Occurrence Mechanism and Risk Assessment of Dynamic of Coal and

Feb 23, 2017 - (5, 6) Therefore, a large number of fissures are generated in CBM ..... mine field by Eichinger, an expert in the former Soviet Union i...
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Occurrence Mechanism and Risk Assessment of Dynamic of Coal and Rock Disasters in the Low-Temperature Oxidation Process of a CoalBed Methane Reservoir Zongqing Tang,†,‡,§ Shengqiang Yang,*,†,‡,§ and Guangyu Wu†,‡,§ †

Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China ‡ State Key Laboratory of Coal Resources and Safety Mining, Xuzhou Jiangsu 221116, China § School of Safety Engineering, China University of Mining and Technology, Xuzhou Jiangsu 221116, China ABSTRACT: The aim of this research is to explore the occurrence mechanism of dynamic disasters of coal and rocks in lowtemperature oxidation of coal-bed methane (CBM) reservoirs. For this purpose, the occurrence probability of dynamic disasters at different oxidizing temperatures was assessed by using the comprehensive predictive index K for dynamic disasters involving coal and rocks. By employing nuclear magnetic resonance (NMR) technology, the evolution law of diameter and quantity of pores inside coal was detected during low-temperature oxidation of coal masses. In addition, fracture development of coal masses was monitored and analyzed by applying various instruments, including an instrument for measuring ΔP, hardness tester, gas chromatograph, and measuring system for rocks using the P-wave. The results showed that both the diameter and quantity of pores inside coal increased with the rise of the oxidizing temperature of the coal masses: the porosity increased by 72.2% as the temperature rose by 200 °C. The gas chromatograph and industrial analytical experiment proved that the whole fracture development process of coal masses was divided into two stages during low-temperature oxidation. In the initial stage of lowtemperature oxidation (30−130 °C), water inside coal masses was lost and evaporated, resulting in the expansion and connection of micropores to mesopores. In the later period of low-temperature oxidation (130−230 °C), mesopores expanded and connected to macropores and microfractures because of oxygenolysis of the macromolecules and volatiles in coal. On the basis of the comprehensive predictive index K for dynamic coal disasters, the occurrence probability of dynamic disasters in lowtemperature oxidation of CBM reservoirs was verified to increase with the increasing oxidizing temperature according to the measured data. In addition, the maximum allowable oxidizing temperature of the CBM reservoir where the specimen was collected was 130 °C. in CBM reservoirs,7 and the generation of fissures increases airleaking passages and leakage rates and therefore provides conditions for low-temperature oxidation of CBM reservoirs that are prone to spontaneous combustion.8 Both stress and pressure on CBM reservoirs increase as the extraction of CBM moves into deep strata, causing more frequent dynamic disasters, especially in spontaneous combustion-prone CBM reservoirs.9−11 Particularly around gas drainage boreholes of CBMs in spontaneous combustion-prone CBM reservoirs, uncertain CBM emissions and the occurrence frequency of coal dynamic disasters are greater than those in other regions. Therefore, studying the disaster-causing mechanism of coupling between low-temperature oxidation and dynamic coal disasters during CBM extraction in deep spontaneous combustion-prone CBM reservoirs is of great significance. It is critical to explore the function mechanism of CBM reservoir low-temperature oxidization during the generation of dynamic disasters because improving CBM extraction and protecting the safety of mineral construction personnel are of great importance.

