Initial Desorption Characteristics of Gas in Tectonic Coal Under Vibration and Its Impact on Coal and Gas Outbursts

Abstract

The rapid desorption of gas in coal is an important cause of gas over-limit and outbursts. In order to explain the causes of coal and gas outbursts induced by vibration, this paper studies the gas desorption experiments of tectonic coal with different particle sizes and different adsorption equilibrium pressures under 0~50 Hz vibration. High-pressure mercury intrusion experiments were used to measure the changes in pore volume and specific surface area of tectonic coal before and after vibration, revealing the control of pore structure changes on the initial desorption capacity of gas. Additionally, from the perspective of energy transformation during coal and gas outbursts, the effect of vibration on the process of coal and gas outbursts in tectonic coal was analyzed. The results showed that tectonic coal has strong initial desorption capacity, desorbing 29.58% to 54.51% of the ultimate desorption volume within 10 min. Vibration with frequencies of 0~50 Hz increased both the gas desorption ratios and desorption volume as the frequency increased. The initial desorption rate also increased with the vibration frequency, and vibration can enhance the initial desorption capacity of tectonic coal and delay the attenuation of desorption rate. Vibration affected the changes in the initial gas desorption rate and desorption rate attenuation coefficient by increasing the pore volume and specific surface area, with the changes in macropores and mesopores primarily affecting the initial desorption rate and 0~10 min desorption ratios, while the changes in micropores and minipores mainly influenced the attenuation rate of the desorption rate. Vibration increased the free gas expansion energy of tectonic coal as the frequency increased. During the incubation and triggering processes of coal and gas outbursts, vibration has been observed to accelerate the fragmentation and destabilisation of the coal body, while simultaneously increasing the gas expansion energy to a point where it reaches the threshold energy necessary for coal transportation, thus inducing and triggering the coal and gas protrusion. The study results elucidate, from an energy perspective, the underlying mechanisms that facilitate the occurrence of coal and gas outbursts, providing theoretical guidance for coal and gas outburst prevention and mine safety production.

1. Introduction

Coal and gas outbursts are extremely complex gas dynamic disasters that occur in underground coal mines, with China accounting for 40% of the total global outburst incidents [1,2,3,4]. As the depth of mining increases, both the gas content and gas pressure in coal seams gradually increase, making coal and gas outbursts a persistent and significant hazard threatening safe coal extraction [5]. The mechanism of coal and gas outbursts essentially involves the coupling of multiple complex physical processes, such as coal mass failure, gas desorption, gas diffusion and seepage, coal transportation in the gas flow field, and two-phase flow propagation [6]. Studies have shown that rapid gas desorption in coal, caused by external disturbances, is a major trigger of gas over-limit and outbursts [7,8]. The hypothesis of the comprehensive effect of coal and gas outburst holds that outburst is the result of the combined action of in situ stress, gas in the coal seam, the physical and mechanical properties of coal, the microstructure and macrostructure of coal, the coal seam structure, and the self-gravity of coal [9]. The adsorbed and free gas in coal directly affects the mechanical properties of coal [10]. The erosion of gas will cause the overall weakening of the mechanical strength of coal, and the greater the gas pressure, the more obvious the deterioration of dynamic parameters [11,12]. The gas in coal provides expansion energy for the destruction, pulverisation, handling, and throwing of coal during the occurrence and development of coal and gas outbursts [13]. Among them, the rapid desorption of gas in pulverised coal can provide a lot of energy for the outburst process [9,14,15]. Therefore, studying the desorption characteristics of gas in coal is an important method for preventing coal and gas outbursts and for summarising the rules governing their occurrence.

The gas desorption and diffusion capacity of coal is influenced by a multitude of factors, which can be classified into two principal categories: internal and external. The internal factors are predominantly influenced by the intrinsic characteristics of the coal itself, such as the level of coalification [16,17,18]. The external factors are primarily determined by external environmental conditions, which can include factors such as temperature [19,20], pressure [21,22], physical fields [23,24,25,26], and moisture [27,28,29]. Prior to coal and gas outbursts, operations that generate vibrations, such as drilling, blasting, and coal cutting, are often involved [30]. Some scholars have suggested that vibration acts as the energy transfer mechanism necessary for coal fragmentation, gas desorption, and the ejection of gas–solid mixtures during the incubation and development stages of coal and gas outbursts [13]. As an important physical field, vibration inevitably influences the gas desorption characteristics of coal. Previous studies have shown that low-frequency mechanical vibration affects coal seam permeability [31], porosity [32], mechanical properties [33], and the gas adsorption–desorption capacity of coal [26,30,34,35]. It was found by Li et al. [34] that the mechanical vibration at 0~40 Hz increases the adsorption sites in coal, and as a consequence of this, the process of gas molecule desorption and diffusion is impeded. Shen et al. [26,30] reported that vibration alters the pore structure of coal, increasing pore volume, specific surface area, and connectivity, which reduces resistance to gas desorption and diffusion, and promotes gas desorption and diffusion in coal. Li’s [36] research indicated that after vibration, the changes in gas desorption and diffusion parameters in acid-treated granular coal were more pronounced compared to untreated coal, suggesting that vibration facilitates the desorption and diffusion of gas in acid-treated granular coal. Zhang [37] showed that vibration significantly enhances gas desorption in gas-bearing coal, increasing both the desorption volume and desorption rate. The closer the vibration frequency is to the natural frequency of the coal mass, the more effective the desorption process. Although these studies have extensively investigated the desorption characteristics of vibrated coal samples, they have not fully analysed the initial desorption characteristics of gas-bearing coal under vibration.

