Effects of Coal Functional Groups on Adsorption Microheat of Coal

Article pubs.acs.org/EF

Effects of Coal Functional Groups on Adsorption Microheat of Coal Bed Methane Fubao Zhou,*,†,‡ Shiqi Liu,‡ Yeqing Pang,‡ Jianlong Li,‡ and Haihui Xin‡ †

Key Lab of Gas and Fire Control for Coal Mines, Ministry of Education, Xuzhou, Jiangsu 221116, China School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China

‡

S Supporting Information *

ABSTRACT: This study measured the adsorption heat for five groups of Chinese coal samples of different ranks at 15 °C and 0.101 MPa using a C80 microcalorimeter. The functional groups of the coal samples were determined by infrared spectroscopy according to quantum chemical theory. The effects of coalification and coal functional groups on the adsorption heat of coal for methane were discussed in terms of energy. As a result, this study has further perfected the adsorption theory of coal bed methane. The results show that the adsorption heat of coal for methane first increases, then decreases with increasing coal rank and reaches a minimum at the fat coal stage. This indicates that coalification has a significant effect on the characteristics of the adsorption heat of coal for methane. These effects can be clearly classified into stages. Coalification influences the adsorption heat for methane by changing the type of coal, the content of oxygen-containing functional groups, and the pore structure. Oxygencontaining functional groups influence the adsorption heat of coal for methane via the adsorption potential of the methane molecules. In the long-flame coal stage, a high content of oxygen-containing functional groups leads to a high adsorption heat of the coal for methane. In the fat coal to coking stages, large numbers of aliphatic series, aliphatic functional groups, and side chains of aromatic condensed nuclei are removed from the coal molecules under the influence of mechanical compression and dehydration. The decrease in oxygen-containing functional groups results in a decrease in the adsorption potential of the coal and a minimum value of its adsorption heat for methane. In the coking coal stage, the adsorption heat for methane changes little because of the weak influences of mechanical compression and dehydration. In the high-rank coal stage (lean coal and anthracite), the internal surface area of the coal increases, with micropores and transition pores caused by the significantly higher degree of aromatization of the coal. This increase in internal surface area improves the adsorption heat of coal for methane to some extent.

1. INTRODUCTION Coal bed methane (CBM) is mainly adsorbed in the pores of coal.1,2 The study of the adsorption properties of coal for methane is essential to exploit CBM, prevent dynamic gas disasters in underground coal mining, and retain CO2 in coal beds to ameliorate global warming. Studies of the adsorption properties of coal for methane have mainly focused on the relationship between the adsorption capacity of coal for methane and the coal characteristics (e.g., coal rank, maceral and mineral composition, pore structure, proximate analysis indices, and functional group content). The existing literature contains extensive reports of the correlation between the adsorbed volume of methane and coal rank using samples from coal basins worldwide. These studies have shown that adsorbed volume of coal for methane first decreases and then increases with increasing coal rank, and reaches a minimum at the vitrinite maximum reflectance of approximately 1.5%;3−6 however some studies showed that it increases with coal rank.3−6 Previous studies have also indicated that the adsorbed volume of coal for methane is related to pore volume and pore surface area, maceral composition, functional group properties of the coal, etc. However, the relationships between the adsorbed volume of coal for methane and these parameters are not currently understood.10−14 Essentially, the adsorption volume of coal for methane is usually used to reflect the adsorption capacity of coal for © XXXX American Chemical Society

methane. It can directly reflect the adsorption capacity of coal for methane and plays a direct quantitative role in the CBM production process. However, the essence of the adsorption of coal for methane is the interactions between the macromolecular structure of the coal and the methane molecules. The adsorption heat of coal and of surface functional groups reflects the energy states of methane molecules on the surface adsorption field of the coal, thereby revealing the essence of the adsorption of the coal. There have been few studies on the heat effects of the adsorption process. The only paper discovered by the authors was by Taraba, in which he tested the adsorption heat of coal for a gas by studying the competitive adsorption of CH4 and CO2 for coal.15 Studies on the relationships between the surface functional groups of coal and its adsorption characteristics have focused on the adsorption capacity of oxygen-containing functional groups and the absorption intensities of CO2, CO, O2, and other gases.16−22 Little research has been conducted on the physical absorption of methane by oxygen-containing functional groups. In particular, studies on the adsorption heat of oxygencontaining functional groups for methane are sparse. Received: December 4, 2014 Revised: February 9, 2015

