Experimental study of the moisture content influence on CH 4 adsorption and deformation characteristics of cylindrical bituminous coal core

Abstract

Moisture content in coal is an important factor affecting the coal seam gas extraction. It directly affects the storage and flow of gas in bituminous coal. In this paper, the cylindrical bituminous coal cores of Xutuan coal mine in Huaibei coal mine group were studied as experimental objects, using the laboratory self-designed experimental device Gas Adsorption and Strain Testing Apparatus system. The influence of the bituminous coal moisture content on gas adsorption characteristics was studied. Drying experiments of coal samples showed that they lose the original moisture content following the exponential decay function of time. At wetting, the saturated moisture content in coal samples increased following the Exponential Association function of time. The experimental results show that the average original moisture content and average saturated moisture content of raw coal samples are 1.3 and 2.4%, respectively. On this basis, the gas adsorption experiments on samples with different moisture contents under different gas pressures were carried out. With the moisture content increase, the gas adsorption capacity and saturation value decreased and the decrease rate gradually reduced. The single exponential decay function describes the gas adsorption capacity dependence on moisture content. Moisture content also affects the adsorption deformation of bituminous coal. At high moisture content, the adsorption deformation of bituminous coal is less.

Keywords

Bituminous coal moisture content gas pressure adsorption strain

Introduction

Improvement of coal permeability and an efficient, safe gas extraction from coal seams is an actual research topic (Long et al., 2017; Zhu et al., 2017). Gas extraction intensity depends on many parameters such as its content and thermodynamic conditions, as well as adsorption and flow characteristics of coal (Li et al., 2017; Liu et al., 2018; Wu et al., 2014). Among them, moisture content is one of the important factors affecting gas adsorption. Therefore, this paper mainly studies the effect of moisture content on CH₄ adsorption and deformation characteristics of bituminous coal.

Many research teams studied coal adsorption of CH₄, CO₂, N_2, and other adsorptive gases (Bae and Bhatia, 2006; Meng and Li, 2017; Ottiger et al., 2008; Song et al., 2015). As a porous medium, coal adsorbs gas molecules through the Van der Waals intermolecular forces attraction, and coal gas adsorption is reversible. The gas adsorption of coal is usually indicated by the adsorption isotherm. Many experimental studies of isothermal adsorption of different coals have been carried out. Several methods for adsorption isotherm tests have been developed: the volumetric method (Chareonsuppanimit et al., 2012; Krooss et al., 2002; Meng et al., 2015; Mohammad et al., 2008; Shen et al., 2015), gravimetric method (Ottiger et al., 2008; Song et al., 2015), direct adsorption method (Hol et al., 2011), etc. The testing methods were compared and improved by Kudasik (2017) and Zhang et al. (2013).

The main factors affecting coal gas adsorption are the moisture content (Nie et al., 2015; Nie et al., 2016; Teng et al., 2017), temperature (Crosdale et al., 2008), gas pressure (Song et al., 2015), the particle size of coal (Yuan et al., 2017; Zhang et al., 2014b), coal rank (Nie et al., 2016; Shen et al., 2015), and ash content, cleat (Jing-Ming et al., 2008). Among them, moisture content is the main factor affecting gas adsorption and desorption (Crosdale et al., 2008; Guo and Guo, 2017; Wang et al., 2014). Water in coal presents in different states (chemically bonded, adsorption, capillary, and free, etc.), but the absorbed moisture influences most strongly on gas adsorption characteristics of coal (Zhang and Ma, 2008). The water vapor adsorption capacity of coal is much higher than the capacity for methane sorption. Obviously, the amount of coal-adsorbed gas decreases if the moisture content in coal is high (Bae and Bhatia, 2006). At the same time, the presence of adsorbed moisture leads to the decrease of pore connectivity of coal and influences the further adsorption of coal on gas (Cheng et al., 2017; Yang et al., 2016).

Philip and De Vries (1957) studied water transfer in porous media and derived equations to determine liquid vs. gaseous state of water. Joubert et al. (1973) believe that moisture beyond the critical point has no further effect on methane adsorption. Some researchers carried out laboratory experiments to find the relationship between the moisture content in coal and the amount of methane adsorbed. Liu et al. (2014) performed isothermal desorption experiment of drying coal sample, liquid water wetting coal sample, and high-pressure water injection into coal sample after methane adsorption, and found that the adsorption capacity of coal samples at high-pressure water injection was significantly higher than that of coal sample at liquid water wetting. He (2015) considered water displacement desorption and inhibiting desorption of gas adsorbed in coal. Wang et al. (2017) studied the effect of inherent moisture on hard coal adsorption–desorption characteristics, found that moisture occupies a certain space of the pore structure and reduces the density of gas adsorption sites in coal. Zhang and Ma (2008) studied the adsorption isotherm of different metamorphic coals with different moisture contents and deduced the correction coefficient of coal moisture effect on its methane adsorptive capacity. Gensterblum et al. (2013) studied sorption isotherms of Australian subbituminous, German high-volatile bituminous, and German anthracite coals in dry and moisture-equilibrated states and concluded that the sorption capacity of dry coals followed a parabolic function of the coal rank while the sorption capacity of moisturized coals depended linearly on it. Nie et al. (2016) performed experiments of sorption characteristics of methane in various rank coal samples at different moisture contents and concluded that the effect of moisture content on a low-rank coal was higher than that on a high-rank coal. Guo et al. (2015) made use of a new water injection and gas adsorption device to conduct isothermal adsorption experiments on low rank coal for different moisture contents and concluded that the inhibition degree of moisture on gas adsorption capacity increase with the increase of moisture content. Clarkson and Bustin (2000) carried out experiments on the effects of moisture content on the adsorption characteristics of a mixture of CH₄ and CO₂ and deduced that the moisture in a coal reduced the selectivity to CO₂. Zhang et al. (2014a) used a modified indirect gravimetric method to test sorption isotherm and found that moisture in a coal affected more on the adsorption capacity of CH₄ than on CO₂ adsorption capacity. Teng et al. (2017) established a fully coupled multi physical model of coal deformation, gas flow and water loss, and got the water loss, which improved the porosity and permeability of coal.

