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. The 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. The 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. The XRD structural parameter of coal was obtained by using the curve fitting method. The changing process of two parameters (interlayer spacing (
The integrally structural arrangement of coal increases with proceeding coalification up to the formation of organic macromolecules of flat shape with carbon atoms in the central part [
In general, the pyrolysis process of coal can be divided into two main stages. One is the depolymerization or decomposition stage where gaseous (e.g., gas and water vapor) and liquid products (e.g., tar) were generated. The other is the condensation or repolymerization stage where the turbostratic lamellar system took place. Amounts of heterogeneous pyrolysis reactions can take place in the aforementioned two types of competitive processes. A better knowledge of coal structure alteration during pyrolysis could promote to comprehend the reaction process of coalification [
In the past couple of decades, the ex situ XRD and FTIR analytical methods were used widely to supply important information on the changes of coal structure under heat treatment [
The FTIR technique focuses on determining composition on the physicochemical structures of coal. Kister et al. [
In conclusion, although ex situ analysis technology can also obtain information on coal structure and its reactivity, but because of the complexity of coal structure transformation during pyrolysis, this analysis method cannot characterize the instantaneous information of coal structure at a certain temperature [
The purpose of this study was (1) probing deeply the structural evolution signatures of different ranked coals during pyrolysis using in situ XRD and FTIR analytical techniques; (2) a better understanding of the relationship between the evolution of coal structural parameters and pyrolysis temperature of coal; (3) the reference standard of structure parameters of coal with different ranks under real-time pyrolysis temperature was proposed.
A set of five different ranked coals having different geological ages was selected from five coal basins located on North China and South China coal-bearing region (Figure
Location and geological ages of Huainan coalfield, Kailuan coalfield, and Pingshuo coalfield in the North China coal-bearing region and Enshi coalfield and Zhijin coalfield in the South China coal-bearing region.
Each coal sample weighed approximately 200 g. These samples were pulverized and sieved to obtain particles of <56
The proximate and ultimate analyses of five coal samples before pyrolysis were executed according to ISO 625: 1975 (E) and ISO 333: 1983 (E), respectively. The results are presented in Table
Results of proximate and ultimate analyses for five coal samples.
Sample number | Proximate analysis (wt.%) | Ultimate analysis (wt.% daf) | Atomic ratio | O + N + S | Coal rank | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|
|
C | H | O | N | S | H/C | O/C | |||
GZ | 1.8 | 21.3 | 17.5 | 91.4 | 3.1 | 4.2 | 0.2 | 1.0 | 0.41 | 0.03 | 5.4 | Anthracite |
HB | 1.2 | 12.4 | 35.0 | 85.8 | 5.0 | 8.1 | 0.3 | 0.7 | 0.70 | 0.07 | 9.1 | Bituminous coal |
HN | 1.9 | 21.6 | 40.7 | 80.1 | 4.5 | 10.5 | 1.4 | 0.3 | 0.67 | 0.10 | 12.2 | Bituminous coal |
KL | 1.2 | 13.2 | 33.5 | 78.5 | 5.6 | 13.7 | 1.2 | 0.3 | 0.85 | 0.13 | 15.2 | Subbituminous coal |
SX | 2.3 | 19.9 | 36.5 | 71.1 | 6.1 | 19.8 | 1.6 | 0.4 | 1.03 | 0.21 | 21.8 | Lignite |
ad: air-dried basis; daf: dried ash-free basis.
For each set of experiments, after the reactor grasped approximately 20 g coal sample, the reactor was sealed and heated in a temperature-programmed furnace. The heat treatment experiment of coal was carried out under oxygen-free conditions. Five rounds of experiments were conducted at difference temperatures, including 100, 200, 300, 400, and 500°C. The furnace temperature increased at 5°C/min and was held for 24 hours isothermally after reaching a chosen temperature. Finally, the in situ FTIR spectrograms of coal were measured at each characteristic temperature.
In this work, the infrared spectral bands selected were 2700 to 3000 cm−1 and 1300 to 1800 cm−1. In the range of 2700 to 3000 cm−1, two characteristic absorption peaks (2920 and 2860 cm−1) were obtained using fitting parameters. Overlapping peaks in 1300 to 1800 cm−1 [
Curve-fitted results of 3000–2800 cm−1 and 1300–1800 cm−1 band for HB coal sample before pyrolysis.
