Changes in the Surface Structure of Coal Caused by Igneous

Subscriber access provided by Kaohsiung Medical University

Fossil Fuels

Changes in Surface Structure of Coal Caused by Igneous Intrusions and Its Effect on the Wettability Quanlin Shi, Botao Qin, Qiang Bi, and Bao Qu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02439 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Changes in Surface Structure of Coal Caused by Igneous Intrusions

2

and Its Effect on the Wettability

3

Quanlin Shi a, Botao Qin a,*, Qiang Bi b, Bao Qu b

4

a

School of Safety Engineering, China University of Mining and Technology (CUMT), Jiangsu 221116, China b

5

Daxing Mine, Liaoning 112700, China

6

ABSTRACT: In this paper, the normal and thermally altered coals were sampled to investigate the surface

7

characteristic changes of coals after igneous intrusions and its effects on the wettability. The chemical

8

compositions, pore structure, surface morphology, and functional groups of coals after igneous intrusions were

9

comparatively analyzed. Contact angle experiment and wetting time test were employed to analyze the

10

wettability of coals. The results showed that the coals after igneous intrusions exhibited much more epigenetic

11

carbonates and higher ash contents compared to the normal coals, which would improve the wettability of these

12

coals. Pore structure analysis revealed that macropore volume and surface roughness of the altered coals was

13

further increased due to igneous intrusions. In addition, the altered coals formed much larger pores and more

14

cracks on their surface, which made these coals more easily wetted by water. XPS analysis revealed the

15

decreased contents of oxygen-containing groups after igneous intrusions. The contact angle and wetting time test

16

results indicated that the altered coals demonstrated the improved wettability than the normal coals. This

17

conclusion could be evidenced by smaller contact angle and shorter wetting time for the altered coals. This paper

18

concluded that the surface structure of bituminous coal was influenced by igneous intrusions obviously, finally

19

enhancing the wettability of the altered coals, which was mainly due to changes in ash contents, minerals, pore

20

structures, and surface morphology.

21 22 23

1. INTRODUCTION

24

Coal, as one kind of the key fossil fuel since ancient times, is not only the main source for generating

25

electricity around the world, but also the important raw materials in coal chemical industry.1-9 Based on the

26

recent report of BP Energy Outlook in 2018, the coal resource is still the most major source for electricity in

27

2040, accounting for nearly 30%. Due to the rapidly growing demand for coal resources, shallow coal resources

28

have been depleted or even exhausted. Thus, with continuously increase in mining depth, more and more thermal

29

altered coal resources affected by igneous intrusions were exploited around the world.10-15 Based on the previous

ACS Paragon Plus 1 Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

studies, the altered coals showed obvious differences with the normal coals of natural coalification in

2

physicochemical structure and properties.11 Many papers discussed the relationship between igneous intrusions

3

and changes in chemical compositions, pore features, coalbed methane reservoir, and combustion behaviors.10-13

4

On the contrary, few researcher has paid attention to the influence of intrusions on the wettability of coals.

5

Wettability is the basic physicochemical property for coals and other materials, which plays an important role

6

for coal mine safety, flotation, CO2-storage, methane recovery and stability of suspensions.16-25 Typically, in the

7

field of coal mine safety, the wettability could be used to guide the control of coal spontaneous combustion and

8

coal dust disaster efficiently.20,26 Previous literatures have revealed the relationship between igneous intrusions

9

and compositions or pore evolution of coals. Mastalerz27 and Jiang12 found that these coals after igneous

10

intrusions showed higher contents of ash and fixed carbon, as well as less moisture and volatile matter than the

11

normal coals. Finkelman10 and Dai11 further studied the changes in the inorganic geochemistry of coals after

12

igneous intrusions, and concluded that hydrothermal fluids during igneous intrusions finally resulted in the

13

secondary enrichment of CaO and MgO in coals. As for the functional groups on the coal surface, some scholars

14

believed intrusions changed the molecular structures, especially destroying oxygen-containing groups in coals.28

15

As for physical structures of coals, many researchers have verified the pore evolution of coals affected by

16

igneous intrusions.12,29 Because of the secondary hydrocarbon generation, many pyrolysis pores and cracks

17

would be formed in the coals after igneous intrusions compared to the normal coals, finally leading to higher

18

surface roughness for the altered coals.12 From the above literatures, it could be clearly found that igneous

19

intrusions significantly destroyed the coals' physicochemical structure, which may further affect the wettability

20

of coals. Nevertheless, as we have seen in other literatures, the relationship between coal structure changes after

21

igneous intrusions and the wettability has not been systematically studied by other scholars, which was

22

extremely important to the safe production during mining and subsequent efficient use.

23

Thus, this paper aimed to comparatively analyze the surface structure distinction between the altered coals and

24

normal coals, finally clarifying the relationship between the wettability changes of coals and igneous intrusions.

