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Structural transformations of coal components on heattreatment and explanation on their abnormal thermal behaviors Shaoqing Wang, Hao Chen, Wei Ma, Penghua Liu, and Zongda Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01426 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017
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Structural transformations of coal components on heat-treatment and explanation on their
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abnormal thermal behaviors
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Shaoqing Wang*, Hao Chen, Wei Ma, Penghua Liu, Zongda Yang
4
College of Geoscience and Surveying Engineering, China University of Mining and Technology
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(Beijing), D11, Xueyuan Road, Beijing 100083, P.R China
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Abstract: The two coal components, barkinite and vitrinite, were selected from the same coal,
7
Mingshan coal in Southern China. The chemical structural transformations of these two
8
components on heat-treatment in situ from 200 to 500 °C were examined by high-resolution
9
transmission electron microscopy (HRTEM) technique. The physical appearance changes were
10
observed by polarized light microscopy with heated stage. At 200 °C, barkinite and vitrinite have
11
no clear change in physical appearance. Most of the layers are poorly orientated in chemical
12
structure. Rounded-off edge of barkinite particle was observed at 250 °C and that of vitrinite was
13
300 °C. The particle sizes were drastically reduced from 450 °C to 500 °C. Massive amount of
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oily material were exuded from the barkinite particle. Meanwhile, the orientation in aromatic layer
15
of barkinite and vitrinite was clearly improved. The changes of aromatic size of barkinite and
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vitrinite on heat-treatment were obvious. The naphthalene abundances decreased from 200 °C to
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350°C and increased at the ranges of 400-500 °C. The abundances of naphthalene reached the
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maximum values at 450 °C. The obvious increase of 3×3 fringe and 4×4 fringe at 200-350 °C and
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decrease of at 400-500 °C were shown. The abundances of 2×2 fringe were different. Furthermore,
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Differences existed on the structural changes of barkinite and vitrinite when heating. Firstly,
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barkinite become obvious orientation of the layer at almost 375 °C and vitrinite at 400 °C.
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Secondly, barkinite has bigger naphthalene abundance and lower 2×2 fringe and 3×3 fringe at 1
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450 °C than vitrinite. The differences in chemical structure of barkinite and vitrinite played a
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significant role in investigations for the different thermal behaviors of these two components and
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bark coal.
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Keywords: vitrinite; barkinite; HRTEM; structure; heat
5 6
Hydrogen-rich bark coal is a special coal type, and has potential commercial applications in
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several technologies due to its high hydrogen content and high volatile matter yield.1,2 Bark coal
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was named as a unique coal component-barkinite. Barkinite is included as one of liptinite
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macerals in the Chinese bituminous maceral classification.3 Barkinite is considered to derive from
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the cortex tissue of stem and root of plants in which the cell wall and filling material apparently
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have become impregnated with suberin substances.3 However, it has not yet been recognized as a
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maceral classification by the International Committee for Coal and Organic Petrology (ICCP). The
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reason is that the chemical structure of barkinite still remains vague to some extent, which is also
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pointed out in the work of Hower et al.4 Once heated, both bark coal and barkinite have some
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special thermal behaviors: an extensive thermal decomposition and extra-high fluidity.5,6 However,
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the reasons remain unclear. In the maceral composition,
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vitrinite and barkinite. Because of pure barkinite and vitrinite can be successfully separated from
18
bark coal,7 so this provide a good way to study the anomalous thermal behaviors of bark coal base
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on the structural transformation of barkinite and vitrinite on heat-treatment.
1, 5, 6
bark coal are mainly composed of
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Structure characterizations of bark coal and barkinite have been discussed using various
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analytical methods, for examples, Fourier transform infrared spectroscopy (FT-IR),8,9
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transmitted-light FT-IR microspectroscopy,10-12 time-of-flight secondary ion mass spectrometry 2
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(TOF-SIMS),13 carbon-13 nuclear magnetic resonance (13C-NMR),9,14 ruthenium ion catalyzed
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oxidation,15 and atomic force microscopy.16,17 The noticeable chemical structural characteristic of
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barkinite is rich aliphatic group, especially CH2 group.2,8,9-12 However, the changes of some
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chemical structural parameters of barkinite on heating, at the most extent, are vague, for instance,
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the orientation of aromatic layer, and the distribution of aromatic ring size. This also led to an
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unsatisfactory explanation on the special thermal behaviors of bark coal and barkinite.
