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Journal of Natural Gas Science and Engineering 57 (2018) 45–51
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering
journal homepage: www.elsevier.com/locate/jngse
Effect of moisture on the desorption and unsteady-state diffusion properties
of gas in low-rank coal
T
Haijun Guoa,b, Liang Yuana, Yuanping Chengc,∗, Kai Wanga,d, Chao Xua,b, Aitao Zhoua,b,
Jie Zanga,b, Jiajia Liue
a
Beijing Key Laboratory for Precise Mining of Intergrown Energy and Resources, China University of Mining and Technology, Beijing, Beijing, 100083, People's Republic of
China
b
State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Henan Polytechnic University, Jiaozuo, 454000, People's Republic of China
c
National Engineering Research Center for Coal Gas Control, China University of Mining and Technology, Xuzhou, 221116, People's Republic of China
d
Hebei State Key Laboratory of Mine Disaster Prevention, North China Institute of Science and Technology, Beijing, 101601, China
e
College of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, 454000, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords:
Low-rank coal
CBM development
Moisture
Unsteady-state diffusion
Non-constant diffusion coefficient (non-CDC)
model
Currently, resource integration in coal-burning facilities and clean energy reconstruction are being strongly
pushed forward in China. As a clean fossil energy, unconventional natural gas, such as coalbed methane (CBM),
exhibits promise as an energy development strategy. In China, there are abundant CBM resources in the considerable reserves of low-rank coal. Therefore, the study of low-rank coal is the key to realize the high-yield and
high-efficiency development of CBM resources in China. In this paper, a new water injection device was designed
to prepare coal samples with a variety of moisture contents, and a novel method to measure the gas desorption
properties in coal containing water was introduced. On this basis, the gas desorption law in low-rank coal with
different moisture contents was tested, and the effect of moisture on the gas diffusion properties in low-rank coal
was analyzed. The results indicate that the initial gas desorption volume of low-rank coal is extremely large and
that the gas desorption law in the moisture-containing low-rank coal can be described by Airey's empirical
equation. It is also observed that the ultimate gas desorption volume, initial gas desorption rate and initial
effective gas diffusion coefficient all shift toward the direction that is beneficial to CBM development. Therefore,
reducing the moisture content in low-rank coals through certain techniques can dramatically enhance the gas
migration behavior and achieve the purpose of high-yield CBM development.
1. Introduction
In China, coal was always the main energy resource for the urban
modernization in the past few decades. However, this has led to severe
environmental pollution. Thus, all over the country, especially in large
and medium-sized cities, resource integration in coal-burning facilities
and clean energy reconstruction have been strongly pushed forward. As
a clean fossil energy, unconventional natural gas, such as coalbed methane (CBM), will certainly become a bright spot for energy development strategies. The total CBM volume is approximately
3.68 × 1013 m3 in China, of which the CBM volume in low-rank coal
seam is around 1.6 × 1013 m3, which accounts for 43% of the total
(Wang et al., 2016). For a long time, most of the research (He et al.,
2010; Xu et al., 2014; Zhao et al., 2013, 2016) was concentrated on
medium-rank and high-rank coals in China. In the 21st century, CBM in
the low-rank coal seam from the Powder River basin of USA, Surat basin
∗
Corresponding author.
E-mail address: cyp620924@hotmail.com (Y. Cheng).
https://doi.org/10.1016/j.jngse.2018.06.045
Received 13 January 2018; Received in revised form 20 June 2018; Accepted 29 June 2018
Available online 30 June 2018
1875-5100/ © 2018 Elsevier B.V. All rights reserved.
of Australia and Alberta basin of Canada has achieved commercial
development (Beaton et al., 2006; Busch and Gensterblum, 2011;
Stricker et al., 1998). By comparison, the study of CBM in the low-rank
coal, especially lignite, still stays in the stage of scientific research and
experimental development.