25

The normal coals and altered coals were sampled from 3 coal seams in Daxing Mine, China. Further, the changes

26

in chemical compositions, surface morphology, pore structure, and functional groups of the normal and altered

27

coals were contrastively analyzed. Moreover, the wettability of coals after igneous intrusions was investigated in

28

this study. The research findings in this study could offer a theoretical instruction and valid references for the

29

prevention and control of coal mine disasters during mining, as well as the subsequent efficient use in the field of

30

coal chemical industry.


Page 2 of 18

Page 3 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

2. EXPERIMENTAL SECTION

2

2.1. The Preparation of Coals

3

Daxing Mine with large-scale igneous intrusions, as an important bituminous coal base in China, was selected

4

as the study area. Six coal samples were collected and sampled at three coal seams in Daxing Mine as shown in

5

Figure 1.

6 7

Figure 1. The study location in China and photograph of collected six coals.

8 9 10

2.2. Chemical Compositions Analysis Proximate analysis of coals was performed by 5E-MAG6600B industrial analyzer, while the ash compositions

11

of coals were tested by the GB/T 1574-2007 (the Chinese standard).

12

2.3. X-ray Photoelectron Spectroscopy Test

13

ESCALAB 250Xi system was used to investigate the element contents (including C, O, Si, Al, N, Ca, Fe, and

14

Mg et al.) and surface functional group characterization of coals. After tests, the binding energies of all coal

15

samples were modified, and set the −CH2−CH2 bond to the binding energy of 284.6 eV.

16

2.4. Pore Structure Test

17

Low-temperature N2 adsorption test was adopted to perform and investigate micropores/mesopores

18

characteristics after igneous intrusions, when MIP mercury intrusion porosimetry were utilized to investigate the

19

changes in mesopores and macropores structure of coals.

20

2.5. Surface Morphology Analysis


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

The surface morphology of these six coals was characterized by employing Quanta 250 scanning electron

2

microscope. Firstly, coal particles around 3 mm were selected and dried for 5 h at about 70 °C. Then, all coal

3

samples would be coated with gold to improve the conductivity of coals. Finally, the SEM test was conducted in

4

high vacuum mode.

5

2.6. Wettability Analysis of Coals

6

2.6.1. Contact Angle Experiment

7

The sessile drop method using distilled water was adopted to measure the contact angle of coals by

8

JGW-360B contact angle analyzer. A high-speed camera would capture the water drop images during entire

9

wetting. For each coal sample, the test was repeated 15 times at the same experiment condition, and the

10

arithmetic mean value of test results was regard as the contact angle value for every sample.

11

2.6.2. Wetting Time Test

12

Wetting time of coals was tested and analyzed by Walker method in this study. Before the test, the coal

13

samples were screened through a 300 mesh sieve. Firstly, 3 wt.‰ sodium dodecyl benzene sulfonate solution

14

was prepared and transferred 50 ml solution to a beaker. And then, place the coal powders (0.2 g) on the solution

15

surface to wet and deposit. The wetting time was obtained at the moment that the coal powders immersed into

16

the solution completely. The test for every coal sample was repeated three times on the same method.

17

3. RESULTS AND DISCUSSION

18

3.1. Chemical Compositions Changes of Coals after Igneous Intrusions

19

Table 1 showed the changes in chemical compositions of coals after igneous intrusions. The altered coals

20

exhibited the decreased volatile matter and moisture contents, and increased ash content and fixed carbon

21

compared to the normal coals. This result revealed that the altered coals have experienced the secondary

22

hydrocarbon generation and dewatered process due to the special geologic condition during igneous intrusions,

23

which resulted in the increase of the maturity level and coal rank.30 Moreover, hydrothermal fluids during

24

igneous intrusions, caused the precipitation of epigenetic minerals in coals. This also elevated the ash contents of

25

coals. Further, the ash compositions of six coals were comparatively analyzed as shown in Table 1. SiO2 and

26

Al2O3 accounted for over 80 % of ash in the normal coals, and the hydrophilicity of these oxides would affect the

27

wettability of coals.31-34 However, after igneous intrusions, CaO and SiO2 were dominant in the ash of the altered

28

coals. Besides, the altered coals possessed higher contents of Fe2O3, MgO, SO3, and MnO2 than the normal coals,

29

which agreed with the previous literature.10,35 It is worth noting that CaO is distinctly enriched in the altered

30

coals due to the precipitation of calcite, with its contents increase from 1.21 % to 32.24 %, from 1.58 % to


Page 4 of 18

Page 5 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

37.83 %, and from 0.61 % to 48.78%, respectively. For the altered coals, the enrichment of CaO, Fe2O3, MgO,

2

and MnO2 meant that the contents of carbonates were obviously increased in these coals. Cheng36 have reported

3

that the increased ash and mineral contents would make coals more easily wetted because of their hydrophilic

4

characteristics. In addition, Gosiewska et al. found the hydrophilicity of inorganic minerals, and concluded that

5

more hydrophilic minerals would lead to the enhanced wettability of coals.37

6

Table 1. Basic information of six coals. Coal

Proximate analysis (%)

