Research on the characteristics of coal-oxygen reaction in the lean

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Research on the characteristics of coal-oxygen reaction in the lean-oxygen environment caused by methane Hongwei Liu, Fei Wang, and Ting Ren Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01753 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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Research on the characteristics of coal-oxygen reaction in the lean-oxygen environment caused by methane Hongwei Liu[a],[b],[c], Fei Wang*[a],[b], Ting Ren[c] [a]College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, 030024, China [b]Center of Shanxi Mine Safety for Graduate Education Innovation, Taiyuan, 030024, China [c]School of Civil, Mining and Environmental Engineering, University of Wollongong, Wollongong, New South Wales 2500, Australia

*Corresponding

to: Fei Wang

Email: [email protected] Tel: +86-18734851429

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Abstract: Coal mine fire often occurs in lean-oxygen environments caused by methane. In order to clarify the characteristics and mechanisms of coal-oxygen reaction under these conditions, the coal-oxygen reaction experiments were carried by using thermogravimetric analysis (TGA)-gas chromatograph (GC) combined technology in the range of 30-800oC. The thermogravimetric-subtraction/derivative thermogravimetricsubtraction (TG-S/DTG-S) curves which can more accurately reflect the coal-oxygen process were obtained by subtracting the TG/DTG curve of coal pyrolysis under the condition of pure N2. Through TG-S/DTG-S curves, the coal-oxygen reaction was divided into low temperature oxidation stage, accelerated oxidation stage and combustion stage. In the low temperature oxidation and accelerated oxidation stages, methane delayed the coaloxygen reaction as an inert gas. Under the same oxygen concentration, the effect of methane on reducing CO, CO2 production and increasing ignition temperature (Ti) of coal was obvious than that of N2. In the combustion stage, the high temperature environment (> 500oC) caused intense combustion and decomposition of the surrounding methane, which consumed a lot of oxygen by direct combustion of methane and combustion of methane decomposition products (C and H2) and released a large amount of CO, CO2 and H2. When the temperature was > 586oC, the oxygen was completely consumed by methane and the coal-oxygen reaction was completely inhibited. With the decrease of initial reaction oxygen concentration (the increase of methane concentration), the production of CO2 decreased gradually at the same temperature, while the changes of CO and H2 were more complex. The research results are of great significance to the prediction and prevention of coal mine fire in goaf. Key words: coal-oxygen reaction; methane; lean-oxygen conditions; TGA-GC experiments

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1 Introduction Coal-oxygen reaction can lead to the self-heating and spontaneous combustion of coal, which is a great threat to coal mining, storage and transportation1. It can burn up a lot of coal resources2, lead to the release of large amounts of greenhouse gases such as CO2 and methane, release a large amount of toxic and harmful substances36

and sometime cause methane and coal dust explosions7-8. Therefore, the coal-oxygen reaction has always been