1. INTRODUCTION The proved reserves of coal-bed methane (CBM) in China are 37 trillion m3 and are ranked third in the world.1,2 Although CBM is a type of governable and clean energy resource and chemical material,3 CBM is also an important predisposing factor for coal mine disasters. Statistical data show that 70% of coal mining accidents consist of gas disasters. CH4, as the main constituent of CBM, is a strong greenhouse gas with a greenhouse effect that is more than 20 times higher than the greenhouse effect of CO2. Therefore, from the perspective of energy utilization, coal mine safety and environmental protection,4 it is necessary to exploit coal seams. However, influenced by geological structures and buried conditions, most CBM reservoirs in China exhibit low permeability. Compact strata result in a low drainage efficiency for CBM. Improving gas permeability through coal seam fracturing is the most effective method to improve the drainage efficiency and drainage quantity of CBM. Application of conventional fracturing measures, including hydraulic cutting, hydraulic fracturing, freeze−thaw damage, and gas injection, can improve the permeability of reservoirs. However, CBM extraction in deep CBM reservoirs, especially in reservoirs with a tendency for spontaneous combustion, damages the structural integrity of reservoirs.5,6 Therefore, a large number of fissures are generated © XXXX American Chemical Society

Received: November 22, 2016 Revised: February 22, 2017 Published: February 23, 2017 A

DOI: 10.1021/acs.energyfuels.6b03106 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

oxidization of coal and by measuring f and Δp of coal at different oxidization stages. This study lays a theoretical foundation and experimental basis for building the codefense system for the spontaneous combustion of coal and dynamic disasters involving coal and rock during CBM extraction from deep spontaneous combustion-prone CBM reservoirs.

Domestic and foreign scholars have carried out numerous studies on low-temperature oxidation of CBM reservoirs and dynamic disasters involving coal and rocks.12,13 For example, Kam14 first put forward the double balance theory of the spontaneous combustion process of coal, which is divided into two parallel reaction sequences: direct reactions between coal and oxygen and a series of reactions due to oxygen adsorption on coal. Qi15 proposed three-sequence reactions of coal by studying the infrared spectrum of the changed functional groups in the process of coal oxidation. He proposed that selfreaction of active groups also exists in the spontaneous combustion of coal apart from the widely acknowledged direct reaction and coal-oxygen composites and decomposition reactions. In addition, Krishnaswamy et al.16 built a kinetic model of the low-temperature oxidation process of coal. The model displays the reaction rates of coal with different particle sizes, surface areas, and humidity. Meanwhile, the kinetic model also considers the influence of water in coal on the lowtemperature oxidation rate of coal. In studying the occurrence mechanism of dynamic disasters of coal, Australian scholar Paterson17 explained the occurrence of cracks in the walls of CBM reservoirs from the perspective of seepage but failed to consider the dynamic processes involved in the deformation and the outburst of coal masses. Litwiniszyn18 from Poland studied the jump condition of the wavefront during coal and gas outbursts by using sparse shock-wave theory. Jiang Chenglin19 and Yu Qixiang put forward the hypothesis of spherical shell destabilization, and they thought that the nature of the outburst process was that coal releases gas after being damaged by crustal stress, and then, the gas expands in fissures and finally destabilizes coal shells. Nuclear magnetic resonance (NMR) is a technology that can be applied to obtain information on the water molecule distribution in coal and rocks to determine the distribution rule for pores with different bore diameters.20 The technology is based on the atomic-scale magnetism property of quanta and the NMR signal that is observed due to the hydrogen atoms in water molecules. Through experiments and modeling, Yao Yanbin et al.21 verified the feasibility of applying the distribution of transverse relaxation time T2 obtained from NMR to manifest the distribution characteristics of pores with different diameters in coal masses. By studying variable quantities of pores with different sizes in coal masses subjected to ultrasonic action by employing NMR, Tang Zongqing et al.7 obtained the evolution law for pore size in coal masses after microwave fracturing. Detailed studies have been carried out regarding the lowtemperature oxidization of CBM reservoirs as well as the occurrence mechanism and evolutionary process of coal and rock dynamic disasters. However, the disaster-causing mechanism of the coupling between low-temperature oxidation and coal dynamic disasters has not been sufficiently investigated. The influence of low-temperature oxidation on the structural strength and development of internal fissures as well as coalrock dynamic disasters during CBM extraction from deep spontaneous combustion-prone CBM reservoirs has not been clarified. To solve these shortcomings, based on NMR, the laws of pore evolution and fissure development in coal masses during low-temperature oxidation of CBM reservoirs were developed by employing a self-developed simulation system for low-temperature oxidation of coal and rocks. In addition, the influence mechanism of low-temperature oxidization in CBM reservoirs on coal and rock dynamic disasters was analyzed through gas chromatography of tail gas during low-temperature

