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Prediction indices and limiting parameters of coal spontaneous combustion in the Huainan mining area in China
Fuel 264 (2020) 116883
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Full Length Article
Prediction indices and limiting parameters of coal spontaneous combustion in the Huainan mining area in China ⁎
Li Maa,c, Li Zoua,c, Li-Feng Rena,b, , Yi-Hong Chungd,e, Peng-Yu Zhanga,c, Chi-Min Shue,
T
⁎
a
School of Safety Science and Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an, Shaanxi 710054, PR China Post-doctoral Research Center of Mining Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an, Shaanxi 710054, PR China c Shaanxi Key Laboratory of Prevention and Control of Coal Fire, 58, Yanta Mid. Rd., Xi’an, Shaanxi 710054, PR China d Doctoral Program, Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology (YunTech), 123, University Rd., Sec. 3, Douliou, Yunlin 64002, Taiwan, ROC e Department of Safety, Health, and Environmental Engineering, YunTech, 123, University Rd., Sec. 3, Douliou, Yunlin 64002, Taiwan, ROC b
A R T I C LE I N FO
A B S T R A C T
Keywords: Limiting parameter Oxygen consumption rate CO production Gas ratio Oxidation index
Hazards due to spontaneous combustion of coal seriously threaten coal mining safety. To explore and evaluate the prediction indices and limiting parameters of coal self-ignition, seven samples from the Huainan mining area in Anhui Province, China were tested. Experiments were conducted using an oil-bath temperature-programmed system. The experimental results indicated that the oxygen consumption rates and CO production increased slowly initially and then rapidly with the increase in temperature. The plots of CO/CO2 ratio versus temperature exhibited an exponential property. The C2H4/C2H6 ratios exhibited an unstable alternation when the temperature was in the range of 100–130 °C. However, the ratios became steady after the temperature increased above 130 °C. The low-temperature oxidation process was divided into three stages according to the ΔCCO /ΔCO2 ratios. The oxidation index DO, a new indicator, was used to precisely forecast the point of spontaneous combustion. The limiting parameter analysis indicated that the minimum float coal thicknesses hmin and limiting oxygen concentrations Cmin first increased and then decreased constantly with an increase in temperature. The parameters reached their maximum value at 50 °C. The maximum air leakage intensity Qmax followed a trend opposite to that of the two aforementioned parameters. With an increase in temperature, Qmax first decreased and then increased. The results provide valuable information for predicting spontaneous combustion of coal, which can help prevent or mitigate risks in industrial mining.
1. Introduction Spontaneous combustion of coal is a major disaster that might occur during coal mining that seriously threatens mining safety [1,2]. Combustion frequently occurs while mining and storing coal and causes severe environmental pollution, resource wastage, and dust or gas explosion if self-ignition is not controlled [3,4]. Moreover, these issues can trigger a series of casualties, economic losses, and social problems [5]. Thus, preemptive warning technology has attracted increasing research attention recently [6]. The Huainan mining area, Anhui, Eastern China, is a rich source of high-quality coal that constitutes approximately 75% of the total coal reserves in Anhui. The Huainan mining area, which has an annual coal output of > 100 million tons, is one of the largest coal mines in China [7]. The risk of coal self-ignition increases with increase in the mining
intensity and depth. There are various coals with different ranks in the Huainan mining area. The self-ignition characteristics and prevention methods are influenced by the coal rank. However, the similarity of coal rank is more vital than that of difference in terms of the mining safety for exploring and evaluating the prediction indices and limiting parameters of coal self-ignition. Therefore, investigating the prediction indicators of spontaneous combustion of coal applicable to the Huainan mining area is crucial. Moreover, the safety for people and the industry in the mining region should be improved. Spontaneous combustion of coal is a physicochemical reaction that involves coal and oxygen, and the continuous accumulation of heat ultimately causes combustion [8,9]. Various gas products, for instance, CO, CO2, CH4, C2H4, and C2H6 are released in the process of coal oxidation and autoignition, and their concentrations vary regularly with raise in coal temperature [10]. Therefore, the stage and trend of coal
⁎ Corresponding authors at: School of Safety Science and Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an, Shaanxi 710054, PR China (L.-F. Ren). E-mail addresses: [email protected] (L.-F. Ren), [email protected] (C.-M. Shu).
https://doi.org/10.1016/j.fuel.2019.116883 Received 19 March 2019; Received in revised form 8 November 2019; Accepted 14 December 2019 Available online 21 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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was used for the low-temperature oxidation of coal. The heating rate and air supply flow rate of the system were set to 0.3 °C min−1 and 30 mL min−1, respectively. Before each experiment, the temperaturecontrol oven was flushed with fresh air for 20 min. Gas composition analysis was conducted at every 10 °C. The obtained gas parameters were used for characterizing the low-temperature oxidation process of the coal samples.
self-ignition can be defined by monitoring the gas indices. Several researchers have attempted to optimize the predictive technology of coal combustion for managing the risks of spontaneous combustion of coal by the variation in the gas indices. Guo et al. [11] predicted the coal temperature during spontaneous combustion by analyzing the CO and C2H4 formation rates. Xiao et al. [12] explored the variation in the hydrocarbon gas indices under different temperature conditions for improving the sensitivity of gas detection. Lei et al. [13] employed a forest prediction approach based on cross-analysis between field and experimental gaseous indices data for predicting the temperature of coal self-ignition. Deng et al. [14] investigated the gas indices for forecasting spontaneous combustion of coal by using a temperatureprogrammed system. They suggested that the CO concentration could be used as a key index for predicting self-ignition. However, the gas concentration can be disturbed by the mine airflow, which influences the prediction accuracy. Thus, other indicator parameters were urgently required to assist the prediction of coal spontaneous combustion. The limiting parameters of coal refer to the essential requirement for coal to spontaneously ignite under certain conditions [15]. Numerous studies demonstrated that the limiting parameters were influenced by the intrinsic properties of coal and other external conditions, such as coal rank [16], ambient temperature [17], and coal particle size [18]. The diversity of coal rank in the Huainan mining area led to the differences in the limiting parameters of spontaneous combustion. Therefore, investigating the similar variation in the self-ignition limiting parameters of coals at different ranks can supply pivotal information for the prevention and control of spontaneous combustion of coal. In this study, seven coal samples obtained from the Huainan mining area were analyzed using an oil-bath temperature-programmed system. The oxygen consumption rate, CO production, and gas ratio were analyzed to define the oxidation stages during spontaneous combustion. The oxidation index, DO, a new indicator, was proposed to precisely predict self-ignition. Furthermore, the limiting parameters of the seven samples were independently determined to determine the relative risk degree of spontaneous combustion. The current results provide valuable information for forecasting spontaneous combustion and improving fire prevention technology in the Huainan mining area.
