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Changes of semifusinite and fusinite surface roughness during heat treatment determined by atomic force microscopy
International Journal of Coal Geology 88 (2011) 218–226
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Changes of semifusinite and fusinite surface roughness during heat treatment determined by atomic force microscopy Rafal Morga Institute of Applied Geology, Silesian University of Technology, Akademicka 2, 44-100 Gliwice, Poland
a r t i c l e
i n f o
Article history: Received 28 May 2011 Received in revised form 11 October 2011 Accepted 14 October 2011 Available online 21 October 2011 Keywords: Coal Inertinite Semifusinite Fusinite Atomic force microscopy Heating experiment
a b s t r a c t This study describes changes of surface roughness of semifusinite and fusinite as an indicator of structural alteration resulting from heat treatment at 400–1200 °C. Surface roughness has been investigated by atomic force microscopy of inertinite concentrates from coking coals (vitrinite reflectance Rr = 1.07%–1.41%) from the Upper Silesian Coal Basin of Poland (Namurian C — Westphalian A). Unheated fusinite has a higher surface roughness than semifusinite from the same coal. The average surface roughness of semifusinite decreases with the Swelling Index of the parent coal. Heating increases the surface roughness of semifusinite and fusinite. Increase in the average surface roughness is stronger for semifusinite than fusinite and correlates to increasing reflectance of these macerals. The surface roughness of semifusinite correlates to the relative mass loss of the inertinite concentrates during heating. After heating to 1200 °C fusinite has a lower average surface roughness than semifusinite from the same coal. Consequently, average surface roughness can be used as a measure of structural alteration of inertinite group macerals during heat treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Atomic force microscopy (AFM) is a promising tool for imaging and characterizing materials at the nanoscale. It is frequently used in physics, chemistry and biology to characterize surface properties of nanomaterials, polymers and metals and as well as tissue imaging (Baker et al., 2000; Dokou et al., 2000; Franz and Puech, 2008; Hansma et al., 1997; Jastrzebska et al., 2010; Muller, 2008; Peng et al., 2001, among many others). AFM was also applied in coal studies (Lawrie et al., 1997; Yang, 1994; Yumura et al., 1993). Lawrie et al. (1997) observed that fusinite has higher surface roughness than vitrinite. Bruening and Cohen (2005) studied coal oxidation effects and found AFM to have a great potential to yield useful information on physical properties of coal macerals. Martin et al. (2005) with the use of AFM revealed different types of coke morphology at the nanometer scale. Rantitsch et al. (2004) applied this method to investigate carbonaceous material from natural graphite. Roughness of super fine coal particles was investigated by Liu et al. (2010). Atomic force microscopy was also used to study nano-ashes formed during high temperature combustion of pulverized coals (Carbone et al., 2011). Different roughness parameters, their definitions and significance were described by Gadelmawla et al. (2002). Inertinite is the most aromatic and most thermally stable of all the maceral groups (Pandolfo et al., 1988; Sun et al., 2003; Vasallo et al., 1991; Wang et al., 2010; White et al., 1989; Xie et al., 1991). In this
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group, the maceral fusinite is more aromatic than semifusinite from the same coal (Morga, 2010) and has lower H/C ratio (Blanc et al., 1991; Diessel, 1992). On the other hand, the amount of mobile hydrogen is higher in semifusinite than in fusinite (Maroto-Valer et al., 1998). The diameter of coherent domains (crystallites) (La) in the studied macerals is on average ca. 1.0–1.4 nm (Malumbazo et al., 2010; Xie et al., 1991), and it is higher in fusinite than in semifusinite (Morga, in press). Mastalerz and Bustin (1993, 1996, 1997) showed that both semifusinite and fusinite can have very varied chemistry. Mastalerz and Bustin (1993, 1997) noticed a very weak correlation between carbon content and reflectance of semifusinite. Machnikowska et al. (2002) found, by the diffuse reflectance infrared fourier transform (DRIFT) spectroscopy, a strong correlation between the condensation of aromatic rings (the CHar/C=Car ratio) and fusinite reflectance. Guo and Bustin (1998) and Bustin and Guo (1999) examined semifusinite, fusinite and modern charcoals by Fourier transform infrared spectroscopy (FTIR) and observed that the abrupt changes in chemical composition of charcoals are coincident with jumps in their reflectance. The distribution of reflectance values and FTIR spectral characteristics of inertinite is consistent with a model of charring that incorporates discrete compositional jumps. Semifusinite is a maceral, which in part is reactive in thermal processing of coal. The lower the reflectance, the higher is the reactivity. Low reflecting semifusinite is wholly fusible, and medium reflectingpartially fusible (ICCP, 2001; Taylor et al., 1998). Fusinite is a nonreactive maceral and does not fuse. The difference in behaviour of fusinite and semifusinite during heat treatment is determined by the structural and chemical properties of these macerals before heating.
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Pyrolysis to the temperature of 600 °C causes primary devolatilization resulting in release of H2O, CO2, hydrocarbons and hydrogen from aliphatic groups and increase in aromaticity of inertinite (Morga, 2010; Pandolfo et al., 1988; Sun et al., 2003; Vasallo et al., 1991; Wang et al., 2010; White et al., 1989; Xie et al., 1991; Zhao et al., 2010). It was observed that at 400–600 °C slight re-ordering of semifusinite structure occurs, which is reflected by reflectance increase (Morga, in press). Alteration of fusinite structure requires high activation energy and starts at higher temperature. As inferred from the micro-FTIR examination, the most significant alteration of the chemical structure of semifusinite occurs at 600 and 800 °C or 800 and 1000 °C, and that of fusinite at 600 and 800 °C (Morga, 2010). The secondary devolatilization at high temperatures (600–1000 °C) is connected mainly with the decomposition of stable oxygen containing functional groups as well with H2 yield due to condensation of aromatic and hydroaromatic structures (Morga, 2010; Wang et al., 2010; Zhao et al., 2010). As revealed from micro-Raman spectroscopy examination, the macromolecular network of semifusinite and fusinite rebuilds at 800 °C. The diameter of coherent domains (La) increases, which is reflected by the reflectance jump found for both macerals (in press; Komorek and Morga, 2007, Morga, 2010). Increase in the La of inertinite and chars from inertinite-rich coals with increasing temperature was also demonstrated by Xie et al. (1991), Wang et al. (2010) and Chabalala et al. (2010) by X-ray diffractometry (XRD). At 1000–1200 °C an increase in the structural organization, accompanied by an increasing reflectance is observed in semifusinite. When heated at 1200 °C, semifusinite has larger coherent domains and they are more ordered than in fusinite, which results in higher reflectance value (Morga, in press). Increase in structural organization of chars from inertinite-rich coals upon heating, was also observed by Malumbazo et al. (2010). Increase in aromaticity and condensation resulting from heating at 400–1200 °C is much higher for semifusinite than fusinite (Morga, 2010). The purpose of this study is to describe changes of surface roughness of semifusinite and fusinite as an indicator of structural alteration of these macerals resulting from experimental heat treatment at 400–1200 °C. 2. Experimental Inertinite concentrates were prepared from four samples of coking coals from seams 405/1 (Westphalian A — sample 1), 409/4 (Namurian C — samples 2 and 3) and 502/1 (Namurian B — sample 4) of the Upper Silesian Coal Basin of Poland. The coals belong to technological types 35.1 to 37.2, according to the Polish Standard (Polish Standard, Polska Norma PN-G-97002, 1982), with a vitrinite reflectance (Rr) ranging from 1.07% to 1.41%. Selected properties of these coals are summarized in Table 1. The concentrates were separated in heavy liquids as described in Morga (2010). About 300 mg of each concentrate were heated in a Carbolite pipe oven at temperature of 400, 600, 800, 1000 and 1200 °C for one hour in an argon atmosphere. Thereafter, polished grain mounts were prepared from the concentrates for the AFM and microscopic examination. Atomic force microscopy examination was carried out on 20 grains of semifusinite and 10–15 grains of fusinite, randomly chosen in each sample, using a Solver P47-Pro atomic force microscope (NT-MDT) in the the semi-contact (tapping) mode. All measurements were
performed at room temperature using rectangular silicon cantilevers of a nominal elastic constant of 12 N/m and a typical resonance frequency of 240 kHz. The free oscillation amplitude was varied in the range of 10–25 nm to get stable images in a measurement area of 1 × 1 μm, similar to that used by Lawrie et al. (1997) and the scanning speed during the acquisitions ranged from 500 to 1000 nm/s. The image was corrected by subtracting the 2nd order trend surface to remove a cylindrical curvature, caused by piezo-tube scanner movement. Measurements of mean semifusinite and fusinite reflectance were carried out under reflected light in immersion oil (no = 1.518 at 23 °C) according to ISO 7404-5/1994, using an Axioskop Zeiss microscope. Relative mass loss of the inertinite concentrate during heating was determined by weighting. The following parameters were calculated: average surface roughness (Ra), root mean square roughness (Rq), surface skewness (Rsk) and coefficient of kurtosis (Rku). Average surfece roughness (Ra) is the most frequently used roughness parameter. It defines average value of the surface roughness within the area analyzed. It is calculated according to the formula: y x X 1 X jzði; jÞ−zmean j Nx Ny i¼1 j¼1
N
Ra ¼
N
ð1Þ
where: y x X 1 X z Nx Ny i¼1 j¼1 ij
N
zmean ¼
N
ð2Þ
and Nx and Ny is a number of points along the X and Y axes. Another roughness parameter is the root mean square roughness (Rq), characterizing the variability of the surface topography. It is determined by: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u Ny Nx X u 1 X zði; jÞ−zmean Þ3 Rq ¼ t Nx Ny i¼1 j¼1
ð3Þ
Surface skewness (Rsk) gives the information on the symmetry of the distribution of the surface data (Eq. (4)). If the value of the parameter equals to zero the distribution is symmetrical. If it is different from zero, the distribution is unsymmetrical. The asymmetry is positive, if the distribution is excessive on the right, which reflects occurence of many high peaks on the surface. The asymmetry is negative, if the distribution is excessive on the left, indicating many deep depressions of the surface. y x X X 1 3 zði; jÞ−zmean Þ 3 Nx Ny Rq i¼1 j¼1
N
Rsk ¼
N
ð4Þ
Surface kurtosis (Rku), characterizes the sample surface excess (Eq. (5)). Kurtosis for normal distribution is equal to 0. Positive kurtosis occurs when the surface data are more concentrated at the mean value than in normal distribution and negative, when they are less concentrated. y x X X 1 4 zði; jÞ−zmean Þ −3 4 Nx Ny Rq i¼1 j¼1
N
Rku ¼ Table 1 Selected properties of the parent coals used in the study.
