The prediction of coal and gas outburst areas is the leading link of the two "four in one" comprehensive outburst prevention measures for gas prevention and control. The accurate determination of prediction indicators and critical values is of great significance for ensuring coal mine safety production and reducing the cost of outburst prevention work. The critical indicators for regional prediction specified in the "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" are gas pressure of 0.74MPa and gas content of 8m3/t. However, there is no corresponding relationship between the critical values of regional prediction indicators for coal seams with different metamorphic degrees. The current gas outburst regional prediction indicator system does not provide corresponding regional prediction indicators and critical values based on different coal qualities. This article presents the critical values of gas content corresponding to the gas pressure of 0.74MPa for the regional prediction index based on the statistics of gas basic parameters of coal mines with different degrees of metamorphism and different regions, combined with the coal rank classification standards. This provides a basis for the determination of the gas content for the regional prediction index of outburst coal seams with different degrees of metamorphism.
Research Article
Discussion on Technical Defects and Improvement of the Current Prediction Index System for Gas Outburst Risk Areas
https://doi.org/10.21203/rs.3.rs-3423406/v1
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gas outburst
Regional prediction
Technical defects
Improvement exploration
The prediction of coal and gas outburst risk areas is the leading link of the two "four in one" aspects in gas prevention and control. The determination of regional prediction indicators and critical values is the top priority of outburst prediction work. The "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" stipulate that coal seam gas pressure and gas content are predictive indicators for coal and gas outburst areas, and provide critical values of 0.74MPa and 8m3/t, respectively. Due to the comprehensive effect of multiple influencing factors on coal and gas outburst, there are differences in the influencing factors of coal seam outburst under different coal qualities and geological conditions, resulting in different applicability of regional prediction indicators and critical values(1). If the regional prediction indicators and critical values required in the "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" are adopted indiscriminately, it may reduce the efficiency of gas outburst area prediction, increase the risk of outburst accidents, and increase the cost of coal mining. Therefore, studying and determining regional outburst risk prediction indicators and critical values that are in line with the actual situation of the mine is of great significance for gas prevention and control work in the mine.
When using coal seam gas pressure as a regional prediction indicator, the critical value of 0.74MPa specified in the "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" is commonly used or determined through empirical formulas. Currently, it is widely believed that using gas pressure of 0.74MPa as a prediction indicator for outburst prevention areas is feasible and reliable(2). The commonly used empirical formulas are: the empirical formulas of the former Soviet Union, Beipiao Mining Bureau, and Chongqing Coal Research Institute. The minimum solidity coefficient and volatile matter of the predicted coal seam in the area can be substituted into the empirical formula to obtain the critical gas pressure value(3).
Many scholars have conducted extensive research on the critical value of coal seam gas content in regional prediction indicators. The main method adopted is to determine the critical value of gas content through laboratory experiments and empirical formula methods, and finally verify the rationality of the critical value of gas content on site. Zhang(3) used historical data statistics and empirical formula laboratory methods to determine the critical value of gas content for the regional prediction index of No. 3 anthracite in Zhongling Coal Mine, which is 10m3/t, through on-site verification; Liu(4) proposed that the critical value of coal seam gas content for regional prediction of lean coal in the No. 21 coal seam of Xin'an Coal Mine is 9m3/t; Deng(5) determined that the predicted gas content in the South Second Well area of Liziya Coal Mine is 7m3/t; Sun(6) clarified in the adsorption and desorption laws and applications of soft and hard coal of different coal ranks that the regional prediction index for gas rich coal in Pingmei No.1 Mine is the coal seam gas content of 5m3/t. It is generally believed that the presence of a certain thickness of structural coal is a necessary condition for outburst occurrence (7), and research on the physical characteristics, thickness, and other aspects of structural coal has received widespread attention. Wang(8) studied the energy conditions during the occurrence of outburst and calculated the critical gas content during outburst under different coal body Proctor coefficients f and in-situ stress conditions. Jia(9) explored the calculation method of critical gas pressure for outburst occurrence under different combinations of coal body structures, and believed that there is a good regularity in the relationship between critical gas pressure and the proportion of soft coal. Zhang(10) believe that the magnitude of the intermolecular forces between structural coal molecules determines the low strength and high adsorption energy of structural coal, thereby controlling the occurrence of outburst disasters. Hu(11) studied the characteristics of different types of structural coal and their control effects on gas outburst, and believed that the development areas of crushed coal, flake coal, and mylonite coal are prone to gas outburst. From the research of the above scholars, it can be concluded that when the relative gas pressure is 0.74MPa, there are differences in the gas content corresponding to coal seams with different degrees of metamorphism. However, the coal seam gas content and critical values in the "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" are not given separately based on coal quality, and there are no corresponding provisions for regional prediction indicators and critical values under different coal body strengths in the structural belt.
