Full-scale pore characteristics in coal and their influence on the adsorption capacity of coalbed methane.

Coalbed methane (CBM) is primarily stored and transported through the pores in the coal matrix, making it essential to study the development of different scales of pores in coal to better understand the evaluation and exploration of CBM. In this study, four coal samples of varying ranks (Ro,max = 0.68%-2.86%) were selected, and different scale pores were obtained through low-pressure CO2 and Ar adsorption (LP-CO2/ArGA) and mercury intrusion porosimetry (MIP) experiments. A full-scale pore evaluation model was established, and the impact of pores on methane adsorption and restriction was analyzed and discussed through high-pressure adsorption experiments. Our results show that (1) at high pressures (> 100 MPa), the MIP technique caused pore compression and overestimated the pore size below 30 nm by up to 47.2%; (2) to obtain a comprehensive pore evaluation, we developed an accurate model that combines LP-CO2/ArGA with NLDFT and BJH and NLDFT models to determine micropore (0.3-1.5 nm) and mesopore (1.5-30 nm) parameters. By combining this model with MIP test results, we can obtain a full-scale pore size in the range of 0.3 nm-200 µm; (3) coal rank affected the development of full-scale pore characteristics. As coal rank increased, the specific surface area (SSA) of micropores and adsorption capacity of methane first decreased, then increased. Micropores were found to be the most important storage space for CBM and control gas adsorption, with a microporous SSA and PV to total SSA and total PV ratio of 97.93% and 63.69%, respectively. (4) We also observed a significant linear relationship between the fractal dimension of micropores and the Langmuir volume (VL) based on fractal theory. As the fractal dimension increased, VL also increased (R2 = 0.8581), indicating that VL is controlled by the complexity of micropores, which is consistent with the comprehensive evaluation index (Dt) and VL (R2 = 0.8744). Based on our predicted model, VL can be estimated using the SSA of micropores and Dt. Our findings shed light on the relationship between pore morphology and CBM occurrence and have practical implications for fields such as catalytic synthesis, E-CBM, and gas purification.

Selection Effect of Liquid Nitrogen Freeze-Thaw Cycles on Full Pore Size Distribution of Different Rank Coals.

In China, the capacity to produce coalbed methane and extract underground gas is restricted by the prevalence of low-permeability coal seams. Liquid nitrogen fracturing is a new low temperature-high-pressure anhydrous fracturing technology that uses low temperature and high frost heave forces to increase coal permeability. To better understand the liquid nitrogen fracturing effect on coal, we conduct the liquid nitrogen freeze-thaw cycle (LNCFT) experiments on different rank coals from Qinghai, Shanxi, and Shaanxi provinces. We combined the low-pressure nitrogen and carbon dioxide adsorption experiment with the non-local density functional theory model and mercury injection porosimetry with compressibility corrections to examine the full pore size distributions of untreated and water-saturated samples before and after LNCFT. The results found that LNCFT can effectively increase the pore volume (PV) and specific surface area of the water-saturated coal sample. Compared with the raw coal, the increased ratio of the full pore size PV is 70.41-100.17%. However, the scale-selective transformation effect on pores during liquid nitrogen fracturing is noticeable. Under the same conditions, LNCFT can significantly increase the pore volume of micropores (>2 nm) and macropores (>50 nm), and the increase ratios are 24.40-44.16 and 103.55-327.93%, respectively. The PV of mesopores (2-50 nm) shows a slightly increasing trend with the increase in metamorphic degree, and the increase ratio is between 8.7 and 56%. Comparing the full pore size distribution curves before and after LNCFT, it is found that the alteration of high-volatile bituminous coal (BLT coal) and anthracite (SH coal) has more significance in the range of less than 2 and 50-20,000 nm, while middle-volatile bituminous coal (YJL coal) varies between 50 and 2000 nm. Meanwhile, the ratio of micropore and mesopore PV to the total decreased gradually before and after LNCFT, while the proportion of macropores increased, indicating that small-scale pores would intersect and connect to form larger-scale pores during the fracturing. The combined effects of temperature gradient, water-ice phase transition, and heat transmission rate are the key factors that determine the impact of LNCFT on pore size distribution. Our results provide new information for enhancing the permeability of low-permeability coal seams of different ranks.