219
N
Sample
Rr %
Vdaf %
SI
Vitrinite %
Liptinite %
Inertinite %
Mineral matter %
3. Results
1 2 3 4
1.07 1.24 1.25 1.41
24.75 21.96 23.14 19.76
8 7.5 9 5
65 62 69 56
2 2 1 1
30 33 28 38
3 3 2 5
3.1. Petrographic characteristics of the inertinite concentrates
ð5Þ
The concentrates contain 80% to 83% inertinite (Table 2). The remaining part consists of vitrinite (14–15%) and small amounts
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R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Table 2 Petrographic properties of the inertinite concentrates used in the study. Concentrate
Sample Sample Sample Sample
V %
1 2 3 4
L %
I %
– 1 – –
15 15 15 14
83 81 80 82
MM %
Sf %
2 3 5 4
F %
54 36 50 47
Id* %
12 20 14 6
Ma %
28 28 24 35
Mi %
4 2 2 2
Fu %
1 2 3 8
Semifusinite
1 12 7 2
Fusinite
Rr %
sr %
Rr %
sr %
2.14 2.10 2.09 1.97
0.42 0.29 0.31 0.24
3.20 3.14 3.09 2.85
0.40 0.36 0.43 0.45
Abbreviations: V — vitrinite, L — liptinite, I — inertinite, MM — mineral matter, Sf — semifusinite, F — fusinite, Id — inertodetrinite, Ma — macrinite, Mi — micrinite, Fu — funginite. *Includes inertinite fragments originated due to crushing of coal (values for the inertinite group macerals are the proportion of total inertinite content); Rsf, ssf — mean reflectance of semifusinite and its standard deviation, Rf, sf — mean reflectance of fusinite and its standard deviation.
of mineral matter (pyrite and clay minerals). The most abundant macerals are semifusinite (36–54% of total inertinite content) and inertodetrinite (24–35%). High inertodetrinite content is likely due
a)
to crushing the coal during preparation of the concentrates. Fusinite proportion is 6–20%. Other macerals of the inertinite group occur in smaller quantities.
c) 0.16
0.1
0.08
Relative frequency
Relative frequency
0.12
0.06
0.04
0.08
0.04 0.02
0
0 1.2
1.6
2
2.4
2.8
3.2
3.6
4
1.2
4.4
1.6
2
2.4
2.8
3.2
3.6
4
4.4
Rr [%]
Rr [%]
d)
b)
0.16
0.12
0.08
Relative frequency
Relative frequency
0.12
0.04
0.08
0.04
0
0 1.2
1.6
2
2.4
2.8
Rr [%]
3.2
3.6
4
4.4
1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
3
3.2 3.4 3.6 3.8 4
Rr [%]
Fig. 1. Reflectograms of semifusinite and fusinite in the examined concentrates: a) — sample 1; b) — sample 2; c) — sample 3; and d) — sample 4.
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Semifusinite in concentrates before heating has mean reflectance from 1.97% (sr = 0.25%) in sample 4 to 2.14% (sr = 0.25%) in sample 1 (Table 2). Fig. 1 shows reflectograms of semifusinite and fusinite. They are multimodal due to the occurrence of different genetic forms of semifusinite and fusinite. In the examined concentrates semifusinite (Fig. 2a) is light grey to white. In low-reflecting grains the cellular structure is usually weakly preserved. Cell lumens are small, circular or oval and often swollen. Relief does not change within an individual grain. Brighter, medium and high-reflecting semifusinite, generally, has better retained cellular structure. It has thin and elongated, oval or lenticular cell lumens. In part of such grains, however, swollen cell lumens are also observed. Relief is usually variable within an individual grain. Fusinite occurs in much smaller quantities than semifusinite. In concentrates its mean reflectance ranges from 2.85% (sr = 0.45%) in sample 4 to 3.20% (sr = 0.40%) in sample 1 (Table 2). Fusinite (Fig. 3a) is white, sometimes with a yellowish tint, and has a well preserved structure. Cell walls are either
221
thick or thin, frequently with variable relief. Cell lumens are usually large. They can be circular, oval, lenticular or elongated, depending on section. Rarely, bogen structure can be seen. 3.2. Surface roughness Fig. 4 shows the change of semifusinite surface topography (scan area 1 × 1 μm) with increasing temperature. In the samples before heating semifusinite has an average surface roughness (Ra) between 0.77 nm (sample 3) and 2.51 nm (sample 4) (Fig. 5a; Table 3). The root mean square roughness (Rq) ranges from 1.05 to 3.22 nm. Surface skewness (Rsk) is usually ~0.20 which indicates slightly unsymmetrical positive distribution of the surface points. Kurtosis (Rku) is close to 0 (samples 1 and 4), which means normal distribution. The average surface roughness of semifusinite tends to decrease with the increasing value of the Swelling Index of a parent coal (Fig. 6). Fusinite in all examined samples before heating has a higher
Fig. 2. Semifusinite from sample 2 before heating (a) and after heat treatment at: 400 °C (b), 600 °C (c), 800 °C (d), 1000 °C (e) and 1200 °C (f).
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Fig. 3. Fusinite from sample 2 before heating (a) and after heat treatment at: 400 °C (b), 600 °C (c), 800 °C (d), 1000 °C (e) and 1200 °C (f).
average surface roughness and root mean square roughness than semifusinite from the same coal (Fig. 5b; Table 4). In three of four examined samples it has lower values of the surface skewness and kurtosis coefficient. Surface skewness determined for fusinite is close to 0 (sample 2, excepting) which means almost symmetrical distribution of the surface points. Kurtosis is generally close to 1, reflecting leptokurtic distribution. When the concentrates are heated at 400 °C the Ra of semifusinite in samples 1 and 3 increases, while in samples 2 and 4 does not change (Fig. 5a; Table 3). These increases are found for the samples characterized by the lowest values of the average surface roughness before heating, and, at the same time, also the highest SI values of the parent coals. When the concentrates are heated at 600 °C, strong increase of the Ra value of semifusinite is observed for all samples. Another increase is found at 800 °C (samples 1, 2 and 4) or 1000 °C (sample 3).
The Ra value of fusinite increases first by heating the concentrates at 600 °C (samples 1 and 4) or 800 °C (samples 2 and 3) (Fig. 5b; Table 4). In all samples a further increase of the surface roughness of fusinite is found at 1000 °C, and in samples 1 and 2 also at 1200 °C. Skewness of semifusinite surface roughness changes from slightly positive for slightly negative values, with increasing temperature but, generally, stays close to 0 (Table 3). Surface skewness values for fusinite are also close to 0, with the exception of samples 3 and 4, when heated at 400 °C (Table 4). This means that distributions of surface points are for both macerals symmetrical. Kurtosis both for semifusinite and fusinite, on general, does not change significantly during the heat treatment. It is close to 0 or stays slightly above this value. After heating the concentrates at 1200 °C fusinite has lower average roughness than semifusinite (Tables 3 and 4). Surface skewness for both macerals is similar and obtains values close to 0. There is no difference in the kurtosis between the two macerals, either.
