Evolution Law of Adsorption and Desorption Characteristics of CH4 in

Sep 10, 2018 - Evolution Law of Adsorption and Desorption Characteristics of CH4 in Coal ... School of Safety Engineering, China University of Mining ...
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Evolution Law of Adsorption and Desorption Characteristics of CH4 in Coal Masses during Coalbed Methane Extraction Zongqing Tang,†,‡,§ Shengqiang Yang,*,†,‡,§ Guang Xu,∥ Mostafa Sharifzadeh,∥ and Cheng Zhai†,‡,§

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†

Key Laboratory of Coal Methane and Fire Control, China University of Mining and Technology, Ministry of Education, Xuzhou 221116, 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 ∥ Department of Mining Engineering and Metallurgical Engineering, Western Australian School of Mines, Curtin University, Kalgoorlie, Australia ABSTRACT: The low-temperature oxidation of coal during coalbed methane extraction is inevitable. To study the evolution of the CH4 adsorption and desorption characteristics in coal masses during the low-temperature oxidation of coal, starting from the evolution of physical and chemical adsorption of CH4 in coal masses during the low-temperature oxidation of coal and combining it with the evolution of free radicals, we constructed a physical model of CH4 adsorption and desorption in the coalbed methane extraction process. This model provides the theoretical basis for improving the efficiency and quantity of coalbed methane extraction. Our results indicate that during coalbed methane extraction, the mesopores, macropores, and overall porosity increase with the rise in oxidation temperature. However, the number of micropores first increases and then decreases during the process, leading to the CH4 physical adsorption capacity showing a trend of first increasing and then decreasing with increasing oxidation temperature; however, the number of −COOH groups shows the opposite trend to that of the number of micropores, resulting in the CH4 chemical adsorption capacity first decreasing and then increasing with the increase of the oxidation temperature; meanwhile, the free radical content increases gradually with the increasing oxidation temperature, leading to the continuous consumption of O2 adsorbed on the coal surface and the reinforcement of the CH4 adsorption capacity. To maximize coalbed methane extraction efficiency, it is necessary to take measures to avoid the lowtemperature oxidation of coal at the initial stage.

1. INTRODUCTION Coalbed methane (CBM) is a hydrocarbon gas that is mainly composed of CH4 and is stored in the coalbed and mainly adsorbed on the surface of the coal matrix particle, while part of it is free in the coal pores or is dissolved in the coalbed water.1−3 CBM is the associated resource of coal and is a type of unconventional gas.4,5 Moreover, it is a clean, high-quality source of energy and chemical raw materials that have become increasingly important over the past decade or two. Generally, CBM is called a “gas”, and its calorific value is 2−5 times larger than that of coal. The calorific value of 1 m3 of pure CBM is equal to that of 1.13 kg of gasoline or 1.21 kg of standard coal. Its calorific value is equivalent to that of natural gas, and these two gases can be mixed together for transport and use. CBM is a super clean energy source because it burns clearly without any exhaust gas.6,7 When the concentration of CBM in the air reaches 5%−16%, it will explode in the presence of a fire, which is the source of gas explosions in a coal mine. If CBM is released to the air directly, its greenhouse effect is 21 times greater than that of CO2, which is extremely destructive to the ecological environment. Hence, from the perspectives of energy utilization, disaster prevention, and environmental protection, it is imperative to improve the efficiency and quantity of CBM extraction. China has rich CBM reserves.8,9 There are approximately 240 trillion cubic meters of CBM reserves globally with a burial depth of less than 2000 m, while China’s CBM reserve is 36.8 © XXXX American Chemical Society

trillion cubic meters, which ranks as third in the world after Russia and Canada. Additionally, CBM reserves with burial depths between 2000 and 4000 m contain approximately 50 trillion cubic meters. Even though China has abundant CBM, most CBM reservoirs have strong compactness and terrible permeability, leading to the low output and extraction efficiency of CBM. Therefore, during CBM extraction in major CBM mining areas, such as the Qinshui Basin and Ordos Basin, methods that increase coalbed fracturing and permeability, such as hydraulic fracturing, hydraulic slotting, and deep hole blasting, are usually used to improve the permeability of the CBM reservoir.10−13 On the one hand, these methods do improve the permeability. On the other hand, their use gives rise to the low-temperature oxidation of CBM reservoirs due to air leaks. Meanwhile, as the extraction proceeds, the CH4 concentration in the coalbed continues to decrease, while that of O2 keeps rising, intensifying the lowtemperature oxidation.14,15 The studies of Kam, Beamish, Wang, Deng, and Tang indicated that the low-temperature oxidation of coal is inevitable during the extraction from a CBM reservoir when the coalbed-fracturing and permeability-increasing methods are used.16−18 However, the physical and chemical properties of Received: July 4, 2018 Revised: September 2, 2018 Published: September 10, 2018 A

