# Influence of Gas from Long-Flame Coal Spontaneous Combustion on Gas Explosion Limit

Authors: Haitao Wang; Yongli Liu; Qiyuan Shan
Journal: International Journal of Chemical Engineering
 (2023)

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## Abstract

To investigate the impact of multiple combustible gases produced by long-flame coal spontaneous combustion on the gas explosion limit and to guide the reoperation of the coal mine goaf and fire area, the influence of gas generated by coal spontaneous combustion on gas explosion limit is investigated in this paper using a temperature-programmed device and a 20 L spherical explosion device. The results show that the volume fraction of CO produced during the spontaneous combustion of coal samples is 0.47%, followed by CH4 and C2H6, and C2H4 has the lowest content. Simultaneously, the coal spontaneous combustion is divided by 30°C, 80°C, and 170°C as the threshold, depending on the different gas characteristics. Organic C2H4 and C2H6 produced by coal spontaneous combustion have a greater impact on the CH4 explosion limit than inorganic CO. The lower and upper limits of CH4 explosion were reduced to 2.98% and 12.2%, respectively, by 0.8% C2H6. C2H4 and C2H6 explosion limits change dramatically when mixed with CH4. The CO and CH4 mixture explosion limit decreases first and then increases. C2H4 and C2H6 have a significant impact on the explosion pressure of mixed gas and the lower explosion limit of gas. The lower explosion limit falls from 5.1% to 4.3% as the explosion pressure rises from 0.25 MPa to 0.29 MPa.


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## Body

## 1. Introduction

Coal remained the richest source of energy in China by the end of 2021, with a reserve of 2078.85 billion tons [1, 2]. Coal production has fallen from a high of 3.974 billion tons in 2013 to 3.411 billion tons in 2016. This has been attributed to economic cycle fluctuations. As a result, while coal demand has increased rapidly, the rate of growth has slowed significantly. Since 2020, COVID-19 has had a greater impact on China’s stable demand for coal energy, highlighting coal’s unshakeable position in China’s energy supply and its inability to be replaced in the short run, as shown in Figure 1.Figure 1
Coal output of China for nearly ten years.Accidents such as coal mine fires and gas explosions have occurred on occasion as coal yields have increased, resulting in massive economic losses and casualties. According to incomplete statistics [3], the number of coal mine fires, gas explosions, and casualties has decreased year by year in the last 20 years due to increased prevention and control of the aforementioned accidents, but serious accidents still occur accidentally, as shown in Figure 2. According to the National Mine Safety Administration’s Accident Data Query System, the root of autoignition fire in coals is mostly coal spontaneous combustion, and gas explosion accidents caused by autoignition fire account for 7% of total gas explosion accidents [4]. Despite its small size, it wields significant destructive power and poses significant governance challenges.Figure 2
Coal mine disasters statistics.Coal spontaneous combustion in coal mine goaf, as we all know, is a serious problem. At the same time, the goaf is a critical location for gas desorption, migration, and accumulation. The occurrence of a gas explosion disaster in a coal spontaneous combustion environment in goaf is the result of a coal spontaneous combustion and gas interaction. Its gas explosion accident will not only result in serious casualties and the destruction of mine equipment and facilities but it may also result in a series of serious secondary disasters, such as secondary coal dust explosion and mine ventilation system disorder. As a result, preventing gas explosions caused by underground coal spontaneous combustion is a problem that coal mine safety must address.Numerous scholars have conducted numerous studies on coal mine fire and gas disaster accidents. Zhou et al. [5–7] investigated the variation characteristic of the velocity field and gas volume fraction field of the vertical section of the roadway in the closed fire area under different wind speeds using COMSOL software. Furthermore, the explosion triangle of the mixed gas was drawn, and the new inert subareas were divided, resulting in a method for determining the dangerous area of methane explosion. Zhai and Lai [8] calculated the possibility and occurrence time of methane explosion based on the variation characteristic of CH4 and O2 volume fraction in a closed fire area in a short time. Luo et al. [9, 10] chose CH4, CO, C2H6, C2H4, and H2 to study the microscopic mechanism of the explosion of multiple flammable gases and analyzed detonation mechanisms and major detonation ways of the joint explosion of these five gases above using Gaussian software to measure thermodynamic and dynamic characteristics of gaseous mixture explosion. Qin et al. [11] used experiments to investigate the explosion concentration range and explosive hazard degree of a mixture of both CH4 and CO in coal spontaneous combustion and theory to investigate the possible area and participation process of a gas explosion caused by coal spontaneous combustion. Li [12] and Gao [13] conducted simulation experiments of gas migration in mined-out areas under different coal autoignition conditions and discovered a new phenomenon of gas accumulation in the coal spontaneous combustion point. Duan et al. [14] established and analyzed an atmospheric pressure fluctuation model and an air leakage model based on continuous monitoring of atmospheric pressure at various monitoring points in multiple mines to evaluate the explosion hazard in closed fire zones. Jiao et al. [15] conducted research on the characteristics of gas explosions caused by closure process fire areas, using a prototype of explosion accidents on the fully mechanized coal mining face. Peng [16] investigated the variation of the explosion-hazard coefficient in the presence of gas exchange in the mined-out area and various air leakage rate coefficients. Li [17] investigated the gas explosion hazard during the fire zone sealing process of the fiery mine, combining theoretical analysis, experimental study, and computer simulation to determine the dangerous gas explosion zone. Deng et al. [18] investigated the spontaneous combustion characteristic parameters using temperature-programmed experiments. The influence of gas on the characteristics of coal oxidation and spontaneous combustion was obtained, and the inhibition of gas on coal spontaneous combustion was revealed based on an analysis of the oxygen consumption rate, CO production rate, CO2 expiration rate, and heat liberation intensity at different coal temperatures. Zhou et al. [19, 20] investigated the current situation of gas and coal spontaneous combustion disasters in 229 pairs of mines in China’s main mining areas and revealed the prevention and control mechanism of symbiotic disasters with low-temperature liquid nitrogen. Xu [21] studied the explosion hazards of multiple combustible gases at different oxygen concentrations using the high temperature-programmed experiment and the 20 L explosion ball experiment and then obtained the explosion limit parameters and explosion risk of different multiple combustible gas ratios at different temperatures. Li investigated the variation of the flow field, concentration field, and temperature field during the fire zone sealing process of the fiery mine, combining theoretical analysis, experimental study, and computer simulation. Shi et al. [22] and Liu [23] developed mathematical models of methane explosion hazard change as a result of CO2 and CO, as well as the evolution of gas composition in the fire zone during the closure process. Zhang et al. [24] investigated the symbiotic disaster coupling relationship from two perspectives: the inhibition of gas on coal oxidation spontaneous combustion and the influence of coal spontaneous combustion on gas explosion limit. The disaster-causing mechanism of symbiotic disasters was revealed using a fluid-solid coupling model of gas extraction and a fluid-solid-thermal coupling model of coal spontaneous combustion. Varghese et al. [25] used the external heating divergent channel method to assess the combined effects of pressure and temperature on the propagation of the CH4-air premixed flame. It was discovered that decreasing the thermal diffusivity of the mixture with pressure aids in lowering the laminar burning rate at high pressure. Khan [26] discovered that when the CO2 concentration was 45%, the mixture of gas and air was least affected by CO2, and the deflagration index was most significant when the CO2 concentration was 35%. The reasons for explosions and flame acceleration caused by mixtures of methane and air were summarized by Sazal et al. [27]. The deflagration and detonation stages of the explosion, as well as the change from deflagration to detonation, were explored. Most mined-out areas contain explosive gas pockets, according to research by Brune et al. [28–30], which can cause mine fires and explosions. The creation of the explosion area can be efficiently controlled by modifying the ventilation settings, choosing the proper ventilation mode, and injecting inert gas as required.The majority of existing studies examine the gas explosion limit from the perspective of a single component gas or a partial component mixed gas or investigate the interaction between a specific section of gas and coal spontaneous combustion, while the influence of mixed gas produced at various stages of coal spontaneous combustion is less studied. In order to address these issues, this paper investigated the gas generation characteristics in the process of coal spontaneous combustion using a coal temperature-programmed experimental device. The gas generated in each stage of gas mixed coal spontaneous combustion was tested using a 20 L spherical explosive device, and the influence of coal spontaneous combustion gas on the gas explosion limit was studied, as well as the disaster-causing characteristics of coal spontaneous combustion and gas compound disaster in goaf. It is useful in predicting and preventing gas explosion accidents caused by coal spontaneous combustion in the early stages of mining and ensuring mine safety.


