Article pubs.acs.org/EF
Deformation Mechanisms and Macromolecular Structure Response of Anthracite under Different Stress Yuzhen Han,† Rongting Xu,† Quanlin Hou,*,† Jin Wang,† and Jienan Pan‡ †
Key Laboratory of Computational Geodynamics, Chinese Academy of Sciences (CAS), College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ‡ School of Resource and Environment, Henan Polytechnic University, Jiaozuo, Henan 454000, People’s Republic of China S Supporting Information *
ABSTRACT: It has been controversial whether tectonic stress affects the chemical structure of coals. To further understand if and how tectonic stress affects the chemical structure of coals, an investigation into the macromolecular-level deformation mechanism is quite necessary. Therefore, we performed deformation experiments on anthracite with axial load both parallel and perpendicular to the bedding plane. Macromolecular structure variations during deformation were revealed by a combination of Raman spectroscopy and high-resolution transmission electron microscopy. Results show that (1) coal deformation behaviors (brittle deformation or ductile deformation) are influenced by stress direction, in addition to the temperature and strain rate, and (2) ductile deformation is related to the growth of defects, which is more favorable under stress parallel to the bedding plane, at higher temperature and lower strain rate, while brittle deformation is presumably due to the direct bond breakage beyond the elastic limit. On the basis of the study of coal deformation mechanisms, we suggest that hydrocarbon generation under stress is possible and that it may become a supplement for the traditional hydrocarbon generation theory.
1. INTRODUCTION The macromolecular structure of anthracite mainly consists of polycyclic aromatic hydrocarbons that exist as aromatic layers.1 Adjacent layers are linked by bridge bonds. It has been generally recognized that the temperature and time are two dominant factors affecting the chemical structure.2,3 Confining pressure is thought to retard the chemical structure changes.4−6 According to coal geology, tectonic stress mainly plays an important role in affecting physical structures, for example, the optical property of vitrinite7,8 and the pore structure.9,10 There have long been speculations whether tectonic stress changes the chemical structure as their physical structure.11−16 In recent years, researchers have found that distinct geochemical differences exist between tectonic-deformed coals (TDCs) and primary coals17−19 and that carbon monoxide was generated in the deformation experiments of anthracite at low temperatures.20 These findings imply that tectonic stress influences the chemical structure as well. However, as far as the mechanism by which tectonic stress works, although some explanations have been proposed, it is still controversial.12,13,15,20−24 For example, some previous researchers thought tectonic stress influenced the coalification via frictional heat, because anomalously high vitrinite reflectance was found in the area adjacent to or within the shear zone of thrust faults.12,13,15 However, as a result of the short duration time and limited generation area, frictional heating only has influence on coals near the faults that move rapidly. In most cases, however, the deformation rate is quite low and the small amount of heat will loss before it has any influences on coals. Thus, frictional heating is not the main cause. To further understand how stress affects the chemical structure of coals, an investigation into the macromolecularlevel deformation mechanism is quite necessary. Coal deformation has been studied by many researchers, and a number of © 2016 American Chemical Society
deformation experiments on coals were conducted aiming at various purposes.7,25−28 However, few addressed the deformation mechanism and the macromolecular structure response. With the development of Raman spectroscopy27−30 and highresolution transmission electron microscopy (HRTEM),31−34 the structural characterization of coals improved a lot. For carbonaceous materials with perfect graphitic structure, only the G band exists in Raman spectra, corresponding to the E2g symmetrical stretching vibration mode in the aromatic layers.35,36 In other carbonaceous material, the D band, which is related to structure defects, also appears.35−37 On the basis of the work of Raman, Li et al.19 showed that defects in ductiledeformed coals exceeded these in brittle-deformed coals. HRTEM (002 bright field mode) is a direct way to visualize aromatic layers.31 In this mode, aromatic layers are imaged as fringes.31−34 Via quantitative analysis, researchers obtained fringe length and tortuosity from HRTEM images.32−34 Fringe length is a measure of the extent of aromatic layers, and fringe tortuosity reflects the wrinkle degree of aromatic layers.34 Studies on graphene indicated that the wrinkle degree of the sheet was related to the appearance of Stone−Wale (SW) defects.38−40 SW defects are induced via in-plane rotation of C−C bonds by 90°, where four hexagons are transformed into two pentagons and two heptagons.41 The propagation and interaction of defects were shown to play an important role in the ductile deformation of nanotube and graphene, while direct bond breakage might result in brittle deformation.42−45 Aromatic layers in anthracite are nearly parallel to the bedding plane under lithostatic pressure.46 To study the deformation Received: December 3, 2015 Revised: January 7, 2016 Published: January 11, 2016 975