In general, coal with a vitrinite reflectance R0 < 0.65% is called
low-rank coal in China (Guo et al., 2015; Wang et al., 2016). The
porosity of low-rank coal is high, its specific surface area is large and it
has many polar functional groups, such as hydroxy and carboxyl;
therefore, it can absorb more water than coals of other ranks (Crosdale
et al., 2008; Fu et al., 2005, 2012; Wang et al., 2011). The statistical
data show that in China, the moisture content of long flame coal is
mostly in the range of 3%–12% and the moisture content of lignite is
typically 10%–28% (Guo et al., 2015; Qiao, 2009). There are four existing forms of moisture in the low-rank coal (Allardice et al., 2003;
Crosdale et al., 2008): 1) In the macropores and fractures in the form of
Journal of Natural Gas Science and Engineering 57 (2018) 45–51
H. Guo et al.
remove the moisture in coal. The density (ρ) of the coal sample was
measured using an Ultrapyc 1200e Automatic Gas Pycnometer for
Density from Quantachrome; the instrument has a resolution and
accuracy of 0.0001 g/mL and 0.02%, respectively. Then, the coal
samples (weighing mcoal) were placed in the coal sample tank. The
volume of gas molecules adsorbed onto the pore surfaces of coal
matrices is extremely small, so it can be ignored. Thus, the volume
of the coal sample in the coal sample tank can be calculated using
Eq. (1):
free water; 2) in the capillary channel of low-rank coal; 3) adsorbed in
the coal pores in the form of multimolecular layer; 4) adsorbed in the
coal pores in the form of monomolecular layer.
Moisture in the low-rank coal matrix has significant impact on the
gas ad-desorption properties and plays an important role in gas migration in coal (Gensterblum et al., 2013; Pan et al., 2010). In the past,
studies on water-containing coal by most scholars (Day et al., 2008;
Gensterblum et al., 2013; Krooss et al., 2002; Liu et al., 2012) have
mainly focused on equilibrium water coals, and the influence of the
variation of moisture content in coal on the CBM development is rarely
taken into account. In addition, some scholars (Crosdale et al., 2008;
Joubert et al., 1974; Moore and Crosdale, 2006; Xie and Chen, 2007;
Zhang et al., 2009) have also studied the gas adsorption properties in
coal with different moisture contents; however, due to limits of the
experimental device, it was difficult to uniformly humidify the experimental coal sample and ensure accurate moisture content when coal
samples with different moisture contents were prepared via artificial
addition of humidity. In this paper, a water injection device that can
uniformly humidify the experimental coal samples was developed to
prepare coal samples with different moisture contents, and the new
device and method to measure the gas desorption properties in coal
containing water were introduced. On this basis, the effect of moisture
on the gas migration mechanism in low-rank coal is studied to provide a
theoretical basis for the CBM development in low-rank coal reservoirs.
Vcoal =
mcoal
ρ
(1)
2) Combining with the volume (V01) of the coal sample tank, the volume (Vf ) of the free gas in the coal sample tank at adsorption
equilibrium can be obtained using the following equation:
Vf = V01 − Vcoal
(2)
Thus, the free gas volume (Vf0 ) under the standard conditions can be
obtained using the following equation (Guo et al., 2016b):
Peq Vf
Teq
=
P0 Vf0
T0
(3)
where Peq is the adsorption equilibrium pressure in the coal sample
tank, MPa; Teq is the temperature of the coal samples within the coal
sample tank, K; P0 and T0 are the standard pressure (0.101325 MPa) and
temperature (273.15 K), respectively.
2. Coal samples and experimental methods
2.1. Coal samples
3) The temperature of the thermostatic oil bath was adjusted to
(60 ± 0.1) °C, and the coal sample tank and coal samples were
outgassed for approximately 48 h under high vacuum to remove air
and other impurities in them. Then, the valve of the coal sample
tank and the switch of the vacuum pump were closed successively.
4) The temperature of the thermostatic oil bath was changed to
(30 ± 0.1) °C, and then the constant-flux pump and the stirring
apparatus were opened to inject the distilled water into the coal
samples in the coal sample tank. The volume of injected water was
calculated in advance according to the requirement for the experiment. After the water injection was completed, the switch of the
constant-flux pump and the corresponding valve of the coal sample
tank were closed. It should be noted that to uniformly humidify the
experimental coal sample, the stirring apparatus should keep continuous stirring for 45–50 min before it is closed (Guo et al., 2016b).
The coal sample in the coal sample tank was equilibrated for 48 h
after the stirring apparatus being closed to allow the injected water
to fully humidify the coal sample.
5) The coal sample tank was filled with methane (> 99.9% purity) and
the gas intake valve was adjusted to let the gas pressure stabilize at
2 MPa when the gas adsorption equilibrium state was achieved. It
was considered that adsorption equilibrium had been reached after
6 h of unchanged pressure in the coal sample tank.