## 2. Experimental Process

### 2.1. Temperature-Programmed Experiments of Coal Spontaneous Combustion

#### 2.1.1. A Theoretical Pattern for Temperature-Programmed Experiments of Coal

On the basis of the law of conservation of energy, any point in the coal temperature-programmed reaction container (regular cylinder) is selected for mathematical modeling, and the mathematical model of the coal temperature-programmed experiment process is analyzed and summarized, which can theoretically explain the adiabatic oxidation experimental principle [31–34].(1)ρCCoal∂T∂t=QA−E/RT+λ∂2T∂r2−HWaterdCWaterdt+ρAirCAirνAir∂T∂χ,where CCoal, CWater, and CAir are the specific heat capacities of coal, water, and air, respectively.We did triple integrals ofx, y, and z axes for formula (1) and sought the heat balance equation of coal in its reactor: real-time heat function equation (2) of coal in reactor varying with time t and temperature T was obtained, and equation (3) was obtained from the integral results of equation (2).(2)∭ρCCoaldxdydz∂T∂t=∭QρdxdydzA−E/RT+∭λdxdydz∂2T∂r2−∭HWaterdxdydzdCWaterdt+∭ρAirCAirνAirdxdydz∂T∂χ.Through equation (3), simple transformations were made, and the heat equation (4) of gas from coal spontaneous combustion was obtained.(3)mCoalCCoal∂T∂t=QmCoalA−E/RT+λmCoal∂2T∂r2−HWatermCoaldCWaterdt+mAirCAirνAir∂T∂t.Through equation (3), simple transformations were made, and the heat equation (4) of gas from coal spontaneous combustion was obtained.(4)mAirCAirνAir∂T∂t=mCoalCCoal∂T∂t−QmCoalA−E/RT−λmCoal∂2T∂r2+HWatermCoaldCWaterdt.We can measure the heat of generated gas by measuring the volume of gas-generated gas because the volume of gas-generated gas is proportional to heat. The macroscopic gas exhaust energy conservation formula for reaction vessels is shown in formula (4). Finally, the volume of gases generated was calculated by observing the function variation of characteristic gases with time and temperature during coal spontaneous combustion.


#### 2.1.2. Experimental Process

The coal seam of Dongrong No. 2 coal mine was sampled in accordance with China National Standard GB/T 482-2008, and the coal sample was produced in accordance with China National Standard GB 474-2008. Then, using an automatic industrial analyzer, a coal industrial analysis experiment was conducted in accordance with China National Standard GB/T 212-2008. To identify the type of coal, the results of the determination were made in accordance with China National Standard GB/T 5751-2009. The content of C, H, O, N, and other components in coal was then determined and analyzed in accordance with China National Standard GB/T 476-2008 and GB/T 214-2007. Finally, the industrial and elemental examination of the coal seam was established. The experimental results are shown in Table1.Table 1
Experimental results of the coal industry and elements.
Coal samplesMadAadVadFCadCadHadOadNadSadDongrong No. 21.627.1242.249.0673.004.0012.002.100.16(1) The Temperature Programmed Experimental Device. The whole experimental measurement system is divided into three parts: gas path, temp-enclosure, and the collection and analysis of the gas sample. It is shown in Figure 3.Figure 3
Temperature-programmed experimental device: 1-gas compression bottle; 2-pressure reducing valve; 3-flowmeter;4-gas preheating tube; 5-intake pipe; 6-reactor;7-thermal insulation layer; 8-coal sample oxidation chamber; 9-thermometer;10-exhaust pipe; 11-temperature acquisition system; 12-gas collector; 13-gas chromatograph; 14-computer.(2) Experimental Conditions. Dongrong NO. 2 coal mine 25 kg coal samples were chosen. The five different types of coal samples with particle sizes of 0∼0.9 mm, 0.9∼3 mm, 3∼5 mm, 5∼7 mm, and 7∼10 mm were separated from the raw coal samples by air crushing and screening. The findings of earlier investigations demonstrate that coal particle size significantly affects the outcomes of trials involving coal spontaneous combustion [35–37]. The prevailing consensus is that coal spontaneous combustion is more likely the smaller the coal particle size. The opposite is true—they are less prone to spontaneous combustion. In a temperature-programmed box, mixed coal samples of 200 g from each of the five coal sample sizes were evaluated to reduce the impact of coal sample particle size on coal temperature programming. The programmed heating box’s experimental circumstances are established in accordance with the experimental conditions of the huge coal spontaneous combustion experimental platform at the Key Laboratory of Western Mine Exploitation and Hazard Prevention. The experimental conditions are shown in Table 2.Table 2
Experimental conditions for heating coal samples.
Coal samplesAverage size (mm)Coal height (cm)Coal weight (g)Coal volume (cm3)Air flow (ml/min)Rate of temperature increase (°C/min)Mixed size4.2015.501000.01225.001200.31 kg of coal was placed into a reactor with a diameter of 10 cm and a length of 22 cm. The coal sample was held in place by a 100 mesh copper wire mesh in order to provide consistent airflow, leaving a 2 cm gap at the top and bottom of the reactor. It was then heated in a temperature-controlled box before being delivered into the preheating air to collect the gas produced at various coal temperatures. When the temperature reached the required level, turn off the furnace, open the door, and let natural convection do the cooling. Finally, an analysis of the gas content and composition of the gas collected at various coal temperatures was performed.(3) Experimental Results. The experimental investigation demonstrates that during spontaneous combustion, coal is heated and oxidized, and various gases are created at various phases. In each stage, representative gases might be chosen in order to characterize the degree of coal spontaneous combustion. Coal spontaneous combustion in the Dongrong No. 2 coal mine is classified into four stages based on the variations in gas composition produced at various times. Figure 4 displays the variation curves of the gas volume fraction with temperature during coal spontaneous combustion.Figure 4
Curve of volume fraction of coal spontaneous combustion gas variation with temperature.The whole process of coal spontaneous combustion was divided into four stages with references [38–40], as shown in Figure 4.(i)
Room temperature stage: A small amount of CH4 and CO was generated with coal temperatures below 30°C, the main cause of which was the presence of free CH4 and CO in coal, and another small amount of CH4 broke free from adsorption to a free state.(ii)
Slow oxidation stage: The production of CH4 and CO increased significantly, and a small amount of C2H6 was produced with coal temperature ranging from 30 to 80°C. It was primarily due to the fact that only a small amount of gas composition and production occurred during the low-temperature oxidation reaction of coal. Furthermore, the majority of the gas transitions from the adsorption state to the free state increases gas production.(iii)
Rapid oxidation stage: CO combustible gas production increased significantly, as did CH4 and C2H6 production, and C2H4 appeared with coal temperatures ranging from 80 to 170°C. Because as the temperature rose, the oxidation reaction of coal spontaneous combustion accelerated and the internal structure reaction of coal became increasingly complicated, resulting in organic gas products such as C2H4 and C2H6.(iv)
Vigorous oxidation stage: CO, CH4, C2H6, and C2H4 tended to grow stably with coal temperature above 170°C. The aromatic ring structure in coal was decomposed under fierce oxidation to produce a large amount of gas due to the intense oxidation of coal spontaneous combustion at high temperature. However, the produced gas was consumed by the reaction with oxygen in the air, resulting in the steady growth of gas above.


### 2.2. The Explosion Experiment of Multiple Combustible Gas Mixtures

#### 2.2.1. Explosion Theoretical Model of Multiple Combustible Gas Mixtures

A complex chemical reaction and flow process lead to a gas explosion in a small space. The description of the mixed gas explosion process is made simpler because it is extremely complex by using the chemical reaction mechanism of the mixed gas ingredient and some assumptions [41–43].(i)
The internal gas in the reaction process possesses all the characteristics of a true gas, satisfying the equation at the gas state. This is an insulated process in the explosion, and there is no heat exchange between the environment and the machinery.(ii)
Each flammable gas could react entirely in a constrained regular space, and multiple gas mixes are thoroughly homogeneously blended.(iii)
Reactive gases like inert gases, which can absorb and release energy, are not taken into account.(iv)
The chain branching reactions between different elements that do not interact with one another and react separately with oxygen in air are what cause gas explosions.Based on the assumptions made above, the basic equation for a 20 L spherical explosion tank’s closed space is as follows:(i)
Continuity equation:(5)∂V∂t+∂Vum∂xm=0.(ii)
Momentum equation:(6)∂∂tVum+∂∂xnVumun−μeff∂um∂xn=∂P∂xn+∂∂xmμeff∂um∂xn−23∂∂xmδnmVk+μeff∂ul∂xl.(iii)
Energy equation:(7)∂∂tVh+∂∂xnVunh−μeffσh∂h∂xn=∂p∂t+Sh.(iv)
Total equilibrium equation:(8)∑i=1N∂ViIi∂t=∑i=1N∂Vi′Ii∂t.


