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Influence of CO2 Exposure on High-Pressure Methane and CO2 Adsorption on Various Rank Coals: Implications for CO2 Sequestration in Coal Seams Qianqian Wang,† Dengfeng Zhang,*,† Haohao Wang,† Wenping Jiang,‡ Xiuping Wu,‡ Jin Yang,† and Peili Huo† †

Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, People’s Republic of China Xi’an Research Institute of China Coal Technology & Engineering Group Corp., Xi’an 710077, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: There exist complex interactions between coal and CO2 during the process of CO2 sequestration in coal seams with enhanced coalbed methane recovery (CO2-ECBM). This work concentrated on the influence of CO2 exposure on highpressure methane and CO2 (up to 10 MPa) adsorption behavior of three types of bituminous coal and one type of anthracite. The possible mechanism of the dependence of CO2 exposure on adsorption performance of coal was also provided. The results indicate that the maximum methane adsorption capacities of various rank coals after CO2 exposure increase by 3.45%−10.37%. However, the maximum CO2 adsorption capacities of various rank coals decrease by 9.99%−23.93%. TG and pore structure analyses do not observe the obvious changes on the inorganic component and pore morphology of the coals after CO2 exposure. In contrast, CO2 exposure makes changes in surface chemistry of the coals, according to the results from FTIR analysis, which is the main reason for increases in the maximum adsorption capacity of methane and decreases in the maximum adsorption capacity of CO2 for the coals after CO2 exposure. The different role of CO2 exposure on methane and CO2 adsorption is detrimental to CO2-ECBM. Thus, the implementation of CO2-ECBM must take into account the influence of CO2 exposure on the adsorption performance of the target coal seams.

1. INTRODUCTION Global warming is mainly caused by the most important anthropogenic greenhouse gas, carbon dioxide (CO2), which is harmful to the environment and human society.1 To mitigate CO2 emissions into atmosphere, CO2 capture and sequestration (CCS) is considered as a potential pathway.2 As a critical ingredient of CCS, onshore geologic sequestration is available for storing CO2 in a geological time scale. The alternative geological formations include deep saline aquifers, depleted petroleum and gas reservoirs, and unminable coal seams.3 Among above, CO2 sequestration in coal seams with enhanced coalbed methane recovery (CO2-ECBM) has received broad attention.4 It is found that the injected CO2 first flows in primary fracture and then is adsorbed in coal matrix. Quantum chemistry calculation shows that the Lennard-Jones (6−12) potential well of CO2 is deeper than that of methane (see Figure S1(a) in the Supporting Information). Thus, the adsorbed CO2 molecules can replace methane when CO2 adsorption occurs (Figure S1(b) in the Supporting Information). The methane recovery from the target coal seams can compensate for the cost of CO2 sequestration. Hitherto, numerous works were performed to study the single or binary adsorption behavior of methane and CO2 on coals under the simulated reservoir conditions. Both adsorption equilibrium and kinetics studies show that the adsorption capacity and the adsorption rate of CO2 are higher than those of methane.5−9 Also, investigations on CO2 injection into methane-saturated coal seams further support the feasibility of methane recovery by CO2 sequestration.10−12 © 2015 American Chemical Society

Adsorption of methane or CO2 on coal will cause coal matrix swelling. The coal matrix swelling induced by CO2 is greater than induced by methane.13 Further studies show that coal matrix swelling is a heterogeneous process and is related to the coal lithotypes.14,15 The coal matrix swelling compresses the cleat space in coal seams, which will result in a decrease in permeability of coal seams and bring harm to CO 2 injection.16,17 The optimum depth is recommended as 800−1000 m for CO2 sequestration in coal seams, where CO2 is denser enough to save the storage space.3 Thus, the temperature and pressure of coal seams corresponding to the optimum depth are usually far above the critical temperature and pressure of CO2 (Tc = 31.05 °C, Pc = 7.3 MPa).18 The injected supercritical CO2 fluid is capable of extracting hydrocarbons from coal matrix, which will show possible influences on coal property. Mazumder et al.19 carried out static and dynamic extraction experiments toward coal exposure to CO2. They proposed that the reaction of coal and CO2 (i.e., coal + CO2 = CO) was a great possibility. Based on density functional theory (DFT), Huang et al.20 found that the formation of hydrogen bonds between CO2 and the oxygen-containing functional groups on coal surface could enhance the interaction of CO2 with coal. Cao et al.21 adopted advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy to explore the effect of CO2 exposure on coal Received: January 12, 2015 Revised: May 4, 2015 Published: May 11, 2015 3785