Following prolonged geological tectonism (faulting, folding, slip, etc.), the primary coal has undergone a range of fracturing processes, including brittle fracturing, crushing, ductile deformation, superposition failure, and even internal chemical composition and structural changes. This results in the formation of tectonic coal with typical low strength and low permeability characteristics, which provide favourable conditions for initiating coal and gas outbursts [38,39,40,41]. Accident statistics have shown that regions prone to coal and gas outbursts almost always exhibit varying thicknesses of tectonic coal, making tectonic coal a necessary condition and significant characteristic of coal and gas outbursts [13,42,43,44]. Compared with primary coal, tectonic coal has lower strength, more developed pore structures, weaker resistance to external stress disturbances, and a higher tendency to pulverise [42,45,46,47]. These factors result in desorption characteristics that differ from those of primary coal. In comparison to primary coal, tectonic coal exhibits a stronger gas adsorption–desorption capacity and a greater initial desorption–diffusion ability [21,48]. This phenomenon gives rise to a rapid rate of gas desorption during the early stages, which, in turn, facilitates the release of gas energy [5,46,47,49,50]. Consequently, tectonic coal is more prone to destruction and pulverisation, and rapid desorption can cause a large accumulation of free gas in a short period, increasing the risk of coal and gas outbursts [15]. According to the survey results of the outburst accident site, almost all of the outburst coal in the outburst accident site is highly broken or pulverised [49,51,52,53]. Studies have shown that most of the outburst coal is fine pulverised coal, which is not pulverised after outburst, but has been pulverised in the coal seam before outburst [14]. Therefore, the selection of granular coal as the research object can better explain the causes of vibration induced coal and gas outbursts. Therefore, it is essential to investigate the effect of vibration on the initial desorption capacity of particle tectonic coal in order to assess the risk of coal and gas outbursts in vibrated tectonic coal.

In conclusion, in order to elucidate the underlying causes of vibration-induced coal and gas outbursts from the standpoint of altered initial gas desorption capacity in vibration-affected tectonic coal, this paper employs representative tectonic coal to conduct isothermal desorption experiments under vibration. The focus is on analysing the changes in desorption parameters during the initial desorption period in vibrated tectonic coal. Furthermore, the study examines the impact of changes in the initial desorption capacity on the occurrence and development stages of coal and gas outbursts from an energy perspective. The results of this study are significant for clarifying the reasons why vibration induces coal and gas outbursts and can provide theoretical guidance for the prevention of coal and gas outbursts in mines.

2. Materials and Methods

2.1. Selection and Preparation of Coal Samples

The pore structure of coal samples with differing particle sizes varies, as does the migration resistance of gas in granular coal [48,54]. Studies have shown that the particle size of pulverised coal thrown out at the outburst site is mainly concentrated within 3 mm [55], and the particle coal in the range of 0.25~1 mm is more likely to be pulverised twice in the process of coal and gas outbursts [14]. Accordingly, the present study has elected to examine tectonic coal with particle sizes of 0.25 to 0.5 mm, 0.5 to 1 mm, and 1 to 3 mm as the subject of its inquiry. Tectonic coal samples were selected from the Wangxingzhuang coal mine (WXZ) in Henan Province. The Wangxingzhuang coal mine is situated within the Xinmi coalfield. As a result of tectonic movement, the entirety of the No. 2-1 coal seam has developed tectonic coal. The WXZ coal samples were procured from the 15,021 working face of the No. 2-1 coal seam. Fresh samples were sealed and transported to the laboratory, where they were subjected to crushing operations to produce particle sizing of 0.25 to 0.5 mm, 0.5 to 1 mm, and 1 to 3 mm. One kilogram of each particle size was prepared for subsequent adsorption–desorption experiments and pore structure analysis. Following the preparation of the coal samples, these were subjected to a 48 h drying process at 70 °C in a vacuum oven. Following this, the samples were cooled and then sealed for storage. Figure 1 illustrates the mesoscopic characteristics of the coal samples, while

Table 1. Basic physical parameters of tectonic coal sample.