Therefore, this article explores the technical shortcomings of the regional prediction system for gas outburst risk, clarifies the non corresponding relationship between the regional prediction indicators of coal seam gas pressure and gas content, and analyzes the reasons for it. The current regional prediction indicator system for gas outburst risk is improved and explored from two aspects: the degree of coal seam metamorphism and coal body strength.
2.1 Prediction index system for gas outburst areas
2.1.1 Regional prediction indicators and critical values specified in the "Detailed Rules for the Prevention and Control of Coal and Gas Outbursts"
The "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" propose that the method of combining coal seam gas parameters with gas geological analysis should be used to predict the risk area of gas outburst, namely: (1) the coal seam gas weathering zone is a non outburst risk area; (2) If the coal seam gas occurrence characteristics, geological structure conditions, outburst distribution patterns, and the detection and prediction results of the coal seam geological structure in the mined area have been accurately grasped, gas geological analysis methods should be used to divide the outburst danger zone. If the outburst area is directly related to the structure, the extension position of the structure and the coal seams on both sides within a certain range are the outburst danger zone; (3) The prediction should be based on the coal seam gas pressure P and the coal seam gas content W in the areas outside the non outburst danger zone classified in the first item and the outburst danger zone classified in the second item. The predicted critical value must be determined based on experimental investigation, and the predicted critical value in Table 1 can be temporarily referred to before determination(12).
|
Gas pressure/MPa |
Gas content/(m3·t − 1) |
Regional category |
|---|---|---|
|
P＜0.74 |
W＜8(Structural beltW＜6) |
No outbursting danger zone |
|
Other situations besides the above |
Outbursting danger zone |
|
2.1.2 Regional prediction indicators and critical values implemented in some provinces
In order to strengthen coal mine safety production measures, effectively prevent and curb the occurrence of major accidents in coal mines, some provinces in China have organized and formulated corresponding methods and regulations in accordance with relevant regulations and standards, and combined with the actual situation of coal mines in their respective provinces.
Shaanxi Province(13) stipulates that when conducting regional prediction for outburst mines, the coal seam gas pressure P < 0.74MPa or the coal seam gas content < 8m3/t (structural zone < 6m3/t) in the area, and there are no other abnormal phenomena such as spray holes and top drilling during the drilling construction process, can the prediction be considered as non hazardous.
Henan Province(14–15) has issued the "Interim Provisions on Strengthening Coal Mine Safety Production in Henan Province" and the "Ten Measures for Preventing and Controlling Coal and Gas Outbursts in Coal Mines in Henan Province", which stipulate that the critical value indicators of parameters for predicting the risk of coal and gas outburst in mining areas should be determined by the chief engineer of coal enterprises in accordance with the "Detailed Rules for Preventing and Controlling Coal and Gas Outbursts", and submitted to the chairman of coal enterprises for signature, And proposed the "double six indicators" for predicting the critical value of the mine area, namely: the coal seam gas pressure is less than 0.6MPa and the gas content is less than 6m3/t, in order to be classified as a non outburst danger zone.
Hebei Province(16) published the Measures for Comprehensive Management of Coal Mine Gas in Hebei Province, which proposed that the regional prediction indicators, coal seam gas pressure P and coal seam gas content W, should be below the critical value of the indicators actually inspected by the mine. Those not inspected can be predicted as the coal seam gas pressure is less than 0.6MPa and the coal seam gas content is less than 6m3/t, and there are no other abnormalities such as blowholes and top drilling in the coal seam during the test construction process, Then the area is a non protruding danger zone.