the focus of research, especially in the spontaneous combustion of coal caused by low temperature oxidation9-17. Initially, the research on coal-oxygen reaction was mainly carried out in air atmosphere. In recent years, it has been found that the coal-oxygen reaction environment in coal mine is significantly different from atmospheric environment. One of the most obvious features is that coal-oxygen reaction is often in the lean-oxygen environment due to the incorporation of other gases. A large number of studies have shown that lean-oxygen environment has significant impact on the generated gases18, kinetic parameters19-20, heat release19, 21 and ignition temperature20, 22 of coal-oxygen reaction. Although the coal-oxygen reaction in lean-oxygen environment has been studied, the lean-oxygen environment in the above research is mostly realized by injecting inert gas (N2 or CO2). At present, there are few researches on coal-oxygen reaction in the lean-oxygen environment caused by methane. As we all know, coal mine methane (CMM) is the associated gas in the process of coal formation, which is released continuously in the process of coal mining23. High concentration methane emission can cause accidents such as methane explosion, combustion and asphyxiation, and most of the time, the presence of methane in mine environment is more dangerous than the self-heating of coal24. Ventilation25-26 and methane extraction27 are often used to prevent methane accumulation in underground mine. However, one place where methane accumulation is really difficult to control is the long-wall goaf of working face. Through a lot of numerical simulation and field testing, Ting et al.28-29 found that the deep part of long-wall goaf is always in the state of methane accumulation. In addition, the fire in long-wall goaf accounts for 60% of the coal spontaneous combustion in China30. Thus, coal-oxygen reaction in the lean-oxygen environment caused by methane is a common phenomenon in coal mine production, and it is necessary to conduct in-depth study on it. Unlike the inert gas of N2 or CO2, the influence of methane on coal-oxygen reaction is reflected in two aspects: reducing oxygen concentration and participating in coal spontaneous combustion. The coal mining is bound to be accompanied by a large amount of methane emission. In the mining process, the long-wall goaf is usually in semi-enclosed state, thus, the emission of methane will lead to the decrease of oxygen concentration with the increase of distance from the working face19. In the long-wall goaf, when the emitted methane and air are mixed, the concentration of methane can reach 30-50 mol%, the concentration of O2 is 10-15 mol%, and the rest are other gases mainly composed of N231. A number of investigators have observed a dependence of the rate of coal oxidation on oxygen concentration in the gas medium, and they suggested that the rate of oxygen consumption over a wide range of oxygen concentration can be expressed as a power of the partial pressure of oxygen in the oxidation medium32. The mixing of methane can slow down the reaction rate of coal oxygen by reducing oxygen partial pressure. On the other hand, Wang et al.33 found that the partial occupation of coal surface by methane molecules may hinder the dissipation of heat generated through the chemisorptions and chemical reaction between O2 and coal, resulting in more heat accumulation on coal surface and accelerate coal spontaneous combustion. Li et al.34 found that the change of CO2 under the condition of methane was different 3


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than of CO. With the increase of the concentration methane, aromatic ―𝐶𝐻 inhibited the spontaneous combustion of coal, while methyl ( ― 𝐶𝐻3) and methylene ( ― 𝐶𝐻2) promoted it. The inhibition effect of alkyl ether ―𝐶 ― 𝑂 ― 𝐶 and aromatic ring 𝐶 = 𝐶 on spontaneous combustion of coal was weakened and enhanced, respectively. Ren et al.35 found that the intense combustion of methane could inhibit the process of coal-oxygen reaction and there was an obvious competitive relationship between methane and coal combustion. Up to now, there is no systematic research on coal-oxygen under the lean-oxygen conditions caused by methane, and the mechanisms of coal-oxygen reaction, methane-oxygen reaction and their interaction in this environment are still unclear. Therefore, it is necessary to study the coal-oxygen reaction process from low temperature oxidation to high temperature combustion in the lean-oxygen environment caused by methane. Adiabaticoxidation36, Basket heating37-38, Thermal analysis39-40, Gas chromatography (GC) or Gas chromatography mass spectrophotometer (GC–MS)41-42, Infrared/Fourier transform infrared (FTIR)43-45, X-ray diffraction (XRD)46, X-ray photoelectron spectroscopy (XPS)47, and Nuclear magnetic resonance (NMR)48 are the common methods to study coal-oxygen reaction. Thermal analysis has the advantages of requiring fewer test samples and a shorter experimental duration and providing good repeatability. The change laws of mass and heat in the heating process with temperature (time) and the characteristics temperature and activation energy can be obtained by suitable analysis methods49. Gas chromatograph (GC) is widely used in the detection of gases produced in the process of coal-oxygen reaction. In this paper, the TGA-GC combined technology and equipment were used to study the characteristics of coal-oxygen reaction in the lean-oxygen environment caused by methane.

2 Experimental section 2.1 Coal sample Xishan coalfield is an important coal-producing base in Shanxi Province, China. The coal seam methane content is high and the coal mine fire is serious, and many serious coal mine fire and methane explosion accidents caused by coal mine fire have occurred in the process of coal mine production. Thus, in the experiments, fresh raw coal sample of 8# coal seam of Xishan coalfield was transported to the laboratory with plastic wrap. The coal sample was milled in laboratory under isolated air condition and sieved into the particle size needed in experiments. Properties of the coal sample are shown in Table 1. Table 1

