Experimental test and particle mechanical analysis of gas adsorption-induced coal rock degradation
Graphical abstract
Introduction
China is the country with the most serious coal and gas outburst in the world. With the increase of mining depth and intensity, the intensity of coal and gas outburst and the proportion of casualties are increasing. The prediction and prevention of coal and gas outburst are very severe [1,2]. Coal is a typical porous medium. The gas in coal exists as adsorbed and free state, and the adsorbed gas has a great influence on the deformation and strength of the coal [3,4]. After adsorbing gas, the coal will undergo changes in mechanical properties and mesoscopic structure. These changes will lead to the change of gas content, permeability and gas emission, which will affect the whole process of occurrence, development and termination of coal and gas outburst. Therefore, the study of the degradation of gas adsorption on coal rock is the basis and necessary condition for exploring the mechanism of coal and gas outburst.
The findings of Larsen indicated that there was a slight expansion in the volume of coal rock after adsorbing gas, and the coal rock internal structure underwent a mesoscopic change [5,6]. This change minimized the surface free energy of the coal rock pores to maintain the stability of the entire coal rock - gas system. Majewska Z and Zietek J [7] found via acoustic emission tests that, gas adsorption caused damage in coal samples. Viete et al. and, Ranjith et al. [8,9] pointed out via experimental analysis that, the effect of gas adsorption on coal rock could be characterized by macro-strength and Young's modulus. On the basis of the Gibbs and Griffith's theories, the material strength will be weakened when the original adsorbent is displaced by the adsorbent with stronger adsorption capacity [10,11]. Yao et al. [12] summarized three typical hypotheses: (1) gas molecules deeply entered into the ultra-micropores of the coal, resulting in coal expansion in the adsorption process; (2) gas molecules entered into the ultra-micropores and wedged open the pores; (3) gas molecules entered into the carbon molecules of the coal, enlarging the molecular spacing and wedging the pores to the similar size of gas molecules. The mechanical relationship of adsorption-induced expansion deformation was qualitatively described by Bangham equation. He et al. [13] used the surface physical and chemistry theory and the fracture mechanics theory to analyze the theoretical model of degradation. Liu et al. [14] studied the characteristics of gas-bearing coal rock based on the theory of damage mechanics and the principle of effective stress.
In terms of physicochemical properties, the degradation degree is related to the adsorption capacities, the change amount of surface free energy in briquette coal particles, and the temperature. From the view of geotechnical mechanical, the degradation degree is related to the adsorption expansion stress/strain of coal particles [15]. The main damages forms of coal rock included tensile damage and compression-shear damage which can be described respectively by the Griffith strength theory featuring crack tip tensile damage and the Mohr-Coulomb shear strength theory emphasizing rock compression-shear damage. Whatever the damage, the change of the mesoscopic mechanical properties must be involved, because the tensile damage is related to the tensile strength of the crack tip, while the shear damage is related to the cohesive force and the internal friction angle. The absorbed gas and the free gas will induce irreversible micro-cracks, resulting in reduction of tensile strength, friction coefficient and cohesive force of coal rock. The macro test results show a decline in coal strength and the Young's modulus, indicating that gas adsorption will change the coal rock from a glassy state to a rubbery state.
It can be seen from the above researches that, the adsorbed gas can alter some of the mechanical parameters of the coal and affect the constitutive relationship of the coal [16]. The change in the macro-mechanical characteristics of gas-bearing coal is caused by the irreversible micro-cracks arising from the gas and the superposition & transmission of the free/absorbed gas force acting on the coal rock particles. Therefore, it can be hypothesized from the mesoscopic view that the characteristic of gas adsorption-induced coal rock degradation originates from the variation in micro parameters, and the degradation effect can be understood by analyzing the micro parameters of coal rock. In view of this, combining the geotechnical particle mechanics with the principle of contact model principle in the commercial particle flow software PFC (Particle Flow Code), this paper integrates the geotechnical mechanical and particle mechanical analysis for the degradation effect of gas absorption on coal rock, and adopts the PFC for the preliminary simulation verification.
Section snippets
Degradation test of gas adsorption on coal rock
A self-developed visual constant-volume gas-solid coupling test system (VCGCTS) is used in the test [17], and its schematic diagram and photo are shown in Fig. 1. Combined with the servo press and flexible chambers, the coupling loading chamber is able to keep constant volume and eliminate load errors due to inner pressure change of the chamber during the pressure loading process. Moreover, equipped with circumferential displacement testing device module and visual windows, the system can
Mechanical model of particle discrete element
PFC's research philosophy coincides with that of geo-mechanics physical model test. The object of the physical model test was a mixture of granular materials (yellow sand, quartz sand, barite powder, etc.) and cementing agent (gypsum, rosin, etc.), so based on the similarity principle, the mechanical behavior of rock and soil mass was simulated through physical test. The object of PFC research was the particle aggregation composed of the particles in certain shape & size and the contact, and
Determination of undetermined coefficients
The PFC2D5.00.21 was selected for particle flow code. The particle size and radius distribution of the tested briquette coal was adopted in the numerical model, as shown in Table 1. The LPBM was selected for the contact model between the particles, and the uniaxial compression test was performed on the specimens by controlling the movement of the upper and lower “wall” on the model, as shown in Fig. 9.
The LPBM was used to simulate the sodium humate cementing agent in the briquette coal
Conclusions
- (1)
Degradation tests of briquette in different equilibrium pressure of gas adsorption were carried out. From three aspects of tensile failure condition, shear failure condition and deformation property, the degradation causes were summarized through mechanical deduction, and the influence of free gas on briquette was eliminated. The strength properties of coal rock can be characterized by the tensile strength of crack tip and the cohesive force, and the deformation properties of coal rock can be
Declaration of Competing Interest
None.
Acknowledgements
This research was funded by National Natural Science Foundation of China (51427804, 41672281), Natural Science Foundation of Shandong Province (ZR2017MEE023,2019GSF111036).
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