6) To test the gas desorption law in coal, the first step was pressure
relief when the gas desorption law in coal was tested. As shown in
Fig. 1, the valve (m) was opened to link the gas sample bag to the
coal sample tank, making the pressure reduce to the atmospheric
pressure. Then, the valve (m) was turned to connect the coal sample
tank with the gas desorption measuring cylinder as soon as possible.
The coal samples were collected from the northern China mining
area. The samples were sealed in a package from the site and prepared
for the proximate analysis, petrographic analysis and porosity tests. The
basic physical parameters of the low-rank coal samples are shown in
Table 1.
2.2. Experimental methods
In general, coal sampled from the mining area is in a pulverized or
broken state; that is, most coal samples are in particulate form.
Therefore, the gas desorption and diffusion properties in coal samples
are usually measured via desorption or sorption methods (Guo et al.,
2016b; Zhang, 2008). In this paper, the desorption method was used. To
prepare coal samples with different moisture contents, a water injection
device that can uniformly humidify experimental coal samples was
developed. Using the device, we can inject water into the coal sample
after vacuum degassing or achieving gas adsorption equilibrium and
then stir the experimental coal sample to humidify it uniformly.
Meanwhile, in this paper, the new experimental device and method to
measure the gas desorption properties were introduced to accurately
obtain experimental results. The schematic diagram of the experimental
device is shown in Fig. 1.
The experimental steps and methods for the analysis are as follows:
1) The coal sample was crushed in a pulverizer and screened to obtain
the desired sample size of 1–3 mm. Then, the specimens were placed
in a vacuum drying oven at 50 °C to completely dry for 72 h to
Table 1
Basic physical parameters of the low-rank coal samples.
Proximate analysis (wt%)
Petrographic composition (vol %)
Aad
Vdaf
Fcad
Vitrinite content
Inertinite content
Liptinite content
Mineral content
9.28
49.53
32.47
95.17
0.59
0
4.24
Maximum vitrinite reflectance (Ro)/%
Porosity/%
0.41
17.82
Aad = ash content on air-dried basis; Vdaf = volatile matter content on dry ash-free basis; Fcad = fixed carbon content on air-dried basis.
46
Journal of Natural Gas Science and Engineering 57 (2018) 45–51
H. Guo et al.
Fig. 1. Schematic diagram of the water injection and
desorption device.
1- High pressure CH4 cylinder; 2- Pressure reducing
valve; 3- Reference tank; 4- Vacuum pump; 5Compound vacuum gauge; 6- Constant-flux pump; 7Blender motor; 8- Stirring apparatus; 9-Water inlet
and gas intake; 10- Gas outlet; 11- Thermostatic oil
bath; 12- Coal sample tank; 13- Agitating vane; 14Gas desorption measuring cylinder; 15- Gas sample
bag; a-e− Pressure gauge; f-i- Valve; j- Four-way
joint; k- Three-way joint; m- Controllable three-way
joint.
considered the coal as an aggregation of separated “blocks” having
different sizes and containing fractures. Thus, on the basis of Darcy's
law, the coal gas emission theory was established and the empirical Eq.
(5) revealing the relationship between the gas desorption volume in
coal and the time was put forward.
The desorbed gas volume at different times (t) was measured by
recording the liquid level in the gas desorption measuring cylinder.
The total gas desorption time was determined according to the gas
desorption volume per unit time. The gas desorption volumes could
be converted into volumes (Vt) at STP based on the atmospheric
pressure and temperature outside the coal sample tank. The gas
volume in the gas sample bag was measured and converted into the
volume (Vco) at STP, and then the revised volume (Qt) of the desorbed gas per unit mass at different times t could be obtained using
the following equation (Guo et al., 2016b):
Qt =
(Vco − Vf0) + Vt
mcoal
n
Qt
− t
= 1 − e ( t0 )
Q∞
(5)
where Q∞ is the ultimate gas desorption volume, mL/g; t0 is the time
constant, s; n is the constant related to the fracture development in coal.
Based on the data in Fig. 2, Eq. (5) was applied to fit the gas desorption volume in low-rank coal with different moisture, and results are
shown in Fig. 3 and Table 2.
From Fig. 3 and Table 2, it is found that the correlation coefficients
of the fitting to the gas desorption volume in low-rank coal with different moisture contents using Airey's empirical equation are all greater
than 0.99, which indicates that the Airey's empirical equation can be
used to quantitatively describe the gas desorption law in low-rank coal.
