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A method for evaluating the spontaneous combustion of coal by monitoring various gases
Process Safety and Environmental Protection 126 (2019) 223–231
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A method for evaluating the spontaneous combustion of coal by monitoring various gases Jun Guo a,b,c,∗ , Hu Wen a,b,c , Xuezhao Zheng a,b,c , Yin Liu a,∗∗ , Xiaojiao Cheng a a b c
School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an, 710054, China Key Laboratory of Western Mine and Hazard Prevention, Ministry of Education of China, Xi’an, 710054, China State Mine Emergency Rescue (Xi’an) Research Center, Xi’an, Shaanxi, 710054, China
a r t i c l e
i n f o
Article history: Received 30 January 2019 Received in revised form 4 April 2019 Accepted 10 April 2019 Available online 17 April 2019 Keywords: Coal spontaneous combustion Coal temperature Evaluating method Various index gases Environmental pollution control
a b s t r a c t Coal spontaneous combustion has always been a worldwide problem, which causes waste of coal resources, greenhouse gas emissions and other atmospheric environmental pollution problems. Although coal temperature monitoring is the most direct and accurate means of predicting the spontaneous combustion of coal, the coal temperature often cannot be directly measured owing to various physical restrictions. As an alternative, the present study assessed the qualitative and quantitative analysis of the CO and C2 H4 formation rates, as well as various gas ratios such as the CO/CO2 ratio and fire coefficient R2 (R2 = 100×CO/O2 ), to predict spontaneous combustion. This method was established based on the temperature-programmed experiments of three different coal rank (including lignite, bituminous coal and anthracite), and was verified using data obtained from on-site monitoring at an actual mine. The results show that the method accuracy is as high as 97% when predicting the coal temperature to within 15 ◦ C (allowable error range of the predicted value). This degree of accuracy should be sufficient for on-site fire prevention and control. This new technique is not only accurate and reliable but also has theoretical significance with regard to the identification of coal spontaneous combustion in goaf and for the development of fire prevention and suppression technologies. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Fires in coal mines can cause significant casualties and resource losses. These incidents involve the production of large quantities of toxic gases, atmospheric environmental pollution, the destruction of equipment and facilities, and the loss of coal resources, and can easily lead to secondary effects such as explosion (Guo et al., 2018; Qi et al., 2017; Chen et al., 2016; Chen et al., 2018). The spontaneous combustion of coal, which accounts for more than 80% of coal mine fires, can develop slowly in concealed locations, and so can be difficult to identify and control (Zhang, 2008; Wen et al., 2016). Greater than 90% of coal spontaneous combustion incidents occur in the goaf area during the mining process (Jin et al., 2014; Avila et al., 2014; Veznikova et al., 2014; Ma et al., 2017), and so there has been a focus on accurately assessing this process.
∗ Corresponding author at: School of Safety Science and Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an, Shaanxi, 710054, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (J. Guo), [email protected] (Y. Liu).
There have been numerous studies of the spontaneous combustion of coal, based on both theory and technological evaluation. As an example, Deng (2006) developed a theoretical model for the molecular structure of coal based on quantum chemistry, and assessed the spontaneous combustion mechanism on a microscopic basis. A spontaneous combustion danger index was proposed on the basis of this prior work (Jin et al., 2014; Wen et al., 2017a; Jin et al., 2015). Xu (2000), Verma and Chaudhari (2016) and Deng et al. (2015) studied the mechanism associated with the spontaneous combustion of coal on the macro and microscopic levels, and developed various theories regarding this process, as well as means of predicting, preventing and controlling combustion, based on a coal-oxygen complex theory. This theoretical basis has since become widely accepted. Cao et al. (2012), Cheng et al. (2016), Yu et al. (2013) and Xu et al. (2014) proposed conditions and methods for the determination of spontaneous combustion danger zones in goafs, based on experimental work and on-site testing, that allow fire prevention and extinguishment. Wen et al. (2017b) established a CO concentration model using experiments and field work, to provide a new method for the early prediction of coal spontaneous combustion. Yu et al. (2005), Zhu (2016), Sahay et al. (2007), Jin et al. (2015), Wen et al. (2015) and Xu et al. (2018) researched
https://doi.org/10.1016/j.psep.2019.04.014 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Fig. 1. The temperature programmed experimental apparatus.
Fig. 2. A flow chart summarizing coal sample processing process.
gaseous spontaneous combustion products and oxygen consumption rates under various conditions, such as at high temperatures and moisture contents. Coal temperature is one of the most direct and accurate indicators of the degree of spontaneous combustion (Yu et al., 2017; Beamish et al., 2016; Taraba et al., 2011). However, the minedout area in a coal mine is a closed space that cannot be entered to measure temperatures. Additionally, coal is a poor conductor of heat and air mobility is low inside a mine. Therefore, it is impossible to accurately determine spontaneous combustion locations by directly measuring coal temperatures in the goaf. A large number of studies (Jin et al., 2014; Lu, 2009; Xiao et al., 2008; Ma et al., 2007; Wu et al., 2012) have shown a correlation between gaseous product concentrations and coal temperature during spontaneous combustion. Niu et al. (2016), Pone et al. (2007) and Parsa et al. (2017) carried out a series of tests involving gases in the Anyuan Mine and developed a more accurate method of predicting various stages of combustion. It appears that the association between the concentrations of gaseous products and temperature during the coal oxidation process could potentially be used to determine the degree of spontaneous combustion, namely coal temperature. In the present work, a temperature-programmed test apparatus was developed and used to examine the spontaneous combustion of coal at low temperatures. In this experiment, three different metamorphic grade coal samples were tested, such as bituminous coal (for example, long-flame coal), lignite and anthracite. The variations in the concentrations of certain gases obtained from coal samples during heating were studied and a method of predicting temperature was constructed. Employing this method, the temperature range of oxidized coal in an actual goaf was estimated.
2. Experimental 2.1. Experimental equipment and principles The experimental work used a temperature-controlled test apparatus (Wen et al., 2017a; Zhao et al., 2019a; Jin et al., 2015; Deng et al., 2016), as shown in Fig. 1. Employing an air pump or compressed air cylinder, a test vessel (diameter of 10 cm and height of 25 cm) preloaded with a coal sample was filled with the desired amount of air. The air flow to the coal was determined using a flow meter and the air was preheated as it flowed through a copper pipe on its way to the coal. The gases resulting from combustion were collected in a tank and quantified using gas chromatography.