Coal	Adsorption Constant		Components			${\mathit{\rho }}_{\mathit{T}\mathit{R}\mathit{D}}$ (g/m3)	${\mathit{\rho }}_{\mathit{A}\mathit{R}\mathit{D}}$ (g/m3)	${\mathit{\phi }}_{\mathit{p}\mathit{o}\mathit{r}\mathit{e}}$ (%)
	a (m3/t)	b (MPa−1)	Mad (%)	Ad (%)	Vdaf (%)
WXZ	26.32	2.74	0.88	7.73	13.81	1.43	1.38	3.19

presents a list of the basic physical parameters.

2.2. Methods

In order to study the gas desorption and diffusion characteristics of granular coal under mechanical vibration, a mechanical vibration isothermal adsorption–desorption platform (Figure 2) of the MVGAD-I type was used. Experiments of the isothermal desorption type were performed on samples of granular coal of differing particle sizes (0.25~0.5 mm, 0.5~1 mm, and 1~3 mm) under varying vibration frequencies, with the equilibrium pressure set at 1 MPa. To examine the differences in gas desorption characteristics under different vibration frequencies and equilibrium pressures, the coal samples with a particle size of 1~3 mm were selected for vibration desorption experiments at different equilibrium pressures (0.5 MPa, 1 MPa, and 1.5 MPa) according to the standard “Determination Method of Gas Desorption Index by Drill Cuttings” (AQ/T 1065-2008) [56]. The adsorbed gas is methane with a purity of 99.9%. The natural frequency of coal has been demonstrated to fall within the 50 Hz range [57], while the mechanical vibrations generated by drilling and tunnelling activities that result in gas-bearing coal seam outbursts have been shown to be below 50 Hz [34]. Additionally, the frequency range of microseismic vibrations has been established to lie between 0 and 35 Hz [30]. In light of the aforementioned vibration frequencies and the objective of this study, the selected experimental frequencies were 0, 25, and 50 Hz, and the vibration time lasted for 60 min. After the desorption experiments, coal samples of 1~3 mm in size underwent degassing treatment and were then subjected to high-pressure mercury intrusion experiments to measure pore structure.

3. Results

3.1. Characteristics of Initial Desorption of Gas in Vibrated Tectonic Coal

To investigate the initial desorption characteristics of gas in vibration-affected tectonic coal under the influence of different adsorption equilibrium pressures and particle sizes, the proportion of desorbed gas volume to the ultimate desorption volume was calculated for the first minute, 1~5 min, 5~10 min, 10~30 min, and 30~60 min to reflect the desorption capacity of the coal samples at different stages. Due to the rapid occurrence of coal and gas outbursts, which typically happen within a few minutes or even seconds, and given that studies indicate that the initial desorption capacity of tectonic coal is significantly greater than that of primary coal, allowing for the desorption of substantial amounts of gas within minutes, this study focuses on analysing the desorption characteristics of coal samples during the intervals of 0 to 1 min (Q_0–1min) and 0 to 10 min (Q_0–10min). The ultimate desorption volume of the coal samples can be calculated using Formula (1) [30]:

$${\mathrm{Q}}_{\infty }=\left({\displaystyle \frac{abP}{1+bP}}-{\displaystyle \frac{ab{P}_0}{1+b{P}_0}}\right)\left(1-W_f-A_f\right)$$

(1)

In Formula (1), the gas adsorption constant a represents the maximum adsorption amount of coal under the specified experimental conditions, mL/g. The value of b, also known as the gas adsorption constant, is MPa⁻¹. The moisture of the coal, designated as W_f is in %; the ash content is indicated by A_f, %. Additionally, P denotes the adsorption equilibrium pressure, while P₀ represents the atmospheric pressure at the given time. It is notable that P₀ is set to 0.1 MPa.

3.1.1. Initial Desorption Characteristics of Gas in Vibration-Affected Tectonic Coal with Different Particle Sizes

Figure 3 illustrates the gas desorption volume and desorption ratio characteristics of coal samples with different particle sizes under vibration. It can be observed from Figure 3 that the gas desorption volume of all coal samples increases monotonically over time, with the rate of increase gradually slowing down. The desorption volume is observed to decrease with increasing particle size for the same duration and vibration frequency. Additionally, the initial desorption volume is seen to increase at an accelerated rate with decreasing particle size. In the context of vibrations in the frequency range of 0–50 Hz, the desorption volume of all coal samples subjected to vibration is observed to exceed that of the non-vibrated samples. Moreover, a positive correlation is evident between the vibration frequency and the desorption volume. This suggests that mechanical vibration plays a facilitating role in the process of gas desorption. A significant rapid desorption process is evident during the initial desorption period of the coal samples. For the tectonic coal with a particle size of 0.25~0.5 mm, the desorption rate Q_{0–1 min} at the first minute under different vibration frequencies reaches 26.2~27%; within 10 min, it can desorb 50.66~54.51% of the ultimate desorption volume, with Q_0~10min at 25 Hz and 50 Hz being 1.03 and 1.08 times that of 0 Hz, respectively. For the tectonic coal with a particle size of 0.5~1 mm, the desorption rate Q_0−1min at the first minute reaches 19.1~22.9%; within 10 min, it can desorb 42.84~49.71% of the ultimate desorption volume, with Q_0−10min at 25 Hz and 50 Hz being 1.09 and 1.16 times that of 0 Hz, respectively. For the tectonic coal with a particle size of 1~3 mm, the desorption rate Q_0−1min at the first minute reaches 18.6–19.9%; within 10 min, it can desorb 31.51~34.67% of the ultimate desorption volume, with Q_0–10min at 25 Hz and 50 Hz being 1.05 and 1.12 times that of 0 Hz, respectively. The desorption rates of all vibrated coal samples within the first 10 min are higher than those of the non-vibrated samples. This suggests that vibration has the effect of enhancing the initial desorption capacity of coal, resulting in the desorption of greater quantities of free gas in a shorter time period. Consequently, there is an increased potential for accumulating larger quantities of gas energy.