The variation of the ultimate gas desorption volume with the
moisture content was plotted in Fig. 4. As can be seen, there is a linear
relationship between the ultimate gas desorption volume and the
moisture content, which can be expressed as follows:
(4)
7) When the desorption experiment ended, the coal sample tank was
opened quickly and coal samples from the upper, middle and lower
portions of the coal sample tank were collected, respectively. Three
coal samples were collected from every portion. Then, the proximate
analysis of the collected coal samples was immediately performed.
The results indicated that the moisture content of every collected
coal sample was basically identical and consistent with that calculated in advance. Thus, the average moisture content of the nine
collected coal samples was considered as the moisture content of the
coal sample in the coal sample tank (Guo et al., 2016b).
Q∞= − 0.62Mad + 21.08
(6)
According to the property of the geometric curve, the slope of a
certain point in the gas desorption curve is the gas desorption rate at
that point. Thus, the Airey's empirical equation is derived over time,
with the result shown below:
3. Experimental results and analysis
3.1. Effect of moisture on the isothermal desorption properties
During the gas desorption experiment on low-rank coal, the gas
adsorption equilibrium pressure and temperature were set as 2 MPa and
30 °C, respectively. The gas desorption experimental results of coal
samples with different moisture contents are shown in Fig. 2.
From Fig. 2, it is found that the initial gas desorption volume of lowrank coal is extremely large, and the gas desorption volume of the lowrank coal from the first 3 min is approximately equal to 80%–90% of
the total gas desorption volume. Then, the desorption curve suddenly
slows down and quickly flattens. Moreover, the gas desorption volume
of coal samples at the same time t decreases significantly with increasing moisture content.
n−1
n ⎛t⎞
Q′t = Q∞ ⎡
⎢ t0 ⋅ t0
⎣ ⎝ ⎠
⎜
⎟
⋅e
n
− t
t0 ⎤
( )⎥
⎦
(7)
Combining the gas desorption data in Fig. 3 and the fitting parameters in Table 2, Eq. (7) was applied to obtain the gas desorption rate
of low-rank coal with different moisture contents. The results are shown
in Fig. 5.
Fig. 5 indicates that under the conditions of the same moisture
content, the gas desorption rate in low-rank coal decreases over time. At
the initial stage of gas desorption in coal, the decreasing trend of the gas
desorption rate is extremely rapid; as time goes on, the decreasing trend
gradually slows down; in the late period, the gas desorption rate almost
no longer changes. From Fig. 5, it is also found that at the initial stage of
gas desorption in coal, the gas desorption rate decreases with increasing
moisture content in coal; as time goes on, the differences among the gas
desorption rates of low-rank coal samples with different moisture
3.2. Effect of moisture on the ultimate gas desorption volume and the gas
desorption rate
During the investigation of coal gas emission, Airey (Airey, 1968)
47
Journal of Natural Gas Science and Engineering 57 (2018) 45–51
H. Guo et al.
Fig. 2. Gas desorption experimental results of low-rank coal samples with different moisture contents.
Fig. 3. Results of the fitting to the gas desorption volume in low-rank coal with
different moisture contents using Airey's empirical equation.
Fig. 4. Relationship between the ultimate gas desorption volume and the
moisture content.
Table 2
Parameters and correlation coefficients of the fitting to the gas desorption volume in low-rank coal with different moisture contents using Airey's empirical
equation.
Mad/%
Q∞/(mL·g−1)
t0 /s
n
R2
0.00
5.13
9.97
15.22
20.46
26.38
21.64
17.12
15.26
10.98
8.98
4.67
3.07
14.97
12.84
17.13
33.59
38.66
0.14
0.17
0.16
0.16
0.17
0.15
0.9949
0.9991
0.9989
0.9942
0.9957
0.9973
contents become smaller and smaller, even showing an irregular
change. The authors believe that it is mainly caused by the anisotropy
of coal.
3.3. Effect of moisture on the unsteady-state diffusion properties of gas in
low-rank coal
Fig. 5. Gas desorption rate of low-rank coal with different moisture contents.