The changing features of the infrared spectral structural parameters with respect to different functional groups were previously summarized by José et al. [
The in situ XRD data collection was performed by Philips X’Pert PRO X-ray powder diffraction, using Ni-filtered Cu K
Some information about the molecular structures of coal can be obtained from an analysis for the elements carbon (C), hydrogen (H), sulfur (S), and nitrogen (N) [
The infrared spectra of coal with the increase of coal rank are shown in Figure
Infrared spectrogram of five coal samples. Attribution of characteristic peak positions is descried as follows: (1) aromatic hydrocarbon CH; (2) aliphatic CH2 or CH3 stretching vibration; (3) aromatic carbonyl/carboxyl groups (C=O); (4) aromatic nucleus (C=C); (5) phenolic deformation C─O─C (stretching); (6) aromatic CH (out-plane bending).
The following conclusions can be drawn in the coal series lignite-subbituminous coal-bituminous coal-anthracite based on Figure The dashed line 2 between 2850 cm−1 and 3000 cm−1 reveals that, with the increase of coal rank, the absorbance of aliphatic CH2 or CH3 stretching vibration dwindles considerably, suggesting the decrease of hydrocarbon content in coal. The change in dashed 1 (aromatic hydrocarbon) was similar to that in dashed line 2, which indicates that hydrogen was gradually removed in higher-rank coal, leading to the decrease of intensity of aromatic ring stretching vibration in coal. In lignite (SX) and subbituminous coal (KL), the absorbance of aromatic carboxyl and carbonyl groups (dashed line 3) was obvious. However, it was absent in bituminous coal (KL and HN) and anthracite (GZ). According to Van Krevelen [ Because of the presence of polar substituents in aromatic rings [ In the range of lignite (SX) to bituminous coal (HN), the absorbance of phenolic deformation C─O─C groups (dashed line 5) decreases obviously. Up to anthracite (GZ), it disappears progressively. The absorbance of aromatic out-of-plane bending (dashed line 6) increases slightly between lignite (SX) and subbituminous coal (KL). Reaching the stage of bituminous coal (HB), it decreases moderately. Up to anthracite (GZ), it decreases to a less content.
As the pyrolysis temperature increases, in situ infrared spectra of five coal samples are shown in Figure
In situ infrared spectra of five coal samples at different temperatures: (a) SX; (b) KL; (c) HN; (d) HB; (e) GZ.
According to Sonibare et al. [
Especially, significant changes in the oxygen-containing functional groups are observed in the range of 1600 to 1800 cm−1 band (Figure
In situ infrared spectra of five coal samples at the same temperature: (a) 100°C; (b) 200°C; (c) 300°C; (d) 400°C; (e) 500°C.
The calculated infrared spectral structural parameters are presented in Table
Statistic results of the infrared spectral structural parameters of five coal samples before and after pyrolysis.
Sample number | Temperature (°C) | Structural parameters | ||||
---|---|---|---|---|---|---|
|
|
|
|
| ||
GZ | Raw coal | – | 1.965 | 0.885 | 1.062 | 0.585 |
100°C | – | 1.941 | 0.880 | 1.063 | 0.592 | |
200°C | – | 1.894 | 0.867 | 1.066 | 0.599 | |
300°C | – | 1.817 | 0.842 | 1.072 | 0.606 | |
400°C | – | 1.693 | 0.794 | 1.087 | 0.612 | |
500°C | – | 1.492 | 0.699 | 1.127 | 0.619 | |
|
||||||
HB | Raw coal | – | 1.843 | 0.830 | 0.997 | 0.546 |
100°C | – | 1.821 | 0.825 | 0.997 | 0.552 | |
200°C | – | 1.777 | 0.813 | 1.000 | 0.559 | |