Vad FCad Mad Aad N1-N 32.82 54.18 6.34 6.66 N1-A 24.72 62.53 3.06 9.69 S5-N 36.75 51.30 5.04 6.91 S5-A 24.60 60.26 2.76 12.38 S2-N 35.16 52.97 3.43 8.44 S2-A 27.77 57.62 1.58 13.03 7 3.2. Pore Structure of Coals

Ash composition (%) SiO2 62.22 34.73 57.92 29.6 53.15 17.68

Al2O3 21.57 13.18 23.71 11.91 34.26 12.21

CaO 1.21 32.24 1.58 37.83 0.61 48.78

MnO2 0.14 0.45 0.06 0.31 0.01 0.13

K2O 2.82 2.39 2.13 1.46 3.07 0.73

P2O5 1.57 0.75 0.34 0.13 0.1 0.09

8

9 10

Figure 2. N2 adsorption/desorption curves of six coals.

11


Fe2O3 6.35 9.10 8.88 10.38 4.78 11.49

Na2O 0.85 0.60 1.05 0.39 0.86 0.29

MgO 0.93 1.90 1.69 2.54 1.47 2.22

SO3 0.42 3.25 0.49 4.14 0.16 5.52

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

As can be seen from Figure 2, during adsorbing experiments, the absorbed N2 volume of three normal coals

2

were 12.20 cm3/g (N1-N coal), 10.30 cm3/g (S5-N coal), and 12.12 cm3/g (S2-N coal) at the highest pressure,

3

whereas the altered coals only absorbed 5.10 cm3/g (N1-A coal), 8.28 cm3/g (S5-A coal), and 6.81 cm3/g (S2-A

4

coal), which revealed higher adsorption volume of the normal coals. Moreover, the monolayer and multilayer

5

adsorption of N2 molecules on micropores and mesopores was developed in all coal samples, as evidenced by

6

adsorption/desorption curves (P/P0 < 0.45). At the relative pressure from 0 to 0.45, the adsorption curves of the

7

altered coals showed a slower rise than the normal coals at the same coal seam, which revealed the decrease of

8

micropores and mesopores in the altered coals due to the special geologic condition during igneous intrusions.

9

The pore structure and surface characteristic are key parameters to the wettability of coals.38,39 Zhou et al.

10

concluded that the decrease in micropores and mesopores resulted in the improved wettability of coals.40 As P/P0

11

increased form 0.45 to 0.99, all curves showed a distinct hysteresis loop due to the capillary condensation, and

12

N2 adsorption did not reach saturation at the end of curves, which verified the existence of mesopores and

13

macropores structure in these six coals. In addition, due to the limitation of N2 adsorption to analyze large pores

14

in coals, mercury intrusion porosimetry (MIP) was employed to analyze macropores in coals.

15 16

Figure 3. Mercury porosimetry results of six coals.

17 18

Figure 3(a) revealed that the intruded mercury volume saturation of all coal samples improved with increasing

19

the intrusion pressure. For the altered coals, the intrusion curves exhibited lower slope, and the hysteresis loops

20

of intrusion-extrusion curves were large in Figure 3(a), which meant the well-developed macropores and poor


Page 6 of 18

Page 7 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

connection of pores in the altered coals.41 This was consistent with the result of N2 adsorption analysis. As

2

shown in Figure 3(b), for the altered coals, the pore distribution peaks with pore diameter > 20 nm were higher

3

compared to the normal coals, which also verified that much larger pores and more cracks were formed in coals

4

after igneous intrusions. Comprehensive pore characteristic parameters of six coals are shown in Table 2

5

obtained from N2 adsorption and MIP. The altered coals exhibited smaller BET surface area, micropore volume,

6

and mesopore volume, indicating that pore structure of the altered coals was destroyed under the impact of the

7

thermal alteration. The phenomenon was ascribed to the coalescence and collapse of neighboring pores in coals,

8

because micropores and mesopores contributed more to BET surface area of coals. Thus, for the altered coals,

9

the significant reduction of BET surface area and larger macropore volume confirmed the pore structure

10

evolution in these coals from micropores and/or mesopores to macropores, due to the remove of water and

11

volatile matter under the impact of the thermal alteration. Moreover, the altered coals demonstrated higher

12

porosity (9.11%, 10.4%, and 9.96%) than the normal coals (7.61%, 7.09%, and 6.17%). The changes in pore

13

structure would influence the wettability of coals. Typically, Zhou at al. found that coals were much easier to be

14

wetted with increasing the porosity of coals.40

15

Table 2. Pore Structure Characteristic Parameter of Six Coals.

