Experimental Analysis of Pore Structure and Fractal Characteristics of Soft and Hard Coals With Same Coalification

Abstract Accurate and quantitative investigation of the physical structure and fractal geometry of coal has important theoretical and practical significance for coal bed methane and the prevention of dynamic disasters such as coal and gas outbursts. This study investigates the pore structure and fractural characteristics of soft and hard coals using nitrogen and carbon dioxide (N2/CO2) adsorption. Coal samples from Pingdingshan Mine in Henan province of China were collected and pulverized to the required size (0.2-0.25mm). N2/CO2 adsorption tests were performed to evaluate the pore size distribution (PSD), specific surface area (SSA), and pore volume (PV). The pore structure was characterized based on fractural theory. The results unveiled that the strength of coal has a significant influence on pore structure and fracture dimensions. The obvious N2-adsorption isotherms of the coals were verified as Type IV (A) and Type II. The shape of the hysteresis loops indicates the presence of slit-shaped pores. There are significant differences in SSA and PV between both coals. The soft coal showed larger SSA and PV than hard coal that shows consistency with adsorption capacity. The fractal dimensions of soft coal are respectively larger than that of hard coal. The greater the value of D1 (complexity of pore surface) of soft coal is, the larger the pore surface roughness and gas adsorption capacity is. The results enable us to conclude that the characterization of pores and fractures of soft and hard coals is different, tending to different adsorption/desorption characteristics and outburst sensitivity. In this regard, results provide a reference for formulating corresponding coal and gas outburst prevention and control measures.

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Study on the Effect of Soft and Hard Coal Pore Structure on Gas Adsorption Characteristics

Advances in Civil Engineering ◽

10.1155/2021/1425227 ◽

2021 ◽

Vol 2021 ◽

pp. 1-10

Author(s):

Xun Zhao ◽

Tao Feng ◽

Ping Wang ◽

Ze Liao

Keyword(s):

Pore Structure ◽

Adsorption Capacity ◽

Residence Time ◽

Coalbed Methane ◽

Gas Adsorption ◽

Nitrogen Adsorption ◽

Hard Coal ◽

Research Results ◽

Adsorption Characteristics ◽

Soft Coal

In order to grasp the effect of soft and hard coal pore structure on gas adsorption characteristics, based on fractal geometry theory, low-temperature nitrogen adsorption and constant temperature adsorption test methods are used to test the pore structure characteristics of soft coal and its influence on gas adsorption characteristics. We used box dimension algorithm to measure the fractal dimension and distribution of coal sample microstructure. The research results show that the initial nitrogen adsorption capacity of soft coal is greater than that of hard coal, and the adsorption hysteresis loop of soft coal is more obvious than that of hard coal. And the adsorption curve rises faster in the high relative pressure section. The specific surface area and pore volume of soft coal are larger than those of hard coal. The number of pores is much larger than that of hard coal. In particular, the superposition of the adsorption force field in the micropores and the diffusion in the mesopores enhance the adsorption potential of soft coal. Introducing the concept of adsorption residence time, it is concluded that more adsorption sites on the surface of soft coal make the adsorption and residence time of gas on the surface of soft coal longer. Fractal characteristics of the soft coal surface are more obvious. The saturated adsorption capacity of soft coal and the rate of reaching saturation adsorption are both greater than those of hard coal. The research results of this manuscript will provide a theoretical basis for in-depth analysis of the adsorption/desorption mechanism of coalbed methane in soft coal seams and the formulation of practical coalbed methane control measures.

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Changes in Pore Structure of Coal Caused by CS2 Treatment and Its Methane Adsorption Response

Geofluids ◽

10.1155/2018/7578967 ◽

2018 ◽

Vol 2018 ◽

pp. 1-11 ◽

Cited By ~ 4

Author(s):

Run Chen ◽

Yong Qin ◽

Pengfei Zhang ◽

Youyang Wang

Keyword(s):