DOI: 10.1021/acs.energyfuels.8b02318 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Maceral and Proximate Analyses of the Sample maceral analysis (vol %)

proximate (wt %)

sampling location

coal type

V

I

E

M

Ro, max (%)

Mad

Aad

Vad

FCad

Shanxi

Subbituminous

52.2

43.5

2.8

1.5

1.256

2.57

14.14

37.65

42.64

the coal are changed after low-temperature oxidation, further strongly influencing the CBM extraction. In particular, using nuclear magnetic resonance (NMR) data, Tang19 researched the development of the law of pore fissures during the entire low-temperature oxidation process and predicted the critical temperature of gas outburst in the coalbed. Deng et al.20 studied the adsorption capacity of multicomponent gases such O2, CO, and CH4 using their in-house-developed natural oxidation warming experiment stage. To assess the macromolecular structure of coal, Zhang et al. tested the CH4 adsorption capacity of different functional groups and concluded that it is not easy for oxygen-containing acid functional groups to adsorb CH4. The previous studies have shown that the low-temperature oxidation of coal during CBM extraction results in changes in the physical and chemical properties, which further influence the processes of physical and chemical CH4 adsorption. Finally, the extraction quantity and efficiency will be affected as well. However, these investigations have focused mainly on either the physical adsorption or chemical adsorption of CH4, whereas an in-depth study of the combined influence of coal pore development and active group change on the coal adsorption of CH4 has not been carried out to date. Therefore, in this work, the influence of changes in physical and chemical properties during the low-temperature oxidation of coal on CH4 adsorption was studied, starting with the influence of coal on the physical absorption of CH4 due to pore fissure development and the impact of coal on the chemical adsorption of CH4 due to active group evolution. We aimed to elucidate the laws governing CH4 adsorption and desorption during CBM extraction in order to provide further scientifically based guidance for CBM extraction to improve the extraction efficiency and amount of CBM.

Table 2. Preparation Standard and Purpose of Coal Samples no.

grain size (mm)

length (mm)

#1 #2 #3 #4

20 0−0.075 0−0.18 0.18−0.25

20

shape

weight (g)

test project

square powder powder powder

52.9 10 10 10

NMR ESR and FTIR IA BET

this coal experiment were set as 30 and 230 °C, respectively. A temperature cycle was carried out every 50 °C, and the oxidation temperature was increased for 250 min (in order to better simulate the low-temperature oxidation of coal during CBM extraction, we intentionally reduced the oxidation rate and increased the oxidation time). NMR, IA Tester, electron spin resonance spectrometry, Fourier transform infrared spectroscopy, and high-pressure gas adsorption instruments were used to monitor the evolution of the physical and chemical properties in a single oxidation temperature increasing cycle (all experimental procedures should be performed in an N 2 environment as much as possible in order to eliminate the experimental error caused by coal oxidation). 2.3. Experimental Facilities. 2.3.1. Simulation Experiment System of Coal Low-Temperature Oxidation. As shown in Figure 1, the experimental system for the simulation of low-temperature coal oxidation is mainly composed of a low-temperature oxidation analyzer, gas chromatograph, constant-voltage steel cylinder, coal sample tank, thermocouple, and gas piping. The low-temperature oxidation analyzer is a ZRJ-2000 coal spontaneous combustion tendency tester with the heating range of 20−600 °C. Its lowest heating rate is 0.01 °C/min, and it uses an LCD digital readout function that can display the temperature, heating time, and rate of coal samples in real time. Additionally, the system can be operated in a steady state for a long time. The gas chromatography instrument is a GC-4100 gas chromatographic analyzer. The coal sample tank is connected to the gas piping, which can directly transport the lowtemperature oxidation gas of coal to the gas chromatograph for the analysis of the gaseous components. 2.3.2. Experimental System of Nuclear Magnetic Resonance Measuring Pores. The nuclear magnetic resonance (NMR) core analyzer used in this work is a Mini MR-60 NMR imaging analysis system manufactured by Shanghai Newmag Electronic Technology Co., Ltd. The main field of this instrument is 0.51 T, the H proton resonance frequency is 21.7 MHz, the radio frequency impulse frequency is 1.0−49.9 MHz, the magnet temperature is controlled in the 25−35 °C range, and the magnet uniformity is 12.0 ppm, with the radio frequency power of 300 W. The parameters of the NMR measurements are as follows: echo time of 0.221 s, waiting time of 5 s, echo number equal to 5000, number of scans equal to 32, and experimental temperature of 32 °C. 2.3.3. Fourier Infrared Spectrometer. A Tensor27 Fourier infrared spectrometer (Bruker, Germany) was used in the experiments. The infrared spectrometer contains a high-throughput RockSolid stereoscopic angle mirror interferometer (Bruker) that ensures the high sensitivity and stability of the instrument. Its basic spectral region is 8000−350 cm−1, with the optional spectral region of 15500−20 cm−1. Thus, far-infrared, mid-infrared, and near-infrared multiband measurements can be carried out. The resolution is better than 0.25 cm−1 and is continuously adjustable. The signal-to-noise ratio is greater than 5000:1. The spectrometer can obtain 40 spectra per second with a wavenumber accuracy of 0.005 cm−1 and absorption accuracy of 0.07%T. 2.3.4. Free Radical Measurement System. A Bruker EMX plus electron paramagnetic resonance (EPR) spectrometer was used in this

