplitude of DmNMR is significantly larger than that of DMNMR, and the variation of three fractal dimensions of DL coal is larger than that of XG coal. The research results can support a reference for coal fire prevention.

Three-dimensional modeling and analysis of macro-pore structure of coal using combined X-ray CT imaging and fractal theory

The porous structure of coal directly determines its gas transport property. The fluid flow behavior of coal is one of the key science questions that will influence the coal energy industry. In this study, the influence of real coal macropore structure on the fluid flow through coal was studied through 3-D coal structure reconstruction by the CT images. Based on the reconstructed coal structure, the micron-scale structure parameters were quantitatively analyzed. A newly programmed Matlab code was established to find the volume fractal dimension, obtain the relationship between porosity/permeability of coal and volume fractal dimension, and estimate the tortuosity fractal dimension by using the 3-D box dimension algorithm. The results show that the volume fractal dimensions of 6 coal samples range from 2.25 to 2.79 and the tortuosity fractal dimensions of capillaries range from 2.15 to 2.73. The 3-D coal structure cannot only quantitatively estimate the real porosity of the coal, but it can be used to characterize the complexity of coal's porous structure through mean deviation of surface porosity. It can be clearly seen from the reconstructed coal that coal specimen-C3 is highly heterogeneous because it has complex pore structure as well as wider pore size distribution and the highest mean deviation of the surface porosity. The volume fractal dimension can be used to quantitatively define the complexity of pores. The larger the porosity of coal, the greater the fractal dimension. The permeability and porosity of coal are negatively correlated with the volume fractal dimension. The tortuosity fractal dimension can effectively characterize coal permeability, but it weakly correlates with coal porosity. The outcome of this study helps to understand the structure-based flow characterization and gas transport behavior in heterogenous coal which will have the implication of the gas extraction from coalbed methane reservoirs and coal mine gas drainage.

Stress-dependent fracture porosity and permeability of fractured coal: An in-situ X-ray tomography study

Coal porosity and permeability are highly stress-sensitive. Although the stress-dependent permeability behaviour of coal has been well documented in the literature, few studies are available which directly observe the dependency of fracture aperture, porosity and connectivity on effective stress. In this study, in-situ X-ray tomography was conducted using a novel X-ray transparent tri-axial core holder to provide a detailed characterization of coal fracture network evolution due to increased effective stress. Both conventional and synchrotron X-ray sources were used to image two fractured anthracite coals (named sample A and sample B) at resolutions of 38.7 and 18.1 μm, respectively. Permeability was measured at effective stresses from 0.5 to 6 MPa for sample A and from 0.5 to 11.0 MPa for sample B. Results show that the permeability for both samples decreased by one order of magnitude following an exponential function. X-ray CT images indicate that large fractures were quickly compressed and isolated to small fractures, which accordingly caused dramatic porosity reduction by a power law. The fracture contact area increases in response to increased stresses, resulting in a decrease of fracture compressibility. Both an exponential decreasing trend and a linear decreasing trend were observed. When the fracture contact area increases, the tortuosity and spatial connectivity of the fractures become important, because these attributes significantly affect permeability. Preliminary studies on the correlation between permeability and CT-resolved porosity and that between permeability and CT-resolved aperture are not consistent with the cubic law. This may indicate that the fractures cannot be adequately represented as smooth parallel plates. A mixture of tube and plate model is proposed to better describe fracture geometry considering its rough nature.

The role of pulverized coal drying temperature in fly ash carbon content and slagging of a tangentially coal-fired boiler

The purpose of this paper is to investigate the effect of the pulverized coal drying temperature on fly ash carbon content and slagging of a 1000 MW unit coal-fired boiler. The pulverized coal was dried at three selected temperatures of 25 °C, 75 °C and 110 °C, and the moisture and porosity of the pulverized coal particles were measured. Based on the porosity range obtained from the measurement, 3-D steady numerical simulation of combustion in tangentially coal-fired furnace was conducted by use of Fluent Software. It was found, (1) The porosity, specific surface area and pore diameter of pulverized coal particles increase with the increase of pulverized coal drying temperature. (2) The increase in the pulverized coal porosity led to the lower flame center and the lower temperature in over-fired air zone. (3) The fly ash particle capture rate and the fly ash carbon content decreased when the pulverized coal porosity increased. (4) The slagging of water wall presents a downward trend with the growth of the pulverized coal porosity. The slagging problem is well resolved in a power plant after applying the strategy of heating up pulverized coal when burning highly slagging tendency coal.