2.2 Experimental setup In the work, TGA-GC experiments of coal-oxygen reaction in the lean-oxygen environment caused by methane were carried out. The experimental setup is shown in Figure 1. The HCT-1 thermogravimetric balance produced by Hengjiu and GC-6890A gas chromatograph produced by Lunan were used in the experiments. In order to better simulate the actual environment of underground coal-oxygen reaction process, different degrees of lean-oxygen conditions were achieved by mixing CH4 (99.999%) and air (21% O2 and 79% N2). The oxygen concentrations (volume fraction) under the conditions of CH4 were set at 21%, 15%, 10%, 5% and 3%. At the same time, the mixture of N2 (99.999%) and air was used as contrast experiment at the same oxygen concentration. After produced by gas distribution system in Figure 1, the methane-containing air entered the

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thermogravimetric balance. The anti-tempering device was used to prevent the tempering of methane. In the thermogravimetric balance, with the increase of temperature, the reaction between methane-containing air and coal sample becomes more and more intense. Then the reacted gases enter the GC, and the gas change in the reaction process is detected and recorded in real time. The coal sample particle size was 0.30-0.45mm, the quantity was 20mg, the gas flow rate was 20ml/min, the temperature range was 30-800℃, and the heating rate was 2℃/min in the range of 30-250℃ and 5℃/min in the range of 250-800℃. In order to ensure the accuracy of the experiments, the experiments in each testing condition were conducted 5 times, and the results of the experiments were taken as the average of the 5 results. Figure 1

3 Results and discussion 3.1 TG-S/DTG-S correction curves of coal-oxygen reaction Coal-oxygen reaction at different temperature is a complex process involving both physical and chemical reactions. However, in the process of coal-oxygen reaction, parallel reactions such as water evaporation50, gas desorption and thermal decomposition51-52 are also included. Previous studies have generally considered the above parallel reactions as a whole to consider the mass change of coal in TGA experiments, which has a certain impact on the study of coal-oxygen reaction process. Zhang et al.53-54 proposed to study the low temperature oxidation process of coal by subtracting the TG/DTG curves in N2 from the TG/DTG curves in Air. This method was also used in this paper to accurately quantify the coal-oxygen reaction. The TG/DTG curves in Air and N2 conditions are shown in Figure 2 and represented by TG(Air)/DTG(Air) and TG(N2)/DTG(N2). After elimination of the effect of water evaporation, gas desorption and thermal decomposition, the mass change caused by coal-oxygen reaction alone is represented by TG-S(Air)/DTG-S(Air). Figure 2

3.2 Division of coal-oxygen reaction stages in the lean-oxygen environment caused by methane The TG-S/DTG-S curves of coal-oxygen reaction in the lean-oxygen environment caused by methane are shown in Figure 3. As can be seen from Figure 3, coal-oxygen reaction has significant staged change in the range of 30-800oC. Previous studies have shown that the mechanism, path and products of coal-oxygen reaction in different stages are greatly different13, 22, 54-55, so the coal-oxygen reaction was divided into different stages by the TG-S/DTG-S curves. In the range of 30-350oC, the change of coal sample mass was small and gentle, and we define it as the low temperature oxidation stage. When the temperature was about 350oC, the value of DTG-S curve was 0. After 350oC, the TG-S and DTG-S curves began to decrease gradually. The coal-oxygen reaction entered the accelerated oxidation stage, and then entered the combustion stage. Ignition temperature (Ti), burnout temperature (Tb), and peak temperature (Tmax) are the characteristic temperatures of coal combustion stage. Ti is defined as the temperature at which samples begin to burn51. After this temperature, coal is rapidly oxidized and

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decomposed, the mass begins to fall sharply, the dense ring aromatic nucleuses in coal are completely cracked, the coal tar is discharged in large quantities and the volatiles begin to burn56-58. Ti can be determined through several methods. Li et al.59 used TGA−DTG tangent method. Wang et al.60 defined Ti as the temperature at which the combustion rate is raised to 1 wt %/min. Tb is the temperature when the coal is burnout, and it also can be defined as the temperature when the combustion rate is reduced to 1 wt %/min61. In this paper, Ti and Tb were obtained by the intersection of DTG-S=-0.1mg/min horizontal line and DTG-S curves (shown in Figure 3). Tmax was the temperature corresponding to the peak of the DTG-S curve60. The coal-oxygen reaction was defined as low temperature oxidation stage (30-350oC), accelerated oxidation stage (350oC-Ti) and combustion stage (Ti-Tb). In order to compare the influence of methane, the TGA-GC experiments of coal-oxygen reaction under the same lean-oxygen conditions caused by N2 were carried out and the TG-S/DTG-S curves are shown in Figure 4. As can be seen from Figure 4, coal-oxygen reaction also has significant staged change in the range of 30-800oC, and the division of the stage is the same as that under methane conditions. Figure 3 Figure 4