2. EXPERIMENTAL SAMPLES AND METHODS 2.1. Experimental Samples. The coal samples were bitumite from Yangquan in Shanxi, China. The detailed vitrinite measurement and proximate analysis parameters are shown in Table 1. The experimental coal samples were taken from an underground coal mine. During their transportation, the coal samples were preserved in N2 to reduce the coal oxidation. Among these, block coal samples with dimensions of 20 mm × 20 mm were prepared using a coring drilling rig and a cutting machine (NMR test); coal fines with particle sizes of 0−0.075 mm (for the ESR and FTIR experiments), 0−0.18 mm (for the IA test), and 0.18−0.25 mm (for the BET experiments) were sifted out from powdery coal samples that fall off naturally due to the effect of a vibrating screen. After the preparation, the coal samples were stored in a dry N2 glass bottle in order to reduce experimental error. The data obtained in the experiments were all obtained using the coal samples listed in Table 2. The samples were used to research the fracture development process and gas adsorption and desorption processes during the low-temperature oxidation of coal. Moreover, their interactions were also studied. 2.2. Experimental Methods. A cyclic temperature programming experiment was designed, as illustrated in Figure 1. In a single temperature programming cycling experiment, coal samples were heated up and oxidized in dry air for 250 min at the heating rate of 0.2 °C/min and the gas flow of 30 mL/min (powdery coal samples) and 50 mL/min (block coal samples). The initial and final temperatures of B

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Figure 1. Experimental system and flowchart.

Figure 2. T2 spectrum during the low-temperature oxidation of coal. work. The EPR experiments were carried out at a room temperature of 20 °C, using the x-waveband to scan continuously. The experimental parameters were as follows: the microwave frequency is 9.389427 GHz, the microwave power is 1.002 × 10−1 mW, the central magnetic field is 3360.000G, the scan width is 200.000G, the resolution is 2000 dots, the time constant is 163.840 ms, the scanning time is 32 s, the modulation frequency is 100.000 kHz, and the modulation amplitude is 1.000G. Since there was no standard sample of coal, DPPH was selected as the standard sample to carry out the experiments under the same experimental conditions as coal. The DPPH signal was used as the EPR parameter of the coal sample.

3. RESULTS AND DISCUSSION 3.1. T2 Spectra and Pore Sizes. Coal composed of pores and fissures.9,21 The complex pore structure that determines chemical characteristics and also affects

chemical properties, such as the mechanical properties, permeability, and gas adsorption and desorption of coal seams. NMR T2 relaxation is due to 3 relaxation mechanisms, namely, free relaxation, surface relaxation, and diffusion relaxation, which jointly affect the T2 relaxation time.22,23 Since coal samples are nonmagnetic and the water fluid is cohesionless, free relaxation can usually be neglected. In addition, for a uniform magnetic field (in which the corresponding magnetic field gradient G is very small) and sufficiently short echo interval (ET), the diffusion relaxation can also be ignored. The T2 relaxation time of water-saturated coal samples can be simplified as the relaxation time of the surface between water and coal, as shown in the following equation.

is a dual medium coal mass has a its physical and its physical and C

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Figure 3. Evolution trend of different pores with oxidation temperature.