N2 Adsorption BET surface area HK micropore volume (m2/g) ×0.001 (cm3/g) N1-N 15.5 5.1 N1-A 3.9 1.2 S5-N 12.7 3.8 S5-A 3.7 1.3 S2-N 15.6 5.7 S2-A 6.0 1.9 16 3.3. SEM Analysis of Coal Samples Sample

BJH mesopore volume ×0.001 (cm3/g) 18.6 6.4 16.1 10.9 17.4 8.52

17


MIP Macropore volume ×0.001 (cm3/g) 8.1 28.4 9.6 26.3 6.5 20.6

Porosity (%) 7.61 9.11 7.09 10.4 6.17 9.96

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

2 3

Figure 4. SEM images of six coals from 3 coal seams.

4 5

Igneous intrusions will change the surface morphology and roughness, which was significant for the

6

wettability of coals. As shown in Figure 4, the normal coals showed the contiguous surface morphology with

7

little obvious large pores and cracks. On the contrary, the surface of altered coals became rougher, and formed

8

much larger pores and more cracks. Wiącek et al. widely investigated the wettability of biomaterials, and

9

concluded that low-temperature plasma modification changed the surface structures and properties, which

10

revealed the importance of surface features on the wettability of materials.42-46 Typically, Xia39 and Niu47

11

investigated the influence of heating and pyrolysis on the coals' structure features and the wettability, concluding

12

that the development of obvious cracks and pores on the surface of coal increased its roughness, and finally

13

enhanced the wettability of coals. From the results of pore structure parameter (Table 2) and surface morphology

14

analysis (Figure 4), it can be deduced that igneous intrusions destroyed the surface structure of coals, which

15

finally increased the roughness of the altered coals. The differences in the surface morphology and roughness

16

between the normal and altered coals from 3 different coal seams were due to the release of hydrocarbon gases

17

(mainly methane) and the formation of pyrolysis pores under the impact of thermal metamorphism.12 More large

18

pores and cracks resulted in the increased roughness, which played the important role during the wetting process


Page 8 of 18

Page 9 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

of coals, finally promoting the water to fill up the porous surface of coals.39,40,47 These changes in surface

2

structure and roughness indicated that the altered coals possessed much better wettability than the normal coals.

3

However, the wettability and hydrophobicity of coals are governed by both the physical pore structure and

4

chemical properties of coals36,47. The chemical structure and characteristic of coals would be studied and

5

analyzed by XPS in Section 3.4.

6

3.4. XPS Analysis

7 8

Figure 5. XPS results of six coals.

9 10

Figure 5 demonstrated X-ray photoelectron spectroscopy (XPS) wide energy spectra for collected six coal

11

samples. The contents of the main elements (C, O, Si, Al, and N) and other elements (Ca, Fe, Mg, and Na) on the

12

surface of coals were summarized in Table 3. For all coals, C and O were the main element on the surface.

13

However, the differences in relative contents of elements are clearly evident. The altered coals showed higher

14

relative contents of C element, where the relative contents of O, Si, and Al elements decreased significantly due

15

to the impact of igneous intrusions. Moreover, the XPS results revealed higher relative contents of Ca, Fe, and

16

Mg elements in the altered coals, which agreed with the conclusion of concentrated carbonates analyzed in ash

17

compositions. For coals, the mineral matters on their surface are more hydrophilic than organic matters.47 The

18

content changes in C and O elements in coals meant that the hydrophobic groups and hydrophilic groups were

19

also altered by igneous intrusions.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Table 3. Elements Relative Contents on the Surface of Six Coals Obtained from XPS Wide Energy Spectra. Sample

2

Page 10 of 18

C N1-N 64.59 N1-A 68.65 S5-N 52.73 S5-A 63.09 S2-N 58.63 S2-A 66.23 The XPS carbon

Mole contents (%) O 25.48 22.37 32.67 24.94 29.19 23.34 (1s) spectra

Si Al N Ca Fe 4.54 2.91 2.16 / / 3.45 2.24 1.61 0.98 / 6.78 5.16 1.41 / / 4.45 3.07 1.52 0.87 0.78 5.96 3.81 1.21 / / 4.14 3.19 1.30 0.97 / were resolved into five individual peaks as exhibited

Mg Na / 0.32 0.41 0.29 0.58 0.67 0.77 0.50 0.58 0.62 0.49 0.33 in Figure 6, including

3

π-π* shake-up satellite peak at 290.4 eV, COO- (carboxyl at 289.0 eV), C=O (carbonyl at 287.5 eV), C-O-

4

(phenolic or ether carbon at 286.1 eV, and C-C/C-H at 284.6 eV corresponding to aromatic/aliphatic carbon.30

5

Among these five functional groups, C-C/C-H and π-π* shake-up satellite peak are hydrophobic groups, while

6

others are hydrophilic oxygen-containing functional groups.48

7 8

Figure 6. C 1s fitting spectra of coal samples.

