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. 2022 Dec 20;8(1):1496–1504. doi: 10.1021/acsomega.2c06994

Experimental Study on Gas Loss Derived from Positive Pressure

Xinjian Li †,∥,⊥, Xiangjun Chen †,‡,§,*, Lin Wang †, Yu Yuan #, Tongyong Yu ∥,⊥
PMCID: PMC9835801  PMID: 36643459

Abstract

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Positive pressure sampling enables the fixed-point and rapid acquisition of coal samples, but the derivation of loss volume during sampling is usually based on the law of gas desorption from granular coal at atmospheric pressure, which seriously affects the reasonableness of loss amounts under positive pressure and thus leads to errors in gas content determination. The gas loss under positive pressure is the key to the accurate determination of the gas content of coal seams. To obtain reliable loss data, under different positive pressures, we tested the gas desorption process of anthracite coal samples with different adsorption equilibrium pressures, analyzed the effect of positive pressure on gas desorption, studied the changes in the gas desorption rate caused by positive pressure, recorded the fluctuation of the amount of gas loss, and compared the values of loss under different conditions. The results show that the positive pressure is the main factor affecting gas desorption compared to the adsorption equilibrium pressure. The positive pressure has an inhibitory influence on gas desorption. Under the same positive pressure, the gas desorption rate shows a decreasing trend over time, and at the same time, the gas desorption rate gradually decreases accompanied by the increasing positive pressure. The gas loss error rate increases with increasing adsorption pressure under the same positive pressure. However, under the same adsorption pressure, the error rate of loss quantity presents a significant increase with positive pressure. The relative error of gas loss under different positive pressures can reach 63–180%, and the positive pressure has an obvious influence on gas loss. This study has experimentally confirmed that positive pressure has a greater effect on gas desorption than adsorption pressure, which will theoretically improve the method of deriving the amount of gas loss and will provide a basis for the accurate determination of gas content under positive pressure in engineering terms.

1. Introduction

Gas is an important factor affecting the safety of coal production. The deeper the coal mining extends, the more difficult a gas disaster prevention will be. At the same time, gas is also a high-quality clean energy and chemical raw material, whose development and utilization can effectively alleviate the shortage of natural gas, with good economic and social benefits.1,2 Gas content is an important parameter of mine gas prevention and control, whose accuracy directly determines the effectiveness of the measures for mine gas prevention and control, thus affecting the safety of coal production. The gas content value is also key for formulating the coalbed methane utilization plan. In the process of gas utilization, resource reserves must be reasonably evaluated based on the gas content, while the utilization mode and scale must be determined according to the evaluation results. It is necessary to avoid investment waste caused by an excessive construction scale due to exaggerated gas content and also to prevent resource waste ascribed to a too small one.3,4 Gas content determination methods are generally divided into the direct method and the indirect method, and the direct one is widely used because of its advantages such as a shorter determination time, simpler process, etc. Current research on gas content determination technology focuses on in situ determination. Scholars have put forward a method to determine technologies such as nuclear magnetic resonance without interfering with the coal seam itself5 and explored the influence of gas components, atmospheric desorption, desorption after crushing, and other factors on gas content.6 The coal seam gas content consists of three parts: loss, desorption, and residual. The desorption and residual can be accurately determined in the laboratory, while the loss can only be calculated by some theoretical methods, and the calculation consequences will directly dominate the accuracy of the gas content. The sampling method has a significant impact on the loss determination results.7 The larger the particle size is, the smaller the loss is.8 The traditional loss calculation method is based on the law of gas desorption under normal pressure,9 which is contradictory to the positive pressure sampling environment in engineering applications, and hence, this method is prone to leading to excessive error in both theory and practical applications.8 When sampling in a positive pressure environment, we drill to the predetermined sampling location, then use the drill bit to grind the coal into pulverized coal in situ, and the coal sample is quickly discharged from the drill bit through the drill pipe under the action of compressed air, with finally the discharged pulverized coal being collected into the coal sample tank. This method has the advantages of a faster sampling speed, more accurate sampling location, and shorter measurement time. Therefore, with the widespread adoption of the positive pressure sampling technology, it is of great significance to study the influence of positive pressure on gas loss to improve the accuracy of gas loss calculation.

