Elsevier

Powder Technology

Volume 350, 15 May 2019, Pages 15-25
Powder Technology

Experimental study of pore structure and fractal characteristics of pulverized intact coal and tectonic coal by low temperature nitrogen adsorption

https://doi.org/10.1016/j.powtec.2019.03.030Get rights and content

Highlights

  • Pore structure characteristics of pulverized intact coal and tectonic coal are measured by N2 adsorption method.

  • Fractal characteristics are used to analyse pore surface and pore structure complexity.

  • The realtionship between fractal characteristics, mineral matter and pore volume and SSA are discussed.

  • The small particle size on gas outburst are analysed.

Abstract

To study the pore structure and fractal characteristics of pulverized intact coal and tectonic coal, proximate analysis, gas adsorption/desorption, and N2 (77K) adsorption experiments were performed. The results show that micropore, minipore and mesopore volumes, as well as specific surface areas (SSAs), are dependent on the particle size and that they all exhibit a positive correlation with decreasing particle size, a correlation which also promotes gas adsorption. Many complex pores became simpler with the destruction of coal, and some long pores were also converted into short pores; that is, the pore structures of coals (referring to changes in pore shapes and lengths) may become increasingly simple. The mineral matter of samples increases with decreasing particle size and contributes more to the mesopore volume and SSA than to the micropore and minipore volumes. The increased D1 and reduced D2 of samples with decreasing particle sizes indicate a greater pore surface roughness and a smaller pore structure anisotropy. Compared with pulverized intact coal, the mineral matter, pore structure and gas adsorption capacity of tectonic coal have significantly increased or decreased. Although they are in the same coal seam, they are no longer the same type. This study of pulverized coal, especially tectonic coal, is of great significance for the further understanding of gas outbursts.

Introduction

As an important fossil fuel, coal has a complex pore structure and a large specific surface area (SSA) [[1], [2], [3]]. Coalbed methane (CBM), which is stored in coal, is different from conventional gas and is widely used throughout the world as a highly efficient and clean energy source. Coal is transformed from plant debris via complex biochemical, physical, chemical and geochemical processes over long periods of geological time [4]. During the formation and storage of coal, it is subjected to one or more periods of tectonic coal stress, resulting in the strong plasticity, ductile deformation and rheological migration of the coal seam. Cracking and wrinkling are then increasingly developed, so tectonic coal is formed.

The tectonic coal underwent multiple stages of geological tectonics, which exhibits a low-strength, weakly cohesive morphology compared to intact coal. Mach research has confirmed that excess coal and gas outbursts can occur in the coal seams where tectonic coal is developed [5], which can seriously damage the safe production of coal mines [6]. In recent years, some scholars have paid more attention to the study of tectonic coal, mainly regarding the pore characteristics, gas adsorption and desorption, gas diffusion, etc. [[7], [8], [9], [10], [11], [12]]. The gas in coal is mainly adsorbed into the pores in the form of their adsorption state, and the development of pore structure directly determines the ability for gas to be adsorbed and diffused [9,13]. According to the Hotot's classification of pore sizes [14], the pores can be divided into micropores (<10 nm), minipores (10–100 nm), mesopores (100–1000 nm) and macropores (>1000 nm).

Tectonism will change the pore characteristics of coal, and the physical properties of CBM reservoirs may change under multiple-periods of geological tectonics [15,16]. In recent years, with the development of several technologies, a series of methods have been applied to study the pore characteristics of tectonic coals, such as the mercury intrusion porosimetry method (MIP), the physisorption method, scanning electron microscopy (SEM), small angle scattering, CT scanning, X-ray computed tomography, nuclear magnetic resonance (NMR), etc. [[17], [18], [19], [20], [21], [22]]. Qu et al. [23] investigated the pore structure and compressibility of tectonic coal, and the results showed that tectonic deformation mainly reformed pores whose sizes were > 100 nm. The “micro-crystal” structures of coals are changed when suffering from strong tectonic stress, causing greater pore volumes analysed by Barrett-Joyner-Halenda (BJH) method, SSAs by Brunauer-Emmett-Teller (BET) method, and adsorption capacities [8]. It is generally believed that micropores, minipores, and mesopores are the main pores that affect the ability of coal to adsorb/desorb gas [24]. Li et al. [25] applied the MIP, the N2 (77K) and the CO2 (273 K) adsorption methods to infer the structural characterization of tectonic coals, and found that seepage-porosity at pore sizes of >100 nm was significantly increased.