DOI: 10.1021/acs.energyfuels.5b02837 Energy Fuels 2016, 30, 975−983
Article
Energy & Fuels
Henan, China. Zhaogu No. 2 coal mine is located in the eastern part of Jiaozuo Colliery (Figure 2a). Structurally, it is monoclinic, striking northwest and dipping to the southwest at an angle of 4−10° (Figure 2b). Zhaogu No. 2 coal mine did not go through complex tectonic movement since its formation. The thick No. 2 coal seam in Shanxi formation of Lower Permian is the main workable seam and retains primary structure. The starting whole anthracite used in this study was collected from it. General information on the starting material is presented in Table 1. Nine cylinders with the same diameter of 20 mm and length of 40 mm were drilled from the whole anthracite. Samples Y04−Y08 were cored along the bedding plane and then were deformed in the apparatus with axial load parallel to the bedding plane. Samples Y09−Y12 were drilled perpendicular to the bedding plane and then were deformed with axial load perpendicular to the bedding plane. Deformation experiments were performed in the State Key Laboratory of Earthquake Dynamics in the Institute of Geology, China Earthquake Administration. The experimental apparatus (Figure 3) included a high-temperature and high-pressure gas medium triaxial test system, with argon as the gas medium. A YAMATAKE DCP30 controller was used to control the temperature, and a digital hydraulic servo machine was used to control the axial pressure. The entire sample, tungsten carbide, and a corundum column were all packaged in an annealing copper tube with a wall thickness of 0.35 mm. The two heads of the copper tube had an O-ring seal to isolate the gas medium and the sample. The area between the copper pipe and the heat furnace was filled with boron nitride powder, which was used for heat transfer and preventing gas convection. The experimental apparatus also included a thermocouple channel. Every deformation experiment terminated when the stress−strain curve displayed a sharp stress drop (i.e., macroscopic fractures occurred) or the strain reached 10%. Experimental conditions for every sample are summarized in Table 2. 2.2. Raman Spectroscopy and Spectra Analysis. Raman spectra of the samples were examined by a Renishaw RM-1000
mechanisms, we performed deformation experiments on anthracite with axial load both parallel and perpendicular to the bedding plane. Raman spectroscopy and HRTEM were used to reveal how macromolecules, especially structure defects, evolved during the deformation. On the basis of the structure characteristics, we discussed the deformation mechanisms and macromolecular structure response under different stress. The possibility of hydrocarbon generation from coals under stress was also discussed.
2. EXPERIMENTAL SECTION 2.1. Deformation Experiments. The starting material used in the deformation experiments was a whole anthracite sample (40 × 60 × 60 cm) (Figure 1) from Zhaogu No. 2 coal mine of Jiaozuo Colliery,
Figure 1. Photograph of the starting material.
Figure 2. Geological backgrounds of the starting material: (a) geological map of Jiaozuo Colliery and (b) structure outline map of Zhaogu No. 2 coal mine. 976
DOI: 10.1021/acs.energyfuels.5b02837 Energy Fuels 2016, 30, 975−983
Article
Energy & Fuels Table 1. General Information on the Starting Material coal rank anthracite
location
geological age
Jiaozuo Colliery Late Permain
Ro,max (%)
vitrinite (%)
inertinite (%)
exinite (%)
mineral (%)
3.21
86.9
4.8
−
8.3
To obtain detailed and fine data of Raman spectra, a curve-fitting procedure was performed within the range of 1000−1800 cm−1 using PeakFit, version 4.12, software. 2.3. HRTEM and Quantitative Analysis. HRTEM observations were performed on a Hitachi H-9000NAR using an acceleration voltage of 100 kV at Peking University. Samples were first examined at moderate magnification to find wedge-shaped fragments that were optically thin at the edge to minimize the superposition of fringes. Several such edge regions were then photographed at high magnifications (350000×) in 002 bright field imaging mode. A total of 15−20 pictures were taken for every sample. The quantitative analytical method, which was used to extract fringe tortuosity from HRTEM images, adopted here was detailed by Yehliu et al.34 The method consists of two parts: image processing and quantitative characterization. The image processing is composed of the following steps: (1) negative transformation (to transform the background to lower intensity pixels and fringes to higher intensity pixels), (2) region of interest (ROI) selection (to select regions containing representative and clear fringe information), (3) contrast enhancement of ROI (to increase the intensity difference between a fringe and a background pixel through a method called histogram equalization), (4) Gaussian lowpass filter (to filter background noise), (5) top-hat transformation (to correct uneven illumination across the image), (6) thresholding to obtain a binary image (to extract fringes from the background using Otsu’s method), (7) fringe morphological opening and closing (to remove joints with three connected neighbors, i.e., Y- and T-shaped links), and (8) skeletonization and removing short fringes (