#### 2.2.2. Experimental Process

CO, CH4, C2H4, and C2H6 are the main combustible gases generated by coal spontaneous combustion in Dongrong No. 2 coal mine, according to the heating experimental process of coal spontaneous combustion. CH4 is the main disaster gas in coal mines, as well as the main gas causing coupling disasters in coal mine fires, due to its high content. The explosion limits of CO, CH4, C2H4, and C2H6 in air were first measured using 20 L spherical explosive devices, and the results were used as a reference for subsequent experiments. Then, the influence of C2H6, C2H4, and CO on CH4 explosion limit was determined by using the explosion limit determination method of combustible gases in air, which provided an experimental and theoretical foundation for the study of gas explosion limit in different stages of coal spontaneous combustion. Based on the detected difference in gas generation at the stage of coal spontaneous combustion, representative gases at four critical points in the coal spontaneous combustion experiment were collected for explosion experiments, and the critical value of gas explosion at the critical point was determined, resulting in the grading warning.(1) Experimental Device. A 20 L spherical explosion test apparatus was used for the experiment, which primarily consists of three parts: a spherical explosion device, a control system, and a data collection system, as shown in Figure 5.Figure 5
Explosion experiment device.(2) Experimental Conditions. The entire test was conducted at standard pressure and temperature, and the humidity of the gas mixtures ranged from 45 to 50% while N2 served as the shielding gas. Premix stabilization time was set to 300 s; constant temperature duration was set to 100 s; accuracy was set to ±10°C; ignition delay was set to 60 ms; solenoid valve opening was set to 10 ms; sampling frequency was set to 5 kH; sampling time was set to 2000 ms.After the ignition delay period was set, the system would automatically begin the experiment. The pressure sensor would now provide the computer with the pressure data generated by the material explosion in the explosion tank. When the explosion’s pressure peak exceeded 7% of the pressure preceding the explosion, it was determined to be an explosion.Reduce or increase the concentration of combustible gas to obtain the lower or upper explosion limit, and then repeat the steps above after it has exploded. If it did not explode, adjust its concentration and repeat the steps above after it did.Start the vacuum pump after each explosion experiment to expel any residual gas in the tank. Open the intake valve to let in fresh air before proceeding to the next experiment.(3) Experimental Results. The heating experiment of coal spontaneous combustion revealed the presence of small amounts of gas such as CO, CH4, C2H4, and C2H6 in the process of coal autoignition. The gas was distributed by the partial pressure method, and the CO, C2H6, and C2H4 gases with volume fractions of 0.2%, 0.4%, 0.6%, and 0.8% were taken for experiments to compare the effect of three combustible gases including CO, C2H4, and C2H6 on the explosion of CH4, combined with the heating experiment result of coal spontaneous combustion. Table 3 and Figure 6 show the final experimental results.Table 3
Statistics of explosion limit test data for combustible gases.
CO explosion limitC2H4 explosion limitC2H6 explosion limitCH4 explosion limitCH4 explosion hazardExplosion limit of mixed gasMixed gas explosion hazardLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limit12.173.82.735.63.214.95.115.5Explosion hazardH = 5.1Explosion hazardH = 12.2Explosion hazardH = 3.6Explosion hazardH = 2.0CO concentrationC2H4 concentrationC2H6 concentration0005.115.52.035.115.52.030.2004.5515.352.374.7515.552.270.4004.4515.252.434.8515.652.230.6004.3015.202.534.9515.802.190.8004.2515.152.565.0515.952.1600.203.814.952.934.015.152.7800.403.714.83.004.115.202.7100.603.414.03.124.014.62.6500.803.013.93.633.814.72.87000.23.7814.252.773.9814.452.63000.43.5214.022.983.9214.422.68000.63.3213.43.043.9214.02.57000.82.9812.23.093.7813.22.49Figure 6
Experimental data analysis: (a) Lower limit of CH4 explosion, (b) upper limit of CH4 explosion, (c) CH4 explosion hazard index, (d) lower limit of mixed gas explosion, (e) lower limit of mixed gas explosion, and (f) mixed gas explosion hazard index.
(a)(b)(c)(d)(e)(f)As shown in Table3, the results of the explosion limit of a single combustible gas under air conditions show that C2H4 has the largest explosion limit range, with the highest risk of 12.2, and the explosion risk is C2H4, CO, C2H6, and CH4. The explosion limit of CH4 varies with the concentrations of C2H4, CO, and C2H6. C2H6 has the most noticeable effect on the gas explosion limit. The minimum value of the upper limit of a CH4 explosion is 12.2, and the minimum value of the lower limit of a CH4 explosion is 2.98. The effect of C2H4 on gas explosion limit is the second, with the lowest value of CH4 explosion limit being 13.9 and 3.0. CO has the smallest effect on the gas explosion limit. The minimum upper limit for a CH4 explosion is 15.15, and the minimum lower limit is 4.25. The explosion risk of a mixture of different concentrations of gas is lower than that of CH4.Figure6(a) shows that the addition of combustible gases lowers the lower explosion limit of CH4, but with different downward trends. The effect of organic combustible gases C2H4 and C2H6 on the lower explosion limit of CH4 with the addition of combustible gases is essentially the same, which is significantly greater than that of the inorganic combustible gas CO. Figure 6(b) shows that the addition of combustible gases lowers the upper explosion limit of CH4, but with different downward trends. With the addition of CO, the variation trend of the upper explosion limit of CH4 becomes more stable. The addition of C2H4 and C2H6 highlights the downward trend in the upper explosion limit of CH4, with C2H6 having a greater impact on the upper explosion limit of CH4 than C2H4. Figure 6(c) shows that the explosion danger degree H of CH4 has been increasing with the addition of combustible gases. CO has less of an effect than other gases, and C2H4 and C2H6 have a more obvious effect on the explosion danger degree H of CH4.As shown in Figure6(d), the variation trend of the lower explosion limit of two-component mixed combustible gas changes with the addition of different types of combustible gases. When the gas addition amount is 0.2%, the variation trend of the lower explosion limit of the mixed gas is the same, and both decrease significantly. The decreasing trend of CO is significantly slower than that of C2H4 and C2H6; after the gas addition amount is 0.2%, the addition of C2H4 and C2H6 makes the downward trend of the lower explosion limit of CH4 become gentle, and the addition of CO makes the lower explosion limit of the mixed gas to rise slowly. As shown in Figure 6(e), the upper explosion limit of mixed gas slowly rises with the addition of CO, while the upper explosion limit of CH4 decreases with the addition of C2H4 and C2H6, and the downward trend of C2H6 is greater than that of C2H4. As shown in Figure 6(f), increasing the addition of three gases causes the explosion danger degree of the mixed gas to increase first and then gradually decrease, and adding C2H4 causes the explosion danger of the mixed gas to fluctuate to some extent. The addition of CO reduces the explosion danger degree of the mixed gas compared to C2H4 and C2H6.