3. Results and discussion 3.1. Gas indices during spontaneous combustion of coal 3.1.1. Oxygen consumption rate The oxygen present in a system is consumed continuously throughout the entire spontaneous combustion process, so the oxygen consumption rate reflects the oxidation rate of a sample [19,20]. Oxygen consumption rates were analyzed to identify the self-ignition behavior of the samples in this study. The oxygen consumption rate during the low-temperature oxidation of coal is expressed as follows [21]:
VO2 (T ) =
Q1 CO02 SnL
ln
CO02 CO12
(1) 3
−1
where Q1 is the air supply volume (m s ); S is the base area of the coal sample tank (m2); n is the porosity of the coal sample (dimensionless); L is the height of the coal sample (m); and CO02 and CO12 are the inlet and outlet oxygen concentrations, respectively (vol.%). Fig. 3 demonstrates that the oxygen consumption rate consistently increased with temperature throughout the spontaneous combustion reaction in each experiment. The rate exhibited an exponential growth trend that first increased slowly and then sharply. In the initial stages (at < 110 °C), the oxygen consumption rate was low because the coal temperature was low, and active functional groups participated in the oxidation reaction in minute amounts. When the coal temperature increased above 110 °C, the chemical adsorption and chemical reactions dominated the low-temperature oxidation process. During this period, a large amount of oxygen was consumed in a short period. The reaction intensity increased, which caused a rapid increase in the oxygen consumption rate. Because of their relatively high maturity, the GE, PE, ZJ, and DJ samples had considerably lower oxygen consumption rates than the other three samples. Fig. 4 demonstrates that the average oxygen consumption rates of the seven coal samples increased with increase in the volatile matter and oxygen content. Furthermore, there are two regions—low- and high-rank coal areas. A high average oxygen consumption rate represents a high reaction rate between coal and oxygen, namely, a high self-heating rate of coal. Thus, as the volatile matter and oxygen contents increase (i.e., the coal rank decreases), the self-ignition rate increases.
2. Experiment and methods 2.1. Coal samples The experimental samples were acquired from the Huainan mining area in Anhui Province, China (Fig. 1). The high-quality coking coal produced in this area is widely applied in the metallurgical industry. Although its coal rank varies, coal is generally prone to self-ignition. The seven coal samples used in this study are as follows: Gu Yi (GY), Gu Er (GE), Gu San (GS), Pan Yi (PY), Pan Er (PE), Zhu Ji (ZJ), and Ding Ji (DJ). These samples were crushed to a particle size of 0–3 mm. Each sample was placed in a vacuum drying oven at 80 °C for 24 h before the testing to eliminate the moisture content. The proximate and ultimate analysis results are presented in Table 1. The samples generally had low ash and sulfur contents with a high amount of volatile organic matter, which is one-third coking coal. According to the range of the high heat values (HHVs), GY, GS, and PY are high heating coal samples and GE, PE, ZJ, and DJ are mid-to-high heating coal samples.
3.1.2. CO production During self-ignition, coal continuously consumes oxygen and generates various gaseous products, such as CO, CO2, C2H4, and C2H6 [22]. CO is a key index that can be used to demarcate the stages of spontaneous combustion; the presence of CO characterizes the beginning of spontaneous combustion [23]. Fig. 5 shows that the CO production of each sample increased with temperature. When the temperature was < 110 °C, low-intensity physical adsorption was the major reaction. Therefore, a low amount of CO was generated by oxidation. The oxidation intensity increased rapidly when the temperature exceeded 110 °C. The increase in the oxidation intensity resulted in the activation of a large number of functional groups that reacted with the oxygen in the system. Because CO is a product of coal oxidation, the amount of CO increased sharply at temperatures > 110 °C. The CO production exhibited a trend similar to the oxygen consumption rate (i.e., an exponential trend). Moreover, CO
2.2. Low-temperature oxidation experiment of the coal samples In this study, an oil-bath temperature-programmed system was used to test the self-ignition characteristics of the samples during low-temperature oxidation. This method was selected because of its thermal stability. The experimental system comprised an air tank, a temperature-control oven, a gas chromatograph, and a data collection system (Fig. 2). The oven provided a temperature range of 30–170 °C, which 2
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Fig. 1. (a) Site of the Huainan mining area in China and (b) the sampling location [16].
trend. The CO/CO2 ratio was < 0.1 when the temperature was below 120 °C and was > 0.1 when the temperature exceeded 120 °C. Hence, the transition and rapid oxidation stages during spontaneous combustion could be distinguished using the CO/CO2 ratio.
was generated throughout the experiment and its concentration was affected by air turbulence and the surrounding rock adsorption. Thus, inaccurate results can be obtained if the CO concentration is only used for predicting the spontaneous combustion phases. Other index parameters are essential for predicting the spontaneous combustion point of coal samples.
3.2.2. C2H4/C2H6 ratio Fig. 7 displays the plots of the C2H4/C2H6 ratio versus temperature for the seven samples. During the low-temperature oxidation of each coal sample, an unstable stage was observed between 100 and 130 °C. However, the C2H4/C2H6 ratio increased steadily when the temperature exceeded 130 °C. When the temperature was < 150 °C, the C2H4/C2H6 ratio was < 1. Then, the C2H4/C2H6 ratio increased slowly and eventually exceeded 1, which indicates that the C2H4 production was higher than the C2H6 production during this phase. The C2H4/C2H6 ratio increased continuously with temperature; however, the curves converged gradually.
3.2. Gas ratios In practice, a is considered to be a major indicator for predicting spontaneous combustion of coal. Gas ratios are affected by gaseous products and air turbulence to a limited extent [24]. Thus, we analyzed the CO/CO2, C2H4/C2H6, and ΔCCO /ΔCO2 ratios during the low-temperature oxidation. 3.2.1. CO/CO2 ratio Fig. 6 indicates that for each coal sample, the CO/CO2 ratio increased as the temperature increased. The increase was slow in the range of 30–80 °C; however, the increase was swift when the temperature exceeded 80 °C. At a given temperature, the GS sample had the highest CO/CO2 ratio, followed by the GY, PY, DJ, ZJ, PE, and GE samples. The CO/CO2 ratios of the seven samples followed exponential growth patterns, which can be expressed using Eq. (2).
y = a exp(T b) + c
3.2.3. ΔCCO /ΔCO2 ratio Fig. 8 displays the plots of the ΔCCO /ΔCO2 ratio versus temperature for the GY sample. The ΔCCO /ΔCO2 ratios for all the seven coal samples are presented in Table 2. These ΔCCO /ΔCO2 ratios followed a fluctuating yet ascending trend with increase in temperature. The other six samples also exhibited a similar tendency. The low-temperature oxidation process was divided into three phases by using two characteristic temperature points: slow oxidation (stage 1), accelerated oxidation (stage 2), and vigorous oxidation (stage 3). The first characteristic
(2)
The fitted results indicate that Eq. (2) suitably matches the variation Table 1 Proximate and ultimate analysis results of the seven coal samples. Sample
GY GE GS PY PE ZJ DJ
Proximate analysis (mass%)
Ultimate analysis (mass%)
Mad
Aad
Vad
FCad
N
C
S
H
O
1.16 1.35 1.47 0.93 1.56 0.97 1.26
11.73 16.16 13.10 11.48 15.75 16.50 20.05
34.22 25.22 32.24 32.03 26.39 26.20 25.04
52.89 57.27 53.19 55.56 56.30 56.33 53.65
1.89 1.82 1.68 1.78 1.59 1.44 1.58
74.38 73.27 73.53 75.51 72.82 73.75 69.82
0.44 0.53 0.63 0.82 1.49 0.46 0.86
5.08 4.93 4.63 4.61 4.45 4.71 4.65
5.32 1.94 4.96 4.87 2.34 2.17 1.78
HHV (MJ/kg)
Coal rank
24.94 23.05 24.43 24.54 23.49 23.67 22.71
HQ MHQ HQ HQ MHQ MHQ MHQ
Notes: The content of O was defined by the subtraction method and compared with the content of C, H, N, S, Aad, and Mad; FCad was determined using the subtraction method and compared with the content of Aad, Mad, and Vad. HHV represents the high heat value of coal; and MHQ and HQ represent mid-to-high heating of coal and high heating of coal, respectively. 3
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Fig. 2. Schematic of the oil-bath temperature-programmed system.
Fig. 3. Curves of the oxygen consumption rate versus temperature for the seven coal samples.
Fig. 5. Curves of CO production versus temperature for the seven coal samples.
Fig. 6. Plots of the CO/CO2 ratio versus temperature for the seven coal samples.