2.2. Experimental conditions and process Coal samples, including bituminous coal (No. 1 long flame coal and No. 2 long flame coal), lignite and anthracite, which were obtained from a freshly-exposed coal face and crushed under ambient air. The preparation process of coal sample is shown in Fig. 2. The coal was subsequently sieved to obtain fractions having particle size ranges of 0–0.9 mm. 0.9–3 mm, 3–5 mm, 5–7 mm and 7–10 mm, after which these fractions were mixed. In each trial, a well-mixed 200 g portion of the coal was placed in the test apparatus, after which the air pipe was connected and the apparatus sealed (GB474-, 2008). After initiating a supply of air, the sample temperature was raised at a rate of 0.3 ◦ C/min and a one-liter sample of the resulting gases was collected at each 10 ◦ C interval for analysis by chromatography. The experimental conditions are summarized in Table 1.
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Table 1 Experimental test conditions. Coal samples
Ventilation volume /(ml/min)
Coal loading height /cm
Coal loading volume /cm3
Bulk density /(N·cm−3 )
Porosity /%
Heating rate /(◦ C/min)
Anthracite Long-flame coal - 1 Long-flame coal -2 Lignite
120 120 120 120
17.50 15.50 16.40 17.00
1373.75 1216.75 1287.40 1334.50
0.73 0.82 0.78 0.75
0.49 0.43 0.46 0.48
0.30 0.30 0.30 0.30
Fig. 3. CO generation as a function of coal temperature.
Fig. 4. CH4 concentration as a function of coal temperature.
3. Results and analysis 3.1. Relationship between specific gases and coal temperature 3.1.1. Relationship between CO and coal temperature CO is one of the most common indicators of the spontaneous combustion of coal (Wen et al., 2017a,b), and the relationship between the CO output and the coal temperature of four coal samples is shown in Fig. 3. For those coal samples, only trace amounts of CO were produced in the initial stage of the experiment, but the CO output began to slowly increase beginning at 60 ◦ C. Above this point, the samples reached the critical temperature necessary for the formation of a coal-oxygen complex, and the CO amount rapidly increased from 60 to 130 ◦ C. Above 130 ◦ C, the amount of CO increased sharply and the rate of CO formation accelerated. This occurred because coal rapidly oxidizes above its cracking temperature, resulting in spontaneous combustion. The cracking temperature is a marker of the rapid development of the coaleoxygen reaction and indicated that coal had entered the stage of intense oxidation. The active groups commenced gradually consuming oxygen and regenerating at this temperature (Zhao et al., 2019b). The temperature at which the rate of CO generation rapidly increases can be used to determine the point at which rapid oxidation occurs and so to qualitatively predict the degree of spontaneous combustion. Fig. 4 plots the CH4 generated as a function of temperature. For four coal samples, the presence of this gas at close to ambient temperature demonstrates the release of CH4 adsorbed in the coal pores. Below 100 ◦ C for long flame coal and lignite, 80 ◦ C for anthracite, the CH4 production is minimal and actually decreases slowly with increasing temperature. This effect is attributed to desorption of adsorbed CH4 , which is gradually depleted at low temperatures. At 100–140 ◦ C, the amount of CH4 begins to slowly increase as the side chains in the coal molecules began to break (Zhao et al., 2019b; Jin et al., 2015; Wen et al., 2017a; Deng et al., 2015). Above 140 ◦ C, the volume fraction of CH4 abruptly increases, primarily because coal-oxygen recombination reactions are pro-
Fig. 5. C2 H4 concentrations as functions of coal temperature.
moted, as well as the more rapid rupture of the side chains in the coal molecules (Zhao et al., 2019b; Jin et al., 2015; Wen et al., 2017a). 3.1.2. Relationship between C2 H4 and C2 H6 and coal temperature Both C2 H4 and C2 H6 are important indicators of spontaneous combustion, and both exhibit increases along with temperature. There is no C2 H4 in the original coal. Only when the coal temperature rising to 80–130 ◦ , can produce C2H4 gas because of the pyrolysis reaction (Liang et al., 2019; Guo et al., 2019a,b; Jin et al., 2015). Therefore, the generation of C2 H4 can be used to quantitatively characterize the degree of spontaneous combustion. The relationships between the generation of C2 H4 and C2 H6 and the coal temperature are plotted in Figs. 5 and 6. From these data it is evident that C2 H4 was not formed at the beginning of the experiment, while a small amount appeared at approximately 80–110 ◦ C. Thus, there was no C2 H4 initially present in the coal samples, and the C2 H4 generated in the high temperature phase can be attributed to the pyrolysis of the coal. In contrast, C2 H6 was generated beginning at about 20–30 ◦ C and gradually increased
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Fig. 6. C2 H6 concentrations as functions of coal temperature.
Fig. 7. CO/CO2 ratio as a function of coal temperature.
with increasing temperature, indicating that the original samples contained some adsorbed C2 H6 . C2 H4 is an unsaturated compound and will not accumulate in coal seams while C2 H6 can be deposited in coal seams, where it is more stable than CH4 (Liang et al., 2019; Guo et al., 2019a,b; Jin et al., 2015; Wen et al., 2017a). Therefore, C2 H6 cannot be used to predict the low temperature oxidation stage of coal. Although trends in C2 H4 and C2 H6 production can be employed to assess the extent of coal pyrolysis, the temperature at which C2 H4 appears is a more accurate indicator. 3.2. The relationship between gas ratios and coal temperature A necessary condition for the spontaneous combustion of coal is a sufficient air supply. The quantity of air available not only affects the reaction intensity but also dilutes the concentrations of the various product gases. This effect directly impacts the accuracy and reliability of predictions based on indicator gases such as CO, C2 H4 and C2 H6 . Using ratios such as CO/CO2 or the so-called Graham coefficient can effectively eliminate any effects of variations in the air volume (Wen et al., 2016, 2017a). Based on prior work, this study used the CO/CO2 ratio, the Graham coefficient (R2 = 100×CO/O2 ) and the alkyl chain ratio (C2 H6 /CH4 ) to establish relationships with coal temperature (Wen et al., 2017a; Wu et al., 2012). 3.2.1. Relationship between CO/CO2 and coal temperature CO/CO2 ratios of those four coal samples are plotted as functions of coal temperature in Fig. 7. This ratio of those four coal samples tends to increase during the spontaneous combustion of coal. In these data, taking long-flame coal-1 for example, the temperature increase from ambient to 60 ◦ C slowly raises the ratio but the value remains less than 0.1, indicating that the coal-oxygen recombination reaction was weak during this stage. Above 60 ◦ C, the CO/CO2 ratio increases rapidly to greater than 0.1 in the range of 80 to 90 ◦ C. At temperatures in the range of 90 to 140 ◦ C, the rate of increase drops slightly and the ratio is between 0.1 and 0.25. This occurs because the volume fractions of CO and CO2 both increase but at slightly different rates. At 140 ◦ C, the ratio suddenly increases again, rapidly rising from 0.25 to more than 0.5. Thus, in this stage, the formation of the coal-oxygen complex increases and the CO relative production rate is much higher than the CO2 relative production rate. This signifies spontaneous combustion of the coal (Jin et al., 2015). 3.2.2. Graham coefficient The Graham coefficient, G, is an important coal spontaneous combustion forecasting indicator, and reflects the relative changes
Fig. 8. Graham coefficient values as a function of coal temperature.