#### 2.2.3. The Influence of Mixed Gas Generated by Coal Spontaneous Combustion on Gas Explosion Characteristics

The research object was gas production at the representative coal temperature during the process of coal spontaneous combustion, and the CH4 gas was then filled with various gases collected at the selected location of coal spontaneous combustion. Table4 shows the specific volume fraction of the gas. Table 4 shows that as the coal spontaneous combustion process progresses, the types of gas-generated gas gradually diversify and the gas volume fraction gradually increases, indicating the coal spontaneous combustion process at all stages. The spontaneous combustion gas was mixed in a 2.5% proportion with the gas intake during the coal spontaneous combustion experiment for the explosion experiment to study the influence of the gas generated at different stages of coal spontaneous combustion on the lower explosion limit of gas explosion. Table 4 displays the experimental results (Table 4).Table 4
Gas volume fraction in different stages of coal spontaneous combustion and its lower limit of gas explosion and explosion pressure.
The process of coal spontaneous combustionVapor volume fraction (%)Explosion pressure (MPa)COCH4C2H4C2H6CO2O2N2CH4 concentration (%)5.1%4.9%4.6%4.3%4.0%Room temperature stage (with coal temperature 30°C)0.00050.0107001.0519.179.70.25NONONONOSlow oxidation stage (with coal temperature 80°C)0.02810.02870.00020.00592.7818.878.40.260.21NONONORapid oxidation stage (with coal temperature 130°C, which is the change node of C2H6 production)0.20300.06190.00110.0122.6917.778.30.280.260.22NONOVigorous oxidation stage (with coal temperature 170°C)0.43500.10420.00710.10182.8215.878.70.290.270.240.21NOIt can be seen from Table4 that the pressure generated by the mixed explosion of gas produced at room temperature, and 5.1% gas was 0.25 MPa, and no explosion occurred with other concentrations of gas, indicating that there is no gas that has an impact on gas explosion at this stage. The pressure generated by the mixed explosion of slow oxidation gas, and 5.1% gas was 0.26 MPa, and 0.21 MPa with 4.9% gas. There was no explosion in the other gas concentrations, indicating that the gas produced during the initial stage of coal spontaneous combustion can reduce the lower explosion limit and improve the explosion pressure. The gas produced in the rapid oxidation stage exploded with gas concentrations of 5.1%, 4.9%, and 4.6%, respectively, at a maximum gas explosion pressure of 0.28 MPa, and there was no explosion with gas concentrations of 4.3% and 4.0%. The gas produced during the coal combustion stage exploded with gas concentrations of 5.1%, 4.9%, 4.6%, and 4.3%, respectively. The maximum gas explosion pressure was 0.29 MPa, and there was no explosion occurred with a gas concentration of 4.0%, indicating that in addition to CO, C2H4 and C2H6 can promote gas combustion and explosion, as well as increase the gas explosion pressure.


## 2.1. Temperature-Programmed Experiments of Coal Spontaneous Combustion

### 2.1.1. A Theoretical Pattern for Temperature-Programmed Experiments of Coal

On the basis of the law of conservation of energy, any point in the coal temperature-programmed reaction container (regular cylinder) is selected for mathematical modeling, and the mathematical model of the coal temperature-programmed experiment process is analyzed and summarized, which can theoretically explain the adiabatic oxidation experimental principle [31–34].(1)ρCCoal∂T∂t=QA−E/RT+λ∂2T∂r2−HWaterdCWaterdt+ρAirCAirνAir∂T∂χ,where CCoal, CWater, and CAir are the specific heat capacities of coal, water, and air, respectively.We did triple integrals ofx, y, and z axes for formula (1) and sought the heat balance equation of coal in its reactor: real-time heat function equation (2) of coal in reactor varying with time t and temperature T was obtained, and equation (3) was obtained from the integral results of equation (2).(2)∭ρCCoaldxdydz∂T∂t=∭QρdxdydzA−E/RT+∭λdxdydz∂2T∂r2−∭HWaterdxdydzdCWaterdt+∭ρAirCAirνAirdxdydz∂T∂χ.Through equation (3), simple transformations were made, and the heat equation (4) of gas from coal spontaneous combustion was obtained.(3)mCoalCCoal∂T∂t=QmCoalA−E/RT+λmCoal∂2T∂r2−HWatermCoaldCWaterdt+mAirCAirνAir∂T∂t.Through equation (3), simple transformations were made, and the heat equation (4) of gas from coal spontaneous combustion was obtained.(4)mAirCAirνAir∂T∂t=mCoalCCoal∂T∂t−QmCoalA−E/RT−λmCoal∂2T∂r2+HWatermCoaldCWaterdt.We can measure the heat of generated gas by measuring the volume of gas-generated gas because the volume of gas-generated gas is proportional to heat. The macroscopic gas exhaust energy conservation formula for reaction vessels is shown in formula (4). Finally, the volume of gases generated was calculated by observing the function variation of characteristic gases with time and temperature during coal spontaneous combustion.


### 2.1.2. Experimental Process

The coal seam of Dongrong No. 2 coal mine was sampled in accordance with China National Standard GB/T 482-2008, and the coal sample was produced in accordance with China National Standard GB 474-2008. Then, using an automatic industrial analyzer, a coal industrial analysis experiment was conducted in accordance with China National Standard GB/T 212-2008. To identify the type of coal, the results of the determination were made in accordance with China National Standard GB/T 5751-2009. The content of C, H, O, N, and other components in coal was then determined and analyzed in accordance with China National Standard GB/T 476-2008 and GB/T 214-2007. Finally, the industrial and elemental examination of the coal seam was established. The experimental results are shown in Table1.Table 1
Experimental results of the coal industry and elements.
Coal samplesMadAadVadFCadCadHadOadNadSadDongrong No. 21.627.1242.249.0673.004.0012.002.100.16(1) The Temperature Programmed Experimental Device. The whole experimental measurement system is divided into three parts: gas path, temp-enclosure, and the collection and analysis of the gas sample. It is shown in Figure 3.Figure 3
Temperature-programmed experimental device: 1-gas compression bottle; 2-pressure reducing valve; 3-flowmeter;4-gas preheating tube; 5-intake pipe; 6-reactor;7-thermal insulation layer; 8-coal sample oxidation chamber; 9-thermometer;10-exhaust pipe; 11-temperature acquisition system; 12-gas collector; 13-gas chromatograph; 14-computer.(2) Experimental Conditions. Dongrong NO. 2 coal mine 25 kg coal samples were chosen. The five different types of coal samples with particle sizes of 0∼0.9 mm, 0.9∼3 mm, 3∼5 mm, 5∼7 mm, and 7∼10 mm were separated from the raw coal samples by air crushing and screening. The findings of earlier investigations demonstrate that coal particle size significantly affects the outcomes of trials involving coal spontaneous combustion [35–37]. The prevailing consensus is that coal spontaneous combustion is more likely the smaller the coal particle size. The opposite is true—they are less prone to spontaneous combustion. In a temperature-programmed box, mixed coal samples of 200 g from each of the five coal sample sizes were evaluated to reduce the impact of coal sample particle size on coal temperature programming. The programmed heating box’s experimental circumstances are established in accordance with the experimental conditions of the huge coal spontaneous combustion experimental platform at the Key Laboratory of Western Mine Exploitation and Hazard Prevention. The experimental conditions are shown in Table 2.Table 2
Experimental conditions for heating coal samples.
Coal samplesAverage size (mm)Coal height (cm)Coal weight (g)Coal volume (cm3)Air flow (ml/min)Rate of temperature increase (°C/min)Mixed size4.2015.501000.01225.001200.31 kg of coal was placed into a reactor with a diameter of 10 cm and a length of 22 cm. The coal sample was held in place by a 100 mesh copper wire mesh in order to provide consistent airflow, leaving a 2 cm gap at the top and bottom of the reactor. It was then heated in a temperature-controlled box before being delivered into the preheating air to collect the gas produced at various coal temperatures. When the temperature reached the required level, turn off the furnace, open the door, and let natural convection do the cooling. Finally, an analysis of the gas content and composition of the gas collected at various coal temperatures was performed.(3) Experimental Results. The experimental investigation demonstrates that during spontaneous combustion, coal is heated and oxidized, and various gases are created at various phases. In each stage, representative gases might be chosen in order to characterize the degree of coal spontaneous combustion. Coal spontaneous combustion in the Dongrong No. 2 coal mine is classified into four stages based on the variations in gas composition produced at various times. Figure 4 displays the variation curves of the gas volume fraction with temperature during coal spontaneous combustion.Figure 4
Curve of volume fraction of coal spontaneous combustion gas variation with temperature.The whole process of coal spontaneous combustion was divided into four stages with references [38–40], as shown in Figure 4.(i)
Room temperature stage: A small amount of CH4 and CO was generated with coal temperatures below 30°C, the main cause of which was the presence of free CH4 and CO in coal, and another small amount of CH4 broke free from adsorption to a free state.(ii)
Slow oxidation stage: The production of CH4 and CO increased significantly, and a small amount of C2H6 was produced with coal temperature ranging from 30 to 80°C. It was primarily due to the fact that only a small amount of gas composition and production occurred during the low-temperature oxidation reaction of coal. Furthermore, the majority of the gas transitions from the adsorption state to the free state increases gas production.(iii)
Rapid oxidation stage: CO combustible gas production increased significantly, as did CH4 and C2H6 production, and C2H4 appeared with coal temperatures ranging from 80 to 170°C. Because as the temperature rose, the oxidation reaction of coal spontaneous combustion accelerated and the internal structure reaction of coal became increasingly complicated, resulting in organic gas products such as C2H4 and C2H6.(iv)
Vigorous oxidation stage: CO, CH4, C2H6, and C2H4 tended to grow stably with coal temperature above 170°C. The aromatic ring structure in coal was decomposed under fierce oxidation to produce a large amount of gas due to the intense oxidation of coal spontaneous combustion at high temperature. However, the produced gas was consumed by the reaction with oxygen in the air, resulting in the steady growth of gas above.


