Sci. Bull. (2015) 60(5):532–540 DOI 10.1007/s11434-015-0747-6

www.scibull.com www.springer.com/scp

Article

Chemistry

The role of volatiles and coal structural variation in coal methane adsorption Wenjing Sun • Ning Wang • Wei Chu Chengfa Jiang

•

Received: 23 December 2014 / Accepted: 19 January 2015 / Published online: 10 February 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015

Abstract We investigated the role of volatiles in the porous structure of coal samples and the corresponding structural deformations that affect the coals’ methane adsorption capacity. For this study, the volatiles in coal were gradually removed by extraction. Changes in the crystal, textural, and porous structures were identified by means of thermogravimetric analysis, X-ray diffraction, and N2 adsorption/desorption. Changes in the methane adsorption behavior before and after volatile removal were investigated. It was found that changes in methane adsorption could be attributed to volatile-related deformations in the coal’s porous structure. Microstructural characterizations indicated that the volatiles could be found in two states within the coal, either trapped in the pores, or cross-linked in the network. The former played an important role in constructing the pore spaces and walls within the coal and affected the accessibility of gases. The latter cross-linked state retained the volatiles within the macromolecular coal structural network. This state affected coal–coal interactions, which was a factor that controlled the crystal structure of coal and contributed to the number of porous deformations.

Electronic supplementary material The online version of this article (doi:10.1007/s11434-015-0747-6) contains supplementary material, which is available to authorized users. W. Sun  N. Wang  W. Chu (&)  C. Jiang Department of Chemical Engineering, Sichuan University, Chengdu 610065, China e-mail: [email protected] W. Sun Guangdong Medical College, Dongguan 523808, China

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Keywords Coal  Volatile components  Structural deformations  Aggregation state  Methane adsorption

1 Introduction Enhanced coal bed methane (ECBM) technologies have developed rapidly as methods to extract methane from coal. During the ECBM process, gases, such as CO2 and N2, are injected into coal seams, where they displace methane by diffusing into the pores and adsorbing onto pore surfaces. This technology is vital for mine safety, environmental protection, and coal mine methane recovery (CMR) [1, 2]. The coal structure plays an important role in methane diffusion and the adsorption–desorption mechanism [3–6]. The pore properties (pore volume and size distribution) determine the methane adsorption capacity of the coal [7–17]. Based on the coal’s storage capacity and the recoverability of coal bed methane (CBM), Shi classified coal pores into two categories: adsorption pores (pore diameters \ 100 nm) and seepage pores (pore diameters [ 100 nm). Adsorption pores consist of micropores (\10 nm) and mesopores (10–100 nm). The mesopores play an important role in coal gas storage and diffusion [18, 19]. The structure of coal is a heterogeneous, three-dimensional, macromolecular network, consisting of fused aromatic ring clusters and a small amount of volatile constituents [20–22]. Bae et al. [23] proposed that the volatile constituents found inside pores play an important role in building the pore spaces and pore walls. These volatile constituents are also unstable and can change the porous system as they degrade. For example, Mastalerz et al. [24] found dehydration and oxidization of the volatile constituents in coal samples stored under weathering