DOI: 10.1021/acs.energyfuels.5b00058 Energy Fuels 2015, 29, 3785−3795

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Energy & Fuels

500 extraction system mainly consists of a pressure-charging unit, an extraction unit, and a collection unit. The pressure-charging unit equipped with a high-pressure P-series pump is capable of generating CO2 fluid with a maximum pressure of 60 MPa and a maximum mass flow rate of 30 g min−1. The extraction unit contains a 500 mL extraction vessel and the maximum operation pressure of the extraction vessel is ∼50 MPa. An automated back-pressure regulator is used to maintain the pressure of the extraction vessel as a constant. The extract phase formed by the extraction process can be obtained from the collection vessel. The inner temperature of both the extraction vessel and the collection vessel can be regulated within a range of room temperature to 150 °C. The SFE-500 extraction system is an automatic device, and all the operations are controlled by a workstation that was installed on the computer. Practical CO2-ECBM is accompanied by CO2 injection in the injection well and by methane recovery from the production well (see Figure S1(b) in the Supporting Information). As illustrated in Figure S1(b) in the Supporting Information, the extraction vessel and collection vessel can be treated as the injection well and the production well, respectively. Thus, the interactions of CO2 with coals operated in the SFE-500 extraction system are similar to CO2ECBM and the conclusions are meaningful to the implementation of CO2-ECBM. Prior to interaction, all the coal samples were dried at 105 °C for 24 h in a vacuum oven. In this work, 100 ± 0.0005 g of coal particles after crushing and dry treatment was used in each experiment. The operating temperature of the extraction vessel was 45 °C, which represented the reservoir temperature corresponding to a depth of ∼1000 m, which is the optimum depth window.4 The pressure of the extraction vessel was 12 MPa, and the mass flow rate of CO2 injection was 10 g min−1. The process of interaction of CO2 with coal is also equivalent to the adsorption process. Goodman et al.26 summarized many adsorption works and concluded that the equilibrium time of CO2 adsorption was between 0.5 h and 12 h. In this work, it is found that 12 h can ensure CO2 adsorption on coal to reach the complete equilibrium state. Thus, 12 h was chosen as the duration of the interaction between coal and CO2. At the end of interaction, all the CO2 exposed coal samples were collected for characterizations and adsorption measurement. 2.3. High-Pressure Adsorption Test. In this work, a volumetric method was applied to test the high-pressure methane and CO2 adsorption behavior of the coal samples before and after CO2 exposure. The main scheme diagram of the entire apparatus is illustrated in Figure S2(a) in the Supporting Information. All the cells, valves, and connected tubings were placed in an air oven. The adsorption measurement is sensitive to temperature and pressure.27,28 Thus, an air oven with temperature fluctuation range of less than ±0.1 °C (UN450, Memmert GmbH & Co. KG, Germany) and highprecision pressure transducers with a precision of 0.05% of the fullscale value 20 MPa (Super TJE, Honeywell International, Inc., USA) were used. The UN450 air oven employs natural convection to ensure the temperature homogeneity and stability. For each run of adsorption testing, all the cells were placed in the same location within the air oven to guarantee the adsorption temperature as a constant. The operation procedure in volumetric method has been presented in our previous work.6 The experimentally measured variable, called Gibbsian surface excess (GSE, mmol g−1), was calculated by26,29