## 2.1.1. A Theoretical Pattern for Temperature-Programmed Experiments of Coal

On the basis of the law of conservation of energy, any point in the coal temperature-programmed reaction container (regular cylinder) is selected for mathematical modeling, and the mathematical model of the coal temperature-programmed experiment process is analyzed and summarized, which can theoretically explain the adiabatic oxidation experimental principle [31–34].(1)ρCCoal∂T∂t=QA−E/RT+λ∂2T∂r2−HWaterdCWaterdt+ρAirCAirνAir∂T∂χ,where CCoal, CWater, and CAir are the specific heat capacities of coal, water, and air, respectively.We did triple integrals ofx, y, and z axes for formula (1) and sought the heat balance equation of coal in its reactor: real-time heat function equation (2) of coal in reactor varying with time t and temperature T was obtained, and equation (3) was obtained from the integral results of equation (2).(2)∭ρCCoaldxdydz∂T∂t=∭QρdxdydzA−E/RT+∭λdxdydz∂2T∂r2−∭HWaterdxdydzdCWaterdt+∭ρAirCAirνAirdxdydz∂T∂χ.Through equation (3), simple transformations were made, and the heat equation (4) of gas from coal spontaneous combustion was obtained.(3)mCoalCCoal∂T∂t=QmCoalA−E/RT+λmCoal∂2T∂r2−HWatermCoaldCWaterdt+mAirCAirνAir∂T∂t.Through equation (3), simple transformations were made, and the heat equation (4) of gas from coal spontaneous combustion was obtained.(4)mAirCAirνAir∂T∂t=mCoalCCoal∂T∂t−QmCoalA−E/RT−λmCoal∂2T∂r2+HWatermCoaldCWaterdt.We can measure the heat of generated gas by measuring the volume of gas-generated gas because the volume of gas-generated gas is proportional to heat. The macroscopic gas exhaust energy conservation formula for reaction vessels is shown in formula (4). Finally, the volume of gases generated was calculated by observing the function variation of characteristic gases with time and temperature during coal spontaneous combustion.


## 2.1.2. Experimental Process

The coal seam of Dongrong No. 2 coal mine was sampled in accordance with China National Standard GB/T 482-2008, and the coal sample was produced in accordance with China National Standard GB 474-2008. Then, using an automatic industrial analyzer, a coal industrial analysis experiment was conducted in accordance with China National Standard GB/T 212-2008. To identify the type of coal, the results of the determination were made in accordance with China National Standard GB/T 5751-2009. The content of C, H, O, N, and other components in coal was then determined and analyzed in accordance with China National Standard GB/T 476-2008 and GB/T 214-2007. Finally, the industrial and elemental examination of the coal seam was established. The experimental results are shown in Table1.Table 1
Experimental results of the coal industry and elements.
Coal samplesMadAadVadFCadCadHadOadNadSadDongrong No. 21.627.1242.249.0673.004.0012.002.100.16(1) The Temperature Programmed Experimental Device. The whole experimental measurement system is divided into three parts: gas path, temp-enclosure, and the collection and analysis of the gas sample. It is shown in Figure 3.Figure 3
Temperature-programmed experimental device: 1-gas compression bottle; 2-pressure reducing valve; 3-flowmeter;4-gas preheating tube; 5-intake pipe; 6-reactor;7-thermal insulation layer; 8-coal sample oxidation chamber; 9-thermometer;10-exhaust pipe; 11-temperature acquisition system; 12-gas collector; 13-gas chromatograph; 14-computer.(2) Experimental Conditions. Dongrong NO. 2 coal mine 25 kg coal samples were chosen. The five different types of coal samples with particle sizes of 0∼0.9 mm, 0.9∼3 mm, 3∼5 mm, 5∼7 mm, and 7∼10 mm were separated from the raw coal samples by air crushing and screening. The findings of earlier investigations demonstrate that coal particle size significantly affects the outcomes of trials involving coal spontaneous combustion [35–37]. The prevailing consensus is that coal spontaneous combustion is more likely the smaller the coal particle size. The opposite is true—they are less prone to spontaneous combustion. In a temperature-programmed box, mixed coal samples of 200 g from each of the five coal sample sizes were evaluated to reduce the impact of coal sample particle size on coal temperature programming. The programmed heating box’s experimental circumstances are established in accordance with the experimental conditions of the huge coal spontaneous combustion experimental platform at the Key Laboratory of Western Mine Exploitation and Hazard Prevention. The experimental conditions are shown in Table 2.Table 2
Experimental conditions for heating coal samples.
Coal samplesAverage size (mm)Coal height (cm)Coal weight (g)Coal volume (cm3)Air flow (ml/min)Rate of temperature increase (°C/min)Mixed size4.2015.501000.01225.001200.31 kg of coal was placed into a reactor with a diameter of 10 cm and a length of 22 cm. The coal sample was held in place by a 100 mesh copper wire mesh in order to provide consistent airflow, leaving a 2 cm gap at the top and bottom of the reactor. It was then heated in a temperature-controlled box before being delivered into the preheating air to collect the gas produced at various coal temperatures. When the temperature reached the required level, turn off the furnace, open the door, and let natural convection do the cooling. Finally, an analysis of the gas content and composition of the gas collected at various coal temperatures was performed.(3) Experimental Results. The experimental investigation demonstrates that during spontaneous combustion, coal is heated and oxidized, and various gases are created at various phases. In each stage, representative gases might be chosen in order to characterize the degree of coal spontaneous combustion. Coal spontaneous combustion in the Dongrong No. 2 coal mine is classified into four stages based on the variations in gas composition produced at various times. Figure 4 displays the variation curves of the gas volume fraction with temperature during coal spontaneous combustion.Figure 4
Curve of volume fraction of coal spontaneous combustion gas variation with temperature.The whole process of coal spontaneous combustion was divided into four stages with references [38–40], as shown in Figure 4.(i)
Room temperature stage: A small amount of CH4 and CO was generated with coal temperatures below 30°C, the main cause of which was the presence of free CH4 and CO in coal, and another small amount of CH4 broke free from adsorption to a free state.(ii)
Slow oxidation stage: The production of CH4 and CO increased significantly, and a small amount of C2H6 was produced with coal temperature ranging from 30 to 80°C. It was primarily due to the fact that only a small amount of gas composition and production occurred during the low-temperature oxidation reaction of coal. Furthermore, the majority of the gas transitions from the adsorption state to the free state increases gas production.(iii)
Rapid oxidation stage: CO combustible gas production increased significantly, as did CH4 and C2H6 production, and C2H4 appeared with coal temperatures ranging from 80 to 170°C. Because as the temperature rose, the oxidation reaction of coal spontaneous combustion accelerated and the internal structure reaction of coal became increasingly complicated, resulting in organic gas products such as C2H4 and C2H6.(iv)
Vigorous oxidation stage: CO, CH4, C2H6, and C2H4 tended to grow stably with coal temperature above 170°C. The aromatic ring structure in coal was decomposed under fierce oxidation to produce a large amount of gas due to the intense oxidation of coal spontaneous combustion at high temperature. However, the produced gas was consumed by the reaction with oxygen in the air, resulting in the steady growth of gas above.