Research on gas loss is based on gas desorption characteristics, where research not only includes single factors such as gas pressure, geological processes, particle size, temperature, pore structure, and water but also covers the coupling of multiple factors. Chen et al. studied the initial gas desorption law of coal samples with different metamorphic degrees under two pressures and used the research results to determine the coal seam outburst risk.10 Yi et al. established an analytical model to calculate the gas diffusion coefficient under different desorption times. Through simulation and experimental research, they found that the ratio of free gas to total gas has a strong linear relationship with the diffusion coefficient and explained the mechanism of gas pressure and desorption time on gas diffusion.11 Geological processes directly affect the integrity of coal seams in the process of coal formation. Lu et al. studied the influence of geological processes on the gas desorption characteristics of coal seams by analyzing the gas desorption characteristics of high-order raw coal and fissured coal and concluded that the diffusion forms of raw coal are different from those of broken ones, and the initial gas desorption volume and speed of broken coal are greater than those of unbroken coal.12 Wen studied the gas desorption law of deformed coal involving distinct damage degrees and determined the desorption characteristics of deformed coal with different particle sizes under diverse equilibrium pressures.13 Ruppel et al. believed that there was no obvious change in the relationship between the particle size and the maximum desorption of coal, and the maximum desorption of samples was roughly the same under the same test conditions, no matter what the particle sizes might be.14 Sun and others studied the influence of temperature on the adsorption characteristics. With the increase in temperature, the adsorption capacity of methane escalates, while the diffusion coefficient decreases first and then increases.15 Pore structure affects the diffusion characteristics of free gas and adsorbed gas in coal seams. Wu et al. analyzed the fractal characteristics of the coal rock fracture network based on high-resolution computer tomography images.16 Pan et al. studied the pore structure of coal by means of scanning electron microscopy.17 Liu et al. focused on the effect of pore irregularity on the gas desorption characteristics of coking coal.18 Ma et al. attached importance to the ash and volatile contents, whose research demonstrated that the adsorption capacity is positively related to pressure and thermal evolution and negatively to temperature, water content, ash, and volatile contents. Based on the research results, they put forward a mathematical model for calculating the coal seam gas adsorption content.19 The influence of water on desorption characteristics has always been the focus of research. Wang and others studied the relationship between the absorption rate and the water content and concluded that during desorption, the rate of pressure drop increases along with the coal water content, and the total amount of gas desorption decreases accordingly, and the gas flow rate changes with the increasing water in a reverse manner.20,21 After triaxial compression and acoustic emission tests, Chen et al. concluded that water can promote the development of damage in gas-bearing coal.22 Pan et al. studied the influence of water on gas migration in coal from the perspective of diffusion and flow.23 Guo et al. studied the desorption and diffusion characteristics of gas in low-rank coal and how the water works.24 With the continuous deepening research on the gas desorption characteristics dominated by the single factor, scholars also pay more attention to the multifactor coupling condition.

In fact, scholars have conducted fruitful research on the law of gas desorption under multifactor coupling. Wen et al. determined the desorption characteristics of deformed coals with distinct particle sizes under different equilibrium pressures and studied the gas desorption laws of deformed coals caused by damage degrees.25 Wang et al. studied the gas desorption process of coal under the condition of temperature–pressure coupling, enriching the theory of freezing sampling to accurately test the gas content.26 Švábová et al. explored the influence of pressure and temperature on the adsorption rate and found that pressure is positively related to the adsorption rate. The introduction of the adsorption rate is a further improvement to the study of the desorption law.27 Nandi et al. considered the degree of metamorphism and other factors at the same time and studied the different diffusion characteristics of methane in anthracite and bituminous coal under the combined action of adsorption amount, methane concentration, temperature, and coal particle size.28 Liu et al. studied the influence of pore structure and water on adsorption characteristics,18 and Zhao et al. introduced pressurized water and superheated steam in the field of gas desorption. By studying the gas desorption characteristics of coal under the coupling conditions of water, pressure, and temperature, it can be concluded that steam can effectively enhance the gas desorption capacity.29 Yue et al., through the coupling experiment of air pressure and water, proved that the lower the air pressure is, the better the wetting effect of micropores and some mesopores is going to be, and the much more easily the replacement of adsorbed gas by water will take place, thus increasing the amount of desorption gas.30