lithotypes. They reported that fusain and vitrain after highpressure CO2 exposure exhibited an increasing trend in the fraction of alkyl carbons but a decreasing trend in aromaticity, whereas this trend was opposite for bright clarain and clarain. Kolak et al.22,23 and our previous work24 studied interactions of high-pressure CO2 with coals using a supercritical fluid extraction apparatus. They documented that supercritical CO2 fluid had the capability to mobilize aliphatic and aromatic hydrocarbons from coal matrix and further emphasized that the resulting environmental issues needed extensive attention for practical CO2-ECBM. For CO2-ECBM, the accurate determination of the amount of methane and CO2 adsorption is helpful to assess the storage capacity of CO2, as well as the amount of recoverable methane of the target coal seams. The above literature reviews show that complex interactions between coal and CO2 do exist, but less is known about the influence of supercritical CO2 exposure on the adsorption performance of coal. Thus, investigations on CO2 exposure dependence of high-pressure methane and CO2 adsorption on various rank coals were conducted in this work. Characterization methods including Fourier transform infrared (FTIR) spectroscopy, thermogravimetry (TG), and pore morphology analyses were also applied to provide information about the possible mechanism behind the effect of CO2 exposure on the adsorption performance of coal.

2. EXPERIMENTAL SECTION 2.1. Samples Collection and Preparation. Four coal samples collected from northwest and north China were used in this study: HB coal (from Hulunbuir League), SM coal (from the city of Yulin), ED coal (from the city of Erdos), and YQ coal (from the city of Yangquan). All the coal samples were preserved in sealed containers full of helium (He) to prevent undesired physical and chemical changes that are due to atmospheric oxidation.25 To accelerate the interaction between coal and CO2, all the coal samples were carefully crushed and sieved to generate coal particles with a diameter range between 125 μm and 150 μm. Proximate analyses (ash, volatile matter, and fixed carbon) were performed according to the China’s National Standard GB/T 212-2008. The equilibrium moisture content and the maximum vitrinite reflectance coefficient, denoted as Ro,max, were determined by following the American Society for Test and Materials (ASTM) standard test methods of ASTM D1412-93 and ASTM D2798, respectively. The analysis results were presented in Table 1. HB coal, SM coal, and ED

Table 1. Characteristics of Coal Samplesa sample

HB

SM

ED

YQ

ash, dry basis (wt %) volatile matter, dry basis (wt %) fixed carbon, dry basis (wt %) equilibrium moisture (wt %) Ro, max (%)

13.26 35.40 51.35 18.18 0.77

10.20 31.47 58.34 9.73 0.88

4.31 30.96 64.73 11.93 0.93

19.42 8.92 71.68 5.31 2.62

a

Table legend: HB, coal from Hulunbuir League, PRC; SM, coal from the city of Yulin, PRC; ED, coal from the city of Erdos, PRC; and YQ, coal from the city of Yangquan, PRC.

ΔGSE =

coal are of high volatile matter content and low level of fixed carbon (Ro,max = 0.77%−0.93%), which can all be classified as bituminous coals. In contrast, YQ coal has the lowest volatile matter content and the highest fixed carbon and Ro,max value, which is categorized as anthracite. 2.2. Exposure of Coal to CO2. The process of coal exposure to supercritical CO2 was simulated using a Model SFE-500 extraction system (Thar Process, Inc., U.S.A.). As depicted in Figure 1, the SFE-

PV PV ⎞ PV 1 ⎛ PV ⎜ 1 V + 2 RC − 3 RC − 4 V ⎟ mRT ⎝ Z1 Z2 Z3 Z4 ⎠

(1)

where m is the mass of the coal sample (given in grams); R is the universal gas constant (R = 8.314 J mol−1 K−1); T is the operation temperature (given in Kelvin) (in this work, each adsorption test was operated at 318.15 K (45 °C)); P1 and P2 are the initial pressures of sample and reference cells, respectively (given in units of Pa); P3 and P4 are the final pressures of reference and sample cells at the adsorption equilibrium state, respectively (given in units of Pa); VRC and VV are the volume of the reference cell and the void volume of 3786


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Figure 1. Schematic diagram of the supercritical CO2 fluid extraction apparatus.

