3.1 Mechanism of gas outburst
People's understanding of the mechanism of outburst is basically consistent with the theory of comprehensive action, and it is generally believed that outburst is the result of the joint action of geological stress, gas, and the physical and mechanical properties of coal. There is also a certain basic pattern of coal and gas outburst in China: (1) The risk of outburst increases with the increase of burial depth. The basic law of coal and gas outburst is that the intensity and frequency of outburst increase with the increase of burial depth, and the initial outburst depth of different outburst coal seams and mines also varies. (2) The danger of outburst increases with the increase of coal seam thickness, and the thicker the coal seam, the greater the danger, manifested in an increase in outburst frequency, intensity, and shallow initial outburst depth. (3) The protrusion is related to the roadway and operation mode. China has the highest number of outburst occurrences in coal seam alleys, and the strongest outburst intensity is observed when exposing coal through crosscuts. (4) Before the outburst, there are mostly signs, such as the sound of thunderbolts, muffled thunder, disordered coal seam bedding, and darkened coal color in the coal body. (5) Most of the protrusions occur in geological structural zones, such as compressive and compressive shear fault zones, syncline axis zones, coal seam bifurcation zones, coal seam and coal seam thickness variation zones, magma intrusion zones, etc., which are all structural areas where protrusions are prone to occur. After understanding the mechanism and basic laws of outburst, suitable prediction indicators can be better selected to reflect the outburst risk of outburst coal seams, and special treatment should be given in geological structural zones to achieve comprehensive and accurate regional prediction. Due to significant differences in the degree of metamorphism and types of damage of coal in different outburst coal seams, the adsorption capacity of coal to gas also varies greatly. Therefore, when predicting gas outburst risk areas, the critical value suitable for the coal seam conditions should be determined based on the adsorption characteristics of the coal seam to avoid the occurrence of low index outburst accidents.
3.2 Differences in adsorption characteristics of coal seams of different coal ranks
The research shows that the factors affecting the gas adsorption capacity of coal seams are: coal rank, water content, temperature, etc. The coal rank is the control factor affecting the gas adsorption capacity of coal seams. The higher the degree of coal metamorphism, the greater the amount of gas generated. Under the same temperature and pressure conditions, coal seams with higher degree of coal metamorphism often contain more gas.
Many scholars have conducted extensive discussions on the methane adsorption characteristics of coal with different degrees of metamorphism. Liu(30) conducted statistical analysis on the adsorption constant test results of hundreds of dry coal samples in China. The analysis showed that the methane adsorption amount showed a "U" shaped arc with the increase of coal rank, and the methane adsorption amount was the lowest in the coking and fertile coal stages. Li(31) studied and analyzed 8 types of coal samples with different metamorphic degrees, and there were significant differences in the adsorption capacity of different coal rank coal samples; As the degree of deterioration deepens, the adsorption capacity shows a "U" pattern of first decreasing and then increasing, which is consistent with the conclusion drawn by Chen(32); Liu(33) selected anthracite, coking coal, gas coal, and long flame coal for mercury intrusion and isothermal adsorption experiments. The experimental results showed that as the degree of coal metamorphism increased, its maximum adsorption capacity showed an overall trend of decreasing increasing.
The essence of the influence of coal rank on adsorption performance is that the proportion of various pores inside the coal body is different. The higher the degree of coal metamorphism, the more developed the pore structure, especially small pores and micropores. As the coal rank increases, the pore structure in coal shows regular changes. The large pores and mesopores in coal gradually close, while the small pores and micropores gradually increase. A large number of micropores and micropores provide more adsorption space for gas, improving the adsorption capacity of coal(32). Zhang(34) conducted mercury intrusion tests on dry coal samples of long flame coal, gas coal, and anthracite to obtain pore structure and connectivity characteristics. The results showed that the gas adsorption capacity of coal is not only related to pore structure, but also to pore connectivity of coal. Coal with low degree of metamorphism has good pore connectivity, while coal with high degree of metamorphism has poor pore connectivity, and conditions with poor pore connectivity are more likely to adsorb gas.
3.3 The optimal characterization index of gas expansion energy
Research has found that the process of coal and gas outburst is not an adiabatic process, but a variable process close to an adiabatic process, and a gas expansion energy formula has been proposed (35):
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Where: W is the gas expansion energy, J/g; P0 is the initial gas pressure, MPa; P is the final gas pressure, MPa; N is the polytropic index of gas, dimensionless, taken as 1.31.
According to Eq. (2), there is a positive correlation between gas expansion energy and coal seam gas pressure, that is, the higher the coal seam gas pressure, the greater the gas expansion energy.
Many scholars have conducted research on the relationship between gas expansion energy and critical indicators for outburst prediction through various methods. Yu(36) calculated the gas expansion energy released by drilling holes based on the theoretical formulas for coal wall release and gas expansion energy released by drilling holes, and analyzed its influencing factors. They believe that the influence of gas pressure is the greatest, followed by the influence of permeability coefficient, and the influence of gas content is the smallest. Jiang(37) conducted relevant experiments by independently developing a gas expansion energy measurement device. The results showed that gas expansion energy is closely related to factors such as coal fractures, gas pressure, temperature, and water content, and a linear relationship between gas expansion energy and gas pressure was measured. Jiang(38) studied the energy dissipation law during coal and gas outburst, and proposed that the initial gas expansion energy released by the coal itself plays a decisive role in the outburst process. The size of the initial gas expansion energy is mainly controlled by the initial desorption characteristics of the coal gas. The higher the gas pressure, the greater the free gas content and initial gas desorption amount in the coal, resulting in a larger initial gas expansion energy released. Liu(39) studied the relationship between gas expansion energy of different metamorphic degrees and outburst prediction indicators, and proposed that the free gas expansion energy of coal is positively correlated with gas pressure and pore volume.
In summary, free gas plays a major role in the triggering stage of outburst, and the expansion energy of free gas is the main energy that constitutes the initial release of gas expansion energy, which is closely related to coal seam gas pressure. The "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" also emphasizes the priority use of gas pressure indicators. Therefore, when predicting the risk of regional outburst, it can be inferred that gas pressure indicators are more sensitive than gas content.
During the geological process, tectonic movements have transformed the primary coal seams, altered the distribution of ground stress and gas occurrence, destroyed the microstructure of the coal body, changed the physical and mechanical properties of the coal body, and accompanied the production of structural coal. Before and after coal seam mining, due to the effect of geological stress, the strength of coal decreases, leading to coal fragmentation. The coal seam state changes from originally blocky coal to many small particle coal, promoting the desorption of gas and providing dynamic support for coal and gas outburst. The strength coefficient of coal reflects the relative strength of the coal itself and is one of the important parameters affecting coal and gas outburst. In general, the larger the solidity coefficient of coal, the greater the internal energy required for outburst occurrence.
Therefore, at a certain mining depth, the solidity coefficient of coal varies, and there are also differences in the gas pressure of the coal seams where outburst occurs. However, the "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" stipulate the critical value of the solidity coefficient of coal (f ≤ 0.5). In the prediction of outburst danger in the structural zone, it is only emphasized that the gas pressure must be less than 0.74MPa and the gas content must be less than 6m3/t in order to be predicted as a non outburst danger zone, The corresponding relationship between the strength coefficient of coal and the pressure required for gas outburst was not considered.