9 10

The fitting parameters and relatively contents of each functional group at a specific binding energy in coals

11

obtained from the peak area were listed in Table 4. For all coals, the C-C/C-H group with the highest content


Page 11 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

dominated the all groups in coals, whereas the π-π* peak content was the lowest among above five peaks. In

2

addition, the altered coals showed higher contents of π-π* peak and C-C/C-H due to the special geologic

3

condition during igneous intrusions. On the contrary, the total contents of oxygen-containing groups (mainly

4

including C-O-, C=O, and COO-) decreased significantly because of thermal metamorphism. The increase in the

5

contents of hydrophobic groups (C-C/C-H and π-π* peak) and the decreased contents of other groups (including

6

C-O-, C=O, and COO-) should have enhanced surface hydrophobicity of these coals after igneous intrusions.48

7

However, the pore structure and SEM analysis results revealed that intrusions improved the surface wettability

8

of coals. Therefore, based on the aforementioned analysis results of physicochemical structure, the final

9

wettability of six coal samples including the normal and altered coals will be discussed and given by contact

10

angle experiment and wetting time test. Table 4. Carbon-oxygen Forms in Six Coals from XPS Analysis.

11 Forms

12

FWHM/eV

Binding energy/eV

C-C/C-H 1.34 284.6 C-O1.40 286.1 C=O 1.40 287.5 COO1.40 289.0 π-π* 1.40 290.4 3.5. Wettability Analysis of Coal Samples

N1-N 78.14 17.32 3.21 1.12 0.21

N1-A 81.06 14.63 1.68 0.94 1.69

Relative contents (%) S5-N S5-A S2-N 75.16 77.94 75.53 18.98 15.35 19.16 3.65 3.81 3.82 1.69 1.47 1.42 0.52 1.43 0.07

S2-A 78.28 17.50 1.35 1.00 1.87

13

14

15 16

Figure 7. Changes of contact angle as a function of time based on optical measurements.

17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Figure 7 demonstrates the contact angle evolution process of coals from 0 s to 10 s, respectively. Typically,

2

the contact angle at 5 s was selected to compare the difference to analyze the wettability of different coal

3

samples. For all coals, after the water droplet dripped on the coal pie, the value of contact angle would change

4

continuously during the whole wetting process until the coal was entirely wetted. The contact angle of all coals

5

was illustrated in Figure 8(a) to further compare the wettability of the normal and altered coals quantitatively.

6

Because of the thermal metamorphism during the process of igneous intrusions, for the coals collected form the

7

same coal seams in Daxing Mine, the contact angles of coals decreased from 61.97° to 44.23°, from 57.72° to

8

46.11°, and from 58.91° to 42.34°, respectively. The lower contact angle meant the better wettability of coals,

9

which indicated the improved wettability of coals after igneous intrusions.

10 11

Figure 8. Contact angle values at 5 s and wetting time of six coals.

12 13

The wetting time of these six coals was presented in Figure 8(b). The coals after igneous intrusions were

14

more easily wetted compared to the normal coals, which was evidenced by the decreased wetting time from 64 s

15

to 43 s, from 58 s to 47 s, and from 59 s to 42 s. Combining with the Section 3.4 XPS analysis, the contents of

16

C-C/C-H and π-π* shake-up satellite peak increased significantly while the total contents of C-O-, C=O, and

17

COO- groups decreased obviously. And results meant surface hydrophobicity of these altered coals should be

18

increased after igneous intrusions. However, the smaller contact angle and shorter wetting time for the altered

19

coal in Figure 8 are contrary to the XPS analysis conclusions. This was because the wettability of the normal and

20

altered coals was also affected by several other aspects, including the chemical compositions, pore structure, and

21

surface morphology of coals.

22

During the process of igneous intrusions, hydrothermal fluids usually caused the precipitation of many

23

inorganic mineral matters in coals due to the special geologic condition, and further increased the ash contents of


Page 12 of 18

Page 13 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

the altered coals, especially leading to the enrichment of carbonates (mainly the calcite), which caused the

2

altered coals more affinity to water.36,37,49 Moreover, thermal metamorphism during igneous intrusions destroyed

3

the original pore structure of coals as evidenced by pore structure parameters analysis (Table 2) and surface

4

morphology analysis (Figure 4). The altered coals experienced the pore evolution process from micropores

5

and/or mesopores to macropores attributed to the remove of water and volatile matter during igneous intrusions.

6

This resulted in the significant reduction of BET surface area, larger macropore volume, and higher porosity in

7

the altered coals, which made the altered coals easier to be wetted.40 In addition, pore evolution process changed

8

the surface morphology of coals in Figure 4. For the altered coals, thermal metamorphism increased the surface

9

roughness by forming more cracks and large pores, which was more easily filled up with water to be wetted.39,47

10

Thus, igneous intrusions finally enhanced the wettability of coals. That is, the altered coals are more hydrophilic

11

and easily wetted than the normal coals.

12

4. CONCLUSIONS

13

It is believed that this research of changes in surface structure and wettability of coals after igneous intrusions

14

could be helpful for controlling coal mine disasters during mining, and provide valid references for the

15

subsequent use in the field of coal chemical industry. In this study, the main findings can be derived from the

16

above analysis as follows:

17

(1) The chemical compositions of coals were changed significantly by igneous intrusions, which could be

18

evidenced by the decreases in volatile matter and moisture contents, as well as the increases in fixed carbon and

19

ash contents. Typically, the epigenetic minerals (mainly carbonates) were distinctly enriched in coals after

20

igneous intrusions compared to the normal coals, which would enhance the wettability of the altered coals.

