Macromolecular evolution and structural defects in tectonically deformed coals

Song, Yu; Jiang, Bo; Meijun, Qu

doi:10.1016/j.fuel.2018.09.080

Cited by 51 publications

(79 citation statements)

References 99 publications

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“…Compared with fal, the fa of vitrinite and inertinite gradually increases (Figure 10b), suggesting that the aliphatic carbons have a process of transforming into aromatic carbons that can also be triggered by tectonic stress. The fal decreases and fa increases, so the fa/fal consequentially increases with the deformation degree ( Figure 10g), leading to an increase in the relative aromatic carbons, which is similar to the findings of Song et al [26]. The fa H , fa B , fa S and fa P show no obvious regularities with change in the deformation degree (Figure 10c-f).…”

Section: Nmr Experiments Resultssupporting

confidence: 85%

“…It has been confirmed that all aliphatic side chains are an critical part of macromolecules in coal [8]. As the aliphatic side chains detach, there will be many structural defects in the macromolecular structure in the TDC [26], gradually causing gaps among macromolecular groups to emerge. From unaltered coal to powdery coal, the CH 2 /CH 3 values of vitrinite and inertinite decrease by 25% and 24%, respectively and when the cone-shaped rigid indenter is applied, there should be more space among the macromolecular groups for the extrusion of the macromolecular groups caused by the indenter, thus reducing the hardness of the tectonic coal (Figure 17b,c).…”

Section: Mechanisms Of Mechanical Property Evolution Controlled By Mamentioning

confidence: 96%

“…Cao et al [8] demonstrated that tectonic stress affects the chemical structure of the coal through stress degradation and stress polycondensation; stress degradation is a process in which some chemical bonds of low activation energy are broken up under tectonic stress and the stress polycondensation indicates that the condensed aromatic nuclei tend to be arranged in parallel under tectonic stress. Song et al [26] revealed that ductile deformation can promote the loss of aliphatic carbons and the degree of macromolecular alignment, while brittle deformation promoted transitions in the aliphatic structure. Liu and Jiang [13] thought that shear tectonic stress caused more stacked aromatic layers and a lower interlayer space.…”

Section: Introductionmentioning

confidence: 99%

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Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A Case Study in Fengfeng Coal Mine of Taihangshan Fault Zone (North China)

et al. 2020

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In order to study the evolution of the mechanical properties and macromolecular structures in different macerals of tectonically deformed coal (TDC), vitrinite and inertinite samples were handpicked from six block TDCs in the same coal seam with an increasing deformation degree (unaltered, cataclastic, porphyroclast, scaly and powdery coal). The micro mechanical properties were tested by the nanoindentation experiment and the macromolecular structures were measured using 13C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results show that the range of hardness and elastic modulus of inertinite is 0.373–1.517 GPa and 4.339–12.158 GPa, respectively, which is significantly higher than that of vitrinite with values of 0.278–0.456 GPa and 4.857–7.810 GPa, respectively. From unaltered coal to powdery coal, the hardness of vitrinite and inertinite gradually decreases, with the difference between these macerals becomes smaller and the elastic modulus of vitrinite shows an increasing trend, while that of inertinite was more variable. Both the NMR and FITR results reveal that the macromolecular structure of inertinite has similar structural transitions as vitrinite. As the degree of deformation increases, the aliphatic side chains become shorter and the aromaticity is increasing. Macromolecular alterations caused by tectonic stress is expected to produce defects in the TDCs, therefore there should be more interspacing among the macromolecular groups for the extrusion of macromolecules caused by the indenter of the nanoindentation experiment, thereby reducing the hardness. The elastic modulus of coal is believed to be related to intermolecular forces, which are positively correlated to the dipole moment. By calculating the dipole moments of the typical aromatic molecular structures with aliphatic side chains, the detachment of the aliphatic side chains and the growth of benzene rings can both increase the dipole moment, which can promote elastic modulus. In addition, the increasing number of benzene rings can create more π-π bonds between the molecules, which can lead to an increase in the intermolecular forces, further increasing the elastic modulus.