Figure 2. Experimental and predictive GSE adsorption isotherms for methane (T = 45 °C).

temperature T, respectively. Based on the relative deviation analysis shown in Figure S3 in the Supporting Information, Wagner and SpanEoS and Span and Wagner-EoS of high predictive accuracy were

sample cell loaded with coal sample calibrated by helium, respectively (given in milliliters); Z1, Z2, Z3, and Z4 are methane or CO2 compressibility factors corresponding to P1, P2, P3, and P4 at 3787


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Figure 3. Experimental and predictive GSE adsorption isotherms for CO2 (T = 45 °C). Considering the independent variables incorporated in the GSE calculation shown in eq 1, the total error of GSE was calculated as follows:

chosen to generate the compressibility factors of methane and CO2, respectively.18,30 The operation procedures of ΔGSE measurement were repeated for gradually increasing the pressure of methane or CO2. Thus, the total amount of the adsorption at the end of the nth step was calculated as

2 2 ⎧ 2 ⎪ ∂ΔGSE ∂ΔGSE ∂ΔGSE σGSE = ⎨ σm2 + σV 2 + σV 2 ⎪ m V V ∂ ∂ ∂ RC V ⎩

n

GSEn =

∑ ΔGSEi i=1

2

(2)

Note that the equilibrium pressure tolerance is 0.5 psia and the equilibrium state lasts for at least 4 h. Pressure stability tests of reference and sample cells (shown in Figures S2(b) and S2(c) in the Supporting Information, respectively) indicate that the adsorption equilibrium state is easy to estimate. Mohammad et al.31 found that ratio of VV/VRC = 2.0 yielded the lowest error for GSE determination. Thus, 40 ± 0.0005 g of dry coal sample before or after CO2 exposure was used to determine methane and CO2 adsorption isotherms. Before the adsorption test, all the samples were entirely degassed under vacuum condition for 24 h at 105 °C. The experimental error of the adsorption data was estimated based on multivariate error propagation theory. For a given function y, the uncertainty in function y, designated as σy, was given by32 n

σy 2 =

∑ i=1

2

∂ΔGSE ∂ΔGSE ∂ΔGSE σT 2 + σP 2 + σP 2 ∂T ∂P1 ∂P2

+

1/2 2 2 ⎫ ⎪ ∂ΔGSE ∂ΔGSE σP 2 + σP 2⎬ ⎪ ∂P3 ∂P4 ⎭

(4) Repeatability test was also performed in our work. As can be seen in Figure S4 in the Supporting Information, the standard deviation of methane adsorption on coal is within a range of 0.0013−0.0103 mmol g−1, which confirms that the experimental repeatability for adsorption measurement is fairly good. 2.4. Thermal Analysis. Thermal analyses of coal samples were carried out using a thermal analyzer (Model STA-449F3, Netzsch Group, Germany). Approximately 10 ± 0.0005 mg of dry coal sample was weighed and loaded into an open alumina crucible for each run. The sample was first heated from ambient temperature to 105 °C. Maintained isothermally after 20 min, the sample was then heated to a final temperature of 815 °C at a fixed heating rate of 30 °C min−1 in an

2

∂y σxi 2 ∂xi

2

+

(3) 3788


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Energy & Fuels air atmosphere. The volume flow rates of air and helium, as a protective gas, were 30 mL min−1 and 15 mL min−1, respectively. 2.5. Mesopore and Macropore Morphology Analysis. The mesopore and macropore structure of the coal sample was determined with a ASAP 2020 system (Micromeritics Instruments, USA), using a low-temperature nitrogen (N2) adsorption and desorption method. Prior to analysis, all the samples were degassed under vacuum at 150 °C for 12 h to effectively remove residual gas and moisture. The N2 adsorption and desorption isotherms at −196.15 °C were collected at relative pressures (P/P0) ranging from 0.005 to 0.995. The pore structure parameters can be calculated based on the N2 adsorption and desorption data. Detailed calculation can be found in ref 33. 2.6. Micropore Morphology Analysis. The ASAP 2020 system was also used for micropore morphology analysis. The micropore morphology of the coal sample was determined by CO2 adsorption at 0 °C. The micropore surface area and volume were estimated using a Dubinin−Rudushkevich (D-R) model. The pore size distribution (PSD) was calculated using a nonlocal density functional theory (NLDFT) model. 2.7. FTIR Analysis. Chemical properties of both raw and CO2 exposed coals were determined using FTIR spectrometry (EQUINOX 55, Bruker Corp., Germany). The coal samples and the dried KBr were ground at a mass ratio of 1:100. The spectra were obtained with 32 scans at a resolution of 4 cm−1.