Anhui Province(17) has issued the "Anhui Province Measures for Comprehensive Control and Utilization of Coal Mine Gas", which stipulates that when the minimum normal distance between the roadway and the outburst coal seam is less than 7 meters, the measured coal seam gas pressure is less than 0.5MPa, and the coal seam gas content is less than 5m3/t. If there are no abnormal phenomena such as spray holes and top drilling during the testing and construction process, the area is considered a non outburst danger zone.
2.1.3 Regional prediction indicators and critical values for some mining areas after investigation
Scholars have conducted a lot of research on the prediction of regional outburst hazards in different mining areas and coal seams, and have clearly pointed out that there is a mismatch between the two indicators of regional prediction of coal seam gas pressure and coal seam gas content specified in the "Detailed Rules for the Prevention and Control of Coal and Gas Outburst". Based on actual investigations in mining areas, the critical value of coal seam gas content corresponding to the coal seam gas pressure of 0.74MPa, a regional prediction indicator, has been analyzed and determined. Chen(18) took the No.3 anthracite coal from Daning Coal Mine in Jincheng Mining Area as the research object, and measured the solidity coefficient and adsorption constant of the coal sample in the laboratory. Using the empirical formula of critical gas pressure and solidity coefficient and Langmuir equation, they proposed a regional prediction of coal seam gas pressure of 0.74MPa, corresponding to a critical value of 10.35m3/t of coal seam gas content; Deng[5] took the K1 lean coal in the South Second Well of Liziya Coal Mine as the research object, and used the methods of laboratory measurement of gas basic parameters and on-site experiments, combined with historical data statistical analysis, to conclude that regional prediction should be based on gas pressure P less than 0.74MPa and coal seam gas content W less than 7m3/t.
2.2 Technical defects of prediction indicators for gas outburst areas
Research has shown that there are significant differences in the degree of metamorphism and types of damage of coal in different outburst coal seams, and the adsorption capacity of coal for gas also varies greatly(19–20). Therefore, the critical value for predicting gas content in gas outburst areas cannot be generalized, but should be determined based on the adsorption characteristics of coal to avoid the occurrence of low index outburst accidents.
2.2.1 The selection of critical values for prediction indicators does not take into account the influence of degree of metamorphism
Coal seam gas pressure and coal seam gas content are two important basic parameters that characterize the occurrence characteristics of coal seam gas, and are also important indicators for predicting coal and gas outburst areas. There is a certain relationship between the two, which can be converted to each other through the Langmuir equation, as shown in Eq. (1). The "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" propose that the critical value of the coal seam gas pressure index P is 0.74MPa, and the critical value of the coal seam gas content index W is 8m3/t (structural zone 6m3/t). However, the gas pressure and gas content of different regions and metamorphic coal seams are not corresponding, that is, the reference critical value of gas pressure (0.74MPa) is not the corresponding reference critical value of gas content (8m3/t or 6m3/t).
Where: W is the coal seam gas content, m3/t; P is the coal seam gas pressure, MPa; a, b is the adsorption constant, m3/t, MPa − 1; Ad is the ash content, %; Mad is the moisture content, %; K is the porosity of coal, m3/m3; γ is the apparent density of coal, t/m3.