Research on gas desorption law under a positive pressure environment is relatively less compared with normal pressure. Zhang and others conducted experimental research on the desorption process of gas-bearing coal under pressure and normal pressure and believed that the determination of whether the gas is minable and the gas extraction time should be based on the desorption law under pressure.31 Ge et al. considered that the pressure drop speed inside coal samples and the pore structure characteristics of coal itself were the main reasons for the distinct desorption law of coal samples, which involved particle sizes of every sort and kind.32

Obviously, research on the gas desorption amount and rate under positive pressure should be enhanced, leading to a large error in the calculation of gas loss under positive pressure, which affects the accuracy of gas content measurement. In this paper, the gas desorption process of anthracite coal samples with adsorption equilibrium pressures of 0.5, 1.0, 1.5, and 2.0 MPa under positive pressures of 0, 0.2, 0.4, and 0.6 MPa were tested, respectively; then, the gas desorption law of granular coal in positive pressure sampling was studied, as well as the influence of positive pressure on the gas desorption amount and rate was analyzed, and the applicability of atmospheric empirical formula to describe the gas desorption law under positive pressure was investigated. In addition, we also discussed the cause of the large error of gas loss and the influence of positive pressure on gas loss through experimental research and theoretical analysis. The research results enrich the relevant theory of gas loss calculation under positive pressure, provide a scientific basis for the accurate determination of coal seam gas content, and provide great guidance for the prevention of gas disasters and the development and utilization of coal seam gas.

2. Research Methods

2.1. Coal Sample Collection and Preparation

The Jincheng mining area is located in the Qinshui Basin in the southeastern part of Shanxi Province, China. The No. 3 coal seam, a typical coal and gas outburst one, in this region is an anthracite coal with a high degree of metamorphism, which possesses a high gas content and belongs to hard coal. With regard to the determination of gas content, positive pressure sampling is commonly used in this locality; therefore, the No. 3 coal seam of the Yonghong coal mine in the Jincheng mining area was selected as the experimental sample for this study. In the process of taking the coal samples, the whole raw coal was collected using the grooving method on a freshly exposed coal wall in accordance with the regulations, processed into granular coal with a particle size of 1–3 mm in the laboratory, and stored in a sealed reserve.

2.2. Determination of Basic Parameters of Coal Samples

A certain amount of coal samples was smashed and sieved by a 0.2 mm standard coal sieve. Industrial analysis of the coal samples was carried out in the laboratory, and the true density, apparent density, and porosity of the coal samples were determined. The solidity coefficient “f” of the coal samples was measured using the falling hammer method, and the gas adsorption constants “a” and “b” were obtained using the high-pressure volumetric method. The results of the testing of the basic parameters of the coal samples are shown in Table 1.

Table 1. Basic Parameters of Coal Samples.

test project moisture Mad (%) ash Aad (%) volatile Vad (%) a(m3/t) b (MPa–1) true density (g/cm3) apparent density (g/cm3) porosity (%) consistent coefficient f
test results 1.5 9.47 4.03 39.873 1.172 1.58 1.47 5.99 1.27
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2.3. Experimental Methods and Equipment

2.3.1. Air Tightness Test

The balanced pressure cylinder and coal sample tank were filled with high-pressure gas (4 MPa) and allowed to stand at a constant temperature for 8 h. If the data of the sensor shows no change in the gas pressure in the device, it indicates that the air tightness of the experimental device is intact, and the experiment can be carried out (Figure 1).

Figure 1.

Figure 1

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Experimental apparatus.

2.3.2. Vacuum Degassing of Coal Samples

The coal samples of 1–3 mm diameter prepared were put into the drying oven at 105 °C to dry, and 100 g was put into the coal sample tank. The coal sample tank was fixed in a constant-temperature (60 ± 1 °C) water bath, and then, the vacuum pump was opened to degas the coal sample tank and the reference cylinder. When the pressure of the vacuum gauge dropped below 20 Pa, the vacuum pump was closed.