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conditions. In this weathered coal, they observed altered porous structures and methane adsorption capacity. Karacan [25], Larsen [26], Kolak and Burruss [27] found that carbon dioxide could also extract polycyclic aromatic hydrocarbon species from the coal bed and change the coal’s adsorption capacity. Therefore, volatile components should be considered when investigating the porous structure of coal. Coal is strained at the molecular level, and intermolecular attractions within the coal’s structural network determine its physical and chemical aggregation states [28, 29]. Physically, the amorphous carbon-based framework of coal is composed of stacked carbon structures. The alignment of these structures generates the coal’s aggregate, crystal structure. The aggregation states reflect the stability of the carbonaceous coal matrix [30, 31]. Chemically, coal consists of three-dimensional, cross-linked macromolecule networks [32]. These networks are associated by covalent bonds and non-covalent interactions, such as hydrogen bonds, ionic bonds, hydrophobic attractions, and dipole– dipole attractions. The strength of these interactions leads to the various chemical aggregation states of coal [33–36]. The different aggregation states and flexibility of the coal structures contribute to the swelling phenomenon observed in many gas-injecting tests. The aggregated coal structure has been investigated using chemical methods. Takanohashi et al. [30, 31] prepared coal samples with different aggregation states by extracting Argonne premium coal with a solvent composed of carbon disulfide, N-methyl-2-pyrrolidinone (CS2–NMP), acetone, and pyridine. They found that the adsorption capacity of the solvent-extracted coal was enhanced due to newly developed pores in these less aggregated structures. This indicates that the porous structure is related to the aggregate structure of the coal. Additionally, Shui et al. [34, 36] showed that lighter constituents inside the coal network play important roles in determining it aggregate structure. The stability of the volatile component of coal is very important because of the role it plays in building coal pore networks and structural deformations. It is still unclear how volatile compounds affect the aggregated structure and pore networks of coal. It is also unclear whether these structural changes influence gas storage in coal. The nature

of these relationships needs to be confirmed. The objective of this study was to investigate methane adsorption behavior in coal and its relationship to coal structure, with a focus on the influence of the volatile coal components. The deformation of the porous coal structures and aggregation states was investigated to examine the effect of varying the volatile components. To accomplish this, volatile components were gradually removed using three solvents with increasing polarity. To monitor the possible structural changes, the structure was characterized using the following techniques. Thermogravimetric analysis (TGA) was employed to measure the amount of volatile components removed, and nitrogen adsorption and desorption at -196 °C were performed to measure the pores’ textural properties, and X-ray diffraction (XRD) was used to detect the crystal structure of the samples. The main factor that dominating the methane adsorption capacity of the coal samples was then identified.

2 Experimental Two types of Chinese bituminous coal (coal A and coal B), whose proximate and ultimate analyses are listed in Table 1, were cracked and sieved to 120–250 lm for use in the present study. Ash in coal composed of carbonate minerals, sulfate, and silicate [37]. These components distribute in the coal pores, blocking them and influencing the methane adsorption capacity [38]. To eliminate the influence of ash on coal porous structures and the methane adsorption capacity, demineralization was carried out before the devolatilization process. The coal samples were first demineralized using HF/HCl (1:3, v:v) to remove the effect of ash. The solids were dried at 100 °C for 8 h before further experimentation to remove moisture. Pyrolysis of volatiles occurs when the temperature was above 200 °C, and thus, the drying process cannot remove the volatile coal constituents. The demineralized coal (denoted as D) samples were extracted with carbon disulfide (CS2), tetrahydrofuran (THF), and pyridine in sequence using a Soxhlet extractor. After extraction, the residue was washed with acetone and

Table 1 Proximate analysis and ultimate analysis of coal Samplea

Proximate analysis (wt%)

Ultimate analysis (wt%)

VM

Ash

FC

C

H

N

S

Ob

Coal A

29.92

1.93

68.15

79.7

5.1

1.6

1.5

12.1

Coal B

21.42

0.81

77.77

89.7

4.0

1.4

0.8

4.1

a b

Demineralized samples By difference

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deionized water. The samples were then dried at 100 °C for 6 h. The extraction yield was calculated based on the weight of the residue. The CS2, THF, and pyridine residues were labeled C, T, and P, respectively. Methane adsorption experiments were carried out using a volumetric method [11–15]. A schematic of the experimental setup is shown in Fig. S1 (online). The accuracy of the pressure transducer was 0.05 % of the full-scale value. The experimental devices were placed in a temperaturecontrolled oven to ensure constant temperature (±0.1 °C) throughout the experiments. Our detailed experimental method is described in the supplemental materials. The crystal structures of the coal samples were characterized using XRD. Tests were performed on a DX-1000 diffraction monochromatic with Cu Ka radiation between 5° and 80° in continuous scanning mode and a scan rate of 2°/min. The mean interlayer spacing was calculated according to the Bragg equation [16]: d002 ¼