Experimental Study on the Effect of Moisture Content on Bituminous Coal Porosity Based on 3D Reconstruction of Computerized Tomography

Bituminous coal in the Xutuan Coal Mine of the Huaibei Mining Bureau (China) is the research object of this study. The influence of moisture content on the porosity of the bituminous coal was investigated from a microscopic perspective by using a high-solution 3D X-ray micro-analyzer. The threshold segmentation method was used to segment the scanning slices of the coal samples. The threshold values of the various media were in the following order (from large to small): minerals, water, matrices, and fractures. The scanning volume and actual volume proportions of the different media in the coal samples with different moisture contents were calculated. The accuracy of the computerized tomography (CT) scanning method in measuring the coal moisture content was verified by comparison with the results obtained using the weighing method. 3D reconstructed coal samples, with different moisture contents, were analyzed, as well as separately reconstructed fractures and water in the coal samples with different moisture contents. The heterogeneity and anisotropy of the coal mass were explained quantitatively by the CT scanning intensity. A commonly used fracture classification method indicated that the primary fracture in the coal sample was a type A fracture. The results of the analysis of water in the coal fracture indicated that the porosity of bituminous coal decreased with the increase in moisture content in conditions of atmospheric pressure and a short immersion period. However, a certain level of porosity remained evident, and the degree of fracture development of the coal samples remained unchanged. This is attributed to the minor volumetric change in the minerals in the coal samples, as the water does not completely occupy the fractures in the coal samples, and the dissolution of the minerals by water is therefore not significant. The reasons for the moisture content affecting gas adsorption, seepage, and strength of a coal body were analyzed from a microscopic perspective. In addition, the types of fractures and water in the coal samples were classified by employing statistics and analyses of volume, surface area, specific surface area, and aspect ratio of the fractures and the water in the coal samples with different moisture contents.

Experiment study on the correlation between the CO2 adsorption capacity and electrical resistivity of coal with temperature effect

AbstractThe main objective of this study is to investigate the relationship between the CO2 adsorption capacity and electrical resistivity of coal. The H‐Sorb 2600 high pressure gas adsorption analyzer is used to measure the CO2 sorption capacity in the temperature range of 25–55°C for anthracite coal. In addition, the TH2811D LCR meter is used to examine the electrical resistivity of raw coal samples and coal briquettes from 15°C to 65°C. The results indicate that there is a critical pressure threshold depending on the coal types and the pattern of changes in CO2 adsorption capacity changes when this threshold is exceeded. The electrical resistivity of raw coal samples decreases linearly and then slightly decreases at temperatures above 45°C and the electrical resistivity decreases approximately linearly from 15°C to 65°C for the coal briquettes due to lower compactness compared with the raw coal samples. The CO2 adsorption capacity has a power function with the electrical conductivity of the raw coal samples and coal briquettes. The power exponent of the power function is related to the porosity of coal. The results of the experiments indicate that it is effective to estimate the CO2 adsorption capacity of coal from the electrical conductivity. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Experimental investigations into stress sensitivity in three Chinese coal samples by NMR

ABSTRACT The coal reservoirs have low mechanical strength and strong stress sensitivity. The study of stress sensitivity is of great significance to the efficient development of coalbed methane (CBM). In order to reveal the stress and strain characteristics of coal reservoirs, the low-field nuclear magnetic resonance (NMR) experiments with variable confining pressures were carried out on three coal samples from China. Based on the T2 spectrum of coal samples under different confining pressures, the pore distribution, connectivity, and porosity variation rules of the coal samples were analyzed. Results show that the peaks of mesopore, macropore-fissure in the coal samples are well developed; however, the micropore peaks are poorly developed. There are differences in the stress deformation of pores and fractures in coals. With increasing confining pressures from 0 to 16 MPa, the peak of fractures decrease most obvious, and the peak of mesopore and macropore-fissure are relatively obvious, while the peak of the small pores is not significantly reduced. With the confining pressure increases, the total NMR porosity of coal decreases as the rule of negative exponential function, which is mainly controlled by changes of the porosity of seepage pore. The effect of adsorption pore on porosity changes is not obvious.

Acetone erosion and its effect mechanism on pores and fractures in coal

Nowadays, coalbed methane recovery faces challenges such as high gas content, micro-porosity and low permeability. An exploratory study on improving coal porosity by acetone treatment was carried out. Pore size distribution and fracture development before and after acetone treatment were evaluated using a suite of integrated diagnostic techniques including rock acoustical test, weight analysis, Scanning Electron Microscope (SEM), Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared Spectroscopy (FT-IR) and mercury intrusion porosimetry (MIP). After acetone treatment, micropores are partially converted into mesopores and macropores. The porosity of coal increases by 2.66% after acetone treatment. Functional groups including hydroxy, methylene, oxygen functional groups and aromatic hydrocarbons reduce significantly and the removal of these hydrophilic groups significantly weakens the hydrophilicity of coal. New fractures appeared within 32.89% of the eroded zone, even large through-going fractures may appear after acetone treatment. The results indicate that acetone treatment is effective in improving gas productivity thus has the potential to become a new coalbed methane reservoir stimulation technology.