3.3 Characteristics of coal-oxygen reaction in the lean-oxygen environment caused by methane 3.3.1 Low temperature oxidation stage In terms of self-heating of coal, the low temperature region is an important stage of coal spontaneous combustion, and has always been the focus of research9-17. In the low temperature oxidation stage (30-350oC), a series of reactions occur between coal and oxygen, including physical and chemical adsorption of oxygen on coal surface, the formation of intermediate complexes, and the decomposition of unstable oxygenated intermediates to gaseous products and other species13, 54-55. At present, it is generally believed that there are two parallel reaction sequences in the low temperature oxidation process of coal: the direct burnoff (formula(1)) and sorption sequences (formula(2))13.

burnoff coal  O2     CO2 , CO and H 2O

(1) CO2 , CO and H 2 O

Unstable solid intermediate sorption coal  O2     carbon  oxygen complexes

Stable solid products, which tend to decompose at

(2)

temperatures above 70 o C

The changes of O2, CO and CO2 concentrations during the low temperature oxidation are shown in Figures 5, 6 and 7. In this stage, with the increase of temperature, oxygen concentration decreased gradually and slowly, accompanied by the formation of CO and CO2. Zhang et al.62 think that active oxygenated species in the coal matrix are the precursors for CO and CO2 emissions63, and these species can be divided into inherent oxygencontaining functional groups, such as COOH , C  O , O  C  OR and C  O  C , and surface oxides, such as C  O  OH and C (O)  O  OH . In the range of 30-70oC, the production of CO and CO2 were small and the lean-oxygen environment caused by methane and N2 had no obvious influence on them. After 70oC, the influence of oxygen concentration on the production of CO and CO2 was gradually significant, and the

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higher the oxygen concentration was, the larger the production of CO and CO2. The reason is that when the temperature is below 70oC, the main process of coal-oxygen reaction is the adsorption of oxygen, at which time the requirement of oxygen concentration is low14, 18. CO and CO2 are mainly produced through the direct decomposition and small amount burn-off reaction of stable oxygenated complexes containing inherent oxygencontaining functional group. The decomposition of the stable oxygenated complexes becomes significant when the temperature is above 70oC, and the demand for oxygen in the coal-oxygen reaction is gradually increasing64. When the temperature is higher than 70oC, the amount of CO and CO2 production depends on the amount of surface oxides produced by the attack of oxygen on the active sites on the coal surface. As the temperature increases further, the direct burnoff reaction begins to play a major role, and the demand for oxygen increases further18. Thus, through the changes of CO and CO2, the low temperature oxidation stage can still be divided into two stages: adsorption stage (30-70oC) and obvious oxidation stage (70-350oC). At present, the low temperature oxidation stage has always been the focus of the research of coal spontaneous combustion, and researchers generally divide the low temperature oxidation stage into different stages18, 49, 65. The results of this paper are basically consistent with previous studies. After 70oC, the lean-oxygen delayed the production of CO and CO2, and the hysteresis effect of methane on coal-oxygen reaction was greater than that of N2. The reason is that the adsorption capacity of coal to different single component gases is significantly different, and the adsorption order is CH4>N2>O266-67. Under the same oxygen concentration condition, the adsorption of methane can more effectively reduce the coal-oxygen contact area, which leads to the reduction of the reaction degree. As shown in Figure 3, in the low temperature oxidation stage, when the oxygen concentrations were 21% and 15%, although the total changes of TG-S and DTG-S curves were not significant, they fluctuated first, then increased gradually, and finally decreased gradually. However, the changes of TG-S and DTG-S curves did not have obvious boundary like CO (shown in Figure 6), and the change of oxygen concentration had no significant effect on them. On the one hand, the above phenomena indicated the sensitivity of CO as an indicator gas of coal-oxygen reaction68-69. On the other hand, the change of coal sample mass consists of the weight added by oxygen absorption and the weight lost by reaction. The coal-oxygen reaction under high oxygen concentration conditions also has a large amount of oxygen absorption while losing more mass. In addition, heating rate has significant effect on the coal-oxygen reaction. If the heating rate is too large, the coal-oxygen reaction time is short, the oxygen consumption is low, and thus the influence of oxygen concentration on the mass of coal sample is not sensitive70. At least, instrument errors can also have an impact on the small mass changes. Combining the changes of TG-S and DTG-S curves in Figure 3 and Figure 4 under other conditions, the conclusion can be drawn that, in the whole 30-350oC stage, the mass loss of coal samples under different oxygen concentrations caused by methane and N2 in these experiments was not significant. Figure 5 Figure 6 Figure 7 Lean-oxygen conditions caused by methane have obvious hysteresis effect on the low-temperature oxidation stage (>70oC), and the hysteresis effect increases with the decrease of oxygen concentration. Under the same oxygen concentration, the hysteresis effect of methane is more obvious than that of N2. In the low temperature oxidation stage, as an inert gas, methane can delay the production of CO and CO2 in the coal-oxygen reaction. The research results have important guiding significance for the prediction of coal mine spontaneous combustion.

