Article pubs.acs.org/EF
Molecular Model of Xishan Bituminous Coal Surface and Its Wetting Properties Zhiqiang Zhang,*,† Qiannan Kang,† Shuai Wei,† Tao Yun,† Guochao Yan,†,‡ and Kefeng Yan§,∥ †
College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, P. R. China Research Center of Coal Resources Safe Mining and Clean Utilization, Liaoning Technical University, Fuxin 123000, P. R. China § Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, P. R. China ∥ Guangzhou Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, P. R. China ‡
ABSTRACT: Microscopic understanding of the coal surface structure offers opportunities to enhance coal preparation and utilization. This study contributes to the construction of a surface model of Xishan bituminous coal based on a recently proposed coal bulk model. An XPS survey spectrum reveals that the surface of this coal sample was partially oxidized by atmospheric oxygen. Therefore, different oxygen-containing groups were randomly incorporated into a 3 nm surface layer of the original surface model to represent the realistic oxidized coal surface based on the XPS and NMR results. We also probed the wetting properties of this model both in water and in methanol. The possible reasons for the discrepancy between the computed and experimental values were also discussed. Although both water and methanol can penetrate and fill micropores in the coal surface layer, methanol is found to swell coal to a greater extent than water. The association of coal molecules in this model is caused mainly by van der Waals energy. However, the wetting of the coal surface layer predominantly decreases the electrostatic interactions of coal−coal. In agreement with the wetting experiments, methanol is more effective to reduce the interaction of coal−coal than water.
1. INTRODUCTION Molecular chemical structure on coal surface could be different from that in bulk. The surface will be oxidized once the reactive radicals in the surface layer of coal particles are exposed to O2 in the air. These radicals could be produced on the freshly created surfaces by mechanochemical effects due to bond ruptures of coal molecules.1,2 On the contrary, the interior of coal particles may seldom be oxidized due to inaccessibility of O2 molecules and lack of reactive radicals. As a result, a significant content of oxygenated species will be formed in the surface layer of coal than in the bulk. X-ray photoelectron spectrometry (XPS) is a surface sensitive technique that enables direct quantification of oxygen as well as other elements at the uttermost atomic layers of the coal (within the range 0.5−5 nm).3 The oxygen enrichment on the coal surface has been confirmed by comparing XPS surface data and elemental bulk composition.4,5 At the molecular level, coal can be depicted as a hydrocarbon matrix that contains various types of functional groups. It was found that C−O groups will be first formed after fission of C−C and C−H bonds in alkyl groups of coal in the oxidation processes. Then, the C−O groups are oxidized to CO groups, and the CO groups are oxidized to O−CO groups.6,7 The type and distribution of all of these oxygencontaining groups in the coal surface layer can be evaluated by XPS spectra quantitatively via the curve-fitting method.8,9 The oxygen-containing functional groups are predominantly the most important groups on the coal surface by providing binding sites for water molecules,10 altering the wetting behavior of coal. This will play a major role in many coal preparation and utilization processes, e.g., coal dust abatement, flotation, dewatering, and preparation of coal−water slurries. © XXXX American Chemical Society
Therefore, studies of oxygen-containing groups over coal surfaces are important for understanding the fundamental nature of coal as well as for improving the water-related coal preparation and utilization processes. Heats of wetting of coal have been shown to be a valuable means of investigating structure and chemistry of coal.11 The heats of wetting of coal with water and methanol have been measured calorimetrically, which is proven to be quite helpful in elucidation and interpretation of wetting tendencies and surface polarity. This technique has been used to obtain fundamental information concerning interactions of the coal surface with flotation reagents.12 The results achieved in the calorimetric experiments correlated well with independent flotation tests. Besides, the calorimetric evaluation is also a useful method to screen inhibitors of spontaneous heating of coal.13 Understanding the coal surface structure at the molecular level offers opportunities to enhance coal preparation and utilization. Ultra-high-resolution electron microscopy is a powerful tool to study properties of materials on the atomic scale. At present, it is possible to achieve a clear image of the element columns in crystal materials projected along the viewing direction using this characterization technique.14,15 However, this new technique is unavailable for coal due to its amorphous nature. XPS and SEM mapping techniques have also been used for elemental analysis of the coal surface.16−18 However, the atom-scale structure of the coal surface cannot be Received: May 9, 2017 Revised: August 16, 2017 Published: August 21, 2017 A
DOI: 10.1021/acs.energyfuels.7b01350 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
structure.21 First, all 62 coal molecules in the bituminous coal bulk model (C7972H4882 O115N50S 30) were randomly packed into a rectangular simulation cell using the tool Packmol.24 The dimensions of the cell were set to 52 × 52 × 250 Å3 (X × Y × Z) with threedimensional periodic boundary conditions applied to encourage the formation of the coal surface in the Z-direction. Then, to achieve structure relaxation of this model, the initial temperature of the simulated annealing algorithm was initially set to 998 K. The temperature decreased once every 50 K, and 50 ps NVT molecular dynamics simulations were performed at each temperature point until the end temperature of 298 K. At this point, the system was reequilibrated for 500 ps. The last configuration was optimized using a steepest descent method and was taken as the initial model for the coal surface (Figure 1a). This model consisted of an approximately 60 Å
obtained by these techniques due to their spatial resolution of μm. Molecular simulation has been proven to be particularly useful in the description of the molecular structure of coal.19,20 A three-dimensional coal molecular surface model is desirable because it not only defines the structural information with a reasonable degree of accuracy but also provides a new way to exploit the interaction mechanism of coal and reagents used in coal preparation and utilization, as well as predict and evaluate the reagent effect. In our previous work,21 a Chinese Xishan bituminous coal bulk model has been constructed using ultimate analysis, HRTEM, XPS, 13C NMR, LD-TOF MS, and molecular simulation. Here, we focused our research on constructing the surface model of this coal and exploring its interactions with wetting liquids. This will bring us to a better understanding of the heterogeneous coal surface and the appropriate coal surface interactions. Furthermore, this model may also be applied to represent other bituminous coal surfaces by proper modification according to their surface characterization results.
2. METHODOLOGY 2.1. Coal Samples. A typical run-of-mine bituminous coal was obtained from Xishan coalfield (Taiyuan, Shanxi, China). This lump coal sample was crushed to 2 mm. Then, the resulting coal particles were dry-ground and wet sieved through a 74 μm sieve. Demineralization was performed using a three-step HCl/HF/HCl acid treatment procedure described in another study22 to minimize the influence of minerals on the surface properties of coal. The precipitate was rinsed with copious distilled water until the pH value of the filtrate became neutral, after which the sample was vacuum-dried. 2.2. Characterization of Coal Surface. To evaluate surface concentration of different oxygen-containing functional groups, XPS measurement has been carried out on the coal sample using an RBD upgraded PerkinElmer PHI-5000C ESCA spectrometer with the monochromatic Al Kα radiation operated at 250 W. To ensure sufficient resolution and sensitivity, a constant pass energy of 93.9 eV was used. The high voltage was kept at 14.0 kV with a detection angle of 54°. The base pressure of the analyzer chamber was about 5 × 10−8 Pa. Binding energies were calibrated based on the graphite C 1s peak at 284.6 eV. Specific surface area of Xishan bituminous coal was determined by CO2 gas adsorption using the Dubinin−Radushkevich equation.23 This experiment was carried out using an automated Micromeritics ASAP 2460 analyzer. About 200 mg of sample was used. The sample was first evacuated at 333 K under vacuum (