To further understand the effect of vibration on the initial desorption capacity of tectonic coal, the desorption rate of vibrated tectonic coal within the first 0~10 min was analysed. The literature indicates that the gas desorption rate over time follows a power function relationship [58]:

$$V_t=V_0\cdot t^{-k}$$

(2)

where V_t is the desorption rate at time t, expressed as mL·g⁻¹·min⁻¹; V₀ is the initial desorption rate, expressed as mL·g⁻¹·min⁻¹; and k is the desorption rate attenuation coefficient. The fitting results of the desorption rate data for vibrated tectonic coal with different particle sizes over 0~10 min are shown in Figure 4.

The results presented in Figure 4 demonstrate that for coal samples subjected to the same vibration frequency, the initial desorption rate (V₀) exhibits a decline as the particle size increases. Conversely, for coal samples of identical particle size, the desorption rate displays an upward trend as the vibration frequency rises. The desorption rate remains relatively high during the initial 3 min, indicating that tectonic coal has a robust initial desorption capacity. The variation in initial desorption rate and desorption rate attenuation coefficient for vibrated coal samples of different particle sizes is illustrated in Figure 5.

Figure 4 and Figure 5 demonstrate that for coal samples at the same vibration frequency, the initial desorption rate decreases as the particle size increases. For coal samples with a particle size of 0.25~0.5 mm, the initial desorption rate at 25 Hz and 50 Hz is 1.01 and 1.03 times that of 0 Hz, respectively; for coal samples with a particle size of 0.5~1 mm, the initial desorption rate at 25 Hz and 50 Hz is 1.15 and 1.20 times that of 0 Hz, respectively; and for coal samples with a particle size of 1~3 mm, the initial desorption rate at 25 Hz and 50 Hz is 1.03 and 1.07 times that of 0 Hz, respectively. The initial desorption rates of all vibrated tectonic coal samples were observed to be higher than those of non-vibrated coal samples. Furthermore, an increase in frequency resulted in a corresponding increase in the desorption rate. In contrast, the desorption rate attenuation coefficient demonstrated a decrease with increasing vibration frequency. This finding suggests that vibration can delay the attenuation of the desorption rate. This evidence serves to reinforce the hypothesis that vibration increases the rate at which tectonic coal can desorb gas, leading to the desorption of greater quantities of gas in a shorter time period. Consequently, conditions are created by vibration that are favourable to the occurrence of both coal and gas outbursts. It thus follows that tectonic coal, which is susceptible to pulverisation, is more likely to experience instances of coal and gas outbursts under the influence of vibration. Furthermore, the occurrence of such outbursts is more probable when the particle sizes are reduced to a smaller size, as this increases the likelihood of reaching the critical conditions required for these outbursts to occur.

3.1.2. Initial Desorption Characteristics of Gas in Vibration-Affected Tectonic Coal Under Varying Adsorption Equilibrium Pressures

Adsorption equilibrium pressure is one of the key factors influencing gas desorption and diffusion. To study the effect of mechanical vibration on the gas desorption characteristics of coal samples under different adsorption equilibrium pressures, coal samples with a particle size of 1~3 mm were selected for vibration desorption experiments under equilibrium pressures of 0.5 MPa, 1 MPa, and 1.5 MPa.

Figure 6 demonstrates the gas desorption volume and desorption rate characteristics of vibration-affected tectonic coal under varying equilibrium pressures for adsorption. As shown in Figure 6, under the same vibration frequency, the cumulative gas desorption volume at the same time increases with the increase in adsorption equilibrium pressure. This is because the larger the adsorption equilibrium pressure, the stronger the concentration gradient driving rapid gas desorption and diffusion, resulting in more gas desorption within the same period. At adsorption equilibrium pressures of 0.5, 1, and 1.5 MPa, the desorption rates (Q_0~1min) of tectonic coal subjected to various vibration frequencies during the first minute were observed to be 12.78% to 23.42%, 18.6% to 19.9%, and 21.72% to 25.82%, respectively. Additionally, for coal samples at different adsorption equilibrium pressures exposed to vibrations at 25 Hz and 50 Hz, the desorption rates over the initial 10 min (Q_0~10min) were found to be 1.05 to 1.26 times and 1.12 to 1.45 times greater than those of the non-vibrated coal samples. Irrespective of the adsorption equilibrium pressure, the volume of gas desorbed and the rate of desorption of all coal samples subjected to vibration are greater than those of the non-vibrated coal samples. This provides further evidence that mechanical vibration enhances the desorption of gas from coal.

