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Chinese Science Bulletin, Volume 62, Issue 21: 2416-2427(2017) https://doi.org/10.1360/N972016-00970

Investigation on the size and fractal dimension of nano-pore in coals by synchrotron small angle X-ray scattering

YiXin ZHAO1,2,*, Lei PENG1,2
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  • ReceivedSep 5, 2016
  • AcceptedDec 12, 2016
  • PublishedFeb 13, 2017

Abstract

Coal is a heterogeneous porous medium with complex pore structures. These pore structures directly influence the adsorption, desorption and migration of gas in coal reservoirs. The characteristics of pore structure play an important role in various aspects of coal utilization including extraction of methane from coal seams, CO2 sequestration in coal and water purification by activated carbon. In this paper, the synchrotron small angle X-ray scattering (SAXS) was applied to test micropore structure (1–100 nm) in six coal samples with different ranks. In the experiment, the particle size of coal powder was 60–80 mesh. The recorded SAXS two dimensional images were processed and quantitatively converted into one dimensional scattering data using FIT2D software. Based on the corrected SAXS data, the pore size distribution and specific surface area were quantitatively estimated. The Porod scattering curves, Guinier scattering curves, pore size distribution, specific surface area, and fractal characteristics of six coal samples were explored and analyzed.

The results indicate that the SAXS data of six coal samples show the positive deviation of Porod theory. This observation proves that there are additional density fluctuations in the coals. Thus, the original SAXS data essentially needs to be corrected to achieve the true scattering results. The polydisperse distribution of micropores is observed based on the Guinier curves of six coal samples. Moreover, the pore size distribution of tested coals was analyzed based on the maximum entropy theory and the “double peaks” feature was observed. Both the average pore diameter and the most probable pore diameter of the tested samples decrease with the increase of coal ranks, which is consistent with the results proved by the methods of mercury injection and low temperature nitrogen adsorption. The fractal dimension of micropores in the tested coal samples was also discussed. According to the double logarithmic curves which shows the realationship between scattering intensity I and the scattering vector q, through different range of q value, method of tangent double logarithm is adopted to get the corresponding q value range of fractal dimension of coal pore. The surface fractal dimensions of micropores were calculated and presented as the scattering vector q stating at the relative lower value. However, the pore fractal dimension related to the distribution of micropores was presented as the scattering vector q rising to the relative higher value. It was found that the surface fractal dimension of micropores experience decreasing firstly, then increasing and finally drop again with the increase of coal ranks. The pore fractal dimension of micropores decreases at beginning and then increases with the rise of coal ranks. It is also found that the minimum value of pore fractal dimension appears as the Ro,max equals to 2.07% based on the current test results. SAXS is a powerful tool to study the micro pore structure of coal, however it is necessary to point out that the small angle scattering data processing methods, different processing methods or theories have advantages and limitations. Therefore, in the follow-up study, increasing the sample number and variety of different metamorphic degree of coal, the pore size, the ratio surface area with the metamorphic degree of variation is more abundant and exact.


Funded by

国家重点研发计划(2016YFC0600708)

国家重点研发计划(2016YFC0801401)


Acknowledgment

感谢中国科学院高能物理研究所同步辐射国家实验室提供相关实验条件及李志宏老师和默广老师给予本论文在数据处理方面的悉心指导. 由衷感谢编委及审稿人对文章的建议与指正.


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  • Figure 1

    (Color online) The diagram and photo of test facilities at 1W2A station. (a) Schematic diagram of equipment principle; (b) sample test area

  • Figure 2

    (Color online) The scattering images of coals with different ranks. (a) Xinzhouyao 11# coal sample; (b) Tangshan 9# coal sample; (c) Yangquhe 2# coal sample; (d) Yuwu 3# coal sample; (e) Changzhen 3# coal sample; (f) Datai 2# coal sample

  • Figure 3

    (Color online) The scattering curves of coals with different ranks

  • Figure 4

    (Color online) Porod curves and related calibrated curves for 6 coal samples. (a) Xinzhouyao 11# coal sample; (b) Tangshan 9# coal sample; (c) Yangquhe 2# coal sample; (d) Yuwu 3# coal sample; (e) Changzhen 3# coal sample; (f) Datai 2# coal sample

  • Figure 5

    (Color online) Guinier curves for 6 coal samples with various ranks

  • Figure 6

    (Color online) Pore size distribution of 6 coal samples based on the maximum entropy probability

  • Figure 7

    (Color online) Relationship between the pore size and the ranks of coals

  • Figure 8

    Relationship between the coal ranks and the specific surface area of the pores

  • Figure 9

    (Color online) Double logarithm curve of 6 coal samples to calculate fractal dimensions of micropores

  • Figure 10

    Relationship between ranks and fractal dimension of 6 coal samples. (a) Relationship between surface fractal dimension and vitrinite reflectance; (b) relationship between pore fractal dimension and vitrinite reflectance

  • Table 1   The mean maximum reflectance of vitrinite in tested coals

    Rmin(%)

    Rmax(%)

    Ro,max(%)

    采样地点

    编号

    镜质组反射率值

    煤阶

    忻州窑11#煤

    A

    0.76

    0.85

    0.81

    气煤

    唐山9#煤

    B

    0.908

    1.129

    1.115

    肥煤

    羊渠河2#煤

    C

    1.349

    1.455

    1.454

    贫瘦煤

    余吾3#煤

    D

    2.23

    1.92

    2.07

    贫煤

    长畛3#煤

    E

    3.164

    4.047

    4.329

    无烟煤

    大台2#煤

    F

    3.268

    6.691

    6.628

    无烟煤

  • Table 2   The statistical table of macerals in different samples

    样品号

    A

    B

    C

    D

    E

    F

    镜质组(%)

    75.4

    61.6

    84.1

    67.8

    66.1

    13.1

    惰质组(%)

    20

    19.3

    1.6

    24.5

    31.4

    81.2

    壳质组(%)

    1.8

    1.6

    0

    0

    0.2

    0

    矿物(%)

    2.8

    17.5

    14.3

    7.7

    2.3

    5.7

  • Table 3   Results of fractal features of 6 coal samples

    采样地点

    试样

    斜率

    分形维数D1

    分形特征

    斜率

    分形维数D2

    分形特征

    忻州窑11#煤

    A

    −3.17

    2.83

    表面分形

    −1.37

    1.37

    孔分形

    唐山9#煤

    B

    −3.46

    2.54

    表面分形

    −2.33

    2.33

    孔分形

    羊渠河2#煤

    C

    −3.47

    2.53

    表面分形

    −1.17

    1.17

    孔分形

    余吾3#煤

    D

    −3.24

    2.76

    表面分形

    −1.08

    1.08

    孔分形

    长畛3#煤

    E

    −3.11

    2.89

    表面分形

    −1.42

    1.42

    孔分形

    大台2#煤

    F

    −3.37

    2.63

    表面分形

    −1.62

    1.62

    孔分形

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