1 S =ρ× T2 V

mesopores increases mainly in the temperature range of 80 °C−180 °C. The number developing law of macropores and micropores is the same as that of mesopores, which means that their numbers gradually increase in the entire oxidation temperature range. Furthermore, the increasing speed rises dramatically with the increase of the oxidation temperature. The temperature range for the increase in the number of the pores is concentrated between 130 °C−230 °C. To better show the dynamic evolution of the pores with different pore sizes and categories with increasing oxidation temperature in the coal oxidation heating stage, the corresponding peak area SR with the corresponding peak area of each pore at the oxidation temperature (30 °C, 80 °C, 130 °C, 180 °C, and 230 °C) is shown (in 30 °C intervals), where ratio I represents the increase/decrease rate between the pore and raw coal at this temperature. Figure 3 shows the evolution trends of different pores in coal with increasing oxidation temperature. Figure 3a shows the change law between pore size and oxidation temperature, while Figure 3b presents the change law between pores with different functions and oxidation temperature. It can be seen from Figure 3a that the number of micropores in the coal first increases and then decreases with the oxidation temperature, which is consistent with the trend observed in Figure 2, and the maximum increase amount is 9.8%; the numbers of mesopores and macropores increase with increasing oxidation temperature, and the rate of increase is positively correlated with the temperature, which means that there is a lower rate of increase at lower temperatures. However, with the increased temperature, the rate of increase rises dramatically. Combined with the analysis of the initial stage of coal oxidation, this means that since there are many gases such as CH4 and O2 adsorbed on the coal surface and in the micropores, the raw coal surface and the micropore wall surface, first oxidation starts with O2, which leads to the dramatic increase in the number of the micropores on the raw coal surface and the increase in the size of protogenous pores in the coal. As the oxidation temperature rises, due to the continuous increase of the micropore size and the connection between the micropores, the number of mesopores keeps rising sharply, while the number of micropores begins to decrease. With the further increase of the oxidation temperature, the increase of the mesopore size and the increasing connections between the micropores and mesopores result in the sharp increase in the number of macropores. Meanwhile, since the rate of mesopore generation from micropores is larger than that of the

(1)

where T2 is the surface relaxation time, ρ is the transverse surface relaxation intensity (μm/ms), and S/V is the ratio of the surface area to the volume of the pore. The S/V values of the micropores are larger than those of the macropores; combined with eq 1, this means that micropores show faster relaxation times, while the relaxation time of the macropores is longer. Therefore, the transverse relaxation time T2 can reflect the pore size distribution of coal samples. A larger pore size corresponds to a longer relaxation time. In contrast, a shorter relaxation time means a smaller pore size. Li et al. found that T2 spectra with the T2 relaxation time shorter than 10 ms are a signature of micropores, while T2 spectra of 10−100 ms are a signature of mesopores, and T2 values larger than 100 ms indicate the presence of macropores and fractures.24 The amplitude of the T2 distribution reflects the number of pores within a certain pore size. A higher amplitude corresponds to a higher number of pores. Xie et al.25 found that the surface relaxation intensity of coal generally ranges from 0.5 × 10−8 to 2 × 10−8 m/ms. Qin et al. found that the methane molecular diameter ranges from 0.34 to 0.37 nm. The diameters of the pores adsorbing the CH4 molecules are approximately 10 nm with the corresponding T2 spectrum of 2 ms.26 Thus, the pores with the T2 spectrum values smaller than 2 ms are defined as adsorbed pores, which play an important role in the gas adsorption of CH4, N2, and O2. These pores have a rather large specific surface area with a very strong gas adsorption capacity. The pores with T2 spectra of larger than 2 ms are defined as seepage macropores, which represent the main passages for gas diffusion and penetration, thus controlling the complexity during CBM extraction. Figure 2 displays the trends in the variation of the T2 spectrum during the low-temperature oxidation of coal (30− 230 °C). It can be seen from the figure that the internal pores of the coal are developing dynamically and continuously in the entire oxidation temperature range. However, the evolution of the pores with different pore sizes and categories is not consistent. During the entire oxidation heating process, the number of micropores first increases (30−180 °C) and then decreases (180−230 °C). Moreover, the T2 relaxation time of the wave peak first increases and then decreases. The number of mesopores increases gradually in the entire oxidation temperature range, and the rate of increase increases rapidly as the oxidation temperature increases. The number of D