2. EXPERIMENTAL THEORIES AND METHODS 2.1. Mechanism of Pore Measurement Using NMR. The permeability of coal depends on the size, quantity, and distribution of its internal pores and fissures. Currently, the common methods for testing the distribution of pores in coal include scanning electron microscopy (SEM), computed tomography (CT), mercury intrusion, and N2/CO2 adsorption (Figure 1). However, these methods have

Figure 1. Measuring ranges (nm) for pores in coal petrophysical characterization methods. disadvantages, including long detecting times, destruction of coal samples, and limited ranges for measuring pore diameters. As an emerging technology that has various advantages, such as a short detecting time, low-destruction of coal samples, and extensive scope for measuring pore diameters, NMR is widely used in the fields of medicine as well as to measure the distribution and pore diameters of rocks. The dipole moment can be expressed as a spectrum of the transverse relaxation time (T2), which can be given as 1 S =ρ× T2 V where S is the pore surface area (m2), V is the pore volume (m3), and ρ is the transverse surface relaxivity (ms−1). NMR is a physical process in which Zeeman splitting occurs at the spin level of the nucleus with a nonzero magnetic moment under the influence of an external magnetic field to resonantly absorb radiofrequency radiation at a certain frequency. NMR uses a magnetic field to create a dipole moment, the amplitude of which is proportional to the number of hydrogen atoms within the fluid and thus is a measure of the pore volume. For coals, T2 < 10 ms corresponds to micropores; T2 from 10−100 ms corresponds to mesopores, and T2 > 100 ms corresponds to macropores and microfractures. The area between the T2 curve and abscissa (x axis) can be used to represent the content of hydrogen atoms at the same relaxation time and in pores with the same sizes. 2.2. Experimental Samples. Bituminous coal collected from Sanhejian Coal Mine, Xuzhou City of Jiangsu Province in China, was used for the test coal samples. Specific industrial parameters are shown in Tables 1 and 2. Three samples were prepared by using a rock coredrilled machine and a square sample with side lengths of 20 mm for NMR, a cylinder sample with a diameter of 50 mm and a length of 50 mm for the P-wave test, and an approximately square sample with side B

DOI: 10.1021/acs.energyfuels.6b03106 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Industrial Analysis of Coal Samplesa testing sites

Mad (%)

Aad (%)

Vad (%)

Vdaf (%)

FCad (%)

5303 working face

2.07

12.09

34.61

36.14

51.23

nitrogen at a normal temperature before the experiment until the weight difference between the dried samples and samples before water retention was less than 0.3%. 2.4. Experimental Equipment. 2.4.1. Simulation System for Low-Temperature Autoxidation of Coal Masses. The simulation system for low-temperature autoxidation of coal masses is displayed in Figure 2b. The system was mainly composed of a coal spontaneous combustion analyzer, gas cylinder for voltage-stabilization, and sample carrier. A ZRJ-2000 tester for coal spontaneous combustion produced by Starr Hengtong Technology Co., Ltd., Beijing, China, was selected as the coal spontaneous combustion analyzer, which can increase the temperature within a range of 20−500 °C (minimum temperature rise rate of 0.1 °C/min). In addition, the equipment had a maximum output power of 2000 W and a digital LCD that showed the real-time temperature, temperature rise time, and temperature rise rate and was able to work steadily for a long time. 2.4.2. Analytical Measurement System for Gas Components. With gas as the mobile phase, the analytical measurement of the gas components was carried out by employing a GC-4100 gas chromatograph. In addition, various components of mixed gases were qualitatively and quantitatively analyzed by using different running speeds of the components in chromatographic columns by using elution chromatography. 2.4.3. Measurement of the P-Wave Velocity. Measurement of the P-wave velocity was conducted by using a HS-YS4 ultrasonic analyzer (see Figure 2c). The propagation velocity of sound waves in the samples was inversely deduced by measuring the time required for the sound waves to pass through samples, and the development disparities of pores in the samples were compared by analyzing the wave velocities. In general, the slower the wave velocity, the greater the number of pores in coal. The HS-YS4 ultrasonic analyzer consists of a pulse generator, signal reception channels, and a computer. 2.4.4. NMR Tests. NMR tests were conducted using a Meso MR23060H-I NMR spectrometer with the following parameters: the echo time, waiting time, number of echoes, and the number of samples were 0.3 ms, 9 s, 5000, and 64, respectively.