300°C | – | 1.705 | 0.790 | 1.005 | 0.565 | |
400°C | – | 1.588 | 0.745 | 1.020 | 0.572 | |
500°C | – | 1.399 | 0.656 | 1.057 | 0.578 | |
|
||||||
HN | Raw coal | – | 1.766 | 0.812 | 0.954 | 0.541 |
100°C | – | 1.721 | 0.776 | 0.931 | 0.555 | |
200°C | – | 1.701 | 0.771 | 0.932 | 0.562 | |
300°C | – | 1.660 | 0.760 | 0.934 | 0.568 | |
400°C | – | 1.592 | 0.738 | 0.939 | 0.575 | |
500°C | – | 1.483 | 0.695 | 0.953 | 0.581 | |
|
||||||
KL | Raw coal | 2.947 | 1.687 | 0.760 | 0.912 | 0.502 |
100°C | 2.916 | 1.667 | 0.756 | 0.913 | 0.508 | |
200°C | 2.852 | 1.626 | 0.745 | 0.915 | 0.514 | |
300°C | 2.743 | 1.561 | 0.723 | 0.920 | 0.520 | |
400°C | 2.559 | 1.454 | 0.682 | 0.934 | 0.526 | |
500°C | – | 1.281 | 0.600 | 0.968 | 0.532 | |
|
||||||
SX | Raw coal | 2.668 | 1.528 | 0.688 | 0.826 | 0.455 |
100°C | 2.641 | 1.510 | 0.684 | 0.827 | 0.461 | |
200°C | 2.583 | 1.473 | 0.674 | 0.829 | 0.466 | |
300°C | 2.484 | 1.413 | 0.655 | 0.833 | 0.472 | |
400°C | 2.317 | 1.316 | 0.617 | 0.845 | 0.477 | |
500°C | – | 1.160 | 0.543 | 0.877 | 0.483 |
The parameter calculation method:
(1) For these samples (GZ, HB, and HN), the contraction vibration intensity of the −CH3 band (2920 cm−1 and 2860 cm−1) is very weak at different temperatures and the peak area of this band cannot be obtained so that the
More generally, coal is amorphous, but its aromatic structure tends to graphitize gradually during coalification. Thus, the XRD pattern of coal presents certain regularity with the increase of the degree of coalification.
Figure
XRD pattern of five coal samples before pyrolysis (a) and the curve fitting result of the SX coal sample before pyrolysis as a function of Gaussian function (b).
Statistic results of the XRD parameters of five coal samples before and after pyrolysis.
Sample number | Temperature (°C) | ▲ (°) | ▼ (nm) | |||
---|---|---|---|---|---|---|
2 |
2 |
|
|
|
||
GZ | Raw coal | 26.15 | 46.16 | 0.3498 | 1.9841 | 3.0507 |
25°C | 26.17 | 44.64 | 0.3496 | 1.9848 | 2.9172 | |
100°C | 26.26 | 45.93 | 0.3485 | 1.9842 | 3.0073 | |
200°C | 26.21 | 45.09 | 0.3491 | 1.9842 | 2.9479 | |
300°C | 26.27 | 45.28 | 0.3483 | 1.9842 | 2.9611 | |
400°C | 26.26 | 44.38 | 0.3485 | 1.9837 | 2.8998 | |
500°C | 26.24 | 44.14 | 0.3487 | 1.9833 | 2.8839 | |
600°C | 26.35 | 44.61 | 0.3474 | 1.9868 | 2.9152 | |
700°C | 26.40 | 45.30 | 0.3468 | 1.9876 | 2.9625 | |
800°C | 26.54 | 45.02 | 0.3451 | 1.9897 | 2.9431 | |
900°C | 26.59 | 45.51 | 0.3444 | 1.9901 | 2.9773 | |
|
||||||
HB | Raw coal | 25.99 | 45.38 | 0.3518 | 1.8797 | 2.9681 |
25°C | 26.04 | 45.26 | 0.3512 | 1.8801 | 2.9598 | |
100°C | 26.00 | 45.53 | 0.3516 | 1.8796 | 2.9787 | |
200°C | 26.00 | 45.25 | 0.3516 | 1.8792 | 2.9591 | |
300°C | 26.00 | 45.26 | 0.3517 | 1.8805 | 2.9598 | |
400°C | 25.96 | 45.16 | 0.3521 | 1.8781 | 2.9528 | |
500°C | 25.93 | 45.60 | 0.3525 | 1.8793 | 2.9837 | |
600°C | 26.19 | 45.53 | 0.3493 | 1.8826 | 2.9787 | |
700°C | 26.25 | 44.98 | 0.3486 | 1.8828 | 2.9404 | |
800°C | 26.40 | 45.12 | 0.3468 | 1.8866 | 2.9500 | |
900°C | 26.43 | 44.96 | 0.3464 | 1.8868 | 2.9390 | |
|
||||||
HN | Raw coal | 25.66 | 45.72 | 0.3561 | 1.6786 | 2.9922 |
25°C | 25.69 | 45.87 | 0.3557 | 1.6796 | 3.0030 | |
100°C | 25.65 | 45.66 | 0.3562 | 1.6786 | 2.9879 | |
200°C | 25.61 | 45.76 | 0.3567 | 1.6772 | 2.9951 | |
300°C | 25.72 | 45.01 | 0.3553 | 1.6781 | 2.9424 | |
400°C | 25.54 | 45.57 | 0.3576 | 1.6816 | 2.9815 | |
500°C | 25.63 | 46.72 | 0.3565 | 1.6806 | 3.0656 | |