In addition to studies of the influence of moisture content in coal on its gas adsorption characteristics, some researchers also analyzed the influence of the moisture content in coal on its volume. Among them, Fry et al. (2009) studied coal swell induced by water adsorption and found that the volume of adsorbed water is linearly related to the pore volume of coal. Van Bergen et al. (2009) believe that moisture is a competitor of gas adsorption. Thus, it affects the coalbed methane extraction.

But the influence of moisture content in coal on gas adsorption characteristics was studied mostly on crushed or briquette coal samples. It is difficult to measure adsorption characteristics of a coal seam. Therefore, in this paper, the bituminous coal of Xutuan coal mine was used as the object of study, employing the laboratory self-designed experimental device Gas Adsorption and Strain Testing Apparatus (GASTA) system. We studied the moisture content of bituminous coal samples and the effect of moisture content on their gas adsorption characteristics. And the coal strain induced by methane adsorption was monitored and analyzed.

Measurement of the moisture content in Xutuan coal mine bituminous coal

Occurrence characteristics of bituminous coal

The Xutuan coal mine belongs to Huaibei mining area (as shown in Figure 1(a)), and the main coal seams are 3₂, 7₁, 7₂, and 8₂ of Permian. The 3₂ coal seam is bituminous rank coal; it mainly includes metabituminous and 1/3 of coking coal. The main water filling source of 3237 working face is the coarse sandstone fissure of the top coal seam and the dynamic water of 3235 goaf in the upper section. Boreholes are arranged at the bottom of 3237 working face. By measuring the coalbed methane pressure and the coal seam gas content, the maximum original gas pressure of 0.53 MPa was found, and the maximum original gas content was 5.80 m³/t. The Xutuan coal mine location in China and the borehole diagram of the longwall face are shown in Figure 1. The industrial analysis results and the gas adsorption parameters of the coal samples from 3237 working face conveyor entry of 3₂ coal seam in Xutuan coal mine are shown in Table 1.

Figure 1.

The Xutuan coal mine location in China (a) and the borehole diagram (b) of the longwall face.

Table 1.

Industrial analysis results and adsorption constants of coal samples from 3₂ coal seam in Xutuan coal mine.

Sampling site	Coal elevation (m)	Industrial analysis			True density (t/m³)	Pseudo density (t/m³)	BET surface area (m²/g)	Porosity (%)	Adsorption constant
Sampling site	Coal elevation (m)	M_ad(%)	A_d(%)	V_daf(%)	True density (t/m³)	Pseudo density (t/m³)	BET surface area (m²/g)	Porosity (%)	a(m³/t)	b(MPa⁻¹)
3237 working face roadway	–620	1.33	15.87	38.06	1.40	1.31	14.89	6.42	23.8853	0.8875

Note: BET (Brunauer-Emmett-Teller)

Preparation of coal samples with different moisture contents

In this paper, the preparation of coal samples with different moisture contents was made according to the method used by Pan and Yang (Pan et al., 2010; Yang et al., 2016). In general, the moisture content in coal means the adsorbed water that is the total moisture (M_t). Besides this, the maximum inherent moisture content of coal (M_HC) means the saturated water content adsorbed in coal capillary pores. It is measured at a temperature of 30°C and relative humidity of 96% conditions (with a saturated K₂SO₄ solution).

The air-drying base moisture (M_ad) refers to the moisture content of coal in the air-drying state. It is generally measured by the air-drying method. According to Chinese National Standards (GB/T 212–2008), M_ad was determined in laboratory experiments using the following relation

M_{a d} = \frac{m_{1}}{m} \times 100

(1)

where M_ad represents the air-drying base moisture, %; m and m₁ are the coal sample weight and its lost after the drying, g.