Table 3. Fitting Results of the Ono−Kondo Lattice Model of Coal for CO2 Adsorption at 45 °C sample HB

SM

ED

YQ

HB

SM

ED

YQ

εA (k)

am (mmol g−1)

q0 (kJ mol−1)a

R2

before CO2 exposure after CO2 exposure

−1153.11

0.41

17.74

0.9989

−1177.64

0.43

17.94

0.9983


−1023.86

0.64

16.66

0.9967

−987.42

0.70

16.36

0.9970


−1087.43

0.63

17.19

0.9966

−1071.12

0.67

17.06

0.9956


−1247.84

0.65

18.52

0.9992

−1219.99

0.71

18.29

0.9973

state

am (mmol g−1)

q0 (kJ mol−1)a

R2

−984.16

1.56

25.33

0.9951

−1180.38

1.40

26.96

0.9932


−1192.71

1.29

27.06

0.9941

−1357.80

0.98

28.43

0.9968


−1167.57

1.35

26.85

0.9985

−1343.44

1.06

28.31

0.9981


−1183.64

1.61

26.98

0.9882

−1332.83

1.31

28.22

0.9956


Isosteric heat of adsorption generated from the expression q0 = qV − εANA,38 where qv is the enthalpy of vaporization at the boiling point and NA is Avogadro’s constant. a

Table 2. Fitting Results of the Ono−Kondo Lattice Model of Coal for Methane Adsorption at 45 °C sample

εA (k)

state

Isosteric heat of adsorption generated from the expression q0 = qV − εANA,38 where qv is the enthalpy of vaporization at the boiling point and NA is Avogadro’s constant. a

3. RESULTS AND DISCUSSION 3.1. GSE Adsorption Isotherms. The GSE isotherms of methane and CO2 adsorption on the coals are shown in Figures 2 and 3, respectively. All the GSE adsorption isotherms for methane show an increasing trend with the equilibrium pressure up to ∼8.50 MPa. Further increases in pressure start to decrease the amount of GSE adsorption slightly. By contrast, a pronounced maximum at a pressure of ∼7.0 MPa is found in the GSE isotherms for CO2 adsorption on coals before and after CO2 exposure (Figure 3). The pronounced maximum has been widely reported,7,34,35 and the detailed explanation can be found elsewhere.6 Although supercritical CO2 exposure makes no change to the shape of the GSE isotherms for both methane

Figure 4. Maximum adsorption capacities of methane and CO2: (a) methane and (b) CO2. 3789


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and CO2 and to determine the equilibrium relationship between fluid and various coals, the Ono−Kondo lattice model was employed. In comparison with the classical adsorption models, such as the Langmuir model, the Brunauer−Emmett−Teller (BET) model, and the Dubinin− Astakhov (D-A) model, the Ono−Kondo lattice model can describe the GSE isotherm directly and is proven to have good predictive accuracy for various supercritical adsorption systems, including methane adsorption on graphon or activated carbon, hydrogen adsorption on activated carbon, and CO2 adsorption on coal.36−38 The form of the simplified Ono−Kondo lattice model was expressed by6 ρb GSE Figure 5. Ash yields of various coals.

2

−1 a

−1 b

3

c

state

SBET (m g )

Vt (cm g )

DA (Å)

HB


4.95 5.21

0.016 0.014

128.35 109.62

SM


5.42 4.71

0.007 0.006

55.11 52.52

ED


5.11 5.15

0.007 0.007

52.90 54.56

YQ


3.85 3.60

0.006 0.006

61.39 65.21

Table 5. Micropore Parameters of Coal Samples

SM

ED

YQ

( kTε )⎤⎦(ρmc − ρb) ε ρmc exp( kT ) + ε 2am⎡⎣1 − exp( kT )⎤⎦ A

specific surface area (m2 g−1)

pore volume (cm3 g−1)