2.3.3. Adsorption Equilibrium

The intake valve was opened, and air was injected into the reference cylinder. After reaching the predetermined pressure, the intake valve was closed and the pressure of the reference cylinder was recorded. The balance valve was opened to allow methane to enter the coal sample tank for adsorption. When the adsorption and balance of methane were accomplished in the coal sample, the pressure of the tank was recorded, the balance valve was closed, the intake valve was opened, and gas was injected into the reference cylinder again. The above steps were repeated until the coal sample adsorption equilibrium pressure reached the default (Figure 2).

Figure 2.

Figure 2

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High-pressure gas injection unit.

2.3.4. Gas Desorption Data Determination

Desorption experiments were carried out under the positive pressures of 0, 0.2, 0.4, and 0.6 MPa, and the desorption capacity and speed were determined when the adsorption equilibrium pressures were 0.5, 1.0, 1.5, and 2.0 MPa. The gas desorption data during the experiment were all acquired under standard atmospheric pressure.

2.3.5. Calculation of Loss

In view of the desorption test data under different positive pressures, the loss during sampling was calculated, and the loss based on the law of atmospheric pressure desorption was obtained by the “√t” method. The loss data obtained by the two methods were compared and analyzed.

3. Experimental Results and Discussions

3.1. Influence of Environmental Positive Pressure on Gas Desorption

The experiment was divided into four groups. In each group, the positive pressure was taken as the variable, and the adsorption equilibrium pressure was kept unchanged. The gas desorption capacity was determined when the positive pressures were 0, 0.2, 0.4, and 0.6 MPa, while the adsorption equilibrium pressures were 0.5, 1.0, 1.5, and 2.0 MPa. The desorption time was 240 min. The gas desorption quantity over time under different positive pressures is shown in Figure 3.

Figure 3.

Figure 3

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Desorption curve of the coal sample at different sampling positive pressures.

As can be seen in Figure 3, under the same adsorption equilibrium pressure, the gas desorption quantity within the same period decreases with the increase of positive pressure. Under different adsorption equilibrium pressure conditions, the positive pressure and desorption quantity have the same trend, and the experimental results prove that positive pressure has an inhibitory effect on gas desorption. To analyze the effect of positive pressure on gas desorption more specifically, gas desorption values under different conditions were collected in detail during the experiment. Taking Figure 3d as an example, when the adsorption equilibrium pressure is 2.0 MPa, the positive experimental pressures are set as 0, 0.2, 0.4, and 0.6 MPa, and the cumulative desorption amount is counted every 60 min, denoted as Q(60). “Q(120) – Q(60)” represents the difference between the desorption amount within 120 and 60 min. The statistical results are shown in Table 2.

Table 2. Accumulated Desorption of Coal Samples in Different Positive Pressures under Adsorption Pressure 2.0 MPa.

positive pressure (MPa) Q(60) (mL/g) Q(120) – Q(60)(mL/g) Q(120) (mL/g) Q(180) – Q(120)(mL/g) Q(180) (mL/g) Q(240) – Q(180)(mL/g) Q(240) (mL/g)
0 10.07 1.74 11.81 1.07 12.88 0.78 13.66
0.2 7.26 1.13 8.39 0.64 9.03 0.44 9.47
0.4 6.02 0.59 6.61 0.43 7.04 0.2 7.24
0.6 5.11 0.32 5.43 0.2 5.63 0.15 5.78
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According to Table 2, it can be concluded that under the same adsorption equilibrium pressure and different statistical times (Q(60), Q(120), Q(180), Q(240)), when the positive pressure rises from 0 to 0.6 MPa, the cumulative gas desorption quantity in different statistical times decreases significantly. Q(60) decreases from 10.07 to 5.11 mL/g, Q(120) from 11.81 to 5.43 mL/g, Q(180) from 12.88 to 5.63 mL/g, and Q(240) from 13.66 to 5.78 mL/g. The drops are 49.25, 54.02, 56.29, and 57.69%, respectively, indicating that the greater the positive pressure is, the smaller the cumulative desorption volume will be, and the greater the fall of desorption volume will be. Under the same positive pressure, the total cumulative amount increases in pace with time. When the positive pressure is 0 MPa, the increased values of desorption amount within 60 min are 1.74, 1.07, and 0.78 mL/g. When the positive pressure is 0.2 MPa, the results are 1.13, 0.64, and 0.44 mL/g. When the positive pressure is 0.4 MPa, the results are 0.59, 0.43, and 0.2 mL/g. When the positive pressure is 0.6 MPa, the results are 0.32, 0.2, and 0.15 mL/g, indicating that the added value gradually decreases in the same time span, and the growth rate of cumulative volume slows down. When the desorption time span is the same, different positive pressures correspond to different desorption increments. The increases in the first 60 min are 1.74, 1.13, 0.59, and 0.32 mL/g; in the second 60 min, the increases are 1.07, 0.64, 0.43, and 0.2 mL/g; and the results of the third 60 min are 0.78, 0.44, 0.2, and 0.15 mL/g; indicating that the growth rate of the cumulant in each stage decreases significantly with the increase of positive pressure during the same desorption time. Under normal pressure, the desorption amount of the coal sample, which is 9.47 mL/g when the positive pressure is 0.2 MPa, is 13.66 mL/g. When the positive pressure rises to 0.6 MPa, the desorption amount drops to 5.78 mL/g, accounting for only 42.3% of the normal. Therefore, if the loss calculation model under positive pressure is established using the atmospheric desorption law, the accuracy of gas content determination will be seriously affected. The experimental data under other equilibrium pressures also support the conclusion fully.