The equilibrium moisture (M_e) is defined as the moisture content when the pore water in the coal sample with 0.18∼0.25 mm pores reaches saturation at a temperature of 30°C and a relative humidity of 96% (using a saturated K₂SO₄ solution). M_e was measured in the laboratory according to Chinese National Standards (GB/T 19560–2008). Its specific formula is as follows

M_{e} = [(1 - \frac{G_{2} - G_{1}}{G_{2}}) \times M_{a d} + \frac{G_{2} - G_{1}}{G_{2}}] \times 100

(2)

where M_e represents the equilibrium moisture of the coal sample, %; G₁ is the mass of the air-drying base coal sample before the water balance, g; G₂ is the mass of the air-drying base coal sample after the water balance, g.

Table 1 shows that the M_ad of the coal is 1.33%, and the M_e of the coal is 2.58%.

Moisture content (M_c) is considered as the relative moisture content (M_rc) and an absolute moisture content (M_ac). The M_rc value represents how the water mass in coal relates to coal mass in percentage (equation (3)). In the natural state (that is the ambient air), M_rc equals to M_ad. In this paper, we used M_rc to indicate the moisture content in the coal samples.

M_{r c} = \frac{m_{w}}{m} \times 100

(3)

where M_rc represents the relative moisture content of coal, %; m_w is the mass of water in the coal, g.

First, we selected lump coal in the Xutuan coal mine then packed it in the sealed plastic bag, transported to the laboratory, and finally prepared coal samples of Φ50 × 50 mm sizes by drilling coal before the experiment. In this paper, HB3, HB4, HB5, and HB6 (HB represent Huaibei) coal samples were selected to investigate the influence of the moisture content on their gas adsorption characteristics. The individual parameters of the experimental coal samples are listed in Table 2. The samples are shown in Figure 2.

Table 2.

Parameters of coal samples.

Coal sample	Diameter (mm)	Height (mm)	Weight (g)
HB3	49.26	50.89	127.24
HB4	49.18	50.50	125.25
HB5	49.28	50.71	130.08
HB6	49.21	50.88	145.12

HB: Huaibei.

Figure 2.

Experimental coal samples. (1) HB3, (2) HB4, (3) HB5, (4) HB6.

For convenience, the samples were put into small steel bowls. After weighing the total mass of the sample and the bowl, both of them were put into a vacuum drying oven with a temperature of 100°C. With the certain period, the sample and the bowl were taken out for weighting and data record. When the sum of the mass of the two is no longer lowered for a long time, the drying was finished. After drying for more than 40 hours, coal samples were dry. The calculated original moisture content values are presented in Table 3. Table 3 shows that the original moisture content was 1.28–1.39% and the average value was about 1.3% (that is consistent with M_ad). The fitting analysis of the experimental data using Origin software provided the decline curve and the formula for the original moisture content as the function of time (as shown in Figure 3 and Table 4), and the fitting precision parameter (R²) reached 0.99. It is known that the original moisture content of coal samples decreases exponentially with time at vacuum heating. The formula for the original moisture content reduction with time at 100°C drying is as followings

M_{r c} = a + b e^{(- t_{1} / c)}

(4)

where t₁ represents the drying time, h; a, b, and c are the fitting parameters.

Table 3.

Coal samples moisture content.

Coal sample	Original quality (g)	Drying Quality (g)	Moisture content (%)
HB3	127.24	125.72	1.38
HB4	125.25	123.50	1.39
HB5	130.08	128.42	1.28
HB6	145.12	143.21	1.32

HB: Huaibei.

Figure 3.

The original moisture content of coal samples as a function of time at heating in vacuum oven. (a) HB3, (b) HB4, (c) HB5, (d) HB6.

Table 4.

The fitting formula and the fitting precision for the moisture content decrease in coal samples.

Coal sample	Fitting formula	Fitting degree
HB3	y=0.02974 + 1.27252e^{(–x/3.61387)}	R²=0.99145
HB4	y=0.04026 + 1.25993e^{(–x/5.24419)}	R²=0.99178
HB5	y=0.0556 + 1.16473e^{(–x/3.96634)}	R²=0.99051
HB6	y=0.04533 + 1.1984e^{(–x/4.78111)}	R²=0.98562

HB: Huaibei.

Firstly, Dried coal samples were placed in a vacuum environment. After the temperature of samples dropped to room temperature, they were put into the vacuum dryer as shown in Figure 4 to speed up wetting and the water was soaked through the lower end of the samples. Finally, samples were weighed through the certain periods until the mass of each sample remained unchanged. Thus, the samples were water saturated after wetting for 250 hours. The saturated moisture content of coal samples was calculated, and the dates are listed in Table 5. It can be seen from Table 5 that the saturated moisture content of coal samples is 2.26–2.55% with an average value of about 2.4%. It can be seen from Figure 2 that HB3 sample is more fractured than the other three samples, so the saturated moisture content of HB3 was higher. The fitting analysis of the experimental data using Origin software provided the curve and the formula of moisture content increase along with the time (as shown in Figure 5 and Table 6), and the fitting precision parameter (R²) reached 0.99. From Figure 5 and equation (5), that the moisture content of coal samples at wetting increases as Exponential Association (Exp Assoc) function. A relation of the moisture content increase at wetting vs. time is obtained

M_{r c} = a + b (1 - e^{(- t_{2} / c)}) + d (1 - e^{(- t_{2} / f)})

(5)

where t₂ is the wetting time of coal sample, h; a, b, c, d, and f are the fitting parameters.