21

Moreover, after thermal metamorphism, higher porosity, larger macropore volume, and rougher surface were

22

found for the altered coals. These coals with much larger pores and more cracks on their surface would be filled

23

up by water more quickly and be wetted more easily.

24

(2) Igneous intrusions affected the functional groups of coals because of thermal metamorphism. Compared

25

with the normal coals, the increased hydrophobic groups (π-π* shake-up satellite peak and C-C/C-H) contents in

26

coals after igneous intrusions should have improved surface hydrophobicity of the altered coals. However, the

27

contact angle and wetting time tests indicated that the altered coals exhibited much better wettability than the

28

normal coals. This meant that the enhanced wettability of the altered coals was mainly governed by the increase

29

in ash contents and minerals, as well as the changes in pore structures and surface morphology, other than the

30

changed surface functional groups.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 18

1

AUTHOR INFORMATION

2

Corresponding Author

3

*Botao Qin

4

Tel: +86-516-83885694.

5

Email address: [email protected]

6

Notes

7

All the authors of this paper declare no competing financial interest.

8

ACKNOWLEDGMENTS

9

This study work was funded by the following funding National Natural Science Foundation of China (No.

10

51476184).

11

REFERENCES

12

(1) Mohanty, M. M.; Pal, B. K., Sorption behavior of coal for implication in coal bed methane an overview.

13

International Journal of Mining Science and Technology 2017, 27, (2), 307-314.

14

(2) Liang, Y.; Tong, Z.; Zhao, Y.; Bo, R.; Hao, X.; Chao, X. U.; Beijing, T., Precise coordinated mining of coal

15

and associated resources:A case of environmental coordinated mining of coal and associated rare metal in Ordos

16

basin. Journal of China University of Mining & Technology 2017, 46, (3), 449-459.

17

(3) Pan, J.; Lv, M.; Bai, H.; Hou, Q.; Li, M.; Wang, Z., Effects of metamorphism and deformation on the coal

18

macromolecular structure by laser Raman spectroscopy. Energy & Fuels 2017, 31, (2), 1136-1146.

19

(4) Shi, Q.; Qin, B.; Bi, Q.; Qu, B., An experimental study on the effect of igneous intrusions on chemical

20

structure and combustion characteristics of coal in Daxing Mine, China. Fuel 2018, 226, 307-315.

21

(5) Hu, Z. X.; Hu, X. M.; Cheng, W. M.; Lu, W., Influence of synthetic conditions on the performance of

22

melamine–phenol–formaldehyde

23

https://doi.org/10.1177/0954008318758489.

24

(6) Zhang, Q.; Hu, X. M.; Wu, M. Y.; Zhao, Y. Y.; Yu, C., Effects of different catalysts on the structure and

25

properties of polyurethane/water glass grouting materials. Journal of Applied Polymer Science 2018, 135, (27),

26

https://doi.org/10.1002/app.46460.

27

(7) Hu, Z. X.; Hu, X. M.; Cheng, W. M.; Zhao, Y. Y.; Wu, M. Y., Performance optimization of one-component

28

polyurethane healing agent for self-healing concrete. Construction and Building Materials 2018, 179, 151-159.

29

(8) Kong, B.; Li, Z.H.; Wang, E.Y.; Yang, Y.L.; Chen, L.; Qi, G.S., An experimental study for characterization

resin

microcapsules.

High


Performance

Polymers

2018,

Page 15 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

the process of coal oxidation and spontaneous combustion by electromagnetic radiation technique. Process

2

Safety and Environmental Protection 2018, 1-27, https://doi.org/10.1016/j.psep.2018.08.002.

3

(9) Kong, B.; Wang, E.Y.; Li, Z.H., The effect of high temperature environment on rock properties - an example

4

of electromagnetic radiation characterization. Environmental Science and Pollution Research 2018, 1-22,

5

https://doi.org/10.1007/s11356-018-2940-z

6

(10) Finkelman, R. B.; Bostick, N. H.; Dulong, F. T.; Senftle, F. E.; Thorpe, A. N., Influence of an igneous

7

intrusion on the inorganic geochemistry of a bituminous coal from Pitkin County, Colorado. International

8

Journal of Coal Geology 1998, 36, (3–4), 223-241.

9

(11) Dai, S. F.; Ren, D. Y., Effects of magmatic intrusion on mineralogy and geochemistry of coals from the

10

Fengfeng−Handan Coalfield, Hebei, China. Energy & Fuels 2007, 21, (3), 1663-1673.

11

(12) Jiang, J. Y.; Cheng, Y. P.; Wang, L.; Li, W.; Wang, L., Petrographic and geochemical effects of sill intrusions

12

on coal and their implications for gas outbursts in the Wolonghu Mine, Huaibei Coalfield, China. International

13

Journal of Coal Geology 2011, 88, (1), 55-66.