The fitting results of the desorption rate for vibration-affected tectonic coal under different equilibrium pressures over 0~10 min are shown in Figure 7. Figure 7 illustrates that for coal samples subjected to the same vibration frequency, the initial desorption rate is observed to increase in conjunction with the adsorption equilibrium pressure. Conversely, for coal samples subjected to the same equilibrium pressure, the initial desorption rate is observed to increase with an increase in vibration frequency.

Figure 8 shows the variation trends of initial desorption rate and desorption rate attenuation coefficient for vibrated coal samples under different adsorption equilibrium pressures. As evidenced in Figure 7 and Figure 8, the initial desorption rate of coal samples at a consistent vibration frequency exhibits a positive correlation with increasing equilibrium pressure. For coal samples at 0.5 MPa, the initial desorption rate at 25 Hz and 50 Hz is 1.46 and 1.79 times that of 0 Hz, respectively; for coal samples at 1 MPa, the initial desorption rate at 25 Hz and 50 Hz is 1.03 and 1.07 times that of 0 Hz, respectively; for coal samples at 1.5 MPa, the initial desorption rate at 25 Hz and 50 Hz is 1.12 and 1.19 times that of 0 Hz, respectively. The initial desorption rates of all vibrated coal samples are higher than those of non-vibrated coal samples, and this increase is observed with increasing frequency. Furthermore, it was found that the higher the equilibrium pressure, the more significant the effect of vibration on the initial desorption rate. The desorption rate attenuation coefficient of vibrated coal samples decreases with increasing frequency; the lower the equilibrium pressure, the more significant the effect of vibration on the desorption rate attenuation coefficient. This is because the greater the adsorption equilibrium pressure, the higher the gas adsorption volume and concentration in the coal pores, which results in greater initial desorption kinetics and slower changes in desorption rate attenuation, thereby making the effect of vibration on gas diffusion kinetics more apparent.

In conclusion, the results of the experimental investigation into the gas desorption of coal samples with varying particle sizes and adsorption equilibrium pressures demonstrate that tectonic coal displays a pronounced capacity for gas desorption during the initial desorption phase. Vibration enhances the desorption process in tectonic coal, leading to an increase in both desorption rate and desorption volume, having a particularly significant impact on desorption capacity within the first 10 min after desorption begins. Therefore, tectonic coal that has been subjected to vibration is more prone to accumulate free gas in a relatively shorter period, which consequently increases the expansion energy of the coal mass.

3.2. Impact of Vibration on the Pore Structure of Tectonic Coals

The pore structure of coal functions as a storage and migration space for gas, and pore structure variations exert a significant influence on gas desorption and diffusion characteristics in coal. To further explore the mechanism by which vibration influences the initial desorption capacity of tectonic coal, this study analyses the control effect of pore structure changes in vibrated tectonic coal on initial gas desorption capacity from a pore structure perspective. Quantitative characterisation of the pore volume and specific surface area of macropores (1000 < d < 10,000 nm), mesopores (100 < d < 1000 nm), minipores (10 < d < 100 nm), and micropores (d < 10 nm) in vibration-affected coal by high pressure mercury intrusion porosimetry was conducted according to the B.B. Hotdot pore classification method [13,59]. The results of the test are presented in Figure 9.

Figure 9 shows that the pore volume of macropores and mesopores dominates in all coal samples, while the specific surface area of micropores is predominant. The pore volume and specific surface area of all pore size categories in vibration-affected coal samples are larger than those in non-vibrated coal samples, and both increase with increasing vibration frequency. This indicates that vibration expands the pore structure of tectonic coal, resulting in an increase in pore volume and specific surface area, which enlarges the storage and migration space for gas in coal and promotes gas desorption and diffusion.

4. Discussion

4.1. Control Effect of Pore Structure Changes on Gas Desorption

In order to elucidate the underlying mechanisms responsible for the alteration in gas desorption capacity in tectonic coal resulting from vibration, this paper presents a detailed analysis from the perspective of pore structure. Correlations were established between the pore structure results (pore volume and specific surface area) of the macropores, mesopores, minipores, and micropores and the desorption parameters of the coal samples (Q_0–10min, Q_0–1min, initial desorption rate V₀, desorption rate attenuation coefficient k), with the fitting results displayed in Figure 10.