in the gas volume proportions (Wen et al., 2017a; Deng et al., 2016). The Graham coefficient eliminates the effect of the air volume and also can reflect the extent of fire spreading. The coefficient is calculated as: G = +CO/(-O2 )×100%
(1)
where +CO is the increment in the volume fraction of CO (%) and O2 is the decrease in the O2 volume fraction (%). The experimental G values are plotted as a function of the coal temperature in Fig. 8. In those plots, variations of the G value of these four coal samples are similar. Taking long-flame coal-1 for example, the G value slowly increases during the initial stage of temperature rise but is below 2 up to 70 ◦ C. This is due to the slow reaction of coal and oxygen at lower temperatures, the slow increase in the CO volume fraction and the weak oxygen consumption of the coal (Zhao et al., 2019b; Jin et al., 2015). As the temperature exceeds approximately 70 ◦ C, the rate of increase in G drops and G remains in the range of 2–3 up to 120 ◦ C. These data are attributed to having exceeded the critical temperature of the coal, which promotes the oxidation reaction along with CO production and oxygen consumption, such that the two gases have basically the same growth rate (Zhong et al., 2010). At 120 ◦ C, the G value increased sharply to above 3. During this stage, the continuous increase in the coal temperature promotes the coal-oxygen complex reaction, and the coal sample enters the spontaneous combustion stage.
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3.3. The determination of coal temperature
Fig. 9. The alkyl chain ratio as a function of coal temperature.
3.2.3. The alkyl chain ratio The alkyl chain ratios (C2 H6 /CH4 ) is the ratio of the volume concentration of ethane to the volume concentration of methane. The alkyl chain ratios are plotted against temperature in Fig. 9. The resulting curve exhibits a certain symmetry, such that the ratio initially increases and then decreases with increasing coal temperature, especially long-flame coal and lignite. The alkane’s ratio of anthracite shows a slow rising trend with increasing coal temperature. Taking long-flame coal-1 for example, the ratio reaches a maximum at 90–100 ◦ C. The ratio initially increases rapidly to about 2.25 at 90–100 ◦ C then decreases between 100 and 120 ◦ C, although remaining above 1.5. At temperatures greater than 120 ◦ C, the release rate of CH4 produced by pyrolysis was continuously higher than that of C2 H6 , and so the alkyl chain ratios undergoes a rapid decrease. Because the alkyl chain ratio data of bituminous coal (long flame coal) and lignite exhibits a symmetrical plot against the coal temperature, there are two temperatures corresponding to each ratio. The ratio increases and decreases with increasing temperature, making it difficult to use as a single predictor. Thus, the other indicators are necessary to determine coal temperature further. On the contrary, the curve of anthracite alkane ratio has a monotone increasing trend, so this ratio can be used as the main prediction index.
The spontaneous combustion of coal is complex and it is difficult to predict the temperature of coal in goaf accurately and quantitatively (Xu, 2000; Wen et al., 2017a; Verma and Chaudhari, 2016; Deng et al., 2016). Based on a qualitative and quantitative analysis of multiple gases, coal temperatures are determined at different confidence levels to allow the accurate identification of the potential for spontaneous combustion. The basic concept is summarized as a flowchart in Fig. 10, in which the main steps are as follows. 1. A single indicator is used to qualitatively determine the potential for spontaneous combustion. 1.1. The CO volume fraction is employed to determine the likelihood of spontaneous combustion. This parameter exhibits a good correlation with coal temperature, such that it increases rapidly above the critical temperature. 1.2. The C2 H4 volume fraction is also assessed to qualitatively evaluate the probability of spontaneous combustion. This gas appeared above approximately 110 ◦ C in the current experimental work and therefore can be used to monitor the coal temperature. 2. The gas ratio is used to determine the probability of spontaneous combustion. 2.1. The CO/CO2 ratio, Graham coefficient and alkyl chain ratio are all determined via on-site measurements. 2.2. Based on correlations derived from experimental testing, the above ratios are used to find the coal temperature. 2.3. A comprehensive analysis of single gases such as CO and C2 H4 as well as gas ratios is employed to calculate the coal temperature. 2.4. The average values of the on-site indicators are calculated so as to obtain the average coal temperature at the site. The prediction accuracy and the confidence intervals are set to the desired values. The degree of confidence for each temperature value obtained from the above process is calculated. This technique allows one to determine the coal temperature based on gas sampling that takes place at the mine. Thus, the tendency towards spontaneous combustion, the temperature range of the coal and the reliability of the temperature range are combined to allow an accurate prediction of the development of spontaneous combustion of the coal. 4. A case study A case of coal spontaneous combustion in the coalmine where the long-flame coal-1 sample came from was used for verifying the
Fig. 10. A flow chart summarizing the new method of determining coal temperature.