Figure 4.

Vacuum dryer.

Table 5.

Saturated moisture content of coal samples.

Coal sample	Original quality (g)	Drying quality (g)	Wetting quality (g)	Saturated moisture content (%)
HB3	127.24	125.72	129.02	2.55
HB4	125.25	123.50	126.64	2.26
HB5	130.08	128.42	131.50	2.29
HB6	145.12	143.21	146.60	2.31

HB: Huaibei.

Figure 5.

Raise curve of coal sample moisture content. (a) HB3, (b) HB4, (c) HB5, (d) HB6.

Table 6.

The fitting formula and the fitting precision for the moisture content increase in coal samples.

Coal sample	Fitting formula	Fitting degree
HB3	y=0.00043 + 0.46369(1–e^{(–x/0.11823)})+2.10158(1–e^{(–x/20.96581)})	R²=0.99871
HB4	y=0.27871 + 0.60557(1–e^{(–x/6.47897)})+1.3695(1–e^{(–x/43.25313)})	R²=0.99363
HB5	y=0.30558 + 1.24499(1–e^{(–x/71.99713)})+0.79888(1–e^{(–x/11.31465)})	R²=0.99394
HB6	y=0.03725 + 1.81057(1–e^{(–x/49.42738)})+0.513(1–e^{(–x/0.58918)})	R²=0.99590

HB: Huaibei.

Using the above experimental method, we obtained bituminous coal samples with 0.0, 0.5, 1.0, 1.5, 2.0, and 2.4% moisture content. We used them for the study of effects of coal moisture content on gas adsorption characteristics.

Experimental system and experimental design

GASTA system

1. Composition of experimental system

The GASTA system mainly consists of the following components: the adsorption cavity system (including reference cylinder and sample cylinder) (withstand pressure 20 MPa), gas decompression system (test gases including CH₄, N₂, He, CO₂), temperature control system (room temperature – 100 ± 0.1°C), vacuum system, acquisition control system, strain testing system, and other systems. Figure 6 is the physical map of GASTA system.

Figure 6.

GASTA system.

2. Experimental principles

The experimental coal samples were placed in a sealed sample cylinder to measure the adsorption volume of CH₄ and other gases when the adsorption equilibrium was reached under different gas pressure conditions (0–12 MPa). According to the Langmuir monolayer adsorption theory, the Langmuir volume (V_L), Langmuir pressure (P_L), and the isothermal adsorption curve, indicating coal adsorption characteristics, were calculated. And during the adsorption process, the strain of the coal was also monitored. Figure 7 is the schematic diagram of GASTA system.

Figure 7.

Schematic diagram of GASTA system.

Figure 8.

Coal samples with strain gauges.

3. Operation process of experimental system

Set the water bath temperature and paste the axial and hoop strain gauges on the coal sample (as shown in Figure 8) and perform zero calibration on the system;

Use N₂ to test the pressure resistance of the system and check the hermeticity of the experimental system;

Vacuum the system and install the coal sample, connect the interface of strain gauge;

The He was used to measure the void space of the sample cylinder under the pressure of 2.0, 3.0, and 4.0 MPa, and the average value was calculated as the void space of sample cylinder;

Vacuum the system and test gas adsorption and adsorption deformation under different pressures;

Vent gas and remove the coal sample.

Experiment principle analysis of coal adsorption

According to the gas pressure and temperature in the reference and sample cylinders before and after adsorption equilibrium, the gas adsorption capacity and saturation under different gas equilibrium pressures were calculated. The adsorption experiments were carried out and the influence of moisture content on coal gas adsorption characteristics is characterized.

Firstly, we measured the pressure in the reference and sample cylinders before and after the same equilibrium pressure at a temperature of T and calculated the related compressibility factors. Then, using the gas equation (equation (6)), the molar numbers (n₁, n₂) of gas in the system before and after adsorption equilibrium were calculated.

The gas equation is as follows

P V = n Z R T

(6)

where P is the gas pressure, MPa; V is the gas volume, cm³; n is the molar number of gas, mol; Z is the gas compressibility factor; R is the molar gas constant, J·mol⁻¹·K⁻¹; T is the equilibrium temperature, K.

The most fundamental operational procedure to quantify gas adsorption is the Gibbs approach. Thus, on the basis of previous studies, this paper decides to adopt Gibbs approach (Gensterblum et al., 2013; Krooss et al., 2002; Zhang et al., 2014c).

The number of moles of the total gas (n_total) in the experimental system is

n_{t o t a l} = \frac{V_{c}}{V_{M c}} + \frac{V_{y}}{V_{M y}}

(7)

where n_total is the total amount-of-substance of gas, mol; V_c and V_y are the volume of the reference cylinder and void space of the sample cylinder, cm³; V_Mc、V_My are the molar volume of gas in the reference and sample cylinders before the adsorption experiment following Soave–Redlich–Kwong (SRK) equation, cm³/mol.