600°C | 25.88 | 46.64 | 0.3532 | 1.6852 | 3.0596 | |
700°C | 25.89 | 45.87 | 0.3531 | 1.6847 | 3.0030 | |
800°C | 26.17 | 45.60 | 0.3496 | 1.6885 | 2.9837 | |
900°C | 26.19 | 45.66 | 0.3493 | 1.6882 | 2.9879 | |
|
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KL | Raw coal | 25.58 | 45.56 | 0.3571 | 1.5786 | 2.9808 |
25°C | 25.65 | 45.64 | 0.3562 | 1.5796 | 2.9865 | |
100°C | 25.58 | 46.20 | 0.3571 | 1.5786 | 3.0270 | |
200°C | 25.47 | 45.31 | 0.3585 | 1.5772 | 2.9632 | |
300°C | 25.54 | 46.55 | 0.3576 | 1.5781 | 3.0529 | |
400°C | 25.80 | 45.36 | 0.3543 | 1.5816 | 2.9667 | |
500°C | 25.73 | 45.36 | 0.3552 | 1.5806 | 2.9667 | |
600°C | 26.07 | 46.60 | 0.3509 | 1.5852 | 3.0566 | |
700°C | 26.03 | 46.20 | 0.3513 | 1.5847 | 3.0270 | |
800°C | 26.31 | 45.59 | 0.3479 | 1.5885 | 2.9830 | |
900°C | 26.29 | 45.70 | 0.3481 | 1.5882 | 2.9908 | |
|
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SX | Raw coal | 25.61 | 45.43 | 0.3567 | 1.3791 | 2.9717 |
25°C | 25.61 | 44.61 | 0.3567 | 1.3791 | 2.9152 | |
100°C | 25.54 | 46.54 | 0.3576 | 1.3781 | 3.0521 | |
200°C | 25.54 | 45.31 | 0.3576 | 1.3781 | 2.9632 | |
300°C | 25.54 | 45.66 | 0.3576 | 1.3781 | 2.9879 | |
400°C | 25.47 | 45.30 | 0.3585 | 1.3772 | 2.9625 | |
500°C | 25.42 | 44.78 | 0.3592 | 1.3765 | 2.9267 | |
600°C | 25.64 | 46.01 | 0.3563 | 1.3794 | 3.0131 | |
700°C | 25.76 | 45.71 | 0.3548 | 1.3812 | 2.9915 | |
800°C | 25.88 | 43.53 | 0.3532 | 1.3826 | 2.8444 | |
900°C | 25.92 | 44.67 | 0.3527 | 1.3832 | 2.9192 |
▲: characteristic parameters of diffraction peak. ▼: crystallite structure parameters.
As shown in Figure
Taking the SX coal sample as an example, its XRD pattern was fitted using Gaussian function (Figure
Table
The in situ XRD patterns of five coal samples during pyrolysis are shown in Figure
In situ XRD pattern of five coal samples at different temperatures: (a) SX; (b) KL; (c) HN; (d) HB; (e) GZ.
As can be seen from Figure
Taking the HB coal sample as example, Origin 8.0 software was used to fit its XRD patterns in the 2
In situ XRD curve fitting results of the HB coal sample at different temperatures: (a) 300°C; (b) 600°C; (c) 900°C.
Table
Variation of XRD structural parameters with temperature.
The structural evolution signatures of different ranked coal during pyrolysis were deeply investigated using in situ XRD and FTIR analytical methods. The main conclusions are as follows: (1) the FTIR spectra analysis shows that when temperature reaches between 400 and 500°C, coal structure occurs in abrupt change in the intensity of absorption peak; (2) the X-ray diffraction analysis illustrates that with the increase of temperature, two parameters (
In this study, the data form mainly includes (1) original spectral data (XRD and FTIR) and (2) spectral parameters. The original spectral data used to support the findings of this study are available from the corresponding author upon request. The spectral parameters data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
We acknowledge support from the Research Projects of Department of Land and Resources of Anhui Province, China (2016-K-2), National Natural Science Foundation of China (No. 41502152), Open Projects of Research Center of Coal Resources Safe Mining and Clean Utilization, Liaoning (LNTU17KF10), Natural Science Foundation General Program of Anhui Program (1808085MA07), and Natural Science Foundation Youth Program of Anhui Program (1808085QE150).