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Fig. 11. CO volume fractions over time. Fig. 12. Gas ratios over time.
feasibility of this prediction method. At this working face, during the mining process, the mined-out area showed signs of undergoing spontaneous combustion. For this reason, a series of fire prevention and extinguishing measures were in place, such as preventing the intrusion of air from the gob area and the injection of colloidal polymeric colloid fire extinguishing agents or yellow mud. These steps were intended to reduce air leakage to the fire zone and to limit the formation of coal-oxygen complexes. Continuous on-site measurements of O2 , CO, CO2 , CH4 and C2 H6 in the goaf at this mine were performed via beam tube monitoring system. 4.1. Analysis of a single gas No C2 H4 was detected during the on-site measurements, and so only the results obtained using CO concentrations are discussed here. The CO values obtained from measurements at the mine over time are plotted in Fig. 11. These data indicate that the CO volume fraction rapidly increased from the initial monitoring stage and reached a peak of 120 ppm on the 11th day. Therefore, the degree of oxidation of the coal left in the goaf was continuously increasing. The CO level subsequently remained steady at about 110 ppm. The varying degrees of fluctuation in the values are attributed to on-site sealing measures, such as grouting and plastic injection. 4.2. Analysis of gas ratios After processing the on-site observation data, the CO/CO2 ratios, Graham coefficients and alkyl chain ratios were plotted as functions of time, as in Fig. 12. In each case, the data show a similar trend, involving a rapid increase from the initial stage of monitoring. The values peaked near the 12th day, at which point the coal-oxygen complex reaction was most pronounced. Over the following 22 days, fire prevention measures such as grouting, plastic injection and inert gas injection were adopted, and the ratios decreased. This result demonstrates the effectiveness of these measures in controlling the spontaneous combustion of the coal. It should be noted that the fire prevention and extinguishment measures were not continuously applied for reasons such as equipment failure, resulting in a rebound phenomenon evident in the data. 4.3. Analysis of coal temperatures Data were collected from the mine over 35 days, and the resulting CO/CO2 ratios Graham coefficients and alkyl chain ratios are plotted in Fig. 13, overlaid with the data from the lab experimentation. In each case, the average, minimum and maximum values
of the three gas ratios are selected for all field data to illustrate the temperature determination process. 4.3.1. Analysis of coal temperature accuracy According to the field data, the average CO/CO2 ratio was 0.04, corresponding to an average coal temperature of 49 ◦ C based on the experimental work. The average Graham coefficient was 1.20, corresponding to an average coal temperature of 52 ◦ C and indicating the average temperature of coal spontaneous combustion predicted using this value. Over 35 days, the average value of the alkyl chain ratio was 0.95, corresponding to an average coal temperature of either 50 or 143 ◦ C. No C2 H4 was observed over this period, indicating that the coal temperature did not reach 110 ◦ C during the entire coal spontaneous combustion development process. Therefore, the temperature of 143 ◦ C was excluded and the average coal temperature predicted by this ratio was approximately 50 ◦ C. This same method could be used to determine the highest and lowest coal temperatures and the predictions are summarized in Table 2. 4.3.2. Reliability analysis To verify the reliability of the proposed method, the temperature values predicted using the three indexes (CO/CO2 ratio, Graham number and alkyl chain ratio) were compared for each day of the on-site monitoring. The aim was to determine whether or not all three indexes gave the same predicted temperature, within an allowable error range of the predicted value of 5, 10, 15 or 20 ◦ C. The results are summarized in Table 3. As shown in Table 3, allowable error range refers to allowable error range of the predicted value of coal temperature. Anastomosis of three indexes refers to the number of data that meets three indexes at the same time. Date of anastomosis refers to the specific day number of field observation above data, which be represent with sets in table. Confidence degree refers to the ratio of the above days to the total number of days. The results in Table 3 demonstrate that the temperatures obtained using the three indicators were within 5 ◦ C of one another 19 days out of the 35, representing 54% of the total days. This degree of uncertainty is considered too great to allow an accurate determination of the coal temperature. Expanding the range to 10 ◦ C, the temperature values from the three parameters agreed on 29 days, representing 83% of the total days, and indicating somewhat reliable predictions. Applying allowable ranges of 15 and 20 ◦ C, the predicated temperatures agreed on 34 and 35 days, representing 97% and 100% of the monitoring days, respectively. This level of agreement is acceptable for temperature analysis. Thus, the temperatures obtained from all three predictive indexes agreed with one another within 20 ◦ C on each day of the monitoring.
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Fig. 13. Comparisons between experimental results and field data.
Table 2 Results for the prediction of coal temperatures. Forecast index CO/CO2 Graham Chain alkanes ratio average value
Average value
Minimum value ◦
Maximum value ◦
Index value
Coal temperature / C
Index value
Coal temperature / C
Index value
Coal temperature /◦ C
0.04 1.20 0.95 —
49.00 52.00 50.00 50.33
0.01 0.47 0.38 —
33.00 32.00 34.00 33.00
0.06 1.74 1.42 —
72.00 71.00 72.00 71.67
Table 3 A comparison of the results obtained from on-site analysis and experimental work. Allowable error range/◦ C
Anastomosis of three indexes
Date of anastomosis
Confidence degree
5 10 15 20
19 29 34 35
U1 ={1,2,3,5,7,9,15,17,18,19,21,22,23,25,26,29,30,33,34} U2 = U1 ∪{4,6,8,11,13,20,24,28,31,35} U3 = U2 ∪{10,12,14,27,32} U4 = U3 ∪{16}
54 83 97 100
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In fact, in terms of coal spontaneous combustion prevention onsite, a range of coal temperatures needs to be determined, not a specific temperature point. Thus, an allowable error range in the 15 ◦ C is suitable. This method could be applied to coal spontaneous combustion monitoring and provide support for the on-site evaluation of the spontaneous combustion status.
5. Conclusions The generation of CO, CH4 , C2 H4 , C2 H6 , and gases ratio (CO/CO2 ratio, Graham coefficient and alkyl chain ratio) have a good correlation with coal temperature, which could be used to predict the temperature range of coal during spontaneous combustion. A method for determining coal temperature was developed, based on index gas (CO and C2 H4 ) and index value (CO/CO2 ratio, Graham coefficient and alkyl chain ratio). On site monitoring of an actual mine found that this technique is 97% accurate within allowable error range of 15 ◦ C. This is a simple method that addresses practical problems such as the effects of air intrusion on gas data. Using the prediction method proposed in this paper, a preliminary qualitative analysis of the risk of coal spontaneous combustion can be performed, which is based on CO, the CO/CO2 ratios, Graham coefficients and C2 H4 . Then, Quantitative predictions of the coal temperature with varying accuracies can be made using the CO/CO2 ratios, Graham coefficients and alkyl chain ratios. This approach can assess the development of spontaneous combustion and allow an evaluation of the effectiveness of fire prevention measures on-site.
Acknowledgments This work was supported by the National Key R&D Program of China (2018YFC0808201); China Postdoctoral Science Foundation (2017M623209); Special Scientific Research Project of Shaanxi Provincial Education Department (17JK0495); Natural Science Basic Research Program of Shaanxi (2018JQ5080; 2018JM5009). We thank Michael D. Judge, MSc, from Liwen Bianji, Edanz Editing China, for editing the English text of a draft of this manuscript.