Pore Structure ◽

Adsorption Capacity ◽

Gas Adsorption ◽

Recovery Process ◽

Micropore Volume ◽

Methane Adsorption ◽

Coal Bed ◽

Methane Recovery ◽

Before And After ◽

Key Issues

The pore structure and gas adsorption are two key issues that affect the coal bed methane recovery process significantly. To change pore structure and gas adsorption, 5 coals with different ranks were treated by CS2 for 3 h using a Soxhlet extractor under ultrasonic oscillation conditions; the evolutions of pore structure and methane adsorption were examined using a high-pressure mercury intrusion porosimeter (MIP) with an AutoPore IV 9310 series mercury instrument. The results show that the cumulative pore volume and specific surface area (SSA) were increased after CS2 treatment, and the incremental micropore volume and SSA were increased and decreased before and after Ro,max=1.3%, respectively; the incremental big pore (greater than 10 nm in diameter) volumes were increased and SSA was decreased for all coals, and pore connectivity was improved. Methane adsorption capacity on coal before and after Ro,max=1.3% also was increased and decreased, respectively. There is a positive correlation between the changes in the micropore SSA and the Langmuir volume. It confirms that the changes in pore structure and methane adsorption capacity due to CS2 treatment are controlled by the rank, and the change in methane adsorption is impacted by the change of micropore SSA and suggests that the changes in pore structure are better for gas migration; the alteration in methane adsorption capacity is worse and better for methane recovery before and after Ro,max=1.3%. A conceptual mechanism of pore structure is proposed to explain methane adsorption capacity on CS2 treated coal around the Ro,max=1.3%.

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Interaction between Supercritical CO2 and Coal Petrography

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.616-618.306 ◽

2012 ◽

Vol 616-618 ◽

pp. 306-309 ◽

Cited By ~ 1

Author(s):

Run Chen

Keyword(s):

Pore Structure ◽

Supercritical Fluid ◽

Gas Adsorption ◽

Coal Bed ◽

Coal Bed Methane ◽

Organic Matters ◽

Coal Swelling ◽

Coal Petrography

CO2enhanced CBM recovery(CO2-ECBM) is an important way for reducing CO2emission into atmosphere and enhancing coal-bed methane (CBM) recovery. The interaction between supercritical CO2and coal petrography has been investigated since the 1990s. Advances in the interaction between supercritical CO2and coal petrography are reviewed in light of certain aspects, such as the competitive multi-component gas adsorption, sorption-induced coal swelling/shrinkage and the fluid-solid coupling between fluids(such as gas, liquid and supercritical fluid) and coal petrography. It is suggested that a comprehensive feasibility demonstration is necessary for a successful application of the technology for CO2-ECBM. At the same time, it also indicated that there are some questions must be discussed in future, such as the influences on pore structure, coal adsorptivity and permeability of the reaction of ScCO2-H2O and rock and small organic matters are extracted by supercritical CO2.

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INVESTIGATION OF FRACTAL CHARACTERISTICS AND METHANE ADSORPTION CAPACITY OF THE UPPER TRIASSIC LACUSTRINE SHALE IN THE SICHUAN BASIN, SOUTHWEST CHINA

Fractals ◽

10.1142/s0218348x19400115 ◽

2019 ◽

Vol 27 (01) ◽

pp. 1940011 ◽

Cited By ~ 2

Author(s):

LEI CHEN ◽

ZHENXUE JIANG ◽

KEYU LIU ◽

WEI YANG ◽

SHU JIANG ◽

...

Keyword(s):

Pore Structure ◽

Adsorption Capacity ◽

Sichuan Basin ◽

Southwest China ◽

Upper Triassic ◽

Methane Adsorption ◽

Relative Pressure ◽

Pore Surface ◽

Fractal Characteristics ◽

The Sichuan Basin

To better understand the nanopore characteristics and their effects on methane adsorption capacity of shales, we performed fractal analysis of nine shale samples collected from the fifth member of Upper Triassic Xujiahe Formation in the Sichuan Basin, southwest China. [Formula: see text] adsorption results show that shales have different adsorption characteristics at relative pressure of 0–0.5 and 0.5–1. Two fractal dimensions [Formula: see text] and [Formula: see text] were calculated using the Frenkel–Halsey–Hill (FHH) equation. Results show that the methane adsorption capacity increases with the increase of [Formula: see text] and [Formula: see text], of which [Formula: see text] has a more significant influence on adsorption capacity than [Formula: see text]. Further studies indicate that [Formula: see text] represents the pore surface fractal characteristics caused by the irregularity of shale surface, whereas [Formula: see text] represents the pore structure fractal characteristics, which is mainly affected by shale components (e.g. TOC, clay minerals) and pore parameters (e.g. average pore diameter, micropores content). A higher [Formula: see text] corresponds to a more irregular pore surface, which provides more space for methane adsorption. While a higher [Formula: see text] indicates a more complex pore structure and a stronger capillary condensation action on the pore surface, which in turn enhances the methane adsorption capacity.