162.49

0.033

153.73

0.032


163.32

0.031

153.32

0.030


134.15

0.026

134.42

0.026


173.39

0.035

164.56

0.033

(5)

where k is the Boltzmann’s constant (k = 1.38 × 10−23 J mol−1 K−1); T is the adsorption temperature (given in Kelvin); εA is the interaction energy between the adsorbate and the adsorbent (given in units of J mol−1); am is the monolayer adsorption capacity; and ρb and ρmc are the bulk density of the adsorbate and the density at the maximum adsorption capacity, respectively (given in units of mol m−3). The adsorption phase is always regarded as a pseudo-liquid state; therefore, the densities of methane (0.421 g mL−1, i.e., 26312.5 mol m−3)39 and CO2 (1.227 g mL−1, i.e., 27886.36 mol m−3)40 at the boiling point under atmospheric pressure are assigned to ρmc. Detailed derivation of eq 5 can be found elsewhere.6 For eq 5, ρb/(ρmc − ρb) and ρb/GSE can be designated as an independent variable and a dependent variable, respectively. Thus, the parameters of am and εA can be easily generated using a linear regression method. Figures 2 and 3 show that the Ono−Kondo lattice model fits the experimental data very well. The fact that all the correlation coefficients (R2) are greater than 0.99 (listed in Tables 2 and 3) indicates that the lattice model can well describe the behaviors of methane and CO2 adsorption on each coal sample. The data regarding the isosteric heat of adsorption (q0) of methane (16.36−18.52 kJ mol−1) and CO2 (25.33−28.43 kJ mol−1) shown in Tables 2 and 3 are very consistent with that obtained from previous investigations.38,41 The fact that the isosteric heat of CO2 is greater than that of methane means that coal has a stronger affinity for adsorption of CO2 than for the adsorption of methane. In addition, compared with the monolayer adsorption capacities (am) of methane and CO2 shown in Tables 2 and 3, the am value of CO2 is larger than that of methane for both the raw and CO2-exposed coals, which means that the adsorption strength of CO2 is superior to that of methane. Because the adsorption temperature and pressure conducted in this work are far beyond the critical parameters of methane and CO2, the adsorption process is categorized as supercritical adsorption. Aranovich et al.38 indicated that the adsorption of a supercritical fluid had a two-layer character. Thus, the maximum adsorption capacity (n0) was given as

a Brunauer, Emmett and Teller (BET) specific surface area. bSinglepoint adsorption total pore volume. cAdsorption average pore width.

HB

2am⎡⎣1 − exp

A

sample

state

ρb ρmc

A

Table 4. Mesopore and Macropore Parameters of Coal Samples

sample

=

and CO2, CO2 exposure has an enormous impact on the adsorption capacity of various coals. Specifically, CO2 exposure strengthens the methane adsorption capacities of all the coals; instead, the CO2 adsorption capacities of all the coals decrease after CO2 exposure. 3.2. Ono−Kondo Lattice Model Fitting. GSE isotherms cannot fully represent the true amount of adsorption. To further obtain the maximum adsorption capacities of methane

n0 = 2am 3790

(6) DOI: 10.1021/acs.energyfuels.5b00058 Energy Fuels 2015, 29, 3785−3795

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Figure 6. Micropore size distribution profiles of coal samples: (a) HB, (b) SM, (c) ED, and (d) YQ.

tion.33,43 Coal, as an extremely complex mixture, is mainly comprised of organic and inorganic structures. Specifically, the three-dimensional macromolecular network of coal contains condensed hydroaromatic and aromatic compounds, which are linked through ether linkages, thioether linkages, and alkyl bridges.44 The inorganic matter mainly refers to mineral or ash. Both organic and inorganic properties determine the adsorption capability of coal. Qu et al.45 revealed that mineral or ash might prevent methane or CO2 from adsorbing on the organic structure of coal. Thus, ash yield analysis was first performed to investigate the effect of CO2 exposure on methane and CO2 adsorption capacity of coal. The ash yields of various coals were determined using a thermogravimetry (TG) analyzer. As can be seen in Figure 5, no obvious change of ash yields is observed between the raw and CO2-exposed coals. Sakurovs et al.42 also observed the same phenomenon in their research work. When compared with most organic components, the solubility of the inorganic components (such as oxides, silica, silicates, carbonates, sulfates, phosphates, aluminosilicates, and pyrites) is not good in supercritical CO2 fluid, which is probably due to the high polarity of metal ions.46 Therefore, there is not much change in the ash yield presented in coal after CO2 exposure. In addition to the inorganic structure of coal, the dependence of CO2 exposure on the organic structure, including pore morphology and surface chemistry, was further studied. Most investigations have shown that the pore structure of coal can strongly influence the fluid adsorption.47,48 In this work, N2 adsorption and desorption at −196.15 °C were carried out for coal pore analysis. The conventional pore morphology characterization of N2 adsorption and desorption only provides mesopore or macropore information for coal, because N2 molecules do not have sufficient energy to enter into the ultrafine micropore or micropore structure of coal, because of the activated diffusion process and shrinkage of pores at the extremely low temperature of −196.15 °C.49 Based on the low-