Fig. 4. Plots of the average oxygen consumption rate versus the volatile matter and oxygen content for the seven coal samples.
4
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Fig. 7. Plots of the C2H4/C2H6 ratios versus temperature for the seven coal samples.
Fig. 9. Plot of the oxidation index versus temperature for the GY sample.
Subsequently, the oxidation intensity increased rapidly with temperature. Thus, according to the aforementioned characteristics of the coal samples during low-temperature oxidation, the ΔCCO /ΔCO2 ratio can be applied as a secondary index for predicting spontaneous combustion of coal. 3.3. Oxidation index According to the observed changes in the oxygen concentration, the oxidation index (DO, min−1), a new index, was proposed for predicting spontaneous combustion of coal. This index represents the ratio between the oxygen consumption per unit time and the total oxygen consumption and indicates the oxidation reaction rate. DO can be calculated using Eq. (3):
DO =
β dCO dT ⎞ dCO dt = 1000 ⎜⎛ ⎟ ΔCO, max ⎝ ΔCO, max ⎠
(3)
where CO is the oxygen concentration (vol%), t is the reaction time (min), T is the temperature (°C), β is the heating rate (°C min−1), 1000 is the amplification factor, and ΔCO, max is the maximum oxygen consumption (vol%). The oxygen concentration was stable at approximately 3 vol% for each experiment with a constant increase in the temperature. Thus, the ΔCO, max value of this study was 18 vol%. Fig. 9 displays the plots of the oxidation index versus temperature for the GY sample. The oxidation
Fig. 8. Plot of the ΔCCO /ΔCO2 ratio versus temperature for the GY sample.
temperature point was at 90 °C. At this point, the coal temperature exceeded the critical temperature, and the oxidation rate began to accelerate. The second characteristic temperature point occurred at 140 °C when the sample entered the vigorous oxidation stage.
Table 2 ΔCCO /ΔCO2 ratios for the seven samples at different temperatures. Temperature (°C)
30 40 50 60 70 80 90 100 110 120 130 140 150 160
ΔCCO ΔCO2 ratio GY
GE
GS
PY
PE
ZJ
DJ
17.72 43.94 68.17 150.10 210.00 196.97 426.41 359.48 402.76 493.22 499.55 699.17 673.19 750.26
46.26 51.16 71.13 82.73 138.75 109.31 325.40 269.41 322.82 336.00 349.16 540.85 514.17 576.99
23.85 84.75 123.75 196.55 353.55 399.28 625.96 556.41 602.05 617.74 635.13 836.15 804.20 839.22
52.75 53.40 66.40 153.14 228.56 189.82 439.25 419.16 443.19 484.43 460.32 904.35 933.96 924.59
19.31 35.71 51.51 94.25 150.50 131.47 365.00 338.63 426.87 460.00 525.89 734.34 678.14 670.44
15.96 56.28 74.10 147.29 243.41 265.55 400.33 388.38 462.50 506.11 591.17 652.30 619.35 666.13
20.40 62.68 81.00 98.91 123.56 165.33 158.89 407.36 350.84 466.83 412.63 863.28 719.36 866.14
5
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Table 3 Oxidation indices for the seven samples at different temperatures. Temperature (°C)
30 40 50 60 70 80 90 100 110 120 130 140 150 160
T VCO =
VO2 (T ) CCO CO2 [1 − exp(−VO2 (T ) V Q1 CO2 )]
Oxidation indexes T VCO = 2
GY
GE
GS
PY
PE
ZJ
DJ
0.47 0.47 0.59 0.82 0.47 1.13 1.22 1.52 5.12 5.0 6.17 5.80 4.28 4.88
0.29 0.31 0.64 0.98 0.42 0.58 1.89 2.65 5.57 5.35 6.88 5.02 5.31 6.04
0.37 0.31 0.42 0.67 0.47 0.73 1.63 1.86 5.78 6.51 6.80 6.15 7.46 6.57
0.20 0.23 0.33 0.78 0.49 1.08 1.76 1.57 5.59 6.75 6.14 8.60 8.31 7.78
0.31 0.31 0.50 0.65 0.77 1.45 1.37 1.58 2.26 3.60 3.83 3.60 3.54 5.33
0.31 0.37 0.26 0.45 0.71 1.47 1.61 1.81 2.83 5.07 4.46 5.71 7.32 4.93
0.24 0.34 0.45 0.63 0.66 1.73 1.94 1.50 2.18 5.14 4.58 4.49 5.83 5.01
h min =
(7)
ρg Cg Q (T − Ty ) q (T )
+
(ρg Cg Q)2 (T − Ty )2 + 8λ e q (T )(T − Ty ) q (T ) (8)
Cmin =
CO02 ⎡ 8 × λ e (T − Ty ) 2 × (T − Ty ) ⎤ + ρg Cg Q 2 ⎥ h q (T ) ⎢ h ⎦ ⎣
Qmax =
h × q (T ) 4λ e − 2 × ρg Cg (T − Ty ) hρg Cg
(9)
(10) −3
where ρg denotes the air density (kg m ), Cg is the specific heat of air (J kg−1 °C−1), T is the coal temperature (°C), Ty is the stratum temperature (°C), and λe is the thermal conductivity of coal (J m−1 s−1 °C−1). 3.4.1. Minimum float coal thickness We analyzed the hmin curves of the seven samples at an air leakage intensity range of 0.001–0.005 m s−1. Fig. 10(a)–(c) demonstrate the plots of hmin versus temperature for the GY, GE, and GS samples, respectively, at various air leakage intensities. For the three samples, hmin increased with temperature but quickly decreased in the range of 50–120 °C. The hmin value remained constant beyond this range. The samples had low oxidation rates in the initial stage (i.e., < 50 °C), which resulted in a limited heat release. A large coal thickness generally indicates a superior thermal storage capacity [31]. Thus, hmin increased consistently when the temperature was < 50 °C. Subsequently, the oxidation reaction intensity increased with temperature, which caused the release of a large amount of heat. Thus, even a very small coal thickness can supply sufficient fuel for the initiation of the spontaneous combustion process. Consequently, the value of hmin further decreased. At a given temperature, hmin increased with increase in the air leakage. The difference between the two adjacent curves first exhibited an increasing trend, followed by a decreasing trend when the maximum value reached approximately 50 °C. Increase in the air leakage may boost heat dissipation, which reduces the heat for self-ignition. Therefore, increase in the coal thickness may increase the heat storage and ensure persistence of spontaneous combustion. The air leakage only had a marginal effect on hmin when the temperature exceeded 140 °C. Fig. 10(d) shows that the hmin curves of the seven samples exhibited similar trends at the same air leakage intensity, which indicates that the samples have similar spontaneous combustion risks.