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Section: Nmr Experiments Resultssupporting

confidence: 85%

Section: Mechanisms Of Mechanical Property Evolution Controlled By Mamentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

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Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A Case Study in Fengfeng Coal Mine of Taihangshan Fault Zone (North China)

et al. 2020

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“…Thus, macerals, newly formed graphite particles, and some transitional matter all coexist [9,26,27]. Han et al [28] and Song et al [29] demonstrated that deformation influences the evolution of coal. González et al [8], Wilks et al [30], and Bustin et al [31] discussed the effect of minerals (e.g., illite and siderite) and deformation on coal graphitization through laboratory investigations at high temperatures and high pressures.…”

Section: Introductionmentioning

confidence: 99%

Strain-Induced Graphitization Mechanism of Coal-Based Graphite from Lutang, Hunan Province, China

et al. 2019

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Anthracite and coal-based graphite (CBG) samples were collected at varying distances from a granite intrusion. Optical microscopy, X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) were used to characterize the structural evolution of CBG at different scales. The results indicated differences in the graphitization rates of coal macerals and crystallization degree of different graphite-like particles. Differentiated graphitization of coal was caused by deformation, which led to the discontinuous distribution of CBG. This indicates that samples located at the same distance from the intrusion were graphitized to different degrees or that CBG with a similar graphitization degree occurred at varying distances from the intrusion. A possible mechanism for graphitization is strain-induced graphitization, where the local stress concentration leads to preferred orientations of the basic structure units (BSUs), as well as the motion and rearrangement of structural defects, resulting in the formation of a locally ordered structure. The graphitization degree is enhanced as the local graphite structure spreads.

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“…We first focus on coal structure and graphitization process seen in experiments. Many techniques, such as transmission electron microscope (TEM), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman spectroscopy, and Fourier Transform Infrared spectroscopy (FTIR), have been applied to determine coal structure, because of its complexity and heterogeneity (Li et al, 2015a(Li et al, , 2015bBaysal et al, 2016;Song et al, 2019). From a chemical point view, coal, in general, mainly consists of aromatic layers where aromatic nucleus are surrounded by peripheral aliphatic chains and oxygen 50 functional groups (Mathews and Chaffee, 2012;Ahamed et al, 2019).…”

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confidence: 99%

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at subseismic slip rates

Fan¹,

Liu²,

Hunfeld³

et al. 2020

Preprint

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Abstract. Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with near 100 % organic content, e.g. coal, and the controlling microscale mechanisms, remain unclear. Here, we report seven velocity stepping (VS) and one slide-hold-slide (SHS) friction experiments performed on simulated fault gouges prepared from bituminous coal, collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 °C, employing sliding velocities of 0.1–100 μm s−1, using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip weakening behaviour at shear displacements beyond ~ 1–2 mm, from a peak friction coefficient approaching ~ 0.5 to (near) steady state values of ~ 0.3, regardless of effective normal stress or whether vacuum dry flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip weakening behaviour can be attributed to the development of R-, B- and Y- shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ~ 0.006. We also determined the rate-dependence of steady state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities > 1 μm s−1 in the coal sample under vacuum dry conditions, but at > 10 μm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. This may be controlled by competition between dilatant granular flow and compaction enhanced by presence of water. Together with our previous work on frictional properties of coal-shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear-out coal seams may promote unstable, seismogenic slip behaviour, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

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Macromolecular evolution and structural defects in tectonically deformed coals

Cited by 51 publications

References 99 publications

Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A Case Study in Fengfeng Coal Mine of Taihangshan Fault Zone (North China)

Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A Case Study in Fengfeng Coal Mine of Taihangshan Fault Zone (North China)

Strain-Induced Graphitization Mechanism of Coal-Based Graphite from Lutang, Hunan Province, China

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at subseismic slip rates

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