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Ra [nm]
a
30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 0
223
Sf 1 Sf 2 Sf 3 Sf 4
200
400
600
800
1000
1200
1400
T [oC]
b
26 24 22 20 18
Ra [nm]
16 14 12 10 8 6 4
F1 F2 F3 F4
2 0
0
200
400
600
800
1000
1200
1400
T [oC] Fig. 5. The average roughness (Ra) of semifusinite (a) and fusinite (b) before and after heat treatment at 400–1200 °C.
or surpasses that of fusinite. A considerable increase is also found for semifusinite at 1200 °C. Finally, reflectance of semifusinite is 8.31–8.83%, while that of fusinite 6.57–7.49%. Semifusinite having the lowest reflectance before heating (sample 4) attains the highest Rr value. 3.4. Relative mass loss The relative mass loss of the inertinite concentrates during heating can be well aproximated by hyperbolic curve. At 400 °C it is very limited (Fig. 8). At 600 °C considerable change is observed, which is due to the first phase of degassing. After that, the rate of the mass loss decreases. After heating the concentrates at 1200 °C the value of the parameter ranges from 18.13% (sample 3) to 20.6% (sample 1). 4. Discussion Fig. 4. Changes of semifusinite surface topography in sample 2 with increasing heat treatment temperature: a) — before heating, b) — 400 °C, c) — 600 °C, d) — 800 °C, e) — 1000 °C, and f) — 1200 °C.
3.3. Mean reflectance Mean reflectance (Rr) of semifusinite and fusinite changes during heat treatment in similar manner for all examined samples. It rises first at 600 °C, and at 800 °C a significant increase in the Rr value is observed (Fig. 7a–b), when semifusinite reflectance is very close to
The average surface roughness of fusinite estimated in this study is lower than the values presented by Lawrie et al. (1997). Changes in the average surface roughness of semifusinite and fusinite upon heating are closely related to physical–chemical alteration of these macerals. This is confirmed by strong correlation between the average roughness and reflectance both for semifusinite (Fig. 9a) and fusinite (Fig. 9b) in the examined temperature range. The correlation coefficients are between 0.90 and 0.97, and between 0.96 and 0.98, respectively, both at significant p b 0.05. There is also a strong
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R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Table 3 Surface roughness parameters of semifusinite before and after heat treatment.
Table 4 Surface roughness parameters of fusinite before and after heat treatment.
Temperature °C
Average roughness nm
Root mean square nm
Surface skewness
Coefficient of kurtosis
Temperature °C
Average roughness nm
Root mean square nm
Surface skewness
Coefficient of kurtosis
Sample 1 25 400 600 800 1000 1200
1.85 ± 1.14 2.41 ± 0.87 11.75 ± 3.70 15.31 ± 8.41 16.67 ± 8.65 18.14 ± 9.06
2.32 ± 1.34 3.10 ± 1.11 15.83 ± 4.98 19.58 ± 10.73 21.82 ± 11.00 23.11 ± 11.52
0.22 0.21 − 0.17 − 0.05 0.06 − 0.03
− 0.19 0.48 − 0.02 0.08 0.68 0.44
Sample 1 25 400 600 800 1000 1200
2.72 ± 1.35 2.15 ± 0.74 7.29 ± 2.43 10.69 ± 7.00 14.73 ± 8.57 16.56 ± 5.41
3.62 ± 1.81 2.73 ± 0.91 9.60 ± 3.54 14.05 ± 9.60 18.28 ± 10.31 21.17 ± 7.05
− 0.03 0.08 0.05 − 0.17 − 0.04 − 0.18
1.15 0.45 − 0.18 0.82 0.57 0.10
Sample 2 25 400 600 800 1000 1200
2.10 ± 1.25 2.04 ± 1.53 3.47 ± 2.34 13.36 ± 4.43 14.71 ± 6.45 16.33 ± 9.91
2.57 ± 1.44 2.60 ± 1.90 4.64 ± 3.10 17.57 ± 6.06 19.30 ± 8.12 20.56 ± 11.43
0.24 − 0.10 0.16 0.07 0.14 − 0.17
1.36 0.58 0.80 0.22 0.86 0.17
Sample 2 25 400 600 800 1000 1200
2.49 ± 2.02 2.56 ± 2.06 2.43 ± 1.28 9.61 ± 5.57 12.33 ± 4.21 14.02 ± 6.59
2.85 ± 2.49 3.14 ± 2.32 3.15 ± 1.62 12.46 ± 7.52 15.99 ± 5.46 18.01 ± 9.33
0.25 − 0.23 − 0.09 0.00 0.16 0.00
0.94 0.02 0.89 0.82 0.75 0.21
Sample 3 25 400 600 800 1000 1200
0.77 ± 0.38 1.90 ± 1.11 5.77 ± 2.13 6.87 ± 3.73 15.34 ± 3.68 14.46 ± 6.57
1.05 ± 0.52 2.68 ± 1.66 8.80 ± 3.16 8.43 ± 4.61 19.24 ± 5.25 16.50 ± 7.83
0.17 1.04 0.34 − 0.04 − 0.08 − 0.07
1.88 2.95 0.94 − 0.22 0.40 0.30
Sample 3 25 400 600 800 1000 1200
1.64 ± 0.75 1.65 ± 1.00 1.83 ± 0.97 5.43 ± 1.07 10.28 ± 2.43 10.47 ± 6.04
2.15 ± 1.03 2.28 ± 1.55 2.47 ± 1.17 6.25 ± 2.36 12.71 ± 3.35 13.31 ± 6.89
− 0.06 0.81 0.12 0.02 − 0.10 0.08
1.02 2.14 2.26 1.13 − 0.32 0.64
Sample 4 25 400 600 800 1000 1200
2.51 ± 0.73 2.59 ± 0.89 6.67 ± 2.78 10.39 ± 4.58 12.97 ± 6.38 13.96 ± 8.25
3.22 ± 1.16 3.18 ± 1.21 8.49 ± 3.62 13.31 ± 5.88 16.42 ± 8.16 17.08 ± 10.43
0.20 0.16 − 0.31 0.01 0.06 − 0.06
− 0.09 0.41 − 0.32 0.32 0.33 0.76
Sample 4 25 400 600 800 1000 1200
2.94 ± 1.02 2.76 ± 1.17 5.23 ± 3.94 7.59 ± 3.39 9.23 ± 3.72 9.01 ± 5.32
3.66 ± 1.21 2.96 ± 1.31 6.68 ± 4.74 9.73 ± 4.31 11.50 ± 4.43 11.10 ± 6.09
0.01 0.56 − 0.40 − 0.12 − 0.09 0.01
− 0.35 0.93 0.43 0.28 − 0.16 − 0.12
correlation between the average roughness of semifusinite and the relative mass loss of the inertinite concentrates (correlation coefficients between 0.89 and 0.99, at p b 0.05) (Fig. 10). Increase in the Ra of semifusinite at 400–600 °C (Fig. 5a, Table 3) is attributed to the partial release of volatiles (first phase of degassing) (Morga, 2010; Pandolfo et al., 1988; Sun et al., 2003; Vasallo et al., 1991; Wang et al., 2010; White et al., 1989; Xie et al., 1991; Zhao et al., 2010), which results in the intense mass loss of the concentrates (Fig. 8). Cell lumens become larger, and the cell walls thinner (Fig. 2b–c). Many small pores appear. More reactive (fusible) semifusinite grains soften and become rounded or melted. These changes are accompanied by a re-ordering of semifusinite structure and a reflectance increase (Komorek and Morga, 2007; Morga, in press). The increase found at 800 °C (samples 1, 2 and 4) or 1000 °C (sample 3) (Fig. 5a, Table 3) is a result of the second phase of degassing (Wang
3.5 3.0 2.5
Sample 4
Ra [nm]
Sample 2
2.0
Sample 1
1.5 1.0
Sample 3
ΔRa ¼ ðRa1200 −Ra25 Þ=Ra25
0.5 0.0 4.5
et al., 2010; Zhao et al., 2010), which causes further mass loss of the concentrates (Fig. 8). The surface of the grains becomes rough, and the porosity increases (Fig. 2d–e). This phase is also connected with an alteration of the chemical structure of semifusinite and rebuilding of its macromolecular network (Morga, 2010, in press), leading to strong reflectance growth (Komorek and Morga, 2007; Morga, 2010, in press). Heating semifusinite at 1200 °C causes further mass loss of the concentrates (Fig. 8). This shows, that the volatiles are still released from the chemical structure, which results in enlargement of cell lumens and pores (Fig. 2f) and increase in surface roughness in three of the examined samples (Fig. 5a, Table 3). The average surface roughness of fusinite at 600 °C (samples 1 and 4) or 800 °C (samples 2 and 3) (Fig. 5b; Table 4) increases together with devolatilization. The cell walls become thinner, and porosity emerges (Fig. 3b–d). The chemical–structural rebuilding at the same temperature range (Morga, 2010, in press) results in a reflectance increase. This initial change occurs at a higher heating temperature than in semifusinite, reflecting more rigid structural framework of fusinite. Fusinite contains less volatiles than semifusinite from the same coal. Change in the Ra value observed at 1000 °C and at 1200 °C (sample 1 and 2) (Fig. 5; Table 4) is a result of final devolatilization of that maceral, which causes that the cell lumens become larger and the surface tortous. Many small pores are observed (Fig. 3e–f). To evaluate the intensity of surface roughness changes during heating, the increase rate (ΔRa) was calculated. That was done according to the formula:
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
SI Fig. 6. The relationship between average roughness (Ra) of semifusinite before heating and the Swelling Index (SI) of a parent coal.
where: – Ra1200 is the average surface roughness after heating the concentrate at 1200 °C – Ra25 is the average surface roughness before heating.