A

DOI: 10.1021/ef502718s Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Complete List and Properties of Coal Samplesa proximate (wt %)

ultimate (%)

maceral analysis (vol %)

coal number

Ro,max (%)

Mad

Aad

Vad

FCad

Oad

Cad

Had

Nad

V

I

E

M

coal rank

sampling position

#1

0.64

5.82

8.58

28.74

56.86

10.50

81.86

4.33

1.25

46.5

50.2

1.9

1.4

#2

1.14

0.99

6.80

32.21

60.00

9.31

83.31

4.49

1.32

58.8

28.1

7.3

5.8

Long flame coal Fat coal

#3

1.49

1.37

15.2

24.49

58.94

9.71

82.82

4.08

1.15

64.6

30.4

1.5

3.5

Coking coal

#4

2.04

1.74

10.41

12.34

75.51

3.44

90.46

2.95

0.72

72.5

25.7

0.9

0.9

Lean coal

#5

2.91

1.01

1.96

8.51

88.52

2.73

93.46

2.25

0.49

83.5

13.5

1.7

1.3

third anthracite

Chenjiashan Coal Mine Shanjiaoshu Coal Mine Weijiadi Coal Mine Gequan Coal Mine Baijigou Coal Mine

a Notes: wt, weight percentage; vol, volume fraction; Mad, moisture, air-drying base; Aad, ash yield, air-drying base; Vad, volatile matter, air-drying base; FCad, fixed carbon content, air-drying base; Odaf, content of oxygen, air-drying base; Cad, content of carbon, air-drying base; Had, content of hydrogen, air-drying base; Nad, content of nitrogen, air-drying base; V, vitrinite; I, inertinite; E, exinite; M, minerals.

a. Size of Coal Samples. The size of coal samples can affect the exothermic characteristics of methane adsorption by the coal. Coal samples with a large particle diameter are characterized by large gaps between the particles. The relatively small specific surface area and large adsorption of the coal space hinder methane adsorption, resulting in poor thermal conductivity. This causes an uneven temperature distribution within the system and reduces the accuracy of the test. On the other hand, smaller coal particles can stack tightly. Therefore, the methane flow among the particles is restricted, resulting in uneven methane adsorption by the coal. On the basis of experience from previous experiments, a 60−80 mesh (0.20−0.25 mm) particle size was chosen as suitable for methane adsorption. b. Quantity of Coal Samples. The exothermic characteristics of the physical adsorption methane by coal are inherent attributes of the coal and have no relationship to the quantity of coal samples. Therefore, the accuracy of the test is mainly considered to determine the quantity of coal samples necessary. According to the working mechanism of the microcalorimeter, a small number of samples is conducive to an even distribution of temperature within the system and sufficient adsorption of methane. A small number of samples can also reduce the temperature gradient as well as the deviation between the programmed sample temperature and the environmental temperature. However, if the sample number is too small, the methane entering the reaction vessels escapes directly from the outlets without adsorbing onto the coal samples. In this case, the methane cannot fully contact the coal samples, and the test errors is large. According to the capacity of the sample cell and experience from previous experiments, and considering the factors discussed above, 1500 mg coal samples were chosen. c. Selection of Desorption Gas. Because they have been exposed to air, the coal samples have adsorbed other gases. These gases could affect the accuracy of the methane adsorption experiment. Therefore, these gases should be removed before the test. Hydrogen atoms have a strong capacity to penetrate into the adsorbent because they have the smallest atomic radius of all atoms. However, according to the adsorption isotherm of H2, the adsorption of coal for H2 is very small and can be ignored. Therefore, H2 was chosen as the desorption gas. Before the test, H2 (purity >99.999%) was injected into the instrument to displace the other gases from the sample cell, the reference cell, and the sample pores. After eliminating instability phenomena, methane was injected into the machine. d. Gas Flow. The C80 microcalorimeter generated results through output signals that are produced by converting the differences between the temperatures (voltages) of the coal samples and the reference. Consequently, as long as the gas flow in the reference cell is the same as that in the sample cell, the influence of the carrier gas flow on the reaction system can be ignored. In addition, in the initial adsorption phase, the methane adsorption of coal is intense, and the amount of liberated heat is large because of competitive adsorption. To guarantee sufficient methane adsorption, the gas flow was set to 30 mL·min−1

In this study, we measured the adsorption heat of typical coal samples in China using a C80 microcalorimeter. The adsorption characteristics of coal for methane are discussed in terms of energy using infrared spectroscopy and quantum chemical theory. This study has further improved our understanding of the theory of the adsorption of methane by coal.