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data of the coal samples was carried out by using Peakfit software. The results of peak-differentiated fitting were compared with the infrared spectrum assignment table of coal, and the peak assignment of functional groups in coal was carried out. Since the content of the aromatic CC bonds does not change substantially during the low-temperature oxidation process, the absorption peak intensity of the aromatic CC bonds can be used as a quantitative criterion. The relative peak intensity was defined as the ratio between the intensities of the peaks of the oxygen-containing functional groups and the intensity of the aromatic CC absorption peaks in order to carry out a quantitative analysis of the changes in the oxygen-containing functional groups. Figure 5 illustrates that as the oxidation temperature is raised, the number of −OH groups first sharply decreases and

generation of macropores from mesopores, the number of mesopores increases at the same time. From Figure 3b, we know that the evolution of adsorbed pore number in the coal with the rising oxidation temperature is basically the same as that of the micropores, which means that at the initial stage of oxidation (30−100 °C), the number of the adsorbed pores first increases sharply, but with the increase of the oxidation temperature, the number of adsorbed pores starts to decrease when the oxidation temperature is higher than 100 °C. The evolution of the seepage macropore number in the coal with the rising oxidation temperature is basically the same as that of the mesopores and macropores, which means that this number increases gradually with the increase of the oxidation temperature. The volume of all pores in the coal increases gradually with increasing oxidation temperature. However, at the later oxidation stage, due to the decrease of the micropore number and volume, the increase rate changes slowly. The adsorbed pores in the coal are the locations of CH4 and O2 adsorption. According to the analysis, at the initial oxidation stage (30−100 °C), due to the increase of the adsorbed pore number and volume, the physical adsorption capacity of coal to CH4 gradually increases. However, when the oxidation temperature is greater than 100 °C, the number and volume of the adsorbed pores decrease, so that the physical adsorption capacity of coal to CH4 gradually decreases. 3.2. Evolution of the Oxygen-Containing Functional Group. Oxygen-containing functional groups are the main active groups in the low-temperature coal oxidation process.6,17,27 Their amounts and types determine the chemical properties of coal and exert a strong effect on the capacity for the chemical adsorption of CH4 during low-temperature oxidation. The coal samples were heated from 30 to 80 °C, 130 °C, 180 °C, and 230 °C while using FTIR to measure the changes in the functional groups during the low-temperature coal oxidation. The FTIR spectra of the coal samples at the five oxidation temperature stages are shown in Figure 4. Previous

Figure 5. Evolution of oxygen-containing functional groups during the low-temperature oxidation of coal.

then decreases at a lower rate. When the oxidation temperature rises from 30 to 80 °C, the −OH amount decreases by 41.2%, accounting for 66.5% of the entire oxidation state. The number of CO groups decreases slowly at first and then increases sharply as the oxidation temperature rises, losing 30.4% when the oxidation temperature rises from 30 to 80 °C. However, when the oxidation temperature rises to 230 °C, compared to the raw coal, the number of CO groups has increased by 121.3%. For the entire low-temperature oxidation stage, the −COOH content first decreases and then increases. When the oxidation temperature rises from 30 to 80 °C, the −COOH content decreases by 24.6%. When the oxidation temperature reaches 230 °C, compared to the raw coal, the −COOH content has increased by 101.2%. The data indicate that −COOH is involved in the coal-oxygen reaction at the initial stage so that the loss rate is rather high, while at low temperatures, the rate of generation is low, giving rise to the continuous decrease of the −COOH content at the initial stage of oxidation. Nonetheless, previous studies have shown that when the oxidation temperature is higher than 80 °C, −OH and CO continuously generate −COOH in the oxidation reaction. The rate of −COOH generation is much faster than the rate of its consumption, resulting in the rapid growth of the −COOH content, as shown in Figure 5. In the early stage of the low-temperature coal oxidation, due to the decrease of the −COOH content, the capacity for the chemical adsorption of CH4 on coal is enhanced. However, when the oxidation temperature exceeds 80 °C, the −COOH content begins to increase, and the capacity for the chemical adsorption capacity of CH4 on coal begins to decrease. When