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High resolution transmission electron microscopy (HRTEM) has been applied to study
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structural characterization of coal and/or coal tar.18-28 HRTEM is an effective method to discuss
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the distribution of the aromatic layers in coal and coal products18-22 and is also method available to
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observe coal structure.18 Direct observation of layer structure of raw coal using high-resolution
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transmission electron microscopy was introduced by Sharma et al.18 The method proposed by
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Mathews et al.23 was used to assign aromatic size to the extracted HRTEM fringes. In the recent,
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Mathews and co-authors have published some important papers to discuss the application of
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HRTEM method on evaluating coal structure.24-27 Niekerk et al.26 applied the HRTEM method to
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determine the size and distribution of aromatic fringes for inertinite-rich and vitrinite-rich South
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African coals. Based on HRTEM lattice fringe micrographs, the aromatic ring size distribution
17
were estimated for Illinois No. 6 Argonne Premium coal. 27 The HRTEM is also used to study coal
18
or carbon structure with combination other techniques, for example, X-ray diffraction (XRD).
19
Sharma et al.28 made a comparison of structural parameters of phenol formaldehyde resin char
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from XRD and HRTEM techniques and obtained a good agreement between these two techniques.
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The structural transformations of different rank coals18 and coals on heating20 were discussed
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using HRTEM technique. However, the observation of structural transformations of coal on 3
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heat-treatment in situ by HRTEM was rarely studied. In this work, the changes of chemical
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structure of barkinite and vitrinite on heat-treatment in situ with HRTEM were studied.
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Considering the temperature ranges of bark coal being special thermal behaviors are
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200-500 °C, this work focus on the structural transformations of barkinite and vitrinite in this
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temperature range. The aims of this work were (1) to study the structural transformation of
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barkinite and vitrinite on heat-treatment from 200 °C to 500 °C; (2) to discuss the reason why bark
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coal and barkinite have some abnormal thermal behaviors.
8 9 10
2. Samples and experimental 2.1 Samples selection
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Barkinite and vitrinite were selected from B3 coal seam in Mingshan mine, Jiangxi province in
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Southern China. Barkinite and vitrinite were separated first by hand picking. Vitrain band was
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selected to obtain pure vitrinite and durain band for barkinite. The samples were crushed to -18
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mesh to make pellets for determining maceral compositions. Maceral compositions were
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determined according to GB/T 8899-1998 standard.29 Petrographic analysis results show that
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vitrain are 87% vitrinite and the durain are 83% barkinite. For obtaining pure barkinite and
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vitrinite, these two macerals were further separated by density gradient centrifugation (DGC)
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method. The detailed procedure for maceral separation was described in the work of Guo et al.7
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The purities of barkinite-separated (BaS) and vitrinite-separated (VS) are above 95% (Vol. %).
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2.2 HRTEM observation
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HRTEM observations were performed on a 200kV transmission electron microscope (JEOL,
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JEM-2010) with a heating system (electric furnace). BaS and VS were analyzed using HRTEM 4
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method. For each test, sample was first diluted with ethanol and sonificated for 20 min to disperse
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the particles. Sample was sprayed over a silicon nitride film. Individual particles were first
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examined at moderate magnification to find the particle with thin sharp edges and such region
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were then magnified to observe lattice fringes. The samples were heated in situ from the
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temperature range of 200- 500 °C for observing their structural changes with increase of
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temperatures used. The temperature desired was controlled depending on using the conversion
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graph of thermo electromotive force and temperature. In the present study, the detailed HRTEM
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acquisition method proposed by Sharma et al.18 was used.
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2.3 Polarized light microscopy with heat stage
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The experiment was carried out by programmed polarized light microscopy with a heated
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stage (DMLP, Leica Company) and with a high-definition camera (MC-D900U(C). To clearly of
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identify macerals, barkinite and vitrinite were separated first by hand picking. The barkinite and
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vitrinite
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polarized light microscopy with heat stage examination, macerals were then further identified
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according to their shapes. Due to the temperature limitations of the instrument, the temperature
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range used was chosen from room temperature to 500°C. To clearly investigate the change of
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morphology of coal particle when heated, two temperature stages conditions were used: from
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room temperature to 200°C at the heating rate of 10°C /min, and from 200 to 500°C at 5°C /min.