3.3.1. Evaluation of the diffusion coefficient
Coal is a porous material, and the pore and fracture system in it is
not only the storage space of CBM but also the channel for CBM
transportation (Zhao et al., 2014, 2016). The gas migration in coal is a
very complex behavior. From the viewpoint of molecular movement, it
is thought that the gas adsorption and/or desorption on the coal pore
surface is completed instantaneously (Guo et al., 2016b). However, due
to the resistance caused by gas diffusion out of various types of pores in
the coal matrices and the gas flow through the fractures in coal, it
usually takes some time for the gas to migrate in coal. In general, the
type of gas migration in coal matrices features as unsteady-state gas
diffusion (Guo et al., 2014, 2016b).
48
Journal of Natural Gas Science and Engineering 57 (2018) 45–51
H. Guo et al.
diffusion coefficients in coal are usually not constant, especially when
the effect of moisture content and temperature is taken into account
(Guo et al., 2016b). Thus, it has become a conundrum for researchers to
obtain the analytical solution of the diffusion equation under the condition of non-constant diffusion coefficients. Recently, some investigators (Jian et al., 2012; Zhang, 2008) have proposed a non-CDC
model by analogy, in which the diffusion coefficient depends on the
time t. According to their ideas, if the gas diffusion coefficient D is only
t
related to the time t, we can define αgas = ∫0 Ddt . Then, Eq. (9) can be
converted as follows (Guo et al., 2016b):
From a mathematical point of view, Fick's law is widely used by
scholars to quantify the gas diffusion process in a porous medium
(Charrière et al., 2010; Guo et al., 2016a; Yang and Wang, 1986; Zhang,
2008):
J = −D
∂cgas
(8)
∂lh
where J is the diffusive flux, kg/(m2·s); D is the diffusion coefficient,
which reflects the diffusion “rate”, m2/s; cgas is the concentration of the
diffusion medium, kg/m3; lh is the diffusion distance, m; and
∂cgas
∂lh
is the
concentration gradient (a vector). The negative sign reflects that the
direction of the concentration gradient is opposite to the direction of
the diffusive flux (i.e., the concentration gradient goes from low to high
concentration, but the diffusive flux is from high to low concentration)
(Guo et al., 2016b; Zhang, 2008).
To apply Fick's law for solving engineering problems, based on
Fick's law, the continuity principle of fluids flow and the mass conservation equation, some scholars (Yang and Wang, 1986; Zhang, 2008)
have derived the diffusion equation (i.e., Fick's second law). If the
diffusion coefficient D is independent of the concentration cgas , the onedimensional diffusion equation can be obtained:
∂cgas
∂t
=D
∂cgas
∑
n=1
(15)
Assuming the following,
∫t Ddt
⎧
⎪x = 0
rc
⎨
Qt
F
=
⎪
Q∞
⎩
(16)
Then, Eq. (15) can be converted into a unary quadratic equation as
follows:
6
x+F=0
π
3x 2 −
2 2t
1 − Dnr 2π
c
e
n2
(14)
t
(9)
∞
∂lh2
t
6 ∫0 Ddt
3∫ Ddt
Qt
=
− 0 2
Q∞
π rc
rc
The structure of coal is extremely complex. To study the gas migration law in coal, the coal particle is considered as a uniform sphere.
Based on this assumption, the following analytical solution for the gas
diffusion coefficient of a spherical coal particle was obtained (Yang and
Wang, 1986; Zhang, 2008):
Qt
6
=1− 2
Q∞
π
∂2cgas
The above equation is equivalent to Eq. (9) by making αgas
equivalent to t and D equal to 1. Therefore, solutions obtained for
constant D may be applied to non-constant D. Thus, the following
equation is obtained (Guo et al., 2016b):
∂2cgas
∂lh2
=
∂αgas
(10)
(17)
Solving the above equation yields the following solution:
where Qt is the gas desorption ratio in coal; and rc is the radius of the
Q∞
spherical coal particle, m.
However, the above equation is an infinite series equation, which is
difficult to be applied to engineering practice (Guo et al., 2016b). Thus,
by introducing the error function, some scholars (Yang and Wang,
1986; Zhang, 2008) solved the analytical solution of the gas diffusion
equation, as presented below:
6
π
x=
−
36
π
− 12F
6
Assuming that y =
1
y = rc2 x 2 = rc2 ⎛⎜
−
⎝ π
=
1
−
π
1
F
−
π
3
(18)
t
0
∫ Ddt , thus,
2
1
F
− ⎞⎟
π
3⎠
(19)
∞
Qt
6 Dt ⎡ 1
nr ⎤ 3Dt
+ 2 ∑ ierfc ⎛ c ⎞ ⎥ − 2
=
Q∞
rc ⎢ π
rc
Dt ⎠ ⎦
⎝
n=1
⎣
⎜
From Eq. (19), it is found that y is a function of F , and F is obtained
by recording the gas desorption volume in coal at the different times t .