3.4. Limiting parameters for low-temperature oxidation of coal Multiple factors affect the spontaneous combustion of coal, such as oxygen concentration, particle size, and ventilation. The limiting values of these essential factors are described as limiting parameters [25,26]. In a mine goaf, the prerequisite conditions for spontaneous combustion are as follows [27]: (4)
where h and hmin are the float coal thickness and minimum float coal thickness (m), respectively; C and Cmin are the corresponding oxygen concentration and oxygen concentration limit (vol%), respectively; and Q and Qmax are the air leakage intensity and maximum air leakage intensity (m s−1), respectively. In this study, the exothermic intensity was calculated as follows [16]: T T T 0 0 T q (T ) = qa [VOT2 − VCO − VCO ] + VCO [(h298 )CO + ΔhCO ] + VCO 2 2 0 0 )CO2 + ΔhCO [ (h298 ] 2
VO2 (T ) CCO2 CO2 [1 − exp(−VO2 (T ) V Q1 CO2 )]
where CO2 is the oxygen concentration (vol%); CCO and CCO2 are the amounts of CO and CO2 production (vol%), respectively; and V is the volume of the coal sample (m3). Three limiting parameters, hmin, Cmin, and Qmax, were calculated using Eqs. (8)–(10), respectively [29,30]. An analysis indicated that these parameters are useful for determining the risk degree of spontaneous combustion in coal mining operations in the Huainan mining area.
index values for the seven coal samples are presented in Table 3. The oxidation index of the seven samples increased continuously with temperature. When the temperature was below approximately 80 °C, the DO value was < 1, indicating a low oxidation rate. Subsequently, the DO value increased gradually and exceeded 2. Above approximately 100 °C, the samples entered the accelerated oxidation phase. Thus, the oxidation index is an excellent parameter for distinguishing between the different stages of low-temperature oxidation. The oxidation index increased rapidly between 100 °C and 110 °C, which indicates the strong intensity of the oxidation reaction in that range. The oxidation index followed an undulating trend when the temperature was > 110 °C. The decrease in the oxygen concentration was considered to be the major cause of the aforementioned phenomenon.
(h > h min ) ∩ (C > Cmin ) ∩ (Q > Qmax )
(6)
(5)
where q(T) is the exothermic intensity (J s−1 m−3); qa is the chemical adsorption heat of coal to oxygen (J mol−1); VOT2 is the oxygen con0 0 )CO and (h298 sumption rate at temperature T (mol s−1 m−3); (h 298 )CO2 are the standard heats of formation of CO and CO2, respectively, at 0 298 K and 1 atm (J mol−1); ΔhCO is the difference between the heat of CO formation at a temperature of T and pressure of 1 atm and the 0 standard heat of CO formation (J mol−1); ΔhCO is the difference be2 tween the heat of CO2 formation at a temperature of T and pressure of T 1 atm and the standard heat of formation of CO2 (J mol−1); and VCO as T are the CO and CO production rates at temperature T well as VCO 2 2 T T and VCO can be cal(mol s−1 m−3), respectively. The parameters VCO 2 culated using Eqs. (6) and (7) [28]:
3.4.2. Limiting oxygen concentration Coal continuously consumed the oxygen present in the system during the spontaneous combustion reaction, so the oxygen concentration decreased gradually. The coal temperature remained constant when the oxygen concentration decreased to a certain value; that is, heat release was equal to heat dissipation. The oxygen concentration at which the coal temperature remains constant is known as the limiting oxygen concentration [32]. We examined the Cmin curves of the seven samples at a coal thickness range of 0.4–0.8 m and an air leakage intensity of 0.0005 m s−1. 6
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Fig. 10. Curves of hmin at various air leakage intensities versus temperature for the (a) GY, (b) GE, and (c) GS samples, and (d) plots of hmin for the seven samples at Q = 0.003 m s−1.
exhibited a similar trend with an increase in temperature.
Fig. 11(a)–(c) shows the plots of Cmin versus temperature for the GY, GE, and GS samples, respectively, at various coal thicknesses under an air leakage intensity of 0.0005 m s−1. For each experiment, Cmin first increased and then decreased with increase in temperature and reached a maximum value at approximately 50 °C. The Cmin values of the GY, GE, and GS samples were < 10% when the temperature exceeded 100 °C, which indicated that the spontaneous combustion risk was high at this stage. When the temperature was constant, Cmin gradually decreased with an increase in coal thickness. This phenomenon occurred because the heat required for spontaneous combustion was constant under certain conditions and an increase in the thickness increased the heat production. Thus, a decrease in oxygen concentration was essential for maintaining a heat balance during oxidation. For the GY, GE, and GS samples, the Cmin values exceeded 21 vol% (i.e., oxygen concentration in the air) when the temperature was in the range of 40–60 °C at a coal thickness of 0.4 m, which indicated that spontaneous combustion did not occur at this stage. Fig. 11(d) illustrates the Cmin curves of the seven samples under the same conditions. The Cmin curves of all the samples
3.4.3. Maximum air leakage intensity When the air leakage intensity increased to a certain value, the heat generated by coal oxidation was completely dissipated by heat conduction and air flow, and the value was defined as the maximum air leakage intensity [33]. In this study, we analyzed the Qmax curves of the seven samples for a coal thickness range of 0.4–0.8 m. Fig. 12(a)–(c) are the plots of Qmax versus temperature for the GY, GE, and GS samples, respectively, at various coal thicknesses. For each experiment, Qmax first decreased and then increased gradually with an increase in temperature. The minimum Qmax value was observed at 50 °C. In the initial stage (i.e., < 50 °C), a small amount of heat was generated by coal oxidation. However, a large air leakage can dissipate heat and reduce the coal temperature. Therefore, a reduction in the air leakage intensity maintains the development of the spontaneous combustion process. The oxygen requirement increased gradually with temperature. Because the oxygen concentration was constant, low air 7
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Fig. 11. Curves of Cmin at various coal thicknesses under an air leakage intensity of 0.005 m s−1 versus temperature for the (a) GY, (b) GE, and (c) GS samples, and (d) plots of Cmin versus temperature for the seven samples (h = 0.5 m, Q = 0.005 m s−1).
4. Conclusions
leakage caused a decrease in the reaction rate. Therefore, an increase in the air leakage ensures the continuation of the combustion process. An increase in air leakage may cause increase in heat loss. Thus, further studies should be conducted on the limiting value of the air leakage for balancing the oxygen supply and heat dissipation. At a given temperature, Qmax increased with coal thickness. The difference between the two adjacent curves of Qmax and coal thickness increased gradually, which indicated that the effect of coal thickness on Qmax increased with temperature. An increase in the coal thickness can increase the heat storage capacity of coal seams, which results in a strong oxidation process and increased oxygen consumption. An increase in the air leakage may provide sufficient oxygen for the occurrence of spontaneous combustion. As displayed in Fig. 12(d), the Qmax curves of all the seven samples exhibited an increasing trend with increase in temperature. The Qmax values of the samples were approximately equal when the temperature was < 90 °C. Above 90 °C, the difference between the Qmax values of the samples increased consistently, which indicates the effect of the coal rank on Qmax as temperature increases.
This study was conducted to generate the prediction indices for the spontaneous combustion process of seven coal samples obtained from the Huainan mining area in Anhui Province, China. The gas indices and ratios during low-temperature oxidation were investigated. The oxygen consumption rate and CO production of the seven samples increased exponentially as the temperature increased. The CO/CO2, C2H4/C2H6, and ΔCCO /ΔCO2 ratios displayed an obvious stage variation characteristic with temperature, which was beneficial to determine the developmental phase of low-temperature oxidation of coal. The oxidation index DO was proposed to further determine the oxidation period during spontaneous combustion of coal. In summary, CO gas can be considered the main indicator for forecasting spontaneous combustion of coal for the Huainan mining area. Gas ratios, such as the CO/CO2, C2H4/C2H6, and ΔCCO /ΔCO2 ratios, and the oxidation index can be applied as auxiliary indicators of coal self-ignition. On the basis of the discussion of the variation law of the three limiting parameters (minimum float coal thickness, maximum air leakage intensity, and 8
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Fig. 12. Curves of Qmax at various coal thicknesses versus the temperature for the (a) GY, (b) GE, and (c) GS samples, and (d) plots of Qmax versus temperature for the seven samples at h = 0.5 m.