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Nitrogen was used for preventing samples from being oxidized. The figures were analyzed by
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Phmias 2008 Cs version 3.0 software. The detailed information was introduced in the works of
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Wang et al.30
samples
were
ground
to
-80
mesh
and
-100
22
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separately.
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3. Results and discussion
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3.1 Orientation changes with different temperatures
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The HRTEM micrographs of BaS and VS were observed and further converted to binary
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images. The lattice fringe was extracted by hand and its length was determined using image
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analyses. HRTEM images and corresponding skeletonized images of barkinite and vitrinite with
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different temperatures are shown in Figures 1 and 2, separately.
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At 200 °C, most of the layers are curved and poorly orientated. With the increasing of
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temperature, slightly better orientated layers are shown. More layers at 400 °C are parallel to each
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other than that at lower temperatures, and more layers also can be seen as forming stacks. After
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this temperature until 500°C, the number of aromatic layers in stacks increased and the orientation
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in aromatic layer was obviously improved.
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From Figures 1 and 2, the two images show a striking difference in the orientation of the layers
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at 200 °C. The layers in Figure 1 are less in orientation, which indicated that barkinite has higher
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disordered in structure than vitrinite. A similar result has been obtained by Wang et al.17, 31 when
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barkinite was studied by Raman spectroscopy and Atomic force microscopy techniques.
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Regardless, with the temperature increased, barkinite become the orientation of the layer more
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quickly than vitrinite, namely, barkinite is 375 °C and vitrinite is 400 °C. This is related to
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barkinite has a rich in aliphatic group in the chemical structure, especially CH2 group.2, 8, 10-12 The
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aliphatic groups are easily decomposed when heated, as shown in the previous works6, and
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relative aromatic group increase and form stacks, just shown in Figures 1 and 2.
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3.2 Length distribution of aromatic size with different temperatures
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The method of aromatic size assigned for extracted fringe was introduced by Mathews et al.23 6
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This method assumed that larger aromatic fringes are in the shape of parallelogram. Some lengths
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(The maximum, minimum, and mean) of a series of parallelogram-shaped aromatic fringes
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ranging from naphthalene to 8×8 aromatic rings were calculated using molecular modeling (Table
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2).26 However, considering the length of the aromatic fringe is dependent on the carbon ring
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catenation and angle of viewing, the mean value was used to assign an aromatic structure to each
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fringe (Table 1). In this work, aromatic parallelogram for each fringe was assigned according to
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the size of mean value. For instance, all fringes between 3.0 Å and 5.4 Å were assigned to
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naphthalene and fringes between 5.5 Å and 7.4 Å were assigned to 2×2 aromatic sheets. All
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fringes smaller than 3 Å were assumed to be noise and ignored. The size distribution of HRTEM
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lattice fringe micrographs of barkinite and vitrinite are also presented in Figures 1(c) and 2(c),
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respectively. The aromatic fringe distributions of barkinite and vitrinite from 200 to 500 °C were
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calculated and shown in Figure 3 and 4, respectively.
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At the beginning, 200 °C, both barkinite and vitrinite are rich in naphthalene abundance. With
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temperature was gradually raised to 350 °C, the decrease in naphthalene abundance and obvious
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increase of 3×3 fringe and 4×4 fringe. But the change of 2×2 fringe between barkinite and vitrinite
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in this temperature range is different. The abundance of 2×2 fringe in barkinite increase and that
17
decrease in vitrinite. After 350 °C, for barkinite and vitrinite, naphthalene abundances increase
18
again and reach the maximum values at 450 °C, and then slightly decrease at 500 °C. The decrease
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in 4×4 fringe and almost no change in 2×2 fringe were observed. The abundance of 3×3 fringe
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decrease during 400-450 °C and increase at 500 °C. Vitrinite has a plastic stage during 400-500 °C.