The amount-of-substance of gas in the void space (n_void) can thus be calculated using the following SRK equation

P V_{v o i d} = n_{v o i d} Z_{S R K} R T

(8)

where, n_void is the amount-of-substance of gas in the void space, mol; V_void is the void volume of the sample cylinder, cm³; Z_SRK is the compressibility coefficient in SRK equation.

The molar number of the gas adsorbed (n_adsorbed) by the coal sample is

n_{a d s o r b e d} = n_{t o t a l} - n_{v o i d}

(9)

The total volume of adsorption gas (V_t) was calculated

V_{t} = n \times 22.4 \times 1000

(10)

The related gas adsorption capacity (V_a) is

V_{a} = \frac{V_{t}}{m}

(11)

The coal adsorption isotherms are usually used to determine the coal adsorption capacity in a laboratory. We applied the Langmuir model in this study. Comparing the adsorption capacity of CH₄ in different moisture content coals, we obtained the influence of bituminous coal moisture content on gas adsorption characteristics. The Langmuir equation is the following

V_{a} = \frac{a b P_{a}}{1 + b P_{a}}

(12)

where V_a is gas adsorption capacity per unit mass of coal sample at a temperature of T and adsorption equilibrium gas pressure of P_a, m³/t; P_a is the adsorption equilibrium gas pressure, MPa; a is an adsorption constant, that is the maximum gas adsorption capacity, also known as the Langmuir volume (V_L), m³/t; b is another adsorption constant, the reciprocal Langmuir pressure (P_L), MPa⁻¹.

According to the ratio of gas adsorption equilibrium pressure to the related adsorption capacity (P/V), we plotted the scatter diagram of P–P/V. The linear regression of these points was found. We calculated the linear equation and correlation coefficient using the least squares method (R²). Assuming that the slope of the line is A and the intercept is B, the following formulas were available

V_L and P_L are the following

V_{L} = \frac{1}{A}

(13)

P_{L} = \frac{B}{A} = V_{L} B

(14)

The relation between V_a and P_a, A, B was obtained

V_{a} = \frac{P_{a}}{B + A P_{a}}

(15)

The coal gas adsorption saturation is the ratio of the adsorbed gas amount to the maximum adsorbed gas amount. According to equation (12), the maximum adsorbed gas amount is a, and the gas adsorption saturation is C

C = \frac{V_{a}}{a} = \frac{b P_{a}}{1 + b P_{a}}

(16)

The effect of coal moisture on its gas adsorption mainly depends on adsorption strength of coal to different gases (equation (17)). Since the adsorption capacity of coal to H₂O exceeds that to CH₄ (Bae and Bhatia, 2006), the influence of coal moisture content on gas adsorption was studied.

H_{2} O > C O_{2} > C H_{4} > N_{2}

(17)

Coal samples deformed during the process of gas adsorption (Van et al., 2009). In the experiment, the axial and hoop strain gauges were pasted on coal samples, and the deformations of samples with different moisture contents were monitored during the process of gas adsorption. The volumetric strain of the coal samples was calculated by equation (18).

ε_{v} = ε_{1} + 2 ε_{2}

(18)

where ε_v is a volume strain, με; ε₁ is an axial strain, με; ε₂ is a hoop strain, με.

Experiment scheme of adsorption

Experiment scheme for adsorption of CH₄ by bituminous coal

The water bath temperature was set at 30°C. Before the experiment, the system withstood pressure test with N₂, and then the free volume of the sample cylinder was measured by helium. The initial pressure in the reference cylinder was 1 MPa, while the initial pressure in the sample cylinder was 0.1 MPa. After 24 hours of CH₄ adsorption, the equilibrium pressure was obtained, and the gas compressibility coefficient, SRK molar volume, and gas adsorption capacity at different gas pressures were calculated. The above procedures were repeated under the conditions of the initial pressure in the reference cylinder of 1.5, 3.0, 4.0, 5.0, and 6.0 MPa, respectively, and the gas adsorption capacity, isothermal adsorption curve, and adsorption parameters were obtained under the conditions of different gas equilibrium pressures. The strain data of coal samples were recorded every 30 minutes. Figure 9 shows the adsorption pressure path.

Figure 9.

The adsorption pressure path.

Experiment scheme for studies of the moisture content influence on CH₄ adsorption characteristics of bituminous coals

Through the analysis of measurements of bituminous coal moisture content, the maximum moisture content investigated in this paper was about 2.4%. Taking sample HB6, we studied the influence of the moisture content on gas adsorption characteristics at six values of the content: 0.0, 0.5, 1.0, 1.5, 2.0, and 2.4%. The selected moisture content for the adsorption experiments and the related actual moisture content in the samples are listed in Table 7. During the experiment, in order to ignore the macroscopic flow velocity of the gas in the reference cylinder as it enters the sample cylinder, we slowly open the connection valve between the two cylinders. And because it is not the gas seepage experiment, therefore, the macroscopic flow velocity of the gas in the sample cylinder is negligible. Thus, the changes of moisture content in the experiment are negligible.