Adsorption equilibrium pressure and positive pressure are the two main variables of this experiment to study the effects of both on gas desorption. Here, we introduce the concept of the decline rate of desorption, which denotes, based on the desorption quantity under positive pressure to 0 MPa, the relative change of desorption quantity within 240 min when the positive pressure is 0.6 MPa. The higher falling rate means a greater influence on desorption, just as reflected by the data in Table 3.

3.1.

where θ is the desorption decline rate, %; Q(0) is the desorption capacity within 240 min under the positive pressure 0 MPa, mL/g; and Q(0.6) is the desorption capacity within 240 min under the positive pressure 0.6 MPa, mL/g.

Table 3. Effect of Positive Pressure on the Decline Rate of Desorption under Different Adsorption Equilibrium Pressures.

experimental conditions 0 MPa 0.2 MPa 0.4 MPa 0.6 MPa θ (%)
0.5 MPa 11.48 8.05 6.23 4.4 61.67
1 MPa 12.27 8.34 6.62 5.57 54.60
1.5 MPa 13.09 8.86 6.82 5.7 56.46
2 MPa 13.53 9.46 7.31 6.22 54.03
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It can be seen from Table 3 that, under the experimental condition of adsorption equilibrium pressure 0.5 MPa, when the positive pressure changes from 0 to 0.6 MPa, the cumulative desorption volume within 240 min can be determined to fall from 11.48 to 4.4 mL/g, and the calculated decline rate of desorption volume reaches 61.67%, which shows that positive pressure has an inhibitory effect on gas desorption. Considering the identical positive pressure span as above, the falling rates of the adsorption equilibrium pressures 1, 1.5, and 2.0 MPa are 54.6, 56.46, and 54.03%, respectively. Under the experimental conditions, the variation range of the desorption decline rate is 54.03–61.67%, and the data is relatively stable within a small fluctuation, indicating that positive pressure, compared with adsorption equilibrium pressure, is the main factor dominating gas desorption.

3.2. Influence of Positive Pressure on Gas Desorption Velocity

To investigate the variation characteristics of desorption velocity in the initial stage of the desorption process, we set the same adsorption equilibrium pressure, and the gas desorption velocity curve of coal samples under different positive pressures in the first 3 min was made, as shown in Figure 4.

Figure 4.

Figure 4

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Desorption speed curve of coal samples in different positive pressures.

Taking Figure 4a as an example for analysis, maintaining the adsorption equilibrium pressure at 0.5 MPa, the gas desorption velocities were determined at 0.5, 1, 1.5, 2, 2.5, and 3 min under different positive pressures, where V(0.5) represents the desorption speed at 0.5 min after desorption, and the statistical duration of the experiment is 3 min. The determination is shown in Table 4.