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Bark coal also has a strong fluidity during this temperature range.5 Most of aliphatic groups and
22
most oxygen, at this stage, have disappeared.32 7
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At the same temperature, barkinite and vitrinite are similar in their distribution of aromatic
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fringes. For both these two samples, three fringes show greatest abundance, namely, naphthalene,
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2×2 fringes and 3×3 fringes, followed by 4×4 fringes and 5×5 fringes, and others three fringes are
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the least: 6×6 fringes, 7×7 fringes, and 8×8 fringes. However, barkinite has a higher abundance of
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naphthalene than vitrinite as well as lower aromatic fringes than barkinite, for instance, 3×3
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fringes, 4×4 fringes and 5×5 fringes, which indicates that vitrinite is more aromatic and
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polycondensed than barkinite.
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3.3 Observation of physical appearance changes when heated
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The physical appearance changes of vitrinite when heated were shown in Figure 5. The overall
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size and shape of coal particle are not greatly changed when heating to 200 °C. The temperature
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was almost 250 °C, white materials of outer edge of coal particle was produced. At 300 °C,
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slightly rounded-off edges were observed. The temperature was raised to 370 °C, rounded-off
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edges increased and the particle size began to become smaller. These changes still continued even
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when the temperature was up to 500 °C. Rounded-off edge was accelerated at 400°C and the
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particle size was drastically reduced from 450 °C to 500 °C. Meanwhile, Flow state of coal
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particle was observed.
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Figure 5 also shows the physical appearance changes of barkinite when heated. The shape of
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barkinite particle was obviously unchanged at around 200 °C. However, when temperature was
19
increased to 250 °C, rounded-off edges of barkinite particle was happened. The edge became dark
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at 300 °C. Yellow trim of barkinite particle was produced and its range gradually increased at
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400 °C or above. When the temperature reached to 450 °C, whole edge of coal particle was
22
rounded-off and the sizes of coal particle were reduced. Flow state of coal particle was happened. 8
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Massive amount of oily material were exuded from the barkinite particle and tarnished the particle
2
edge at 500 °C.
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Comparing the changes of physical structure of barkinite and vitrinite, the temperature of
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rounded-off of barkinite began at 250 °C and that of vitrinite is 300 °C. This is a similar result
5
obtained from HRTEM, as shown in section 3.1. Therefore, the changes of physical appearance
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(shape, size, and color) of barkinite and vitrinite when heated are relative of their chemical
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structure changes with heating. The rounded-off of coal particle possible results from the increase
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of orientation of layers in chemical structure. The sizes of molecules and their alignment possible
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lead to the change of size of coal particle, which indicated from the distribution of length with
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increasing of temperature, just as shown in Figures 3 and 4.
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It should be pointed out that the temperature used is the range of temperature, but not a strict
12
temperature point, because barkinite particle was quickly reacted when heating, especially at
13
temperature ranges 300-500 °C. Barkinite began a thermal decomposition at almost 350°C, and
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the thermal reactions continued to 550 °C. Similar results were obtained from the results of
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thermogravimetric analysis of barkinite.5, 6 Besides, the flow state and oily material of barkinite
16
and vitrinite were observed from 450 to 500 °C, this is not to say that no oily material was formed
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below 450 °C, only because the extensive oily material was formed at the range of 450-500 °C
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and was clearly noticeable under the microscope.