Table 7.

Designed and actual moisture content for adsorption experiments.

Moisture content of design (%)	Moisture content of actual (%)
0.0	0.14
0.5	0.57
1.0	1.08
1.5	1.57
2.0	2.00
2.4	2.24

Experimental results and analysis of the coal gas adsorption

Experimental results and analysis of coal adsorption under the same moisture content

Isothermal gas adsorption curve

By equations (7) to (12), the gas adsorption mole number, gas adsorption volume, and gas adsorption capacity of coal samples with different moisture content under different equilibrium gas pressures were obtained. The P/V of different equilibrium gas pressures was obtained as well, and the scatter plot of P/V–V was drawn. Then, the linear regression of these scatter points was evaluated. Using the least square method, linear equation and the correlation coefficient (R²) were calculated. The P/V–V diagram of coal samples with different moisture contents is shown in Figure 10. Linear fitting equation parameters and the correlation coefficient (R²) of P/V–V diagram with different moisture contents are presented in Table 8.

Figure 10.

P–P/V diagram of coal samples with different moisture contents.

Table 8.

Linear fitting equation parameters and correlation coefficient of P/V–P diagram.

Moisture content (%)	Linear fitting formula	Fitting degree (R²)
0.0	y=0.1031x+0.3386	0.9873
0.5	y=0.0804x+0.6983	0.9549
1.0	y=0.0815x+0.8491	0.977
1.5	y=0.0805x+0.9924	0.9638
2.0	y=0.1015x+1.1878	0.9416
2.4	y=0.0864x+1.4305	0.9877

The slope and the intercept of the obtained linear equation were substituted into equations (13) and (14), and the Langmuir volume (V_L) and pressure (P_L) were calculated. The adsorption constants a and b are obtained and presented in Table 9. Because the adsorption time in this experiment is 24 h (it meets the experiment design requirements), so the adsorption under the related gas pressure is insufficient, and the adsorption constant a is lower than the value of a in Table 1. The scatter diagram of gas adsorption capacity at different gas equilibrium pressures and different moisture contents was plotted. The adsorption constants a and b were substituted into Langmuir equation (equation (12)) and the adsorption isotherms for different moisture content were obtained as shown in Figure 11. The divergence between the gas adsorption capacity obtained experimentally and calculated by Langmuir equation is small. At fixed moisture content, the gas adsorption capacity increased with the gas equilibrium pressure increase, and the gas adsorption capacity growth rate gradually decreased. When the gas equilibrium pressure approaches infinity, the gas adsorption increase rate tends to 1. Thus, the gas adsorption capacity reaches the limit and no longer increases if the gas equilibrium pressure approaches infinity.

Table 9.

Gas adsorption parameters.

Moisture content (%)	V_L (ml/g)	P_L (MPa)	a (ml/g)	b (MPa^–1)
0.0	9.6993	3.2841	9.6993	0.3044
0.5	12.4378	8.6853	12.4378	0.1151
1.0	12.2699	10.4184	12.2699	0.0959
1.5	12.4223	12.3279	12.4223	0.0811
2.0	9.8522	11.7024	9.8522	0.0854
2.4	11.5740	16.5567	11.5740	0.0603

Figure 11.

Adsorption isotherms for different moisture contents.

2. Adsorption saturation

According to equation (16), the gas adsorption saturation is the ratio of the gas adsorption capacity to the adsorption capacity limit. When the adsorption constant b was substituted into equation (16), the gas adsorption saturation related to different gas pressures at fixed moisture content was obtained, as shown in Figure 12. The gas adsorption saturation increases gradually with the increase of gas pressure, but the growth rate decreases. When the gas pressure approaches infinity, the gas adsorption saturation approaches unit that is the adsorption saturation limit.

Figure 12.

Relationship between the adsorption saturation and gas pressure at fixed moisture content.

According to equation (16), the gas adsorption saturation depends on the adsorption constant b and the gas pressure. It can be seen from Table 9, the value of P_L depends on the moisture content: it increases with the moisture content increase, while the related value of the adsorption constant b decreases. As illustrated in Figure 12, constant b is the slope of the curve at the starting point P = 0. With the decrease of b, the rate of gas adsorption saturation growth reduces and more gas pressure is required to reach the adsorption saturation limit.

3. Variation of strains at gas adsorption

In the experiment, the tensile strain was positive while the compressive strain was negative. According to the axial and hoop strain values monitored in the experiment, the corresponding volume strain was calculated by equation (18). We chose bituminous coal samples with the moisture content of 0.5% to draw the curves of its axial, hoop, and volume strains at different gas pressure condition (Figure 13).

Figure 13.

Adsorption strain curve of coal sample.