Table 4. Gas Desorption Speed in Different Positive Pressures under the Adsorption Pressure 0.5 MPa.

experimental condition V(0.5) (mL/g·min–1) V(1.0) (mL/g·min–1) V(1.5) (mL/g·min–1) V(2.0) (mL/g·min–1) V(2.5) (mL/g·min–1) V(3.0) (mL/g·min–1)
0 MPa 4.8 3.2 2.53 2.1 1.8 1.57
0.2 MPa 4.2 2.8 2.13 1.75 1.51 1.3
0.4 MPa 3.6 2.4 1.93 1.6 1.39 1.19
0.6 MPa 3.12 2.17 1.61 1.35 1.16 1.03
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From the second row of Table 4, when the adsorption equilibrium pressure is 0.5 MPa, the positive pressure is maintained at 0 MPa, and the desorption speed is recorded every 0.5 min, whose corresponding figures are 4.8, 3.2, 2.53, 2.1, 1.8, and 1.57 mL/g·min–1; it can be concluded that there is an obvious decreasing trend, and the gas desorption rate decreases over time under the same positive pressure condition when the equilibrium adsorption pressure is constant. Taking column 5 of Table 4 (the data at 2 min after the desorption) as an example, the desorption velocities of different positive pressures are 2.1, 1.75, 1.6, and 1.35 mL/g·min–1, which are opposite to the change trend of positive pressure; thus, we suppose that, at the same time, the gas desorption velocity gradually decreases with increasing positive pressure. The experiment confirmed the inhibitory effect of positive pressure on gas desorption speed and revealed the difference of gas emission characteristics between normal and positive pressures. According to the measurement, the desorption speed of atmospheric desorption at any time, during the whole process of coal sample desorption, is higher than that of positive pressure. Therefore, the rule of atmospheric desorption cannot reflect the actual situation of gas desorption under positive pressure, and the calculation of loss based on the rule of atmospheric desorption will seriously affect the accuracy of gas content measurement. In addition, it is proved that the determination of loss should be based on the determined results under positive pressure instead of normal pressure. The same conclusion can be drawn from the experimental data under other equilibrium pressures, as shown in Table 4.

3.3. Influence of Positive Pressure on Gas Loss

The gas loss refers to the gas desorption amount of coal samples within the period when the samples are loaded into the special tank from the exposed state on-site. The gas loss is usually calculated by the theoretical method. At present, the “√t” method is the most common one, which is based on the law of atmospheric pressure desorption; however, this is not in accord with the practical case. In fact, throughout the process of positive pressure sampling, coal samples are always in the positive pressure desorption environment, completely different from the atmospheric pressure, during the period from being exposed to the air to being collected in the coal sample tank. As mentioned above, due to the great discrepancy between the positive pressure desorption law and the normal, the loss based on the constant positive pressure desorption data in the laboratory and the loss calculated by the “√t” method were compared and analyzed to study the influence of positive pressure on the gas loss.

The concept of relative error “η” is introduced here, which refers to the error rate between the calculated value of the “√t” method and the determined result under diverse positive pressures based on the laboratory data. The larger the value is, the lower the reliability originating from the traditional method will be. We gathered the relevant calculation in Table 5 when the positive pressure is 0.4 MPa and in Table 6 when it is 0.6 MPa.

3.3.

where η is the relative error of loss, 100%; C(t) is the loss calculated by the “√t” method, mL/g; and C(p) is the loss amount based on the measured data at the positive pressure “p” MPa, mL/g.

Table 5. Loss Calculation and Laboratory Test Results (0.4 MPa).

positive pressure (MPa) adsorption pressure (MPa) C(t) (mL/g) C(0.4) (mL/g) C(t) – C(0.4)(mL/g) η (%)
0.4 0.5 0.2 0.11 0.09 82
1 0.31 0.19 0.12 63
1.5 0.44 0.26 0.18 69
2 0.56 0.28 0.28 100
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Table 6. Loss Calculation and Laboratory Test Results (0.6 MPa).