Note: FCad, Mad, Aad and Vad represent mass fractions of fixed carbon, moisture, ash content, and volatiles in air-dried samples (%), respectively. a

Table 2. Processing Criterion and Test Items of Coal Samples serial number no. no. no. no. no.

1 2 3 4 5

particle size (mm)

length (mm)

20 50 20−30 0−0.18 0.18−0.25

20 50 20−30

shape

mass (g)

test items

square cylinder block powder powder

52.9 129.5 50 10 10

NMR P-wave f IA Δp

lengths of 20−30 mm for the f test. Coal powders with particle sizes of 0−0.18 mm (the analytical test of the industry) and 0.18−0.25 mm (Δp test) were separately sifted out by employing a grinding machine and vibrating screen (Δp test). All of the data were obtained from these coal samples. By analyzing the data obtained, the evolution law for pore diameters in coal during low-temperature oxidation of spontaneous combustion-prone CBM reservoirs with time was determined, and the development law for fissures and risk assessment for dynamic disasters involving coal and rock were discussed. 2.3. Test Process. A cyclic temperature programming experiment was designed, as illustrated in Figure 2a. In a temperature-programmed cycle, coal samples were heated to undergo oxidation for 125 min in dry air at a rising temperature rate of 0.4 °C/min and a gas flow rate of 50 mL/min. Changes in the coal samples within one cycle of 125 min were tested by applying P-wave detection, a tester for the initial speed of the methane emission, a coal-rock hardness tester, and NMR. In addition, the coal samples were centrifuged for 10 min at a revolving speed of 6000 r/min and then dried for 8 h in a vacuum drying oven at 60 °C to eliminate water from all permeable pores. The first NMR test was carried out after the corresponding coal samples had reached an irreducible water condition (Sir). After the first NMR test, the coal samples were subjected to vacuum for 8 h at a constant pressure of 0.01 MPa and then placed in distilled water for 8 h for water retention to achieve 100% water-saturation (Sw) before conducting the other NMR tests. The coal samples subjected to the NMR test were dried in

3. EXPERIMENTAL RESULTS AND ANALYSIS 3.1. Distribution Changes in the T2 Curve. As a porous medium, coal has numerous microscopic pore properties, including porosity as well as distributed pore diameters and pore volumes.22−24 By using the pore diameters, pores in coal masses can be classified as micropores, mesopores, and macropores. Micropores, as important sites for adsorbing

Figure 2. Experimental system and test process. C

DOI: 10.1021/acs.energyfuels.6b03106 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels gases, have a sizable specific surface area that endows them with a strong gas adsorption capacity. Mesopores and macropores are primary channels of gas diffusion and permeation and are among the important parameters that influence the structural strength of coal masses. The pore diameters of pores in coal are directly proportional to the relaxation time, while the functional relationship between the pore diameters and relaxation time is not clarified. Generally, relaxation times of 0−10, 10−100, and 100−10000 ms correspond to micropores, mesopores, and macropores, respectively. Figure 3 displays the T2 distribution curves of one sample subjected to a 200 °C temperature rise across four programmed