14

(13) Presswood, S. M.; Rimmer, S. M.; Anderson, K. B.; Filiberto, J., Geochemical and petrographic alteration

15

of rapidly heated coals from the Herrin (No. 6) Coal Seam, Illinois Basin. International Journal of Coal Geology

16

2016, 165, 243-256.

17

(14) Yang, H.; Xu, Y.; Peng, G.; Yu, G.; Chen, M.; Duan, W.; Zhu, Y.; Cui, Y.; Wang, X., Particle swarm

18

optimization and its application to seismic inversion of igneous rocks. International Journal of Mining Science

19

and Technology 2018, 47, (5), 183-193.

20

(15) Ye, T.; Wei, A. J.; Peng, J. S.; Gao, K. S.; Deng, H.; Lu, F. T.; Guo, Y.; Ren, Y. P., Characteristcs and

21

reservoir-forming differences of volcanic lithofacies in Yixian formation of Cretaceous in Bohai Bay area.

22

Journal of China University of Mining & Technology 208, (1), 112-120.

23

(16) Xia, W.; Yang, J., Effect of pre-wetting time on oxidized coal flotation. Powder Technology 2013, 250, (12),

24

63-66.

25

(17) Wang, J.; Liu, J.; Wang, S.; Cheng, J., Slurrying property and mechanism of coal–coal gasification

26

wastewater–slurry. Energy & Fuels 2018, 32(4), 4833-4840.

27

(18) Ibrahim, A. F.; Nasreldin, H. A., Effect of water salinity on coal wettability during CO2 sequestration in coal

28

seams. Energy & Fuels 2016, 30, (9), 7532-7542.

29

(19) Xi, Z.; Feng, Z.; Li, A., Synergistic coal dust control using aqueous solutions of thermoplastic powder and

30

anionic surfactant. Colloids & Surfaces A Physicochemical & Engineering Aspects 2017, 520, 864-871.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

(20) Cheng, W.; Xue, J.; Xie, J.; Zhou, G.; Nie, W., A model of lignite macromolecular structures and its effect

2

on the wettability of coal – A case study. Energy & Fuels 2017, 31, (12), 13834-13841.

3

(21) Arif, M.; Jones, F.; Barifcani, A.; Iglauer, S., Influence of surface chemistry on interfacial properties of low

4

to high rank coal seams. Fuel 2017, 194, 211-221.

5

(22) Cheng, W.; Hu, X.; Xie, J.; Zhao, Y., An intelligent gel designed to control the spontaneous combustion of

6

coal: Fire prevention and extinguishing properties. Fuel 2017, 210, 826-835.

7

(23) Xu, G.; Chen,Y. C.; Eksteen, J.; Xu, J. L., Surfactant-aided coal dust suppression: A review of evaluation

8

methods and influencing factors. Science of the Total Environment 2018, 639, 1060-1076.

9

(24) Czech, B.; Oleszczuk, P.; Wiącek, A., Advanced oxidation (H2O2 and/or UV) of functionalized carbon

10

nanotubes (CNT-OH and CNT-COOH) and its influence on the stabilization of CNTs in water and tannic acid

11

solution. Environmental pollution 2015, 200, 161-167.

12

(25) Czech, B.; Oleszczuk, P.; Wiącek, A. E.; Barczak, M., Water treatment by H2O2 and/or UV affects carbon

13

nanotube (CNT) properties and fate in water and tannic acid solution. Environmental Science and Pollution

14

Research 2015, 22, (24), 20198-20206.

15

(26) Shi, Q.; Qin, B.; Bi, Q.; Qu, B., Fly ash suspensions stabilized by hydroxypropyl guar gum and xanthan

16

gum for retarding spontaneous combustion of coal. Combustion Science and Technology 2018,

17

https://doi.org/10.1080/00102202.2018.1491845.

18

(27) Mastalerz, M.; Drobniak, A.; Schimmelmann, A., Changes in optical properties, chemistry, and micropore

19

and mesopore characteristics of bituminous coal at the contact with dikes in the Illinois Basin. International

20

Journal of Coal Geology 2009, 77, (3–4), 310-319.

21

(28) Wu, D.; Liu, G.; Sun, R.; Fan, X., Investigation of structural characteristics of thermally metamorphosed

22

coal by FTIR spectroscopy and X-ray diffraction. Energy & Fuels 2013, 27, (10), 5823-5830.

23

(29) Yao, Y.; Liu, D., Effects of igneous intrusions on coal petrology, pore-fracture and coalbed methane

24

characteristics in Hongyang, Handan and Huaibei coalfields, North China. International Journal of Coal

25

Geology 2012, 96–97, (3), 72-81.

26

(30) Shi, Q.; Qin, B.; Liang, H.; Gao, Y.; Bi, Q.; Qu, B., Effects of igneous intrusions on the structure and

27

spontaneous combustion propensity of coal: A case study of bituminous coal in Daxing Mine, China. Fuel 2018,

28

216, 181-189.