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
a
a
10
24 Sf 1: r = 0.8995, p = 0.0146 Sf 2: r = 0.9606, p = 0.0023 Sf 3: r = 0.9160, p = 0.0103 Sf 4: r = 0.9679, p = 0.0015
22
9
20 18
7
16
Ra [nm]
8
Rr [%]
6 5
225
14 12 10 8
4
6 3
4 Sf 1 Sf 2 Sf 3 Sf 4
2 1 0
200
400
600
800
1000
1200
Sf 1 Sf 2 Sf 3 Sf 4
2 0 1
1400
2
3
4
5
T [ C]
b
b9
7
8
9
10
20 F 1: r = 0.9595, p = 0.0024 F 2: r = 0.9834, p = 0.0004 F 3: r = 0.9684, p = 0.0015 F 4: r = 0.9789, p = 0.0007
18 8
16 14
Ra [nm]
7
Rr [%]
6
Rr [%]
o
6 5
12 10 8 6
4 4 3 2 0
200
400
600
800
1000
1200
F1 F2 F3 F4
2
F1 F2 F3 F4
0 2
1400
3
4
5
T [ C] Fig. 7. The mean reflectance (Rr) of semifusinite (a) and fusinite (b) before and after heat treatment at 400–1200 °C.
7
8
Fig. 9. The relationship between the average roughness (Ra) and reflectance (Rr) of semifusinite (a) and fusinite (b) before and after heat treatment at 400–1200 °C.
increase was observed for semifusinite from distinctly lower SI (Fig. 11). For fusinite ΔRa attains the values between 266% (sample 4) and 538% (sample 3). Thus, there is the same relation here as in the case of semifusinite. Higher rate of roughness increase during heat treatment observed for semifusinite corresponds with higher reactivity of this maceral, compared with fusinite.
22
22
20
20
18
18
Sf 1: r = 0.9882, p = 0.0015 Sf 2: r = 0.8999, p = 0.0374 Sf 3: r = 0.8925, p = 0.0416 Sf 4: r = 0.9754, p = 0.0046
16
16
14 14
Ra [nm]
Relative mass loss [%]
ΔRa determined for semifusinite substantially vary, from 456% (sample 4) to 1778% (sample 3), which indicates varied reactivity of this maceral. The highest rate of the average roughness increase was found for semifusinite coming from coal characterized by the highest value of the Swelling Index, while the lowest rate of Ra
6
Rr [%]
o
12
12 10
10
8
8
6 4
6 Sample 1 Sample 2 Sample 3 Sample 4
4 2
400
600
800
1000
1200
T [oC] Fig. 8. The relative mass loss of the inertinite concentrates during heat treatment.
Sf 1 Sf 2 Sf 3 Sf 4
2 0
0
2
4
6
8
10
12
14
16
18
20
Relative mass loss [%] Fig. 10. The relationship between the average roughness (Ra) of semifusinite and the relative mass loss of the inertinite concentrates during heat treatment.
226
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
2000 Sample 3
1800
Ra [%]
1600 1400 1200 1000
Sample 1
800
Sample 2
600 Sample 4
400 4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
SI Fig. 11. The relationship between the rate of average roughness increase (ΔRa) determined for semifusinite and the Swelling Index (SI) of a parent coal.
5. Conclusions Atomic force microscopy is a useful tool for examination of surface properties of coal macerals. The study presents the first data on the evolution of surface roughness of semifusinite and fusinite during experimental heating. The results of the study broaden the general knowledge about properties of semifusinite and fusinite and their behaviour under the influence of heating. Fusinite has a higher average surface roughness and root mean square roughness than semifusinite from the same coal. The average surface roughness of semifusinite decreases with the increase in the Swelling Index. Experimental heating causes an increase in average roughness and root mean square roughness of both semifusinite and fusinite. The rate of this increase is higher for semifusinite than for fusinite, which corresponds to higher thermal reactivity of the former maceral. Changes of the average roughness of semifusinite and fusinite during heating are strongly correlated to the reflectance increase of these macerals. There is a strong correlation between the average roughness of semifusinite and the relative mass loss of the inertinite concentrates. After heating the concentrates at 1200 °C fusinite has a lower average roughness than semifusinite from the same coal. The average roughness can be used as a measure of structural alteration of inertinite group macerals during heat treatment. Acknowledgments The study was financially supported by the Ministry of Science and Higher Education of Poland grant no. 3631/B/T02/2008/35. References Baker, A.A., Helbert, W., Sugiyama, J., Miles, M.J., 2000. New insight into cellulose structure by atomic force microscopy shows the Iα crystal phase at near-atomic resolution. Biophysical Journal 79, 1135–1139. Blanc, P., Valisolalao, J., Albrecht, P., Kohut, J.P., Muller, J.F., Duchene, J.M., 1991. Comparative geochemical study of three maceral groups from a high-volatile bituminous coal. Energy & Fuels 5, 875–884. Bruening, F.A., Cohen, A.D., 2005. Measuring surface properties and oxidation of coal macerals using the atomic force microscope. International Journal of Coal Geology 63, 195–204. Bustin, R.M., Guo, Y., 1999. Abrupt changes (jumps) in reflectance values and chemical compositions of artificial charcoals and inertinite in coals. International Journal of Coal Geology 38, 237–260. Carbone, F., Beretta, F., D'Anna, A., 2011. A flat premixed flame reactor to study nanoash formation during high temperature pulverized coal combustion and oxygen firing. Fuel 90, 369–375. Chabalala, V.P., Wagner, N., Potgieter-Vermaak, S., 2010. Investigation into the evolution of char structure using Raman spectroscopy in conjunction with coal petrography; Part 1. Fuel Processing Technology 92, 750–756.
Diessel, C.F.K., 1992. Coal-bearing depositional systems. Springer-Verlag. Dokou, E., Stangland, E.E., Andres, R.P., Delgass, W.N., Barteau, M.A., 2000. Comparison of AFM and HRTEM to determine the metal particle morphology and loading of an Au/TiO2 catalyst. Catalysis Letters 70, 1–7. Franz, C.M., Puech, P.-H., 2008. Atomic Force Microscopy: a versatile tool for studying cell morphology, adhesion and mechanics. Cellular and Mechanical Bioengineering 1, 289–300. Gadelmawla, E.C., Koura, M.M., Maksoud, T.M.A., Elewa, I.M., Soliman, H.H., 2002. Roughness parameters. Journal of Materials Processing Technology 123, 133–145. Guo, Y., Bustin, R.M., 1998. FTIR spectroscopy and reflectance of modern charcoals and fungal decayed woods: implications for studies of inertinite in coals. International Journal of Coal Geology 37, 29–53. Hansma, H.G., Kim, K.J., Laney, D.E., Garcia, R.A., Argaman, M., Allen, M.J., Parsons, S.M., 1997. Properties of biomolecules measured from Atomic Force Microscope images: a review. Journal of Structural Biology 119, 99–108. International Committee for Coal and Organic Petrology, 2001. The new inertinite classification (ICCP System 1994). Fuel 80, 459–471. Jastrzebska, M., Mróz, I., Barwiński, B., Wrzalik, R., Boryczka, S., 2010. AFM investigations of self-assembled DOPA-melanin nano-aggregates. Journal of Materials Science 45, 5302–5308. Komorek, J., Morga, R., 2007. Evolution of optical properties of vitrinite, sporinite and semifusinite in response to heating under inert conditions. International Journal of Coal Geology 71, 389–404. Lawrie, G.A., Gentle, I.R., Fong, C., Glickson, M., 1997. Atomic force microscopic studies of Bowen Basin coal macerals. Fuel 76, 1519–1526. Liu, J., Jiang, X., Huang, X., Wua, S., 2010. Morphological characterization of super fine pulverized coal particle. Part 2. AFM investigation of single coal particle. Fuel 89, 3884–3891. Machnikowska, H., Krztoń, A., Machnikowski, J., 2002. The characterization of coal macerals by diffuse reflectance infrared spectroscopy. Fuel 81, 245–252. Malumbazo, N., Wagner, N.J., Bunt, J.R., Van Niekerk, D., Assumption, H., 2010. Structural analysis of chars generated from South African inertinite coals in a pipe-reactor combustion unit. Fuel Processing Technology 92, 743–749. Maroto-Valer, M.M., Taulbee, D.N., Andresen, J.M., Hower, J.C., Snape, C.E., 1998. The role of semifusinite in plasticity development for a coking coal. Energy & Fuels 12, 1040–1046. Martin, N., Viniegra, M., Zarate, R., Espinosa, G., Batina, N., 2005. Coke characterization for an industrial Pt–Sn/γ-Al2O3 reforming catalyst. Catalysis Today 107–108, 719–725. Mastalerz, M., Bustin, R.M., 1993. Variation in elemental composition of macerals; an example of the application of electron microprobe to coal studies. International Journal of Coal Geology 22, 83–99. Mastalerz, M., Bustin, R.M., 1996. Application of reflectance micro-Fourier Transform infrared analysis to the study of coal