References
limiting oxygen concentration), the essential external conditions for spontaneous combustion of coal were obtained. These conditions can provide valuable information for preventing and controlling coal selfignition in the Huainan mining area. The three limiting parameters considered in this study had a comprehensive influence on the self-ignition of coal. Therefore, further research is required on the risk areas of spontaneous combustion of coal, particularly the limiting value of air leakage for balancing oxygen supply and heat dissipation.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by National Key R&D Program of China (No. 2018YFC0808104), National Natural Science Foundation of China (Nos. 5157-4193, 5167-4191, 5180-4247). 9
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Full Length Article
Prediction indices and limiting parameters of coal spontaneous combustion in the Huainan mining area in China ⁎
Li Maa,c, Li Zoua,c, Li-Feng Rena,b, , Yi-Hong Chungd,e, Peng-Yu Zhanga,c, Chi-Min Shue,
T
⁎
a
School of Safety Science and Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an, Shaanxi 710054, PR China Post-doctoral Research Center of Mining Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an, Shaanxi 710054, PR China c Shaanxi Key Laboratory of Prevention and Control of Coal Fire, 58, Yanta Mid. Rd., Xi’an, Shaanxi 710054, PR China d Doctoral Program, Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology (YunTech), 123, University Rd., Sec. 3, Douliou, Yunlin 64002, Taiwan, ROC e Department of Safety, Health, and Environmental Engineering, YunTech, 123, University Rd., Sec. 3, Douliou, Yunlin 64002, Taiwan, ROC b
A R T I C LE I N FO
A B S T R A C T
Keywords: Limiting parameter Oxygen consumption rate CO production Gas ratio Oxidation index
Hazards due to spontaneous combustion of coal seriously threaten coal mining safety. To explore and evaluate the prediction indices and limiting parameters of coal self-ignition, seven samples from the Huainan mining area in Anhui Province, China were tested. Experiments were conducted using an oil-bath temperature-programmed system. The experimental results indicated that the oxygen consumption rates and CO production increased slowly initially and then rapidly with the increase in temperature. The plots of CO/CO2 ratio versus temperature exhibited an exponential property. The C2H4/C2H6 ratios exhibited an unstable alternation when the temperature was in the range of 100–130 °C. However, the ratios became steady after the temperature increased above 130 °C. The low-temperature oxidation process was divided into three stages according to the ΔCCO /ΔCO2 ratios. The oxidation index DO, a new indicator, was used to precisely forecast the point of spontaneous combustion. The limiting parameter analysis indicated that the minimum float coal thicknesses hmin and limiting oxygen concentrations Cmin first increased and then decreased constantly with an increase in temperature. The parameters reached their maximum value at 50 °C. The maximum air leakage intensity Qmax followed a trend opposite to that of the two aforementioned parameters. With an increase in temperature, Qmax first decreased and then increased. The results provide valuable information for predicting spontaneous combustion of coal, which can help prevent or mitigate risks in industrial mining.
1. Introduction Spontaneous combustion of coal is a major disaster that might occur during coal mining that seriously threatens mining safety [1,2]. Combustion frequently occurs while mining and storing coal and causes severe environmental pollution, resource wastage, and dust or gas explosion if self-ignition is not controlled [3,4]. Moreover, these issues can trigger a series of casualties, economic losses, and social problems [5]. Thus, preemptive warning technology has attracted increasing research attention recently [6]. The Huainan mining area, Anhui, Eastern China, is a rich source of high-quality coal that constitutes approximately 75% of the total coal reserves in Anhui. The Huainan mining area, which has an annual coal output of > 100 million tons, is one of the largest coal mines in China [7]. The risk of coal self-ignition increases with increase in the mining
intensity and depth. There are various coals with different ranks in the Huainan mining area. The self-ignition characteristics and prevention methods are influenced by the coal rank. However, the similarity of coal rank is more vital than that of difference in terms of the mining safety for exploring and evaluating the prediction indices and limiting parameters of coal self-ignition. Therefore, investigating the prediction indicators of spontaneous combustion of coal applicable to the Huainan mining area is crucial. Moreover, the safety for people and the industry in the mining region should be improved. Spontaneous combustion of coal is a physicochemical reaction that involves coal and oxygen, and the continuous accumulation of heat ultimately causes combustion [8,9]. Various gas products, for instance, CO, CO2, CH4, C2H4, and C2H6 are released in the process of coal oxidation and autoignition, and their concentrations vary regularly with raise in coal temperature [10]. Therefore, the stage and trend of coal
⁎ Corresponding authors at: School of Safety Science and Engineering, Xi’an University of Science and Technology, 58, Yanta Mid. Rd., Xi’an, Shaanxi 710054, PR China (L.-F. Ren). E-mail addresses: [email protected] (L.-F. Ren), [email protected] (C.-M. Shu).
https://doi.org/10.1016/j.fuel.2019.116883 Received 19 March 2019; Received in revised form 8 November 2019; Accepted 14 December 2019 Available online 21 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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was used for the low-temperature oxidation of coal. The heating rate and air supply flow rate of the system were set to 0.3 °C min−1 and 30 mL min−1, respectively. Before each experiment, the temperaturecontrol oven was flushed with fresh air for 20 min. Gas composition analysis was conducted at every 10 °C. The obtained gas parameters were used for characterizing the low-temperature oxidation process of the coal samples.
self-ignition can be defined by monitoring the gas indices. Several researchers have attempted to optimize the predictive technology of coal combustion for managing the risks of spontaneous combustion of coal by the variation in the gas indices. Guo et al. [11] predicted the coal temperature during spontaneous combustion by analyzing the CO and C2H4 formation rates. Xiao et al. [12] explored the variation in the hydrocarbon gas indices under different temperature conditions for improving the sensitivity of gas detection. Lei et al. [13] employed a forest prediction approach based on cross-analysis between field and experimental gaseous indices data for predicting the temperature of coal self-ignition. Deng et al. [14] investigated the gas indices for forecasting spontaneous combustion of coal by using a temperatureprogrammed system. They suggested that the CO concentration could be used as a key index for predicting self-ignition. However, the gas concentration can be disturbed by the mine airflow, which influences the prediction accuracy. Thus, other indicator parameters were urgently required to assist the prediction of coal spontaneous combustion. The limiting parameters of coal refer to the essential requirement for coal to spontaneously ignite under certain conditions [15]. Numerous studies demonstrated that the limiting parameters were influenced by the intrinsic properties of coal and other external conditions, such as coal rank [16], ambient temperature [17], and coal particle size [18]. The diversity of coal rank in the Huainan mining area led to the differences in the limiting parameters of spontaneous combustion. Therefore, investigating the similar variation in the self-ignition limiting parameters of coals at different ranks can supply pivotal information for the prevention and control of spontaneous combustion of coal. In this study, seven coal samples obtained from the Huainan mining area were analyzed using an oil-bath temperature-programmed system. The oxygen consumption rate, CO production, and gas ratio were analyzed to define the oxidation stages during spontaneous combustion. The oxidation index, DO, a new indicator, was proposed to precisely predict self-ignition. Furthermore, the limiting parameters of the seven samples were independently determined to determine the relative risk degree of spontaneous combustion. The current results provide valuable information for forecasting spontaneous combustion and improving fire prevention technology in the Huainan mining area.