k ; 2 sinðh002 Þ

where h is the Bragg angle of the diffraction maximum, ˚. k = 1.5418 A The porous structure of the demineralized coal and coal residues were analyzed on a Quantachrome NOVA 1000e auto-sorption instrument. We used N2 adsorption at -196 °C after the samples were degassed at 110 °C for 14 h. The total pore volume (Vtotal) was derived from the amount of vapor adsorbed at relative pressure P/P0 ? 1, when pores are assumed to fill with liquid adsorbate [39]. The specific surface area and the pore size distribution (PSD) were calculated using the Brunauer–Emmett–Teller (BET) equation and nonlocal density functional theory (NLDFT), respectively [23, 40]. The porous structure fractal dimension (DPSD) was calculated using the fractal Frenkel– Halsey–Hill (FHH) method with an N2 isotherm adsorption/ desorption at relative pressures of 0.5–1 [19]. The results are shown in the Fig. S2 (online) and Table S1 (online). TGA of the coal was performed using a Setaram SETSYS 12 thermogravimetric analyzer. The samples were heated at a rate of 10 °C/min in a nitrogen atmosphere.

tetrahydrofuran and 26.4 % in pyridine. This is because coal A contained a greater proportion of polar groups such as oxygen-containing functional groups, including hydroxyl groups, which were dissolved by the polar solvents [41]. Data from the differential thermogravimetric (DTG) analysis show the evolution of volatile organics (Fig. 1). In all samples, weight was lost in the range of 450–500 °C corresponding to the release of volatile organic components [27, 42–45]. The data in Fig. 1 show that carbon disulfide treatment removed a negligible amount of volatiles. However, the peak at about 450 °C shrank for tetrahydrofuran- and pyridine-treated samples. This indicates that the two extraction methods each removed a portion of the volatile content in the raw coal [46]. This also indicates that the quantity of the volatiles removed was proportional to the observed extraction yield. The main peak from coal B (Fig. 1b) appears at a higher temperature than found from coal A (Fig. 1a). According to Takanohashi et al.’s [47] work, this is because there are higher molecular weight volatile constituents in coal B, which are released at higher temperatures. Evaluation of the coal aggregation states (crystal structures) was performed using XRD measurements. Figure 2 shows the XRD profiles from the coal samples before and after solvent treatment. In all samples, an asymmetric, broad band was observed at 10°–30°. This indicated that the samples had low graphitization and were amorphous [48]. The broad band can be considered a combination of two peaks located at 20° and 26° originating from alkyl and aromatic assemblies, respectively [48–51]. Carbon disulfide treatment had less impact on the stacked carbon coal structures than treatment with the other solvents. THF and pyridine treatment appeared to loosen the p–p stacking and intensify the alkyl entanglement [50]. The XRD peak of coal

3 Results Table 1 shows the proximate and ultimate analyses of the two original coal samples. Each coal sample contained a different quantity of volatiles: 29.92 % in coal A and 21.42 % in coal B. The two samples were extracted to remove these volatiles. The extraction yields from coal B are: 2.1 %, 5.8 %, and 10.1 % for carbon disulfide, tetrahydrofuran, and pyridine solvents, respectively. Coal A had a higher extraction yield from polar solvents, 14.8 % in

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Fig. 1 DTG curves of coal A (a) and coal B (b) samples

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Fig. 2 XRD spectra of the parent coal and their derivatives. a Coal A, b coal B

A extracted with pyridine (AP sample) was broader than those of the other samples, indicating a reduction in the degree of crystallinity in this sample. This reduction in crystalline tendency was not observed for the coal B series. The (002) band of coal B was sharper, suggesting that the p–p stacking structure in coal B displayed better crystalline alignment and a more highly aggregated structure than coal A [50, 51]. ˚ in the The interplanar crystal spacing was close to 3.523 A two raw samples. The broad (002) peaks around 20° and 26° were almost unchanged for the AC, AT, BC, and BT samples, but shift to a slightly lower angle for the AP and BP samples. ˚ Along with this shift, the d002 values increased from 3.523 A ˚ ˚ ˚ (AD) to 3.626 A (AP) and from 3.522 A (BD) to 3.538 A (BP). The increase in the d002 value for these coal samples may be attributed to two different phenomena: (1) the expansion of the space between aromatic layers caused by weakening of the intermolecular forces in the direction of d002 [41, 50] and (2) newly developed slit-shaped micropores [47].