The results presented in Figure 10 demonstrate a positive correlation between Q_0–10min, Q_0–1min, and the initial desorption rate V₀ with the pore volume and specific surface area of each pore size category. Conversely, the desorption rate attenuation coefficient k exhibits a negative correlation with pore volume and specific surface area. The pore volume and specific surface area of macropores and mesopores exert a more pronounced influence on Q_0–10min, Q_0−1min, and V₀ than those of minipores and micropores. Conversely, the pore volume and specific surface area of micropores and minipores exert a more pronounced influence on k. This phenomenon can be attributed to the preferential desorption of gases in macropores and mesopores to the environment during the initial stages of desorption. Additionally, the resistance to the transport of free gases in macropores and mesopores is lower than that in minipores and micropores. As the desorption time progresses, the gas adsorbed in the minipores and micropores gradually desorbs and diffuses into the environment. Since gas molecules must pass through complex pore channels to diffuse from micropores to the environment, the larger the pore volume and specific surface area of micropores and minipores, the greater the connectivity of internal pores and the lower the complexity of pore channels, and the smaller the resistance to gas molecule diffusion, leading to a slower desorption rate attenuation. Vibration has been demonstrated to increase the pore volume and specific surface area of all pore size categories in tectonic coal. This enhances the initial desorption capacity and results in a higher initial desorption rate, which, in turn, leads to the accumulation of greater quantities of gas desorption.

4.2. Effect of Vibration on the Incubation and Triggering Stages of Coal and Gas Outbursts

4.2.1. The Energy Relationship in Coal and Gas Outbursts Process

The process of coal and gas outburst can be divided into four stages: inoculation, excitation, development, and termination [9,60,61]. Gas plays an important role in the whole process of outburst [9]. The energy conversion form in the process of outburst is mainly the elastic energy and gas expansion energy of coal into the crushing work and handling work of coal body, which follows the principle of energy conservation:

$${\mathrm{W}}_1+{\mathrm{W}}_2={\mathrm{W}}_3+{\mathrm{W}}_4+{\mathrm{W}}_5$$

(3)

where W₁ is the gas expansion energy, MJ; W₂ is the elastic energy of coal, MJ; W₃ is crushing work, MJ; W₄ is handling work, MJ; and W₅ is the residual kinetic energy of gas, MJ.

Zhao [13] and Wang [62] proposed that the crushing work is primarily derived from the elastic energy of the coal, while the expansion energy of the gas is mainly converted into the transportation work of the coal and the kinetic energy of the residual gas, as shown in:

$${\mathrm{W}}_1{=\mathrm{W}}_4{+\mathrm{W}}_5$$

(4)

The expansion energy of the gas in coal can be calculated using the following equation:

$${\mathrm{W}}_1={\displaystyle \frac{{\mathrm{p}}_{\mathrm{e}}{\mathrm{V}}_0}{\mathrm{n}-1}}\left[{\left({\displaystyle \frac{{\mathrm{p}}_1}{{\mathrm{p}}_{\mathrm{e}}}}\right)}^{{\displaystyle \frac{\mathrm{n}-1}{\mathrm{n}}}}-1\right]$$

(5)

where P_e is the gas pressure in the roadway after the coal is ejected, MPa; V₀ is the volume of gas involved in performing the work, m³; P₁ is the outburst gas pressure, in MPa; and n is the adiabatic coefficient, n = 1.31.

The gas involved in performing the work can be divided into two parts: free gas and desorbed gas during the outburst process. Thus, Equation (5) can be transformed into [62]:

$${\mathrm{W}}_1={\mathrm{W}}_{\mathrm{a}}+{\mathrm{W}}_{\mathrm{f}}={\displaystyle \frac{{\mathrm{p}}_{\mathrm{e}}}{\mathrm{n}-1}}\left({\mathrm{V}}_{\mathrm{a}}+{\mathrm{V}}_{\mathrm{f}}\right)\left[{\left({\displaystyle \frac{{\mathrm{p}}_1}{{\mathrm{p}}_{\mathrm{e}}}}\right)}^{{\displaystyle \frac{\mathrm{n}-1}{\mathrm{n}}}}-1\right]$$

(6)

where V_a and V_f represent the volumes of adsorbed gas and free gas involved in the work, respectively, m³; and W_a and W_f represent the energy contributed by the adsorbed gas and free gas, respectively, MJ.

In actual calculations, we can determine the original gas pressure and the volume of the original free gas, but it is difficult to determine the exact volume of gas involved in performing the work. Therefore, Equation (6) must be further transformed into:

$${\mathrm{W}}_1={\displaystyle \frac{{\mathrm{p}}_{\mathrm{e}}}{\mathrm{n}-1}}\left({\mathrm{V}}_{\mathrm{a}}^{\prime }+{\mathrm{V}}_{\mathrm{f}}^{\prime }\right){\left({\displaystyle \frac{{\mathrm{p}}_1}{{\mathrm{p}}_{\mathrm{e}}}}\right)}^{{\displaystyle \frac{1}{\mathrm{n}}}}\left[{\left({\displaystyle \frac{{\mathrm{p}}_1}{{\mathrm{p}}_{\mathrm{e}}}}\right)}^{{\displaystyle \frac{\mathrm{n}-1}{\mathrm{n}}}}-1\right]$$

(7)

where V_a′ and V_f′ represent the volumes of adsorbed gas and free gas before the outburst occurs, respectively.