2. METHODOLOGY 2.1. Collection and Preparation of Coal Samples. Five groups of Chinese coal samples of different ranks were systematically collected: long-flame coal from the Chenjiasshan Coal Mine, fat coal from the Shanjiaoshu Coal Mine, coking coal from the Weijiadi Coal Mine, lean coal from the Gequan Coal Mine, and third anthracite from the Baijigou Coal Mine (Table 1). These coal samples were respectively assigned the numbers 1−5 to reflect the higher coal rank. The collection, retention, and preparation of the coal samples followed the relevant national standards GB/T 19222-2003 (sampling of coal petrolog) and GB/T 16773-2008 (method of preparing coal samples for coal petrographic analysis). The determination of the mean maximum reflectance of vitrinite (Ro,max, %) in the coal samples was performed in compliance with the national standard GB/T 69482008 (method of determining microscopically the reflectance of vitrinite in coal) and was recorded by an Axio Imager.M1m microscope produced by the Carl Zesiss Foundation Group. Coal samples were hermetically sealed in bags at 5 °C. The key properties of those coal samples are shown in Table 1. 2.2. Experimental Method. 2.2.1. Determination of the Characteristics of the Adsorption Heat of Coal. In this study, we investigated the exothermic characteristics of the adsorption process using a C80 microcalorimeter. The C80 microcalorimeter was produced by Setaram Company, France, and consisted of a C80 host and a gas circulating pool. On the basis of 3D sensor technology according to the Calvet calorimetric principle, the C80 microcalorimeter reflects the thermal properties of the sample very accurately. The testing temperature ranges from room temperature to 300 °C, and the constant temperature control accuracy is ±0.001 °C with a resolution of 0.1 μw. The gas circulating pool is composed of two reaction vessels (the sample cell and the reference cell) that have the same shape, size, and material composition and have a high thermal conductivity. The volume of the gas circulating pool is 12.5 mL. The gas circulating pool has an inlet and an outlet. Gas enters the inlet and discharges from the outlet. During the test, experimental conditions (temperature, gas flow, gas type, etc.) in the sample cell and reference cell are the same. To reflect the adsorption heat of coal for methane more accurately and completely, the experimental temperature and ambient pressure were set to 15 °C and 0.101 MPa. Moreover, the size and quantity of coal samples, the desorption gas type, and the gas flow rate were taken into account in determining the experimental parameters. B

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Energy & Fuels Table 2. Proportion of Typical Functional Groups of Five Coal Samples (%) methyl groups and other saturated groups

aromatic nucleus

coal number

−CH3/−CH2−/CH

CC

−OH

−COOH−

−C−O−

−COO−

CO

#1 #2 #3 #4 #5

18 28 26 35 36

26 18 21 9 19

0 15 10 12 3

0 0 8 1 1

35 33 34 30 29

13 1 1 7 5

8 5 0 6 7

considering experimental accuracy and efficiency, experience from previous experiments, and the factors described above. 2.2.2. Determination of Functional Group Types and Contents. The types and ratios of the molecular functional groups of coal, the adsorption potential of the methane molecules, and the characteristics of the adsorption heat of the coal molecules were analyzed with a combination of infrared spectroscopy and quantum chemical theory. a. Fourier Transform Infrared Spectroscopy Experiment. The infrared spectroscopy experiments used a NCOLET 6700 Fourier transform infrared spectrometer. The instrument consists of six main components: an interferometer, an infrared source, a sample chamber, the diaphragm setting, the detector, and an infrared reflector. To obtain accurate infrared spectrograms, the coal samples used in the experiment were no bigger than 200 mesh ( single-bond oxygen-containing functional groups > methyl groups and other saturated groups > aromatic nuclei. The adsorption potential of carbon−oxygen double-bond functional groups for methane molecules can be as high as 26.07 kJ·mol−1, whereas that for aromatic nuclei is only 1.35 kJ·mol−1 (Table 4). Combining the measured adsorption heat of the five groups of coal samples for methane, a larger content of oxygen-containing functional groups, especially carbon−oxygen double-bond functional groups, contributes to an increase in the adsorption heat for methane (Figure 3).