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3.3.2 Accelerated oxidation stage In the accelerated oxidation stage (350oC-Ti), ignition temperature (Ti) was obtained by the intersection point of DTG-S=-0.1mg/min horizontal line and DTG-S curves under different conditions. The Ti values at different oxygen concentrations under the conditions of methane and N2 are shown in Table 2. With the decrease (from 21% to 10%) of O2 concentration, Ti gradually increased under the conditions of methane and N2. Lean-oxygen environment could effectively delay the ignition temperature of coal-oxygen reaction, and the effect of methane under the same oxygen concentration was better. In this stage, CO and CO2 production showed rapidly increasing trend. The formation of CO and CO2 were inhibited in the lean-oxygen environment, and the inhibition effect of methane was more obvious than that of N2 under the same oxygen concentrations. In this stage, when the temperature is high enough, coal is rapidly oxidized and decomposed, the mass begins to fall sharply, the dense ring aromatic nucleuses in coal are completely cracked, the coal tar is discharged in large quantities and the volatiles begin to burn56-58. The oxygen concentration becomes the main factor that limits the combustion of coal21. The decrease of oxygen concentration reduces the contact between coal surface and oxygen. Thus, the devolatilization rate, as well as the oxidation rate of volatiles and fixed carbon of coal, decrease and result in higher Ti35. The lean-oxygen condition delayed the combustion of coal, and the hysteresis effect increased with the decrease of oxygen concentration. At the same time, due to the adsorption order is CH4>N2>O266, different inerting environments have different hysteresis effects under the same oxygen concentration. However, when the O2 concentrations were 5% and 3% under methane conditions, the values of Ti suddenly increased, reaching 535oC and 536oC, respectively. And the Ti still increased gradually with the decrease of oxygen concentration (from 21% to 3%) under N2 condition. The reason is that in the lean-oxygen environment (< 5%), the Ti was close to the combustion temperature of methane, and the combustion of methane further decreased the oxygen concentration rapidly, thereby causing the degree of coal oxygen reaction to decrease rapidly. Table 2

3.3.3 Combustion stage In the combustion stage (Ti-Tb), Ti, Tmax and Tb values at different oxygen concentrations under the conditions of methane and N2 are shown in Table 2. As shown in Table 2, in the conditions of N2, with the decrease of O2 concentration, Tb gradually increased to more than 800oC. In the process of decreasing oxygen concentration from 21% to 10%, Tmax increased from 552oC to 560oC, but decreased to about 540oC when oxygen concentrations were 5% and 3%. Under the conditions of methane, the Tb values were much smaller than that of N2, and their values were roughly around 580oC. The Tmax did not change with the change of oxygen concentration, but remained around 540oC. In the combustion stage, the concentration of oxygen began to drop sharply. As shown in Figure 5, under N2 conditions, when the initial oxygen concentrations were 21%, 15% and 10%, the oxygen concentration first decreased and then gradually returned to the initial value, which indicated that when the oxygen concentration was >10%, the coal samples were fully burned. As the initial oxygen concentration decreased, the temperature at which the lowest oxygen concentration appeared gradually increased, indicating that the maximum reaction rate of coal-oxygen was delayed. The above phenomenon was consistent with the TG-S/DTG-S variation rule of coal samples in Figure 4. In the initial stage of coal sample combustion, under the same initial oxygen concentrations, the oxygen concentration curves under methane and 8