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
A method for evaluating the spontaneous combustion of coal by monitoring various gases Jun Guo a,b,c,∗ , Hu Wen a,b,c , Xuezhao Zheng a,b,c , Yin Liu a,∗∗ , Xiaojiao Cheng a a b c
School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an, 710054, China Key Laboratory of Western Mine and Hazard Prevention, Ministry of Education of China, Xi’an, 710054, China State Mine Emergency Rescue (Xi’an) Research Center, Xi’an, Shaanxi, 710054, China
a r t i c l e
i n f o
Article history: Received 30 January 2019 Received in revised form 4 April 2019 Accepted 10 April 2019 Available online 17 April 2019 Keywords: Coal spontaneous combustion Coal temperature Evaluating method Various index gases Environmental pollution control
a b s t r a c t Coal spontaneous combustion has always been a worldwide problem, which causes waste of coal resources, greenhouse gas emissions and other atmospheric environmental pollution problems. Although coal temperature monitoring is the most direct and accurate means of predicting the spontaneous combustion of coal, the coal temperature often cannot be directly measured owing to various physical restrictions. As an alternative, the present study assessed the qualitative and quantitative analysis of the CO and C2 H4 formation rates, as well as various gas ratios such as the CO/CO2 ratio and fire coefficient R2 (R2 = 100×CO/O2 ), to predict spontaneous combustion. This method was established based on the temperature-programmed experiments of three different coal rank (including lignite, bituminous coal and anthracite), and was verified using data obtained from on-site monitoring at an actual mine. The results show that the method accuracy is as high as 97% when predicting the coal temperature to within 15 ◦ C (allowable error range of the predicted value). This degree of accuracy should be sufficient for on-site fire prevention and control. This new technique is not only accurate and reliable but also has theoretical significance with regard to the identification of coal spontaneous combustion in goaf and for the development of fire prevention and suppression technologies. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Fires in coal mines can cause significant casualties and resource losses. These incidents involve the production of large quantities of toxic gases, atmospheric environmental pollution, the destruction of equipment and facilities, and the loss of coal resources, and can easily lead to secondary effects such as explosion (Guo et al., 2018; Qi et al., 2017; Chen et al., 2016; Chen et al., 2018). The spontaneous combustion of coal, which accounts for more than 80% of coal mine fires, can develop slowly in concealed locations, and so can be difficult to identify and control (Zhang, 2008; Wen et al., 2016). Greater than 90% of coal spontaneous combustion incidents occur in the goaf area during the mining process (Jin et al., 2014; Avila et al., 2014; Veznikova et al., 2014; Ma et al., 2017), and so there has been a focus on accurately assessing this process.
∗ Corresponding author at: School of Safety Science and Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an, Shaanxi, 710054, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (J. Guo), [email protected] (Y. Liu).
There have been numerous studies of the spontaneous combustion of coal, based on both theory and technological evaluation. As an example, Deng (2006) developed a theoretical model for the molecular structure of coal based on quantum chemistry, and assessed the spontaneous combustion mechanism on a microscopic basis. A spontaneous combustion danger index was proposed on the basis of this prior work (Jin et al., 2014; Wen et al., 2017a; Jin et al., 2015). Xu (2000), Verma and Chaudhari (2016) and Deng et al. (2015) studied the mechanism associated with the spontaneous combustion of coal on the macro and microscopic levels, and developed various theories regarding this process, as well as means of predicting, preventing and controlling combustion, based on a coal-oxygen complex theory. This theoretical basis has since become widely accepted. Cao et al. (2012), Cheng et al. (2016), Yu et al. (2013) and Xu et al. (2014) proposed conditions and methods for the determination of spontaneous combustion danger zones in goafs, based on experimental work and on-site testing, that allow fire prevention and extinguishment. Wen et al. (2017b) established a CO concentration model using experiments and field work, to provide a new method for the early prediction of coal spontaneous combustion. Yu et al. (2005), Zhu (2016), Sahay et al. (2007), Jin et al. (2015), Wen et al. (2015) and Xu et al. (2018) researched
https://doi.org/10.1016/j.psep.2019.04.014 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Fig. 1. The temperature programmed experimental apparatus.
Fig. 2. A flow chart summarizing coal sample processing process.
gaseous spontaneous combustion products and oxygen consumption rates under various conditions, such as at high temperatures and moisture contents. Coal temperature is one of the most direct and accurate indicators of the degree of spontaneous combustion (Yu et al., 2017; Beamish et al., 2016; Taraba et al., 2011). However, the minedout area in a coal mine is a closed space that cannot be entered to measure temperatures. Additionally, coal is a poor conductor of heat and air mobility is low inside a mine. Therefore, it is impossible to accurately determine spontaneous combustion locations by directly measuring coal temperatures in the goaf. A large number of studies (Jin et al., 2014; Lu, 2009; Xiao et al., 2008; Ma et al., 2007; Wu et al., 2012) have shown a correlation between gaseous product concentrations and coal temperature during spontaneous combustion. Niu et al. (2016), Pone et al. (2007) and Parsa et al. (2017) carried out a series of tests involving gases in the Anyuan Mine and developed a more accurate method of predicting various stages of combustion. It appears that the association between the concentrations of gaseous products and temperature during the coal oxidation process could potentially be used to determine the degree of spontaneous combustion, namely coal temperature. In the present work, a temperature-programmed test apparatus was developed and used to examine the spontaneous combustion of coal at low temperatures. In this experiment, three different metamorphic grade coal samples were tested, such as bituminous coal (for example, long-flame coal), lignite and anthracite. The variations in the concentrations of certain gases obtained from coal samples during heating were studied and a method of predicting temperature was constructed. Employing this method, the temperature range of oxidized coal in an actual goaf was estimated.
2. Experimental 2.1. Experimental equipment and principles The experimental work used a temperature-controlled test apparatus (Wen et al., 2017a; Zhao et al., 2019a; Jin et al., 2015; Deng et al., 2016), as shown in Fig. 1. Employing an air pump or compressed air cylinder, a test vessel (diameter of 10 cm and height of 25 cm) preloaded with a coal sample was filled with the desired amount of air. The air flow to the coal was determined using a flow meter and the air was preheated as it flowed through a copper pipe on its way to the coal. The gases resulting from combustion were collected in a tank and quantified using gas chromatography.