The criterion for whether desorbed gas participates in the outburst work can be expressed as:

$$W_f<W_4+W_5$$

(8)

At this point, the adsorbed gas needs to desorb and convert into free gas to supplement the energy required. The energy that needs to be supplemented is:

$$W_{ad}=W_4+W_5-W_f$$

(9)

Based on the above analysis, when the expansion energy of the original free gas cannot meet the energy requirements of the outburst, the adsorbed gas must desorb to compensate. The rapid initial desorption of gas has been identified as a crucial factor in the genesis of coal and gas outbursts, with evidence suggesting that the energy imparted by the initial release of free gas inadequate to satisfy the energetic requirements of outbursts [15,63]. Jin et al. [64] found that the rapid desorption of gas in pulverised coal provides 73.3% to 89.95% of the energy required for outbursts. Thus, it is evident that newly desorbed gas provides the primary energy for coal and gas outbursts.

4.2.2. Gas Expansion Energy of Vibration-Affected Tectonic Coal

The results presented in Section 3.1 of this study demonstrate that tectonic coal exhibits robust initial desorption capacity. Furthermore, it has been observed that tectonic coal accumulates greater quantities of free gas in a relatively brief span of time when subjected to vibration, resulting in an augmented expansion energy of the coal mass. Therefore, granular coal formed after tectonic coal breakage is more likely to experience a sudden increase in gas energy under the influence of vibration, and it is easier to stimulate coal and gas outbursts, which become more likely. To accurately determine the incremental gas expansion energy generated by free gas in tectonic coal under vibration, it is necessary to use desorption experimental data and calculate the corresponding gas expansion energy based on Equation (5), in order to quantitatively analyse the role of the initial gas desorption capacity of tectonic coal, influenced by vibration, in promoting the occurrence and development of coal and gas outbursts.

The statistical evidence derived from a multitude of documented instances of coal and gas outbursts indicates that the duration of such occurrences is typically less than one minute. In most cases, the outbursts last from a few seconds to several tens of seconds [8,42,47,60,65]. This research paper presents a calculation of the gas expansion energy involved in the process of coal and gas outbursts for a variety of coal samples of varying particle sizes and equilibrium pressures, with the influence of different vibration frequencies taken into account. The following assumptions were made [62]: (1) The duration of the coal and gas outbursts is assumed to be 0.5 min; (2) the gas desorbed in the initial 0.5 min of the desorption phase is assumed to be the sole gas involved in the outburst; (3) the environmental gas pressure is assumed to be atmospheric pressure, and the gas pressure prior to the coal and gas outbursts is assumed to be the adsorption equilibrium pressure. The results of the calculated gas expansion energy are presented in Figure 11.

As illustrated in Figure 11, under the equilibrium pressure conditions of 1 MPa and consistent particle size, the gas expansion energy of coal samples with particle sizes of 0.25~0.5 mm, subjected to vibrations at 25 Hz and 50 Hz, increased from 0.4480 to 0.5210 J/g, representing an enhancement of 8.62% to 16.29%, respectively, in comparison to the 0 Hz samples. The gas expansion energy of coal samples with particle sizes of 0.5~1 mm exhibited an increase from 0.3640 J/g to 0.4476 J/g, representing an 8.96% to 22.97% rise. In the case of coal samples with particle sizes of 1~3 mm, the gas expansion energy increased from 0.2961 J/g to 0.3496 J/g under similar conditions, reflecting an increase of 12.39% to 18.07%. Additionally, for coal samples measuring between 1 and 3 mm in size, when subjected to an equilibrium adsorption pressure of 0.5 MPa, there was a considerable increase in gas expansion energy. From 0.1606 J/g to 0.3139 J/g, a 51.81% to 95.45% rise was observed. The gas expansion energy exhibited a comparable increase from 0.4361 J/g to 0.5170 J/g at the adsorption equilibrium pressure of 1.5 MPa. This corresponds to a notable 12.08% to 18.55% enhancement, respectively.

The above results indicate that, regardless of particle size and equilibrium pressure during adsorption, the energy of gas expansion rises with increasing frequency. After the coal mass is broken into granular coal during the incubation and development stages of an outburst, vibration significantly increases the energy of free gas expansion. This indicates that tectonic coal, which has high gas pressure and is susceptible to fracturing and pulverisation, can generate greater gas expansion energy within a shorter timeframe when subjected to mechanical vibration, thereby facilitating the fulfilment of the outburst triggering criteria and consequently inducing an outburst. Furthermore, elevated gas expansion energy will facilitate enhanced transportation of the pulverised coal throughout the outburst development process, resulting in a proportional increase in outburst intensity and severity. This provides a partial explanation for why mechanical vibration represents a significant external factor contributing to the occurrence of coal and gas outbursts.