Thus, y is also a function of t . Given the value of y , the diffusion
coefficient D can be calculated as follows:
⎟
(11)
( )
nr
where ierfc Dtc is the integrated error function.
In terms of engineering applications (such as coal mining), it still
meets the requirements of engineering accuracy if the error function is
ignored. Therefore, the above equation can be expressed as follows
(Guo et al., 2016b):
Qt
6 Dt
3Dt
=
− 2
Q∞
π rc
rc
D=
Qt
Q∞
(20)
Based on the non-CDC model, the value of y at the different times t
is obtained by the gas desorption experiment first; then, the functional
relationship between y and t can be obtained by the function fitting;
finally, the gas diffusion coefficient D at the different time can be obtained by deriving the function of y for t (Guo et al., 2016b).
(12)
When the fraction desorbed is less than 0.5, the diffusion equation
can be simplified as Eq. (13) to give an estimate of the diffusivity for
short duration (t < 600s) (Charrière et al., 2010; Smith and Williams,
1984a; b).
Qt
6 Dt
=
=B t
Q∞
π rc
dy
dt
3.3.2. Effect of moisture on the gas diffusion properties in low-rank coal
Based on Fig. 2 and Table 2, Qt (i.e., the value of F ) can be calQ∞
culated. Thus, the value of y at the different time can be obtained using
Eq. (19). The results are shown in Fig. 6.
It is found that the exponential formula as follows can describe the
curves in Fig. 6. The fitting results are shown in Table 3.
(13)
From Eq. (13), it is found that there is a linear relationship between
and t if the gas desorption time in coal is very short. In the Car-
y = y0 + A1 e−t / t1 + A2 e−t / t2 + A3 e−t / t3
tesian coordinate system, a linear fit of Qt versus t yields a value of K;
Q∞
this allows the calculation of the gas diffusion coefficient D.
However, the above method assumes a constant diffusion coefficient
and is only accurate for short desorption times, but in fact, the gas
(21)
where y0 , A1 , A2 , A3 , t1, t2, t3 are the fitting parameters.
Taking the derivative of Eq. (21), the following expression can be
obtained:
49
Journal of Natural Gas Science and Engineering 57 (2018) 45–51
H. Guo et al.
Fig. 6. Variation of y with time under the condition of different moisture
contents.
D=
dy
A
A
A
= − 1 e−t / t1 − 2 e−t / t2 − 3 e−t / t3
dt
t1
t2
t3
Fig. 7. Variation of effective diffusion coefficients of the low-rank coal with
different moisture contents along with time.
(22)
In practical application, the effective diffusion coefficient De as
follows is used widely.
De = D / rc2
(23)
From Table 3, it is found that the correlation coefficients are all
greater than 0.99 by fitting the curves in Fig. 6 using Eq. (21), which
indicates that the non-CDC model can quantitatively describe the gas
diffusion characteristics in low-rank coal well.
Substituting the data in Table 3 into Eq. s (22) and (23), the changes
of effective diffusion coefficients of the low-rank coal with different
moisture contents along with time were obtained, as shown in Fig. 7.
Fig. 7 indicates that under the condition of the same moisture
content, the initial effective diffusion coefficients of low-rank coal are
relatively larger; as time goes on, the effective diffusion coefficients
drop sharply. It is also found that the lower the moisture content, the
smaller the effective gas diffusion coefficients for low-rank coal, which
indicates that the gas diffusibility in low-rank coal decreases with increasing moisture content.
Fig. 8. Variation of the ultimate desorption volume, initial desorption rate and
initial effective diffusion coefficient with the moisture content.
with the moisture content were summarized and presented in Fig. 8.
From Fig. 8, it is found that if the moisture content in low-rank coal
decreases from 26.38% to 0%, the ultimate desorption volume, initial
desorption rate and initial effective diffusion coefficient increase 363%,
309% and 367%, respectively. It is worth noting that all parameters will
transform to the direction of enhancing CBM development when the
moisture content in low-rank coal decreases.
The pores in low-rank coal are well connected which provides a
huge space for coal to adsorb moisture and gas (Guo et al., 2015).