|
Coal rank |
Coal sample source |
Adsorption constant |
K/ % |
γ/ (t·m − 3) |
Industrial analysis of Coal |
Gas content corresponding to 0.74MPa/(m3/t) |
Gas pressure corresponding to 8m3·t − 1/(MPa) |
|||
|---|---|---|---|---|---|---|---|---|---|---|
|
a/ (m3·t − 1) |
b/ (MPa − 1) |
Mad/% |
Aad/% |
Vdaf/% |
||||||
|
gas coal |
Zouzhuang Coal Mine |
20.65 |
0.35 |
7.86 |
1.29 |
1.15 |
10.36 |
36.06 |
3.22 |
2.60 |
|
fat coal |
Kongzhuang Coal Mine |
17.15 |
1.06 |
2.88 |
1.39 |
1.59 |
10.25 |
32.36 |
4.62 |
2.53 |
|
coking coal |
Pingdingshan No.5 Mine |
25.02 |
0.68 |
8.33 |
1.32 |
1.12 |
12.57 |
20.64 |
5.83 |
1.21 |
|
Meagre coal |
Xiangshui Coal Mine |
26.12 |
1.32 |
3.42 |
1.41 |
1.06 |
10.04 |
14.35 |
8.82 |
0.68 |
|
Anthracite coal |
Lujiacun Coal Mine |
34.44 |
1.14 |
3.55 |
1.36 |
0.6 |
8.71 |
10.32 |
12.24 |
0.38 |
Select the gas basic parameter data of 5 different coal mines and coal seams with different metamorphic degrees as shown in Table 2, and convert the corresponding gas content and gas pressure according to Eq. (1) Langmuir Eq. (21–24). From Table 2, it can be seen that the gas pressure and gas content converted according to Eq. (1) are closely related to the degree of coal metamorphism. That is, as the degree of metamorphism increases, the gas content corresponding to gas pressure of 0.74MPa gradually increases, and the gas pressure corresponding to gas content of 8m3/t shows a decreasing trend. According to the relationship between gas pressure, gas content, and degree of metamorphism, anthracite with higher degree of metamorphism is characterized by low gas pressure and high gas content coal seams, while gas rich coal with lower degree of metamorphism is characterized by high gas pressure and low gas content coal seams. If we do not consider the degree of coal seam metamorphism and directly use the gas pressure indicators in the "Detailed Rules for the Prevention and Control of Coal and Gas Outburst", the "Measures" and "Regulations" formulated by various provinces as the critical values, it will greatly increase the cost of gas control, the amount of outburst prevention work, and the period of outburst elimination in low metamorphic coal mines. However, high metamorphic coal mines may have blind spots in predicting gas outburst areas, increasing the risk of gas outburst.
2.2.2 Unreasonable selection of prediction indicators
Coal and gas outburst accidents include gas dominated outburst, geostress dominated outburst, and mixed outburst dominated by both gas and geostress. It is generally believed that coal and gas outburst is the result of a comprehensive effect of factors such as geostress, gas, and the inherent properties of coal. At present, the selection of regional prediction indicators is mostly related to gas, and there are few indicators that reflect in-situ stress. Ground stress is the power and energy source of middling coal rock failure in the process of outburst. However, due to the difficulty in measuring ground stress, it is still difficult to take stress as a quantitative outburst prediction index. Research has shown that in-situ stress increases with the increase of mining depth, with lateral stress greater than vertical stress. Therefore, the mining depth can be used as a predictive indicator to reflect in-situ stress. However, research on the depth of initial outburst in many domestic and foreign mines has shown that different coal seams have different depths of initial outburst, and there is no universal prediction index for the depth of initial outburst. At the same time, it indicates that tectonic stress may be one of the important factors affecting the distribution of outburst areas(25).
Related research and statistical data indicate that(26–27) geological structure is the dominant factor controlling the distribution of coal and gas outburst, and coal and gas outburst almost always follows areas with strong deformation along translational, reverse, or normal faults. Tectonic movements cause coal seams to be destroyed into fragmented coal, granular coal, or mylonite coal, resulting in a decrease in coal strength and the formation of a certain thickness of soft coal zone, creating necessary conditions for outburst and facilitating the accumulation of high-pressure gas. Under the influence of geological structure, there are coal thickness variation zones in local areas of the coal seam in the mine, where stress and energy are concentrated. Compared with the surrounding coal bodies of the thickening zone, the gas content in the coal seam generally increases, with a maximum increase of more than twice.
Lei(28) constructed an energy equation for the outburst process with variables such as coal seam burial depth, gas pressure, and coal body solidity coefficient, verifying that under low gas conditions in the outburst source area, abnormal coal seam gas pressure and coal body solidity coefficient play a dominant role. Sun(29) found in his research that a large number of coal seam units are prone to instability due to disturbance, rapid gas desorption, and an instantaneous increase in coal seam gas pressure, which can reach a relatively high level. He named this phenomenon "gas explosion". For the coal seam at the structural location, due to the low solidity coefficient of the coal seam, the anti instability performance of the coal seam unit is poor, and it is easy to be disturbed and cause coal and gas outburst. For structural coal in low gas mines, as the mining depth increases, the solidity coefficient decreases, and the gas pressure required for outburst also decreases accordingly. If the solidity coefficient of the constructed coal is not considered and the gas pressure of 0.74MPa specified in the "Detailed Rules for Preventing Coal and Gas Outburst" is directly used as the critical value, it will lead to incorrect prediction of low gas mines and increase the risk of low gas mine outburst. High gas, coal and gas outburst mines require a considerable amount of anti outburst engineering and anti outburst period investment, seriously affecting normal underground production.