The N2 adsorption/desorption isotherms are shown in Fig. S3 (online). These isotherms can be divided into two groups. The samples in group A (samples AD, AC, AT, and BP) exhibited hysteresis loops in higher pressure ranges (P/P0 = 0.5–1). Bottle-shaped pores usually cause this type of hysteresis loop. In contrast, the samples in group B (samples BD, BC, and BT) showed no capillary condensation, which can be attributed to cylindrical pores with dead ends [52]. The N2 adsorption/desorption isotherms and pore parameters summarized in Table 2 clearly indicate that porous deformations occurred. For the coal A series, hysteresis loop area gradually decreased and vanished as volatiles were removed. The BET surface area (SBET), Vtotal, and micropore volume (Vmic) were all increased by volatile removal. Meanwhile, Vmic (0–10 nm) decreased as a percentage of Vtotal from 79.2 % for AD, to 65.0 % for AC, to 65.2 % for AT, and 64.1 % for AP. This trend indicates that interconnected micropores were opened to form cylindrical pores during extraction. The adsorption isotherms for samples BD and BC were similar to each other, but both were different from sample BT. The Vmic/Vtotal percentage notably increased from 2.7 % for BD, to 74.8 % for BC, and finally to 78.1 % to BT. For AP, however, the hysteresis loop remained unclosed for the whole relative pressure range, while SBET dropped to 0.799 cm2/g. This decrease may have been induced by porous shrinkage. Data used to evaluate the porous structure of the coal samples are shown in Fig. 3a, b. For sample AC, the volume of the sample with pore sizes between 2 and 4 nm was enhanced, while the volume of the sample with pore sizes \2 nm decreased. The Vmic of sample AC increased by 0.945 9 10-3 cm3/g, while its DPSD decreased to 2.687, and this indicated that the pores in this sample became less heterogeneous and were enlarged. For the AT sample, the increased PSD intensity mainly occurred for pore sizes of

Table 2 Porous structure parameters of coal samples Sample

SBET (cm2/g)

AD

4.181

7.16

6.85

5.673

79.2

2.757

AC

4.834

10.18

8.42

6.618

65.0

2.687

AT AP

8.055 5.441

14.26 10.02

5.72 7.37

9.301 6.427

65.2 64.1

2.801 2.739

BD

3.121

3.711

4.78

0.102

2.7

2.651

BC

2.545

3.471

5.46

2.596

74.8

2.651

BT

1.99

2.622

5.27

2.047

78.1

2.642

BP

0.799

2.010

1.11

0.895

44.6

2.535

a

Vtotal (10-3 cm3/g)

D (nm)

Vamic (10-3 cm3/g)

Vbmic (%)

DcPSD

Calculated by the NLDFT cumulative pore volume method

b

Volume percentage of micropores, %

c

Method of calculations are provided in the supporting material, Fig. S2 (online) and Table S1 (online)

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Fig. 3 Pore size distributions by fitting the NLDFT equation to N2 adsorption data. a Coal A, b coal B

