Effect of nitric acid on the pore structure and fractal characteristics of coal based on the low-temperature nitrogen adsorption method

Guanhua, Ni; Li, Shang; Rahman, Sheik S.; Meng, Xianglong; Wang, Hui; Xu, Yongzhao; Xie, Heping

doi:10.1016/j.powtec.2020.04.011

Cited by 129 publications

(46 citation statements)

References 51 publications

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“…The principle of low-temperature nitrogen adsorption is based on physical adsorption caused by intermolecular forces [26]. At -195.8°C, the energy of nitrogen molecules is reduced, and nitrogen molecules reach the adsorption equilibrium of approximate monolayer on the surface of solid material under the action of van der Waals forces.…”

Section: Introductionmentioning

confidence: 99%

Pore Structure Characteristics and Evolution Law of Different-Rank Coal Samples

et al. 2021

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In this study, the full-size pore structure characteristics of six different-rank coal samples were investigated and analyzed from three perspectives, namely, pore shape, pore volume, and pore specific surface area, by performing a high-pressure mercury injection experiment and a low-temperature nitrogen adsorption experiment. Next, the full-size pore volumes and pore specific surface areas of the six coal samples were accurately characterized through a combination of the two experiments. Furthermore, the relationships between volatile matter content and pore volume and between volatile matter content and pore specific surface area were fitted and analyzed. Finally, the influences of metamorphic degree on pore structure were discussed. The following conclusions were obtained. The pore shapes of different-rank coal samples differ significantly. With the increase of metamorphic degree, the full-size pore volume and pore specific surface area both decrease first and then increase. Among the pores with various sizes, micropores are the largest contributor to the full-size pore volume and pore specific surface area. The fitting curves between volatile matter content and pore volume and between volatile matter content and pore specific surface area can well reflect the influence and control of metamorphic degree on pore volume and pore specific surface area, respectively. With the increase of volatile matter content, the pore volume and the pore specific surface area both vary in a trend resembling a reverse parabola.

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Section: Introductionmentioning

confidence: 99%

Pore Structure Characteristics and Evolution Law of Different-Rank Coal Samples

et al. 2021

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“…The T 2 spectrum corresponding to diffusion pores is generally in the range of 0.05–2.5 ms, while that corresponding to seepage pores–microfractures is generally larger than 2.5 ms, which matches previous research results. 8 Therefore, in the following comprehensive evaluation of pore–fracture systems, 2.5 ms is taken as the critical value for distinguishing diffusion pores from seepage pores–microfractures. Furthermore, the spectral area enclosed by the spectral peaks and changes in morphology of T 2 spectra were used to characterize the percentage of porous volumes of diffusion pores, seepage pores, and microfractures and the connectivity between pores and fractures.…”

Section: Results and Discussionmentioning

confidence: 99%

Classification of Pore–fracture Combination Types in Tectonic Coal Based on Mercury Intrusion Porosimetry and Nuclear Magnetic Resonance

Zhao

Wang

et al. 2020

ACS Omega

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Differences in content, distribution, and connectivity of pores and fractures with different sizes in coal lead to different modes of gas migration. An accurate classification of pore−fracture combination types in coal can lay a foundation for studying gas migration. High-pressure mercury intrusion and nuclear magnetic resonance (NMR) experiments were conducted on coal samples collected from the Changping coal mine in Jincheng City, Shanxi Province, and Pingdingshan no. 4 mine in Pingdingshan City, Henan Province, China. The fractal dimensions of pores with different sizes were calculated using the Menger model. By combining the data with T 2 spectra obtained by NMR, critical values for distinguishing diffusion pores from seepage pores−microfractures were determined. In addition, the main parameters affecting development of diffusion pores and seepage pores−microfractures and pore−fracture connectivity were analyzed, and a comprehensive evaluation index system for pores and fractures was established by selecting eight indices. Based on the method combining the analytical hierarchy process with multiparameter superposition, a method for determining critical values, establishing the evaluation index system, and classifying pore−fracture combination types was formed. The pore−fracture combination types in the test coal samples were classified according to the experimental data. The results indicate that the critical values for distinguishing diffusion pores from seepage pores−microfractures based on fractal dimensions obtained through mercury intrusion porosimetry and T 2 spectra obtained by NMR are 72 nm and 2.5 ms, respectively. The studied coal samples can be classified into three combination types, separately characterized by high diffusivity and permeability and poor pore−fracture connectivity; low diffusivity, high permeability, and good pore−fracture connectivity; and low diffusivity and permeability and good pore−fracture connectivity. In the coal samples from the Changping coal mine, diffusion pores and seepage pores−microfractures are developed, while the connectivity between pores and fractures is poor. The coal samples from Pingdingshan no. 4 mine have undeveloped diffusion pores and seepage pores− microfractures but good connectivity between pores and fractures. The research results provide a method for classifying pore− fracture combination types in coal samples taken from different regions.

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“…Figure 3c reveals that the channels caused by pickling deeply penetrated into the sample surface, which means the ash is embedded in the coal and plays the role of skeleton of pore structure. In the research of Ni et al [26], nitric acid can increase the surface area of coal, but nitric acid shows poor effect on the RC-I. Hydrofluoric acid removed the ash content and led to the collapse of RC-I pore structure.…”

Section: Pore Structurementioning

confidence: 96%

“…Figure 3g illustrates that RC-II has a more stable structure, and ash removal contributes to the pore development, which is absolutely different with RC-I. The ash dissolution produced a large amount of pores during acid pickling [26], which resulted in an increase in surface area for AFC-II char. The larger surface area promoted the mass transfer of CO 2 ; therefore, the carbon conversion of AFC-II was significantly higher than that of RC-II.…”

Section: Pore Structurementioning

confidence: 99%

Influences of Ash-Existing Environments and Coal Structures on CO2 Gasification Characteristics of Tri-High Coal

et al. 2020

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Two kinds of tri-high coals were selected to determine the influences of ash-existing environments and coal structures on CO2 gasification characteristics. The TGA results showed that the gasification of ash-free coal (AFC) chars was more efficient than that of corresponding raw coal (RC) chars. To uncover the reasons, the structures of RCs and AFCs, and their char samples prepared at elevated temperatures were investigated with SEM, BET, XRD, Raman and FTIR. The BET, SEM and XRD results showed that the Ash/mineral matter is associated with coal, carbon forms the main structural framework and mineral matters are found embedded in the coal structure in the low-rank tri-high coal. The Raman and FTIR results show that the ash can hinder volatile matters from exposing to the coal particles. Those results indicate that the surface of AFC chars has more free active carbon sites than raw coal chars, which are favorable for mass transfer between C and CO2, thereby improving reactivity of the AFC chars. However, the gasification reactivity was dominated by pore structure at elevated gasification temperatures, even though the microcrystalline structure, functional group structure, and increase in the disorder carbon were improved by acid pickling.

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Effect of nitric acid on the pore structure and fractal characteristics of coal based on the low-temperature nitrogen adsorption method

Cited by 129 publications

References 51 publications

Pore Structure Characteristics and Evolution Law of Different-Rank Coal Samples

Pore Structure Characteristics and Evolution Law of Different-Rank Coal Samples

Classification of Pore–fracture Combination Types in Tectonic Coal Based on Mercury Intrusion Porosimetry and Nuclear Magnetic Resonance

Influences of Ash-Existing Environments and Coal Structures on CO2 Gasification Characteristics of Tri-High Coal

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