Moreover, there are many polar and hydrophilic functional groups in
low-rank coal (Crosdale et al., 2008; Fu et al., 2005, 2012), which results in the water molecules being adsorbed by low-rank coal pores
more easily than gas molecules. On the one hand, water in low-rank
coal occupies the adsorption sites of gas molecules, which sharply reduces the gas adsorption and desorption volume in low-rank coal. On
the other hand, water molecules block some pore channels in low-rank
4. Discussion
CBM drainage is a process of diffusion and permeation of methane
in the pores of the coal reservoir. During this process, the key parameters that have a great effect on the CBM development are the effective diffusion coefficient and permeability, which are seriously influenced by the moisture content in coal. As shown in the previous
studies (Guo et al., 2015), the moisture content in low-rank coal is high,
and especially, the moisture content of some lignite can even be as high
as 40%–50%. Therefore, the influence of moisture must be considered
during the development of CBM in low-rank coal seams. Based on the
above experimental results, the variation of the ultimate desorption
volume, initial desorption rate and initial effective diffusion coefficient
Table 3
Fitting results of curves between y and time under the different moisture contents.
Mad/%
y0/(10−8 m2)
A1 /( × 10−8)
t1
A2 /( × 10−8)
t2
A3 /( × 10−7)
t3
R2
0.00
5.13
9.97
15.22
20.46
26.38
3.02
2.84
2.63
2.44
2.29
1.91
−9.57
−9.69
−7.97
−7.54
−6.79
−6.68
14.70
18.71
18.60
19.20
42.46
49.61
−5.64
−5.47
−5.07
−4.83
−4.44
−3.57
275.91
335.04
342.64
371.88
421.12
459.69
−1.49
−1.52
−1.33
−1.21
−1.17
−0.89
4900.47
4826.05
4612.22
4496.33
4661.92
4677.47
0.9995
0.9998
0.9999
0.9999
0.9999
0.9999
50
Journal of Natural Gas Science and Engineering 57 (2018) 45–51
H. Guo et al.
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5. Conclusion
In the paper, low-rank coal was studied as a subject. A water injection device was developed to prepare coal samples with a variety of
moisture contents, and a novel method to measure the gas desorption
properties in coal containing water were introduced. Then, the effect of
moisture on the gas desorption and diffusion properties in low-rank
coal was tested. Conclusions are as follows:
1) The initial gas desorption volume of low-rank coal is relatively
large; next, the desorption curve suddenly slows and quickly flattens. Moreover, the gas desorption volume of coal samples at the
same time t decreases significantly with increasing moisture content. The Airey's empirical equation can be used to quantitatively
describe the gas desorption law in low-rank coal.
2) Under the conditions of the same moisture content, the gas desorption rate in low-rank coal decreases over time. At the initial stage
of gas desorption in coal, the decreasing trend of the gas desorption
rate is extremely rapid, but over time, this decreasing trend gradually slows. In the late period, the gas desorption rate almost stay
changed. At the initial stage of gas desorption in coal, the gas desorption rate decreases with increasing moisture content in coal, and
over time, the differences among the gas desorption rates of lowrank coal samples with different moisture contents are increasingly
smaller, even showing an irregular change.
3) Under the condition of the same moisture content, the initial effective diffusion coefficients of low-rank coal are relatively larger,
and over time, the effective diffusion coefficients drop sharply. The
lower the moisture content, the smaller are the effective gas diffusion coefficients in low-rank coal.
4) The ultimate desorption volume, initial desorption rate and initial
effective diffusion coefficient will transform to the direction of enhancing CBM development when the moisture content in low-rank
coal decreases. Therefore, to enhance the gas migration in CBM
development, it is very important to reduce the moisture content in
low-rank coal by applying certain technical measures.
Notes
The authors declare no competing financial interest.
Acknowledgments
The authors are grateful to the financial support from the State Key
Laboratory Cultivation Base for Gas Geology and Gas Control (Henan
Polytechnic University) (WS2018B06), the State Key Research
Development Program of China (No. 2016YFC0600708), the Open
Funds of Hebei State Key Laboratory of Mine Disaster Prevention (No.
KJZH2017K02), the Yue Qi Distinguished Scholar Project, China
University of Mining & Technology, Beijing, and the National Natural
Science Foundation of China (No. 51474219, 51604278 and
51604101).
51
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