Elsevier

Powder Technology

Volume 363, 1 March 2020, Pages 559-568
Powder Technology

The influence of pore structure of coal on characteristics of dust generation during the process of conical pick cutting

https://doi.org/10.1016/j.powtec.2019.12.039Get rights and content

Highlights

  • Pore structure of coal sample was tested by a low field nuclear magnetic resonance.

  • The cumulative proportion of respirable dust reduced with increasing porosity.

  • The amount of total dust generation grew with increasing porosity.

  • The effect of fractal dimension of pore on dust generation is contrary to porosity.

Abstract

To reveal the influence of pore structure of coal on the characteristics of coal dust generation during roadheader cutting, a self-developed coal cutting system was used to generate dust. A low field nuclear magnetic resonance was utilized to measure the pore system of coal. Results show that the porosity had negative correlations with the cumulative proportions of both respirable dust and fine particulate matter (PM2.5), but the positive influence on the mass ratio of dust generation (MRD) of total dust and slight effect on MRD of respirable dust and PM2.5 were observed. The correlations of the fractal dimension (Df) of pore structure on dust generation were inverse compared with that of porosity. The negative relationship between porosity and Df was the reason for the opposite correlations. This study can provide basic support for the estimate of dust hazard and the development of dust control technologies.

Introduction

Coal is one of the essential parts of energy supply in the world, accounting for 29.25% of total energy consumption [1]. In China, coal accounted for over 50% of energy consumption [2,3], and will remain to be the crucial energy source until at least 2050 [4]. Over 90% of coal mines are underground in China [5] requiring the tunnels to be headed firstly to form the coal mining working face. With the increasing utilization of roadheader, the heading working face has become one of the primary dust sources [6]. Dust can cause the coal workers' pneumoconiosis (CWP) due to a long time working in the environment with a high concentration of dust [7,8]. According to a statistic, CWP caused 69,377 deaths during 34 years from 1970 to 2004 in the U.S. [9]. Over $39 billion was paid to workers who suffered from pneumoconiosis between 1980 and 2005 [9] and more than $5.67 billion from 2000 to 2013 [10]. In China, the total number of pneumoconiosis patients exceeds 440,000. CWP caused 6 times death more than other disasters in coal mines[11]. In addition, high concentrations of coal dust can also cause coal dust explosions. In 2014, 26 people were killed in a coal dust explosion in Hengda Coal Mine, Fuxin Mining Group, Liaoning Province.

Although several dust control technologies, such as water spray, foam and dust fan [12], have been rapidly developed, the residual concentration of dust can hardly reach the standard [13,14]. Additionally, the concentration of dust generation varies from different mines. The concentration of total dust was 300 mg/m3 and respirable dust of 70 mg/m3 [15], while it was 1200 mg/m3 and 700 mg/m3, respectively in another coal mine [16]. Few studies about the characteristics of dust generation are the fundamental reason for the unqualified residual concentration of dust, which makes the coal mine managers and workers cannot select the targeted and effective dust control methods. Thus, it is essential to study the mechanism of dust generation.

Coal is a heterogeneous material with complex pore structure [17]. The influence of pore system on dust generation can be divided into two parts. On the one hand, the original dust exists inside the pores and fractures of coal [14], which is one of the primary dust sources suppressed by water coal seam infusion [18]. The amount of original dust would grow with the increasing development of pores and fractures. On the other hand, pore system could affect the physical and mechanical properties [[19], [20], [21], [22], [23], [24]]. The process of coal bulk breaking into fragments and dust is primarily the transformation of physical form, indicating that pore system would also influence dust generation by affecting mechanical properties [[25], [26], [27], [28]]. Therefore, pore system has remarkable effect on dust generation. Additionally, fractal theory, a useful mathematical tool to quantify the complex pore characteristics in coal and carbon [[29], [30], [31], [32], [33]], was used to investigate the relationship between mechanical property and pore structure [34,35]. However, there are limited studies focusing on the effect of pore structure in coal on dust production when cutting by the roadheader. Therefore, it is of great necessity and importance to study the influence of coal pore on dust generation for enhancing basic understanding on the mechanism of dust generation.

Existing techniques for measuring the pore characteristics of coal include nitrogen gas adsorption analysis, mercury intrusion porosimetry (MIP), microscopic observation, nuclear magnetic resonance (NMR), X-ray computed tomography (CT), etc. [[36], [37], [38], [39], [40]]. The liquid nitrogen method is used to detect micropores or mesopores. MIR can measure the structure of mesopores and macropores, while the high pressure intrusion could destroy the internal coal structure and develop more fractures and pores [41,42]. Microscopic observations can only observe local microscopic images of coal surface [36]. X-ray CT could provide accurate measurement when testing the small size sample, such as the diameter and length of 2 mm and 5 mm, respectively [43]. However, the pore and fracture development in the small size sample are likely diverse from that with a big size. Although the X-ray CT can test a larger sample, it will give numerous 2D scanned images, which is a time-consuming process to obtain the results. Low field NMR can measure a large coal block, and the measurement process has no damage to coal samples [39]. Therefore, this paper uses the microscopic NMR method to measure the coal specimen.

In this study, the influence of the pore structure on the characteristics of dust generation was investigated. Six types of coal samples were selected from 8 coal mines. A self-developed coal cutting system was utilized to produce dust with a conical pick as the cutting tool, similar to that used in the roadheader. The pore structure was tested by a low field NMR spectrometer. Then, the fractal method was adopted to calculate the fractal dimensions of coal samples for quantifying the pore size distribution. The dust particle size distributions were measured through a laser particle size analyzer.