3. Results and discussion 3.1. Gas indices during spontaneous combustion of coal 3.1.1. Oxygen consumption rate The oxygen present in a system is consumed continuously throughout the entire spontaneous combustion process, so the oxygen consumption rate reflects the oxidation rate of a sample [19,20]. Oxygen consumption rates were analyzed to identify the self-ignition behavior of the samples in this study. The oxygen consumption rate during the low-temperature oxidation of coal is expressed as follows [21]:
VO2 (T ) =
Q1 CO02 SnL
ln
CO02 CO12
(1) 3
−1
where Q1 is the air supply volume (m s ); S is the base area of the coal sample tank (m2); n is the porosity of the coal sample (dimensionless); L is the height of the coal sample (m); and CO02 and CO12 are the inlet and outlet oxygen concentrations, respectively (vol.%). Fig. 3 demonstrates that the oxygen consumption rate consistently increased with temperature throughout the spontaneous combustion reaction in each experiment. The rate exhibited an exponential growth trend that first increased slowly and then sharply. In the initial stages (at < 110 °C), the oxygen consumption rate was low because the coal temperature was low, and active functional groups participated in the oxidation reaction in minute amounts. When the coal temperature increased above 110 °C, the chemical adsorption and chemical reactions dominated the low-temperature oxidation process. During this period, a large amount of oxygen was consumed in a short period. The reaction intensity increased, which caused a rapid increase in the oxygen consumption rate. Because of their relatively high maturity, the GE, PE, ZJ, and DJ samples had considerably lower oxygen consumption rates than the other three samples. Fig. 4 demonstrates that the average oxygen consumption rates of the seven coal samples increased with increase in the volatile matter and oxygen content. Furthermore, there are two regions—low- and high-rank coal areas. A high average oxygen consumption rate represents a high reaction rate between coal and oxygen, namely, a high self-heating rate of coal. Thus, as the volatile matter and oxygen contents increase (i.e., the coal rank decreases), the self-ignition rate increases.
2. Experiment and methods 2.1. Coal samples The experimental samples were acquired from the Huainan mining area in Anhui Province, China (Fig. 1). The high-quality coking coal produced in this area is widely applied in the metallurgical industry. Although its coal rank varies, coal is generally prone to self-ignition. The seven coal samples used in this study are as follows: Gu Yi (GY), Gu Er (GE), Gu San (GS), Pan Yi (PY), Pan Er (PE), Zhu Ji (ZJ), and Ding Ji (DJ). These samples were crushed to a particle size of 0–3 mm. Each sample was placed in a vacuum drying oven at 80 °C for 24 h before the testing to eliminate the moisture content. The proximate and ultimate analysis results are presented in Table 1. The samples generally had low ash and sulfur contents with a high amount of volatile organic matter, which is one-third coking coal. According to the range of the high heat values (HHVs), GY, GS, and PY are high heating coal samples and GE, PE, ZJ, and DJ are mid-to-high heating coal samples.
3.1.2. CO production During self-ignition, coal continuously consumes oxygen and generates various gaseous products, such as CO, CO2, C2H4, and C2H6 [22]. CO is a key index that can be used to demarcate the stages of spontaneous combustion; the presence of CO characterizes the beginning of spontaneous combustion [23]. Fig. 5 shows that the CO production of each sample increased with temperature. When the temperature was < 110 °C, low-intensity physical adsorption was the major reaction. Therefore, a low amount of CO was generated by oxidation. The oxidation intensity increased rapidly when the temperature exceeded 110 °C. The increase in the oxidation intensity resulted in the activation of a large number of functional groups that reacted with the oxygen in the system. Because CO is a product of coal oxidation, the amount of CO increased sharply at temperatures > 110 °C. The CO production exhibited a trend similar to the oxygen consumption rate (i.e., an exponential trend). Moreover, CO
2.2. Low-temperature oxidation experiment of the coal samples In this study, an oil-bath temperature-programmed system was used to test the self-ignition characteristics of the samples during low-temperature oxidation. This method was selected because of its thermal stability. The experimental system comprised an air tank, a temperature-control oven, a gas chromatograph, and a data collection system (Fig. 2). The oven provided a temperature range of 30–170 °C, which 2
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Fig. 1. (a) Site of the Huainan mining area in China and (b) the sampling location [16].
trend. The CO/CO2 ratio was < 0.1 when the temperature was below 120 °C and was > 0.1 when the temperature exceeded 120 °C. Hence, the transition and rapid oxidation stages during spontaneous combustion could be distinguished using the CO/CO2 ratio.
was generated throughout the experiment and its concentration was affected by air turbulence and the surrounding rock adsorption. Thus, inaccurate results can be obtained if the CO concentration is only used for predicting the spontaneous combustion phases. Other index parameters are essential for predicting the spontaneous combustion point of coal samples.
3.2.2. C2H4/C2H6 ratio Fig. 7 displays the plots of the C2H4/C2H6 ratio versus temperature for the seven samples. During the low-temperature oxidation of each coal sample, an unstable stage was observed between 100 and 130 °C. However, the C2H4/C2H6 ratio increased steadily when the temperature exceeded 130 °C. When the temperature was < 150 °C, the C2H4/C2H6 ratio was < 1. Then, the C2H4/C2H6 ratio increased slowly and eventually exceeded 1, which indicates that the C2H4 production was higher than the C2H6 production during this phase. The C2H4/C2H6 ratio increased continuously with temperature; however, the curves converged gradually.
3.2. Gas ratios In practice, a is considered to be a major indicator for predicting spontaneous combustion of coal. Gas ratios are affected by gaseous products and air turbulence to a limited extent [24]. Thus, we analyzed the CO/CO2, C2H4/C2H6, and ΔCCO /ΔCO2 ratios during the low-temperature oxidation. 3.2.1. CO/CO2 ratio Fig. 6 indicates that for each coal sample, the CO/CO2 ratio increased as the temperature increased. The increase was slow in the range of 30–80 °C; however, the increase was swift when the temperature exceeded 80 °C. At a given temperature, the GS sample had the highest CO/CO2 ratio, followed by the GY, PY, DJ, ZJ, PE, and GE samples. The CO/CO2 ratios of the seven samples followed exponential growth patterns, which can be expressed using Eq. (2).
y = a exp(T b) + c
3.2.3. ΔCCO /ΔCO2 ratio Fig. 8 displays the plots of the ΔCCO /ΔCO2 ratio versus temperature for the GY sample. The ΔCCO /ΔCO2 ratios for all the seven coal samples are presented in Table 2. These ΔCCO /ΔCO2 ratios followed a fluctuating yet ascending trend with increase in temperature. The other six samples also exhibited a similar tendency. The low-temperature oxidation process was divided into three phases by using two characteristic temperature points: slow oxidation (stage 1), accelerated oxidation (stage 2), and vigorous oxidation (stage 3). The first characteristic
(2)
The fitted results indicate that Eq. (2) suitably matches the variation Table 1 Proximate and ultimate analysis results of the seven coal samples. Sample
GY GE GS PY PE ZJ DJ
Proximate analysis (mass%)
Ultimate analysis (mass%)
Mad
Aad
Vad
FCad
N
C
S
H
O
1.16 1.35 1.47 0.93 1.56 0.97 1.26
11.73 16.16 13.10 11.48 15.75 16.50 20.05
34.22 25.22 32.24 32.03 26.39 26.20 25.04
52.89 57.27 53.19 55.56 56.30 56.33 53.65
1.89 1.82 1.68 1.78 1.59 1.44 1.58
74.38 73.27 73.53 75.51 72.82 73.75 69.82
0.44 0.53 0.63 0.82 1.49 0.46 0.86
5.08 4.93 4.63 4.61 4.45 4.71 4.65
5.32 1.94 4.96 4.87 2.34 2.17 1.78
HHV (MJ/kg)
Coal rank
24.94 23.05 24.43 24.54 23.49 23.67 22.71
HQ MHQ HQ HQ MHQ MHQ MHQ
Notes: The content of O was defined by the subtraction method and compared with the content of C, H, N, S, Aad, and Mad; FCad was determined using the subtraction method and compared with the content of Aad, Mad, and Vad. HHV represents the high heat value of coal; and MHQ and HQ represent mid-to-high heating of coal and high heating of coal, respectively. 3
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Fig. 2. Schematic of the oil-bath temperature-programmed system.
Fig. 3. Curves of the oxygen consumption rate versus temperature for the seven coal samples.