1–2, 2–4, and 4–6 nm. Vmic for the AT sample increased by 3.628 9 10-3 cm3/g, and DPSD increased to 2.801. For the AP sample, the volume of pores with sizes under 2 nm was reduced, and the volume of pores in the range of 2–4 nm was increased. DPSD decreased to 2.739, and the porous structure became more homogeneous. No significant changes were observed in the PSD for the BC and BT samples (Fig. 3b). Vtotal decreased to 3.471 % for sample BC and to 2.622 % for sample BT, compared with 3.711 % for the original coal. However, micropore volume slightly increased for these two samples. The DPSD values were 2.651 and 2.642 for samples BC and BT, respectively, suggesting that the PSD was more homogeneous. The PSD in sample BP suggests that significant pore shrinkage and collapse occurred as shown by the decrease in the volume of pores with sizes in the range of 5–10 nm and the generation of pores smaller than 2 nm. The methane adsorption results are shown in Fig. 4 and Table 3. The Langmuir volume (nL) ranged from 0.430 to 0.948 mmol/g for coal A. The methane adsorption capacity increased after the removal of volatile components. The nL value ranged from 0.281 to 0.532 mmol/g for coal B. The methane adsorption capacity was slightly enhanced for the BC and BT samples, while it was almost unchanged for the BP sample. Significant changes in adsorption capacity indicate that structural changes occurred, and the details of these changes are discussed below.

4 Discussion The removal of volatiles caused observable structural changes in the crystal structure and porous structures of the coal samples. We found that the changes in the crystal structure are too slight to estimate their relationship to the

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Fig. 4 Methane adsorption isotherms corresponding to coal samples. a Coal A, b coal B

methane adsorption capacity. For example, the crystal structure remained unchanged, but the methane adsorption capacity increased for these samples. Therefore, porous deformation may be the dominant factor that caused the changes in methane storage capacity. The histogram in Fig. 5a–c relates the methane adsorption capacities to the parameters SBET, Vtotal, and Vmic. This demonstrates the relationship between the methane adsorption capacity and the porous structure. In most cases, the

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Table 3 Methane sorption parameters of the Langmuir sorption equation for the coal samples Sample

nL (mmol/g)

PL (MPa)

Mean relative deviation d (%)

AD

0.430

1.579

0.68

AC

0.580

2.105

1.25

AT AP

0.808 0.948

1.778 1.828

0.76 1.05

BD

0.281

3.121

0.65

BC

0.357

3.754

1.57

BT

0.532

3.650

0.79

BP

0.376

2.566

1.46

methane adsorption capacity can be positively correlated with SBET, Vtotal, and Vmic. There are some exceptions to this trend. For example, the SBET and Vtotal from the BP sample are the lowest, but its methane adsorption capacity is higher

than the BC and BD samples. This may due to the fact that the PSD in the BP sample is the most homogeneous (the DPSD is 2.535, which is the lowest in the B series samples). Also, the SBET, Vtotal, and Vmic for sample AP are lower than sample AT, while its methane adsorption capacity is higher than sample AT. The DPSD of sample AP is 2.739, lower than that of AT (2.801), indicating that the pore structures in sample AP are more homogeneous than in AT. These results demonstrate that the methane adsorption capacity of coal depends not only on SBET, Vtotal, and Vmic, but also on the PSD. These results are consistent with those reported by Luo et al. [11], Yao et al. [19], and Sevilla et al. [40], who suggested that a narrower and more homogeneous PSD could reduce the liquid/gas surface tension and improve methane adsorption capacity. In summary, the enhancement in methane adsorption is caused by: (1) the pore opening size; (2) the formation of micropores; and (3) a narrower and more homogeneous PSD.

Fig. 5 Adsorbed amount of methane as a function of the specific surface area (a), total volume (b), and micropore volume (c)

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538

Except for deformations in the porous structures, obvi˚ ) ocous changes in the crystal structure (Dd002 [ 0.05 A curred for samples AP and BP. Figure 6 exhibits the distinct difference in methane adsorption capacity when the crystal structure changed. In Fig. 6, samples AD, AC, AT, BD, BC, and BT are classified as stage I, a stage where only porous deformations occurred. The enhancement of methane adsorption capacity in this stage was mainly caused by increased pore space. Volatile release was the main reason for the formation of new creased spaces. In Fig. 6, samples AP and BP are classified as stage II, a stage where obvious structural relaxation occurred. In this stage, the DPSD decreased while the Dd002 increased, because structural relaxations gave rise to a rearrangement of porous structures. Thus, methane adsorption behavior showed the largest changes in this stage. The decrease in the crosslink density of coal samples is the main reason for the change. Obviously, the two different structural deformations are related with the state of volatiles within the samples. Figure 7 illustrates the structural deformations and their relationship to the volatiles in coal. Raw coal is strained and highly aggregated. Volatiles exist in two states: trapped