Figure 4. Changes in the growth rates of pores with different diameters with the oxidizing temperature.

micropores, mesopores, and macropores and microfractures increased by 30.2%, 34.9%, and 276.5%, respectively. In addition, the growth rates of the quantity of micropores and mesopores in coal first increased and then decreased during the temperature-rise during oxidation. However, the growth rates of macropores and microfractures constantly increased with the increase in the oxidizing temperature. Therefore, the growth rates of macropores and microfractures were far larger than the growth rates of micropores and mesopores with the temperature-rise during oxidation. However, the coal structure was damaged due to the growth of the quantity of macropores and microfractures, so the structural strength of the coal largely declined with the rising temperature. In addition, by observing the differences in the growth rates of pores with different diameters shown in Figure 4, the development process for fissures in coal during the temperature rise can be obtained through inversion. Figure 4 displays the initial stage of temperature-rise oxidation, showing that the number of micropores inside the coal first significantly increased, and then, micropores became mesopores after expansion and connection. This caused a large increase in the quantity of mesopores, which then expanded and connected to form macropores and microfractures with the further increase in oxidizing temperature, resulting in an increase in the quantity of macropores. The above results demonstrate that during temperature-rise oxidation of coal, fissures developed step-bystep, and pores with different diameters developed in sequential order from micropores to mesopores and finally macropores. In the later stage of the temperature rise, there were large quantities of macropores and microfractures inside coal that damaged the coal structure and resulted in broken internal structures. Therefore, the structural strength of coal declined significantly. 3.2. Changes in Porosity and P-Wave Velocity. During the temperature-rise oxidation of coal, the intrinsic pore structure inside the coal was destroyed due to water evaporation, transformation of inorganic matter, and macromolecule decomposition. In the later stage of low-temperature oxidation, accelerated combustion of the coal further intensified the damage, resulting in increased coal porosity. The red curve in Figure 5 shows that the porosity of the coal improved by 72.2% as the temperature of the low-temperature oxidation rose from 30 to 230 °C. In addition, the growth rate of the porosity constantly increased with the increase in oxidizing temperature. In the later stage of the oxidation, the porosity increased exponentially. Figure 4 shows that the internal changes

Figure 3. T2 distribution curves.

temperature gradients (30−80, 80−130, 130−180, and 180− 230 °C). Figure 3 shows that the quantities of pores in coal with different diameters increased with the increase of the oxidizing temperature of coal. Within the first 100 °C temperature rise (30−130 °C) in oxidation of the coal masses, the growth rates of the quantities of both micropores and mesopores were higher than the growth rate of macropores. However, within the later 100 °C temperature rise (130−230 °C) during oxidation, the quantity of macropores greatly increased at a growth rate higher than the growth rate of micropores and mesopores. In addition, the quantity of micropores significantly increased as the temperature rose from 30 to 230 °C, while the quantity of mesopores mainly increased within the range of 80−180 °C. Compared with micropores, the quantity of mesopores increased at the higher temperature, which showed that pores with different diameters in coal gradually developed from micropores to mesopores during low-temperature oxidation. Mesopores developed from the expansion and connection of micropores. The quantity of macropores increased dramatically when the oxidation temperature increased to 130 °C. At the same time, the distribution range of pore diameters and maximum diameters improved and the fissures inside the coal started to extend and connect. Thus, fissure networks were preliminarily built, damaging the structural integrity of the coal. Figure 3 shows the growth rates of pores with different diameters in coal samples during the 200 °C temperature rise, which consisted of four programmed temperature gradients (30−80, 80−130, 130−180, and 180−230 °C). The quantities of pores with different diameters are represented by the areas between the corresponding T2 curves and the x axis. Figure 4 shows that the quantities of pores with various diameters in raw coal all increased with the increasing oxidizing temperature. As the temperature rose from 30 to 230 °C, the quantities of D