As illustrated in Figure 13, at the fixed gas pressure, the variation trends of the axial and hoop strains are basically the same, and they increase with the increase of adsorption time. The strain growth rate decreases gradually, and the strain tends to a certain value. The results show that the coal possesses swelling deformation while adsorbing gas, but the expansion slows down gradually with the decrease of adsorption rate. The hoop strain is larger than the axial strain (ε₂>ε₁) which indicates that the expansion of the sample in the hoop direction is larger than in the axial direction. The axial and hoop strains of coal samples increase much with the increase of gas pressure. It has the same conclusion with Meng and Li (2017). And Feng et al. (2016) and Zhou et al. (2017) used a method combining SEM-EDS (Scanning Electron Microscopy and Energy Dispersive Spectrometer) and micro-CT (Computed Tomography) scan to verify and explain the volume expansion of coal in methane adsorption from microcosmic angle. At the beginning of the experiment, the strain value was negative indicating that the external gas pressure exceeded the internal one. The strain of coal sample decreases with the increase of gas pressure in the system. However, due to the large pressure difference outside and in the samples, the gas quickly permeates into the coal, and the strain begins to increase after a short time of decrease. Since the volume strain is the sum of the axial strain and the double hoop strain, the volume strain is larger than both axial, and hoop strains and the growth rate of the volume strain at gas adsorption is higher than that of the axial and hoop strains.

Experimental results and analysis of gas adsorption of coal samples with different moisture contents

Gas adsorption curve

The adsorption capacity and the moisture content under the same gas pressure condition are plotted in Figure 14. And the curve fitting of the scatter plot also carried out as shown in Figure 14. The parameters of the fitting equation were adjusted, so that the fitting formula describes the actual relationship between the gas adsorption capacity and the moisture content. The fitting of the experimental relation between the gas adsorption and the moisture content satisfies the single exponential attenuation function (equation (19)). The curve fitting formula parameters and the correlation coefficient of the moisture content are shown in Table 10. Using the fitting formula parameters in Table 10, the maximum moisture content corresponding to the minimum gas adsorption capacity of 0.001 ml/g was obtained as shown in Table 11.

Figure 14.

Relationship between the adsorption capacity and moisture content under fixed gas pressure.

Table 10.

The fitting formula coefficients of adsorption capacity and moisture content under fixed gas pressure.

Gas pressure (MPa)	Fitting formula	Correlation coefficient (R²)
0.5	y=1.33079×e^{(–x/1.58862)}	0.79689
1	y=2.35725×e^{(–x/1.90694)}	0.90694
2	y=3.64755×e^{(–x/2.13822)}	0.92875
3	y=4.60688×e^{(–x/2.36622)}	0.95145
4	y=5.36092×e^{(–x/2.63368)}	0.9504
5	y=6.07499×e^{(–x/2.77779)}	0.94832

Table 11.

Adsorption amount limit and the related moisture content.

Gas pressure (MPa)	Moisture content (%)	Adsorption capacity (ml/g)
0.5	8	0.0086
1	11	0.0073
2	13	0.0083
3	15	0.0081
4	17	0.0084
5	18	0.0093

It can be seen from Figure 14, with the moisture content increase, the gas adsorption capacity and its rate gradually decreased. The adsorption capacity in dry condition is the maximum adsorption capacity under the certain gas pressure. When the moisture content increases to a certain magnitude, the gas adsorption capacity tends to 0, and the decrease rate approaches to 1. All the adsorption sites in coal are occupied by water in this case. As illustrated in Table 11, with the gas pressure increase, the maximum moisture content related to the minimum gas adsorption capacity increases (the default minimum order is 10⁻³ ml/g). When the gas pressure was 0.5 MPa, the maximum moisture content was 8%, while at the gas pressure of 5 MPa, the maximum moisture content increased to 18%.

The fitting formula for the gas adsorption capacity and the relative moisture content is

V_{a} = α \times e^{(- M_{r c} / β)}

(19)

where α and β are the variation parameters, they change with the change of gas pressure.

The Langmuir volume and Langmuir pressure of the coal samples with different moisture content are presented in Table 9. The variation trends of the Langmuir volume and Langmuir pressure were analyzed with the increase of moisture content as shown in Figure 15. It can be seen from Figure 15, the Langmuir volume is almost insensitive to the moisture content increase (that is, the maximum gas adsorption capacity is unchanged), while the Langmuir pressure increases linearly with the moisture content increase.

Figure 15.

Relationship between V_L, P_L, and moisture content.

Linear fits of parameters α and β in the fitting formula (equation (19)) at different gas pressures (Table 10) are shown in Figure 16. It can be seen from Figure 16, there is a good fitting relationship between α, β, and the gas pressure, which shows that α and β increase linearly with the gas pressure growth. At low gas pressure, the parameter α is less than β (α < β), while at the higher gas pressure the parameter α is larger than β (α > β). The growth rate of α is larger than the growth rate of β with the gas pressure increase.

Figure 16.

Relationship between parameter α , β, and gas pressure.

2. Adsorption saturation

The dependencies of the gas adsorption saturation on moisture content at different gas pressures are shown in Figure 17. As illustrated in Figure 17, with the increase of the moisture content, the gas adsorption saturation and its rate decrease. The gas adsorption saturation is the largest at zero moisture content.

Figure 17.

Relationship between adsorption saturation and moisture content at different gas pressures.