Fig. 5. Curves of CO production versus temperature for the seven coal samples.
Fig. 6. Plots of the CO/CO2 ratio versus temperature for the seven coal samples.
Fig. 4. Plots of the average oxygen consumption rate versus the volatile matter and oxygen content for the seven coal samples.
4
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Fig. 7. Plots of the C2H4/C2H6 ratios versus temperature for the seven coal samples.
Fig. 9. Plot of the oxidation index versus temperature for the GY sample.
Subsequently, the oxidation intensity increased rapidly with temperature. Thus, according to the aforementioned characteristics of the coal samples during low-temperature oxidation, the ΔCCO /ΔCO2 ratio can be applied as a secondary index for predicting spontaneous combustion of coal. 3.3. Oxidation index According to the observed changes in the oxygen concentration, the oxidation index (DO, min−1), a new index, was proposed for predicting spontaneous combustion of coal. This index represents the ratio between the oxygen consumption per unit time and the total oxygen consumption and indicates the oxidation reaction rate. DO can be calculated using Eq. (3):
DO =
β dCO dT ⎞ dCO dt = 1000 ⎜⎛ ⎟ ΔCO, max ⎝ ΔCO, max ⎠
(3)
where CO is the oxygen concentration (vol%), t is the reaction time (min), T is the temperature (°C), β is the heating rate (°C min−1), 1000 is the amplification factor, and ΔCO, max is the maximum oxygen consumption (vol%). The oxygen concentration was stable at approximately 3 vol% for each experiment with a constant increase in the temperature. Thus, the ΔCO, max value of this study was 18 vol%. Fig. 9 displays the plots of the oxidation index versus temperature for the GY sample. The oxidation
Fig. 8. Plot of the ΔCCO /ΔCO2 ratio versus temperature for the GY sample.
temperature point was at 90 °C. At this point, the coal temperature exceeded the critical temperature, and the oxidation rate began to accelerate. The second characteristic temperature point occurred at 140 °C when the sample entered the vigorous oxidation stage.
Table 2 ΔCCO /ΔCO2 ratios for the seven samples at different temperatures. Temperature (°C)
30 40 50 60 70 80 90 100 110 120 130 140 150 160
ΔCCO ΔCO2 ratio GY
GE
GS
PY
PE
ZJ
DJ
17.72 43.94 68.17 150.10 210.00 196.97 426.41 359.48 402.76 493.22 499.55 699.17 673.19 750.26
46.26 51.16 71.13 82.73 138.75 109.31 325.40 269.41 322.82 336.00 349.16 540.85 514.17 576.99
23.85 84.75 123.75 196.55 353.55 399.28 625.96 556.41 602.05 617.74 635.13 836.15 804.20 839.22
52.75 53.40 66.40 153.14 228.56 189.82 439.25 419.16 443.19 484.43 460.32 904.35 933.96 924.59
19.31 35.71 51.51 94.25 150.50 131.47 365.00 338.63 426.87 460.00 525.89 734.34 678.14 670.44
15.96 56.28 74.10 147.29 243.41 265.55 400.33 388.38 462.50 506.11 591.17 652.30 619.35 666.13
20.40 62.68 81.00 98.91 123.56 165.33 158.89 407.36 350.84 466.83 412.63 863.28 719.36 866.14
5
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Table 3 Oxidation indices for the seven samples at different temperatures. Temperature (°C)
30 40 50 60 70 80 90 100 110 120 130 140 150 160
T VCO =
VO2 (T ) CCO CO2 [1 − exp(−VO2 (T ) V Q1 CO2 )]
Oxidation indexes T VCO = 2
GY
GE
GS
PY
PE
ZJ
DJ
0.47 0.47 0.59 0.82 0.47 1.13 1.22 1.52 5.12 5.0 6.17 5.80 4.28 4.88
0.29 0.31 0.64 0.98 0.42 0.58 1.89 2.65 5.57 5.35 6.88 5.02 5.31 6.04
0.37 0.31 0.42 0.67 0.47 0.73 1.63 1.86 5.78 6.51 6.80 6.15 7.46 6.57
0.20 0.23 0.33 0.78 0.49 1.08 1.76 1.57 5.59 6.75 6.14 8.60 8.31 7.78
0.31 0.31 0.50 0.65 0.77 1.45 1.37 1.58 2.26 3.60 3.83 3.60 3.54 5.33
0.31 0.37 0.26 0.45 0.71 1.47 1.61 1.81 2.83 5.07 4.46 5.71 7.32 4.93
0.24 0.34 0.45 0.63 0.66 1.73 1.94 1.50 2.18 5.14 4.58 4.49 5.83 5.01
h min =
(7)
ρg Cg Q (T − Ty ) q (T )
+
(ρg Cg Q)2 (T − Ty )2 + 8λ e q (T )(T − Ty ) q (T ) (8)
Cmin =
CO02 ⎡ 8 × λ e (T − Ty ) 2 × (T − Ty ) ⎤ + ρg Cg Q 2 ⎥ h q (T ) ⎢ h ⎦ ⎣
Qmax =
h × q (T ) 4λ e − 2 × ρg Cg (T − Ty ) hρg Cg
(9)
(10) −3
where ρg denotes the air density (kg m ), Cg is the specific heat of air (J kg−1 °C−1), T is the coal temperature (°C), Ty is the stratum temperature (°C), and λe is the thermal conductivity of coal (J m−1 s−1 °C−1). 3.4.1. Minimum float coal thickness We analyzed the hmin curves of the seven samples at an air leakage intensity range of 0.001–0.005 m s−1. Fig. 10(a)–(c) demonstrate the plots of hmin versus temperature for the GY, GE, and GS samples, respectively, at various air leakage intensities. For the three samples, hmin increased with temperature but quickly decreased in the range of 50–120 °C. The hmin value remained constant beyond this range. The samples had low oxidation rates in the initial stage (i.e., < 50 °C), which resulted in a limited heat release. A large coal thickness generally indicates a superior thermal storage capacity [31]. Thus, hmin increased consistently when the temperature was < 50 °C. Subsequently, the oxidation reaction intensity increased with temperature, which caused the release of a large amount of heat. Thus, even a very small coal thickness can supply sufficient fuel for the initiation of the spontaneous combustion process. Consequently, the value of hmin further decreased. At a given temperature, hmin increased with increase in the air leakage. The difference between the two adjacent curves first exhibited an increasing trend, followed by a decreasing trend when the maximum value reached approximately 50 °C. Increase in the air leakage may boost heat dissipation, which reduces the heat for self-ignition. Therefore, increase in the coal thickness may increase the heat storage and ensure persistence of spontaneous combustion. The air leakage only had a marginal effect on hmin when the temperature exceeded 140 °C. Fig. 10(d) shows that the hmin curves of the seven samples exhibited similar trends at the same air leakage intensity, which indicates that the samples have similar spontaneous combustion risks.