## 2.2. The Explosion Experiment of Multiple Combustible Gas Mixtures

### 2.2.1. Explosion Theoretical Model of Multiple Combustible Gas Mixtures

A complex chemical reaction and flow process lead to a gas explosion in a small space. The description of the mixed gas explosion process is made simpler because it is extremely complex by using the chemical reaction mechanism of the mixed gas ingredient and some assumptions [41–43].(i)
The internal gas in the reaction process possesses all the characteristics of a true gas, satisfying the equation at the gas state. This is an insulated process in the explosion, and there is no heat exchange between the environment and the machinery.(ii)
Each flammable gas could react entirely in a constrained regular space, and multiple gas mixes are thoroughly homogeneously blended.(iii)
Reactive gases like inert gases, which can absorb and release energy, are not taken into account.(iv)
The chain branching reactions between different elements that do not interact with one another and react separately with oxygen in air are what cause gas explosions.Based on the assumptions made above, the basic equation for a 20 L spherical explosion tank’s closed space is as follows:(i)
Continuity equation:(5)∂V∂t+∂Vum∂xm=0.(ii)
Momentum equation:(6)∂∂tVum+∂∂xnVumun−μeff∂um∂xn=∂P∂xn+∂∂xmμeff∂um∂xn−23∂∂xmδnmVk+μeff∂ul∂xl.(iii)
Energy equation:(7)∂∂tVh+∂∂xnVunh−μeffσh∂h∂xn=∂p∂t+Sh.(iv)
Total equilibrium equation:(8)∑i=1N∂ViIi∂t=∑i=1N∂Vi′Ii∂t.


### 2.2.2. Experimental Process

CO, CH4, C2H4, and C2H6 are the main combustible gases generated by coal spontaneous combustion in Dongrong No. 2 coal mine, according to the heating experimental process of coal spontaneous combustion. CH4 is the main disaster gas in coal mines, as well as the main gas causing coupling disasters in coal mine fires, due to its high content. The explosion limits of CO, CH4, C2H4, and C2H6 in air were first measured using 20 L spherical explosive devices, and the results were used as a reference for subsequent experiments. Then, the influence of C2H6, C2H4, and CO on CH4 explosion limit was determined by using the explosion limit determination method of combustible gases in air, which provided an experimental and theoretical foundation for the study of gas explosion limit in different stages of coal spontaneous combustion. Based on the detected difference in gas generation at the stage of coal spontaneous combustion, representative gases at four critical points in the coal spontaneous combustion experiment were collected for explosion experiments, and the critical value of gas explosion at the critical point was determined, resulting in the grading warning.(1) Experimental Device. A 20 L spherical explosion test apparatus was used for the experiment, which primarily consists of three parts: a spherical explosion device, a control system, and a data collection system, as shown in Figure 5.Figure 5
Explosion experiment device.(2) Experimental Conditions. The entire test was conducted at standard pressure and temperature, and the humidity of the gas mixtures ranged from 45 to 50% while N2 served as the shielding gas. Premix stabilization time was set to 300 s; constant temperature duration was set to 100 s; accuracy was set to ±10°C; ignition delay was set to 60 ms; solenoid valve opening was set to 10 ms; sampling frequency was set to 5 kH; sampling time was set to 2000 ms.After the ignition delay period was set, the system would automatically begin the experiment. The pressure sensor would now provide the computer with the pressure data generated by the material explosion in the explosion tank. When the explosion’s pressure peak exceeded 7% of the pressure preceding the explosion, it was determined to be an explosion.Reduce or increase the concentration of combustible gas to obtain the lower or upper explosion limit, and then repeat the steps above after it has exploded. If it did not explode, adjust its concentration and repeat the steps above after it did.Start the vacuum pump after each explosion experiment to expel any residual gas in the tank. Open the intake valve to let in fresh air before proceeding to the next experiment.(3) Experimental Results. The heating experiment of coal spontaneous combustion revealed the presence of small amounts of gas such as CO, CH4, C2H4, and C2H6 in the process of coal autoignition. The gas was distributed by the partial pressure method, and the CO, C2H6, and C2H4 gases with volume fractions of 0.2%, 0.4%, 0.6%, and 0.8% were taken for experiments to compare the effect of three combustible gases including CO, C2H4, and C2H6 on the explosion of CH4, combined with the heating experiment result of coal spontaneous combustion. Table 3 and Figure 6 show the final experimental results.Table 3
Statistics of explosion limit test data for combustible gases.
CO explosion limitC2H4 explosion limitC2H6 explosion limitCH4 explosion limitCH4 explosion hazardExplosion limit of mixed gasMixed gas explosion hazardLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limit12.173.82.735.63.214.95.115.5Explosion hazardH = 5.1Explosion hazardH = 12.2Explosion hazardH = 3.6Explosion hazardH = 2.0CO concentrationC2H4 concentrationC2H6 concentration0005.115.52.035.115.52.030.2004.5515.352.374.7515.552.270.4004.4515.252.434.8515.652.230.6004.3015.202.534.9515.802.190.8004.2515.152.565.0515.952.1600.203.814.952.934.015.152.7800.403.714.83.004.115.202.7100.603.414.03.124.014.62.6500.803.013.93.633.814.72.87000.23.7814.252.773.9814.452.63000.43.5214.022.983.9214.422.68000.63.3213.43.043.9214.02.57000.82.9812.23.093.7813.22.49Figure 6
Experimental data analysis: (a) Lower limit of CH4 explosion, (b) upper limit of CH4 explosion, (c) CH4 explosion hazard index, (d) lower limit of mixed gas explosion, (e) lower limit of mixed gas explosion, and (f) mixed gas explosion hazard index.
(a)(b)(c)(d)(e)(f)As shown in Table3, the results of the explosion limit of a single combustible gas under air conditions show that C2H4 has the largest explosion limit range, with the highest risk of 12.2, and the explosion risk is C2H4, CO, C2H6, and CH4. The explosion limit of CH4 varies with the concentrations of C2H4, CO, and C2H6. C2H6 has the most noticeable effect on the gas explosion limit. The minimum value of the upper limit of a CH4 explosion is 12.2, and the minimum value of the lower limit of a CH4 explosion is 2.98. The effect of C2H4 on gas explosion limit is the second, with the lowest value of CH4 explosion limit being 13.9 and 3.0. CO has the smallest effect on the gas explosion limit. The minimum upper limit for a CH4 explosion is 15.15, and the minimum lower limit is 4.25. The explosion risk of a mixture of different concentrations of gas is lower than that of CH4.Figure6(a) shows that the addition of combustible gases lowers the lower explosion limit of CH4, but with different downward trends. The effect of organic combustible gases C2H4 and C2H6 on the lower explosion limit of CH4 with the addition of combustible gases is essentially the same, which is significantly greater than that of the inorganic combustible gas CO. Figure 6(b) shows that the addition of combustible gases lowers the upper explosion limit of CH4, but with different downward trends. With the addition of CO, the variation trend of the upper explosion limit of CH4 becomes more stable. The addition of C2H4 and C2H6 highlights the downward trend in the upper explosion limit of CH4, with C2H6 having a greater impact on the upper explosion limit of CH4 than C2H4. Figure 6(c) shows that the explosion danger degree H of CH4 has been increasing with the addition of combustible gases. CO has less of an effect than other gases, and C2H4 and C2H6 have a more obvious effect on the explosion danger degree H of CH4.As shown in Figure6(d), the variation trend of the lower explosion limit of two-component mixed combustible gas changes with the addition of different types of combustible gases. When the gas addition amount is 0.2%, the variation trend of the lower explosion limit of the mixed gas is the same, and both decrease significantly. The decreasing trend of CO is significantly slower than that of C2H4 and C2H6; after the gas addition amount is 0.2%, the addition of C2H4 and C2H6 makes the downward trend of the lower explosion limit of CH4 become gentle, and the addition of CO makes the lower explosion limit of the mixed gas to rise slowly. As shown in Figure 6(e), the upper explosion limit of mixed gas slowly rises with the addition of CO, while the upper explosion limit of CH4 decreases with the addition of C2H4 and C2H6, and the downward trend of C2H6 is greater than that of C2H4. As shown in Figure 6(f), increasing the addition of three gases causes the explosion danger degree of the mixed gas to increase first and then gradually decrease, and adding C2H4 causes the explosion danger of the mixed gas to fluctuate to some extent. The addition of CO reduces the explosion danger degree of the mixed gas compared to C2H4 and C2H6.