2.2. Experimental conditions and process Coal samples, including bituminous coal (No. 1 long flame coal and No. 2 long flame coal), lignite and anthracite, which were obtained from a freshly-exposed coal face and crushed under ambient air. The preparation process of coal sample is shown in Fig. 2. The coal was subsequently sieved to obtain fractions having particle size ranges of 0–0.9 mm. 0.9–3 mm, 3–5 mm, 5–7 mm and 7–10 mm, after which these fractions were mixed. In each trial, a well-mixed 200 g portion of the coal was placed in the test apparatus, after which the air pipe was connected and the apparatus sealed (GB474-, 2008). After initiating a supply of air, the sample temperature was raised at a rate of 0.3 ◦ C/min and a one-liter sample of the resulting gases was collected at each 10 ◦ C interval for analysis by chromatography. The experimental conditions are summarized in Table 1.
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Table 1 Experimental test conditions. Coal samples
Ventilation volume /(ml/min)
Coal loading height /cm
Coal loading volume /cm3
Bulk density /(N·cm−3 )
Porosity /%
Heating rate /(◦ C/min)
Anthracite Long-flame coal - 1 Long-flame coal -2 Lignite
120 120 120 120
17.50 15.50 16.40 17.00
1373.75 1216.75 1287.40 1334.50
0.73 0.82 0.78 0.75
0.49 0.43 0.46 0.48
0.30 0.30 0.30 0.30
Fig. 3. CO generation as a function of coal temperature.
Fig. 4. CH4 concentration as a function of coal temperature.
3. Results and analysis 3.1. Relationship between specific gases and coal temperature 3.1.1. Relationship between CO and coal temperature CO is one of the most common indicators of the spontaneous combustion of coal (Wen et al., 2017a,b), and the relationship between the CO output and the coal temperature of four coal samples is shown in Fig. 3. For those coal samples, only trace amounts of CO were produced in the initial stage of the experiment, but the CO output began to slowly increase beginning at 60 ◦ C. Above this point, the samples reached the critical temperature necessary for the formation of a coal-oxygen complex, and the CO amount rapidly increased from 60 to 130 ◦ C. Above 130 ◦ C, the amount of CO increased sharply and the rate of CO formation accelerated. This occurred because coal rapidly oxidizes above its cracking temperature, resulting in spontaneous combustion. The cracking temperature is a marker of the rapid development of the coaleoxygen reaction and indicated that coal had entered the stage of intense oxidation. The active groups commenced gradually consuming oxygen and regenerating at this temperature (Zhao et al., 2019b). The temperature at which the rate of CO generation rapidly increases can be used to determine the point at which rapid oxidation occurs and so to qualitatively predict the degree of spontaneous combustion. Fig. 4 plots the CH4 generated as a function of temperature. For four coal samples, the presence of this gas at close to ambient temperature demonstrates the release of CH4 adsorbed in the coal pores. Below 100 ◦ C for long flame coal and lignite, 80 ◦ C for anthracite, the CH4 production is minimal and actually decreases slowly with increasing temperature. This effect is attributed to desorption of adsorbed CH4 , which is gradually depleted at low temperatures. At 100–140 ◦ C, the amount of CH4 begins to slowly increase as the side chains in the coal molecules began to break (Zhao et al., 2019b; Jin et al., 2015; Wen et al., 2017a; Deng et al., 2015). Above 140 ◦ C, the volume fraction of CH4 abruptly increases, primarily because coal-oxygen recombination reactions are pro-
Fig. 5. C2 H4 concentrations as functions of coal temperature.
moted, as well as the more rapid rupture of the side chains in the coal molecules (Zhao et al., 2019b; Jin et al., 2015; Wen et al., 2017a). 3.1.2. Relationship between C2 H4 and C2 H6 and coal temperature Both C2 H4 and C2 H6 are important indicators of spontaneous combustion, and both exhibit increases along with temperature. There is no C2 H4 in the original coal. Only when the coal temperature rising to 80–130 ◦ , can produce C2H4 gas because of the pyrolysis reaction (Liang et al., 2019; Guo et al., 2019a,b; Jin et al., 2015). Therefore, the generation of C2 H4 can be used to quantitatively characterize the degree of spontaneous combustion. The relationships between the generation of C2 H4 and C2 H6 and the coal temperature are plotted in Figs. 5 and 6. From these data it is evident that C2 H4 was not formed at the beginning of the experiment, while a small amount appeared at approximately 80–110 ◦ C. Thus, there was no C2 H4 initially present in the coal samples, and the C2 H4 generated in the high temperature phase can be attributed to the pyrolysis of the coal. In contrast, C2 H6 was generated beginning at about 20–30 ◦ C and gradually increased
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Fig. 6. C2 H6 concentrations as functions of coal temperature.
Fig. 7. CO/CO2 ratio as a function of coal temperature.
with increasing temperature, indicating that the original samples contained some adsorbed C2 H6 . C2 H4 is an unsaturated compound and will not accumulate in coal seams while C2 H6 can be deposited in coal seams, where it is more stable than CH4 (Liang et al., 2019; Guo et al., 2019a,b; Jin et al., 2015; Wen et al., 2017a). Therefore, C2 H6 cannot be used to predict the low temperature oxidation stage of coal. Although trends in C2 H4 and C2 H6 production can be employed to assess the extent of coal pyrolysis, the temperature at which C2 H4 appears is a more accurate indicator. 3.2. The relationship between gas ratios and coal temperature A necessary condition for the spontaneous combustion of coal is a sufficient air supply. The quantity of air available not only affects the reaction intensity but also dilutes the concentrations of the various product gases. This effect directly impacts the accuracy and reliability of predictions based on indicator gases such as CO, C2 H4 and C2 H6 . Using ratios such as CO/CO2 or the so-called Graham coefficient can effectively eliminate any effects of variations in the air volume (Wen et al., 2016, 2017a). Based on prior work, this study used the CO/CO2 ratio, the Graham coefficient (R2 = 100×CO/O2 ) and the alkyl chain ratio (C2 H6 /CH4 ) to establish relationships with coal temperature (Wen et al., 2017a; Wu et al., 2012). 3.2.1. Relationship between CO/CO2 and coal temperature CO/CO2 ratios of those four coal samples are plotted as functions of coal temperature in Fig. 7. This ratio of those four coal samples tends to increase during the spontaneous combustion of coal. In these data, taking long-flame coal-1 for example, the temperature increase from ambient to 60 ◦ C slowly raises the ratio but the value remains less than 0.1, indicating that the coal-oxygen recombination reaction was weak during this stage. Above 60 ◦ C, the CO/CO2 ratio increases rapidly to greater than 0.1 in the range of 80 to 90 ◦ C. At temperatures in the range of 90 to 140 ◦ C, the rate of increase drops slightly and the ratio is between 0.1 and 0.25. This occurs because the volume fractions of CO and CO2 both increase but at slightly different rates. At 140 ◦ C, the ratio suddenly increases again, rapidly rising from 0.25 to more than 0.5. Thus, in this stage, the formation of the coal-oxygen complex increases and the CO relative production rate is much higher than the CO2 relative production rate. This signifies spontaneous combustion of the coal (Jin et al., 2015). 3.2.2. Graham coefficient The Graham coefficient, G, is an important coal spontaneous combustion forecasting indicator, and reflects the relative changes
Fig. 8. Graham coefficient values as a function of coal temperature.