Figure 4. Evolution of infrared spectra during the low-temperature oxidation of coal.

studies have shown that −COOH inside the coal has the greatest effect on the chemical adsorption of CH4 on coal and that there is a chemical repulsion between −COOH and CH4, that is, the smaller the amount of −COOH, the stronger is the capacity for the chemical adsorption of CH4 on coal.28 FTIR was used to measure the changes in the functional groups during the entire low-temperature coal oxidation process, and the peak-differentiating fitting of the obtained infrared spectral E

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Figure 6. Evolution of adsorption and desorption of CH4 at different oxidation temperatures.

the oxidation temperature further increases, exceeding 130 °C, the −COOH number exceeds the −COOH number in the raw coal, and the capacity for chemical adsorption of CH4 on coal is further reduced and is even lower than that of the raw coal. 3.3. Evolution of CH4 Adsorption and Desorption Characteristics. CH4 is adsorbed on the surface and internal pores of coal by physical adsorption and chemical adsorption.29,30 Among these, physical adsorption accounts for more than 95% of the total adsorption amount, dominating the measured adsorption capacity of CH4 on coal.31,32 Figure 6 illustrates the evolution process of the adsorption and desorption of CH4 due to the low-temperature coal oxidation in the CBM extraction process and the demonstration of fissure development in coal during the entire low-temperature oxidation stage. It can be seen from the figure that as the oxidation temperature of the coal masses continues to increase, the adsorption and desorption curve of CH4 begins to change. Combining this result with the observed development process of the pores in coal, it can be seen that at 30 °C, no or little coal has been oxidized. The coal surface is smooth, with some micropores and microfissures inside it, and CH4 is adsorbed on

the surface and micropores. When the oxidation temperature reaches 80 °C, the coal surface is oxidized, and part of the organic matter is decomposed, with the surface becoming rugged. The volume and number of internal micropores and microfissures increase. Thus, the specific surface area increases as well, resulting in the adsorption of more CH4. When the oxidation temperature is further increased to 130 and 180 °C, the organic matter and carbon-containing compounds on the coal surface begin to decompose quickly. The micropores connect to each other, forming mesopores and macropores and resulting in the sharp decrease of the coal specific surface area suitable for CH4 adsorption. Hence, the amount of the adsorbed CH4 begins to decrease. When the oxidation temperature is eventually increased to 230 °C, the organic matter on the coal surface has reacted almost completely. The diameter of the coal matrix is greatly reduced, and a large number of micropores develop into mesopores and macropores, which are no longer suitable for CH4 adsorption. The specific surface area suitable for CH4 adsorption is drastically reduced, leading to a further decrease of the CH4 adsorption amount. F

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Figure 7. Adsorption and desorption curves of CH4 during low-temperature oxidation of coal.

where A is the ESR spectrogram area, N is the spin number of the samples, g is the g value, and S denotes the spin numbers S = 1/2; ℏ···ℏ = h/2π, h = Planck’s constant, h = 6.62620 × 10−34J·s; β = Bohr magnetron, β = 9.27410 × 10−28JG·s−1; h1 = amplitude of the microwave field, h1 = 3.5Gs; ω = microwave frequency, ω = 2πf, f = 9059 × 106s−1; k = Boltzmann’s constant, k = 1.380662 × 10−23J·K−1; and T = measuring temperature. We calculate the spectrogram area ratio between the coal samples and the standard sample Tempol by integration. On the basis of the free radical concentration of the standard sample and coal sample dosage (5 vmg), we can obtain the free radical concentration of coal samples. g-Values can be directly calculated by hv = gβH. The changing curves of the free radical concentration and g-values during low-temperature oxidation are shown in Figure 8.

Figure 7 displays the adsorption and desorption curves of CH4 during the entire low-temperature coal oxidation process. According to the data in the figure, the amount of adsorbed CH4 on coal shows the trend of first increasing and then decreasing with the increase of the oxidation temperature. When the oxidation temperature is approximately 80 °C, the amount of adsorbed CH4 is the largest, while it is the smallest at 230 °C. The desorption and trends for CH4 in coal masses were almost the same. The analysis of the data in the figure shows that at the initial oxidation stage (initial temperature → 80 °C) during the CBM extraction, due to the continuous increase in the number and volume of micropores within the coal and the continuous decrease of −COOH, its adsorption capacity of CH4 gradually increases, leading to more difficulties and the lower efficiency of CBM extraction. However, when the oxidation temperature exceeds 80 °C, due to the continuous decrease in the number and volume of the micropores within the coal and the sharp increase of the − COOH content, the adsorption of CH4 decreases gradually, resulting in less difficulty and higher efficiency and larger quantity of CBM extraction. Nevertheless, according to previous studies, the probability of coal spontaneous combustion and gas outburst accidents in CBM reservoirs increases significantly. Therefore, controlling the oxidation temperature of coal at the initial temperature is highly beneficial for maintaining the efficiency and safety of CBM extraction.