macerals: an example from the Late Jurassic to Early Cretaceous coals of the Mist Mountain Formation, British Columbia, Canada. International Journal of Coal Geology 32, 55–67. Mastalerz, M., Bustin, R.M., 1997. Variation in the chemistry of macerals in coals of the Mist Mountain Formation, Elk Valley coalfield, British Columbia, Canada. International Journal of Coal Geology 33, 43–59. Morga, R., 2010. Chemical structure of semifusinite and fusinite of steam and coking coal from the Upper Silesian Coal Basin (Poland) and its changes during heating as inferred from micro-FTIR analysis. International Journal of Coal Geology 84, 1–15. Morga, R., in press. Micro-Raman spectroscopy of carbonized semifusinite and fusinite. International Journal of Coal Geology 87, 253–267. Muller, D.J., 2008. AFM: a nanotool in membrane biology. Biochemistry 47, 7986–7998. Pandolfo, A.G., Johns, R.B., Dyrkacz, G.R., Buchanan, A.S., 1988. Separation and preliminary characterization of high-purity maceral group fractions from an Australian bituminous coal. Energy & Fuels 2, 657–662. Peng, X.L., Barber, Z.H., Clyne, T.W., 2001. Surface roughness of diamond-like carbon films prepared using various techniques. Surface and Coatings Technology 138, 23–32. Polish Standard (Polska Norma) PN-G-97002, 1982. Węgiel kamienny — Typy. (in Polish) . Rantitsch, G., Grogger, W., Teichert, Ch., Ebner, F., Hofer, Ch., Maurer, E.-M., Schaffer, B., Toth, M., 2004. Conversion of carbonaceous material to graphite within the Greywacke Zone of the Eastern Alps. International Journal of Earth Sciences 93, 959–973. Sun, Q., Li, W., Chen, H., Li, B., 2003. The variation of structural characteristics of macerals during pyrolysis. Fuel 82, 669–676. Taylor, G.H., Teichmüller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, S., 1998. Organic Petrology. Gebruder Borntraeger, Berlin – Stuttgart. Vasallo, A.M., Liu, Y.L., Pang, L.S.K., Wilson, M.A., 1991. Infrared spectroscopy of coal maceral concentrates at elevated temperatures. Fuel 70, 635–639. Wang, J., Du, J., Chang, L., Xie, K., 2010. Study on the structure and pyrolysis characteristics of Chinese western coals. Fuel Processing Technology 91, 430–433. White, A., Davies, M.R., Jones, S., 1989. Reactivity and characterization of coal maceral concentrates. Fuel 68, 511–519. Xie, K.-C., Zhang, Y.-F., Li, C.-Z., Ling, D.-Q., 1991. Pyrolysis characteristics of macerals separated from a single coal and their artificial mixture. Fuel 70, 474–479. Yang, Q., 1994. Study of coal structure using STM and AFM. Kexue Tongbao (Chinese Science Bulletin) 39, 941–944. Yumura, M., Ohshima, S., Kuriki, Y., Ikazaki, F., Suzuki, Y., 1993. Atomic force microscopy observations of coals. Proceedings of International Conference on Coal Science, 1, pp. 394–397. Zhao, Y., Hu, H., Jin, L., He, X., Wu, B., 2010. Pyrolysis behavior of vitrinite and inertinite from Chinese Pingshuo coal by TG-MS and in a fixed bed reactor. Fuel Processing Technology 92, 780–786.
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International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo
Changes of semifusinite and fusinite surface roughness during heat treatment determined by atomic force microscopy Rafal Morga Institute of Applied Geology, Silesian University of Technology, Akademicka 2, 44-100 Gliwice, Poland
a r t i c l e
i n f o
Article history: Received 28 May 2011 Received in revised form 11 October 2011 Accepted 14 October 2011 Available online 21 October 2011 Keywords: Coal Inertinite Semifusinite Fusinite Atomic force microscopy Heating experiment
a b s t r a c t This study describes changes of surface roughness of semifusinite and fusinite as an indicator of structural alteration resulting from heat treatment at 400–1200 °C. Surface roughness has been investigated by atomic force microscopy of inertinite concentrates from coking coals (vitrinite reflectance Rr = 1.07%–1.41%) from the Upper Silesian Coal Basin of Poland (Namurian C — Westphalian A). Unheated fusinite has a higher surface roughness than semifusinite from the same coal. The average surface roughness of semifusinite decreases with the Swelling Index of the parent coal. Heating increases the surface roughness of semifusinite and fusinite. Increase in the average surface roughness is stronger for semifusinite than fusinite and correlates to increasing reflectance of these macerals. The surface roughness of semifusinite correlates to the relative mass loss of the inertinite concentrates during heating. After heating to 1200 °C fusinite has a lower average surface roughness than semifusinite from the same coal. Consequently, average surface roughness can be used as a measure of structural alteration of inertinite group macerals during heat treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Atomic force microscopy (AFM) is a promising tool for imaging and characterizing materials at the nanoscale. It is frequently used in physics, chemistry and biology to characterize surface properties of nanomaterials, polymers and metals and as well as tissue imaging (Baker et al., 2000; Dokou et al., 2000; Franz and Puech, 2008; Hansma et al., 1997; Jastrzebska et al., 2010; Muller, 2008; Peng et al., 2001, among many others). AFM was also applied in coal studies (Lawrie et al., 1997; Yang, 1994; Yumura et al., 1993). Lawrie et al. (1997) observed that fusinite has higher surface roughness than vitrinite. Bruening and Cohen (2005) studied coal oxidation effects and found AFM to have a great potential to yield useful information on physical properties of coal macerals. Martin et al. (2005) with the use of AFM revealed different types of coke morphology at the nanometer scale. Rantitsch et al. (2004) applied this method to investigate carbonaceous material from natural graphite. Roughness of super fine coal particles was investigated by Liu et al. (2010). Atomic force microscopy was also used to study nano-ashes formed during high temperature combustion of pulverized coals (Carbone et al., 2011). Different roughness parameters, their definitions and significance were described by Gadelmawla et al. (2002). Inertinite is the most aromatic and most thermally stable of all the maceral groups (Pandolfo et al., 1988; Sun et al., 2003; Vasallo et al., 1991; Wang et al., 2010; White et al., 1989; Xie et al., 1991). In this
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group, the maceral fusinite is more aromatic than semifusinite from the same coal (Morga, 2010) and has lower H/C ratio (Blanc et al., 1991; Diessel, 1992). On the other hand, the amount of mobile hydrogen is higher in semifusinite than in fusinite (Maroto-Valer et al., 1998). The diameter of coherent domains (crystallites) (La) in the studied macerals is on average ca. 1.0–1.4 nm (Malumbazo et al., 2010; Xie et al., 1991), and it is higher in fusinite than in semifusinite (Morga, in press). Mastalerz and Bustin (1993, 1996, 1997) showed that both semifusinite and fusinite can have very varied chemistry. Mastalerz and Bustin (1993, 1997) noticed a very weak correlation between carbon content and reflectance of semifusinite. Machnikowska et al. (2002) found, by the diffuse reflectance infrared fourier transform (DRIFT) spectroscopy, a strong correlation between the condensation of aromatic rings (the CHar/C=Car ratio) and fusinite reflectance. Guo and Bustin (1998) and Bustin and Guo (1999) examined semifusinite, fusinite and modern charcoals by Fourier transform infrared spectroscopy (FTIR) and observed that the abrupt changes in chemical composition of charcoals are coincident with jumps in their reflectance. The distribution of reflectance values and FTIR spectral characteristics of inertinite is consistent with a model of charring that incorporates discrete compositional jumps. Semifusinite is a maceral, which in part is reactive in thermal processing of coal. The lower the reflectance, the higher is the reactivity. Low reflecting semifusinite is wholly fusible, and medium reflectingpartially fusible (ICCP, 2001; Taylor et al., 1998). Fusinite is a nonreactive maceral and does not fuse. The difference in behaviour of fusinite and semifusinite during heat treatment is determined by the structural and chemical properties of these macerals before heating.