Table 6. Fractal Dimension Values (D) of Coal Samples sample

state

D

R2

HB


2.5014 2.5232

0.9871 0.9843

SM


2.6800 2.6831

0.9499 0.9700

ED


2.7267 2.7137

0.9518 0.9761

YQ


2.5760 2.5897

0.9571 0.9239

The maximum adsorption capacities of methane and CO2 are plotted in Figure 4. Figure 4a shows that the maximum methane adsorption capacities of various coals after CO2 exposure increase by 3.45%−10.37%. However, for CO2 adsorption on various coals (Figure 4b), CO2 exposure leads to a decrease in the maximum adsorption capacity (by 9.99%− 23.93%). The effect of CO2 exposure on the adsorption performance of coal is greatly dependent on the exposure pressure of CO2. For CO2 exposure pressure of 12 MPa, the supercritical CO2 fluid has great extraction power, which will affect the adsorption performance of coal. In contrast, Sakurovs et al.42 observed no significant effect of CO2 exposure on the adsorption performance of coal when the exposure pressure was maintained as low as 0.3 MPa. 3.3. Effect of CO2 Exposure on Coal Property. Most works that are focused on adsorption science have shown that the properties of the adsorbent and the adsorbate, as well as the operating parameters (such as temperature and pressure), affect adsorption. Among them, the pore structure and the surface chemistry of adsorbent play important roles during adsorp3791


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Figure 7. FTIR spectra of various coals: (a) HB, (b) SM, (c) ED, and (d) YQ.

Figure 8. Possible effect of CO2 exposure on CO2 sequestration into methane-saturated coal seams.

obtained from N2 adsorption and desorption isotherms. The surface fractal dimension was given by the Avnir equation:56

temperature N2 adsorption and desorption isotherms, the mesopore and macropore morphology parameters of coal samples, including specific surface area, pore volume, and pore size, could be generated. The data listed in Table 4 show that CO2 exposure has only a weak influence on the mesoporous and macroporous structure of various coals. Combining the results of Table 5 and Figure 6, it is also shown that CO2 exposure cannot significantly change the micropore morphological properties of coal, such as specific surface area, pore volume, and pore size distribution. The pore system of coal is extremely complex.50,51 Thus, a comprehensive evaluation tool is still in demand. In this work, fractal theory was adopted to describe the complicated random structure within a piece of coal.52 A fractal object is of scale invariance and self-similarity.53 Hitherto, fractal theory has been used widely to describe coal structure.54,55 The surface fractal dimension, which is a critical parameter in fractal theory, can be

⎡ ⎛ P ⎞⎤ D − 3 V = K ⎢ln⎜ 0 ⎟⎥ ⎣ ⎝ P ⎠⎦ Vm

(7)

where V is the adsorption volume of N2 at a relative pressure of P/P0 (given in units of cm3 g−1); Vm is the monolayer adsorption volume of N2 calculated from the BET model (given in units of cm3 g−1); K is a characteristic constant; P and P0 are the partial pressure and saturated vapor pressure at −196.15 °C, respectively; and D is the surface fractal dimension. Table 6 shows that the multiple correlation coefficients (R2) of various coal samples are >0.9239. The fractal dimension (D) of each coal sample varies between 2.5014 and 2.7267, which means that the pore morphology of the coal surface is irregular and fractal.57 The fractal dimension of coal after CO2 exposure 3792


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Figure 9. Preferential adsorption ratio of CO2 to methane: (a) HB, (b) SM, (c) ED, and (d) YQ.