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The influence factors for gas adsorption with different ranks of coals

Adsorption Science & Technology ◽

10.1177/0263617417730186 ◽

2017 ◽

Vol 36 (3-4) ◽

pp. 904-918 ◽

Cited By ~ 6

Author(s):

Deyong Guo ◽

Xiaojie Guo

Keyword(s):

Moisture Content ◽

Specific Surface ◽

Pore Structure ◽

Adsorption Capacity ◽

Surface Area ◽

Specific Surface Area ◽

Pore Volume ◽

Gas Adsorption ◽

Total Pore Volume ◽

Factors Influencing

In this paper, scanning electron microscopy, low-temperature N2 adsorption and CH4 isothermal adsorption experiments were performed on 11 coal samples with Ro,max between 0.98 and 3.07%. The pore structure characteristics of coals (specific surface area, total volume distribution) were studied to assess the gas adsorption capacity. The results indicate that there is significant heterogeneity on coal surface, containing numerous channel-like pores, bottle-shaped pores and wedge-shaped pores. Both Langmuir volume (VL) and Langmuir pressure (PL) show a stage change trend with the increase of coalification degree. For different coalification stages, there exist different factors influencing the VL and PL values. For low-rank coals (Ro,max < 1.1%), the increase of VL values and decrease of PL values are mainly due to the abundant primary pore and fracture within coal. For middle-rank coals (1.1% < Ro,max < 2.1%), the moisture content, vitrinite content and total pore volume are all the factors influencing VL, and the reduction of PL is mainly attributed to the decrease of moisture content and inertinite content. Meanwhile, this result is also closely related to the pore shape. For high-rank coals (Ro,max > 2.1%), VL values gradually increase and reach the maximum. When the coal has evolved into anthracite, liquid hydrocarbon within pore begins pyrolysis and gradually disappears, and a large number of macropores are converted into micropores, leading to the increase of specific surface area and total pore volume, corresponding to the increase of VL. In addition, the increase of vitrinite content within coal also contributes to the increase of VL. PL, reaches the minimum, indicating that the adsorption rate reaches the largest at the low pressure stage. The result is mainly controlled by the specific surface area and total pore volume of coal samples. This research results will provide a clearer insight into the relationship between adsorption parameters and coal rank, moisture content, maceral composition and pore structure, and it is of great significance for better assessing the gas adsorption capacity.

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Nanopore Structure of Different Rank Coals and Its Quantitative Characterization

Journal of Nanoscience and Nanotechnology ◽

10.1166/jnn.2021.18728 ◽

2021 ◽

Vol 21 (1) ◽

pp. 22-42

Author(s):

Xiangchun Li ◽

Zhongbei Li ◽

Fan Zhang ◽

Qi Zhang ◽

Baisheng Nie ◽

...

Keyword(s):