in the gas volume proportions (Wen et al., 2017a; Deng et al., 2016). The Graham coefficient eliminates the effect of the air volume and also can reflect the extent of fire spreading. The coefficient is calculated as: G = +CO/(-O2 )×100%
(1)
where +CO is the increment in the volume fraction of CO (%) and O2 is the decrease in the O2 volume fraction (%). The experimental G values are plotted as a function of the coal temperature in Fig. 8. In those plots, variations of the G value of these four coal samples are similar. Taking long-flame coal-1 for example, the G value slowly increases during the initial stage of temperature rise but is below 2 up to 70 ◦ C. This is due to the slow reaction of coal and oxygen at lower temperatures, the slow increase in the CO volume fraction and the weak oxygen consumption of the coal (Zhao et al., 2019b; Jin et al., 2015). As the temperature exceeds approximately 70 ◦ C, the rate of increase in G drops and G remains in the range of 2–3 up to 120 ◦ C. These data are attributed to having exceeded the critical temperature of the coal, which promotes the oxidation reaction along with CO production and oxygen consumption, such that the two gases have basically the same growth rate (Zhong et al., 2010). At 120 ◦ C, the G value increased sharply to above 3. During this stage, the continuous increase in the coal temperature promotes the coal-oxygen complex reaction, and the coal sample enters the spontaneous combustion stage.
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3.3. The determination of coal temperature
Fig. 9. The alkyl chain ratio as a function of coal temperature.
3.2.3. The alkyl chain ratio The alkyl chain ratios (C2 H6 /CH4 ) is the ratio of the volume concentration of ethane to the volume concentration of methane. The alkyl chain ratios are plotted against temperature in Fig. 9. The resulting curve exhibits a certain symmetry, such that the ratio initially increases and then decreases with increasing coal temperature, especially long-flame coal and lignite. The alkane’s ratio of anthracite shows a slow rising trend with increasing coal temperature. Taking long-flame coal-1 for example, the ratio reaches a maximum at 90–100 ◦ C. The ratio initially increases rapidly to about 2.25 at 90–100 ◦ C then decreases between 100 and 120 ◦ C, although remaining above 1.5. At temperatures greater than 120 ◦ C, the release rate of CH4 produced by pyrolysis was continuously higher than that of C2 H6 , and so the alkyl chain ratios undergoes a rapid decrease. Because the alkyl chain ratio data of bituminous coal (long flame coal) and lignite exhibits a symmetrical plot against the coal temperature, there are two temperatures corresponding to each ratio. The ratio increases and decreases with increasing temperature, making it difficult to use as a single predictor. Thus, the other indicators are necessary to determine coal temperature further. On the contrary, the curve of anthracite alkane ratio has a monotone increasing trend, so this ratio can be used as the main prediction index.
The spontaneous combustion of coal is complex and it is difficult to predict the temperature of coal in goaf accurately and quantitatively (Xu, 2000; Wen et al., 2017a; Verma and Chaudhari, 2016; Deng et al., 2016). Based on a qualitative and quantitative analysis of multiple gases, coal temperatures are determined at different confidence levels to allow the accurate identification of the potential for spontaneous combustion. The basic concept is summarized as a flowchart in Fig. 10, in which the main steps are as follows. 1. A single indicator is used to qualitatively determine the potential for spontaneous combustion. 1.1. The CO volume fraction is employed to determine the likelihood of spontaneous combustion. This parameter exhibits a good correlation with coal temperature, such that it increases rapidly above the critical temperature. 1.2. The C2 H4 volume fraction is also assessed to qualitatively evaluate the probability of spontaneous combustion. This gas appeared above approximately 110 ◦ C in the current experimental work and therefore can be used to monitor the coal temperature. 2. The gas ratio is used to determine the probability of spontaneous combustion. 2.1. The CO/CO2 ratio, Graham coefficient and alkyl chain ratio are all determined via on-site measurements. 2.2. Based on correlations derived from experimental testing, the above ratios are used to find the coal temperature. 2.3. A comprehensive analysis of single gases such as CO and C2 H4 as well as gas ratios is employed to calculate the coal temperature. 2.4. The average values of the on-site indicators are calculated so as to obtain the average coal temperature at the site. The prediction accuracy and the confidence intervals are set to the desired values. The degree of confidence for each temperature value obtained from the above process is calculated. This technique allows one to determine the coal temperature based on gas sampling that takes place at the mine. Thus, the tendency towards spontaneous combustion, the temperature range of the coal and the reliability of the temperature range are combined to allow an accurate prediction of the development of spontaneous combustion of the coal. 4. A case study A case of coal spontaneous combustion in the coalmine where the long-flame coal-1 sample came from was used for verifying the
Fig. 10. A flow chart summarizing the new method of determining coal temperature.