4. INFLUENCE OF FREE RADICALS ON CH4 DESORPTION AND ADSORPTION During the CBM extraction process, the chemical structure of the coal is destroyed, and a large number of free radicals produced by covalent bond exercise show strong activity, which can easily set off a chemical reaction with O2 in the air and have a certain promoting effect on coal oxygen adsorption.33,34 The experiments were performed on a Bruker EMX Plus electron paramagnetic resonance spectrometer at a room temperature of 20 °C. The experimental parameters are discussed in section 2.3.4. The ESR spectrogram records the first-order differential curve. It is necessary to carry out the double integration of the spectrogram to calculate the spectrogram area. The relationship between the spectrogram and spin number is expressed in the following equation: A=N

gβS(S + 1)ℏh12πω 2 6kT

Figure 8. Evolution of free radicals during CBM extraction.

Figure 8 shows that as the oxidation temperature increases, the concentration of free radicals Ng within the coal increases, while the g-values of the free radical species first increase and then decrease. Since free radicals will chemically react with the O2 adsorbed by the coal and as the oxidation temperature increases, the concentration of the free radicals increases, the corresponding amount of O2 consumed also increases, resulting in the increase of the capacity and amount of adsorption of CH4 on coal. Hence, from the perspective of free radicals, it is further concluded that only the effective control

(2) G

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of low-temperature coal oxidation during CBM extraction could maximize CBM extraction efficiency and quantity.

REFERENCES

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5. CONCLUSIONS In this article, based on the low-temperature coal oxidation system, NMR, IA tester, electron spin resonance spectrometry, Fourier transform infrared spectroscopy, and high-pressure gas adsorption measurements were used together to study the effect of low-temperature oxidation on the adsorption and desorption of CH4 in coal masses in the CBM extraction process. The intrinsic mechanism of this effect was explained in three aspects, namely, the CH4 physical adsorption and chemical adsorption in coal and the evolution of free radicals. The results show that during the CBM extraction process, at the initial oxidation stage (initial temperature → 80 °C), due to the continuous increase in the number and volume of the micropores within the coal and the continuous decrease in the −COOH amount, the CH4 adsorption capacity was enhanced. Meanwhile, the extraction difficulty is a tougher problem, with a lower extraction efficiency obtained. However, when the oxidation temperature exceeds 80 °C, the decrease in the number and volume of the micropores within the coal and the dramatic increase of the number of −COOH molecules lead to the lower CH4 adsorption capacity. Thus, the difficulty of the CBM extraction is reduced, enhancing the extraction efficiency and quantity. However, at this time, the spontaneous combustion of coal and the probability of gas outburst accidents are greatly increased. The number of free radicals in coal continues to increase during the entire CBM extraction process, which results in free radicals and O2 adsorbed on the coal surface consuming the coal in large quantities. Moreover, the adsorption capacity and quantity of CH4 on coal is further improved. CBM extraction will inevitably lead to low-temperature coal oxidation. After low-temperature oxidation, the capacity for CH4 adsorption and the amount of the increases at the initial stage of oxidation lead to low CBM extraction efficiency. As the oxidation temperature rises, the adsorption capacity and quantity of coal for CH4 adsorption gradually decrease, while the desorption rate and amount increase greatly. However, the probability of coal spontaneous combustion and gas outburst accidents increases significantly at the same time. Therefore, in order to maximize CBM extraction efficiency, inhibitory measures should be taken to avoid low-temperature oxidation of coal during its initial stage.

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Article

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 research was supported by the Fundamental Research Funds for the Central Universities (No. 2018BSCXA03) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX18_1907). H

DOI: 10.1021/acs.energyfuels.8b02318 Energy Fuels XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.energyfuels.8b02318 Energy Fuels XXXX, XXX, XXX−XXX