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N2 conditions had obvious separation. When the methane was contained, the oxygen concentration in the coaloxygen reaction environment dropped sharply and remained stable at the position close to 0, and the final value was consistent under different initial oxygen concentration conditions. Under the conditions of N2, CO and CO2 were mainly produced by the combustion of fixed carbon contained in coal. With the increase of temperature, the concentration of CO increased first and then decreased, and its production was proportional to the initial oxygen concentration. The peak temperature of CO concentration increased with the increase of oxygen concentration. When the oxygen concentrations were 21%, 15% and 10%, the change trend of CO2 was similar to that of CO. When oxygen concentrations were 5% and 3%, CO2 remained unchanged after 650oC, which was due to the slow coal-oxygen reaction. As shown in Figure 6 and Figure 7, under the conditions of methane, the variation of CO and CO2 were significantly different from that under N2 conditions. The above phenomena fully showed that the way of oxygen consumption had changed obviously under the conditions of methane. Previous studies have proved that the main reaction of coal is the combustion of fixed carbon in this stage, and the reaction equation can be expressed by formulas (3) and (4)51, 71. In addition, volatile combustion often occurs in the early stage of combustion35. In the conditions of methane, the mass loss and mass loss rate of coal samples were significantly reduced, and the lower the oxygen concentration, the more obvious the reduction. At the same time, under the same oxygen concentration conditions, the mass loss of coal samples under methane conditions was much smaller than that of N2. The changes of CH4 and H2 in the combustion stage are shown in Figure 8 and Figure 9. Although coal-oxygen reaction can produce CH466 and H276, the amount of CH4 and H2 produced by coal-oxygen reaction under the conditions of N2 was very small. So, the changes of CH4 and H2 under N2 conditions were not shown in Figure 8 and Figure 9. As shown in Figure 8 and Figure 9, with the decrease of methane concentration, a large amount of H2 was produced. This phenomenon indicates that methane decomposition occurred at high temperature, and the decomposition progress produced C and H2 (𝐶𝐻4→𝐶 + 2𝐻2). C and H2 generated by methane decomposition burnt at high temperature and consumed a large amount of oxygen while producing CO and CO2 (2𝐻2 + 𝑂2→2𝐻2𝑂; 𝐶 + 𝑂2 →𝐶𝑂2; 𝐶 + 1 2𝑂2→𝐶𝑂 )23, 35. Combining with the variation of O2, CO, CO2, CH4 and H2, it can be found that the methane combustion occurred at high temperature. The combustion of methane also consumed a large amount of oxygen while producing CO and CO2 (𝐶𝐻4 + 2𝑂2→𝐶𝑂2 +2𝐻2𝑂; 𝐶𝐻4 + 3 2𝑂2→𝐶𝑂 + 2𝐻2𝑂). Pinilla et al.72 found that high temperature can favor the decomposition of methane and accelerate the combustion. Therefore, the inhibition effect of methane on coal-oxygen reaction increased with the increase of temperature, which was reflected in the TGA-GC experiments that the coal-oxygen reaction stopped when the temperature was higher than 586oC. In addition, previous studies have demonstrated that a variety of carbon materials, including carbon black (CB) and activated carbon (AC), can catalyze the decomposition of methane7374.

Thus, the coal in the coal-oxygen reaction environment can in turn catalyze the decomposition of methane. Figure 8 Figure 9 The chemical reactions that occur during the combustion stage can involve the combustion of volatile, the

combustion of fixed carbon, the combustion of methane, the decomposition of methane and the combustion of H2. The characteristics of coal-oxygen reaction under methane conditions can be well explained by formulas (3)-(8). When the coal is on fire, the high temperature environment (> 500oC) will cause intense combustion and

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decomposition of the surrounding methane, thereby inhibiting the combustion of coal, but at the same time releasing a large amount of CO, CO2 and H2.