DOI: 10.1021/acs.energyfuels.6b03106 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

and temperature rise rates increase when the temperature reaches the critical self-heating value of coal. Finally, coal is burned after it reaches its ignition temperature. This phenomenon and process are called spontaneous combustion of coal, and the period before reaching the ignition temperature is called low-temperature oxidation of coal. As the spontaneous combustion of coal in the natural state requires a long time (in general, 1−3 months) and because heat is difficult to accumulate, simulation of the spontaneous combustion oxidation process of coal masses in the field is commonly achieved by temperature programming. Analytical determination of industrial parameters was performed on coal specimens at various temperature stages during low-temperature oxidation. The results showed that the contents of coal ash and fixed carbon basically remained unchanged at different temperatures in low-temperature oxidation, while the content of moisture (Mad) and volatiles (Vad) was significantly changed, as shown in Figure 6. The

Figure 5. Changes of porosity and P-wave velocity with the oxidizing temperature.

resulting from the increase in coal porosity occurred at different stages of the temperature rise. In the initial stage (30−130 °C), the growth of the quantity and diameters of micropores and mesopores were the primary factors leading to the increase in the coal porosity. However, in the later stage of the temperature rise (130−230 °C), the growth of the quantity and dimensions of macropores and microfractures became the main factors of the increase of the coal porosity with the rise in temperature. As a new technique developed in recent decades, acoustic wave measurement of rocks employs acoustic waves as information carriers. The technique is designed to study physical and mechanical parameters as well as structural properties of rock masses. To accomplish this measurement, the acoustic wave measurement needs to analyze the propagation characteristics of acoustic waves in rock masses as well as changes in the acoustic parameters (wave velocity, attenuation coefficient, amplitude, frequency spectrum, etc.). On the basis of the P-wave rock test technique, the developmental conditions of the pores inside the specimens were revealed by measuring the wave velocity of the P-wave passing through the specimens. In general, the slower the wave velocity, the better the development of fractures inside the coal. The blue curve in Figure 5 displays the changing trends of the P-wave velocity in coal with the rise of the oxidizing temperature at different temperature rising stages during oxidation. The curve shows that the P-wave velocity decreased by 52.7% over the entire temperature rise process in coal during low-temperature oxidation (30−230 °C). The decreasing rate increased first and then declined with the rise of the oxidizing temperature. The P-wave velocity test showed that fractures inside the coal constantly developed with the temperature rise during low-temperature oxidation. In the midterm of the temperature rise, the velocity rate of the P-wave decreased rapidly, indicating that the pore structure inside the coal changed dramatically and that the coal structures were seriously damaged. 3.3. Development Mechanism of Fractures during Temperature-Rise Oxidation of Coal. At a normal temperature, the interactions between coal and oxygen in air produces a small amount of heat through physical adsorption, chemical adsorption, and oxidation. In addition, under certain conditions, the rate of heat production through oxidation is larger than the rate of heat dissipation to the surroundings.25−28 Therefore, accumulation of generated heat causes a slow and steady rise in the temperature of coal, and then, the oxidation

Figure 6. Changes of Mad and Vad with the oxidizing temperature.

figure displays the changing tendencies of the contents of moisture and volatiles in coal during low-temperature oxidation. The figure shows that the contents of moisture and volatiles in coal decreased constantly with the gradually increasing oxidizing temperature of coal. The rate of the moisture content decrease declined with the increasing oxidizing temperature, while the rate of the volatile content decrease increased with the increase in the oxidizing temperature. At the initial stage of the low-temperature oxidation (30− 130 °C), the content of volatiles in coal was basically unchanged, while the moisture content decreased significantly from 2.07% to 0.51% as the temperature rose by 100 °C. The decrease at this stage accounted for 83.9% of the decreasing amplitude during the whole temperature rise stage (30−230 °C). At a later stage of low-temperature oxidation (130−230 °C), the moisture content in the coal masses was basically unchanged, while the volatile content declined significantly from 32.98% to 25.32%, as the temperature rose by 100 °C. This reduction accounted for 82.5% of the decreasing amplitude during the whole temperature rise stage (30−230 °C). Determination of the industrial parameters suggested that during the initial stage of oxidation (30−130 °C), fracture development in coal was attributed to water evaporation, dehydration of compounds containing water of crystallization, and decomposition and volatilization of some of the volatiles inside the coal. However, in the later stage of low-temperature oxidation of coal (130−230 °C), as the oxidizing temperature E