3.4. Limiting parameters for low-temperature oxidation of coal Multiple factors affect the spontaneous combustion of coal, such as oxygen concentration, particle size, and ventilation. The limiting values of these essential factors are described as limiting parameters [25,26]. In a mine goaf, the prerequisite conditions for spontaneous combustion are as follows [27]: (4)
where h and hmin are the float coal thickness and minimum float coal thickness (m), respectively; C and Cmin are the corresponding oxygen concentration and oxygen concentration limit (vol%), respectively; and Q and Qmax are the air leakage intensity and maximum air leakage intensity (m s−1), respectively. In this study, the exothermic intensity was calculated as follows [16]: T T T 0 0 T q (T ) = qa [VOT2 − VCO − VCO ] + VCO [(h298 )CO + ΔhCO ] + VCO 2 2 0 0 )CO2 + ΔhCO [ (h298 ] 2
VO2 (T ) CCO2 CO2 [1 − exp(−VO2 (T ) V Q1 CO2 )]
where CO2 is the oxygen concentration (vol%); CCO and CCO2 are the amounts of CO and CO2 production (vol%), respectively; and V is the volume of the coal sample (m3). Three limiting parameters, hmin, Cmin, and Qmax, were calculated using Eqs. (8)–(10), respectively [29,30]. An analysis indicated that these parameters are useful for determining the risk degree of spontaneous combustion in coal mining operations in the Huainan mining area.
index values for the seven coal samples are presented in Table 3. The oxidation index of the seven samples increased continuously with temperature. When the temperature was below approximately 80 °C, the DO value was < 1, indicating a low oxidation rate. Subsequently, the DO value increased gradually and exceeded 2. Above approximately 100 °C, the samples entered the accelerated oxidation phase. Thus, the oxidation index is an excellent parameter for distinguishing between the different stages of low-temperature oxidation. The oxidation index increased rapidly between 100 °C and 110 °C, which indicates the strong intensity of the oxidation reaction in that range. The oxidation index followed an undulating trend when the temperature was > 110 °C. The decrease in the oxygen concentration was considered to be the major cause of the aforementioned phenomenon.
(h > h min ) ∩ (C > Cmin ) ∩ (Q > Qmax )
(6)
(5)
where q(T) is the exothermic intensity (J s−1 m−3); qa is the chemical adsorption heat of coal to oxygen (J mol−1); VOT2 is the oxygen con0 0 )CO and (h298 sumption rate at temperature T (mol s−1 m−3); (h 298 )CO2 are the standard heats of formation of CO and CO2, respectively, at 0 298 K and 1 atm (J mol−1); ΔhCO is the difference between the heat of CO formation at a temperature of T and pressure of 1 atm and the 0 standard heat of CO formation (J mol−1); ΔhCO is the difference be2 tween the heat of CO2 formation at a temperature of T and pressure of T 1 atm and the standard heat of formation of CO2 (J mol−1); and VCO as T are the CO and CO production rates at temperature T well as VCO 2 2 T T and VCO can be cal(mol s−1 m−3), respectively. The parameters VCO 2 culated using Eqs. (6) and (7) [28]:
3.4.2. Limiting oxygen concentration Coal continuously consumed the oxygen present in the system during the spontaneous combustion reaction, so the oxygen concentration decreased gradually. The coal temperature remained constant when the oxygen concentration decreased to a certain value; that is, heat release was equal to heat dissipation. The oxygen concentration at which the coal temperature remains constant is known as the limiting oxygen concentration [32]. We examined the Cmin curves of the seven samples at a coal thickness range of 0.4–0.8 m and an air leakage intensity of 0.0005 m s−1. 6
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Fig. 10. Curves of hmin at various air leakage intensities versus temperature for the (a) GY, (b) GE, and (c) GS samples, and (d) plots of hmin for the seven samples at Q = 0.003 m s−1.
exhibited a similar trend with an increase in temperature.
Fig. 11(a)–(c) shows the plots of Cmin versus temperature for the GY, GE, and GS samples, respectively, at various coal thicknesses under an air leakage intensity of 0.0005 m s−1. For each experiment, Cmin first increased and then decreased with increase in temperature and reached a maximum value at approximately 50 °C. The Cmin values of the GY, GE, and GS samples were < 10% when the temperature exceeded 100 °C, which indicated that the spontaneous combustion risk was high at this stage. When the temperature was constant, Cmin gradually decreased with an increase in coal thickness. This phenomenon occurred because the heat required for spontaneous combustion was constant under certain conditions and an increase in the thickness increased the heat production. Thus, a decrease in oxygen concentration was essential for maintaining a heat balance during oxidation. For the GY, GE, and GS samples, the Cmin values exceeded 21 vol% (i.e., oxygen concentration in the air) when the temperature was in the range of 40–60 °C at a coal thickness of 0.4 m, which indicated that spontaneous combustion did not occur at this stage. Fig. 11(d) illustrates the Cmin curves of the seven samples under the same conditions. The Cmin curves of all the samples
3.4.3. Maximum air leakage intensity When the air leakage intensity increased to a certain value, the heat generated by coal oxidation was completely dissipated by heat conduction and air flow, and the value was defined as the maximum air leakage intensity [33]. In this study, we analyzed the Qmax curves of the seven samples for a coal thickness range of 0.4–0.8 m. Fig. 12(a)–(c) are the plots of Qmax versus temperature for the GY, GE, and GS samples, respectively, at various coal thicknesses. For each experiment, Qmax first decreased and then increased gradually with an increase in temperature. The minimum Qmax value was observed at 50 °C. In the initial stage (i.e., < 50 °C), a small amount of heat was generated by coal oxidation. However, a large air leakage can dissipate heat and reduce the coal temperature. Therefore, a reduction in the air leakage intensity maintains the development of the spontaneous combustion process. The oxygen requirement increased gradually with temperature. Because the oxygen concentration was constant, low air 7
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Fig. 11. Curves of Cmin at various coal thicknesses under an air leakage intensity of 0.005 m s−1 versus temperature for the (a) GY, (b) GE, and (c) GS samples, and (d) plots of Cmin versus temperature for the seven samples (h = 0.5 m, Q = 0.005 m s−1).
4. Conclusions
leakage caused a decrease in the reaction rate. Therefore, an increase in the air leakage ensures the continuation of the combustion process. An increase in air leakage may cause increase in heat loss. Thus, further studies should be conducted on the limiting value of the air leakage for balancing the oxygen supply and heat dissipation. At a given temperature, Qmax increased with coal thickness. The difference between the two adjacent curves of Qmax and coal thickness increased gradually, which indicated that the effect of coal thickness on Qmax increased with temperature. An increase in the coal thickness can increase the heat storage capacity of coal seams, which results in a strong oxidation process and increased oxygen consumption. An increase in the air leakage may provide sufficient oxygen for the occurrence of spontaneous combustion. As displayed in Fig. 12(d), the Qmax curves of all the seven samples exhibited an increasing trend with increase in temperature. The Qmax values of the samples were approximately equal when the temperature was < 90 °C. Above 90 °C, the difference between the Qmax values of the samples increased consistently, which indicates the effect of the coal rank on Qmax as temperature increases.
This study was conducted to generate the prediction indices for the spontaneous combustion process of seven coal samples obtained from the Huainan mining area in Anhui Province, China. The gas indices and ratios during low-temperature oxidation were investigated. The oxygen consumption rate and CO production of the seven samples increased exponentially as the temperature increased. The CO/CO2, C2H4/C2H6, and ΔCCO /ΔCO2 ratios displayed an obvious stage variation characteristic with temperature, which was beneficial to determine the developmental phase of low-temperature oxidation of coal. The oxidation index DO was proposed to further determine the oxidation period during spontaneous combustion of coal. In summary, CO gas can be considered the main indicator for forecasting spontaneous combustion of coal for the Huainan mining area. Gas ratios, such as the CO/CO2, C2H4/C2H6, and ΔCCO /ΔCO2 ratios, and the oxidation index can be applied as auxiliary indicators of coal self-ignition. On the basis of the discussion of the variation law of the three limiting parameters (minimum float coal thickness, maximum air leakage intensity, and 8
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Fig. 12. Curves of Qmax at various coal thicknesses versus the temperature for the (a) GY, (b) GE, and (c) GS samples, and (d) plots of Qmax versus temperature for the seven samples at h = 0.5 m.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by National Key R&D Program of China (No. 2018YFC0808104), National Natural Science Foundation of China (Nos. 5157-4193, 5167-4191, 5180-4247). 9
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