Pore Structure ◽

Adsorption Capacity ◽

Pore Volume ◽

Coal Sample ◽

Gas Adsorption ◽

Low Rank ◽

Small Range ◽

Coal Rank ◽

Scattering Intensity ◽

The Difference

Based on gas adsorption theory, high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen gas adsorption (LT-N2GA), CO2 adsorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and small-angle X-ray scattering (SAXS) techniques were used to analyze the pore structures of six coal samples with different metamorphisms in terms of pore volume, specific surface area (SSA), pore size distribution (PSD) and pore shape. Combined with the gas adsorption constant a, the influence and mechanism of the pore structure of different coal ranks on gas adsorption capacity were analyzed. The results show that there are obvious differences in the pore structure of coals with different ranks, which leads to different adsorption capacities. To a large extent, the pore shapes observed by SEM are consistent with the LT-N2GA isotherm analysis. The pore morphology of coal samples with different ranks is very different, indicating the heterogeneity among the coal surfaces. Adsorption analysis revealed that mesopore size distributions are multimodal and that the pore volume is mainly composed of mesopores of 2–15 nm. The adsorption capacity of the coal body micropores depends on the 0.6–0.9 nm and 1.5–2.0 nm aperture sections. The influence of coal rank on gas desorption and diffusion is mainly related to the difference in pore structure. The medium metamorphic coal sample spectra show that the number of peaks in the high-wavenumber segment is small and that it is greater in the high metamorphic coal. The absorption intensity of the C–H stretching vibration peak of naphthenic or aliphatic hydrocarbons varies significantly among the coal samples. Over a small range of angles, as the scattering angle increases, the scattering intensity of each coal sample gradually decreases, and as the degree of metamorphism increases, the scattering intensity gradually increases. That is, the degree of metamorphism of coal samples is directly proportional to the scattering intensity. The influence of coal rank on gas adsorption capacity is mainly related to the difference in pore structure. The gas adsorption capacity shows an asymmetric U-shaped relationship with coal rank. For higher rank coals (Vdaf < 15%), the gas adsorption consistently decreases significantly with increasing Vdaf. In the middle and low rank coal stages (Vdaf > 15%), it increases slowly with the increase of Vdaf. We believe that the results of this study will provide a theoretical basis and practical reference value for effectively evaluating coal-rock gas storage capacity, revealing the law of CBM enrichment and the development and utilization of CBM resources.

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Characterization of Pores and Fractures in Soft Coal from the No. 5 Soft Coalbed in the Chenghe Mining Area

Processes ◽

10.3390/pr7010013 ◽

2018 ◽

Vol 7 (1) ◽

pp. 13 ◽

Cited By ~ 4

Author(s):

Pan Wei ◽

Yunpei Liang ◽

Song Zhao ◽

Shoujian Peng ◽

Xuelong Li ◽

...

Keyword(s):

Low Temperature ◽

Pore Structure ◽

Pore Volume ◽

Fractal Dimensions ◽

Nitrogen Adsorption ◽

Mining Area ◽

Hard Coal ◽

Structure Characteristics ◽

Soft Coal

The characteristics of the pore structure and gas migration in soft coalbeds are the premise of evaluating gas discharge in soft coalbeds. To explore the pore structure characteristics of soft coal masses, the No. 5 soft coalbed in the eastern zone of Chenghe Mining Area, was investigated and compared with the No. 5 hard coalbed in the western zone. By using a mercury intrusion method, low-temperature liquid nitrogen adsorption, and scanning electron microscopy (SEM), the pore structure characteristics of the No. 5 coalbed were explored. Moreover, based on fractal theory, the pore structure of coal was characterized. The results showed the pores in soft coal mainly appeared as small pores and micropores in which the small pores accounted for nearly half of the total pore volume. Mesopores and macropores were also distributed throughout the soft coal. The mercury-injection and mercury-ejection curves of soft coal showed significant hysteresis loops, implying that pores in coal samples were mainly open while the mercury-injection curve of hard coal was consistent with its mercury-ejection curve, showing no hysteresis loop while having an even segment, which indicated that closed pores occupied the majority of the pore volume in the coal samples. The curves of low-temperature nitrogen adsorption of soft coal all follow an IV-class isotherm. Moreover, the fractal dimensions of soft coal are respectively larger than the fractal dimensions of hard coal. It can be seen that the characterization of pores and fractures of the soft coal was different from the hard coal in the western distinct of the old mining area. The gas prevention and control measures of soft coal should be formulated according to local conditions.

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Study on an Analytical Solution of Gas Seepage Equation Considering the Adsorption Effect

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.744-746.1654 ◽

2015 ◽

Vol 744-746 ◽

pp. 1654-1661

Author(s):

Zhi Gang Zhang

Keyword(s):

Analytical Solution ◽

Numerical Solutions ◽

Gas Adsorption ◽

Development Planning ◽

Practical Significance ◽

Coal Bed ◽

Adsorption Effect ◽

Gas Seepage ◽

Standard Solution ◽

Adsorption Quantity

The functional relationship between rock and coal permeability and gas adsorption quantity has been determined, based on earlier experimental results. Then, by method of mathematical physics, the authors derived an analytical solution for a gas seepage equation in which the adsorption effect has been considered. This analytical solution could be used for developing the theory of Seepage Mechanics and Computational Fluid Dynamics, and for checking and correcting various numerical solutions as a standard solution, and for inspiring various calculating techniques such as difference schemes, grid generation, and others as well. The analytical solution derived in this paper has extremely important practical significance in conducting the arrangement of gas extraction operation in coal mine and the CBM (coal bed methane) development planning.