DOI: 10.1021/acs.energyfuels.6b03106 Energy Fuels XXXX, XXX, XXX−XXX

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4. RISK ASSESSMENT FOR DYNAMIC DISASTERS INVOLVING COAL AND ROCKS IN THE LOW-TEMPERATURE OXIDATION PROCESS OF CBM RESERVOIRS Dynamic disasters involving coal and rocks refer to phenomena that have dynamic effects and disastrous consequences and occur over a short time under high external stress and internal gas stress.33 In a dynamic disaster, coal and gas outbursts from this type of phenomenon are from fractured coal and gas and are suddenly ejected out of coal to mining areas; the gas stress and crustal stress are the most commonly observed dynamic disasters involving coal and rocks. Most CBM reservoirs in China exhibit low permeability, so some fracturing methods, such as hydraulic fracturing and hydraulic cutting, must be applied to crush the coal masses before extracting CBM. These methods increase the permeability of the CBM reservoirs. However, hydraulic fracturing and hydraulic cutting also promote air-leakage in the coal masses. As a result of the application of these methods, some areas without lowtemperature oxidation can start to be heated and oxidized while the oxidation rate of areas with the possibility of lowtemperature oxidation increases significantly. The initial velocity of gas diffusion (Δp), put forward and introduced to the coal mine field by Eichinger, an expert in the former Soviet Union in the 1970’s, is used to measure the speed of gas transformation from the adsorption state to the free state when coal-containing gas is exposed. In addition, Δp is one of the quality indices of coal and characterizes the microstructure of coal. Additionally, Δp reflects not only the capacity of the gas diffusing from coal but also the law of gas permeation and flow, therefore playing an important role in the prediction of regional outbursts. A new comprehensive index K (K = Δp/f) for predicting gas outbursts formed by combining the index with the firmness coefficient f of coal is one of the most common indices in the field of predicting coal and gas outbursts. Figure 8 displays the changing tendencies of Δp, f, and K as the oxidizing temperature of the coal masses rises. The figure

rose, the changes in the pores triggered by water evaporation could be ignored. In addition, decomposition and volatilization of volatiles were the major factors that led to the change in the quantity and dimensions of pores at this stage. Coal is a nonhomogeneous mixture with a complex microstructure.29−31 Some of the active materials contained in coal react easily with oxygen in air to produce peroxides and heat.32 In addition, the peroxides are further decomposed to produce various gaseous products (such as CO and CO2, shown in Figure 7). The gaseous products are representative of

Figure 7. Changes of the concentrations of CO, CO2, and C2H4 with the rise in the oxidizing temperature.

the active materials and volatiles inside coal that start to be decomposed. The intensity of the oxidation reaction inside coal at this moment can be inferred by detecting the gas concentration. The gas composition and concentration of each component generated at various temperature-rise stages during low-temperature oxidation were detected by using gas chromatography, as shown in Figure 7. The figure shows that during the oxidation and temperature rise of coal, the contents of separated CO, CO2, and C2H4 increased from slow to sharply increasing as the oxidizing temperature rose. Among the three gases, CO was generated to the highest degree in the early stage, followed by CO2 and C2H4. In addition, the CO concentration dramatically increased as the oxidizing temperature rose to 100 °C, while CO2 and C2H4 were not generated at large quantities before the oxidizing temperature rose to 140 °C. Gas chromatography analysis showed that oxygenolysis occurred in coal masses as soon as the oxidizing temperature rose to 30 °C. However, the oxygenolysis rate was so slow as to probably be negligible. As the oxidizing temperature constantly rose, coal started to be oxidized slowly and decomposed as the temperature rose to 100 °C when the CO concentration and rate of coal oxygenolysis increased significantly. With the further increase of the oxidizing temperature (higher than 140 °C), the coal masses began to undergo vigorous oxygenolysis as the concentrations of CO, CO 2, and C2 H4 increased dramatically, as did the rate of coal oxygenolysis. The results were consistent with the results from the industrial analysis experiment. At the initial stage of low-temperature oxidation, pore growth was caused by water evaporation, dehydration of compounds containing water of crystallization, and decomposition and volatilization of some of the volatiles in coal. However, at the later stage of the oxidation, oxygenolysis of macromolecules and volatiles in coal played a dominant role in the development of coal fractures.