Effect of compressive stresses on permeability of coal: Experimental and modeling estimates

Research findings on permeability of fractured coking fat coal from Tikhov Mine are presented. It is shown that rock pressure has considerable effect on coal permeability under various gas pressure gradients. In particular it is found that an increase in the triaxial compression from 1 to 8 MPa reduces coal permeability by 8–1 times if the gas pressure gradient is 0.67–1.34 MP/m and by 6.1–7 times if the higher gradient is equal to 2–3.3 MPa/m. Parameters of cleats and sizes of joints (blocks) in coal matrix are studied before and after loading of coal specimens. It is obtained that the size of the coal matrix blocks in the unloaded state is 530 μm at the width (opening) of cleats of 12.7 μm, which decreases by 1.8 times under the triaxial compression equal to 8 MPa. The measured parameters of cleats and coal matrix blocks are used to assess permeability of coal using theoretical models relating coal permeability with porosity or effective stress (compression) of coal. The comparison of the theoretical estimates and experimental studies of fractured coal shows that the measured permeability of coal under compression from 1 to 5 MPa agrees with the estimates by the permeability–porosity model. If the compression is higher than 5 MPa the estimates by different models coincide. It is found that the highest disagreement between the measured and calculated permeabilities takes place at the low gradient of gas pressure which is probably connected with the Kinkenberg effect.

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The effect of adsorption-induced swelling on porosity based on the transient coal swelling model

Coal swelling induced by gas adsorption is a well-known phenomenon. In order to enhance coalbed methane production and greenhouse gases sequestration, adsorption phase should be understood deeply, and it is the most basic and the most important. Experimental measurements performed in this work on three samples from Kaiping Basin using carbon dioxide. Basing on Gibbs adsorption model, the steady-state swelling model is developed, which also assume that the change of the coal surface free energy induced by adsorption is equal to the change of energy caused by adsorption deformation. The mass balance equation of gas in porous media is combined with the steady-state swelling model. The dynamic variation of porosity and internal gas pressure of coal are integrated into the transient swelling model. The steady-state model and transient model are applied to describe the experimental volumetric strain. The results show good agreement between the models and the experimental strain data. The coal pore tests with different probe scales are also used to investigate the true pore size distribution and porosity, which provide more accurate parameter values. According to the transient swelling model, the effects of porosity on adsorption-induced swelling are further analyzed.

Investigating Permeability of Coal Samples of Various Porosities under Stress Conditions

Among the numerous factors that have an impact on coal permeability, coal porosity is one of the main parameters. A change in the mechanical stress applied to coal results in a change of porosity. The main objective of the conducted research was to answer the following question: is a decline in coal permeability a direct effect of a decrease in coal porosity, and does mechanical stress result solely in a porosity change? A study of coal porosity under mechanical stress conditions was conducted using a uniquely constructed measurement stand. The coal samples used were briquettes prepared from a granular coal material (middle-rank coal of type B—meta bituminous, upper carboniferous formation) from the “Zofiówka” coal mine, in Poland. In order to describe coal permeability, the Klinkenberg equation was used, as it takes into consideration the slippage effect, typical of porous media characterized by low permeability. On the basis of the obtained results, it was established that the values of the Klinkenberg permeability coefficient decrease as the mechanical stress and the corresponding reduction in porosity become greater. As the briquette porosity increased, the Klinkenberg slippage effect: (i) disappeared in the case of nitrogen, (ii) and was minor for methane. The briquettes used were characterized by various porosities and showed that mechanical stress results mainly in a change in coal porosity, which, in turn, reduces coal permeability.

Experimental investigation on coal pore and fracture characteristics based on fractal theory

Coal is an extremely complex porous medium. To study the pore and fracture characteristics of coal, samples from northeastern China were collected and crushed into eight kinds of particle sizes to conduct the experiments using a combination of basic physical parameters, mercury intrusion porosimetry (MIP) and N2 (77 K)/CO2 (273 K) adsorption pore structure characterization. At the same time, the scale characteristics of the coal pores and fractures were analyzed by fractal theory. The results indicate that the pores and fractures in coal become increasingly simpler and are more favorable for gas storage and migration during the crushing of the coal samples. The fractal curves of the pores and fractures in coal samples with different particle sizes determined from MIP were all divided into two parts by an inflection point. According to fractal theory, the width that corresponds to the inflection points of the fractal curves are considered to be the critical value for determining the pores and fractures in coal. On this basis, the matrix porosity and the fracture porosity in coal were obtained by combining the data from the MIP and N2 (77 K)/CO2 (273 K) adsorption. It is believed that this study is of great significance to analyze the coal permeability evolution law affected by the dual-porosity coal structure characteristics.