### 2.2.3. The Influence of Mixed Gas Generated by Coal Spontaneous Combustion on Gas Explosion Characteristics

The research object was gas production at the representative coal temperature during the process of coal spontaneous combustion, and the CH4 gas was then filled with various gases collected at the selected location of coal spontaneous combustion. Table4 shows the specific volume fraction of the gas. Table 4 shows that as the coal spontaneous combustion process progresses, the types of gas-generated gas gradually diversify and the gas volume fraction gradually increases, indicating the coal spontaneous combustion process at all stages. The spontaneous combustion gas was mixed in a 2.5% proportion with the gas intake during the coal spontaneous combustion experiment for the explosion experiment to study the influence of the gas generated at different stages of coal spontaneous combustion on the lower explosion limit of gas explosion. Table 4 displays the experimental results (Table 4).Table 4
Gas volume fraction in different stages of coal spontaneous combustion and its lower limit of gas explosion and explosion pressure.
The process of coal spontaneous combustionVapor volume fraction (%)Explosion pressure (MPa)COCH4C2H4C2H6CO2O2N2CH4 concentration (%)5.1%4.9%4.6%4.3%4.0%Room temperature stage (with coal temperature 30°C)0.00050.0107001.0519.179.70.25NONONONOSlow oxidation stage (with coal temperature 80°C)0.02810.02870.00020.00592.7818.878.40.260.21NONONORapid oxidation stage (with coal temperature 130°C, which is the change node of C2H6 production)0.20300.06190.00110.0122.6917.778.30.280.260.22NONOVigorous oxidation stage (with coal temperature 170°C)0.43500.10420.00710.10182.8215.878.70.290.270.240.21NOIt can be seen from Table4 that the pressure generated by the mixed explosion of gas produced at room temperature, and 5.1% gas was 0.25 MPa, and no explosion occurred with other concentrations of gas, indicating that there is no gas that has an impact on gas explosion at this stage. The pressure generated by the mixed explosion of slow oxidation gas, and 5.1% gas was 0.26 MPa, and 0.21 MPa with 4.9% gas. There was no explosion in the other gas concentrations, indicating that the gas produced during the initial stage of coal spontaneous combustion can reduce the lower explosion limit and improve the explosion pressure. The gas produced in the rapid oxidation stage exploded with gas concentrations of 5.1%, 4.9%, and 4.6%, respectively, at a maximum gas explosion pressure of 0.28 MPa, and there was no explosion with gas concentrations of 4.3% and 4.0%. The gas produced during the coal combustion stage exploded with gas concentrations of 5.1%, 4.9%, 4.6%, and 4.3%, respectively. The maximum gas explosion pressure was 0.29 MPa, and there was no explosion occurred with a gas concentration of 4.0%, indicating that in addition to CO, C2H4 and C2H6 can promote gas combustion and explosion, as well as increase the gas explosion pressure.


## 2.2.1. Explosion Theoretical Model of Multiple Combustible Gas Mixtures

A complex chemical reaction and flow process lead to a gas explosion in a small space. The description of the mixed gas explosion process is made simpler because it is extremely complex by using the chemical reaction mechanism of the mixed gas ingredient and some assumptions [41–43].(i)
The internal gas in the reaction process possesses all the characteristics of a true gas, satisfying the equation at the gas state. This is an insulated process in the explosion, and there is no heat exchange between the environment and the machinery.(ii)
Each flammable gas could react entirely in a constrained regular space, and multiple gas mixes are thoroughly homogeneously blended.(iii)
Reactive gases like inert gases, which can absorb and release energy, are not taken into account.(iv)
The chain branching reactions between different elements that do not interact with one another and react separately with oxygen in air are what cause gas explosions.Based on the assumptions made above, the basic equation for a 20 L spherical explosion tank’s closed space is as follows:(i)
Continuity equation:(5)∂V∂t+∂Vum∂xm=0.(ii)
Momentum equation:(6)∂∂tVum+∂∂xnVumun−μeff∂um∂xn=∂P∂xn+∂∂xmμeff∂um∂xn−23∂∂xmδnmVk+μeff∂ul∂xl.(iii)
Energy equation:(7)∂∂tVh+∂∂xnVunh−μeffσh∂h∂xn=∂p∂t+Sh.(iv)
Total equilibrium equation:(8)∑i=1N∂ViIi∂t=∑i=1N∂Vi′Ii∂t.


## 2.2.2. Experimental Process

CO, CH4, C2H4, and C2H6 are the main combustible gases generated by coal spontaneous combustion in Dongrong No. 2 coal mine, according to the heating experimental process of coal spontaneous combustion. CH4 is the main disaster gas in coal mines, as well as the main gas causing coupling disasters in coal mine fires, due to its high content. The explosion limits of CO, CH4, C2H4, and C2H6 in air were first measured using 20 L spherical explosive devices, and the results were used as a reference for subsequent experiments. Then, the influence of C2H6, C2H4, and CO on CH4 explosion limit was determined by using the explosion limit determination method of combustible gases in air, which provided an experimental and theoretical foundation for the study of gas explosion limit in different stages of coal spontaneous combustion. Based on the detected difference in gas generation at the stage of coal spontaneous combustion, representative gases at four critical points in the coal spontaneous combustion experiment were collected for explosion experiments, and the critical value of gas explosion at the critical point was determined, resulting in the grading warning.(1) Experimental Device. A 20 L spherical explosion test apparatus was used for the experiment, which primarily consists of three parts: a spherical explosion device, a control system, and a data collection system, as shown in Figure 5.Figure 5
Explosion experiment device.(2) Experimental Conditions. The entire test was conducted at standard pressure and temperature, and the humidity of the gas mixtures ranged from 45 to 50% while N2 served as the shielding gas. Premix stabilization time was set to 300 s; constant temperature duration was set to 100 s; accuracy was set to ±10°C; ignition delay was set to 60 ms; solenoid valve opening was set to 10 ms; sampling frequency was set to 5 kH; sampling time was set to 2000 ms.After the ignition delay period was set, the system would automatically begin the experiment. The pressure sensor would now provide the computer with the pressure data generated by the material explosion in the explosion tank. When the explosion’s pressure peak exceeded 7% of the pressure preceding the explosion, it was determined to be an explosion.Reduce or increase the concentration of combustible gas to obtain the lower or upper explosion limit, and then repeat the steps above after it has exploded. If it did not explode, adjust its concentration and repeat the steps above after it did.Start the vacuum pump after each explosion experiment to expel any residual gas in the tank. Open the intake valve to let in fresh air before proceeding to the next experiment.(3) Experimental Results. The heating experiment of coal spontaneous combustion revealed the presence of small amounts of gas such as CO, CH4, C2H4, and C2H6 in the process of coal autoignition. The gas was distributed by the partial pressure method, and the CO, C2H6, and C2H4 gases with volume fractions of 0.2%, 0.4%, 0.6%, and 0.8% were taken for experiments to compare the effect of three combustible gases including CO, C2H4, and C2H6 on the explosion of CH4, combined with the heating experiment result of coal spontaneous combustion. Table 3 and Figure 6 show the final experimental results.Table 3
Statistics of explosion limit test data for combustible gases.
CO explosion limitC2H4 explosion limitC2H6 explosion limitCH4 explosion limitCH4 explosion hazardExplosion limit of mixed gasMixed gas explosion hazardLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limitLower explosive limitUpper explosive limit12.173.82.735.63.214.95.115.5Explosion hazardH = 5.1Explosion hazardH = 12.2Explosion hazardH = 3.6Explosion hazardH = 2.0CO concentrationC2H4 concentrationC2H6 concentration0005.115.52.035.115.52.030.2004.5515.352.374.7515.552.270.4004.4515.252.434.8515.652.230.6004.3015.202.534.9515.802.190.8004.2515.152.565.0515.952.1600.203.814.952.934.015.152.7800.403.714.83.004.115.202.7100.603.414.03.124.014.62.6500.803.013.93.633.814.72.87000.23.7814.252.773.9814.452.63000.43.5214.022.983.9214.422.68000.63.3213.43.043.9214.02.57000.82.9812.23.093.7813.22.49Figure 6
Experimental data analysis: (a) Lower limit of CH4 explosion, (b) upper limit of CH4 explosion, (c) CH4 explosion hazard index, (d) lower limit of mixed gas explosion, (e) lower limit of mixed gas explosion, and (f) mixed gas explosion hazard index.
(a)(b)(c)(d)(e)(f)As shown in Table3, the results of the explosion limit of a single combustible gas under air conditions show that C2H4 has the largest explosion limit range, with the highest risk of 12.2, and the explosion risk is C2H4, CO, C2H6, and CH4. The explosion limit of CH4 varies with the concentrations of C2H4, CO, and C2H6. C2H6 has the most noticeable effect on the gas explosion limit. The minimum value of the upper limit of a CH4 explosion is 12.2, and the minimum value of the lower limit of a CH4 explosion is 2.98. The effect of C2H4 on gas explosion limit is the second, with the lowest value of CH4 explosion limit being 13.9 and 3.0. CO has the smallest effect on the gas explosion limit. The minimum upper limit for a CH4 explosion is 15.15, and the minimum lower limit is 4.25. The explosion risk of a mixture of different concentrations of gas is lower than that of CH4.Figure6(a) shows that the addition of combustible gases lowers the lower explosion limit of CH4, but with different downward trends. The effect of organic combustible gases C2H4 and C2H6 on the lower explosion limit of CH4 with the addition of combustible gases is essentially the same, which is significantly greater than that of the inorganic combustible gas CO. Figure 6(b) shows that the addition of combustible gases lowers the upper explosion limit of CH4, but with different downward trends. With the addition of CO, the variation trend of the upper explosion limit of CH4 becomes more stable. The addition of C2H4 and C2H6 highlights the downward trend in the upper explosion limit of CH4, with C2H6 having a greater impact on the upper explosion limit of CH4 than C2H4. Figure 6(c) shows that the explosion danger degree H of CH4 has been increasing with the addition of combustible gases. CO has less of an effect than other gases, and C2H4 and C2H6 have a more obvious effect on the explosion danger degree H of CH4.As shown in Figure6(d), the variation trend of the lower explosion limit of two-component mixed combustible gas changes with the addition of different types of combustible gases. When the gas addition amount is 0.2%, the variation trend of the lower explosion limit of the mixed gas is the same, and both decrease significantly. The decreasing trend of CO is significantly slower than that of C2H4 and C2H6; after the gas addition amount is 0.2%, the addition of C2H4 and C2H6 makes the downward trend of the lower explosion limit of CH4 become gentle, and the addition of CO makes the lower explosion limit of the mixed gas to rise slowly. As shown in Figure 6(e), the upper explosion limit of mixed gas slowly rises with the addition of CO, while the upper explosion limit of CH4 decreases with the addition of C2H4 and C2H6, and the downward trend of C2H6 is greater than that of C2H4. As shown in Figure 6(f), increasing the addition of three gases causes the explosion danger degree of the mixed gas to increase first and then gradually decrease, and adding C2H4 causes the explosion danger of the mixed gas to fluctuate to some extent. The addition of CO reduces the explosion danger degree of the mixed gas compared to C2H4 and C2H6.