Nanopore Structure and Origin of Lower Permian Transitional Shale, Eastern Ordos Basin, China.

Nanopores in the shale play a vital role in methane adsorption, and their structural characteristics and origins are of great significance for revealing the mechanism of methane adsorption, desorption, and diffusion. In this paper, through low-temperature ashing and low-pressure gas adsorption experiments, the nanopore structure of original shales and ashed shales was quantitatively characterized, and the nanopore origins in the transitional shale of lower Permian in eastern Ordos Basin were analyzed. The results show that the pore volume (PV) and specific surface area (SSA) of nanopores in transitional shale reservoirs are 0.0217-0.0449 cm3/g and 13.91-51.20 m2/g, respectively. The average contribution rates of micropores (<2 nm), mesopores (2-50 nm), and macropores (50-100 nm) to PV are 18.78, 72.26, and 8.96%, respectively, and the average contribution rates to SSA are 66.19, 33.10, and 0.71%, respectively. In addition, it is found that the average contribution rates of inorganic minerals and organic matter to the SSA of micropores are 55.9 and 44.1%, respectively, and the average contribution rates to the SSA of mesopores are 92.3 and 7.7%, respectively. Combining the adsorption properties of the main clay minerals and kerogen in shale, it is concluded that organic pores control the adsorption of methane with an absolute advantage in transitional shales. It is of great significance to understand the mechanism of methane occurrence, desorption, and diffusion in shales by clarifying the origins of multiscale pores.

Influence of Nanopore Structure Deformation on Gas Migration in Coal.

To better understand the influence and control of nanopore characteristics on gas migration, three kinds of coal samples with different metamorphic degrees were selected for the experiments including high-pressure isothermal gas adsorption, low-pressure CO2 adsorption, and low-pressure Ar adsorption. The changes of the pore volume (PV) and specific surface area (SSA) of coal samples before and after adsorption-desorption were compared and analyzed. The adsorption data of all coal samples at a low pressure stage (<8 MPa) conformed to the Langmuir equation, and the adsorption capacity of powdered coal samples was higher than that of columnar coal samples. Some adsorption data deviated from the original fitting curve at a high pressure stage (>8 MPa), and this was the most remarkable in columnar coal samples. There was a positive correlation between the cumulative SSA of pores and adsorption capacity of coal samples. When the adsorption time was more than 10 min, the adsorption efficiency of 200 mesh coal samples from YJL was lower than those of 200 mesh coal samples from CZ and WY, which was due to the good development and connectivity of micro-fissures and nanopores in YJL coal samples. The pore size distribution of coal samples had changed after adsorption-desorption, and the cumulative deformation of the nanopore structure was anisotropic. As a result of the swelling or shrinkage deformation of the coal matrix, the PV and SSA with the same pore size presented many forms, such as almost unchanged, increased, or decreased. There are two types of deformation mechanisms: the whole collaborative deformation and partial deformation. Both gas adsorption and desorption can lead to the shrinkage or swell deformation of nanopores and fissures. In brief, the research provides theoretical and technical support for reservoir evaluation, fine drainage, and efficient development of coalbed methane.

Effects of Early Pollution Control Measures on Secondary Species of PM2.5 in Jiaozuo, China.

Various measures for reducing air pollution have been promulgated since 2013 in China. To investigate the synergistic results of emission control and meteorological environment, PM2.5 samples collected from October 2013 to July 2016 and November 2018 to October 2019 in Jiaozuo city were analyzed for their compositions, secondary species (Ss) variations, and factors changing for Ss formation. The results showed that the concentrations of sulfate, nitrate, ammonium, and secondary organic aerosols (SOAs) generally decreased over the same seasonal period during these years. In addition, the concentrations and proportions of each Ss increased with the increase in the PM2.5 level in these years, implying that although PM2.5 levels have been reduced by various control policies, Ss formation would remain the major contributor to PM elevations. The enhanced effects of gas-phase reactions on intensification of sulfate, SOA, and PM were observed in 2018-2019, which was consistent with the elevation of nitrate and SOA at PM levels of >150 µg/m3. Only sulfate in all PM levels sharply decreased after 2015, showing the fine effect of coal-related pollution control and the importance of collaborative control of NO x , volatile organic compounds, and organic aerosol emissions with SO2 emissions in the future.