Figure 8. Changes in Δp, f, and K with oxidizing temperature.

shows that as the temperature of the coal masses rises from 30 to 230 °C, Δp increases by 145.1%, f declines by 50.3%, and the comprehensive index K increases by 388.9%. Obviously, f shows a gentle decrease, maintaining a basically constant decreasing rate at different oxidizing stages. However, the growth rate of Δp increases first and then decreases, followed by an increase in the rising temperature after the oxidizing temperature rose to 130 °C. The rising rate of K slowly increased at the initial stage of coal oxidation (30−130 °C), F

DOI: 10.1021/acs.energyfuels.6b03106 Energy Fuels XXXX, XXX, XXX−XXX

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while the K value increased significantly at the later stage of the oxidation (130−230 °C). Figure 8 shows that the large increase in the K value was caused by the increase in Δp. By analyzing the comprehensive index K value of coal and gas outbursts, the occurrence probability of dynamic disasters involving coal and rocks constantly increases throughout the process of lowtemperature oxidation of CBM reservoirs. After the oxidizing temperature of the coal masses rose to 130 °C, the occurrence probability of dynamic disasters largely increased. The result provides a theoretical basis for guiding safe underground drainage of CBM and preventing the occurrence of dynamic disasters involving coal and rocks caused by coal lowtemperature oxidation.

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5. CONCLUSIONS Physical changes in coal during low-temperature oxidation, especially the process for development of fractures, were detected by employing NMR technology and gas chromatography. The results showed that the diameters and quantities of pores inside coal increased with the increasing oxidizing temperature. The porosity increased by 72.2% as the temperature rose from 30 to 230 °C. The whole development process of fractures was divided into two stages. First, in the initial stage of low-temperature oxidation (30−130 °C), because of water evaporation, dehydration of compounds containing water of crystallization, and decomposition and volatilization of some of the volatiles inside the coal, micropores in coal expanded and connected to mesopores. In the later stage of the oxidation (130−230 °C), macromolecular compounds and volatiles inside the coal were oxidized and decomposed, resulting in the expansion and connection of mesopores to macropores and microfractures. Because the quantity and dimension of pores increased during lowtemperature oxidation of coal, the structural strength of the coal constantly declined, while the initial velocity of gas diffusion largely increased. The results led to a constant increase in the comprehensive index K for predicting coal and gas outbursts during low-temperature oxidation. As the temperature rose to 130 °C, K increased significantly, indicating that the occurrence probability of dynamic disasters in the coal also increased significantly. Therefore, this study shows that in CBM extraction, if the temperature in a part of or in the whole CBM reservoir rises to 130 °C, proper measures need to be taken to inhibit oxidation of coal; otherwise, the risk of dynamic disasters involving coal in the region is likely to be multiplied.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shengqiang Yang: 0000-0002-5968-0982 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by The National Natural Science Foundation of China (Grant 51174198) and State Key Laboratory of Coal Resource and Safe Mining Independent Project Funding (SKLCRSM11 × 01). G

DOI: 10.1021/acs.energyfuels.6b03106 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b03106 Energy Fuels XXXX, XXX, XXX−XXX