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):
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.
Research has found that the degree of coal metamorphism and coal strength have a significant impact on gas outburst. Usually, the higher the degree of coal metamorphism, the higher the gas content; The lower the strength of the coal body, the higher the risk of gas outburst. Based on the above analysis of the technical defects and reasons for the main indicators of regional prediction, it can be seen that there is no clear division in the regional prediction indicator system. Therefore, this article will improve from these two aspects.
4.1 The degree of coal metamorphism
4.1 Classification of coal rank
Classification of coal rank Industrial analysis of coal quality is a common method widely used to determine the basic composition of coal samples. The data results obtained from experiments play an important role in evaluating coal quality characteristics and classifying coal rank, and are of great significance. The coal rank of coal samples can be divided based on the volatile content of coal measured in industrial analysis experiments. The coal rank classification standards(40) are shown in Table 3.
|
Coal rank |
Volatile substance range(%) |
|---|---|
|
Anthracite coal |
< 10 |
|
Meagre coal |
10–12 |
|
Lean coal |
12–18 |
|
Coking coal |
18–26 |
|
Fat coal |
26–35 |
|
Gas coal |
35–44 |
4.1.2 Critical values of gas content prediction indicators for coal with different metamorphic degrees
The gas pressure of 0.74MPa, as the critical value for regional outburst prevention measures, has achieved good application results. Using the basic gas parameter data of 74 coal seams with different metamorphic degrees, including Yangquan mining area and Pingdingshan mining area, the gas content corresponding to gas pressure of 0.74MPa was indirectly calculated according to Langmuir equation. The relationship between volatile matter and gas content was plotted, as shown in Fig. 1.
From Fig. 1, it can be seen that when the critical value of gas pressure is 0.74MPa, the higher the degree of coal metamorphism, the higher the corresponding gas content in the coal seam. Based on the numerical division results of volatile matter in the coal rank classification standard, the corresponding gas content of coal with different metamorphic degrees at a gas pressure of 0.74MPa can be obtained, as shown in Table 4.
|
Coal rank |
Gas content corresponding to 0.74MPa(m3/t) |
|---|---|
|
Anthracite coal |
10.698 ~ 16.122 |
|
Meagre coal |
8.765 ~ 10.616 |
|
Lean coal |
7.210 ~ 8.667 |
|
Coking coal |
5.483 ~ 7.129 |
|
Fat coal |
4.487 ~ 5.383 |
|
Gas coal |
2.673 ~ 4.423 |
4.2 Impact of coal firmness coefficient on regional prediction index gas content
The firmness coefficient f of coal is the main indicator of coal strength, used to characterize the impact resistance of coal. The smaller the firmness coefficient, the smaller the impact resistance of coal. Cai(41) studied the crushing work theory of coal seam outburst danger and found that there is an important relationship between the crushing work of outburst coal and the solidity coefficient: W = 9.18 × 10-3f, which means that the lower the strength of the coal, the less energy is required to be crushed, and the easier it is to break. It is generally believed that the development of high-energy gas in the structural coal body is a necessary condition for the occurrence of coal and gas outburst, and the distribution pattern of coal and gas outburst is determined by the distribution of structurally soft coal with low crushing power. Bai(42) proposed in their analysis of gas dynamic phenomena at low critical values that newly excavated exposed surfaces with fully developed structural soft coal are prone to forming gas pressure differences, even when the gas pressure is less than 0.74MPa and the gas content is less than 8m3/t, coal and gas outburst can occur.
The solidity coefficient of coal samples collected from the Three Soft Coalfields in western Henan is generally below 0.4. For example, the Daping Coal Mine experiences outburst under low gas pressure (0.33MPa), while the solidity coefficient of coal is below 0.2. The gas content of the completed mine is generally 7m3/t. The solidity coefficient f value of most sampling points is around 0.25, and dynamic phenomena have also occurred(43–44). The "Detailed Rules for the Prevention and Control of Coal and Gas Outburst" only stipulate a critical value of 0.5 for the solidity coefficient in the identification of outburst coal seams, and coal seams less than or equal to 0.5 have outburst danger, without specific classification.