4.3. The Role of Vibration in Triggering Coal and Gas Outbursts in Tectonic Coal Seams

Tectonic coal has lower mechanical strength than primary coal and is more susceptible to pulverisation under external forces. During the incubation stage of outbursts, the original tectonic coal is subjected to ground stress, leading to compressive deformation, shear, and fracturing, resulting in the formation of a fractured coal mass region. The original structure is damaged, and initial gas desorption occurs. When the fractured tectonic coal deposits in this region are subjected to mechanical vibration, they continue to undergo damage and breakage due to the combined effects of ground stress and vibration stress. This process leads to the formation of smaller coal particles. Concurrently, mechanical vibration facilitates the desorption of gas from these coal particles, resulting in a rapid increase and accumulation of free gas, thereby providing augmented gas expansion energy to precipitate coal and gas outbursts. As the amount of desorbed gas increases, the gas expansion energy gradually rises. When the gas expansion energy reaches the energy threshold required to transport the coal mass and the particle size of the broken coal mass decreases to the critical average size for outbursts, the crushed coal mass rapidly loses its stability under the external disturbance caused by mechanical vibration. The significant amounts of free gas can form a high-rate gas flow that carries the fractured and pulverised coal mass, completing the ejection work and triggering a coal and gas outburst. Therefore, mechanical vibration plays a role in both inducing and triggering coal and gas outbursts, as shown in Figure 12.

In this paper, only one kind of tectonic coal is used, and the difference between different tectonic briquettes (cataclastic coal, granulitic coal, mylonitic coal) is not considered. The evolution characteristics of pore structure and the variation characteristics of adsorption and desorption capacity of different types of tectonic coal under vibration should be further studied. Then, using fracture mechanics, gas adsorption theory, surface physical chemistry and other theories, the mechanism of the change of pore structure, adsorption and desorption capacity and diffusion capacity of tectonic coal caused by vibration will be deeply explored and clarified. Based on the above research results, the theory and method of increasing the output of coalbed methane by means of vibration excitation are sought to promote the efficient extraction of coalbed methane.

5. Conclusions

(1)

The desorption experiments on tectonic coal under vibration show that tectonic coal has a high initial desorption capacity, with 29.58~54.51% of the ultimate desorption volume being desorbed in the first 10 min. Additionally, the smaller the particle size and the greater the adsorption equilibrium pressure, the higher the desorption ratios in the first 10 min. Regardless of particle size and adsorption equilibrium pressure, vibration at frequencies of 0~50 Hz led to increased gas desorption rate and volume as the frequency increased, indicating that vibration promotes gas desorption in tectonic coal.

(2)

The initial desorption rate of tectonic coal increases as the particle size decreases and adsorption equilibrium pressure increases. For coal samples of different particle sizes and adsorption equilibrium pressures, the initial desorption rate after vibration was 1.01~1.2 times and 1.03~1.79 times that of the non-vibrated samples, respectively. Furthermore, the initial desorption rate demonstrated a positive correlation with vibration frequency. Furthermore, vibration also resulted in a delay in the attenuation of the desorption rate, as evidenced by a decrease in the desorption rate attenuation coefficient with increasing vibration frequency. Vibration has been demonstrated to enhance the initial desorption capacity of tectonic coal, thereby facilitating the rapid desorption of substantial quantities of free gas within a relatively brief period following the onset of desorption.

(3)

The results of the high-pressure mercury intrusion experiments on vibration-affected tectonic coal demonstrate that vibration causes pore expansion in tectonic coal. This leads to an increase in pore volume and specific surface area, which subsequently affects the initial gas desorption rate and desorption rate attenuation coefficient. The desorption rate in the first minute, the desorption rate within 10 min, and the initial desorption rate are positively correlated with the pore volume and specific surface area of each pore size in coal, with the influence of macropores and mesopores being greater than that of minipores and micropores. The desorption rate attenuation coefficient k is negatively correlated with pore volume and specific surface area, with minipores and micropores having a greater influence than macropores and mesopores.

(4)

Vibration causes the free gas expansion energy of tectonic coal to increase with vibration frequency. During the incubation and development stages of an outburst, when the tectonic coal mass is broken into granular coal and subjected to vibration, it may more easily reach the critical energy needed to trigger an outburst due to the generation of higher gas expansion energy within a short period. In the incubation and triggering process of coal and gas outbursts, vibration accelerates coal mass fragmentation and destabilisation to a certain extent, promotes the rapid desorption of gas from the coal mass, and leads to the accumulation of more gas expansion energy, thus causing the gas expansion energy of the broken tectonic coal mass to reach the energy threshold required for transporting the coal mass, thereby inducing and triggering coal and gas outbursts.