An experimental study on the evolution of aggregate structure in coals of different ranks by in situ X-ray diffractometry

Zhang, Wenyong; Chen, Shancheng; Han, Feng; Wu, Daxu

doi:10.1039/c5ay01922b

Cited by 21 publications

(5 citation statements)

References 26 publications

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“…Our XRD results on the unsheared samples (S * and S0) show the structure of the graphite-like crystalline carbon in bituminous coal, yielding an interlayer spacing (d 002 ) of ∼ 3.56 Å, a crystallite diameter (L c ) of ∼ 13.5 Å, a stacked height (L a ) of ∼ 19 Å and aromaticity (f a ) of ∼ 0.47. These parameter values are consistent with those of similar bituminous coal reported by K. , Okolo et al (2015) and Zhang et al (2015). Note that the parameter values of L c and L a that were calculated using Scherrer's equation may be larger than the real size (Lu et al, 2001) and that the presence of mineral peaks, particularly in the 2θ range of 27-50 • , may also influence the fitting process (i.e., fitting a Gaussian peak to the 002, γ and 10 bands) and may accordingly influence the accuracy of the structure parameter values.…”

Section: Development Of the Molecular Structure And Maturity Of Coal ...supporting

confidence: 92%

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

et al. 2020

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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 nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS) friction experiment 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 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ∼ 1–2 mm, from a peak friction coefficient approaching ∼0.5 to (nearly) steady-state values of ∼0.3, regardless of effective normal stress or whether vacuum-dry or 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 behavior 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. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the 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 behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

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Section: Development Of the Molecular Structure And Maturity Of Coal ...supporting

confidence: 92%

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

et al. 2020

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“…show the structure of the graphite-like crystalline carbon in bituminous coal, yielding the interlayer spacing (d002) of ~3.56 Å, the crystallite diameter (Lc) of ~13.5 Å, the stacked height (La) of ~19 Å, and aromaticity (fa) of ~0.47. These parameter values are consistent with those of the similar bituminous coal reported by K. Li et al (2015a), Okolo et al (2015) and Zhang et al (2015). Note that the parameter values of d002, Lc and La that were calculated using Scherrer's equation may be 375 larger than the real size (Lu et al 2001), and that the presence of mineral peaks particularly in the 2θ range of 27-50° may also influence the fitting process (i.e.…”

Section: Development Of Molecular Structure and Maturity Of Coal In Tsupporting

confidence: 89%

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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“…ese parameters can be obtained using the Scherrer equation and fitting parameters. For the XRD study of coal structure under pyrolysis conditions, Zhang et al [18] presented the correlation between coal element composition and coal structure and the evolution of XRD structural parameters of coal with the change of temperature. However, they did not discuss the changes of XRD structural parameters of coal with temperature but also did not study the changes of macromolecular functional groups of coal with temperature, especially oxygen-containing functional groups.…”

Section: Introductionmentioning

confidence: 99%

Evolution Mechanism of Macromolecular Structure in Coal during Heat Treatment: Based on FTIR and XRD In Situ Analysis Techniques

Zhang

2019

Journal of Spectroscopy

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Owing to the complexity and heterogeneity of coal during pyrolysis, the ex situ analytical techniques cannot accurately reflect the real coal pyrolysis process. In this study, according to the joint investigation of Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), the structural evolution characteristics of lignite-subbituminous coal-bituminous coal-anthracite series under heat treatment were discussed in depth. e results of the infrared spectrum of coal show that the different functional groups of coal show different changes with the increase of coal rank before pyrolysis experiment. Based on in situ infrared spectroscopy experiments, it was found that the infrared spectrum curves of the same coal sample have obvious changes at different pyrolysis temperatures. As a whole, when the pyrolysis temperature is between 400 and 500°C, the coal structure can be greatly changed. By fitting the infrared spectrum curve, the infrared spectrum parameters of coal were obtained. With the change of temperature, these parameters show regular changes in coal with different ranks. In the XRD study of coal, the absorption intensity of the diffraction peak (002) of coal increases with increasing coal rank. e XRD patterns of coal have different characteristics at different pyrolysis temperatures. Overall, the area of (002) diffraction peak of the same coal sample increases obviously with the increase of temperature. e XRD structural parameter of coal was obtained by using the curve fitting method. e changing process of two parameters (interlayer spacing (d 002 ) and stacking height (L c )) can be divided into two main stages, but the average lateral size (L a ) does not change significantly and remains at the 2.98 ± 0.09 nm. In summary, the above two technologies complement each other in the study of coal structure. e temperature range of both experiments is different, but the XRD parameters of coal with different ranks are reduced within the temperature range of less than 500°C, which reflects that the size of coal-heated aromatic ring lamellae is reduced and the distance between lamellae is also reduced, indicating that the degree of condensation of coal aromatic nuclei may be increased. Correspondingly, the FTIR parameters of coal also reflect that, with increasing temperature, the side chains of coal are constantly cracked, the oxygen-containing functional groups are reduced, and the degree of aromatization of coal may be increased.

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An experimental study on the evolution of aggregate structure in coals of different ranks by in situ X-ray diffractometry

Abstract: The XRD profiles of raw coal.

Cited by 21 publications

References 26 publications

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

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

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

Evolution Mechanism of Macromolecular Structure in Coal during Heat Treatment: Based on FTIR and XRD In Situ Analysis Techniques

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