The effects of the variations in particle size on the pore structure of coal are obvious [[26], [27], [28]]. Mastalerz et al. [28] applied different analytical particle sizes in N2 and CO2 adsorption of the high volatile bituminous coal and found that the 0.25 mm particle size is optimal, and the most practical size for low pressure gas adsorption. Jin et al. [29] analysed the pore characteristics of different pulverized coals and their effects on gas adsorption and diffusion, and the results demonstrated that pore structure was evidently modified, while the pulverization process significantly increased SSA by BET method and pore volume by the density functional theory (DFT) method and BJH method (as measured by N2 adsorption); however, the effects on the micropore structure (as measured by CO2 adsorption) were irregular. A study by Hou et al. [30] showed a similar feature nature, which was the lack of an obvious linear correlation between the micropore structure (as measured by CO2 adsorption) and decreasing particle size. In addition, some scholars have also found that the pore volumes analysed by Dubibin-Astakhov (D-A) equation whose sizes are <2 nm and SSAs by Dubinin-Radushkevich (D-R) equation of tectonic coal did not increase significantly, or even decrease [31], which may be attributed to the collapse of some micropores (measured by CO2 adsorption) [30]. Mineral matter exhibited a positive correlation with decreasing particle size. The increased mineral matter usually decreased pore volume by D-A equation with sizes <2 nm and SSAs by D-R equation [1,30]. However, the relationship between mineral matter and pore volume by D-A equation <2 nm is indeed complex, and both positive and negative relationships are observed [30,32].

During the formation process, coal accumulation basins in China were affected by the superposition and transformation of multi-stage tectonic movements, and experienced complex sedimentation, uplift and stratigraphic tectonic deformation. As a result, most of the coal seams have deep burial depths and complicated geological conditions. The deformation of coal seams was dominated by brittle failure and subsequent movement along fracture planes [33]. To deeply analyse the pore structure evolution characteristics of tectonic coal, variations in mineral matter and geological genesis, intact coal were placed in a pulverizer to obtain differentially pulverized intact coal samples (1–3 mm, 0.5–1 mm, 0.25–0.5 mm, 0.2–0.25 mm, 0.074–0.2 mm and < 0.074 mm), which were similar to the geological evolutionary process of coal seams and were compared to tectonic coals of the same size. Our aims in this paper are to (i) compare and analyse the basic physical parameters of pulverized intact and tectonic coals, and the effects of mineral matter on pore structure characteristics, (ii) investigate the variation in pore structure of pulverized intact and tectonic coals with particle size and the difference between them, (iii) discuss the relationship between the fractal characteristics, the pore volumes and SSAs, (iv) analyse the small particle sizes upon gas outburst.

Section snippets

Coal samples

The Xinjing Coal Mine is located in the west Yangquan Coalfield at the Qinshui Basin, Shanxi Province, China. Coal samples are collected from working faces 3215 and 3216 of coal seam No. 3, representing different extents of deformation, and the sample morphology is shown in Fig. 1. Once a coal sample was freshly obtained from a working face, it was sealed and immediately sent to the laboratory to remove waste rocks for further testing. It can be seen that coals from working face 3215 (NJY) are

Coal basic physical characteristics

Basic physical parameters of Coals NJY and BJR were tested and listed in Table 1, from which it can be noticed that the true densities and indices of the initial gas diffusion rate all show a positive correlation with the decrease of particle sizes. For the same sample, the contents of moisture, ash, volatile and fixed carbon all changed with decreasing particle size. The moisture content of Coal NJY increased from 1.75% to 1.87%, except for samples whose sizes was 0.25–0.5 mm (1.63%), and that

Conclusions

In this paper, some experiments were conducted to compare the differences in the properties of pulverized intact coal and tectonic coal. This study leads to the following conclusions:

  • (1)

    As the particle size decreases, the moisture content, ash content, and the VL values of samples all increased. In the process, the increase ratio of the minimum particle size to the maximum particle size is not much different, but the results of each parameter of tectonic coal are significantly larger than that of

Acknowledgments

The authors are grateful to the Fundamental Research Funds for the Central Universities (No. 2017XKZD01) and the National Natural Science Foundation of China (No. 51574229).

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