Section snippets

NMR experiments

In this paper, a low field NMR spectrometer MesoMR23-060 h-i (Suzhou Niumag Analytical Instrument Corporation) was conducted to characterize the pore system in coal with a magnetic field strength of 0.5 T. The 1H nucleus is magnetic and can generate magnetic resonance signals by interacting with an external magnetic field. After adjusting the measurement parameters, only the signal of 1H in the pore fluid was detected instead of those in solid skeleton [44]. In the low field conditions, the

T2 spectrum and fractal dimensions

The T2 spectrum ranges from 0.1 ms to 10,000 ms under completely distilled water-saturated condition. According to Hodot [55], the coal pores can be classified into three types based on the pore size: micropores (smaller than 100 nm), mesopores (100–1000 nm) and macropores and micro-fractures (larger than 1000 nm). T2 values <10 ms correspond to micropores, T2 values ranging from 10 ms to 100 ms corresponding to mesopores, and T2 values >100 ms correspond to either macropores or micro-fractures

Conclusion

The pore structure of the coal sample was measured by NMR. The dust generation test was performed through a self-developed cutting system with a conical pick as the cutting tool. The mass and size distribution of dust were tested. According to the experimental results and analyses, the conclusions that can be drawn as follows:

  • (1)

    The distinct pore structures in coal samples were displayed in the T2 spectrums. In general, the T2 spectrum showed three-peak distributions, except for HS which showed

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was funded by the National Natural Science Foundation of China (grant number 51874290), the National Key R&D Program of China (grant number 2017YFC0805201), the Young Elite Scientists Sponsorship Program by CAST (grant number: 2017QNRC001) and the Jiangsu Province “Qinglan Project” (2019).

References (65)

  • Y. Zhang et al.

    Pore characteristics and mechanical properties of sandstone under the influence of temperature

    Appl. Therm. Eng.

    (2017)
  • A. Fakhimi et al.

    Discrete element analysis of the effect of pore size and pore distribution on the mechanical behavior of rock

    Int. J. Rock Mech. Min.

    (2011)
  • X. Chen et al.

    Influence of porosity on compressive and tensile strength of cement mortar

    Constr. Build. Mater.

    (2013)
  • S.J. Page et al.

    Coal proximate analyses correlation with airborne respirable dust

    Fuel.

    (1993)
  • S.P. Singh

    Brittleness and the mechanical winning of coal

    Min. Sci. Technol.

    (1986)
  • S. Zheng et al.

    Characterizations of full-scale pore size distribution, porosity and permeability of coals: a novel methodology by nuclear magnetic resonance and fractal analysis theory

    Int. J. Coal Geol.

    (2018)
  • G.H. Coetzee et al.

    Pore development during gasification of south African inertinite-rich chars evaluated using small angle X-ray scattering

    Carbon.

    (2015)
  • G. Lee et al.

    The effect of pore structures on fractal characteristics of meso/macroporous carbons synthesised using silica template

    Carbon.

    (2005)
  • P.A. Gauden et al.

    The new correlation between microporosity of strictly microporous activated carbons and fractal dimension on the basis of the Polanyi–Dubinin theory of adsorption

    Carbon.

    (2001)
  • X. Ji et al.

    Fractal model for simulating the space-filling process of cement hydrates and fractal dimensions of pore structure of cement-based materials

    Cement Concrete Res.

    (1997)
  • S. Jin et al.

    Fractal analysis of relation between strength and pore structure of hardened mortar

    Constr. Build. Mater.

    (2017)
  • K.S.W. Sing

    Characterization of porous materials: past, present and future

    Colloids Surf. A Physicochem. Eng. Asp.

    (2004)
  • Y. Yao et al.

    Fractal characterization of adsorption-pores of coals from North China: an investigation on CH4 adsorption capacity of coals

    Int. J. Coal Geol.

    (2008)
  • Y. Yao et al.

    Non-destructive characterization of coal samples from China using microfocus X-ray computed tomography

    Int. J. Coal Geol.

    (2009)
  • Y. Yao et al.

    Petrophysical characterization of coals by low-field nuclear magnetic resonance (NMR)

    Fuel.

    (2010)
  • J. Busse et al.

    Image processing based characterisation of coal cleat networks

    Int. J. Coal Geol.

    (2017)
  • E.M. Suuberg et al.

    Elastic behaviour of coals studied by mercury porosimetry

    Fuel

    (1995)
  • Y. Cai et al.

    Petrophysical characterization of Chinese coal cores with heat treatment by nuclear magnetic resonance

    Fuel.

    (2013)
  • D. Chen et al.

    The diffusion of dust in a fully-mechanized mining face with a mining height of 7 m and the application of wet dust-collecting nets

    J. Clean. Prod.

    (2018)
  • J. Xu et al.

    Investigation of temperature effects from LCO2 with different cycle parameters on the coal pore variation based on infrared thermal imagery and low-field nuclear magnetic resonance

    Fuel.

    (2018)
  • H. Wang et al.

    Experimental investigation on the wettability of respirable coal dust based on infrared spectroscopy and contact angle analysis

    Adv. Powder Technol.

    (2017)
  • L. Liu et al.

    Experimental investigation on the relationship between pore characteristics and unconfined compressive strength of cemented paste backfill

    Constr. Build. Mater.

    (2018)
  • Cited by (0)

    1

    Co-first author: These authors contributed equally to this work.

    View full text