C  O2  CO2

H (298)  398 kJ / mol

C  1 / 2O2  CO

H (298)  110 kJ / mol

CH 4  2O2  CO2  2 H 2 O

CH 4  C  2 H 2

(4)

H (298)  802 kJ / mol

CH 4  3 / 2O2  CO  2 H 2 O

2 H 2  O2  2 H 2 O

(3)

(5)

H (298)  607 kJ / mol

H (298)  75 kJ / mol H (298)  483 kJ / mol

(6)

(7) (8)

4 Conclusions The experiments of coal-oxygen reaction in the lean-oxygen environment caused by methane and N2 were carried out by the TGA-GC method in this paper. The TG-S/DTG-S correction curves were introduced to reflect the process of coal-oxygen reaction more accurately. The coal-oxygen reaction was defined as low temperature oxidation stage, accelerated oxidation stage and combustion stage, and the effect of methane on them is different. In the low temperature oxidation stage, as an inert gas, methane can delay the production of CO and CO2 in the coal-oxygen reaction. So, there is a certain delay effect when CO is used as the index of coal spontaneous combustion in the actual production process. In the accelerated oxidation stage, the lean-oxygen environment caused by methane can slow down the rate of coal-oxygen reaction, and the ignition temperature (Ti) of coal sample increases with the decrease of oxygen concentration. In the low temperature oxidation and accelerated oxidation stages, the inhibitory effect of methane on coal-oxygen reaction at the same oxygen concentration is better than that of N2. In the combustion stage, when the temperature is > 500oC, methane will undergo combustion and decomposition reactions. The direct combustion of methane and the combustion of C and H2 produced by methane decomposition consume a lot of oxygen, thus inhibiting or even terminating the coaloxygen reaction process. In this paper, the characteristics of coal-oxygen reaction in different lean-oxygen environments caused by methane are systematically studied and analyzed, which makes up for the shortcomings of previous studies and has important guiding significance for the prediction and prevention of coal mine fire in goaf.

Acknowledgements This work was supported by Key Research and Development (R&D) Projects of Shanxi Province (No.201803D31053); Major Technological Research Projects of Shanxi Coking Coal Group Co. LTD.(No.201812xs06); National Natural Science Foundation of China (No.51804212); High-end Foreign Experts’ Intelligence Introduction Project of the State Administration of Foreign Affairs (GDW20181400424); Opening Project of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology) (No.KFJJ19-03M).

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Table captions Table 1

Properties of the coal sample

Table 2

Ti, Tmax and Tb values under different lean-oxygen conditions caused by methane and N2

Table 1 proximate analysis (W/%)

ultimate analysis (Wdaf/%)

Mad

Aad

Vdaf

C

H

O

N

S

0.96

6.52

15.13

88.10

3.83

4.82

1.12

2.13

Table 2 Atmosphere conditions

Ti(oC)

Tmax(oC )

Tb(oC )

21%O2+79%N2

403

552

612

15%O2+85%N2

406

555

656

10%O2+90%N2

410

560

743

5%O2+95%N2

433

540

>800

3%O2+97%N2

453

541

>800

15%O2+29%CH4+56%N2

409

541

586

10%O2+52%CH4+38%N2

418

544

582

5%O2+76%CH4+19%N2

535

536

580

3%O2+86%CH4+11%N2

536

536

576

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Energy & Fuels

Figure captions Figure 1

Experimental setup

Figure 2

TG/DTG curves in Air and N2 conditions and the calculate of TG-S(Air)/DTG-S(Air)

Figure 3

TG-S/DTG-S curves of coal-oxygen reaction in the lean-oxygen environment caused by methane

Figure 4

TG-S/DTG-S curves of coal-oxygen reaction in the lean-oxygen environment caused by N2

Figure 5

Changes of O2 concentration in the lean-oxygen environment caused by methane and N2

Figure 6

Changes of CO concentration in the lean-oxygen environment caused by methane and N2

Figure 7

Changes of CO2 concentration in the lean-oxygen environment caused by methane and N2

Figure 8

Changes of methane in the lean-oxygen environment caused by methane

Figure 9

Changes of H2 (detected by GC) in the lean-oxygen environment caused by methane

Figure 1

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Figure 2

Figure 3

Figure 4

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Energy & Fuels

Figure 5

Figure 6

Figure 7

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Figure 8

Figure 9

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Research on the characteristics of coal-oxygen reaction in the lean

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