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Pyrolysis to the temperature of 600 °C causes primary devolatilization resulting in release of H2O, CO2, hydrocarbons and hydrogen from aliphatic groups and increase in aromaticity of inertinite (Morga, 2010; Pandolfo et al., 1988; Sun et al., 2003; Vasallo et al., 1991; Wang et al., 2010; White et al., 1989; Xie et al., 1991; Zhao et al., 2010). It was observed that at 400–600 °C slight re-ordering of semifusinite structure occurs, which is reflected by reflectance increase (Morga, in press). Alteration of fusinite structure requires high activation energy and starts at higher temperature. As inferred from the micro-FTIR examination, the most significant alteration of the chemical structure of semifusinite occurs at 600 and 800 °C or 800 and 1000 °C, and that of fusinite at 600 and 800 °C (Morga, 2010). The secondary devolatilization at high temperatures (600–1000 °C) is connected mainly with the decomposition of stable oxygen containing functional groups as well with H2 yield due to condensation of aromatic and hydroaromatic structures (Morga, 2010; Wang et al., 2010; Zhao et al., 2010). As revealed from micro-Raman spectroscopy examination, the macromolecular network of semifusinite and fusinite rebuilds at 800 °C. The diameter of coherent domains (La) increases, which is reflected by the reflectance jump found for both macerals (in press; Komorek and Morga, 2007, Morga, 2010). Increase in the La of inertinite and chars from inertinite-rich coals with increasing temperature was also demonstrated by Xie et al. (1991), Wang et al. (2010) and Chabalala et al. (2010) by X-ray diffractometry (XRD). At 1000–1200 °C an increase in the structural organization, accompanied by an increasing reflectance is observed in semifusinite. When heated at 1200 °C, semifusinite has larger coherent domains and they are more ordered than in fusinite, which results in higher reflectance value (Morga, in press). Increase in structural organization of chars from inertinite-rich coals upon heating, was also observed by Malumbazo et al. (2010). Increase in aromaticity and condensation resulting from heating at 400–1200 °C is much higher for semifusinite than fusinite (Morga, 2010). The purpose of this study is to describe changes of surface roughness of semifusinite and fusinite as an indicator of structural alteration of these macerals resulting from experimental heat treatment at 400–1200 °C. 2. Experimental Inertinite concentrates were prepared from four samples of coking coals from seams 405/1 (Westphalian A — sample 1), 409/4 (Namurian C — samples 2 and 3) and 502/1 (Namurian B — sample 4) of the Upper Silesian Coal Basin of Poland. The coals belong to technological types 35.1 to 37.2, according to the Polish Standard (Polish Standard, Polska Norma PN-G-97002, 1982), with a vitrinite reflectance (Rr) ranging from 1.07% to 1.41%. Selected properties of these coals are summarized in Table 1. The concentrates were separated in heavy liquids as described in Morga (2010). About 300 mg of each concentrate were heated in a Carbolite pipe oven at temperature of 400, 600, 800, 1000 and 1200 °C for one hour in an argon atmosphere. Thereafter, polished grain mounts were prepared from the concentrates for the AFM and microscopic examination. Atomic force microscopy examination was carried out on 20 grains of semifusinite and 10–15 grains of fusinite, randomly chosen in each sample, using a Solver P47-Pro atomic force microscope (NT-MDT) in the the semi-contact (tapping) mode. All measurements were
performed at room temperature using rectangular silicon cantilevers of a nominal elastic constant of 12 N/m and a typical resonance frequency of 240 kHz. The free oscillation amplitude was varied in the range of 10–25 nm to get stable images in a measurement area of 1 × 1 μm, similar to that used by Lawrie et al. (1997) and the scanning speed during the acquisitions ranged from 500 to 1000 nm/s. The image was corrected by subtracting the 2nd order trend surface to remove a cylindrical curvature, caused by piezo-tube scanner movement. Measurements of mean semifusinite and fusinite reflectance were carried out under reflected light in immersion oil (no = 1.518 at 23 °C) according to ISO 7404-5/1994, using an Axioskop Zeiss microscope. Relative mass loss of the inertinite concentrate during heating was determined by weighting. The following parameters were calculated: average surface roughness (Ra), root mean square roughness (Rq), surface skewness (Rsk) and coefficient of kurtosis (Rku). Average surfece roughness (Ra) is the most frequently used roughness parameter. It defines average value of the surface roughness within the area analyzed. It is calculated according to the formula: y x X 1 X jzði; jÞ−zmean j Nx Ny i¼1 j¼1
N
Ra ¼
N
ð1Þ
where: y x X 1 X z Nx Ny i¼1 j¼1 ij
N
zmean ¼
N
ð2Þ
and Nx and Ny is a number of points along the X and Y axes. Another roughness parameter is the root mean square roughness (Rq), characterizing the variability of the surface topography. It is determined by: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u Ny Nx X u 1 X zði; jÞ−zmean Þ3 Rq ¼ t Nx Ny i¼1 j¼1
ð3Þ
Surface skewness (Rsk) gives the information on the symmetry of the distribution of the surface data (Eq. (4)). If the value of the parameter equals to zero the distribution is symmetrical. If it is different from zero, the distribution is unsymmetrical. The asymmetry is positive, if the distribution is excessive on the right, which reflects occurence of many high peaks on the surface. The asymmetry is negative, if the distribution is excessive on the left, indicating many deep depressions of the surface. y x X X 1 3 zði; jÞ−zmean Þ 3 Nx Ny Rq i¼1 j¼1
N
Rsk ¼
N
ð4Þ
Surface kurtosis (Rku), characterizes the sample surface excess (Eq. (5)). Kurtosis for normal distribution is equal to 0. Positive kurtosis occurs when the surface data are more concentrated at the mean value than in normal distribution and negative, when they are less concentrated. y x X X 1 4 zði; jÞ−zmean Þ −3 4 Nx Ny Rq i¼1 j¼1
N
Rku ¼ Table 1 Selected properties of the parent coals used in the study.
219
N
Sample
Rr %
Vdaf %
SI
Vitrinite %
Liptinite %
Inertinite %
Mineral matter %
3. Results
1 2 3 4
1.07 1.24 1.25 1.41
24.75 21.96 23.14 19.76
8 7.5 9 5
65 62 69 56
2 2 1 1
30 33 28 38
3 3 2 5
3.1. Petrographic characteristics of the inertinite concentrates
ð5Þ
The concentrates contain 80% to 83% inertinite (Table 2). The remaining part consists of vitrinite (14–15%) and small amounts
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R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Table 2 Petrographic properties of the inertinite concentrates used in the study. Concentrate
Sample Sample Sample Sample
V %
1 2 3 4
L %
I %
– 1 – –
15 15 15 14
83 81 80 82
MM %
Sf %
2 3 5 4
F %
54 36 50 47
Id* %
12 20 14 6
Ma %
28 28 24 35
Mi %
4 2 2 2
Fu %
1 2 3 8
Semifusinite
1 12 7 2
Fusinite
Rr %
sr %
Rr %
sr %
2.14 2.10 2.09 1.97
0.42 0.29 0.31 0.24
3.20 3.14 3.09 2.85
0.40 0.36 0.43 0.45
Abbreviations: V — vitrinite, L — liptinite, I — inertinite, MM — mineral matter, Sf — semifusinite, F — fusinite, Id — inertodetrinite, Ma — macrinite, Mi — micrinite, Fu — funginite. *Includes inertinite fragments originated due to crushing of coal (values for the inertinite group macerals are the proportion of total inertinite content); Rsf, ssf — mean reflectance of semifusinite and its standard deviation, Rf, sf — mean reflectance of fusinite and its standard deviation.
of mineral matter (pyrite and clay minerals). The most abundant macerals are semifusinite (36–54% of total inertinite content) and inertodetrinite (24–35%). High inertodetrinite content is likely due
a)
to crushing the coal during preparation of the concentrates. Fusinite proportion is 6–20%. Other macerals of the inertinite group occur in smaller quantities.
c) 0.16
0.1
0.08
Relative frequency
Relative frequency
0.12
0.06
0.04
0.08
0.04 0.02
0
0 1.2
1.6
2
2.4
2.8
3.2
3.6
4
1.2
4.4
1.6
2
2.4
2.8
3.2
3.6
4
4.4
Rr [%]
Rr [%]
d)
b)
0.16
0.12
0.08
Relative frequency
Relative frequency
0.12
0.04
0.08
0.04
0
0 1.2
1.6
2
2.4
2.8
Rr [%]
3.2
3.6
4
4.4
1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
3
3.2 3.4 3.6 3.8 4
Rr [%]
Fig. 1. Reflectograms of semifusinite and fusinite in the examined concentrates: a) — sample 1; b) — sample 2; c) — sample 3; and d) — sample 4.
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Semifusinite in concentrates before heating has mean reflectance from 1.97% (sr = 0.25%) in sample 4 to 2.14% (sr = 0.25%) in sample 1 (Table 2). Fig. 1 shows reflectograms of semifusinite and fusinite. They are multimodal due to the occurrence of different genetic forms of semifusinite and fusinite. In the examined concentrates semifusinite (Fig. 2a) is light grey to white. In low-reflecting grains the cellular structure is usually weakly preserved. Cell lumens are small, circular or oval and often swollen. Relief does not change within an individual grain. Brighter, medium and high-reflecting semifusinite, generally, has better retained cellular structure. It has thin and elongated, oval or lenticular cell lumens. In part of such grains, however, swollen cell lumens are also observed. Relief is usually variable within an individual grain. Fusinite occurs in much smaller quantities than semifusinite. In concentrates its mean reflectance ranges from 2.85% (sr = 0.45%) in sample 4 to 3.20% (sr = 0.40%) in sample 1 (Table 2). Fusinite (Fig. 3a) is white, sometimes with a yellowish tint, and has a well preserved structure. Cell walls are either
221
thick or thin, frequently with variable relief. Cell lumens are usually large. They can be circular, oval, lenticular or elongated, depending on section. Rarely, bogen structure can be seen. 3.2. Surface roughness Fig. 4 shows the change of semifusinite surface topography (scan area 1 × 1 μm) with increasing temperature. In the samples before heating semifusinite has an average surface roughness (Ra) between 0.77 nm (sample 3) and 2.51 nm (sample 4) (Fig. 5a; Table 3). The root mean square roughness (Rq) ranges from 1.05 to 3.22 nm. Surface skewness (Rsk) is usually ~0.20 which indicates slightly unsymmetrical positive distribution of the surface points. Kurtosis (Rku) is close to 0 (samples 1 and 4), which means normal distribution. The average surface roughness of semifusinite tends to decrease with the increasing value of the Swelling Index of a parent coal (Fig. 6). Fusinite in all examined samples before heating has a higher
Fig. 2. Semifusinite from sample 2 before heating (a) and after heat treatment at: 400 °C (b), 600 °C (c), 800 °C (d), 1000 °C (e) and 1200 °C (f).