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3.4 Understanding of special thermal behaviors of barkinite and bark coal
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Some special thermal behaviors of barkinite, vitrinite, and bark coal were discussed in some
21
previous works.5, 6, 33 Barkinite, vitrinite, and bark coal have intensive thermal decomposition,32
22
and bark coal has an extra-high Gieseler thermoplastic property: the values of the maximum 9
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fluidity exceeded 180,000 dial divisions per minute (ddpm).5 This is certainly related to chemical
2
composition and chemical structure of bark coal and barkinite, as discussed in other papers.2
3
Table 2 shows some critical temperatures of thermal behaviors of barkinite, vitrinite and bark
4
coal. The temperatures of maximum volatile matter loss (Tmax) of barkinite and vitrinit are almost
5
the same, around 450 °C. However, the maximum rate of mass loss of barkinite is bigger than that
6
of vitrinite. According to the results of HRTEM, naphthalene abundances in aromatic size of
7
barkinite and vitrinite are richer than 3×3 fringe and 4×4 fringe at 450 °C. Furthermore, barkinite
8
has a bigger intensity of naphthalene than vitrinite. On the other hand, the abundances of 3×3
9
fringe and 4×4 fringe of barkinite are less than those of vitrinite at this temperature. In the
10
temperature ranges of 300-500°C, the broken of the amorphous materials weakly bonded to
11
aromatic layers were reacted, such as the aliphatic side chains and oxygen-containing functional
12
groups.34 Barkinite has high aliphatic groups 2,8,10-12 and it is easily decomposed. Large amount of
13
the volatile matters (such as CH4, CO2, H2O, etc) are released.6 Meanwhile, with increasing of
14
temperature, aromatization in chemical structure of barkinite and vitrinite enhanced. In 400-550°C,
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the enormous free radicals were produced because bark coal has super-high fluidity.5 The relative
16
higher molecules aromatic fringe, 3×3 fringe and 4×4 fringe, decomposed into small molecules
17
aromatic fringe, for examples, naphthalene, as shown in Figures 3 and 4. The results of flash
18
pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) also show that the pyrolysates of
19
barkinite are mainly characterized by the presence of alkylbenzenes, alkylnaphthalenes, and
20
phenanthrene [unpublished]. Therefore, the anomalous thermal behaviors of barkinite and vitrinite
21
are related to the distribution of aromatic size in chemical structure, especially naphthalene
22
abundance. 10
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Bark coal has super-high Giesler fluidity.5 The temperatures of maximum fluidity of bark coals
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concentrate on about 440 °C (Table 2). The γ-compound theory suggested that low molecular
3
weight products that play an important role in thermoplasticity are derived from the coal chemical
4
network.35 The development of the thermoplasticity of coal involves the chemical and physical
5
changes in the coal structure. The plasticity of coal can be seen from the structural changes
6
because plastic coal develops anisotropic structure on heat-treatment.20 The anisotropic structure
7
was termed as molecular-orientated domains by Oberlin et al.36, 37 Barkinite and vitrinite show the
8
development of ordered structure at 250 °C and 300 °C, separately. At about 400 °C, bark coal
9
begin to soften (Table 2). Softening of coal takes place in this temperature ranges by the extensive
10
molecular disruption and considerable evolution of volatile matter.34 For vitrinite, stacking in
11
chemical structure was produced at 400 °C, and for barkinite, this characterization is more clear.
12
When temperature is increased to around 450 °C, both these two coal components have obvious
13
stacking in structure, and bark coals also reach the maximum fluidity values at that temperature
14
range.
15
On the other hand, the abnormal thermal behaviors of barkinite and bark coal are also related to
16
their rich in aliphatic structures concentration, especially CH2 group.2,8,10-12 The results of
17
Py-GC/MS also showed that a strong dominance of the C7-C29 n-alkane/alkene series in the
18
pyrolysated of barkinite [unpublished]. These aliphatic functional groups are easily decomposed
19
when heated, as indicated from the results of thermogravimetry coupled with mass spectrometry
20
and Fourier transform infrared spectroscopy (TG/MS/FTIR). 6 Also because of barkinite’s higher
21
aliphatic content, barkinite has higher disordered in structure than vitrinite at 200°C. With
22
increasing of temperature, the orientation of the layer of barkinite become more quickly than 11
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vitrinite, as shown in Figures 1 and 2.
2
4. Conclusions
3
The structural transformations of barkinite and vtirinite with heating in situ from 200-500 °C
4
were examined by HRTEM. The physical appearance changes of these two coal components when
5
heated were observed by polarized light microscopy with a heated stage. The changes obtained in
6
structure on heat-treatment were used to further discuss the special thermal behaviors of bark coal
7
and barkinite.
8
The overall size and shape of barkinite and vitrinite were greatly influenced by temperatures
9
used. At 200 °C, there is no clear change in physical appearance for both barkinite and vitrinite.
10
Rounded-off edge of barkinite particle was observed at 250 °C and that of vitrinite was 300 °C.
11
After this temperature, rounded-off edge was accelerated at 400 °C and the particle size was
12
drastically reduced from 450 °C to 500 °C. Flow states of these two coal component were
13
observed, and massive amount of oily materials were exuded from barkinite particle.
14
For barkinite and vitrinite, at 200 °C, most of the layers are curved and poorly orientated.