At fixed gas pressure value, the adsorption saturation decreases with the moisture increase from a dry state to lower moisture content state. For example, when the moisture content increases from dry to 0.5% the adsorption saturation decreases by nearly 50%. But with the increase of gas pressure, the decrease in adsorption saturation slightly reduces. Therefore, the influence of the moisture content on the gas adsorption reduces if the gas pressure is high.

3. Variation of strains at gas adsorption

As illustrated in Figure 13, the variation trends of axial and hoop strains are the same. Therefore, in this work, we studied the influence of moisture content on the adsorption strain by investigation of variation trend of the axial strain of bituminous coal samples with different moisture contents under fixed gas pressure. The axial strain curve of samples with different moisture contents under the gas pressure of 0.5 MPa is drawn in Figure 18. Since the strain gauges were pasted on coal samples before the experiment, the initial volume of coal samples differs, and it increases with the moisture content increase. The initial strain of samples at the beginning of the experiment was different. The scatter diagram of the initial axial strain of samples with different moisture contents is plotted in Figure 19.

Figure 18.

Axial strain curves of coal with different moisture contents.

Figure 19.

Initial axial strain of coal with different moisture contents.

As illustrated in Figure 18, the axial strain curves of coal samples with different moisture contents have the same trend for an increase with the adsorption time increase, while strain increase rate reduces with the adsorption time. At fixed adsorption time, the axial strain value of coal sample decreases with the increase of moisture content, that is, with the moisture content increase the adsorption capacity of coal samples decreases, and the expansion deformation decreases gradually. That result is consistent with the conclusions from Figure 14. As shown in Figure 19, the initial axial strain of the coal sample is larger if the moisture content is higher. Thus, the initial compression deformation decreases with the moisture content increase. Therefore, it can be concluded that the adsorption deformation of bituminous coal is strongly related to the moisture content. When the moisture content is high, the adsorption capacity and adsorption deformation of bituminous coal are less.

Conclusions

The average original moisture content and average saturated moisture content of coal samples from the Xutuan coal mine are 1.3 and 2.4%, respectively. At drying the original moisture content decreased following the exponential decay function of time, while at wetting the moisture content rose up according to Exp Assoc function.

Using the designed GASTA system for adsorption experiments, the influence of bituminous coal moisture content on gas adsorption characteristics was investigated. At fixed moisture content the gas adsorption capacity and saturation increased along with an increase of equilibrium gas pressure, but the growth rate gradually decreased, and coal samples were found to have developed adsorption swelling. When the gas equilibrium pressure tends to infinity, the gas adsorption capacity does not rise, and the gas adsorption saturation tends to 1. With the adsorption constant b drop, the rising rate of the gas adsorption saturation curve decreases, and more gas pressure is required to reach the adsorption saturation limit of 1. When the gas pressure was fixed, the strain gradually increased with the adsorption time increase.

With the moisture content increase, the gas adsorption capacity, as well as the adsorption saturation, reduced and their decrease rates gradually lowered. The swell deformation decreased gradually as well. The gas adsorption capacity and the moisture content followed the single exponential decay function. The gas adsorption capacity and saturation under fixed gas pressure were the largest in the dry condition. When the moisture content increases to a certain magnitude the gas adsorption capacity tends to zero, and the reduction rate approaches 1. With the gas pressure increase, the moisture content related the minimum gas adsorption capacity increased. At high gas pressure, the influence of moisture content on gas absorption was less. The adsorption deformation of bituminous coal is closely related to the moisture content. At high moisture content, the adsorption deformation of bituminous coal is less.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This research is supported by National Natural Science Foundation of China (51704274, 51704278, 51502339), Beijing Natural Science Foundation (8184082), the Young Talent Lifting Project from CAST, the Open Projects of State Key Laboratory of Coal Resources and Safe Mining CUMT (SKLCRSM16KF10, SKLCRSM16KF06), the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) (WS2017B02), Key Laboratory of Coal-based CO₂ Capture and Geological Storage, Jiangsu Province CUMT (NO: 2016B09), and the Priority Academic Programme Development of Higher Education Institutions in Jiangsu Province, Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJZZ16_0272).

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Experimental study of the moisture content influence on CH 4 adsorption and deformation characteristics of cylindrical bituminous coal core

Abstract

Keywords

Introduction

Measurement of the moisture content in Xutuan coal mine bituminous coal

Occurrence characteristics of bituminous coal

Preparation of coal samples with different moisture contents

Experimental system and experimental design

GASTA system

Experiment principle analysis of coal adsorption

Experiment scheme of adsorption

Experiment scheme for adsorption of CH4 by bituminous coal

Experiment scheme for studies of the moisture content influence on CH4 adsorption characteristics of bituminous coals

Experimental results and analysis of the coal gas adsorption

Experimental results and analysis of coal adsorption under the same moisture content

Experimental results and analysis of gas adsorption of coal samples with different moisture contents

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References

Experiment scheme for adsorption of CH₄ by bituminous coal

Experiment scheme for studies of the moisture content influence on CH₄ adsorption characteristics of bituminous coals