222
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Fig. 3. Fusinite from sample 2 before heating (a) and after heat treatment at: 400 °C (b), 600 °C (c), 800 °C (d), 1000 °C (e) and 1200 °C (f).
average surface roughness and root mean square roughness than semifusinite from the same coal (Fig. 5b; Table 4). In three of four examined samples it has lower values of the surface skewness and kurtosis coefficient. Surface skewness determined for fusinite is close to 0 (sample 2, excepting) which means almost symmetrical distribution of the surface points. Kurtosis is generally close to 1, reflecting leptokurtic distribution. When the concentrates are heated at 400 °C the Ra of semifusinite in samples 1 and 3 increases, while in samples 2 and 4 does not change (Fig. 5a; Table 3). These increases are found for the samples characterized by the lowest values of the average surface roughness before heating, and, at the same time, also the highest SI values of the parent coals. When the concentrates are heated at 600 °C, strong increase of the Ra value of semifusinite is observed for all samples. Another increase is found at 800 °C (samples 1, 2 and 4) or 1000 °C (sample 3).
The Ra value of fusinite increases first by heating the concentrates at 600 °C (samples 1 and 4) or 800 °C (samples 2 and 3) (Fig. 5b; Table 4). In all samples a further increase of the surface roughness of fusinite is found at 1000 °C, and in samples 1 and 2 also at 1200 °C. Skewness of semifusinite surface roughness changes from slightly positive for slightly negative values, with increasing temperature but, generally, stays close to 0 (Table 3). Surface skewness values for fusinite are also close to 0, with the exception of samples 3 and 4, when heated at 400 °C (Table 4). This means that distributions of surface points are for both macerals symmetrical. Kurtosis both for semifusinite and fusinite, on general, does not change significantly during the heat treatment. It is close to 0 or stays slightly above this value. After heating the concentrates at 1200 °C fusinite has lower average roughness than semifusinite (Tables 3 and 4). Surface skewness for both macerals is similar and obtains values close to 0. There is no difference in the kurtosis between the two macerals, either.
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
Ra [nm]
a
30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 0
223
Sf 1 Sf 2 Sf 3 Sf 4
200
400
600
800
1000
1200
1400
T [oC]
b
26 24 22 20 18
Ra [nm]
16 14 12 10 8 6 4
F1 F2 F3 F4
2 0
0
200
400
600
800
1000
1200
1400
T [oC] Fig. 5. The average roughness (Ra) of semifusinite (a) and fusinite (b) before and after heat treatment at 400–1200 °C.
or surpasses that of fusinite. A considerable increase is also found for semifusinite at 1200 °C. Finally, reflectance of semifusinite is 8.31–8.83%, while that of fusinite 6.57–7.49%. Semifusinite having the lowest reflectance before heating (sample 4) attains the highest Rr value. 3.4. Relative mass loss The relative mass loss of the inertinite concentrates during heating can be well aproximated by hyperbolic curve. At 400 °C it is very limited (Fig. 8). At 600 °C considerable change is observed, which is due to the first phase of degassing. After that, the rate of the mass loss decreases. After heating the concentrates at 1200 °C the value of the parameter ranges from 18.13% (sample 3) to 20.6% (sample 1). 4. Discussion Fig. 4. Changes of semifusinite surface topography in sample 2 with increasing heat treatment temperature: a) — before heating, b) — 400 °C, c) — 600 °C, d) — 800 °C, e) — 1000 °C, and f) — 1200 °C.
3.3. Mean reflectance Mean reflectance (Rr) of semifusinite and fusinite changes during heat treatment in similar manner for all examined samples. It rises first at 600 °C, and at 800 °C a significant increase in the Rr value is observed (Fig. 7a–b), when semifusinite reflectance is very close to
The average surface roughness of fusinite estimated in this study is lower than the values presented by Lawrie et al. (1997). Changes in the average surface roughness of semifusinite and fusinite upon heating are closely related to physical–chemical alteration of these macerals. This is confirmed by strong correlation between the average roughness and reflectance both for semifusinite (Fig. 9a) and fusinite (Fig. 9b) in the examined temperature range. The correlation coefficients are between 0.90 and 0.97, and between 0.96 and 0.98, respectively, both at significant p b 0.05. There is also a strong
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Table 3 Surface roughness parameters of semifusinite before and after heat treatment.
Table 4 Surface roughness parameters of fusinite before and after heat treatment.
Temperature °C
Average roughness nm
Root mean square nm
Surface skewness
Coefficient of kurtosis
Temperature °C
Average roughness nm
Root mean square nm
Surface skewness
Coefficient of kurtosis
Sample 1 25 400 600 800 1000 1200
1.85 ± 1.14 2.41 ± 0.87 11.75 ± 3.70 15.31 ± 8.41 16.67 ± 8.65 18.14 ± 9.06
2.32 ± 1.34 3.10 ± 1.11 15.83 ± 4.98 19.58 ± 10.73 21.82 ± 11.00 23.11 ± 11.52
0.22 0.21 − 0.17 − 0.05 0.06 − 0.03
− 0.19 0.48 − 0.02 0.08 0.68 0.44
Sample 1 25 400 600 800 1000 1200
2.72 ± 1.35 2.15 ± 0.74 7.29 ± 2.43 10.69 ± 7.00 14.73 ± 8.57 16.56 ± 5.41
3.62 ± 1.81 2.73 ± 0.91 9.60 ± 3.54 14.05 ± 9.60 18.28 ± 10.31 21.17 ± 7.05
− 0.03 0.08 0.05 − 0.17 − 0.04 − 0.18
1.15 0.45 − 0.18 0.82 0.57 0.10
Sample 2 25 400 600 800 1000 1200
2.10 ± 1.25 2.04 ± 1.53 3.47 ± 2.34 13.36 ± 4.43 14.71 ± 6.45 16.33 ± 9.91
2.57 ± 1.44 2.60 ± 1.90 4.64 ± 3.10 17.57 ± 6.06 19.30 ± 8.12 20.56 ± 11.43
0.24 − 0.10 0.16 0.07 0.14 − 0.17
1.36 0.58 0.80 0.22 0.86 0.17
Sample 2 25 400 600 800 1000 1200
2.49 ± 2.02 2.56 ± 2.06 2.43 ± 1.28 9.61 ± 5.57 12.33 ± 4.21 14.02 ± 6.59
2.85 ± 2.49 3.14 ± 2.32 3.15 ± 1.62 12.46 ± 7.52 15.99 ± 5.46 18.01 ± 9.33
0.25 − 0.23 − 0.09 0.00 0.16 0.00
0.94 0.02 0.89 0.82 0.75 0.21
Sample 3 25 400 600 800 1000 1200
0.77 ± 0.38 1.90 ± 1.11 5.77 ± 2.13 6.87 ± 3.73 15.34 ± 3.68 14.46 ± 6.57
1.05 ± 0.52 2.68 ± 1.66 8.80 ± 3.16 8.43 ± 4.61 19.24 ± 5.25 16.50 ± 7.83
0.17 1.04 0.34 − 0.04 − 0.08 − 0.07
1.88 2.95 0.94 − 0.22 0.40 0.30
Sample 3 25 400 600 800 1000 1200
1.64 ± 0.75 1.65 ± 1.00 1.83 ± 0.97 5.43 ± 1.07 10.28 ± 2.43 10.47 ± 6.04
2.15 ± 1.03 2.28 ± 1.55 2.47 ± 1.17 6.25 ± 2.36 12.71 ± 3.35 13.31 ± 6.89
− 0.06 0.81 0.12 0.02 − 0.10 0.08
1.02 2.14 2.26 1.13 − 0.32 0.64
Sample 4 25 400 600 800 1000 1200
2.51 ± 0.73 2.59 ± 0.89 6.67 ± 2.78 10.39 ± 4.58 12.97 ± 6.38 13.96 ± 8.25
3.22 ± 1.16 3.18 ± 1.21 8.49 ± 3.62 13.31 ± 5.88 16.42 ± 8.16 17.08 ± 10.43
0.20 0.16 − 0.31 0.01 0.06 − 0.06
− 0.09 0.41 − 0.32 0.32 0.33 0.76
Sample 4 25 400 600 800 1000 1200
2.94 ± 1.02 2.76 ± 1.17 5.23 ± 3.94 7.59 ± 3.39 9.23 ± 3.72 9.01 ± 5.32
3.66 ± 1.21 2.96 ± 1.31 6.68 ± 4.74 9.73 ± 4.31 11.50 ± 4.43 11.10 ± 6.09
0.01 0.56 − 0.40 − 0.12 − 0.09 0.01
− 0.35 0.93 0.43 0.28 − 0.16 − 0.12
correlation between the average roughness of semifusinite and the relative mass loss of the inertinite concentrates (correlation coefficients between 0.89 and 0.99, at p b 0.05) (Fig. 10). Increase in the Ra of semifusinite at 400–600 °C (Fig. 5a, Table 3) is attributed to the partial release of volatiles (first phase of degassing) (Morga, 2010; Pandolfo et al., 1988; Sun et al., 2003; Vasallo et al., 1991; Wang et al., 2010; White et al., 1989; Xie et al., 1991; Zhao et al., 2010), which results in the intense mass loss of the concentrates (Fig. 8). Cell lumens become larger, and the cell walls thinner (Fig. 2b–c). Many small pores appear. More reactive (fusible) semifusinite grains soften and become rounded or melted. These changes are accompanied by a re-ordering of semifusinite structure and a reflectance increase (Komorek and Morga, 2007; Morga, in press). The increase found at 800 °C (samples 1, 2 and 4) or 1000 °C (sample 3) (Fig. 5a, Table 3) is a result of the second phase of degassing (Wang