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Fig. 11. CO volume fractions over time. Fig. 12. Gas ratios over time.
feasibility of this prediction method. At this working face, during the mining process, the mined-out area showed signs of undergoing spontaneous combustion. For this reason, a series of fire prevention and extinguishing measures were in place, such as preventing the intrusion of air from the gob area and the injection of colloidal polymeric colloid fire extinguishing agents or yellow mud. These steps were intended to reduce air leakage to the fire zone and to limit the formation of coal-oxygen complexes. Continuous on-site measurements of O2 , CO, CO2 , CH4 and C2 H6 in the goaf at this mine were performed via beam tube monitoring system. 4.1. Analysis of a single gas No C2 H4 was detected during the on-site measurements, and so only the results obtained using CO concentrations are discussed here. The CO values obtained from measurements at the mine over time are plotted in Fig. 11. These data indicate that the CO volume fraction rapidly increased from the initial monitoring stage and reached a peak of 120 ppm on the 11th day. Therefore, the degree of oxidation of the coal left in the goaf was continuously increasing. The CO level subsequently remained steady at about 110 ppm. The varying degrees of fluctuation in the values are attributed to on-site sealing measures, such as grouting and plastic injection. 4.2. Analysis of gas ratios After processing the on-site observation data, the CO/CO2 ratios, Graham coefficients and alkyl chain ratios were plotted as functions of time, as in Fig. 12. In each case, the data show a similar trend, involving a rapid increase from the initial stage of monitoring. The values peaked near the 12th day, at which point the coal-oxygen complex reaction was most pronounced. Over the following 22 days, fire prevention measures such as grouting, plastic injection and inert gas injection were adopted, and the ratios decreased. This result demonstrates the effectiveness of these measures in controlling the spontaneous combustion of the coal. It should be noted that the fire prevention and extinguishment measures were not continuously applied for reasons such as equipment failure, resulting in a rebound phenomenon evident in the data. 4.3. Analysis of coal temperatures Data were collected from the mine over 35 days, and the resulting CO/CO2 ratios Graham coefficients and alkyl chain ratios are plotted in Fig. 13, overlaid with the data from the lab experimentation. In each case, the average, minimum and maximum values
of the three gas ratios are selected for all field data to illustrate the temperature determination process. 4.3.1. Analysis of coal temperature accuracy According to the field data, the average CO/CO2 ratio was 0.04, corresponding to an average coal temperature of 49 ◦ C based on the experimental work. The average Graham coefficient was 1.20, corresponding to an average coal temperature of 52 ◦ C and indicating the average temperature of coal spontaneous combustion predicted using this value. Over 35 days, the average value of the alkyl chain ratio was 0.95, corresponding to an average coal temperature of either 50 or 143 ◦ C. No C2 H4 was observed over this period, indicating that the coal temperature did not reach 110 ◦ C during the entire coal spontaneous combustion development process. Therefore, the temperature of 143 ◦ C was excluded and the average coal temperature predicted by this ratio was approximately 50 ◦ C. This same method could be used to determine the highest and lowest coal temperatures and the predictions are summarized in Table 2. 4.3.2. Reliability analysis To verify the reliability of the proposed method, the temperature values predicted using the three indexes (CO/CO2 ratio, Graham number and alkyl chain ratio) were compared for each day of the on-site monitoring. The aim was to determine whether or not all three indexes gave the same predicted temperature, within an allowable error range of the predicted value of 5, 10, 15 or 20 ◦ C. The results are summarized in Table 3. As shown in Table 3, allowable error range refers to allowable error range of the predicted value of coal temperature. Anastomosis of three indexes refers to the number of data that meets three indexes at the same time. Date of anastomosis refers to the specific day number of field observation above data, which be represent with sets in table. Confidence degree refers to the ratio of the above days to the total number of days. The results in Table 3 demonstrate that the temperatures obtained using the three indicators were within 5 ◦ C of one another 19 days out of the 35, representing 54% of the total days. This degree of uncertainty is considered too great to allow an accurate determination of the coal temperature. Expanding the range to 10 ◦ C, the temperature values from the three parameters agreed on 29 days, representing 83% of the total days, and indicating somewhat reliable predictions. Applying allowable ranges of 15 and 20 ◦ C, the predicated temperatures agreed on 34 and 35 days, representing 97% and 100% of the monitoring days, respectively. This level of agreement is acceptable for temperature analysis. Thus, the temperatures obtained from all three predictive indexes agreed with one another within 20 ◦ C on each day of the monitoring.
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Fig. 13. Comparisons between experimental results and field data.
Table 2 Results for the prediction of coal temperatures. Forecast index CO/CO2 Graham Chain alkanes ratio average value
Average value
Minimum value ◦
Maximum value ◦
Index value
Coal temperature / C
Index value
Coal temperature / C
Index value
Coal temperature /◦ C
0.04 1.20 0.95 —
49.00 52.00 50.00 50.33
0.01 0.47 0.38 —
33.00 32.00 34.00 33.00
0.06 1.74 1.42 —
72.00 71.00 72.00 71.67
Table 3 A comparison of the results obtained from on-site analysis and experimental work. Allowable error range/◦ C
Anastomosis of three indexes
Date of anastomosis
Confidence degree
5 10 15 20
19 29 34 35
U1 ={1,2,3,5,7,9,15,17,18,19,21,22,23,25,26,29,30,33,34} U2 = U1 ∪{4,6,8,11,13,20,24,28,31,35} U3 = U2 ∪{10,12,14,27,32} U4 = U3 ∪{16}
54 83 97 100
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In fact, in terms of coal spontaneous combustion prevention onsite, a range of coal temperatures needs to be determined, not a specific temperature point. Thus, an allowable error range in the 15 ◦ C is suitable. This method could be applied to coal spontaneous combustion monitoring and provide support for the on-site evaluation of the spontaneous combustion status.
5. Conclusions The generation of CO, CH4 , C2 H4 , C2 H6 , and gases ratio (CO/CO2 ratio, Graham coefficient and alkyl chain ratio) have a good correlation with coal temperature, which could be used to predict the temperature range of coal during spontaneous combustion. A method for determining coal temperature was developed, based on index gas (CO and C2 H4 ) and index value (CO/CO2 ratio, Graham coefficient and alkyl chain ratio). On site monitoring of an actual mine found that this technique is 97% accurate within allowable error range of 15 ◦ C. This is a simple method that addresses practical problems such as the effects of air intrusion on gas data. Using the prediction method proposed in this paper, a preliminary qualitative analysis of the risk of coal spontaneous combustion can be performed, which is based on CO, the CO/CO2 ratios, Graham coefficients and C2 H4 . Then, Quantitative predictions of the coal temperature with varying accuracies can be made using the CO/CO2 ratios, Graham coefficients and alkyl chain ratios. This approach can assess the development of spontaneous combustion and allow an evaluation of the effectiveness of fire prevention measures on-site.
Acknowledgments This work was supported by the National Key R&D Program of China (2018YFC0808201); China Postdoctoral Science Foundation (2017M623209); Special Scientific Research Project of Shaanxi Provincial Education Department (17JK0495); Natural Science Basic Research Program of Shaanxi (2018JQ5080; 2018JM5009). We thank Michael D. Judge, MSc, from Liwen Bianji, Edanz Editing China, for editing the English text of a draft of this manuscript.
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