Applicability Analysis of Determination Models for Nanopores in Coal Using Low-Pressure CO2 and N2 Adsorption Methods.

The development characteristics of nanopores (with pore sizes <200 nm) in coal are a key factor affecting the accumulation and migration of coalbed methane (CBM). Thus, an appropriate determination method and calculation model are essential for accurate nanopore representation. Based on the experiments of low-pressure CO2 adsorption (LP-CO2GA) at 273 K and low-pressure N2 adsorption (LP-N2GA) at 77 K on four coals with different ranks, the abilities of different models (e.g., Langmuir, Dubinin-Radushkevich (D-R), Dubinin-Astakhov (D-A), Brunauer-Emmett-Teller (BET) and nonlocal density functional theory (NLDFT)) to accurately predict the pore parameters were analyzed. The results showed that (1) for LP-N2GA, the Langmuir model is only suitable for gas adsorptions at low relative pressure conditions (P/P0 < 0.01), and its error value increased with the relative adsorption pressure. The fitting results of the D-R model showed good agreement with the D-A model under low relative pressure of LP-CO2GA (P/P0 < 0.01), and the D-A model had more accurate fitting results. The BET model is more accurate than the other models (φ = -1.2733%) only in the interval of LP-N2GA with 0.05 < P/P0 < 0.35. The data also showed that the NLDFT model can maintain a higher fitting accuracy for LPCO2/N2GA processes at relative adsorption pressures from 0.001-0.9996. (2) Using LP-CO2GA with the Langmuir, D-R, D-A, and NLDFT models, the micropore specific surface area (SSA; 66.9570-248.6736 m²/g) and pore volume (0.0201- 0.0997 cm³/g) were obtained, while the values of meso-/macropore SSA (0.0007-2.3398 m²/g) and pore volume (0.0036-0.04 cm³/g) were calculated by LP-N2GA with the BET and NLDFT models. The results showed that the fitting accuracy in descending order was the D-R, D-A, Langmuir and NLDFT models. (3) In combination with the applicable model range, LP-CO2GA with the NLDFT model was recommended for micropore analysis of the coal pore sizes from 0.36-1.1 nm, while LP-N2GA combined with the NLDFT model was recommended for nanopore analysis of pore sizes from 1.1-200 nm. (4) The characteristics of pore development in the Beiloutian coal were analyzed using LP-CO2/N2GA combined with the NLDFT model. It was found that a pore volume and SSA less than 1.0 nm accounted for 88.82% of the total pore volume and 98.05% of the total SSA, indicating that micropores in coal are the main space for CBM storage and are key physical factors for the occurrence and migration of coalbed methane. The conclusions of this article will provide a basis for the accurate calculation of nanopores in coal.

Control Mechanism of the Effective Stress on Nano-Micro Pores and the Permeability of High-Rank Coals.

To study the change and main control factors of the high-rank coal reservoir permeability in deep coal seams, permeability tests under different stresses and gas pressures were carried out in the laboratory. The development and distribution of nano-micro pores and fractures in the coal matrix were analyzed and observed by mercury intrusion porosimetry, gas adsorption, scanning electron microscope and computed tomography to reveal the permeability variation mechanism. The results showed that the initial permeability of the coal samples ranged from 0.0114 mD to 0.2349 mD when the effective stress was 0 MPa, and it clearly varied among different samples. The permeability of all the coal samples was very sensitive to the effective stress and decreased exponentially with the increase of the effective stress. The increase of the pore pressure also led to a decrease of the permeability, whereas the impact of the pore pressure on permeability was less obvious compared with the effective stress. Sub-nanopores, nanopores, micro-fractures and larger fractures are all developed in the coal samples. Connected larger fractures were the main gas migration channels in permeability determination, and the narrowing, disconnection, and closure of the fractures caused by the increase of the effective stress were the most important reasons for significant reduction of permeability.