Professor Yu Qixiang from China University of Mining and Technology(45), based on the relevant parameters of coal seam outburst locations in 26 outburst mining areas in China, used the mining statistical method to study the empirical formula for the minimum outburst pressure: the minimum gas pressure pmin for gas outburst in coal seams is related to the volatile matter V of coal and the solidity coefficient f of coal, and the relationship formula is:
pmin=5（0.1+0.017Vf） （3）
Where, pmin is the minimum gas pressure at which gas outburst occurs in the coal seam, MPa; V is the volatile matter of coal,%; F is the solidity coefficient of coal.
On the basis of coal rank classification, the firmness coefficients f were selected with values of 0.5, 0.4, 0.3, 0.2, and 0.1, respectively. Using Eq. (3), the critical values of outburst pressure corresponding to different coal ranks and f values were calculated. The trend diagram of the relationship between the minimum outburst pressure and the degree of metamorphism under different f values was drawn, as shown in Fig. 2 (a) and (b).
From Figure a, it can be seen that the larger the volatile matter and solidity coefficient f of coal, the higher the critical value of gas pressure for outburst danger. Under the same coal rank, as the strength of the coal increases, the critical value of gas pressure also increases. Use Eq. (1) Langmuir equation to calculate the corresponding outburst hazard gas content value as shown in Figure (b). From Figure (b), it can be seen that as the degree of metamorphism decreases, the gas content corresponding to the minimum outburst pressure shows a decreasing trend. Under the same coal rank, as the strength of the coal increases, the critical value of gas content gradually decreases. When the solidity coefficient is between 0.1 and 0.2, anthracite with higher degree of metamorphism exhibits obvious "low gas pressure and high gas content". At this time, gas content indicators and critical values should be prioritized based on the actual situation of the mine as an effective basis for regional outburst risk. Gas coal with lower degree of metamorphism is "high gas pressure and low gas content", The calculated minimum outburst gas pressure critical value clearly exceeds the 0.74MPa specified in the "Regulations on the Prevention and Control of Coal and Gas Outbursts", but the corresponding gas content is only 3.968m3/t. To ensure safe production, the gas pressure and critical value of 0.74MPa should be preferred for regional prediction indicators. Analysis shows that if the impact of coal strength is not considered in the prediction indicators of regional outburst risk in geological structural zones, it is easy to make regional prediction errors in high and low metamorphic mining areas, leading to the occurrence of "high gas outburst, low gas outburst".
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Based on the provisions of regional prediction index systems in various standards, regulations, and methods, as well as the research of scholars, it has been found that current research on regional prediction indicators has not fully considered the impact of coal body strength in different geological structure zones of mines on the prediction of regional outburst risk, and there is a lack of specific requirements for the corresponding relationship between gas pressure and gas content in regional prediction indicators of coal seams with different metamorphic degrees;
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After analyzing the reasons for the technical defects of the regional prediction index system, it is clear that gas pressure is the main factor in predicting the risk of regional outburst. When a high-pressure gas pressure gradient can be formed, gas can independently trigger outburst. The coal structure and mechanical properties of the structural zone are necessary conditions for the occurrence of coal and gas outburst;
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Based on the results of coal rank division, the gas content corresponding to 0.74MPa of coal with different metamorphic degrees was calculated using Langmuir Eq. (10−16m3/t for anthracite, 8−10m3/t for lean coal, 7−8m3/t for lean coal, 5−7m3/t for coking coal, 4−5m3/t for fat coal, and 2−4m3/t for gas coal);
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In structural coal seams with the same degree of metamorphism, the strength of the coal body is one of the main factors for coal and gas outburst. The higher the strength of the coal body, the greater the critical pressure for outburst occurrence, and the corresponding gas content also increases. When the strength of the coal body is the same, as the degree of metamorphism decreases, the critical value of the Vais outburst pressure gradually increases, and the corresponding gas content critical value shows a downward trend.
The authors want to thank the anonymous reviewers for their valuable suggestions. The authors also express their appreciation of the funding provided by the national natural science foundation of China (No.51274090), Supported by the University Science and Technology Innovation Team Support Program of Henan Province (171RSTHN030), and Key Research Funding Projects for Higher Education Institutions of Henan Province (19B440002).
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and publication of this article
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No competing interests reported.