adsorption heat ΔE (kJ·mol−1) 1.35 1.37 11.78 26.07

molecular weight of functional groups, and morphotropism of functional groups.23,24 Larger intermolecular forces and larger molecular polarity of the functional groups of coal are associated with a greater adsorption potential, stability of adsorption, and adsorption heat.23,24 Aromatic nuclei and methane molecules are both nonpolar with polar bonds. They repel each other. Their intermolecular forces, stability of adsorption, and adsorption heat are small. However, if the aromatic nuclei has a side chain composed of methyl groups and other saturated groups, single-bond oxygencontaining functional groups, and carbon−oxygen double-bond functional groups, it is no longer a nonpolar molecule, but a polar molecule instead. If the side chain length of the aromatic condensed nuclei is increased, the polarity of the coal molecule and the intermolecular forces increase, along with its dispersion and induction forces, resulting in increased adsorption heat. Methyl groups and other saturated groups are polar molecules. Their intermolecular forces associated with methane are larger than those of aromatic nuclei. Their stability of adsorption and adsorption heat are also larger than those of aromatic nuclei. Carbon−oxygen double-bond groups and single-bond oxygencontaining groups are both polar molecules. Their polarities and adsorption potentials are stronger than those of aromatic nuclei. Oxygen, with its high electronegativity in carbon− oxygen double-bond groups and single-bond oxygen-containing groups, can easily form hydrogen bonds (−O−H) with the hydrogen in methane molecules. Hydrogen bonds have strong polarity and greater stability than intermolecular forces. Therefore, carbon−oxygen double-bond groups and singlebond oxygen-containing groups have larger adsorption heats than aromatic nuclei, methyl groups and other saturated groups. In addition, single-bonded oxygen (i.e., a hydroxyl group) involves SP3 hybridization, whereas the carbon−oxygen double bond involves SP2 hybridization.25−28 The electronegativity of a carbon−oxygen double bond is greater than that of single-bonded oxygen. Therefore, the stability of adsorption and adsorption heat of a carbon−oxygen double bond are greater than those of single-bonded oxygen. In conclusion, under similar conditions the adsorption heat of coal molecules and methane increases with increasing side chain length of aromatic condensed nuclei. For aromatic condensed nuclei with side chains of similar length, the adsorption heats can be ranked in descending order: carbon− oxygen double-bond groups > single-bond oxygen-containing groups > methyl groups and other saturated groups. 4.2. Important Factors in Coalification. 4.2.1. LongFlame Coal Stage. During the early stages of coalification (Ro,max < 0.65%, the long-flame coal stage, coal sample #1), which is the first iteration of coalification, the aromatic layer of coal is small and randomly distributed.29−34 The density of carbon atoms per unit of internal surface area is low, and the content of oxygen-containing functional groups (−OH, −C O, and −COOH, etc.) is large (Table 2). The adsorption heat

Figure 3. Relationships between typical functional groups and adsorption heat.

4. DISCUSSION The changes in the adsorption heat of the five groups of coal samples considered with the coal ranks and the relationships with typical functional groups indicate the stability of the functional groups when they combine with methane molecules and the important factors of coalification to achieve the desired functional group content and adsorption heat for methane. 4.1. Stability of Molecular Association. The adsorption of methane by coal is physical absorption and relies on an intermolecular force (i.e., the van der Waals force). The intermolecular force has a bearing on molecular polarity, the E

DOI: 10.1021/ef502718s Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels is high at this stage because of the high content of oxygencontaining functional groups. 4.2.2. Fat Coal and Coking Coal Stages. During the gas coal and fat coal stages (Ro,max = 0.65−1.2%, coal sample #2), with an increase in stress and the occurrence of gelation, aliphatic series, aliphatic functional groups, and side chains of aromatic condensed nuclei such as −COOH decrease considerably.29−34 At the coking coal stage (Ro,max = 1.3− 1.7%, coal sample #3), humic gelation completes dehydration, and the effects of stress and dehydration gradually lessen,29−34 and the oxygen-containing functional groups in coal almost disappear. The combination of methane molecules and aromatic nuclei is less stable than the combination of methane molecules and hydrocarbons. Therefore, at the fat coal stage (coal sample #2), with the sharp decline in the oxygen-containing functional groups, the adsorption potential of the coal molecules for methane molecules decreases, causing a great decrease of the adsorption heat of coal for methane (Table 2). During the coking coal stage (coal sample #3), compaction and dehydration are low, resulting little changes in the adsorption heat of coal for methane. 4.2.3. High-Rank Coal Stage. During the high-rank coal stage (Ro,max > 1.9%, lean coal, anthracite, coal samples #4 and #5), the degree of coalification further improves, and changes in the chemical structure of coal molecules are caused mainly by temperature with a significant increase in the degree of aromatization;29−34 the aromatic ring expands, and the adsorption heat of the coal for methane decreases. However, the aromatic ring has a directional alignment, forming a series of micropores and transition pores, mainly intermolecular pores.29−34 Figure 4 shows that the micropore and transition