positive pressure (MPa) adsorption pressure (MPa) C(t) (mL/g) C(0.6) (mL/g) C(t) – C(0.6)(mL/g) η (%)
0.6 0.5 0.2 0.08 0.12 150
1 0.31 0.14 0.17 121
1.5 0.44 0.18 0.26 144
2 0.56 0.2 0.36 180
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According to Tables 5 and 6, compared with the calculated results under atmospheric pressure, the loss determined in the laboratory is significantly smaller when the positive pressure is maintained by 0.4 and 0.6 MPa, respectively, during the experiment, which further confirms the inhibitory effect of positive pressure on gas desorption. Under positive pressure 0.4 MPa, the difference between them is 0.09–0.28 mL/g, and when the positive pressure is 0.6 MPa, the difference is 0.12–0.36 mL/g. With the increase of adsorption pressure, the absolute difference between them shows an enlarging inclination, indicating that the stronger the adsorbability of the coal seam is, the greater the absolute error value of loss will be. The relative error rate of gas loss is up to 63–100% under the positive pressure 0.4 MPa, and it is 121–180% under 0.6 MPa. The relative error rate of gas loss increases significantly with the uplift of positive pressure, which proves that the higher the positive pressure used in the sampling process is, the greater the relative error of gas loss will be. The experimental results show that the relative error of gas loss is different under different positive pressures, and the determination value of gas loss directly affects the reliability of the gas content. The greater the adsorption pressure is, the higher the positive pressure will be, and consequently, the greater will be the determination error. The large error value is also caused by the inconsistent conditions between the laboratory and the production site. It means that the working environment in the coal mine is usually airflow with a certain speed, while in the laboratory condition, it does not exist. This experiment fully proves that the “√t” method brings about a large error compared with the laboratory determination, and the calculations will seriously affect the accuracy of gas content determination. When measuring the gas content of coal seams under positive pressure, we first take coal samples under the coal mine and then measure the desorption characteristics of these samples under different positive pressures in the laboratory. The setting of positive pressure is determined according to the actual situation of the coal mine and the sampling site. According to the experimental results, a calculation model will be established for the gas loss in the area. The same calculation model can be selected for coal seams in the same mining area or areas with similar gas storage conditions. When the occurrence conditions of coal seams change, the loss calculation model needs to be reconstructed. Although the loss calculation method based on the determined data is more realistic, it takes into account the fact that the adsorbability of coal seams in different areas is not consistent, and the positive pressure of coal debris during the migration process of sampling is changing; meanwhile, this experiment did not involve the study on the law of gas desorption and its influencing factors under dynamic positive pressure. Therefore, although the method of determining the amount of gas loss based on the determined data proposed in the experiment is in progress, more and more detailed studies are needed to obtain the accurate amount of gas loss.

4. Conclusions

By means of experimental research, we found that positive pressure, which has an inhibitory effect on gas desorption speed, is the main factor affecting gas desorption. Under the same positive pressure, gas desorption speed shows a decreasing trend over time. At an identified moment, the gas desorption velocity gradually decreases, which is accompanied by the increase of positive pressure. The larger the positive pressure is, the smaller the gas desorption quantity and desorption speed of coal will become within the same time span.

Under the same positive pressure, the absolute error value of gas loss increases along with the adsorption pressure, and the stronger the adsorbability of the coal seam is, the greater the absolute error value of the gas loss will be. Under the same adsorption pressure, the relative error rate of loss increases obviously with the growing positive pressure, and the higher the positive pressure that is provided, the greater the relative error of the loss will become. The relative errors of the loss amount under different positive pressures vary greatly, and the error value of the loss amount, which is positively correlated with adsorption pressure and positive pressure, directly affects the reliability of the gas content determination results.

The loss should be calculated based on the desorption rule under positive pressure when positive pressure sampling is used to determine the gas content. This study enriches and improves the theory of loss calculation, promotes the accuracy of coal seam gas content determination effectively, and can provide a scientific basis for the prevention and control of mine gas disasters and the development and utilization of coalbed methane resources. When determining the gas content of coal seams in different mining areas and occurrence conditions, we must establish a gas content calculation model suitable for that particular area on the basis of measurements and determine the applicability of the model in time with the progress of production. In future research, an airflow environment similar to the production site can be set in the laboratory, and dynamic positive pressure can be provided in the desorption experiment so as to obtain more reasonable calculation results of loss.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (Nos. 51874122 and 52074105), the Key R&D and Extension Projects of Henan Province (Nos. 202102310223 and 222102320017), the Doctoral Foundation of Henan Polytechnic University (B2021-7), the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) (No. WS2021A06), the Key Scientific Research Projects of Colleges and Universities in Henan Province (No. 22B620002), and the Key Science and Technology Project of Henan Province (No. 222102320017).

The authors declare no competing financial interest.

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