3.5 3.0 2.5
Sample 4
Ra [nm]
Sample 2
2.0
Sample 1
1.5 1.0
Sample 3
ΔRa ¼ ðRa1200 −Ra25 Þ=Ra25
0.5 0.0 4.5
et al., 2010; Zhao et al., 2010), which causes further mass loss of the concentrates (Fig. 8). The surface of the grains becomes rough, and the porosity increases (Fig. 2d–e). This phase is also connected with an alteration of the chemical structure of semifusinite and rebuilding of its macromolecular network (Morga, 2010, in press), leading to strong reflectance growth (Komorek and Morga, 2007; Morga, 2010, in press). Heating semifusinite at 1200 °C causes further mass loss of the concentrates (Fig. 8). This shows, that the volatiles are still released from the chemical structure, which results in enlargement of cell lumens and pores (Fig. 2f) and increase in surface roughness in three of the examined samples (Fig. 5a, Table 3). The average surface roughness of fusinite at 600 °C (samples 1 and 4) or 800 °C (samples 2 and 3) (Fig. 5b; Table 4) increases together with devolatilization. The cell walls become thinner, and porosity emerges (Fig. 3b–d). The chemical–structural rebuilding at the same temperature range (Morga, 2010, in press) results in a reflectance increase. This initial change occurs at a higher heating temperature than in semifusinite, reflecting more rigid structural framework of fusinite. Fusinite contains less volatiles than semifusinite from the same coal. Change in the Ra value observed at 1000 °C and at 1200 °C (sample 1 and 2) (Fig. 5; Table 4) is a result of final devolatilization of that maceral, which causes that the cell lumens become larger and the surface tortous. Many small pores are observed (Fig. 3e–f). To evaluate the intensity of surface roughness changes during heating, the increase rate (ΔRa) was calculated. That was done according to the formula:
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
SI Fig. 6. The relationship between average roughness (Ra) of semifusinite before heating and the Swelling Index (SI) of a parent coal.
where: – Ra1200 is the average surface roughness after heating the concentrate at 1200 °C – Ra25 is the average surface roughness before heating.
R. Morga / International Journal of Coal Geology 88 (2011) 218–226
a
a
10
24 Sf 1: r = 0.8995, p = 0.0146 Sf 2: r = 0.9606, p = 0.0023 Sf 3: r = 0.9160, p = 0.0103 Sf 4: r = 0.9679, p = 0.0015
22
9
20 18
7
16
Ra [nm]
8
Rr [%]
6 5
225
14 12 10 8
4
6 3
4 Sf 1 Sf 2 Sf 3 Sf 4
2 1 0
200
400
600
800
1000
1200
Sf 1 Sf 2 Sf 3 Sf 4
2 0 1
1400
2
3
4
5
T [ C]
b
b9
7
8
9
10
20 F 1: r = 0.9595, p = 0.0024 F 2: r = 0.9834, p = 0.0004 F 3: r = 0.9684, p = 0.0015 F 4: r = 0.9789, p = 0.0007
18 8
16 14
Ra [nm]
7
Rr [%]
6
Rr [%]
o
6 5
12 10 8 6
4 4 3 2 0
200
400
600
800
1000
1200
F1 F2 F3 F4
2
F1 F2 F3 F4
0 2
1400
3
4
5
T [ C] Fig. 7. The mean reflectance (Rr) of semifusinite (a) and fusinite (b) before and after heat treatment at 400–1200 °C.
7
8
Fig. 9. The relationship between the average roughness (Ra) and reflectance (Rr) of semifusinite (a) and fusinite (b) before and after heat treatment at 400–1200 °C.
increase was observed for semifusinite from distinctly lower SI (Fig. 11). For fusinite ΔRa attains the values between 266% (sample 4) and 538% (sample 3). Thus, there is the same relation here as in the case of semifusinite. Higher rate of roughness increase during heat treatment observed for semifusinite corresponds with higher reactivity of this maceral, compared with fusinite.
22
22
20
20
18
18
Sf 1: r = 0.9882, p = 0.0015 Sf 2: r = 0.8999, p = 0.0374 Sf 3: r = 0.8925, p = 0.0416 Sf 4: r = 0.9754, p = 0.0046
16
16
14 14
Ra [nm]
Relative mass loss [%]
ΔRa determined for semifusinite substantially vary, from 456% (sample 4) to 1778% (sample 3), which indicates varied reactivity of this maceral. The highest rate of the average roughness increase was found for semifusinite coming from coal characterized by the highest value of the Swelling Index, while the lowest rate of Ra
6
Rr [%]
o
12
12 10
10
8
8
6 4
6 Sample 1 Sample 2 Sample 3 Sample 4
4 2
400
600
800
1000
1200
T [oC] Fig. 8. The relative mass loss of the inertinite concentrates during heat treatment.
Sf 1 Sf 2 Sf 3 Sf 4
2 0
0
2
4
6
8
10
12
14
16
18
20
Relative mass loss [%] Fig. 10. The relationship between the average roughness (Ra) of semifusinite and the relative mass loss of the inertinite concentrates during heat treatment.
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2000 Sample 3
1800
Ra [%]
1600 1400 1200 1000
Sample 1
800
Sample 2
600 Sample 4
400 4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
SI Fig. 11. The relationship between the rate of average roughness increase (ΔRa) determined for semifusinite and the Swelling Index (SI) of a parent coal.
5. Conclusions Atomic force microscopy is a useful tool for examination of surface properties of coal macerals. The study presents the first data on the evolution of surface roughness of semifusinite and fusinite during experimental heating. The results of the study broaden the general knowledge about properties of semifusinite and fusinite and their behaviour under the influence of heating. Fusinite has a higher average surface roughness and root mean square roughness than semifusinite from the same coal. The average surface roughness of semifusinite decreases with the increase in the Swelling Index. Experimental heating causes an increase in average roughness and root mean square roughness of both semifusinite and fusinite. The rate of this increase is higher for semifusinite than for fusinite, which corresponds to higher thermal reactivity of the former maceral. Changes of the average roughness of semifusinite and fusinite during heating are strongly correlated to the reflectance increase of these macerals. There is a strong correlation between the average roughness of semifusinite and the relative mass loss of the inertinite concentrates. After heating the concentrates at 1200 °C fusinite has a lower average roughness than semifusinite from the same coal. The average roughness can be used as a measure of structural alteration of inertinite group macerals during heat treatment. Acknowledgments The study was financially supported by the Ministry of Science and Higher Education of Poland grant no. 3631/B/T02/2008/35. References Baker, A.A., Helbert, W., Sugiyama, J., Miles, M.J., 2000. New insight into cellulose structure by atomic force microscopy shows the Iα crystal phase at near-atomic resolution. Biophysical Journal 79, 1135–1139. Blanc, P., Valisolalao, J., Albrecht, P., Kohut, J.P., Muller, J.F., Duchene, J.M., 1991. Comparative geochemical study of three maceral groups from a high-volatile bituminous coal. Energy & Fuels 5, 875–884. Bruening, F.A., Cohen, A.D., 2005. Measuring surface properties and oxidation of coal macerals using the atomic force microscope. International Journal of Coal Geology 63, 195–204. Bustin, R.M., Guo, Y., 1999. Abrupt changes (jumps) in reflectance values and chemical compositions of artificial charcoals and inertinite in coals. International Journal of Coal Geology 38, 237–260. Carbone, F., Beretta, F., D'Anna, A., 2011. A flat premixed flame reactor to study nanoash formation during high temperature pulverized coal combustion and oxygen firing. Fuel 90, 369–375. Chabalala, V.P., Wagner, N., Potgieter-Vermaak, S., 2010. Investigation into the evolution of char structure using Raman spectroscopy in conjunction with coal petrography; Part 1. Fuel Processing Technology 92, 750–756.
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