3.4. Implications for CO2 Sequestration in Coal Seams. CO2-ECBM will not only benefit mitigating CO2 emissions, but also increase methane recovery. However, in consideration of the characteristics of CO2 fluid and the target coal seams, complex interactions of coal and CO2 should be considered. On one hand, previous studies have found that CO2 adsorption on the micropore surface contained in coal matrix can induce coal matrix swelling, which will result in a decrease in the permeability of the reservoir system and the CO2 injection rate.66−68 On the other hand, Kolak et al.22 and our previous works24 showed that the CO2 stored within the coal seams was a supercritical fluid and had the ability to free toxic alkane and polycyclic aromatic hydrocarbons (PAHs) from the coal matrix. Thus, the possible environmental safety and health (ES&H) issues stemming from CO2 sequestration are of great concern. In this work, single-component adsorption tests testify that CO2 exposure acts in different roles, with regard to the methane and CO2 adsorption capacities of various coals. Based on this conclusion, it is deduced that the injected high-pressure CO2 will improve in situ methane adsorption and weaken CO2 adsorption performance of coal, which may hinder CO2 adsorption and methane desorption, as shown in positions 1, 2, 3, 4, and 5 in Figure 8. It is accepted that GSE does not represent the actual amount of adsorption. Therefore, the actual amount of adsorption, which is called the absolute adsorption amount, is very important for CO2-ECBM. To integrate the above different role of CO2 exposure on methane and CO2 adsorption, the preferential adsorption ratio of CO2 to methane using the absolute adsorption amount was adopted. The absolute adsorption amount (nabs) was defined as

is approximate to that of the raw coal. Thus, exposure to CO2 cannot distinctly change the pore structure of the coal. This conclusion is consistent with other investigations. Applying both adsorption and scanning electron microscopy (SEM) methods, Gathitu et al.58 found that CO2 exposure cannot significantly change the micropore surface and volume of dry bituminous coal. Kutchko et al.59 also concluded that CO2 exposure made no obvious change to the pore structure of Pittsburgh and Sewickly coal samples, according to fieldemission scanning electron microscopy (FE-SEM) analysis. Both inorganic components and pore morphology of coals change weakly, because of CO2 exposure; thus, the effect of CO2 exposure on adsorption capability of coal is most probably related to the alteration of surface chemistry. Lu et al.60 found that some functional groups (−H, −NH2, −OH, −COOH) could significantly affect methane and CO2 adsorption on carbon materials at pressure up to 20 MPa. Based on the results of molecular simulation, Liu et al.61 also indicated that the oxygen-containing functional groups could influence CO2 adsorption in microporous and mesoporous carbons at pressures up to 25 MPa. Thus, FTIR analysis was used to determine the surface chemistry of various coals. Figure 7 indicates that the oxygen-containing functional groups (wavenumbers of 1000−1800 cm−1)62 of the CO2 exposed coals are less than that of the raw samples. The decrease in the oxygencontaining functional groups is probably due to the extracting ability or reactivity of supercritical CO2 fluid, and further work will be performed to verify this deduction. For methane adsorption, previous investigations have found that the oxygencontaining functional groups on coal surface do not favor methane adsorption.60,63 However, the opposite relationship is applicable for CO2 adsorption reported by Nishino64 and Liu et al.65 Thus, the effect of CO2 exposure on the oxygen-containing functional groups on coal surface may account for the interchange of the maximum adsorption capacities of methane and CO2.

nabs

3793

⎡ xb ⎢ = 2am⎢ ⎣ xb + (1 − xb) exp

( kTε ) A

⎤ ⎥ ⎥ ⎦

(8)


Article

Energy & Fuels

(Grant No. KKSY201205160). We appreciate Mr. Zengmin Lun and Mrs. Xia Zhou (Exploration and Production Research Institute, SINOPEC) for their assistance. We also appreciate Dr. Yanping He (Faculty of Chemical Engineering, Kunming University of Science and Technology) and Dr. Chuigang Fan (State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences) for their help with the English editing of this paper.

where

xb =

ρb ρmc

(9)

Figure 9 presents the preferential adsorption ratio of CO2 to methane, which is defined as nabs,CO2/nabs,methane (calculated from eq 8). Although all the preferential adsorption ratios of CO2 to methane are >1, the preferential adsorption ratios of the coals after CO2 exposure decrease, when compared to the raw samples (except HB coal at equilibrium pressures of

Influence of CO2 Exposure on High-Pressure ... - ACS Publications

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