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Influence of reservoir properties on the methane adsorption capacity and fractal features of coal and shale in the upper Permian coal measures of the South Sichuan coalfield, China

Energy Exploration & Exploitation ◽

10.1177/0144598719877527 ◽

2019 ◽

Vol 38 (1) ◽

pp. 57-78 ◽

Cited By ~ 3

Author(s):

Yuan Bao ◽

Yiwen Ju ◽

Zhongshan Yin ◽

Jianlong Xiong ◽

Guochang Wang ◽

...

Keyword(s):

Pore Structure ◽

Adsorption Capacity ◽

Gas Adsorption ◽

Methane Adsorption ◽

Structure Characterization ◽

Upper Permian ◽

Organic Carbon Content ◽

Reservoir Properties ◽

Fracture Width ◽

Coal Measures

Pore structure plays an essential role in the reservoir heterogeneity and methane adsorption capacity. Significant progress has been made in the pore structure classification of porous materials (such as coal and shale). Considering the pore structure characterization of the coal measures and the measuring range of high-pressure mercury intrusion porosimetry and low-pressure N2/CO2 gas adsorption, an integrated classification for coal and shale is provided. They are micropore (<2 nm), mesopore (2–100 nm), macropore A (100 nm–1 µm), macropore B (1–10 µm), and micro-fracture (>10 µm). For coal and shale samples from Guxu mining area, the micropores and mesopores largely control the gas adsorption while micro-fractures and macropore B are significant for the storage and flow of free gas. The fractal dimensions calculated from limited N2 adsorption data are not suitable for the coal samples which are not developed in mesopore and macropore A; these samples are precisely corresponding to the N2 adsorption/desorption isotherms of group B (reversible isotherm). Furthermore, the main factors influencing the methane adsorption capacity of coal and shale in the coal measures are micropore frequency, micro-fracture width, clay mineral composition, and total organic carbon content.

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Influence of intramolecular interactions on carbon dioxide gas adsorption capacity in Mesoporous Imine polymers

10.26434/chemrxiv.5826435 ◽

2018 ◽

Author(s):

Jaya Prakash Madda ◽

Pilli Govindaiah ◽

Sushant Kumar Jena ◽

Sabbhavat Krishna ◽

Rupak Kishor

Keyword(s):

Carbon Dioxide ◽

Adsorption Capacity ◽

Density Functional ◽

Gas Adsorption ◽

Ambient Pressure ◽

Condensation Reaction ◽

Intramolecular Interactions ◽

Carbon Dioxide Adsorption ◽

Nitrogen Gas ◽

Adsorption Characteristic

Covalent organic Imine polymers with intrinsic meso-porosity were synthesized by condensation reaction between 4,4-diamino diphenyl methane and (para/meta/ortho)-phthaladehyde. Even though these polymers were synthesized from precursors of bis-bis covalent link mode, the bulk materials were micrometer size particles with intrinsic mesoporous enables nitrogen as well as carbon dioxide adsorption in the void spaces. These polymers were showed stability up to 260o centigrade. Nitrogen gas adsorption capacity up to 250 cc/g in the ambient pressure was observed with type III adsorption characteristic nature. Carbon dioxide adsorption experiments reveal the possible terminal amine functional group to carbamate with CO2 gas molecule to the polymers. One of the imine polymers, COP-3 showed more carbon dioxide sorption capacity and isosteric heat of adsorption (Qst) than COP-1 and COP-2 at 273 K even though COP-3 had lower porosity for nitrogen gas than COP-1 and COP-2. We explained the trends in gas adsorption capacities and Qst values as a consequence of the intra molecular interactions confirmed by Density Functional Theory computational experiments on small molecular fragments.

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