Changes in the Surface Structure of Coal Caused by Igneous

aSchool of Safety Engineering, China University of Mining and Technology ... field of coal mine safety, the wettability could be used to guide the con...
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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

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Changes in Surface Structure of Coal Caused by Igneous Intrusions

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and Its Effect on the Wettability

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

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a

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

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Daxing Mine, Liaoning 112700, China

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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structures, and surface morphology.

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1. INTRODUCTION

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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coal chemical industry.

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2. EXPERIMENTAL SECTION

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2.1. The Preparation of Coals

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

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

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Figure 1.

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

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

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

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2.3. X-ray Photoelectron Spectroscopy Test

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

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

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

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2.4. Pore Structure Test

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

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

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changes in mesopores and macropores structure of coals.

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2.5. Surface Morphology Analysis

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

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

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

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high vacuum mode.

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2.6. Wettability Analysis of Coals

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2.6.1. Contact Angle Experiment

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

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

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

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

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2.6.2. Wetting Time Test

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

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

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

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

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

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3. RESULTS AND DISCUSSION

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3.1. Chemical Compositions Changes of Coals after Igneous Intrusions

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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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

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

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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,

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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

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

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

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

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

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

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

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

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

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

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

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

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Figure 3. Mercury porosimetry results of six coals.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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wetted with increasing the porosity of coals.40

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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

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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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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analyzed by XPS in Section 3.4.

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3.4. XPS Analysis

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Figure 5. XPS results of six coals.

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

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

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

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

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

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

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

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

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

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also altered by igneous intrusions.

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

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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

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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-

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

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

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others are hydrophilic oxygen-containing functional groups.48

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

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

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

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

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

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

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

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

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

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

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

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

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

11 Forms

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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

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

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

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

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

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

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

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

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

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

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which indicated the improved wettability of coals after igneous intrusions.

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

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

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

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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

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

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

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

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

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

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surface morphology of coals.

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

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

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

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

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

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

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

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

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

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

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

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

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and easily wetted than the normal coals.

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4. CONCLUSIONS

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

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

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

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above analysis as follows:

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

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

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

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

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

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

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up by water more quickly and be wetted more easily.

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

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

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

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

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

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

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changed surface functional groups.

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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.

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ACKNOWLEDGMENTS

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

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51476184).

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