Fig. 6 Structural deformations of coal

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in the pores and bonded in the macromolecular structure (Fig. 7a). These two types of volatiles play different roles in porous deformations. Volatiles blocked in the pores lead to imperfect connections within the microporous network inside the coal. The motion and removal of volatile components mainly occur in the pores that are\10 nm, leading to the deformation of porous structures (Fig. 7b). The newly created pore spaces become the physical diffusion sites that contribute to the enhancement of methane adsorption capacity [30, 31]. This is consistent with data from Bae et al. [23], who indicated that volatiles produce energy barriers that prevent adsorbed molecules from passing through the pores. In contrast, the volatiles that are crosslinked within the coal’s macromolecular network play an important role in coal–coal interactions. Removing them makes the macromolecular chains in the coal more mobile, and structural rearrangement (relaxation) may then occur (Fig. 7c). This is consistent with data from Shui et al. [34], who demonstrated that lighter constituents inside the coal network cross-link as active sites within the coal molecules and act like a ‘‘lock’’ removal of the lock lead to the relaxation of the coal molecules. Additionally, coal A is more sensitive to pore deformation than coal B, which can be mainly attributed to the different aggregate structures of the two samples. After pyridine treatment, the PSD of sample AP became more homogeneous, while the pores of sample BP collapsed. The XRD tests also distinguished the structures of coal A and coal B and showed that coal A was less aggregated with more flexible macromolecular chains. Such structures are more vulnerable to relaxation or expansion. The structure of coal B was less flexible and more rigid and therefore had a higher resistance to deformation. These results indicate that porous deformations are constrained by the aggregation state of coal. Our results are similar to those of Shui et al. [34] and Khandare et al. [53], who reported that materials with less flexible and more rigid structures have higher resistance to deformation in their thermal expansion. Therefore, the porous structure is the essential factor that affects the methane adsorption capacity of coal. However, other structural properties, including composition and

Fig. 7 Stages of structural deformations. A, aromatic cluster; B, low molecular weight constituents embedded in the pores matrix; and C1, C2, new created micropores

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aggregation state, are all related to the stability of porous structure. Importantly, pore deformations are determined by the state of the volatile components and the deformations constrict with the aggregation state. In many laboratories, gas adsorption measurements are carried out on coal samples to estimate the methane adsorption storage capacity of coalfields. During the CBM/coal-exploiting process, the gas–coal systems are dynamic and coal structural deformations are likely to occur. Therefore, the raw aggregation state of the coal and the volatile features need to be considered to estimate coal gas adsorption capacity.

5 Conclusions In summary, the changes in the methane adsorption behavior of coal mainly depend on the porous parameters: specific surface area, micropore volume, and PSD. Volatiles play a key role in determining the porous structure and the aggregation state of coal and hence affect the coal’s methane adsorption behavior. This is because volatiles exist in two main states: trapped in the pores or connected to coal–coal interactions. The volatiles in the pores impact the pore space and accessibility. Thus, pore opening or new pore formation could be caused by the motion/removal of these volatile components. The volatiles that are connected to the coal–coal interactions act on the mobility of macromolecular chains. Once these volatiles are removed, structural rearrangement and structural relaxation could occur. The pore volume could either increase or decrease, which is depended on the aggregation state of the raw coal. These results suggest that the selection of ‘‘assistant gases’’ in CMB applications needs to be considered according to the coal structure to optimize the process. Acknowledgments This work was supported by the National Basic Research Program of China (2011CB201202). Conflict of interest of interest.

The authors declare that they have no conflict

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