Selection and Validation of Appropriate Reference Genes for Quantitative Real-Time PCR Normalization in Staminate and Perfect Flowers of Andromonoecious Taihangia rupestris.

Quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR) is the most commonly used and powerful method for gene expression analysis due to its high sensitivity, specificity, and high throughput, and the accuracy of this approach depends on the stability of reference genes used for normalization. Taihangia rupestris Yu and Li (Rosaceae), an andromonoecious plant, produces both bisexual flowers and unisexual male flowers within the same individual. Using qRT-PCR technique, investigation of the gene expression profiling in staminate and perfect flowers would improve our understanding of the molecular mechanism in regulation of flower formation and sex differentiation in andromonoecious T. rupestris. To accurate normalize the gene expression level in Taihangia flower, 16 candidate reference genes, including 10 traditional housekeeping genes, and 6 newly stable genes, were selected based on transcriptome sequence data and previous studies. The expressions of these genes were assessed by qRT-PCR analysis in 51 samples, including 30 staminate and perfect flower samples across developmental stages and 21 different floral tissue samples from mature flowers. By using geNorm, NormFinder, BestKeeper, and comprehensive RefFinder algorithms, ADF3 combined with UFD1 were identified as the optimal reference genes for staminate flowers, while the combination of HIS3/ADF3 was the most accurate reference genes for perfect floral samples. For floral tissues, HIS3, UFD1, and TMP50 were the most suitable reference genes. Furthermore, two target genes, TruPI, and TruFBP24, involved in floral organ identity were selected to validate the most and least stable reference genes in staminate flowers, perfect flowers, and different floral tissues, indicating that the use of inappropriate reference genes for normalization will lead to the adverse results. The reference genes identified in this study will improve the accuracy of qRT-PCR quantification of target gene expression in andromonoecious T. rupestris flowers, and will facilitate the functional genomics studies on flower development and sex differentiation in the future.

Contents and occurrence of cadmium in the coals from Guizhou province, China.

Eleven raw coal samples were collected from Liuzhi, Suicheng, Zunyi, Xingren, Xingyi, and Anlong districts in Guizhou Province, Southwest China. The content of cadmium (Cd) in coal was determined using inductively coupled plasma mass-spectrometry (ICP-MS). Cd contents ranged from 0.146 to 2.74 ppm (whole coal basis), with an average of 1.09 ppm. In comparison with the arithmetic means of Cd in Chinese coal (0.25 ppm), this is much higher. In order to find its occurrence in coal, float-sink analysis and a coal flotation test by progressive release were conducted on two raw coal samples. The content of the Cd and ash yield of the flotation products were determined. The organic matter was removed by low-temperature ashing (LTA). X-ray diffraction (XRD) was used to differentiate the main, minor, and trace minerals in the LTA from different flotation subproducts. Quartz, kaolinite, pyrite, and calcite were found to dominate the mineral matters, with a proportion of anatase, muscovite, and illite. Then quantitative analysis of minerals in LTA was conducted using material analysis using diffraction (MAUD) based on the Rietveld refinement method. Results show that Cd has a strong association with kaolinite.

The studying of washing of arsenic and sulfur from coals having different ranges of arsenic contents.

To study the effectiveness of washing in removal of arsenic and sulfur from coals with different ranges of arsenic concentration, coal was divided into three groups on the basis of arsenic content: 0-5.5 mg/kg, 5.5 mg/kg-8.00 mg/kg, and over 8.00 mg/kg. The result shows that the arsenic in coals with higher arsenic content occurs mainly in an inorganic state and can be relatively easily removed. Arsenic removal is very difficult and less complete when the arsenic content is lower than 5.5 mg/kg because most of this arsenic is in an organic state. There is no relationship between washing rate of total sulfur and arsenic content, but the relationship between the washing rate of total sulfur and percent of organic sulfur is very strong.