Figure 5. Relationships between volume and specific surface area of micropores and transition pore and adsorption heat. The transition pore and micropore content are obtained from mercury injection experiment.

for methane can obviously be classified into stages. At the soft coal stage, the potential adsorption of coal for methane molecules, which is determined by the types and contents of oxygen-containing functional groups, is the major factor affecting the adsorption heat for methane. At the high-rank coal stage, the total quantity of methane adsorbed, which is determined by molecular structural changes in the coal is the primary factor that causes the increase in adsorption heat of the coal.

5. CONCLUSIONS This study measured the adsorption heats for five groups of typical coal samples via microcalorimetry. The adsorption characteristics of coal for methane regarding energy were discussed using quantum chemical theory and infrared spectroscopy experiments. The following conclusions can be drawn from this study. (1) The adsorption heat of coal for methane first increases and then decreases with increasing coal rank. It reaches a peak in the long-flame coal stage and a minimum in the fat coal stage, rebounding afterward in the lean coal and anthracite stages. Coalification exerts significant effects on the adsorption heat characteristics of methane. These effects can be obviously classified into stages. Coalification influences the adsorption heat of coal for methane according to the coal type, the functional group content and the pore structure. (2) The adsorption heat of coal for methane is significantly affected by the type and ratio of oxygen-containing functional groups. The higher the content of oxygen-containing functional groups, the higher the adsorption heat of coal for methane. Oxygen-containing functional groups influence the adsorption heat of coal for methane through the adsorption potential of methane molecules by coal. (3) In the long-flame coal stage, the high content of oxygencontaining functional groups increase the adsorption potential of methane by coal and the adsorption heat of coal. In the fat coal and coking coal stages, under the influence of mechanical compaction and dehydration, a decreasing number of oxygencontaining functional groups reduces the adsorption potential of the coal and the adsorption heat of the coal for methane. At the high-rank coal stage (lean coal, anthracite), micropores and transition pores caused by the increase in the degree of aromatization increase the adsorption of the coal quantity for methane and the adsorption heat of the coal for methane.

Figure 4. Relationships between transition pore and micropore content and coal rank. The transition pore and micropore content are obtained from mercury injection experiment.

pore content of five coal samples greatly increases at the highrank coal stage (coal samples #4 and #5). The increase in micropore and transition pore content effectively increases the pore volume and the internal surface area (Figure 5). Therefore, the adsorption quantity of coal for methane and the adsorption heat of the coal increases (Figure 5). In summary, during the coalification process, the coal molecular structure changes with an increase in the coal rank. The changes in the adsorption heat for the five groups of coal samples, along with their coal ranks and their relationships with the types and contents of oxygen-containing functional groups, reveal important factors of coalification that can be used to modify the adsorption heat of coal for methane. Meanwhile, coalification factors with respect to the adsorption heat of coal F

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ASSOCIATED CONTENT

S Supporting Information *

The low-temperature liquid nitrogen adsorption method was used for analyzing pore sizes from 0.7 to 12 nm. The liquid nitrogen adsorption experiment was performed on an automated surface area analyzer (ASAP 2000) produced by Micromeritics Instrument, according to ISO 15901-2:2006 and ISO 15901-3:2007. This research used a pore structure classification system put forward by Hodot, who divided coal pores into micropores (less than 10 nm in diameter), transition pores (10−100 nm in diameter), mesopores (100−1000 nm in diameter), and macropores (larger than 1000 nm in diameter). The Supporting Information shows the surface areas obtained from the mercury injection experiment and the low-temperature liquid nitrogen adsorption method that were used for verifying the accuracy of the calculated results for the adsorption heats for the functional groups (Table S1), and optimized structures of molecules before and after adsorption (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-516-83995053. Fax: +86-516-83995053. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Nos. 51325403, 41402135) and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13098).

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DOI: 10.1021/ef502718s Energy Fuels XXXX, XXX, XXX−XXX