15
Barkinite has higher disordered in structure than vitrinite. When the temperature is up to 375 °C,
16
barkinite become obvious orientation of the layer, which is early than vitrinite, 400 °C. After
17
400 °C, the orientation in aromatic layer of these two components was obviously improved.
18
The chemical structures of barkinite and vitrinite were significantly changed on heat-treatment
19
from 200 to 500 °C. Barkinite and vitrinite show the decreases in naphthalene abundance and
20
obvious increases in 3×3 fringe and 4×4 fringe at 200-350 °C. Naphthalene abundance increase
21
from 400 to 500 °C and reach the maximum values at 450 °C. Regardless, the abundance of 2×2
22
fringe increase for barkinite from 200 to 350 °C and that decrease for vitrinite. No great change in 12
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2×2 fringe abundance at 400-500 °C was observed. The distributions of aromatic size in chemical
2
structure of barkinite and vitrinite, especially naphthalene abundance, have great influence on the
3
special thermal behaviors of barkinite and bark coal.
4 5
Acknowledgement
6
The authors gratefully thank the National Natural Science Foundation of China for financial
7
support (Research Project No. 41102097; 41472132). The authors also wish to acknowledge Jin Ju
8
for her useful suggestions. We would like to express our appreciation to Peking University for
9
providing the measurement of HRTEM.
10 11
References
12
[1]
Hsieh, C.Y. Bull. Geol. Soc. China 1933, 12(4), 469-490.
13
[2]
Wang, S.Q.; Tang, Y.G.; Schobert, H.H.; Jiang, D.;Guo, X.; Huang, F.;Guo, Y.N.; Su,, Y.F. Fuel 2014, 126, 116-121.
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[3]
China. 2001, p.1-7(Chinese).
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[4]
[5]
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Table 1 Assignment of parallelogram-shaped aromatic fringes from the HRTEM fringe data26 Aromatic sheet Naphthalene 2×2 3×3 4×4 5×5 6×6 7×7 8×8
Mean length（Å） 3.9 6.0 9.3 12.7 16.0 19.4 22.8 26.1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 16
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Grouping（Å） 3.0-5.4 5.5-7.4 7.5-11.4 11.5-14.4 14.5-17.4 17.5-20.4 20.5-24.4 24.5-28.4
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Table 2 Some critical temperatures of thermal behaviors of barkinite, vitrinite, and bark coals Thermogravimetric analysis Tmax (°C)
MR(%/°C)
Ts(°C)
MFT(°C)
Tr(°C)
LP5
424
0.86
407
441
481
LP-2
5
411
0.52
390
443
484
LP-4
5
415
1.11
412
436
-
33
Lp5-1
450
0.40
-
-
-
33
454
1.08
-
-
-
33
453
0.39
-
-
-
Barkinite Vitrinite 2 3
Gieseler fluidity
Coal ID#
Tmax, temperature of the maximum volatile matter loss; MR, the maximum rate of mass loss ; Ts, softening temperature; MFT, the temperature of maximum fluidity; Tr, resolidification temperature
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 17
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Figure Captions
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Fig. 1 HRTEM images and the corresponding skeletonized images of barkinite
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Fig. 2 HRTEM images and the corresponding skeletonized images of vitrinite
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Fig. 3 Aromatic fringe distributions for barkinite with different temperatures
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Fig. 4 Aromatic fringe distributions for vitrinite with different temperatures
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Fig. 5 Physical feature changes of barkinite (Ba) and vitrinite (V) with different temperatures
7
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Fig. 1 HRTEM images and the corresponding skeletonized images of barkinite ((a,b,c) 200°C; (d,e,f) 250°C; (g,h,i) 300°C; (j,k.l) 350°C; (m,n,o) 400°C;(p, q,r) 450°C; (s,t,u) 500°C)
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Fig. 2 HRTEM images and the corresponding skeletonized images of vitrinite ((a,b,c) 200°C; (d,e,f) 250°C; (g,h,i) 300°C; (j,k.l) 350°C; (m,n,o) 400°C;(p, q,r) 450°C; (s,t,u) 500°C)
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Fig. 3 Aromatic fringe distributions for barkinite with different temperatures
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Fig. 4 Aromatic fringe distributions for vitrinite with different temperatures
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2 3
Fig. 5 Physical feature changes of barkinite (Ba) and vitrinite (V) with different temperatures
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