## 2.2.3. The Influence of Mixed Gas Generated by Coal Spontaneous Combustion on Gas Explosion Characteristics

The research object was gas production at the representative coal temperature during the process of coal spontaneous combustion, and the CH4 gas was then filled with various gases collected at the selected location of coal spontaneous combustion. Table4 shows the specific volume fraction of the gas. Table 4 shows that as the coal spontaneous combustion process progresses, the types of gas-generated gas gradually diversify and the gas volume fraction gradually increases, indicating the coal spontaneous combustion process at all stages. The spontaneous combustion gas was mixed in a 2.5% proportion with the gas intake during the coal spontaneous combustion experiment for the explosion experiment to study the influence of the gas generated at different stages of coal spontaneous combustion on the lower explosion limit of gas explosion. Table 4 displays the experimental results (Table 4).Table 4
Gas volume fraction in different stages of coal spontaneous combustion and its lower limit of gas explosion and explosion pressure.
The process of coal spontaneous combustionVapor volume fraction (%)Explosion pressure (MPa)COCH4C2H4C2H6CO2O2N2CH4 concentration (%)5.1%4.9%4.6%4.3%4.0%Room temperature stage (with coal temperature 30°C)0.00050.0107001.0519.179.70.25NONONONOSlow oxidation stage (with coal temperature 80°C)0.02810.02870.00020.00592.7818.878.40.260.21NONONORapid oxidation stage (with coal temperature 130°C, which is the change node of C2H6 production)0.20300.06190.00110.0122.6917.778.30.280.260.22NONOVigorous oxidation stage (with coal temperature 170°C)0.43500.10420.00710.10182.8215.878.70.290.270.240.21NOIt can be seen from Table4 that the pressure generated by the mixed explosion of gas produced at room temperature, and 5.1% gas was 0.25 MPa, and no explosion occurred with other concentrations of gas, indicating that there is no gas that has an impact on gas explosion at this stage. The pressure generated by the mixed explosion of slow oxidation gas, and 5.1% gas was 0.26 MPa, and 0.21 MPa with 4.9% gas. There was no explosion in the other gas concentrations, indicating that the gas produced during the initial stage of coal spontaneous combustion can reduce the lower explosion limit and improve the explosion pressure. The gas produced in the rapid oxidation stage exploded with gas concentrations of 5.1%, 4.9%, and 4.6%, respectively, at a maximum gas explosion pressure of 0.28 MPa, and there was no explosion with gas concentrations of 4.3% and 4.0%. The gas produced during the coal combustion stage exploded with gas concentrations of 5.1%, 4.9%, 4.6%, and 4.3%, respectively. The maximum gas explosion pressure was 0.29 MPa, and there was no explosion occurred with a gas concentration of 4.0%, indicating that in addition to CO, C2H4 and C2H6 can promote gas combustion and explosion, as well as increase the gas explosion pressure.


## 3. Conclusions

The following results are reached by theoretical investigation and experimental inquiry.(i)
The gas produced by coal spontaneous combustion is primarily CO, CH4, C2H4, and C2H6. The gas content of various components increases with time and temperature. CO has the highest volume fraction, which is 0.47%. The volume fractions of C2H6 and CH4 are similar, with C2H4, having the lowest volume fraction. Simultaneously, coal spontaneous combustion is divided into four stages based on the characteristics of these four gases produced at different times: room temperature stage (with coal temperature below 30°C), slow oxidation stage (with coal temperature ranging from 30 to 80°C), rapid oxidation stage (with coal temperature ranging from 80 to 170°C), and combustion stage (with coal temperature above 170°C).(ii)
The effect of organic combustible C2H4 and C2H6 on the explosion limit of CH4 is more obvious as the gas volume fraction increases than that of inorganic combustible CO. 0.8% C2H6, C2H4, and CO, in particular, reduced the lower explosion limit of CH4 from 5.1% to 2.98%, 3.0%, and 4.25%, respectively. The addition of 0.8% C2H6, C2H4, and CO reduced the upper explosion limit of CH4 from 15.5% to 12.2%, 13.9%, and 15.15%. The upper and lower explosion limits of the C2H6/CH4 mixture are the lowest, at 13.2% and 3.78%, respectively. The upper and lower explosion limits of a C2H4/CH4 mixture are 14.7% and 3.8%, respectively. It demonstrates that C2H4 and C2H6 have a significant impact on the explosion limit of mixed gas. The CO/CH4 mixture explosion limit decreases first and then increases.(iii)
The maximum pressure of gas and CH4 mixed explosion in each stage of coal spontaneous combustion increases with the continuous process, from 0.25 MPa to 0.29 MPa, while the lower limit of CH4 explosion gradually decreases from 5.1% to 4.3%. The C2H4 and C2H6 generated in the initial stage of coal spontaneous combustion are very few and then gradually increase with the process of coal spontaneous combustion, indicating that C2H4 and C2H6 have a significant influence on the explosion pressure of mixed gas and the lower limit of gas explosion in the process of coal spontaneous combustion.As a result, monitoring of goaf temperature and CO, CH4, C2H4, C2H6, and other gases should be improved in order to detect and prevent them in time. Simultaneously, the experimental results of various combustible gas explosions are improved, and the results are substituted into the simulation to predict and warn the gas explosion, and gas explosion risk judgment criteria in goaf suitable for coal spontaneous combustion environment are formulated.



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