29

(31) Wiącek, A. E., Changes in wetting properties of silica surface treated with DPPC in the presence of

30

phospholipase A2 enzyme. Applied Surface Science 2010, 256, (24), 7672-7677.


Page 16 of 18

Page 17 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

(32) Wiącek, A. E., Changes in wetting properties of alumina surface treated with DPPC in the presence of

2

phospholipase A2 enzyme. Colloids and Surfaces B: Biointerfaces 2011, 87, (1), 54-60.

3

(33)

4

dipalmitoylphosphatidylcholine and lipase enzyme. Powder technology 2011, 212, (2), 332-339.

5

(34) Wiącek, A. E., Investigations of DPPC effect on Al2O3 particles in the presence of (phospho) lipases by the

6

zeta potential and effective diameter measurements. Applied Surface Science 2011, 257, (9), 4495-4504.

7

(35) Chen, J.; Liu, G.; Li, H.; Wu, B., Mineralogical and geochemical responses of coal to igneous intrusion in

8

the Pansan Coal Mine of the Huainan coalfield, Anhui, China. International Journal of Coal Geology 2014, 124,

9

(4), 11-35.

Wiącek,

A.

E.,

The

wetting and

interfacial

properties

of

alumina

surface

treated

with

10

(36) Cheng, W.; Xue, J.; Zhou, G.; Nie, W.; Wen, J., Research on the relationship between bituminous coal dust

11

wettability and inorganic mineral content. Journal of China University of Mining & Technology 2016, 45(3),

12

462-468.

13

(37) Gosiewska, A.; Drelich, J.; Laskowski, J. S.; Pawlik, M., Mineral matter distribution on coal surface and its

14

effect on coal wettability. Journal of Colloid & Interface Science 2002, 247, (1), 107-116.

15

(38) Ji, H.; Cheng, W.; Hu, S.; Li, J.; Chen, J., Effects of organic micromolecules in coal on its wettability.

16

Journal of Dispersion Science & Technology 2017, 1-7.

17

(39) Xia, W.; Li, Y., Role of Roughness Change on wettability of Taixi anthracite coal surface before and after

18

the heating process. Energy & Fuels 2016, 30, (1), 281-284.

19

(40) Zhou, G.; Qiu, H.; Zhang, Q.; Xu, M.; Wang, J.; Wang, G., Experimental investigation of coal dust

20

wettability based on surface contact angle. Journal of Chemistry,2016, 2016, 2016, 1-8.

21

(41) Li, X.; Kang, Y.; Haghighi, M., Investigation of pore size distributions of coals with different structures by

22

nuclear magnetic resonance (NMR) and mercury intrusion porosimetry (MIP). Measurement 2018, 116,

23

122-128.

24

(42) Wiącek, A. E.; Terpiłowski, K.; Jurak, M.; Worzakowska, M., Low-temperature air plasma modification of

25

chitosan-coated PEEK biomaterials. Polymer Testing 2016, 50, 325-334.

26

(43) Wiącek, A. E.; Terpiłowski, K.; Jurak, M.; Worzakowska, M., Effect of low-temperature plasma on

27

chitosan-coated PEEK polymer characteristics. European Polymer Journal 2016, 78, 1-13.

28

(44) Jurak, M.; Wiącek, A. E.; Terpiłowski, K., Properties of PEEK-supported films of biological substances

29

prepared by the Langmuir-Blodgett technique. Colloids and Surfaces A: Physicochemical and Engineering

30

Aspects 2016, 510, 263-274.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

(45) Jurak, M.; Wiącek, A. E., Wettability of Hybrid chitosan/phospholipid coating. Progress on Chemistry and

2

Application of Chitin and its Derivatives 2017, 22, 66-76.

3

(46) Wiącek, A. E.; Gozdecka, A.; Jurak, M.; Przykaza, K.; Terpiłowski, K., Wettability of plasma modified glass

4

surface with bioglass layer in polysaccharide solution. Colloids and Surfaces A: Physicochemical and

5

Engineering Aspects 2018, 551, 185-194.

6

(47) Niu, C.; Xia, W.; Xie, G., Effect of low-temperature pyrolysis on surface properties of sub-bituminous coal

7

sample and its relationship to flotation response. Fuel 2017, 208, 469-475.

8

(48) Zhou, F.; Cheng, J.; Liu, J.; Wang, Z.; Zhou, J.; Cen, K., Activated carbon and graphite facilitate the

9

upgrading of Indonesian lignite with microwave irradiation for slurryability improvement. Fuel 2016, 170,

10

39-48.

11

(49) Fu, H. U., Study on influence of coal composition upon hydrophilicity of coal dust. Mining Safety &

12

Environmental Protection 2014, 41(6), 19-22.


Page 18 of 18

Changes in the Surface Structure of Coal Caused by Igneous

Recommend Documents